Battery

Abstract

The present invention provides a battery with excellent charging and discharging characteristics even in low-temperature environments, and a method for producing the same. The battery has a positive electrode, a negative electrode, and an electrolytic solution, wherein the positive electrode has a positive electrode active material layer, the positive electrode active material layer has positive electrode active material particles containing magnesium, nickel, aluminum, and lithium cobalt oxide, the negative electrode has a negative electrode active material layer, the negative electrode active material layer has graphite particles and silicon particles, the electrolytic solution contains lithium salt, fluorinated cyclic carbonate, and fluorinated chain carbonate, and the capacity remaining in the negative electrode is between 5% and 15%, inclusive, when the battery is in the discharged state.

Description

battery

[0001] One embodiment of the present invention relates to a lithium-ion battery (also referred to as a lithium-ion secondary battery). The present invention is not limited to the above fields, but also relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, a vehicle, and a manufacturing method thereof. The above-described semiconductor device, display device, light-emitting device, power storage device, lighting device, electronic device, and vehicle can use a lithium-ion battery, which is one embodiment of the present invention, as a necessary power source. For example, the above-described electronic device includes an information terminal device equipped with a lithium-ion battery. Furthermore, the above-described power storage device includes a stationary power storage device.

[0002] In recent years, there has been active development of various types of storage batteries, including lithium-ion batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries. Demand for high-power, high-capacity lithium-ion batteries has expanded rapidly in line with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0003] It is known that the discharge capacity of lithium-ion batteries varies depending on the temperature during discharge, and therefore there is a demand for lithium-ion batteries that have excellent battery characteristics even in low-temperature environments (see, for example, Patent Document 1).

[0004] Furthermore, in order to increase the capacity of lithium-ion batteries at room temperature and improve their charge-discharge cycle characteristics, various research and development efforts are being conducted on both the positive and negative electrodes. Lithium cobalt oxide, which has a stable crystal structure, has been investigated as a positive electrode active material (see, for example, Patent Document 2).

[0005] Furthermore, fluorides such as fluorite (calcium fluoride) have long been used as fluxes in iron manufacturing and the like, and their physical properties have been studied (for example, Non-Patent Document 1).

[0006] Furthermore, as a negative electrode active material, silicon-based materials are known to have a higher capacity than graphite-based materials, and negative electrodes using silicon-based materials are being investigated (see, for example, Patent Document 3).

[0007] JP 2015-026608 A, WO2020 / 026078 Pamphlet, JP 2019-165005 A

[0008] W. E. Counts, R. Roy, and E. F. Osborn, “Fluoride Model Systems: II, The Binary Systems CaF▲2▼-BeF▲2▼, MgF▲2▼-BeF▲2▼, and LiF-MgF▲2▼”, Journal of the American Ceramic Society, 36 [1] 12-17 (1953).

[0009] Patent Document 1 describes that a lithium ion battery capable of operating in a low-temperature environment (for example, below 0° C.) has been realized by using the electrolyte solution described in Patent Document 1. However, even the lithium ion battery described in Patent Document 1 does not have a large discharge capacity when discharged in a low-temperature environment at the time of filing, and further improvement is desired.

[0010] Furthermore, in order to realize a lithium-ion battery with excellent charge / discharge characteristics even in a low-temperature environment, it is necessary to develop not only an electrolyte but also a positive electrode and a negative electrode suitable for a lithium-ion battery that can operate in a low-temperature environment. Furthermore, it is desirable for the positive electrode to have a positive electrode active material that can be charged and discharged at a high voltage. By using a positive electrode active material that can be charged and discharged at a high voltage, a lithium-ion battery with a large charge capacity and / or a large charge energy density can be obtained.

[0011] Therefore, an object of one embodiment of the present invention is to provide a lithium-ion battery having excellent charge / discharge characteristics even in a low-temperature environment. Specifically, an object is to provide a positive electrode, a negative electrode, an electrolyte, etc. that can be used in a lithium-ion battery that has a large discharge capacity even when discharged in a low-temperature environment. Alternatively, an object is to provide a positive electrode, a negative electrode, an electrolyte, etc. that can be used in a lithium-ion battery that has a large charge capacity and / or a large charge energy density even when charged in a low-temperature environment.

[0012] Note that the description of these problems does not preclude the existence of other problems. Furthermore, one embodiment of the present invention does not necessarily solve all of these problems. Furthermore, problems other than these can be extracted from the description of this specification, drawings, claims, etc.

[0013] One embodiment of the present invention is a battery including a positive electrode, a pre-doped negative electrode, and an electrolyte solution. By using the pre-doped negative electrode, reaction resistance between a negative electrode active material and lithium ions can be reduced, and reaction unevenness in the battery can be suppressed.

[0014] Specifically, in the case of a battery using a pre-doped negative electrode, since a certain amount of lithium (for example, 5% to 15%) is present in the negative electrode active material even when the battery is in a discharged state, the reaction resistance between the negative electrode active material and lithium ions is low, and charging can be performed smoothly and evenly. In particular, when silicon is used as the negative electrode active material, alloying with lithium greatly improves the lithium ion conductivity in the negative electrode active material, so that pre-doping is particularly preferable in the case of a battery using a silicon-containing negative electrode.

[0015] One aspect of the present invention is a battery including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a positive electrode active material layer, and the positive electrode active material layer includes positive electrode active material particles including magnesium, nickel, aluminum, and lithium cobalt oxide; the negative electrode includes a negative electrode active material layer, and the negative electrode active material layer includes graphite particles and silicon particles; and the electrolyte includes a lithium salt, a fluorinated cyclic carbonate, and a fluorinated chain carbonate; and when a test battery is fabricated using lithium metal and a portion of the negative electrode removed from a battery that has completed a first discharge, the charge capacity of the test battery in a first charge is 5% to 15% of the discharge capacity of the first discharge, as measured per area of ​​the portion of the negative electrode.

[0016] In the above, the first discharge is a discharge treatment in which the battery is charged to a voltage of 4.50 V and then discharged at a constant current until the voltage reaches 2.50 V, and the first charge is a charge treatment in which, after the test battery is produced, the battery is charged at a constant current until the voltage reaches 1.00 V, and it is preferable that the first discharge and first charge are carried out in an environment of 25°C.

[0017] Alternatively, in the above, the electrolyte of the test battery preferably contains a lithium salt, a fluorinated cyclic carbonate, and a fluorinated chain carbonate.

[0018] Alternatively, in the above, it is preferable that the current value of the first discharge is 20 mA / g based on the weight of the positive electrode active material particles, and the current value of the first charge is 75.4 mA / g based on the weight of the graphite particles and silicon particles.

[0019] Alternatively, in the above, it is preferable that the capacity when the battery is discharged in a -30°C environment is 70% or more of the capacity when the battery is discharged in a 25°C environment.

[0020] According to one embodiment of the present invention, a lithium-ion battery having excellent charge / discharge characteristics even in a low-temperature environment can be provided. Specifically, a positive electrode, a negative electrode, an electrolyte, etc. applicable to a lithium-ion battery having a high discharge capacity and / or a high discharge energy density even when discharged in a low-temperature environment can be provided. Alternatively, a positive electrode, a negative electrode, an electrolyte, etc. applicable to a lithium-ion battery having a high charge capacity and / or a high charge energy density even when charged in a low-temperature environment can be provided.

[0021] Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily have all of these effects. Effects other than these can be extracted from the description in the specification, drawings, and claims.

[0022] FIG. 1A is a cross-sectional view illustrating the internal structure of a lithium ion battery, and FIGS. 1B and 1C are views illustrating pre-doping of a lithium ion battery. FIG. 2 is a diagram illustrating an example of a production flow of a secondary battery showing one embodiment of the present invention. FIGS. 3A1 and 3A2 are perspective views of a positive electrode, FIG. 3A3 is a cross-sectional view of the positive electrode, FIGS. 3B1 and 3B2 are perspective views of a pre-doping electrode, and FIG. 3B3 is a cross-sectional view of the pre-doping electrode. FIG. 4A1 is a perspective view of a positive electrode, FIG. 4A2 is a perspective view of a pre-doping electrode, FIG. 4A3 is a perspective view of a negative electrode, and FIG. 4B is a perspective view of a stack and an outer casing. FIG. 5A is a side view of the liquid injection process, FIG. 5B is a top view of a secondary battery cell, and FIG. 5C is a cross-sectional view of the secondary battery cell. FIG. 6A is a side view showing the outer casing during cutting, and FIG. 6B is a side view immediately after the pre-doping electrode has been pulled out. FIG. 7A is a perspective view of a secondary battery, and FIG. 7B is a cross-sectional view of the secondary battery. FIG. 8 is a cross-sectional schematic view of a positive electrode active material layer. FIGS. 9A to 9D are diagrams illustrating a method for producing a positive electrode active material. FIG. 10A is a cross-sectional schematic view of a heating furnace, FIG. 10B is a top view of a lid, and FIG. 10C is a cross-sectional schematic view illustrating the heights of a container, a lid, and an object to be heated. FIG. 11A is a diagram illustrating an example of a manufacturing apparatus, and FIG. 11B is a diagram illustrating the arrangement of rollers. FIG. 12 is a diagram illustrating an example of a manufacturing apparatus. FIG. 13 is a diagram illustrating a method for producing a positive electrode active material. FIGS. 14A to 14C are diagrams illustrating a method for producing a positive electrode active material. FIG. 15 is a phase diagram showing the relationship between the composition and temperature of lithium fluoride and magnesium fluoride. FIG. 16 is a diagram illustrating the results of DSC analysis. FIG. 17 is a diagram illustrating a method for producing a negative electrode active material. FIGS. 18A and 18B are cross-sectional views illustrating a positive electrode active material. Figures 19A to 19F are cross-sectional views illustrating a positive electrode active material. Figure 20 is a diagram illustrating the crystalline structure of a positive electrode active material. Figure 21 is a diagram illustrating the crystalline structure of a conventional positive electrode active material. Figure 22 is a diagram illustrating an XRD pattern calculated from the crystalline structure. Figure 23 is a diagram illustrating an XRD pattern calculated from the crystalline structure. Figure 24 is a diagram illustrating a method for processing a film. Figures 25A to 25E are diagrams illustrating a method for processing a film.Figures 26A and 26B are diagrams explaining a method for processing a film, and Figure 26C is a perspective view of a curved battery. Figures 27A to 27H are diagrams explaining an example of an electronic device. Figures 28A to 28D are diagrams explaining an example of an electronic device. Figures 29A to 29C are diagrams explaining an example of an electronic device. Figures 30A to 30C are diagrams explaining an example of a vehicle. Figures 31A to 31D are diagrams explaining an example of space equipment. Figures 32A and 32B are graphs explaining the low-temperature characteristics of the battery of the example. Figures 33A to 33C are graphs explaining the charge-discharge cycle characteristics of the battery of the example.

[0023] The following description will explain the embodiments of the present invention with reference to the accompanying 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 changes can be made in form and detail without departing from the spirit and scope of the present invention. Therefore, in the embodiments described below, the same reference numerals are used in different drawings to indicate the same objects.

[0024] Furthermore, in each of the embodiments and examples described below, unless otherwise specified, it is possible to implement the embodiments and examples described in this specification and the like in appropriate combinations.

[0025] In this specification, a low-temperature environment refers to 0° C. or below, and 0° C. or below may be referred to as below freezing. When referring to a low-temperature environment in this specification, any temperature below 0° C. can be selected. For example, when referring to a low-temperature environment in this specification, one selected from 0° C. or below, −10° C. or below, −20° C. or below, −30° C. or below, −40° C. or below, −50° C. or below, −60° C. or below, −80° C. or below, and −100° C. or below can be selected.

[0026] In this specification and the like, excellent charge / discharge characteristics in a low temperature environment means that the discharge capacity in a low temperature environment decreases at a smaller rate than the discharge capacity at 25°C.

[0027] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. Individual planes representing crystal planes are expressed using ( ). In crystallography, space groups, crystal planes, and crystal directions are expressed by adding a superscript bar to the numbers. However, due to formatting constraints, in this specification, instead of adding a bar above the numbers, a minus sign (-) may be added before the numbers. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. Furthermore, trigonal crystals represented by the space group R-3m are generally expressed as a hexagonal complex hexagonal lattice for ease of understanding the structure, and unless otherwise specified, the space group R-3m will be expressed as a complex hexagonal lattice in this specification. In addition, Miller indices may be (hkil) instead of (hkl), where i is -(h+k).

[0028] Any integer greater than or equal to 1 may be represented by letters such as h, k, i, l, etc. For example, (00l) includes (001), (003), and (006).

[0029] The space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, the terms "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0030] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. ２ The theoretical capacity of LiNiO is 274 mAh / g per weight. ２ The theoretical capacity of LiMn is 275 mAh / g per weight. ２ O ４ The theoretical capacity of the battery is 148 mAh / g by weight.

[0031] In addition, the amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ CoO ２In the case of a positive electrode active material of a lithium ion battery, x can be expressed as (theoretical capacity - charging capacity) / theoretical capacity. For example, LiCoO ２ When a lithium ion battery using this as the positive electrode active material is charged at 219.2 mAh / g per weight of the positive electrode active material, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２ The state where x is small is, for example, x≦0.24, and in consideration of the practical range when used as a positive electrode active material for a lithium ion battery, it is assumed that 0.1<x≦0.24, for example.

[0032] When the lithium cobalt oxide satisfies the stoichiometric ratio, LiCoO ２ and x = 1. In addition, the lithium ion battery after discharge also has LiCoO ２ In other words, x=1. ２ In a lithium-ion battery using a lithium cobalt oxide battery, the discharge voltage drops sharply before it reaches 2.5 V. For this reason, in this specification and the like, the state in which the voltage reaches 2.5 V (counter electrode is lithium) at a current of 100 mA / g or less per weight of positive electrode active material is regarded as the end of discharge, and x = 1. Therefore, to obtain lithium cobalt oxide when x = 0.2, for example, charging at 219.2 mAh / g per weight of positive electrode active material is sufficient.

[0033] Li ｘ CoO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above calculations be measured under conditions where there is little or no influence of short circuit and / or decomposition of the electrolyte. For example, it is not preferable to use data from a lithium-ion battery that has experienced a sudden voltage change that is considered to be a short circuit in calculating x.

[0034] In this specification and the like, the term "carbonate" refers to a compound having at least one carbonate ester in its molecular structure, and unless otherwise specified, includes "cyclic carbonates" and "chain carbonates." Furthermore, "chain" includes both linear and branched chains.

[0035] In this specification and the like, the phrase "having A and / or B" may be used, which means having A, having B, or having A and B.

[0036] In this specification, a full cell refers to a battery cell assembled with different electrodes, such as a positive electrode / negative electrode unit cell, and a half cell refers to a battery cell assembled with lithium metal as the negative electrode (counter electrode).

[0037] Embodiment 1 In this embodiment, a lithium ion battery having excellent charge / discharge characteristics even in a low-temperature environment will be described.

[0038] [Lithium-ion battery] A lithium-ion battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte. The battery also includes a separator between the positive electrode and the negative electrode. The battery also includes an exterior body that houses the positive electrode, the negative electrode, the electrolyte, and the like. While a method for manufacturing a lithium-ion battery will be described in this embodiment, the order in which the positive electrode, the negative electrode, and the electrolyte are manufactured is not limited to the order described in this embodiment.

[0039] In this embodiment, the description will focus on the configuration of a lithium ion battery required to realize a lithium ion battery having excellent charge / discharge characteristics even in a low temperature environment (e.g., 0° C., preferably −20° C., more preferably −30° C., more preferably −40° C.) Specifically, the description will focus on the positive electrode active material contained in the positive electrode, the negative electrode active material layer, and the electrolyte solution.

[0040] FIG. 1A is a cross-sectional schematic diagram illustrating the internal structure of a battery 10. The battery 10 includes a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11 includes a positive electrode current collector 21 and a positive electrode active material layer 22 on the positive electrode current collector 21, while the negative electrode 12 includes a negative electrode current collector 31 and a negative electrode active material layer 32. As shown in the figure, the positive electrode active material layer 22 and the negative electrode active material layer 32 face each other with the separator 13 interposed therebetween. Although not shown in FIG. 1A , the battery 10 includes an electrolyte in the voids of the positive electrode active material layer 22, the voids of the separator, and the voids of the negative electrode active material layer 32.

[0041] FIG. 1B is a characteristic curve showing the relationship between capacity and potential of the positive electrode active material layer 22 and the negative electrode active material layer 32 of the positive electrode 11 and the negative electrode 12 having the same area and facing each other in the region surrounded by the dashed line A in FIG. 1A.

[0042] In the positive electrode characteristic curve 25 shown in FIG. 1B, the positive electrode capacity value is the value obtained by subtracting the capacity C1 from the capacity C2. In the negative electrode characteristic curve 35 shown in FIG. 1B, the negative electrode capacity value is the capacity C2. Note that the battery 10 of one embodiment of the present invention has a negative electrode 12 that is pre-doped up to the capacity C1 in the negative electrode characteristic curve 35 shown in FIG. 1B. Note that the pre-doped negative electrode 12 contains an amount of lithium corresponding to the pre-doping capacity. Therefore, in the characteristic curve shown in FIG. 1B, the region indicated by diagonal hatching is the operating range B of the battery 10 of one embodiment of the present invention.

[0043] In this specification, the charging process for pre-doping (pre-doping process) is not limited to doping of lithium ions into the negative electrode active material, but refers to the application of current to add lithium ions to the secondary battery in order to replenish the lithium ions that have been consumed and reduced due to film formation or side reactions during the initial charge, and to obtain the designed capacity.

[0044] For comparison, the characteristic curve of a battery using a non-pre-doped negative electrode is shown in Figure 1C. As shown in Figure 1C, in the positive electrode characteristic curve 25r, the positive electrode capacity value is capacity C3. Also, as shown in Figure 1C, in the negative electrode characteristic curve 35r, the negative electrode capacity value is capacity C4. Therefore, in the characteristic curve shown in Figure 1C, the area indicated by diagonal hatching is the operating range C of a battery using a non-pre-doped negative electrode.

[0045] 1B and the characteristic curve shown in FIG. 1C on the assumption that the positive electrode capacity values ​​are equal, the difference between the positive electrode potential and the negative electrode potential in the operating range B is larger than that in the operating range C, and therefore the operating range B is wider than the operating range C. In other words, the battery 10 of one embodiment of the present invention using the pre-doped negative electrode 12 can charge and discharge more energy.

[0046] In addition, in the case of a battery using a non-pre-doped negative electrode, since there is almost no lithium in the negative electrode active material in the discharged state of the battery, the reaction resistance between the negative electrode active material and lithium ions is high. Therefore, there is a concern that the negative electrode active material and lithium ions are unlikely to react or that uneven reaction may occur. On the other hand, in the case of a battery using a pre-doped negative electrode, since a certain amount of lithium (e.g., 5% or more) is present in the negative electrode active material even in the discharged state of the battery, the reaction resistance between the negative electrode active material and lithium ions is low, and charging can be performed smoothly and evenly. In particular, when silicon is used as the negative electrode active material, alloying with lithium greatly improves the lithium ion conductivity in the negative electrode active material, so pre-doping is particularly preferable for batteries using a silicon-containing negative electrode.

[0047] <Method for Manufacturing Battery> A method for manufacturing the 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.

[0048] FIG. 2 is a flow chart showing an example of a method for manufacturing the battery 10.

[0049] First, the positive electrode 11, the negative electrode 12, and the pre-doping electrode 45 are prepared. In the present embodiment, an example of a 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 exterior body 50, but this is not particularly limited, and a process in which a double-sided coated positive electrode or negative electrode is used and a plurality of sets are enclosed in a single exterior body can also be used.

[0050] The positive electrode 11 and the pre-doping electrode 45 can be formed by coating one side of a current collector with a positive electrode active material layer 22. As shown in FIG. 2, 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. Furthermore, aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, can be used. The current collector can be in the form of a foil, plate (sheet), mesh, punched metal, expanded metal, or the like, as appropriate. The current collector has a thickness of 5 μm or more and 30 μm or less.

[0051] 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.

[0052] 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.

[0053] 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. 3A2 and 3A3. FIG. 3A3 is a cross-sectional view of the positive electrode taken along the chain line X1-X2 in FIG. 3A2. In FIGS. 3A1 and 3A2, a positive electrode active material layer is formed on the lower surface of the positive electrode current collector 21, and is therefore not shown.

[0054] 3A1 and 3A2 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. 3A2. 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. 3A2 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.

[0055] Each current collector is provided with a protruding region (called a tab region) for connection to a lead electrode, and the positive electrode active material layer 22 or the like is not formed in that region, leaving the conductive surface exposed.

[0056] The pre-doping electrode 45 is also wrapped in a second film 47a in the same manner as the positive electrode 11. FIGS. 3B1 and 3B2 show perspective views of the pre-doping electrode 45 and the second film 47a. As shown in FIG. 3B1, 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 the state shown in FIG. 3B2. The cross-sectional view taken along the dashed line X3-X4 in FIG. 3B2 corresponds to FIG. 3B3. 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.

[0057] 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. Then, the negative electrode lead electrode 33 is ultrasonically bonded to the tab region of the negative electrode current collector 31 where the negative electrode active material layer 32 is not formed.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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 may contain at least one of silicon, silicon oxide, and silicon alloy.

[0062] 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.

[0063] 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 the conductive material (acetylene black).

[0064] 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.

[0065] 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 electrode 23 is bonded to the tab region of the positive electrode 11, the negative electrode lead electrode 33 is bonded to the tab region of the negative electrode 12, and the pre-doping lead electrode 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.

[0066] Note that Figure 4A1 shows an oblique view before connecting the positive electrode lead electrode 23, Figure 4A2 shows an oblique view before connecting the pre-doping lead electrode 48, and Figure 4A3 shows an oblique view before connecting the negative electrode lead electrode 33, and the dotted lines in the figures are aligned so that they overlap.

[0067] 4B shows a perspective view of one laminate film 50a that will become the exterior body 50 before folding. The negative electrode has a larger area than the positive electrode, and they are stacked with their centers aligned, with their respective ends almost, but not perfectly, aligned. As shown in FIG. 4B, 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 second film 47a is designed to have a larger area than the first film 13a so that it can be used as a gripping area.

[0068] When the periphery is thermocompressed using a heat bar sealer, the thermocompression is performed leaving one side for later filling with the electrolyte. For example, as shown in FIG. 4B, the above-mentioned laminate is placed in a predetermined position inside a laminate film 50a folded in half. Then, as shown in FIG. 5A, thermocompression is performed in the first compression region 52a (step S101). After thermocompression, the laminate film 50a becomes a bag-shaped exterior body 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 exterior body 50. This state is also called three-sided sealing.

[0069] Then, the bag-shaped exterior body 50 is fixed with the portion containing the electrolyte solution 60 facing up, and the electrolyte solution 60 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 contained 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.

[0070] 5A shows a side view of the injection process of the electrolyte solution 60. 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.

[0071] After the injection of the electrolyte solution 60 is completed, a vacuum sealing process (step S103) is performed.

[0072] 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 electrolytic solution 60 used, excessive pressure reduction may cause evaporation, so the reduced pressure value is adjusted depending on the electrolytic solution 60 used.

[0073] After the injection of the electrolytic solution 60, an impregnation treatment may be performed to facilitate impregnation of the electrolytic solution 60 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.

[0074] 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.

[0075] 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.

[0076] A secondary battery cell having a pre-doping electrode is fabricated through the above steps, and Fig. 5B shows a top view of the secondary battery cell. Fig. 5C is a cross-sectional view taken along the chain line X5-X6 in Fig. 5B.

[0077] As shown in Fig. 5C, an electrolyte solution 60 is sealed inside the exterior case 50, and a laminate is disposed therein. The laminate disposed inside the exterior case 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.

[0078] 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 is calculated in advance based on an estimated value (design capacity) of the capacity of the battery to be finally manufactured, and the charging amount can be determined, and is calculated 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.

[0079] 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%.

[0080] During the charging process for pre-doping, the pre-doping lead electrode 48 is connected to an external power source, and the negative electrode lead electrode 33 is connected to the external power source to energize the battery. 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, charging to a capacity exceeding the target remaining capacity may be performed, and then a discharging process may be performed so that the target remaining capacity is reached. Furthermore, charging and discharging may be repeated one or more times before the charging process for pre-doping is performed.

[0081] 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 lead electrode 48 is also cut. Note that Fig. 6A shows a side view of the exterior body when cutting, and the pre-doping lead electrode 48 is fixed in a position where it faces upward to prevent leakage of the internal electrolyte. The dashed line 49 in Fig. 6A is the cut line, as shown.

[0082] Then, in an argon gas atmosphere, a process (step S106) is performed to pull out the pre-doping electrode 45 and the separator 47. Fig. 6B 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 lead electrode 48. 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.

[0083] 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.

[0084] In this manner, the battery 10 according to one embodiment of the present invention can be fabricated using the pre-doped negative electrode 12 .

[0085] 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.

[0086] The fabricated 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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).

[0093] The above steps produce the secondary battery 500. An external view of the produced secondary battery is shown in Fig. 7A, and a cross section taken along the chain line A1-A2 in Fig. 7A is shown in Fig. 7B.

[0094] The battery 10 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 that surround the sealed region 53 .

[0095] 8, the positive electrode active material layer 22 includes a positive electrode active material 100 and a conductive material 41. Although not shown, the positive electrode active material layer 22 may include a binder in addition to the positive electrode active material 100 and the conductive material 41.

[0096] Furthermore, the voids in the positive electrode active material layer 22 are preferably filled with the electrolyte. For example, it is preferable that 60% or more of the voids in the positive electrode active material layer 22 be filled with the electrolyte, more preferably 70% or more of the voids, even more preferably 70% or more of the voids, even more preferably 80% or more of the voids, even more preferably 90% or more of the voids, even more preferably 95% or more of the voids, and most preferably 99% or more of the voids. The voids in the positive electrode active material layer 22 refer to regions in the positive electrode active material layer 22 other than the solid components (such as the positive electrode active material and the conductive material).

[0097] Although detailed description will be omitted, similar to the description of the positive electrode active material layer 22 above, the voids in the negative electrode active material layer 32 may also be filled with the electrolyte. For example, it is preferable that 60% or more of the voids in the negative electrode active material layer 32 be filled with the electrolyte, more preferably 70% or more of the voids, more preferably 80% or more of the voids, more preferably 90% or more of the voids, more preferably 95% or more of the voids, and most preferably 99% or more of the voids. The voids in the negative electrode active material layer 32 refer to regions in the negative electrode active material layer 32 other than those filled with solid components (negative electrode active material, conductive material, etc.).

[0098] In this way, by filling every corner of the positive electrode active material layer 22 and the negative electrode active material layer 32 with the electrolyte, it is possible to increase the area where the positive electrode active material and the negative electrode active material come into contact with the electrolyte, thereby providing a lithium ion battery with excellent charge and discharge characteristics in a low-temperature environment.

[0099] Furthermore, during charging in a low-temperature environment, the energy barrier for desorbing lithium ions from the positive electrode active material tends to be high. In other words, the lower the temperature of the charging environment, the greater the overvoltage required to desorb lithium ions from the positive electrode active material. In other words, the positive electrode active material may be exposed to a high voltage during charging in a low-temperature environment. In other words, if the positive electrode active material is not exposed to a high voltage during charging in a low-temperature environment, the charge capacity may be reduced.

[0100] 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.

[0101] 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).

[0102] 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.

[0103] [Positive Electrode] The positive electrode has a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer has a positive electrode active material, and may further have at least one of a conductive material and a binder.

[0104] <Positive Electrode Active Material> The positive electrode active material has the function of absorbing and releasing lithium ions during charge and discharge. The positive electrode active material used in one embodiment of the present invention can be a material that exhibits little deterioration (or little increase in resistance) during charge and / or discharge (hereinafter also referred to as "charge and discharge") in a low-temperature environment, even at high charge voltages (unless otherwise specified, this refers to a voltage value based on lithium metal; hereinafter, this will also be referred to as "high charge voltage"). Specifically, it is preferable to use a positive electrode active material (composite oxide) having a particle size (strictly speaking, a median diameter (D50)) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less) obtained by the preparation method described in embodiment 1. Of course, a positive electrode active material having a particle size of more than 12 μm and 20 μm or less may also be used. This positive electrode active material is a material containing the additive element A described in embodiment 1. In this embodiment, the additive element A is divided into additive element X, additive element Y, and additive element Z, and detailed description will be given. In other words, the positive electrode active material described in this embodiment contains one or more of the additional element X, the additional element Y, and the additional element Z. The additional element X, the additional element Y, and the additional element Z will be described in detail in <Containing Elements>. The additional element X described in this embodiment corresponds to the additional element A1 described in Embodiment 1. The additional element Y and the additional element Z described in this embodiment correspond to the additional element A2 described in Embodiment 1.

[0105] The particle size can be measured using a particle size distribution analyzer using a laser diffraction / scattering method. The median diameter (D50) is the particle diameter when the cumulative amount in the cumulative curve of the particle size distribution measurement results accounts for 50%. The measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the major axis of the particle cross section may also be measured by analysis using a scanning electron microscope (hereinafter referred to as SEM) or a transmission electron microscope (hereinafter referred to as TEM). Note that, as a method for measuring the median diameter (D50) using analysis using SEM or TEM, for example, 20 or more particles are measured, a cumulative curve is created, and the particle diameter when the cumulative amount accounts for 50% can be taken as the median diameter (D50).

[0106] As an evaluation of low-temperature characteristics, it is preferable that the discharge capacity value in a low-temperature environment is 50% or more (preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and most preferably 90% or more) of the discharge capacity value at 20° C. Note that the above numerical values ​​should be obtained assuming that the measurement conditions other than the environmental temperature are the same.

[0107] Alternatively, even at a high charging voltage, by using a material that is less prone to deterioration during charging and discharging (or a material that is less prone to increase in resistance) as the positive electrode active material, it is possible to obtain a large discharge capacity even at low temperature environments.

[0108] More specifically, the discharge capacity when charging and discharging at -40°C is preferably 60% or more, more preferably 65% ​​or more, more preferably 70% or more, and even more preferably 75% or more, of the discharge capacity when charging and discharging at 25°C. Note that although -40°C is used, this temperature may be any low temperature and may be interpreted as other low temperatures such as -20°C or -30°C. For example, the discharge condition may be discharge at a current rate of 0.1C (where 1C = 200mA / g (per weight of positive electrode active material)). In the evaluation of low-temperature characteristics as described above, evaluation may be performed at a low rate as long as the measurement conditions other than the environmental temperature are consistent.

[0109] As another evaluation of low-temperature characteristics, the discharge energy density value in a low-temperature environment is preferably 50% or more (preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and most preferably 90% or more) of the discharge energy density value at 25°C.

[0110] As used herein, the term "ambient temperature" refers to the temperature of a lithium-ion battery. When measuring battery characteristics using a thermostatic chamber, the ambient temperature can be considered to be the temperature set in the thermostatic chamber. Therefore, after placing the battery to be measured (e.g., a test battery or half-cell) in the thermostatic chamber, it is recommended to wait a sufficient amount of time (e.g., one hour or more) until the test cell reaches the same temperature as the thermostatic chamber before starting the measurement, but this method is not necessarily limited to this.

[0111] A positive electrode active material 100 according to one embodiment of the present invention will be described with reference to FIGS. 18 and 19. The positive electrode active material 100 exhibits little deterioration due to repeated charging and discharging at a high voltage relative to lithium metal, and therefore can provide sufficient battery characteristics even in a low-temperature environment. In this embodiment, the high voltage is 4.6 V, preferably 4.65 V, and more preferably 4.7 V, relative to lithium metal.

[0112] 18A and 18B are cross-sectional views of a positive electrode active material 100 according to one embodiment of the present invention. Enlarged views of the vicinity of A-B in FIG. 18B are shown in FIGS. 19A to 19C. Enlarged views of the vicinity of C-D in FIG. 18B are shown in FIGS. 19D to 19F.

[0113] 18A, positive electrode active material 100 has surface layer portion 100a and interior portion 100b. In these figures, the boundary between surface layer portion 100a and interior portion 100b is indicated by a dashed line, but the boundary is not clearly defined.

[0114] 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. A narrow region extending from the surface toward the interior, specifically within 20 nm, is called a shell. Note that "approximately perpendicular" includes perpendicular, specifically, 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 or near-surface region.

[0115] The region of the positive electrode active material deeper than the surface layer 100a is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0116] Furthermore, when positive electrode active material 100 has a layered rock salt crystal structure of space group R-3m, surface layer portion 100a has edge region 100a1 and basal region 100a2 as shown in FIG. 18B.

[0117] 18A and 18B, the straight line labeled (00l) represents the (00l) plane. The basal region 100a2 has a particle surface (called a basal plane) parallel or approximately parallel to the (00l) plane. The particle surface other than the basal plane is called an edge plane, and the region having the edge plane is called an edge region 100a1. When lithium cobalt oxide is used in the positive electrode active material 100, lithium ions can be inserted and removed at the edge plane.

[0118] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer portion 100a and the inner portion 100b. ２ O ３ This does not include metal oxides having no lithium sites that can contribute to charging and discharging, such as carbonates and hydroxyl groups that are chemically adsorbed after the preparation of the positive electrode active material. Note that the attached metal oxides refer to, for example, metal oxides whose crystal orientation does not match that of the interior 100b.

[0119] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron beam diffraction patterns, and the like. The determination can also be made based on the FFT pattern of a TEM image, the FFT pattern of a STEM image, etc. Furthermore, XRD (X-ray diffraction), neutron diffraction, etc. can also be used as materials for the determination.

[0120] The positive electrode active material does not include the electrolytic solution, decomposition products of the electrolyte, organic solvent, binder, conductive material, or compounds derived therefrom that are attached to the positive electrode active material 100. In other words, the electrolytic solution, decomposition products of the electrolyte, organic solvent, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material are removed.

[0121] 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 may be the surface of the positive electrode active material. Therefore, surfaces resulting from slippage, cracks, and / or fissures are also included in the surface of the positive electrode active material. When analyzing the positive electrode active material, a protective film may be applied 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, or the like.

[0122] <Containing Elements> The positive electrode active material 100 contains lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material 100 contains lithium cobalt oxide (LiCoO ２ However, the positive electrode active material 100 of one embodiment of the present invention may have a crystal structure described later. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0123] The additive elements contained in the positive electrode active material 100 are preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium.

[0124] The additive element is preferably present in the form of a solid solution in the positive electrode active material 100. The additive element further stabilizes the crystal structure of the positive electrode active material 100, as will be described later.

[0125] 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, etc., are further enhanced. The weight of manganese contained in the cathode active material 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0126] The surface layer 100a, especially the edge region having the edge surface, is the region where lithium ions are first desorbed during charging, and is the region where the lithium concentration is likely to be lower than that of the interior 100b. In addition, in the surface layer 100a where lithium ions are desorbed, especially in the edge region, some of the bonds of the atoms on the surface of the particles of the positive electrode active material 100 are broken. Therefore, the surface layer 100a is likely to become unstable, and is the region where deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer 100a, especially the edge region, can be sufficiently stabilized, Li ｘ CoO ２ Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 100b made of octahedrons of cobalt and oxygen can be made less likely to break. Furthermore, if the surface layer portion 100a, particularly the edge region, can be made sufficiently stable, it is possible to suppress displacement of the layer made of octahedrons of cobalt and oxygen in the inner portion 100b.

[0127] To provide the surface layer 100a with a stable composition and crystal structure, the surface layer 100a preferably contains the above-described additive elements, and more preferably contains a plurality of additive elements. The surface layer 100a preferably has a higher concentration of one or more selected from the additive elements than the interior 100b. The edge region 100a1 preferably has a higher concentration of one or more selected from the additive elements than the basal region 100a2.

[0128] 19A to 19C are enlarged views of the vicinity of A-B in FIG. 18B and are views illustrating the edge region 100a1 of the positive electrode active material 100. Also, FIGS. 19D to 19F are enlarged views of the vicinity of C-D in FIG. 18B and are views illustrating the basal region 100a2 of the positive electrode active material 100.

[0129] For example, some of the additive elements, such as magnesium, fluorine, and titanium, preferably have a concentration gradient that increases from the interior 100b toward the surface. In Figures 19A and 19D, the concentration gradient is expressed using the density of the hatching. An additive element having such a concentration gradient will be referred to as additive element X. However, the concentrations of magnesium, fluorine, titanium, and the like may be higher in the edge region 100a1 than in the basal region 100a2.

[0130] Another additive element, such as aluminum, preferably has a concentration gradient and a concentration peak in a region deeper than the additive element X shown in FIGS. 19B and 19E. In FIGS. 19B and 19E, the concentration gradient and peak region are expressed using hatch darkness. The concentration peak 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 in a region of 5 nm to 30 nm from the surface toward the interior. An additive element having such a concentration gradient will be referred to as additive element Y. However, the concentration of aluminum, etc., may be higher in the edge region 100a1 than in the basal region 100a2.

[0131] Another additive element, such as nickel, may be clearly present in the edge region 100a1 but substantially absent in the basal region 100a2, as indicated by the presence or absence of hatching and the density of the hatching in Figures 19C and 19F. The concentration of nickel or the like may be higher in the edge region 100a1 than in the basal region 100a2. Note that "clearly present" here refers to a case in which a characteristic X-ray energy spectrum of the element is detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 100. An additive element having such a distribution will be referred to as additive element Z.

[0132] Furthermore, "substantially free" refers to a case where the characteristic X-ray energy spectrum of the element is not detected in a cross-sectional STEM-EDX analysis of the positive electrode active material 100. This also refers to the element being below the lower limit of detection in the STEM-EDX analysis. In this case, it also refers to the element being below the lower limit of detection in the STEM-EDX analysis.

[0133] For example, magnesium, which is one of the additive elements X, is divalent, and magnesium is more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, so it is easy to enter the lithium site. When magnesium is present at an appropriate concentration at the lithium site in the surface layer 100a, it becomes easier to maintain the layered rock-salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by CoO ２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２ When x is, for example, 0.24 or less, the desorption of oxygen from the periphery of magnesium can be suppressed.

[0134] At appropriate concentrations, magnesium does not adversely affect lithium intercalation and deintercalation 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 or 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 a resistive component in lithium-ion batteries. Furthermore, increasing the magnesium concentration of the positive electrode active material can decrease the discharge capacity of the positive electrode active material. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0135] 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 0.001 to 0.1 times the number of cobalt atoms, more preferably more than 0.01 to less than 0.04 times, and even more preferably about 0.02 times. 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.

[0136] Furthermore, aluminum, one of the additive elements Y, can exist at the cobalt site in the 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. Aluminum also suppresses the elution of surrounding cobalt and improves continuous charging durability. Furthermore, because the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when used in lithium-ion batteries. Furthermore, a positive electrode active material 100 can be obtained whose crystal structure is less likely to collapse even with repeated charging and discharging. On the other hand, excessive aluminum may adversely affect lithium insertion and extraction.

[0137] 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.

[0138] Nickel, which is one of the additional elements Z, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, it has a lower oxidation-reduction potential than cobalt, which leads to an increase in discharge capacity, which is preferable.

[0139] 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 volume change 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 can also be prevented from shifting to CoO ２This 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.

[0140] On the other hand, an excess of nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and may also adversely affect the insertion and extraction of lithium.

[0141] 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 greater than 0% and not greater 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, it is preferably greater than 0% and not greater than 4%. Alternatively, it is preferably greater than 0% and not greater than 2%. Alternatively, it is preferably 0.05% to 7.5%. Alternatively, it is preferably 0.05% to 2%. Alternatively, it is preferably 0.1% to 7.5%. Alternatively, it is preferably 0.1% to 4%. 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.

[0142] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When a portion of the oxygen in the surface layer 100a is replaced by fluorine, the lithium desorption energy decreases. This is because the valence of the cobalt ion changes with lithium desorption (from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine), resulting in different redox potentials. Therefore, when a portion of the oxygen in the surface layer 100a of the positive electrode active material 100 is replaced by fluorine, it can be said that desorption and insertion of lithium ions near the fluorine occurs more smoothly. Therefore, when used in a lithium ion battery, charge / discharge characteristics, rate characteristics, low-temperature characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer 100a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance against hydrofluoric acid. Furthermore, as will be described in a later embodiment, when the melting point of a fluoride, such as lithium fluoride, is lower than the melting point of other additive element sources, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element sources.

[0143] Furthermore, titanium oxide, which is one of the additive elements X, is known to have superhydrophilicity. Therefore, by providing the cathode active material 100 with titanium oxide in the surface layer portion 100a, it is possible that the cathode active material 100 has good wettability with highly polar solvents. When used in a secondary battery, this improves the contact at the interface between the cathode active material 100 and a highly polar electrolyte, which may suppress an increase in internal resistance.

[0144] Furthermore, when the surface layer 100a shown in FIG. 19A has magnesium and the surface layer 100a shown in FIG. 19C has nickel, that is, when both magnesium and nickel are present, there is a possibility that divalent nickel can exist more stably near the divalent magnesium. ｘ CoO 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.

[0145] 19C and 19F , the additive element Z may be contained in a large amount in the edge region 100a1 (also referred to as being contained preferentially or selectively), which is preferable because it improves the stability of the crystal structure of the edge region 100a1 where lithium ions enter and exit the positive electrode active material 100 during charging and discharging of the lithium ion battery. Furthermore, when the additive element Z has the above-described distribution, for example, when the positive electrode active material 100 is lithium cobalt oxide, it is preferable because it can minimize the effects of adding the additive element Z, such as a decrease in discharge voltage or a decrease in discharge capacity.

[0146] As described above, when multiple additive elements are present, the effects of each additive element are synergistic, which can contribute to further stabilization of the surface layer portion 100a. In particular, the presence of magnesium, nickel, and aluminum is highly effective in achieving a stable composition and crystal structure, making it preferable. In particular, it is preferable that the surface layer portion 100a of the positive electrode active material 100 has a region where aluminum is distributed more inward than magnesium. Furthermore, in addition to the region where magnesium and aluminum are distributed, it is most preferable that the surface layer portion 100a of the positive electrode active material 100 has a region where the nickel distribution and the magnesium distribution overlap in the edge region 100a1.

[0147] <Crystalline Structure> One aspect of the present invention is to provide a lithium-ion battery with improved battery characteristics in a low-temperature environment, but XRD measurements and the like that identify the crystalline structure and the like are performed at room temperature.

[0148] <Li ｘ CoO ２ When x is 1 in the positive electrode active material 100 according to one embodiment of the present invention, the positive electrode active material 100 is in a discharged state, that is, Li ｘ CoO ２ In the case where x = 1 in the formula (I), it is preferable that the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. The layered rock-salt type composite oxide has a high discharge capacity, has two-dimensional lithium ion diffusion paths, is suitable for lithium ion insertion / extraction reactions, and is excellent as a positive electrode active material for lithium ion batteries. Therefore, it is 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.

[0149] 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 octahedra of cobalt and oxygen in the inner portion 100b is not destroyed even when lithium is released 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 the inner portion 100b of the cathode active material 100, such as oxygen release, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100.

[0150] To provide a reinforcing function, the surface layer portion 100a may have a different crystal structure from the interior portion 100b. For example, 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 may have a rock salt crystal structure. Alternatively, the surface layer portion 100a may have both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a may have characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0151] 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.

[0152] Furthermore, it is preferable that some of the added elements, especially magnesium, have a higher concentration in the surface layer 100a than in the interior 100b, and that they are present randomly and dilutely in the interior 100b. Furthermore, when aluminum is present at an appropriate concentration at the lithium sites in the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 100b, it can suppress the deviation of the layered structure consisting of octahedra of cobalt and oxygen, as described above. Furthermore, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, and a synergistic effect of suppressing magnesium elution can be expected.

[0153] Furthermore, it is preferable that the crystal structure continuously changes from the interior 100b toward the surface due to the magnesium concentration gradient described above, or that the crystal orientation of the surface layer 100a and the interior 100b roughly coincide.

[0154] 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 cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a distorted lattice structure, and the orientation of the crystals may be roughly the same.

[0155] 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, although cation or anion defects may occur.

[0156] The rock salt crystal structure does not distinguish between cation sites, but the layered rock salt crystal structure has two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by the transition metal M. 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 and layered rock salt structures.

[0157] The layered rock salt crystal structure and the anions in the rock salt crystal structure form a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in the O3' crystal, which will be described later, also form a cubic close-packed structure. Therefore, when the layered rock salt crystal structure and the rock salt crystal structure come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0158] 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.

[0159] However, the space group of the layered rock salt type crystal structure and the O3' type crystal structure described later is R-3m, which is different from the space group Fm-3m of the rock salt type crystal structure (the space group of a general rock salt type crystal), and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal structure, the O3' type crystal structure, and the rock salt type crystal structure. In this specification, when the orientations of the cubic close-packed structures formed by anions are aligned in the layered rock salt type crystal structure, the O3' type crystal structure, and the rock salt type crystal structure, it may be said that the crystal orientations are approximately the same.

[0160] <Li ｘ CoO ２ The positive electrode active material 100 according to one embodiment of the present invention has the above-described magnesium distribution and / or crystal structure, and therefore, is in a discharged state (Li ｘ CoO ２ The crystal structure in the state where x is small (where x is small) is different from that of conventional positive electrode active materials.

[0161] Using Figures 20 to 23, Li ｘ CoO ２ The 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.

[0162] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 21. The conventional positive electrode active material shown in FIG. 21 is a lithium cobalt oxide (LiCoO ２ )

[0163] Figure 21 shows the R-3m O3 and Li ｘCoO ２ The crystal structure of lithium cobalt oxide with x=1 in the figure 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.

[0164] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0165] 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.

[0166] 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 ２ It can also be said that this crystal structure is a structure in which the structure of and the structure of are stacked alternately. For this reason, this crystal structure is sometimes called an H1-3 crystal structure. In reality, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in this specification, including Figure 21, the c-axis of the H1-3 crystal structure is shown as half the unit cell to make it easier to compare with other crystal structures.

[0167] As an example of the H1-3 type crystal structure, the coordinates of cobalt and oxygen in the 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 should be 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 sufficient to adopt a unit cell that has a small GOF (goodness of fit) value, specifically one that is close to 1.

[0168] 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.

[0169] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 21, 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.

[0170] Furthermore, the difference in volume between these two crystal structures is large: per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure is greater than 3.5%, typically 3.9% or more.

[0171] 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.

[0172] 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.

[0173] On the other hand, in the positive electrode active material 100 of one embodiment of the present invention shown in FIG. ｘ CoO 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, specifically, x is 0.2 (this may be referred to as a 20% Li existence probability), is smaller than that of conventional positive electrode active materials. More specifically, 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 CoO 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 CoO 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 lithium ion battery is further improved, which is preferable.

[0174] Li ｘ CoO ２ The crystal structure of the inner portion 100b of the positive electrode active material 100 when x is approximately 1 or 0.2 is shown in FIG. 20. The inner portion 100b occupies the majority of the volume of the positive electrode active material 100 and is the portion that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0175] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0176] 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 0.12, which results in an H1-3 type crystal structure.

[0177] 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 layer symmetry is the same as that of O3. Therefore, this crystal structure is referred to as an O3'-type crystal structure. Furthermore, although the positive electrode active material 100 of one embodiment of the present invention when x = approximately 0.2 does not have a spinel structure, a pattern similar to a spinel structure may appear in the XRD pattern, and this crystal structure may be referred to as a pseudo-spinel structure. This crystal structure is shown in Figure 20 with the notation R-3m O3'.

[0178] 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 −１ nm), and 2.807≦a≦2.827 (×10 −１ nm) is more preferable, and typically a=2.817 (×10 −１ nm). The c-axis is 13.681≦c≦13.881 (×10 −１ nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 −１ nm).

[0179] In the O3' type crystal structure, ions of cobalt, magnesium, etc. occupy the hexacoordinated oxygen positions. Light elements such as lithium may occupy the tetracoordinated oxygen positions.

[0180] As shown by the dotted line in FIG. 20, the difference between R-3m(O3) in the discharged state and the O3′-type crystal structure is ２ There is almost no layer misalignment.

[0181] 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%.

[0182] As described above, in the positive electrode active material 100 according to one embodiment of the present invention, Li ｘ CoO 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 lithium-ion battery with a high discharge capacity per weight and per volume can be fabricated.

[0183] The positive electrode active material 100 is Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, it may have an O3' type crystal structure, and it is estimated that even when x is more than 0.24 and 0.27 or less, it has an O3' type crystal structure. However, the crystal structure is ｘ CoO 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.

[0184] Therefore, the positive electrode active material 100 is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the entire interior 100b of the positive electrode active material 100 does not have to have the O3′-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0185] Also Li ｘ CoO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘCoO ２ The state where x is small can be rephrased as a state where the battery is charged at a high charging voltage.

[0186] 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 4.6 V or higher at 25° C. In other words, it is preferable because it can adopt an O3′-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower at 25° C.

[0187] In some cases, the H1-3 crystal structure is finally observed when the charge voltage is further increased, even in the positive electrode active material 100. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the electrolyte, and the like. Therefore, even when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or higher and lower than 4.6 V at 25° C., the positive electrode active material 100 of one embodiment of the present invention may be able to adopt the O3′ crystal structure.

[0188] In addition, when graphite is used as the negative electrode active material in a lithium ion battery, the voltage of the lithium ion battery is lower than the voltage described above by the amount of the potential of the graphite. The potential of graphite is approximately 0.05 V to 0.2 V relative to the potential of lithium metal. Therefore, in the case of a lithium ion battery using graphite as the negative electrode active material, the battery 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.

[0189] In addition, in O3' of FIG. 20, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, or, for example, in the monoclinic O1 (Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0190] In order to form an O3'-type crystal structure, it is preferable that the magnesium concentration gradient be similar at multiple locations in the surface layer 100a of the positive electrode active material 100. In other words, it is preferable that reinforcement derived from magnesium is uniformly present in the surface layer 100a. Even if a portion of the surface layer 100a is reinforced, if there is a portion without reinforcement, stress may concentrate in the unreinforced portion. If stress concentrates in a portion of the positive electrode active material 100, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity. However, it is not necessary for magnesium to have a similar concentration gradient throughout the entire surface layer 100a of the positive electrode active material 100.

[0191] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ It can be said that the structure is one in which layers and lithium layers are alternately stacked parallel to the (001) plane. Therefore, the diffusion path of lithium ions also exists parallel to the (001) plane. Again, the (001) plane is called the basal plane, and the planes other than the (001) plane where the diffusion path of lithium ions is exposed are called edge planes.

[0192] CoO 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.

[0193] On the other hand, the diffusion paths of lithium ions are exposed on surfaces other than the (001) plane. Therefore, the surfaces other than the (001) plane and the surface layer portion 100a having such surfaces are important regions for maintaining the diffusion paths of lithium ions, and at the same time, they are the regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surfaces other than the (001) plane and the surface layer portion 100a having such surfaces is extremely important for maintaining the crystal structure of the entire positive electrode active material 100.

[0194] <Analysis method> A certain positive electrode active material is ｘ CoO 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 crystal structure by Li ｘCoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x in the positive electrode active material 100 using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. Among XRD methods, powder XRD is preferred because it can provide diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100.

[0195] 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 generated when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 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.

[0196] Furthermore, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, it may change from an O3'-type crystal structure to an H1-3-type crystal structure. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0197] Furthermore, whether or not the distribution of the additive elements contained in the positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0198] 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 .

[0199] Charging for determining whether a composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be performed by fabricating 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. The coin cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0200] 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.

[0201] The counter electrode can be made of lithium metal.

[0202] The lithium salt was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7 is used as the electrolyte, and vinylene carbonate (VC) can be used as an additive mixed in an amount of 2 wt % with respect to the mixed solvent.

[0203] The separator may be a 25 μm thick porous polypropylene film.

[0204] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0205] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 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 per weight of positive electrode active material. CV charging can be terminated at 2 mA / g or more and 10 mA / g or less per weight of positive electrode active material. Charging at such a low current value is desirable to observe the phase change of the positive electrode active material. The temperature is set to 25°C. After charging in this manner, the coin cell can be disassembled in a glove box under an argon atmosphere and the positive electrode removed to obtain a positive electrode active material with the desired charge capacity. When performing various subsequent analyses, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed in a sealed container under an argon atmosphere. After the charging is completed, the positive electrode is preferably taken out and analyzed promptly, preferably within 1 hour, more preferably within 30 minutes.

[0206] Furthermore, 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 up to an arbitrary voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V) at a current value of 20 mA / g or more and 100 mA / g or less per weight of the positive electrode active material, followed by constant voltage charging until the current value becomes 2 mA / g or more and 10 mA / g or less per weight of the positive electrode active material, and then discharging by constant current discharging at 2.5 V at a current value of 20 mA / g or more and 100 mA / g or less.

[0207] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at, for example, 2.5 V, with a current value of 20 mA / g or more and 100 mA / g or less per weight of the positive electrode active material.

[0208] <XRD> The apparatus and conditions for XRD measurement are not particularly limited as long as they are properly adjusted and calibrated. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray: Cu Kα ２ 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): Set to 0.01° Counting time: 1 second / step Sample stage rotation: 15 rpm The standard sample used for adjustment and calibration may be, for example, NIST (National Institute of Standards and Technology) standard aluminum oxide sintered plate SRM 1976.

[0209] If the measurement sample is a powder, it can be set by placing it on a glass sample holder, or by sprinkling the sample on a greased silicone anti-reflective plate, etc. 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.

[0210] The characteristic X-rays may be monochromated using a filter or by using software for XRD data analysis after obtaining an XRD pattern. For example, CuKα ２ Excluding the peak due to the line, CuKα １ It is possible to extract only the line peaks. The software can also be used to remove background noise.

[0211] In this specification, data processing when referring to the 2θ value of a certain diffraction peak will be described. First, a calculation model is fitted to the XRD pattern using crystal structure analysis software to obtain a calculated pattern. In the calculated pattern, the 2θ value at which the peak top of the diffraction peak appears is referred to as the 2θ value of the diffraction peak. The crystal structure analysis software used for fitting is not particularly limited, but for example, TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker) can be used.

[0212] The ideal powder XRD patterns calculated from the O3' type crystal structure and the H1-3 type crystal structure model using CuKα1 radiation are shown in Figures 22 and 23. For comparison, Li ｘ CoO ２ LiCoO where x=1 ２ The ideal XRD patterns calculated from the crystal structure of LiCoO3 and the trigonal O1 with x = 0 are also shown. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), based on the crystal structure information obtained from ICSD (Inorganic Crystal Structure Database). The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The XRD pattern of the H1-3 type crystal structure was created by the same method as above, based on the information on the H1-3 type crystal structure shown in FIG. 23. The XRD pattern of the O3' type crystal structure was created by estimating the crystal structure from the XRD pattern of the positive electrode active material of one embodiment of the present invention, fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and creating an XRD pattern in the same manner as the others.

[0213] As shown in FIG. 22, in the O3′ type crystal structure, diffraction peaks appear at 2θ=19.25±0.12° (19.13° or more and 19.37° or less) and 2θ=45.47±0.10° (45.37° or more and 45.57° or less).

[0214] However, as shown in FIG. 23, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and 19.37° or less) and 2θ = 45.47 ± 0.10° (45.37° or more and 45.57° or less) 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.

[0215] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ of 42° or more and 46° or less, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0216] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ CoO ２When x in the formula is small, the material has an O3'-type crystal structure, but not all of it needs to be an O3'-type crystal structure. It may contain other crystal structures, or a portion may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0217] 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. The half-width varies depending on the XRD measurement conditions or the value of 2θ, even for peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for peaks observed at 2θ=43° or more and 46° or less, the half-width is, for example, preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. A narrow half-width and high crystallinity contribute to the stabilization of the crystal structure after charging. On the other hand, in conventional LiCoO ２ In this case, even if a part of the crystal structure is similar to the O3' type crystal structure, the crystallite size will be small and the peak will be broad and small.

[0218] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, using monochromatic aluminum Kα rays as X-rays allows analysis of a region from the surface to a depth of approximately 2 to 8 nm (usually 5 nm or less), making it possible to quantitatively analyze the concentration of each element in a region approximately half the depth of the surface layer 100a. Furthermore, narrow scan analysis can be used to analyze the bonding state of elements. The quantitative accuracy of XPS is often approximately ±1 atomic %, and the lower detection limit is approximately 1 atomic %, depending on the element.

[0219] 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 of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 or more, more preferably 0.500 or more, more preferably 0.600 or more, more preferably 0.700 or more, more preferably 0.800 or more, more preferably 0.900 or more, and more preferably 1.000 or more. Furthermore, Mg / Co is preferably 2.000 or less, more preferably 1.500 or less, more preferably 1.400 or less, preferably 1.300 or less, or preferably 1.200 or less.

[0220] The ratio of the number of atoms of nickel to cobalt, Ni / Co, as determined by XPS analysis, is preferably 0.05 or more, more preferably 0.06 or more, more preferably 0.07 or more, more preferably 0.08 or more, and even more preferably 0.09 or more. Ni / Co is preferably 0.200 or less, preferably 0.150 or less, preferably 0.140 or less, preferably 0.130 or less, preferably 0.120 or less, or preferably 0.110 or less.

[0221] Furthermore, the ratio of the number of fluorine atoms to the number of cobalt atoms, F / Co, as determined by XPS analysis, is preferably 0.100 or more, more preferably 0.200 or more, more preferably 0.300 or more, more preferably 0.400 or more, more preferably 0.500 or more, more preferably 0.600 or more, and more preferably 0.700 or more. Furthermore, F / Co is preferably 1.500 or less, preferably 1.200 or less, preferably 1.100 or less, preferably 1.000 or less, and preferably 0.900 or less.

[0222] The above range indicates that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100, but are widely distributed at preferred concentrations in the surface layer portion 100a of the positive electrode active material 100. In other words, as a result of the XPS analysis of the positive electrode active material 100, the above range indicates that the crystalline structure is less likely to collapse even when charging and discharging are repeated so that x is 0.24 or less, and excellent cycle characteristics can be achieved.

[0223] 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 fluorine and another element is preferably greater than or equal to 682 eV and less than 685 eV, and more preferably about 684.3 eV, which is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV).

[0224] 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 1304 eV, and more preferably about 1303 eV, which is a value different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0225] <EDX> It is preferable that 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 a focused ion beam (FIB) or the like and analyzing the cross section using energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0226] 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.

[0227] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the additive element in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material 100. Furthermore, EDX analysis can analyze the concentration distribution and maximum value of the additive element. Furthermore, analysis after thinning the sample using FIB or the like 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.

[0228] Therefore, when EDX area analysis or EDX point analysis is performed on the positive electrode active material 100 of one embodiment of the present invention, it can be confirmed that the concentration of each additional element, particularly the additional element X, in the surface layer portion 100a is higher than that in the interior portion 100b.

[0229] 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 on the surface 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 transition metal M inside and the average value of the detected amount of the characteristic X-rays of the transition metal M in the background. 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 oxygen inside and the average value of the detected amount of the characteristic X-rays of oxygen in the background 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 oxygen inside and the average value of the detected amount of the characteristic X-rays of oxygen in the background, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen attached to the surface. 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 point where the amount of characteristic X-rays detected inside the material is the largest can be used as the surface position of the positive electrode active material. ＡＶＥ and M ＢＧ The reference point can be determined using the following formula:

[0230] The average value M of the background of the transition metal M ＢＧ can be obtained by averaging 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 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 counts of the transition metal M and oxygen are saturated and stable, for example, the region where the detected amount of the transition metal M starts to increase. ＢＧ and the average value of the amount of oxygen detected inside O ＡＶＥ can also be found in the same way.

[0231] Furthermore, the surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image or the like is the boundary between an area where an image derived from the crystalline structure of the positive electrode active material is observed and an area where it is not observed, and is the outermost 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 observed. The surface in an STEM image or the like may be determined in conjunction with an analysis with higher spatial resolution.

[0232] 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.

[0233] 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 analysis is performed, the magnesium concentration peak in the surface layer 100a preferably exists within a depth of 3 nm from the surface toward the center of the cathode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Alternatively, it is preferably within ±1 nm from the surface. Furthermore, the magnesium concentration preferably decays to 60% or less of the peak at a depth of 1 nm from the peak. Furthermore, it preferably decays to 30% or less of the peak at a depth of 2 nm from the peak. Note that the concentration peak here refers to the maximum concentration. Due to the influence of spatial resolution in EDX analysis, the position where the magnesium concentration peak exists may take a negative value as a depth from the surface toward the interior.

[0234] In addition, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration and the peak of the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0235] Furthermore, when EDX-ray analysis is performed, the fluorine concentration peak of the surface layer portion 100a preferably exists within a depth of 3 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Alternatively, it is preferable that the fluorine concentration peak is within ±1 nm from the surface. Furthermore, it is more preferable that the magnesium concentration peak is located slightly inward from the fluorine concentration peak, as this increases resistance to hydrofluoric acid. For example, it is more preferable that the magnesium concentration peak is located at least 0.5 nm inward from the fluorine concentration peak, and even more preferable that it is located at least 1.5 nm inward.

[0236] Furthermore, in the positive electrode active material 100 containing nickel as an additive element, the nickel concentration peak in the surface layer 100a preferably exists within a depth of 3 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Alternatively, it is preferable that it exists within ±1 nm from the surface. Furthermore, in the positive electrode active material 100 containing magnesium and nickel, the nickel distribution preferably overlaps with the magnesium distribution. For example, the difference in depth between the nickel concentration peak and the magnesium concentration peak is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0237] Furthermore, when the cathode active material 100 contains aluminum as an additive element, it is preferable that the peak of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak of the aluminum concentration in the surface layer portion 100 a when EDX-ray analysis is performed. For example, the peak of the aluminum concentration is preferably present at a depth of 0.5 nm to 50 nm, more preferably 3 nm to 30 nm, from the surface to the center of the cathode active material 100.

[0238] 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 0.01 or more and 0.6 or less, more preferably 0.05 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, more preferably 0.05 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.

[0239] <Washing> Various analyses have been described, but before subjecting the sample to analysis, such as 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 from these adhering 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 atomic ratio of the added element is not affected.

[0240] <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 onto the 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 21.

[0241] As the current collector, a highly conductive material such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, 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 heat resistance, such as 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.

[0242] <Positive Electrode Binder> Binders that can be used in the positive electrode will be described.

[0243] 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.

[0244] 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.

[0245] 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.

[0246] 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.

[0247] <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.

[0248] 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.

[0249] 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.

[0250] In view of these considerations, it is preferable to set the weight ratio of acetylene black in the negative electrode to be equal to or less than 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.

[0251] 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.

[0252] 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.

[0253] 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.

[0254] 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.

[0255] <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.

[0256] 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.

[0257] 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(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO２ ) (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.

[0258] 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), and dinitrile compounds such as succinonitrile and adiponitrile. The concentration of the additive may be, for example, 0.1 wt % to 5 wt % of the mixed solvent containing the lithium salt.

[0259] <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.

[0260] 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.

[0261] 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 carbonates (also referred to as fluorinated chain carbonates).

[0262] 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.

[0263] The following structural formula (H10) is the structural formula of FEC: In FEC, the electron-withdrawing substituent is an F group.

[0264]

[0265] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. 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.

[0266]

[0267] An example of a fluorinated chain carbonate 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.

[0268]

[0269] An example of a fluorinated chain carbonate 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.

[0270]

[0271] An example of a fluorinated chain carbonate 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.

[0272]

[0273] <FEC and MTFP> The mixed solvent described in this embodiment preferably contains FEC and MTFP. The reason for this will be described below.

[0274] 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, MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, but may form a solvate with lithium ions when used in the electrolyte solution.

[0275] 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 ) or water (H ２O). It is preferable that the content of ) is low and highly purified. It is also 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.

[0276] 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.

[0277] 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.

[0278] The measured values ​​of the HOMO level, solvation energy, and melting point are summarized in the table below.

[0279]

[0280] 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, the mixed solvent should contain more MTFP than FEC. Note that the above volume ratio may be the volume ratio measured before mixing the mixed solvents, and the ambient air temperature when mixing the mixed solvents may be room temperature (typically 25°C). A mixed solvent containing FEC and MTFP is preferable because it exhibits a viscosity that allows operation as a lithium-ion battery and maintains an appropriate viscosity even in a low-temperature environment.

[0281] 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.

[0282] 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.

[0283] Although MTFP has been described as a representative example above, any of the organic compounds described as fluorinated chain carbonates 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, if the mixed solvent of one embodiment of the present invention contains a fluorinated cyclic carbonate and a fluorinated chain carbonate, it is possible to provide a lithium ion battery that can be charged and discharged in a low-temperature environment.

[0284] <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, the ethyl methyl carbonate, and the dimethyl carbonate is x:y:100-x-y (where 5≦x≦35 and 0<y<65) when the total content of the ethylene carbonate, the ethyl methyl carbonate, and the 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).

[0285] 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.

[0286] 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.

[0287] 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.

[0288] 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.

[0289] 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 may be, for example, 0.1 wt % to 5 wt % relative to the solvent.

[0290] In the electrolyte solution example 2, the lithium salt may be the same as that described in the electrolyte solution example 1. Also, the additive may be the same as that described in the electrolyte solution example 1.

[0291] 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.

[0292] [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.

[0293] <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.

[0294] 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.

[0295] 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.

[0296]

[0297] 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.

[0298] 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.

[0299] 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.

[0300] 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.

[0301] Such polyglutamic acid is hydrophilic, so deionized water can be used as a solvent, which is suitable for forming a slurry.

[0302] The structural formula of polyacrylic acid is shown below.

[0303]

[0304] As is clear from the structural formula, polyacrylic acid has a carboxy group.

[0305] 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.

[0306] <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.

[0307] 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.

[0308] 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.

[0309] 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).

[0310] 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 Shimadzu Corporation).

[0311] 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.

[0312] 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).

[0313] 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 is less than 1 μm. The silicon particles may be any silicon-based material, and specifically, may include at least one of silicon, silicon oxide, and silicon alloy.

[0314] 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 Shimadzu Corporation).

[0315] In one embodiment of the present invention, the negative electrode active material contains both graphite particles and silicon particles, thereby achieving a lithium ion battery with a high discharge capacity. 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 low support amount can improve the output characteristics of the lithium ion battery, but a low 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.

[0316] In one embodiment of the present invention, the weight ratio of the graphite particles in the negative electrode active material layer is preferably greater than the weight ratio of the silicon particles, for example, the weight ratio of the graphite particles is preferably 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 is preferably 7.5 wt % to 37.5 wt %.

[0317] Furthermore, a conductive material may be added when forming the negative electrode active material layer.

[0318] 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.

[0319] 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.

[0320] 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 %.

[0321] <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.

[0322] <Method for Producing Negative Electrode Active Material Layer> Here, a method for producing a negative electrode active material layer will be described. The slurry for the negative electrode according to one embodiment of the present invention may be prepared by mixing graphite particles, silicon particles, and a binder having a carboxy group, followed by adding a solvent and mixing. 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 producing the slurry, the graphite particles, silicon particles, the binder having a carboxy group, and the solvent can also be mixed simultaneously. Furthermore, when producing the slurry, a conductive material can also be mixed simultaneously. The conductive material may be any of the above-mentioned conductive materials, and acetylene black, for example, may be used.

[0323] An example of a flow for producing the negative electrode active material layer is shown in FIG.

[0324] 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.

[0325] <Step S60> The above-mentioned raw materials are each weighed, and the first mixing is performed in step S60 of Figure 17. 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.

[0326] 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.

[0327] <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.

[0328] <Step S61> After adding the solvent 405, the second mixing is performed in step S61 of Fig. 17 to prepare a slurry 406. The second mixing is sometimes called slurry preparation.

[0329] 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.

[0330] Then, in step S62 of FIG. 17, the slurry 406 is applied onto the negative electrode current collector 407. Thereafter, in step S63 of FIG. 17, drying is performed. As drying conditions, preliminary drying and main drying may be performed. That is, two drying steps are performed, with the first drying step being performed under milder conditions. For example, the slurry may be dried in a dryer at 40° C. to 60° C. for 10 minutes to 1 hour, which may be referred to as preliminary drying. Next, as main drying, the slurry may be dried in a dryer at 60° C. to 90° C. for 30 minutes to 1.5 hours. Pressing may be performed simultaneously with drying.

[0331] After drying, a pressing process is performed as step S64 in FIG. 17. 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 should 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.

[0332] 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.

[0333] A lithium ion battery using the negative electrode 408 thus obtained has a large discharge capacity and exhibits excellent cycle characteristics.

[0334] It is advisable 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.

[0335] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0336] [Separator] A separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those made of cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0337] 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).

[0338] 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.

[0339] 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.

[0340] 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.

[0341] [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.

[0342] 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.

[0343] 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.

[0344] [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.

[0345] 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.

[0346] 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.

[0347] Embossing, which is a type of press working, will be explained below.

[0348] Fig. 24 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.

[0349] 24 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.

[0350] 24 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.

[0351] 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.

[0352] In Figure 24, 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.

[0353] 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.

[0354] Next, a film having a plurality of convex portions with shapes different from those shown in Fig. 24 will be described with reference to Fig. 25A to Fig. 25E. By changing the convex shapes of the embossing roll 95 and the embossing roll 96 shown in Fig. 24 to shapes different from those shown in Fig. 24, embossing with various cross-sectional shapes shown in Fig. 25A to Fig. 25E can be performed.

[0355] Fig. 25A is a cross-sectional schematic diagram of an embossment having a wavy shape, and Figs. 25B to 25E are modifications of Fig. 25A. Figs. 25B and 25C are diagrams showing an example in which the wavy shape is formed in a staircase pattern, Fig. 25D is a diagram showing an example in which the wavy shape is formed in a rectangular shape, and Fig. 25E is a diagram showing an example in which the wavy shape is formed with acute-angled valley shapes and trapezoidal peak shapes.

[0356] 26A and 26B are perspective views showing the resulting shape when the embossing process shown in FIGS. 24 to 25E 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, 81c) having the embossed shape (which can be referred to as a cross-wave shape) shown in FIGS. 26A and 26B . The film 81a having the cross-wave shape shown in FIG. 26A 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 multiple films (films 81b and 81c) having the cross-wave shape shown in FIG. 26B represent the shape used when fabricating a secondary battery using two films (films 81b and 81c), and the films 81b and 81c can be stacked together.

[0357] 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.

[0358] 26C is a perspective view showing a curved state of battery 10 manufactured using a laminate film embossed with a unidirectional wave pattern as exterior body 50. In this way, using a laminate film embossed with a unidirectional wave pattern as exterior body makes it possible to produce a battery that is easy to bend in one direction.

[0359] In the above-described configuration example of the secondary battery, the exterior body on one side of the secondary battery and the exterior body on the other side have the same embossed shape. However, the configuration of the secondary battery of one embodiment of the present invention is not limited to this. For example, the secondary battery may have an embossed shape on the exterior body on one side and no embossed shape on the exterior body on the other side. Furthermore, the exterior body on one side of the secondary battery may have different embossed shapes.

[0360] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0361] 9 to 14, a method for manufacturing a positive electrode active material applicable to a lithium ion battery having excellent charge-discharge characteristics even in a low-temperature environment will be described. Features of the positive electrode active material applicable to a lithium ion battery will be described in Embodiment 2.

[0362] <Example 1 of Method for Manufacturing Positive Electrode Active Material> An example of a method for manufacturing a positive electrode active material that can be used as one embodiment of the present invention (Example 1 of Method for Manufacturing Positive Electrode Active Material) will be described with reference to FIGS. 9A to 9D .

[0363] First, in step S10, lithium cobalt oxide is prepared as a starting material. The starting lithium cobalt oxide may have a particle size (strictly speaking, a median diameter (D50)) of 10 μm or less (preferably 8 μm or less). The lithium cobalt oxide having a median diameter (D50) of 10 μm or less may be a known or commonly used (in short, commercially available) lithium cobalt oxide, or may be a lithium cobalt oxide prepared through steps S11 to S14 shown in FIG. 9B . A representative example of a commercially available lithium cobalt oxide having a median diameter (D50) of 10 μm or less is lithium cobalt oxide (product name "CellSeed C-5H") manufactured by Nippon Chemical Industry Co., Ltd. CellSeed C-5H has a median diameter (D50) of approximately 7 μm. A method for obtaining lithium cobalt oxide having a median diameter (D50) of 10 μm or less through steps S11 to S14 will be described below.

[0364] <Step S11> In step S11 shown in FIG. 9B, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0365] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0366] As the cobalt source, it is preferable to use a compound containing cobalt, such as tricobalt tetroxide or cobalt hydroxide. The cobalt source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is increased, and the reliability of the secondary battery is improved.

[0367] <Step S12> Next, in step S12 shown in FIG. 9B , the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed in a dry or wet manner. Wet pulverization and mixing allows for smaller pulverization, which is preferable for obtaining lithium cobalt oxide with a median diameter (D50) of 10 μm or less as a starting material. When performing the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). However, it is preferable to use an aprotic solvent that does not react easily with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the transition metal source with dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity reduces the amount of impurities that may be present.

[0368] <Step S13> Next, in step S13 shown in Fig. 9B, the mixed material is heated. The heating temperature is preferably 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably about 950°C or lower (1000°C or lower). If the temperature is too low, the decomposition and melting of the lithium source and transition metal source may be insufficient. On the other hand, if the temperature is too high, lithium may evaporate from the lithium source and / or cobalt may be excessively reduced, which may result in defects.

[0369] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. Therefore, the heating time should be 1 hour or more and 100 hours or less, preferably 2 hours or more and 20 hours or less, and more preferably 2 hours or more and 10 hours or less.

[0370] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate should be 200° C. / h.

[0371] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２ The impurity concentrations of the above should be 5 ppb (parts per billion) or less.

[0372] The heating atmosphere is preferably an atmosphere containing oxygen. For example, dry air may be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method in which oxygen is continuously introduced into the reaction chamber and flows through the reaction chamber is called "flow."

[0373] When the heating atmosphere is an atmosphere containing oxygen, a method of not allowing oxygen to flow may be used. For example, a method of depressurizing the reaction chamber and then filling it with oxygen to prevent the oxygen from entering or leaving the reaction chamber may be used, which is called purging. For example, the reaction chamber may be depressurized to -970 hPa (differential pressure gauge) and then filled with oxygen to 50 hPa.

[0374] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0375] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0376] The container for containing the object to be heated during heating is preferably an aluminum oxide crucible or an aluminum oxide setter (also called a sheath). An aluminum oxide crucible is a material that is almost free of impurities. In this embodiment, a setter made of aluminum oxide with a purity of 99.9% is used. Note that it is preferable to place a lid on the crucible or setter before heating, as this prevents the material from volatilizing.

[0377] After the heating is completed, the mixture may be crushed and sieved as necessary.

[0378] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ ) can be synthesized. ２ ) is an oxide containing multiple metal elements in its structure, and therefore can be called a composite oxide. In this specification and the like, the term "composite oxide" refers to an oxide containing multiple metal elements in its structure. After step S13, a crushing step and a classification step are carried out to adjust the particle size distribution, and then lithium cobalt oxide (LiCoO ２ ) may be obtained.

[0379] Although the composite oxide is produced by the solid phase method in steps S11 to S14, the composite oxide may be produced by a coprecipitation method or a hydrothermal method.

[0380] Through steps S11 to S14, lithium cobalt oxide can be obtained as a starting material for obtaining a positive electrode active material that can be used in lithium ion batteries and has excellent charge-discharge characteristics even in low-temperature environments. Specifically, lithium cobalt oxide having a median diameter (D50) of 10 μm or less can be obtained as the starting lithium cobalt oxide.

[0381] 9A, the lithium cobalt oxide starting material is heated. The heating in step S15 is sometimes referred to as initial heating in this specification, etc., because it is the first heating of the lithium cobalt oxide. Alternatively, because it is heating performed before step S31 described below, it is sometimes referred to as preheating or pretreatment.

[0382] The initial heating causes lithium compounds and the like unintentionally remaining on the surface of the lithium cobalt oxide to be desorbed. It is also expected to have the effect of enhancing the internal crystallinity. Although impurities may be present in the lithium source and / or cobalt source prepared in step S11, etc., the initial heating can reduce the impurities from the lithium cobalt oxide starting material. The effect of enhancing the internal crystallinity is, for example, the effect of alleviating distortion, misalignment, and the like resulting from differential shrinkage of the lithium cobalt oxide prepared in step S14.

[0383] Furthermore, initial heating has the effect of smoothing the surface of the lithium cobalt oxide. Initial heating also has the effect of mitigating cracks, crystal defects, and the like that the lithium cobalt oxide has. In this specification and elsewhere, a "smooth" surface refers to a surface with few irregularities, a rounded overall surface, and rounded corners. Alternatively, a state in which there is little foreign matter attached to the surface is also referred to as "smooth." Foreign matter is considered to be a cause of irregularities, so it is preferable not to allow it to adhere to the surface.

[0384] In this initial heating, it is not necessary to separately prepare a material that functions as a lithium source, an additive element source, or a flux.

[0385] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. An appropriate heating time range can be selected, for example, from the heating conditions described in step S13. The heating temperature in step S15 is preferably lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in step S15 is preferably shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating is preferably performed at a temperature of 700°C or higher and 1000°C or lower (more preferably, 800°C or higher and 900°C or lower) for 1 hour or higher and 20 hours or lower (more preferably, 1 hour or higher and 5 hours or lower).

[0386] The heating in step S13 can cause a temperature difference between the surface and the interior of the lithium cobalt oxide. This temperature difference can induce a shrinkage difference. It is also thought that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a shrinkage difference. The energy associated with the shrinkage difference causes a difference in internal stress in the lithium cobalt oxide. This difference in internal stress is also called strain, and this energy is sometimes called strain energy. It is thought that the internal stress is removed by the initial heating in step S15; in other words, the strain energy is homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain in the lithium cobalt oxide is relaxed. This smooths the surface of the lithium cobalt oxide. Alternatively, it can be said that the surface is improved. In other words, by going through step S15, the shrinkage difference that occurred in the lithium cobalt oxide is alleviated, resulting in a smooth surface for the composite oxide.

[0387] Furthermore, the shrinkage difference may cause microscopic misalignment, such as crystal misalignment, in the lithium cobalt oxide. To reduce this misalignment, it is preferable to perform step S15. By performing step S15, it is possible to equalize the misalignment of the composite oxide (alleviate the misalignment of crystals, etc., that has occurred in the composite oxide, or align the crystal grains). As a result, the surface of the composite oxide becomes smooth.

[0388] As described above, in step S10, pre-synthesized lithium cobalt oxide having a median diameter (D50) of 12 μm or less, preferably 10 μm or less, and more preferably 8 μm or less may be used. In this case, steps S11 to S13 can be omitted. It is useful to perform step S15 on pre-synthesized lithium cobalt oxide, and this is a preferred step because it allows lithium cobalt oxide with a smooth surface to be obtained.

[0389] Note that step S15 is not an essential configuration in one aspect of the present invention, and therefore an aspect in which step S15 is omitted is also included in one aspect of the present invention.

[0390] The positive electrode active material of one embodiment of the present invention preferably contains an additional element A. A method for adding the additional element A in the following steps will be described.

[0391] <Step S20> Next, details of step S20 for preparing the additional element A as the A source will be described with reference to FIGS. 9C and 9D.

[0392] <Step S21> Step S20 shown in FIG. 9C includes steps S21 to S23. In step S21, an additive element A is prepared. Specific examples of the additive element A include one or more elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium. FIG. 9C illustrates a case where a magnesium source (Mg source) and a fluorine source (F source) are prepared. In step S21, a lithium source may be separately prepared in addition to the additive element A.

[0393] When magnesium is selected as the additional element A, the source of the additional element A can be called a magnesium source. As the magnesium source, magnesium fluoride (MgF ２ ), magnesium oxide (MgO), magnesium hydroxide (Mg(OH) ２ ), or magnesium carbonate (MgCO ３) etc. A plurality of magnesium sources may be used.

[0394] When fluorine is selected as the additional element A, the source of the additional element A can be called a fluorine source. Examples of the fluorine source include lithium fluoride (LiF) and magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF ２ , CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among these, lithium fluoride is preferred because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

[0395] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can also be used as a lithium source. Other lithium sources that can be used in step S21 include lithium carbonate.

[0396] The fluorine source may also be a gas, such as fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O６ F ２ , O ２ F) or the like may be used and mixed into the atmosphere in the heating step described below. A plurality of fluorine sources may be used.

[0397] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and magnesium source. ２ ) is prepared. In addition, when the melting point of a fluorine compound (sometimes called a fluoride) such as lithium fluoride is lower than the melting point of the other additive element source, the fluorine compound can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source. ２ As shown in FIG. 15 (quoted and added from FIG. 13 of Non-Patent Document 1), ２ The eutectic point P of is around 742°C (T1). Therefore, LiF and MgF ２ When a mixed fluoride having the formula (I) is used as the source of the additive element, it is preferable to set the heating temperature to 742° C. or higher in the heating step after mixing the additive element.

[0398] Here, the differential scanning calorimetry (DSC) measurement of the mixed fluoride and the mixture will be explained with reference to Fig. 16. The curve labeled "mixed fluoride" in Fig. 16 is the curve of LiF and MgF ２ The mixed fluoride is LiF:MgF. ２ The curve labeled "Mixture" in FIG. 16 represents the mixture of lithium cobalt oxide, LiF, and MgF. ２ The mixture was mixed with LiCoO ２ :LiF:MgF ２ The components were mixed in a molar ratio of 100:0.33:1.

[0399] As shown in Fig. 16, an endothermic peak is observed in the mixed fluoride at around 735°C. Also, an endothermic peak is observed in the mixed material at around 830°C. Therefore, the heating temperature after mixing the additive element (e.g., step S33 described later) is preferably 742°C or higher, more preferably 830°C or higher. Alternatively, it may be 800°C (T2 in Fig. 15) or higher, which is between these temperatures.

[0400] Lithium fluoride and magnesium fluoride are also known as LiF:MgF ２ When the ratio of lithium fluoride to magnesium fluoride is about 65:35, the effect of lowering the melting point is maximized. In addition, if the ratio of lithium fluoride is too high, there is a concern that the lithium will be excessive, which may deteriorate the cycle characteristics. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = 0.33 or its vicinity) is more preferable. In this specification and the like, "near a certain value" means a value that is greater than 0.9 times and smaller than 1.1 times that value, unless otherwise specified.

[0401] 9C, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0402] 9C, the pulverized and mixed materials are collected to obtain a source of the additional element A. The source of the additional element A shown in step S23 includes a plurality of starting materials and can also be called a mixture.

[0403] The particle size of the mixture is preferably such that the median diameter (D50) is 100 nm or more and 10 μm or less, more preferably 300 nm or more and 5 μm or less. Even when a single material is used as the source of the additive element A, the median diameter (D50) is preferably 100 nm or more and 10 μm or less, more preferably 300 nm or more and 5 μm or less.

[0404] The mixture (including the case where only one type of additive element is contained) pulverized in step S22 is likely to be uniformly adhered to the surface of the lithium cobalt oxide when mixed with the lithium cobalt oxide in a later step. If the mixture is uniformly adhered to the surface of the lithium cobalt oxide, the additive element can be easily distributed or diffused uniformly in the surface layer portion 100a of the composite oxide after heating, which is preferable.

[0405] <Step S21> A process different from that shown in Fig. 9C will be described with reference to Fig. 9D. Step S20 shown in Fig. 9D includes steps S21 to S23.

[0406] In step S21 shown in Fig. 9D, four types of additive element A sources to be added to lithium cobalt oxide are prepared. That is, Fig. 9D differs from Fig. 9C in the types of additive element A sources. In addition to the additive element A sources, a lithium source may be separately prepared.

[0407] As sources of four types of additive element A, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described with reference to FIG. 9C . Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0408] <Step S22> and <Step S23> Next, step S22 and step S23 shown in FIG. 9D are the same as step S22 and step S23 described with reference to FIG. 9C.

[0409] <Step S31> Next, in step S31 shown in FIG. 9A , the lithium cobalt oxide that has undergone step S15 (initial heating) is mixed with an additive element A source (Mg source). Here, the ratio of the number of cobalt atoms Co in the lithium cobalt oxide that has undergone step S15 to the number of magnesium atoms Mg in the additive element A is preferably Co:Mg = 100:y (0.1≦y≦6), and more preferably Co:Mg = 100:y (0.3≦y≦3). Note that adding the additive element A to the lithium cobalt oxide that has undergone initial heating allows the additive element A to be added evenly. For this reason, it is preferable to add the additive element A after initial heating (step S15), rather than adding the additive element A and then performing initial heating (step S15).

[0410] Furthermore, when nickel is selected as the additional element A, it is preferable to perform the mixing in step S31 so that the number of nickel atoms in the nickel source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15. Furthermore, when aluminum is selected as the additional element A, it is preferable to perform the mixing in step S31 so that the number of aluminum atoms in the aluminum source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15.

[0411] The mixing in step S31 is preferably performed under milder conditions than the pulverization and mixing in step S12 so as not to destroy the shape of the lithium cobalt oxide. For example, it is preferable to perform the mixing under conditions of a lower rotation speed or a shorter time than in step S12. Also, dry mixing is preferable. For example, a particle compositer, a ball mill, a bead mill, etc. can be used for mixing.

[0412] Known particle compositing devices include Mechanofusion (registered trademark) and Nobilta (registered trademark) manufactured by Hosokawa Micron Corporation. Mechanofusion has a fixed blade inside a cylindrical container, and the rotation of the cylindrical container applies mechanical energy to the powder, thereby enabling mixing. Nobilta has a rotating blade inside a cylindrical container, and the rotation of the blade applies mechanical energy to the powder, thereby enabling mixing. In this embodiment, mixing is performed using Nobilta at 3000 rpm for 10 minutes.

[0413] <Step S32> Next, in step S32 of Fig. 9A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0414] <Step S33> Next, in step S33 shown in FIG. 9A, the mixture 903 is heated. The heating temperature in step S33 is preferably 800°C or higher and 1100°C or lower, more preferably 800°C or higher and 950°C or lower, and even more preferably 850°C or higher and 900°C or lower. The heating time in step S33 may be 1 hour or higher and 100 hours or lower, and preferably 1 hour or higher and 10 hours or lower. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element A source proceeds. The temperature at which the reaction proceeds may be any temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element A source occurs, and may be lower than the melting temperature of these materials. For example, taking an oxide as an example, the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs, so the heating temperature in step S33 may be 500° C. or higher.

[0415] The reaction is more likely to proceed when the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 903. For example, LiF and MgF are used as the source of the additive element A. ２ As described above, when ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0416] Also, LiCoO ２ :LiF:MgF ２ As described above, the mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830°C in differential scanning calorimetry (DSC measurement). Therefore, the lower limit of the heating temperature is more preferably 830°C or higher.

[0417] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0418] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Therefore, the upper limit of the heating temperature is preferably 1000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower.

[0419] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0420] In the manufacturing method described in this embodiment, some materials, for example, LiF as a fluorine source, may function as a flux, which allows the heating temperature to be lowered to a temperature lower than the decomposition temperature of lithium cobalt oxide, for example, 742° C. to 950° C., and allows additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0421] Incidentally, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF may volatilize or sublime when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. In this case, the function as a flux will be weakened. Therefore, it is preferable to heat the mixture while suppressing the volatilization or sublimation of LiF.

[0422] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization or sublimation of LiF in the mixture 903.

[0423] Furthermore, the heating in this step is preferably performed so as not to cause adhesion between particles of the mixture 903. If the particles of the mixture 903 adhere to each other during heating, the contact area with oxygen in the atmosphere will decrease, and the route along which the additive elements (e.g., fluorine) diffuse will be blocked, which may result in poor distribution of the additive elements (e.g., magnesium and fluorine) in the surface layer portion.

[0424] Furthermore, when the additive element (e.g., fluorine) is uniformly distributed in the surface layer portion, a smooth cathode active material with few irregularities can be obtained. Therefore, in this process, in order to maintain or further smooth the surface by heating in step S15, it is preferable that the particles of mixture 903 do not adhere to each other.

[0425] An example in which step S33 is performed in a heating furnace is shown in FIG. 10A.

[0426] The heating furnace 220 shown in FIG. 10A includes a furnace space 202, a hot plate 204, a pressure gauge 221, a heater unit 206, and a heat insulator 208. A container 216 and a lid 218 are shown as setters for accommodating the object to be heated. It is preferable to heat the container 216 with the lid 218 attached. FIG. 10B shows a top view of the lid 218, and FIG. 10C shows a cross-sectional schematic diagram of the container 216 and the lid 218. Simply placing the lid 218 on the container 216 creates a sealed space, but because it is not completely sealed, the pressure inside the container does not become abnormally high, ensuring safety. Because a source of additive element A (typically fluoride) is added to the container 216 in advance, a fluoride-containing atmosphere can be created inside the space 219 defined by the container 216 and the lid 218. During heating, by keeping the concentration of gasified fluoride in the space 219 constant or preventing it from decreasing by covering the space 219, it is possible to contain additive elements A, such as fluorine and magnesium, near the particle surfaces of the mixture 903. Because the volume of the space 219 is smaller than that of the heating furnace space 202, a small amount of fluoride volatilizes, thereby creating an atmosphere containing fluoride. In other words, the atmosphere of the reaction system can be made to contain fluoride without significantly reducing the amount of fluoride contained in the mixture 903. Furthermore, by using the lid 218, the mixture 903 can be heated in an atmosphere containing fluoride simply and inexpensively.

[0427] Furthermore, before heating in the heating furnace space 202, a step of creating an oxygen-containing atmosphere in the heating furnace space 202 and a step of placing the container 216 containing the mixture 903 in the heating furnace space 202 are performed. By performing these steps in this order, the mixture 903 can be heated in an atmosphere containing oxygen and fluoride. For example, heating can be performed while a gas is flowing (flow). The gas can be introduced from the bottom of the heating furnace space 202 and exhausted to the top. Furthermore, the heating furnace space 202 can be sealed during heating to create a closed space to prevent the gas from being transported to the outside (purging).

[0428] There are no particular limitations on the method for creating an oxygen-containing atmosphere in the heating furnace space 202, but examples include a method of evacuating the heating furnace space 202 and then introducing an oxygen-containing gas such as oxygen gas or dry air, or a method of infusing an oxygen-containing gas such as oxygen gas or dry air for a certain period of time. Among these, it is preferable to evacuate the heating furnace space 202 and then introduce oxygen gas (oxygen substitution). Note that the air in the heating furnace space 202 may be considered to be an oxygen-containing atmosphere.

[0429] Furthermore, a source of the additive element A (typically a fluoride) that has been impregnated into the inner walls of the container 216 and the lid 218 can be re-emitted by heating and attached to the mixture 903 .

[0430] There is no particular limitation on the process for heating the heating furnace 220. Heating may be performed using a heating mechanism provided in the heating furnace 220.

[0431] The conditions for placing the mixture 903 in the container 216 will be described with reference to FIG. 10C . As shown in FIG. 10C , it is preferable to place the mixture 903 so that the top surface of the mixture 903 is flat relative to the bottom surface of the container 216, in other words, so that the height H of the top surface of the mixture 903 is uniform. The height H of the top surface of the mixture 903 is preferably 4.0 mm or less, and more preferably 2.0 mm or less. By setting the height H of the top surface of the mixture 903 to the above conditions, oxygen in the atmosphere can reach the mixture 903 near the bottom surface of the container 216. On the other hand, if the height H of the top surface of the mixture 903 is higher than 4.0 mm, the amount of oxygen reaching the mixture 903 near the bottom surface of the container 216 will be insufficient, resulting in reduced battery characteristics when the cathode active material subjected to this process is used in a battery. Furthermore, if the height H of the top surface of the mixture 903 is too low, the amount of mixture 903 that can be placed in the container 216 will be reduced, resulting in reduced productivity. Therefore, the height H of the upper surface of the mixture 903 is preferably 0.5 mm or more, or 1.0 mm or more. To summarize the above, the height H of the upper surface of the mixture 903 is preferably 0.5 mm or more and 4.0 mm or less, and more preferably 1.0 mm or more and 2.0 mm or less.

[0432] The heating in step S33 is preferably performed while controlling the pressure inside the furnace with the pressure gauge 221. The inside of the furnace is preferably kept at atmospheric pressure or a pressurized state. For example, it is thought that when exposed to a pressurized state, the surface of lithium cobalt oxide is more likely to melt. Therefore, LiF and MgF ２ The surface of the lithium cobalt oxide heated together can be melted by applying pressure.

[0433] Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluorine compounds resulting from the fluorine source or the like within an appropriate range. The partial pressure can also be controlled by heating the container used in this step with a lid on. As described above, the lid can prevent the material from volatilizing or sublimating.

[0434] In the manufacturing method described in this embodiment, a material for the source of the additive element A (typically a fluoride), for example, LiF as a fluorine source, may function as a flux. This function allows the heating temperature to be lowered to a temperature lower than the decomposition temperature of lithium cobalt oxide, for example, to a temperature between 742°C and 950°C, and allows the additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0435] However, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF will volatilize or sublime when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Note that even if LiF is not used as the fluorine source, Li on the surface of the lithium cobalt oxide may react with F in the fluorine source, producing LiF, which may then volatilize. Therefore, even if a fluorine compound with a higher melting point than LiF is used, it is still necessary to suppress volatilization.

[0436] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of LiF in the mixture 903. In order to suppress the volatilization of LiF, it is also preferable to place a lid on the setter container. Since the setter container and the lid are exposed to high temperatures, if they are made of materials with different thermal expansion coefficients, there is a risk that the gap between the setter container and the lid will become large. Therefore, it is preferable that the setter container and the lid are made of the same material.

[0437] <Roller hearth kiln> The manufacturing apparatus according to one embodiment of the present invention may be a roller hearth kiln that continuously processes materials contained in containers. Fig. 11A is a cross-sectional schematic diagram of a roller hearth kiln 150. Fig. 11B is a diagram illustrating rollers 152 of the roller hearth kiln.

[0438] The roller hearth kiln 150 includes a kiln body 151, a plurality of rollers 152, heating means 153a and 153b, an atmosphere control means 154, and adhesion suppression means 155a, 155b, and 155c. The roller hearth kiln 150 also preferably includes one or more baffle plates 157 and measuring devices 120a and 120b. Figure 11A shows an example having three baffle plates 157 (shown as baffle plate 157a, baffle plate 157b, and baffle plate 157c).

[0439] The kiln body 151 is tunnel-shaped. A plurality of rollers 152 serve to transport a container 160 containing an object to be heated 161. The container 160 is transported by the plurality of rollers 152 through the tunnel-shaped kiln body 151 to the outside. For the container 160 and the object to be heated 161, the description of the container 216 and the mixture 903 described in FIG. 10C can be referred to. That is, for the object to be heated 161 contained in the container 160, the height of the top surface of the object to be heated 161 relative to the bottom surface of the container 160 is preferably 0.5 mm to 4.0 mm, and more preferably 1.0 mm to 2.0 mm.

[0440] The kiln body 151 has an upstream portion and a downstream portion along the conveying direction of the multiple rollers 152. The kiln body 151 has a heating means 153a in the upstream portion and a heating means 153b in the downstream portion. A shield plate 157b may be provided between the upstream portion and the downstream portion. By providing the shield plate 157b, the atmosphere in the upstream portion and the downstream portion can be controlled separately. In addition, the shield plate 157b may be provided near the entrance of the kiln body 151, and the shield plate 157c may be provided near the exit. By providing these, it becomes easier to control the atmosphere inside the kiln body 151.

[0441] The adhesion suppression means 155 of the roller hearth kiln 150 is, for example, a means for vibrating the container 160. For example, as shown in FIG. 11A , the three adhesion suppression means 155 (shown as adhesion suppression means 155a, adhesion suppression means 155b, and adhesion suppression means 155c) may be rod-shaped or plate-shaped devices disposed between the rollers 152. The adhesion suppression means 155a, adhesion suppression means 155b, and adhesion suppression means 155c may be fixed, or may move to vibrate the container 160. Although FIG. 11A illustrates a configuration in which three adhesion suppression means 155 are provided, this is not a limitation of one aspect of the present invention. One or two adhesion suppression means 155, or four or more adhesion suppression means 155 may be provided.

[0442] The sticking prevention means of the roller hearth kiln 150 may be a plurality of rollers 152 with different inclinations, as shown in FIG. 11B.

[0443] 11A can be referred to for the heating means 153a and 153b, the atmosphere control means 154, etc. Also, for the measuring device 120a and the measuring device 120b, the description of FIG. 11A can be referred to.

[0444] The roller hearth kiln 150 is preferable because it has high productivity since it continuously processes the materials to be treated.

[0445] <Cooling Section of Roller Hearth Kiln> The roller hearth kiln may be provided with a cooling section.

[0446] 12 shows an example of a roller hearth kiln 150b that, in addition to the configuration of the roller hearth kiln 150 shown in FIG. 11A, also has a temperature-raising zone 121, a first cooling zone 124, and a second cooling zone 125. The area located upstream and heated by heating means 153a is referred to as the first holding zone 122, and the area located downstream and heated by heating means 153b is referred to as the second holding zone 123.

[0447] The atmosphere control means 154 preferably has a function of controlling the atmosphere in each of the five zones (heating zone 121, first holding zone 122, second holding zone 123, first cooling zone 124, and second cooling zone 125). For example, gas is introduced into each of the five zones from the atmosphere control means 154. The gas introduced into each of the five zones from the atmosphere control means 154 may differ in type, temperature, flow rate, etc.

[0448] 12 shows an example in which five zones are separated by shield plates 157, but a configuration in which no shield plate is provided between adjacent zones may also be used. For example, a configuration in which no shield plate is provided between the temperature increasing zone 121 and the first holding zone 122 may also be used. Furthermore, for example, a configuration in which no shield plate is provided between the first cooling zone and the second cooling zone may also be used.

[0449] The temperature increasing zone 121 includes a heating means 153j. Here, it is preferable that the temperature of the heating means 153j varies depending on the region. For example, it is preferable that the temperature gradually increases from the upstream side to the downstream side. Specifically, for example, the heating means 153j may have a plurality of blocks, each block provided with a heater, and the heater temperature may increase sequentially from the upstream block toward the downstream side.

[0450] The first cooling zone 124 includes a heating means 153k. The heating means 153k may have different temperatures in different regions. For example, the temperature may gradually decrease from the upstream side toward the downstream side. Specifically, the heating means 153j may have multiple blocks, each of which is provided with a heater, and the heater temperature may decrease from the upstream block toward the downstream block.

[0451] The second cooling zone 125 is, for example, a room temperature region. By performing cooling at room temperature, the temperature drop rate can be increased.

[0452] Cooling may be performed using cooling water in the first cooling zone 124 and the second cooling zone 125. By using cooling water, the temperature drop rate can be increased.

[0453] In addition, in one embodiment of the roller hearth kiln of the present invention, either the first cooling zone 124 or the second cooling zone 125 may be omitted.

[0454] For example, the first cooling zone 124 may not be provided, and the second holding zone 123 may be connected to the second cooling zone, and cooling may be performed at room temperature immediately after the temperature holding step, thereby increasing the temperature drop rate.

[0455] As the heating means 153j and the heating means 153k, for example, a silicon carbide heater, a carbon heater, a metal heater, a molybdenum disilicide heater, or the like can be used.

[0456] <Step S34> Next, in step S34 shown in FIG. 9A , the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sieve the recovered positive electrode active material 100. Through the above steps, a positive electrode active material 100 (composite oxide) having a median diameter (D50) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less) can be produced. Note that the positive electrode active material 100 contains the additive element A.

[0457] 13 and 14 , another example of a method for producing a positive electrode active material that can be used as one embodiment of the present invention (Example 2 of Method for Producing a Positive Electrode Active Material) will be described. Example 2 of Method for Producing a Positive Electrode Active Material differs from Example 1 of Method for Producing a Positive Electrode Active Material described above in the number of times that additive elements are added and the mixing method, but the other descriptions in Example 1 of Method for Producing a Positive Electrode Active Material can be applied.

[0458] 13, steps S10 and S15 are performed in the same manner as in FIG. 9A to prepare lithium cobalt oxide that has undergone initial heating. Note that step S15 is not an essential configuration in one aspect of the present invention, and therefore an aspect in which step S15 is omitted is also included in one aspect of the present invention.

[0459] <Step S20a> Next, as shown in step S20a, a first additive element A1 source (A1 source) is prepared. Details of step S20a will be described with reference to FIG.

[0460] <Step S21> In step S21 shown in Fig. 14A, a first additive element A1 source (A1 source) is prepared. The A1 source can be selected from the additive elements A described in step S21 shown in Fig. 9C and used. For example, the additive element A1 can be one or more selected from magnesium, fluorine, and calcium. Fig. 14A illustrates an example in which a magnesium source (Mg source) and a fluorine source (F source) are used as the additive element A1.

[0461] Steps S21 to S23 shown in Fig. 14A can be fabricated under the same conditions as steps S21 to S23 shown in Fig. 9C. As a result, an additional element A1 source (A1 source) can be obtained in step S23.

[0462] Steps S31 to S33 shown in FIG. 13 can be performed under the same conditions as steps S31 to S33 shown in FIG. 9A.

[0463] <Step S34a> Next, the material heated in step S33 is recovered to obtain lithium cobalt oxide containing the additional element A1. Here, this is also referred to as the second composite oxide to distinguish it from the lithium cobalt oxide (first composite oxide) that has been subjected to step S15.

[0464] <Step S40> In step S40 shown in Fig. 13, a second additive element A2 source (A2 source) is prepared. Step S40 will be described with reference to Figs. 14B and 14C.

[0465] <Step S41> In step S40 shown in Figure 14B, a second additive element A2 source (A2 source) is prepared. The A2 source can be selected from the additive elements A described in step S20 shown in Figure 9C and used. For example, the additive element A2 can be any one or more selected from nickel, titanium, boron, zirconium, and aluminum. Figure 14B illustrates an example in which a nickel source and an aluminum source are used as the additive element A2.

[0466] Steps S41 to S43 shown in Fig. 14B can be performed under the same conditions as steps S21 to S23 shown in Fig. 9C. As a result, an additional element A2 source (A2 source) can be obtained in step S43.

[0467] Steps S41 to S43 shown in Figure 14C are a modified example of Figure 14B. In step S41 shown in Figure 14C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are independently pulverized. As a result, in step S43, a plurality of second additional element A2 sources (A2 sources) are prepared. Thus, step S40 in Figure 14C differs from step S40 in Figure 14B in that the additional element sources are independently pulverized in step S42a.

[0468] <Steps S51 to S53> Next, steps S51 to S53 shown in FIG. 13 can be performed under the same conditions as steps S31 to S34 shown in FIG. 9A. The conditions for step S53, which relates to the heating step, are preferably the same as or lower than the heating temperature of step S33 shown in FIG. 13. The heating time is preferably shorter than that of step S33. Specifically, the heating temperature is preferably 800°C or higher and 950°C or lower, more preferably 820°C or higher and 870°C or lower, and even more preferably 850°C±10°C. The heating time is preferably 0.5 hours or higher and 8 hours or lower, and more preferably 1 hour or higher and 5 hours or lower.

[0469] When nickel is selected as the additional element A2, it is preferable to perform the mixing in step S51 so that the number of nickel atoms in the nickel source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15. When aluminum is selected as the additional element A2, it is preferable to perform the mixing in step S51 so that the number of aluminum atoms in the aluminum source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15.

[0470] <Step S54> Next, in step S54 shown in Figure 13, the heated material is recovered and crushed as necessary to obtain a positive electrode active material 100. Through the above steps, a positive electrode active material 100 (composite oxide) having a median diameter (D50) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less) can be produced. Alternatively, a positive electrode active material 100 applicable to lithium-ion batteries having excellent charge / discharge characteristics even in low-temperature environments can be produced. The positive electrode active material 100 contains the additive element A1 and the additive element A2.

[0471] In the above-described Example 2 of the manufacturing method, the additive element into the lithium cobalt oxide is introduced separately as a first additive element A1 and a second additive element A2, as shown in Figures 13 and 14. By introducing the additive elements separately, the distribution of each additive element in the depth direction can be changed.

[0472] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0473] Embodiment 3 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted on an electronic device will be described with reference to FIGS. 27A to 29C.

[0474] 27A to 27G show examples in which the secondary battery having the positive electrode active material described in the above embodiment is mounted in an electronic device. Examples of electronic devices to which the secondary battery is applied include television devices (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines.

[0475] Furthermore, a secondary battery having a flexible shape can be incorporated along the curved surfaces of the inner or outer walls of houses, buildings, etc., or the interior or exterior of automobiles.

[0476] 27A illustrates an example of a mobile phone. The mobile phone 7400 includes a display portion 7402 built into a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 includes a secondary battery 7407. By using the secondary battery of one embodiment of the present invention as the secondary battery 7407, a lightweight mobile phone with a long lifetime can be provided.

[0477] Figure 27B shows the mobile phone 7400 in a bent state. When the mobile phone 7400 is deformed by an external force and bent as a whole, the secondary battery 7407 provided inside is also bent. Figure 27C shows the state of the bent secondary battery 7407 at that time. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in a bent state. The secondary battery 7407 has lead electrodes electrically connected to the current collectors.

[0478] FIG. 27D shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display portion 7102, operation buttons 7103, and a secondary battery 7104. FIG. 27E shows a bent secondary battery 7104. When the secondary battery 7104 is worn on a user's arm in a bent state, the housing deforms, causing a change in the curvature of part or the entire secondary battery 7104. Note that the degree of curvature at any point on the curve, expressed as the radius of the corresponding circle, is called the radius of curvature, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or the entire main surface of the housing or the secondary battery 7104 changes when the radius of curvature is in the range of 40 mm to 150 mm. High reliability can be maintained when the radius of curvature of the main surface of the secondary battery 7104 is in the range of 40 mm to 150 mm. By using the secondary battery of one embodiment of the present invention as the secondary battery 7104, a lightweight and long-life portable display device can be provided.

[0479] 27F shows an example of a wristwatch-type portable information terminal 7200. The portable information terminal 7200 includes a housing 7201, a display portion 7202, a band 7203, a buckle 7204, operation buttons 7205, an input / output terminal 7206, and the like.

[0480] The portable information terminal 7200 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0481] The display surface of the display portion 7202 is curved, and display can be performed along the curved display surface. The display portion 7202 also includes a touch sensor, and can be operated by touching the screen with a finger or a stylus. For example, an application can be started by touching an icon 7207 displayed on the display portion 7202.

[0482] The operation button 7205 can have various functions, such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 7205 can be freely set by an operating system incorporated in the portable information terminal 7200.

[0483] The portable information terminal 7200 is also capable of performing standardized short-range wireless communication. For example, hands-free conversation is possible by communicating with a wirelessly enabled headset.

[0484] The portable information terminal 7200 also includes an input / output terminal 7206, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal 7206. Note that charging may be performed wirelessly without using the input / output terminal 7206.

[0485] The display portion 7202 of the portable information terminal 7200 includes the secondary battery of one embodiment of the present invention. By using the secondary battery of one embodiment of the present invention, a lightweight portable information terminal with a long life can be provided. For example, the secondary battery 7104 shown in FIG. 27E can be incorporated into the housing 7201 in a curved state or into the band 7203 in a bendable state.

[0486] The portable information terminal 7200 preferably has a sensor. For example, a fingerprint sensor, a pulse sensor, a human body sensor such as a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted as the sensor.

[0487] 27G illustrates an example of an armband-type display device. The display device 7300 includes a display portion 7304 and the secondary battery of one embodiment of the present invention. The display device 7300 can also be provided with a touch sensor in the display portion 7304 and can also function as a portable information terminal.

[0488] The display surface of the display portion 7304 is curved, and display can be performed along the curved display surface. The display state of the display device 7300 can be changed by short-range wireless communication according to a communication standard.

[0489] The display device 7300 also includes an input / output terminal, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal. Note that charging may be performed wirelessly without using the input / output terminal.

[0490] By using the secondary battery of one embodiment of the present invention as the secondary battery included in the display device 7300, a lightweight display device with a long lifetime can be provided.

[0491] An example in which the secondary battery having good cycle characteristics described in the above embodiment is mounted on an electronic device will be described with reference to FIGS. 27H to 29C. FIG.

[0492] By using a secondary battery of one embodiment of the present invention as a secondary battery in daily electronic devices, products that are lightweight and have a long life can be provided. For example, daily electronic devices include electric toothbrushes, electric shavers, and electric beauty devices. For secondary batteries in these products, a stick-shaped secondary battery that is easy for users to hold, small, lightweight, and has a large discharge capacity is desired.

[0493] FIG. 27H is a perspective view of a device also known as a tobacco-containing smoking device (electronic cigarette). In FIG. 27H, the electronic cigarette 7500 includes an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 that includes a liquid supply bottle, a sensor, and the like. To enhance safety, a protection circuit that prevents overcharging and / or over-discharging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in FIG. 27H has external terminals so that it can be connected to a charging device. Because the secondary battery 7504 is the tip portion when held, it is desirable that the total length be short and the weight be light. The secondary battery of one embodiment of the present invention has a high discharge capacity and good cycle characteristics, making it possible to provide a compact and lightweight electronic cigarette 7500 that can be used for a long period of time.

[0494] 28A shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used at home or outdoors, there is a demand for a wearable device that can be charged wirelessly as well as via a wired connection with an exposed connector.

[0495] For example, the secondary battery of one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 28A . The eyeglasses-type device 4000 includes a frame 4000a and a display unit 4000b. Mounting the secondary battery on the temples of the curved frame 4000a makes it possible to provide the eyeglasses-type device 4000 with a lightweight design, a good weight balance, and a long continuous use time. By including the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to a smaller housing can be realized.

[0496] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the headset device 4001. The headset device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b and / or the earphone unit 4001c. By including the secondary battery according to one embodiment of the present invention, a configuration that can accommodate space saving due to a miniaturized housing can be realized.

[0497] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. By providing the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0498] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. By providing the secondary battery according to one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0499] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted inside the belt portion 4006a. By including the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0500] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided on the display portion 4005a or the belt portion 4005b. By providing the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0501] The display unit 4005a can display not only the time but also various other information such as incoming emails and phone calls.

[0502] Furthermore, since the wristwatch-type device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0503] FIG. 28B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0504] 28C shows a side view of the display portion 4005a. Fig. 28C shows a state in which a secondary battery 913 is built inside the display portion 4005a. The secondary battery 913 is the secondary battery described in Embodiment 3. The secondary battery 913 is provided at a position overlapping with the display portion 4005a, and is small and lightweight.

[0505] 28D shows an example of a wireless earphone. Here, the wireless earphone is shown having a pair of main bodies 4100a and 4100b, but this does not necessarily have to be a pair.

[0506] The main bodies 4100a and 4100b each include a driver unit 4101, an antenna 4102, and a secondary battery 4103. They may also include a display portion 4104. They also preferably include a substrate on which a circuit such as a wireless IC is mounted, a charging terminal, and the like. They may also include a microphone.

[0507] The case 4110 has a secondary battery 4111. It is preferable that the case 4110 also has a board on which circuits such as a wireless IC and a charge control IC are mounted, and a charging terminal. It may also have a display unit, buttons, and the like.

[0508] The main units 4100a and 4100b can wirelessly communicate with other electronic devices such as smartphones. This allows sound data and the like sent from other electronic devices to be played back on the main units 4100a and 4100b. If the main units 4100a and 4100b have microphones, the sound picked up by the microphones can be sent to the other electronic device, and the sound data after processing by the electronic device can be sent back to the main units 4100a and 4100b for playback. This allows the main units 4100a and 4100b to be used as, for example, a translation device.

[0509] Furthermore, the secondary battery 4103 included in the main body 4100a can be charged from the secondary battery 4111 included in the case 4110. The coin-type secondary battery, the cylindrical secondary battery, or the like described in the above embodiments can be used as the secondary battery 4111 and the secondary battery 4103. A secondary battery using the positive electrode active material 100 obtained in Embodiment 1 as a positive electrode has high energy density. By using the secondary battery 4103 and the secondary battery 4111 as the secondary battery, a structure that can accommodate space saving due to miniaturization of wireless earphones can be realized.

[0510] 29A shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0511] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 to determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component therein. By using the secondary battery 6306 according to one embodiment of the present invention in the cleaning robot 6300, the cleaning robot 6300 can be a highly reliable electronic device with a long operating time.

[0512] Fig. 29B shows an example of a robot. The robot 6400 shown in Fig. 29B includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a transmitting / receiving device, a computing device, etc.

[0513] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0514] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0515] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0516] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component inside the robot 6400. By using the secondary battery according to one embodiment of the present invention in the robot 6400, the robot 6400 can be a highly reliable electronic device with a long operating time.

[0517] Fig. 29C shows an example of an air vehicle. The air vehicle 6500 shown in Fig. 29C has a propeller 6501, a camera 6502, a secondary battery 6503, and the like, and has the function of autonomous flight.

[0518] For example, image data captured by the camera 6502 is stored in the electronic component 6504. The electronic component 6504 can analyze the image data and detect the presence or absence of an obstacle when moving. Furthermore, the electronic component 6504 can estimate the remaining battery charge from a change in the storage capacity of the secondary battery 6503. The flying object 6500 includes the secondary battery 6503 according to one embodiment of the present invention therein. By using the secondary battery according to one embodiment of the present invention in the flying object 6500, the flying object 6500 can be an electronic device with a long operating time and high reliability.

[0519] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0520] Embodiment 4 In this embodiment, an example in which a secondary battery including the positive electrode active material of one embodiment of the present invention is mounted on a vehicle will be described.

[0521] By installing a secondary battery in a vehicle, next-generation clean energy vehicles such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs) can be realized.

[0522] FIG. 30 illustrates an example of a vehicle using a secondary battery according to one embodiment of the present invention. An automobile 8400 illustrated in FIG. 30A is an electric automobile using an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. By using one embodiment of the present invention, a vehicle with a long cruising distance can be realized. Furthermore, the automobile 8400 includes a secondary battery. For example, secondary battery modules can be arranged on the floor of the interior of the vehicle. The secondary battery not only drives the electric motor 8406 but also supplies power to light-emitting devices such as a headlight 8401 and a room light (not shown).

[0523] The secondary battery can also supply power to display devices such as a speedometer and a tachometer included in the automobile 8400. The secondary battery can also supply power to semiconductor devices such as a navigation system included in the automobile 8400.

[0524] The automobile 8500 shown in FIG. 30B can charge its secondary battery by receiving power from an external charging facility using a plug-in system and / or a wireless power supply system. FIG. 30B shows a state in which a ground-mounted charging device 8021 charges a secondary battery 8024 mounted on the automobile 8500 via a cable 8022. The charging method and connector specifications may be determined as appropriate using a predetermined system, such as CHAdeMO (registered trademark) or Combo. The charging device 8021 may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge the secondary battery 8024 mounted on the automobile 8500 using external power supply. Charging can be performed by converting AC power to DC power using a conversion device, such as an AC-DC converter.

[0525] Although not shown, a power receiving device can be mounted on a vehicle and can be charged by receiving power contactlessly from a ground-based power transmitting device. In the case of this contactless power supply method, by incorporating a power transmitting device into the road and / or exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between vehicles. Furthermore, a solar cell can be installed on the exterior of the vehicle, and the secondary battery can be charged while the vehicle is stopped and / or moving. For such contactless power supply, an electromagnetic induction method and / or a magnetic field resonance method can be used.

[0526] 30C shows an example of a two-wheeled vehicle using the secondary battery of one embodiment of the present invention. A scooter 8600 shown in FIG. 30C includes a secondary battery 8602, a side mirror 8601, and a turn signal light 8603. The secondary battery 8602 can supply electricity to the turn signal light 8603.

[0527] 30C can store a secondary battery 8602 in under-seat storage 8604. Even if under-seat storage 8604 is small, secondary battery 8602 can be stored in under-seat storage 8604. Secondary battery 8602 is removable, and when charging, secondary battery 8602 can be carried indoors, charged, and then stored before riding.

[0528] According to one aspect of the present invention, the cycle characteristics of the secondary battery are improved, and the discharge capacity of the secondary battery can be increased. Therefore, the secondary battery itself can be made smaller and lighter. If the secondary battery itself can be made smaller and lighter, it will contribute to reducing the weight of the vehicle, thereby improving the cruising range. Furthermore, the secondary battery installed in the vehicle can also be used as a power supply source for purposes other than the vehicle. In this case, for example, it is possible to avoid using a commercial power source during peak power demand. Avoiding the use of a commercial power source during peak power demand can contribute to energy conservation and reduction of carbon dioxide emissions. Furthermore, if the cycle characteristics are good, the secondary battery can be used for a long period of time, and the amount of rare metals used, such as cobalt, can be reduced.

[0529] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0530] Embodiment 5 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted on space equipment will be described.

[0531] 31A shows an artificial satellite 6800 as an example of space equipment. The artificial satellite 6800 has a body 6801, a solar panel 6802, an antenna 6803, and a secondary battery 6805. The solar panel is sometimes called a solar cell module.

[0532] When sunlight is irradiated onto the solar panel 6802, the power required for the operation of the satellite 6800 is generated. However, for example, in a situation where sunlight is not irradiated onto the solar panel or in a situation where the amount of sunlight irradiating the solar panel is small, the generated power is small. Therefore, there is a possibility that the power required for the operation of the satellite 6800 will not be generated. In order to operate the satellite 6800 even in a situation where the generated power is small, it is preferable to provide a secondary battery 6805 in the satellite 6800. By using the positive electrode active material of the present invention for the secondary battery, a secondary battery with high discharge capacity and excellent cycle characteristics can be obtained.

[0533] The satellite 6800 can generate a signal. The signal is transmitted via an antenna 6803, and can be received by, for example, a receiver installed on the ground or another satellite. By receiving the signal transmitted by the satellite 6800, for example, the position of the receiver that received the signal can be measured. As described above, the satellite 6800 can constitute, for example, a satellite positioning system.

[0534] Alternatively, the artificial satellite 6800 may be configured to include a sensor. For example, by including a visible light sensor, the artificial satellite 6800 may have the function of detecting sunlight reflected from an object on the ground. Alternatively, by including a thermal infrared sensor, the artificial satellite 6800 may have the function of detecting thermal infrared rays emitted from the earth's surface. As described above, the artificial satellite 6800 may function as, for example, an earth observation satellite.

[0535] FIG. 31B shows a probe 6900 with a solar sail (also called a sun sail) as an example of space equipment. The probe 6900 includes a body 6901, a solar sail 6902, and a secondary battery 6905. By using the positive electrode active material of the present invention in the secondary battery, it is possible to obtain a secondary battery with high discharge capacity and excellent cycle characteristics. When photons emitted from the sun strike the surface of the solar sail 6902, momentum is transferred to the solar sail 6902. Therefore, the surface of the solar sail 6902 preferably has a thin film with high reflectivity and preferably faces the sun.

[0536] The solar sail 6902 may also be designed to be folded up small until it leaves the atmosphere, and then deployed into a large sheet shape outside the Earth's atmosphere (outer space) as shown in Figure 31B.

[0537] FIG. 31C shows a spacecraft 6910 as an example of space equipment. The spacecraft 6910 has a body 6911, a solar panel 6912, and a secondary battery 6913. By using the positive electrode active material of the present invention in the secondary battery, it is possible to obtain a secondary battery with high discharge capacity and excellent cycle characteristics. The body 6911 can have, for example, a pressurized compartment and a non-pressurized compartment. The pressurized compartment may be designed to accommodate a crew member. Electricity generated by sunlight irradiating the solar panel 6912 can be charged into the secondary battery 6913.

[0538] 31D shows a rover 6920 as an example of space equipment. The rover 6920 has a body 6921 and a secondary battery 6923. By using the positive electrode active material of the present invention for the secondary battery, the secondary battery can have a high discharge capacity and excellent cycle characteristics. The rover 6920 may have a solar panel 6922.

[0539] The rover 6920 may be designed to allow a crew member to ride in. The secondary battery 6923 may be charged with electricity generated by sunlight irradiating the solar panel 6912, or the secondary battery 6923 may be charged with electricity generated by other power sources, such as a fuel cell, a radioisotope thermoelectric converter, or the like.

[0540] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0541] In this example, a battery having a pre-doped negative electrode according to one embodiment of the present invention was fabricated, and low-temperature characteristics and charge / discharge cycle characteristics were measured.

[0542] <Preparation of Positive Electrode Active Material> The positive electrode active material used in this example will be described with reference to the preparation method shown in FIGS.

[0543] Lithium cobalt oxide (LiCoO ２Commercially available lithium cobalt oxide (Cellseed C-5H, manufactured by Nippon Chemical Industry Co., Ltd.) containing cobalt as the transition metal M and no additional elements was prepared and sieved using an automatic sieving machine. The automatic sieving process can be performed using, for example, an electromagnetic vibration sieving machine MS-200 manufactured by Ito Seisakusho, with a sieve opening of 53 μm and tapping balls. For the initial heating in step S15, the lithium cobalt oxide was placed in a setter, the lid was closed, and the firing was performed at 850°C for 2 hours using a roller hearth kiln simulator furnace (manufactured by Noritake Company, Ltd.). Air (compressed air, thoroughly dried) was flowed through the furnace at a rate of 10 L / min. The flow rate, specifically the opening width of the exhaust port, was adjusted so that the furnace differential pressure gauge read 5 Pa, creating a positive pressure inside the furnace. After the initial heating, the inside of the furnace was cooled at a rate of 200°C / hour, and the air flow was not stopped until the temperature reached 200°C.

[0544] In this example, first, according to step S20a shown in Fig. 14A, an Mg source and an F source were added as additive elements. According to step S21 shown in Fig. 14A, LiF was prepared as an F source, and MgF ２ LiF:MgF was prepared. ２ The mixture was weighed out so that the molar ratio was 1:3, and mixed in dehydrated acetone at a rotation speed of 500 rpm for 20 hours. The mixture was sieved through a sieve with 300 μm openings to prepare an additive element source (A1 source) with a uniform particle size.

[0545] Next, in step S31 shown in Fig. 13, magnesium in the Al source was weighed out so that it accounted for 1 mol % of cobalt in the lithium cobalt oxide. The Al source and the initially heated lithium cobalt oxide were stirred for 10 minutes at a rotation speed of 3000 rpm using a Picobond (manufactured by Hosokawa Micron Co., Ltd.), yielding a mixture 903 (step S32). A Nobilta rotor was used for the Picobond. Prior to the next step S33, the mixture 903 was sieved using an automatic sieving machine.

[0546] Next, in step S33, the mixture 903 was heated at 850° C. for 10 hours. During heating, the mixture 903 was placed in a setter and covered with a lid.

[0547] The setter was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Company, Ltd.) and heated at the above-mentioned heating temperature. Oxygen was flowed through the furnace at a rate of 10 L / min (O ２ Flow). The flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace reached 5 Pa, creating a positive pressure inside the furnace. After the initial heating, the furnace was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C. In this way, a composite oxide containing Mg and F was obtained (step S34a).

[0548] Next, in step S40, a composite oxide and an additive element source (A2 source) were prepared. First, according to step S41 shown in FIG. 14C, nickel hydroxide that had undergone a pulverization process was prepared as the nickel source, and aluminum hydroxide that had undergone a pulverization process was prepared as the aluminum source, and these were used as the additive element source (A2 source). In the pulverization process, nickel hydroxide and aluminum hydroxide were mixed in dehydrated acetone at a rotation speed of 500 rpm for 20 hours. Then, the mixture was sieved through a sieve with 300 μm openings.

[0549] As the A2 source, nickel hydroxide was weighed so that the nickel content of the nickel hydroxide was 0.5 mol % of the cobalt content, and aluminum hydroxide was weighed so that the aluminum content of the aluminum hydroxide was 0.5 mol % of the cobalt content. This was mixed with a composite oxide containing Mg and F using a Picobond (manufactured by Hosokawa Micron) at a rotation speed of 3000 rpm for 10 minutes to obtain a mixture 904 (step S52). A Nobilta rotor was used for the Picobond. Prior to the next step S53, the mixture 904 was sieved using an automatic sieving machine.

[0550] Next, in step S53, the mixture 904 was heated at 850° C. for 2 hours. During heating, the mixture 904 was placed in a setter and covered with a lid.

[0551] The setter was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Company, Ltd.) and heated at the above-mentioned heating temperature. Oxygen was flowed through the furnace at a rate of 10 L / min (O ２The flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace reached 5 Pa, creating a positive pressure inside the furnace. After heating, the furnace was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C.

[0552] In this way, lithium cobalt oxide containing Mg, F, Ni, and Al was obtained (step S54).

[0553] <Preparation of Positive Electrode> The above positive electrode active material was prepared as the positive electrode active material, acetylene black (AB) was prepared as the conductive material, and polyvinylidene fluoride (PVDF) was prepared as the binder. PVDF was prepared by dissolving it in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, the positive electrode active material, AB, and PVDF were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum positive electrode current collector. NMP was used as the solvent for the slurry. After the slurry was applied to the positive electrode current collector, the solvent was evaporated.

[0554] Thereafter, in order to increase the density of the positive electrode active material layer on the positive electrode current collector, a pressing process was performed using a roll press machine. The pressing process was performed under a linear pressure of 210 kN / m. The upper and lower rolls of the roll press machine were both set at 120°C.

[0555] By the above steps, a positive electrode having each positive electrode active material was obtained. ２ 11mg / cm or more ２ The values ​​were adjusted to the following ranges:

[0556] <Fabrication of Negative Electrode> The negative electrode used in this example will be described with reference to the fabrication method shown in FIG.

[0557] According to FIG. 17, graphite particles having an average particle size of 20 μm (Formula BT 1520, manufactured by Superior Graphite) were prepared as graphite particles in this example.

[0558] According to FIG. 17, the silicon particles of this example have a specific surface area of ​​12.7715 m2 measured by the BET method. ２Silicon particles (manufactured by Aldrich: product number 633097) having an average particle size of 100 nm and a molecular weight of 1000 kJ / g were prepared. The 100 nm silicon particles are called nanosilicon particles.

[0559] According to FIG. 17, polyacrylic acid (10CL manufactured by Fujifilm Wako Chemical Co., Ltd.) was used as the binder in this example.

[0560] In accordance with FIG. 17, acetylene black (AB) was prepared as the conductive material in this example.

[0561] 17, graphite particles, silicon particles, AB, and PAA were mixed in a weight ratio of 72:8:6:14 to prepare a mixture 404. The amount of the negative electrode active material supported was 3.66 mg / cm. ２ 3.85mg / cm or more ２ The values ​​were adjusted to the following ranges:

[0562] According to FIG. 17, deionized water was prepared as a solvent.

[0563] According to step S61 of FIG. 17, deionized water was added to the mixture 404 and mixed to obtain a slurry 406.

[0564] According to FIG. 17, copper foil was prepared as a negative electrode current collector, and the above slurry was applied to the copper foil according to step S62.

[0565] 17 , the negative electrode 408 was obtained by drying the negative electrode 408. The negative electrode 408 was pre-dried by being sandwiched between a hot plate heated to 50° C. for 30 minutes, and then was fully dried by being placed in a forced-air dryer at 80° C. for 45 minutes.

[0566] <Preparation of Pre-doping Electrode> An electrode for pre-doping identical to the above-mentioned positive electrode was separately prepared.

[0567] <Fabrication of Battery> A laminated battery (Cell 1) was fabricated using the above-described positive electrode, negative electrode, and pre-doped electrode. The positive electrode was a single-sided coated positive electrode, and the area of ​​the positive electrode active material layer (coated portion) was approximately 21 cm2. ２ The negative electrode was a single-sided coated negative electrode, and the area of ​​the negative electrode active material layer (coated portion) was approximately 24 cm ２ The battery was fabricated with reference to the descriptions of FIGS.

[0568] Using a positive electrode and a first film, as shown in Figures 3A1 and 3A2, the first film was fixed so that the area other than the tab region of the positive electrode was covered by the first film. Also, using a pre-doping electrode and a second film, as shown in Figures 3B1 and 3B2, the second film was fixed so that the area other than the tab region of the pre-doping electrode was covered by the second film. Note that as the first film and the second film, a porous polypropylene film having a thickness of 25 μm was used, which functions as a battery separator.

[0569] Next, the tab region of the positive electrode and the positive electrode lead electrode were fixed as shown in FIG. 4A1. Furthermore, the tab region of the pre-doping electrode and the pre-doping lead electrode were fixed as shown in FIG. 4A2. Furthermore, the tab region of the negative electrode and the negative electrode lead electrode were fixed as shown in FIG. 4A3. An ultrasonic welding device was used for the above fixation. These were stacked in the order of the negative electrode, the pre-doping electrode, and the positive electrode, and placed inside a laminate film with one side folded as shown in FIG. 4B. An aluminum laminate film was used as the laminate film. The aluminum laminate film had a configuration in which a polypropylene film was placed on one side of a 40 μm thick aluminum foil and a nylon film on the other side. The above-mentioned negative electrode, pre-doping electrode, and positive electrode were placed on the side of the polypropylene film that could be thermocompressed.

[0570] Next, as shown in Fig. 5A, three sides of the aluminum laminate film were thermocompression-bonded (first compression-bonded region 52a) to form a three-sided sealed state, after which an electrolyte solution was poured into the one side that was not thermocompression-bonded.

[0571] The electrolyte solution was a mixed solvent containing FEC and MTFP in a volume ratio of FEC:MTFP = 20:80. ６ Here, LiPF ６ The amount of the electrolyte was set to 1 mol / L relative to the mixed solvent of FEC and MTFP. No additive was used. This electrolyte solution was called electrolyte solution A.

[0572] After injecting the electrolyte A, the one side that was not thermocompression bonded was thermocompression bonded (second bonding region 52e). This thermocompression bonding was performed in a reduced pressure environment (approximately -60 kPa). The four-sided sealed battery was then left to stand for 24 hours. The excess laminate film was then cut off. In this way, a battery with an aluminum laminate film as an exterior body was produced. This battery is referred to as a battery before pre-doping.

[0573] [Pre-doping] The pre-doped battery prepared above was used to carry out pre-doping treatment.As pre-doping treatment, the capacity of 60mAh / g was charged relative to the weight of the positive electrode active material of the battery.In pre-doping treatment, the pre-doping electrode lead electrode was connected to the positive terminal of the charging device, and the negative electrode lead electrode was connected to the negative terminal of the charging device, and constant current charging was carried out at a current value of 2mA / g.It should be noted that this charging was carried out in an environment of 25 ° C.

[0574] After the pre-doping process, one side of the outer casing was cut and the pre-doping electrode was pulled out, as described in Figures 6A and 6B. The cut side of the outer casing was then thermocompression bonded (third compression bonding region 52d) to form the battery shape shown in Figures 7A and 7B. The thermocompression bonding was performed in a reduced pressure environment (approximately -60 kPa).

[0575] In this way, a battery having a pre-doped negative electrode was produced, and this battery is called cell 1. Note that a plurality of cells 1 were produced for the measurements described below.

[0576] [Initial Charge / Discharge] Initial charge / discharge was performed on the plurality of cells 1 prepared above. The initial charge / discharge method is shown in Table 2. In the initial charge / discharge in Table 2, steps A6 and A7 were repeated a total of three times. In addition, steps A8 and A9 were repeated a total of three times.

[0577] In this example, 1C was set to 200 mA / g per weight of the positive electrode active material.

[0578]

[0579] [Confirmation of negative electrode capacity during discharge] Two of the multiple cells 1 that had undergone the above initial charge and discharge were used to investigate how much capacity remained in the negative electrode in the discharged state of the cell 1. For the investigation, the discharged cell 1 was disassembled, the negative electrode was removed, and a test battery was fabricated using a part of the negative electrode, and its capacity was measured. The test battery was a half cell with lithium metal as the counter electrode.

[0580] Before disassembly, charge and discharge were performed to confirm the capacity of Cell 1. Charging was performed by constant current charging at a current value of 0.1 C up to 4.50 V, followed by constant voltage charging at 4.50 V until the charging current became 0.01 C or less. Discharging was performed by constant current discharging at a current value of 0.1 C down to 2.50 V.

[0581] After the above-described charging and discharging, cell 1 was disassembled in a glove box filled with argon gas. The negative electrode was removed from the disassembled cell 1, and a portion of the negative electrode was cut into a circle with a diameter of 12 mm. The cutting position was such that one surface of the cut-out circle had the negative electrode active material layer covering the entire surface, and the other surface had the negative electrode current collector exposed over the entire surface. In this way, a test battery positive electrode was obtained.

[0582] Next, a test battery was fabricated using the above-described test battery positive electrode. As described above, lithium metal was used as the counter electrode (test battery negative electrode). The above-described electrolyte solution A was used as the electrolyte. The test battery was a coin-type battery (coin cell) with a CR2032 type shape (diameter 20 mm, height 3.2 mm).

[0583] The test battery prepared above was charged. This charging allowed the remaining capacity (or Li amount) in the test battery's positive electrode (a portion of the negative electrode removed from the discharged battery) to be determined. In other words, the charge capacity of the test battery corresponds to the remaining capacity in the negative electrode removed from Cell 1 in a discharged state.

[0584] The test battery was charged to 1.00 V at a constant current of 75.4 mA / g relative to the weight of the active material in the positive electrode of the test battery. The ambient temperature was 25°C. Table 3 shows the charge capacity of this test battery and the discharge capacity of Cell 1 before disassembly. Table 3 also shows the negative electrode area of ​​Cell 1, the positive electrode area of ​​the test battery, and their respective unit area capacities. Table 3 also shows a ratio of the first unit area capacity obtained by dividing the discharge capacity of Cell 1 by the area of ​​the region containing the negative electrode active material layer in the negative electrode of Cell 1 (negative electrode area), to the second unit area capacity obtained by dividing the charge capacity of the test battery by the area of ​​the positive electrode of the test battery.

[0585] In this measurement, two cells 1 were used, and two test batteries were made from each of them, resulting in four test batteries, Test Battery 1 to Test Battery 4, and Table 3 shows the measurement results for each.

[0586]

[0587] As shown in Table 3, it was found that in a battery (Cell 1) according to one embodiment of the present invention having a pre-doped negative electrode, in a discharged state after initial charge and discharge, 7% to 10% of the discharge capacity of Cell 1 remained in the negative electrode. More specifically, it was found that the second unit area capacity obtained by dividing the charge capacity of the test battery by the area of ​​the positive electrode of the test battery was 7% to 10% of the first unit area capacity obtained by dividing the discharge capacity of Cell 1 by the area of ​​the region having the negative electrode active material layer (negative electrode area). In other words, the capacity remaining in the negative electrode of Cell 1 during discharge was in the range of 5% to 15%.

[0588] <Low-Temperature Charge-Discharge Test> A low-temperature charge-discharge test was conducted using Cell 1 that had undergone the initial charge-discharge cycle described above. The low-temperature charge-discharge test measured the charge and discharge capacities under each temperature environment, in the following order: 5 cycles of charge-discharge in a 25°C environment, 2 cycles of charge-discharge in a 0°C environment, 2 cycles of charge-discharge in a -10°C environment, 2 cycles of charge-discharge in a -20°C environment, 2 cycles of charge-discharge in a -30°C environment, and 2 cycles of charge-discharge in a -40°C environment. The results of the final charge-discharge cycles at each temperature are shown in Figures 32A and 32B and Table 4.

[0589] Under each temperature environment, charging was performed by constant current charging at a charging current of 0.1 C until the voltage reached 4.50 V, followed by constant voltage charging at 4.50 V until the charging current fell to 0.01 C or less. Discharging was performed by constant current discharging at a discharge rate of 0.1 C until the voltage reached 2.75 V (cutoff voltage). Note that a current of 0.1 C can be considered to be a current of 20 mA / g per weight of positive electrode active material, and a current of 0.01 C can be considered to be a current of 2 mA / g per weight of positive electrode active material.

[0590] Fig. 32A is a graph showing charge and discharge curves in 25°C, 0°C, and -10°C environments. Fig. 32B is a graph showing charge and discharge curves in -20°C, -30°C, and -40°C environments. In Figs. 32A and 32B, the horizontal axis represents charge capacity or discharge capacity, and the vertical axis represents voltage. Table 4 shows the capacity values ​​of the charge and discharge curves shown in Figs. 32A and 32B.

[0591]

[0592] In Table 4, the first column shows the temperature conditions, the second column shows the charge capacity of cell 1, the third column shows the discharge capacity of cell 1, the fourth column shows the charge capacity in each temperature environment as a ratio (%) to 25°C when the charge capacity at 25°C is set to 100%, and the fifth column shows the discharge capacity in each temperature environment as a ratio (%) to 25°C when the discharge capacity at 25°C is set to 100%.

[0593] As a result of charge / discharge measurements in low-temperature environments, Cell 1, a battery according to one embodiment of the present invention, achieved the following favorable results, assuming that the discharge capacity measured during charge and discharge in a 25°C environment was 100%. The discharge capacity measured during charge and discharge in a −40°C environment was 44.8%, a favorable result of 40% or more. The discharge capacity measured during charge and discharge in a −30°C environment was 74.9%, a favorable result of 70% or more. The discharge capacity measured during charge and discharge in a −20°C environment was 87.8%, a favorable result of 85% or more. The discharge capacity measured during charge and discharge in a −10°C environment was 95.1%, a favorable result of 90% or more. The discharge capacity measured during charge and discharge in a 0°C environment was 97.2%, a favorable result of 97% or more.

[0594] As shown in the above examples, it has become clear that a battery including a pre-doped negative electrode, a positive electrode active material, a negative electrode, and an electrolyte solution obtained by the manufacturing methods described in Embodiments 1 and 2, can perform excellent charging and discharging operations in a temperature range of −40° C. or higher and 25° C. or lower.

[0595] <Charge / Discharge Rate Test> A 25° C. charge / discharge rate test and a −30° C. charge / discharge rate test were performed using the cell 1 that had undergone the above initial charge / discharge. Note that a plurality of cells 1 were produced under the same conditions, and measurements were performed using different cells 1 in each of the low-temperature charge / discharge test, the 25° C. charge / discharge rate test, and the −30° C. charge / discharge rate test.

[0596] [25°C Charge / Discharge Rate Test] In the 25°C charge / discharge rate test, a charge rate test and a discharge rate test were performed using one cell 1 in a temperature environment of 25°C. The conditions and results of the charge rate test are shown in Table 5, and the conditions and results of the discharge rate test are shown in Table 6. As shown in Tables 5 and 6, the discharge conditions were fixed in the charge rate test, and the charge conditions were fixed in the discharge rate test.

[0597]

[0598]

[0599] [-30°C Charge / Discharge Rate Test] In the -30°C charge / discharge rate test, a charge rate test and a discharge rate test were performed in a temperature environment of -30°C using one cell 1. The conditions and results of the charge rate test are shown in Table 7, and the conditions and results of the discharge rate test are shown in Table 8. As shown in Tables 7 and 8, the discharge conditions were fixed in the charge rate test, and the charge conditions were fixed in the discharge rate test.

[0600]

[0601]

[0602] <Charge-Discharge Cycle Test> A charge-discharge cycle test was carried out using the cell 1 prepared as described above and subjected to the initial charge-discharge cycle. In the charge-discharge cycle test, the measurement environment temperature was set to three conditions of 25°C, 0°C, and -30°C.

[0603] In the charge-discharge cycle test, charging was performed under conditions of constant current charging at a current value of 0.1 C until the battery voltage reached 4.50 V, followed by constant voltage charging at 4.50 V until the current value reached 0.01 C or less (constant current-constant voltage charging (CC-CV charging)). Discharging was performed under conditions of constant current discharging at a current value of 0.1 C (CC discharging) until the battery voltage reached 2.75 V.

[0604] The results of the charge-discharge cycle test are shown in Figures 33A to 33C. In the graphs of Figures 33A to 33C, the horizontal axis represents the number of cycles, and the vertical axis represents the discharge capacity in Figure 33A, the discharge capacity retention rate when the maximum discharge capacity in the charge-discharge cycle is set to 100% in Figure 33B, and the discharge energy (the value obtained by multiplying the discharge capacity by the average discharge voltage) in Figure 33C.

[0605] <Battery Freezing Test> A battery freezing test was conducted using the cell 1 prepared as described above and having undergone initial charge and discharge. In this test, to investigate the effect of freezing of the battery, specifically freezing of the electrolyte, on the battery characteristics, the charge and discharge characteristics of the battery before freezing were compared with those of the battery after freezing and thawing.

[0606] In the battery freezing test, the fully charged cell 1 was immersed in liquid nitrogen, and after the battery temperature reached −175° C. and stabilized, the cell was kept frozen for 10 minutes. Thereafter, the cell was thawed in a room temperature environment.

[0607] Charge-discharge tests were conducted before and after the freezing treatment, and the results of comparing the charge-discharge characteristics are shown in Table 9. Charging was performed under conditions of constant current charging at a current value of 0.1 C until the battery voltage reached 4.50 V, followed by constant voltage charging at 4.50 V until the current value reached 0.01 C or less (constant current-constant voltage charging (CC-CV charging)). Discharging was performed under conditions of constant current discharging at a current value of 0.1 C (CC discharging) until the battery voltage reached 2.75 V. All charge-discharge tests were conducted in a 25°C environment.

[0608]

[0609] As shown in Table 9, when the charge capacity, discharge capacity, and impedance (values ​​at 1 kHz) before the freezing treatment were compared with the charge capacity, discharge capacity, and impedance after the freezing treatment (thawing), no significant differences were observed. In other words, it can be considered that the battery of one embodiment of the present invention does not have problems such as a significant decrease in capacity even when stored in an extremely low temperature environment below the temperature at which the electrolyte freezes.

[0610] 10: battery, 11: positive electrode, 12: negative electrode, 13: separator, 13a: first film, 21: positive electrode current collector, 22: positive electrode active material layer, 23: positive electrode lead electrode, 25: positive electrode characteristic curve, 25r: positive electrode characteristic curve, 31: negative electrode current collector, 32: negative electrode active material layer, 33: negative electrode lead electrode, 35: characteristic curve, 35r: negative electrode characteristic curve, 41: conductive material, 45: pre-doping electrode, 46: pre-doping active material layer, 47: separator, 47a: second film, 48: pre-doping lead electrode, 50: outer casing, 50a: laminate film, 51a: adhesive region, 51b: adhesive region, 52a: first pressure-bonded region, 52e: second pressure-bonded region, 52b: third pressure-bonded region, 52c: fourth pressure-bonded region, 53: sealing region, 60: electrolyte, 81: film, 81a: film, 81b: film, 81c: film, 90: film, 91: traveling direction, 95: embossing roll, 95a: convex portion, 96: embossing roll, 96a: convex portion, 100: positive electrode active material, 100a: surface layer portion, 100b: interior, 120a: measuring device, 120b : measuring device, 121: temperature rising zone, 122: first holding zone, 123: second holding zone, 124: first cooling zone, 125: second cooling zone, 150: roller hearth kiln, 150b: roller hearth kiln, 151: kiln body, 152: roller, 153a: heating means, 153b: heating means, 153j: heating means, 153k: heating means, 154: atmosphere control means, 155: sticking suppression means, 155a: sticking suppression means, 155b: sticking suppression means, 155c: sticking suppression means, 157: shutoff Plate, 157a: shielding plate, 157b: shielding plate, 157c: shielding plate, 160: container, 161: object to be heated, 202: space inside heating furnace, 204: hot plate, 206: heater part, 208: heat insulating material, 216: container, 218: lid, 219: space, 220: heating furnace, 400: graphite particles, 401: silicon particles, 402: binder, 403: conductive material, 404: mixture, 405: solvent, 406: slurry, 407: negative electrode current collector, 408: negative electrode, 500: secondary battery, 903: mixture, 904: mixture, 913: secondary battery

Claims

A battery having a positive electrode, a negative electrode, and an electrolytic solution, wherein the positive electrode has a positive electrode active material layer, the positive electrode active material layer has positive electrode active material particles containing magnesium, nickel, aluminum, and lithium cobaltate, the negative electrode has a negative electrode active material layer, the negative electrode active material layer has graphite particles and silicon particles, the electrolytic solution has a lithium salt, a fluorinated cyclic carbonate, and a fluorinated chain carbonate, when a test battery is fabricated using a part of the negative electrode taken out from the battery after the first discharge and lithium metal, the charge capacity in the first charge of the test battery is 5% or more and 15% or less as the capacity per unit area of the part of the negative electrode with respect to the discharge capacity of the first discharge, a battery. 　 In claim 1, the first discharge is a discharge process in which the battery is charged to a voltage of 4.50 V and then discharged at a constant current until the voltage reaches 2.50 V, the first charge is a charge process in which, after the test battery is fabricated, it is directly charged at a constant current to a voltage of 1.00 V, the first discharge and the first charge are carried out in an environment at 25°C, a battery. 　 In claim 1, the electrolytic solution of the test battery has a lithium salt, a fluorinated cyclic carbonate, and a fluorinated chain carbonate, a battery. 　 In claim 1, the current value of the first discharge is 20 mA / g with respect to the weight of the positive electrode active material particles, the current value of the first charge is 75.4 mA / g with respect to the weight of the graphite particles and the silicon particles, a battery. 　 In claim 1, the capacity of the battery when discharged in an environment at -30°C is 70% or more with respect to the capacity of the battery when discharged in an environment at 25°C, a battery.

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