Positive electrode active material and secondary battery

Abstract

To provide a positive electrode active material for a lithium ion secondary battery that has high capacity and excellent charge / discharge cycle characteristics.SOLUTION: A positive electrode active material includes lithium, cobalt, and oxygen, and has a spin density caused by divalent cobalt ions and tetravalent cobalt ions within a predetermined range. The positive electrode active material preferably further includes magnesium. The appropriate magnesium concentration is expressed as the concentration relative to cobalt. The positive electrode active material preferably further contains fluorine.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] One embodiment of the present invention relates to an article, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, or an electronic device, or a manufacturing method thereof. In particular, the present invention relates to a positive electrode active material that can be used in a secondary battery, a secondary battery, and an electronic device having a secondary battery.

[0002] In this specification, the term "power storage device" refers to all elements and devices having a power storage function, including, for example, storage batteries (also called secondary batteries) such as lithium ion secondary batteries, lithium ion capacitors, and electric double layer capacitors.

[0003] In this specification and the like, the term "electronic device" refers to any device having a power storage device, and electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like are all electronic devices. [Background technology]

[0004] In recent years, the development of various energy storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, and air batteries, has been actively pursued. Demand for high-power, high-energy-density lithium-ion secondary batteries, in particular, is rapidly expanding along with the development of the semiconductor industry, as they are used in mobile information terminals (PDAs) such as mobile phones, smartphones, tablets, and notebook computers, as well as portable music players, digital cameras, medical devices, and next-generation clean energy vehicles (hybrid electric vehicles (HEVs), electric vehicles (EVs), plug-in hybrid electric vehicles (PHEVs), etc.). Furthermore, lithium-ion secondary batteries have become indispensable in today's information society as a rechargeable energy source.

[0005] The characteristics required of lithium ion secondary batteries include higher energy density, improved cycle characteristics, safety in various operating environments, and improved long-term reliability.

[0006] Therefore, improvements to the positive electrode active material have been investigated with the aim of improving the cycle characteristics and increasing the capacity of lithium-ion secondary batteries (Patent Documents 1 and 2). Research is also being conducted on the crystal structure of the positive electrode active material (Non-Patent Documents 1 to 3).

[0007] X-ray diffraction (XRD) is one of the techniques used to analyze the crystalline structure of positive electrode active materials. XRD data can be analyzed using the Inorganic Crystal Structure Database (ICSD) described in Non-Patent Document 5. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-216760 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-261132 [Non-patent literature]

[0009] [Non-Patent Document 1] Toyoki Okumura et al, “Correlation of lithium ion distribution and X-ray absorption near-edge structure in O3-and O2-lithium cobalt oxides from first-principle calculation”, Journal of Materials Chemistry, 2012, 22, p.17340-17348 [Non-patent document 2] Motohashi, T. et al, “Electronic phase diagram of the layered cobalt oxide system LiXCoO2(0.0≦X≦1.0)”, Physical Review B, 80(16);165114 [Non-patent document 3] Zhaohui Chen et al, “Staging Phase Transitions in LiXCoO2”, Journal of The Electrochemical Society, 2002, 149(12) A1604-A1609 [Non-patent document 4] WE Counts et al, Journal of the American Ceramic Society,(1953) 36[1] 12-17. Fig.01471 [Non-patent document 5] Belsky, A. et al., “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Cryst., (2002) B58 364-369. Summary of the Invention [Problem to be solved by the invention]

[0010] An object of one embodiment of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that has high capacity and excellent charge-discharge cycle characteristics, and a manufacturing method thereof. Another object is to provide a manufacturing method of the positive electrode active material with high productivity. Another object is to provide a positive electrode active material that, when used in a lithium ion secondary battery, suppresses a decrease in capacity during charge-discharge cycles. Another object is to provide a high-capacity secondary battery. Another object is to provide a secondary battery that has excellent charge-discharge characteristics. Another object is to provide a positive electrode active material in which elution of a transition metal such as cobalt is suppressed even when the battery is maintained in a charged state at a high voltage for a long period of time. Another object is to provide a secondary battery that is safe or highly reliable.

[0011] Another object of one embodiment of the present invention is to provide a novel substance, active material particles, a power storage device, or a manufacturing method thereof.

[0012] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description in the specification, drawings, and claims. [Means for solving the problem]

[0013] One aspect of the present invention is a semiconductor device comprising lithium, cobalt, and oxygen, wherein the spin density resulting from divalent cobalt ions and tetravalent cobalt ions is 2.0×10 17 spins / g or more 1.0×10 18 The positive electrode active material has a density of 0.1 to 1.0 spins / g or less.

[0014] The positive electrode active material preferably further contains magnesium, and the magnesium concentration is preferably 0.1 atomic % to 6.0 atomic % relative to the cobalt.

[0015] The above-mentioned positive electrode active material preferably further contains fluorine.

[0016] In the positive electrode active material mentioned above, the lattice constant of the a axis is 2.8155 × 10 -10 m or more 2.8175×10 -10 m, and the lattice constant of the c axis is 14.045×10 -10 m or more 14.065×10 -10 It is preferable that the length is m or less.

[0017] One embodiment of the present invention is a secondary battery including a positive electrode having the above-described positive electrode active material and a negative electrode. [Effects of the Invention]

[0018] According to one embodiment of the present invention, a positive electrode active material for a lithium ion secondary battery having high capacity and excellent charge / discharge cycle characteristics, and a manufacturing method thereof, can be provided. Furthermore, a manufacturing method of the positive electrode active material with high productivity can be provided. Furthermore, a positive electrode active material that, when used in a lithium ion secondary battery, suppresses a decrease in capacity during charge / discharge cycles can be provided. Furthermore, a high-capacity secondary battery can be provided. Furthermore, a secondary battery with excellent charge / discharge characteristics can be provided. Furthermore, a positive electrode active material in which elution of a transition metal such as cobalt is suppressed even when maintained in a charged state at a high voltage for a long period of time can be provided. Furthermore, a secondary battery with high safety or reliability can be provided. Furthermore, a novel material, active material particles, a power storage device, or a manufacturing method thereof can be provided. [Brief explanation of the drawings]

[0019] [Figure 1] 1A and 1B are schematic diagrams illustrating the configuration of the positive electrode active material. [Figure 2] 2A and 2B are schematic diagrams illustrating the configuration of the positive electrode active material. [Figure 3] FIG. 3 is a diagram illustrating the depth of charge and the crystal structure of a positive electrode active material according to one embodiment of the present invention. [Figure 4]FIG. 4 is a diagram illustrating the state of charge and the crystal structure of a conventional positive electrode active material. [Figure 5] FIG. 5 is an XRD pattern calculated from the crystal structure. [Figure 6] 6A and 6B are diagrams illustrating the crystal structure and magnetism of a positive electrode active material according to one embodiment of the present invention. [Figure 7] 7A and 7B are diagrams illustrating the crystal structure and magnetism of a conventional positive electrode active material, respectively. [Figure 8] FIG. 8 illustrates an example of a method for manufacturing a positive electrode active material of one embodiment of the present invention. [Figure 9] FIG. 9 illustrates an example of a method for manufacturing a positive electrode active material of one embodiment of the present invention. [Figure 10] FIG. 10 illustrates an example of a method for manufacturing a positive electrode active material of one embodiment of the present invention. [Figure 11] 11A and 11B are cross-sectional views of an active material layer in which a graphene compound is used as a conductive additive. [Figure 12] 12A, 12B, and 12C are diagrams illustrating a method for charging a secondary battery. [Figure 13] 13A, 13B, 13C, and 13D are diagrams illustrating a method for charging a secondary battery. [Figure 14] FIG. 14 is a diagram illustrating a method for discharging a secondary battery. [Figure 15] 15A, 15B, and 15C are diagrams illustrating a coin-type secondary battery. [Figure 16] 16A, 16B, 16C, and 16D are diagrams illustrating a cylindrical secondary battery. [Figure 17] 17A and 17B are diagrams illustrating an example of a secondary battery. [Figure 18] 18A1, 18A2, 18B1, and 18B2 are diagrams illustrating examples of secondary batteries. [Figure 19] 19A and 19B are diagrams illustrating an example of a secondary battery. [Figure 20] FIG. 20 is a diagram illustrating an example of a secondary battery. [Figure 21] 21A, 21B, and 21C are diagrams illustrating a laminated secondary battery. [Figure 22] 22A and 22B are diagrams illustrating a laminated secondary battery. [Figure 23] FIG. 23 is a diagram showing the appearance of a secondary battery. [Figure 24] FIG. 24 is a diagram showing the appearance of a secondary battery. [Figure 25] 25A, 25B, and 25C are diagrams for explaining a method for manufacturing a secondary battery. [Figure 26] 26A, 26B1, 26B2, 26C, and 26D are diagrams illustrating a bendable secondary battery. [Figure 27] 27A and 27B are diagrams illustrating a bendable secondary battery. [Figure 28] 28A, 28B, 28C, 28D, 28E, 28F, 28G, and 28H are diagrams illustrating an example of an electronic device. [Figure 29] 29A, 29B, and 29C are diagrams illustrating an example of an electronic device. [Figure 30] FIG. 30 is a diagram illustrating an example of an electronic device. [Figure 31] 31A, 31B, and 31C are diagrams illustrating an example of a vehicle. [Figure 32] FIG. 32 shows the results of ESR measurements. [Figure 33] FIG. 33 shows the results of ESR measurements. [Figure 34] 34A and 34B are graphs showing spin density. [Figure 35] 35A and 35B are diagrams showing the correlation between the amount of magnesium added and the spin density. [Figure 36] FIG. 36 shows the results of XRD measurement. [Figure 37] 37A and 37B are diagrams showing the results of XRD measurement. [Figure 38] FIG. 38 shows the results of XRD measurement. [Figure 39] 39A and 39B are diagrams showing lattice constants. [Figure 40] 40A and 40B are graphs showing cycle characteristics. [Figure 41] 41A and 41B are graphs showing cycle characteristics. [Figure 42] 42A and 42B are graphs showing continuous charging characteristics. [Figure 43] 43A and 43B are diagrams showing the results of ESR measurements. [Figure 44] 44A and 44B are graphs showing spin density. [Figure 45] 45A and 45B are diagrams showing the correlation between the amount of magnesium added and the spin density. [Figure 46] 46A and 46B are graphs showing cycle characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0021] In this specification, crystal planes and directions are indicated by Miller indices. In crystallography, crystal planes and directions are indicated by a superscript bar after the number; however, in this specification, due to limitations in application notation, the number may be expressed by a minus sign (-) before it instead of a bar above it. Furthermore, individual directions indicating directions within a crystal are expressed with [ ], collective directions indicating all equivalent directions with < >, individual planes indicating crystal faces with ( ), and collective planes with equivalent symmetry with {}.

[0022] In this specification and the like, segregation refers to a phenomenon in which a certain element (for example, B) is spatially distributed non-uniformly in a solid composed of multiple elements (for example, A, B, and C).

[0023] In this specification, the particle surface layer of an active material or the like refers to the region from the surface to a depth of about 10 nm. Surfaces formed by cracks or fissures may also be considered the surface. The region deeper than the particle surface layer is referred to as the particle interior.

[0024] In this specification, the layered rock-salt type crystal structure of a composite oxide containing lithium and a transition metal refers to a crystal structure having a rock-salt type ion arrangement in which cations and anions are alternately arranged, 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. Strictly speaking, the layered rock-salt type crystal structure may have a distorted rock-salt type crystal lattice.

[0025] In this specification and the like, the rock salt type crystal structure refers to a structure in which cations and anions are arranged alternately, and it is also possible for there to be deficiencies of cations or anions.

[0026] In this specification and the like, the pseudospinel crystal structure of a composite oxide containing lithium and a transition metal refers to a crystal structure that has the space group R-3m and is not a spinel crystal structure, but in which ions of cobalt, magnesium, etc. occupy hexacoordinated oxygen positions and the arrangement of cations has a symmetry similar to that of a spinel structure. Note that in the pseudospinel crystal structure, light elements such as lithium may occupy tetracoordinated oxygen positions, and in this case, the arrangement of ions also has a symmetry similar to that of a spinel structure.

[0027] The pseudospinel crystal structure can be said to be similar to the CdCl2 crystal structure, although it has random lithium between the layers. This CdCl2-like crystal structure was observed when lithium nickel oxide was charged to a depth of charge of 0.94 (Li 0.06The crystal structure is similar to that of lithium cobaltate (NiO2), but it is known that pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt do not usually adopt this crystal structure.

[0028] The anions in layered rock salt crystals and rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in pseudospinel crystals also have a cubic close-packed structure. When these crystals contact, there are crystal planes where the cubic close-packed structures formed by the anions are aligned. However, the space group of layered rock salt crystals and pseudospinel crystals is R-3m, which is different from the space groups of rock salt crystals, Fm-3m (the space group of general rock salt crystals) and Fd-3m (the space group of rock salt crystals with the simplest symmetry). Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between layered rock salt crystals and pseudospinel crystals and rock salt crystals. In this specification, when the cubic close-packed structures formed by the anions are aligned in layered rock salt crystals, pseudospinel crystals, and rock salt crystals, the crystal orientations may be said to be approximately aligned.

[0029] The roughly identical orientation of the crystals in the two regions can be determined from transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), high-angle annular dark field (HAADF-STEM), and annular bright field (ABF-STEM) images. X-ray diffraction (XRD), electron diffraction, and neutron diffraction can also be used for this determination. In TEM images, the arrangement of cations and anions can be observed as repeated bright and dark lines. When the orientation of the cubic close-packed structure in the layered rock salt crystal and the rock salt crystal is aligned, the angle between the repeated bright and dark lines between the crystals can be observed to be 5 degrees or less, preferably 2.5 degrees or less. In some cases, light elements such as oxygen and fluorine cannot be clearly observed in TEM images, but in such cases, the alignment of the orientation can be determined from the arrangement of the metal elements.

[0030] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable lithium contained in the positive electrode active material is deintercalated. For example, the theoretical capacity of LiCoO2 is 274 mAh / g, the theoretical capacity of LiNiO2 is 274 mAh / g, and the theoretical capacity of LiMn2O4 is 148 mAh / g.

[0031] In this specification and the like, the depth of charge when all intercalable / deintercalable lithium is intercalated is defined as 0, and the depth of charge when all intercalable / deintercalable lithium contained in the positive electrode active material is deintercalated is defined as 1.

[0032] In this specification, charging refers to the transfer of lithium ions from the positive electrode to the negative electrode within a battery and the transfer of electrons from the negative electrode to the positive electrode in an external circuit. Regarding positive electrode active materials, charging refers to the removal of lithium ions. A positive electrode active material with a charge depth of 0.74 to 0.9, more specifically, 0.8 to 0.83, is considered to be a positive electrode active material charged at a high voltage. For example, a LiCoO2 positive electrode active material charged at 219.2 mAh / g is considered to be a positive electrode active material charged at a high voltage. Furthermore, a positive electrode active material charged at a constant current of 4.525 V to 4.65 V (for a lithium counter electrode) at 25°C, followed by constant voltage charging at 0.01 C or until the current drops to approximately 1 / 5 to 1 / 100 of the current value during constant current charging, is also considered to be a positive electrode active material charged at a high voltage.

[0033] Similarly, discharging refers to the transfer of lithium ions from the negative electrode to the positive electrode within the battery and the transfer of electrons from the positive electrode to the negative electrode in an external circuit. For a positive electrode active material, discharging refers to the insertion of lithium ions. A fully discharged positive electrode active material is defined as a positive electrode active material with a charge depth of 0.06 or less, or a positive electrode active material that has been discharged from a high-voltage charged state to 90% or more of its charge capacity. For example, a LiCoO2 positive electrode active material with a charge capacity of 219.2 mAh / g is considered to be in a high-voltage charged state. A fully discharged positive electrode active material is defined as a positive electrode active material that has been discharged from this state to 90% of its charge capacity (197.3 mAh / g or more). Furthermore, a LiCoO2 positive electrode active material that has been discharged at a constant current until the battery voltage reaches 3 V or less (when using a lithium counter electrode) at 25°C is also defined as a fully discharged positive electrode active material.

[0034] In this specification, a non-equilibrium phase change refers to a phenomenon that causes a non-linear change in a physical quantity. For example, a non-equilibrium phase change occurs around the peak in the dQ / dV curve obtained by differentiating capacitance (Q) with voltage (V), and it is believed that the crystal structure changes significantly.

[0035] (Embodiment 1)

[0036] In this embodiment, a positive electrode active material of one embodiment of the present invention will be described.

[0037] One embodiment of the present invention is a positive electrode active material containing lithium, cobalt, and oxygen. The positive electrode active material of one embodiment of the present invention preferably further contains magnesium. The presence of magnesium stabilizes the crystal structure, and can prevent the crystal structure from collapsing during repeated charge and discharge. In addition, the positive electrode active material of one embodiment of the present invention is a positive electrode active material containing lithium cobalt oxide (LiCoO2) containing a portion of Li. + Mg 2+ is substituted by Co 3+ is reduced to Co 2+ (See Figures 1A and 2A.) 3+ Mg 2+ is substituted by Co 3+ is oxidized to Co 4+ (See FIGS. 1B and 2A.) Therefore, the positive electrode active material according to one embodiment of the present invention is 2+ and Co 4+ In addition, the positive electrode active material has either one or both of the following: 2+ and Co 4+ The spin density due to 17 spins / g or more 1.0×10 18 It is preferable that the spin density is not more than 1000 spins / g. By using a cathode active material having the above spin density, the crystal structure is stabilized, particularly in the charged state, which is preferable. Note that FIG. 2A is a schematic diagram illustrating the structure of a cathode active material that does not contain magnesium, and FIGS. 1A and 1B are schematic diagrams illustrating the structure of a cathode active material that contains magnesium, which is one embodiment of the present invention. In addition, if the amount of magnesium added is too large, Co 2+ and Co 4+ The spin density may be reduced due to this (see FIG. 2B). Furthermore, by using the positive electrode active material of one embodiment of the present invention for a secondary battery, the secondary battery can have excellent cycle characteristics and rate characteristics.

[0038] The magnesium concentration in the positive electrode active material is preferably 0.1 atomic % or more and 6.0 atomic % or less, more preferably 0.5 atomic % or more and 5.0 atomic % or less, and even more preferably 1.0 atomic % or more and 4.0 atomic % or less, relative to the cobalt atoms. Note that the magnesium concentration refers to the average value for all particles of the positive electrode active material.

[0039] The spin density in the positive electrode active material can be analyzed using, for example, electron spin resonance (ESR), etc. The average magnesium concentration in all particles of the positive electrode active material can be analyzed using, for example, inductively coupled plasma-mass spectrometry (ICP-MS).

[0040] The positive electrode active material according to one embodiment of the present invention preferably further contains fluorine. The presence of fluorine can improve corrosion resistance to hydrofluoric acid generated by decomposition of the electrolyte. The components of the positive electrode active material can be measured, for example, by X-ray photoelectron spectroscopy (XPS). The average fluorine concentration of the entire particles of the positive electrode active material can be analyzed, for example, by inductively coupled plasma mass spectrometry (ICP-MS).

[0041] The positive electrode active material according to one embodiment of the present invention has an a-axis lattice constant of 2.8155×10 -10 m or more 2.8175×10 -10 m, and the c-axis lattice constant is 14.045×10 -10 m or more 14.065×10 -10 It is preferable that the length is m or less.

[0042] [Positive electrode active material structure] Using Figures 3 and 4, we will explain a cathode active material 100 according to one embodiment of the present invention and a conventional cathode active material, and discuss the differences between them. Figures 3 and 4 describe a case in which cobalt is used as the transition metal in the cathode active material. Figure 3 shows the cathode active material 100 according to one embodiment of the present invention. Figure 4 shows a conventional cathode active material. The conventional cathode active material described in Figure 4 is a simple lithium cobalt oxide (LiCoO2) that has not been processed by adding elements other than lithium, cobalt, and oxygen to the inside or coating the surface layer of the cathode active material particles.

[0043] <Conventional positive electrode active materials> The crystal structure of lithium cobalt oxide LiCoO2, one of the conventional positive electrode active materials, changes depending on the depth of charge, as described in Non-Patent Documents 1 and 2. A typical crystal structure of lithium cobalt oxide is shown in Figure 4.

[0044] As shown in Figure 4, lithium cobalt oxide at a depth of charge of 0 (discharged state) has a region with a crystal structure of space group R-3m, with three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3-type crystal structure. Note that a CoO2 layer is an octahedral structure in which cobalt is six-coordinated with oxygen, and the layers are connected in a plane with edge sharing.

[0045] When the charge depth is 1, the crystal structure has the space group P-3m1, and there is one CoO2 layer in the unit cell. Therefore, this crystal structure is sometimes called the O1-type crystal structure.

[0046] At a charge depth of approximately 0.88, lithium cobalt oxide has a crystal structure of the space group R-3m. This structure can be described as a structure in which a CoO2 structure such as P-3m1(O1) and a LiCoO2 structure such as R-3m(O3) are alternately stacked. Therefore, this crystal structure is sometimes referred to as an H1-3 crystal structure. In reality, the H1-3 crystal structure has twice the number of cobalt atoms per unit cell compared to other structures. However, in Figure 4 and other parts of this specification, the c-axis of the H1-3 crystal structure is shown as half the unit cell to facilitate comparison with other structures.

[0047] When lithium cobalt oxide is repeatedly charged and discharged at high voltages to a depth of charge of approximately 0.88 or more, it undergoes repeated changes in its crystal structure (i.e., a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m(O3) structure in the discharged state.

[0048] However, these two crystal structures have a large deviation in the CoO2 layers. As shown by the dotted line and double arrows in Figure 4, in the H1-3 type crystal structure, the CoO2 layers are significantly deviated from the R-3m(O3) structure. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0049] Furthermore, the difference in volume between the H1-3 and O3 crystal structures is large: when compared per the same number of cobalt atoms, the difference in volume between the H1-3 and O3 crystal structures in the discharged state is more than 3.5%.

[0050] In addition, the H1-3 type crystal structure, which has continuous CoO2 layers such as P-3m1(O1), is likely to be unstable.

[0051] Therefore, repeated high-voltage charging and discharging causes the crystalline structure of lithium cobalt oxide to collapse, which leads to a deterioration in cycle characteristics. This is thought to be because the collapse of the crystalline structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0052] <Positive Electrode Active Material of One Embodiment of the Present Invention> <Inside the particle> In the positive electrode active material 100 according to one embodiment of the present invention, the change in crystal structure and the difference in volume per the same number of transition metal atoms are small between a fully discharged state and a high-voltage charged state (a charge depth of 0.8 or more and 0.83 or less).

[0053] The crystal structure of the positive electrode active material 100 before and after charge and discharge is shown in FIG. 3. The positive electrode active material 100 according to one embodiment of the present invention is a composite oxide containing lithium and cobalt. It preferably contains magnesium in addition to the above. It also preferably contains a halogen, such as fluorine or chlorine.

[0054] The crystal structure at a charge depth of 0 (discharged state) shown in FIG. 3 is the same as that shown in FIG. 4 , R-3m(O3). On the other hand, a cathode active material 100 according to one embodiment of the present invention has a different crystal structure from that shown in FIG. 4 when fully charged to a charge depth of approximately 0.88. This crystal structure of space group R-3m is referred to herein as a pseudo-spinel crystal structure. In the pseudo-spinel crystal structure shown in FIG. 3 , lithium is omitted to explain the symmetry of the cobalt atoms and the oxygen atoms. However, in reality, approximately 12 atomic % of lithium is present relative to cobalt between the CoO2 layers. In both the O3 crystal structure and the pseudo-spinel crystal structure, magnesium is preferably present in a dilute form between the CoO2 layers, i.e., at the lithium sites. Furthermore, it is preferable that halogens such as fluorine are present randomly at the oxygen sites.

[0055] In the positive electrode active material 100, when a large amount of lithium is released during high-voltage charging, the change in the crystal structure is suppressed more than in conventional LiCoO. For example, as shown by the dotted line in Figure 3, there is almost no displacement of the CoO layers in these crystal structures.

[0056] In the positive electrode active material 100, the difference in volume per unit cell between the O3 crystal structure at a charge depth of 0 and the pseudospinel crystal structure at a charge depth of 0.88 is 2.5% or less, more specifically 2.2% or less.

[0057] Therefore, the crystal structure of the positive electrode active material 100 is not easily broken even when it is repeatedly charged and discharged at a high voltage.

[0058] The pseudospinel crystal structure can be expressed by the coordinates of cobalt and oxygen in the unit cell being Co(0,0,0.5), O(0,0,x), where 0.20≦x≦0.25.

[0059] Magnesium, which is present randomly and dilutely between the CoO2 layers, i.e., at the lithium sites, has the effect of suppressing the displacement of the CoO2 layers. Therefore, the presence of magnesium between the CoO2 layers facilitates the formation of a pseudo-spinel crystal structure. Furthermore, it is preferable that magnesium be distributed throughout the particles of the positive electrode active material 100. To distribute magnesium throughout the particles, it is preferable to perform a heat treatment during the manufacturing process of the positive electrode active material 100.

[0060] However, if the heat treatment temperature is too high, cation mixing occurs, increasing the possibility that magnesium will enter the cobalt site. The presence of magnesium in the cobalt site may reduce the effect of maintaining the R-3m structure. Furthermore, if the heat treatment temperature is too high, there are concerns that adverse effects such as the layered rock-salt structure becoming unstable and lithium evaporating may occur.

[0061] Therefore, it is preferable to add a halogen compound such as a fluorine compound to lithium cobalt oxide before heat treatment to distribute magnesium throughout the particles. Adding a halogen compound lowers the melting point of lithium cobalt oxide. Lowering the melting point facilitates distributing magnesium throughout the particles at a temperature where cation mixing is unlikely to occur. However, the positive electrode active material may be corroded by hydrofluoric acid produced by decomposition of the electrolyte. The presence of fluorine in the positive electrode active material 100, which is one embodiment of the present invention, can improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0062] In this specification and the like, an electrolyte refers to a substance having electrical conductivity. The electrolyte is not limited to a liquid, but may be a gel or a solid. A liquid electrolyte may be called an electrolyte solution, which can be prepared by dissolving a solute in a solvent. A solid electrolyte may be called a solid electrolyte.

[0063] Although the positive electrode active material 100 has been described above as a composite oxide containing lithium, cobalt, and oxygen, it may also contain nickel in addition to cobalt. In this case, the ratio Ni / (Co+Ni) of the number of nickel atoms (Ni) to the sum of the numbers of cobalt and nickel atoms (Co+Ni) is preferably less than 0.1, and more preferably 0.075 or less.

[0064] If the battery is charged at a high voltage for a long period of time, transition metals may leach out of the positive electrode active material into the electrolyte, causing the crystal structure to collapse. However, by including nickel in the above proportions, it may be possible to suppress the leaching of transition metals from the positive electrode active material 100.

[0065] The addition of nickel reduces the charge / discharge voltage, so that the same capacity can be achieved at a lower voltage, potentially suppressing the elution of transition metals and the decomposition of the electrolyte. Here, the charge / discharge voltage refers to the voltage ranging from zero to a predetermined charge depth.

[0066] <Particle surface layer> While magnesium is preferably distributed throughout the particles of the positive electrode active material 100, it is more preferable that the magnesium concentration in the particle surface layer be higher than the average throughout the particles. The magnesium concentration in the particle surface layer can be measured, for example, by X-ray photoelectron spectroscopy (XPS). The average magnesium concentration throughout the particles can be measured, for example, by inductively coupled plasma mass spectrometry (ICP-MS) or glow discharge mass spectrometry (GDMS). The particle surface is essentially a region of crystal defects, and lithium is released from the particle surface during charging, making the lithium concentration in this region more likely to be lower than in the particle interior. Therefore, the particle surface is prone to instability and the crystalline structure is easily disrupted. A high magnesium concentration in the particle surface layer can more effectively suppress changes in the crystalline structure. Furthermore, a high magnesium concentration in the particle surface layer can be expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0067] It is also preferable that the concentration of halogens such as fluorine is higher in the particle surface layer portions of the positive electrode active material 100 than the average for the entire particle. The presence of halogens in the particle surface layer portions, which are the regions in contact with the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid.

[0068] Thus, the particle surface layer portion of the positive electrode active material 100 preferably has a different composition from the particle interior, with higher concentrations of magnesium and fluorine than the particle interior. Furthermore, the particle surface layer portion preferably has a stable crystal structure at room temperature. Therefore, the particle surface layer portion may have a different crystal structure from the particle interior. For example, at least a portion of the particle surface layer portion of the positive electrode active material 100 may have a rock salt crystal structure. Furthermore, when the particle surface layer portion and the particle interior have different crystal structures, it is preferable that the crystal orientations of the particle surface layer portion and the particle interior are roughly the same.

[0069] However, if the particle surface layer is composed only of MgO or only of a solid solution of MgO and CoO(II), it becomes difficult to insert and extract lithium. Therefore, the particle surface layer must contain at least cobalt, and in the discharged state, it must also contain lithium, providing a path for lithium insertion and extraction. In addition, it is preferable that the concentration of cobalt is higher than that of magnesium.

[0070] <Grain boundary> The magnesium or halogen contained in the positive electrode active material 100 may be present randomly and dilutely inside the particles, but it is more preferable that a portion of it is segregated at the grain boundaries.

[0071] In other words, the magnesium concentration at and near the grain boundaries of the positive electrode active material 100 is preferably higher than that in other regions inside the particles, and the halogen concentration at and near the grain boundaries is preferably higher than that in other regions inside the particles.

[0072] Like particle surfaces, grain boundaries are also planar defects. Therefore, grain boundaries are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the magnesium concentration at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0073] When the magnesium and halogen concentrations are high at and near the grain boundaries, even if cracks occur along the grain boundaries of the particles of the positive electrode active material 100, the magnesium and halogen concentrations will be high near the cracked surface. Therefore, even after cracks occur, the corrosion resistance to hydrofluoric acid of the positive electrode active material can be improved.

[0074] In this specification and the like, the vicinity of the grain boundary refers to a region up to about 10 nm from the grain boundary.

[0075] <Particle size> If the particle size of the positive electrode active material 100 is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector arise. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector and excessive reaction with the electrolyte occur. Therefore, the average particle size (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less.

[0076] In this specification, the average particle size (D50) refers to the particle size at a cumulative 50% on a volume basis. The average particle size (D50) may also be referred to as the median size.

[0077] <Analysis method> Whether a certain positive electrode active material is the positive electrode active material 100 of one embodiment of the present invention that exhibits a pseudospinel crystal structure when charged at a high voltage can be determined by analyzing the positive electrode charged at a high voltage using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), magnetization measurement, etc. XRD is particularly preferred because it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, and it can obtain sufficient accuracy even when measuring a positive electrode obtained by disassembling a secondary battery.

[0078] As described above, the cathode active material 100 of one embodiment of the present invention is characterized by minimal change in crystal structure between a high-voltage charged state and a discharged state. Materials with a crystal structure that exhibits a large change between a high-voltage charged state and a discharged state at 50 wt% or more are undesirable because they cannot withstand high-voltage charging and discharging. It should be noted that the desired crystal structure may not be achieved simply by adding elements. For example, even if both materials share the common feature of being lithium cobalt oxide containing magnesium and fluorine, there are cases in which the pseudo-spinel crystal structure accounts for 60 wt% or more and cases in which the H1-3 crystal structure accounts for 50 wt% or more when charged at a high voltage. Furthermore, at a certain voltage, the pseudo-spinel crystal structure may be nearly 100 wt%, and further increasing the voltage may result in the H1-3 crystal structure. Therefore, crystal structure analysis, such as XRD, is required to determine whether a material is the cathode active material 100 of one embodiment of the present invention.

[0079] However, when positive electrode active materials are charged or discharged at high voltage, their crystal structure may change when exposed to air. For example, they may change from a pseudospinel crystal structure to an H1-3 crystal structure. Therefore, it is recommended that all samples be handled in an inert atmosphere such as an argon atmosphere.

[0080] <Charging method> High-voltage charging for determining whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be performed, for example, by preparing a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) with a lithium counter electrode and charging it.

[0081] 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 additive, and a binder.

[0082] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, voltages and potentials in this specification refer to the potential of the positive electrode.

[0083] The electrolyte used in the electrolytic solution is 1 mol / L lithium hexafluorophosphate (LiPF6), and the solvent is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 2 wt% vinylene carbonate (VC).

[0084] The separator can be made of polypropylene with a thickness of 25 μm.

[0085] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0086] The coin cell fabricated under the above conditions was charged at a constant current of 4.6 V and 0.5 C, followed by constant voltage charging until the current reached 0.01 C. Here, 1 C corresponds to 137 mA / g. The temperature was 25°C. After charging in this manner, the coin cell was disassembled in an argon-filled glove box and the positive electrode was removed to obtain a positive electrode active material charged at high voltage. When various analyses were performed, it was preferable to seal the cell in an argon-filled container to prevent reactions with external components. For example, XRD could be performed by sealing the cell in an argon-filled container.

[0087] <xrd> Figure 5 shows ideal powder XRD patterns calculated from the pseudospinel crystal structure and H1-3 crystal structure model using CuKα1 radiation. For comparison, ideal XRD patterns calculated from the crystal structures of LiCoO2(O3) at a charge depth of 0 and CoO2(O1) at a charge depth of 1 are also shown. The LiCoO2(O3) and CoO2(O1) patterns were created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), from crystal structure information obtained from the ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 5). The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562×10. -10 m and λ2 were not set, and Monochromator was set to single. A pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. For the pseudospinel pattern, the crystal structure was estimated from the XRD pattern of a positive electrode active material according to one embodiment of the present invention, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and an XRD pattern was created in the same manner as for the others.

[0088] As shown in FIG. 5, the pseudospinel crystal structure exhibits diffraction peaks at 2θ = 19.30 ± 0.20° (19.10° to 19.50°) and 2θ = 45.55 ± 0.10° (45.45° to 45.65°). More specifically, sharp diffraction peaks appear at 2θ = 19.30 ± 0.10° (19.20° to 19.40°) and 2θ = 45.55 ± 0.05° (45.50° to 45.60°). However, peaks do not appear at these positions in the H1-3 crystal structure and CoO2(P-3m1, O1). Therefore, the appearance of peaks at 2θ = 19.28 ± 0.60° and 2θ = 45.55 ± 0.20° when charged at a high voltage is a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0089] This can also be said to mean that the positions at which XRD diffraction peaks appear are close between the crystal structure at a charge depth of 0 and the crystal structure after high-voltage charging. More specifically, the difference in the positions at which two or more, preferably three or more, of the main diffraction peaks of both structures appear is 2θ=0.7 or less, more preferably 2θ=0.5 or less.

[0090] Although the positive electrode active material 100 of one embodiment of the present invention has a pseudo-spinel crystal structure when charged at a high voltage, not all of the particles need to have a pseudo-spinel crystal structure. Other crystal structures may be included, or some particles may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the pseudo-spinel crystal structure is preferably 50 wt % or more, more preferably 60 wt % or more, and even more preferably 66 wt % or more. A positive electrode active material having a pseudo-spinel crystal structure of 50 wt % or more, more preferably 60 wt % or more, and even more preferably 66 wt % or more can have sufficiently excellent cycle characteristics.

[0091] Even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the pseudospinel crystal structure preferably accounts for 35 wt% or more, more preferably 40 wt% or more, and even more preferably 43 wt% or more.

[0092] The pseudo-spinel crystallite size of the positive electrode active material particles only decreases to about 1 / 10 of that of LiCoO2(O3) in a discharged state. Therefore, even under the same XRD measurement conditions as for the positive electrode before charging and discharging, a clear pseudo-spinel crystal structure peak can be confirmed after high-voltage charging. On the other hand, with simple LiCoO2, even if some of the material adopts a structure similar to the pseudo-spinel crystal structure, the crystallite size becomes smaller and the peak becomes broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0093] In the layered rock-salt crystal structure of the particles of the discharged positive electrode active material, which can be estimated from the XRD pattern, a small c-axis lattice constant is preferred. The c-axis lattice constant increases when foreign elements substitute for lithium sites or when cobalt occupies the oxygen tetracoordination site (A site). Therefore, it is believed that by first creating a composite oxide with a layered rock-salt crystal structure with few foreign element substitutions and a spinel-type crystal structure, i.e., with few defects, and then mixing it with a magnesium source and a fluorine source to insert magnesium into the lithium site, a positive electrode active material with good cycle performance can be produced.

[0094] The lattice constant of the a-axis in the crystal structure of the positive electrode active material in the discharged state is 2.8155 × 10 -10 m or more 2.8175×10 -10 m, and the c-axis lattice constant is 14.045×10 -10 m or more 14.065×10 -10 It is preferable that the length is m or less.

[0095] To keep the c-axis lattice constant within the above range, the amount of impurities is preferably small, and in particular, the amount of transition metals other than cobalt, manganese, and nickel is preferably small, specifically, 3000 ppm (wt) or less is preferable, and 1500 ppm (wt) or less is more preferable. Also, the amount of cation mixing between lithium and cobalt, manganese, or nickel is preferably small.

[0096] The characteristics revealed by the XRD pattern are characteristics of the internal structure of the positive electrode active material. In a positive electrode active material with an average particle diameter (D50) of approximately 1 μm to 100 μm, the volume of the particle surface layer is very small compared to the internal volume. Therefore, even if the particle surface layer of the positive electrode active material 100 has a crystal structure different from that of the particle internal volume, it is unlikely to appear in the XRD pattern.

[0097] <esr> Here, we will explain the case where ESR is used to determine the difference between a pseudo-spinel crystal structure and other crystal structures, using Figures 6 and 7. In a pseudo-spinel crystal structure, cobalt exists in a site where oxygen is six-coordinated, as shown in Figures 3 and 6A. In cobalt with six-coordinated oxygen, the 3d orbital is e g Orbit and t 2g In cobalt with 6-coordinated oxygen atoms, the orbital in the direction of the oxygen atoms is e g Compared to the orbital, the orbital that avoids the direction where oxygen exists is t 2g Orbitals are low in energy, t 2g The orbitals are in the ground state. Some of the cobalt present in the oxygen hexacoordinated sites is Co 3+ and the ground state Co 3+ is t 2g It is diamagnetic (spin quantum number S=0) with all orbitals filled. However, some of the cobalt atoms present in the oxygen hexacoordinated sites are Co 2+ or Co 4+ may be the ground state Co 2+ or Co 4+ is paramagnetic (spin quantum number S=1 / 2). This paramagnetic cobalt is Co 2+ and Co 4+ In both cases, there is one unpaired electron (spin quantum number S=1 / 2), so they cannot be distinguished by ESR.

[0098] On the other hand, some conventional positive electrode active materials have been described as having a spinel-type crystal structure that does not contain lithium in the particle surface layer when charged. In this case, they have the spinel-type crystal structure Co3O4 shown in Figure 7A.

[0099] When spinel is described by the general formula A[B2]O4, element A is tetracoordinated with oxygen and element B is hexacoordinated with oxygen. Therefore, in this specification and elsewhere, the tetracoordinated oxygen site may be referred to as the A site, and the hexacoordinated oxygen site may be referred to as the B site.

[0100] In the spinel-type crystal structure of Co3O4, cobalt exists not only in the B site with 6 oxygen atoms but also in the A site with 4 oxygen atoms. As shown in Figure 7B, in the cobalt with 4 oxygen atoms, the 3d orbital is split and e g Orbit and t 2g Among the orbitals, e g The orbital energy is low, e g The orbital is in the ground state. Therefore, the Co atom with 4 oxygen atoms is 2+ , Co 3+ and Co 4+ In the ground state, all of these have unpaired electrons and are paramagnetic. Therefore, if particles containing sufficient spinel-type Co3O4 are analyzed by ESR, etc., paramagnetic Co3O4 with 4-coordinated oxygen atoms will be found. 2+ (spin quantum number S=3 / 2), Co 3+ (spin quantum number S=1) or Co 4+ A signal due to the spin quantum number S=1 / 2 should be detected.

[0101] However, in the positive electrode active material 100 of one embodiment of the present invention, the signal due to the paramagnetic cobalt with four oxygen coordinates is so small that it cannot be detected. Therefore, unlike the positive spinel, the pseudospinel referred to in this specification does not contain an amount of cobalt with four oxygen coordinates that can be detected by ESR. Therefore, compared to conventional positive electrode active materials, the positive electrode active material 100 of one embodiment of the present invention may have a small or even undetectable signal due to spinel-type Co3O4 that can be detected by ESR or the like. Because spinel-type Co3O4 does not contribute to charge / discharge reactions, the less spinel-type Co3O4, the better. Thus, even from ESR analysis, it can be determined that the positive electrode active material 100 is different from conventional positive electrode active materials.

[0102] The positive electrode active material according to one embodiment of the present invention is Co 2+ and Co 4+ In addition, the positive electrode active material according to one embodiment of the present invention includes Co. 2+ and Co 4+ The spin density due to 17 spins / g or more 1.0×10 18 spins / g or less is preferable, 2.5×10 17 spins / g or more 9.5×10 17 spins / g or less is more preferable, and 3.0×10 17 spins / g or more 9.0×10 17 spins / g or less is more preferable, and 3.5×10 17 spins / g or more 8.5×10 17 The spin density of the positive electrode active material can be evaluated by, for example, ESR analysis. 2+ and Co 4+ The ESR signal caused by this is observed at a g-value of around 2.15. The spin density mentioned above refers to the value obtained by ESR analysis at room temperature and is the number of spins per weight of the positive electrode active material. The spin density mentioned above can be calculated by dividing the number of spins obtained by ESR analysis by the weight of the sample used in the ESR analysis.

[0103] The positive electrode active material according to one embodiment of the present invention is Co 2+ and Co 4+ The spin density due to -5 spins / Co atom or more 1.6×10 -4 spins / Co atom or less is preferred, 4.1×10 -5 spins / Co atoms or more 1.5×10 -4 spins / Co atom or less is more preferable, and 4.9 × 10 -5 spins / Co atoms or more 1.5×10 -4 spins / Co atom or less is more preferable, and 5.7 × 10 -5 spins / Co atom or more 1.4×10 -4 More preferably, the spin density is less than 1 / Co atom. The spin density is a value obtained by ESR analysis at room temperature and is the number of spins per cobalt atom in the positive electrode active material. The spin density can be calculated by dividing the number of spins obtained by ESR analysis by the number of cobalt atoms in the positive electrode active material used in the ESR analysis. For example, in the case of lithium cobalt oxide, the composition is LiCoO2, and the number of cobalt atoms in the positive electrode active material can be calculated from its molecular weight of 97.87 and the weight of the positive electrode active material used in the ESR analysis.

[0104] The positive electrode active material having the above spin density stabilizes the crystal structure, and the crystal structure can be prevented from collapsing during repeated charge and discharge. Furthermore, by using the positive electrode active material according to one embodiment of the present invention in a secondary battery, the secondary battery can have excellent cycle characteristics and rate characteristics. Furthermore, the positive electrode active material having the above spin density may have a pseudo-spinel crystal structure in a charged state.

[0105] <xps> X-ray photoelectron spectroscopy (XPS) can analyze regions from the surface to a depth of approximately 2 to 8 nm (usually about 5 nm), allowing quantitative analysis of the concentration of each element in approximately half of the particle surface. 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.

[0106] When XPS analysis is performed on the positive electrode active material 100, the relative value of the magnesium concentration is preferably 0.4 to 1.5, and more preferably 0.45 to less than 1.00, when the cobalt concentration is set to 1. The relative value of the halogen concentration such as fluorine is preferably 0.05 to 1.5, and more preferably 0.3 to 1.00.

[0107] When the positive electrode active material 100 is analyzed by XPS, the peak showing the bond energy between fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably about 684.3 eV. This value is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV). In other words, when the positive electrode active material 100 contains fluorine, it is preferable that the bond be other than that of lithium fluoride or magnesium fluoride.

[0108] Furthermore, when the positive electrode active material 100 is subjected to XPS analysis, the peak showing the bond energy between magnesium and other elements is preferably 1302 eV or more and less than 1304 eV, and more preferably about 1303 eV. This value is different from the bond energy of magnesium fluoride, 1305 eV, and is closer to the bond energy of magnesium oxide. In other words, when the positive electrode active material 100 contains magnesium, the bond is preferably other than that of magnesium fluoride.

[0109] <edx> The concentrations of various elements in the interior of the particles, in the particle surface layer, and in the vicinity of the crystal grain boundaries can be evaluated, for example, using energy dispersive X-ray spectroscopy (EDX). Among EDX measurements, when measuring while scanning within a region and evaluating the region two-dimensionally, it may be called EDX surface analysis. Also, from the surface analysis of EDX, when extracting data of a linear region and evaluating the distribution of atomic concentrations within the positive electrode active material particles, it may be called line analysis.

[0110] By EDX surface analysis (for example, elemental mapping), the concentrations of magnesium and fluorine in the interior of the particles, in the particle surface layer, and in the vicinity of the crystal grain boundaries can be quantitatively analyzed. Also, by EDX line analysis, the peaks of the concentrations of magnesium and fluorine can be analyzed.

[0111] When performing EDX line analysis on the positive electrode active material 100, the peak of the magnesium concentration in the particle surface layer preferably exists up to a depth of 3 nm from the surface of the positive electrode active material 100 toward the center, more preferably up to a depth of 1 nm, and even more preferably up to a depth of 0.5 nm.

[0112] The distribution of fluorine possessed by the positive electrode active material 100 preferably overlaps with the distribution of magnesium. Therefore, when performing EDX line analysis, the peak of the fluorine concentration in the particle surface layer preferably exists up to a depth of 3 nm from the surface of the positive electrode active material 100 toward the center, more preferably up to a depth of 1 nm, and even more preferably up to a depth of 0.5 nm.

[0113] When performing line analysis or surface analysis on the positive electrode active material 100, the ratio (Mg / Co) of the number of atoms of magnesium and cobalt in the vicinity of the crystal grain boundaries is preferably 0.020 or more and 0.50 or less. Further, it is preferably 0.025 or more and 0.30 or less. Further, it is preferably 0.030 or more and 0.20 or less.

[0114] <dQ / dV vs V curve> When the positive electrode active material of one embodiment of the present invention is charged at a high voltage and then discharged at a low rate of, for example, 0.2 C or less, a characteristic voltage change may occur near the end of discharge. This change can be clearly confirmed by the presence of at least one peak in the range of 3.5 V to 3.9 V in the dQ / dV vs. V curve obtained from the discharge curve.

[0115] (Embodiment 2)

[0116] In this embodiment, an example of a method for manufacturing a positive electrode active material of one embodiment of the present invention will be described.

[0117] [Method 1 for preparing positive electrode active material] An example of a method for manufacturing the positive electrode active material 100 according to one embodiment of the present invention will be described with reference to FIGS.

[0118] <Step S11> In step S11, a halogen source and a magnesium source, which are materials for the first mixture, are prepared (step S11 in FIGS. 8 and 9). It is also preferable to prepare a lithium source. Furthermore, if the subsequent mixing and grinding steps are performed wet, a solvent is prepared.

[0119] As the magnesium source, for example, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. can be used. As the lithium source, for example, lithium fluoride, lithium carbonate can be used.

[0120] Materials containing fluorine, chlorine, or the like can be used as the halogen source. Materials containing halogen and magnesium can be used as the halogen source and magnesium source. Materials containing halogen and lithium can be used as the halogen source and lithium source. For example, lithium fluoride can be used as the fluorine-containing halogen source and lithium source. For example, magnesium fluoride can be used as the fluorine-containing halogen source and magnesium source. Among these, lithium fluoride has a relatively low melting point of 848°C and is easily melted in the annealing step described below, and is therefore suitable for use as the halogen source and lithium source. For example, lithium chloride can be used as the chlorine-containing halogen source and lithium source. For example, magnesium chloride can be used as the chlorine-containing halogen source and magnesium source.

[0121] Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. It is more preferable to use an aprotic solvent that is less likely to react with lithium, and for example, acetone can be suitably used.

[0122] This will be explained in detail using an example in which lithium fluoride (LiF) is used as the halogen source and lithium source, and magnesium fluoride (MgF2) is used as the halogen source and magnesium source. The melting point is most effectively lowered when lithium fluoride and magnesium fluoride are mixed in a molar ratio of approximately LiF:MgF2 = 65:35 (Non-Patent Document 4). On the other hand, if the amount of lithium fluoride is too high, there is a risk of excess lithium and deterioration of cycle characteristics. Therefore, the molar ratio of lithium fluoride (LiF) to magnesium fluoride (MgF2) is preferably LiF:MgF2 = x:1 (0≦x≦1.9), more preferably LiF:MgF2 = x:1 (0.1≦x≦0.5), and even more preferably LiF:MgF2 = x:1 (near x = 0.33). In this specification, "near" refers to a value greater than 0.9 times but less than 1.1 times the value.

[0123] <Step S12> Next, in step S12, the materials and solvent prepared in step S11 are mixed and pulverized (step S12 in FIGS. 8 and 9). Mixing can be done either dry or wet, but wet mixing is preferred because it allows for finer pulverization. For example, a ball mill, bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the medium. It is preferable to thoroughly perform this mixing and pulverization process to finely pulverize the materials.

[0124] <Steps S13 and S14> The materials mixed and pulverized in step S12 are collected (step S13 in FIGS. 8 and 9), and a first mixture is obtained (step S14 in FIGS. 8 and 9).

[0125] The particle size of the first mixture is preferably, for example, an average particle size (D50) of 600 nm to 20 μm, more preferably 1 μm to 10 μm. Such a finely pulverized first mixture facilitates uniform adhesion of the first mixture to the surface of the composite oxide particles when mixed with a composite oxide containing lithium and a transition metal in a subsequent process. Uniform adhesion of the first mixture to the surface of the composite oxide particles is preferred because it facilitates thorough distribution of halogen and magnesium throughout the surface layer of the composite oxide particles by heat treatment. If there are regions in the particle surface that do not contain halogen and magnesium, it may be difficult to form the pseudospinel crystal structure described below in the charged state.

[0126] <Step S21> In step S21, a lithium source and a transition metal source that are materials for a composite oxide containing lithium and a transition metal are prepared (step S21 in FIG. 8).

[0127] As the lithium source, for example, lithium carbonate, lithium fluoride, etc. can be used.

[0128] Cobalt is preferably used as the transition metal, and either or both of aluminum and nickel may be further contained.

[0129] As the transition metal source, oxides, hydroxides, etc. of the above transition metals can be used. As the cobalt source, for example, cobalt oxide, cobalt hydroxide, etc. can be used. As the manganese source, manganese oxide, manganese hydroxide, etc. can be used. As the nickel source, nickel oxide, nickel hydroxide, etc. can be used. As the aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used.

[0130] <Step S22> Next, in step S22, the lithium source and transition metal source prepared in step S21 are mixed (step S22 in FIG. 8). Mixing can be performed by a dry method or a wet method. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconia balls, for example, as media.

[0131] <Step S23> Next, in step S23, the material mixed in step S22 is subjected to a heat treatment (step S23 in FIG. 8). This step is sometimes referred to as firing or the first heat treatment to distinguish it from subsequent heat treatments. The heat treatment is preferably carried out at a temperature of 800°C or higher but lower than 1100°C, more preferably 900°C or higher but lower than 1000°C, and even more preferably around 950°C. If the heat treatment temperature is too low, the starting materials may not be sufficiently decomposed or melted. On the other hand, if the heat treatment temperature is too high, defects may occur due to excessive reduction of transition metals or evaporation of lithium.

[0132] The heat treatment time is preferably 2 hours or more and 20 hours or less. The heat treatment atmosphere is preferably dry air or the like with little water (for example, a dew point of -50°C or less, more preferably -100°C or less). For example, heating can be performed at 1000°C for 10 hours, with a temperature increase rate of 200°C / h. The flow rate of the dry air is preferably set so that the partial pressure of the gas desorbed from the material by the heat treatment is sufficiently low. By setting the flow rate so that the partial pressure of the desorbed gas is sufficiently low, the reaction occurring in the material can be promoted.

[0133] After the heat treatment, the material can be cooled to room temperature. For example, it is preferable to set the cooling time from the specified temperature to room temperature to between 10 hours and 50 hours. However, cooling to room temperature in step S23 is not essential. If there is no problem in performing the subsequent steps S24, S25, and S31 to S34, cooling to a temperature higher than room temperature may be performed.

[0134] <Steps S24 and S25> Next, the material fired in step S23 is recovered (step S24 in FIG. 8), and a composite oxide containing lithium and a transition metal is obtained (step S25 in FIG. 8). Specifically, lithium cobalt oxide or lithium cobalt oxide containing aluminum or nickel is obtained.

[0135] Note that a composite oxide containing lithium and a transition metal that has been synthesized in advance may be used in step S25 (step S25 in FIG. 9), in which case steps S21 to S24 can be omitted.

[0136] When using a composite oxide containing lithium and a transition metal that has been synthesized in advance, it is preferable to use one with few impurities. In this specification, the composite oxide containing lithium and a transition metal and the positive electrode active material are defined as having lithium, cobalt, nickel, aluminum, and oxygen as the main components, and elements other than the main components are defined as impurities. For example, when analyzed by glow discharge mass spectrometry, the total impurity concentration is preferably 10,000 ppm (wt) or less, and more preferably 5000 ppm (wt) or less.

[0137] For example, lithium cobalt oxide particles (product name: Cellseed C-10N) manufactured by Nippon Chemical Industry Co., Ltd. can be used as pre-synthesized lithium cobalt oxide. This lithium cobalt oxide has an average particle size (D50) of approximately 12 μm, and impurity analysis by glow discharge mass spectrometry (GD-MS) shows that the magnesium and fluorine concentrations are 50 ppm (wt) or less, the calcium, aluminum, and silicon concentrations are 100 ppm (wt) or less, the nickel concentration is 150 ppm (wt) or less, the sulfur concentration is 500 ppm (wt) or less, the arsenic concentration is 1100 ppm (wt) or less, and the concentrations of other elements other than lithium, cobalt, and oxygen are 150 ppm (wt) or less.

[0138] As pre-synthesized lithium cobalt oxide, lithium cobalt oxide particles (product name: Cellseed C-5H) manufactured by Nippon Chemical Industry Co., Ltd. can also be used. This is lithium cobalt oxide with an average particle size (D50) of approximately 6.5 μm, and in impurity analysis by GD-MS, the concentrations of elements other than lithium, cobalt, and oxygen are similar to or lower than those of C-10N.

[0139] The composite oxide containing lithium and a transition metal in step S25 preferably has a layered rock-salt crystal structure with few defects and strain. Therefore, it is preferable that the composite oxide contains few impurities. If the composite oxide containing lithium and a transition metal contains a large amount of impurities, it is likely to have a crystal structure with many defects or strains.

[0140] <Step S31> Next, in step S31, the first mixture obtained in step S14 is mixed with the composite oxide containing lithium and a transition metal obtained in step S25 (step S31 in FIGS. 8 and 9). The transition metal TM in the composite oxide containing lithium and a transition metal and the magnesium Mg in the first mixture Mix1 are mixed. Mix1 The atomic ratio of TM:Mg Mix1 = 1:y (0.0005≦y≦0.03), and TM:Mg Mix1 = 1:y (0.001≦y≦0.01), and TM:Mg Mix1 A ratio of about 1:0.005 is even more preferable.

[0141] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the composite oxide particles. For example, the mixing conditions are preferably lower in rotation speed or shorter in time than those in step S12. It can also be said that dry mixing conditions are milder than wet mixing. For example, a ball mill, bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media.

[0142] <Steps S32 and S33> Next, the materials mixed in step S31 are collected (step S32 in FIGS. 8 and 9) to obtain a second mixture (step S33 in FIGS. 8 and 9).

[0143] Although this embodiment describes a method of adding a mixture of lithium fluoride and magnesium fluoride to lithium cobalt oxide with few impurities, one embodiment of the present invention is not limited thereto. Instead of the second mixture in step S33, a lithium cobalt oxide starting material to which a magnesium source and a fluorine source have been added and then calcined may be used. In this case, there is no need to separate steps S11 to S14 from steps S21 to S25, resulting in a simple and highly productive process.

[0144] Alternatively, lithium cobalt oxide to which magnesium and fluorine have been added in advance may be used. If lithium cobalt oxide to which magnesium and fluorine have been added is used, the steps up to step S32 can be omitted, which is simpler.

[0145] Furthermore, a magnesium source and a fluorine source may be further added to lithium cobalt oxide to which magnesium and fluorine have been added in advance.

[0146] <Step S34> Next, in step S34, the second mixture obtained in step S33 is subjected to a heat treatment (step S34 in FIGS. 8 and 9). This step may be referred to as annealing or a second heat treatment to distinguish it from the previous heat treatment.

[0147] The annealing is preferably performed at an appropriate temperature and time. The appropriate temperature and time vary depending on conditions such as the size and composition of the composite oxide particles containing lithium and a transition metal in step S25. If the particles are small, a lower temperature or shorter time may be more preferable than if the particles are large.

[0148] For example, when the average particle size (D50) of the particles in step S25 is about 12 μm, the annealing temperature is preferably, for example, 600° C. or more and 950° C. or less. The annealing time is, for example, preferably 3 hours or more, more preferably 10 hours or more, and even more preferably 60 hours or more.

[0149] On the other hand, when the average particle size (D50) of the particles in step S25 is about 5 μm, the annealing temperature is preferably, for example, 600° C. to 950° C. The annealing time is preferably, for example, 1 hour to 10 hours, more preferably about 2 hours.

[0150] The temperature drop time after annealing is preferably, for example, 10 hours or more and 50 hours or less.

[0151] When the second mixture is annealed, it is believed that the material with a low melting point in the first mixture (e.g., lithium fluoride, melting point 848°C) melts first and distributes to the surface layer of the composite oxide particles. The presence of this molten material then lowers the melting points of other materials, which then melts them. For example, magnesium fluoride (melting point 1263°C) melts and distributes to the surface layer of the composite oxide particles.

[0152] It is believed that the elements contained in the first mixture distributed in the surface layer portion of the particles are dissolved in the composite oxide containing lithium and a transition metal.

[0153] The diffusion of elements contained in this first mixture is faster in the particle surface and near the grain boundaries than in the interior of the composite oxide particles. Therefore, magnesium and halogens are concentrated at higher concentrations in the particle surface and near the grain boundaries than in the particle interior. As will be described later, a high magnesium concentration in the particle surface and near the grain boundaries can more effectively suppress changes in the crystal structure.

[0154] <Step S35> Next, in step S35, the material annealed in step S34 is collected to obtain the positive electrode active material 100 according to one embodiment of the present invention.

[0155] By fabricating the cathode active material 100 using the method shown in FIGS. 8 and 9, it is possible to fabricate a cathode active material 100 that has a pseudo-spinel crystal structure with few defects when charged at a high voltage. Furthermore, by using the cathode active material 100 according to one embodiment of the present invention in a secondary battery, it is possible to obtain a secondary battery with excellent cycle characteristics and rate characteristics. For example, by making the proportion of the pseudo-spinel crystal structure in the cathode active material 50% or more, it is possible to obtain a secondary battery with excellent cycle characteristics and rate characteristics. The proportion of the pseudo-spinel crystal structure in the cathode active material can be confirmed, for example, by performing Rietveld analysis in XRD measurement.

[0156] To produce a positive electrode active material with a pseudo-spinel crystal structure after high-voltage charging, it is effective to have the positive electrode active material contain magnesium and fluorine and anneal it at an appropriate temperature and time. The magnesium source and fluorine source may be added to the starting material for the composite oxide. However, if the melting points of the magnesium source and fluorine source are higher than the calcination temperature, the magnesium source and fluorine source may not melt and may not diffuse sufficiently. This may result in numerous defects or distortions in the layered rock-salt crystal structure. Therefore, defects or distortions may also occur in the pseudo-spinel crystal structure after high-voltage charging.

[0157] Therefore, it is preferable to first obtain a composite oxide having a layered rock-salt crystal structure with few impurities and few defects or distortion. Then, in a subsequent step, it is preferable to mix the composite oxide with a magnesium source and a fluorine source, and anneal the mixture to form a solid solution of magnesium and fluorine in the surface layer of the composite oxide particles. This preparation method allows for the production of a positive electrode active material that has a pseudo-spinel structure with few defects or distortion after high-voltage charging.

[0158] The positive electrode active material 100 produced in the above steps may be further coated with another material, and may be further subjected to a heat treatment.

[0159] For example, the positive electrode active material 100 can be mixed with a compound containing phosphoric acid. After mixing, a heat treatment can be performed. By mixing the compound containing phosphoric acid, the positive electrode active material 100 can be obtained in which the elution of transition metals such as cobalt is suppressed even when the positive electrode active material 100 is maintained in a charged state at a high voltage for a long period of time. Furthermore, by performing a heat treatment after mixing, the phosphoric acid can be more uniformly coated.

[0160] Examples of compounds having phosphoric acid include lithium phosphate and ammonium dihydrogen phosphate. The mixing can be carried out by, for example, a solid phase method. The heating can be carried out at 800°C or higher for 2 hours.

[0161] [Method 2 for preparing positive electrode active material] A method for producing a positive electrode active material 100, which is different from the above-described [Method 1 for producing a positive electrode active material], will be described with reference to FIG.

[0162] <Step S41> In step S41, a lithium source, a transition metal source, a halogen source, and a magnesium source, which are materials for the mixture, are prepared (step S41 in FIG. 10). In addition, if the subsequent mixing and pulverization steps are performed wet, a solvent is prepared.

[0163] Regarding the lithium source, transition metal source, halogen source, magnesium source and solvent, the above description of [Method 1 for preparing positive electrode active material] can be referred to, and therefore detailed description thereof will be omitted.

[0164] <Step S42> Next, in step S42, the materials and solvent prepared in step S41 are mixed and pulverized (step S42 in FIG. 10). For the mixing and pulverization, the description in the above [Method 1 for producing positive electrode active material] can be referred to, and therefore a detailed description thereof will be omitted.

[0165] <Steps S43 and S44> The materials mixed and pulverized in step S42 are collected (step S43 in FIG. 10) to obtain a mixture (step S44 in FIG. 10).

[0166] <Step S45> Next, in step S45, the mixture obtained in step S44 is subjected to a first heat treatment (step S45 in FIG. 10). The first heat treatment is preferably performed at a temperature of 800°C or higher but lower than 1100°C, more preferably 900°C or higher but lower than 1000°C, and even more preferably around 950°C. If the temperature of the first heat treatment is too low, the starting materials may not be sufficiently decomposed or melted. On the other hand, if the temperature of the first heat treatment is too high, defects may occur due to excessive reduction of the transition metal or evaporation of lithium.

[0167] The first heat treatment time is preferably 2 hours or more and 20 hours or less. The atmosphere for the first heat treatment is preferably dry air or the like with little water (for example, a dew point of -50°C or less, more preferably -100°C or less). For example, it is preferable to heat at 1000°C for 10 hours, with a temperature rise rate of 200°C / h and a dry air flow rate of 10 L / min.

[0168] After the first heat treatment, the material can be cooled to room temperature. For example, it is preferable to set the temperature drop time from the specified temperature to room temperature to between 10 hours and 50 hours. However, cooling to room temperature in step S45 is not essential. If there is no problem in performing the subsequent steps S46 and S47, the material may be cooled to a temperature higher than room temperature.

[0169] <Step S46> Next, in step S46, the fired mixture is collected (step S46 in FIG. 10).

[0170] <Step S47> Next, in step S47, the second mixture obtained in step S46 is subjected to a heat treatment (step S47 in FIG. 10).

[0171] The second heat treatment is preferably carried out at an appropriate temperature and time, which vary depending on the particle size and composition of the mixture. If the particles are small, a lower temperature or shorter time may be preferable than if the particles are large.

[0172] For example, when the average particle size (D50) of the particles of the mixture is about 12 μm, the temperature of the second heat treatment is preferably, for example, 600° C. or more and 950° C. or less. The time period of the second heat treatment is, for example, preferably 3 hours or more, more preferably 10 hours or more, and even more preferably 60 hours or more.

[0173] On the other hand, when the average particle size (D50) of the particles of the mixture is about 5 μm, the temperature of the second heat treatment is preferably, for example, from 600° C. to 950° C. The time period of the second heat treatment is preferably, for example, from 1 hour to 10 hours, and more preferably about 2 hours.

[0174] The temperature-lowering time after the second heat treatment is preferably, for example, 10 hours or more and 50 hours or less.

[0175] <Step S48> Next, in step S48, the material annealed in step S34 is collected to obtain positive electrode active material 100 according to one embodiment of the present invention.

[0176] The above is the method for producing the positive electrode active material 100 according to one embodiment of the present invention.

[0177] (Embodiment 3) In this embodiment, examples of materials that can be used in a secondary battery having the positive electrode active material 100 described in the previous embodiment will be described. In this embodiment, a secondary battery in which a positive electrode, a negative electrode, and an electrolyte are enclosed in an exterior body will be described as an example.

[0178] [Positive electrode] The positive electrode has a positive electrode active material layer and a positive electrode current collector.

[0179] <Cathode active material layer> The positive electrode active material layer contains at least a positive electrode active material. In addition to the positive electrode active material, the positive electrode active material layer may contain other substances such as a coating on the surface of the active material, a conductive additive, or a binder.

[0180] The positive electrode active material may be the positive electrode active material 100 described in the previous embodiment. By using the positive electrode active material 100 described in the previous embodiment, a secondary battery with high capacity and excellent cycle characteristics can be obtained.

[0181] The conductive additive may be a carbon material, a metal material, a conductive ceramic material, or the like. Alternatively, a fibrous material may be used as the conductive additive. The content of the conductive additive relative to the total amount of the active material layer is preferably 1 wt% to 10 wt%, more preferably 1 wt% to 5 wt%.

[0182] The conductive additive can form an electrically conductive network in the active material layer. The conductive additive can maintain an electrical conduction path between the positive electrode active materials. By adding the conductive additive to the active material layer, an active material layer with high electrical conductivity can be realized.

[0183] Examples of the conductive additive include natural graphite, artificial graphite such as mesocarbon microbeads, and carbon fibers. Examples of the carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Examples of the carbon fibers that can be used include carbon nanofibers and carbon nanotubes. Carbon nanotubes can be produced by, for example, vapor phase growth methods. Examples of the conductive additive include carbon materials such as carbon black (e.g., acetylene black (AB)), graphite particles, graphene, and fullerene. Examples of the conductive additive include metal powders and metal fibers such as copper, nickel, aluminum, silver, and gold, and conductive ceramic materials.

[0184] A graphene compound may be used as the conductive additive.

[0185] Graphene compounds may have excellent electrical properties, such as high electrical conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds also have a planar shape. Graphene compounds enable surface contact with low contact resistance. Even thin graphene compounds can have very high electrical conductivity, allowing a small amount to efficiently form a conductive path within an active material layer. Therefore, using a graphene compound as a conductive additive is preferable because it increases the contact area between the active material and the conductive additive. Using a spray-drying device is preferable to form a coating of the graphene compound, which serves as a conductive additive, covering the entire surface of the active material. This is also preferable because it may reduce electrical resistance. Here, graphene, multigraphene, or RGO are particularly preferable as graphene compounds. Here, RGO refers to a compound obtained by reducing graphene oxide (GO), for example.

[0186] When using an active material with a small particle size, for example, an active material with a particle size of 1 μm or less, the specific surface area of ​​the active material is large, and more conductive paths connecting the active material particles are required. Therefore, the amount of conductive additive tends to be large, which tends to result in a relative decrease in the amount of active material supported. A decrease in the amount of active material supported results in a decrease in the capacity of the secondary battery. In such cases, using a graphene compound as a conductive additive is particularly preferable because even a small amount of the graphene compound can efficiently form conductive paths without reducing the amount of active material supported.

[0187] As an example, a cross-sectional configuration example in which a graphene compound is used as a conductive additive in the active material layer 200 will be described below.

[0188] FIG. 11A shows a longitudinal cross-sectional view of an active material layer 200. The active material layer 200 includes granular positive electrode active material 100, a graphene compound 201 as a conductive additive, and a binder (not shown). Here, graphene or multi-graphene may be used as the graphene compound 201, for example. Here, the graphene compound 201 preferably has a sheet-like shape. Alternatively, the graphene compound 201 may be a sheet-like shape formed by partially overlapping a plurality of multi-graphenes and / or a plurality of graphenes.

[0189] 11B, in a longitudinal cross section of the active material layer 200, sheet-like graphene compounds 201 are dispersed approximately uniformly inside the particles of the active material layer 200. In FIG. 11B, the graphene compounds 201 are schematically represented by thick lines, but in reality, they are thin films having the thickness of a single layer or multiple layers of carbon molecules. The plurality of graphene compounds 201 are formed so as to partially cover the plurality of granular positive electrode active material 100 or to adhere to the surfaces of the plurality of granular positive electrode active material 100, and are therefore in surface contact with each other.

[0190] Here, a plurality of graphene compounds are bonded together to form a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net). When an active material is covered with a graphene net, the graphene net can also function as a binder that binds the active materials together. Therefore, the amount of binder can be reduced or can be eliminated, thereby improving the ratio of the active material to the electrode volume or weight. In other words, the capacity of the secondary battery can be increased.

[0191] Here, it is preferable to use graphene oxide as the graphene compound 201, mix it with an active material to form a layer that becomes the active material layer 200, and then reduce it. By using graphene oxide, which has extremely high dispersibility in a polar solvent, to form the graphene compound 201, it is possible to disperse the graphene compound 201 approximately uniformly inside the particles of the active material layer 200. The solvent is volatilized and removed from the dispersion medium containing the uniformly dispersed graphene oxide, and the graphene oxide is reduced. Therefore, the graphene compound 201 remaining in the active material layer 200 is dispersed to the extent that it partially overlaps and is in surface contact with each other, thereby forming a three-dimensional conductive path. Note that the reduction of the graphene oxide may be performed, for example, by heat treatment or using a reducing agent.

[0192] Therefore, unlike a granular conductive additive such as acetylene black that makes point contact with the active material, the graphene compound 201 enables surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular positive electrode active material 100 and the graphene compound 201 with a smaller amount than that of a typical conductive additive. This makes it possible to increase the ratio of the positive electrode active material 100 in the active material layer 200. This allows the discharge capacity of the secondary battery to be increased.

[0193] By using a spray dryer in advance, a graphene compound serving as a conductive additive can be formed as a coating that covers the entire surface of the active material, and further a conductive path can be formed between the active material particles by the graphene compound.

[0194] 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. Also, fluororubber can be used as the binder.

[0195] It is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, as well as starch. It is even more preferable to use these water-soluble polymers in combination with the aforementioned rubber material.

[0196] 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.

[0197] The binder may be used in combination with two or more of the above.

[0198] For example, a material with particularly excellent viscosity adjusting effect may be used in combination with other materials. For example, while rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with particularly excellent viscosity adjusting effect. Furthermore, as water-soluble polymers with particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, cellulose derivatives such as regenerated cellulose, and starch may be used.

[0199] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium salts or ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with active materials and other components when preparing electrode slurry. In this specification, the cellulose and cellulose derivatives used as electrode binders also include their salts.

[0200] Fluorine-based resins have advantages such as excellent mechanical strength, high chemical resistance, and high heat resistance. PVDF, one of the fluororesins, has extremely excellent properties among fluororesins, including mechanical strength, excellent processability, and high heat resistance.

[0201] On the other hand, PVDF may gel or become insolubilized if the slurry prepared when applying the active material layer becomes alkaline. The gelation or insolubilization of the binder may reduce the adhesion between the current collector and the active material layer. The use of a positive electrode active material according to one embodiment of the present invention is preferable because it may be possible to lower the pH of the slurry and inhibit gelation or insolubilization.

[0202] The thickness of the positive electrode active material layer is, for example, 10 μm or more and 200 μm or less, or 50 μm or more and 150 μm or less. For example, when the positive electrode active material has a material having a layered rock salt crystal structure containing cobalt, the amount of the positive electrode active material layer is 1 mg / cm. 2 More than 50mg / cm 2 or less than 5 mg / cm 2 More than 30mg / cm 2 For example, when the positive electrode active material has a material having a layered rock salt crystal structure containing cobalt, the density of the positive electrode active material layer is 2.2 g / cm 3 More than 4.9g / cm 3 or less. Or 3.8 g / cm 3 More than 4.5g / cm 3 The following is the result.

[0203] <Positive electrode current collector> The positive electrode current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The positive electrode current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the form of a foil, plate (sheet), mesh, punched metal, or expanded metal, as appropriate. It is preferable to use a current collector with a thickness of 5 μm to 30 μm.

[0204] [Negative electrode] The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also include a conductive additive and a binder.

[0205] <Negative electrode active material> As the negative electrode active material, for example, an alloy-based material or a carbon-based material can be used.

[0206] The negative electrode active material can be an element capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium. For example, materials containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium can be used. These elements have a larger capacity than carbon, and silicon, in particular, has a high theoretical capacity of 4200 mAh / g. For this reason, silicon is preferred as the negative electrode active material. Compounds containing these elements can also be used. Examples include SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn. Here, elements that can undergo charge-discharge reactions through alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes called alloy-based materials.

[0207] In this specification and the like, SiO refers to, for example, silicon monoxide. Alternatively, SiO refers to SiO x Here, x preferably has a value close to 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0208] Examples of carbonaceous materials that can be used include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0209] 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 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.

[0210] When lithium ions are inserted into graphite (when lithium-graphite intercalation compounds are formed), graphite exhibits a low potential similar to that of metallic lithium (0.05 V to 0.3 V vs. Li / Li + ) This allows lithium-ion secondary batteries to exhibit high operating voltages. Furthermore, graphite is preferred because it has advantages such as a relatively high capacity per unit volume, a relatively small volume expansion, low cost, and higher safety compared to lithium metal.

[0211] The negative electrode active materials are titanium dioxide (TiO2) and lithium titanium oxide (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li X C6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), molybdenum oxide (MoO2), and other oxides can be used.

[0212] The negative electrode active material is a composite nitride of lithium and transition metals, which has a Li3N structure. 3-x M x N (M=Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 N3 has a large charge / discharge capacity (900mAh / g, 1890mAh / cm 3 ) and is preferred.

[0213] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, and therefore it can be preferably combined with a material that does not contain lithium ions, such as V2O5 or Cr3O8, as the positive electrode active material. Even when a material containing lithium ions is used as the positive electrode active material, the composite nitride of lithium and a transition metal can be used as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0214] Materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as the negative electrode active material. Materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS 0.89 It also occurs with sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N and Ge3N4, phosphides such as NiP2, FeP2 and CoP3, and fluorides such as FeF3 and BiF3.

[0215] As the conductive additive and binder that can be contained in the negative electrode active material layer, the same materials as the conductive additive and binder that can be contained in the positive electrode active material layer can be used.

[0216] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, but it is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0217] [Electrolyte] The electrolytic solution contains a solvent and an electrolyte. The solvent for the electrolytic solution is preferably an aprotic organic solvent, such as 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, or sultone, or any combination and ratio of two or more of these.

[0218] By using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the secondary battery from exploding or catching fire even if the internal temperature rises due to an internal short circuit or overcharging. Ionic liquids are composed of cations and anions, including organic cations and anions. 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.

[0219] Examples of the electrolyte to be dissolved in the solvent include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B 10 Cl 10 , Li2B 12 Cl 12 Lithium salts such as LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, etc. can be used alone or in any combination and ratio of two or more of these.

[0220] The electrolyte used in the secondary battery is preferably a highly purified electrolyte with a low content of granular dust and 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, preferably 0.1% or less, and more preferably 0.01% or less.

[0221] Additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalato)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile may be added to the electrolyte solution. The concentration of the added material may be, for example, 0.1 wt% to 5 wt% of the total solvent.

[0222] A polymer gel electrolyte in which a polymer is swollen with an electrolytic solution may also be used.

[0223] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0224] Examples of polymers that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel.

[0225] Examples of polymers that can be used include polymers having a polyalkylene oxide structure, such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0226] Instead of an electrolyte solution, a solid electrolyte containing inorganic materials such as sulfides or oxides, or a solid electrolyte containing polymeric materials such as polyethylene oxide (PEO) can be used. When a solid electrolyte is used, the installation of a separator or spacer is unnecessary. In addition, since the entire battery can be solidified, there is no risk of leakage, dramatically improving safety.

[0227] [Separator] The secondary battery preferably has a separator. The separator may be made of, for example, paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, or polyurethane. The separator is preferably processed into an envelope shape and disposed so as to encase either the positive electrode or the negative electrode.

[0228] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials that can be used include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials that can be used include PVDF and polytetrafluoroethylene. Examples of polyamide materials that can be used include nylon and aramid (meta-aramid, para-aramid).

[0229] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging 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.

[0230] 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.

[0231] 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.

[0232] [Outer casing] As the outer casing of the secondary battery, for example, a metal material such as aluminum or a resin material can be used. Also, a film-shaped outer casing can be used. As the film, for example, on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, polyamide, etc., a metal thin film with excellent flexibility such as aluminum, stainless steel, copper, nickel, etc. is provided, and further on the metal thin film, an insulating synthetic resin film such as a polyamide-based resin or a polyester-based resin is provided as the outer surface of the outer casing, and a three-layer structure film can be used.

[0233] [Charging and discharging method] The charging and discharging of the secondary battery can be performed, for example, as follows.

[0234] [CC charging] First, as one of the charging methods, constant current (CC: Constant current) charging will be described. CC charging is a charging method in which a constant current is passed through the secondary battery throughout the charging period and the charging is stopped when a predetermined voltage is reached. Assume the secondary battery to be an equivalent circuit of an internal resistance R and a secondary battery capacity C as shown in Fig. 12A. In this case, the secondary battery voltage V B is the sum of the voltage V R across the internal resistance R and the voltage V C across the secondary battery capacity C.

[0235] During CC charging, as shown in Fig. 12A, the switch is turned on and a constant current I flows through the secondary battery. During this time, since the current I is constant, according to Ohm's law of V R =R×I, the voltage V R across the internal resistance R is also constant. On the other hand, the voltage V C across the secondary battery capacity C increases with time. Therefore, the secondary battery voltage V B increases with time.

[0236] And the secondary battery voltage V B When it reaches a predetermined voltage, for example, 4.3V, charging stops. When CC charging stops, as shown in Fig. 12B, the switch turns off and the current I = 0. Therefore, the voltage V R across the internal resistance R becomes 0V. Therefore, the secondary battery voltage V B drops.

[0237] Examples of the secondary battery voltage V B and the charging current during CC charging and after CC charging stops are shown in Fig. 12C. The secondary battery voltage V B which was rising during CC charging, is shown to drop slightly after CC charging stops.

[0238] <CCCV Charging> Next, constant current constant voltage (CCCV) charging, which is a charging method different from the above, will be described. CCCV charging first charges up to a predetermined voltage by CC charging, and then charges until the current flowing during CV (constant voltage) charging decreases, specifically until it reaches the termination current value.

[0239] During CC charging, as shown in Fig. 13A, the switch of the constant current power supply is on and the switch of the constant voltage power supply is off, and a constant current I flows into the secondary battery. During this period, since the current I is constant, according to Ohm's law of V R =R×I, the voltage V R across the internal resistance R is also constant. On the other hand, the voltage V C across the secondary battery capacity C rises with the passage of time. Therefore, the secondary battery voltage V B rises with the passage of time.

[0240] And when the secondary battery voltage V B reaches a predetermined voltage, for example, 4.3V, it switches from CC charging to CV charging. During CV charging, as shown in Fig. 13B, the switch of the constant voltage power supply is on and the switch of the constant current power supply is off, and the secondary battery voltage V B becomes constant. On the other hand, the voltage V across the secondary battery capacity C C increases with the passage of time. V B = V R + V C Since it is, the voltage V applied to the internal resistance R R decreases with the passage of time. The voltage V applied to the internal resistance R R As the voltage V applied to the internal resistance R decreases, R According to Ohm's law of V = R×I, the current I flowing through the secondary battery also decreases.

[0241] And when the current I flowing through the secondary battery reaches a predetermined current, for example, a current equivalent to 0.01C, charging is stopped. When CCCV charging is stopped, as shown in Fig. 13C, all switches turn off and the current I = becomes 0. Therefore, the voltage V applied to the internal resistance R R becomes 0V. However, since the voltage V applied to the internal resistance R by CV charging R is sufficiently small, even when the voltage drop across the internal resistance R disappears, the secondary battery voltage V B hardly drops.

[0242] Examples of the secondary battery voltage V B and the charging current during CCCV charging and after CCCV charging is stopped are shown in Fig. 13D. It is shown that even when CCCV charging is stopped, the secondary battery voltage V B hardly drops.

[0243] <CC Discharge> Next, CC discharge, which is one of the discharge methods, will be described. CC discharge is a discharge method in which a constant current flows from the secondary battery throughout the discharge period, and the discharge is stopped when the secondary battery voltage V B reaches a predetermined voltage, for example, 2.5V.

[0244] Examples of the secondary battery voltage V B and the discharge current during CC discharge are shown in Fig. 14. As the discharge progresses, it is shown that the secondary battery voltage V B decreases.

[0245] Next, we will explain the discharge rate and charge rate. The discharge rate is the relative ratio of the current during discharge to the battery capacity, and is expressed in units of C. For a battery with a rated capacity of X [Ah], the current equivalent to 1C is X [A]. If the battery is discharged at a current of 2X [A], it is said to have been discharged at 2C, and if the battery is discharged at a current of 0.2X [A], it is said to have been discharged at 0.2C. The same applies to the charge rate; if the battery is charged at a current of 2X [A], it is said to have been charged at 2C, and if the battery is charged at a current of 0.2X [A], it is said to have been charged at 0.2C.

[0246] (Fourth embodiment) In this embodiment, an example of the shape of a secondary battery including the positive electrode active material 100 described in the previous embodiment will be described. The description in the previous embodiment can be referred to for materials used in the secondary battery described in this embodiment.

[0247] [Coin-type secondary battery] First, an example of a coin-type secondary battery will be described. Fig. 15A is an external view of a coin-type (single-layer flat) secondary battery, and Fig. 15B is a cross-sectional view thereof.

[0248] In a coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector.

[0249] It is to be noted that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 each only need to have an active material layer formed on one side.

[0250] Positive electrode can 301 and negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, it is preferable to coat them with nickel, aluminum, or the like. Positive electrode can 301 is electrically connected to positive electrode 304, and negative electrode can 302 is electrically connected to negative electrode 307.

[0251] These negative electrode 307, positive electrode 304, and separator 310 are impregnated with an electrolyte, and as shown in FIG. 15B, the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downwards, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303 to produce a coin-type secondary battery 300.

[0252] By using the positive electrode active material described in the above embodiment for the positive electrode 304, the coin-type secondary battery 300 can have a high capacity and excellent cycle characteristics.

[0253] Here, we will explain the current flow during charging of a secondary battery using Figure 15C. When a lithium-based secondary battery is considered as a closed circuit, the movement of lithium ions and the flow of current are in the same direction. In lithium-based secondary batteries, the anode (positive electrode) and cathode (negative electrode) are interchanged during charging and discharging, and the oxidation and reduction reactions are alternated. Therefore, the electrode with the higher reaction potential is called the positive electrode, and the electrode with the lower reaction potential is called the negative electrode. Therefore, in this specification, whether during charging, discharging, when a reverse pulse current is applied, or when a charging current is applied, the positive electrode will be called the "positive electrode" or "+ electrode," and the negative electrode will be called the "negative electrode" or "- electrode." Using the terms anode (positive electrode) and cathode (negative electrode), which are related to oxidation and reduction reactions, could lead to confusion because their meanings are reversed during charging and discharging. Therefore, the terms anode (positive electrode) and cathode (negative electrode) will not be used in this specification. If the terms anode and cathode are used, it should be clearly stated whether they are used during charging or discharging, and whether they correspond to the positive or negative pole.

[0254] 15C is connected to a charger to charge the secondary battery 300. As the charging of the secondary battery 300 progresses, the potential difference between the electrodes increases.

[0255] [Cylindrical secondary battery] Next, an example of a cylindrical secondary battery will be described with reference to Fig. 16. Fig. 16A shows an external view of a cylindrical secondary battery 600. Fig. 16B is a schematic diagram showing a cross section of the cylindrical secondary battery 600. As shown in Fig. 16B, the cylindrical secondary battery 600 has a positive electrode cap (battery lid) 601 on the top surface, and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0256] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. Although not shown, the battery element is wound around a center pin. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of a metal that is corrosion-resistant to the electrolyte, such as nickel, aluminum, or titanium, or an alloy of these or an alloy of these with another metal (e.g., stainless steel). To prevent corrosion by the electrolyte, the battery can 602 is preferably coated with nickel, aluminum, or the like. Inside the battery can 602, the wound battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. A nonaqueous electrolyte (not shown) is poured into the battery can 602, in which the battery element is provided. The nonaqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0257] Because the positive and negative electrodes used in cylindrical storage batteries are wound, it is preferable to form active materials on both sides of the current collector. A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be made of a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 612, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 612 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature rises, and the increased resistance limits the amount of current to prevent abnormal heat generation. The PTC element can be made of barium titanate (BaTiO3)-based semiconductor ceramics or the like.

[0258] 16C, a module 615 may be configured by sandwiching a plurality of secondary batteries 600 between conductive plates 613 and 614. The plurality of secondary batteries 600 may be connected in parallel, in series, or in series after being connected in parallel. By configuring a module 615 having a plurality of secondary batteries 600, a large amount of power can be extracted.

[0259] FIG. 16D is a top view of module 615. For clarity, conductive plate 613 is shown with dotted lines. As shown in FIG. 16D, module 615 may have conductors 616 that electrically connect multiple secondary batteries 600. A conductive plate can be superimposed on the conductors 616. Furthermore, a temperature control device 617 may be provided between multiple secondary batteries 600. When a secondary battery 600 overheats, it can be cooled by the temperature control device 617, and when a secondary battery 600 is too cold, it can be heated by the temperature control device 617. This makes the performance of module 615 less susceptible to the influence of the outside air temperature. It is preferable that the heat medium in temperature control device 617 is insulating and non-flammable.

[0260] By using the positive electrode active material described in the above embodiment for the positive electrode 604, the cylindrical secondary battery 600 can have high capacity and excellent cycle characteristics.

[0261] [Example of secondary battery structure] Another structural example of the secondary battery will be described with reference to FIGS.

[0262] 17A and 17B are diagrams showing the appearance of the battery pack. A secondary battery 913 is connected to an antenna 914 via a circuit board 900. A label 910 is attached to the secondary battery 913. The circuit board 900 is fixed to the label 910 with a sticker 915. Furthermore, as shown in FIG. 17B, the secondary battery 913 is connected to a terminal 951 and a terminal 952.

[0263] The circuit board 900 has a terminal 911 and a circuit 912. The terminal 911 is connected to a terminal 951, a terminal 952, an antenna 914, and the circuit 912. Note that a plurality of terminals 911 may be provided, and each of the plurality of terminals 911 may be used as a control signal input terminal, a power supply terminal, or the like.

[0264] The circuit 912 may be provided on the back surface of the circuit board 900. The antenna 914 is not limited to a coil shape and may be, for example, a wire shape or a plate shape. Also, antennas such as a planar antenna, an aperture antenna, a traveling wave antenna, an EH antenna, a magnetic field antenna, and a dielectric antenna may be used. Alternatively, the antenna 914 may be a flat conductor. This flat conductor can function as one of the conductors for electric field coupling. In other words, the antenna 914 may function as one of the two conductors of a capacitor. This allows power to be exchanged not only by electromagnetic fields and magnetic fields but also by electric fields.

[0265] The battery pack has a layer 916 between the antenna 914 and the secondary battery 913. The layer 916 has a function of, for example, shielding an electromagnetic field generated by the secondary battery 913. The layer 916 can be made of, for example, a magnetic material.

[0266] The structure of the battery pack is not limited to that shown in FIG.

[0267] For example, as shown in Figures 18A1 and 18A2, an antenna may be provided on each of a pair of opposing surfaces of secondary battery 913 shown in Figures 17A and 17B. Figure 18A1 is an external view showing one of the pair of surfaces, and Figure 18A2 is an external view showing the other of the pair of surfaces. Note that the description of the secondary battery shown in Figures 17A and 17B can be used as appropriate for the same parts as those of the secondary battery shown in Figures 17A and 17B.

[0268] 18A1, an antenna 914 is provided on one of a pair of surfaces of a secondary battery 913 with a layer 916 sandwiched therebetween, and as shown in Fig. 18A2, an antenna 918 is provided on the other of the pair of surfaces of the secondary battery 913 with a layer 917 sandwiched therebetween. The layer 917 has a function of, for example, being able to shield an electromagnetic field caused by the secondary battery 913. For example, a magnetic material can be used as the layer 917.

[0269] The above structure allows the sizes of both the antenna 914 and the antenna 918 to be increased. The antenna 918 has a function of, for example, performing data communication with an external device. For example, an antenna having a shape applicable to the antenna 914 can be used as the antenna 918. As a communication method between the secondary battery and other devices via the antenna 918, a response method that can be used between the secondary battery and other devices, such as NFC (near field wireless communication), can be used.

[0270] Alternatively, as shown in Fig. 18B1, a display device 920 may be provided on the secondary battery 913 shown in Figs. 17A and 17B. The display device 920 is electrically connected to the terminal 911. Note that the label 910 does not need to be provided on the portion where the display device 920 is provided. Note that the description of the secondary battery shown in Figs. 17A and 17B can be used as appropriate for the same portions as those of the secondary battery shown in Figs. 17A and 17B.

[0271] The display device 920 may display, for example, an image indicating whether charging is in progress or an image indicating the amount of stored power. For example, electronic paper, a liquid crystal display device, an electroluminescence (EL) display device, or the like can be used as the display device 920. For example, by using electronic paper, the power consumption of the display device 920 can be reduced.

[0272] 18B2, a sensor 921 may be provided in the secondary battery 913 shown in Figures 17A and 17B. The sensor 921 is electrically connected to the terminal 911 via a terminal 922. Note that the description of the secondary battery shown in Figures 17A and 17B can be used as appropriate for the same parts as those of the secondary battery shown in Figures 17A and 17B.

[0273] The sensor 921 may have a function of measuring, for example, displacement, position, speed, acceleration, angular velocity, rotation speed, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, power, radiation, flow rate, humidity, gradient, vibration, odor, or infrared ray. By providing the sensor 921, for example, data indicating the environment in which the secondary battery is placed (such as temperature) can be detected and stored in the memory in the circuit 912.

[0274] Furthermore, an example of the structure of the secondary battery 913 will be described with reference to FIGS.

[0275] A secondary battery 913 shown in Fig. 19A has a wound body 950 in which terminals 951 and 952 are provided inside a housing 930. The wound body 950 is impregnated with an electrolyte inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that in Fig. 19A, for convenience, the housing 930 is shown separated, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (such as aluminum) or a resin material.

[0276] 19B, the housing 930 shown in Fig. 19A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 19B has housings 930a and 930b bonded together, and a wound body 950 is provided in the area surrounded by the housings 930a and 930b.

[0277] The housing 930a can be made of an insulating material such as organic resin. In particular, using a material such as organic resin on the surface on which the antenna is formed can prevent the secondary battery 913 from blocking the electric field. Note that if the electric field blocking effect of the housing 930a is small, an antenna such as the antenna 914 may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material.

[0278] 20 shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the separator 933 sandwiched therebetween, and the laminated sheet is wound. Note that a plurality of layers of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0279] 17 via one of the terminal 951 and the terminal 952. The positive electrode 932 is connected to the terminal 911 shown in FIG.

[0280] By using the positive electrode active material described in the above embodiment for the positive electrode 932, the secondary battery 913 can have high capacity and excellent cycle characteristics.

[0281] [Laminated secondary battery] Next, examples of laminated secondary batteries will be described with reference to Figures 21 to 27. If a laminated secondary battery has a flexible configuration, and is mounted in an electronic device having at least a flexible portion, the secondary battery can also be bent in accordance with deformation of the electronic device.

[0282] A laminated secondary battery 980 will be described using Fig. 21. The laminated secondary battery 980 has a wound body 993 shown in Fig. 21A. The wound body 993 has a negative electrode 994, a positive electrode 995, and a separator 996. Similar to the wound body 950 described in Fig. 20, the wound body 993 is formed by stacking the negative electrode 994 and the positive electrode 995 on top of each other with the separator 996 sandwiched therebetween, and winding the laminated sheet.

[0283] The number of layers of the negative electrode 994, the positive electrode 995, and the separator 996 may be appropriately designed depending on the required capacity and element volume. The negative electrode 994 is connected to a negative electrode current collector (not shown) via one of the lead electrodes 997 and 998, and the positive electrode 995 is connected to a positive electrode current collector (not shown) via the other of the lead electrodes 997 and 998.

[0284] 21B, a film 981 serving as an exterior body and a film 982 having a recess are bonded together by thermocompression or the like to form a space, and the above-described wound body 993 is stored in the space, thereby producing a secondary battery 980 as shown in Fig. 21C. The wound body 993 has lead electrodes 997 and 998, and is impregnated with an electrolyte solution between the film 981 and the film 982 having a recess.

[0285] For example, a metal material such as aluminum or a resin material can be used for film 981 and film 982 having recesses. If a resin material is used as the material for film 981 and film 982 having recesses, film 981 and film 982 having recesses can be deformed when an external force is applied, and a flexible storage battery can be produced.

[0286] Although an example using two films is shown in FIGS. 21B and 21C, a space may be formed by folding one film, and the wound body 993 described above may be housed in that space.

[0287] By using the positive electrode active material described in the above embodiment for the positive electrode 995, the secondary battery 980 can have a high capacity and excellent cycle characteristics.

[0288] Figure 21 describes an example of a secondary battery 980 having a wound body in a space formed by a film that serves as an outer casing. However, as shown in Figure 22, for example, a secondary battery may also be used that has multiple rectangular positive electrodes, separators, and negative electrodes in a space formed by a film that serves as an outer casing.

[0289] 22A includes a positive electrode 503 having a positive electrode current collector 501 and a positive electrode active material layer 502, a negative electrode 506 having a negative electrode current collector 504 and a negative electrode active material layer 505, a separator 507, an electrolyte 508, and an exterior body 509. The separator 507 is disposed between the positive electrode 503 and the negative electrode 506 provided in the exterior body 509. The exterior body 509 is filled with the electrolyte 508. The electrolyte solution described in Embodiment 2 can be used as the electrolyte solution 508.

[0290] 22A , the positive electrode current collector 501 and the negative electrode current collector 504 also serve as terminals for electrical contact with the outside. Therefore, the positive electrode current collector 501 and the negative electrode current collector 504 may be arranged so as to be partially exposed to the outside from the exterior body 509. Alternatively, the positive electrode current collector 501 and the negative electrode current collector 504 may not be exposed to the outside from the exterior body 509, and a lead electrode may be used to ultrasonically bond the positive electrode current collector 501 or the negative electrode current collector 504 to the outside, thereby exposing the lead electrode to the outside.

[0291] In the laminated secondary battery 500, the exterior body 509 can be a three-layer laminate film having a highly flexible metal thin film made of aluminum, stainless steel, copper, nickel, or the like provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and further having an insulating synthetic resin film made of polyamide-based resin, polyester-based resin, or the like provided on the metal thin film as the outer surface of the exterior body.

[0292] Fig. 22B shows an example of the cross-sectional structure of laminated secondary battery 500. For simplicity, Fig. 22A shows an example configured with two current collectors, but in reality, as shown in Fig. 22B, it is configured with multiple electrode layers.

[0293] In FIG. 22B, the number of electrode layers is 16 as an example. Note that even if the number of electrode layers is 16, the secondary battery 500 remains flexible. FIG. 22B shows a structure with a total of 16 layers, including eight layers of negative electrode current collectors 504 and eight layers of positive electrode current collectors 501. Note that FIG. 22B also shows a cross section of the negative electrode lead-out portion, in which eight layers of negative electrode current collectors 504 are ultrasonically bonded. Of course, the number of electrode layers is not limited to 16 and may be more or less. When the number of electrode layers is large, a secondary battery with a larger capacity can be obtained. Furthermore, when the number of electrode layers is small, a secondary battery can be made thinner and have excellent flexibility.

[0294] 23 and 24 show an example of an external view of a laminated secondary battery 500. The battery 500 includes a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0295] FIG. 25A shows an external view of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and a positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. The positive electrode 503 also has a region where the positive electrode current collector 501 is partially exposed (hereinafter referred to as a tab region). The negative electrode 506 has a negative electrode current collector 504, and a negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., a tab region. The areas and shapes of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 25A.

[0296] [Method for manufacturing laminated secondary batteries] Here, an example of a method for manufacturing the laminated secondary battery whose external view is shown in FIG. 23 will be described with reference to FIGS. 25B and 25C.

[0297] First, the negative electrode 506, the separator 507, and the positive electrode 503 are stacked. FIG. 25B shows the stacked negative electrode 506, the separator 507, and the positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0298] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are placed on the exterior body 509 .

[0299] Next, as shown in Fig. 25C, exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of exterior body 509 is joined. For the joining, for example, thermocompression bonding or the like may be used. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of exterior body 509 so that electrolyte 508 can be introduced later.

[0300] Next, electrolyte 508 (not shown) is introduced into the inside of exterior body 509 through an inlet provided in exterior body 509. The introduction of electrolyte 508 is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, laminated secondary battery 500 can be produced.

[0301] By using the positive electrode active material described in the above embodiment for the positive electrode 503, the secondary battery 500 can have a high capacity and excellent cycle characteristics.

[0302] [Bendable secondary battery] Next, an example of a bendable secondary battery will be described with reference to FIGS. 26 and 27. FIG.

[0303] FIG. 26A shows a schematic top view of a bendable secondary battery 250. FIGS. 26B1, 26B2, and 26C are schematic cross-sectional views taken along the lines C1-C2, C3-C4, and A1-A2 in FIG. 26A, respectively. The secondary battery 250 includes an exterior housing 251 and an electrode stack 210 housed within the exterior housing 251. The electrode stack 210 has a structure in which at least a positive electrode 211a and a negative electrode 211b are stacked. A lead 212a electrically connected to the positive electrode 211a and a lead 212b electrically connected to the negative electrode 211b extend outside the exterior housing 251. In addition to the positive electrode 211a and the negative electrode 211b, an electrolyte (not shown) is enclosed within the area surrounded by the exterior housing 251.

[0304] The positive electrode 211a and the negative electrode 211b of the secondary battery 250 will be described with reference to Fig. 27. Fig. 27A is a perspective view illustrating the stacking order of the positive electrode 211a, the negative electrode 211b, and the separator 214. Fig. 27B is a perspective view showing the lead 212a and the lead 212b in addition to the positive electrode 211a and the negative electrode 211b.

[0305] 27A, secondary battery 250 has a plurality of rectangular positive electrodes 211a, a plurality of rectangular negative electrodes 211b, and a plurality of separators 214. Positive electrode 211a and negative electrode 211b each have a protruding tab portion and a portion other than the tab. A positive electrode active material layer is formed on one surface of positive electrode 211a in the portion other than the tab, and a negative electrode active material layer is formed on one surface of negative electrode 211b in the portion other than the tab.

[0306] The positive electrode 211a and the negative electrode 211b are stacked so that the surfaces of the positive electrode 211a on which the positive electrode active material layer is not formed and the surfaces of the negative electrode 211b on which the negative electrode active material layer is not formed are in contact with each other.

[0307] A separator 214 is provided between the surface of the positive electrode 211a on which the positive electrode active material is formed and the surface of the negative electrode 211b on which the negative electrode active material is formed. In Fig. 27, the separator 214 is indicated by a dotted line for ease of viewing.

[0308] 27B, the positive electrodes 211a and the lead 212a are electrically connected at a joint 215a, and the negative electrodes 211b and the lead 212b are electrically connected at a joint 215b.

[0309] Next, the exterior body 251 will be described with reference to FIGS. 26B1, 26B2, 26C, and 26D.

[0310] The exterior body 251 has a film-like shape and is folded in two to sandwich the positive electrode 211a and the negative electrode 211b. The exterior body 251 has a folded portion 261, a pair of sealing portions 262, and a sealing portion 263. The pair of sealing portions 262 are provided to sandwich the positive electrode 211a and the negative electrode 211b, and can also be called side seals. The sealing portion 263 has a portion that overlaps with the lead 212a and the lead 212b, and can also be called a top seal.

[0311] The exterior body 251 preferably has a wave shape in which ridge lines 271 and valley lines 272 are alternately arranged in the portions overlapping the positive electrode 211a and the negative electrode 211b. Furthermore, the seal portions 262 and 263 of the exterior body 251 are preferably flat.

[0312] Fig. 26B1 is a cross section taken at a portion overlapping with ridge line 271, and Fig. 26B2 is a cross section taken at a portion overlapping with valley line 272. Fig. 26B1 and Fig. 26B2 both correspond to widthwise cross sections of secondary battery 250, positive electrode 211a, and negative electrode 211b.

[0313] Here, the distance La is defined as the distance between the widthwise ends of the positive electrode 211a and the negative electrode 211b, i.e., the ends of the positive electrode 211a and the negative electrode 211b, and the seal portion 262. When the secondary battery 250 is deformed, such as by bending, the positive electrode 211a and the negative electrode 211b deform so as to be displaced from each other in the longitudinal direction, as described below. In this case, if the distance La is too short, the exterior body 251 may rub strongly against the positive electrode 211a and the negative electrode 211b, resulting in damage to the exterior body 251. In particular, if the metal film of the exterior body 251 is exposed, the metal film may be corroded by the electrolyte. Therefore, it is preferable to set the distance La as long as possible. On the other hand, if the distance La is made too large, the volume of the secondary battery 250 increases.

[0314] It is preferable that the distance La between the positive electrode 211a and the negative electrode 211b and the seal portion 262 is increased as the total thickness of the stacked positive electrode 211a and the negative electrode 211b increases.

[0315] More specifically, when the total thickness of the stacked positive electrode 211a, negative electrode 211b, and separator 214 (not shown) is t, the distance La is preferably 0.8 to 3.0 times, more preferably 0.9 to 2.5 times, and even more preferably 1.0 to 2.0 times the thickness t. By setting the distance La in this range, a compact battery with high reliability against bending can be realized.

[0316] When the distance between the pair of seal portions 262 is distance Lb, it is preferable to set distance Lb sufficiently larger than the width of the positive electrode 211a and the negative electrode 211b (here, width Wb of the negative electrode 211b). This allows parts of the positive electrode 211a and the negative electrode 211b to shift in the width direction even if the positive electrode 211a and the negative electrode 211b come into contact with the exterior body 251 when deformation such as repeated bending is applied to the secondary battery 250, thereby effectively preventing the positive electrode 211a and the negative electrode 211b from rubbing against the exterior body 251.

[0317] For example, it is preferable that the difference between the distance Lb between the pair of seal portions 262 and the width Wb of the negative electrode 211b is 1.6 to 6.0 times, preferably 1.8 to 5.0 times, and more preferably 2.0 to 4.0 times the thickness t of the positive electrode 211a and the negative electrode 211b.

[0318] In other words, it is preferable that the distance Lb, the width Wb, and the thickness t satisfy the relationship of the following mathematical formula 1.

[0319]

number

[0320] Here, a satisfies the range of 0.8 to 3.0, preferably 0.9 to 2.5, and more preferably 1.0 to 2.0.

[0321] 26C is a cross section including lead 212a, and corresponds to a cross section in the longitudinal direction of secondary battery 250, positive electrode 211a, and negative electrode 211b. As shown in FIG. 26C, it is preferable that a space 273 be formed between exterior body 251 and the ends of positive electrode 211a and negative electrode 211b in the longitudinal direction at bent portion 261.

[0322] Fig. 26D shows a schematic cross-sectional view of the bent secondary battery 250. Fig. 26D corresponds to the cross section taken along the cutting line B1-B2 in Fig. 26A.

[0323] When the secondary battery 250 is bent, a portion of the exterior body 251 located on the outside of the bend expands, and another portion located on the inside contracts. More specifically, the portion located on the outside of the exterior body 251 deforms so that the wave amplitude becomes smaller and the wave period becomes larger. On the other hand, the portion located on the inside of the exterior body 251 deforms so that the wave amplitude becomes larger and the wave period becomes smaller. In this way, the deformation of the exterior body 251 relieves the stress applied to the exterior body 251 due to bending, so the material that constitutes the exterior body 251 itself does not need to expand or contract. As a result, the exterior body 251 does not break, and the secondary battery 250 can be bent with a small force.

[0324] As shown in FIG. 26D, when secondary battery 250 is bent, positive electrode 211a and negative electrode 211b are misaligned relative to each other. At this time, because one end of each of the stacked positive electrodes 211a and negative electrodes 211b on the sealing portion 263 side is fixed by fixing member 217, the amount of misalignment increases toward bent portion 261. This reduces stress applied to positive electrode 211a and negative electrode 211b, eliminating the need for positive electrode 211a and negative electrode 211b themselves to expand and contract. As a result, secondary battery 250 can be bent without damaging positive electrode 211a and negative electrode 211b.

[0325] By providing the space 273 between the positive electrode 211a and the negative electrode 211b and the exterior body 251, the positive electrode 211a and the negative electrode 211b located on the inner side when bent can be relatively displaced without coming into contact with the exterior body 251.

[0326] 26 and 27 is a battery that is resistant to damage to the exterior body, the positive electrode 211a, and the negative electrode 211b, etc., even when repeatedly bent and stretched, and the battery characteristics are also resistant to deterioration. By using the positive electrode active material described in the previous embodiment for the positive electrode 211a of the secondary battery 250, it is possible to obtain a battery with even better cycle characteristics.

[0327] (Embodiment 5) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted on an electronic device will be described.

[0328] 28A to 28G show examples of electronic devices incorporating the bendable secondary battery described in part of Embodiment 4. Examples of electronic devices that use the bendable secondary battery include television devices (also called televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines.

[0329] A secondary battery having a flexible shape can be incorporated into the inner or outer wall of a house or building, or along the curved surface of the interior or exterior of an automobile.

[0330] 28A 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 life can be provided.

[0331] FIG. 28B 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 installed inside is also bent. FIG. 28C 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 a lead electrode electrically connected to the current collector. For example, the current collector is copper foil, and a portion of the current collector is alloyed with gallium to improve adhesion with the active material layer in contact with the current collector, resulting in a configuration with high reliability when the secondary battery 7407 is bent.

[0332] FIG. 28D shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display unit 7102, operation buttons 7103, and a secondary battery 7104. FIG. 28E 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.

[0333] 28F 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.

[0334] 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.

[0335] The display surface of the display portion 7202 is curved, and a display can be performed along the curved display surface. The display portion 7202 is also provided with a touch sensor, and can be operated by touching the screen with a finger, a stylus, or the like. For example, an application can be started by touching an icon 7207 displayed on the display portion 7202.

[0336] The operation button 7205 can be provided with 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 mobile information terminal 7200.

[0337] The mobile information terminal 7200 is capable of performing short-range wireless communication according to a communication standard. For example, it is also possible to make a hands-free call by communicating with a wirelessly enabled headset.

[0338] The portable information terminal 7200 includes an input / output terminal 7206 and can directly exchange data with other information terminals via a connector. Charging can also be performed via the input / output terminal 7206. Note that charging may be performed by wireless power supply without using the input / output terminal 7206.

[0339] The display portion 7202 of the mobile 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 mobile information terminal with a long life can be provided. For example, the secondary battery 7104 shown in FIG. 28E can be installed in a curved state inside the housing 7201 or in a bendable state inside the band 7203.

[0340] The portable information terminal 7200 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, or other human body sensor, a touch sensor, a pressure sensor, or an acceleration sensor.

[0341] 28G 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 function as a portable information terminal.

[0342] 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.

[0343] The display device 7300 has an input / output terminal and can directly exchange data with other information terminals via a connector. Charging can also be performed via the input / output terminal. Note that charging may also be performed by wireless power supply without using the input / output terminal.

[0344] 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.

[0345] An example in which the secondary battery having good cycle characteristics shown in the above embodiment is mounted in an electronic device will be described with reference to FIGS. 28H, 29, and 30. FIG.

[0346] 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 small, lightweight, and large-capacity secondary battery with a stick shape is desired, taking into account ease of holding by users.

[0347] FIG. 28H is a perspective view of a device also known as a tobacco-containing smoking device (electronic cigarette). In FIG. 28H, electronic cigarette 7500 includes atomizer 7501 including a heating element, secondary battery 7504 that supplies power to the atomizer, and cartridge 7502 including a liquid supply bottle, a sensor, and the like. To enhance safety, a protection circuit that prevents overcharging and overdischarging of secondary battery 7504 may be electrically connected to secondary battery 7504. Secondary battery 7504 shown in FIG. 28H has external terminals so that it can be connected to a charging device. Because secondary battery 7504 is the tip portion when held, it is desirable that its total length be short and its weight be light. The secondary battery of one embodiment of the present invention has high 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.

[0348] Next, an example of a foldable tablet terminal is shown in FIGS. 29A and 29B. The tablet terminal 9600 shown in FIGS. 29A and 29B includes a housing 9630a, a housing 9630b, a movable portion 9640 connecting the housings 9630a and 9630b, a display portion 9631 having display portions 9631a and 9631b, switches 9625 to 9627, a fastener 9629, and an operation switch 9628. A flexible panel can be used for the display portion 9631 to provide a tablet terminal with a larger display area. FIG. 29A shows the tablet terminal 9600 in an open state, and FIG. 29B shows the tablet terminal 9600 in a closed state.

[0349] The tablet terminal 9600 includes a power storage unit 9635 inside the housing 9630a and the housing 9630b. The power storage unit 9635 passes through the movable portion 9640 and is provided across the housing 9630a and the housing 9630b.

[0350] The entire or part of the display portion 9631 can be a touch panel area, and data can be input by touching an image including an icon, text, an input form, etc. displayed in the area. For example, keyboard buttons may be displayed on the entire surface of the display portion 9631a on the housing 9630a side, and information such as text and images may be displayed on the display portion 9631b on the housing 9630b side.

[0351] A keyboard may be displayed on the display portion 9631b of the housing 9630b, and information such as text and images may be displayed on the display portion 9631a of the housing 9630a. Alternatively, a keyboard display switch button of a touch panel may be displayed on the display portion 9631, and the keyboard may be displayed on the display portion 9631 by touching the button with a finger, a stylus, or the like.

[0352] A touch input can be simultaneously made to the touch panel area of ​​the display portion 9631a on the housing 9630a side and the touch panel area of ​​the display portion 9631b on the housing 9630b side.

[0353] The switches 9625 to 9627 may be interfaces capable of switching various functions in addition to interfaces for operating the tablet terminal 9600. For example, at least one of the switches 9625 to 9627 may function as a switch for turning the tablet terminal 9600 on and off. Furthermore, for example, at least one of the switches 9625 to 9627 may have a function for switching the display orientation, such as portrait or landscape, or a function for switching between monochrome and color display. Furthermore, for example, at least one of the switches 9625 to 9627 may have a function for adjusting the brightness of the display unit 9631. The brightness of the display unit 9631 can be optimized depending on the amount of external light detected by an optical sensor built into the tablet terminal 9600 during use. The tablet terminal may also have built-in not only an optical sensor but also other detection devices, such as a gyroscope, an acceleration sensor, or other sensors for detecting tilt.

[0354] 29A shows an example in which the display area of ​​the display portion 9631a on the housing 9630a side and the display portion 9631b on the housing 9630b side are substantially the same, but the display areas of the display portion 9631a and the display portion 9631b are not particularly limited, and one size and the other size may be different, and the display quality may also be different. For example, one may be a display panel that can display at a higher resolution than the other.

[0355] 29B shows a tablet terminal 9600 folded in half, and the tablet terminal 9600 includes a housing 9630, a solar cell 9633, and a charge / discharge control circuit 9634 including a DC-DC converter 9636. The power storage unit 9635 is a power storage unit according to one embodiment of the present invention.

[0356] As described above, the tablet terminal 9600 can be folded in half, and therefore, the housing 9630a and the housing 9630b can be folded together when not in use. By folding, the display portion 9631 can be protected, thereby improving durability of the tablet terminal 9600. Furthermore, the power storage unit 9635 using the secondary battery of one embodiment of the present invention has high capacity and favorable cycle characteristics, and therefore, the tablet terminal 9600 can be used for a long period of time.

[0357] In addition, the tablet terminal 9600 shown in Figures 29A and 29B can have the function of displaying various information (still images, videos, text images, etc.), the function of displaying a calendar, date or time on the display unit, a touch input function of touch inputting or editing information displayed on the display unit, and the function of controlling processing using various software (programs), etc.

[0358] A solar cell 9633 attached to the surface of the tablet terminal 9600 can supply power to a touch panel, a display unit, a video signal processor, or the like. The solar cell 9633 can be provided on one or both sides of the housing 9630, and can be configured to efficiently charge the power storage unit 9635. Use of a lithium-ion battery as the power storage unit 9635 has the advantage of enabling miniaturization.

[0359] The configuration and operation of the charge / discharge control circuit 9634 shown in Fig. 29B will be described with reference to a block diagram in Fig. 29C. Fig. 29C shows a solar cell 9633, a power storage unit 9635, a DC-DC converter 9636, a converter 9637, switches SW1 to SW3, and a display unit 9631. The power storage unit 9635, the DC-DC converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the charge / discharge control circuit 9634 shown in Fig. 29B.

[0360] First, an example of operation when power is generated by the solar cell 9633 using external light will be described. The power generated by the solar cell is stepped up or down by a DC-DC converter 9636 to a voltage for charging a power storage unit 9635. When power from the solar cell 9633 is used to operate the display unit 9631, a switch SW1 is turned on, and the converter 9637 steps up or steps down the voltage to a voltage required for the display unit 9631. When no display is to be performed on the display unit 9631, SW1 is turned off and SW2 is turned on to charge the power storage unit 9635.

[0361] Note that the solar cell 9633 is shown as an example of a power generating means, but is not particularly limited thereto, and the power storage unit 9635 may be charged by other power generating means such as a piezoelectric element (piezo element) or a thermoelectric conversion element (Peltier element). For example, a contactless power transmission module that transmits and receives power wirelessly (contactlessly) for charging, or a combination of other charging means may be used.

[0362] FIG. 30 illustrates an example of another electronic device. In FIG. 30, a display device 8000 is an example of an electronic device using a secondary battery 8004 according to one embodiment of the present invention. Specifically, the display device 8000 corresponds to a display device for receiving TV broadcasts and includes a housing 8001, a display portion 8002, a speaker portion 8003, a secondary battery 8004, and the like. The secondary battery 8004 according to one embodiment of the present invention is provided inside the housing 8001. The display device 8000 can receive power from a commercial power source or can use power stored in the secondary battery 8004. Therefore, even when power cannot be supplied from the commercial power source due to a power outage or the like, the display device 8000 can be used by using the secondary battery 8004 according to one embodiment of the present invention as an uninterruptible power source.

[0363] The display unit 8002 can be a liquid crystal display device, a light-emitting device in which each pixel has a light-emitting element such as an organic EL element, an electrophoretic display device, a semiconductor display device such as a DMD (Digital Micromirror Device), a PDP (Plasma Display Panel), or an FED (Field Emission Display).

[0364] The display device includes all display devices for displaying information, such as those for receiving TV broadcasts, those for personal computers, and those for displaying advertisements.

[0365] 30 , a stationary lighting device 8100 is an example of an electronic device using a secondary battery 8103 according to one embodiment of the present invention. Specifically, the lighting device 8100 includes a housing 8101, a light source 8102, a secondary battery 8103, and the like. Although FIG. 30 illustrates the case where the secondary battery 8103 is provided inside a ceiling 8104 on which the housing 8101 and the light source 8102 are installed, the secondary battery 8103 may be provided inside the housing 8101. The lighting device 8100 can receive power from a commercial power source or can use power stored in the secondary battery 8103. Therefore, even when power cannot be supplied from the commercial power source due to a power outage or the like, the lighting device 8100 can be used by using the secondary battery 8103 according to one embodiment of the present invention as an uninterruptible power supply.

[0366] Note that although Figure 30 illustrates an example of a stationary lighting device 8100 provided on the ceiling 8104, the secondary battery of one embodiment of the present invention can also be used in a stationary lighting device provided in places other than the ceiling 8104, such as a side wall 8105, a floor 8106, or a window 8107, or can also be used in a tabletop lighting device.

[0367] An artificial light source that artificially obtains light using electric power can be used as the light source 8102. Specifically, examples of the artificial light source include discharge lamps such as incandescent lamps and fluorescent lamps, and light-emitting elements such as LEDs and organic EL elements.

[0368] 30 , an air conditioner including an indoor unit 8200 and an outdoor unit 8204 is an example of an electronic device using a secondary battery 8203 of one embodiment of the present invention. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, a secondary battery 8203, and the like. Although FIG. 30 illustrates the case where the secondary battery 8203 is provided in the indoor unit 8200, the secondary battery 8203 may be provided in the outdoor unit 8204. Alternatively, the secondary battery 8203 may be provided in both the indoor unit 8200 and the outdoor unit 8204. The air conditioner can receive power from a commercial power source or can use power stored in the secondary battery 8203. In particular, when the secondary battery 8203 is provided in both the indoor unit 8200 and the outdoor unit 8204, the air conditioner can be used by using the secondary battery 8203 of one embodiment of the present invention as an uninterruptible power supply even when power cannot be supplied from a commercial power source due to a power outage or the like.

[0369] Note that although FIG. 30 illustrates an example of a separate-type air conditioner including an indoor unit and an outdoor unit, a secondary battery according to one embodiment of the present invention can also be used in an integrated-type air conditioner that has the functions of both the indoor unit and the outdoor unit in a single housing.

[0370] 30 , an electric refrigerator-freezer 8300 is an example of an electronic device using a secondary battery 8304 of one embodiment of the present invention. Specifically, the electric refrigerator-freezer 8300 includes a housing 8301, a refrigerator door 8302, a freezer door 8303, a secondary battery 8304, and the like. In FIG. 30 , the secondary battery 8304 is provided inside the housing 8301. The electric refrigerator-freezer 8300 can receive power from a commercial power source or can use power stored in the secondary battery 8304. Therefore, even when power cannot be supplied from the commercial power source due to a power outage or the like, the electric refrigerator-freezer 8300 can be used by using the secondary battery 8304 of one embodiment of the present invention as an uninterruptible power source.

[0371] Among the electronic devices described above, electronic devices such as microwave ovens and other high-frequency heating devices and electric rice cookers require a large amount of power for a short period of time. Therefore, by using a secondary battery according to one embodiment of the present invention as an auxiliary power source for supplementing the power that cannot be supplied by the commercial power source, it is possible to prevent the breaker of the commercial power source from tripping when the electronic device is in use.

[0372] By storing power in the secondary battery during time periods when electronic devices are not in use, particularly during time periods when the ratio of the amount of power actually used to the total amount of power that can be supplied by the commercial power supplier (referred to as the power usage rate) is low, it is possible to prevent the power usage rate from increasing outside of these time periods. For example, in the case of electric refrigerator-freezer 8300, power is stored in secondary battery 8304 during the night when the temperature is low and refrigerator door 8302 and freezer door 8303 are not opened or closed. Then, during the daytime when the temperature rises and refrigerator door 8302 and freezer door 8303 are opened and closed, secondary battery 8304 is used as an auxiliary power source, thereby making it possible to keep the daytime power usage rate low.

[0373] According to one embodiment of the present invention, the cycle characteristics of the secondary battery can be improved, and the reliability can be improved. Furthermore, according to one embodiment of the present invention, a high-capacity secondary battery can be obtained, and therefore the characteristics of the secondary battery can be improved, and therefore the secondary battery itself can be made smaller and lighter. Therefore, by incorporating the secondary battery according to one embodiment of the present invention in the electronic device described in this embodiment, the electronic device can have a longer life and be lighter. This embodiment can be implemented in combination with other embodiments as appropriate.

[0374] (Sixth embodiment) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted on a vehicle will be described.

[0375] By installing a secondary battery in a vehicle, next-generation clean energy vehicles such as hybrid electric vehicles (HEV), electric vehicles (EV), and plug-in hybrid electric vehicles (PHEV) can be realized.

[0376] FIG. 31 illustrates an example of a vehicle using a secondary battery according to one embodiment of the present invention. An automobile 8400 illustrated in FIG. 31A 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. The secondary battery may be arranged in the form of secondary battery modules shown in FIGS. 16C and 16D on the floor of the vehicle. Alternatively, a battery pack including a combination of a plurality of secondary batteries as shown in FIG. 19 may be installed on the floor 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).

[0377] The secondary battery can 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.

[0378] The automobile 8500 shown in FIG. 31B can charge its secondary battery by receiving power from an external charging facility using a plug-in method, a wireless power supply method, or the like. FIG. 31B shows a state in which a secondary battery 8024 mounted on the automobile 8500 is being charged via a cable 8022 from a ground-mounted charging device 8021. The charging method and connector specifications may be determined appropriately using a predetermined method 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.

[0379] 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 a road or exterior wall, charging can be performed not only while the vehicle is stopped but also while it is moving. This contactless power supply method can also be used to transmit and receive power between vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. Electromagnetic induction and magnetic resonance methods can be used for such contactless power supply.

[0380] 31C is an example of a two-wheeled vehicle using the secondary battery of one embodiment of the present invention. A scooter 8600 shown in FIG. 31C 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.

[0381] 31C 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 stored before riding.

[0382] According to one aspect of the present invention, the cycle characteristics of the secondary battery are improved, and the 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.

[0383] This embodiment mode can be implemented in appropriate combination with other embodiment modes. [Example]

[0384] In this example, lithium cobalt oxide (samples A1 to A3) was prepared as a positive electrode active material according to one embodiment of the present invention, and XRD analysis and ESR analysis were performed. A commercially available lithium cobalt oxide (sample A4) was used as a comparative example. Secondary batteries were also prepared using these lithium cobalt oxides, and their cycle characteristics under high-voltage charging and continuous charging durability were evaluated. Samples A1 to A3 each contained different amounts of magnesium and halogen.

[0385] [Method for producing positive electrode active material] <sample A1> Sample A1 was produced using a halogen source, a lithium source, a magnesium source, and a composite oxide containing lithium and a transition metal as starting materials (see FIG. 9).

[0386] First, the starting materials were weighed. Magnesium fluoride (MgF2) was used as the halogen source and magnesium source. Lithium fluoride (LiF) was used as the halogen source and lithium source.

[0387] Next, a composite oxide containing lithium and a transition metal was weighed. CellSeed C-10N, a lithium cobalt oxide (LiCoO2) manufactured by Nippon Chemical Industry Co., Ltd., was used as the composite oxide containing lithium and a transition metal. CellSeed C-10N is a lithium cobalt oxide with a D50 of approximately 12 μm and low impurity content.

[0388] Specifically, 0.47585 g (7.6378 mmol) of MgF2, 0.06602 g (2.54526 mmol) of LiF, and 149.45813 g (1527.109 mmol) of CellSeed C-10N were weighed out. This amount is equivalent to a ratio of 0.5 atomic % of magnesium atoms to cobalt atoms. The molar ratio of LiF to MgF2 was also 0.33.

[0389] In this specification and the like, the amount of magnesium added refers to the ratio of the number of magnesium atoms to the number of cobalt atoms in the starting material.

[0390] Next, the starting materials were mixed. A wet ball mill was used for mixing. Specifically, the mixing was dry mixing. The mixing was carried out in a ball mill using zirconia balls at 150 rpm for 1 hour.

[0391] Next, the mixture was subjected to a heat treatment. The mixture was placed in an alumina crucible and treated in a muffle furnace in an oxygen atmosphere at 850°C for 60 hours. The alumina crucible was covered during the heat treatment. The oxygen flow rate was 10 L / min. The temperature was increased at a rate of 200°C / hr and decreased over 10 hours.

[0392] Next, the heat-treated material was cooled to room temperature and collected to obtain sample A1.

[0393] <sample A2> For sample A2, 0.12553 g (2.0149 mmol) of MgF2, 0.01742 g (0.67146 mmol) of LiF, and 9.85705 g (100.716 mmol) of CellSeed C-10N were weighed out. This amount corresponds to a ratio of 2.0 atomic % of magnesium atoms to cobalt atoms. The molar ratio of LiF to MgF2 was also 0.33. The other steps can be referenced from the description of sample A1, so a detailed explanation is omitted.

[0394] <sample A3> For sample A3, 0.36613 g (5.8767 mmol) of MgF2, 0.05080 g (1.9583 mmol) of LiF, and 9.58307 g (97.9163 mmol) of CellSeed C-10N were weighed. This amount corresponds to a ratio of 6.0 atomic % of magnesium atoms to cobalt atoms. The molar ratio of LiF to MgF2 was also 0.33. The other steps can be found in the description for sample A1, so a detailed explanation is omitted.

[0395] <sample A4> A commercially available lithium cobalt oxide (Cellseed C-10N) that was not subjected to any particular treatment was used as sample A4 (comparative example).

[0396] [ESR analysis] Next, ESR analysis was performed on sample A1 to sample A4. The ESR analysis was performed with a 9.15 GHz high-frequency power (microwave power) of 1 mW, a magnetic field sweep from 0 mT to 800 mT, and measurement temperatures of 300 K (approximately 27°C), 200 K (approximately -73°C), and 113 K (approximately -160°C). The weight of the sample used in the ESR analysis was 0.005 g for each of sample A1 to sample A4. 2+ The magnetic field and detection sensitivity were corrected using markers. TEMPOL (4-Hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) was used as a standard sample to calculate the number of spins.

[0397] The ESR measurement results for samples A1 to A4 are shown in Figure 32. In Figure 32, the horizontal axis represents the magnetic field, and the vertical axis represents the ESR signal intensity. Note that the signal intensity represents the first derivative of the amount of microwave absorption.

[0398] As shown in Figure 32, a sharp signal was observed around 305 mT (g = 2.15) in all samples. The signal around 305 mT (g = 2.15) was due to Co 2+ and Co 4+ The line width ΔHpp of the signal near 305 mT (g = 2.15) was approximately 4 mT. Here, the line width ΔHpp refers to the difference in magnetic field between the maximum and minimum values ​​of the signal. Furthermore, no signal was observed near 130 mT (g = 5.1) due to Co3O4. It was confirmed that there was no Co3O4 or very little Co3O4 in samples A1 to A4.

[0399] Next, ESR analysis was performed by sweeping the magnetic field from 200 mT to 400 mT. The ESR analysis was performed at temperatures of 300 K (approximately 27°C), 200 K (approximately -73°C), and 113 K (approximately -160°C).

[0400] The ESR measurement results for samples A1 to A4 are shown in Figure 33. In Figure 33, the horizontal axis represents the magnetic field, and the vertical axis represents the ESR signal intensity. Note that the signal intensity represents the first derivative of the amount of microwave absorption.

[0401] As shown in Figure 33, a sharp signal was observed near 305 mT (g = 2.15) in all samples. The spin numbers calculated from the signal intensity near 305 mT (g = 2.15) are shown in Table 1. Table 1 also shows spin densities 1 and 2 calculated from the spin numbers. Spin density 1 shown in Table 1 is the value obtained by dividing the spin number by the weight (0.005 g) of the sample used in the ESR analysis. Spin density 2 shown in Table 1 is the value obtained by dividing the spin number by the number of cobalt atoms calculated from the molecular weight of 97.87, assuming that the composition of each sample is LiCoO2. Spin densities 1 and 2 of samples A1 to A4 are shown in Figure 34A and Figure 34B, respectively. In Figures 34A and 34B, the horizontal axis indicates the sample name, and the vertical axis indicates spin density 1 and spin density 2, respectively.

[0402] [Table 1]

[0403] As shown in Table 1 and Figures 34A and 34B, samples A1 to A3, which are embodiments of the present invention, show paramagnetism, with their spin densities increasing as the ESR measurement temperature decreases, in accordance with the Curie-Weiss law. On the other hand, sample A4, a comparative sample, showed little dependence of its spin density on the measurement temperature, demonstrating behavior different from paramagnetism.

[0404] The correlation between spin density 1, obtained by ESR analysis at 300 K (approximately 27°C), and the amount of magnesium added relative to cobalt is shown in Figure 35A. The correlation between spin density 2 and the amount of magnesium added relative to cobalt is shown in Figure 35B. In Figures 35A and 35B, the horizontal axis represents the amount of magnesium added, and the vertical axis represents spin density 1 and spin density 2, respectively.

[0405] As shown in Figures 35A and 35B, as the amount of magnesium added increases, 2+ and Co 4+ It was also confirmed that the spin density due to the addition of too much magnesium increases. 2+ and Co 4+ It was confirmed that the spin density due to the addition of magnesium was reduced. In addition, in samples A1, A2, and A3, a small signal was observed on the lower magnetic field side of g = 2.1, which indicates the presence of anisotropy in the crystal field caused by the substitution of magnesium.

[0406] As shown in FIG. 35A, in sample A4 to which magnesium was not added, the Co content per weight of the positive electrode active material was 2+ and Co 4+ The spin density due to 17 In contrast, the spin density of sample A1, which had a magnesium addition of 0.5 atomic %, was 2.8 × 10 17 spins / g, and the spin density of sample A2 with a magnesium addition of 2.0 atomic% is 4.8 × 10 17 spins / g, and the spin density of sample A3 with a magnesium addition of 6.0 atomic% is 2.1 × 10 17 spins / g, 2.0×10 17 spins / g or more 1.0×10 18 spins / g or less.

[0407] As shown in FIG. 35B, in sample A4 to which magnesium was not added, the Co per Co atom 2+ and Co 4+ The spin density due to -5 In contrast, the spin density of sample A1, which contains 0.5 atomic % magnesium, is 4.6 × 10 -5 The spin density of sample A2, which has 2.0 atomic% magnesium doping and spins / Co atom, is 7.8 × 10 -5 spins / Co atom, the spin density of sample A3 with magnesium doping of 6.0 atomic% is 3.5 × 10 -5 spins / Co atom, 3.3×10 -5 spins / Co atom or more 1.6×10 -4 spins / Co atom or less.

[0408] [XRD analysis] Next, XRD analysis was carried out on sample A1 to sample A4, using CuKα radiation with a wavelength of 0.15418 nm as the X-ray source for the XRD analysis.

[0409] The XRD measurement results for sample A1 to sample A4 are shown in Figure 36. It was confirmed that the XRD peaks observed in sample A1 to sample A4 were almost all attributable to LiCoO2.

[0410] Enlarged views of the graph in Figure 36 are shown in Figures 37A, 37B, and 38. In Figures 36, 37A, 37B, and 38, the horizontal axis represents the diffraction angle 2θ, and the vertical axis represents the intensity of the diffracted X-rays.

[0411] As shown in Figures 37A, 37B, and 38, differences in XRD peak positions were confirmed depending on the amount of magnesium added. In sample A3, a peak not attributable to LiCoO2 was confirmed near 2θ = 37 deg (see Figure 38). As the amount of magnesium added increased, for example, two Mg 2+ are Li + , Co 3+ If the Mg concentration is too high, it may be difficult to induce magnetic spin.

[0412] The Rietveld analysis was performed on the XRD measurement results shown in Figure 36 to determine the crystallite size and crystal lattice. For the Rietveld analysis, Bruker's software (TOPAS) was used, and the R-3m structure was used as the space group.

[0413] The crystallite sizes and crystal lattices of samples A1 to A4 obtained by Rietveld analysis are shown in Table 2. The lattice constants are shown in Figures 39A and 39B. In Figures 39A and 39B, the horizontal axis represents the amount of magnesium added, and the vertical axis represents the lattice constants a and c, respectively.

[0414] [Table 2]

[0415] As shown in Table 2, in samples A1 to A4, the lattice constant of the a-axis is 2.8155×10 -10 m or more 2.8175×10 -10 m, and the c-axis lattice constant is 14.045×10 -10 m or more 14.065×10 -10 The lattice constant tended to increase with increasing magnesium content, but the difference in lattice constant between samples was small.

[0416] Subsequently, secondary batteries were fabricated using sample A1 to sample A4, and the cycle characteristics were evaluated.

[0417] [Method for manufacturing secondary batteries] Positive electrodes were fabricated using sample A1 to sample A4 as the positive electrode material.

[0418] [Cycle characteristics 1] Next, the cycle characteristics of samples A1 to A4 were evaluated. 2 The upper limit of charging voltage was set to 4.5V.

[0419] First, two cycles were measured at 25°C, with charging at CCCV (rate 0.05C, 4.5V, cut-off current 0.005C) and discharging at CC (0.05C, 2.5V). Then, at 25°C, charging and discharging were repeated at CCCV (rate 0.2C, 4.5V, cut-off current 0.02C) and CC (0.2C, 2.5V) to evaluate the cycle characteristics.

[0420] The cycle characteristics of samples A1 to A4 are shown in Figures 40A and 40B. In Figure 40A, the horizontal axis represents the cycle number, and the vertical axis represents the discharge capacity. In Figure 40B, the horizontal axis represents the cycle number, and the vertical axis represents the discharge capacity retention rate. The discharge capacity retention rate is the ratio of the capacity at each cycle to the maximum discharge capacity.

[0421] 40A and 40B, samples A1 to A3 containing magnesium exhibited better cycle characteristics than sample A4 containing no magnesium. In particular, samples A1 and A2 exhibited high capacity and excellent cycle characteristics.

[0422] [Cycle characteristics 2] Next, the loading of the positive electrode was increased to 20 mg / cm 2 The upper limit voltage of charging was set to 4.6 V, and the cycle characteristics were evaluated.

[0423] First, two cycles were measured at 25°C, with charging at CCCV (rate 0.05C, 4.6V, cut-off current 0.005C) and discharging at CC (rate 0.05C, 2.5V). Then, at 25°C, charging and discharging were repeated at CCCV (rate 0.2C, 4.6V, cut-off current 0.02C) and CC (rate 0.2C, 2.5V) to evaluate the cycle characteristics.

[0424] The cycle characteristics of samples A1 to A4 are shown in FIGS. 41A and 41B.

[0425] 41A and 41B, samples A1 to A3 containing magnesium exhibited better cycle characteristics than sample A4 containing no magnesium. In particular, samples A1 and A2 exhibited high capacity and excellent cycle characteristics.

[0426] [Continuous charging resistance] Next, the continuous charge durability of each secondary battery using each of the prepared positive electrode active materials was evaluated. First, two cycles were measured at 25°C under the conditions of CCCV charging (rate 0.05C, 4.5V or 4.6V, cut-off current 0.005C) and CC discharging (rate 0.05C, 2.5V).

[0427] Thereafter, charging was performed at 60°C using CCCV (rate 0.05C). The upper voltage limit was set to 4.55V or 4.65V, and the termination condition was measured as the time until the secondary battery voltage dropped below the upper voltage limit minus 0.01V (4.54V for 4.55V). If the secondary battery voltage falls below the upper voltage limit, a phenomenon such as a short circuit may have occurred. 1C was set to 200mA / g.

[0428] The evaluation results of continuous charge resistance are shown in Figures 42A and 42B. Figure 42A shows the evaluation results of continuous charge resistance when the charging voltage was 4.55 V. Figure 42B shows the evaluation results of continuous charge resistance when the charging voltage was 4.65 V. In Figures 42A and 42B, the horizontal axis represents time, and the vertical axis represents current.

[0429] 42A and 42B, the continuous charge tolerance increased with an increase in the amount of added magnesium. It was also confirmed that sample A3, which had an added amount of magnesium of 6.0 atomic %, tended to have a lower continuous charge tolerance. [Example]

[0430] In this example, lithium cobalt oxide (sample B1), an embodiment of the present invention, was prepared using a method different from that of Example 1, and ESR analysis was performed. As a comparative example, lithium cobalt oxide (sample B2) without magnesium addition was also prepared. Secondary batteries were also fabricated using these lithium cobalt oxides, and their cycle characteristics during high-voltage charging were evaluated.

[0431] [Method for producing positive electrode active material] <sample B1> Sample B1 was prepared using a halogen source, a lithium source, a magnesium source, and a cobalt source as starting materials (see FIG. 10).

[0432] First, the starting materials were weighed: lithium carbonate (Li2CO3) as the lithium source, tricobalt tetroxide (Co3O4) as the cobalt source, magnesium oxide (MgO) as the magnesium source, and lithium fluoride (LiF) as the fluorine source.

[0433] Specifically, 3.1489 g (42.62 mmol) of Li2CO3, 6.7726 g (28.13 mmol) of Co3O4, 0.0344 g (0.85 mmol) of MgO, and 0.0442 g (1.70 mmol) of LiF were weighed out. This amount corresponds to a ratio of 1.0 atomic % of magnesium atoms to the number of cobalt atoms. The Li2CO3 used was manufactured by Kojundo Chemical Laboratory Co., Ltd. (catalog number: LIH06XB). The MgO used was manufactured by Kojundo Chemical Laboratory Co., Ltd. (catalog number: MGO12PB). The LiF used was manufactured by Kojundo Chemical Laboratory Co., Ltd. (LIH10XB).

[0434] Next, the starting materials were mixed. A wet ball mill was used for mixing. Specifically, 3 mm diameter balls were used, and acetone was used as the solvent. The materials were milled and mixed at a rotation speed of 300 rpm for 2 hours.

[0435] Next, the mixed material was subjected to a first heat treatment. In the first heat treatment, a muffle furnace was used, and the temperature was raised from room temperature to 1000°C at a rate of 200°C / hr, and the material was heated at 1000°C for 10 hours. The atmosphere for the first heat treatment was dry air, and the flow rate of the dry air was 10 L / min.

[0436] Next, the material that had undergone the first heat treatment was cooled to room temperature. After cooling, the material was subjected to a crushing treatment to reduce the particle size of the material. A 53 μm mesh was used for the crushing treatment.

[0437] Next, the material was subjected to a second heat treatment. For the second heat treatment, a muffle furnace was used, and the temperature was raised from room temperature to 800°C at a rate of 200°C / hr, and the material was heated at 800°C for 2 hours. The atmosphere for the second heat treatment was dry air, and the flow rate of the dry air was 10 L / min.

[0438] Next, the heat-treated material was cooled to room temperature and collected to obtain sample B1.

[0439] <sample B2> Sample B2 was prepared using a lithium source and a cobalt source as starting materials. Sample B2, which is a comparative example, did not use a magnesium source or a halogen source.

[0440] First, the starting materials were weighed: lithium carbonate (Li2CO3) as the lithium source, tricobalt tetroxide (Co3O4) as the cobalt source, magnesium oxide (MgO) as the magnesium source, and lithium fluoride (LiF) as the fluorine source.

[0441] Specifically, 3.1521 g (42.66 mmol) of Li2CO3 and 6.8479 g (28.44 mmol) of Co3O4 were weighed. For other steps, the description of sample B1 can be referred to, and detailed explanations will be omitted.

[0442] [ESR analysis] Next, ESR analysis was performed on sample B1 and sample B2. The ESR analysis was performed with a 9.15 GHz high-frequency power (microwave power) of 1 mW, a magnetic field sweep from 0 mT to 500 mT, and measurement temperatures of 300 K (approximately 27°C), 200 K (approximately -73°C), and 113 K (approximately -160°C). The weights of the samples used in the ESR analysis were 0.0044 g for sample B1 and 0.0045 g for sample B2.

[0443] The ESR measurement results for sample B1 and sample B2 are shown in Figure 43A. An enlarged version of Figure 43A is shown in Figure 43B. In Figures 43A and 43B, the horizontal axis represents the magnetic field, and the vertical axis represents the ESR signal intensity. The signal intensity represents the first derivative of the amount of microwave absorption.

[0444] As shown in Figure 43A, a broad peak was observed around 130 mT (g = 5.1) in sample B2, a comparative example to which no magnesium was added. This result suggests the presence of Co with 4-coordinated oxygen atoms in sample B2.

[0445] As shown in Figures 43A and 43B, a sharp signal was observed around 305 mT (g = 2.15) in all samples. The signal around 305 mT (g = 2.15) was due to Co 2+ and Co 4+ This is due to the fact that the spin number calculated from the signal intensity around 305 mT (g = 2.15) is shown in Table 3. Table 3 also shows spin density 1 and spin density 2 calculated from the spin number. Spin density 1 shown in Table 1 is the value obtained by dividing the spin number by the weight (0.005 g) of the sample used in the ESR analysis. Spin density 2 shown in Table 3 is the value obtained by dividing the spin number by the number of cobalt atoms calculated from the molecular weight of 97.87, assuming that the composition of each sample is LiCoO2. Spin density 1 of sample B1 and sample B2 is shown in Figure 44A, and spin density 2 of sample B2 is shown in Figure 44B. In Figures 44A and 44B, the horizontal axis indicates the sample name, and the vertical axis indicates spin density 1 (Spin Density 1) and spin density 2 (Spin Density 2), respectively.

[0446] [Table 3]

[0447] As shown in Table 3 and Figures 44A and 44B, sample B1, one embodiment of the present invention, exhibited paramagnetic properties, with its spin density increasing as the ESR measurement temperature decreased, in accordance with the Curie-Weiss law. On the other hand, sample B2, a comparative sample, was paramagnetic but had a low spin density.

[0448] The correlation between spin density 1, obtained by ESR analysis at 300 K (approximately 27°C), and the amount of magnesium added relative to cobalt is shown in Figure 45A. The correlation between spin density 2 and the amount of magnesium added relative to cobalt is shown in Figure 45B. In Figures 45A and 45B, the horizontal axis represents the amount of magnesium added, and the vertical axis represents spin density 1 and spin density 2, respectively.

[0449] As shown in FIG. 45A, in sample B2 to which magnesium was not added, the Co content per weight of the positive electrode active material was 2+ and Co 4+ The spin density due to 17 In contrast, the spin density of sample B1, which had a magnesium addition of 1.0 atomic %, was 2.1 × 10 17 spins / g, 2.0×10 17 spins / g or more 1.0×10 18 spins / g or less.

[0450] As shown in FIG. 45B, in sample B2 to which magnesium was not added, the Co per Co atom 2+ and Co 4+ The spin density due to -5 In contrast, the spin density of sample B1, which contains 1.0 atomic% magnesium, is 3.4 × 10 -5 spins / Co atom, 3.3×10 -5 spins / Co atom or more 1.6×10 -4 spins / Co atom or less.

[0451] [Method for manufacturing secondary batteries] Positive electrodes were fabricated using sample B1 and sample B2 as positive electrode materials.

[0452] [Cycle characteristics] Next, the cycle characteristics of sample B1 and sample B2 were evaluated. 2 The upper limit of charging voltage was set to 4.6V.

[0453] First, two cycles were measured at 25°C, with charging at CCCV (0.05C, 4.6V, cut-off current 0.005C) and discharging at CC (0.05C, 2.5V). Then, at 25°C, charging at CCCV (0.2C, 4.6V, cut-off current 0.02C) and discharging at CC (0.2C, 2.5V) were repeated to evaluate the cycle characteristics.

[0454] The cycle characteristics of samples A1 to A4 are shown in Figures 46A and 46B. In Figure 46A, the horizontal axis represents the cycle number, and the vertical axis represents the discharge capacity. In Figure 46B, the horizontal axis represents the cycle number, and the vertical axis represents the discharge capacity retention rate. The discharge capacity retention rate is the ratio of the capacity at each cycle to the maximum discharge capacity.

[0455] As shown in FIGS. 46A and 46B, it was confirmed that sample B1, to which magnesium was added, had better cycle characteristics than sample B2, to which magnesium was not added. [Explanation of symbols]

[0456] 100: positive electrode active material, 200: active material layer, 201: graphene compound, 210: electrode laminate, 211a: positive electrode, 211b: negative electrode, 212a: lead, 212b: lead, 214: separator, 215a: joint portion, 215b: joint portion, 217: fixing member, 250: secondary battery, 251: exterior body, 261: folded portion, 262: sealing portion, 263: sealing portion, 271: ridge line, 272: valley line, 273: space, 300: secondary battery, 301: positive electrode can, 302: negative electrode can, 303: gasket, 304: positive electrode, 305: positive electrode current collector, 306: positive electrode active material layer, 307: negative electrode, 308: negative electrode current collector, 309: negative electrode active material layer, 310: separator, 500: secondary battery, 501: positive electrode current collector, 502: positive electrode active material layer, 503: positive electrode, 504: negative electrode current collector, 505: negative electrode active material layer, 506: negative electrode, 507: separator, 508: electrolyte, 509: exterior body, 510: positive electrode lead electrode, 511: negative electrode lead electrode, 600: secondary battery, 601: positive electrode cap, 602: battery can, 603: positive electrode terminal, 604: positive electrode, 605: separator, 606: negative electrode, 607: negative electrode terminal, 608: insulating plate, 609: insulating plate, 611: PTC element, 612: safety valve Mechanism, 613: conductive plate, 614: conductive plate, 615: module, 616: conductive wire, 617: temperature control device, 900: circuit board, 910: label, 911: terminal, 912: circuit, 913: secondary battery, 914: antenna, 915: seal, 916: layer, 917: layer, 918: antenna, 920: display device, 921: sensor, 922: terminal, 930: housing, 930a: housing, 930b: housing, 931: negative electrode, 932: positive electrode, 933: separator, 950: wound body, 951: terminal, 952: terminal, 980: secondary battery, 981: film, 982: film, 993: Wound body, 994: negative electrode, 995: positive electrode, 996: separator, 997: lead electrode, 998: lead electrode, 7100: portable display device, 7101: housing, 7102: display unit, 7103: operation button, 7104: secondary battery, 7200: portable information terminal, 7201: housing, 7202: display unit, 7203: band, 7204: buckle, 7205: operation button, 7206: input / output terminal, 7207: icon, 7300: display device, 7304: display unit, 7400: mobile phone, 7401: housing, 7402: display unit, 7403: operation button, 7404: external connection port,7405: Speaker, 7406: Microphone, 7407: Secondary battery, 7500: Electronic cigarette, 7501: Atomizer, 7502: Cartridge, 7504: Secondary battery, 8000: Display device, 8001: Housing, 8002: Display unit, 8003: Speaker unit, 8004: Secondary battery, 8021: Charging device, 8022: Cable, 8024: Secondary secondary battery, 8100: lighting device, 8101: housing, 8102: light source, 8103: secondary battery, 8104: ceiling, 8105: side wall, 8106: floor, 8107: window, 8200: indoor unit, 8201: housing, 8202: air outlet, 8203: secondary battery, 8204: outdoor unit, 8300: electric refrigerator-freezer, 8301: housing, 8302: refrigerator compartment door, 8303: Freezer door, 8304: Secondary battery, 8400: Automobile, 8401: Headlight, 8406: Electric motor, 8500: Automobile, 8600: Scooter, 8601: Side mirror, 8602: Secondary battery, 8603: Turn signal light, 8604: Under-seat storage, 9600: Tablet terminal, 9625: Switch, 9627: Switch, 9628: Operation switch, 9629: Fastener, 9630: Housing, 9630a: Housing, 9630b: Housing, 9631: Display unit, 9631a: Display unit, 9631b: Display unit, 9633: Solar cell, 9634: Charge / discharge control circuit, 9635: Power storage unit, 9636: DCDC converter, 9637: Converter, 9640: Moving part,< / edx> < / xps> < / esr> < / xrd>

Claims

1. A positive electrode active material including positive electrode active material particles having lithium, cobalt, oxygen, magnesium, and fluorine, the orientation of the crystals in the surface layer portions of the positive electrode active material particles is approximately the same as the orientation of the crystals in the interior of the positive electrode active material particles, The spin density at g = 2.15 by ESR analysis was 2.0 × 10 １７ spins / g or more 1.0×10 １８ spins / g or less, The concentration of the magnesium relative to the cobalt is 0.1 atomic % or more and 6.0 atomic % or less in the positive electrode active material.

2. A positive electrode active material including positive electrode active material particles having lithium, cobalt, oxygen, and magnesium, the orientation of the crystals in the surface layer portions of the positive electrode active material particles is approximately the same as the orientation of the crystals in the interior of the positive electrode active material particles, The spin density at g = 2.15 by ESR analysis was 2.0 × 10 １７ spins / g or more 1.0×10 １８ spins / g or less, the concentration of the magnesium is 0.1 atomic % or more and 6.0 atomic % or less relative to the cobalt; The lattice constant of the a-axis is 2.8155 × 10 －１０ m or more 2.8175×10 －１０ m, The c-axis lattice constant is 14.045 × 10 －１０ m or more 14.065×10 －１０ The positive electrode active material has a molecular weight of 1000 or less.

3. A positive electrode active material including positive electrode active material particles having lithium, cobalt, oxygen, magnesium, and fluorine, the orientation of the crystals in the surface layer portions of the positive electrode active material particles is approximately the same as the orientation of the crystals in the interior of the positive electrode active material particles, The spin density at g = 2.15 by ESR analysis was 2.0 × 10 １７ spins / g or more 1.0×10 １８ spins / g or less, the concentration of the magnesium is 0.1 atomic % or more and 6.0 atomic % or less relative to the cobalt; The lattice constant of the a-axis is 2.8155 × 10 －１０ m or more 2.8175×10 －１０ m, The c-axis lattice constant is 14.045 × 10 －１０ m or more 14.065×10 －１０ The positive electrode active material has a molecular weight of 1000 or less.

4. A secondary battery comprising a positive electrode having the positive electrode active material according to claim 1 and a negative electrode.

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