Positive electrode active material, method for manufacturing positive electrode active material, and secondary battery

Abstract

A high-capacity cathode active material with excellent cycle characteristics is provided. The crystalline structure of this cathode active material changes little between the charging and discharging states. For example, the cathode active material exhibits a layered rock-salt type crystalline structure in the discharging state and a pseudo-spinel type crystalline structure in the charging state at a high voltage of approximately 4.6V. The changes in crystalline structure and volume before and after charging are less than those of conventional cathode active materials.

Description

Technical Field

[0001] One embodiment of the present invention relates to an article, method, or manufacturing method. Additionally, the present invention relates to a process, machine, manufacture, or composition of matter. One embodiment of the present invention relates to a semiconductor device, display device, light-emitting device, energy storage device, lighting device, electronic device, or a method for manufacturing the same. In particular, one embodiment of the present invention relates to a positive electrode active material for use in secondary batteries, a secondary battery, and an electronic device comprising a secondary battery.

[0002] Note that in this specification, "energy storage device" refers to all components and devices with energy storage function. For example, lithium-ion secondary batteries (also known as rechargeable batteries), lithium-ion capacitors, and double-layer capacitors are all included in the scope of energy storage devices.

[0003] Note that in this specification, electronic device refers to all devices with energy storage devices, such as electro-optical devices with energy storage devices and information terminal devices with energy storage devices. Prior Technology

[0004] In recent years, the development of various energy storage devices, such as lithium-ion rechargeable batteries, lithium-ion capacitors, and air batteries, has been very active. In particular, with the development of the semiconductor industry for portable information terminals such as mobile phones, smartphones, and laptops, portable music players, digital cameras, medical devices, and next-generation clean energy vehicles (hybrid electric vehicles (HEVs), electric vehicles (EVs), or plug-in hybrid electric vehicles (PHEVs), the demand for high-output, high-energy-density lithium-ion rechargeable batteries has surged, making them a necessity in modern information society as a rechargeable energy source.

[0005] The characteristics currently required for lithium-ion rechargeable batteries include: higher energy density, improved cycle performance, and enhanced safety and long-term reliability under various operating environments.

[0006] Therefore, research is underway on positive electrode active materials aimed at improving the cycle characteristics and achieving high capacity in lithium-ion secondary batteries (Patent Document 1, Patent Document 2, and Non-Patent Document 1). Additionally, research is being conducted on the crystal structure of the positive electrode active materials (Non-Patent Documents 2 to 4).

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2006-164758 [Patent Document 2] Japanese Patent Application Publication No. 2014-523840

[0008] [Non-patent document 1] Jae-Hyun Shim et al, “Characterization of Spinel LixCo2O4-Coated LiCoO2 Prepared with Post-Thermal Treatment as a Cathode Material for Lithium Ion Batteries”, CHEMISTRY OF MATERIALS, 2015, 27, pp.3273-3279 [Non-patent document 2] 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, pp.17340-17348 [Non-Patent Literature 3] T. Motohashi 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 Literature 4] Zhaohui Chen et al, “Staging Phase Transitions in LixCoO2”, Journal of The Electrochemical Society, 2002, 149(12)A1604-A1609 Summary of the Invention

[0009] One objective of one embodiment of the present invention is to provide a high-capacity positive electrode active material for lithium-ion secondary batteries that exhibits excellent charge-discharge cycle characteristics. Another objective of one embodiment of the present invention is to provide a positive electrode active material that, when used in lithium-ion secondary batteries, suppresses capacity reduction during charge-discharge cycles. Another objective of one embodiment of the present invention is to provide a high-capacity secondary battery. Another objective of one embodiment of the present invention is to provide a secondary battery with excellent charge-discharge characteristics. Another objective of one embodiment of the present invention is to provide a secondary battery with high safety or reliability.

[0010] In addition, one objective of one embodiment of the present invention is to provide a novel substance, active material particles, energy storage device or method of manufacturing the same.

[0011] Note that the description of these objectives does not preclude the existence of other objectives. One embodiment of the present invention does not need to achieve all of the above objectives. Furthermore, objectives other than those described above can be extracted from the specification, drawings, and claims.

[0012] To achieve the above objectives, the positive electrode active material of one embodiment of the present invention exhibits minimal changes in its crystal structure between the charging and discharging states.

[0013] One embodiment of the present invention is a secondary battery comprising a positive electrode and a negative electrode, wherein when the XRD pattern of the positive electrode is analyzed by Rietveld analysis, the positive electrode has a pseudo-spinel crystal structure and the proportion of the pseudo-spinel crystal structure is 60 wt% or more.

[0014] Another embodiment of the present invention is a positive electrode active material comprising lithium, cobalt, magnesium, oxygen, and fluorine. When a lithium-ion secondary battery using the positive electrode active material as the positive electrode and lithium metal as the negative electrode is charged at 25°C until the battery voltage reaches 4.6V and the current is sufficiently low, powder X-ray diffraction analysis using CuKα1 lines reveals diffraction peaks at 2θ of 19.30 ± 0.20° and 2θ of 45.55 ± 0.10°.

[0015] Another embodiment of the present invention is a positive electrode active material comprising lithium, cobalt, magnesium, oxygen, and fluorine, wherein the difference between the volume per unit cell of the crystalline structure comprising 60 wt% or more in the positive electrode active material with a depth of charge of 0.8 or more and the volume per unit cell of the crystalline structure comprising 60 wt% or more in the positive electrode active material with a depth of charge of 0.06 or less is within 2.5%.

[0016] The aforementioned positive electrode active material preferably contains at least one of Ti and Al.

[0017] According to one embodiment of the present invention, a high-capacity positive electrode active material for lithium-ion secondary batteries with excellent charge-discharge cycle characteristics can be provided. Furthermore, according to one embodiment of the present invention, a positive electrode active material that suppresses capacity reduction during charge-discharge cycles when used in lithium-ion secondary batteries can be provided. Furthermore, according to one embodiment of the present invention, a high-capacity secondary battery can be provided. Furthermore, according to one embodiment of the present invention, a secondary battery with excellent charge-discharge characteristics can be provided. Furthermore, according to one embodiment of the present invention, a secondary battery with high safety or reliability can be provided. According to one embodiment of the present invention, a novel material, active material particles, energy storage device, or method for manufacturing the same can be provided.

[0018] Note that the description of the above effects does not preclude the existence of other effects. Furthermore, one embodiment of the present invention does not necessarily require all of the above effects. Additionally, effects other than those described above are clearly present in the specification, drawings, and claims, and such effects can be derived from the description in the specification, drawings, and claims. Simple Explanation of the Diagram

[0019] In the diagram: [Figure 1] is a diagram illustrating the charging depth and crystal structure of the positive electrode active material according to one embodiment of the present invention; [Figure 2] is a diagram illustrating the depth of charge and crystal structure of a conventional positive electrode active material; [Figure 3] shows the XRD pattern calculated from the crystal structure; [Figure 4A] and [Figure 4B] are diagrams illustrating the crystal structure and magnetism of the positive electrode active material according to one embodiment of the present invention; [Figure 5A] and [Figure 5B] are diagrams illustrating the crystal structure and magnetism of conventional positive electrode active materials; [Figure 6A] and [Figure 6B] are cross-sectional views illustrating the active material layer when graphene compounds are used as conductive additives; [Figures 7A to 7C] are diagrams illustrating the charging method of a secondary battery; [Figures 8A to 8D] are diagrams illustrating the charging method of secondary batteries; Figure 9 is a diagram illustrating the discharge method of a secondary battery; [Figures 10A to 10C] are illustrations of coin-type secondary batteries; [Figures 11A to 11D] are illustrations of a cylindrical secondary battery; Figures 12A and 12B are diagrams illustrating examples of secondary batteries; Figures 13A1, 13A2, 13B1, and 13B2 are illustrations of examples of secondary batteries; Figures 14A and 14B are illustrations of examples of secondary batteries; [Figure 15] is a diagram illustrating an example of a secondary battery; [Figures 16A to 16C] are diagrams illustrating laminated secondary batteries; [Figure 17A] and [Figure 17B] are diagrams illustrating laminated secondary batteries; [Figure 18] is a diagram showing the appearance of a secondary battery; [Figure 19] is a diagram showing the appearance of a secondary battery; [Figures 20A to 20C] are diagrams illustrating a method for manufacturing a secondary battery; Figures 21A, 21B1, 21B2, 21C, and 21D illustrate a flexible secondary battery. [Figure 22A] and [Figure 22B] are diagrams illustrating a flexible secondary battery; Figures 23A to 23H are illustrations of an example of an electronic device; Figures 24A to 24C are illustrations of an example of an electronic device; [Figure 25] is a diagram illustrating an example of an electronic device; [Figures 26A to 26C] are figures illustrating an example of an electronic device; [Figure 27] is the XRD pattern of the positive electrode active material of one embodiment of the present invention in Example 1; [Figure 28] is the XRD pattern of the positive electrode active material of one embodiment of the present invention in Example 1; [Figure 29] is the XRD pattern of the positive electrode active material of one embodiment of the present invention in Example 1; [Figure 30] is the XRD pattern of the positive electrode active material of one embodiment of the present invention in Example 1; [Figure 31] is the XRD pattern of the positive electrode active material of the comparative example of Example 1; [Figure 32] is the XRD pattern of the positive electrode active material of the comparative example of Example 1; [Figure 33] is the XRD pattern of the positive electrode active material of one embodiment of the present invention in Example 1; [Figure 34] is the XRD pattern of the positive electrode active material of one embodiment of the present invention in Example 1; [Figure 35] is the XRD pattern of the positive electrode active material of the comparative example of Example 1; [Figure 36] is the XRD pattern of the positive electrode active material of an embodiment of the present invention and a comparative example of Example 1; [Figure 37] is a graph showing the volume change rate of the positive electrode active material in one embodiment of the present invention, Example 1; [Figure 38A] and [Figure 38B] show the cycle characteristics of a secondary battery according to an embodiment of the present invention and a comparative example of Example 1; [Figure 39] shows the ESR signal of the positive electrode active material in Example 2 of the present invention, as well as a comparative example; [Figures 40A to 40C] show the ESR signals of the positive electrode active material in one embodiment of the present invention and a comparative example of Example 2; [Figures 41A1], [Figures 41A2], [Figures 41B1] and [Figures 41B2] are the crystal structure models used in the calculations of Example 3; [Figure 42A] and [Figure 42B] are graphs illustrating the calculation results of Example 3; [Figure 43] is a graph illustrating the calculation results of Example 3. Implementation

[0020] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the following description, and those skilled in the art will readily understand that its methods and details can be varied in various forms. Furthermore, the present invention should not be construed as being limited solely to the contents described in the following embodiments.

[0021] In this specification, Miller indices are used to represent crystal planes and orientations. In crystallography, superscript lines are added to numbers to indicate crystal planes and orientations. However, in this specification, due to the symbol limitations in the patent application, sometimes a - (negative number sign) is added before the digits to indicate crystal planes and orientations instead of superscript lines. In addition, "[ ]" indicates the individual orientation of an orientation within a crystal, "〈〉" indicates the collective orientation of all equivalent crystal orientations, "()" indicates an individual crystal plane, and "{}" indicates a collective plane with equivalent symmetry.

[0022] In this specification, segregation refers to the phenomenon that a certain element (e.g., B) is not spatially uniformly distributed in a solid containing multiple elements (e.g., A, B, C).

[0023] In this specification, the surface portion of particles such as active materials refers to the region extending from the surface to approximately 10 nm. The surface where cracks or fissures occur may also be referred to as the surface. Furthermore, the region deeper than the surface portion is referred to as the interior.

[0024] In this specification, the layered rock-salt type crystal structure of composite oxides containing lithium and transition metals refers to a crystal structure with a rock-salt type ionic arrangement of alternating cations and anions, where transition metals and lithium are arranged regularly to form a two-dimensional plane, allowing lithium to diffuse in two dimensions. Additionally, it may include defects such as vacancies of cations or anions. Strictly speaking, the layered rock-salt type crystal structure is sometimes a lattice deformation structure of rock-salt type crystals.

[0025] Furthermore, in this specification, a rock-salt type crystal structure refers to a structure in which cations and anions are arranged alternately. It may also include vacancies of cations or anions.

[0026] Furthermore, in this specification, the pseudo-spinel crystal structure of composite oxides containing lithium and transition metals refers to a structure with space group R-3m; although not a spinel-type crystal structure, ions such as cobalt and magnesium occupy six-coordinate positions of oxygen, and the arrangement of the ions has a symmetry similar to that of a spinel-type structure. Additionally, sometimes light elements such as lithium occupy four-coordinate positions of oxygen in the pseudo-spinel crystal structure, in which case the arrangement of the ions has a symmetry similar to that of a spinel-type structure.

[0027] Alternatively, it can be said that the pseudo-spinel-type crystal structure, although irregularly containing Li in the interlayer, is similar to the CdCl2-type crystal structure. It is known that the aforementioned crystal structure similar to the CdCl2-type crystal structure is similar to the crystal structure of lithium nickel oxide charged to a depth of 0.94 (Li0.06NiO2), but layered rock salt-type cathode active materials containing large amounts of pure lithium cobalt oxide or cobalt typically do not possess this crystal structure.

[0028] Layered rock salt crystals and rock salt crystals contain anions that form a cubic closest-packed structure (face-centered cubic lattice structure). It is speculated that the anions in pseudo-spinel crystals also possess a cubic closest-packed structure. When layered rock salt crystals and rock salt crystals come into contact, there are aligned crystal faces formed by the cubic closest-packed structure of anions. The space group for layered rock salt crystals and pseudo-spinel crystals is R-3m, which differs from the space group Fm-3m (the space group for general rock salt crystals) and Fd-3m (the space group for rock salt crystals with the simplest symmetry) of rock salt crystals. Therefore, the Miller indices of the crystal faces satisfying the above conditions in layered rock salt crystals and pseudo-spinel crystals differ from those in rock salt crystals. In this specification, sometimes the alignment of the cubic closest-packed structure formed by anions in layered rock salt crystals, pseudo-spinel crystal structures, and rock salt crystals refers to approximately identical crystal orientation.

[0029] The crystal orientation of two regions can be roughly determined to be consistent based on TEM (transmission electron microscopy), STEM (scanning transmission electron microscopy), HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy), and ABF-STEM (annular bright-field scanning transmission electron microscopy) images. Additionally, X-ray diffraction (XRD), electron diffraction, and neutron diffraction can be used as diagnostic criteria. When the crystal orientation is roughly consistent, the difference in the direction of the alternating linear arrangement of cations and anions observed in TEM images should be less than 5 degrees, preferably less than 2.5 degrees. Note that sometimes light elements such as oxygen and fluorine cannot be clearly observed in TEM images; in such cases, the consistency of orientation can be determined based on the arrangement of the metal elements.

[0030] Furthermore, in this specification, the theoretical capacity of the positive electrode active material refers to the amount of electricity required when all lithium ions that can be inserted and detached from the positive electrode active material are detached. 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] Furthermore, in this specification, the depth of charge is 0 when all lithium ions that can be inserted and detached are inserted, and the depth of charge is 1 when all lithium ions that can be inserted and detached contained in the positive electrode active material are detached.

[0032] Furthermore, in this specification, charging refers to the process of moving lithium ions from the positive electrode to the negative electrode within the battery, and moving electrons from the negative electrode to the positive electrode in an external circuit. Charging of the positive electrode active material refers to the process of removing lithium ions. Additionally, a positive electrode active material with a depth of charge exceeding 0.5 is referred to as a charged positive electrode active material. Furthermore, a positive electrode active material with a depth of charge of 0.8 or more is referred to as a high-voltage charged positive electrode active material. Therefore, for example, in LiCoO2, if it is charged to 219.2 mAh / g or more, it can be considered a high-voltage charged positive electrode active material. Furthermore, a positive electrode active material in lithium cobalt oxide with impurity elements of 5 at% or less (here, impurity elements refer to elements other than lithium, cobalt, and oxygen) that has undergone constant current charging at 25°C until the battery voltage reaches 4.6V (for the counter electrode lithium), and then constant voltage charging until the current value reaches 0.01C, is also referred to as a high-voltage charged positive electrode active material.

[0033] Similarly, discharge refers to the movement of lithium ions from the negative electrode to the positive electrode within the battery, and the movement of electrons from the positive electrode to the negative electrode in an external circuit. Discharging of the positive electrode active material refers to the insertion of lithium ions. Furthermore, positive electrode active materials with a depth of charge of 0.5 or less are referred to as discharged positive electrode active materials. Additionally, positive electrode active materials with a depth of charge of 0.06 or less, or positive electrode active materials that have discharged more than 90% of their capacity from a high-voltage charged state, are referred to as fully discharged positive electrode active materials. For example, a charging capacity of 219.2 mAh / g in LiCoO2 refers to a high-voltage charged state; a positive electrode active material that has discharged more than 90% of its capacity (197.3 mAh / g) from this state is a fully discharged positive electrode active material. In addition, the positive electrode active material that has been fully discharged is also called the positive electrode active material that is in lithium cobalt oxide with impurity elements of less than 5 at% (here, impurity elements refer to elements other than lithium, cobalt, and oxygen) and subjected to constant current discharge at 25°C until the battery voltage becomes less than 3V (when the counter electrode is lithium).

[0034] Implementation Method 1 [Structure of the positive electrode active material] First, Figures 1 and 2 are used to illustrate an embodiment of the positive electrode active material 100 of the present invention and a conventional positive electrode active material, and the differences between the two are explained. Note that the conventional positive electrode active material described in this embodiment is simple lithium cobalt oxide (LiCoO2), in which no elements other than lithium, cobalt, and oxygen are added internally and no elements other than lithium, cobalt, and oxygen are coated on the surface.

[0035] <Commonly Known Positive Electrode Active Materials> Lithium cobalt oxide is a well-known example of a positive electrode active material. As described in Non-Patent Literature 2 and Non-Patent Literature 3, the crystal structure of lithium cobalt oxide varies depending on the depth of charge. Figure 2 shows a typical crystal structure of lithium cobalt oxide.

[0036] As shown in Figure 2, LiCoO2 with a depth of charge of 0 (discharge state) includes a region with a crystalline structure having space group R-3m, comprising three CoO2 layers in a unit cell. This crystalline structure is sometimes referred to as an O3-type crystalline structure. Note that the CoO2 layer refers to the structure where the octahedral structure formed by cobalt and six coordinated oxygen atoms maintains a shared edge configuration on a single plane.

[0037] At a depth of charge of 1, it has a crystalline structure with space group P-3m1, and the unit cell includes a CoO2 layer. Therefore, this crystalline structure is sometimes referred to as the O1 type crystalline structure.

[0038] At a depth of charge of approximately 0.88, LiCoO2 exhibits a crystal structure with space group R-3m. This structure can also be described as an alternating layering of CoO2 structures such as P-3m1(O1) and LiCoO2 structures such as R-3m(O3). Therefore, this crystal structure is sometimes referred to as the H1-3 type crystal structure. In reality, the number of cobalt atoms per unit cell in the H1-3 type crystal structure is twice that of other structures. However, in this specification, as shown in Figure 2, for ease of comparison with other structures, the c-axis in the H1-3 type crystal structure is represented as half the unit cell.

[0039] When repeatedly charged and discharged at high voltages with a depth of charge of around 0.88 or higher, the crystal structure of LiCoO2 changes repeatedly between the H1-3 type crystal structure and the R-3m(O3) structure in the discharge state.

[0040] However, the CoO2 layer deviates significantly in both of the above-mentioned crystal structures. As shown by the dashed line and arrow in Figure 2, in the H1-3 crystal structure, the CoO2 layer deviates significantly from R-3m(O3). Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0041] Furthermore, the volume difference is also significant. Details will be explained in Example 1. When comparing the same number of cobalt atoms, the volume difference between the H1-3 type crystal structure and the O3 type crystal structure in the discharge state is more than 3.5%.

[0042] In addition to the above, the continuous CoO2 layer structure of H1-3 type crystal structure, such as P-3m1(O1), is more likely to be unstable.

[0043] Therefore, the crystalline structure of lithium cobalt oxide collapses under repeated high-voltage charge-discharge cycles. This collapse leads to a deterioration in cycle performance. This is because the collapse reduces the number of stable lithium sites, making lithium insertion and extraction more difficult.

[0044] Positive electrode active material according to one embodiment of the present invention "internal" In contrast, the crystal structure and volume of the positive electrode active material 100 in one embodiment of the present invention have a smaller difference when fully discharged and when charged with a high voltage.

[0045] Figure 1 shows the crystal structure of the positive electrode active material 100 before and after charge and discharge. The positive electrode active material 100 contains lithium, cobalt, and oxygen. Preferably, it also contains magnesium in addition to the above. Furthermore, it preferably contains halogens such as fluorine and chlorine. Additionally, it preferably contains at least one of titanium and aluminum.

[0046] The crystal structure of the depth of charge 0 (discharge state) in Figure 1 is the same as that in Figure 2, R-3m (O3). On the other hand, when the depth of charge is approximately 0.88, the positive electrode active material 100 of one embodiment of the present invention includes a different crystal structure than that in Figure 2. In this description, the crystal structure of the space group R-3m is referred to as a pseudo-spinel crystal structure. In addition, to illustrate the symmetry of cobalt atoms and oxygen atoms, the representation of lithium is omitted in the pseudo-spinel crystal structure shown in Figure 1, but in fact, there is approximately 12 atomic percent lithium relative to cobalt in the CoO2 interlayer. In addition, in both the O3 type crystal structure and the pseudo-spinel crystal structure, it is preferable that a small amount of magnesium is present in the CoO2 interlayer, i.e., at the lithium sites. In addition, it is preferable that a small amount of halogens such as fluorine is present in the oxygen sites. Furthermore, it is preferable that at least one of aluminum and titanium is present in the cobalt sites.

[0047] The changes in the crystal structure during lithium desorption in the positive electrode active material 100 are suppressed. For example, as shown by the dashed line in Figure 1, there is almost no deviation of the CoO2 layer in the above crystal structure.

[0048] Furthermore, the details will be explained in Example 1. In the positive electrode active material 100, the volume difference per unit cell between the O3 type crystal structure with a charging depth of 0 and the pseudo-spinel type crystal structure with a charging depth of 0.88 is less than 2.5%, specifically less than 2.2%.

[0049] Therefore, even with repeated charging and discharging at high voltage, the crystal structure is not easily collapsed.

[0050] The coordinates of cobalt and oxygen in each unit cell of the pseudo-spinel crystal structure can be represented by Co(0,0,0.5) and O(0,0,x) (0.20≤x≤0.25), respectively.

[0051] The presence of a small amount of magnesium between the CoO2 layers has the effect of suppressing the deviation of the CoO2 layers. Therefore, the presence of magnesium between the CoO2 layers facilitates the formation of a pseudo-spinel crystal structure. Thus, it is preferable that magnesium is also distributed within the particles of the positive electrode active material 100. Furthermore, to ensure that magnesium is distributed within these particles, it is preferable to perform a heat treatment during the fabrication process of the positive electrode active material 100.

[0052] However, at excessively high heat treatment temperatures, the likelihood of cation mixing and magnesium intrusion into cobalt sites increases. When magnesium is present at cobalt sites, it does not maintain the R-3m effect. Furthermore, excessively high heat treatment temperatures also raise concerns about adverse effects such as cobalt reduction to divalent form and lithium diffusion.

[0053] Therefore, it is preferable to add halogen compounds such as fluorine compounds to lithium cobalt oxide before performing the heat treatment used to distribute magnesium within the particles. By adding halogen compounds, the melting point of lithium cobalt oxide is lowered. This lowering of the melting point allows magnesium to be easily distributed throughout the particles at temperatures where cation mixing is less likely. The presence of fluorine compounds also suggests improved resistance to corrosion from hydrofluoric acid produced by electrolyte decomposition.

[0054] When at least one of titanium and aluminum is present in small amounts at the cobalt position of the positive electrode active material 100, changes in the crystal structure can be further suppressed.

[0055] The magnesium distributed within the positive electrode active material 100 has the effect of suppressing the deviation of the CoO2 layer. However, at the same time, the cobalt surrounding the magnesium is easily reduced to divalent to maintain charge balance. Therefore, there is a concern that when magnesium is in excess, some particles of the positive electrode active material 100 may form a MgO and CoO(II) solid solution structure. Within the MgO and CoO(II) solid solution structural region, there are no pathways for lithium insertion and extraction.

[0056] However, titanium is most stable in the tetravalent state, followed by the trivalent state, while aluminum is stable in the trivalent state. Both titanium and aluminum are unstable in the divalent state. Therefore, titanium or aluminum present at cobalt sites is not easily reduced to the divalent state even if magnesium is present at the surrounding lithium sites. Thus, it can be considered that when titanium or aluminum is present in small amounts at cobalt sites, it is not easy to form a solid solution structure of MgO and CoO(II).

[0057] Furthermore, when at least one of titanium and aluminum is included, especially in the charging state, oxygen is not easily removed. In other words, because the activity of oxygen bonded to titanium or aluminum decreases, the catalytic effect on the oxidative decomposition of the electrolyte decreases, and the oxidative decomposition of the electrolyte is less likely to occur on the surface of the positive electrode active material.

[0058] Surface layer Magnesium is preferably distributed throughout the particles of the positive electrode active material 100; however, the magnesium concentration at the particle surface is preferably higher than the average concentration of the particles. The particle surface is essentially a region of crystalline defects, making it prone to instability and changes in crystal structure. A higher magnesium concentration at the surface can more effectively suppress changes in crystal structure. Furthermore, a higher magnesium concentration at the surface is expected to improve resistance to corrosion from hydrofluoric acid generated by electrolyte decomposition.

[0059] Furthermore, it is preferable that the concentration of fluorine in the surface portion of the positive electrode active material 100 is also higher than the average concentration of the entire particle. By having fluorine present in the surface portion of the area in contact with the electrolyte, the corrosion resistance to hydrofluoric acid can be effectively improved.

[0060] Furthermore, it is preferable that the concentration of either titanium or aluminum in the surface layer is higher than the overall particle average. The presence of a large amount of either titanium or aluminum in regions with high magnesium concentration enhances the effect of suppressing changes in the CoO2 layer. Additionally, oxidative decomposition of the electrolyte on the surface of the positive electrode active material is less likely to occur.

[0061] Preferably, the concentration of at least one of magnesium, fluorine, titanium, and aluminum in the surface layer of the positive electrode active material 100 is higher than that in the interior; thus, it has a different composition from the interior. This composition preferably employs a crystalline structure that is stable at room temperature. Therefore, the surface layer can also have a different crystalline structure from the interior. For example, at least a portion of the surface layer of the positive electrode active material 100 can have a rock salt-type crystalline structure. Note that when the surface layer has a different crystalline structure from the interior, the orientation of the crystals in the surface layer and the interior is preferably substantially the same.

[0062] Note that when the positive electrode active material 100 contains magnesium and titanium, the peak value of the titanium concentration is preferably located in a region deeper than the peak value of the magnesium concentration. Since titanium can be tetravalent or trivalent, the distance between titanium and oxygen varies depending on the valence of titanium. Therefore, even if the distance between the metal and oxygen is not uniform, the area around the titanium atoms is easily stabilized. For example, when the surface layer of the positive electrode active material 100 has a rock salt-type crystalline structure, the region containing titanium acts as a buffer zone, contributing to the stabilization of the internal crystalline structure.

[0063] However, when the surface layer contains only MgO or only a solid solution of MgO and CoO(II), as mentioned above, there is no pathway for lithium insertion and extraction. Therefore, the surface layer needs to contain at least cobalt and, during discharge, also lithium to provide pathways for lithium insertion and extraction. Furthermore, the concentration of cobalt is preferably higher than that of magnesium.

[0064] Crystal Boundary Magnesium, halogens, cobalt, aluminum or titanium contained in the positive electrode active material 100 may exist irregularly and in small amounts inside, but more preferably, some of them are segregated at the grain boundaries.

[0065] In other words, the magnesium concentration at and near the crystal boundaries of the positive electrode active material 100 is preferably higher than that in other internal regions. Additionally, the fluorine concentration at and near the crystal boundaries is also preferably high. Furthermore, the concentration of either titanium or aluminum at and near the crystal boundaries is also preferably high.

[0066] Similar to particle surfaces, grain boundaries are also planar defects. Therefore, they are prone to instability and changes in the crystalline structure are easily initiated. Consequently, a high magnesium concentration at and near the grain boundaries can more effectively suppress changes in the crystalline structure. Furthermore, a high concentration of either titanium or aluminum at and near the grain boundaries can strongly suppress changes in the CoO2 layer.

[0067] Furthermore, when the concentrations of magnesium and fluorine at and near the crystal boundaries are high, even when cracks occur along the crystal boundaries of the particles of the positive electrode active material 100, the concentrations of magnesium and fluorine near the surface where the cracks occur also increase. Therefore, the corrosion resistance of the positive electrode active material to hydrofluoric acid can also be improved after the cracks occur.

[0068] Note that in this specification, the vicinity of the crystal boundary refers to the region from the crystal boundary to approximately 10 nm.

[0069] Particle size When the particle size of the positive electrode active material 100 is too large, the following problems arise: lithium diffusion becomes difficult; the surface of the active material layer is too rough when coated on the current collector, etc. On the other hand, when the particle size of the positive electrode active material 100 is too small, the following problems arise: the active material layer is not easily supported when coated on the current collector; excessive reaction with the electrolyte, etc. Therefore, D50 (also known as median particle size) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less.

[0070] ˂Analytical Methods˃ To determine whether a material exhibits a pseudo-spinel-type crystal structure in the positive electrode active material 100 of one embodiment of the present invention when charged at a high voltage, the positive electrode charged at a high voltage can be determined by analysis using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. In particular, XRD is preferred because it offers advantages such as: high-resolution analysis of the crystal structure of the positive electrode active material; comparison of crystallinity and crystal orientation; analysis of lattice periodic distortion and grain size; and sufficient accuracy when directly measuring the positive electrode obtained by disassembling a secondary battery.

[0071] As described above, the positive electrode active material 100 of one embodiment of the present invention is characterized by minimal change in crystal structure between the high-voltage charging state and the discharge state. Materials with a large change in crystal structure between high-voltage charging and discharging (more than 50%) are not preferred because they cannot withstand high-voltage charging and discharging. Details will be explained in Example 1. Note that sometimes the desired crystal structure cannot be achieved simply by adding elements. For example, lithium cobalt oxide containing magnesium and fluorine sometimes has a pseudo-spinel crystal structure of more than 60 wt%, and sometimes has an H1-3 type crystal structure of more than 50%. Furthermore, the pseudo-spinel crystal structure becomes almost 100% when using a specified voltage, and an H1-3 type crystal structure sometimes occurs when the specified voltage is further increased. Therefore, when determining whether it is the positive electrode active material 100 of one embodiment of the present invention, analysis of the crystal structure, such as XRD, is required.

[0072] Charging Methods The high-voltage charging used to make the above judgment can be performed, for example, by using a coin battery (CR2032 type, 20mm in diameter and 3.2mm in height) made of lithium as the counter electrode.

[0073] More specifically, the positive electrode can be a positive electrode made by coating an aluminum foil positive current collector with a slurry made by mixing positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) in a ratio of positive electrode active material:AB:PVDF=95:3:2 (by weight).

[0074] Lithium metal can be used as the counter electrode. Note that the potential of the positive electrode differs from that of a secondary battery when a material other than lithium metal is used as the counter electrode. For example, considering the potential of the positive electrode, a 4.5V charge with a graphite counter electrode is roughly equivalent to a 4.6V charge with a lithium counter electrode. Unless otherwise specified, the voltages and potentials in this manual, etc., refer to the potential of the positive electrode.

[0075] The electrolyte used is 1 mol / L lithium hexafluorophosphate (LiPF6). Alternatively, an electrolyte solution can be used, which is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, along with 2 wt% ethylene carbonate (VC).

[0076] Polypropylene with a thickness of 25 μm can be used as an insulator.

[0077] The positive and negative electrode containers can be made of stainless steel (SUS).

[0078] The coin cell battery manufactured under the above conditions was charged at a constant current of 4.6V and 0.5C, and then charged at a constant voltage until the current value reached 0.01C. Here, 1C was set to 137mA / g. The temperature was set to 25°C. After charging as described above, the coin cell battery was disassembled and the positive electrode was removed, thereby obtaining the positive electrode active material charged at a high voltage. In order to prevent the removed positive electrode active material from reacting with external components during subsequent analyses, it is preferable to perform the analysis under an argon atmosphere. For example, XRD can be performed under conditions of a sealed container under an argon atmosphere.

[0079] XRD Figure 3 shows the ideal powder XRD patterns, represented by CuKα1 lines, calculated from models of pseudo-spinel and H1-3 type crystal structures. Additionally, for comparison, Figure 3 also shows the ideal XRD patterns calculated from the crystal structures of LiCoO2(O3) with a charge depth of 0 and CoO2(O1) with a charge depth of 1. The patterns of LiCoO2(O3) and CoO2(O1) were calculated using Reflex Powder Diffraction, one of the modules in Materials Studio (BIOVIA), based on crystal structure information obtained from the ICSD (Inorganic Crystal Structure Database). The range of 2θ was set to 15° to 75°, step size = 0.01, wavelength λ1 = 1.540562 × 10⁻¹⁰ m, λ2 was not set, and the Monochromator was set to single. The pattern of the H1-3 type crystal structure was calculated similarly based on the crystal structure information described in Non-Patent Document 4. The XRD pattern of the pseudo-spinel crystal structure was deduced from the XRD pattern of the positive electrode active material of one embodiment of the present invention using TOPAS version 3 manufactured by Bruker AXS Rietveld analysis software, and the XRD pattern was prepared in the same manner as others. Note that the XRD pattern of the positive electrode active material of one embodiment of the present invention is shown in Example 1.

[0080] As shown in Figure 3, in the pseudo-spinel crystal structure, diffraction peaks occur at 2θ of 19.30 ± 0.20° (above 19.10° and below 19.50°) and 2θ of 45.55 ± 0.10° (above 45.45° and below 45.65°). More specifically, sharp diffraction peaks appear at 2θ of 19.30 ± 0.10° (above 19.20° and below 19.40°) and 2θ of 45.55 ± 0.05° (above 45.50° and below 45.60°). However, the H1-3 type crystal structure and CoO2 (P-3m1, O1) do not exhibit peaks at the above positions. Therefore, it can be said that the presence of peak values ​​at 2θ of 19.30±0.20° and 2θ of 45.55±0.10° when charged at high voltage is a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0081] Note that the positive electrode active material 100 of one embodiment of the present invention has a pseudo-spinel crystalline structure when charged with a high voltage, but it is not necessary for all particles to have a pseudo-spinel crystalline structure. Other crystalline structures are possible, and some particles may also be amorphous. Note that when performing Rietwald analysis on the XRD pattern, the pseudo-spinel crystalline structure is preferably 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more. When the pseudo-spinel crystalline structure is 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more, a positive electrode active material with sufficiently excellent cycle characteristics can be achieved.

[0082] Furthermore, the grain size of the pseudo-spinel crystal structure in the positive electrode active material is reduced to only about 1 / 10 of that in the discharged state of LiCoO2(O3). Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, a distinct peak value of the pseudo-spinel crystal structure can be confirmed after high-voltage charging. On the other hand, even if a portion of pure LiCoO2 may possess a structure similar to the pseudo-spinel crystal structure, the grain size will be smaller, and its peak value will be wider and smaller. The grain size can be determined from the half-width at half-maximum (HWHM) of the XRD peak value.

[0083] The XRD pattern reveals the structural characteristics of the internal structure of the positive electrode active material. In positive electrode active materials with a particle size (D50) of approximately 1 μm to 100 μm, the surface layer is very small compared to the interior. Therefore, even if the surface layer of the positive electrode active material 100 has a different crystalline structure than the interior, it may not be reflected in the XRD pattern.

[0084] ESR As shown in Figures 1 and 4A, in the positive electrode active material 100 with a pseudo-spinel crystal structure, cobalt exists at the oxygen six-coordinate position. As shown in Figure 4B, in the oxygen six-coordinated cobalt, the 3d orbital domain splits into the eg orbital domain and the t2g orbital domain, with the t2g orbital domain, which is configured to avoid the direction of oxygen presence, having low energy. Part of the cobalt present at the oxygen six-coordinate position is diamagnetic Co3+ cobalt, where the t2g orbital domain is completely filled. The other part of the cobalt present at the oxygen six-coordinate position can also be paramagnetic Co2+ or Co4+ cobalt. The aforementioned paramagnetic Co2+ or Co4+ cobalt both include an unpaired electron, so they cannot be determined using ESR, but can be determined based on the valence of the surrounding elements.

[0085] On the other hand, it has been documented that some conventional positive electrode active materials, when charged, can possess a spinel-type crystalline structure in which the surface layer does not contain lithium. In this case, Co3O4 exhibits the spinel-type crystalline structure shown in Figure 5A.

[0086] When spinel is represented by the general formula A[B2]O4, element A is tetracoordinated with oxygen and element B is hexacoordinated with oxygen. Therefore, in this specification, the position of the tetracoordinated oxygen is sometimes referred to as the A position, and the position of the hexacoordinated oxygen is sometimes referred to as the B position.

[0087] In spinel-type Co3O4, cobalt exists not only at the B site (six-coordinated with oxygen) but also at the A site (tetra-coordinated with oxygen). As shown in Figure 5B, in the tetra-coordinated cobalt, the eg orbital and t2g orbital domains split, with the eg orbital domain having lower energy. Therefore, the four-coordinated Co2+, Co3+, and Co4+ all contain unpaired electrons and are paramagnetic. Thus, when analyzing particles that fully contain spinel-type Co3O4 using ESR or similar methods, a peak of paramagnetic cobalt originating from Co2+, Co3+, or Co4+ will definitely be detected at the four-coordinated oxygen sites.

[0088] However, in one embodiment of the present invention, the peak value of cobalt derived from oxygen-tetracoordinated paramagnetic cobalt in the positive electrode active material 100 is so small that it cannot be confirmed. In other words, compared with conventional examples, the peak value of spinel-type Co3O4 in the positive electrode active material of one embodiment of the present invention, which can sometimes be detected by ESR or the like, is small or so small that it cannot be confirmed. Spinel-type Co3O4 does not contribute to the charge-discharge reaction and is thermally unstable, so the less spinel-type Co3O4, the better. From this point of view, it can also be said that the positive electrode active material 100 is different from conventional examples.

[0089] XPS X-ray photoelectron spectroscopy (XPS) can analyze from the surface down to a depth of approximately 2 to 8 nm (typically around 5 nm), allowing for quantitative analysis of elemental concentrations in about half of the surface region. Furthermore, narrow-scan analysis can reveal elemental bonding states. XPS measurement accuracy is often around ±1 atom%, although the detection limit varies depending on the element, but is also around 1 atom%.

[0090] When performing XPS analysis on the positive electrode active material 100, the relative value of the magnesium concentration when the cobalt concentration is 1 is preferably 0.4 or higher and 1.5 or lower, more preferably 0.45 or higher and less than 1.00. Furthermore, the relative value of the fluorine concentration is preferably 0.05 or higher and 1.5 or lower, more preferably 0.3 or higher and 1.00 or lower. Additionally, the relative value of the concentration of either titanium or aluminum is preferably 0.05 or higher and 0.4 or lower, more preferably 0.1 or higher and 0.3 or lower.

[0091] Furthermore, when analyzing the positive electrode active material 100 using XPS, it is preferable to show a peak value of fluorine's bonding energy with other elements that is 682 eV or higher and less than 685 eV, more preferably around 684.3 eV. This value differs from the bonding energy of 685 eV for LiF and 686 eV for magnesium fluoride. In other words, when the positive electrode active material 100 contains fluorine, it is preferable to use bonds other than lithium fluoride and magnesium fluoride.

[0092] Furthermore, when performing XPS analysis on the positive electrode active material 100, it is preferable to show a peak value of the bonding energy between magnesium and other elements that is above 1302 eV and below 1304 eV, more preferably around 1303 eV. This value differs from the 1305 eV bonding energy of magnesium fluoride and is close to the bonding energy of MgO. In other words, when the positive electrode active material 100 contains magnesium, it is preferable to use bonds other than magnesium fluoride.

[0093] EDX In EDX measurements, the method of measuring along the edge of a scanning area and performing a two-dimensional evaluation within that area is sometimes called EDX surface analysis. Alternatively, the method of extracting data from linear regions from EDX surface analysis to evaluate the atomic concentration distribution within the positive electrode active material particles is sometimes called line analysis.

[0094] EDX surface analysis (e.g., elemental mapping) allows for the quantitative analysis of the concentrations of magnesium, fluorine, titanium, or aluminum in the interior, surface, and near grain boundaries. Additionally, EDX beam analysis can be used to analyze the peak concentrations of magnesium, fluorine, titanium, or aluminum.

[0095] When performing EDX analysis on the positive electrode active material 100, the peak concentration of magnesium in the surface layer preferably appears in the range from the surface of the positive electrode active material 100 to a depth of 3 nm towards the center, more preferably in the range to a depth of 1 nm, and even more preferably in the range to a depth of 0.5 nm.

[0096] Furthermore, the fluorine distribution of the positive electrode active material 100 preferably overlaps with the magnesium distribution. Therefore, during EDX analysis, the peak concentration of fluorine in the surface layer preferably appears in the range from the surface of the positive electrode active material 100 to a depth of 3 nm towards the center, more preferably in the range to a depth of 1 nm, and even more preferably in the range to a depth of 0.5 nm.

[0097] In addition, during EDX analysis, the peak concentration of at least one of titanium and aluminum in the surface layer of the positive electrode active material 100 preferably appears in the range of 0.2 nm or more and 10 nm or less from the surface of the positive electrode active material 100 toward the center, and more preferably appears in the range of 0.5 nm or more and 3 nm or less.

[0098] Furthermore, when performing line or surface analysis of the positive electrode active material 100, the ratio of magnesium to cobalt atoms (Mg / Co) near the grain boundaries is preferably 0.020 or higher and 0.50 or lower. More preferably, it is 0.025 or higher and 0.30 or lower. Even more preferably, it is 0.030 or higher and 0.20 or lower.

[0099] [Manufacturing method of positive electrode active material] Next, an example of a method for manufacturing the positive electrode active material 100 according to one embodiment of the present invention will be described.

[0100] Step S11: Preparation of Starting Materials First, lithium and cobalt sources are prepared as starting materials. Additionally, magnesium and fluorine sources are also preferably prepared as starting materials.

[0101] Lithium can be used as a lithium source, for example, lithium carbonate or lithium fluoride. Magnesium oxide can be used as a cobalt source, for example, magnesium oxide. Magnesium oxide, magnesium fluoride, magnesium hydroxide, lithium carbonate, etc., can be used as a magnesium source, for example, magnesium oxide or magnesium fluoride. In other words, lithium fluoride can be used as both a lithium and a fluorine source.

[0102] The atomic weight of magnesium contained in the magnesium source is preferably 0.001 or more and 0.1 or less when the atomic weight of cobalt is 1, more preferably 0.005 or more and 0.02 or less, and even more preferably around 0.01.

[0103] The fluorine in the fluorine source is preferably more than 1.0 times and less than 4 times the magnesium in the magnesium source (atomic ratio), and more preferably more than 1.5 times and less than 3 times.

[0104] Step S12: Mixing of starting materials Next, the above starting materials are mixed. Mixing can be done using, for example, a ball mill or a sand mill. When using a ball mill, zirconia balls are preferably used as the mixing medium.

[0105] Step S13: First heat treatment Next, the material mixed in step S12 is heated. This step is sometimes referred to as firing or the first heating treatment. Heating is preferably performed at 800°C or higher but less than 1100°C, more preferably at 900°C or higher but less than 1000°C, and even more preferably at around 950°C. At excessively low temperatures, there is concern about decomposition of the starting material and insufficient melting. On the other hand, at excessively high temperatures, there is concern about Co becoming divalent due to reduction of Co and evaporation of Li.

[0106] The heating time is preferably 2 hours or more and 20 hours or less. Firing is preferably carried out in an atmosphere such as dry air. Preferably, heating is performed at 1000°C for 10 hours, with a heating rate of 200°C / h and a dry atmosphere flow rate of 10 L / min. Then, the heated material is cooled to room temperature. For example, the cooling time from the holding temperature to room temperature is preferably 10 hours or more and 50 hours or less.

[0107] Lithium cobalt oxide can be synthesized by heating in step S13. When the starting material contains magnesium and fluorine, it becomes particles of a composite oxide in which magnesium and fluorine are distributed in lithium cobalt oxide.

[0108] Alternatively, pre-synthesized composite oxide particles containing lithium, cobalt, fluorine, and magnesium can be used as starting materials. In this case, steps S12 and S13 can be omitted. For example, lithium cobalt oxide particles (trade name: C-20F) manufactured by NIPPON CHEMICAL INDUSTRIAL CO.,LTD. can be used as one of the starting materials. These particles are lithium cobalt oxide particles with a particle size of about 20 μm and contain fluorine, magnesium, calcium, sodium, silicon, sulfur, and phosphorus from the surface to the region that can be analyzed by XPS.

[0109] Step S14: Cover with a material containing at least one of titanium and aluminum. Next, it is preferable to cover the surface of the lithium cobalt oxide particles with a material containing at least one of titanium and aluminum. As a method for performing the coating, liquid-phase methods such as sol-gel, solid-phase methods, sputtering, evaporation, CVD (chemical vapor deposition), and PLD (pulsed laser deposition) can be used. In this embodiment, the case of using the sol-gel method, which is expected to achieve uniform coating and can be processed at atmospheric pressure, will be described.

[0110] First, dissolve titanium alkoxide, aluminum alkoxide, or a mixture thereof in alcohol, and then mix in lithium cobalt oxide particles.

[0111] For example, titanium tetraisopropoxide (TTIP) can be used as a titanium alkoxide. For example, aluminum isopropoxide can be used as an aluminum alkoxide. Additionally, isopropanol can be used as a solvent.

[0112] The required amount of metal alkoxide varies depending on the particle size of lithium cobalt oxide. For example, when using TTIP and the lithium cobalt oxide particle size (D50) is approximately 20 μm, it is preferable to add 0.004 ml / g or more and 0.01 ml / g or less of TTIP to the lithium cobalt oxide particles. When using aluminum isopropoxide and with the same particle size, it is preferable to add 0.0279 g / g or more and 0.0697 g / g or less of aluminum isopropoxide to the lithium cobalt oxide particles.

[0113] Next, the mixture of the metal alkoxide alcohol solution and the lithium cobalt oxide is stirred in an atmosphere containing water vapor. For example, a magnetic stirrer can be used. The stirring time is sufficient for the hydrolysis and condensation reaction between the water in the atmosphere and the metal alkoxide; for example, it can be stirred at 25°C for 4 hours at 90% RH (relative humidity).

[0114] By reacting the metal alkoxide with water vapor in the atmosphere, the sol-gel reaction can proceed more slowly compared to adding liquid water. Furthermore, by reacting the metal alkoxide with water at room temperature, for example, compared to heating at a temperature exceeding the boiling point of the alcohol in the solvent, the sol-gel reaction can proceed more slowly. This slow sol-gel reaction allows for the formation of a uniform and high-quality coating layer.

[0115] Collect the precipitate from the mixture after the above treatment. Collection methods may include filtration, centrifugation, evaporation, and drying / solidification. The precipitate can be washed with an alcohol that is the same solvent used to dissolve the metal alkoxide.

[0116] Next, the collected residue is dried. For example, it can be dried under vacuum or ventilation at 70°C for more than 1 hour but less than 4 hours.

[0117] Step S15: Second heat treatment Next, the lithium cobalt oxide particles coated with a material containing titanium or aluminum, produced in step S14, are heated. This step is sometimes referred to as the second heat treatment.

[0118] During heating, the time for maintaining the temperature is preferably 1 hour or more but less than 50 hours, and more preferably 2 hours or more but less than 20 hours. If the heating time is too short, there is a concern that insufficient segregation may occur in the surface layer and near the grain boundaries when magnesium and fluorine are added. However, if the heating time is too long, there is a concern that excessive diffusion of the aforementioned metals may reduce the concentration in the surface layer and near the grain boundaries when titanium or aluminum is used as a coating.

[0119] The optimal operating temperature is 500℃ or higher and 1200℃ or lower, more preferably 700℃ or higher and 920℃ or lower, and even more preferably 800℃ or higher and 900℃ or lower. There is a concern that magnesium segregation may not occur if the operating temperature is too low. However, there are also concerns if the operating temperature is too high: Mg may also be distributed at Co sites; Co2+ (not Co3+ as in LiCO2) may become more stable, leading to the inability to maintain the layered structure of CoO2, etc.

[0120] The second heat treatment is preferably carried out in an oxygen-containing atmosphere. However, at low oxygen partial pressures, there is a concern about Co reduction if the heating temperature is not further reduced.

[0121] In this embodiment, heating is performed under the following conditions: the temperature is maintained at 800°C; the holding time is 2 hours; the heating rate is 200°C / h; and the oxygen flow rate is 10L / min.

[0122] Setting a longer cooling time after heating makes it easier to stabilize the crystal structure, which is preferable. For example, the cooling time from the holding temperature to room temperature is preferably more than 10 hours and less than 50 hours.

[0123] Thus, it is preferable to perform multiple heating processes as in the first heating treatment (step S13) and the second heating treatment (step S15). In the first heating treatment, to ensure sufficient reaction between the starting materials, heating is performed at a temperature higher than the melting point of Co3O4 (895°C) and the melting point of Li2Co3 (723°C). In the second heating treatment, to ensure magnesium distribution between the CoO2 layers, heating is performed at a temperature lower than that of the first heating treatment. Specifically, according to the Ellingham diagram, the temperature at which Co3+ is more stable than Co2+ in the atmosphere is 920°C, so the second heating treatment is preferably performed at a temperature below 920°C.

[0124] Step S16: Collect Next, the cooled particles are collected. Preferably, the particles are sieved. Through the above process, a positive electrode active material 100 according to one embodiment of the present invention can be manufactured.

[0125] Furthermore, after step S16, the sol-gel coating process from steps S14 to S16 can be repeated multiple times. This can be repeated once or more. By repeatedly performing the sol-gel process and heat treatment, cracks can be reduced when they occur in the lithium cobalt oxide particles.

[0126] Furthermore, the types of metal alkoxides used in multiple sol-gel treatments can be the same or different. For example, when using different metal alkoxides, titanium alkoxides can be used in the first sol-gel treatment and aluminum alkoxides in the second.

[0127] Note that this embodiment describes the use of a material containing lithium, cobalt, and oxygen as the positive electrode active material 100, but one embodiment of the present invention is not limited to this. For example, the transition metal included in the positive electrode active material 100 is not limited to cobalt, and may also include at least one of nickel and manganese in very small amounts. In addition, in addition to the aforementioned transition metals, a very small amount of aluminum may be added to the starting material.

[0128] Furthermore, in one embodiment of the present invention, it is sufficient to suppress the change in the crystal structure of the positive electrode active material between when it is fully charged and when it is fully discharged. Therefore, it may not have the pseudo-spinel type crystal structure defined in this specification, nor may it contain elements such as magnesium, fluorine, titanium, or aluminum.

[0129] In addition, the positive electrode active material 100 may also contain other elements such as carbon, sulfur, silicon, sodium, calcium, and zirconium.

[0130] This embodiment can be used in combination with other embodiments as appropriate.

[0131] Implementation Method 2 In this embodiment, an example of a material that can be used in a secondary battery including the positive electrode active material 100 described in the above embodiment will be described. In this embodiment, a secondary battery in which the positive electrode, negative electrode, and electrolyte are surrounded by an outer packaging body will be used as an example.

[0132] [positive electrode] The positive electrode includes the positive electrode active material layer and the positive electrode current collector.

[0133] <Positive Electrode Active Material Layer> The positive electrode active material layer contains at least the positive electrode active material. In addition to the positive electrode active material, the positive electrode active material layer may also contain other substances such as a coating, conductive additives, or adhesives on the surface of the active material.

[0134] The positive electrode active material 100 described in the above embodiments can be used as the positive electrode active material. By using the positive electrode active material 100 described in the above embodiments, a secondary battery with high capacity and excellent cycle characteristics can be realized.

[0135] Carbon materials, metallic materials, or conductive ceramic materials can be used as conductive additives. Furthermore, fibrous materials can also be used. The proportion of the conductive additive in the total amount of the active material layer is preferably 1 wt% or more and 10 wt% or less, more preferably 1 wt% or more and 5 wt% or less.

[0136] By using conductive additives, a conductive network can be formed in the active material layer. By using conductive additives, conductive pathways between the positive electrode active materials can be maintained. By adding conductive additives to the active material layer, an active material layer with high conductivity can be achieved.

[0137] As conductive additives, examples include natural graphite, artificial graphite such as mesophase carbon microspheres, and carbon fibers. For carbon fibers, examples include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Carbon nanofibers or carbon nanotubes can also be used. For example, carbon nanotubes can be manufactured using methods such as vapor phase growth. As conductive additives, examples include carbon materials such as carbon black (acetylene black (AB), graphite (lead black) particles, graphene, or fullerenes. Furthermore, metal powders or fibers of copper, nickel, aluminum, silver, gold, etc., and conductive ceramic materials can also be used.

[0138] In addition, graphene compounds can also be used as conductive additives.

[0139] Graphene compounds sometimes possess excellent electrical properties such as high conductivity, as well as excellent physical properties such as high flexibility and high mechanical strength. Furthermore, graphene compounds have a planar shape. Graphene compounds can form surface contacts with low contact resistance. Graphene compounds sometimes exhibit very high conductivity even when thin, thus allowing conductive pathways to be formed efficiently in small quantities within the active material layer. Therefore, using graphene compounds as conductive additives increases the contact area between the active material and the conductive additive, which is preferable. Preferably, the graphene compound used as a conductive additive can be formed by using a spray drying device to cover the entire surface of the active material. Furthermore, resistance can be reduced, which is also preferable. Particularly preferred here are graphene, multilayer graphene, or RGO (Reduced Graphene Oxide) used as the graphene compound. Here, RGO refers, for example, to a compound obtained by reducing graphene oxide (GO).

[0140] When using active materials with small particle sizes, such as those with a particle size of less than 1 μm, the specific surface area of ​​the active material is large, thus requiring more conductive paths connecting the active materials. Therefore, the amount of conductive additive tends to increase, resulting in a relative decrease in the content of the active material. When the content of the active material decreases, the capacity of the secondary battery also decreases. In this case, as a conductive additive, graphene compounds, which can efficiently form conductive paths even in small amounts, are particularly preferred because it is not necessary to reduce the content of the active material.

[0141] The following is an example illustrating the cross-sectional structure of the active material layer 200 containing graphene compounds as a conductive additive.

[0142] Figure 6A is a longitudinal cross-sectional view of the active material layer 200. The active material layer 200 includes granular positive electrode active material 100, graphene compound 201 used as a conductive additive, and a binder (not shown). Here, as the graphene compound 201, graphene or multilayer graphene can be used, for example. Furthermore, the graphene compound 201 is preferably in a sheet-like form. The graphene compound 201 can be formed into a sheet by partially overlapping multiple multilayer graphene or (and) multiple monolayer graphene.

[0143] As shown in Figure 6B, in the longitudinal cross-section of the active material layer 200, sheet-like graphene compounds 201 are generally uniformly dispersed within the active material layer 200. Although graphene compounds 201 are schematically represented by thick lines in Figure 6B, they are actually thin films with a single or multiple layers of carbon molecules. Since multiple graphene compounds 201 are formed either by covering a portion of multiple granular positive electrode active materials 100 or by adhering to the surface of multiple granular positive electrode active materials 100, the graphene compounds 201 form surface contact with the positive electrode active materials 100.

[0144] Here, by bonding multiple graphene compounds together, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound mesh or graphene network) can be formed. When the graphene network covers the active material, it can be used as a binder to bind the compounds together. Therefore, the amount of binder can be reduced or eliminated, thereby increasing the proportion of active material in the electrode volume or weight. In other words, the capacity of the secondary battery can be increased.

[0145] Preferably, graphene oxide is used as the graphene compound 201. This graphene oxide is mixed with an active material to form a layer that will become the active material layer 200, and then reduction is performed. By using highly dispersible graphene oxide in a polar solvent in the formation of the graphene compound 201, the graphene compound 201 can be dispersed substantially uniformly in the active material layer 200. The solvent is evaporated from the dispersion medium containing the uniformly dispersed graphene oxide, and the graphene oxide is reduced. Therefore, the graphene compounds 201 remaining in the active material layer 200 partially overlap each other, dispersing in a surface-contact manner, thereby forming a three-dimensional conductive path. Alternatively, the reduction of graphene oxide can also be performed, for example, by heat treatment or by using a reducing agent.

[0146] Therefore, unlike granular conductive additives such as acetylene black that form point contacts with the active material, graphene compound 201 can form surface contacts with low contact resistance. Thus, the conductivity between the granular positive electrode active material 100 and graphene compound 201 can be improved with less graphene compound 201 than with typical conductive additives. This increases the proportion of positive electrode active material 100 in the active material layer 200, thereby increasing the discharge capacity of the secondary battery.

[0147] In addition, by using a spray drying device in advance, a graphene compound that serves as a conductive additive for coating can be formed in a way that covers the entire surface of the active material, and the conductive path between the active materials is formed by the graphene compound.

[0148] Preferred adhesives include, for example, styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-isoprene-styrene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Fluororubber can also be used as an adhesive.

[0149] Furthermore, water-soluble polymers are preferably used as adhesives. Examples of water-soluble polymers include polysaccharides. Among polysaccharides, cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, as well as starch, can be used. It is even more preferable to use these water-soluble polymers in combination with the aforementioned rubber material.

[0150] Alternatively, materials preferred as adhesives include polystyrene, polymethyl acrylate, 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 monomer (EPDM), polyvinyl acetate, and nitrocellulose.

[0151] As an adhesive, multiple of the above materials can also be used in combination.

[0152] For example, materials with particularly high viscosity-regulating properties can be combined with other materials. For instance, while materials like rubber have high adhesive strength and elasticity, viscosity regulation can sometimes be difficult when mixed in a solvent. In such cases, it is preferable to mix them with materials that have particularly high viscosity-regulating properties. For example, water-soluble polymers can be used as materials with particularly high viscosity-regulating properties. Furthermore, polysaccharides with particularly good viscosity-regulating properties can be used, such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, and starch.

[0153] Note that cellulose derivatives such as carboxymethyl cellulose, for example, by being converted into sodium or ammonium salts of carboxymethyl cellulose, have increased solubility and thus readily function as viscosity modifiers. Due to the increased solubility, the dispersibility of the active material with other components can be improved when forming the electrode slurry. In this specification, cellulose and cellulose derivatives used as binders for electrodes include their salts.

[0154] By dissolving water-soluble polymers in water to stabilize their viscosity, active materials and other materials used as binders, such as styrene-butadiene rubber, can be stably dispersed in aqueous solutions. Because water-soluble polymers possess functional groups, they are expected to readily and stably adhere to the surface of active materials. Cellulose derivatives, such as carboxymethyl cellulose, often possess functional groups such as hydroxyl and carboxyl groups. Due to these functional groups, the polymers are expected to interact and extensively cover the surface of active materials.

[0155] When an adhesive forms a film covering or contacting the surface of the active material, it is also desirable for it to function as a passivation film to suppress electrolyte decomposition. Here, the passivation film is a film with no conductivity or extremely low conductivity; for example, when a passivation film is formed on the surface of the active material, it can suppress electrolyte decomposition at the battery reaction potential. More preferably, the passivation film can transport lithium ions while suppressing conductivity.

[0156] Positive current collector As a positive electrode current collector, highly conductive materials such as stainless steel, gold, platinum, aluminum, titanium, and their alloys can be used. Furthermore, the material used for the positive electrode current collector is preferably one that does not dissolve due to the positive electrode potential. Additionally, aluminum alloys with added elements to improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, can also be used. Alternatively, metal elements that react with silicon to form silicates can be used. Metal elements that react with silicon to form silicates include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can appropriately have shapes such as foil, plate (sheet), mesh, perforated metal mesh, and expanded metal mesh. The thickness of the current collector is preferably 5 μm or more and 30 μm or less.

[0157] [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 contain conductive additives and binders.

[0158] <Negative Electrode Active Material> As the negative electrode active material, alloy materials or carbon materials can be used, for example.

[0159] As the negative electrode active material, elements capable of undergoing charge-discharge reactions through alloying / dealloying with lithium can be used. 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 higher capacity than carbon, especially silicon, which has a theoretical capacity of 4200 mAh / g. Therefore, silicon is preferred as the negative electrode active material. Alternatively, 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. Elements that can undergo charge-discharge reactions through alloying / dealloying with lithium, and compounds containing such elements, are sometimes referred to as alloy materials.

[0160] In this specification, SiO refers to silicon monoxide, for example. Alternatively, SiO may be represented as SiOx. Here, x is preferably a value close to 1. For example, x is preferably 0.2 or higher and 1.5 or lower, more preferably 0.3 or higher and 1.2 or lower.

[0161] As carbon-based materials, graphite, easily graphitized carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), carbon nanotubes, graphene, carbon black, etc. can be used.

[0162] Examples of graphite include synthetic graphite and natural graphite. Examples of synthetic graphite include mesophase carbon microspheres (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Spherical graphite with a spherical shape can be used as synthetic graphite. For example, MCMB sometimes has a spherical shape, which is preferred. Furthermore, MCMB is sometimes preferred because it is easier to reduce its surface area. Examples of natural graphite include flake graphite and spheroidized natural graphite.

[0163] When lithium ions are intercalated in graphite (during the formation of lithium-graphite intercalation compounds), graphite exhibits a low potential similar to that of lithium metal (above 0.05V and below 0.3V vs. Li / Li+). Therefore, lithium-ion secondary batteries can exhibit high operating voltages. Graphite also has the following advantages: higher capacity per unit volume; smaller volume expansion; lower cost; and higher safety compared to lithium metal, making it a superior choice.

[0164] In addition, oxides such as titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O12), lithium-graphite intercalation compound (LixC6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), and molybdenum oxide (MoO2) can be used as negative electrode active materials.

[0165] Alternatively, as the negative electrode active material, Li3-xMxN (M=Co, Ni, Cu) with a Li3N-type structure containing lithium and transition metal nitrides can be used. For example, Li2.6Co0.4N3 exhibits a large charge-discharge capacity (900 mAh / g, 1890 mAh / cm3) and is therefore preferred.

[0166] When lithium- and transition metal-containing nitrides are used as negative electrode active materials, the negative electrode active material contains lithium ions. Therefore, it is preferable to combine this negative electrode active material with materials that do not contain lithium ions, such as V₂O₅ and Cr₃O₈, which are used as positive electrode active materials. Note that when lithium-ion-containing materials are used as positive electrode active materials, lithium- and transition metal-containing nitrides can also be used as negative electrode active materials by pre-degrading the lithium ions contained in the positive electrode active material.

[0167] In addition, materials that induce the conversion reaction can also be used as negative electrode active materials. 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 negative electrode active materials. Other materials that induce the conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3; sulfides such as CoS0.89, NiS, and CuS; nitrides such as Zn3N2, Cu3N, and Ge3N4; phosphides such as NiP2, FeP2, and CoP3; and fluorides such as FeF3 and BiF3.

[0168] The conductive additives and binders that can be included in the negative electrode active material layer can be the same materials that can be included in the positive electrode active material layer.

[0169] Negative current collector As the negative electrode current collector, the same material as the positive electrode current collector can be used. Furthermore, it is preferable to use a material that does not alloy with carrier ions such as lithium as the negative electrode current collector.

[0170] Electrolyte The electrolyte comprises a solvent and an electrolyte. Preferably, a non-protic organic solvent is used as the solvent for the electrolyte, such as one of the following: ethylene carbonate (EC), propylene carbonate (PC), butenyl carbonate, vinyl chloride carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethyl glycol ether (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, cyclobutane, sulfolactone, etc., or two or more of the above can be used in any combination and ratio.

[0171] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, even if the internal temperature of the secondary battery rises due to internal short circuits, overcharging, etc., it can prevent the secondary battery from rupturing or catching fire. 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 strontium cations, and quaternary phosphonium cations, or aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methylide anions, fluorosulfonic acid anions, perfluoroalkyl sulfonic acid anions, tetrafluoroborate anions, perfluoroalkyl borate anions, hexafluorophosphate anions, or perfluoroalkyl phosphate anions.

[0172] In addition, as the electrolyte dissolved in the above solvents, one of the following lithium salts can be used, for example: LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, Li2B10Cl10, Li2B12Cl12, LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, or two or more of the above can be used in any combination and ratio.

[0173] As an electrolyte for secondary batteries, it is preferable to use a highly purified electrolyte with low content of particulate dust or elements other than the constituent elements of the electrolyte (hereinafter referred to as "impurities"). Specifically, the proportion of impurities in the weight of the electrolyte is 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0174] In addition, additives such as vinylene carbonate, propanesulfonate lactone (PS), terebutylbenzene (TBB), fluoroethylene carbonate (FEC), lithium dioxoborate (LiBOB), or dinitrile compounds such as succinate and adiponitrile can be added to the electrolyte. The concentration of the added material can be set, for example, to be more than 0.1 wt% and less than 5 wt% in the total solvent.

[0175] Alternatively, a polymer gel electrolyte in which the polymer has been swollen by an electrolyte solution can also be used.

[0176] Furthermore, the use of polymer gel electrolytes improves safety in the event of liquid leaks. It also allows for the reduction in the size and weight of the secondary device.

[0177] As a gelling polymer, silicone gel, acrylic acid gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, fluoropolymer gel, etc. can be used.

[0178] As polymers, polymers having a polyoxyalkylene structure, such as polyethylene oxide (PEO), PVDF and polyacrylonitrile, and copolymers containing these can be used. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. Furthermore, the resulting polymer can also have a porous shape.

[0179] Furthermore, solid electrolytes containing inorganic materials such as sulfides or oxides, or solid electrolytes containing polymeric materials such as PEO (polyethylene oxide), can be used instead of liquid electrolytes. When using solid electrolytes, there is no need to install separators or spacers. In addition, since the entire battery can be solidified, there is no concern about liquid leakage, thus significantly improving safety.

[0180] [Isolation] Furthermore, the secondary battery preferably includes a separator. The separator can be made of materials such as paper, non-woven fabric, glass fiber, ceramic, or synthetic fibers containing nylon (polyamide), vinylon (polyvinyl alcohol fibers), polyester, acrylic resin, polyolefin, or polyurethane. Preferably, the separator is processed into a bag shape and configured to surround either the positive or negative electrode.

[0181] The separator can have a multilayer structure. For example, ceramic materials, fluorinated materials, polyamide materials, or mixtures thereof can be coated onto organic materials such as polypropylene and polyethylene. Examples of ceramic materials include alumina particles and silicon oxide particles. Examples of fluorinated materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aromatic polyamides (meta-aromatic polyamides and para-aromatic polyamides).

[0182] Coating with ceramic materials can improve oxidation resistance, thereby suppressing the degradation of the separator during high-voltage charging and discharging, and thus improving the reliability of the secondary battery. Coating with fluorine-based materials facilitates a tighter connection between the separator and the electrodes, thereby improving output characteristics. Coating with polyamide materials (especially aromatic polyamides) can improve heat resistance, thereby improving the safety of the secondary battery.

[0183] For example, a mixture of alumina and aromatic polyamine can be coated on both sides of a polypropylene film. Alternatively, the side of the polypropylene film in contact with the positive electrode can be coated with a mixture of alumina and aromatic polyamine, while the side in contact with the negative electrode can be coated with a fluorinated material.

[0184] By employing a multi-layered separator, the safety of the secondary battery can be ensured even if the total thickness of the separator is small, thus increasing the capacity per unit volume of the secondary battery.

[0185] [Outer Packaging] The outer packaging of a secondary battery can be made of materials such as metals like aluminum and resins. Alternatively, a thin-film outer packaging can be used. For example, a three-layer film can be used: a flexible metal film such as aluminum, stainless steel, copper, or nickel is placed on a film made of materials such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide; and an insulating synthetic resin film such as polyamide or polyester resin can be placed on this metal film as the outer surface of the outer packaging.

[0186] [Charging and discharging method] The charging and discharging of a secondary battery can be performed, for example, as described below.

[0187] CC Charging First, let's explain CC charging as a charging method. CC charging refers to a charging method in which a constant current flows through the secondary battery throughout the entire charging period, and charging stops when the voltage of the secondary battery reaches a predetermined voltage. As shown in Figure 7A, the secondary battery is assumed to be an equivalent circuit of internal resistance R and secondary battery capacity C. In this case, the secondary battery voltage VB is the sum of the voltage VR applied to the internal resistance R and the voltage VC applied to the secondary battery capacity C.

[0188] During CC charging, as shown in Figure 7A, the switch is turned on, and a constant current I flows through the secondary battery. During this period, because the current I is constant, the voltage VR applied to the internal resistor R is constant according to Ohm's law of VR = R × I. On the other hand, the voltage VC applied to the secondary battery capacity C increases over time. Therefore, the secondary battery voltage VB increases over time.

[0189] Furthermore, charging stops when the secondary battery voltage VB reaches a specified voltage, such as 4.3V. When CC charging stops, as shown in Figure 7B, the switch closes, and the current I = 0. Therefore, the voltage VR applied to the internal resistor R becomes 0V. Thus, the decrease in the secondary battery voltage VB corresponds to the portion of the voltage across the internal resistor R that no longer decreases.

[0190] Figure 7C shows an example of the secondary battery voltage VB and charging current during and after CC charging. As can be seen from Figure 7C, the secondary battery voltage VB, which rises during CC charging, decreases slightly after CC charging is stopped.

[0191] CCCV Charging Next, a different charging method, namely CCCV charging, will be explained. CCCV charging refers to first performing CC charging to a specified voltage, and then performing CV (constant voltage) charging to reduce the current flowing through it; more specifically, charging to the point where the current reaches the termination value.

[0192] During CC charging, as shown in Figure 8A, the constant current switch is on and the constant voltage switch is off, thus a constant current I flows through the secondary battery. During this period, because the current I is constant, the voltage VR applied to the internal resistor R is constant according to Ohm's law of VR = R × I. On the other hand, the voltage VC applied to the secondary battery capacity C increases over time. Therefore, the secondary battery voltage VB increases over time.

[0193] Furthermore, when the secondary battery voltage VB reaches a predetermined voltage, such as 4.3V, the charging process switches from CC charging to CV charging. During CV charging, as shown in Figure 8B, the constant current switch is on and the constant voltage switch is off, thus keeping the secondary battery voltage VB constant. On the other hand, the voltage VC applied to the secondary battery capacity C increases over time. Because VB = VR + VC, the voltage VR applied to the internal resistor R decreases over time. As the voltage VR applied to the internal resistor R decreases, the current I flowing through the secondary battery decreases according to Ohm's law of VR = R × I.

[0194] Furthermore, charging stops when the current I flowing through the secondary battery reaches a predetermined current, for example, equivalent to 0.01C. When CCCV charging stops, as shown in Figure 8C, all switches are closed, and the current I = 0. Therefore, the voltage VR applied to the internal resistor R becomes 0V. However, because the voltage VR applied to the internal resistor R is sufficiently reduced by CV charging, the secondary battery voltage VB hardly decreases even if the voltage of the internal resistor R no longer decreases.

[0195] Figure 8D shows an example of the secondary battery voltage VB and charging current during and after CCCV charging. As can be seen from Figure 8D, the secondary battery voltage VB hardly decreases even after CCCV charging is stopped.

[0196] CC Charging Next, we will explain one of the discharge methods, CC discharge. CC discharge refers to a discharge method in which a constant current is released from the secondary battery throughout the entire discharge period, and the discharge stops when the secondary battery voltage VB reaches a specified voltage, such as 2.5V.

[0197] Figure 9 shows an example of the secondary battery voltage VB versus discharge current during CC discharge. As can be seen from Figure 9, the secondary battery voltage VB decreases as the discharge progresses.

[0198] Here, we will explain the discharge rate and charge rate. The discharge rate refers to the ratio of the discharge current to the battery capacity, and is expressed in units of C. In a battery with a rated capacity of X (Ah), the current equivalent to 1C is X (A). When discharging at a current of 2X (A), it can be said to be discharging at 2C, and when discharging at a current of X / 5 (A), it can be said to be discharging at 0.2C. Similarly, the charge rate is the same; when charging at a current of 2X (A), it can be said to be charging at 2C, and when charging at a current of X / 5 (A), it can be said to be charging at 0.2C.

[0199] Implementation Method 3 In this embodiment, an example of the shape of a secondary battery including the positive electrode active material 100 described in the above embodiments will be described. The materials used in the secondary battery described in this embodiment can be found in the description of the above embodiments.

[0200] [Coin-type rechargeable battery] First, let's illustrate an example of a coin-type secondary battery. Figure 10A is an external view of a coin-type (single-layer flat) secondary battery, and Figure 10B is a cross-sectional view of it.

[0201] In the coin-type secondary battery 300, the positive electrode container 301, which also serves as the positive terminal, and the negative electrode container 302, which also serves as the negative 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 disposed in contact with it. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 disposed in contact with it.

[0202] The active material layers included in the positive electrode 304 and negative electrode 307 of the coin-type secondary battery 300 can be formed on only one surface of the positive electrode and the negative electrode.

[0203] As the positive electrode container 301 and the negative electrode container 302, metals such as nickel, aluminum, and titanium, their alloys, or alloys of them with other metals (e.g., stainless steel) that are resistant to electrolyte corrosion can be used. Furthermore, to prevent corrosion caused by the electrolyte, the positive electrode container 301 and the negative electrode container 302 are preferably covered with nickel or aluminum. The positive electrode container 301 is electrically connected to the positive electrode 304, and the negative electrode container 302 is electrically connected to the negative electrode 307.

[0204] A coin-shaped secondary battery 300 is manufactured by immersing the negative electrode 307, positive electrode 304 and separator 310 in an electrolyte, as shown in Figure 10B, and stacking the positive electrode 304, separator 310, negative electrode 307 and negative electrode 302 in sequence with the positive electrode can 301 positioned below, and pressing the positive electrode can 301 and negative electrode can 302 together with a gasket 303.

[0205] By using the positive electrode active material described in the above embodiments in the positive electrode 304, a coin-type secondary battery 300 with high capacity and excellent cycle characteristics can be realized.

[0206] Here, referring to FIG10C, we will explain how current flows during the charging of a secondary battery. When a lithium-ion secondary battery is considered as a closed circuit, the direction of lithium ion migration is the same as the direction of current flow. Note that in a lithium-ion secondary battery, since the anode and cathode, oxidation reaction and reduction reaction are reversed depending on whether it is charging or discharging, 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, even when charging, discharging, supplying a reverse pulse current, and supplying a charging current, the positive electrode is referred to as the "positive electrode" or "+ electrode," and the negative electrode is referred to as the "negative electrode" or "- electrode." If the terms anode and cathode related to oxidation and reduction reactions are used, the anode and cathode are reversed during charging and discharging, which may cause confusion. Therefore, the terms anode and cathode are not used in this specification. When the terms anode and cathode are used, it is clearly indicated whether it is during charging or discharging, and it is shown whether it corresponds to the positive electrode (+ electrode) or the negative electrode (- electrode).

[0207] The two terminals shown in Figure 10C are 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.

[0208] Cylindrical secondary battery Next, an example of a cylindrical secondary battery will be described with reference to Figures 11A to 11D. As shown in Figure 11A, the cylindrical secondary battery 600 has a positive electrode cover (battery cover) 601 on the top surface and battery canisters (outer canisters) 602 on the sides and bottom surface. The positive electrode cover and the battery canisters (outer canisters) 602 are insulated from each other by a gasket (insulating gasket) 610.

[0209] Figure 11B is a schematic cross-sectional view of a cylindrical secondary battery. A battery element is disposed inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a strip-shaped negative electrode 606 are wound around an insulator 605. Although not shown, the battery element is wound around a central pin. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of metals such as nickel, aluminum, and titanium, alloys thereof, or alloys of these metals with other metals (e.g., stainless steel), which are resistant to electrolyte corrosion. Furthermore, to prevent corrosion caused by the electrolyte, the battery can 602 is preferably covered with nickel or aluminum. Inside the battery can 602, the battery element, consisting of the wound positive electrode, negative electrode, and insulator, is held between a pair of opposing insulating plates 608 and 609. Additionally, a non-aqueous electrolyte (not shown) is injected into the interior of the battery can 602 containing the battery element. As a non-aqueous electrolyte, the same electrolyte as that used in coin-type secondary batteries can be used.

[0210] Because the positive and negative electrodes of the cylindrical secondary battery are wound, the active material is preferably formed on both surfaces of the current collector. The positive electrode 604 is connected to the positive terminal (positive current collector wire) 603, and the negative electrode 606 is connected to the negative terminal (negative current collector wire) 607. Both the positive terminal 603 and the negative terminal 607 can be made of metal materials such as aluminum. The positive terminal 603 is resistor-welded to the safety valve mechanism 612, and the negative terminal 607 is resistor-welded to the bottom of the battery canister 602. The safety valve mechanism 612 is electrically connected to the positive cover 601 via a PTC (Positive Temperature Coefficient) element 611. When the internal pressure of the battery rises above a specified critical value, the safety valve mechanism 612 disconnects the electrical connection between the positive cover 601 and the positive electrode 604. Furthermore, the PTC element 611 is a heat-sensitive resistor whose resistance increases with temperature, and the increased resistance limits the current to prevent abnormal heating. As a PTC element, barium titanate (BaTiO3) type semiconductor ceramics can be used.

[0211] Alternatively, as shown in Figure 11C, multiple secondary batteries 600 can be sandwiched between conductive plates 613 and 614 to form a module 615. The multiple secondary batteries 600 can be connected in parallel, in series, or in parallel followed by series connection. By constructing a module 615 including multiple secondary batteries 600, a greater amount of power can be extracted.

[0212] Figure 11D is a top view of module 615. For clarity, the conductive plate 613 is indicated by dashed lines. As shown in Figure 11D, module 615 may include wires 616 that electrically connect multiple secondary batteries 600. The conductive plate may be disposed on the wires 616 in a manner overlapping the wires 616. Additionally, a temperature control device 617 may be included between the multiple secondary batteries 600. The temperature control device 617 can cool a secondary battery 600 when it is overheated and heat it when it is undercooled. Therefore, the performance of module 615 is less susceptible to external temperature fluctuations. The heat transfer medium included in the temperature control device 617 is preferably insulating and non-flammable.

[0213] By using the positive electrode active material described in the above embodiments in the positive electrode 604, a cylindrical secondary battery 600 with high capacity and excellent cycle characteristics can be realized.

[0214] [Example of a secondary battery structure] Other structural examples of secondary batteries are illustrated with reference to Figures 12A to 16C.

[0215] Figures 12A and 12B are external views of the secondary battery. The secondary battery 913 is connected to antennas 914 and 915 via circuit board 900. A label 910 is attached to the secondary battery 913. Furthermore, as shown in Figure 12B, the secondary battery 913 is connected to terminals 951 and 952.

[0216] The circuit board 900 includes terminals 911 and circuits 912. Terminals 911 are connected to terminals 951, 952, antennas 914 and 915, and circuits 912. Alternatively, multiple terminals 911 can be provided, and these terminals 911 can be used as control signal input terminals, power supply terminals, etc.

[0217] Circuit 912 can also be disposed on the back side of circuit board 900. Furthermore, the shapes of antennas 914 and 915 are not limited to coil shape; for example, they can also be wire-shaped or plate-shaped. Additionally, planar antennas, aperture antennas, traveling wave antennas, EH antennas, magnetic field antennas, or dielectric antennas can be used. Alternatively, antenna 914 or antenna 915 can also be a planar conductor. This planar conductor can also be used as one of the conductors for electric field coupling. In other words, antenna 914 or antenna 915 can also be used as one of the two conductors in a capacitor. Thus, not only electromagnetic and magnetic fields can be utilized, but also electric fields can be used to exchange power.

[0218] The linewidth of antenna 914 is preferably greater than that of antenna 915. This increases the electrical force on antenna 914.

[0219] The secondary battery includes a layer 916 between antenna 914 and antenna 915 and the secondary battery 913. Layer 916, for example, has the function of shielding electromagnetic fields from the secondary battery 913. As layer 916, a magnetic material can be used, for example.

[0220] The structure of a secondary battery is not limited to the structures shown in Figures 12A and 12B.

[0221] For example, as shown in Figures 13A1 and 13A2, antennas can also be provided on a pair of opposing surfaces of the secondary battery 913 shown in Figures 12A and 12B. Figure 13A1 is an external view showing one side of one of the aforementioned pairs of surfaces, and Figure 13A2 is an external view showing one side of the other of the aforementioned pairs of surfaces. Furthermore, the parts identical to those of the secondary batteries shown in Figures 12A and 12B can be appropriately referenced from the descriptions of the secondary batteries shown in Figures 12A and 12B.

[0222] As shown in Figure 13A1, an antenna 914 is disposed on one of the two surfaces of the secondary battery 913, with a layer 916 sandwiched between them. As shown in Figure 13A2, an antenna 918 is disposed on the other of the two surfaces of the secondary battery 913, with a layer 917 sandwiched between them. The layer 917, for example, has the function of shielding electromagnetic fields from the secondary battery 913. A magnetic material can be used as the layer 917, for example.

[0223] By adopting the above structure, the size of both antenna 914 and antenna 918 can be increased. Antenna 918, for example, has the function of data communication with external devices. As antenna 918, for example, an antenna with a shape that can be applied to antenna 914 can be used. As a communication method between the secondary battery using antenna 918 and other devices, a response method that can be used between the secondary battery and other devices, such as NFC (Near Field Communication), can be used.

[0224] Alternatively, as shown in Figure 13B1, a display device 920 may be provided on the secondary battery 913 shown in Figures 12A and 12B. The display device 920 is electrically connected to the terminal 911. Furthermore, the label 910 may not be affixed to the portion where the display device 920 is provided. Additionally, the descriptions of the secondary batteries shown in Figures 12A and 12B can be appropriately referenced for the portions identical to those shown in Figures 12A and 12B.

[0225] The display device 920 can display, for example, an image indicating whether charging is in progress, or an image indicating the battery level. The display device 920 can be, for example, electronic paper, a liquid crystal display, or an electroluminescent (EL) display. For example, using electronic paper can reduce the power consumption of the display device 920.

[0226] Alternatively, as shown in FIG13B2, a sensor 921 may be provided in the secondary battery 913 shown in FIG12A and FIG12B. The sensor 921 is electrically connected to the terminal 911 via terminal 922. Furthermore, the same parts as those in the secondary batteries shown in FIG12A and FIG12B can be appropriately referenced from the description of the secondary batteries shown in FIG12A and FIG12B.

[0227] Sensor 921 may have the function of measuring factors such as: displacement, position, velocity, acceleration, angular velocity, number of rotations, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, power, radiation, flow rate, humidity, slope, vibration, odor, or infrared radiation. By setting sensor 921, for example, data (temperature, etc.) of the environment in which a secondary battery is installed can be detected and stored in the memory of circuit 912.

[0228] Furthermore, a structural example of the secondary battery 913 will be described with reference to Figures 14A, 14B, and 15.

[0229] The secondary battery 913 shown in Figure 14A includes a wound body 950 with terminals 951 and 952 disposed inside a housing 930. The wound body 950 is immersed in electrolyte inside the housing 930. Terminals 952 are in contact with the housing 930, while terminals 951 are not in contact with the housing 930 due to insulating material or the like. Note that, for convenience, although the housing 930 is shown separately in Figure 14A, the wound body 950 is actually covered by the housing 930, and terminals 951 and 952 extend to the outside of the housing 930. The housing 930 can be made of metal (e.g., aluminum) or resin.

[0230] Alternatively, as shown in FIG14B, multiple materials can be used to form the housing 930 shown in FIG14A. For example, in the secondary battery 913 shown in FIG14B, the housings 930a and 930b are bonded together, and a winding body 950 is provided in the area surrounded by the housings 930a and 930b.

[0231] As the outer casing 930a, insulating materials such as organic resin can be used. In particular, by using materials such as organic resin to form the antenna surface, the electric field shielding caused by the secondary battery 913 can be suppressed. Furthermore, if the electric field shielding caused by the outer casing 930a is small, an antenna such as antenna 914 or antenna 915 can be installed inside the outer casing 930a. As the outer casing 930b, for example, a metal material can be used.

[0232] Furthermore, Figure 15 shows the structure of the wound body 950. The wound body 950 includes a negative electrode 931, a positive electrode 932, and an insulator 933. The wound body 950 is formed by sandwiching the insulator 933, overlapping the negative electrode 931 and the positive electrode 932 to form a laminate, and then winding the laminate. Alternatively, multiple laminates of negative electrode 931, positive electrode 932, and insulator 933 can be further stacked.

[0233] The negative terminal 931 is connected to terminal 911 shown in Figures 12A and 12B via one of terminals 951 and 952. The positive terminal 932 is connected to terminal 911 shown in Figures 12A and 12B via the other of terminals 951 and 952.

[0234] By using the positive electrode active material described in the above embodiments in the positive electrode 932, a secondary battery 913 with high capacity and excellent cycle characteristics can be realized.

[0235] [Laminated secondary battery] Next, an example of a laminated secondary battery will be described with reference to Figures 16A to 22B. When a flexible laminated secondary battery is installed in at least a portion of a flexible electronic device, the secondary battery can be bent along the deformation of the electronic device.

[0236] The laminated secondary battery 980 is described with reference to Figures 16A to 16C. The laminated secondary battery 980 includes a wound body 993 as shown in Figure 16A. The wound body 993 includes a negative electrode 994, a positive electrode 995, and an insulator 996. Similar to the wound body 950 described in Figure 15, the wound body 993 is formed by sandwiching the insulator 996, overlapping the negative electrode 994 and the positive electrode 995 to form a laminate, and then winding the laminate.

[0237] Furthermore, the number of layers in the stack consisting of negative electrode 994, positive electrode 995, and insulator 996 can be appropriately designed according to the required capacity and component volume. Negative electrode 994 is connected to negative current collector (not shown) via one of wire electrode 997 and wire electrode 998, and positive electrode 995 is connected to positive current collector (not shown) via the other of wire electrode 997 and wire electrode 998.

[0238] As shown in Figure 16B, the wound body 993 is housed in the space formed by bonding a film 981 (which will become the outer packaging body) and a film 982 with recesses through hot pressing or other means, thereby manufacturing the secondary battery 980 shown in Figure 16C. The wound body 993 includes wire electrodes 997 and 998, and the space formed by the film 981 and the film 982 with recesses is immersed in an electrolyte.

[0239] Thin film 981 and thin film 982 with recesses are made of, for example, a metallic material such as aluminum or a resin material. When a resin material is used as the material for thin film 981 and thin film 982 with recesses, thin film 981 and thin film 982 with recesses can be deformed when force is applied from the outside, thereby manufacturing a flexible secondary battery.

[0240] Furthermore, examples of using two films are shown in Figures 16B and 16C, but it is also possible to bend one film to form a space and accommodate the aforementioned winding 993 in that space.

[0241] By using the positive electrode active material described in the above embodiments in the positive electrode 995, a high-capacity secondary battery 980 with excellent cycle characteristics can be achieved.

[0242] Although Figures 16A to 16C show an example of a secondary battery 980 in which a wound body is included in the space formed by the film that serves as the outer packaging, it is also possible to use a secondary battery in which a plurality of rectangular positive electrodes, separators and negative electrodes are included in the space formed by the film that serves as the outer packaging, as shown in Figures 17A and 17B.

[0243] The laminated secondary battery 500 shown in Figure 17A includes: a positive electrode 503 comprising a positive current collector 501 and a positive active material layer 502; a negative electrode 506 comprising a negative current collector 504 and a negative active material layer 505; a separator 507; an electrolyte 508; and an outer packaging 509. The separator 507 is disposed between the positive electrode 503 and the negative electrode 506 within the outer packaging 509. Furthermore, the outer packaging 509 is filled with the electrolyte 508. The electrolyte shown in Embodiment 2 can be used as the electrolyte 508.

[0244] In the laminated secondary battery 500 shown in Figure 17A, the positive current collector 501 and the negative current collector 504 also serve as terminals for electrical contact with the outside. Therefore, it is also possible to configure a portion of the positive current collector 501 and the negative current collector 504 to be exposed to the outside of the outer casing 509. Alternatively, the wire electrode can be ultrasonically welded to the positive current collector 501 or the negative current collector 504 using a wire electrode to expose the wire electrode to the outside of the outer casing 509, without exposing the positive current collector 501 and the negative current collector 504 to the outside of the outer casing 509.

[0245] In the laminated secondary battery 500, the outer packaging 509 can be, for example, a three-layer film structure as follows: a highly flexible metal film such as aluminum, stainless steel, copper, nickel, etc. is provided on a film made of materials such as polyethylene, polypropylene, polycarbonate, ionomer, polyamide, etc., and an insulating synthetic resin film such as polyamide resin, polyester resin, etc. is provided on the outer surface of the metal film as the outer packaging.

[0246] Additionally, Figure 17B shows an example of a cross-sectional structure of a laminated secondary battery 500. For simplicity, Figure 17A shows an example including two current collectors, but in reality, as shown in Figure 17B, the battery includes multiple electrode layers.

[0247] An example in Figure 17B includes 16 electrode layers. Furthermore, even with 16 electrode layers, the secondary battery 500 is flexible. Figure 17B shows a structure with a total of 16 layers, including an 8-layer negative electrode current collector 504 and an 8-layer positive electrode current collector 501. Additionally, Figure 17B shows a cross-section of the negative electrode extraction section, where the 8-layer negative electrode current collector 504 is ultrasonically welded. Of course, the number of electrode layers is not limited to 16; it can be more than 16 or less than 16. With a larger number of electrode layers, a secondary battery with greater capacity can be manufactured. Furthermore, with a smaller number of electrode layers, a secondary battery that is thinner and has excellent flexibility can be manufactured.

[0248] Here, Figures 18 and 19 show an example of the external appearance of the laminated secondary battery 500. Figures 18 and 19 include: a positive electrode 503; a negative electrode 506; a separator 507; an outer packaging 509; a positive electrode lead 510; and a negative electrode lead 511.

[0249] Figure 20A shows the external appearance of the positive electrode 503 and the negative electrode 506. The positive electrode 503 includes a positive current collector 501, and a positive active material layer 502 is formed on the surface of the positive current collector 501. Additionally, the positive electrode 503 has a portion of the positive current collector 501 exposed (hereinafter referred to as the tab region). The negative electrode 506 has a negative current collector 504, and a negative active material layer 505 is formed on the surface of the negative current collector 504. Furthermore, the negative electrode 506 has a portion of the negative current collector 504 exposed, i.e., the tab region. The area or shape of the tab regions of the positive and negative electrodes is not limited to the example shown in Figure 20A.

[0250] [Manufacturing method of laminated secondary batteries] Here, an example of a method for manufacturing a laminated secondary battery, the appearance of which is shown in FIG18, will be described with reference to FIGS. 20B and 20C.

[0251] First, the negative electrode 506, the insulator 507, and the positive electrode 503 are stacked. Figure 20B shows the stacked negative electrode 506, insulator 507, and positive electrode 503. Here, an example using 5 sets of negative electrodes and 4 sets of positive electrodes is shown. Next, the tab regions of the positive electrode 503 are joined together, and the positive electrode wire 510 is joined to the tab region of the outermost positive electrode. As a joining method, ultrasonic welding can be used, for example. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode wire 511 is joined to the tab region of the outermost negative electrode.

[0252] Next, a negative electrode 506, an isolator 507, and a positive electrode 503 are configured on the outer packaging 509.

[0253] Next, as shown in Figure 20C, the outer packaging body 509 is folded along the portion indicated by the dashed line. Then, the outer periphery of the outer packaging body 509 is joined. As a joining method, for example, heat pressing can be used. At this time, in order to inject the electrolyte 508 later, an area (hereinafter referred to as the inlet) that is not joined to a part (or an edge) of the outer packaging body 509 is provided.

[0254] Next, electrolyte 508 (not shown) is introduced into the inside of the outer packaging 509 through an inlet provided in the outer packaging 509. Preferably, electrolyte 508 is introduced under reduced pressure or an inert gas atmosphere. Finally, the inlet is closed. In this way, a laminated secondary battery 500 can be manufactured.

[0255] By using the positive electrode active material described in the above embodiments in the positive electrode 503, a secondary battery 500 with high capacity and excellent cycle characteristics can be realized.

[0256] [Flexible rechargeable battery] Next, an example of a flexible secondary battery will be described with reference to Figures 21A, 21B1 and 21B2, 21C and 21D, and 22A and 22B.

[0257] Figure 21A shows a top view of the flexible secondary battery 250. Figures 21B1, 21B2, and 21C are cross-sectional views along cut lines C1-C2, C3-C4, and A1-A2 in Figure 21A, respectively. The secondary battery 250 includes an outer casing 251 and a positive electrode 211a and a negative electrode 211b housed inside the outer casing 251. A wire 212a electrically connected to the positive electrode 211a and a wire 212b electrically connected to the negative electrode 211b extend outside the outer casing 251. Furthermore, an electrolyte (not shown) is sealed in the area surrounded by the outer casing 251, in addition to the positive electrode 211a and the negative electrode 211b.

[0258] Referring to Figures 22A and 22B, the positive electrode 211a and negative electrode 211b included in the secondary battery 250 are described. Figure 22A is a perspective view illustrating the stacking order of the positive electrode 211a, negative electrode 211b, and separator 214. Figure 22B is a perspective view showing wires 212a and 212b in addition to the positive electrode 211a and negative electrode 211b.

[0259] As shown in Figure 22A, the secondary battery 250 includes multiple rectangular positive electrodes 211a, multiple rectangular negative electrodes 211b, and multiple separators 214. The positive electrodes 211a and 211b each include a protruding tab portion and a portion other than the tab. A positive electrode active material layer is formed on the portion of one side of the positive electrode 211a outside the tab, and a negative electrode active material layer is formed on the portion of one side of the negative electrode 211b outside the tab.

[0260] The positive electrode 211a and the negative electrode 211b are stacked in such a way that the surfaces of the positive electrode 211a that do not form a positive electrode active material layer are in contact with each other, and the surfaces of the negative electrode 211b that do not form a negative electrode active material layer are in contact with each other.

[0261] Furthermore, an insulator 214 is provided between the surface of the positive electrode 211a where the positive electrode active material layer is formed and the surface of the negative electrode 211b where the negative electrode active material layer is formed. For convenience, the insulator 214 is indicated by a dashed line in Figures 22A and 22B.

[0262] As shown in Figure 22B, multiple positive electrodes 211a are electrically connected to wires 212a in a junction 215a. Furthermore, multiple negative electrodes 211b are electrically connected to wires 212b in a junction 215b.

[0263] Next, the outer packaging body 251 will be described with reference to Figures 21B1, 21B2, 21C, and 21D.

[0264] The outer packaging 251 has a film shape and is folded in half to sandwich the positive electrode 211a and the negative electrode 211b. The outer packaging 251 includes a folded portion 261, a pair of sealing portions 262 and 263. The pair of sealing portions 262 are arranged to sandwich the positive electrode 211a and the negative electrode 211b and can also be referred to as side seals. In addition, the sealing portion 263 includes a portion that overlaps with the wires 212a and 212b and can also be referred to as top seals.

[0265] The outer packaging body 251 preferably has a wave-like shape with alternating ridge lines 271 and valley lines 272 in the portions overlapping with the positive electrode 211a and the negative electrode 211b. Furthermore, the sealing portions 262 and 263 of the outer packaging body 251 are preferably flat.

[0266] Figure 21B1 is a cross-section cut off at the portion overlapping with the ridge line 271, and Figure 21B2 is a cross-section cut off at the portion overlapping with the valley line 272. Figures 21B1 and 21B2 both correspond to cross-sections in the width direction of the secondary battery 250 and the positive electrode 211a and negative electrode 211b.

[0267] Here, the distance La is the distance between the ends of the positive electrode 211a and the negative electrode 211b in the width direction and the sealing portion 262. When the secondary battery 250 is bent or deformed, as described later, the positive electrode 211a and the negative electrode 211b are deformed in a staggered manner in the length direction. At this time, if the distance La is too short, the outer packaging 251 may rub strongly against the positive electrode 211a and the negative electrode 211b, causing damage to the outer packaging 251. In particular, when the metal film of the outer packaging 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 too long, the volume of the secondary battery 250 will increase.

[0268] Furthermore, it is preferable that the greater the total thickness of the stacked positive electrode 211a and negative electrode 211b, the longer the distance La between the positive electrode 211a and negative electrode 211b and the sealing part 262.

[0269] More specifically, when the total thickness of the stacked positive electrode 211a, negative electrode 211b, and separator 214 (not shown) is thickness t, the distance La is at least 0.8 times and less than 3.0 times the thickness t, preferably at least 0.9 times and less than 2.5 times, and more preferably at least 1.0 times and less than 2.0 times the thickness t. By keeping the distance La within the above range, a compact battery with high reliability against bending can be achieved.

[0270] Furthermore, when the distance between the pair of sealing portions 262 is a distance Lb, it is preferable that the distance Lb is sufficiently larger than the width of the positive electrode 211a and the negative electrode 211b (here, the width Wb of the negative electrode 211b). Therefore, when the secondary battery 250 is repeatedly bent or deformed, even if the positive electrode 211a and the negative electrode 211b come into contact with the outer packaging 251, a portion of the positive electrode 211a and the negative electrode 211b can be offset in the width direction, thus effectively preventing friction between the positive electrode 211a and the negative electrode 211b and the outer packaging 251.

[0271] For example, the difference between the distance La between a pair of sealing portions 262 and the width Wb of the negative electrode 211b is more than 1.6 times and less than 6.0 times the thickness t of the positive electrode 211a and the negative electrode 211b, preferably more than 1.8 times and less than 5.0 times, and more preferably more than 2.0 times and less than 4.0 times.

[0272] In other words, the distance Lb, width Wb, and thickness t are preferably satisfied by the following formula 1.

[0273] [Formula 1]

[0274] Here, a is 0.8 or higher and 3.0 or lower, preferably 0.9 or higher and 2.5 or lower, and even better 1.0 or higher and 2.0 or lower.

[0275] Additionally, Figure 21C is a cross-section including the wire 212a, corresponding to the cross-section along the length of the secondary battery 250, the positive electrode 211a, and the negative electrode 211b. As shown in Figure 21C, preferably, a space 273 is included between the ends of the positive electrode 211a and the negative electrode 211b along the length direction and the outer packaging body 251 in the folded portion 261.

[0276] Figure 21D shows a cross-sectional view of the battery 250 when it is bent. Figure 21D corresponds to a cross-section along the cut-off line B1-B2 in Figure 21A.

[0277] When the secondary battery 250 is bent, a portion of the outer packaging 251 located on the outer side of the bend deforms into an extension, while another portion of the outer packaging 251 located on the inner side of the bend deforms into a contraction. More specifically, the portion of the outer packaging 251 located on the outer side of the bend deforms with a small wave amplitude and a large wave period. On the other hand, the portion of the outer packaging 251 located on the inner side of the bend deforms with a large wave amplitude and a small wave period. By deforming the outer packaging 251 in this manner, the stress applied to the outer packaging 251 by bending can be mitigated, thus the material constituting the outer packaging 251 does not necessarily need to be elastic. As a result, the secondary battery 250 can be bent with relatively small force without damaging the outer packaging 251.

[0278] Furthermore, as shown in Figure 21D, when the secondary battery 250 is bent, the positive electrode 211a and the negative electrode 211b are respectively offset relative to each other. At this time, since the ends of the multiple stacked positive electrodes 211a and negative electrodes 211b on the sealing part 263 side are fixed by the fixing member 217, they are offset in such a way that the offset increases as they get closer to the folded part 261. As a result, the stress applied to the positive electrode 211a and the negative electrode 211b can be mitigated, and the positive electrode 211a and the negative electrode 211b themselves do not necessarily need to be stretchable. As a result, the secondary battery 250 can be bent without damaging the positive electrode 211a and the negative electrode 211b.

[0279] Furthermore, since there is a space 273 between the positive electrode 211a and the negative electrode 211b and the outer packaging body 251, the positive electrode 211a and the negative electrode 211b located on the inside can be staggered relative to each other in a way that they do not come into contact with the outer packaging body 251 when the device is bent.

[0280] The secondary battery 250 illustrated in Figures 21A, 21B1, 21B2, 21C, 21D, 22A, and 22B is a battery whose outer packaging and the positive electrode 211a and negative electrode 211b are not easily damaged even with repeated bending and stretching, and whose battery characteristics are not easily degraded. By using the positive electrode active material described in the above embodiments in the positive electrode 211a included in the secondary battery 250, a battery with high capacity and excellent cycle characteristics can be achieved.

[0281] Implementation Method 4 In this embodiment, an example of installing a secondary battery according to one embodiment of the present invention into an electronic device is described.

[0282] First, Figures 23A to 23G show examples of installing the flexible secondary battery described in part of Embodiment 3 in an electronic device. Examples of electronic devices that use flexible secondary batteries include television sets (also called televisions or television receivers), monitors for computers, digital cameras, digital camcorders, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game consoles, portable information terminals, audio playback devices, pinball machines, and other large game machines.

[0283] In addition, flexible secondary batteries can be assembled along the curved surfaces of the interior or exterior walls of houses and high-rise buildings, or the interior or exterior of automobiles.

[0284] Figure 23A shows an example of a mobile phone. In addition to a display unit 7402 assembled in a housing 7401, the mobile phone 7400 also includes operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, etc. Furthermore, the mobile phone 7400 has a secondary battery 7407. By using a secondary battery according to one embodiment of the present invention as the aforementioned secondary battery 7407, a lightweight mobile phone with a long service life can be provided.

[0285] Figure 23B shows the mobile phone 7400 in a bent state. When the mobile phone 7400 is deformed and bent as a whole by external force, the secondary battery 7407 located inside it is also bent. Figure 23C shows the state of the bent secondary battery 7407 at this time. The secondary battery 7407 is a thin-film rechargeable battery. The secondary battery 7407 is fixed in the bent state. The secondary battery 7407 has wire electrodes 7408 that are electrically connected to a current collector. For example, the current collector is a copper foil, part of which is alloyed with gallium to improve the adhesion to the active material layer in contact with the current collector, thereby improving the reliability of the secondary battery 7407 in the bent state.

[0286] Figure 23D shows an example of a bracelet-type display device. The portable display device 7100 includes a housing 7101, a display section 7102, operation buttons 7103, and a secondary battery 7104. Figure 23E shows a bent secondary battery 7104. When the bent secondary battery 7104 is worn on the user's arm, the housing of the secondary battery 7104 deforms, causing a change in the curvature of part or all of the secondary battery 7104. The radius of curvature is the value representing the degree of curvature at any point on a curve, expressed as the radius of an equivalent circle, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or all of the housing or the main surface of the secondary battery 7104 deforms within a radius of curvature of 40 mm or more and 150 mm or less. High reliability can be maintained as long as the radius of curvature in the main surface of the secondary battery 7104 is within the range of 40 mm or more and 150 mm or less. By using a secondary battery according to one embodiment of the present invention as the aforementioned secondary battery 7104, a lightweight and long-lasting portable display device can be provided.

[0287] Figure 23F shows an example of a watch-type portable information terminal. The portable information terminal 7200 includes a housing 7201, a display unit 7202, a strap 7203, a buckle 7204, operation buttons 7205, input / output terminals 7206, etc.

[0288] The portable information terminal 7200 can run various applications such as mobile phone, email, article reading and writing, music playback, Internet communication, and computer games.

[0289] The display surface of the display unit 7202 is curved, allowing for display along the curved surface. Furthermore, the display unit 7202 is equipped with a touch sensor, allowing operation by touching the screen with a finger or stylus. For example, by touching the icon 7207 displayed on the display unit 7202, an application can be launched.

[0290] In addition to setting the time, the operation button 7205 can also have various functions such as power switch, wireless communication switch, setting and canceling silent mode, and setting and canceling power saving mode. For example, the functions of the operation button 7205 can be freely configured by using the operating system assembled in the portable information terminal 7200.

[0291] In addition, the portable information terminal 7200 can perform short-range wireless communication according to communication standards. For example, it can make hands-free calls by communicating with a wireless headset.

[0292] In addition, the portable information terminal 7200 has an input / output terminal 7206, which allows it to directly send data to or receive data from other information terminals via a connector. It can also be charged via the input / output terminal 7206. Alternatively, charging can be performed wirelessly without using the input / output terminal 7206.

[0293] The display unit 7202 of the portable information terminal 7200 includes a secondary battery according to one embodiment of the present invention. By using the secondary battery according to one embodiment of the present invention, a lightweight and long-lasting portable information terminal can be provided. For example, the secondary battery 7104 shown in FIG. 23E in a bent state can be assembled inside the housing 7201, or the secondary battery 7104 can be assembled inside the strap 7203 in a bendable state.

[0294] The portable information terminal 7200 preferably includes sensors. These sensors may include, for example, human body sensors such as fingerprint sensors, pulse sensors, and body temperature sensors, touch sensors, pressure sensors, and accelerometers.

[0295] Figure 23G shows an example of a sleeve-type display device. The display device 7300 includes a display unit 7304 and a secondary battery according to one embodiment of the present invention. The display device 7300 may also include a touch sensor in the display unit 7304 and be used as a portable information terminal.

[0296] The display surface of the display unit 7304 is curved, allowing for display along the curved surface. Furthermore, the display device 7300 can change the display state using short-range wireless communication, which is standardized for communication.

[0297] The display device 7300 has input / output terminals, allowing it to directly send data to or receive data from other information terminals via connectors. It can also be charged via the input / output terminals. Alternatively, charging can be performed wirelessly without using the input / output terminals.

[0298] By using a secondary battery according to one embodiment of the present invention as the secondary battery included in the display device 7300, a lightweight display device with a long service life can be provided.

[0299] In addition, an example of installing the secondary battery with excellent cycle characteristics shown in the above embodiment into an electronic device will be described with reference to Figures 23H, 24A to 24C and 25.

[0300] By using the secondary battery of one embodiment of the present invention as a secondary battery for everyday electronic devices, lightweight products with long service life can be provided. Examples of such everyday electronic devices include electric toothbrushes, electric shavers, and electric beauty devices. The secondary batteries in these products are expected to be rod-shaped, small, lightweight, and high-capacity for easy handling by the user.

[0301] Figure 23H is a perspective view of a device referred to as an e-liquid-containing smoking device (electronic cigarette). In Figure 23H, the electronic cigarette 7500 includes: an atomizer 7501 including a heating element; a secondary battery 7504 powering the atomizer; and a cartridge 7502 including a liquid supply container and sensors. For improved safety, protection circuitry to prevent overcharging and over-discharging of the secondary battery 7504 can be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in Figure 23H includes external terminals for connection to a charger. When removed, the secondary battery 7504 is located at the top, thus preferably having a shorter overall length and lighter weight. Since the secondary battery of one embodiment of the present invention has high capacity and excellent cycle characteristics, a small and lightweight electronic cigarette 7500 that can be used for extended periods can be provided.

[0302] Next, Figures 24A and 24B show an example of a tablet terminal capable of folding. The tablet terminal 9600 shown in Figures 24A and 24B includes a housing 9630a, a housing 9630b, a movable part 9640 connecting the housings 9630a and 9630b, a display unit 9631, a display mode switching switch 9626, a switch 9627, a switching switch 9625, a fastener 9629, and an operation switch 9628. By using a flexible panel for the display unit 9631, a tablet terminal with a larger display unit can be achieved. Figure 24A shows the tablet terminal 9600 in the open state, and Figure 24B shows the tablet terminal 9600 in the closed state.

[0303] The tablet terminal 9600 has a battery 9635 inside the housings 9630a and 9630b. The battery 9635 is disposed in the housings 9630a and 9630b through the movable part 9640.

[0304] In the display unit 9631, an area that can be used entirely or partially as a touch panel can be used to input data by touching images, text, input boxes, etc., of the icons displayed in the aforementioned area. For example, the entire surface of the display unit 9631 on the housing 9630a side can be used to display a keyboard, and the display unit 9631 on the housing 9630b side can be used to display information such as text and images.

[0305] Additionally, it can be used to display the keyboard on the display unit 9631 on the side of the casing 9630b and to display information such as text and images on the display unit 9631 on the side of the casing 9630a. Furthermore, keyboard buttons can also be displayed on the display unit 9631 by touching it with a finger or stylus by displaying the keyboard display switch button on the touch panel on the display unit 9631.

[0306] In addition to being used as the interface for operating the tablet terminal 9600, switches 9625 to 9627 can also be used as interfaces for switching various functions. For example, at least one of switches 9625 to 9627 can be used as a switch to turn the power of the tablet terminal 9600 on / off. Furthermore, at least one of switches 9625 to 9627 can have functions such as switching between portrait and landscape display orientations, and switching between black-and-white and color display. Additionally, at least one of switches 9625 to 9627 can have a function to adjust the brightness of the display unit 9631. Furthermore, the brightness of the display unit 9631 can be optimized based on the amount of external light detected by the light sensor built into the tablet terminal 9600. Note that in addition to the light sensor, the tablet terminal can also incorporate other detection devices such as a gyroscope and an accelerometer for detecting tilt.

[0307] Figure 24B shows the 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 with a DC-DC converter 9636. A secondary battery according to one embodiment of the present invention is used as the energy storage device 9635.

[0308] Furthermore, as described above, the tablet terminal 9600 is foldable, so when not in use, the outer casings 9630a and 9630b can be folded over each other. By folding the outer casings 9630a and 9630b, the display unit 9631 can be protected, thereby improving the durability of the tablet terminal 9600. Moreover, since the energy storage element 9635 of the secondary battery using one embodiment of the present invention has high capacity and excellent cycle characteristics, a tablet terminal 9600 that can be used for extended periods can be provided.

[0309] In addition, the tablet terminal 9600 shown in Figures 24A and 24B can also have the following functions: displaying various types of information (static images, dynamic images, text images, etc.); displaying calendars, dates, or times on the display; touch input for touch input operations or editing of information displayed on the display; and control processing through various software (programs), etc.

[0310] By utilizing a solar cell 9633 mounted on the surface of the tablet terminal 9600, power can be supplied to the touch panel, display unit, or image signal processing unit. Note that the solar cell 9633 can be disposed on one or both surfaces of the casing 9630, allowing for efficient charging of the energy storage device 9635. Using a lithium-ion battery as the energy storage device 9635 offers advantages such as miniaturization.

[0311] Furthermore, the structure and operation of the charge / discharge control circuit 9634 shown in FIG24B will be described with reference to the block diagram shown in FIG24C. FIG24C shows the solar cell 9633, the energy storage unit 9635, the DC-DC converter 9636, the converter 9637, the switches SW1 to SW3, and the display unit 9631. The energy 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 FIG24B.

[0312] First, an example of operation when the solar cell 9633 generates electricity using external light will be explained. A DC-DC converter 9636 is used to boost or buck the power generated by the solar cell to make it a voltage used to charge the energy storage unit 9635. Furthermore, when the display unit 9631 is operated using power from the solar cell 9633, switch SW1 is turned on, and converter 9637 boosts or bucks the voltage to the voltage required by the display unit 9631. Alternatively, a structure can be adopted where switch SW1 is turned off and switch SW2 is turned on when the display unit 9631 is not displaying anything, thereby charging the energy storage unit 9635.

[0313] Note that while the solar cell 9633 is shown as an example of a power generation unit, it is not limited to this. Other power generation units, such as piezoelectric elements or thermoelectric conversion elements, can also be used to charge the energy storage unit 9635. For example, a contactless power transmission module capable of wireless (contactless) power transmission and reception, or a combination of other charging methods, can also be used for charging.

[0314] Figure 25 shows examples of other electronic devices. In Figure 25, the display device 8000 is an example of an electronic device using a secondary battery 8004 according to an embodiment of the present invention. Specifically, the display device 8000 is equivalent to a television broadcast receiving display device, including a housing 8001, a display section 8002, a speaker section 8003, and a secondary battery 8004, etc. The secondary battery 8004 according to an embodiment of the present invention is disposed inside the housing 8001. The display device 8000 can receive power from commercial power sources and also use the power stored in the secondary battery 8004. Therefore, even when power supply from commercial power sources cannot be received due to power outages, the display device 8000 can be used as an uninterruptible power supply system by using the secondary battery 8004 according to an embodiment of the present invention.

[0315] As the display unit 8002, semiconductor display devices such as liquid crystal display devices, light-emitting devices having light-emitting elements such as organic EL elements in each pixel, electrophoretic display devices, DMD (Digital Micromirror Device), PDP (Plasma Display Panel), and FED (Field Emission Display) can be used.

[0316] In addition to display devices used for receiving television broadcasts, display devices also include all display devices used for displaying information, such as display devices for personal computers or display devices for advertising.

[0317] In Figure 25, the recessed lighting device 8100 is an example of an electronic device using a secondary battery 8103 according to an embodiment of the present invention. Specifically, the lighting device 8100 includes a housing 8101, a light source 8102, and a secondary battery 8103. Although Figure 25 illustrates a case where the secondary battery 8103 is disposed inside a ceiling 8104 containing the housing 8101 and the light source 8102, the secondary battery 8103 can also be disposed inside the housing 8101. The lighting device 8100 can receive power from a commercial power source and also utilize the power stored in the secondary battery 8103. Therefore, even when power from a commercial power source cannot be received due to a power outage, the lighting device 8100 can be used as an uninterruptible power supply system by using the secondary battery 8103 according to an embodiment of the present invention.

[0318] Furthermore, although Figure 25 illustrates an inlaid lighting device 8100 installed in the ceiling 8104, the secondary battery according to one embodiment of the present invention can be used for inlaid lighting devices installed outside the ceiling 8104, such as side walls 8105, floor 8106, or windows 8107, and can also be used for tabletop lighting devices, etc.

[0319] Furthermore, as the light source 8102, an artificial light source that artificially generates light using electricity can be used. Specifically, examples of the aforementioned artificial light sources include incandescent bulbs, fluorescent lamps, and other discharge lamps, as well as light-emitting elements such as LEDs or organic EL elements.

[0320] In Figure 25, an air conditioner having an indoor unit 8200 and an outdoor unit 8204 is an example of an electronic device using a secondary battery 8203 according to an embodiment of the present invention. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, and a secondary battery 8203, etc. Although Figure 25 illustrates the case where the secondary battery 8203 is provided in the indoor unit 8200, the secondary battery 8203 can also be provided in the outdoor unit 8204. Alternatively, the secondary battery 8203 can be provided in both the indoor unit 8200 and the outdoor unit 8204. The air conditioner can receive power from a commercial power source and can also use the 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, even when power from a commercial power source cannot be received due to a power outage or other reasons, the air conditioner can be used as an uninterruptible power supply system by using the secondary battery 8203 according to an embodiment of the present invention.

[0321] In addition, although a split-type air conditioner consisting of an indoor unit and an outdoor unit is illustrated in Figure 25, a secondary battery according to an embodiment of the present invention can also be used in an integrated air conditioner that has the functions of an indoor unit and an outdoor unit in one casing.

[0322] In Figure 25, the electric refrigerator / freezer 8300 is an example of an electronic device using a secondary battery 8304 according to an embodiment of the present invention. Specifically, the electric refrigerator / freezer 8300 includes a housing 8301, a refrigerator door 8302, a freezer door 8303, and a secondary battery 8304, etc. In Figure 25, the secondary battery 8304 is disposed inside the housing 8301. The electric refrigerator / freezer 8300 can receive power from a commercial power source and can also use the power stored in the secondary battery 8304. Therefore, even when power from a commercial power source cannot be received due to a power outage, the electric refrigerator / freezer 8300 can be utilized by using the secondary battery 8304 according to an embodiment of the present invention as an uninterruptible power supply system.

[0323] Among the aforementioned electronic devices, high-frequency heating devices such as microwave ovens and electric cookers require high power for short periods of time. Therefore, by using an energy storage device according to an embodiment of the present invention as an auxiliary power source to supplement the power supply when commercial power cannot provide sufficient power, the main power switch of the commercial power supply can be prevented from tripping when the electronic device is in use.

[0324] Furthermore, during periods when electronic devices are not used, especially when the ratio of actual electricity used to the total electricity available from commercial power sources (referred to as electricity usage rate) is low, electricity is stored in secondary batteries, thereby suppressing increased electricity usage during periods outside of these periods. For example, in the case of an electric refrigerator / freezer 8300, electricity is stored in secondary batteries 8304 at night when the temperature is low and the refrigerator door 8302 or freezer door 8303 is not opened or closed. During the day when the temperature is high and the refrigerator door 8302 or freezer door 8303 is opened or closed, secondary batteries 8304 are used as auxiliary power, thereby suppressing daytime electricity usage.

[0325] By employing one embodiment of the present invention, the cycle characteristics and reliability of the secondary battery can be improved. Furthermore, by employing one embodiment of the present invention, a high-capacity secondary battery can be achieved, thereby improving the characteristics of the secondary battery and enabling the secondary battery itself to be miniaturized and lightweight. Therefore, by installing the secondary battery of one embodiment of the present invention into the electronic device described in this embodiment, a longer service life and a lighter electronic device can be provided. This embodiment can be implemented in suitable combinations with other embodiments.

[0326] Implementation Method 5 In this embodiment, an example is shown of installing a secondary battery according to one embodiment of the present invention in a vehicle.

[0327] When a secondary battery is installed in a vehicle, it can enable a new generation of clean energy vehicles such as hybrid electric vehicles (HEV), electric vehicles (EV), or plug-in hybrid electric vehicles (PHEV).

[0328] Figures 26A to 26C illustrate a vehicle using a secondary battery according to one embodiment of the present invention. The car 8400 shown in Figure 26A is an electric vehicle that uses an electric motor as its power source for driving. Alternatively, the car 8400 is a hybrid vehicle that can appropriately use an electric motor or an engine as its power source for driving. By using the secondary battery according to one embodiment of the present invention, a vehicle with a long driving range can be realized. Furthermore, the car 8400 is equipped with a secondary battery. As a secondary battery, small secondary battery modules as shown in Figures 11C and 11D can be arranged in the floor portion of the vehicle interior for use. Alternatively, a battery pack composed of multiple secondary batteries shown in Figures 16A to 16C can be installed in the floor portion of the vehicle interior. The secondary battery not only drives the electric motor 8406, but also supplies power to lighting devices such as headlights 8401 or interior lights (not shown).

[0329] In addition, the secondary battery can supply power to display devices such as the speedometer and tachometer in the car 8400. Furthermore, the secondary battery can supply power to semiconductor devices such as the navigation system in the car 8400.

[0330] In the vehicle 8500 shown in Figure 26B, the secondary battery of the vehicle 8500 can be charged by receiving power from an external charging device using a plug-in or contactless power supply method. Figure 26B shows the charging of the secondary batteries 8024 and 8025 installed in the vehicle 8500 from a ground-mounted charging device 8021 via a cable 8022. When charging, the charging method and connector specifications can be appropriately determined according to the specifications of CHAdeMO (registered trademark) or the "Combined Charging System". The charging device 8021 can also use power from charging stations in commercial facilities or household power sources. For example, the secondary batteries 8024 and 8025 installed in the vehicle 8500 can be charged by supplying power from an external source using plug-in technology. The AC power can be converted to DC power using a conversion device such as an AC / DC converter for charging.

[0331] Alternatively, although not illustrated, the receiving device can be installed in the vehicle and charged by receiving power from a ground-based power supply device without contact. When using a contactless power supply method, by assembling the power supply device in the road or exterior wall, charging can be performed both while the vehicle is parked and while it is in motion. Furthermore, this contactless power supply method can also be used for power transmission and reception between vehicles. Moreover, solar panels can be installed on the exterior of the vehicle to charge the secondary battery when parked or in motion. Such contactless power supply can be achieved using electromagnetic induction or magnetic field resonance.

[0332] Figure 26C is an example of a two-wheeled vehicle using a secondary battery according to one embodiment of the present invention. The small motorcycle 8600 shown in Figure 26C includes a secondary battery 8602, a rearview mirror 8601, and turn signals 8603. The secondary battery 8602 can power the turn signals 8603.

[0333] Furthermore, in the small motorcycle 8600 shown in Figure 26C, the secondary battery 8602 can be stored in the under-seat storage box 8604. Even if the under-seat storage box 8604 is small, the secondary battery 8602 can still be stored in it. The secondary battery 8602 is removable, so it can be moved indoors to charge when needed, and stored away before riding.

[0334] By employing one embodiment of the present invention, the cycle characteristics and capacity of the secondary battery can be improved. This allows for a smaller and lighter secondary battery. Furthermore, the ability to make the secondary battery smaller and lighter contributes to vehicle weight reduction, thereby extending driving range. Additionally, the secondary battery installed in the vehicle can be used as a power source outside the vehicle. This, for example, avoids the use of commercial power during peak electricity demand periods. Avoiding the use of commercial power during peak electricity demand periods helps save energy and reduce carbon dioxide emissions. Moreover, excellent cycle characteristics allow for longer-term use of the secondary battery, thereby reducing the use of rare metals such as cobalt.

[0335] This implementation method can be appropriately combined with other implementation methods. Example 1

[0336] In this embodiment, the positive electrode active material 100 of one embodiment of the present invention and the lithium cobalt oxide of a comparative example are described, and the results of analysis using XRD are presented.

[0337] [Manufacturing of positive electrode active materials] Sample 01 A sample 01 of the positive electrode active material of one embodiment of the present invention is formed by adding magnesium and fluorine to the starting material to produce lithium cobalt oxide particles, and then heating them.

[0338] In sample 01, as described in step S11 of embodiment 1, lithium carbonate as a lithium source, cobalt oxide as a cobalt source, magnesium oxide as a magnesium source, and lithium fluoride as a fluorine source are prepared. The elements are weighed in such a ratio that Li 1.02 Co 0.99 Mg 0.01 O 1.98 F 0.02.

[0339] Next, as step S12, the starting materials are mixed. Mixing is performed for 2 hours at 250 rpm using a ball mill with zirconia balls.

[0340] Next, as step S13, the mixed material is placed in a furnace made of alumina (hereinafter referred to as an alumina furnace) and heated. This heating is performed under the following conditions: a muffle furnace is used; the flow rate of the dry air atmosphere is 10 L / min; the holding temperature is 950°C (heating rate is 200°C / hour); and the holding temperature is maintained for 10 hours. The cooling time from the holding temperature to room temperature is set to be 10 hours or more and 15 hours or less.

[0341] Since no titanium and aluminum coating treatment is performed, step S14 is not performed.

[0342] Next, as step S15, the lithium cobalt oxide particles containing magnesium and fluorine synthesized in step S13 are placed in an alumina furnace and heated. This heating is performed under the following conditions: a muffle furnace is used; the oxygen atmosphere flow rate is 10 L / min; the holding temperature is 900°C (heating rate is 200°C / hour); and the holding temperature is maintained for 2 hours. The cooling time from the holding temperature to room temperature is set to be between 10 and 15 hours.

[0343] Next, a grinding process is performed. The grinding process is carried out by sieving, using a sieve with a pore size of 53 μm.

[0344] Finally, the particles were collected to obtain the positive electrode active material of sample 01. It is known that the concentration of magnesium and fluorine in the surface layer of the positive electrode active material manufactured under the above conditions is higher than that in the interior.

[0345] Sample 02 A sample 02 of the positive electrode active material in one embodiment of the present invention is formed by heating lithium cobalt oxide particles containing magnesium and fluorine.

[0346] In Sample 02, lithium cobalt oxide particles (manufactured by Nippon Chemical Industries, Ltd., trade name: C-20F) were used as the starting material. Therefore, steps S12 and S13 described in Embodiment 1 are omitted in Sample 02. The aforementioned lithium cobalt oxide particles have a particle size (D50) of approximately 20 μm and contain fluorine, magnesium, calcium, sodium, silicon, sulfur, and phosphorus in the region analyzable by XPS. Furthermore, since titanium and aluminum coating treatment is not performed, step S14 is omitted.

[0347] Next, as step S15, lithium cobalt oxide particles are placed in an alumina furnace for heating. This heating is performed under the following conditions: a muffle furnace is used; the flow rate of dry air atmosphere is 5 L / min; the holding temperature is 800°C (heating rate is 200°C / hour); and the holding temperature is maintained for 2 hours. The cooling time from the holding temperature to room temperature is set to be between 10 and 15 hours. The samples are then collected by screening in the same manner as sample 01. It is known that the concentrations of magnesium and fluorine in the surface layer of the positive electrode active material manufactured under the above conditions are also higher than those in the interior.

[0348] Sample 03 As an embodiment of the present invention, sample 03 is a positive electrode active material containing magnesium and fluorine, which is coated with titanium and formed by the sol-gel method.

[0349] Lithium cobalt oxide particles (manufactured by Nippon Chemical Industries, Ltd., trade name: C-20F) were also used as the starting material for sample 03. Therefore, steps S12 and S13 are omitted.

[0350] Next, as step S14, the lithium cobalt oxide particles are coated with a titanium-containing material. Specifically, TTIP is dissolved in isopropanol to prepare an isopropanol solution of TTIP. The lithium cobalt oxide particles are mixed in this solution. The TTIP is mixed such that the ratio of TTIP to lithium cobalt oxide containing magnesium and fluorine is 0.004 ml / g.

[0351] The mixture was stirred for 72 hours at 25°C with a magnetic stirrer without a lid, under 90% RH humidity. This process caused water in the atmosphere to undergo hydrolysis and condensation reactions with TTIP, thereby forming a titanium-containing layer on the surface of the lithium cobalt oxide particles containing magnesium and fluorine.

[0352] The mixture after the above treatment was centrifuged, and the precipitate was collected. Centrifugation was performed at 3000 rpm for 1 minute, and isopropanol was used for washing.

[0353] The collected precipitate was dried at 70°C for 3 hours in a circulating drying oven.

[0354] Next, as step S15, lithium cobalt oxide particles covered with titanium-containing material are placed in an alumina furnace for heating. This heating is performed under the following conditions: a muffle furnace is used; the oxygen atmosphere flow rate is 10 L / min; the holding temperature is 800°C (heating rate is 200°C / hour); and the holding temperature is maintained for 2 hours. The cooling time from the holding temperature to room temperature is set to be between 10 and 15 hours. The samples are then collected by screening in the same manner as sample 01. It is known that the concentrations of titanium, magnesium, and fluorine in the surface layer of the positive electrode active material manufactured under the above conditions are higher than those in the interior. Furthermore, it is also known that the peak concentration of titanium is located in a region deeper than the peak concentration of magnesium.

[0355] Sample 04 As an embodiment of the present invention, sample 04 is a positive electrode active material formed by a sol-gel method, comprising lithium cobalt oxide particles containing magnesium and fluorine coated with aluminum.

[0356] Lithium cobalt oxide particles (manufactured by Nippon Chemical Industries, Ltd., trade name: C-20F) were also used as the starting material for sample 04. Therefore, steps S12 and S13 are omitted.

[0357] Next, as step S14, the lithium cobalt oxide particles are coated with an aluminum-containing material. Specifically, aluminum isopropoxide is dissolved in isopropanol to prepare an isopropanol solution of aluminum isopropoxide. The lithium cobalt oxide particles are then mixed with this solution. The mixing is carried out such that the ratio of aluminum isopropoxide to lithium cobalt oxide containing magnesium and fluorine is 0.0279 g / g.

[0358] The mixture was stirred for 8 hours at 25°C with a magnetic stirrer without a lid, under 90% RH humidity. This process caused the water in the atmosphere to undergo hydrolysis and condensation reactions with aluminum isopropoxide, thereby forming an aluminum-containing layer on the surface of the lithium cobalt oxide particles containing magnesium and fluorine.

[0359] The treated mixture was filtered, and the residue was collected. Kiriyama filter paper (No. 4) was used as the filter, and isopropanol was used for washing.

[0360] The collected residue was dried at 70°C for 1 hour in a vacuum bell jar.

[0361] Next, as step S15, lithium cobalt oxide particles covered with aluminum-containing material are placed in an alumina furnace for heating. This heating is performed under the following conditions: a muffle furnace is used; the oxygen atmosphere flow rate is 10 L / min; the holding temperature is 800°C (heating rate is 200°C / hour); and the holding temperature is maintained for 2 hours. The cooling time from the holding temperature to room temperature is set to be between 10 and 15 hours. The samples are then collected by screening in the same manner as sample 01. It is known that the concentrations of aluminum, magnesium, and fluorine in the surface layer of the positive electrode active material manufactured under the above conditions are higher than those in the interior. Furthermore, it is known that the peak concentration of aluminum is located in a region deeper than the peak concentration of magnesium.

[0362] Sample 05 As a comparative example, Sample 05 did not undergo sol-gel treatment or heat treatment, but directly used lithium cobalt oxide (manufactured by Nippon Chemical Industries Co., Ltd., trade name: C-20F) containing magnesium and fluorine.

[0363] Sample 06 As a comparative example, Sample 06 uses aluminum to cover lithium cobalt oxide particles that do not contain magnesium or fluorine using the sol-gel method.

[0364] In Sample 06, lithium cobalt oxide particles (manufactured by Nippon Kagaku Kogyo Co., Ltd., trade name: C-5H) were used as the starting material. Therefore, steps S12 and S13 are omitted. Note that the aforementioned lithium cobalt oxide particles are those with a particle size of approximately 5 μm (D50) and for which magnesium is not detectable in XPS or similar tests.

[0365] Next, as step S14, the lithium cobalt oxide particles are coated with an aluminum-containing material. Specifically, aluminum isopropoxide is dissolved in isopropanol to prepare an isopropanol solution of aluminum isopropoxide. The lithium cobalt oxide particles are then mixed with this solution. The mixing is carried out such that the ratio of aluminum isopropoxide to lithium cobalt oxide containing magnesium and fluorine is 0.0917 g / g.

[0366] The same stirring, collection, and drying processes were performed as for sample 04.

[0367] Next, as step S15, the lithium cobalt oxide particles covered by the aluminum-containing material are heated, cooled, and collected. Except for the heating temperature being set to 500°C, the manufacturing process is carried out under the same conditions as for sample 04.

[0368] Table 1 shows the manufacturing conditions for samples 01 to 06.

[0369]

[0370] [Manufacturing of secondary batteries] A coin-shaped secondary battery (diameter: 20 mm, height: 3.2 mm) of CR2032 was manufactured using the positive electrode active materials manufactured as Samples 01 to 06.

[0371] As the positive electrode, a positive electrode manufactured in the following manner is used: a slurry made by mixing the positive electrode active material (LCO): acetylene black (AB): polyvinylidene fluoride (PVDF) manufactured above in a weight ratio of 95:3:2 is coated onto the current collector.

[0372] Lithium metal is used as the counter electrode.

[0373] The electrolyte used is 1 mol / L lithium hexafluorophosphate (LiPF6). The electrolyte is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, along with 2 wt% ethylene carbonate (VC).

[0374] 25μm thick polypropylene is used as the separator.

[0375] The positive and negative electrode containers are made of stainless steel (SUS).

[0376] [XRD after initial charge] Secondary batteries using the positive electrode active materials of samples 01 to 06 were subjected to CCCV charging at a specified voltage. Specifically, constant current charging at 0.5C was performed until the specified voltage was reached, followed by constant voltage charging at 0.01C. Next, the charged secondary batteries were disassembled and the positive electrode removed in an argon-atmospheric glove box, and the electrolyte was removed by washing with DMC (dimethyl carbonate). Then, powder XRD analysis was performed using CuKα1 lines. In this analysis, a fully automated multi-object X-ray diffraction system D8 ADVANCE manufactured by Bruker AXS was used. Although the XRD system was set to powder sample mode, the sample height was set to the measurement surface required by the system. Furthermore, the sample was set flat without bending.

[0377] Figure 27 shows the XRD pattern of the positive electrode of a secondary battery using the positive electrode active material of sample 01 after charging at 4.6V. For comparison, patterns of the pseudo-spinel crystal structure and the H1-3 type crystal structure, similar to those in Figure 3, are also shown. Furthermore, it can be seen that sample 01, when charged at 4.6V, contains a mixture of pseudo-spinel crystal structures and H1-3 type crystal structures. Additionally, Rietwald analysis suggests that it contains 66 wt% pseudo-spinel crystal structure.

[0378] Figure 28 shows the XRD patterns of the positive electrode of a secondary battery using the positive electrode active material of sample 02 after charging at 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, 4.7V, and 4.8V. It can be seen that sample 02 charged at 4.6V has a pseudo-spinel crystal structure. Furthermore, it is speculated that sample 02 charged at 4.7V or higher has a structure different from the pseudo-spinel crystal structure, with a wider peak width and reduced crystallinity.

[0379] Figure 29 shows the XRD patterns of the positive electrode of a secondary battery using the positive electrode active material of sample 03 after charging at 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, 4.7V, and 4.8V. It can be seen that sample 03, charged at 4.6V, has a pseudo-spinel crystal structure. Furthermore, sample 03 charged at 4.6V shows a more pronounced pattern, suggesting that there are fewer non-pseudo-spinel crystal structures compared to sample 02 charged at 4.6V. Additionally, it is speculated that sample 03 charged at 4.7V or higher has a crystal structure different from pseudo-spinel, with a wider peak width and reduced crystallinity.

[0380] Figure 30 shows the XRD patterns of the positive electrode of a secondary battery using the positive electrode active material of sample 04 after charging at 4.6V, 4.7V, and 4.8V. It can be seen that sample 04, when charged at 4.6V and 4.7V, has a pseudo-spinel crystal structure. Furthermore, it is speculated that sample 04, when charged at 4.8V, has a different crystal structure from the pseudo-spinel, and the peak width becomes wider while the crystallinity decreases.

[0381] Figure 31 shows the XRD patterns of the positive electrode of a secondary battery using the positive electrode active material of Comparative Example Sample 05 after charging at 4.1V, 4.2V, 4.3V, 4.4V, 4.5V, 4.6V, 4.7V, and 4.8V. It can be seen that Comparative Example Sample 05 exhibits an H1-3 type crystal structure but not a pseudo-spinel type crystal structure when charged at 4.6V or higher but below 4.7V (the peak from 43.5° to 46° (2θ) is particularly characteristic). Furthermore, it can be seen that the crystal structure changes to H1-3 type between 4.5V and 4.6V. Additionally, it is speculated that Sample 05, when charged at 4.8V, has a crystal structure different from both the pseudo-spinel type and H1-3 crystal structures, and the peak width becomes wider while the crystallinity decreases.

[0382] Figure 32 shows the XRD pattern of the positive electrode of a secondary battery using the positive electrode active material of the comparative example, sample 06, after charging at 4.6V. It can be seen that sample 06 has an H1-3 type crystal structure when charged at 4.6V.

[0383] [XRD after multiple charges] Next, samples 02, 03, and the comparative example 05 were charged multiple times at 4.6V and analyzed by XRD. Specifically, samples that underwent CCCV charging at 4.6V were considered as samples that were charged once. Additionally, samples that underwent CCCV charging at 4.6V until the discharge voltage reached 2.5V, followed by constant current discharge (CC discharge), and then CCCV charging at 4.6V were considered as samples that were charged twice. Similarly, some samples were charged nine times.

[0384] Figure 33 shows the XRD patterns of the positive electrode of a secondary battery using the positive electrode active material of sample 02 after being charged once at 4.6V and after being charged twice. It is inferred that during the first charge, the crystallinity is very low due to the presence of H1-3 type crystal structures and other structures besides the pseudo-spinel crystal structure. However, during the second charge, the crystallinity increases compared to the first charge because the structures other than the pseudo-spinel crystal structure are reduced.

[0385] Figure 34 shows the XRD patterns of the positive electrode of a secondary battery using the positive electrode active material of sample 03 after being charged at 4.6V once, twice, and nine times. It is inferred that sample 03 also exhibits low crystallinity during the first charge due to the presence of H1-3 type crystal structures and other structures besides the pseudo-spinel crystal structure. However, during subsequent charges, the pseudo-spinel crystal structure maintains high crystallinity because the structures other than the pseudo-spinel crystal structure are reduced.

[0386] Figure 35 shows the XRD patterns of the positive electrode of a secondary battery using the positive electrode active material of the comparative example, Sample 05, after being charged once and twice at 4.6V. Sample 05 exhibits an H1-3 type crystal structure in both the first and second charges. Its characteristics are also evident when considering the peak values ​​present in the range of 43.5° to 46° (2θ).

[0387] [XRD pattern after multiple discharges] Next, samples 02, 03, and the comparative example sample 05 were charged ten times and analyzed by XRD. Specifically, the charge-discharge cycle of CCCV charging (4.6V) followed by CC discharging (2.5V) was repeated ten times. The discharged secondary battery was then disassembled, the positive electrode was removed, and analyzed by XRD.

[0388] Figure 36 shows the XRD patterns of the positive electrodes of samples 02, 03, and the comparative example sample 05 after ten discharge cycles. For comparison, patterns of the same LiCoO2(O3), pseudo-spinel crystal structure, and H1-3 type crystal structure as in Figure 3 are also shown. Samples 02, 03, and 05 all have a LiCoO2(O3) structure. However, in sample 05, the crystallinity decreases due to the widening of the diffraction peaks from the (003) and (006) planes perpendicular to the c-axis in LiCoO2(O3). In contrast, it is speculated that samples 02 and 03 maintain high crystallinity due to the absence of CoO2 layer deviation, and therefore show little degradation after ten charge cycles.

[0389] [Changes in volume] Next, the lattice constant and crystal structure were inferred from the XRD patterns of each charging depth of sample 03. Then, the volume per unit cell of each crystal structure was calculated and compared with the volume before charging. In addition, to facilitate comparison with other crystal structures, the c-axis of the H1-3 type crystal structure was calculated using half the value of the unit cell.

[0390] Table 2 shows the lattice constants and crystal structures inferred from the XRD patterns of sample 03 at various charging depths.

[0391]

[0392] It is speculated that when charged at 4.1V or higher and 4.5V or lower, they all exhibit a two-phase crystalline structure belonging to space group R-3m. This may be due to the difference in charging depth generated within or between the particles of each positive electrode active material. In Table 2, they are represented as R-3m(1) and R-3m(2).

[0393] When charged at 4.6V, it is inferred that a mixture of pseudo-spinel and H1-3 type crystal structures exists. In addition, based on Rietwald analysis, it is inferred that the pseudo-spinel crystal structure accounts for more than 77 wt%.

[0394] Furthermore, when charged at 4.7V, it is speculated that the H1-3 type crystal structure and the O1 type crystal structure coexist.

[0395] Furthermore, the volume change rate of the pseudo-spinel crystal structure from the O3 type crystal structure is less than 2.5%, more specifically less than 2.2%, while the volume change rate of the H1-3 type crystal structure from the O3 type crystal structure is more than 3.5%.

[0396] Figure 37 shows the case where the volume change rate shown in Table 2 is represented as a graph. Only the marker indicating O3: depth of charge 0 refers to the horizontal axis at the top of the graph. In addition, in Figure 37, each marker indicates that the crystal structure is presumed to be any of the following structures: R-3m(1)(2); pseudo-spinel crystal structure; H1-3 type crystal structure; and O1 type crystal structure.

[0397] As shown in Table 2 and Figure 37, the volume change per unit cell of the pseudo-spinel crystal structure is less than that of the H1-3 type crystal structure. Furthermore, it can be seen that sample 03, charged at 4.6V, exhibits over 77wt% pseudo-spinel crystal structure, indicating that its crystal structure and volume changes are suppressed.

[0398] [Cyclic Characteristics] Next, the cycle characteristics of the secondary batteries using samples 01, 03, and 05 were evaluated.

[0399] Note that the cycling characteristics of samples 01 and 03, which were evaluated, were from different batches than those analyzed by XRD, and the manufacturing conditions were slightly different, hence the asterisks in the figures. However, as characteristics of the positive electrode active material, there were no significant differences. Specifically, sample 01 underwent a first heat treatment at 1000°C. Sample 03 used 0.01 ml / g of TTIP for sol-gel treatment and underwent a second heat treatment in a dry air atmosphere.

[0400] The coin battery is made by mixing positive electrode active material (LCO), acetylene black (AB), and polyvinylidene fluoride (PVDF) in a weight ratio of LCO:AB:PVDF=95:2.5:2.5, with other manufacturing conditions being the same.

[0401] Cyclic testing was conducted at 25°C, with charging at CCCV (0.5C, 4.6V, termination current 0.01C) and discharging at CC (0.5C, 2.5V). Here, 1C was set to 137mA / g, which is the current value per unit mass of the positive electrode active material.

[0402] Figure 38A shows the discharge capacity of samples 01, 03, and 05, and Figure 38B shows the discharge capacity retention. In the comparative example, sample 05's discharge capacity retention decreased to 40.9% after 40 cycles. On the other hand, the initial capacity of the positive electrode active material in one embodiment of the present invention is approximately as high as that of the comparative example, and samples 01 and 03 both exhibit excellent cycling characteristics, with sample 03 retaining 78.4% after 100 cycles and sample 01 retaining 67.5% after 70 cycles.

[0403] It can be seen that the positive electrode active material of one embodiment of the present invention also exhibits good cycle characteristics when charged and discharged at a high voltage such as 4.6V.

[0404] Thus, it can be seen that in samples 01 to 04 of the positive electrode active material according to one embodiment of the present invention, more than 60% of the pseudo-spinel crystal structure is present when charged at 4.6V. The difference in crystal structure and volume between the pseudo-spinel crystal structure and the crystal structure in the discharge state is smaller than that of the H1-3 type crystal structure, so it is not easily degraded by repeated charge and discharge. Therefore, when charged at high voltage, as long as the positive electrode active material has a pseudo-spinel crystal structure, its cycle characteristics are excellent even when charged and discharged at high voltage.

[0405] In contrast, it can be seen that in the comparative examples, samples 05 and 06, when charged at 4.6V, the pseudo-spinel crystal structure is absent or very rare, and the main crystal structure is H1-3. The difference in crystal structure and volume between the H1-3 and O3 types is significant, making them prone to degradation. Therefore, samples 05 and 06 are materials that cannot withstand high-voltage charging, and their discharge capacity is significantly reduced.

[0406] Furthermore, compared to sample 01, sample 05 contains both magnesium and fluorine, but when charged at 4.6V, it primarily exhibits an H1-3 type crystal structure, resulting in deteriorated cycle performance. Thus, it can be seen that a characteristic of one embodiment of the present invention is that the crystal structure of the positive electrode active material undergoes less change during charging and discharging, and the aforementioned characteristics cannot be determined solely based on the elements it contains. Example 2

[0407] In this embodiment, the positive electrode active material 100 of one embodiment of the present invention and lithium cobalt oxide of a comparative example are described, and the results of analysis using ESR are presented.

[0408] [Manufacturing of positive electrode active materials] Sample 11A and Sample 11B The positive electrode active material formed by adding magnesium and fluorine to the starting material and performing a first heat treatment is designated as sample 11A, and the positive electrode active material that undergoes a second heat treatment is designated as sample 11B.

[0409] In steps S11 and S12, lithium carbonate, cobalt oxide, magnesium oxide, and lithium fluoride were weighed in the following ratio: Li 1.02 Co 0.99 Mg 0.01 O 1.98 F 0.02, and then mixed. The first heating in step S13 was performed under the following conditions: an alumina furnace was used; the flow rate of the dry air atmosphere was 10 L / min; the holding temperature was 1000°C (heating rate was 200°C / hour); and the holding temperature was maintained for 10 hours. The cooling time from the holding temperature to room temperature was set to be between 10 and 15 hours. The lithium cobalt oxide particles containing magnesium and fluorine synthesized by the first heat treatment were used as sample 11A.

[0410] Next, the lithium cobalt oxide particles containing magnesium and fluorine from sample 11A were placed in an alumina furnace for the second heating treatment in step S15. This heating was performed under the following conditions: a dry air flow rate of 10 L / min; a holding temperature of 800°C (heating rate of 200°C / hour); and a holding temperature of 2 hours. The cooling time from the holding temperature to room temperature was set to be between 10 and 15 hours. The particles produced by the above steps were designated as sample 11B.

[0411] Sample 12B Sample 12B is a comparative example of a positive electrode active material manufactured by first and second heat treatments without the addition of magnesium and fluorine.

[0412] Except that lithium carbonate and cobalt oxide are weighed in the same manner as in sample 11B, with each element weighed in the proportion of Li1Co1O2.

[0413] [ESR] Figures 39 and 40A to 40C show the results of ESR analysis of samples 11A, 11B, and 12B. Figure 39 shows the signal measured at room temperature. Figures 40A to 40C are magnified graphs of the results measured at low temperature (10K) to compare with the sharp signal near 320 mT.

[0414] As shown in Figure 39, a broad signal centered at 120 mT to 150 mT was detected in samples 12B and 11A, but in sample 11B, the signal was below the detection limit. This signal corresponds to oxygen-tetracoordinated Co (position A in Figures 5A and 5B).

[0415] Therefore, it can be concluded that: sample 12B, which does not contain magnesium and fluorine, and sample 11A, which contains magnesium and fluorine but has not undergone the second heat treatment, have a spinel-type crystal structure of Co3O4; the Co3O4 with a spinel-type crystal structure in sample 11B, which contains magnesium and fluorine and has undergone the second heat treatment, is below the detection limit.

[0416] Furthermore, as shown in Figures 40A to 40C, a sharp peak centered around 320 mT was detected in all samples. This signal corresponds to oxygen-hexacoordinated Co (position B in Figures 5A and 5B).

[0417] In Figure 40B, sample 11A, and in Figure 40C, sample 11B, a shoulder peak was observed near 312 mT, but it was not observed in sample 12B of Figure 40A. This peak indicates the presence of Mg near Co. Therefore, it can be concluded that Mg can also be determined from ESR. Example 3

[0418] In this embodiment, calculations were used to determine which elements, when dissolved in solid solution, are more likely to exhibit a spinel-like crystalline structure when charged at high voltage.

[0419] As shown in Figure 2, the H1-3 type crystal structure is a structure in which CoO2 structures such as P-3m1(O1) and LiCoO2 structures such as R-3m(O3) are stacked alternately.

[0420] Therefore, it can be concluded that by increasing the proportion of P-3m1 structures to about half, it is easy to obtain an H1-3 type crystal structure; conversely, by making the proportion of R-3m structures more than 50%, it is easy to obtain a pseudo-spinel type crystal structure of R-3m. Thus, the crystal structure models of P-3m1 and R-3m are used to recreate the positive electrode active material under high-voltage charging state to calculate the stability energy when Mg, Al, and Ti are included.

[0421] As a crystal structure model for the high-voltage charging state, the structure with all Li removed from R-3m(O3) and P-3m1(O1) illustrated in Figure 2 were used. Next, as described below, the most stable positions of Mg, Al, or Ti embedded in the CoO2 layers and the cases where Co positions are replaced by Mg, Al, or Ti were calculated respectively.

[0422] Figure 41A1 shows the crystal structure model of P-3m1 when Mg, Al, or Ti is located in the CoO2 interlayer, and Figure 41A2 shows the crystal structure model of R-3m when Mg, Al, or Ti is located in the CoO2 interlayer. Figure 41B1 shows the crystal structure model of P-3m1 when Mg, Al, or Ti is located in the Co site, and Figure 41B2 shows the crystal structure model of R-3m when Mg, Al, or Ti is located in the Co site. Table 3 shows the calculation conditions.

[0423]

[0424] When Mg is embedded in the CoO2 interlayer, the energy difference ΔE (eV) between the space group P-3m1 structure and the space group R-3m structure is calculated using the following formula. The element energy used for embedding and substitution is the monomer energy of the atom.

[0425] [Formula 2] The energy of a crystal structure with space group R-3m and a model in which a Mg atom is embedded between CoO2 layers (equivalent to structure b in Figure 41A2) is... The energy of the crystal structure of space group P-3m1 with a Mg atom embedded in the CoO2 interlayer (equivalent to structure a in Figure 41A1)

[0426] Similarly, when Co is replaced by Mg, the energy difference is calculated using the following formula.

[0427] [Formula 3] The energy of the crystal structure of space group R-3m with Co replaced by one Mg atom (equivalent to structure d in Figure 41B2) The energy of the crystal structure of space group P-3m1 with Co replaced by one Mg atom (equivalent to structure c in Figure 41B1)

[0428] Figures 42A and 42B show the calculation results for other elements performed in the same manner as described above. For comparison, results are also shown for the absence of embedding or substitution, and for embedding Li into the CoO2 interlayer.

[0429] Figure 42A is a graph showing the stability energies ΔE when Al, Ti, Mg, or Li are embedded in the CoO2 interlayer. The ΔE values ​​for all elements are negative, meaning that the structure in space group R-3m is more stable than P-3m1. Furthermore, the values ​​for all elements are lower than those for Li. In other words, positive electrode active materials containing these elements in the CoO2 interlayer are more likely to form an R-3m structure under high-voltage charging conditions than pure LiCoO2. It can be seen that Mg is the most effective among Al, Ti, and Mg.

[0430] Figure 42B is a graph of the stability energies ΔE when Co is substituted at the position with Al, Ti, and Mg. ΔE becomes positive for all elements, indicating that the P-3m1 structure is more stable. The values ​​for Al and Ti are lower than in the unsubstituted case (with Co present), but the value for Mg is higher.

[0431] Therefore, it can be concluded that Mg present in the CoO2 layer has a high effect on maintaining R-3m; conversely, Mg present in the Co site does not have the above effect.

[0432] Next, the R-3m(O3) crystal structure with all lithium ions intercalated in the discharge state was calculated to determine which of the Li and Co sites is more stable when Al, Ti, or Mg substitutes for them. The calculation method is the same as in Figures 42A and 42B. The results are shown in Figure 43.

[0433] When either the Li or Co site is substituted, the ΔE values ​​for Al and Ti become approximately the same negative value. Furthermore, the ΔE for Ti is smaller. This indicates that both Al and Ti tend to readily dissolve in LiCoO2, with Ti being more readily soluble.

[0434] On the other hand, comparing Mg at Li and Co sites, the Li sites are larger and more stable. This indicates that Mg is more likely to enter the Li sites compared to the Co sites. Furthermore, ΔE is positive for each substitution, suggesting that Mg tends to be less readily dissolved in LiCoO2. This tendency explains the segregation of some Mg in the surface layer and near grain boundaries.

[0435] The above explanation demonstrates that when Mg is present in the interlayer of CoO2 (Li sites), the R-3m structure is easily maintained even under high-voltage charging conditions where multiple Li layers are removed, and a pseudo-spinel crystal structure is readily formed. Therefore, it is important that, in the manufacturing process including the second heat treatment, Mg, which readily enters LiCoO2, is reliably located at Li sites, rather than at Co sites.

[0436] 100: Positive electrode active material 200: Active substance layer 201: Graphene Compounds 211a: Positive electrode 211b: Negative electrode 212a: Conductor 212b: Conductor 214: Isolation body 215a: Joint 215b: Joint 217: Fixed components 250: Secondary battery 251: Outer packaging 261: Folded section 262: Sealing part 263: Sealing part 271: Edge 272: Valley Bottom Line 273: Space 300: Secondary battery 301: Positive electrode container 302: Negative electrode container 303: Gasket 304: Positive electrode 305: Positive Current Collector 306: Positive electrode active material layer 307: Negative electrode 308: Negative current collector 309: Negative electrode active material layer 310: Isolation body 500: Secondary battery 501: Positive Current Collector 502: Positive electrode active material layer 503: Positive electrode 504: Negative current collector 505: Negative electrode active material layer 506: Negative electrode 507: Isolation 508: Electrolyte 509: Outer Packaging 510: Positive electrode wire 511: Negative electrode wire 600: Secondary battery 601: Positive electrode cover 602: Battery Canister 603: Positive extreme 604: Positive electrode 605: Isolation Body 606: Negative electrode 607: Negative extreme sub-particle 608: Insulation Board 609: Insulation Board 611: PTC Component 612: Safety Valve Mechanism 613: Conductive plate 614: Conductive plate 615: Module 616: Wire 617: Temperature control device 900: Circuit board 910: Signature 911:Terminal 912: Circuit 913: Secondary battery 914: Antenna 915: Antenna 916: Floor 917: Floor 918: Antenna 920: Display device 921: Sensor 922:Terminal 930: Outer shell 930a: Outer casing 930b: Casing 931: Negative electrode 932: Positive electrode 933: Isolation 950: Wound body 951:Terminal 952:Terminal 980: Secondary battery 981:Film 982:Film 993: Wound Body 994: Negative electrode 995: Positive electrode 996: Isolation Body 997: Wire Electrode 998: Wire Electrode 7100: Portable display device 7101: Outer casing 7102: Display Unit 7103: Operation Button 7104: Secondary battery 7200: Portable Information Terminal 7201: Outer casing 7202: Display Unit 7203: Ribbon 7204: Buckle 7205: Operation Button 7206: Input / output terminals 7207: Illustration 7300: Display device 7304: Display Unit 7400: Mobile Phone 7401: Outer casing 7402: Display Unit 7403: Operation button 7404: External connection port 7405: Speaker 7406: Microphone 7407: Secondary battery 7408: Wire Electrode 7500: Electronic cigarettes 7501: Atomizer 7502: Smoke cartridge 7504: Secondary battery 8000: Display device 8001: Casing 8002: Display Unit 8003: Speaker Section 8004: Secondary battery 8021: Charging device 8022: Cable 8024: Secondary battery 8025: Secondary battery 8100: Lighting equipment 8101: Outer casing 8102: Light source 8103: Secondary battery 8104: Ceiling 8105: Sidewall 8106: Floor 8107: Window 8200: Indoor unit 8201: Outer casing 8202: Air vent 8203: Secondary battery 8204: Outdoor unit 8300: Electric Refrigeration and Freezing Box 8301: Outer casing 8302: Refrigerator door 8303: Freezer door 8304: Secondary battery 8400: Car 8401: Headlights 8406: Electric motor 8500: Car 8600: Miniature Motorcycle 8601: Rearview Mirror 8602: Secondary battery 8603: Turn signals 8604: Under-seat storage box 9600: Tablet Terminal 9625: Switch 9626: Switch 9627: Switch 9628: Operation switch 9629: Fasteners 9630: Outer casing 9630a: Outer casing 9630b: Casing 9631: Display Unit 9633: Solar Cell 9634: Charge / Discharge Control Circuit 9635: Storage element 9636: DC-DC converter 9637: Converter 9640: Movable Part

Claims

1. A lithium-ion secondary battery comprising a positive electrode, wherein the positive electrode comprises a positive electrode active material, the positive electrode active material comprises positive electrode active material particles, wherein the positive electrode active material particles comprise lithium cobalt oxide, and wherein the positive electrode active material particles comprise magnesium, aluminum, and fluorine. In the EDX line analysis of the positive electrode active material particles, the peak of aluminum is located in a deeper region than that of magnesium. When manufacturing a battery containing this positive electrode, lithium metal as the counter electrode, an electrolyte solution, and a polypropylene separator, the battery is charged in a charging state. The positive electrode is removed from the battery charged in this charging state, sealed in a sealed container, and powder X-ray diffraction measurement is performed on the positive electrode sealed in the sealed container. The X-ray diffraction pattern of the positive electrode includes at least a first diffraction peak at 2θ = 19.30 ± 0.20° and a second diffraction peak at 2θ = 45.55 ± 0.10°. The electrolyte solution contains: 1 mol / L lithium hexafluorophosphate as the electrolyte; and a solution of ethylene carbonate and diethyl carbonate in a volume ratio of 3:7 and 2 wt% ethylene carbonate (VC). The charging state includes: The battery is charged at a constant current of 0.5C at 25°C until the voltage reaches 4.6V; and after the constant current charging, it is charged at a constant voltage of 4.6V at 25°C until the current reaches 0.01C, wherein 1C is set to 137 mA / g. The positive electrode is removed from the battery under this charging state and sealed in the sealed container, which is then placed in an argon atmosphere glove box. The powder X-ray diffraction measurement is performed using CuKα1 lines.

2. As in request item 1, a lithium-ion secondary battery, wherein, When the X-ray diffraction pattern of the cathode was analyzed by Ritwald analysis, it was shown that the ratio of the crystal structure of the first diffraction peak to the second diffraction peak was greater than or equal to 50 wt%.

3. As in request item 1, a lithium-ion secondary battery, wherein, When the X-ray diffraction pattern of the cathode was analyzed by Ritwald analysis, it was shown that the ratio of the crystal structure of the first diffraction peak to the second diffraction peak was greater than or equal to 60 wt%.

4. As in request item 1, a lithium-ion secondary battery, wherein, When the X-ray diffraction pattern of the cathode was analyzed by Ritwald analysis, the ratio of the crystal structure of the first diffraction peak to the second diffraction peak was greater than or equal to 66 wt%.

5. The lithium-ion secondary battery of claim 1, wherein the positive electrode active material is a material that, when subjected to X-ray photoelectron spectroscopy, shows a peak value of at least 1302 eV and less than 1304 eV for the bonding energy between magnesium and other elements.

6. The lithium-ion secondary battery of claim 1, wherein the positive electrode active material particles contain magnesium within the positive electrode active material particles.

7. The lithium-ion secondary battery of claim 1, wherein the positive electrode active material includes a region in which aluminum is present at the cobalt sites of the positive electrode active material.

8. The lithium-ion secondary battery of claim 1, wherein the positive electrode active material particles comprise titanium, and wherein the positive electrode active material comprises a region in which titanium is present at the cobalt position of the positive electrode active material.

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