Secondary battery

Abstract

Provided are a secondary battery excellent in charge / discharge characteristics and impact resistance and a method for manufacturing the same. The secondary battery includes a positive electrode, a negative electrode, and an exterior body that houses the positive electrode and the negative electrode, wherein: on one side of the exterior body, the positive electrode and the exterior body are connected at at least two locations and the negative electrode and the exterior body are connected at at least two locations; the positive electrode includes a positive electrode active material layer; the positive electrode active material layer contains lithium cobalt oxide; the lithium cobalt oxide has, in a surface layer part, magnesium, titanium, aluminum, and nickel; the surface layer part is a region within 50 nm from the surface of the lithium cobalt oxide; and when STEM-EDX line analysis is performed in the depth direction of the surface layer part, the aluminum has a peak closer to the inside of the lithium cobalt oxide than the magnesium.

Description

secondary battery

[0001] One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, or a manufacturing method thereof.

[0002] In this specification, the term "electronic device" refers to any device having a power storage device, and includes electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like.

[0003] In this specification, the term "energy storage device" refers to elements and devices in general that have a power storage function, and includes, for example, energy storage devices such as lithium ion batteries (also called secondary batteries), lithium ion capacitors, and electric double layer capacitors.

[0004] Development of various secondary battery energy storage devices, such as lithium-ion batteries, lithium-ion capacitors, and air batteries, has been actively pursued. Lithium-ion batteries, which have high power output and high energy density, are particularly well-suited for use in portable information terminals (PDAs) such as mobile phones, smartphones, tablets, and laptop computers, portable music players, digital cameras, medical devices, clean-energy vehicles (e.g., hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs)), agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, ships, submarines, aircraft, rockets, satellites, space probes, planetary probes, and spacecraft. Demand for lithium-ion batteries has thus rapidly expanded in tandem with the development of the semiconductor industry, making them an essential rechargeable energy source in today's information-driven society.

[0005] In recent years, wearable devices have been actively developed. Because wearable devices are worn, they are preferably curved to fit the curves of the body or to bend in accordance with the movement of the body. Therefore, it is preferable that the secondary battery installed in a wearable device is flexible, just like the display or other housing.

[0006] Furthermore, even in devices other than wearable devices, it is preferable for secondary batteries to be flexible, since being able to deform the secondary battery when it is installed can increase the efficiency of utilization of the internal space of the device.

[0007] As examples of flexible secondary batteries, Patent Documents 1 and 2 disclose electrochemical devices (e.g., secondary batteries, capacitors, etc.) that are covered with a metal laminate and have a structure that allows them to be easily curved and maintained in a curved state.

[0008] There is also an increasing demand for electronic devices that can withstand harsh environments, specifically low temperatures (e.g., −50° C.) or high temperatures (e.g., 100° C.), and are intended for use over a wide range of operating temperatures. Examples include observation equipment around the Antarctic or Arctic, seismometers installed around the craters of active volcanoes, artificial satellites orbiting the Earth, and probes intended for use in investigating other planets.

[0009] It is also desirable for the seismometer or probe to be highly resistant to shocks such as those caused by being dropped. It would be convenient if the seismometer could be dropped from an unmanned aircraft and installed near a crater where it would be dangerous for humans to carry the measuring device. There are also plans to drop a penetrator from a satellite orbiting the planet to install a seismometer from the planet's surface to its interior. A penetrator is an exploration probe equipped with a dual-axis seismometer.

[0010] Conventionally, 18650 type cylindrical can batteries have been used as power sources for such electronic devices. Conventionally, primary batteries have been used for one-time use, and are disposable after a short period of use and function as a power source.

[0011] However, when considering long-term measurements or weight restrictions during transportation, it is desirable to use secondary batteries that can be charged using solar cells. It is also desirable to be able to collect the measuring equipment after installation and reuse it.

[0012] Secondary batteries that are strong and bend even when an external force is applied are disclosed in Patent Documents 3 and 4.

[0013] Furthermore, there is a high demand for secondary batteries with a large discharge capacity per unit weight and excellent cycle characteristics. To meet these demands, improvements to the positive electrode active materials of the positive electrodes of secondary batteries have been actively pursued (e.g., Patent Documents 5 and 6). Research on the crystalline structure of positive electrode active materials has also been conducted (Non-Patent Documents 1 to 3).

[0014] X-ray diffraction (XRD) is one of the techniques used to analyze the crystal structure of positive electrode active materials. XRD data can be analyzed using the Inorganic Crystal Structure Database (ICSD) introduced in Non-Patent Document 4. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 5 can be referenced from the ICSD. For Rietveld analysis, the analysis program RIETAN-FP (Non-Patent Document 6) can be used. VESTA (Non-Patent Document 7) can be used as software for drawing crystal structures.

[0015] Also, image processing software such as ImageJ (Non-Patent Documents 8 to 10) is known. By using this software, for example, the shape of the positive electrode active material can be analyzed.

[0016] Microelectron beam diffraction is also effective for identifying the crystalline structure of the positive electrode active material, particularly the crystalline structure of the surface layer. For example, the analysis program ReciPro (Non-Patent Document 11) can be used to analyze the electron beam diffraction pattern.

[0017] JP 2004-241250 A, JP 2018-6336 A, JP 2015-233004 A, JP 2016-139609 A, JP 2018-206747 A, JP 2022-070247 A

[0018] Toyoki Okumura et al.,“Correlation of lithium ion distribution and X−ray absorption near−edge structure in O3−and O2−lithium cobalt oxides from first−principle calculation”,Journal of Materials Chemistry,2012,22,p.17340−17348T.Motohashi,et al.,“Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”,Physical Review B,80(16);165114Zhaohui Chen et al.,“Staging Phase Transitions in Li▲x▼CoO▲2▼”,Journal of The Electrochemical Society,2002,149(12)A1604−A1609A.Belsky,et al.,“New developments in the Inorganic Crystal Structure Database(ICSD):accessibility in support of materials research and design”,Acta Cryst.,(2002)B58 364−369.J.Akimoto,Y.Gotoh,Y.Oosawa“Synthesis and structure refinement of LiCoO▲2▼ single crystals”Journal of Solid State Chemistry(1998)141,p.298−302.F.Izumi and K.Momma,,“Three−Dimensional Visualization in Powder Diffraction”Solid State Phenom.130,15−20(2007)K.Momma and F.Izumi,”VESTA 3 for three−dimensional visualization of crystal,Volumetric and Morphology Data” J. Appl. Cryst. (2011). 44, 1272-1276 Rasband, W. S., ImageJ, U.S. National Institutes of Health,Bethesda,Maryland,USA,http: / / rsb.info.nih.gov / ij / ,1997−2012.Schneider,C.A.,Rasband,W.S.,Eliceiri,K.W.”NIH Image to ImageJ: 25 years of image analysis”.Nature Methods 9, 671-675, 2012. Abramoff, M. D. , Magelhaes, P. J. , Ram, S. J. "Image Processing with ImageJ". Biophotonics International, volume 11, issue 7, pp. 36-42, 2004. Seto, Y. &Ohtsuka, M. ”ReciPro: free and open-source multipurpose crystallographic software integrating a crystal model database and viewer, diffraction and microscopy simulators, and diffraction data analysis tools” (2022). J. Appl. Cryst. 55.,

[0019] One type of astronomical observation device is a rocket-shaped probe called a penetrator, which performs autonomous observations. The secondary batteries installed in penetrators are required to have high impact resistance. While it is possible to enclose the secondary batteries with thick metal plates to provide impact resistance, this would significantly increase the total weight of the penetrator.

[0020] Conventional 18650-type cylindrical lithium-ion batteries are constructed by filling a cylindrical can with electrolyte and connecting the wound body inside the can to external terminals. Because a large amount of electrolyte is required to fill the can, there is a risk that the wound body may shift position due to impact. Temperature changes also pose a risk of the electrolyte expanding or gasifying. Therefore, secondary batteries using conventional cylindrical cans are vulnerable to drop impacts and are at risk of leakage.

[0021] Furthermore, because all-solid-state batteries are at least partially made of ceramics, they are brittle when subjected to impact, and if they crack, they lose conductivity and may become unusable.

[0022] Therefore, a laminated battery that uses a lightweight laminate film as an exterior body, rather than a cylindrical or coin-shaped battery, is preferable as a secondary battery with high impact resistance. However, even in a laminated battery, depending on the battery structure, problems such as electrodes being displaced inside the battery when subjected to a strong impact, poor adhesion between the metal foil and the active material layer, causing the active material layer to peel off from the current collector in the electrode inside the battery, and gaps between electrodes being partially widened inside the battery may occur.

[0023] In view of the above problems, an object of one embodiment of the present invention is to provide a secondary battery having high impact resistance.

[0024] Furthermore, secondary batteries have room for improvement in various aspects, such as discharge capacity, cycle characteristics, reliability, safety, and cost. For example, in order to suppress changes in the crystalline structure of the surface of the positive electrode active material, the surface of the positive electrode active material may be coated with an inert oxide, but this coating may inhibit the intercalation and deintercalation of lithium ions. If the intercalation and deintercalation of lithium ions is inhibited, there is a concern that the characteristics of the secondary battery may deteriorate, such as a decrease in rate characteristics and a decrease in charge / discharge capacity in a low-temperature environment.

[0025] In view of the above, an object of one embodiment of the present invention is to provide a positive electrode active material that can be used in a lithium-ion secondary battery and that promotes insertion and desorption of lithium ions. Another object is to provide a positive electrode active material in which a decrease in discharge capacity in a low-temperature environment is suppressed. Another object is to provide a positive electrode active material in which a decrease in discharge capacity during high-rate discharge is suppressed. Another object is to provide a positive electrode active material in which a decrease in discharge capacity during charge-discharge cycles is suppressed. Another object is to provide a positive electrode active material whose crystal structure is not easily destroyed even after repeated charge-discharge. Another object is to provide a positive electrode active material with a large discharge capacity.

[0026] Another object of one embodiment of the present invention is to provide a secondary battery which has high impact resistance and in which a decrease in discharge capacity in a low-temperature environment is suppressed.

[0027] Another object of one embodiment of the present invention is to provide a secondary battery with a novel structure. Specifically, an object of one embodiment of the present invention is to provide a secondary battery with a novel flexible structure. Another object of one embodiment of the present invention is to provide a novel power storage device, an electronic device equipped with the novel secondary battery, or the like.

[0028] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily need to solve all of these problems. Note that problems other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other problems from the description in the specification, drawings, claims, etc.

[0029] In order to solve the above problems, a secondary battery according to one embodiment of the present invention has a structure in which a positive electrode and an outer casing are connected to each other at two positions and a negative electrode and an outer casing are connected to each other at two positions.

[0030] In order to solve the above problems, a secondary battery according to one embodiment of the present invention includes, as a positive electrode active material, lithium cobalt oxide having magnesium, titanium, aluminum, and nickel in a surface layer portion.

[0031] One aspect of the present invention is a secondary battery comprising: a positive electrode; a negative electrode; and an exterior housing that houses the positive electrode and the negative electrode; the positive electrode and the exterior housing are connected at least two locations on one side of the exterior housing; and the negative electrode and the exterior housing are connected at least two locations on one side of the exterior housing; the positive electrode has a positive electrode active material layer that includes lithium cobalt oxide, and the lithium cobalt oxide has magnesium, titanium, aluminum, and nickel in a surface layer portion, the surface layer portion being a region within 50 nm from a surface of the lithium cobalt oxide; and when STEM-EDX ray analysis is performed in the depth direction of the surface layer portion, the aluminum has a peak closer to the interior of the lithium cobalt oxide than the magnesium.

[0032] In the above, the lithium cobalt oxide has a layered rock-salt crystal structure of space group R-3m, and the surface layer portion has a basal region having a surface parallel to the (001) plane of the crystal structure and an edge region having a surface in a direction intersecting the (001) plane, the edge region containing titanium, and when STEM-EDX ray analysis is performed in the depth direction of the surface layer portion, it is preferable that the edge region has a region where the distribution of magnesium and the distribution of titanium overlap.

[0033] In the above, the edge region preferably contains nickel, and when STEM-EDX ray analysis is performed in the depth direction of the surface layer portion, the edge region preferably has a region where the distribution of titanium and the distribution of nickel overlap.

[0034] Furthermore, in the above, when STEM-EDX ray analysis is performed in the depth direction of the surface layer portion, it is preferable that the edge region has a region in which the difference between the depth of the peak of the detected amount of magnesium and the depth of the peak of the detected amount of titanium is within 3 nm.

[0035] According to one embodiment of the present invention, a secondary battery having high impact resistance can be provided.

[0036] According to one embodiment of the present invention, a positive electrode active material that can be used in a lithium ion secondary battery and that promotes lithium ion insertion and desorption can be provided. Alternatively, a positive electrode active material in which a decrease in discharge capacity in a low-temperature environment is suppressed can be provided. Alternatively, according to one embodiment of the present invention, a positive electrode active material in which a decrease in discharge capacity during high-rate discharge is suppressed can be provided. Alternatively, according to one embodiment of the present invention, a positive electrode active material in which a decrease in discharge capacity during charge-discharge cycles is suppressed can be provided. Alternatively, according to one embodiment of the present invention, a positive electrode active material whose crystalline structure is not easily destroyed even after repeated charge-discharge can be provided. Alternatively, according to one embodiment of the present invention, a positive electrode active material with a large discharge capacity can be provided.

[0037] Alternatively, one embodiment of the present invention can provide a secondary battery which has high impact resistance and in which a decrease in discharge capacity in a low-temperature environment is suppressed.

[0038] According to one embodiment of the present invention, a secondary battery with a novel structure can be provided. More specifically, a secondary battery with a novel flexible structure can be provided. According to one embodiment of the present invention, a novel power storage device, an electronic device including the novel secondary battery, or the like can be provided.

[0039] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other effects from the description in the specification, drawings, claims, etc.

[0040] 1A and 1B are perspective views illustrating an example of the configuration of a secondary battery. FIG. 2A is a top view illustrating an example of the configuration of a secondary battery, and FIG. 2B is a cross-sectional view illustrating an example of the configuration of a secondary battery. FIGS. 3A, 3B, and 3C are cross-sectional views illustrating an example of the configuration of a secondary battery. FIGS. 4A, 4B, and 4C are top views illustrating an example of the configuration of a secondary battery. FIG. 5A is a top view illustrating an example of the configuration of a positive electrode, and FIGS. 5B and 5C are cross-sectional views illustrating an example of the configuration of a positive electrode. FIG. 6A is a top view illustrating an example of the configuration of a negative electrode, and FIGS. 6B and 6C are cross-sectional views illustrating an example of the configuration of a negative electrode. FIG. 7 is a diagram illustrating an example of a production flow of a coating layer. FIG. 8 is a diagram illustrating an example of a production flow of a secondary battery. FIGS. 9A1 and 9A2 are perspective views of a positive electrode, FIG. 9A3 is a cross-sectional view of the positive electrode, FIGS. 9B1 and 9B2 are perspective views of a pre-doping electrode, and FIG. 9B3 is a cross-sectional view of the pre-doping electrode. FIG. 10A1 is a perspective view of a positive electrode, FIG. 10A2 is a perspective view of a pre-doping electrode, FIG. 10A3 is a perspective view of a negative electrode, and FIG. 10B is a perspective view of a laminate and an exterior body. FIG. 11A is a side view of the liquid injection process, FIG. 11B is a top view of a secondary battery cell, and FIG. 11C is a cross-sectional view of the secondary battery cell. FIG. 12A is a side view showing the exterior body during cutting, and FIG. 12B is a side view immediately after the pre-doping electrode has been pulled out. FIG. 13A is a perspective view of a secondary battery, and FIG. 13B is a cross-sectional view of the secondary battery. FIGS. 14A, 14B, and 14C are diagrams illustrating a method for fabricating a secondary battery. FIGS. 15A and 15B are diagrams illustrating an example of a connection between a secondary battery and an electronic device. FIGS. 16A, 16B, and 16C are diagrams illustrating an example of a connection when a secondary battery is mounted on a penetrator. FIG. 17 is a diagram illustrating an example of a configuration of a current collector. FIG. 18 is a diagram illustrating a method for producing a negative electrode active material layer. FIG. 19 is a diagram illustrating a method for producing a negative electrode active material layer. FIG. 20 is a diagram illustrating a method for processing a film. FIGS. 21A, 21B, 21C, 21D, and 21E are diagrams illustrating a method for processing a film. FIGS. 22A and 22B are diagrams illustrating a method for processing a film. FIG. 23A is a cross-sectional view of a positive electrode active material, and FIGS. 23B and 23C are diagrams illustrating the distribution of added elements in the positive electrode active material. FIGS. 24A and 24B are diagrams illustrating the results of DSC analysis.FIG. 25 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 26A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 26B is an FFT pattern of the region of the rock salt crystal RS, and FIG. 26C is an FFT pattern of the region of the layered rock salt crystal LRS. FIG. 27 is a diagram illustrating the crystal structure of a positive electrode active material. FIG. 28 is a diagram illustrating the crystal structure of a conventional positive electrode active material. FIG. 29 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 30 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIGS. 31A and 31B are diagrams illustrating XRD patterns calculated from the crystal structure. FIGS. 32A, 32B, 32C, 32D, 32E, 32F, and 32G are diagrams illustrating the positional relationship of distributions in EDX-ray analysis. FIG. 33 is a cross-sectional view of a positive electrode active material. FIG. 34 is a diagram illustrating a method for producing a positive electrode active material. 35A, 35B, and 35C are diagrams illustrating a method for preparing a positive electrode active material. FIGS. 36A and 36B are diagrams illustrating a method for preparing a positive electrode active material. FIGS. 37A, 37B, 37C, 37D, 37E, 37F, 37G, and 37H are diagrams illustrating an example of an electronic device. FIGS. 38A, 38B, 38C, and 38D are diagrams illustrating an example of an electronic device. FIGS. 39A, 39B, and 39C are diagrams illustrating an example of an electronic device. FIGS. 40A, 40B, and 40C are diagrams illustrating an example of a vehicle. FIGS. 41A, 41B, 41C, and 41D are diagrams illustrating an example of space equipment. FIGS. 42A and 42B are photographs of the secondary battery of Example 1. FIG. 43 is a graph showing charge / discharge curves of the secondary battery of Example 1. FIG. 44 is a graph showing charge / discharge curves of the secondary battery of Example 2. 45A and 45B are graphs showing the low temperature characteristics of Example 3. FIGS. 46A and 46B are graphs showing the charge / discharge rate characteristics of Example 3. FIGS. 47A and 47B are graphs showing the charge / discharge rate characteristics of Example 3. FIGS. 48A and 48B are graphs showing the charge / discharge cycle characteristics of Example 3. FIGS. 49A and 49B are graphs showing the charge / discharge cycle characteristics of Example 3. FIGS. 50A and 50B are graphs showing the results of a freeze test of Example 3. FIGS. 51A and 51B are diagrams explaining the method of an impact test of Example 3.52A and 52B are diagrams illustrating the results of the impact test of Example 3. Figures 53A and 53B are diagrams illustrating the results of the impact test of Example 3. Figure 54 is a graph showing the results of the bending and stretching test of Example 3.

[0041] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0042] For ease of understanding, the position, size, range, etc. of each component shown in the drawings etc. may not represent the actual position, size, range, etc. Therefore, the disclosed invention is not necessarily limited to the position, size, range, etc. disclosed in the drawings etc.

[0043] In addition, in the drawings illustrating the present invention, some components (e.g., the ratio of the size and thickness of the electrodes) may be exaggerated to facilitate understanding, and some components may be omitted to avoid cluttering the drawings.

[0044] Ordinal numbers such as "first," "second," and "third" are used to avoid confusion of components.

[0045] In this specification, "parallel" refers to a state in which two straight lines are arranged at an angle of -10° or more and 10° or less. Therefore, it also includes a state in which the angle is -5° or more and 5° or less. Furthermore, "substantially parallel" or "roughly parallel" refers to a state in which two straight lines are arranged at an angle of -30° or more and 30° or less.

[0046] In this specification, "perpendicular" refers to a state in which two straight lines are arranged at an angle of 80° or more and 100° or less. Therefore, it also includes a state in which the angle is 85° or more and 95° or less. Furthermore, "substantially perpendicular" or "roughly perpendicular" refers to a state in which two straight lines are arranged at an angle of 60° or more and 120° or less.

[0047] In this specification, flexibility refers to the property of an object being soft and bendable. It refers to the property of an object being able to deform in response to an external force applied to the object, regardless of whether it is elastic or has the ability to return to its original shape before deformation. Deforming in response to an external force means that an average adult's hand can deform the object without requiring excessive force. Flexibility can be quantified as the deformation of an object in response to an external force using a testing machine (such as a tensile testing machine or a compression testing machine) capable of measuring stress-strain.

[0048] Furthermore, in this specification, when an object is described as being flexible, it means that at least a part of the object is flexible. In other words, a flexible object may have an inflexible part (also called a hard part).

[0049] In this specification, "high flexibility" means that when two objects are deformed by the same external force, the object that deforms more is said to be the object with high flexibility. Also, when a first part and a second part of an object are deformed by the same external force, the part that deforms more is said to be the part with high flexibility.

[0050] Embodiment 1 In this embodiment, with reference to FIGS. 1 to 6C , a structural example of a flexible secondary battery (also referred to as a flexible battery, a bendable battery, a curved battery, or a bendable battery) according to one embodiment of the present invention, which has high impact resistance, will be described.

[0051] 1A to 6C are schematic diagrams of a secondary battery 10 according to one embodiment of the present invention. Fig. 1A is a perspective view of the secondary battery 10. Fig. 1B is a perspective view showing the internal structure of the secondary battery 10 in a dashed line portion A in Fig. 1A.

[0052] 1A is a schematic diagram of a flexible secondary battery 10, showing the secondary battery 10 curved in one direction. The secondary battery 10 has a first positive electrode lead 23A, a second positive electrode lead 23B, a first negative electrode lead 33A, and a second negative electrode lead 33B extending from the inside to the outside of a space (also referred to as the internal space of the secondary battery 10) enclosed by an exterior body 50 and a sealing portion 52 formed by bonding the exterior bodies 50 together.

[0053] The first positive electrode lead 23A, the second positive electrode lead 23B, the first negative electrode lead 33A, and the second negative electrode lead 33B are connected to the sealing portion 52 on the first side of the exterior body 50.

[0054] The exterior body 50 has a second side opposite the first side. The secondary battery 10 has a fixing portion 55 that clamps the exterior body 50 from the exterior side of the secondary battery 10 in an area including the second side. Note that the fixing portion 55 preferably fixes the exterior body 50 so as to eliminate the internal space of the secondary battery 10 that overlaps with the fixing portion 55. Fixing the exterior body 50 in this manner by the fixing portion can reduce excess internal space of the secondary battery 10.

[0055] Fig. 1B is a perspective view showing the internal structure of the secondary battery 10 at the dashed line portion A in Fig. 1A. In Fig. 1B, the exterior body 50 and the like are omitted. Fig. 2A is a top view of the secondary battery 10, and Fig. 2B is a cross-sectional view showing a cut surface between the dashed dotted line X1-X2 in Fig. 2A.

[0056] 1B, 2A, and 2B, the secondary battery 10 has a positive electrode 11, a negative electrode 12, and a separator (not shown) in an internal space. The positive electrode 11 and the negative electrode 12 are stacked with the separator interposed therebetween, and the stacked positive electrode 11, negative electrode 12, and separator are referred to as a laminate 60.

[0057] 1B and 2A, the laminate 60 has, in the internal space of the secondary battery 10, a first protrusion 11A and a second protrusion 11B formed by protruding portions of the positive electrode 11, and a third protrusion 11C and a fourth protrusion 11D formed by protruding portions of the negative electrode 12. The first protrusion 11A, the second protrusion 11B, the third protrusion 11C, and the fourth protrusion 11D protrude from the laminate 60 toward the first side. It is preferable that the first protrusion 11A, the second protrusion 11B, the third protrusion 11C, and the fourth protrusion 11D do not overlap with each other.

[0058] 1B and 2A , the positive electrode 11 is connected to the first positive electrode lead 23A at the first protrusion 11A and to the second positive electrode lead 23B at the second protrusion 11B. The negative electrode 12 is connected to the first negative electrode lead 33A at the third protrusion 11C and to the second negative electrode lead 33B at the fourth protrusion 11D. In other words, the positive electrode 11 is connected to the sealing portion 52 of the exterior body 50 via the first positive electrode lead 23A at the first protrusion 11A and to the sealing portion 52 of the exterior body 50 via the second positive electrode lead 23B at the second protrusion 11B. Similarly, the negative electrode 12 is connected to the sealing portion 52 of the exterior body 50 via the first negative electrode lead 33A at the third protrusion 11C, and is connected to the sealing portion 52 of the exterior body 50 via the second negative electrode lead 33B at the fourth protrusion 11D. A sealant 56 may be provided at the connection between the lead and the sealing portion 52. In this case, the lead and the sealing portion 52 are connected via the sealant 56.

[0059] In other words, the positive electrode 11 is connected to the exterior body 50 at two points on the first side via the first positive electrode lead 23A and the second positive electrode lead 23B. Similarly, the negative electrode 12 is connected to the exterior body 50 at two points on the first side via the first negative electrode lead 33A and the second negative electrode lead 33B.

[0060] 2A and 2B , the fixing portion 55 fixes the exterior body 50 in a region including a second side opposite to the first side. At this time, the fixing portion 55 is preferably provided so as not to overlap with the laminate 60. In other words, the fixing portion 55 preferably does not fix the laminate 60. In other words, the fixing portion 55 preferably does not fix the positive electrode 11, the negative electrode 12, or the separator (not shown).

[0061] 3A to 3C, the effect of providing fixing portion 55 in the configuration of secondary battery 10 will be described. Fig. 3A is an enlarged schematic cross-sectional view of dashed line portion B in Fig. 2B. Figs. 3B and 3C are schematic cross-sectional views of a case where fixing portion 55 is not provided at dashed line portion B.

[0062] As shown in Figure 3B, when fixing portion 55 is not provided, space C is present at the position indicated by the dashed line. Because exterior body 50 of one embodiment of the present invention uses a flexible material, this space C may deform into a shape like space C' shown in Figure 3C when the secondary battery is subjected to an external force such as a large impact. If a space like space C' is formed inside the secondary battery, as shown in Figure 3C, a part of the electrode (positive electrode or negative electrode) inside the secondary battery may deform so as to separate from stack 60, which may inhibit the electrochemical reaction (also referred to as battery reaction) inside the secondary battery.

[0063] On the other hand, as shown in FIG. 3A , the secondary battery 10 of one embodiment of the present invention has the fixing portion 55 as described above, and therefore has a small excess space such as the space C shown in FIG. 3B . Therefore, even if the secondary battery 10 is subjected to an external force such as a large impact, deformation of the internal space of the secondary battery 10 is suppressed. Therefore, deformation of a part of the electrode (positive electrode or negative electrode) so as to separate from the stack 60 can be suppressed. As a result, even if the secondary battery 10 of one embodiment of the present invention is subjected to an external force such as a large impact, inhibition of the battery reaction can be suppressed. In other words, the secondary battery 10 of one embodiment of the present invention can have high impact resistance by having the fixing portion 55.

[0064] Using Figures 4A and 4B, the effects of connecting the positive electrode 11 and the outer casing 50 at two points and connecting the negative electrode 12 and the outer casing 50 at two points as described above in the configuration of the secondary battery 10 will be explained.

[0065] FIG. 4A is a top view schematic diagram partially illustrating the internal structure of a secondary battery in which an electrode (negative electrode 12a) and an exterior body 50 are connected at one location. The double-headed arrow in FIG. 4A indicates the direction in which the electrode position changes when the secondary battery is subjected to an impact. In such a case, the electrode position may change in a direction rotating around the point where the electrode and the exterior body are connected. If the electrode position changes as shown in FIG. 4A , there is a risk of a malfunction in which the positive electrode and the negative electrode do not face each other. There is also a risk of a malfunction in which the positive electrode and the negative electrode come into direct contact with each other, causing an internal short circuit in the secondary battery.

[0066] On the other hand, as shown in FIG. 4B , the secondary battery 10 of one embodiment of the present invention has a structure in which the electrode (negative electrode 12) and the exterior body 50 are connected at two locations, as described above. Therefore, it is possible to prevent the position of the electrode from changing in the direction indicated by the double arrow in FIG. 4B . As a result, the secondary battery 10 of one embodiment of the present invention can prevent inhibition of the battery reaction even when subjected to an external force such as a large impact. In other words, the secondary battery 10 of one embodiment of the present invention has a structure in which the electrode and the exterior body are connected at two locations, thereby enabling it to have high impact resistance. Note that, as shown in FIG. 2A , the positions at which the positive electrode and the negative electrode are connected may be arranged in the following order on one side of the exterior body, from one end point to the other end point: the first positive electrode lead 23A, the first negative electrode lead 33A, the second positive electrode lead 23B, and the second negative electrode lead 33B. In other words, it is preferable that the two positions at which the positive electrode and the negative electrode are connected are spaced apart rather than adjacent to each other, thereby enabling it to have high impact resistance.

[0067] Furthermore, it is preferable that one side of the exterior body fixed to the electrodes be arranged parallel to the direction in which a large impact is applied. In other words, it is preferable that the protrusions to which the positive electrode lead and the negative electrode lead are connected be arranged perpendicular to the direction in which a large impact is applied. By arranging the secondary battery 10 in this manner, even when subjected to an external force such as a large impact, changes in the positions of the positive and negative electrodes are suppressed, and the laminate 60 is less likely to be distorted.

[0068] Furthermore, as a configuration of secondary battery 10 with high impact resistance, laminate 60 may be fixed using adhesive tape 61, as shown in Fig. 4C . In this case, as shown in Fig. 4C , it is preferable that adhesive tape 61 not be provided in laminate 60 at a position close to the second side of exterior body 50.

[0069] <Electrodes> The secondary battery 10 has a positive electrode 11 and a negative electrode 12 as electrodes. Figures 5A to 5C are diagrams illustrating the positive electrode 11, and Figures 6A to 6C are diagrams illustrating the negative electrode 12. Figure 5A is a top view of the positive electrode 11, and Figures 5B and 5C are cross-sectional views taken along dashed line P1-P2 in Figure 5A. Figure 6A is a top view of the negative electrode 12, and Figures 6B and 6C are cross-sectional views taken along dashed line N1-N2 in Figure 6A.

[0070] As shown in FIG. 5A , the positive electrode 11 includes a positive electrode current collector 21 and a positive electrode active material layer 22. In the positive electrode 11, the positive electrode active material layer 22 may be provided directly on the positive electrode current collector 21 as shown in FIG. 5B . Alternatively, as shown in FIG. 5C , an intermediate layer 24 may be provided between the positive electrode current collector 21 and the positive electrode active material layer 22. The intermediate layer 24 may also be referred to as an undercoat layer, anchor coat layer, or coat layer. The intermediate layer 24 includes a conductive material and / or a binder. Providing an appropriate intermediate layer 24 is preferable because it improves adhesion and electronic conductivity between the positive electrode current collector 21 and the positive electrode active material layer 22.

[0071] As shown in FIG. 6A , the negative electrode 12 includes a negative electrode current collector 31 and a negative electrode active material layer 32. In the negative electrode 12, the negative electrode active material layer 32 may be provided directly on the negative electrode current collector 31 as shown in FIG. 6B . Alternatively, as shown in FIG. 6C , an intermediate layer 34 may be provided between the negative electrode current collector 31 and the negative electrode active material layer 32. The intermediate layer 34 may also be referred to as an undercoat layer, anchor coat layer, or coat layer. The intermediate layer 34 includes a conductive material and a binder. Providing an appropriate intermediate layer 34 is preferable because it improves adhesion and electronic conductivity between the negative electrode current collector 31 and the negative electrode active material layer 32.

[0072] <Method of Producing Coating Layer> An example of a method of producing the coating layer (intermediate layer 24, intermediate layer 34) will be described with reference to FIG.

[0073] In step S1 of FIG. 7, a conductive material, a binder, a thickener, and a solvent are prepared.

[0074] As the conductive material, for example, one or more of carbon black such as acetylene black (AB) and furnace black, graphite such as artificial graphite and natural graphite, carbon fiber such as carbon nanofiber and carbon nanotube (CNT), graphene, and graphene compounds can be used.

[0075] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0076] Furthermore, as the binder, it is preferable to use materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate (PMMA)), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0077] It may be preferable to use a thickener in addition to the binder. For example, a water-soluble polymer is preferably used as the thickener. For example, a polysaccharide can be used as the water-soluble polymer. For example, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, or starch can be used as the polysaccharide. If a thickener is not required, no thickener is prepared in step S1.

[0078] Next, as shown in step S2 of Fig. 7, the materials prepared in step S1 are mixed. A rotation / revolution type agitator can be used for mixing. The mixing method is not limited to the above method, and any known mixing method can be used as appropriate. Furthermore, ultrasonic treatment may be performed to improve the dispersibility of the conductive material during mixing. The ultrasonic treatment can be performed at a frequency of 25 kHz to 40 kHz.

[0079] By carrying out the above-mentioned mixing, the mixture of step S3 in Fig. 7 is obtained. The mixture of step S3 is also called a slurry.

[0080] Next, as shown in step S4 of Fig. 7, a metal foil is prepared. The type of metal foil is appropriately selected depending on whether the current collector with a coating layer prepared in this preparation flow is to be used for a positive electrode or a negative electrode. For example, when preparing a current collector with a coating layer to be used for a positive electrode, aluminum foil may be prepared in step S4. Alternatively, when preparing a current collector with a coating layer to be used for a negative electrode, copper foil may be prepared in step S4. Details of current collectors that can be used for positive and negative electrodes will be described later.

[0081] Next, as shown in step S5 of Fig. 7, the mixture (slurry) from step S3 is applied to the metal foil prepared in step S4. For application, a slot die method, a gravure method, a blade method, or a combination thereof can be used. Alternatively, a continuous coater or the like can be used for application.

[0082] Next, as shown in step S6 of Fig. 7, drying is performed to remove the solvent from the coated mixture. The drying method is not particularly limited, and the mixture can be subjected to ventilation drying, reduced pressure (vacuum) drying, or the like.

[0083] Through the above steps, a current collector with a coating layer can be obtained as shown in step S7 of Fig. 7. In addition to the coating method described above, the coating layer may also be formed by spraying the mixture of step S3.

[0084] 5A and 5C , the current collector with a coating layer used for the positive electrode has a configuration including a positive electrode current collector 21 and an intermediate layer 24. When a positive electrode active material layer 22 is provided on the positive electrode current collector 21, the adhesion between the positive electrode current collector 21 and the positive electrode active material layer 22 can be increased when the intermediate layer 24 is present compared to when the intermediate layer 24 is not present.

[0085] 6A and 6C , the current collector with a coating layer used for the negative electrode has a configuration including a negative electrode current collector 31 and an intermediate layer 34. When a negative electrode active material layer 32 is provided on the negative electrode current collector 31, the adhesion between the negative electrode current collector 31 and the negative electrode active material layer 32 can be increased when the intermediate layer 34 is present compared to when the intermediate layer 34 is not present.

[0086] The adhesion at the interface between the current collector and the active material layer can be evaluated using, for example, an electrode resistance measuring device. By using this evaluation method, the interfacial resistance between the current collector and the active material layer can be calculated, and it can be determined that the lower the interfacial resistance between the current collector and the active material layer, the higher the adhesion.

[0087] <Electrode Resistance Measurement> As the electrode resistance measurement device, an "Electrode Resistance Measuring Instrument XF057" or "Electrode Resistance Measurement System RM2610" manufactured by Hioki E.E. Corporation can be used, but the device is not limited to these as long as similar measurements and analyses can be performed.

[0088] The electrode resistance measurement can be carried out, for example, using the following equipment and conditions: Measuring equipment: Hioki E.E. Corporation electrode resistance measuring instrument XF057 Measuring probe: 46 pins Measuring current: 1 mA Voltage range: 0.5 V Measuring speed: Normal

[0089] In the above-described electrode resistance measurement, when a current is passed between two of the multiple measurement probes in contact with the surface of the active material layer of the electrode sample, a potential distribution occurs on the surface of the electrode sample due to the current distribution. This surface potential distribution is measured at multiple points using multiple measurement probes other than the two measurement probes described above, thereby obtaining the actual potential. This actual potential is obtained from various directions by changing the combination of the two measurement probes through which the current is passed.

[0090] An inverse problem analysis is performed using the actual potential measured in this way and a calculated potential calculated using an electrode model consisting of a current collector layer, an active material layer, and an interfacial resistance layer. In the electrode model, the thickness of the current collector, the thickness of the active material layer, and the volume resistivity of the current collector are assumed to be known values, but the volume resistivity and interfacial resistance of the active material layer are assumed to be unknown values.

[0091] Specifically, the analysis method involves performing an inverse problem analysis in which unknown values ​​of the volume resistivity and interfacial resistance of the active material layer are appropriately changed to obtain a calculated potential, and the difference between the calculated potential and the measured potential is reduced. In this way, the volume resistivity and interfacial resistance of the active material layer of the electrode sample can be calculated.

[0092] As described above, the value of the interface resistance can be obtained by performing an inverse problem analysis using the measured potential obtained by measuring the surface of the active material layer of the electrode sample using multiple measurement probes and the calculated potential calculated by an electrode model consisting of a collector layer, an active material layer, and an interface resistance layer.

[0093] The value of the interface resistance obtained by such measurements and analysis is 3.0 × 10 −３ Ω cm ２ Preferably, it is 2.0 × 10 or less. −３ Ω cm ２ An electrode having such an interface resistance value can be said to have good adhesion between the current collector and the active material layer and high electronic conductivity, and is suitable as an electrode for use in a secondary battery having high impact resistance.

[0094] A method for manufacturing the secondary battery 10 described above will be described with reference to FIGS. 8 to 14C . FIGS. 8 to 14C illustrate a method for manufacturing a secondary battery having a pre-doped negative electrode as an example of a method for manufacturing a secondary battery. However, the secondary battery 10 of one embodiment of the present invention may use a negative electrode that is not pre-doped. Note that in this specification and the like, pre-doping refers to a process performed so that carrier ions (e.g., lithium ions or sodium ions) remain in the negative electrode of the secondary battery even when the secondary battery is in a discharged state.

[0095] <Method for Manufacturing Battery> A method for manufacturing the secondary battery 10 of one embodiment of the present invention using the above-described pre-doped negative electrode 12 will be described with reference to FIGS. 8 to 14C.

[0096] FIG. 8 is a flow chart showing an example of a method for manufacturing the secondary battery 10.

[0097] First, prepare a positive electrode 11 having a first protrusion and a second protrusion, a negative electrode 12 having a third protrusion and a fourth protrusion, and a pre-doping electrode 45. In this embodiment, an example of a secondary battery 10 is shown in which a single-sided coated positive electrode or negative electrode is used and a set of the positive electrode 11 and the negative electrode 12 is enclosed in an outer casing 50, but this is not particularly limited, and a process in which a double-sided coated positive electrode or negative electrode is used and multiple sets are enclosed in a single outer casing is also possible.

[0098] The positive electrode 11 and the pre-doping electrode 45 can be formed by coating a positive electrode active material layer 22 on one side of each current collector. Here, the first protrusion and the second protrusion are provided on the positive electrode current collector 21 as shown in FIG. 9A1. It is preferable that the first protrusion and the second protrusion of the positive electrode 11 do not have the positive electrode active material layer 22. As shown in FIG. 8, the pre-doping electrode 45 can be formed in step S21, and the positive electrode 11 can be formed separately in step S31. However, if the same material and size are used, the positive electrode 11 and the pre-doping electrode 45 can be fabricated in the same process. The current collectors used for the positive electrode 11 and the pre-doping electrode 45 can be made of highly conductive materials such as metals such as stainless steel, gold, platinum, aluminum, and titanium, and alloys thereof. Furthermore, when a current collector is used for the positive electrode 11, it is preferable that it does not dissolve at the potential of the positive electrode 11. Alternatively, an aluminum alloy containing an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can be used. The current collector may be in any suitable shape, such as a foil, plate (sheet), mesh, punched metal, or expanded metal. The current collector used has a thickness of 5 μm or more and 30 μm or less.

[0099] The positive electrode active material layer 22 may be made of a positive electrode active material, a conductive material, and a binder, which will be described later.

[0100] An example of the positive electrode active material layer 22 will be described. For example, the film thickness of the positive electrode active material layer 22 is determined by the viscosity or components of the slurry to be applied to the positive electrode current collector 21 or the film-forming conditions of the application device. When preparing the slurry, a positive electrode active material, a binder, a solvent, and a conductive material are mixed. Polyvinylidene fluoride (PVDF) is used as the binder, N-methyl-2-pyrrolidone (NMP) is used as the solvent, and acetylene black is used as the conductive material. The film thickness of the positive electrode active material layer 22 also varies depending on the heating conditions or pressing conditions after application.

[0101] Then, a first film 13a is provided to encase the positive electrode 11. The first film 13a is processed into a bag shape and fixed with tape or the like. This stage is shown in FIGS. 9A2 and 9A3. Also, FIG. 9A3 is a cross-sectional view of the positive electrode taken along the chain line X1-X2 in FIG. 9A2. In FIGS. 9A1 and 9A2, a positive electrode active material layer is formed on the lower surface of the positive electrode current collector 21, and is therefore not shown.

[0102] 9A1 and 9A2 show perspective views of the positive electrode current collector 21 and the separator 13. A single first film 13a that will become the separator 13 is folded, and the end of the first film 13a is fixed as an adhesive region 51a, with the tab region of the positive electrode current collector 21 protruding from the first film 13a, resulting in the state shown in FIG. 9A2. The separator 13 refers to the region sandwiched between the positive electrode and the negative electrode, and is part of the first film. While FIG. 9A2 shows an example in which the separator 13 is fixed as an adhesive region 51a on two sides, it may also be fixed by providing adhesive regions on four sides.

[0103] Each current collector is provided with a protruding portion (also called a protruding region, tab portion, or tab region) for connection to a lead, and the positive electrode active material layer 22 or the like is not formed in that region, leaving the conductive surface exposed.

[0104] The pre-doping electrode 45 is also wrapped in a second film 47a in the same manner as the positive electrode 11. FIGS. 9B1 and 9B2 show perspective views of the pre-doping electrode 45 and the second film 47a. As shown in FIG. 9B1, the pre-doping electrode 45 is formed by folding a single second film 47a, fixing the end of the second film 47a as an adhesive region 51b, and causing the tab region of the pre-doping electrode 45 to protrude from the second film 47a, resulting in a shape having a separator 47 as shown in FIG. 9B2. The cross-sectional view taken along the dashed line X3-X4 in FIG. 9B2 corresponds to FIG. 9B3. For the subsequent extraction process, the second film 47a may have a larger area than the first film 13a of the positive electrode, and a gripping margin may be provided. Since the pre-doping electrode 45 is an electrode that is removed during the production of the secondary battery 10, it is possible for it to have only one protrusion, but it is preferable for it to have two protrusions, as shown in Figures 9B1 and 9B2, in order to make it easier to remove the pre-doping electrode 45.

[0105] Furthermore, it is preferable that the negative electrode current collector 31 used in the negative electrode 12 does not alloy with carrier ions such as lithium. Specifically, copper or a copper alloy is used as the material for the negative electrode current collector 31. Furthermore, as shown in step S11, the negative electrode active material layer 32 is formed by coating one side of the negative electrode current collector 31. Here, the third protrusion and the fourth protrusion are provided on the negative electrode current collector 31 as shown in FIG. 10A3. It is preferable that the negative electrode 12 does not have the negative electrode active material layer 32 on the third protrusion and the fourth protrusion. Then, as shown in FIG. 10A3, the negative electrode leads 33 (negative electrode leads 33A and 33B) are ultrasonically bonded to the protrusions of the negative electrode current collector 31 on which the negative electrode active material layer 32 is not formed.

[0106] The negative electrode active material is a mixture of carbon particles and a silicon-based material. Silicon has a theoretical capacity of 4200 mAh / g, more than 10 times that of graphite (372 mAh / g). However, a negative electrode made solely of silicon suffers from rapid cycle degradation due to expansion and contraction during charging and discharging. To improve cycle degradation, nanosilicon, which is made by miniaturizing silicon particles, is preferably used.

[0107] A polymer compound having a carboxy group is used as the binder for the negative electrode 12. Specifically, a polymer having polyacrylic acid is used as the binder.

[0108] As the carbon particles, graphite, carbon having a layer structure like graphite, amorphous carbon, and hard carbon are used. The carbon particles used in this specification specifically refer to graphite particles, which are abundant in nature and therefore inexpensive, making them preferable as a negative electrode active material.

[0109] In the above configuration, the silicon particles refer to silicon powder used as a negative electrode active material for lithium-ion secondary batteries, and have an average particle diameter of approximately 100 nm, which are sometimes called nanosilicon particles. The silicon particles used are preferably prepared by pulverizing silicon raw materials to adjust the particle diameter to a uniform size. The silicon particles preferably contain at least one of silicon, silicon oxide, and silicon alloy.

[0110] A conductive material may also be added when forming the negative electrode active material layer 32. A typical example of a carbon material used as a conductive material is acetylene black (also called AB). Acetylene black is a bulky particle with an average particle diameter of several tens to several hundreds of nanometers, making it difficult to form surface contact with other materials, and tends to form point contact. Therefore, when an active material is mixed with acetylene black, the contact resistance between the active material and acetylene black increases. Using a large amount of acetylene black to reduce the contact resistance reduces the ratio of active material to the entire electrode, resulting in a decrease in the discharge capacity of the secondary battery. Carbon fiber may also be used as a conductive material.

[0111] Acetylene black is a material that tends to aggregate, so it is preferable to mix it so that it is uniformly dispersed. The weight ratio of acetylene black is less than or equal to the weight ratio of silicon particles. Of course, it is possible to prepare a negative electrode without adding a conductive material (such as acetylene black).

[0112] Through the above steps, the positive electrode 11 wrapped in the first film 13a, the pre-doping electrode 45 wrapped in the second film 47a, and the negative electrode 12 can be prepared.

[0113] Next, in step S100, the positive electrode 11 wrapped in the first film 13a, the pre-doping electrode 45 wrapped in the second film 47a, and the negative electrode 12 are stacked and aligned, and then fixed with adhesive tape to form a laminate. The positive electrode lead 23 (positive electrode lead 23A and positive electrode lead 23B) is bonded to the tab region of the positive electrode 11, the negative electrode lead 33 (negative electrode lead 33A and negative electrode lead 33B) is bonded to the tab region of the negative electrode 12, and the pre-doping lead 48 is bonded to the tab region of the pre-doping electrode by ultrasonic bonding. Next, the periphery of the above-mentioned laminate is thermocompressed using a heat bar sealer so as to sandwich it between laminate films that will become the exterior body.

[0114] 10A1 shows an oblique view before connecting the positive electrode lead 23 (positive electrode lead 23A and positive electrode lead 23B), FIG. 10A2 shows an oblique view before connecting the pre-doping lead 48 (48A and 48B), and FIG. 10A3 shows an oblique view before connecting the negative electrode lead 33 (negative electrode lead 33A and negative electrode lead 33B), and alignment is performed so that the dashed lines in the figures overlap.

[0115] 10B shows a perspective view before laminating the laminate film 50a and the laminate film 50b that form the outer casing 50. The negative electrode has a larger area than the positive electrode, and they are stacked with their centers aligned, with their ends almost, but not perfectly, aligned. As shown in FIG. 10B, the ends of the positive electrode 11, the pre-doping electrode 45, and the negative electrode 12 are stacked so that they almost coincide, while the end of the separator 47 is designed to have a larger area than the separator 13 so that it can be gripped.

[0116] When thermocompression bonding is performed around the periphery using a heat bar sealer, thermocompression bonding is performed, leaving one side for later insertion of the electrolyte. For example, as shown in FIG. 10B, the laminate is placed in a predetermined position inside the laminate film 50a and laminate film 50b, which are folded in half. It is preferable to use an embossed laminate film on one of the laminate films 50a and 50b, and a flat laminate film that is not embossed on the other, but an embossed laminate film on both sides can also be used. Then, as shown in FIG. 11A, thermocompression bonding is performed in the first compression region 52a (step S101). After thermocompression bonding, the laminate film 50a and laminate film 50b become a bag-shaped outer casing 50, and the positive electrode 11, the pre-doping electrode 45, and the negative electrode 12 are placed inside, aligned, and sealed with the bag-shaped outer casing 50. This state is also called three-sided sealing.

[0117] Then, the bag-shaped exterior body 50 is fixed with the portion containing the electrolyte solution 40 facing up, and the electrolyte solution 40 is poured into the exterior body 50 (step S102) in an argon gas atmosphere. It is preferable to dry the exterior body 50 and the laminated body enclosed therein before the pouring process. For example, the drying conditions may be a drying process at 80°C for 10 hours in a reduced pressure environment. The drying conditions are not limited to those described above, as long as the moisture adsorbed in the exterior body and the laminated body can be removed.

[0118] 11A shows a side view of the injection process of the electrolyte solution 40. The exterior body 50 is fixed in place by the first crimping region 52a. The portion of the laminate film folded to form one side does not need to be crimped, but is crimped in this embodiment. The first crimping region 52a, which overlaps with the folded portion of the laminate film, functions as a guide for aligning the exterior body and the laminate, thereby preventing misalignment of the overlapping electrodes.

[0119] After the injection of the electrolyte solution 40 is completed, a vacuum sealing process (step S103) is performed.

[0120] The reduced pressure sealing is a sealing process performed in a reduced pressure environment, and the reduced pressure environment is preferably 50,000 Pa or less, and more preferably 40,000 Pa or less, 30,000 Pa or less, 20,000 Pa or less, 10,000 Pa or less, 5,000 Pa or less, or 1,000 Pa or less. Depending on the type of electrolyte solution 40 used, excessive pressure reduction may cause evaporation, so the reduced pressure value is adjusted depending on the electrolyte solution 40 used.

[0121] After the injection of the electrolytic solution 40, an impregnation treatment may be performed to facilitate impregnation of the electrolytic solution 40 into the pores of the positive electrode active material layer 22, the negative electrode active material layer 32, and the separator 13. As the impregnation treatment, a decompression treatment (also referred to as a vacuuming treatment) is preferably performed, and the decompression treatment and the pressure recovery treatment may be performed multiple times.

[0122] The environmental pressure in the decompression treatment is preferably about −60 kPa as a gauge pressure (40,000 Pa as an absolute pressure). The exterior body can be sealed at the same environmental pressure as the decompression treatment described above, or at an environmental pressure different from that of the decompression treatment described above.

[0123] During the reduced pressure sealing shown in step S103, a second thermocompression bonding is performed, and the second compression bonding region 52e is provided to close the opening of the bag-shaped exterior body 50. Thereafter, excess laminate film is cut off.

[0124] A secondary battery cell having a pre-doping electrode is fabricated through the above steps, and Fig. 11B shows a top view of the secondary battery cell. Fig. 11C shows a cross-sectional view taken along the chain line X5-X6 in Fig. 11B.

[0125] 11C , an electrolyte solution 40 is sealed inside the exterior package 50, and a laminate is disposed therein. The laminate disposed inside the exterior package 50 is formed by laminating the negative electrode 12, the pre-doping electrode 45, and the positive electrode 11 in this order. The positive electrode 11 has a positive electrode active material layer 22 formed on a positive electrode current collector 21, and is wrapped in a separator 13, which is a first film 13a. The pre-doping electrode 45 has a pre-doping active material layer 46 formed on the pre-doping electrode 45, and is wrapped in a separator 47, which is a second film 47a. The negative electrode 12 has a negative electrode active material layer 32 formed on a negative electrode current collector 31.

[0126] Then, in step S104, a charging process for pre-doping is performed. The charging process for pre-doping is performed so that the battery capacity is 10% to 40%, preferably 30%. The pre-doping capacity may be calculated in advance based on an estimated value (design capacity) of the capacity of the battery to be finally manufactured, and the charge amount may be determined, and the calculation is performed taking into account the material and weight of the positive electrode active material layer 22 and the configuration of the negative electrode. In this embodiment, the configuration of the negative electrode 12 is a mixture of graphite and nanosilicon at a rate less than that of graphite, specifically, the weight ratio of graphite: nanosilicon: AB: polyacrylic acid = 72:8:6:14, and the pre-doping capacity is optimally 30% of the capacity of the battery to be finally manufactured.

[0127] The finally manufactured battery is generally subjected to initial charge and discharge (also referred to as aging or conditioning) before normal use. In the battery of one embodiment of the present invention that has undergone initial charge and discharge, the capacity remaining in the negative electrode in a discharged state (also referred to as remaining capacity) is preferably 1% to 30%, more preferably 5% to 15%, and even more preferably 7% to 10%.

[0128] During the charging process for pre-doping, the pre-doping lead 48 (48A or 48B) is connected to an external power source, and the negative electrode lead 33 (negative electrode lead 33A or negative electrode lead 33B) is connected to the external power source to energize. The charging process for pre-doping may be performed so that the target remaining capacity is reached in a single charging process, but is not limited to this. As the charging process for pre-doping, after charging to a capacity exceeding the target remaining capacity, a discharging process may be performed so that the target remaining capacity is reached. Moreover, the charging process for pre-doping may be performed after repeating charging and discharging one or more times.

[0129] After the charging process for pre-doping is completed, the pre-doping electrode 45 is no longer needed, so a process of cutting a portion of the exterior body 50 to remove it (step S105) is performed. When cutting, a portion of the pre-doping leads 48 (48A and 48B) is also cut. Note that FIG. 12A shows a side view of the exterior body when cutting, and the pre-doping leads 48 (48A and 48B) are fixed in a position where they are facing upward to prevent leakage of the internal electrolyte. The dashed line 49 in FIG. 12A is the cut line, as shown.

[0130] Then, in an argon gas atmosphere, a process of pulling out the pre-doping electrode 45 and the separator 47 (step S106) is performed. Fig. 12B shows a side view immediately after the pre-doping electrode 45 is pulled out, illustrating the state in which the pre-doping electrode 45 and the separator 47 are pulled out together with the partially cut pre-doping leads 48 (48A and 48B). It is important to use insulating tweezers to quickly or smoothly pull out the separator 47 and the pre-doping electrode 45 so that the positions of the positive electrode and the negative electrode do not shift.

[0131] Then, a third thermocompression bonding (third compression bonding region 52b) is performed under reduced pressure to close the opening, and reduced pressure sealing is performed in step S107. Note that an electrolyte solution can be added before the third thermocompression bonding. The added electrolyte solution may have the same composition as the initially injected electrolyte solution, or a different composition. For example, the added electrolyte solution may contain an electrolyte solution with a different type of additive from the initially injected electrolyte solution. Note that the additive contained in the added electrolyte solution preferably includes an additive that acts on the positive electrode side.

[0132] In this manner, the secondary battery 10 according to one embodiment of the present invention using the pre-doped negative electrode 12 can be fabricated.

[0133] In addition, when removing the pre-doping electrode 45, rather than completely aligning the positions of the positive electrode 11 and the pre-doping electrode 45 of the same size, by shifting the positions of the positive electrode 11 and the pre-doping electrode 45 by about 3 mm, the separator 47 also protrudes by 3 mm, so that this portion can be grasped and smoothly removed. Also, even when fixing the positive electrode 11 and the separator 13 with tape to prevent misalignment, by shifting them by 3 mm, the overlapping portion between the adhesive tape and the separator 47 is reduced, making it possible to remove them smoothly. When accurately aligning multiple metal foils, it is preferable to use a guide or jig for alignment.

[0134] The fabricated secondary battery 10 is preferably subjected to initial charge and discharge (also referred to as aging treatment). As the aging treatment, for example, the following methods described as Aging 1 to Aging 4 may be used. Note that the aging treatment method is not limited to the following method, and other methods may also be used.

[0135] Next, after maintaining the battery at an ambient temperature of 25°C for 24 hours, aging treatments (Aging 1 and Aging 2) are performed in step S108. The first aging treatment (Aging 1) is performed under the conditions of 0.01C (when 1C is 200mA / g) constant current (CC) charging, stopping at 15mAh / g. The second aging treatment (Aging 2) is performed under the conditions of 0.1C CC charging, stopping at 105mAh / g.

[0136] After the high temperature is maintained, a degassing process (step S109) is performed. The high temperature is maintained at 60° C. for 24 hours. The degassing process is performed to release gas that is generated by charging and discharging.

[0137] A portion of the exterior body (including the second crimped region 52e) is cut to provide an opening, gas is released, and then the opening is resealed (step S110). The side to be cut is the portion that was subjected to the third thermocompression bonding. Under reduced pressure, a fourth thermocompression bonding is performed to close the opening that was released in the fourth crimped region 52c. The reduced pressure is then returned to atmospheric pressure.

[0138] Then, in step S111, aging processes (aging 3 and 4) are performed. The third aging (aging 3) involves an ambient temperature of 25° C., conditions of 0.1 C, constant current / constant voltage (CC / CV) upper limit voltage of 4.5 V, cutoff at 0.01 C, followed by discharge at an ambient temperature of 25° C., conditions of 0.2 C, CC, and cutoff at a lower limit voltage of 2.5 V. A rest period may be provided between discharge and the next charge.

[0139] The fourth aging (aging 4) involved cutting off at 0.02C under conditions of 0.2C at an ambient temperature of 25°C, a CC / CV upper limit voltage of 4.5V, and discharging at 25°C under conditions of 0.2C, CC, and a lower limit voltage of 2.75V.

[0140] Here, the rates of charging and discharging the power storage device will be explained. For example, when a secondary battery with a capacity of X [Ah] is charged at a constant current, a charge rate of 1C is a current value I [A] at which charging is completed in exactly one hour, and a charge rate of 0.2C is a current value I / 5 [A] (i.e., a current value at which charging is completed in exactly five hours). Similarly, a discharge rate of 1C is a current value I [A] at which discharging is completed in exactly one hour, and a discharge rate of 0.2C is a current value I / 5 [A] (i.e., a current value at which discharging is completed in exactly five hours).

[0141] The above steps produce the secondary battery 10a before providing the fixing portion 55. An external view of the produced secondary battery 10a is shown in Fig. 13A, and a cross section taken along the chain line A1-A2 in Fig. 13A is shown in Fig. 13B.

[0142] The secondary battery 10 a shown in FIGS. 13A and 13B has an exterior body 50 sealed with a first crimping region 52 a , a third crimping region 52 b , and a fourth crimping region 52 c so as to surround a sealing region 53 .

[0143] Next, a process of providing a fixing portion 55 on the secondary battery 10a will be described with reference to FIGS. 14A to 14C . FIG. 14A is a schematic diagram showing a cross section of the secondary battery 10a taken along dashed line B1-B2 in FIG. 13A . When the secondary battery 10 according to one embodiment of the present invention is used as a curved secondary battery, the secondary battery 10a is curved before providing the fixing portion 55, as shown in FIG. 14B . Then, as shown in FIG. 14C , the fixing portion 55 is provided in a region including the second side of the exterior body 50. At this time, the exterior body 50 is sandwiched and deformed from the outside of the secondary battery 10a so as to compress the excess space, and the fixing portion 55 is fixed so as to maintain the deformed shape (step S112). The fixing portion 55 may have a shape that sandwiches the exterior body 50 by combining two rectangular parallelepiped-shaped components, as shown in FIGS. 1A , 14C , and the like. The two components may be fixed together using screws, adhesive, joints, sawtooth fixing portions, etc. Plastic, metal, etc. may be used as the material for fixing portion 55. Note that the shape, structure, and material of fixing portion 55 are not limited to the above examples, as long as fixing portion 55 can hold exterior body 50 as described in this specification.

[0144] Through the above steps, the secondary battery 10 of one embodiment of the present invention can be manufactured.

[0145] Note that the secondary battery 10 of one embodiment of the present invention may have a configuration in which the first positive electrode lead 23A and the second positive electrode lead 23B are integrated together on the outside of the sealing portion 52 of the secondary battery 10 as shown in FIG. 15A , or similarly, the secondary battery 10 may have a configuration in which the first negative electrode lead 33A and the second negative electrode lead 33B are integrated together.

[0146] Furthermore, the secondary battery 10 of one embodiment of the present invention may be connected to a circuit board 70 as shown in FIG. 15B . FIG. 15B illustrates an example in which a first positive electrode lead 23A, a second positive electrode lead 23B, a first negative electrode lead 33A, and a second negative electrode lead 33B are each connected to the circuit board 70. The circuit board 70 may have, for example, a positive electrode potential wiring connected to the first positive electrode lead 23A and the second positive electrode lead 23B, and a negative electrode potential wiring connected to the first negative electrode lead 33A and the second negative electrode lead 33B. Such a configuration can be fabricated by using a laminated printed circuit board for the circuit board 70. Furthermore, as shown in FIG. 15B , a positive electrode potential wire 71A and a negative electrode potential wire 71B may be configured to extend from the inside to the outside of the circuit board 70. Furthermore, the secondary battery 10 can be discharged to a battery device or the like that incorporates the secondary battery 10 and can be charged through the positive electrode potential line 71A and the negative electrode potential line 71B.

[0147] 16A to 16C show an example of a penetrator 1000 equipped with a secondary battery 10 according to one embodiment of the present invention, as an example of a battery device equipped with a secondary battery 10. Fig. 16A is a perspective view of the penetrator 1000, and Fig. 16B is a cross-sectional schematic diagram illustrating the arrangement of the secondary battery 10 inside the penetrator 1000.

[0148] 16A, the secondary battery 10 is connected to a main unit 1010 inside the penetrator 1000. The main unit 1010 includes a calculation unit, a memory unit, a communication unit, a sensor unit, etc., and can be driven by power supplied from the secondary battery 10. Although not shown, the penetrator 1000 may also include power generation means such as a solar cell device or a temperature difference power generation device.

[0149] 16A and 16B , the secondary battery 10 of one embodiment of the present invention is flexible and can therefore be installed in a curved state inside the penetrator 1000. When installing the secondary battery 10 in such a shape, the secondary battery 10 may be curved to fit the internal shape of the penetrator 1000, and then fixed with a fixing portion 55 as shown in FIG. 16B . The fixing portion 55 may also be configured to be connected to the inside of the penetrator 1000.

[0150] The secondary battery 10 according to one embodiment of the present invention is not limited to the configuration described above, and may have a configuration in which two secondary batteries 10 are clamped and fixed by one fixing portion 55, as shown in Fig. 16C, for example. In addition, Fig. 16A shows an example in which the secondary battery 10 is mounted on the cylindrical portion at the rear of the penetrator 1000, but the secondary battery 10 may also be mounted on the conical portion at the tip of the penetrator 1000.

[0151] 16A to 16C may be used in outer space, on celestial bodies other than Earth, in cold regions on Earth, etc. Therefore, it is preferable to use a positive electrode active material that can withstand high voltage and obtain a high charge capacity when charging in a low-temperature environment as a positive electrode active material for a lithium-ion battery that has excellent charge and discharge characteristics even in a low-temperature environment.

[0152] Furthermore, it is preferable that the electrolyte of a lithium ion battery that has excellent charge and discharge characteristics even in a low-temperature environment uses a material that has excellent lithium ion conductivity even when charging and / or discharging (charging and discharging) in a low-temperature environment (e.g., 0°C, preferably −20°C, more preferably −30°C, more preferably −40°C).

[0153] A preferred positive electrode, negative electrode, and electrolyte for a lithium ion battery having excellent charge and discharge characteristics even in a low-temperature environment will be described in detail below.

[0154] [Positive Electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and may further include at least one of a conductive material and a binder. Positive electrode active materials suitable for one aspect of the present invention will be described in Embodiments 2 and 3.

[0155] <Positive electrode current collector> A metal foil can be used as the positive electrode current collector. The positive electrode can be formed by applying a slurry to a metal foil and drying it. Note that pressing may be performed after drying. The positive electrode is formed by forming an active material layer on the positive electrode current collector.

[0156] As the current collector, a highly conductive material such as a metal such as stainless steel, gold, platinum, aluminum, titanium, or iron, or an alloy thereof, can be used. Furthermore, it is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Furthermore, an aluminum alloy containing an element that improves physical properties, such as iron, silicon, titanium, neodymium, scandium, or molybdenum, can be used. The current collector can be appropriately shaped, such as a foil, plate, sheet, mesh, punched metal, or expanded metal. It is preferable to use a current collector with a thickness of 5 μm or more and 30 μm or less.

[0157] As shown in FIG. 17 , a laminated current collector 17 having a structure in which metal layers 16 are formed on both sides of an organic material film 15 can be used as the current collector. The organic material film 15 can be made of an organic material film such as polypropylene, polyethylene, nylon, or polyethylene terephthalate. The metal layer 16 can be made of a highly conductive material such as stainless steel, gold, platinum, aluminum, titanium, or an alloy thereof. The laminated current collector 17 can be fabricated by bonding an organic material film to a metal foil (metal layer 16). In this case, an adhesive layer is provided between the organic material film and the metal layer. Alternatively, the laminated current collector 17 can be fabricated by forming the metal layers 16 on both sides of the organic material film 15 by sputtering, vapor deposition, or the like. When the laminated current collector 17 is used as a positive electrode current collector 21, it is preferable to use aluminum as the metal layer 16. Another example of a laminated current collector configuration is one in which a graphene layer is used instead of the metal layer.

[0158] <Positive Electrode Binder> Binders that can be used in the positive electrode will be described.

[0159] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0160] Furthermore, as the binder, it is preferable to use materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate (PMMA)), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0161] It may be preferable to use a thickener in addition to the binder. For example, a water-soluble polymer is preferably used as the thickener. For example, a polysaccharide can be used as the water-soluble polymer. For example, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, or starch can be used as the polysaccharide.

[0162] When the binder covers the surface of the active material, or when the binder in contact with the surface forms a film, it can serve as a passive film and is expected to have the effect of suppressing decomposition of the electrolyte. Here, the "passive film" refers to a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0163] <Conductive Material> The conductive material that can be used in the positive electrode and the negative electrode is also called a conductivity imparting agent or conductive material, and is preferably a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers a part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the active material is electrically connected even when not in contact with each other.

[0164] As the conductive material, for example, one or more of carbon black such as acetylene black (AB) and furnace black, graphite such as artificial graphite and natural graphite, carbon fiber such as carbon nanofiber and carbon nanotube, graphene, and graphene compounds can be used.

[0165] Acetylene black is difficult to bring into surface contact with other active materials, and tends to result in point contact. Therefore, when an active material is mixed with acetylene black, it is possible to use a large amount of acetylene black to reduce contact resistance, but this reduces the proportion of active material, resulting in a decrease in the discharge capacity of the secondary battery. In addition, acetylene black is a material that tends to aggregate, so it is preferable to form a slurry using a dispersant or the like to ensure uniform dispersion.

[0166] In view of these considerations, it is preferable that the weight ratio of acetylene black in the negative electrode be less than or equal to the weight ratio of silicon particles used in the negative electrode active material. In other words, by satisfying this weight ratio, acetylene black can be mixed to exhibit high dispersibility without reducing the proportion of silicon particles. This can increase the discharge capacity of the secondary battery.

[0167] As the carbon fiber, for example, mesophase pitch-based carbon fiber, isotropic pitch-based carbon fiber, or the like can be used. Furthermore, as the carbon fiber, carbon nanofiber or carbon nanotube can be used. Carbon nanotube can be produced by, for example, vapor phase growth method. VGCF (registered trademark) can also be used as the carbon fiber.

[0168] The above-mentioned graphene includes graphene, multilayer graphene, multigraphene, etc. Furthermore, the above-mentioned graphene compounds include graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene refers to a substance containing carbon, having a shape such as a plate or sheet, and having a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be referred to as a carbon sheet. Furthermore, graphene is preferably rigid and has a curved shape. Graphene compounds may have holes in the carbon rings or may have more than six rings, and may also have functional groups. Furthermore, because graphene compounds are soft, they may be rolled up into, for example, carbon nanofibers.

[0169] Since graphene or a graphene compound can be in surface contact with an active material, a smaller amount of graphene or a graphene compound is required than a conventional conductive material. This allows the proportion of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the secondary battery.

[0170] Although carbon fibers are in surface contact with active materials, their long axis is longer than their short axis, allowing for an appropriate electrical path between active materials that are spaced apart. This allows for a smaller amount of carbon fiber than a typical conductive material. This allows for a larger proportion of active material in the active material layer, thereby increasing the discharge capacity of the secondary battery.

[0171] <Electrolyte> As one form of the electrolyte, an electrolyte solution having a solvent and an electrolyte dissolved in the solvent can be used. The solvent is preferably an aprotic solvent, and for example, one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., or two or more of these can be used in any combination and ratio. When two or more types are used, it may be referred to as a mixed solvent.

[0172] As another form of electrolyte, one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) can be used as the solvent. In this case, even if the internal temperature of the power storage device rises due to an internal short circuit or overcharging, the power storage device can be prevented from exploding or catching fire. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0173] The electrolyte (also called lithium salt) dissolved in the solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(FSO ２ ) ２ (LiFSI), LiN(CF ３ SO ２ )２ (LiTFSI), LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ , lithium bis(oxalato)borate (Li(C) ２ O ４ ) ２ , LiBOB), or two or more of these can be used in any combination and ratio.

[0174] An additive may be mixed into the mixed solvent containing the lithium salt. Examples of the additive include vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), LiFSI, LiTFSI, and dinitrile compounds such as succinonitrile and adiponitrile. The concentration of the additive is preferably 0.1 wt % or more and 5 wt % or less of the mixed solvent containing the lithium salt.

[0175] <Electrolyte Solution Example 1> As the mixed solvent used in one embodiment of the present invention, a material having excellent lithium ion conductivity can be used even when charging and / or discharging (charging and discharging) in a low-temperature environment.

[0176] An example of the electrolyte solution will be described below. The electrolyte solution described in this embodiment is a mixed solvent in which a lithium salt is dissolved, and the mixed solvent is liquid at room temperature. The mixed solvent is not limited to being liquid at room temperature, and a solid electrolyte that becomes solid at room temperature can also be used. Alternatively, a semi-solid electrolyte that contains both liquid and solid at room temperature can also be used. The semi-solid electrolyte includes a gel-like electrolyte.

[0177] The mixed solvent of the electrolyte solution according to one embodiment of the present invention may contain two or more selected from fluorinated cyclic carbonates (also referred to as fluorinated cyclic carbonates) and fluorinated chain esters (also referred to as fluorinated chain esters).

[0178] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC includes isomers such as cis-4,5 and trans-4,5. Since all of these fluorinated cyclic carbonates have electron-withdrawing substituents, they are believed to have low solvation energies for lithium ions.

[0179] The following structural formula (H10) is the structural formula of FEC: In FEC, the electron-withdrawing substituent is an F group.

[0180]

[0181] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain ester. The following structural formula (H22) is the structural formula of methyl 3,3,3-trifluoropropionate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." In MTFP, the electron-withdrawing substituent is CF ３ It is the base.

[0182]

[0183] An example of a fluorinated chain ester is trifluoromethyl 3,3,3-trifluoropropionate. The following structural formula (H23) is the structural formula of trifluoromethyl 3,3,3-trifluoropropionate. The electron-withdrawing substituent is CF ３ It is the base.

[0184]

[0185] An example of a fluorinated chain ester is trifluoromethyl propionate. The following structural formula (H24) is the structural formula of trifluoromethyl propionate. The electron-withdrawing substituent is CF ３ It is the base.

[0186]

[0187] An example of a fluorinated chain ester is methyl 2,2-difluoropropionate. The following structural formula (H25) is the structural formula of methyl 2,2-difluoropropionate. The electron-withdrawing substituent is CF ２ It is the base.

[0188]

[0189] <FEC and MTFP> The mixed solvent described in this embodiment preferably contains FEC and MTFP. The reason for this will be described below.

[0190] FEC is a cyclic carbonate with a high dielectric constant, and when used in an organic solvent, it promotes the dissociation of lithium salts. On the other hand, because FEC has electron-withdrawing substituents, it is more likely to desolvate with lithium ions than ethylene carbonate (EC). Specifically, the solvation energy of lithium ions in FEC is lower than that of EC without electron-withdrawing substituents. Therefore, it is easier to separate lithium ions from the surfaces of the positive and negative electrode active materials, thereby reducing the internal resistance of the secondary battery. Furthermore, because FEC has a deep highest occupied molecular orbital (HOMO), it is less susceptible to oxidation and has improved oxidation resistance. On the other hand, there are concerns about the high viscosity of FEC. Therefore, it is recommended to use a mixed organic solvent containing MTFP in addition to FEC alone in the electrolyte. MTFP is a type of chain carbonate, and can have the effect of reducing the viscosity of the electrolyte solution or maintaining the viscosity at room temperature (typically 25° C.) even at low temperatures (typically 0° C.). Furthermore, although MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, it may form a solvate with lithium ions when used in the electrolyte solution.

[0191] The organic solvent described above is free from granular dust or molecules other than the constituent molecules of the organic solvent (hereinafter simply referred to as "impurities"), and oxygen (O ２ ), water (H ２0) or water. ) content is preferably low and highly purified. Furthermore, it is preferable that reaction by-products during synthesis are suppressed through appropriate purification. Specifically, the impurities in the electrolyte are 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. The concentration of water among the impurities can be detected by Karl Fischer titration.

[0192] Furthermore, it is preferable that the above-mentioned organic solvent has almost no peaks due to impurities that can be confirmed by NMR measurement or the like. "Almost no peaks can be confirmed" means that the ratio of the integrated area of ​​the peak due to the impurity to the integrated area of ​​the peak due to the main component (simply referred to as "integral ratio") is 0.005 or less, preferably 0.002 or less. The device used for NMR measurement is not particularly limited, but for example, Bruker's "AVANCE III 400" can be used. Furthermore, among the five peaks of acetonitrile derived from acetonitrile-d3 used as a solvent in 1H-NMR measurement, the central peak can be located at 1.94 ppm.

[0193] For example, in the case of MTFP, it is known that when 1H-NMR is measured using acetonitrile-d3 solvent, four peaks appear at δ between 3.29 ppm and 3.43 ppm. However, if other peaks appear in this vicinity, for example, if a peak appears at δ between 3.24 ppm and 3.29 ppm, the peak is considered to be derived from impurities. Therefore, if the ratio (integral ratio) of the peak area between 3.24 ppm and 3.29 ppm to the peak area between 3.29 ppm and 3.43 ppm is 0.005 or less, preferably 0.002 or less, it can be said that peaks due to impurities are almost impossible to confirm.

[0194] The measured values ​​of the HOMO level, solvation energy, and melting point are summarized in the table below.

[0195]

[0196] FEC and MTFP having such physical properties can be mixed and used in a volume ratio of x:100-x (where 5≦x≦30, preferably 10≦x≦20), assuming the total content of these two mixed solvents to be 100 vol%. That is, it is preferable to mix them so that there is more MTFP than FEC in the mixed solvent. Note that the above volume ratio may be the volume ratio measured before mixing the mixed solvent, and the outside air may be room temperature (typically 25°C) when mixing the mixed solvent. A mixed solvent obtained by mixing FEC and MTFP is preferable because it exhibits a viscosity that allows it to operate as a lithium-ion battery and maintains an appropriate viscosity even in a low-temperature environment.

[0197] Because typical solvents used in lithium ion batteries freeze at around −20° C., it is difficult to fabricate a lithium ion battery that can be charged and discharged at −30° C., preferably −40° C. However, the mixed solvent described as an example in this embodiment allows the freezing point to be −30° C. or lower, preferably −40° C. or lower, making it possible to realize a lithium ion battery that can be charged and discharged even in low-temperature environments. As a result, it is possible to realize a lithium ion battery that can be charged and discharged over a wide temperature range, including at least low-temperature environments.

[0198] Although FEC has been described above as a representative example, any of the organic compounds described as fluorinated cyclic carbonates has the effect of promoting the dissociation of lithium salts, has small solvation energy so that the bond between the lithium ion and the solvent is easily broken, and has high viscosity, making it difficult to use it alone below freezing point.

[0199] Although MTFP has been described above as a representative example, any of the organic compounds described as fluorinated chain esters can be said to have the effect of reducing or maintaining the viscosity of the electrolyte solution of one embodiment of the present invention. Therefore, as long as the mixed solvent of one embodiment of the present invention contains a fluorinated cyclic carbonate and a fluorinated chain ester, it is possible to provide a lithium ion battery that can be charged and discharged in a low-temperature environment.

[0200] <Electrolyte Solution Example 2> A mixed solvent for an electrolyte solution according to another embodiment of the present invention may contain ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and the volume ratio of the ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is x:y:100-x-y (where 5≦x≦35 and 0<y<65) when the total content of the ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 100 vol%. More specifically, a mixed solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC=30:35:35 may be used. Note that the above volume ratio may be the volume ratio before the mixed solvent is mixed, and the ambient air temperature when the mixed solvent is mixed may be room temperature (typically, 25°C).

[0201] EC is a cyclic carbonate with a high dielectric constant, which promotes the dissociation of lithium salts. However, EC has a high viscosity and a high freezing point (melting point) of 38°C, making it difficult to use EC alone as a solvent in low-temperature environments. Therefore, a solvent specifically described as one embodiment of the present invention further includes EMC and DMC, rather than EC alone. EMC is a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −54°C. DMC is also a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −43°C. An electrolyte prepared using a mixed solvent containing EC, EMC, and DMC with such physical properties, in a volume ratio of x:y:100−x−y (where 5≦x≦35 and 0<y<65), where the total content of the mixed solvent is 100 vol%, is characterized by a freezing point of −40°C or lower.

[0202] A typical electrolyte used in lithium ion batteries freezes at about −20° C., making it difficult to fabricate a battery that can be charged and discharged at −40° C. The electrolyte described as an example in this embodiment has a freezing point of −40° C. or lower, making it possible to realize a lithium ion battery that can be charged and discharged even in a low-temperature environment of −40° C.

[0203] The lithium salt dissolved in the solvent may be, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ , lithium bis(oxalate)borate (LiBOB), or two or more of these can be used in any combination and ratio. The lithium salt dissolved in the solvent is preferably 0.5 mol / L or more and 1.5 mol / L or less, more preferably 0.7 mol / L or more and 1.3 mol / L or less, and more preferably 0.8 mol / L or more and 1.2 mol / L or less, relative to the volume of the solvent. A specific example of use is LiPF ６ is preferably 0.5 mol / L or more and 1.5 mol / L or less, more preferably 0.7 mol / L or more and 1.3 mol / L or less, and even more preferably 0.8 mol / L or more and 1.2 mol / L or less.

[0204] Furthermore, the mixed solvent is preferably highly purified, with a low content of granular dust or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is preferably 1% or less, more preferably 0.1% or less, and even more preferably 0.01% or less.

[0205] Furthermore, for the purpose of improving safety, etc., an additive such as vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), or a dinitrile compound such as succinonitrile or adiponitrile may be added to the electrolyte to form a coating (solid electrolyte interphase film) at the interface between the electrode (active material layer) and the electrolyte. The concentration of the additive is preferably, for example, 0.1 wt % to 5 wt % relative to the solvent.

[0206] In the electrolyte example 2, the lithium salt may be the same as that described in the electrolyte example 1. The additive may also be the same as that described in the electrolyte example 1.

[0207] As described above, examples of the electrolyte solution that can be used in the lithium-ion battery of one embodiment of the present invention have been described, but the electrolyte solution that can be used in the lithium-ion battery of one embodiment of the present invention is not limited to this example. Other materials can also be used as long as they have excellent lithium-ion conductivity even during charge and discharge in a low-temperature environment.

[0208] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector, and the negative electrode active material layer has a negative electrode active material.

[0209] <Negative Electrode Binder> A polymer having a carboxy group is preferably used as the binder for the negative electrode, which is one embodiment of the present invention. The carboxy group can be said to have two basic oxygen atoms, one acidic hydrogen atom, and one electrophilic carbon atom. The carboxy group can also be said to have a hydroxyl group (OH) and a carbonyl group (C=O), and thus be a polar group. When the binder has a polar group such as a carboxy group, it is expected to interact with lithium ions, which are carrier ions. For example, the binder may attract lithium ions, thereby assisting the insertion of lithium ions into the negative electrode active material. The carboxy group can be identified using FT-IR or the like.

[0210] Examples of polymers having carboxy groups include polyglutamic acid (sometimes referred to as PGA), polyacrylic acid (sometimes referred to as PAA), and alginic acid (sometimes referred to as polysaccharide). Polyamino acids may also be used as polymers having carboxy groups, and specifically, polyornithine and polysarcosine may be used as binders. Furthermore, polyaspartic acid may also be used as a binder for polymers having ketone groups. Furthermore, binary copolymers (copolymers) may also be used as polymers having ketone groups, and copolymers of acrylic acid and maleic acid, or copolymers of acrylic acid and sulfonic acid may also be used as binders. Using these as binders for the negative electrode also has the effect of reducing the amount of binder mixed in the negative electrode.

[0211] Among the above polymers, polyglutamic acid or polyacrylic acid is particularly preferred as a binder for use in a negative electrode. The structural formula of polyglutamic acid is shown below.

[0212]

[0213] As is clear from the structural formula, polyglutamic acid contains nitrogen in addition to carboxyl groups, and since the nitrogen has an unshared electron pair, it is expected to interact with lithium ions, which are carrier ions. For example, the unshared electron pair may attract lithium ions and assist their insertion into the negative electrode active material.

[0214] Furthermore, as is clear from the structural formula, polyglutamic acid has a carbonyl group, C=O, in addition to the carbonyl group. If the binder has a polar group such as a carbonyl group, it is expected to interact with lithium ions, which are carrier ions, and may, for example, assist the insertion and desorption of lithium ions in the negative electrode active material.

[0215] Either linear γ-polyglutamic acid or cross-linked γ-polyglutamic acid may be used as the binder. These are collectively referred to as a structure mainly composed of γ-polyglutamic acid. Cross-linked γ-polyglutamic acid is more suitable for binders because it has a network structure. Furthermore, the molecular weight of polyglutamic acid should be 1 million or more, preferably 3 million or more, and more preferably 10 million to 50 million.

[0216] Depending on the method for preparing polyglutamic acid, it can be said that the structure is mainly composed of γ-glutamic acid containing other elements (e.g., Ca, Al, Na, Mg, Fe, Si, S). That is, polyglutamic acid may be neutralized with alkali metal ions, such as lithium ions or sodium ions.

[0217] Such polyglutamic acid is hydrophilic, so deionized water can be used as a solvent, which is suitable for forming a slurry.

[0218] The structural formula of polyacrylic acid is shown below.

[0219]

[0220] As is clear from the structural formula, polyacrylic acid has a carboxy group.

[0221] A material obtained by cross-linking polyacrylic acid may also be used. This is preferable because it can form a cross-linked structure, i.e., a network structure, which may enhance the function as a binder.

[0222] <Negative Electrode Active Material> A negative electrode according to one embodiment of the present invention includes both carbon particles and silicon particles as negative electrode active materials. Examples of the carbon particles include graphite, carbon having a layer structure similar to graphite, amorphous carbon, and hard carbon. Specifically, graphite particles are preferably used as the carbon particles used in this specification.

[0223] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is therefore preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0224] The graphite particles according to one embodiment of the present invention preferably have an average particle size of 1 μm or more, preferably 5 μm or more, preferably 10 μm or more, more preferably 20 μm or more. The graphite particles may be mixed with silicon particles before use in the negative electrode.

[0225] The average particle size of the graphite particles can be measured by a particle size distribution analyzer using a laser diffraction / scattering method, etc. In this specification and the like, the average particle size of the graphite particles can be determined as the median diameter (D50).

[0226] The specific surface area of ​​the graphite particles is 0.5 m ２ / g or more 3m ２ / g or less. The specific surface area can be measured by the BET method. The specific surface area by the BET method is a value measured by the BET single-point method using nitrogen gas adsorption, and can be measured using an automatic specific surface area / pore distribution measuring device, Tristar II 3020 (manufactured by Micromeritics).

[0227] Silicon particles with an average particle size of 100 nm or thereabouts are preferably used, and these are sometimes called nanosilicon particles. Silicon has a capacity of 4200 mAh / g per weight, which is more than 10 times the capacity of graphite (372 mAh / g per weight of active material). However, silicon has the problem of rapid cycle degradation due to expansion and contraction during charge and discharge. Therefore, to improve cycle degradation, nanosilicon particles, in which silicon is refined to the above average particle size, are suitable.

[0228] The average particle size of silicon particles can be measured by a particle size distribution analyzer using a laser diffraction / scattering method, etc. In this specification, the average particle size of silicon particles can be determined as the median diameter (D50).

[0229] The silicon particles are preferably prepared by pulverizing silicon raw materials and adjusting the particle size to a uniform size. Through this adjustment, silicon particles having an average particle size of less than 1 μm can be obtained. Note that if the average particle size is large, the negative electrode active material layer may become thick, so it is preferable that the average particle size be less than 1 μm. The silicon particles contain at least one of silicon, silicon oxide, and silicon alloy.

[0230] The specific surface area of ​​silicon particles is 10 m ２ / g or more 35m ２ / g or less, preferably 10m ２ / g or more 15m ２ / g or less. The specific surface area can be measured by the BET method. The specific surface area by the BET method is a value measured by the BET single-point method using nitrogen gas adsorption, and can be measured using an automatic specific surface area / pore distribution measuring device, Tristar II 3020 (manufactured by Micromeritics).

[0231] In one embodiment of the present invention, when the negative electrode active material contains both graphite particles and silicon particles, a lithium ion battery with a high discharge capacity can be realized. Furthermore, since the average particle diameter of the graphite particles is different from that of the silicon particles, mixing these particles and using them in the negative electrode can increase the amount of the negative electrode active material supported. In this specification, the amount of support refers to the weight of the negative electrode active material per unit surface area of ​​the negative electrode current collector. The amount of the negative electrode active material supported can be determined according to the capacity of the positive electrode. A small amount of support can improve the output characteristics of the lithium ion battery, but a small amount will reduce the discharge capacity. Therefore, the amount of the negative electrode active material supported is set to 1.5 mg / cm. ２ The above is preferable.

[0232] In one embodiment of the present invention, the weight ratio of the graphite particles in the negative electrode active material layer may be greater than the weight ratio of the silicon particles, for example, the weight ratio of the graphite particles may be 5 to 15 times the weight ratio of the silicon particles. In other words, the weight ratio of silicon to the total weight of the powder material constituting the negative electrode active material may be 7.5 wt % to 37.5 wt %.

[0233] Furthermore, a conductive material may be added when forming the negative electrode active material layer.

[0234] In a lithium-ion battery, a negative electrode active material layer can be formed on one or both sides of a negative electrode current collector. The negative electrode active material layer is completed by applying a slurry onto the negative electrode current collector and then drying it.

[0235] In this specification, the weight ratio of each raw material may be regarded as the blending ratio of each raw material when preparing the slurry. That is, the weight ratio of the negative electrode active material is the blending ratio (wt%) of the negative electrode active material to the total weight of the negative electrode active material and binder in the slurry, or to the total weight of the negative electrode active material, binder, and conductive material. The weight ratio and blending ratio can be understood by replacing the negative electrode active material with the binder.

[0236] The weight ratio of the binder is preferably smaller than the weight ratio of the graphite particles, and in order to exert the effect as a binder, the weight ratio of the binder is preferably greater than 5 wt %.

[0237] <Negative electrode current collector> In addition to the same materials as the positive electrode current collector, copper can also be used for the negative electrode current collector. Note that metals that alloy with lithium ions, such as aluminum, cannot be used for the negative electrode current collector.

[0238] Similarly to the positive electrode current collector, the laminated current collector can be used as the negative electrode current collector 31. When the laminated current collector 17 is used as the negative electrode current collector 31, it is preferable to use copper for the metal layer 16.

[0239] <Method for Producing Negative Electrode Active Material Layer> Here, a method for producing a negative electrode active material layer will be described. A negative electrode slurry according to one embodiment of the present invention can be prepared by mixing graphite particles, silicon particles, and a binder having a carboxy group, followed by adding a solvent. In the slurry according to one embodiment of the present invention, the graphite particles, silicon particles, and the binder having a carboxy group can be mixed simultaneously, which is preferable because it can shorten the process. Furthermore, when preparing the slurry, the graphite particles, silicon particles, the binder having a carboxy group, and the solvent can also be mixed simultaneously. Furthermore, when preparing the slurry, a conductive material can also be mixed simultaneously. The conductive material described above can be used, and for example, acetylene black can be used.

[0240] An example of a flow for producing the negative electrode active material layer is shown in FIG.

[0241] First, there are prepared graphite particles 400, silicon particles 401, a binder 402, and a conductive material 403. A polymer having a carboxy group is used as the binder.

[0242] <Step S60> The above-mentioned raw materials are each weighed, and the first mixing is performed in step S60 of Figure 18. Specifically, the weight ratio of silicon particles 401 to the total weight of the powders mixed in the first mixing ranges from 7.5 wt% to 37.5 wt%, and the weight ratio of binder 402 to the total weight ranges from 10 wt% to 50 wt%. Furthermore, the weight ratio of conductive material 403 to the total weight ranges from 0 wt% to 20 wt%. Note that acetylene black is preferably used as conductive material 403 to satisfy the above weight ratios.

[0243] For example, the silicon particles 401, graphite particles 400, binder 402, and conductive material 403 are weighed out to a weight ratio of 3:5:1:1. Alternatively, without using the conductive material, the silicon particles 401, graphite particles 400, and binder 402 are weighed out to a weight ratio of 3:5:1. Alternatively, the graphite particles 400, silicon particles 401, and binder 402 may be weighed out to a weight ratio of 9:1:1.

[0244] <Mixing of mixture 404 and solvent 405> In one aspect of the present invention, since all raw materials are powders in step S60, they are mixed before adding the solvent to obtain mixture 404. Mixing the powders together allows for a uniform mixture. After that, solvent 405 can be added. Deionized water is preferably used as solvent 405.

[0245] <Step S61> After adding the solvent 405, the second mixing is performed in step S61 of Fig. 18 to prepare a slurry 406. The second mixing is sometimes called slurry preparation.

[0246] The slurry 406 is a material liquid used to form an active material layer on a current collector, and contains at least an active material, a binder, and a solvent, and may further contain a conductive material. The slurry is sometimes called an electrode slurry or an active material slurry.

[0247] Then, in step S62 of FIG. 18 , the slurry 406 is applied onto the negative electrode current collector 407. It is preferable to use the coated current collector described with reference to FIGS. 6 and 7 as the negative electrode current collector 407. Thereafter, in step S63 of FIG. 18 , the negative electrode current collector 407 is dried. Pre-drying and main drying may be performed as drying conditions. That is, two drying steps are performed, with the first drying step being performed under milder conditions. For example, the slurry 406 may be dried in a dryer at a temperature of 40° C. to 60° C. for 10 minutes to 1 hour, which may be referred to as pre-drying. Next, as main drying, the slurry 406 may be dried in a dryer at a temperature of 60° C. to 90° C. for 30 minutes to 1.5 hours. Pressing may be performed simultaneously with drying.

[0248] After drying, a pressing process is performed as step S64 in FIG. 18. A roll press can be used for the pressing process, but the upper and lower rollers can be heated to a temperature of 100°C or higher and 150°C or lower. In other words, heating can be performed simultaneously with the pressing process. The linear pressure during pressing can be 0.3 MPa or higher and 1 MPa or lower. Of course, the lithium-ion battery can be operated even if the pressing process is omitted.

[0249] Through the above steps, the negative electrode 408 having the negative electrode active material layer over the negative electrode current collector 407 can be manufactured.

[0250] A lithium ion battery using the negative electrode 408 thus obtained has a large discharge capacity and exhibits excellent cycle characteristics.

[0251] It is preferable to prevent the silicon particles from being oxidized. For example, when preparing a slurry, it is preferable to carry out a mixing process so that the silicon particles are not oxidized.

[0252] In the above-described method for producing a negative electrode active material layer, a method for producing a negative electrode active material layer containing both graphite particles and silicon particles has been described. Fig. 19 shows a method for producing a negative electrode active material layer using graphite particles as the negative electrode active material.

[0253] The method for producing a negative electrode active material layer shown in Fig. 19 differs from the method described in Fig. 18 in that graphite particles, a binder, a thickener, a conductive material, and a solvent are prepared as materials. Also, Fig. 19 shows step S160, which is different from step S60, as a process corresponding to step S60 in Fig. 18. In the mixing in step S160, the ratio of graphite particles:binder:thickener:conductive material can be, for example, 96:2:1:1 by weight, but is not limited to this ratio and may be changed as appropriate.

[0254] Steps S161, S162, and S163 in FIG. 19 can be processed in the same manner as steps S62, S63, and S64 in FIG. 18, respectively.

[0255] In this manner, a negative electrode active material layer using graphite particles as the negative electrode active material can be produced.

[0256] [Separator] A separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those formed from cellulose fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or porous films made of nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, polyimide, or polyurethane. The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0257] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0258] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0259] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0260] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the secondary battery can be increased.

[0261] [Exterior Body] The exterior body of the battery can be made of a metal material such as aluminum, stainless steel, or titanium, or a resin material. A film-like exterior body can also be used. Examples of films that can be used include a three-layer structure in which a highly flexible metal thin film or metal foil such as aluminum, stainless steel, titanium, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the metal thin film as the outer surface of the exterior body. Such a multilayer structure film can be called a laminate film. In this case, the laminate film may be referred to as an aluminum (aluminum) laminate film, a stainless steel laminate film, a titanium laminate film, a copper laminate film, a nickel laminate film, or the like, using the name of the material of the metal layer.

[0262] The material or thickness of the metal layer of the laminate film may affect the flexibility of the battery. For example, an aluminum laminate film having a polypropylene layer, an aluminum layer, and nylon is preferably used as an exterior body for a battery with excellent flexibility (flexibility). Here, the thickness of the aluminum layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and more preferably 20 μm or less. If the aluminum layer is thinner than 10 μm, there is a concern that pinholes in the aluminum layer may reduce the gas barrier properties, so the thickness of the aluminum layer is preferably 10 μm or more.

[0263] Alternatively, a graphene sheet may be used as the laminate film instead of the metal layer. The graphene sheet may be a multilayer graphene sheet having a thickness of 100 nm to 30 μm, preferably 200 nm to 20 μm. The graphene sheet is flexible, has an interlayer distance of 0.34 nm, and has gas barrier properties, making it suitable for use as an exterior body for a secondary battery.

[0264] [Method for Processing a Film Having Concave and Convex Portions] Next, a method for processing a film that can be used for an exterior body will be described. The above-mentioned laminate film can be used as the film.

[0265] For example, a laminate film can be used as the laminate film. For example, a laminate film having a heat seal layer on one or both surfaces of a metal film can be used as the laminate film. For the adhesive layer, a heat-sealable resin film containing polypropylene, polyethylene, or the like can be used. In this embodiment, an aluminum laminate film is used which has a nylon resin on the surface of an aluminum foil and an acid-resistant polypropylene film and a polypropylene film laminate on the back surface of the aluminum foil.

[0266] The film is then embossed, resulting in a film with a concave-convex pattern. The film has a plurality of concave-convex portions, giving it a visible wavy pattern.

[0267] Embossing, which is a type of press working, will be explained below.

[0268] Fig. 20 is a cross-sectional view showing an example of embossing. Embossing is a type of press processing, and refers to a process in which an embossing roll with an uneven surface is pressed against a film to form unevenness in the film corresponding to the unevenness of the embossing roll. The embossing roll is a roll with a pattern engraved on its surface.

[0269] 20 shows an example of embossing on both sides of a film, and a method of forming a film with convex portions having peaks on one side.

[0270] 20 shows a film 90 being sandwiched between an embossing roll 95 in contact with one side of the film and an embossing roll 96 in contact with the other side, and being fed in a film traveling direction 91. A pattern is formed on the film surface by pressure or heat. Alternatively, a pattern may be formed on the film surface by both pressure and heat.

[0271] As the embossing roll, a metal roll, a ceramic roll, a plastic roll, a rubber roll, an organic resin roll, a wooden roll, or the like can be used as appropriate.

[0272] In Figure 20, embossing is performed using an embossing roll 96, which is a male-pattern embossing roll, and a female-pattern embossing roll 95. The male-pattern embossing roll 96 has multiple convex portions 96a. These convex portions correspond to the convex portions to be formed on the film to be processed. The female-pattern embossing roll 95 has multiple convex portions 95a. Adjacent convex portions 95a form recesses that fit into the convex portions to be formed on the film by the convex portions 96a provided on the male-pattern embossing roll 96.

[0273] By successively performing embossing to raise a portion of the film 90 and blank pressing to depress a portion of the film 90, it is possible to continuously form convex portions and flat portions. As a result, a pattern can be formed on the film 90.

[0274] Next, a film having a plurality of convex portions with shapes different from those shown in Fig. 20 will be described with reference to Fig. 21A to Fig. 21E. By changing the convex shapes of the embossing roll 95 and the embossing roll 96 shown in Fig. 20 to shapes different from those shown in Fig. 20, embossing with various cross-sectional shapes shown in Fig. 21A to Fig. 21E can be performed.

[0275] Fig. 21A is a cross-sectional schematic diagram of an embossment having a wavy shape, and Figs. 21B to 21E are modified examples of Fig. 21A. Figs. 21B and 21C are diagrams showing an example in which the wavy shape is formed in a stepped shape, Fig. 21D is a diagram showing an example in which the wavy shape is formed in a rectangular shape, and Fig. 21E is a diagram showing an example in which the wavy shape is formed with acute-angled valley shapes and trapezoidal peak shapes.

[0276] 22A and 22B are perspective views showing the resulting shape when the embossing process shown in FIGS. 20 to 21E is performed twice, with the film 90 rotated in different directions. Specifically, the film 90 is embossed in a first direction, and then embossed in a second direction rotated 90 degrees from the first direction, resulting in a film 81 (81a, 81b) having the embossed shape (which may be referred to as a cross-wave shape) shown in FIGS. 22A and 22B . The film 81a having the cross-wave shape shown in FIG. 22A represents the shape used when fabricating a secondary battery using a single film 81a, and can be folded in half along the dashed line. The film 81b having the cross-wave shape and the unembossed film 81c shown in FIG. 22B represent the shape used when fabricating a secondary battery using two films (film 81b, film 81c), and the film 81b and the film 81c can be stacked together.

[0277] As described above, by performing processing using an embossing roll, it is possible to miniaturize the device. Furthermore, since processing can be performed without cutting the film, it is excellent in mass productivity. Note that processing is not limited to using an embossing roll, and for example, the film may be processed by pressing a pair of embossing plates with uneven surfaces against the film. In this case, one of the embossing plates may be flat, and processing may be performed in multiple steps.

[0278] In the above-described configuration example of the secondary battery, an example is shown in which the exterior body on one side of the secondary battery has an embossed shape and the exterior body on the other side does not have an embossed shape. However, the configuration of a secondary battery of one embodiment of the present invention is not limited to this. For example, the exterior body on one side of the secondary battery and the exterior body on the other side may have the same embossed shape. Furthermore, the exterior body on one side of the secondary battery and the exterior body on the other side may have different embossed shapes.

[0279] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0280] Embodiment 2 In this embodiment, a positive electrode active material of one embodiment of the present invention will be described with reference to FIGS.

[0281] A positive electrode active material of one embodiment of the present invention includes any one or more of “lithium cobalt oxide to which magnesium and titanium are added,” “lithium cobalt oxide to which magnesium, titanium, and fluorine are added,” “lithium cobalt oxide to which magnesium, nickel, and titanium are added,” “lithium cobalt oxide to which magnesium, nickel, titanium, and fluorine are added,” “lithium cobalt oxide to which magnesium, nickel, aluminum, and titanium are added,” and “lithium cobalt oxide to which magnesium, nickel, aluminum, titanium, and fluorine are added.” In other words, a positive electrode active material of one embodiment of the present invention includes any one or more of “lithium cobalt oxide having magnesium and titanium,” “lithium cobalt oxide having magnesium, titanium, and fluorine,” “lithium cobalt oxide having magnesium, nickel, titanium, and fluorine,” “lithium cobalt oxide having magnesium, nickel, aluminum, and titanium,” and “lithium cobalt oxide having magnesium, nickel, aluminum, titanium, and fluorine.” In particular, the positive electrode active material of one embodiment of the present invention more preferably includes lithium cobalt oxide to which magnesium, nickel, aluminum, titanium, and fluorine are added (lithium cobalt oxide having magnesium, nickel, aluminum, titanium, and fluorine). Note that in this specification, "lithium cobalt oxide to which magnesium and titanium are added" may also be referred to as "lithium cobalt oxide having magnesium and titanium" as described above, and the same may also be referred to as "a positive electrode active material having magnesium, titanium, and lithium cobalt oxide," "positive electrode active material particles having magnesium, titanium, and lithium cobalt oxide," or the like. The region in which the added element is present in the positive electrode active material of one embodiment of the present invention will be described with reference to FIGS. 23A , 23B , and 23C .

[0282] Fig. 23A is a cross-section of a positive electrode active material 100 of one embodiment of the present invention. Fig. 23B is a schematic diagram of element distribution when measurement is performed from the surface toward the inside in a cross-section analysis including the surface and a surface layer portion of the positive electrode active material 100. Measurement from the surface toward the inside is also referred to as measurement in the depth direction, and the X1-X2 and Y1-Y2 arrows in Fig. 23A are examples of the depth direction.

[0283] As shown in Fig. 23A, the positive electrode active material 100 has a surface layer portion 100a and an inner portion 100b. In Fig. 23A, the boundary between the surface layer portion 100a and the inner portion 100b is indicated by a dashed line. In the figure, (001) represents LiMO. ２ This shows the (001) plane of LiMO. ２ belongs to the space group R-3m.

[0284] In this specification, the surface layer 100a of the positive electrode active material 100 refers to, for example, a region extending from the surface toward the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface toward the interior. Note that approximately perpendicular refers to an angle of 80° to 100°. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0285] The region deeper than the surface layer 100a of the positive electrode active material is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0286] Furthermore, when the positive electrode active material 100 has a layered rock salt crystal structure of the space group R-3m, the surface layer portion 100a has a basal region and an edge region.

[0287] The basal region has a surface parallel to the (001) plane (also referred to as a (001)-oriented surface), and the region extending from the surface inward perpendicularly or approximately perpendicularly from the surface to within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm is called the basal region. Note that "parallel" here means that the angle formed between the perpendicular to the first plane (the (001) plane) and the normal to the second plane (the surface of the positive electrode active material 100) is 0 degrees or more and less than 10 degrees, preferably 0 degrees or more and 5 degrees or less, more preferably 0 degrees or more and 2.5 degrees or less.

[0288] The edge region is a region of the surface layer portion 100a other than the basal region. In other words, the edge region has a surface exposed in a direction intersecting with the (001) plane (also referred to as a surface other than the (001) orientation), and the region extending from the surface toward the interior, perpendicular or substantially perpendicular to the surface, is referred to as the edge region. Note that, intersecting here means that the angle formed between the perpendicular to the first surface (the (001) plane) and the normal to the second surface (the surface of the positive electrode active material 100) is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less, and even more preferably 50 degrees or more and 90 degrees or less.

[0289] The surface of the positive electrode active material 100 refers to the surface of the positive electrode active material particles, including the surface layer 100a and the interior 100b. Therefore, the positive electrode active material 100 does not include metal oxides, such as aluminum oxide, that do not have lithium sites that can contribute to charging and discharging and that are attached to the particle surface, carbonates that are chemically adsorbed after the preparation of the positive electrode active material, hydroxyl groups, etc. The attached metal oxide refers to, for example, a metal oxide whose crystal structure orientation does not match that of the interior 100b, i.e., a metal oxide that does not have continuity in terms of the ordered atomic arrangement. In other words, the oxide region (e.g., a region having a rock salt-type crystal structure) that is continuous with the interior 100b of the positive electrode active material 100 in terms of the ordered atomic arrangement is included in the positive electrode active material 100.

[0290] Furthermore, the positive electrode active material 100 does not include electrolytes, organic solvents, binders, conductive materials, or compounds derived from these materials that are attached to the positive electrode active material 100 .

[0291] Although not shown, the positive electrode active material 100 may have a crystal grain boundary. The crystal grain boundary refers to, for example, a portion where particles of the positive electrode active material 100 adhere to each other, a portion where the crystal orientation changes within the positive electrode active material 100, i.e., a portion where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. Furthermore, the crystal defect refers to a defect that can be observed in a cross-sectional TEM image, a cross-sectional STEM image, etc., i.e., a structure in which other atoms have entered between the lattices, a cavity, etc. The crystal grain boundary can be considered to be one type of planar defect. Furthermore, the vicinity of the crystal grain boundary refers to a region within 10 nm from the crystal grain boundary.

[0292] <Containing Elements> The positive electrode active material 100 contains lithium, a transition metal M, oxygen, and an additive element. The transition metal M is at least one selected from cobalt, nickel, and manganese. Alternatively, the positive electrode active material 100 may be a composite oxide of lithium and the transition metal M (LiMO). ２ However, the positive electrode active material 100 of one embodiment of the present invention preferably has a distribution or crystal structure of the additive element described later. Therefore, the composition is not strictly limited to Li:M:O=1:1:2 (atomic ratio).

[0293] The positive electrode active material of a lithium-ion secondary battery must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material 100 of one embodiment of the present invention preferably uses cobalt as the transition metal M responsible for the oxidation and reduction reaction. It is preferable that the positive electrode active material 100 contains 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals M contained in the positive electrode active material 100, because of many advantages such as relatively easy synthesis and handling, and excellent cycle characteristics.

[0294] Furthermore, when cobalt is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metal M in the positive electrode active material 100, lithium nickel oxide (LiNiO ２Compared to composite oxides in which nickel accounts for the majority of the transition metal, such as lithium nickel oxide (LiNiO), the stability when a large amount of lithium is released upon charging is superior. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides in which octahedral-coordinated low-spin nickel(III) accounts for the majority of the transition metal M, such as lithium nickel oxide, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen more susceptible to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions and are closer in size to lithium ions. Therefore, layered rock-salt composite oxides in which nickel accounts for the majority of the transition metal M, such as lithium nickel oxide, have the problem of prone to cation mixing between nickel and lithium.

[0295] The additive elements contained in the positive electrode active material 100 are preferably one or more selected from magnesium, titanium, nickel, aluminum, fluorine, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0296] That is, the positive electrode active material 100 can have "lithium cobalt oxide to which magnesium and titanium have been added," "lithium cobalt oxide to which magnesium, titanium, and fluorine have been added," "lithium cobalt oxide to which magnesium, nickel, and titanium have been added," "lithium cobalt oxide to which magnesium, nickel, titanium, and fluorine have been added," "lithium cobalt oxide to which magnesium, nickel, aluminum, and titanium have been added," "lithium cobalt oxide to which magnesium, nickel, aluminum, titanium, and fluorine have been added," and the like.

[0297] The additive element is preferably dissolved in the positive electrode active material 100. Therefore, for example, when a line analysis is performed using STEM-EDX (Energy Dispersive X-ray Spectroscopy), the depth at which the amount of the additive element detected increases is preferably located deeper than the depth at which the amount of the transition metal M detected increases, i.e., closer to the interior of the positive electrode active material 100.

[0298] In this specification and the like, the depth at which the amount of a certain element detected increases in STEM-EDX line analysis refers to the depth at which measurement values ​​that can be determined not to be noise in terms of intensity, spatial resolution, etc. are continuously obtained.

[0299] These added elements further stabilize the crystal structure of the positive electrode active material 100, as will be described later.

[0300] It is not necessary for the additive elements to contain all of fluorine, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0301] For example, if the cathode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, are further enhanced. The weight of manganese contained in the cathode active material 100 is preferably 600 ppm or less, more preferably 100 ppm or less.

[0302] The positive electrode active material 100 has the above-described additive element in the surface layer portion 100a. It is more preferable that the positive electrode active material 100 has a plurality of additive elements. It is also preferable that the surface layer portion 100a has a higher concentration of one or more selected from the additive elements than the inner portion 100b. Alternatively, it is preferable that the surface layer portion 100a has a higher detected amount of one or more selected from the additive elements than the inner portion 100b. It is also preferable that the one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient.

[0303] In some cases, it may be more preferable for the distribution of the positive electrode active material 100 to differ depending on the added element. For example, it may be more preferable for the depth of the peak of the detected amount in the surface layer 100a from the surface or from a reference point in EDX-ray analysis, which will be described later, to differ depending on the added element. The peak of the detected amount here refers to the maximum value (maximum value) of the detected amount at a depth of 50 nm or less from the surface of the surface layer 100a. The detected amount refers to, for example, the number of characteristic X-rays counted in EDX-ray analysis.

[0304] [Titanium] The presence of titanium as an additive element in the surface layer portion 100 a can promote the insertion and desorption of lithium ions into and from the positive electrode active material 100 .

[0305] Although titanium exists stably in the octahedral sites of hexavalent oxygen atoms in the oxide, titanium oxide and lithium titanate cannot form a stable layered rock salt type crystal structure. For example, TiO ２ The rutile crystal structure is the most stable, and Li ４ Ti ５ O １２ has a spinel-type crystal structure. Therefore, when a small amount of titanium is dissolved in the surface layer 100a having a layered rock salt type or rock salt type crystal structure, defects occur in part of the crystal structure of the surface layer 100a.

[0306] Lithium cobalt oxide is in a discharged state (i.e., Li ｘ CoO ２ When x is 1 in the formula (1), the band gap is large and the electrical resistance is high. Therefore, by introducing titanium defects into a part of the surface layer 100a, the band gap can be narrowed and the resistance can be reduced to a desirable degree.

[0307] Cobalt is also present in the oxide. ３＋ is the most stable, while titanium is Ti ４＋ Therefore, the charge on the lithium ion around titanium is relatively lower than that around cobalt. ４＋Defects may be induced in the cation sites near titanium, and such defects reduce the diffusion resistance of cations, especially lithium ions. Therefore, the diffusion resistance of lithium ions is reduced around titanium. In addition, some of the cobalt around titanium is converted into Co due to the presence of titanium. ２＋ It may be the case.

[0308] Therefore, the presence of titanium in the surface layer portion 100a can reduce the diffusion resistance of lithium ions at the interface between the electrolyte and the positive electrode active material 100. Furthermore, it is believed that the titanium present in the surface layer portion 100a does not significantly reduce the electrical resistance (e.g., powder resistance) of the positive electrode active material 100. A relatively high electrical resistance of the positive electrode active material 100 is preferable because it contributes to safety in the event of a short circuit in the secondary battery.

[0309] If the amount of titanium is too small, the effect of promoting the insertion and desorption of lithium ions described above is not fully exhibited. On the other hand, if the amount of titanium is too large, it may form a different phase from other added elements such as magnesium (for example, MgTiO with an ilmenite-type crystal structure). ３ Furthermore, magnesium may be lost in the formation of a different phase, which may reduce the magnesium concentration in the surface layer 100a, which has a crystal structure roughly identical to that of the interior 100b. ＋ For the positive electrode active material 100 of one embodiment of the present invention, which is intended for high-voltage charging exceeding 100 V, magnesium present at a high concentration in the surface layer portion 100a is essential for suppressing phase change. Therefore, the loss of magnesium due to heterophase formation is a significant disadvantage. Furthermore, if there are too many titanium defects, there is a concern that oxygen may be more likely to be desorbed from the surface. Therefore, titanium, together with magnesium, fluorine, and the like, is preferably present at a lower concentration than magnesium in the surface and near-surface regions of the surface layer portion 100a.

[0310] Specifically, the Ti / Co ratio of the entire positive electrode active material is preferably 0.0001 (atomic ratio) or more, more preferably 0.001 (atomic ratio) or more, and is preferably less than 0.0075 (atomic ratio), more preferably 0.004 (atomic ratio) or less, and even more preferably 0.003 (atomic ratio) or less.

[0311] Furthermore, in EDX ray analysis of the surface layer portion 100a, when the denominator is the sum of carbon, oxygen, fluorine, magnesium, sodium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, gallium, zirconium, and molybdenum, the maximum titanium concentration is preferably 0.3 at% or more and 3 at% or less, and more preferably 0.5 at% or more and 2 at% or less.

[0312] Furthermore, in an XPS analysis for analyzing the surfaces of a plurality of particles of the positive electrode active material 100, when the denominator is the sum of carbon, oxygen, fluorine, magnesium, sodium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, gallium, zirconium, and molybdenum, the titanium concentration is preferably 0.5 at% or more and 5.0 at% or less, and more preferably 1.0 at% or more and 2.0 at% or less.

[0313] In addition, in an EPMA analysis of the particle cross section of the positive electrode active material 100, it is preferable that titanium is not detected (below the lower detection limit) or is present at a level similar to the lower detection limit in the inner portion 100b.

[0314] The titanium source is not particularly limited, but may be, for example, TiO ２ Titanium oxide such as Li ２ TiO ３ , LiTiO ２ , Li ４ Ti ５ O １２ In particular, when a heating step is carried out after mixing the titanium source, it is possible that part of the lithium evaporates during heating, and therefore it is preferable to use the lithium titanate described above that contains lithium.

[0315] Furthermore, the element that can be added to promote the insertion and desorption of lithium ions is not limited to titanium. Other elements may also be added. When using elements other than titanium, it is preferable that the element stably exists at the hexacoordinated octahedral oxygen site in the oxide, but that the oxide of lithium and the element does not form a stable layered rock-salt type crystal structure. Furthermore, elements that are stable at a higher oxidation number than cobalt are preferable. For example, niobium, manganese, tungsten, chromium, molybdenum, rhenium, tantalum, etc. can be used.

[0316] [Distribution] To achieve the above-described effects, it is preferable that the concentration of at least magnesium (Mg), nickel (Ni), and titanium (Ti) among the additive elements is higher in the surface layer 100a than in the interior 100b, as shown in FIG. 23B . Alternatively, it is preferable that the detected amount in the surface layer 100a is greater than that in the interior 100b. Furthermore, it is preferable that the detected amount peak in the surface layer 100a be closer to the surface. For example, it is preferable that the detected amount peak be on the surface or within 3 nm from the reference point. It is also preferable that the distributions of magnesium and nickel have an overlapping region. Note that in FIG. 23B , the distributions of magnesium, nickel, and titanium are shown as a single curve to avoid cluttering the drawing, but this is not intended to limit the configuration in which the distribution positions and concentrations of the above elements coincide. For example, the detected amount peaks of magnesium and nickel may be at the same depth, or the magnesium peak may be closer to the interior, or the nickel peak may be closer to the interior. In other words, the detected amount peaks of magnesium and nickel may be at the same depth, or the magnesium peak may be closer to the surface, or the nickel peak may be closer to the surface.

[0317] The difference in depth between the peak of detected nickel and the peak of detected magnesium is preferably within 3 nm, more preferably within 1 nm. The half-width of the detected amounts is preferably narrow. In this specification, the half-width refers to the full width at half maximum.

[0318] Similarly, it is preferable to have a region where the distributions of magnesium and titanium overlap. The peaks of the detected amounts of magnesium and titanium may be at the same depth, or the magnesium peak may be closer to the inside, or the titanium peak may be closer to the inside. In other words, the peaks of the detected amounts of magnesium and titanium may be at the same depth, or the magnesium peak may be closer to the surface, or the titanium peak may be closer to the surface. In other words, the difference in depth between the peak of the detected amount of titanium and the peak of the detected amount of magnesium is preferably within 3 nm, and more preferably within 1 nm. In addition, it is preferable that the half-width of the detected amount is narrow.

[0319] Similarly, it is preferable to have a region where the distributions of titanium and nickel overlap. The peaks of the detected amounts of titanium and nickel may be at the same depth, or the titanium peak may be closer to the inside, or the nickel peak may be closer to the inside. The difference in depth between the peak of the detected amount of titanium and the peak of the detected amount of nickel is preferably within 3 nm, more preferably within 1 nm. In addition, it is preferable that the half-width of the detected amount is narrow.

[0320] In other words, it is preferable that the distributions of magnesium, nickel, and titanium overlap. The difference in depth between the peaks of the detected amount of titanium, nickel, and magnesium is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detected amount is narrow.

[0321] The above-mentioned overlapping regions of magnesium and nickel, the overlapping regions of magnesium and titanium, or the overlapping regions of magnesium, nickel, and titanium are preferably located in the edge regions of the surface layer portion 100a where lithium ions are inserted and desorbed. In other words, the distribution of the additive elements shown in the schematic diagram of FIG. 23B is preferably observed in the direction of the X1-X2 arrow in FIG. 23A. On the other hand, the above-mentioned overlapping regions are not necessarily required in the basal region of the surface layer portion 100a. The horizontal axis of FIG. 23B indicates the distance from X1 (direction of the arrow).

[0322] Furthermore, the amount of nickel detected in the inner portion 100b is very small compared to the surface portion 100a, or may not be detected, or may be 1 atomic % or less.

[0323] Similarly to magnesium, nickel, and titanium, it is preferable that the amount of fluorine detected in the surface layer 100a is greater than the amount detected inside. It is also preferable that the peak of the detection amount be in a region closer to the surface of the surface layer 100a. For example, it is preferable that the peak of the detection amount be on the surface or within 3 nm from the reference point.

[0324] 23C , an example of the distribution of the additive elements in the positive electrode active material 100 of one embodiment of the present invention, lithium cobalt oxide to which magnesium (Mg), nickel (Ni), aluminum (Al), titanium (Ti), and fluorine (F) have been added, may have a distribution as shown in FIG. 23C . Here, the peaks of the detected amounts of magnesium and fluorine are at the same depth (first position), and the peaks of the detected amounts of nickel and titanium are at the same depth (second position), and the second position is located more inward than the first position. Here, "peaks at the same depth" means that the positions of two or more peaks to be compared differ by 0.3 nm or less.

[0325] 23B and 23C, it is preferable that at least aluminum (Al) among the additive elements has a maximum detectable amount closer to the interior of the positive electrode active material particle than magnesium and titanium. As shown in FIGS. 23B and 23C, the distributions of magnesium and titanium and aluminum may have overlapping regions, but the overlapping region may be minimal. The maximum detectable amount of aluminum may be present in the surface layer portion 100a or may be deeper than the surface layer portion 100a. For example, it is preferable that the peak be present on the surface or in a region of 5 nm to 30 nm from the reference point toward the interior.

[0326] The distribution of aluminum as described above can further stabilize the layered rock-salt crystal structure of the positive electrode active material 100. For example, it is expected that the change from the layered rock-salt crystal structure to the spinel crystal structure in the surface layer 100a of the positive electrode active material 100 can be suppressed. The spinel crystal structure generated in the layered rock-salt crystal structure may move or expand due to the transfer of charge of the transition metal M. Furthermore, defects such as grain boundaries in the positive electrode active material 100 can serve as diffusion paths for lithium ions in the c-axis direction. Additional elements such as aluminum may be present near the defects.

[0327] The reason why aluminum is distributed deeper than magnesium and titanium is thought to be because aluminum has a faster diffusion rate than magnesium. On the other hand, the amount of aluminum detected in the region closest to the surface is low, presumably because aluminum exists more stably in regions where magnesium and other elements are not present in solid solution at high concentrations than in regions where they are not.

[0328] More specifically, in the region of the layered rock salt type of space group R-3m or the cubic rock salt type where magnesium is dissolved at a high concentration, layered rock salt type LiAlO ２ Compared to the case of cobalt, the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. ＋ is Mg ２＋ The valence change due to substitution to Co ３＋ From Co ２＋ This compensates for the cation balance and allows for a balance. However, since aluminum can only be trivalent, it is thought that it is difficult for it to exist stably near magnesium in a layered rock salt structure. For the same reason that aluminum can only be trivalent, it is thought that it is difficult for aluminum to exist stably in a rock salt structure.

[0329] However, the added element does not necessarily have to have the same concentration gradient or distribution throughout the entire surface layer portion 100 a of the positive electrode active material 100 .

[0330] The surface of the basal region of the positive electrode active material 100 (also referred to as the (001)-oriented surface) may have a different distribution of additive elements than the other surfaces. For example, the (001)-oriented surface and its surface layer 100a may have a lower detectable amount of one or more elements selected from the additive elements compared to surfaces other than the (001)-oriented surface. Specifically, the detectable amount of one or more elements selected from magnesium, nickel, and titanium may be low. Alternatively, the (001)-oriented surface and its surface layer 100a may have no detectable amount of one or more elements selected from the additive elements, or the detectable amount of such elements may be 1 atomic % or less. Specifically, the detectable amount of nickel may be no detectable amount or 1 atomic % or less. In particular, in the case of an analytical method that detects characteristic X-rays such as EDX, the energy of Kβ of cobalt and Kα of nickel are close, making it difficult to detect trace amounts of nickel in a material in which cobalt is the primary element. Alternatively, the (001) oriented surface and its surface layer 100a may have peaks of one or more detected elements selected from the additive elements that are shallower from the surface than those of surfaces other than the (001) oriented surface. Specifically, the peaks of the detected amounts of magnesium and aluminum may be shallower than those of other surfaces.

[0331] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is composed of alternately stacked layers and lithium layers parallel to the (001) plane, and the diffusion path of lithium ions is also parallel to the (001) plane.

[0332] MO ２ Since the layer is relatively stable, it is more stable if the surface of the positive electrode active material 100 has a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0333] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (001) orientation. Therefore, the surface and the surface layer 100a other than the (001) orientation are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surface and the surface layer 100a other than the (001) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 100.

[0334] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, it is important that the distribution of the additive elements in the surface other than the (001) orientation and in the surface layer 100a thereof is, for example, as shown in FIG. 23B . Of the additive elements, nickel is particularly preferably detected in the surface other than the (001) orientation and in the surface layer 100a thereof. On the other hand, as described above, the concentration of the additive element in the (001)-oriented surface and in the surface layer 100a thereof may be low or absent.

[0335] High-purity LiMO, which will be described in a later embodiment ２ In the manufacturing method in which the additive element is mixed and heated after the (001) orientation, the additive element spreads mainly through the diffusion path of lithium ions, and therefore the distribution of the additive element in the surface other than the (001) orientation and in the surface layer portion 100a thereof can be easily controlled to a preferred range.

[0336] [Magnesium] Magnesium ions are divalent, and since magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock salt crystal structure, they are more likely to enter the lithium site. When magnesium is present at an appropriate concentration at the lithium site in the surface layer 100a, the layered rock salt crystal structure can be easily maintained. This is because magnesium present at the lithium site is easily absorbed by the MO ２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２When x in the formula (1) is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100. Furthermore, a high magnesium concentration in the surface layer portion 100a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0337] At appropriate concentrations, magnesium does not adversely affect the lithium intercalation and deintercalation processes during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect lithium intercalation and deintercalation. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying cobalt sites in addition to lithium sites. Furthermore, excess magnesium compounds (e.g., oxides and fluorides) that do not substitute for either the lithium or cobalt sites may segregate on the surface of the positive electrode active material and become resistance components in secondary batteries. Furthermore, as the magnesium concentration in the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0338] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.002 to 0.06 times (0.2% to 6.0%) the number of cobalt atoms, more preferably 0.005 to 0.03 times (0.5% to 3.0%), and even more preferably about 0.01 times (0.1%). The amount of magnesium contained in the entire positive electrode active material 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0339] [Nickel] Nickel is LiMO ２In the layered rock salt crystal structure of the present invention, nickel can exist on either the cobalt site or the lithium site. When present on the cobalt site, its redox potential is lower than that of cobalt, making it easier to release lithium and electrons during charging. This can be expected to result in faster charge and discharge speeds. Therefore, even at the same charge voltage, a greater charge and discharge capacity can be obtained when the transition metal M is nickel than when it is cobalt.

[0340] Furthermore, when nickel exists at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the change in volume caused by charging and discharging is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel existing at the lithium site also acts as a catalyst for MO ２ This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0341] In addition, the distance between the cations and anions of nickel oxide (NiO) is closer to that of LiCoO than that of rock salt MgO and rock salt CoO. ２ The average distance between the cations and anions is close to that of LiCoO ２ The orientation is likely to match.

[0342] In addition, the order of ionization tendency is lowest for magnesium, aluminum, cobalt, and nickel (Mg>Al>Co>Ni). Therefore, nickel is thought to be less likely to dissolve into the electrolyte than the other elements listed above during charging. Therefore, it is thought to be highly effective in stabilizing the crystalline structure of the surface layer in the charged state.

[0343] Furthermore, nickel is Ni ２＋ , Ni ３＋ , Ni ４＋ Of which Ni ２＋ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. Therefore, nickel is thought to have the effect of suppressing the phase change from the layered rock salt type to the spinel-type crystal structure.

[0344] On the other hand, excessive nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and excessive nickel may also adversely affect the insertion and extraction of lithium.

[0345] Therefore, it is preferable that the entire positive electrode active material 100 contains an appropriate amount of nickel. For example, the number of nickel atoms contained in the positive electrode active material 100 is preferably more than 0% but not more than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, more than 0% but not more than 4% is preferable. Alternatively, more than 0% but not more than 2% is preferable. Alternatively, 0.05% to 7.5% is preferable. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 7.5% is preferable. Alternatively, 0.1% to 4% is preferable. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0346] [Aluminum] Aluminum can also be present at the cobalt site in a layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Therefore, as described below, even if the positive electrode active material 100 experiences a force that causes it to expand and contract in the c-axis direction due to the insertion and desorption of lithium ions, i.e., even if a force that causes it to expand and contract in the c-axis direction due to changes in the charge depth or charge rate occurs, deterioration of the positive electrode active material 100 can be suppressed.

[0347] Aluminum also has the effect of suppressing the elution of surrounding cobalt and improving continuous charge durability. Furthermore, since the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen from the aluminum's surroundings. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when the positive electrode active material 100 is used in a secondary battery. Furthermore, the positive electrode active material 100 can be made to have a crystal structure that is resistant to collapse even after repeated charge and discharge.

[0348] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0349] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material 100 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2%, and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire positive electrode active material 100 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0350] [Fluorine] Fluorine is a monovalent anion, and when a portion of the oxygen in the surface layer portion 100a is substituted with fluorine, the lithium desorption energy decreases. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. That is, in the absence of fluorine, cobalt ions change from trivalent to tetravalent upon lithium desorption. On the other hand, when fluorine is present, cobalt ions change from divalent to trivalent upon lithium desorption. The redox potential of cobalt ions differs between the two. Therefore, when a portion of the oxygen in the surface layer portion 100a of the positive electrode active material 100 is substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine easily occurs. Therefore, when the positive electrode active material 100 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer portion 100a, which has a surface that contacts the electrolyte, or the adhesion of fluoride to the surface, can suppress excessive reaction between the positive electrode active material 100 and the electrolyte. Furthermore, corrosion resistance to hydrofluoric acid can be effectively improved.

[0351] Here, differential scanning calorimetry (DSC measurement) for fluorides and mixtures will be described with reference to Figures 24A and 24B. In both cases, the horizontal axis represents the temperature of the sample, and the vertical axis represents the difference in heat flow with respect to the reference. The DSC measurement device and conditions are not particularly limited, but in this embodiment, measurements were performed using the following device and conditions. DSC device: EVO2 DSC8271 manufactured by Rigaku Corporation Heating rate: 20°C / min The obtained measurement results were subjected to background correction using the analysis software Thermo Plus EVO, and the extrapolated initial melting temperature T ｉｍ , melting peak temperature T ｐｍ and the melting end temperature T ｅｍ The heat flow was calculated as the heat flow rate per sample weight.

[0352] FIG. 24A shows a fluoride mixture of lithium fluoride and magnesium fluoride, LiF:MgF ２ Fig. 24B shows the DSC measurement results of the mixture obtained by adding lithium cobalt oxide (C-10N manufactured by Nippon Chemical Industry Co., Ltd.) to the above mixture. ２ :LiF:MgF ２ The components were mixed so that the molar ratio was 100:0.33:1.

[0353] As shown in FIG. 24A, the initial melting temperature T ｉｍ is 731 ° C, the melting peak temperature T ｐｍ is 739°C, and the melting end temperature T ｅｍ The T of the mixture of lithium cobalt oxide, lithium fluoride, and magnesium fluoride was 746°C. ｉｍ is 779°C, T ｐｍ is 815℃, T ｅｍ The temperature was 826°C.

[0354] Therefore, the heating temperature after mixing the additive element is preferably 731°C or higher, and more preferably 826°C or higher. Alternatively, it may be set to 800°C or higher, which is between these temperatures. A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is high. However, if the heating temperature is too high, there is a risk of decomposition of lithium cobalt oxide. Therefore, the upper limit of the heating temperature is set to be lower than the decomposition temperature of lithium cobalt oxide (1130°C). At temperatures near the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit in a small amount. Therefore, a temperature of 1000°C or lower is more preferable.

[0355] [Other additive elements] When phosphorus is contained in the surface layer portion 100a, Li ｘ CoO ２ When the value of x in the graph is kept small, short circuits can be prevented, which is preferable. For example, it is preferable that the graphite oxide is present in the surface layer portion 100a as a compound containing phosphorus and oxygen.

[0356] When the positive electrode active material 100 contains phosphorus, the phosphorus reacts with hydrogen fluoride generated by decomposition of the electrolytic solution or electrolyte, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable.

[0357] The electrolyte is LiPF ６ In the case where the electrolyte contains the above-mentioned compound, hydrogen fluoride may be generated by hydrolysis. Furthermore, hydrogen fluoride may be generated by a reaction between polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, and an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 104 may be suppressed. Furthermore, a decrease in adhesion due to gelation and / or insolubilization of PVDF may be suppressed.

[0358] When the positive electrode active material 100 contains phosphorus together with magnesium, Li ｘ CoO ２This is preferable because stability is extremely high when x is small in the positive electrode active material 100. When the positive electrode active material 100 contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Alternatively, 1% to 10% is preferable. Alternatively, 1% to 8% is preferable. Alternatively, 2% to 20% is preferable. Alternatively, 2% to 8% is preferable. Alternatively, 3% to 20% is preferable. Alternatively, 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and more preferably 0.7% to 4%. Alternatively, 0.1% to 5% is preferable. Alternatively, 0.1% to 4% is preferable. Alternatively, 0.5% to 10% is preferable. Alternatively, 0.5% to 4% is preferable. Alternatively, 0.7% to 10% is preferable. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire cathode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on values ​​of the composition of raw materials in the process of producing the cathode active material 100.

[0359] Furthermore, when the positive electrode active material 100 has cracks, the progression of the cracks can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the cracks on the surface, for example, in the embedded portion.

[0360] [Synergistic Effect of Multiple Added Elements] Furthermore, when the surface layer portion 100a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. ｘ MO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0361] For the same reason, when adding an additive element to lithium cobalt oxide in the manufacturing process, it is preferable to add magnesium in a step before adding nickel. Alternatively, it is preferable to add magnesium and nickel in the same step. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide regardless of the step in which it is added, whereas nickel can diffuse widely into the interior of lithium cobalt oxide in the absence of magnesium. Therefore, if nickel is added before magnesium, there is a concern that nickel will diffuse into the interior of the lithium cobalt oxide and not remain in the surface layer in the desired amount.

[0362] Furthermore, when the surface layer 100a contains both nickel and titanium, the presence of tetravalent titanium near divalent nickel may further stabilize the surrounding crystal structure. ｘ MO ２ Even when the value of x in the formula is small, the elution of oxygen (O) can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0363] Furthermore, when the surface layer 100a contains magnesium, nickel, and titanium, the presence of nickel results in the formation of MgTiO with an ilmenite-type crystal structure. ３ The formation of heterophases such as MgTiO is suppressed. The ilmenite crystal structure has hexagonal close-packed anions, which differs from the rock salt and layered rock salt crystal structures, which have cubic close-packed anions, and therefore requires a certain amount of activation energy for the phase transition. NiO(II) is a compound with low chemical activity, so MgTiO ３ It has the effect of suppressing the production of

[0364] For the above reasons, when adding an additional element to lithium cobalt oxide in the manufacturing process, if titanium is added in a process later than magnesium and nickel, the effect of nickel in suppressing the formation of heterophases is particularly strong.

[0365] Furthermore, having additive elements with different distributions in combination is preferable because it can stabilize the crystal structure over a wider region. For example, if the positive electrode active material 100 has both magnesium, nickel, and titanium distributed in a region closer to the surface of the surface layer 100a and aluminum distributed in a deeper region, it can stabilize the crystal structure over a wider region than if it only had one of these elements. In this way, when the positive electrode active material 100 has additive elements with different distributions in combination, aluminum is not essential on the surface because surface stabilization can be sufficiently achieved by magnesium, nickel, etc. Rather, it is preferable for aluminum to be widely distributed slightly inward from the surface. For example, it is preferable for aluminum to be continuously detected in a region from 0 nm to 100 nm in the depth direction from the surface, preferably a region from 0.5 nm to 50 nm from the surface, and more preferably a region from 1 nm to 25 nm from the surface. In this way, a wide distribution of aluminum is preferable because it can stabilize the crystal structure over a wider region.

[0366] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 100 a. In particular, when magnesium, nickel, aluminum, and titanium are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0367] However, if the surface layer 100a is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 100a to be occupied only by MgO or a structure in which MgO and MO(II) are solid-solved. Therefore, the surface layer 100a must contain at least a transition metal M such as cobalt, and also contain lithium in the discharged state, so that it has a path for the insertion and extraction of lithium.

[0368] In order to ensure sufficient lithium insertion / extraction paths, the surface layer portion 100a preferably has a higher cobalt concentration than magnesium. For example, when measured from the surface of the positive electrode active material 100 using XPS, the ratio Mg / Co of the number of magnesium atoms Mg to the number of cobalt atoms Co is preferably 0.4 or more and 1.5 or less. The surface layer portion 100a preferably has a higher cobalt concentration than titanium, and the ratio Ti / Co of the number of titanium atoms Ti to the number of cobalt atoms Co is preferably 0.05 or more and 0.2 or less. The surface layer portion 100a preferably has a higher cobalt concentration than nickel, and the ratio Ni / Co of the number of nickel atoms Ni to the number of cobalt atoms Co is preferably 0.05 or more and 0.2 or less. The surface layer portion 100a preferably has a higher cobalt concentration than aluminum. The surface layer portion 100a preferably has a higher cobalt concentration than fluorine.

[0369] Furthermore, since an excessive amount of nickel may inhibit the diffusion of lithium, it is preferable that the concentration of magnesium is higher than that of nickel in the surface layer portion 100 a. For example, when measured from the surface of the positive electrode active material 100 by XPS, the number of nickel atoms is preferably 1 / 6 or less of the number of magnesium atoms.

[0370] Furthermore, while it is preferable that some of the added elements, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, they are also preferably present randomly and in a sparse manner in the interior 100b. The presence of magnesium and aluminum at appropriate concentrations at the lithium sites in the interior 100b has the effect of making it easier to maintain the layered rock-salt crystal structure. Furthermore, the presence of nickel at an appropriate concentration in the interior 100b can suppress the shift in the layered structure consisting of cobalt and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, a synergistic effect of suppressing magnesium elution can be expected, as described above.

[0371] It is preferable that the crystal structure continuously changes from the interior 100b toward the surface due to the concentration gradient of the added element as described above, or that the crystal orientation of the surface layer 100a and the interior 100b roughly coincide.

[0372] For example, it is preferable that the crystal structure continuously change from the interior 100b of the layered rock salt type toward the surface and surface layer 100a, which has a rock salt type or both a rock salt type and a layered rock salt type crystal structure. Alternatively, it is preferable that the crystal orientation of the surface layer 100a, which has the characteristics of the rock salt type or both the rock salt type and the layered rock salt type, and the interior 100b of the layered rock salt type are roughly the same.

[0373] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as vacancies of cations or anions may also be present. Furthermore, the layered rock-salt crystal structure may be considered to be a structure in which the lattice of a rock-salt crystal is distorted.

[0374] The rock salt crystal structure refers to a cubic crystal structure, such as a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately, and may contain cation or anion defects.

[0375] The fact that it has both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0376] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is taken as the origin (000), the bright spot closest to the central spot is, for example, the (111) plane in the ideal rock salt type, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO ２The distance between the bright spots on the (003) plane of LiCoO is observed to be about half the distance between the bright spots on the (111) plane of MgO. ２ In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0377] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0378] Layered rock salt crystals and the anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in the O3'-type and monoclinic O1(15) crystals described below also have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is aligned.

[0379] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0380] However, the space group of the layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space group Fm-3m (space group of general rock salt crystals) of the rock salt crystals, and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt crystals and O3'-type crystals and the rock salt crystals. In this specification, when the orientations of the cubic close-packed structures formed by anions in the layered rock salt crystals, O3'-type and rock salt crystals are aligned, it may be said that the crystal orientations are approximately the same. In addition, having a three-dimensional structural similarity such that the crystal orientations are approximately the same, or having the same crystallographic orientation, is called topotaxis.

[0381] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM images, STEM images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron diffraction patterns, etc. It can also be determined from FFT patterns of TEM images and FFT patterns of STEM images, etc. Furthermore, XRD, neutron diffraction, etc. can also be used as materials for determination.

[0382] 25 shows an example of a TEM image in which the orientations of the layered rock salt crystals LRS and the rock salt crystals RS are roughly the same. Images reflecting the crystal structure can be obtained from TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0383] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicularly to the c-axis of a layered rock-salt type composite hexagonal lattice, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in FIG. 25) are not clearly distinguishable from each other. ＲＳ and L ＬＲＳWhen the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly aligned.

[0384] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers appearing brighter. For example, in the case of layered rock-salt lithium cobaltate belonging to the space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the cobalt atoms, and the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobaltate with a layered rock-salt crystal structure is observed perpendicular to the c-axis, the arrangement of the cobalt atoms perpendicular to the c-axis is observed as a bright line or an arrangement of highly bright dots, while the arrangements of lithium and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium cobaltate contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0385] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5 degrees or less or 2.5 degrees or less, it can also be determined that the crystal orientations are roughly consistent.

[0386] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to the atomic number is obtained, so the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0387] Figure 26A shows an example of an STEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS roughly coincide. Figure 26B shows the FFT pattern of the region of the rock salt crystal RS, and Figure 26C shows the FFT pattern of the region of the layered rock salt crystal LRS. The left side of Figures 26B and 26C shows the composition, JCPDS card number, and d value and angle calculated from the JCPDS card data. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction.

[0388] The spot marked A in Figure 26B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 26C is derived from the 0003 reflection of the layered rock salt type. From Figures 26B and 26C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly coincide. In other words, it can be seen that the line passing through AO in Figure 26B is roughly parallel to the line passing through AO in Figure 26C. Here, "roughly coincident" and "roughly parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

[0389] In this way, in the FFT pattern and the electron beam diffraction pattern, when the orientations of the layered rock salt type crystal and the rock salt type crystal are roughly the same, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point being spot-like and not continuous with other reciprocal lattice points means high crystallinity.

[0390] Furthermore, as described above, when the orientation of the cubic 11-1 reflection and the orientation of the layered rock salt 0003 reflection are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the layered rock salt 0003 reflection may be observed in a reciprocal lattice space different from the orientation of the layered rock salt 0003 reflection. For example, the spot marked B in Figure 26C is originating from the layered rock salt 10-14 reflection. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point originating from the layered rock salt 0003 reflection (A in Figure 26C), and at a point where d is 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be identical. For example, a reciprocal lattice point equivalent to 0003 and 10-14 may also be used.

[0391] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the orientation where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 26B is originating from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 26B) originating from the 11-1 reflection of the cubic crystal. Note that this index is merely an example and does not necessarily have to match this. For example, reciprocal lattice points equivalent to the 11-1 and 200 reflections of the cubic crystal may also be used.

[0392] It is known that layered rock salt type positive electrode active materials, such as lithium cobalt oxide, tend to have the (0003) plane and its equivalent planes, as well as the (10-14) plane and its equivalent planes, as crystal planes. Therefore, when observing the (0003) plane using a TEM or the like, it is preferable to first select positive electrode active material particles in which a crystal plane expected to be the (0003) plane is observed using a SEM or the like, and then thin-section the positive electrode active material particles using a FIB (Focused Ion Beam) or the like so that the (0003) plane can be observed using a TEM or the like with an electron beam [12-10] incident. When it is desired to determine the coincidence of crystal orientations, it is preferable to thin-section the layered rock salt type (0003) plane so that it is easy to observe.

[0393] <Crystal structure> <Li ｘ MO ２ When x is 1 in the positive electrode active material 100 of one embodiment of the present invention is in a discharged state, that is, Li ｘ MO ２ When x = 1 in the formula, the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. Layered rock-salt type composite oxides have high discharge capacity, have two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion / extraction reactions, making them excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the inner portion 100b, which occupies the majority of the volume of the positive electrode active material 100, has a layered rock-salt type crystal structure. Figure 27 shows the layered rock-salt type crystal structure, labeled R-3m O3. R-3m O3 has lattice constants a = 2.81610, b = 2.81610, c = 14.05360, α = 90.0000, β = 90.0000, and γ = 120.0000, and the coordinates of lithium, cobalt, and oxygen in the unit cell are Li(0,0,0), Co(0,0,0.5), and O(0,0,0.23951) (Non-Patent Document 5).

[0394] On the other hand, the surface layer portion 100a of the cathode active material 100 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 100b so that the layered structure of the transition metal M and oxygen octahedra is not destroyed even when lithium is removed from the cathode active material 100 upon charging. Alternatively, the surface layer portion 100a preferably functions as a barrier film for the cathode active material 100. Alternatively, the surface layer portion 100a, which is the outer periphery of the cathode active material 100, preferably reinforces the cathode active material 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 100a and inner portion 100b of the cathode active material 100, such as oxygen desorption and / or shifting of the layered structure of the transition metal M and oxygen octahedra. And / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100.

[0395] Therefore, the surface layer portion 100a preferably has a different crystal structure from the interior portion 100b. Furthermore, the surface layer portion 100a preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 100b. For example, at least a portion of the surface layer portion 100a of the positive electrode active material 100 of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0396] The surface layer 100a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the interior 100b. In addition, it can be said that the atoms on the surface of the particles of the positive electrode active material 100 in the surface layer 100a are in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and is a region where deterioration of the crystal structure is likely to begin. For example, if the crystal structure of the layered structure consisting of transition metal M and oxygen octahedra in the surface layer 100a is displaced, this effect will be transmitted to the interior 100b, causing the layered crystal structure in the interior 100b to also be displaced, which is thought to lead to deterioration of the crystal structure of the entire positive electrode active material 100. On the other hand, if the surface layer 100a can be sufficiently stabilized, Li ｘ MO ２ Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 100b made of the transition metal M and oxygen octahedra can be made less likely to break. Furthermore, displacement of the layer made of the transition metal M and oxygen octahedra in the inner portion 100b can be suppressed.

[0397] Furthermore, the interior 100b of the positive electrode active material 100 preferably has a low density of defects, including dislocations. Furthermore, the positive electrode active material 100 preferably has a large crystallite size as measured by XRD. In other words, the interior 100b preferably has high crystallinity. Furthermore, the surface of the positive electrode active material 100 preferably has a smooth surface. These characteristics are important factors supporting the reliability of the positive electrode active material 100 when used in a secondary battery. If the reliability of the positive electrode active material is high, the upper limit of the charging voltage of the secondary battery can be increased, resulting in a secondary battery with a high charge / discharge capacity.

[0398] Dislocations in the interior 100b can be observed, for example, using a TEM. If the density of defects, including dislocations, is sufficiently low, they may not be observed within a specific 1 μm square of the observation sample. Note that dislocations are a type of crystal defect and are different from vacancy defects.

[0399] The larger the crystallite size, the greater the amount of Li ｘ CoO ２ When x is small, the O3' type crystal structure is easily maintained, and contraction of the c-axis length is easily suppressed.

[0400] It is believed that the fewer defects, including dislocations, observed by TEM, the larger the crystallite size measured by XRD.

[0401] When calculating the crystallite size, the XRD diffraction pattern is preferably obtained from the positive electrode active material alone. However, it may also be obtained from the positive electrode, which includes the positive electrode active material, a current collector, a binder, a conductive material, and the like. However, in the positive electrode state, the positive electrode active material may be oriented due to the influence of pressure, etc., during the manufacturing process. Strong orientation may prevent accurate calculation of the crystallite size. Therefore, it is more preferable to obtain the pattern by removing the positive electrode active material layer from the positive electrode, removing some of the binder, etc., in the positive electrode active material layer using a solvent, etc., and then loading the sample into a sample holder. Another method is to apply grease to a silicon anti-reflective plate and then attach the powder sample.

[0402] The crystallite size is calculated using, for example, a Bruker D8 ADVANCE and CuKα X-rays. １The diffraction pattern obtained using a LYNXEYE XE-T detector with a 2θ range of 15° to 90° increments of 0.005 and ICSD Coll. Code 172909 as the literature value for lithium cobalt oxide can be used. Analysis can be performed using DIFFRAC. TOPAS ver. 6 crystal structure analysis software, with the following settings, for example: Emission Profile: CuKa5. lam Background: Chebychev polynomial, 5th Instrument Primary radius: 280mm Secondary radius: 280mm Linear PSD 2Th angular range: 2.9 FDS angle: 0.3 Full Axial Convolution Filament length: 12mm Sample length: 15mm Receiving Slit length: 12mm Primary Sollers: 2.5 Secondary Sollers: 2.5 Corrections Specimen displacement: Refine LP Factor: 0

[0403] It is preferable to use the value of LVol-IB, which is the crystallite size corrected by the integral width standard calculated by the above method, as the crystallite size. Note that if the calculated preferred orientation is less than 0.8, the orientation of the sample may be too strong and it may not be suitable for determining the crystallite size.

[0404] <Li ｘ MO ２ The positive electrode active material 100 of one embodiment of the present invention has the above-described distribution of the additive element and / or the crystal structure in a discharged state, and therefore, ｘ MO ２ The crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means 0.1<x≦0.24.

[0405] 27 to 31, Li ｘ MO ２The change in the crystal structure accompanying the change in x in the positive electrode active material 100 will be described by comparing a conventional positive electrode active material with the positive electrode active material 100 of one embodiment of the present invention.

[0406] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 28. The conventional positive electrode active material shown in FIG. 28 is lithium cobalt oxide (LiCoO ２ In particular, changes in the crystal structure of lithium cobalt oxide that does not contain any added elements are described in Non-Patent Documents 1 to 3, etc.

[0407] Figure 28 shows the R-3m O3 and Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in Fig. 1 shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a plane with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0408] It is also known that conventional lithium cobalt oxide has a crystal structure that belongs to the monoclinic space group P2 / m when x is about 0.5, and the lithium ions are aligned to increase the symmetry. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0409] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0410] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２It can also be said that the structure of and the structure of are stacked alternately. Therefore, this crystal structure is sometimes called an H1-3 crystal structure. Note that, since the actual insertion and desorption of lithium does not necessarily occur uniformly within the positive electrode active material and the lithium concentration may become uneven, the H1-3 crystal structure is experimentally observed from about x = 0.25. In fact, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in Figure 28 and other parts of this specification, to facilitate comparison with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0411] As an example of the H1-3 type crystal structure, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in a unit cell can be expressed as Co (0,0,0.42150±0.00016), O1 (0,0,0.27671±0.00045), and O2 (0,0,0.11535±0.00045). O1 and O2 are each oxygen atoms. Which unit cell is used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is preferable to adopt a unit cell that results in a small GOF (goodness of fit) value.

[0412] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0413] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 28, in the H1-3 type crystal structure, CoO ２ The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0414] Furthermore, the difference in volume between these two crystal structures is large, so that, when compared per equal number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure exceeds 3.5%, typically 3.9% or more.

[0415] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0416] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0417] On the other hand, in the positive electrode active material 100 of one embodiment of the present invention shown in FIG. ｘ MO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. ｘ MO ２ When x is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than conventional positive electrode active materials. ｘ MO ２ When the value of x in the formula (1) is kept at 0.24 or less, short circuits are unlikely to occur. In such a case, the safety of the secondary battery is further improved, which is preferable.

[0418] Li ｘ MO ２The crystal structure of the interior 100b of the positive electrode active material 100 when x is 1, approximately 0.2, and approximately 0.15 is shown in FIG. 27. The interior 100b occupies the majority of the volume of the positive electrode active material 100 and is the part that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0419] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0420] However, the positive electrode active material 100 has a crystal structure different from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.15, which gives the H1-3 type crystal structure.

[0421] The positive electrode active material 100 according to one embodiment of the present invention when x is about 0.2 has a crystal structure belonging to the trigonal space group R-3m. ２ The symmetry of the layers is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 27 with the notation R-3m O3'.

[0422] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１０ m), and 2.807≦a≦2.827(×10 −１０ m) is more preferable, and typically a=2.817(×10 −１０ The c-axis is 13.681 ≤ c ≤ 13.881 (× 10 −１０ m), and 13.751≦c≦13.811 (×10 −１０ m) is more preferable, and typically c=13.781 (×10 −１０ m).

[0423] The positive electrode active material 100 of one embodiment of the present invention when x is about 0.15 may have a crystal structure belonging to the monoclinic space group P2 / m. ２There is one layer. In addition, the lithium present in the positive electrode active material 100 at this time is about 15 atomic % in the discharged state. Therefore, this crystal structure is called a monoclinic O1(15) type crystal structure. This crystal structure is shown in Figure 27 with the P2 / m monoclinic O1(15) attached.

[0424] The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as follows: Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X Ｏ１ , 0, Z Ｏ１ ), 0.23≦X Ｏ１ ≦0.24, 0.61≦Z Ｏ１ ≦0.65, O2(X Ｏ２ , 0.5, Z Ｏ２ ), 0.75≦X Ｏ２ ≦0.78, 0.68≦Z Ｏ２ The lattice constant of the unit cell is a = 4.880 ± 0.05 (× 10 −１０ m), b=2.817±0.05(×10 −１０ m), c=4.839±0.05(×10 −１０ m), α = 90°, β = 109.6 ± 0.1°, γ = 90°.

[0425] In addition, this crystal structure can also show the lattice constant in the space group R-3m if a certain degree of error is allowed. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z Ｏ ), 0.21≦Z Ｏ The lattice constant of the unit cell is a = 2.817 ± 0.02 (× 10 −１０ m), c=13.68±0.1(×10 −１０ m).

[0426] In both the O3' and monoclinic O1(15) crystal structures, ions of cobalt, nickel, magnesium, etc. occupy hexacoordinated oxygen sites, although lighter elements such as lithium and magnesium may occupy tetracoordinated oxygen sites.

[0427] As shown by the dotted line in FIG. 27, the CoO ２ There is almost no layer misalignment.

[0428] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0429] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the monoclinic O1(15) crystal structure is 3.3% or less, more specifically 3.0% or less, typically 2.5%.

[0430] Table 2 shows the difference in volume per cobalt atom between discharged R-3m O3, O3', monoclinic O1(15), H1-3, and trigonal O1. For the lattice constants of the crystal structures of discharged R-3m O3 and trigonal O1 used in the calculations in Table 2, ICSD Coll. Code. 172909 and 88721 can be referenced. For H1-3, Non-Patent Document 3 can be referenced. For O3' and monoclinic O1(15), calculations can be made from experimental XRD values.

[0431]

[0432] As described above, in the positive electrode active material 100 according to one embodiment of the present invention, Li ｘ MO ２ When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the positive electrode active material 100 is resistant to collapse of its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0433] The positive electrode active material 100 is Li ｘ MO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, the O3' type crystal structure may be present, and it is presumed that even when x is more than 0.24 and 0.27 or less, the O3' type crystal structure is present. ｘ MO ２ It has been confirmed that when x is greater than 0.1 and less than 0.2, typically when x is 0.15 or more and less than 0.17, the crystal structure may be monoclinic O1(15) type. However, the crystal structure is Li ｘ MO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0434] Therefore, the positive electrode active material 100 is Li ｘ MO ２ When x is greater than 0.1 and equal to or less than 0.24, the positive electrode active material 100 may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, not all of the particles in the interior 100b of the positive electrode active material 100 have the O3' type and / or the monoclinic O1(15) type crystal structure. Other crystal structures may be included, or some may be amorphous.

[0435] Also, Li ｘ MO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ MO ２ A state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.60 V or higher relative to the potential of lithium metal in an environment of 25°C, a H1-3 crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.60 V or higher relative to the potential of lithium metal can be said to be a high charging voltage. Unless otherwise specified, in this specification and the like, charging voltages are expressed relative to the potential of lithium metal.

[0436] Therefore, in other words, the positive electrode active material 100 of one embodiment of the present invention is preferable because it can maintain a crystal structure having the symmetry of R-3m O3 even when charged at a high charging voltage, for example, a voltage of 4.60 V or higher at 25°C.

[0437] Even with the positive electrode active material 100, an H1-3 crystal structure may be finally observed when the charge voltage is further increased. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, the electrolyte, and the like. Therefore, when the charge voltage is lower, for example, even when the charge voltage is 4.50 V or more and less than 4.60 V at 25° C., the positive electrode active material 100 of one embodiment of the present invention may be able to adopt an O3′ crystal structure. Similarly, when charged at a voltage of 4.65 V or more and 4.70 V or less at 25° C., the positive electrode active material 100 may be able to adopt a monoclinic O1(15) crystal structure.

[0438] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.20 V relative to the potential of lithium metal. Therefore, a secondary battery using graphite as the negative electrode active material has a similar crystal structure at a voltage obtained by subtracting the potential of graphite from the voltage when lithium metal is used as the negative electrode active material.

[0439] In addition, in O3' and monoclinic O1(15) in FIG. 27, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist biasedly at some lithium sites, or, for example, in monoclinic O1(Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0440] The O3' and monoclinic O1(15) crystal structures have random lithium between layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0441] <Grain Boundaries> In addition to the above distribution, at least a portion of the additive element contained in the positive electrode active material 100 of one embodiment of the present invention is preferably unevenly distributed in and near the grain boundaries.

[0442] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the presence of a mixture of high-concentration and low-concentration regions.

[0443] For example, it is preferable that the magnesium concentration at and near the grain boundaries of the positive electrode active material 100 is higher than that in other regions of the interior 100b. It is also preferable that the fluorine concentration at and near the grain boundaries is higher than that in other regions of the interior 100b. It is also preferable that the nickel concentration at and near the grain boundaries is higher than that in other regions of the interior 100b. It is also preferable that the aluminum concentration at and near the grain boundaries is higher than that in other regions of the interior 100b.

[0444] Grain boundaries are a type of planar defect. Therefore, like particle surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of added elements at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0445] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries, even if cracks occur along the grain boundaries of the positive electrode active material 100 of one embodiment of the present invention, the magnesium concentration and fluorine concentration are high near the surface where the cracks occur. Therefore, even after the cracks occur, the corrosion resistance to hydrofluoric acid of the positive electrode active material can be improved. Furthermore, even after the cracks occur, the side reaction between the electrolyte and the positive electrode active material can be suppressed.

[0446] <Particle diameter> If the particle diameter of the positive electrode active material 100 of one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium, excessive roughness of the surface of the active material layer when applied to a current collector, etc. On the other hand, if the particle diameter is too small, problems such as excessive reaction with the electrolyte solution may occur.

[0447] The particle diameter of the positive electrode active material 100 can be measured, for example, by a laser diffraction particle size distribution analyzer. The particle diameter (D50) of the positive electrode active material 100 measured by the laser diffraction particle size distribution analyzer is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Alternatively, 1 μm or more and 40 μm or less is preferable. Alternatively, 1 μm or more and 30 μm or less is preferable. Alternatively, 2 μm or more and 100 μm or less is preferable. Alternatively, 2 μm or more and 30 μm or less is preferable. Alternatively, 5 μm or more and 100 μm or less is preferable. Alternatively, 5 μm or more and 40 μm or less is preferable.

[0448] D50 is the particle size at which the integrated amount in the cumulative curve of the particle size distribution measurement results accounts for 50%. Similarly, D10 is the particle size at which the integrated amount in the cumulative curve of the particle size distribution measurement results accounts for 10%. D90 is the particle size at which the integrated amount in the cumulative curve of the particle size distribution measurement results accounts for 90%.

[0449] Furthermore, when particles having different particle sizes are mixed and used in the positive electrode, the electrode density can be increased, which is preferable because it allows for a secondary battery with a high energy density. The positive electrode active material 100 having a relatively small particle size is expected to have high charge / discharge rate characteristics. The positive electrode active material 100 having a relatively large particle size is expected to have high charge / discharge cycle characteristics and maintain a high discharge capacity.

[0450] <Analysis method> A certain positive electrode active material is ｘ MO ２ When x in the formula (I) is small, it can be determined whether the positive electrode active material 100 of one embodiment of the present invention has an O3′-type and / or monoclinic O1(15)-type crystal structure by Li ｘ MO ２This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0451] In particular, XRD is preferable in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material 100, which occupies the majority of the volume of the positive electrode active material 100.

[0452] When analyzing the crystallite size by powder XRD, it is preferable to measure the size while excluding the influence of the orientation of the positive electrode active material particles due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then measure the size.

[0453] As described above, the positive electrode active material 100 according to one embodiment of the present invention is ｘ MO ２ The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at high voltage accounts for 50% or more of the crystal structure are not preferable because they cannot withstand repeated high-voltage charging and discharging.

[0454] It should also be noted that simply adding an additive element may not result in an O3' or monoclinic O1(15) crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additive element, Li ｘ MO ２ In some cases, x is 0.24 or less and the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more, and in other cases, the H1-3 type crystal structure accounts for 50% or more.

[0455] Furthermore, even in the positive electrode active material 100 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be formed when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.90 V. Therefore, to determine whether or not the positive electrode active material 100 of one embodiment of the present invention is the positive electrode active material 100, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0456] However, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, the crystal structure may change from O3'-type or monoclinic O1(15)-type to H1-3-type. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0457] However, even when handled in an argon atmosphere, the crystal structure of the positive electrode may change over time after the secondary battery is disassembled. Therefore, it is preferable to adopt a method that allows for rapid measurement as needed.

[0458] In the case of XRD, the detector and / or measurement start angle can be appropriately changed in order to quickly observe the main peak. For example, it is preferable to measure the main peak within 13 minutes from the start of measurement.

[0459] Furthermore, whether or not the distribution of the added elements contained in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, EDX, electron probe microanalyzer (EPMA), or the like.

[0460] The crystal structure of the surface layer 100 a and the grain boundaries can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0461] <Charging Method> To determine whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention, for example, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) using the composite oxide as a positive electrode and lithium metal as a counter electrode can be fabricated as a test battery and charged. The coin cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0462] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0463] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the voltage value of the secondary battery and the potential value of the positive electrode will be different. Unless otherwise specified, the voltage and potential in this specification refer to the potential of the positive electrode.

[0464] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 with 2 wt % vinylene carbonate (VC).

[0465] The separator may be a 25 μm thick porous polypropylene film.

[0466] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0467] Note that the above coin cell (test battery) may be fabricated using the positive electrode removed by disassembling a battery (for example, a battery in an initial state mounted on a commercially available product), and charging may be performed to determine whether or not the coin cell is the positive electrode active material 100 of one embodiment of the present invention described below.

[0468] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.60 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V). The charging method is not particularly limited as long as charging is performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be terminated at 2 mA / g or more and 10 mA / g or less. To observe the phase change of the positive electrode active material, it is desirable to charge at such a small current value. On the other hand, if the current does not reach 2 mA / g or more and 10 mA / g or less even after long-term CV charging, it is considered that the current is being consumed not for charging the positive electrode active material but for decomposing the electrolyte. Therefore, CV charging may be terminated after a sufficient time has elapsed since the start of the charging. In this case, a sufficient time can be, for example, 1.5 hours or more and 3 hours or less. The temperature is set to 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere and the positive electrode is removed to obtain a positive electrode active material with a desired charge capacity. When various analyses are performed thereafter, it is preferable to seal the cell in an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container under an argon atmosphere. Furthermore, it is preferable to quickly remove the positive electrode and subject it to analysis after charging is complete. Specifically, it is preferable to do so within one hour after charging is complete, and more preferably within 30 minutes.

[0469] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions for charge / discharge. For example, charging may be performed by constant current charging at a current value of 20 mA / g or more and 100 mA / g or less up to a given voltage (e.g., 4.60 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V), followed by constant voltage charging until the current value reaches 2 mA / g or more and 10 mA / g or less, and then discharging by constant current discharging at 2.50 V and 20 mA / g or more and 100 mA / g or less.

[0470] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at 2.50 V with a current value of 20 mA / g or more and 100 mA / g or less.

[0471] <XRD> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker Corporation X-ray: CuKα １ Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° Setting counting time: 1 second / step Sample stage rotation: 15 rpm

[0472] If the measurement sample is a powder, it can be placed in a glass sample holder or sprinkled on a greased silicone anti-reflective plate. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device. If the positive electrode active material layer is higher than the measurement surface required by the device, the diffraction pattern will be shifted overall to higher angles; if it is lower, the diffraction pattern will be shifted overall to lower angles. In this case, it is preferable to correct the diffraction pattern shift using crystal structure analysis software or similar.

[0473] CuKα calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model １ Ideal powder XRD patterns based on the line are shown in Figures 29, 30, 31A, and 31B. ｘ MO ２ LiCoO where x=1 ２ Also shown are ideal XRD patterns calculated from the crystal structure of O3 and trigonal O1 with x = 0. Figures 31A and 31B show the XRD patterns of the O3'-type crystal structure, the monoclinic O1(15)-type crystal structure, and the H1-3-type crystal structure, with Figure 31A showing an enlarged view of the region where 2θ is in the range of 18° to 21°, and Figure 31B showing an enlarged view of the region where 2θ is in the range of 42° to 46°. ２ (O3) and CoO ２The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), from the crystal structure information obtained from ICSD (see Non-Patent Document 4). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０ m and λ2 were not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. The patterns of the O3' type and monoclinic O1(15) type crystal structures were estimated from the XRD pattern of the positive electrode active material, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and XRD patterns were created in the same way as the others.

[0474] As shown in Figures 29, 31A and 31B, the O3' type crystal structure has diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°).

[0475] Furthermore, the monoclinic O1(15) type crystal structure has diffraction peaks at 2θ=19.47±0.10° (19.37° or more and 19.57° or less) and 2θ=45.62±0.05° (45.57° or more and 45.67° or less).

[0476] However, as shown in Figures 30, 31A and 31B, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of peaks at 19.13° or more and less than 19.37° and / or 19.37° or more and less than 19.57°, and at 45.37° or more and less than 45.57° and / or 45.57° or more and less than 45.67° when x is small can be said to be a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0477] This means that in the positive electrode active material 100 of one embodiment of the present invention, the positions of XRD diffraction peaks are close between the crystal structures where x = 1 and where x ≦ 0.24. More specifically, among the main diffraction peaks of the crystal structures where x = 1 and where x ≦ 0.24, the difference in 2θ between peaks that appear at 2θ of 42° to 46° is 0.7° or less, more preferably 0.5° or less.

[0478] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ CoO ２ When x in the formula is small, the particles may have an O3'-type and / or monoclinic O1(15)-type crystal structure, but not all of the particles may have an O3'-type and / or monoclinic O1(15)-type crystal structure. Other crystal structures may be included, or some may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type and / or monoclinic O1(15)-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, the positive electrode active material can have sufficiently excellent cycle characteristics.

[0479] Similarly, when Rietveld analysis is performed, the H1-3 type and O1 type crystal structures are preferably less than 50%, more preferably 34% or less, and even more preferably substantially not observed.

[0480] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type and / or monoclinic O1(15) type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0481] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. For example, a narrow half-width is preferable. Even for peaks arising from the same crystalline phase, the half-width varies depending on the XRD measurement conditions and the value of 2θ. Under the above-mentioned measurement conditions, for peaks observed between 2θ = 43° and 46°, the half-width is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after sufficient charging.

[0482] The crystallite size of the O3'-type and monoclinic O1(15) crystal structures of the positive electrode active material 100 is approximately equal to that of LiCoO in a discharged state. ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘ CoO ２ When x in the formula is small, the peaks of the O3' type and / or monoclinic O1(15) crystal structure can be clearly observed. ２ In this case, even if some of the crystal structure resembles the O3' type and / or monoclinic O1(15) crystal structure, the crystallite size will be small and the peaks will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0483] As described above, the positive electrode active material 100 of one embodiment of the present invention preferably has a small influence of the Jahn-Teller effect. As long as the influence of the Jahn-Teller effect is small, the positive electrode active material 100 may contain a transition metal such as nickel or manganese as an additive element in addition to cobalt.

[0484] For example, when the nickel concentration is 5% or 7.5%, the a-axis / c-axis tends to change significantly, and when the nickel concentration is 7.5%, the distortion of the a-axis increases. This distortion may be due to Jahn-Teller distortion of trivalent nickel. Therefore, it is preferable that the nickel content of the transition metal M contained in the positive electrode active material 100 is less than 7.5 atomic %.

[0485] Furthermore, it is suggested that when the manganese concentration is 5% or more, the behavior of the change in lattice constant is different and does not follow Vegard's law. Therefore, of the transition metals M contained in the positive electrode active material 100, manganese is preferably 4 atomic % or less.

[0486] The above ranges of nickel concentration and manganese concentration do not necessarily apply to the surface layer 100a, that is, the concentrations in the surface layer 100a may be higher than the above ranges.

[0487] As a result of considering a preferable range of the lattice constant from the above, it was found that in the positive electrode active material of one embodiment of the present invention, the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge and discharge are performed or in a discharged state, which can be estimated from the XRD pattern, has an a-axis lattice constant of 2.814 × 10 −１０ m is greater than 2.817 x 10 −１０ m and the lattice constant of the c-axis is 14.05 × 10 −１０ m or larger, 14.07 x 10 −１０ It has been found that the value is preferably smaller than m. The state in which no charge and discharge are performed may be, for example, the state of powder before the positive electrode of the secondary battery is produced.

[0488] Alternatively, in the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, it is preferable that the value obtained by dividing the lattice constant of the a-axis by the lattice constant of the c-axis (a-axis / c-axis) is greater than 0.20000 and smaller than 0.20049.

[0489] Alternatively, when XRD analysis is performed on the layered rock salt type crystal structure of the positive electrode active material 100 in a state where no charge / discharge is performed or in a discharged state, a first peak may be observed at 2θ of not less than 18.50° and not more than 19.30°, and a second peak may be observed at 2θ of not less than 38.00° and not more than 38.80°.

[0490] <<Resistance DCIR>> In the above <<Charging Method>>, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) is prepared with a lithium counter electrode and charged as a charging method for determining whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention. Using a coin cell prepared in a similar manner, it is possible to measure the low resistance DCIR, which is one of the characteristics of the positive electrode active material 100 of one embodiment of the present invention.

[0491] A coin cell prepared under the same conditions as in the above charging method is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.58 V, 4.60 V, 4.62 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V). The charging method is not particularly limited as long as charging can be performed at the desired voltage for a sufficient period of time. Note that, in this specification, "approximately 4.6 V" refers to a voltage of 4.58 V or higher and 4.62 V or lower. For example, when charging by CCCV, the CC charging current can be 20 mA / g or higher and 100 mA / g or lower. CV charging can be terminated at a current of 2 mA / g or higher and 10 mA / g or lower.

[0492] The charged coin cell is left in a resting state (a state in which neither charging nor discharging is performed) for 10 minutes, and then the resistance DCIR (Direct Current Internal Resistance, also referred to as DC resistance) is measured. The resistance DCIR is measured by discharging for 1 second at a current of 2 mA / g to 10 mA / g (for example, 6 mA / g) per weight of the positive electrode active material (lithium cobalt oxide). The DCIR can be calculated by dividing the difference between the open circuit voltage before discharge and the voltage during discharge by the current (for example, 6 mA / g).

[0493] For example, when the open circuit voltage before discharge (after charge) is 4.50 V or more and 4.60 V or less, the resistance DCIR of a coin cell including the positive electrode active material 100 of one embodiment of the present invention is preferably 60 Ω or less, more preferably 50 Ω or less, more preferably 40 Ω or less, and even more preferably 35 Ω or less. Note that a series of measurements of the resistance DCIR are preferably performed in an environment of 45°C.

[0494] XPS: In the case of inorganic oxides, XPS can analyze a region from the surface to a depth of approximately 1 to 8 nm (usually 5 nm or less) by using monochromatic aluminum Kα rays as the X-ray source. Furthermore, the depth of the analysis region can be varied by changing the photoelectron take-off angle (also called the take-off angle or detector angle) relative to the surface to be measured. For example, a take-off angle of 45° can analyze a region from the surface to a depth of approximately 5 nm, and a take-off angle of 15° can analyze a region from the surface to a depth of approximately 2 nm. By performing measurements under multiple conditions with different detection depths in XPS analysis, it is possible to estimate the distribution of additive elements in the depth direction of the surface layer 100a. Furthermore, by combining this with digging into the surface to be measured by Ar ion sputtering, it is also possible to measure the distribution of additive elements in the depth direction to approximately several μm. For example, an XPS device such as the Quantera II or Quantera SXM manufactured by ULVAC-PHI, Inc. can be used.

[0495] Furthermore, narrow scan analysis can be used in the above measurements to analyze the bonding state of elements.The quantitative accuracy of XPS is often about ±1 atomic %, and the detection limit is about 0.1 to 1 atomic %, depending on the element.

[0496] In the cathode active material 100 according to one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer portion 100a than in the interior portion 100b. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer portion 100a is preferably higher than the average concentration throughout the cathode active material 100. Therefore, for example, it can be said that the concentration of one or more selected additive elements in the surface layer portion 100a measured by XPS or the like is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by ICP-MS, GD-MS, or the like. For example, the magnesium concentration of at least a portion of the surface layer portion 100a measured by XPS or the like is preferably higher than the average magnesium concentration throughout the cathode active material 100. Furthermore, the titanium concentration of at least a portion of the surface layer portion 100a is preferably higher than the average titanium concentration throughout the cathode active material 100. Furthermore, the nickel concentration of at least a portion of the surface layer portion 100a is preferably higher than the average nickel concentration throughout the cathode active material 100. It is also preferable that the aluminum concentration in at least a part of the surface layer portion 100a is higher than the average aluminum concentration in the entire positive electrode active material 100. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 100a is higher than the average fluorine concentration in the entire positive electrode active material 100.

[0497] Note that the surface and surface layer portion 100a of the positive electrode active material 100 according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 100. Furthermore, the surface of the positive electrode active material 100 also does not contain the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 100. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0498] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the ratio of the number of atoms of the added element is not affected.

[0499] The concentration of the added element may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and the like chemisorbed after the preparation of the positive electrode active material. For example, the ratio Mg / Co of the number of atoms of magnesium (Mg) to cobalt (Co) measured by XPS analysis is preferably 0.4 or more and 1.5 or less. The ratio Ti / Co of the number of atoms of titanium (Ti) to cobalt (Co) measured by XPS analysis is preferably 0.05 or more and 0.20 or less. The ratio Ni / Co of the number of atoms of nickel (Ni) to cobalt (Co) measured by XPS analysis is preferably 0.05 or more and 0.20 or less. Meanwhile, the ratio Mg / Co of the number of atoms of nickel (Ni) to cobalt (Co) measured by ICP-MS analysis is preferably 0.002 or more and 0.06 or less.

[0500] Furthermore, it is more preferable that aluminum is widely distributed in a deep region, for example, the surface or a region at a depth of 5 nm to 50 nm from the reference point. Therefore, it is more preferable that aluminum is detected in an analysis of the entire cathode active material 100 using ICP-MS, GD-MS, or the like, but the aluminum concentration is not detected by XPS or the like, or the aluminum concentration is 1 atomic % or less.

[0501] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as X-rays. The take-off angle may be, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions. Measurement equipment: Quantera II manufactured by ULVAC-PHI, Inc. X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: range of approximately 4 to 5 nm from the surface (detector angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0502] When the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 688 eV.

[0503] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1306 eV, and more preferably about 1304 eV.

[0504] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak showing the bond energy between titanium and another element is preferably greater than or equal to 455 eV and less than 460 eV, and the valence of Ti is preferably tetravalent.

[0505] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak showing the bond energy between nickel and another element is preferably 855 eV or more and less than 859 eV. ２＋ It is preferable that the energy is shifted to a higher energy side than 854 eV, which is the bond energy of —O.

[0506] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. It is more preferable that the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using an FIB or the like and analyzing the cross section using EDX, EPMA, or the like.

[0507] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0508] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0509] Since the positive electrode active material 100 is a compound containing a transition metal and oxygen capable of lithium insertion / extraction, the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and oxygen is present and a region where it is not present is defined as the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multilayer film of carbon, metal, oxide, resin, etc.

[0510] In STEM-EDX-ray analysis or the like, in principle or due to measurement errors, the graph of the detected amount of characteristic X-rays of the element does not change sharply, and it may be difficult to precisely determine the surface. Therefore, when referring to the depth direction in STEM-EDX-ray analysis or the like, the detected amount of characteristic X-rays of the transition metal M is the average value M of the detected amount of characteristic X-rays of the internal transition metal M. ＡＶＥ and the average M of the detected amount of characteristic X-rays of the transition metal M in the background. ＢＧ The point where the detected amount of oxygen characteristic X-rays is 50% of the sum of the detected amount of oxygen characteristic X-rays and the average value O ＡＶＥ and the average value O of the detected amount of characteristic X-rays of oxygen in the background ＢＧThe reference point is the point where the detected amount of the characteristic X-rays of the transition metal M is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal transition metal M and the average value of the detected amount of the characteristic X-rays of the background transition metal M. If the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal oxygen and the average value of the detected amount of the characteristic X-rays of the background oxygen is different from the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal oxygen and the average value of the detected amount of the characteristic X-rays of the background oxygen, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen attached to the surface, and therefore, the detected amount of the characteristic X-rays of the transition metal M is different from the point where ... oxygen is different from the sum of the average value of the detected amount of the characteristic X-rays of the internal transition metal M. ＡＶＥ and the average M of the detected amount of characteristic X-rays of the transition metal M in the background. ＢＧ In the case of a positive electrode active material containing a plurality of transition metals M, the M of the element with the largest amount of characteristic X-rays detected inside can be used as the reference point. ＡＶＥ and M ＢＧ The reference point can be determined using the following formula:

[0511] The average value M of the detected amount of characteristic X-rays of the transition metal M in the background ＢＧ can be obtained by averaging, for example, a range of 2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material, avoiding the vicinity where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＡＶＥ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 30 nm or more, preferably more than 50 nm, from the region where the detected amount of characteristic X-rays of the transition metal M and oxygen becomes saturated and stable, for example, the region where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＢＧ and the average value O of the detected amount of characteristic X-rays of oxygen inside ＡＶＥ can also be found in the same way.

[0512] Furthermore, the surface of the positive electrode active material 100 in a cross-sectional STEM image or the like is the boundary between an area where an image derived from the crystal structure of the positive electrode active material is observed and an area where an image is not observed, and is the outermost area of ​​an area where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material are confirmed.

[0513] In addition, a peak in STEM-EDX-ray analysis refers to a convex maximum value that appears in a graph of the characteristic X-ray intensity for each element, or the maximum value of the characteristic X-ray for each element. Note that noise in STEM-EDX-ray analysis may be a measured value with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0514] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured after six scans can be used as the detection value for each element. The number of scans is not limited to six, and more scans can be performed and the integrated value can be used as the detection value for each element.

[0515] The STEM-EDX analysis can be performed, for example, as follows: First, a protective film is vapor-deposited on the surface of the positive electrode active material. For example, carbon can be vapor-deposited using an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0516] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-section sample. For example, the slice processing can be performed using an FIB-SEM device (Hitachi High-Tech XVision 200TBS). Pickup is performed using an MPS (microprobing system), and the finishing conditions can be, for example, an acceleration voltage of 10 kV.

[0517] STEM-EDX ray analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) and an EDAX Octane T Ultra W EDX detector. An example of conditions for EDX ray analysis using the Hitachi High-Tech HD-2700 is to set the emission current of the STEM device to 6 μA or more and 10 μA or less, and measure a portion of the thinned sample with minimal depth and unevenness. The magnification is, for example, approximately 150,000 times. The conditions for EDX ray analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and six or more frames.

[0518] In order to increase the spatial resolution in STEM-EDX ray analysis, it is preferable that the beam diameter of the electron beam (also referred to as beam diameter, probe diameter, or probe diameter) is small. The beam diameter in STEM-EDX ray analysis is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. Furthermore, in order to increase the analytical sensitivity in STEM-EDX ray analysis, it is preferable to increase the beam current of the electron beam (also referred to as probe current). Therefore, it is preferable that the device used in STEM-EDX ray analysis is equipped with a spherical aberration corrector (Cs corrector) that can reduce the beam diameter and increase the beam current.

[0519] When EDX area analysis or EDX point analysis is performed on the positive electrode active material 100 of one embodiment of the present invention, the concentration of each additive element, particularly any one or more of magnesium, nickel, and titanium, in the surface layer portion 100a is preferably higher than the concentration of the element in the interior portion 100b. Alternatively, the detected amount of each additive element, particularly any one or more of magnesium, nickel, and titanium, in the surface layer portion 100a is preferably higher than the detected amount of the element in the interior portion 100b.

[0520] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, the magnesium concentration in the surface layer 100a is preferably higher than the magnesium concentration in the interior 100b. Furthermore, when EDX ray analysis is performed, the peak of the magnesium concentration in the surface layer 100a is preferably present at a depth of 3 nm from the surface or reference point toward the center of the cathode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, the magnesium concentration preferably decays to 60% or less of the peak at a depth of 1 nm from the peak top. Furthermore, it is preferable that the magnesium concentration decays to 30% or less of the peak at a depth of 2 nm from the peak top. Alternatively, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, the detected amount of characteristic X-rays of magnesium in the surface layer 100a is preferably higher than the detected amount of characteristic X-rays of magnesium in the interior 100b. Furthermore, when EDX-ray analysis is performed, the peak of the detected amount of characteristic X-rays of magnesium in the surface layer portion 100a preferably exists on the surface of the positive electrode active material 100 or at a depth of 3 nm from the reference point toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, the detected amount of characteristic X-rays of magnesium preferably decays to 60% or less of the peak at a depth of 1 nm from the peak top. Furthermore, it preferably decays to 30% or less of the peak at a depth of 2 nm from the peak top. The concentration peak top here refers to the maximum value or the position where the maximum value appears in a convex shape that appears in the concentration distribution graph for each element. Furthermore, the detected amount peak top refers to the maximum value or the position where the maximum value appears in a convex shape that appears in the detection amount distribution graph for each element.

[0521] Furthermore, when EDX-ray analysis is performed, the maximum value of the magnesium concentration (detected amount of magnesium / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.5 Atomic% or more and 10 Atomic% or less, and more preferably 1 Atomic% or more and 7 Atomic% or less, or 1 Atomic% or more and 5 Atomic% or less.

[0522] Furthermore, when EDX-ray analysis is performed, the maximum value of the titanium concentration (detected amount of titanium / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.2 Atomic% or more and 10 Atomic% or less, and more preferably 0.5 Atomic% or more and 6 Atomic% or less, or 0.5 Atomic% or more and 5 Atomic% or less.

[0523] Furthermore, when EDX-ray analysis is performed, the maximum value of the nickel concentration (detected amount of nickel / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.2 Atomic% or more and 5 Atomic% or less, and more preferably 0.5 Atomic% or more and 4 Atomic% or less, or 0.5 Atomic% or more and 3 Atomic% or less.

[0524] Furthermore, when EDX-ray analysis is performed, the maximum value of the fluorine concentration (detected amount of fluorine / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.5 Atomic% or more and 10 Atomic% or less, and more preferably 1 Atomic% or more and 6 Atomic% or less.

[0525] In addition, in the positive electrode active material 100 having magnesium and fluorine as additive elements, the distribution of fluorine preferably has a region overlapping with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration or detection amount and the peak of the magnesium concentration or detection amount is preferably within 10 nm, more preferably within 3 nm, even more preferably within 1 nm, even more preferably within 0.5 nm, and even more preferably within 0.3 nm.

[0526] Furthermore, in the positive electrode active material 100 containing nickel as an additive element, the peak of the nickel concentration or detectable amount in the surface layer 100a is preferably present at a depth of 3 nm from the surface or reference point toward the center of the positive electrode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, in the positive electrode active material 100 containing magnesium and nickel, the nickel distribution preferably has a region overlapping with the magnesium distribution. For example, the difference in depth between the peak of the nickel concentration or detectable amount and the peak of the magnesium concentration or detectable amount is preferably within 3 nm, more preferably within 1 nm.

[0527] Furthermore, in the cathode active material 100 containing titanium as an additive element, the peak of the titanium concentration or detectable amount in the surface layer 100a is preferably present at a depth of 3 nm from the surface or reference point toward the center of the cathode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, in the cathode active material 100 containing magnesium and titanium, the distribution of titanium preferably has a region overlapping with the distribution of magnesium. For example, the difference in depth between the peak of the titanium concentration or detectable amount and the peak of the magnesium concentration or detectable amount is preferably within 3 nm, more preferably within 1 nm.

[0528] Furthermore, when the positive electrode active material 100 contains aluminum as an additive element, it is preferable that, when EDX-ray analysis is performed, the peak of the magnesium, nickel, or fluorine concentration or detected amount is closer to the surface than the peak of the aluminum concentration or detected amount in the surface layer portion 100a. In other words, it is preferable that the peak of the aluminum concentration or detected amount in the surface layer portion 100a is closer to the interior than the peak of the magnesium, nickel, or fluorine concentration or detected amount. For example, it is preferable that the peak of the aluminum concentration or detected amount is present on the surface of the positive electrode active material 100 or at a depth of 0.5 nm to 50 nm from the reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0529] Here, how to express the positional relationship of element distribution when EDX-ray analysis is performed will be explained using Figures 32A to 32G. Figures 32A to 32F are schematic diagrams showing the concentration distribution or detection amount distribution of a first element e1 and a second element e2. Figure 32G is a schematic diagram showing the concentration distribution or detection amount distribution of a first element e1, a second element e2, and a third element e3.

[0530] For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 32A, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. Also, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 32B, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. Also, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 32C, the position where the concentration or detectable amount of the first element e1 is maximum is said to be located more inward than the position where the concentration or detectable amount of the second element e2 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 32D, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 32E, the position where the concentration or detectable amount of the first element e1 is maximum is said to be located more inward than the position where the concentration or detectable amount of the second element e2 is maximum. For example, when the concentration distribution or detectable amount distribution of the first element e1 and the second element e2 has a shape as shown in Figure 32F, the position where the concentration or detectable amount of the second element e2 is maximum is said to be located more inward than the position where the concentration or detectable amount of the first element e1 is maximum.

[0531] The expression "having an overlapping region" will be explained using an example in which the concentration distributions or detection amount distributions of a first element e1, a second element e2, and a third element e3 have the positional relationship shown in FIG. 32G. In this specification, "having an overlapping region" means, for example, that the position of maximum value in the concentration distribution or detection amount distribution of at least one element is located in a range in which the concentration or detection amount in the concentration distribution or detection amount distribution of the other element is 1 / 5 or more of the maximum value. Note that when the background detection intensity in EDX-ray analysis is equal to or greater than the above-mentioned "1 / 5 of the maximum value," the "1 / 5 of the maximum value" in the above sentence shall be referred to as the "background detection intensity (also referred to as the detection limit)."

[0532] 32G , the position (P2) at which the second element e2 has a maximum value in its concentration distribution or detectable amount distribution is located in a range (hatched area in the figure) where the first element e1 has a concentration or detectable amount equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the first element e1's concentration distribution or detectable amount distribution, and therefore the first element e1 and the second element e2 have an overlapping distribution area. Furthermore, the position (P3) at which the third element e3 has a maximum value in its concentration distribution or detectable amount distribution is not located in a range (hatched area in the figure) where the first element e1 has a concentration or detectable amount equal to or greater than 1 / 5 of the maximum value (or the lower detection limit) in the first element e1's concentration distribution or detectable amount distribution, and therefore the first element e1 and the third element e3 do not have an overlapping distribution area.

[0533] 32G, the distribution of the second element e2 and the distribution of the third element e3 can be said to be located more inward than the distribution of the first element e1. Alternatively, the distribution of the second element e2 and the distribution of the third element e3 can be said to be biased more inward than the distribution of the first element e1.

[0534] When the positive electrode active material 100 contains magnesium, titanium, nickel, and aluminum as described above, a battery using the positive electrode active material 100 can achieve both "high cycle characteristics" that can suppress deterioration in discharge capacity due to repeated high-voltage charging (for example, charging with an upper limit of 4.60 V) and discharging, and "high low-temperature characteristics" that can obtain a large discharge capacity at low temperatures (for example, 0°C, -20°C, -40°C).

[0535] Furthermore, when EDX-ray analysis, area analysis, or point analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of magnesium (Mg) to cobalt (Co) (Mg / Co) at the peak of the magnesium concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum (Al) to cobalt (Co) (Al / Co) at the peak of the aluminum concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel (Ni) to cobalt (Co) (Ni / Co) at the peak of the nickel concentration is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine (F) to cobalt (Co) (F / Co) at the peak of the fluorine concentration is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0536] Furthermore, when the positive electrode active material 100 is subjected to linear or area analysis, the ratio of the number of atoms of the additional element A to the cobalt Co in the vicinity of the grain boundaries (A / Co) is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less.

[0537] For example, when the additive element is magnesium, when linear or area analysis is performed on the positive electrode active material 100, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) near the grain boundaries is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less. Furthermore, when the ratio is within the above range at multiple locations, for example, three or more locations, on the positive electrode active material 100, it can be said that this indicates that the additive element is not attached to a narrow area on the surface of the positive electrode active material 100, but is widely distributed at a preferred concentration in the surface layer portion 100a of the positive electrode active material 100.

[0538] <<EPMA>> The concentration of the additive element in the positive electrode active material 100 can be analyzed using EDX, but it can also be analyzed using EPMA. EPMA has a higher detection capability (also referred to as a lower detection limit) than EDX when analyzing elements present in trace amounts in a sample. Therefore, it is preferable to use EPMA when analyzing a region where a trace amount of the additive element is present.

[0539] In the EPMA analysis, a cross section of the positive electrode active material 100 is exposed by mechanical polishing, ion polishing, FIB, etc., and the cross section is analyzed. As an EPMA device, for example, an electron probe microanalyzer JXA-iHP200F manufactured by JEOL Ltd. can be used.

[0540] EPMA uses a wavelength-dispersive detector, so it has a higher ability to detect trace elements than EDX, which uses an energy-dispersive detector. On the other hand, the spatial resolution of EPMA analysis is inferior to that of EDX (especially STEM-EDX). Therefore, STEM-EDX is suitable for analysis focusing on the detailed distribution of added elements in the surface layer 100a of the positive electrode active material 100, while EPMA analysis is suitable for analysis of trace amounts of added elements in the interior 100b. Note that EPMA and EDX use different analytical methods, so the concentration values ​​obtained when analyzing the same region using each analytical method may not match.

[0541] The additive elements in the interior 100b of the positive electrode active material 100 may preferably contain trace amounts of magnesium and nickel. For example, in EPMA analysis, the ratio Mg / Co of the number of magnesium (Mg) and cobalt (Co) atoms in the interior 100b is preferably 0.003 or more and 0.01 or less. Furthermore, in EPMA analysis, the ratio Ni / Co of the number of nickel (Ni) and cobalt (Co) atoms in the interior 100b is preferably 0.003 or more and 0.01 or less. Meanwhile, in EPMA analysis, the number of titanium atoms in the interior 100b is preferably less than the lower limit of detection. The lower limit of detection for the number of titanium atoms is 0.2 at% when analyzing the interior of lithium cobalt oxide using, for example, a JXA-iHP200F.

[0542] <Raman Spectroscopy> As described above, in the positive electrode active material 100 according to one embodiment of the present invention, at least a portion of the surface layer 100a preferably has a rock salt crystal structure. Therefore, when the positive electrode active material 100 and a positive electrode including the positive electrode active material 100 are analyzed by Raman spectroscopy, it is preferable to observe not only the layered rock salt crystal structure but also cubic crystal structures such as rock salt crystal structure. In the STEM image and the electron microbeam diffraction pattern described below, unless cobalt is substituted at lithium positions with a certain frequency in the depth direction during observation, or cobalt is present at the oxygen tetracoordination positions, they cannot be detected as bright spots in the STEM image and the electron microbeam diffraction pattern. On the other hand, because Raman spectroscopy is an analysis that captures the vibrational modes of bonds such as Co—O, peaks of the wavenumbers of the corresponding vibrational modes may be observed even when the amount of the corresponding Co—O bond is small. Furthermore, Raman spectroscopy can be performed on a surface layer having an area of ​​several μm ２ Since it is possible to measure a range of about 1 μm in depth, it is possible to capture with high sensitivity the state that exists only on the particle surface.

[0543] For example, when the laser wavelength is 532 nm, layered rock salt LiCoO ２ So, 470 cm −１ ~490cm −１ , 580 cm −１ ~600cm −１ Peak at (vibration mode: E ｇ , A １ｇ ) is observed. On the other hand, cubic CoO ｘ (0<x<1) (rock salt type Co １−ｙ O (0<y<1) or spinel type Co ３ O ４ ) is 665 cm −１ ~685cm −１ Peak at (Vibration mode: A １ｇ ) is observed.

[0544] Therefore, 470 cm −１ ~490cm −１ The integrated intensity of I1, 580 cm −１ ~600cm −１ The integrated intensity of I2, 665 cm −１ ~685cm −１When the integrated intensity of the above is I3, the value of I3 / I2 is preferably 1% or more and 10% or less, and more preferably 3% or more and 9% or less.

[0545] If a cubic crystal structure such as a rock salt type is observed within the above range, it can be said that the surface layer 100a of the positive electrode active material 100 has a rock salt type crystal structure within a preferred range.

[0546] <<Electron Diffraction Pattern>> As with Raman spectroscopy, it is preferable that the characteristics of the rock salt-type crystal structure are observed in the electron diffraction pattern as well as the layered rock salt crystal structure. However, in the STEM image and the electron diffraction pattern, taking into account the above-mentioned difference in sensitivity, it is preferable that the characteristics of the rock salt-type crystal structure are not too strong in the surface layer portion 100a, particularly in the outermost surface (for example, 1 nm deep from the surface). This is because, rather than the outermost surface being covered with a rock salt-type crystal structure, it is preferable that the lithium layer has a layered rock salt-type crystal structure and an additive element such as magnesium is present in the lithium layer, which can ensure a lithium diffusion path and have a stronger function of stabilizing the crystal structure.

[0547] Therefore, for example, when a micro-electron beam diffraction pattern is obtained from a region having a depth of 1 nm or less from the surface and a micro-electron beam diffraction pattern from a region having a depth of 3 nm to 10 nm, it is preferable that the difference in lattice constant calculated from these patterns is small.

[0548] For example, the difference in lattice constant calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm to 10 nm is 0.1 × 10 for the a-axis. −１０ m or less, and the c-axis is preferably 1.0 × 10 −１０ It is preferable that the a-axis is 0.05×10 −１０ m or less, and the c-axis is preferably 0.6×10 −１０ It is more preferable that the a-axis is 0.04×10 −１０ m or less, and the c-axis is more preferably 0.3 × 10 −１０ It is more preferable that the length is m or less.

[0549] <Surface Roughness and Specific Surface Area> The positive electrode active material 100 of one embodiment of the present invention preferably has a smooth surface with little unevenness. A smooth surface with little unevenness indicates that the effect of the flux described below is sufficiently exerted, and the surface of the additive element source and the lithium cobalt oxide are melted. Therefore, this is one factor indicating that the distribution of the additive element in the surface layer portion 100 a is good.

[0550] Whether the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional SEM image or cross-sectional TEM image of the positive electrode active material 100, the specific surface area of ​​the positive electrode active material 100, or the like.

[0551] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 as follows.

[0552] First, the cathode active material 100 is processed using an FIB or the like to expose a cross section. At this time, it is preferable to cover the cathode active material 100 with a protective film, protective agent, or the like. Next, an SEM image of the interface between the protective film or the like and the cathode active material 100 is taken. The SEM image is subjected to noise processing using image processing software. For example, Gaussian blurring (σ = 2) is performed, followed by binarization. Interface extraction is then performed using image processing software. The interface line between the protective film or the like and the cathode active material 100 is selected using an automatic selection tool or the like, and the data is extracted into a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed from a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness at least within 400 nm of the outer periphery of the particle of the cathode active material.

[0553] On the particle surfaces of the positive electrode active material 100 of this embodiment, the root mean square (RMS) surface roughness, which is an index of roughness, is preferably less than 3 nm, more preferably less than 1 nm, and even more preferably less than 0.5 nm.

[0554] The image processing software for performing noise processing, interface extraction, etc. is not particularly limited, but for example, "ImageJ" described in Non-Patent Documents 8 to 10 can be used. Furthermore, the spreadsheet software is also not particularly limited, but for example, Microsoft Office Excel can be used.

[0555] For example, the actual specific surface area S measured by the gas adsorption method using the constant volume method Ｒ and the ideal specific surface area S ｉ The smoothness of the surface of the positive electrode active material 100 can also be quantified from the ratio of

[0556] Ideal specific surface area S ｉ is calculated assuming that all particles have the same diameter as D50, the same weight, and an ideal sph...

Claims

a positive electrode, a negative electrode, and an exterior body that houses the positive electrode and the negative electrode; The positive electrode and the outer casing are connected at least at two points on one side of the outer casing, The negative electrode and the outer casing are connected at least at two points on one side of the outer casing, the positive electrode has a positive electrode active material layer, the positive electrode active material layer contains lithium cobalt oxide, the lithium cobalt oxide has magnesium, titanium, aluminum, and nickel in a surface layer portion; the surface layer portion is a region within 50 nm from the surface of the lithium cobalt oxide, The secondary battery, wherein, when STEM-EDX ray analysis is performed in the depth direction of the surface layer portion, the aluminum has a peak closer to the inside of the lithium cobalt oxide than the magnesium. 　 In claim 1, The lithium cobalt oxide has a layered rock salt crystal structure of space group R-3m, the surface layer portion has a basal region having a surface parallel to a (001) plane of the crystal structure, and an edge region having a surface in a direction intersecting the (001) plane, the edge region comprises the titanium; a secondary battery having a region in the edge region where the distribution of magnesium and the distribution of titanium overlap when STEM-EDX ray analysis is performed in the depth direction of the surface layer portion; 　 In claim 2, the edge region comprises the nickel; a secondary battery having a region in the edge region where the distribution of titanium and the distribution of nickel overlap when STEM-EDX ray analysis is performed in the depth direction of the surface layer portion; 　 In claim 3, a secondary battery having a region in the edge region where, when STEM-EDX ray analysis is performed in the depth direction of the surface layer portion, the difference between the depth of the peak of the detected amount of magnesium and the depth of the peak of the detected amount of titanium is within 3 nm.

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