Secondary battery

Abstract

To provide a secondary battery that can freely be curved.SOLUTION: A secondary battery 10 according to one aspect of the invention comprises a positive electrode 43, a negative electrode 41, and an outer jacket 23 storing the positive electrode 43 and the negative electrode 41. The rigidity of the outer jacket 23 is lesser than the rigidity of a collector included in the positive electrode 43, or lesser than the rigidity of a collector included in the negative electrode 41. It is preferable that the secondary battery further comprises a protective material 40, which is a sheet-like polyimide impregnated with an electrolyte 45, provided between the outer jacket 23 and the negative electrode 41.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] One embodiment of the present invention relates to a secondary battery. Note that one embodiment of the present invention is not limited to the above fields, and relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, or a manufacturing method thereof. [Background technology]

[0002] From the viewpoint of improving the fit, a curved structure for the lithium ion secondary battery mounted on the wearable electronic device is being considered. For example, a lithium ion secondary battery with a protective material disposed on the inside of the exterior body has been proposed (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-233003 A Summary of the Invention [Problem to be solved by the invention]

[0004] Although it is possible to curve the lithium ion secondary battery according to the above Patent Document 1 etc., there is room for further study on the lithium ion secondary battery. For example, the exterior body of the lithium ion secondary battery uses a laminated structure including metal foil to prevent the intrusion of water and atmospheric components, but this exterior body hinders the flexibility of the lithium ion secondary battery.

[0005] Furthermore, in terms of the design of electronic devices, the area provided for mounting the lithium ion secondary battery may be limited, and it is therefore desirable for the lithium ion secondary battery to be able to bend freely within the provided area.

[0006] In consideration of the above-mentioned circumstances, an object of the present invention is to provide a secondary battery that can be curved freely and an exterior body suitable for the secondary battery.

[0007] Note that the description of the above problems does not prevent the existence of other problems. Furthermore, problems other than the above problems can be extracted from the description of the specification, drawings, and claims. One embodiment of the present invention does not need to solve all of the above problems, but solves at least one of the problems. [Means for solving the problem]

[0008] One embodiment of the present invention is a secondary battery that includes a positive electrode, a negative electrode, and an exterior body that houses the positive electrode and the negative electrode, in which the rigidity of the exterior body is smaller than the rigidity of a current collector of the positive electrode.

[0009] Another embodiment of the present invention is a secondary battery having a positive electrode, a negative electrode, and an exterior body that houses the positive electrode and the negative electrode, the rigidity of the exterior body being smaller than the rigidity of a current collector of the negative electrode.

[0010] Another aspect of the present invention is a secondary battery having a positive electrode, a negative electrode, an exterior body that houses the positive electrode and the negative electrode, and a protective material located between the exterior body and the negative electrode, wherein the rigidity of the exterior body is smaller than the rigidity of a current collector of the positive electrode, and the protective material has a sheet-like member impregnated with an electrolyte.

[0011] Another aspect of the present invention is a secondary battery having a positive electrode, a negative electrode, an exterior body that houses the positive electrode and the negative electrode, and a protective material located between the exterior body and the positive electrode, wherein the rigidity of the exterior body is smaller than the rigidity of a current collector of the negative electrode, and the protective material has a sheet-like member impregnated with an electrolyte.

[0012] In the present invention, the protective material preferably comprises a sheet-like polyimide impregnated with an electrolyte.

[0013] In the present invention, the secondary battery has a separator, and the separator preferably has polyimide.

[0014] In the present invention, the electrolyte preferably contains an ionic liquid. Effect of the Invention

[0015] According to one embodiment of the present invention, a secondary battery and an exterior body that can be freely curved can be provided.

[0016] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract effects other than these from the description in the specification, drawings, claims, etc. [Brief description of the drawings]

[0017] [Figure 1] 1(A) and 1(B) show examples of a curved secondary battery. [Diagram 2] 2A to 2C are cross-sectional structure examples of a curved secondary battery. [Diagram 3] 3A to 3E show examples of the structure of a curved secondary battery. [Figure 4] 4A to 4D are examples of the cross-sectional structure of a positive electrode or a negative electrode. [Diagram 5] 5A1 to 5B2 show examples of the positive electrode active material. [Figure 6] FIG. 6 is a diagram illustrating the crystal structure of the positive electrode active material. [Figure 7] FIG. 7 is a graph showing the diffraction peaks of the positive electrode active material. [Figure 8] FIG. 8 is a graph showing the diffraction peaks of the positive electrode active material. [Figure 9] 9(A) and 9(B) are graphs showing the diffraction peaks of the positive electrode active material. [Figure 10] 10A to 10C2 show examples of the positive electrode active material. [Figure 11] 11(A) to 11(D) are an example of a flowchart showing a method for producing a positive electrode active material. [Figure 12] FIG. 12 is an example of a flowchart showing a method for producing a positive electrode active material. [Figure 13]13(A) and 13(B) are an example of a flowchart showing a method for producing a positive electrode active material. [Figure 14] Figures 14(A) to 14(D) are diagrams illustrating an example of a transportation vehicle, and Figure 14(E) is a diagram illustrating an example of an artificial satellite. [Figure 15] FIG. 15(A) is a diagram showing an electric bicycle, FIG. 15(B) is a diagram showing a secondary battery of the electric bicycle, and FIG. 15(C) is a diagram showing a scooter. [Figure 16] 16A to 16D are diagrams illustrating examples of electronic devices. [Figure 17] 17A to 17C are diagrams illustrating examples of electronic devices. [Figure 18] FIG. 18(A) and FIG. 18(B) are photographs showing the appearance of the sample. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be easily understood by those skilled in the art that the form and details of the present invention can be modified in various ways. Furthermore, the present invention is not to be interpreted as being limited to the description of the embodiments shown below.

[0019] In this specification, the space group is expressed using short notation of the international notation (or Hermann-Mauguin notation). Furthermore, the crystal plane and crystal direction are expressed using Miller indices. In crystallography, the notation of the space group, crystal plane, and crystal direction is expressed by adding a superscript bar to the number, but in this specification, due to format restrictions, instead of adding a bar above the number, a - (minus sign) may be added before the number. Furthermore, an individual direction indicating a direction within a crystal is expressed by [ ], a collective direction indicating all equivalent directions is expressed by < >, an individual face indicating a crystal face is expressed by ( ), and a collective face having equivalent symmetry is expressed by {}. Furthermore, for ease of understanding of the structure, the trigonal crystal represented by the space group R-3m is generally expressed as a composite hexagonal lattice of a hexagonal crystal, and unless otherwise specified, the space group R-3m will be expressed as a composite hexagonal lattice in this specification. Furthermore, not only (hkl) but also (hkil) may be used as the Miller indices. Here, i is -(h+k).

[0020] In this specification and the like, the space group is identified by X-ray diffraction (XRD), electron beam diffraction, neutron beam diffraction, etc. Therefore, belonging to a certain space group, belonging to a certain space group, or being a certain space group can be rephrased as being identified with a certain space group.

[0021] In this specification, if the arrangement of anions is roughly close to cubic close-packed, it can be considered as cubic close-packed. The arrangement of anions in cubic close-packed refers to a state in which the second layer of anions is arranged on the voids of the anions packed in the first layer, and the third layer of anions is arranged directly above the voids of the second layer of anions, but not directly above the anions in the first layer. Therefore, the anions do not have to be strictly cubic lattices. In addition, since real crystals always have defects, the analysis results do not necessarily have to be theoretical. For example, in an FFT (fast Fourier transform) pattern such as an electron beam diffraction pattern or a TEM image, a spot may appear at a position slightly different from the theoretical position. For example, if the deviation between the theoretical position and the orientation is 5 degrees or less, or 2.5 degrees or less, it can be said that the structure has a cubic close-packed structure.

[0022] In the present specification, the layered rock salt type crystal structure refers to a crystal structure having a rock salt type ion arrangement in which cations and anions are alternately arranged, in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as deficiencies of cations or anions may be present. Strictly speaking, the layered rock salt type crystal structure may have a structure in which the lattice of the rock salt type crystal is distorted.

[0023] In this specification, the rock salt type crystal structure refers to a structure having a cubic crystal structure such as the space group Fm-3m, in which cations and anions are arranged alternately. Note that there may be a deficiency of cations or anions.

[0024] In this specification, the term "particle" is not limited to referring to only spherical shapes (cross-sectional shape is circular), and the cross-sectional shape of each particle may be an ellipse, a rectangle, a trapezoid, a cone, a square with rounded corners, an asymmetric shape, etc., and further, each particle may be irregular in shape.

[0025] In this specification, the smooth surface of an active material means that the surface roughness of a cross section of the active material is at least 10 nm or less when surface irregularity information is quantified from measurement data. In this specification, the cross section is, for example, a cross section obtained when observing with a STEM (Scanning Transmission Electron Microscope) image.

[0026] In this specification, the positive electrode active material refers to a compound having a transition metal and oxygen that can insert and remove lithium. Carbonates, hydroxyl groups, and the like that are adsorbed after the preparation of the positive electrode active material are not included in the positive electrode active material. Lithium salts, organic solvents, binders, conductive materials, or compounds derived from these that are attached after the preparation of the positive electrode active material are also not included in the positive electrode active material.

[0027] In this specification and the like, the theoretical capacity of the positive electrode active material refers to the amount of electricity when all the lithium that can be inserted and extracted from the positive electrode active material has been extracted. For example, the theoretical capacity of LiCoO2 is 274 mAh / g, the theoretical capacity of LiNiO2 is 275 mAh / g, and the theoretical capacity of LiMn2O4 is 148 mAh / g.

[0028] In this specification and the like, the degree to which lithium remains in the positive electrode active material compared to the theoretical capacity is represented by x in the composition formula, for example, x in Li x CoO2, or x in Li x MO2. Here, M means a transition metal that undergoes oxidation-reduction with the insertion and extraction of lithium. In this specification and the like, Li x CoO2 can be appropriately read as Li x MO2. In the case of the positive electrode active material in a secondary battery, x = (theoretical capacity - charge capacity) / theoretical capacity can be used. For example, when a secondary battery using LiCoO2 as the positive electrode active material is charged to 219.2 mAh / g, it can be said that Li 0.2 CoO2 or x = 0.2. A small x in Li x CoO2 means, for example, 0.1 < x ≤ 0.24. In some cases, the degree to which the lithium extracted from the positive electrode active material is relative to the theoretical capacity is called the depth of charge. In this specification and the like, the depth of charge corresponds to 1 - x.

[0029] Li x The charge capacity and discharge capacity used for calculating x in LiCoO2 are preferably measured under conditions where there is no short circuit and the influence of lithium salt decomposition is absent or minimal. For example, data of a secondary battery in which a rapid change in capacity considered to be a short circuit has occurred should not be used for calculating x.

[0030] When lithium cobalt oxide approximately satisfies the stoichiometric ratio, it is LiCoO2 and the occupancy rate of Li on the lithium site is x=1. The lithium cobalt oxide contained in the secondary battery after discharge is also LiCoO2 and x=1. The end of discharge here means, for example, a state in which the voltage is 2.5V (counter electrode lithium) or less at a current of 100mA / g per weight of the positive electrode active material. In a lithium ion secondary battery, when the occupancy rate of lithium on the lithium site is x=1 and no more lithium is inserted, the voltage drops sharply. At this point, it can be said that the discharge is completed. In general, in a lithium ion secondary battery using LiCoO2, the discharge voltage drops sharply before it reaches 2.5V, so the discharge is considered to be completed under the above conditions. In addition, when the positive electrode after discharge is analyzed using XRD patterns, it can be confirmed that it has a general LiCoO2 crystal structure.

[0031] In this specification, homogeneity refers to a phenomenon in which, in a solid consisting of multiple elements (e.g., A, B, C), a certain element (e.g., A) is distributed with similar characteristics in a specific region. Note that it is sufficient that the concentration of the element in the specific regions is substantially the same. For example, it is sufficient that the difference in element concentration between the specific regions is within 10%. Examples of the specific region include a surface layer, a surface, a convex portion, a concave portion, and a bulk.

[0032] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the presence of a mixture of areas with high concentration and areas with low concentration.

[0033] In the present specification, when describing the characteristics of the positive electrode active material, not all of the positive electrode active materials in the secondary battery need to have that characteristic. For example, when describing the characteristics of the coating of the positive electrode active material, if 50% or more, preferably 70% or more, more preferably 90% or more of the total number of randomly selected three or more positive electrode active materials have the characteristics of the coating, it can be said that there is an effect of sufficiently improving the characteristics of the positive electrode active material and the secondary battery having it. Regarding the surface on which the coating is formed, if 50% or more, preferably 70% or more, more preferably 90% or more of the positive electrode active material has the characteristics of the coating, it can be said that there is an effect of sufficiently improving the characteristics of the positive electrode active material and the secondary battery having it.

[0034] In this specification, unless otherwise specified, the materials (positive electrode active material, negative electrode active material, lithium salt, etc.) of the secondary battery will be described in a state before deterioration. Note that a decrease in discharge capacity due to aging treatment and burn-in treatment in the secondary battery manufacturing stage is not called deterioration. For example, when the secondary battery has a discharge capacity of 97% or more of the rated capacity, it can be said to be in a state before deterioration. The rated capacity conforms to JIS C 8711:2019. Note that in this specification, the state of the materials of the secondary battery before deterioration is referred to as an initial product or initial state, and the state after deterioration (when the secondary battery has a discharge capacity of less than 97% of the rated capacity) may be referred to as a product in use or a state in use, or a used product or a used state.

[0035] In this specification, the lithium ion secondary battery refers to a battery using lithium ions as carrier ions, but the carrier ions of the present invention are not limited to lithium ions. For example, alkali metal ions or alkaline earth metal ions can be used as the carrier ions of the present invention, and specifically, sodium ions can be applied. In this case, the present invention can be understood by reading lithium ions as sodium ions. In addition, when there is no limitation on the carrier ions, it may be referred to as a secondary battery.

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

[0037] In this specification, the (001) plane and the (003) plane may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may be referred to as the C plane, the basal plane, or the like. In addition, in lithium cobalt oxide, lithium has a two-dimensional diffusion path. That is, it can be said that the lithium diffusion path exists along the plane. In this specification, a plane where the lithium diffusion path is exposed, that is, a plane other than the plane where lithium is inserted and removed (specifically, the (001) plane), may be referred to as an edge plane.

[0038] Secondary particles refer to particles formed by agglomeration of primary particles. In this specification, agglomeration includes a state of gathering, and does not matter what kind of bonding force acts between multiple primary particles. In other words, it may be any of covalent bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and other intermolecular interactions, or multiple bonding forces may be acting. In this specification, primary particles may also be called single particles.

[0039] In the present specification, etc., the secondary battery that can be freely curved includes a secondary battery fixed in a curved state (curved state) and a secondary battery that can change between a curved state and an extended state by following the casing of the secondary battery. The term "curved" may be read as "bendable." In the present specification, etc., a secondary battery in an extended state refers to a state in which the exterior body of the secondary battery is straight, for example, in a cross-sectional view, the long side of the exterior body of the secondary battery forms 180 degrees. In the present specification, etc., a secondary battery in a curved state refers to a state in which the exterior body of the secondary battery is curved at a certain angle, for example, in a cross-sectional view, the long side of the exterior body of the secondary battery satisfies an angle of 0 degrees or more and less than 360 degrees at a certain point. The certain point may be called a curved point, and the curved point may be confirmed at multiple points in a cross-sectional view. In the curved state, the long side of the exterior body may be read as a short side of the exterior body. Furthermore, when the exterior body does not distinguish between the long side and the short side, the long side of the exterior body may be read as a certain side.

[0040] In this specification, the term "SOC (State Of Charge)" refers to the state of charge (also called the charging rate), and is an index in which a fully charged state is 100% and a fully discharged state is 0%. In this specification, the term "OCV (Open Circuit Voltage)" refers to the voltage when a battery is in an electrochemical equilibrium state.

[0041] In this specification, Young's modulus can be measured with reference to ISO527, JISK7161, JISK7162, JISK7127, ASTM D638, ASTM D882, etc.

[0042] In this specification and the like, the description "A and / or B" may be used, but this is an example of the description including only A, only B, or both A and B.

[0043] (Embodiment 1) In this embodiment, a secondary battery will be described.

[0044] 1(A) and 1(B) show a secondary battery 10 in a curved state, and the secondary battery 10 is bent so that there is a curved point on the long side of the secondary battery 10. FIG. 1(A) shows a secondary battery 10 in which the first lead electrode 21 and the second lead electrode 22 are located on the long side of the exterior body 23, and FIG. 1(B) shows a secondary battery 10 in which the first lead electrode 21 and the second lead electrode 22 are located on the short side of the exterior body 23. Note that, although FIGS. 1(A) and 1(B) show a configuration in which the first lead electrode 21 and the second lead electrode 22 are both provided on the same side of the exterior body 23, in the secondary battery 10 in FIGS. 1(A) and 1(B), the first lead electrode 21 may be provided on one side of the exterior body 23, and the second lead electrode 22 may be provided on the other side of the exterior body 23. The one side may be a side facing the other side, or may be a side adjacent to the other side.

[0045] The secondary battery 10 shown in Fig. 1(A) and Fig. 1(B) has an exterior body 23. Arrow 24 indicates the direction along the short side of the exterior body 23. Although not shown in Fig. 1(A) and Fig. 1(B), at least a positive electrode, a separator, and a negative electrode are housed in the exterior body 23. In this specification and the like, the structure housed in the exterior body 23 may be called an internal structure. The first lead electrode 21 is electrically connected to the positive electrode, and the second lead electrode 22 is electrically connected to the negative electrode. In the region where the first lead electrode 21 is located, the positive electrode current collector has a protruding region. In the region where the second lead electrode is located, the negative electrode current collector has a protruding region.

[0046] The secondary battery 10 shown in Fig. 1(A) and Fig. 1(B) can be said to be curved so as to intersect with the direction of the arrow 24. In this case, the configuration in which the first lead electrode 21 and the second lead electrode 22 are arranged as shown in Fig. 1(A) and Fig. 1(B) is preferable because the first lead electrode 21 and the second lead electrode 22 are less likely to wrinkle. As described above, the current collector has a protruding region, and therefore the protruding region may wrinkle or even break. Therefore, in this specification and the like, the wrinkles of the first lead electrode 21 and the second lead electrode 22 include wrinkles of the current collector in the region where the first lead electrode 21 and the second lead electrode 22 are bonded, in addition to the wrinkles of the first lead electrode 21 and the second lead electrode 22.

[0047] The first lead electrode 21 and the second lead electrode 22 can be electrically connected to an external circuit, allowing charging or discharging from the external circuit of the secondary battery 10. A protection circuit for preventing overcharging or overdischarging may be electrically connected to the first lead electrode 21 and the second lead electrode 22.

[0048] As a result of intensive research by the present inventors into the secondary battery 10, it was found that the flexibility of the secondary battery 10 can be improved by lowering the rigidity of the exterior body 23, not the internal structure. The physical properties that indicate the flexibility of the exterior body 23 include rigidity. Rigidity is a value that indicates hardness, and is expressed by the formula Rigidity (MPa·mm) = Young's modulus (MPa) × thickness (mm). If the flexibility of the exterior body 23 is high, the rigidity value becomes small. It is preferable that the rigidity of the exterior body 23 is 0.005 MPa·mm or more and 20 MPa·mm or less. In addition, it is preferable that the Young's modulus of the exterior body 23 is 1 MPa or more and 69 GPa or less, and preferably 1 MPa or more and 100 MPa or less. An exterior body that satisfies such a Young's modulus can be designed to be smaller than the rigidity of the positive electrode current collector or the negative electrode current collector. The rigidity and Young's modulus of the exterior body 23 described in this specification and the like are preferably satisfied by the exterior body obtained by dismantling the secondary battery 10, but may be satisfied by the exterior body before assembling the secondary battery 10, and the secondary battery 10 can be freely curved by the exterior body before assembling the secondary battery 10 satisfying the rigidity and Young's modulus. In other words, the exterior body obtained by dismantling the secondary battery 10 may be affected by side reactions such as electrolyte, and therefore the rigidity and Young's modulus may not be accurately determined. Furthermore, a secondary battery having a discharge capacity of 97% or more of the rated capacity is not affected by the side reactions or is only slightly affected if any, so it is desirable that the exterior body obtained by dismantling a secondary battery having a discharge capacity of 97% or more of the rated capacity satisfies the rigidity and Young's modulus. Furthermore, in a secondary battery 10 that allows movement between a curved state and a straight state, the exterior body 23 may be affected by the movement, but a secondary battery having a discharge capacity of 97% or more of the rated capacity may be considered not to be affected by the movement.

[0049] Such an outer casing 23 may have a lower rigidity than the positive electrode, typically the positive electrode current collector. Alternatively, the outer casing 23 may have a lower rigidity than the negative electrode, typically the negative electrode current collector. Specifically, the rigidity of the positive electrode current collector or the negative electrode current collector may be 0.005 MPa·mm or more and 500 MPa·mm or less, preferably 0.01 MPa·mm or more and 50 MPa·mm or less. In addition, if the Young's modulus of the positive electrode current collector or the negative electrode current collector is 5 MPa or more and 5000 MPa or less, and further 20 MPa or more and 3000 MPa or less, it is preferable because it is easy to design the rigidity of the outer casing 23 to be smaller than these values. Note that, although it is desirable that the rigidity and Young's modulus of the current collector described in this specification and the like are satisfied by the current collector obtained by disassembling the secondary battery 10, they may be satisfied by the current collector before the active material layer is applied. There may be an influence of pressing during application, and the rigidity and Young's modulus may not be accurately determined. Furthermore, like the rigidity and Young's modulus of the exterior body 23, it is desirable that the rigidity and Young's modulus of the current collector be satisfied by the current collector obtained by dismantling the secondary battery 10, but since the current collector obtained by dismantling the secondary battery 10 may also be affected by side reactions of the electrolyte, etc., it may not be possible to accurately determine the rigidity and Young's modulus. Furthermore, since a secondary battery having a discharge capacity of 97% or more of the rated capacity is not affected by the side reactions or is only slightly affected, it is desirable that the current collector obtained by dismantling a secondary battery having a discharge capacity of 97% or more of the rated capacity satisfy the rigidity and Young's modulus.

[0050] In order for the exterior body 23 to have flexibility, the exterior body 23 is preferably of a single-layer structure rather than a laminated structure. If the exterior body 23 has a laminated structure, no metal foil is used for the exterior body 23, and five or less layers are laminated. In the case of a single-layer structure, the exterior body 23 may be made of an organic material exhibiting insulating properties. Examples of organic materials exhibiting insulating properties include silicone resin, polyimide resin, polyamide-based resin, and polyester-based resin. Polyimide is a polymer containing imide bonds and is an example of a polyimide resin. A polyamide-based resin may contain polyamide, and nylon is a fiber made from polyamide and is an example of a polyamide-based resin. A polyester-based resin may contain polyester, and polyethylene terephthalate (PET) is a fiber made from PET, polyethylene naphthalate (PEN) is a fiber made from PEN, polytrimethylene terephthalate (PTT) is a fiber made from PTT, and polybutylene terephthalate (PBT) is a fiber made from PBT, all of which are examples of polyester-based resins.

[0051] The laminated exterior body 23 may be made of any two or more of the above organic materials applicable to the single layer structure, for example, two or more organic materials selected from the above silicone resin, polyimide resin, polyamide resin, and polyester resin may be laminated in order. The laminated exterior body 23 may also be made of an organic material applicable to the single layer structure with a coating layer. For example, an inorganic material such as silicon oxide (also called silica) or silicon nitride may be coated on an organic material applicable to the single layer structure to a thickness of 50 nm or less, preferably 20 nm or less, and more preferably 5 nm or less to form a laminated structure.

[0052] If the exterior body 23 is thick, it becomes difficult to bend the secondary battery 10, but by imparting flexibility to the exterior body 23, it becomes possible to freely bend the secondary battery 10 even if the long side of the exterior body 23 is 5 cm or more, preferably 10 cm or more, and more preferably 12 cm or more. The short side of the exterior body 23 is not particularly limited, but may be 1 / 2 to 3 / 4, preferably 1 / 3 to 4 / 5 of the long side.

[0053] When the long side is 5 cm or more, the area of ​​the exterior body 23 is 20 cm 2 The above should be satisfied. The area of ​​the positive electrode should be 2 / 3 or more and 9 / 10 or less, more preferably 7 / 8 or more and 9 / 10 or less, of the area of ​​the exterior body 23. Since the positive electrode is housed in the exterior body 23, it will be smaller than the area of ​​the exterior body 23. Furthermore, the area of ​​the negative electrode should be larger than the area of ​​the positive electrode, and should be 2 / 3 or more and 9 / 10 or less, more preferably 7 / 8 or more and 9 / 10 or less, of the area of ​​the exterior body 23. Since the negative electrode is housed in the exterior body 23, it will be smaller than the area of ​​the exterior body 23.

[0054] If the exterior body 23 is thick, the secondary battery 10 is less likely to bend, but by imparting flexibility to the exterior body 23, the thickness of the exterior body 23 can be set to 5 μm or more and 50 μm or less, preferably 10 μm or more and 20 μm or less. If the thickness of the exterior body 23 is large, the intrusion of impurities can be suppressed. When the exterior body 23 has a single-layer structure, the thickness of the exterior body 23 is equal to the thickness of the single-layer structure, and when the exterior body 23 has a multi-layer structure, the thickness of the exterior body 23 is equal to the thickness of the multi-layer structure.

[0055] 1(A) and 1(B), exterior body 23 is exemplified as a polygon having two long sides and two short sides, that is, a rectangle. A rectangle means that exterior body 23 has four corners, and the corners may be right angles, notched, or rounded. The corners of exterior body 23 may all have the same shape, or may have different shapes selected from right angles, notched shapes, and rounded shapes.

[0056] It can be said that the secondary battery 10 shown in Figures 1(A) and 1(B) has curved long sides (direction intersecting with arrow 24) that intersect with the short sides (arrow 24). A secondary battery 10 that can be freely bent includes a battery that can be bent not only in the same direction, but also in different directions. For example, if a secondary battery 10 with curved long sides (direction intersecting with arrow 24) can also be bent in the short sides (arrow 24), it can be said to be a secondary battery 10 that can be freely bent.

[0057] Furthermore, the secondary battery 10 that can be freely curved may include not only one that is fixed in the curved state, but also one that can be changed from the curved state to a straight state, and of course one that can be changed from a straight state to the curved state.

[0058] The secondary battery 10 that can be curved freely may be described using the radius of curvature. The radius of curvature is the reciprocal of the curvature, and the curvature is a quantity that indicates the ease of bending a curve or a curved surface. When the radius of curvature is used for the secondary battery 10, the thickness of the secondary battery 10 may be taken into consideration. If the exterior body 23 located on the outside of the curved secondary battery 10 in a cross-sectional view is regarded as a curve and the radius of curvature can be calculated, the secondary battery 10 can be said to be able to be curved freely if the radius of curvature satisfies 50 mm or less, preferably 20 mm or less. Note that when describing the secondary battery 10 that can be curved freely, the thickness of the secondary battery 10 can be ignored. This is because the secondary battery 10 can be regarded as a curve in a cross-sectional view. If the center of the curved secondary battery 10 in a cross-sectional view has a radius of curvature that satisfies 50 mm or less, preferably 20 mm or less, the secondary battery 10 can be said to be able to be curved freely.

[0059] Furthermore, the secondary battery 10 that can be freely curved may include one that can be changed from a curved state that satisfies the above-mentioned radius of curvature to a straight state, and of course, may also include one that can be changed from a straight state to a curved state that satisfies the above-mentioned radius of curvature.

[0060] By using the secondary battery 10 that can be freely bent as described above, it becomes possible to mount the secondary battery in an area even if the area is limited by design. The secondary battery may be mounted in the area in a rolled state. The secondary battery may also be mounted in the area in a folded state. When the secondary battery is rolled up, it is difficult to identify the bending points because they are continuous. When the secondary battery is folded, two or more bending points may be identified.

[0061] In the secondary battery 10, each side of the exterior body 23 is bonded to accommodate the internal structure. Although not shown, polyimide film tape or the like may be used to strengthen the bonding of each side. The tape is called a sealing member, and is not included in the thickness of the exterior body 23. For example, when the secondary battery 10 is curved along the long side (the direction intersecting with the arrow 24) as shown in FIG. 1(A) and FIG. 1(B), it is preferable to provide the sealing member only on the short side rather than on the long side and the short side. More preferably, it is preferable to provide the sealing member on one of the short sides rather than on the two opposing short sides. When the first lead electrode 21 and the second lead electrode 22 are arranged on the short side as shown in FIG. 1(B), it is preferable to provide the sealing member on the other short side. When the sealing member is provided on the side on which the first lead electrode 21 and the second lead electrode 22 are arranged, it is necessary to provide the sealing member only between the first lead electrode 21 and the second lead electrode 22. The rigidity of the sealing member is preferably equal to or smaller than the rigidity of the exterior body 23. The Young's modulus of the sealing member is preferably equal to or smaller than the Young's modulus of the exterior body 23. The above rigidity and Young's modulus are preferred examples for preventing the flexibility of the sealing member from being hindered when the sealing member overlaps the curved point. Therefore, if the sealing member can be arranged so as not to overlap the curved point, the rigidity and Young's modulus of the sealing member do not need to be limited as described above. The rigidity and Young's modulus of the sealing member only need to be satisfied before assembling the secondary battery, and the sealing member obtained by disassembling the secondary battery may be affected by side reactions, and the exact rigidity and Young's modulus may not be accurately determined. Furthermore, since a secondary battery having a discharge capacity of 97% or more of the rated capacity is not affected by the above side reactions or only slightly affected, it is desirable that the sealing member obtained by disassembling the secondary battery having a discharge capacity of 97% or more of the rated capacity satisfies the above rigidity and Young's modulus.

[0062] <Cross-sectional structure of secondary battery> FIG. 2(A) shows an XY cross-sectional view of the secondary battery 10. The secondary battery 10 is the one described in FIG. 1(A) and FIG. 1(B). FIG. 2(A) illustrates an example in which the secondary battery 10 has a positive electrode 43, a separator 42, a negative electrode 41, and an electrolyte 45 as an internal structure. The positive electrode 43 and the negative electrode 41 can be stacked to increase the capacity of the secondary battery. When stacking the positive electrode 43 and the negative electrode 41, a plurality of separators 42 are prepared. Note that a solid electrolyte layer may be used instead of the electrolyte 45, but the electrolyte 45 makes it easier to bend the secondary battery 10. An organic solvent and an ionic liquid can be used for the electrolyte 45, but the ionic liquid is preferable. This is because the ionic liquid has excellent electrochemical stability and can suppress the intrusion of impurities (typically water or moisture) into the secondary battery 10. The ionic liquid will be described later.

[0063] FIG. 2(B) shows an excerpt of the negative electrode 41 of the curved secondary battery 10. The bending stress (load) that the laminated negative electrodes 41 receive may differ depending on the direction of bending of the secondary battery 10. Therefore, in the curved state as shown in FIG. 2(B), the negative electrode 41 on the inside of the secondary battery 10 may have a different thickness from the negative electrode on the outside of the secondary battery 10. For example, the thickness of the negative electrode may be made thinner. In order to make the negative electrode thinner, there are configurations in which the negative electrode current collector is made thinner and / or the negative electrode active material layer is made thinner, and either may be applied. When three or more layers of negative electrodes 41 are laminated, the thickness of the negative electrode 41 may be gradually made thinner. In addition, the negative electrode may be replaced with a positive electrode to understand the configuration in which the thickness is made thinner. Furthermore, the negative electrode may be replaced with a separator to understand the configuration in which the thickness is made thinner.

[0064] As shown in Fig. 2(A), an adhesive region 48 is provided on the outer periphery or edge of the exterior body 23. The adhesive region 48 is an area that is bonded to each side of the exterior body 23. Although the lead electrodes are not shown in Fig. 2(A), the sides on which the lead electrodes are arranged are also bonded. For example, when bonding each side of the exterior body 23 using a thermocompression method, the exterior body 23 in the adhesive region 48 does not need to exhibit flexibility. In other words, the exterior body 23 in the adhesive region 48 may not satisfy the above-mentioned rigidity, thickness, etc.

[0065] When the dimensions of exterior body 23 are 160 mm×135 mm×3 mm, it is preferable to inject electrolyte 45 into exterior body 23 so that the liquid volume is 15 mL to 30 mL, and preferably 18 mL to 22 mL.

[0066] Fig. 2(C) is an XY cross-sectional view of the secondary battery 10, which is an example of a case where the amount of electrolyte 45 injected is larger than that of the secondary battery 10 shown in Fig. 2(A). When the dimensions of the exterior body 23 are 160 mm × 135 mm × 3 mm, the amount of electrolyte 45 injected into the exterior body 23 is 25 mL to 40 mL, preferably 28 mL to 32 mL, and this can be said to be a configuration in which the amount of electrolyte injected is large.

[0067] By increasing the amount of electrolyte 45 injected, the contact area or contact points between the negative electrode 41 and the exterior body 23 can be reduced compared to the configuration of Fig. 2(A). Avoiding contact between the negative electrode 41 and the exterior body 23 includes having an area where the negative electrode 41 and the exterior body 23 are separated, and a preferable example is a case where the negative electrode 41 and the exterior body 23 are entirely separated as shown in Fig. 2(C), but also includes a case where the negative electrode 41 and the exterior body 23 are partially in contact with each other and the rest are separated. Having an area where the negative electrode 41 and the exterior body 23 are separated is preferable because it reduces friction between the negative electrode 41 and the exterior body 23 when the secondary battery 10 is changed from a curved state to a straight state.

[0068] The secondary battery 10 in Fig. 2(C) can also be curved as in Fig. 2(B). In this case, when it is desired to reduce friction between the outer negative electrode and the exterior body 23, as an application example of Fig. 2(C), the inner negative electrode 41 may be in contact with the exterior body 23, and only the outer negative electrode may be separated from the exterior body 23. In order to maintain contact between the inner negative electrode 41 and the exterior body 23, it is preferable to adhere the negative electrode 41 to the inside of the exterior body 23 using, for example, a polyimide film tape or the like.

[0069] The exterior body 23 described above may have an uneven shape. For example, an exterior body 23 having an uneven shape can be obtained by embossing. The uneven shape can be confirmed as convex and concave portions of the exterior body 23 in a cross-sectional view of the secondary battery 10. The interval between adjacent convex portions (sometimes referred to as the pitch of the convex portions) is preferably 0.5 mm or more and 5 mm or less. The interval between adjacent concave portions (sometimes referred to as the pitch of the concave portions) is preferably 0.5 mm or more and 5 mm or less, and may be equal to or different from the pitch of the convex portions described above. However, when the secondary battery 10 is changed from a curved state to a straight state, cracks in the exterior body 23 may not be suppressed, and in this case, an exterior body 23 without an uneven shape is preferable.

[0070] <Protective material> The secondary battery 10 having the above-mentioned exterior body 23 is characterized in that it can be curved freely, but as a more preferred embodiment, a protective material is placed inside the exterior body 23. The protective material is not essential, but it can be said to be one of the preferred embodiments because it can suppress friction when the secondary battery 10 is deformed. Furthermore, the protective material can cause the secondary battery 10 to exhibit good cycle characteristics. The protective material can be said to be one of the internal structures.

[0071] FIG. 3(A) illustrates a secondary battery 10 having a protective material 40. The protective material 40 may be provided between the exterior body 23 and the negative electrode 41. FIG. 3(A) illustrates an example in which a protective material 40 having the same area as the negative electrode 41 is provided. The configuration in which the protective material 40 has the same area as the negative electrode 41 includes a configuration in which the protective material 40 has an area of ​​90% to 110% of the area of ​​the negative electrode 41 when viewed from above. In FIG. 3(A), the protective material 40 is provided on the upper and lower sides of the secondary battery 10, but the protective material 40 may be provided on only one side of the secondary battery 10, for example, only on the upper side or only on the lower side. The secondary battery 10 in FIG. 3(A) can also be curved as in FIG. 2(B). In this case, if it is desired to suppress friction between the outer negative electrode and the exterior body 23, the protective material 40 may be provided on the upper side (corresponding to the outside) of the secondary battery 10.

[0072] FIG. 3B shows an example in which protective material 40a smaller than the area of ​​the negative electrode 41 is disposed on the upper and lower sides of the secondary battery 10. The configuration in which the protective material 40a has an area smaller than the area of ​​the negative electrode 41 includes an area in which the protective material 40a is less than 90% of the area of ​​the negative electrode 41 in a top view. FIG. 3B shows an example in which the protective material 40a is extended along the short side of the exterior body 23 so as to fill 5% to 10% of the area of ​​the negative electrode 41. In this case, it is preferable that the protective material 40a on the upper side of the secondary battery 10 is disposed so as to overlap with the protective material 40b on the lower side, but they may be disposed so that their centers are offset from each other, and in this case, it is preferable that the offset of the centers is within 1 cm. It is preferable that the protective material 40a is extended along the short side of the exterior body 23 in a top view, but it is also acceptable that the protective material 40a is dotted. In Fig. 3(B), the protective material 40a is disposed on the upper and lower sides of the secondary battery 10, but the protective material 40a may be disposed on only one side of the secondary battery 10, for example, only on the upper side or only on the lower side. The secondary battery 10 in Fig. 3(B) can also be curved as in Fig. 2(B), but in this case, if it is desired to suppress friction between the outer negative electrode and the exterior body 23, the protective material 40a may be provided on the upper side (corresponding to the outside) of the secondary battery 10.

[0073] FIG. 3(C) shows an example in which a protective material 40b smaller than the area of ​​the negative electrode 41 is arranged so as to overlap with the center line of the secondary battery 10 in a top view. The protective material 40b may extend along the center line, but may be scattered. In FIG. 3(C), the protective material 40 is arranged on the upper side of the secondary battery 10, but may be arranged on the upper and lower sides. The secondary battery 10 in FIG. 3(C) can also be curved as in FIG. 2(B). In this case, if it is desired to suppress friction between the outer negative electrode and the exterior body 23 and friction between the inner negative electrode and the exterior body 23, the protective material 40a may be provided on the upper side (corresponding to the outside) and lower side (corresponding to the inside) of the secondary battery 10.

[0074] It is preferable that the protective material 40 bends together with the secondary battery 10. Therefore, the rigidity of the protective material 40 is preferably equal to or smaller than the rigidity of the exterior body 23. The Young's modulus of the protective material 40 is preferably equal to or smaller than the Young's modulus of the exterior body 23. The above rigidity and Young's modulus are preferable examples for the protective material 40 not to inhibit flexibility. Therefore, if the protective material 40 does not inhibit flexibility, the rigidity and Young's modulus of the protective material 40 do not need to be limited as described above. The rigidity and Young's modulus of the protective material 40 only need to be satisfied before assembling the secondary battery, and the protective material 40 obtained by disassembling the secondary battery may be affected by side reactions, and the exact rigidity and Young's modulus may not be accurately determined. Furthermore, a secondary battery having a discharge capacity of 97% or more of the rated capacity is not affected by the above side reactions or only slightly, so it is preferable that the protective material 40 obtained by disassembling a secondary battery having a discharge capacity of 97% or more of the rated capacity satisfies the above rigidity and Young's modulus.

[0075] As an example of a configuration in which the protective material 40 does not impair flexibility, a sheet-like member impregnated with the electrolyte 45 is used as the protective material 40. If the protective material 40 is impregnated with the electrolyte 45, friction at the interface between the protective material 40 and the adjacent member is suppressed, and the protective material 40 is easily bent together with the secondary battery 10, and it can be said that the protective material 40 is an example of a configuration in which the flexibility is not impaired. In this specification, impregnation includes the electrolyte 45 being located in the voids of the protective material 40. The member or structure described in the description of the separator 42 above can be applied to the sheet-like member used for the protective material 40. The protective material 40 can be arbitrarily selected from the members described in the description of the separator 42, and a member different from the separator 42 may be used.

[0076] It is more preferable that the protective material 40 shown in Figs. 3(A) to 3(C) is fixed in position, for example, to the exterior body 23. An adhesive, specifically a polyimide film tape, is used as a fixing means. The rigidity of the adhesive should be equal to or smaller than the rigidity of the exterior body 23. The Young's modulus of the adhesive should be equal to or smaller than the Young's modulus of the exterior body 23. The above rigidity and Young's modulus are preferred examples for not impeding flexibility when the adhesive overlaps a curved point. Therefore, as long as the adhesive can be positioned so as not to overlap a curved point, the rigidity and Young's modulus of the adhesive do not need to be limited as described above.

[0077] <Positive and negative electrodes> Next, the positive electrode 43 used in the secondary battery 10 will be described. As shown in FIG. 4(A), the positive electrode 43 has a positive electrode current collector 50 and a positive electrode active material layer 51 having a positive electrode active material. The positive electrode active material layer 51 is preferably thicker than the positive electrode current collector 50. As shown in FIG. 4(B), the positive electrode 43 may have a first positive electrode active material layer 51a and a second positive electrode active material layer 51b on both sides of the positive electrode current collector 50. In this specification and the like, FIG. 4(A) may be referred to as a single-sided coated positive electrode 43, and FIG. 4(B) may be referred to as a double-sided coated positive electrode 43. In a secondary battery 10 that can be freely curved, a single-sided coated positive electrode 43 may be preferable to a double-sided coated positive electrode 43.

[0078] Next, the negative electrode 41 used in the secondary battery 10 will be described. As shown in FIG. 4(C), the negative electrode 41 has a negative electrode current collector 53 and a negative electrode active material layer 54 having a negative electrode active material. The negative electrode active material layer 54 is preferably thicker than the negative electrode current collector 53. As shown in FIG. 4(D), the negative electrode 41 may have a first negative electrode active material layer 54a and a second negative electrode active material layer 54b on both sides of the negative electrode current collector 53. In this specification and the like, FIG. 4(C) may be referred to as a single-sided coated negative electrode 41, and FIG. 4(D) may be referred to as a double-sided coated negative electrode 41. In the secondary battery 10 that can be freely curved, the single-sided coated negative electrode 41 may be preferable to the double-sided coated negative electrode 41.

[0079] Again, it may be preferable to use a single-sided coated positive electrode 43 and a single-sided coated negative electrode 41 in the secondary battery 10. In order to further increase the capacity of the secondary battery 10, the positive electrode 43 and the negative electrode 41 may be laminated. When a laminated structure is formed using a single-sided coated positive electrode 43, the positive electrode current collectors 50 may be laminated so as to face each other as shown in FIG. 4(E). When a laminated structure is formed using a single-sided coated positive electrode 43, the negative electrode current collectors 53 may be laminated so as to face each other as shown in FIG. 4(F). By laminating the current collectors so as to face each other, the internal structure can be appropriately displaced, making it easier to bend the secondary battery 10. In this specification and the like, a configuration in which current collectors are laminated so as to face each other may be called current collector facing or back-to-back.

[0080] When the positive electrode 43 is laminated, the first lead electrode 21 is also laminated, and when these are fixed, it is considered that the fixing area is likely to wrinkle when the secondary battery 10 is bent. Since the positive electrode current collector protrudes at the position overlapping the fixing area, wrinkles may also occur in the positive electrode current collector. Similarly, when the negative electrode 41 is laminated, the second lead electrode 22 is also laminated, and when these are fixed, it is considered that the fixing area is likely to wrinkle when the secondary battery 10 is bent. Since the negative electrode current collector protrudes at the position overlapping the fixing area, wrinkles may also occur in the negative electrode current collector. The positive electrode 43 and the negative electrode 41 to which the above-mentioned current collector matching structure is applied are suitable for the secondary battery 10 to be bent because the amount of deviation of the above-mentioned fixing area is small.

[0081] <separator> The separator 42 used in the secondary battery 10 will be described. The separator 42 is made of a material that has a function of being impregnated with the electrolyte 45 and exhibits insulating properties. In this specification and the like, impregnation includes the electrolyte 45 being located in the voids of the separator 42. In order to facilitate impregnation with the electrolyte 45, the separator 42 may have a plurality of voids. The separator 42 is provided so that the positive electrode 43 and the negative electrode 41 are not short-circuited, and by shifting the positions of the plurality of voids present in the thickness direction of the separator 42 from each other, it is possible to prevent dendrites of the negative electrode 41 from penetrating the separator 42 and reaching the positive electrode 43.

[0082] The separator 42 may be processed into a thin film or an envelope. The separator 42 in Fig. 2(A) to Fig. 3(C) corresponds to an envelope-shaped separator. In the figures, the envelope-shaped separator 42 houses a positive electrode 43. The negative electrode 41 is disposed on the outside of the separator 42. When the area of ​​the positive electrode 43 is smaller than the area of ​​the negative electrode 41, it is preferable to house the positive electrode 43 in the separator 42.

[0083] Of course, the envelope-shaped separator 42 may house the negative electrode 41. In that case, the positive electrode 43 is disposed on the outside of the separator 42. When the area of ​​the negative electrode 41 is smaller than the area of ​​the positive electrode 43, it is preferable to house the negative electrode 41 in the separator 42.

[0084] When the secondary battery 10 is curved, a misalignment may occur between the stacked positive electrodes 43. Therefore, the area of ​​the separator 42 is preferably larger than the area of ​​the accommodated positive electrode 43. Similarly, the area of ​​the separator 42 is preferably larger than the area of ​​the accommodated negative electrode 41.

[0085] The separator 42 may be made of paper, nonwoven fabric, ceramics, glass fiber, or synthetic fiber. As the synthetic fiber, nylon (also called polyamide resin), polyimide (also called polyimide resin), vinylon, polyester, acrylic, polyolefin, or polyurethane may be used. Synthetic fibers using polyimide are preferred because they are easily impregnated with ionic liquid. As the polyamide, nylon or aramid (meta-aramid or para-aramid) may be used. The separator 42 may have a single layer structure or a laminate structure of the above-mentioned materials.

[0086] When a laminated structure is applied, for example, a structure in which an organic material used for synthetic fibers is coated with a polyamide material, a ceramic material, or a fluorine material may be applied. The polyamide material may be nylon or aramid (meta-aramid or para-aramid). The ceramic material may be aluminum oxide or silicon oxide. For example, the organic material used for synthetic fibers may be processed into a sheet, and aluminum oxide or silicon oxide may be processed into particles and sprinkled on the sheet-like material. For example, polyvinylidene fluoride (PVDF) or polytetrafluoroethylene may be used as the fluorine material. Viscous fluorine materials may be applied to the sheet-like material.

[0087] Coating the separator 42 with a ceramic material improves the oxidation resistance of the separator 42, suppressing the deterioration of the separator 42 during high-voltage charging and discharging, and improving the reliability of the secondary battery 10. Coating the separator 42 with a fluorine material also facilitates adhesion between the separator 42 and the positive electrode 43 and the negative electrode 41, improving the output characteristics of the secondary battery 10. Coating the separator 42 with a polyamide material, particularly aramid, improves the heat resistance of the separator 42, improving the safety of the secondary battery 10. For example, polypropylene may be processed into a sheet and both sides of the sheet may be coated with a mixed material of aluminum oxide and aramid. Polypropylene may also be processed into a sheet and the surface in contact with the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface in contact with the negative electrode may be coated with a fluorine material.

[0088] The thickness of the single-layer separator 42 is preferably 10 μm or more and 100 μm or less, and more preferably 12 μm or more and 50 μm or less. The thickness of the laminated separator 42 is preferably 10 μm or more and 80 μm or less, and more preferably 12 μm or more and 45 μm or less. When a laminated separator is used, the safety of the secondary battery 10 can be maintained even if the overall thickness of the separator is thin, so that the weights of the positive electrode 43 and the negative electrode 41 can be increased, and the discharge capacity per volume of the secondary battery 10 can be increased.

[0089] It is preferable that the protective material 40, the protective material 40a, and the protective material 40b shown in Fig. 3(A) to Fig. 3(C) are bent together with the secondary battery 10. Although it has been described that the protective material 40 has the same area as the negative electrode 41, it is desirable that such a protective material 40 be freely curved together with the secondary battery 10. For example, by using a sheet-like member impregnated with an electrolyte 45 for the protective material 40, the protective material 40a, and the protective material 40b (collectively referred to as the protective material 40), the protective material 40 becomes easier to bend together with the secondary battery 10. In this specification and the like, the state in which the protective material 40 is impregnated with the electrolyte 45 includes the electrolyte being located in the voids of the protective material, and it is sufficient that the electrolyte is located in the voids of the protective material at least when the secondary battery 10 is in a curved state.

[0090] The sheet-like member used for the protective material 40 can be the member or structure described in the description of the separator 42 above. Members applicable to the separator 42 are preferable because they have high impregnation with the electrolyte 45. Specifically, the protective material 40 can be arbitrarily selected from the members described in the description of the separator 42. Furthermore, the protective material 40 may be made of the same member as the separator 42, or a different member.

[0091] When the dimensions of the exterior body 23 are 160 mm×135 mm×3 mm, it is preferable to inject the electrolyte into the exterior body 23 so that the amount of electrolyte is 15 mL to 30 mL, preferably 18 mL to 22 mL. By injecting the electrolyte in this amount, the protective material 40 can be impregnated with the electrolyte, and friction when the secondary battery 10 is curved is suppressed. Furthermore, after the electrolyte 45 is injected into the exterior body 23, the reduced pressure state and the normal pressure state are repeated without sealing. This operation allows the electrolyte 45 to be sufficiently impregnated into the protective material 40. Thereafter, the exterior body 23 is placed in a reduced pressure state in order to bond the periphery thereof. The reduced pressure state is preferably a pressure of -150 kPa or more, preferably -120 kPa or more. Specifically, the atmosphere may be reduced so that a differential pressure gauge connected to a chamber used for injecting the electrolyte is -50 kPa to -120 kPa. By placing the battery in such an atmosphere, the battery characteristics in a vacuum environment are improved.

[0092] This embodiment can be used in combination with other embodiment modes.

[0093] (Embodiment 2) In this embodiment, an ionic liquid that is preferably used in the secondary battery 10 will be described.

[0094] <Ionic liquid> An ionic liquid according to one embodiment of the present invention will be described. Ionic liquids are sometimes referred to as room temperature molten salts, and have cations and anions. The basic skeleton of the cation is imidazolium-based, ammonium-based, pyrrolidinium-based, piperidinium-based, pyridinium-based, or phosphonium-based. Ionic liquids having an imidazolium-based basic skeleton of the cation have a lower viscosity than ammonium-based ionic liquids. When the viscosity is low, the conductivity of the carrier ions tends to increase. Furthermore, physical properties such as viscosity can be controlled by the alkyl group on the side chain of the cation. Ionic liquids are preferable electrolytes because they do not generate hydrofluoric acid, unlike organic solvents, even if moisture or water is mixed into the secondary battery 10. Such ionic liquids are less likely to deteriorate the exterior body 23, and are suitable as electrolytes.

[0095] <Anion> The anion of the ionic liquid according to one embodiment of the present invention will now be described. The anion may be a halide ion, tetrafluoroborate, hexafluorophosphate, bis(trifluoromethylsulfonyl)amide, bis(fluorosulfonyl)imide, or the like.

[0096] Specific anions that can be used include one or more of a monovalent amide anion, a monovalent methide anion, a fluorosulfonate anion, a perfluoroalkylsulfonate anion, a tetrafluoroborate anion, a perfluoroalkylborate anion, a hexafluorophosphate anion, a perfluoroalkylphosphate anion, and a tetrafluoroborate anion.

[0097] Monovalent amide anions are represented by the general formula (C n F 2n+1SO2)2N - (n is 0 to 3).

[0098] In the above general formula, when n is 0, it is a bis(fluorosulfonyl)imide anion, which is represented by structural formula (H11). The abbreviation for bis(fluorosulfonyl)imide anion is FSI or FSA.

[0099] [ka]

[0100] In the above general formula, when n is 1, it is a bis(trifluoromethanesulfonyl)imide anion, which is represented by structural formula (H12). The abbreviation for bis(trifluoromethanesulfonyl)imide anion is TFSI or TFSA.

[0101] [ka]

[0102] One of the monovalent cyclic amide anions is 4,4,5,5-tetrafluoro-1,3,2-dithiazolidinetetraoxide anion, which is represented by structural formula (H13).

[0103] [ka]

[0104] Monovalent methide anions are represented by the general formula (C n F 2n+1 SO2)3C - (n is 0 to 3).

[0105] One of the monovalent cyclic methide anions is 4,4,5,5-tetrafluoro-2-[(trifluoromethyl)sulfonyl]-1,3-dithiolane tetraoxide anion, which is represented by structural formula (H14).

[0106] [ka]

[0107] The fluoroalkylsulfonate anion is represented by the general formula (C m F 2m+1 SO3) - (m is 0 to 4).

[0108] In the above general formula, when m is 0, the anion is a fluorosulfonate anion, and when m is 1, 2, 3, or 4, the anion is a perfluoroalkylsulfonate anion.

[0109] Fluoroalkylborate anions have the general formula {BF n (C m H k F 2m+1-k ) 4-n} - (n is between 0 and 3, m is between 1 and 4, and k is between 0 and 2m).

[0110] The fluoroalkyl phosphate anion has the general formula {PF n (C m H k F 2m+1-k ) 6-n} - (n is between 0 and 5, m is between 1 and 4, and k is between 0 and 2m).

[0111] One or more of these anions can be used.

[0112] <General formula of cation> The cation of the ionic liquid according to one embodiment of the present invention will be described.

[0113] The cation of the ionic liquid according to one embodiment of the present invention has an imidazolium-based cation represented by general formula (G1). - represents any one of the above anions, for example, FSI anion or TFSI anion.

[0114] [ka]

[0115] In the above general formula (G1), R 1 represents an alkyl group having 1 to 10 carbon atoms, and R 2 ~R 4 each independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms; R 5 represents an alkyl group having 1 to 6 carbon atoms, or an ether group, thioether group, or siloxane having a main chain composed of two or more atoms selected from C, O, Si, N, S, and P atoms.

[0116] The ionic liquid according to one embodiment of the present invention has a pyridinium-based cation represented by general formula (G2). - represents any one of the above anions, for example, FSI anion or TFSI anion.

[0117] [ka]

[0118] In the above general formula (G2), R 6 R has a main chain composed of two or more selected from an alkyl group having 1 to 6 carbon atoms, or C, O, Si, N, S, and P atoms. 7 ~R 11 each independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. 8 or R 9 may represent a hydroxyl group.

[0119] The ionic liquid of one embodiment of the present invention may have a quaternary ammonium cation. For example, the ionic liquid has a quaternary ammonium cation represented by General Formula (G3). In General Formula (G3), A - represents any one of the above anions, for example, FSI anion or TFSI anion.

[0120] [ka]

[0121] In the above general formula (G3), R 28 ~R 31 each independently represents an alkyl group having 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethyl group, or a hydrogen atom.

[0122] The ionic liquid of one embodiment of the present invention has a cation represented by general formula (G4). - represents any one of the above anions, for example, FSI anion or TFSI anion.

[0123] [ka]

[0124] In the above general formula (G4), R 12 and R 17 R each independently represents an alkyl group having 1 to 3 carbon atoms. 13 ~R 16 each independently represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms.

[0125] The ionic liquid of one embodiment of the present invention has a cation represented by General Formula (G5). - represents any one of the above anions, for example, FSI anion or TFSI anion.

[0126] [ka]

[0127] In the above general formula (G5), R 18 and R 24R each independently represents an alkyl group having 1 to 3 carbon atoms. 19 ~R 23 each independently represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms.

[0128] The ionic liquid of one embodiment of the present invention has a cation represented by General Formula (G6). - represents an anion as defined above, for example the FSI anion or the TFSI anion.

[0129] [ka]

[0130] In the above general formula (G6), n and m are 1 or more and 3 or less, α is 0 or more and 6 or less, β is 0 or more and 6 or less, and X or Y represents, as a substituent, a linear or side chain alkyl group having 1 to 4 carbon atoms, a linear or side chain alkoxy group having 1 to 4 carbon atoms, or a linear or side chain alkoxyalkyl group having 1 to 4 carbon atoms.

[0131] The ionic liquid of one embodiment of the present invention has a tertiary sulfonium cation represented by general formula (G7). - represents an anion as defined above, for example the FSI anion or the TFSI anion.

[0132] [ka]

[0133] In the above general formula (G7), R 25 ~R 27 each independently represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. 25 ~R 27 each independently has a main chain composed of two or more atoms selected from C, O, Si, N, S, and P atoms.

[0134] The ionic liquid of one embodiment of the present invention has a quaternary phosphonium cation represented by the following general formula (G8). - represents an anion as defined above, for example the FSI anion or the TFSI anion.

[0135] [ka]

[0136] In the above general formula (G8), R 32 ~R 35 each independently represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. 32 ~R 35 each independently has a main chain composed of two or more atoms selected from C, O, Si, N, S, and P atoms.

[0137] <Cation> Specific examples of the cation of general formula (G1) include structural formulae (111) to (174). Structural formula (111) is a 1-ethyl-3-methylimidazolium cation, abbreviated as EMI. Since EMI is insoluble in water, it is believed that the use of EMI in the electrolyte can impart water barrier properties to the secondary battery 10. Structural formula (113) is a 1-butyl-3-methylimidazolium cation, abbreviated as BMI.

[0138] [ka]

[0139] [ka]

[0140] [ka]

[0141] [ka]

[0142] [ka]

[0143] [ka]

[0144] Specific examples of the cation of the above general formula (G2) include, for example, structural formulae (701) to (719).

[0145] [ka]

[0146] [ka]

[0147] Specific examples of the cation of the above general formula (G4) include, for example, structural formulae (501) to (520).

[0148] [ka]

[0149] Specific examples of the cation of the above general formula (G5) include, for example, structural formulae (601) to (630).

[0150] [ka]

[0151] [ka]

[0152] Specific examples of the cation of the above general formula (G6) include, for example, structural formulae (301) to (309) and structural formulae (401) to (419).

[0153] [ka]

[0154] [ka]

[0155] In addition, structural formulae (301) to (309) and structural formulae (401) to (419) show examples in which m is 1 in general formula (G6), but in structural formulae (301) to (309) and structural formulae (401) to (419), m may be changed to 2 or 3.

[0156] Specific examples of the cation of the above general formula (G7) include, for example, structural formulae (201) to (215).

[0157] [ka]

[0158] Since such ionic liquids are liquids consisting only of ions, they have strong electrostatic interactions and exhibit excellent thermal stability, and secondary batteries using these ionic liquids as electrolytes can exhibit excellent battery characteristics at high temperatures of 25°C or higher.

[0159] <Lithium salt> The lithium salt dissolved in the non-aqueous solvent of one embodiment of the present invention is preferably a lithium salt having a halogen. It is more preferably a fluorine-containing imide lithium salt. The fluorine-containing imide lithium salt may be Li(CF3SO2)2N (hereinafter, also referred to as "LiTFSI" or "LiTFSA"), Li(C2F5SO2)2N (hereinafter, also referred to as "LiBETI"), or Li(SO2F)2N (hereinafter, also referred to as "LiFSI" or "LiFSA"), or the like.

[0160] Also, other lithium salts having halogens such as LiPF6, LiBF4, and LiClO4 can be used.

[0161] Furthermore, LiBOB (lithium bis(oxalato)borate) may be used as another halogen-free lithium salt.

[0162] These lithium salts may be used alone or in combination.

[0163] (Embodiment 3) In this embodiment, a positive electrode active material that is preferably used for the secondary battery 10 will be described.

[0164] 5(A1) and 5(A2) are cross-sectional views of a positive electrode active material 100 according to one embodiment of the present invention. The positive electrode active material 100 in FIG. 5(A1) has a surface layer 100a and an inner portion 100b. The positive electrode active material in FIG. 5(A2) has a surface layer 100a and an inner portion 100b, and further has defects 100c such as cracks and grain boundaries 101, which will be described later. Enlarged views of the vicinity of AB in FIG. 5(A1) are shown in FIG. 5(B1) and FIG. 5(B2).

[0165] In the present specification, the surface layer 100a of the positive electrode active material 100 refers to, for example, a region within 50 nm from the surface toward the inside, more preferably within 35 nm from the surface toward the inside, even more preferably within 20 nm from the surface toward the inside, and most preferably within 10 nm from the surface toward the inside, perpendicular or approximately perpendicular from the surface. Note that approximately perpendicular is 80° or more and 100° or less. Surfaces caused by cracks and / or cracks may also be called the surface. The surface layer 100a is synonymous with the surface vicinity, surface vicinity region, or shell.

[0166] The region deeper than the surface layer 100a of the positive electrode active material is referred to as the inner portion 100b, which is synonymous with the inner region or the core.

[0167] Since the positive electrode active material 100 is a compound containing oxygen and a transition metal capable of inserting and extracting lithium, the interface between a region in which the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced with the insertion and extraction of lithium and oxygen is present and a region in which they are not present is defined as the surface of the positive electrode active material. Surfaces caused by slips, cracks, and / or cracks may also be considered to be the surface of the positive electrode active material.

[0168] The surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image or the like is the boundary between an area where an image derived from the crystal structure of the positive electrode active material is observed and an area where it is not observed, and is the outermost area where an atomic column derived from the atomic nucleus of a metal element having a larger atomic number than lithium among the metal elements constituting the positive electrode active material is confirmed. Alternatively, it is the intersection point of a tangent drawn to a brightness profile from the surface to the bulk in a STEM image and an axis in the depth direction. The surface in a STEM image or the like may be determined in conjunction with an analysis with higher spatial resolution.

[0169] Figure 5 (B1) shows the concentration distribution (sometimes referred to as a profile) of added element X, and Figure 5 (B2) shows the concentration distribution of added element Y. The concentration distribution of added elements can be measured by STEM-EDX. The spatial resolution of STEM-EDX is at least about 1 nm. Therefore, the maximum value of the added element profile may deviate by about 1 nm. For example, even if the maximum value of the added element profile of magnesium or the like is located outside the surface obtained above, it can be considered an error as long as the difference between the maximum value and the surface is less than 1 nm.

[0170] In addition, a peak in STEM-EDX line analysis refers to the detected intensity in each element profile or the maximum value of the characteristic X-rays for each element. Note that noise in STEM-EDX line analysis can be considered to be a measurement value with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0171] The effect of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated values ​​measured over six scans can be used as the profile for each element. The number of scans is not limited to six, and it is also possible to perform more than six scans and use the average as the profile for each element.

[0172] The STEM-EDX line analysis can be carried out, for example, as follows: First, a protective film is deposited on the surface of the positive electrode active material. For example, carbon can be deposited using a carbon coating unit of an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0173] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-section sample. For example, the slice processing can be performed using a FIB-SEM device (Hitachi High-Tech XVision200TBS). In this case, the pickup is performed using an MPS (micro-probing system), and the finishing processing conditions can be, for example, an acceleration voltage of 10 kV.

[0174] For STEM-EDX analysis, for example, a STEM device (Hitachi High-Tech HD-2700) can be used, and an EDX detector can be EDAX Octane T Ultra W (Dual EDS). During EDX analysis, the acceleration voltage of the STEM device is set to 200 kV, the emission current to between 6 μA and 10 μA, and a portion of the thinned sample with minimal irregularities is measured. The magnification is, for example, about 150,000 times. The conditions for EDX analysis can be a beam diameter of 0.2 nmφ, drift correction, line width of 42 nm, pitch of 0.2 nm, and frame number of 6 or more.

[0175] The crystal grain boundary 101 refers to, for example, a portion where particles of the positive electrode active material 100 are stuck together, a portion where the crystal orientation changes inside the positive electrode active material 100, i.e., a portion where the repetition of bright and dark lines in a STEM image or the like becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. The crystal defect refers to a defect that can be observed in a cross-sectional TEM (transmission electron microscope) or cross-sectional STEM image, i.e., a structure in which other atoms have entered between the lattices, a cavity, etc. The crystal grain boundary 101 can be said to be one of the planar defects. The vicinity of the crystal grain boundary 101 refers to a region within 10 nm of the crystal grain boundary 101.

[0176] <Contained elements> The positive electrode active material 100 includes lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material 100 may include lithium cobalt oxide (LiCoO2) to which an additive element has been added. However, the positive electrode active material 100 may have a crystal structure described below. Therefore, the composition of the lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0177] The positive electrode active material of a lithium ion secondary battery must have a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material 100 preferably mainly uses cobalt as the transition metal responsible for the oxidation and reduction reaction. In addition to cobalt, at least one or two selected from nickel and manganese may be used. If the positive electrode active material 100 contains 75 atomic % or more of cobalt, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metals, it is preferable because it has many advantages such as being relatively easy to synthesize, being easy to handle, and having excellent cycle characteristics.

[0178] In addition, when the transition metals in the positive electrode active material 100 are 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, the transition metals in the positive electrode active material 100 are more favorable than those in composite oxides in which nickel accounts for the majority of the transition metals, such as lithium nickel oxide (LiNiO2). x The stability is better when x in CoO2 is small. This is thought to be because the influence of distortion due to the Jahn-Teller effect is smaller in cobalt than in nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. In layered rock-salt type composite oxides in which low-spin nickel (III) in octahedral coordination accounts for the majority of the transition metal, such as lithium nickel oxide, the influence of the Jahn-Teller effect is large, and distortion is likely to occur in the octahedral layers of nickel and oxygen. This raises concerns about the collapse of the crystal structure during charge-discharge cycles. In addition, nickel ions are larger than cobalt ions and are close to the size of lithium ions. Therefore, in layered rock-salt type composite oxides in which nickel accounts for the majority of the transition metal, such as lithium nickel oxide, there is an issue that cation mixing of nickel and lithium is likely to occur.

[0179] The additive element contained in the positive electrode active material 100 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. The sum of the transition metals among the additive elements is preferably less than 25 atomic %, more preferably less than 10 atomic %, and even more preferably less than 5 atomic %.

[0180] The additive element is preferably dissolved in the positive electrode active material 100. Therefore, for example, when performing a line analysis by STEM-EDX, the depth at which the amount of the additive element detected increases is preferably located deeper than the depth at which the amount of the transition metal M detected increases, i.e., on the inner side of the positive electrode active material 100.

[0181] In this specification, the depth at which the amount of a certain element detected in a STEM-EDX line analysis increases refers to the depth at which measurement values ​​that can be determined to be not noise in terms of intensity, spatial resolution, etc., are successively obtained.

[0182] As described below, these additive elements further stabilize the crystal structure of the positive electrode active material 100. In other words, the presence of the additive elements in the surface layer portion 100a and / or the interior portion 100b can stabilize the crystal structure. In this specification and the like, the additive elements are synonymous with a mixture or a part of a raw material.

[0183] The additive element does not necessarily have to include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium.

[0184] For example, if the positive electrode active material 100 is substantially free of manganese, the above-mentioned advantages of being relatively easy to synthesize and handle, and having excellent cycle characteristics, are even greater. The concentration of manganese contained in the positive electrode active material 100 is preferably, for example, 600 ppm wt or less, more preferably 100 ppm wt or less.

[0185] 〔magnesium〕 For example, magnesium, which is one of the added elements X, is divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, and therefore more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 100a makes it easier to maintain the layered rock-salt crystal structure. Magnesium may also be present at an appropriate concentration at the lithium site in the interior 100b. This is presumably because magnesium present at the lithium site functions as a pillar supporting the CoO2 layers. The presence of magnesium also makes it easier to maintain the Li x When x in CoO2 is, for example, 0.24 or less, the desorption of oxygen around magnesium can be suppressed. In addition, the presence of magnesium is expected to increase the density of the positive electrode active material 100. In addition, if the magnesium concentration in the surface layer portion 100a is high, it is expected that the corrosion resistance against hydrofluoric acid generated by decomposition of the electrolyte solution will be improved.

[0186] At an appropriate concentration, magnesium does not adversely affect the insertion and desorption of lithium during charging and discharging, and the above benefits can be enjoyed. However, excessive magnesium may adversely affect the insertion and desorption of lithium. Furthermore, the effect of stabilizing the crystal structure may be reduced. This is thought to be because magnesium enters the cobalt site in addition to the lithium site. In addition, unnecessary magnesium compounds (oxides, fluorides, etc.) that do not substitute for either the lithium site or the cobalt site may segregate on the surface of the positive electrode active material, and may become resistance components of the secondary battery. In addition, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be because too much magnesium enters the lithium site, reducing the amount of lithium that contributes to charging and discharging.

[0187] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.002 to 0.06 times the number of cobalt atoms, more preferably 0.005 to 0.03 times, and even more preferably about 0.01 times. The amount of magnesium contained in the entire positive electrode active material 100 may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0188] 〔nickel〕 Nickel, which is one of the additive elements X, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, it has a lower redox potential than cobalt, which leads to an increase in discharge capacity, which is preferable.

[0189] In addition, when nickel is present at the lithium site, the shift in the layered structure consisting of octahedra of cobalt and oxygen can be suppressed. The change in volume caused by charging and discharging is also suppressed. The elastic modulus also increases, that is, the battery becomes harder. This is presumably because the nickel present at the lithium site also functions as a pillar supporting the CoO2 layers. Therefore, it is expected that the crystal structure will become more stable, especially at high temperatures, for example, at 45°C or higher when the battery is in a charged state.

[0190] In addition, the distance between cations and anions in nickel oxide (NiO) is closer to the average distance between cations and anions in LiCoO2 than are those in MgO and CoO, making it easier for the orientation to match that of LiCoO2.

[0191] In addition, the order of ionization tendency is smallest for magnesium, aluminum, cobalt, and nickel. Therefore, nickel is considered to be less likely to dissolve into the electrolyte during charging than the other elements mentioned above. Therefore, it is considered to have a high effect of stabilizing the crystal structure of the surface layer in the charged state. The presence of nickel in the interior 100b also has the effect of stabilizing the crystal structure inside.

[0192] Furthermore, nickel is Ni 2+ , Ni 3+ , Ni 4+ Of which, Ni 2+ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel crystal structure. Therefore, nickel is thought to have the effect of suppressing the phase change from the layered rock salt type to the spinel type crystal structure.

[0193] On the other hand, an excess of nickel is undesirable because it increases the influence of distortion due to the Jahn-Teller effect, and too much nickel can also adversely affect the insertion and extraction of lithium.

[0194] Therefore, it is preferable that the amount of nickel contained in the entire positive electrode active material 100 is appropriate. For example, the number of nickel atoms contained in the positive electrode active material 100 is preferably higher than 0% and not more than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferably higher than 0% and not more than 4%. Alternatively, it is preferably higher than 0% and not more than 2%. Alternatively, it is preferably 0.05% to 7.5%. Alternatively, it is preferably 0.05% to 2%. Alternatively, it is preferably 0.1% to 7.5%. Alternatively, it is preferably 0.1% to 4%. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0195] 〔aluminum〕 Aluminum, which is one of the additive elements Y, can be present at the cobalt site in the layered rock salt crystal structure. Since aluminum is a typical trivalent element and its valence does not change, lithium around the aluminum is unlikely to move even during charging and discharging. Therefore, aluminum and the lithium around it function as pillars and can suppress changes in the crystal structure. Aluminum also has the effect of suppressing the elution of surrounding cobalt and improving continuous charging resistance. In addition, since the Al-O bond is stronger than the Co-O bond, it can suppress the detachment of oxygen around the aluminum. These effects improve the thermal stability. Therefore, if aluminum is included as an additive element, the safety of the positive electrode active material 100 when used in a secondary battery can be improved. In addition, the positive electrode active material 100 can be made to have a crystal structure that is unlikely to collapse even when repeatedly charged and discharged.

[0196] On the other hand, an excess of aluminum may adversely affect the insertion and extraction of lithium.

[0197] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material 100 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2% and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire positive electrode active material 100 here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0198] [Fluorine] In addition, fluorine, which is one of the additive elements X, is a monovalent anion, and when part of the oxygen in the surface layer portion 100a is replaced by fluorine, the lithium desorption energy is reduced. This is because the redox potential of the cobalt ion accompanying lithium desorption differs depending on the presence or absence of fluorine. In other words, when there is no fluorine, the cobalt ion changes from trivalent to tetravalent as lithium desorption occurs. On the other hand, when there is fluorine, the cobalt ion changes from divalent to trivalent as lithium desorption occurs. The redox potential of the cobalt ion differs between the two. Therefore, when part of the oxygen in the surface layer portion 100a of the positive electrode active material 100 is replaced by fluorine, it can be said that the desorption and insertion of lithium ions near the fluorine is likely to occur smoothly. Therefore, when the positive electrode active material 100 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved. In addition, the presence of fluorine in the surface layer portion 100a having the surface that is in contact with the electrolyte can effectively improve corrosion resistance against hydrofluoric acid. As described in the following embodiment, when the melting point of a fluoride such as lithium fluoride is lower than that of other additive element sources, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element sources. When the fluorine compound contains LiF and MgF2, since LiF and MgF2 are around 742°C, it is preferable to set the heating temperature to 742°C or higher in the heating step after mixing the additive elements.

[0199] 〔titanium〕 In addition, titanium oxide, which is one of the additive elements X, is known to have superhydrophilicity. Therefore, by making the surface layer portion 100a of the positive electrode active material 100 have titanium oxide, the wettability to a highly polar solvent may be improved. When the positive electrode active material 100 is used as a secondary battery, the contact at the interface between the positive electrode active material 100 and the highly polar electrolyte may be improved, and an increase in internal resistance may be suppressed.

[0200] Also, if the positive electrode active material 100 has additive elements with different distributions, it is preferable that the crystal structure of a wider region can be stabilized. For example, if the positive electrode active material 100 has both magnesium and nickel, which are additive elements A, and aluminum, which is additive element B, it can stabilize the crystal structure of a wider region than if it has only one of additive elements A and additive element B. In this way, when the positive electrode active material 100 has additive elements A and B together, additive element B, such as aluminum, is not essential for the surface, since the surface can be sufficiently stabilized by additive element A, such as magnesium or nickel. Rather, it is preferable that aluminum is widely distributed in a deeper region. For example, it is preferable that aluminum is continuously detected in a region from the surface to a depth of 1 nm to 25 nm. It is preferable that aluminum is widely distributed in a region from 0 nm to 100 nm from the surface, preferably from 0.5 nm to 50 nm from the surface, because the crystal structure of a wider region can be stabilized.

[0201] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 100a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high, and this is preferable.

[0202] In addition, although it is preferable that some of the added elements, particularly magnesium, nickel and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, it is also preferable that magnesium, nickel and aluminum are present randomly and dilutely in the interior 100b. When magnesium and aluminum are present at an appropriate concentration in the lithium site of the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure as described above. When nickel is present at an appropriate concentration in the interior 100b, it is possible to suppress the deviation of the layered structure consisting of octahedra of cobalt and oxygen as described above. When magnesium and nickel are present together, a synergistic effect of suppressing the elution of magnesium can be expected as described above.

[0203] It is also preferable that the crystal structure changes continuously from the inside 100b toward the surface due to the concentration gradient of the added element as described above, or that the crystal orientation of the surface layer 100a and the inside 100b is roughly the same.

[0204] For example, it is preferable that the crystal structure changes continuously from the inside 100b of the layered rock salt type toward the surface and surface layer portion 100a having characteristics of the rock salt type or both the rock salt type and the layered rock salt type. It is also preferable that the orientation of the surface layer portion 100a having characteristics of the rock salt type or both the rock salt type and the layered rock salt type and the inside 100b of the layered rock salt type are approximately the same.

[0205] In this specification, the layered rock-salt type crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal such as cobalt, refers to a crystal structure having a rock-salt type ion arrangement in which cations and anions are alternately arranged, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as deficiencies of cations or anions may be present. Strictly speaking, the layered rock-salt type crystal structure may have a structure in which the lattice of the rock-salt type crystal is distorted.

[0206] The rock-salt crystal structure is a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cations or anions may be missing.

[0207] Moreover, the fact that it has both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron beam diffraction, TEM images, cross-sectional STEM images, etc.

[0208] Layered rock-salt crystals and the anions of rock-salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions of the O3' type and monoclinic O1(15) crystals described below also have a cubic close-packed structure. Therefore, when a layered rock-salt crystal and a rock-salt crystal come into contact, there are crystal faces where the cubic close-packed structure composed of anions is aligned.

[0209] Alternatively, it can be explained as follows. The anions on the {111} plane of the cubic crystal structure have a triangular lattice. The layered rock-salt type has a space group of R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally represented as a compound hexagonal lattice, and the (0001) plane of the layered rock-salt type has a hexagonal lattice. The triangular lattice of the cubic {111} plane has the same atomic arrangement as the hexagonal lattice of the (0001) plane of the layered rock-salt type. When the two lattices are compatible, it can be said that the orientation of the cubic close-packed structure is aligned.

[0210] However, the space group of the layered rock salt crystal and O3' type crystal is R-3m, which is different from the space group Fm-3m (the space group of general rock salt crystals) of the rock salt crystal, so the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt crystal and O3' type crystal and the rock salt crystal. In this specification, when the orientation of the cubic close-packed structure formed by anions in the layered rock salt crystal, O3' type and rock salt crystal is aligned, it may be said that the crystal orientation is approximately the same. In addition, the three-dimensional structural similarity in which the crystal orientation is approximately the same, or the same crystallographic orientation, is called topotaxis.

[0211] The approximate alignment of the crystal orientations in the two regions can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron diffraction patterns, etc. It can also be determined from the FFT patterns of TEM images and the FFT patterns of STEM images, etc. Furthermore, XRD (X-ray Diffraction), neutron diffraction, etc. can also be used as materials for determination.

[0212] <Crystal Structure> ≪Li x <<When x in LiCoO2 is small>> Due to the positive electrode active material 100 having the distribution of additive elements and / or crystal structure as described above, the crystal structure in the state where x in Li x CoO2 is small is stable. This point is different from the conventional positive electrode active material. Note that when x is small, it refers to 0.1 < x ≤ 0.24, which is considered to correspond to high-voltage charging of lithium cobaltate. In other words, the positive electrode active material 100 has the change in crystal structure during high-voltage charging and the crystal structure during discharging suppressed, and this point is also different from the conventional positive electrode active material.

[0213] Using FIG. 6, the change in crystal structure accompanying the change in x in Li x CoO2 will be explained while comparing the conventional positive electrode active material and the positive electrode active material 100. The conventional positive electrode active material is lithium cobaltate (LiCoO2) that does not particularly have additive elements.

[0214] In FIG. 6, R-3m O3 is attached to Li xThe crystal structure of lithium cobalt oxide with x=1 in CoO2 is shown. In this crystal structure, lithium occupies the octahedral site, and there are three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3 type crystal structure. Note that a CoO2 layer is an octahedral structure in which cobalt is coordinated with six oxygen atoms, and the edges are shared to form a continuous plane. This is sometimes called a layer made of octahedra of cobalt and oxygen.

[0215] In addition, conventional lithium cobalt oxide is known to have a crystal structure that belongs to the monoclinic space group P2 / m when the symmetry of lithium increases when x is about 0.5. This structure has one CoO2 layer in the unit cell. Therefore, it is sometimes called O1 type or monoclinic O1 type.

[0216] Also, when x=0, conventional positive electrode active materials have a trigonal space group P-3m1 crystal structure, and one CoO2 layer also exists in the unit cell. Therefore, this crystal structure is sometimes called O1 type, or trigonal O1 type. In addition, the trigonal structure may be converted to a composite hexagonal lattice and called hexagonal O1 type.

[0217] Furthermore, when x=0.12, conventional lithium cobalt oxide has a crystal structure of space group R-3m. This structure can be said to be a structure in which a trigonal O1-type CoO2 structure and an R-3m O3-type LiCoO2 structure are alternately stacked. Therefore, this crystal structure is sometimes called an H1-3-type crystal structure. Note that the actual insertion and desorption of lithium does not necessarily occur uniformly in the positive electrode active material, and the lithium concentration may become spotty, so experimentally, an H1-3-type crystal structure is observed from about x=0.25. In fact, the number of cobalt atoms per unit cell in the H1-3-type crystal structure is twice that of other structures. However, in FIG. 6 and other parts of this specification, in order to make it easier to compare with other crystal structures, the c-axis of the H1-3-type crystal structure is shown as 1 / 2 of the unit cell.

[0218] As an example of the H1-3 type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co(0,0,0.42150±0.00016), O1(0,0,0.27671±0.00045), and O2(0,0,0.11535±0.00045). O1 and O2 are oxygen atoms. Which unit cell should be used to express the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt the unit cell with the smallest GOF (goodness of fit) value.

[0219] Li x When the charge and discharge cycle is repeated such that x in CoO2 is 0.24 or less, conventional lithium cobalt oxide undergoes repeated crystal structure changes (i.e., non-equilibrium phase changes) between the H1-3 crystal structure and the R-3m O3 structure in the discharged state. In the H1-3 crystal structure, the CoO2 layer is significantly displaced from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0220] Furthermore, the difference in volume between these two crystal structures is large: per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3m O3 crystal structure exceeds 3.5%, typically 3.9% or more.

[0221] In addition, the H1-3 crystal structure, which has continuous CoO2 layers like the trigonal O1 type, is likely to be unstable.

[0222] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. The collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult to insert and remove lithium.

[0223] On the other hand, in the positive electrode active material 100 shown in FIG. xThe change in the crystal structure between the discharge state where x in CoO2 is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the deviation of the CoO2 layer between the state where x is 1 and the state where x is 0.24 or less can be reduced. Also, the change in volume compared per cobalt atom can be reduced. Therefore, the positive electrode active material 100 is less likely to lose its crystal structure even when repeated charging and discharging such that x is 0.24 or less can be achieved, and excellent cycle characteristics can be realized. In addition, the positive electrode active material 100 is Li x When x in CoO2 is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than the conventional positive electrode active material. x When the state where x in CoO2 is kept at 0.24 or less is maintained, short circuits are unlikely to occur. In such a case, the safety of the secondary battery is further improved, which is preferable.

[0224] Li x 6 shows the crystal structures of the interior 100b of the positive electrode active material 100 when x in CoO2 is 1, approximately 0.2, and approximately 0.15. The interior 100b occupies most of the volume of the positive electrode active material 100 and is the part that contributes greatly to charge and discharge, so it can be said that the displacement of the CoO2 layer and changes in volume are the most problematic parts.

[0225] The positive electrode active material 100 has a crystal structure different from that of conventional lithium cobalt oxide having an H1-3 type crystal structure when x is 0.24 or less, for example, about 0.2 or about 0.15.

[0226] The positive electrode active material 100 when x=0.2 has a crystal structure belonging to the trigonal space group R-3m. The symmetry of the CoO2 layer is the same as that of O3. Therefore, this crystal structure is called an O3' type crystal structure. This crystal structure is shown in FIG. 6 with R-3m O3'.

[0227] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constants of the unit cell are preferably 2.797≦a≦2.837 (Å), more preferably 2.807≦a≦2.827 (Å), and typically a=2.817 (Å). The lattice constants of the c-axis are preferably 13.681≦c≦13.881 (Å), more preferably 13.751≦c≦13.811 (Å), and typically c=13.781 (Å). Å is a unit cell constant of 10 -10 (m).

[0228] Furthermore, when x=0.15, the positive electrode active material 100 has a crystal structure belonging to the monoclinic space group P2 / m. In this case, one CoO2 layer exists in the unit cell. In addition, the lithium present in the positive electrode active material 100 at this time is about 15 atomic % in the discharged state. Therefore, this crystal structure is called a monoclinic O1(15) type crystal structure. This crystal structure is shown in Figure 6 with the P2 / m monoclinic O1(15) attached.

[0229] The monoclinic O1(15) crystal structure has the coordinates of cobalt and oxygen in the unit cell as follows: Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X O1 ,0,Z O1 ), 0.23≦X O1 ≦0.24, 0.61≦Z O1 ≦0.65, O2(X O2 ,0.5,Z O2 ), 0.75≦X O2 ≦0.78, 0.68≦Z O2 The lattice constant of the unit cell is in the range of ≦0.71. a = 4.880 ± 0.05 Å, b = 2.817 ± 0.05 Å, c = 4.839 ± 0.05 Å, α=90°, β=109.6±0.1°, γ=90°.

[0230] It is possible to show the lattice constants of this crystal structure in the space group R-3m if a certain degree of error is allowed. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z O ), 0.21≦Z O ≦0.23. The lattice constant of the unit cell is a = 2.817 ± 0.02 Å, c=13.68±0.1 Å.

[0231] In both the O3' and monoclinic O1(15) crystal structures, ions of cobalt, nickel, magnesium, etc. occupy 6-coordinate oxygen sites, although lighter elements such as lithium and magnesium may occupy 4-coordinate oxygen sites.

[0232] As shown by the dotted line in FIG. 6, there is almost no deviation in the CoO2 layer between R-3m O3 in the discharged state and O3' and the monoclinic O1(15) type crystal structure.

[0233] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0234] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the monoclinic O1(15) type crystal structure is 3.3% or less, more specifically 3.0% or less, typically 2.5%.

[0235] In this way, in the positive electrode active material 100, Li xWhen x in CoO2 is small, that is, when a large amount of lithium is released, the change in the crystal structure is suppressed more than in conventional positive electrode active materials. In addition, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the crystal structure of the positive electrode active material 100 is not easily broken even when charging and discharging are repeated such that x is 0.24 or less. Therefore, the decrease in discharge capacity during charge and discharge cycles is suppressed in the positive electrode active material 100. In addition, since more lithium can be stably used than in conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be manufactured.

[0236] The positive electrode active material 100 is Li x It has been confirmed that when x in CoO2 is 0.15 or more and 0.24 or less, it may have an O3' type crystal structure, and it is presumed that even when x is more than 0.24 and 0.27 or less, it has an O3' type crystal structure. x It has been confirmed that when x in CoO2 is greater than 0.1 and less than 0.2, typically between 0.15 and 0.17, it may have a monoclinic O1(15) type crystal structure. However, the crystal structure is Li x Since it is affected by not only x in CoO2 but also the number of charge / discharge cycles, charge / discharge current, temperature, etc., it is not necessarily limited to the above range of x.

[0237] Therefore, the positive electrode active material 100 is Li x When x in CoO2 is more than 0.1 and not more than 0.24, it may have only O3' type, may have only monoclinic O1(15) type, or may have both crystal structures. All of the particles in the inside 100b of the positive electrode active material 100 do not have to have O3' type and / or monoclinic O1(15) type crystal structures. It may contain other crystal structures, or may be partially amorphous.

[0238] Also Li x To make the x in CoO2 small, it is generally necessary to charge at a high charging voltage. xThe state where x in CoO2 is small can be said to be a state where the battery is charged at a high charging voltage. For example, when the battery is charged by CC / CV at a voltage of 4.6 V or more based on the potential of lithium metal in an environment of 25°C, the H1-3 type crystal structure appears in the conventional positive electrode active material. Therefore, a charging voltage of 4.6 V or more based on the potential of lithium metal can be said to be a high charging voltage. In this specification and the like, unless otherwise specified, the charging voltage is expressed based on the potential of lithium metal.

[0239] Therefore, the positive electrode active material 100 can be said to be preferable because it can maintain a crystal structure having the symmetry of R-3m O3 even when charged at a high charging voltage, for example, a voltage of 4.6 V or more at 25° C. In other words, it can be said to be preferable because it can adopt an O3'-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or more and 4.7 V or less at 25° C. In other words, it can be said to be preferable because it can adopt a monoclinic O1(15)-type crystal structure when charged at an even higher charging voltage, for example, a voltage of more than 4.7 V and 4.8 V or less at 25° C.

[0240] Even with the positive electrode active material 100, when the charging voltage is further increased, the H1-3 type crystal may finally be observed. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, etc., so when the charging voltage is lower, for example, even when the charging voltage is 4.5 V or more and less than 4.6 V at 25° C., the positive electrode active material 100 may be able to have the O3' type crystal structure. Similarly, when the counter electrode is lithium metal and charging at a voltage of 4.65 V or more and 4.7 V or less at 25° C., the positive electrode active material 100 may be able to have the monoclinic O1(15) type crystal structure.

[0241] In addition, when graphite is used as the negative electrode active material in the secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V based on the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the battery has the same crystal structure at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0242] In addition, in the O3' and monoclinic O1(15) of FIG. 6, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist biasedly at some lithium sites, for example, in the monoclinic O1(Li 0.5 The lithium distribution can be analyzed, for example, by neutron diffraction.

[0243] It can also be said that the O3' and monoclinic O1(15) types have random lithium between the layers, but are similar to the CdCl2 type crystal structure. 0.06 Although the crystal structure is close to that when charged to NiO2, it is known that pure lithium cobalt oxide or layered rock salt type positive electrode active materials containing a large amount of cobalt do not usually adopt a CdCl2 type crystal structure.

[0244] <Analysis method> A certain positive electrode active material is Li x When x in CoO2 is small, whether the positive electrode active material 100 has an O3' type and / or monoclinic O1(15) type crystal structure is determined by Li x This can be determined by analyzing a positive electrode having a positive electrode active material with a small x in CoO2 using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0245] In particular, XRD is preferred in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, can compare the degree of crystallinity and the orientation of the crystals, can analyze the periodic distortion of the lattice and the crystallite size, can obtain sufficient accuracy even when the positive electrode obtained by disassembling the secondary battery is measured as it is, etc. Among XRD, powder XRD can obtain diffraction peaks that reflect the crystal structure of the inside 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100.

[0246] When analyzing the crystallite size by powder XRD, it is preferable to perform the measurement while excluding the influence of orientation due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then perform the measurement.

[0247] As described above, the positive electrode active material 100 is Li x A characteristic of this material is that there is little change in the crystal structure when x in CoO2 is 1 and when it is 0.24 or less. Materials that undergo large changes in crystal structure when charged at high voltage and have a crystal structure that accounts for 50% or more are not desirable because they cannot withstand high-voltage charging and discharging.

[0248] It should also be noted that the addition of an additive element alone may not result in an O3' or monoclinic O1(15) crystal structure. For example, even if lithium cobalt oxide with magnesium and fluorine or lithium cobalt oxide with magnesium and aluminum has something in common, depending on the concentration and distribution of the additive element, Li x When x in CoO2 is 0.24 or less, the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more, and when the H1-3 type crystal structure accounts for 50% or more.

[0249] Furthermore, even in the case of the positive electrode active material 100, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9 V, the H1-3 type or trigonal O1 type crystal structure may be generated. Therefore, in order to determine whether or not a material is the positive electrode active material 100, analysis of the crystal structure, such as XRD, and information such as the charging capacity or charging voltage are required.

[0250] However, when the positive electrode active material has a small x, its crystal structure may change when it is exposed to air. For example, the crystal structure may change from O3' type or monoclinic O1(15) type to H1-3 type. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0251] Furthermore, whether or not the distribution of the additive elements in a certain positive electrode active material is in the state described above can be determined by analyzing it using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0252] The crystal structure of surface layer 100a, crystal grain boundaries 101, etc. can be analyzed by electron beam diffraction of a cross section of positive electrode active material 100, etc.

[0253] ≪Charging method≫ Charging to determine whether a certain composite oxide is the positive electrode active material 100 can be performed, for example, by fabricating a coin-type secondary battery (CR2032 type, diameter 20 mm, height 3.2 mm) as a half cell serving as the counter electrode lithium and charging it.

[0254] More specifically, for the positive electrode, a slurry in which a positive electrode active material, a conductive material, and a binder are mixed is applied to a positive electrode current collector made of aluminum foil and used.

[0255] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery is different from the potential of the positive electrode. Unless otherwise specified, the voltage and potential in this specification are the potential of the positive electrode.

[0256] The lithium salt in the electrolyte is 1 mol / L lithium hexafluorophosphate (LiPF6), and the mixed solvent in the electrolyte is ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7. The electrolyte can be made by adding 2 wt% vinylene carbonate (VC) as an additive to the mixed solvent to which the lithium salt has been added.

[0257] The separator can be a 25 μm thick porous polypropylene film.

[0258] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0259] The coin-type secondary battery prepared under the above conditions is charged at an arbitrary voltage (for example, 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V or 4.8V). The charging method is not particularly limited as long as the charging can be performed at an arbitrary voltage for a sufficient time. For example, when charging by CCCV, the current in CC charging can be 20mA / g or more and 100mA / g or less per weight of the positive electrode active material. CV charging can be terminated at 2mA / g or more and 10mA / g or less per weight of the positive electrode active material. In order to observe the phase change of the positive electrode active material, it is desirable to charge at such a small current value. The temperature is 25°C or 45°C. After charging in this way, the coin-type secondary battery is disassembled in a glove box in an argon atmosphere and the positive electrode is taken out, and a positive electrode active material with an arbitrary charging capacity can be obtained. When various analyses are performed after this, it is preferable to seal in an argon atmosphere in order to suppress reactions with external components. For example, XRD can be performed by sealing in an airtight container in an argon atmosphere. After the charging is completed, the positive electrode is preferably taken out and analyzed promptly, preferably within 1 hour, and more preferably within 30 minutes.

[0260] In addition, when analyzing the crystal structure in the charged state after multiple charging and discharging, the multiple charging and discharging conditions may be different from the above charging conditions. For example, charging can be performed by constant current charging up to an arbitrary voltage (e.g., 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V) with a current value of 20mA / g to 100mA / g per weight of the positive electrode active material, followed by constant voltage charging until the current value becomes 2mA / g to 10mA / g per weight of the positive electrode active material, and discharging can be performed by constant current discharging at 2.5V and 20mA / g to 100mA / g per weight of the positive electrode active material.

[0261] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at, for example, 2.5 V, with a current value of 20 mA / g or more and 100 mA / g or less per weight of the positive electrode active material.

[0262] <XRD> The XRD measurement apparatus and conditions are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD equipment: Bruker AXS, D8 ADVANCE X-ray source: CuKα1 ray Output: 40kV, 40mA Divergence angle: Div.Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0263] When the measurement sample is a powder, this is sometimes called powder X-ray diffraction, and the sample can be set up by placing it in a glass sample holder, or by sprinkling the sample on a greased silicon anti-reflective plate, etc. When the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0264] The ideal powder XRD patterns calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model are shown in Figures 7, 8, 9(A), and 9(B). xThe ideal XRD patterns calculated from the crystal structure of LiCoO2O3 with x=1 in CoO2 and trigonal O1 with x=0 are also shown. Figures 9(A) and 9(B) show the XRD patterns of the O3' type crystal structure, monoclinic O1(15) type crystal structure, and H1-3 type crystal structure, with Figure 9(A) showing an enlarged view of the region where 2θ is in the range of 18° to 21°, and Figure 9(B) showing an enlarged view of the region where 2θ is in the range of 42° to 46°. The patterns of LiCoO2(O3) and CoO2(O1) were created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from crystal structure information obtained from ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 5). The range of 2θ was 15° to 75°, Step size=0.01, and wavelength λ1=1.540562×10 -10 m and λ2 were not set, and monochromator was set to single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. The patterns of the O3' type and monoclinic O1(15) type crystal structures were estimated from the XRD pattern of the positive electrode active material 100, and fitting was performed using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker), and XRD patterns were created in the same manner as the others.

[0265] As shown in Figures 7, 9(A) and 9(B), in the O3' type crystal structure, diffraction peaks appear at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°).

[0266] In addition, in the monoclinic O1(15) crystal structure, diffraction peaks appear at 2θ = 19.47 ± 0.10° (19.37° to 19.57°) and 2θ = 45.62 ± 0.05° (45.57° to 45.67°).

[0267] However, as shown in Figures 8, 9(A) and 9(B), no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. x It can be said that it is a characteristic of the positive electrode active material 100 that peaks appear at 19.13° or more and less than 19.37° and / or 19.37° or more and 19.57° or less, and at 45.37° or more and less than 45.57° and / or 45.57° or more and 45.67° or less when x in CoO2 is small.

[0268] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ between 42° and 46°, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0269] The positive electrode active material 100 is Li x When x in CoO2 is small, it has O3'-type and / or monoclinic O1(15)-type crystal structure, but not all of the particles may have O3'-type and / or monoclinic O1(15)-type crystal structure. It may contain other crystal structures, or may be partially amorphous. However, when Rietveld analysis is performed on the XRD pattern, it is preferable that the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0270] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type and / or monoclinic O1(15) type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0271] Similarly, when Rietveld analysis is performed, it is preferable that the H1-3 type and O1 type crystal structures account for 50% or less.

[0272] The sharpness of the diffraction peaks in the XRD pattern indicates the high crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, that is, the half-width, for example, the full width at half maximum is narrow. The half-width varies depending on the XRD measurement conditions and the value of 2θ, even for peaks generated from the same crystal phase. In the case of the above-mentioned measurement conditions, the full width at half maximum is preferably, for example, 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less, in peaks observed at 2θ=43° or more and 46° or less. It is not necessary for all peaks to meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystal phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after charging sufficiently.

[0273] In addition, the crystallite size of the O3' type and monoclinic O1(15) crystal structures of the positive electrode active material 100 is reduced to only about 1 / 20 of that of LiCoO2(O3) in the discharged state. Therefore, even if the XRD measurement conditions are the same as those of the positive electrode before and after charging and discharging, the Li x When x in CoO2 is small, clear peaks of O3' type and / or monoclinic O1(15) crystal structure can be confirmed. On the other hand, in conventional LiCoO2, even if some parts have a structure similar to O3' type and / or monoclinic O1(15) crystal structure, the crystallite size becomes small and the peak becomes broad and small. The crystallite size can be calculated from the half-width of the XRD peak.

[0274] In addition, the surface of the positive electrode active material 100 is preferably smooth and has few irregularities, but this is not necessarily the case for the entire surface of the positive electrode active material 100. A composite oxide having a layered rock salt type crystal structure of R-3m is prone to slippage in a plane parallel to the (001) plane, for example, a plane in which lithium is arranged. For example, as shown in FIG. 10(A), when a (001) plane is present, slippage may occur parallel to the (001) plane as shown by the arrow in FIG. 10(B) by a process such as pressing, resulting in deformation.

[0275] In this case, the additional element may not be present on the surface newly generated as a result of the slip and its surface layer 100a. EF in FIG. 10(B) is an example of the surface newly generated as a result of the slip and its surface layer 100a. Enlarged views of the vicinity of EF are shown in FIG. 10(C1) and FIG. 10(C2). Unlike FIG. 5(B1) and FIG. 10(B2), additional element A and additional element B are not distributed in FIG. 10(C1) and FIG. 10(C2).

[0276] However, since slippage tends to occur parallel to the (001) plane, the newly formed surface and its surface layer 100a tend to have a (001) orientation. In this case, the diffusion path of lithium ions is not exposed and it is relatively stable, so there is almost no problem even if no additive element is present.

[0277] As mentioned above, in a composite oxide with a layered rock-salt structure of R-3m and a composition of LiCoO2, the cobalt atoms are arranged parallel to the (001) plane. In addition, in a HAADF-STEM image, the brightness of cobalt, which has the highest atomic number in LiCoO2, is the highest. Therefore, in a HAADF-STEM image, the arrangement of atoms with high brightness can be considered to be the arrangement of cobalt atoms. The repetition of this arrangement with high brightness is synonymous with crystal fringes or lattice fringes.

[0278] <Grain Boundaries> It is more preferable that the additive element contained in the positive electrode active material 100 is distributed as described above, and at least a part of the additive element is unevenly distributed in the crystal grain boundaries 101 and their vicinity.

[0279] For example, it is preferable that the magnesium concentration at and near the grain boundaries 101 of the positive electrode active material 100 is higher than that in other regions of the interior 100b. It is also preferable that the fluorine concentration at and near the grain boundaries 101 is higher than that in other regions of the interior 100b. It is also preferable that the nickel concentration at and near the grain boundaries 101 is higher than that in other regions of the interior 100b. It is also preferable that the aluminum concentration at and near the grain boundaries 101 is higher than that in other regions of the interior 100b.

[0280] The grain boundary 101 is one of the planar defects. Therefore, like the grain surface, it is easily unstable and the crystal structure is easily changed. Therefore, if the concentration of the added element at and near the grain boundary 101 is high, the change in the crystal structure can be more effectively suppressed.

[0281] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries 101, even if cracks occur along the grain boundaries 101 of the positive electrode active material 100, the magnesium concentration and fluorine concentration are high near the surface created by the cracks. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after cracks occur.

[0282] <Particle size> If the particle size of the positive electrode active material 100 is too large, there are problems such as difficulty in diffusing lithium, and the surface of the active material layer becomes too rough when applied to a current collector. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector, and excessive reaction with the electrolyte may occur. Therefore, the median diameter (D50) of the positive electrode active material is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Or preferably 1 μm or more and 40 μm or less. Or preferably 1 μm or more and 30 μm or less. Or preferably 2 μm or more and 100 μm or less. Or preferably 2 μm or more and 30 μm or less. Or preferably 5 μm or more and 100 μm or less. Or preferably 5 μm or more and 40 μm or less. It is preferable to mix the median diameter (D50) of the positive electrode active material that satisfies the large particle size and the small particle size. The large particle size and the small particle size are determined relatively, and typically the large particle size is 5 to 10 times the small particle size, preferably 7 to 9 times the small particle size. A positive electrode active material layer using both the large particle size and the small particle size has a higher electrode density than a positive electrode active material layer using only the large particle size or only the small particle size. When the electrode density is high, the capacity per volume can be increased.

[0283] As described above, it is preferable that the influence of the Jahn-Teller effect is small in the positive electrode active material 100. As long as the influence of the Jahn-Teller effect is small, the positive electrode active material 100 may contain a transition metal such as nickel or manganese as an additive element in addition to cobalt.

[0284] In the positive electrode active material, the range of nickel and manganese ratios and lattice constants in which the influence of the Jahn-Teller effect is assumed to be small is considered using XRD analysis.

[0285] This embodiment can be used in combination with other embodiment modes.

[0286] (Embodiment 4) In this embodiment mode, a manufacturing method of the positive electrode active material shown in the above embodiment mode will be described with reference to FIGS.

[0287] <Example 1 of a method for producing a positive electrode active material> An example of a method for manufacturing a positive electrode active material that can be used as one embodiment of the present invention (example 1 of a method for manufacturing a positive electrode active material) will be described with reference to FIGS.

[0288] First, in step S10, lithium cobalt oxide is prepared as a starting material. The lithium cobalt oxide as a starting material can be divided into two or more types depending on the range of the median diameter (D50). For example, lithium cobalt oxide having a median diameter (D50) of more than 10 μm, preferably 12 μm or more, and lithium cobalt oxide having a median diameter (D50) of 10 μm or less, preferably 8 μm or less, may be called lithium cobalt oxide A and lithium cobalt oxide B, respectively. Lithium cobalt oxide A and lithium cobalt oxide B may be commercially available lithium cobalt oxide. A representative example of lithium cobalt oxide A is lithium cobalt oxide (product name "Cellseed C-10N") manufactured by Nippon Chemical Industry Co., Ltd. A representative example of lithium cobalt oxide B is lithium cobalt oxide (product name "Cellseed C-5H") manufactured by Nippon Chemical Industry Co., Ltd. Lithium cobalt oxide B may exhibit good low-temperature characteristics.

[0289] Alternatively, lithium cobalt oxide manufactured through steps S11 to S14 shown in FIG. 11B may be used.

[0290] <Step S11> Step S11 shown in FIG. 11(B) is a step of preparing a lithium source (Li source) and a cobalt source (Co source) as starting materials, ie, lithium and transition metal materials, respectively.

[0291] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, lithium fluoride, etc. It is preferable that the lithium source has high purity, and for example, it is preferable to use a material with a purity of 99.99% or more.

[0292] As the cobalt source, it is preferable to use a compound containing cobalt, for example, tricobalt tetroxide, cobalt hydroxide, etc. The cobalt source is preferably high in purity, for example, a material with a purity of 3N (99.9%) or more, preferably 4N (99.99%) or more, more preferably 4N5 (99.995%) or more, and even more preferably 5N (99.999%) or more is used. By using a material with high purity, it is possible to control impurities in the positive electrode active material. As a result, the capacity of the secondary battery is increased, and the reliability of the secondary battery is improved.

[0293] <Step S12> Next, in step S12 shown in FIG. 11(B), the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed in a dry or wet manner. Wet pulverization and mixing can be performed to obtain lithium cobalt oxide B as a starting material because it can be crushed into smaller pieces. In addition, when performing the wet method, it is recommended to prepare a solvent. As the solvent, ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. can be used. It is preferable to use an aprotic solvent as a solvent that does not easily react with lithium. For example, it is preferable to mix the lithium source and the cobalt source in dehydrated acetone with a purity of 99.5% or more, in which the water content is reduced to 10 ppm or less, and then perform the pulverization and mixing.

[0294] <Step S13> Next, in step S13 shown in FIG. 11B, the mixed material is heated. The heating temperature is preferably 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably about 950°C or lower and 1000°C or lower. If the temperature is too low, the decomposition and melting of the lithium source and the cobalt source may be insufficient. On the other hand, if the temperature is too high, lithium may evaporate from the lithium source and / or cobalt may be excessively reduced, causing defects. For example, it is known that cobalt changes from trivalent to divalent, inducing oxygen defects in lithium cobalt oxide.

[0295] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, the productivity will decrease. Therefore, the heating time should be 1 hour or more and 100 hours or less, preferably 2 hours or more and 20 hours or less, and more preferably 2 hours or more and 10 hours or less.

[0296] The rate of temperature rise depends on the heating temperature reached, but it is recommended to set it to 80°C / h or more and 250°C / h or less. For example, if heating at 1000°C for 10 hours, the temperature rise should be set to 200°C / h.

[0297] The heating atmosphere is preferably an atmosphere with little water, such as dry air, for example, with a dew point of -50° C. or less, more preferably with a dew point of -80° C. or less. In this embodiment, the heating is performed in an atmosphere with a dew point of -93° C. In order to suppress impurities that may be mixed into the material, the impurity concentrations of CH4, CO, CO2, H2, and the like in the heating atmosphere should each be 5 ppb (parts per billion) or less.

[0298] The heating atmosphere is preferably an atmosphere containing oxygen. For example, there is a method of continuously introducing dry air into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and having oxygen flow through the reaction chamber is called flow.

[0299] When the heating atmosphere is an atmosphere containing oxygen, a method that does not allow the oxygen to flow may be used. For example, the reaction chamber may be depressurized and then filled with oxygen to prevent the oxygen from entering or leaving the reaction chamber, which is called purging. For example, the reaction chamber may be depressurized to -970 hPa and then filled with oxygen to 50 hPa.

[0300] After heating, the material may be naturally cooled, but it is preferable that the time required for the material to cool from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not always necessary, and it is sufficient to cool the material to a temperature that is acceptable for the next step.

[0301] The heating in this step may be performed using a rotary kiln or a roller hearth kiln. Heating using a rotary kiln may be performed while stirring in either a continuous or batch type.

[0302] The container used for heating is preferably a crucible made of zirconium oxide or aluminum oxide, or a sheath made of zirconium oxide or aluminum oxide. The crucible made of aluminum oxide is a material that is almost free of impurities. It is preferable to place a lid on the crucible or sheath before heating, since this can prevent the material from sublimating.

[0303] In addition, in heating steps other than step S13 (to be described later), the same heating conditions as those in step S13 can be applied.

[0304] After the heating is completed, the mixture may be crushed and sieved as necessary. The crushing step or sieving step can provide lithium cobalt oxide with an adjusted particle size distribution.

[0305] <Step S14> By the above steps, lithium cobalt oxide (LiCoO2) shown in step S14 in FIG. 11(B) can be synthesized. Lithium cobalt oxide (LiCoO2) shown in step S14 is an oxide containing multiple metal elements in its structure, so it can be called a composite oxide. In this specification, the term "composite oxide" includes oxides containing multiple metal elements in their structure.

[0306] Although the example of producing the composite oxide by the solid phase method as in steps S11 to S14 has been shown, the composite oxide may be produced by a coprecipitation method or a hydrothermal method.

[0307] Through steps S11 to S14, lithium cobalt oxide having a median diameter (D50) of 10 μm or less can be obtained as a starting material. A positive electrode active material using lithium cobalt oxide having the above median diameter (D50) as a starting material is preferable because it can exhibit excellent low-temperature characteristics.

[0308] <Step S15> Next, in step S15 shown in Fig. 11(A), the lithium cobalt oxide starting material is heated. The heating in step S15 is sometimes referred to as initial heating in this specification and the like because it is the first heating of the lithium cobalt oxide. Alternatively, since it is heating before step S31 described below, it is sometimes referred to as preheating or pretreatment.

[0309] The initial heating causes lithium compounds and the like that remain unintentionally on the surface of the lithium cobalt oxide to be desorbed. Although impurities may be mixed into the lithium source and / or cobalt source prepared in step S11, etc., the initial heating makes it possible to reduce the impurities from the lithium cobalt oxide starting material. In addition, the effect of increasing the internal crystallinity can be expected. The effect of increasing the internal crystallinity is, for example, the effect of mitigating distortion, displacement, etc. resulting from shrinkage differences, etc., that the lithium cobalt oxide prepared in step S14 has.

[0310] The initial heating has the effect of smoothing the surface of lithium cobalt oxide. In this specification, the term "smooth" refers to a surface with few irregularities, a rounded surface overall, and rounded corners. Alternatively, the term "smooth" also refers to a state in which there is little foreign matter attached to the surface. Foreign matter is considered to be a cause of irregularities, so it is preferable not to allow it to adhere to the surface. In addition, the initial heating has the effect of mitigating cracks and crystal defects that lithium cobalt oxide has.

[0311] The heating in step S13 may cause a temperature difference between the surface and the inside of the lithium cobalt oxide. The temperature difference may induce a shrinkage difference. It is also considered that the temperature difference causes the shrinkage difference because the fluidity of the surface and the inside is different. The energy related to the shrinkage difference gives the lithium cobalt oxide a difference in internal stress. The internal stress difference is also called strain, and the energy is sometimes called strain energy. The internal stress is removed by the initial heating in step S15, or in other words, the strain energy is considered to be homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain of the lithium cobalt oxide is relaxed. As a result, the surface of the lithium cobalt oxide becomes smooth. Or, it can be said that the surface is improved. In other words, by going through step S15, the shrinkage difference caused in the lithium cobalt oxide is relaxed, and the surface of the composite oxide can be made smooth.

[0312] Furthermore, the shrinkage difference may cause microscopic deviations in the lithium cobalt oxide, such as deviations in crystals. In order to reduce these deviations, it is preferable to carry out step S15. By carrying out step S15, it is possible to equalize the deviations in the composite oxide (alleviate deviations in crystals, etc., that have occurred in the composite oxide, or align the crystal grains). As a result, the surface of the composite oxide becomes smooth.

[0313] In this initial heating, it is not necessary to separately prepare a material that functions as a lithium compound source, an additive element source, or a flux.

[0314] If the heating time in this step is too short, sufficient effects are not obtained, but if it is too long, productivity decreases. The appropriate heating time range can be selected, for example, from the heating conditions described in step S13. The heating temperature in step S15 is preferably lower than the temperature in step S13 in order to maintain the crystal structure of the complex oxide. The heating time in step S15 is preferably shorter than the time in step S13 in order to maintain the crystal structure of the complex oxide. For example, heating is preferably performed at a temperature of 700°C to 1000°C (more preferably, 800°C to 900°C) for 1 hour to 20 hours (more preferably, 1 hour to 5 hours).

[0315] As described above, in step S10, lithium cobalt oxide having a median diameter (D50) of 10 μm or less that is synthesized in advance may be used. In this case, steps S11 to S13 can be omitted. It is useful to carry out step S15 on lithium cobalt oxide that is synthesized in advance.

[0316] Note that step S15 is not an essential configuration in one aspect of the present invention, and therefore an aspect in which step S15 is omitted is also included in one aspect of the present invention.

[0317] <Step S20> Next, details of step S20 of preparing the additive element A as the A source will be described with reference to FIG. 11(C) and FIG. 11(D).

[0318] <Step S21a> Step S20a shown in FIG. 11(C) is an example of step S20, and includes steps S21a to S23. Step S21a is a step of preparing an additive element A. Specific examples of the additive element A include one or more elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. Alternatively, one or more elements selected from bromine and beryllium may be used. In step S21a, a lithium source may be prepared separately in addition to the additive element A.

[0319] When magnesium is selected as the additive element A, the source of the additive element A can be called a magnesium source. As the magnesium source, magnesium fluoride (MgF2), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCO3), etc. can be used. A plurality of magnesium sources may be used.

[0320] When fluorine is selected as the additive element A, the additive element A source can be called a fluorine source. As the fluorine source, for example, lithium fluoride (LiF), magnesium fluoride (MgF2), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (CoF2, CoF3), nickel fluoride (NiF2), zirconium fluoride (ZrF4), vanadium fluoride (VF5), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF2), calcium fluoride (CaF2), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF2), cerium fluoride (CeF3, CeF4), lanthanum fluoride (LaF3), or sodium aluminum hexafluoride (Na3AlF6) can be used. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

[0321] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can be used as a lithium source. Other lithium sources that can be used in step S21 include lithium carbonate.

[0322] The fluorine source may be a gas, such as fluorine (F2), carbon fluoride, sulfur fluoride, or oxygen fluoride (OF2, O2F2, O3F2, O4F2, O5F2, O6F2, O2F), which may be mixed into the atmosphere in the heating step described below. A plurality of fluorine sources may be used.

[0323] In step S21a shown in Fig. 11(C), magnesium fluoride (MgF2) is prepared as a fluorine source and a magnesium source, and lithium fluoride (LiF) is prepared as a fluorine source. In addition, when the melting point of a fluorine compound (sometimes called a fluoride) such as lithium fluoride is lower than the melting point of other additive element sources, the fluorine compound or the like can function as a flux (also called a flux agent) that lowers the melting point of the other additive element sources. Since the eutectic point of LiF and MgF2 is around 742°C, it is preferable to set the heating temperature to 742°C or higher in the heating step (such as step S33 described later) after mixing the additive element A and lithium cobalt oxide.

[0324] In addition, when lithium fluoride and magnesium fluoride are mixed at a molar ratio of LiF:MgF2=65:35, the effect of lowering the melting point is maximized. In addition, if the ratio of lithium fluoride is too large, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride and magnesium fluoride is preferably LiF:MgF2=x:1 (0≦x≦1.9), more preferably LiF:MgF2=x:1 (0.1≦x≦0.5), and even more preferably LiF:MgF2=x:1 (x=0.33 or nearby). In this specification, unless otherwise specified, the term "nearby" refers to a value that is greater than 0.9 times and less than 1.1 times the value.

[0325] <Step S22> In step S22 shown in FIG. 11(C), the magnesium source and the fluorine source are pulverized and mixed. Step S22 can be performed by selecting from the pulverization and mixing conditions described in step S12. When the additive element A source pulverized in step S22 is mixed with lithium cobalt oxide in a later step, the mixture is easily attached uniformly to the surface of the lithium cobalt oxide. If the mixture is attached uniformly to the surface of the lithium cobalt oxide, it is preferable because the additive element A is easily distributed or diffused uniformly after heating. Therefore, in step S22, it is preferable to perform pulverization and mixing so that the median diameter (D50) is 100 nm or more and 10 μm or less, preferably 300 nm or more and 5 μm or less. Also, even when one type of material is prepared as the additive element A source, it is preferable to pulverize it so that the median diameter (D50) is 100 nm or more and 10 μm or less, preferably 300 nm or more and 5 μm or less.

[0326] <Step S23> In step S23 shown in FIG. 11(C), the material crushed and mixed as described above can be collected to obtain a source of additive element A (A source).

[0327] <Step S21> A process different from that shown in Fig. 11(C) will be described with reference to Fig. 11(D). Step S20b shown in Fig. 11(D) includes steps S21b to S23.

[0328] In step S21b shown in Fig. 11(D), four types of additive element A sources to be added to lithium cobalt oxide are prepared. That is, Fig. 11(D) differs from Fig. 11(C) in the type of additive element A source. In addition to the additive element A source, a lithium source may be prepared separately.

[0329] As sources of four kinds of additive elements A, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described in FIG. 11(C). Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0330] <Step S22> and <Step S23> Next, steps S22 and S23 shown in FIG. 11(D) are similar to steps S22 and S23 described in FIG. 11(C).

[0331] <Step S31> Next, in step S31 shown in FIG. 11(A), the lithium cobalt oxide that has been subjected to step S15 (initial heating) is mixed with the additive element A source. Here, the ratio of the number of cobalt atoms Co in the lithium cobalt oxide that has been subjected to step S15 to the number of magnesium atoms Mg in the additive element A is preferably Co:Mg=100:y (0.1≦y≦6), and more preferably Co:Mg=100:y (0.3≦y≦3). If the additive element A is added to the lithium cobalt oxide that has been subjected to the initial heating, the additive element A can be added evenly. For this reason, it is preferable to add the additive element A after the initial heating (step S15), rather than adding the additive element A and then performing the initial heating (step S15).

[0332] Furthermore, when nickel is selected as the additive element A, it is preferable to perform the mixing in step S31 so that the number of nickel atoms in the nickel source is 0.05% to 4% of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15. Furthermore, when aluminum is selected as the additive element A, it is preferable to perform the mixing in step S31 so that the number of aluminum atoms in the aluminum source is 0.05% to 4% of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15.

[0333] In order not to destroy the shape of the lithium cobalt oxide, the mixing in step S31 is preferably performed under milder conditions than the grinding and mixing in step S12. For example, it is preferable to perform the mixing under conditions of a lower rotation speed or a shorter time than in step S12. It can also be said that the dry method is a milder method than the wet method. For example, a ball mill, a bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the media.

[0334] Also, a composite treatment using mechanical energy can be applied in step S31. For example, Picobond manufactured by Hosokawa Micron can be used in step S31.

[0335] <Step S32> 11(A), the mixed material is collected to obtain a mixture 903. When collecting the material, it may be crushed and then sieved, if necessary.

[0336] <Step S33> Next, in step S33 shown in FIG. 11(A), the mixture 903 is heated. The heating temperature in step S33 is preferably 800° C. or more and 1100° C. or less, more preferably 800° C. or more and 950° C. or less, and even more preferably 850° C. or more and 900° C. or less. The heating time in step S33 may be 1 hour or more and 100 hours or less, and preferably 1 hour or more and 10 hours or less. The lower limit of the heating temperature in step S33 must be a temperature at which the reaction between the lithium cobalt oxide and the additive element A source proceeds. The temperature at which the reaction proceeds may be a temperature at which mutual diffusion of elements contained in the lithium cobalt oxide and the additive element A source occurs, and may be lower than the melting temperature of these materials. For example, in the case of an oxide, the melting temperature T m 0.757 times (Tammann temperature T d) solid-phase diffusion occurs, so the heating temperature in step S33 may be 500° C. or higher. When solid-phase diffusion occurs, it is preferable that a part of the surface of the lithium cobalt oxide melts, and the source of the additive element A can be uniformly distributed.

[0337] The reaction proceeds more easily when the temperature is equal to or higher than the melting temperature of one or more of the materials contained in the mixture 903. For example, when LiF and MgF2 are contained as the source of the additive element A, the eutectic point of LiF and MgF2 is around 742°C as described above, so that the lower limit of the heating temperature in step S33 is preferably 742°C or higher.

[0338] In addition, the mixture 903 obtained by mixing LiCoO2:LiF:MgF2=100:0.33:1 (molar ratio) shows an endothermic peak at about 830° C. in differential scanning calorimetry (DSC measurement) as described above. Therefore, the lower limit of the heating temperature is more preferably 830° C. or higher.

[0339] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and the productivity is high.

[0340] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit in a small amount. Therefore, the heating temperature is preferably 1000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower.

[0341] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0342] In the manufacturing method described in this embodiment, some materials, for example, LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be lowered to a temperature lower than the decomposition temperature of lithium cobalt oxide, for example, to a temperature between 742°C and 950°C, and additive elements such as magnesium can be distributed in the surface layer to manufacture a positive electrode active material with good characteristics.

[0343] However, since LiF has a lower specific gravity in a gaseous state than oxygen, LiF may sublime when heated, and the amount of LiF in the mixture 903 decreases when it sublimes. In this case, the function as a flux is weakened. Therefore, it is preferable to heat the mixture while suppressing the sublimation of LiF.

[0344] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace, or to place a lid on the container containing the mixture 903. For example, when heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by placing a lid on the container containing the mixture 903. By such heating, it is possible to suppress the sublimation of LiF in the mixture 903.

[0345] Moreover, the heating in this step is preferably performed so as not to stick the mixture 903 together. If the mixture 903 sticks together during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the added element (e.g., fluorine) diffuses is blocked, which may result in poor distribution of the added element (e.g., magnesium and fluorine) in the surface layer.

[0346] Furthermore, when the additive element (e.g., fluorine) is uniformly distributed in the surface layer portion, a smooth positive electrode active material with few irregularities can be obtained. Therefore, in this process, in order to maintain the surface smooth or to make it even smoother by heating in step S15, it is better that the mixture 903 does not stick to each other.

[0347] <Step S34> Next, in step S34 shown in Fig. 11(A), the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sieve the recovered positive electrode active material 100. Through the above steps, it is possible to produce a positive electrode active material 100 having a median diameter (D50) of 12 µm or less, preferably 10 µm or less, and more preferably 8 µm or less. The positive electrode active material 100 is lithium cobalt oxide containing an additive element A.

[0348] <Example 2 of a method for producing a positive electrode active material> Another example of the method for producing a positive electrode active material (Example 2 of the method for producing a positive electrode active material) will be described with reference to Figures 12 and 13. Example 2 of the method for producing a positive electrode active material differs from Example 1 of the method for producing a positive electrode active material described above in the number of times the additive elements are added and the mixing method, but the rest of the description in Example 1 of the method for producing a positive electrode active material can be applied.

[0349] 12, steps S10 and S15 are performed in the same manner as in Fig. 11(A) to prepare lithium cobalt oxide that has been subjected to initial heating. Note that step S15 is not an essential configuration in one embodiment of the present invention, and therefore an embodiment in which step S15 is omitted is also included in one embodiment of the present invention.

[0350] <Step S20> Next, as shown in step S20, a first additive element A1 source (A1 source) is prepared. Details of step S20 can be fabricated under the same conditions as in FIG.

[0351] <Steps S31 to S33> Steps S31 to S33 shown in FIG. 12 can be fabricated under the same conditions as steps S31 to S33 shown in FIG.

[0352] <Step S34a> Next, in step S33, the heated material is recovered to obtain lithium cobalt oxide having the additive element A1. In order to distinguish it from the lithium cobalt oxide (first composite oxide) that has been subjected to step S15, the lithium cobalt oxide having the additive element A1 may also be called a second composite oxide.

[0353] <Step S40> In step S40 shown in Fig. 12, a source of the second additive element A2 (A2 source) is prepared. Step S40 will be described with reference to Figs. 13(A) and 13(B).

[0354] <Step S41> In step S41 shown in FIG. 13(A), a second additive element A2 source (A2 source) is prepared. The A2 source can be selected from the additive elements A described in step S20 shown in FIG. 11(C). The additive element A2 is preferably different from the additive element A1, and the additive element A2 can be one or more selected from nickel, titanium, boron, zirconium, and aluminum. FIG. 13(A) illustrates an example in which a nickel source (Ni source) and an aluminum source (Al source) are used as the additive element A2.

[0355] Steps S41 to S43 shown in Fig. 13A can be performed under the same conditions as steps S21 to S23 shown in Fig. 11C. As a result, the additive element A2 source (A2 source) can be obtained in step S43.

[0356] Steps S41 to S43 shown in Fig. 13(B) are a modified example of Fig. 13(A). In step S41 shown in Fig. 13(B), a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are individually pulverized. As a result, in step S43, a plurality of second additive element A2 sources (A2 sources) are prepared. Thus, step S40 in Fig. 13(B) differs from step S40 in Fig. 13(A) in that the additive element sources are individually pulverized in step S42a.

[0357] <Steps S51 to S53> Next, steps S51 to S53 shown in Fig. 12 can be produced under the same conditions as steps S31 to S34 shown in Fig. 11(A). The conditions for step S53 relating to the heating process are preferably a lower temperature and / or a shorter time than those for step S33 shown in Fig. 12. Specifically, the heating temperature is preferably 800°C or higher and 950°C or lower, more preferably 820°C or higher and 870°C or lower, and further preferably 850°C±10°C. The heating time is preferably 0.5 hours or higher and 8 hours or lower, and more preferably 1 hour or higher and 5 hours or lower.

[0358] When nickel is selected as the additive element A2, it is preferable to perform the mixing in step S51 so that the number of nickel atoms in the nickel source is 0.05% to 4% of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15. When aluminum is selected as the additive element A2, it is preferable to perform the mixing in step S51 so that the number of aluminum atoms in the aluminum source is 0.05% to 4% of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15.

[0359] <Step S54> 12, the heated material is collected and crushed as necessary to obtain the positive electrode active material 100. Through the above steps, the positive electrode active material 100 (composite oxide) having a median diameter (D50) of 12 μm or less, preferably 10 μm or less, and more preferably 8 μm or less can be produced. The positive electrode active material 100 is lithium cobalt oxide containing the additive element A1 and the additive element A2.

[0360] In the above-described example 2 of the preparation method, as shown in FIG. 12 and FIG. 13, the additive element to the lithium cobalt oxide is introduced separately as a first additive element A1 and a second additive element A2. By introducing them separately, the profile of each additive element in the depth direction can be changed. For example, the first additive element can be profiled so that the concentration is higher in the surface layer than in the inside, and the second additive element can be profiled so that the concentration is higher in the inside than in the surface layer. Since multiple types of additive element A sources are added in multiple steps, the preparation cost is relatively high, but it is preferable because the profile of each additive element A in the depth direction can be controlled more accurately. On the other hand, the positive electrode active material 100 prepared through the steps of FIG. 11(A) and FIG. 11(D) has the advantage that it can be prepared at low cost because multiple types of additive element A sources are added at once.

[0361] The contents of this embodiment mode can be freely combined with the contents of other embodiment modes.

[0362] (Embodiment 5) In this embodiment, in addition to the above-described materials, materials that can be used in the secondary battery of one embodiment of the present invention will be described.

[0363] [Cathode active material] The positive electrode includes a positive electrode active material layer and a positive electrode current collector as described above. The positive electrode active material layer includes a positive electrode active material, and may further include a conductive material and a binder. The positive electrode active material having a stable crystal structure described in the above embodiment may be used as the positive electrode active material. The positive electrode active material according to one embodiment of the present invention may be mixed with another positive electrode active material.

[0364] Other examples of the positive electrode active material include composite oxides having an olivine type crystal structure, a layered rock salt type crystal structure, or a spinel type crystal structure, such as LiFePO4, LiFeO2, LiNiO2, LiMn2O4, V2O5, Cr2O5, and MnO2.

[0365] In addition, as another positive electrode active material, a lithium-containing material having a spinel-type crystal structure containing manganese such as LiMn2O4 is preferably mixed with lithium nickelate (LiNiO2 or LiNi 1-x M x O2(0 < x < 1) (M = Co, Al, etc.)). By adopting such a configuration, the characteristics of the secondary battery can be improved.

[0366] In addition, as another positive electrode active material, a lithium manganese composite oxide represented by the composition formula Li a Mn b M c O d can be used. Here, the element M is preferably a metal element selected from those other than lithium and manganese, or silicon or phosphorus, and more preferably nickel. When measuring the entire particle of the lithium manganese composite oxide, it is preferable to satisfy 0 < a / (b + c) < 2, c > 0, and 0.26 ≦ (b + c) / d < 0.5 (where a, b, c, and d are non-zero) during discharge. The composition of metals, silicon, phosphorus, etc. in the entire particle of the lithium manganese composite oxide can be measured using, for example, ICP-MS (inductively coupled plasma mass spectrometer). Also, the oxygen composition of the entire particle of the lithium manganese composite oxide can be measured using, for example, EDX (energy dispersive X-ray analysis method). In addition, it can be determined by using valence evaluation of melting gas analysis and XAFS (X-ray absorption fine structure) analysis in combination with ICP-MS analysis. The lithium manganese composite oxide refers to an oxide containing at least lithium and manganese, and may contain one or more elements selected from the group consisting of chromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium, silicon, and phosphorus.

[0367] [Positive electrode current collector] As the positive electrode current collector, a material having high electrical conductivity, such as metals such as stainless steel, gold, platinum, aluminum, and titanium, and alloys thereof, can be used. In addition, it is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. In addition, an aluminum alloy to which an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, is added can be used. In addition, it may be formed of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be appropriately used in a shape such as a foil, plate, sheet, net, punched metal, or expanded metal. It is preferable to use a current collector having a thickness of 5 μm or more and 30 μm or less, and when the current collector is wrinkled, the thickness of the current collector refers to the thickness of the part without wrinkles.

[0368] [Conductive material] The conductive material in the positive electrode active material layer has a function of assisting a current path between the active material and a current collector, or a current path between a plurality of active materials. The conductive material is also called a conductive assistant or a conductive agent because of its function. To perform this function, the conductive material may have a material with a lower resistance than the positive electrode active material.

[0369] The conductive material is typically a carbon material or a metal material. Particulate conductive materials include carbon black (furnace black, acetylene black, or graphite). Most carbon blacks have a smaller particle size than the positive electrode active material, and are often amorphous.

[0370] Fibrous conductive materials are sometimes called carbon fibers. Carbon fibers include carbon nanotubes (CNTs), carbon nanofibers, and VGCF (registered trademark). CNTs have a layer of carbon atoms, and when there is a single layer, they are called single-wall nanotubes, and when there are multiple layers, they are called multi-wall nanotubes, and multi-wall nanotubes include double-wall nanotubes, which have two layers. Carbon fibers can also be entangled because of their large long axis or fiber length, and this state is called an aggregate. An entangled state includes a state in which one carbon fiber is entangled, or a state in which multiple carbon fibers are entangled.

[0371] The specific surface area of ​​VGCF (registered trademark) is 100m 2 / g or less, preferably 60m 2 / g or more, more preferably 20m 2 / g or more. The specific surface area of ​​CNT is 500m 2 / g or more, preferably 650m 2 / g or more, more preferably 800m 2 The specific surface area is, for example, a value measured by the BET method.

[0372] The long axis or fiber length of VGCF (registered trademark) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 20 μm or less. The long axis or fiber length of CNT is preferably 100 μm or more and 600 μm or less, more preferably 200 μm or more and 500 μm or less. In addition, if the long axis or fiber length is larger than the median diameter (D50) of the positive electrode active material, the carbon fiber can be arranged across multiple positive electrode active materials, so it is not limited to the above numerical value. Furthermore, since carbon fibers are entangled, the long axis or fiber length of one carbon fiber is not so important, and the long axis when entangled is also important as a conductive material.

[0373] Furthermore, when the cross section of one carbon fiber can be regarded as a circle, the average diameter of the carbon fiber is preferably 1 nm to 180 nm, and more preferably 2 nm to 150 nm. VGCF (registered trademark) can satisfy the average diameter of 100 nm to 180 nm, and more preferably 130 nm to 160 nm, and can be said to have a large average diameter. VGCF (registered trademark) with a large average diameter shows high dispersibility. CNT can satisfy the average diameter of 1 nm to 100 nm, and more preferably 1 nm to 50 nm, and more preferably 3 nm to 5 nm, and can be said to have a small average diameter.

[0374] When considering the volume resistivity of VGCF powder, the volume resistivity of carbon fiber is 1×10 -3 Ω cm or less and / or 1×10 at a pressure of 13 MPa -2 It is preferable that the volume resistivity is Ω·cm or less. When referring to the volume resistivity of CNT powder, the volume resistivity of carbon fiber is 1×10 -2 Ω cm or less and / or 3×10 at a pressure of 13 MPa -2 It is preferable that the volume resistivity is Ω·cm or less. Furthermore, when the volume resistivity of CNT powder is taken into consideration, the volume resistivity of carbon fiber is 1×10 -2 Ω cm or less 1×10 -3 Ω cm and / or 3×10 at a pressure of 13 MPa -2 Ω cm or less 9×10 -3 A value larger than Ω·cm is preferable.

[0375] The sheet-like conductive material may be graphene or a graphene compound, and may appear thread-like in the cross section of the positive electrode.

[0376] In this specification and the like, graphene refers to a material that has carbon, has a shape such as a plate or sheet, and has a two-dimensional structure formed of six-membered carbon rings. The two-dimensional structure formed of six-membered carbon rings may be called a carbon sheet. In this specification and the like, graphene includes multi-layer graphene and multi-graphene. Multi-layer graphene has a structure in which two or more graphenes are stacked.

[0377] In the present specification and the like, the graphene compound includes graphene oxide, multi-layer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multi-layer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. That is, the graphene compound may have a functional group. Furthermore, the interlayer distance of the graphene compound is preferably greater than 0.34 nm and less than 0.44 nm.

[0378] As described above, the graphene or graphene compound has a sheet shape and may be partially bent. Also, the graphene or graphene compound may be rolled, and rolled graphene is sometimes called carbon nanofiber.

[0379] In this specification and the like, graphene oxide refers to a material that contains carbon and oxygen, has a sheet-like shape, and has a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.

[0380] In this specification and the like, reduced graphene oxide refers to a material having carbon and oxygen, a sheet-like shape, and a two-dimensional structure formed of six-membered carbon rings. Reduced graphene oxide preferably has a portion in which the carbon concentration is greater than 80 atomic % and the oxygen concentration is 2 atomic % to 15 atomic %. By setting such carbon and oxygen concentrations, it can function as a conductive material with high conductivity even in a small amount. In addition, reduced graphene oxide preferably has an intensity ratio G / D of the G band to the D band in a Raman spectrum of 1 or more. Reduced graphene oxide with such an intensity ratio can function as a conductive material with high conductivity even in a small amount. For example, the electrical conductivity of reduced graphene oxide is 0.1 S / cm to 10 7 In addition, the graphene compound may have holes large enough to allow carrier ions, typically lithium ions, to pass through.

[0381] Fluorine-containing graphene may be used as the graphene compound. The fluorine in the graphene compound may be adsorbed on the surface. Fluorine-containing graphene may be produced by contacting graphene with a fluorine compound (called fluorination treatment). Fluorine (F2) or a fluorine compound may be used for the fluorination treatment. As the fluorine compound, hydrogen fluoride, halogen fluoride (ClF3, IF5, etc.), gaseous fluoride (BF3, NF3, PF5, SiF4, SF6, etc.), metal fluoride (LiF, NiF2, AlF3, MgF2, etc.), etc. are preferable. For the fluorination treatment, gaseous fluoride is preferably used, and the gaseous fluoride may be diluted with an inert gas. The temperature of the fluorination treatment is preferably room temperature, but is preferably 0°C or higher and 250°C or lower, which includes the room temperature. When the fluorination treatment is performed at 0°C or higher, fluorine can be adsorbed on the surface of graphene.

[0382] The graphene compound may have excellent electrical properties such as high electrical conductivity, and excellent physical properties such as high flexibility and high mechanical strength. The graphene compound may have very high electrical conductivity even when thin, and can efficiently form a conductive path in the active material layer with a small amount. Therefore, by using the graphene compound as a conductive material, the contact area between the active material and the graphene compound can be increased. The graphene compound may cover 80% or more of the area of ​​the active material. In addition, the graphene compound may be located along the positive electrode active material, or may cover a part of the positive electrode active material.

[0383] Here, it is preferable to use graphene oxide as the graphene or graphene compound, mix it with an active material to form an active material layer, and then reduce it. In other words, it is preferable that the completed active material layer has reduced graphene oxide. By using graphene oxide, which has extremely high dispersibility in a polar solvent, to form graphene or graphene compound, it is possible to disperse graphene or graphene compound approximately uniformly inside the active material layer. Since the solvent is volatilized and removed from the dispersion medium containing uniformly dispersed graphene oxide and graphene oxide is reduced, graphene or graphene compound remaining in the active material layer is partially overlapped and dispersed to such an extent that they are in surface contact with each other, thereby forming a three-dimensional conductive path. Note that the reduction of graphene oxide may be performed, for example, by heat treatment or by using a reducing agent.

[0384] A mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed by bonding multiple graphenes or graphene compounds together. When the graphene net covers the positive electrode active material, it can also function as a binder. The graphene net can also be applied to the negative electrode active material layer, and the negative electrode active material can cover small particles such as nanosilicon, and if the negative electrode active material is a large particle such as graphite particles, it can be attached across adjacent graphite particles. Therefore, the amount of binder can be reduced or no binder can be used, so the ratio of active material to the electrode volume and electrode weight can be improved. That is, the discharge capacity of the secondary battery can be increased.

[0385] In addition, by using a spray dryer, a graphene compound can be formed as a coating portion for a positive electrode active material, and further, a conductive path can be formed between adjacent particles of the active material by the graphene compound.

[0386] In addition, particles used as a catalyst when forming the graphene compound may be mixed with the graphene compound. Examples of catalysts when forming the graphene compound include silicon oxide (SiO2, SiO x (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particle diameter (D50) of the catalyst is preferably 1 μm or less, and more preferably 100 nm or less.

[0387] The particulate conductive material can enter the gaps between the positive electrode active material and the like, and is also prone to aggregation. Therefore, the particulate conductive material can assist the conductive path between the positive electrode active material arranged nearby. The fibrous conductive material and the sheet-like conductive material can have a bent region, but have a longer axis larger than that of the positive electrode active material. Therefore, the fibrous conductive material and the sheet-like conductive material can assist the conductive path between the adjacent positive electrode active materials as well as between the distant positive electrode active materials.

[0388] It is preferable to mix two or more conductive materials having different shapes. For example, it is preferable to use multi-layer graphene as the sheet-shaped conductive material and carbon black as the particulate conductive material. In this case, it is preferable that the weight of the carbon black in the slurry state in which the multi-layer graphene and the carbon black are mixed is 1.5 to 20 times, preferably 2 to 9.5 times, the weight of the multi-layer graphene.

[0389] When the mixing ratio of the multi-layer graphene and the carbon black is within the above range, the carbon black does not aggregate and is easily dispersed. Furthermore, when the mixing ratio of the multi-layer graphene and the carbon black is within the above range, the electrode density can be made higher than when only carbon black is used as the conductive material. By increasing the electrode density, the capacity per unit volume can be increased. Furthermore, when the mixing ratio of the multi-layer graphene and the carbon black is within the above range, rapid charging can be supported.

[0390] [Binder] The positive electrode active material layer has a binder. The binder is necessary to strengthen the adhesion of the powdered active material without covering the surface of the active material. Furthermore, the binder must be adhesive to the current collector. In other words, the binder should have a material that exhibits a binding component, and a thickener may be used. Furthermore, in consideration of the expansion of the active material, the binder should exhibit sufficient flexibility and be able to respond to changes in the state of the active material. The binder must also be compatible with the electrolyte. Furthermore, since extremely strong oxidation and reduction reactions occur in secondary batteries, a binder that does not deteriorate or has low reactivity to these reactions is desired.

[0391] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Also, as the binder, fluororubber can be used.

[0392] In addition, it is preferable to use, for example, a water-soluble polymer as the thickener. For example, polysaccharides can be used as the water-soluble polymer. As the polysaccharide, one or more of cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, and starch can be used. It is more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0393] Alternatively, it is preferable to use materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, PVDF, polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, nitrocellulose, and glutamic acid as the binder.

[0394] The binder may be a combination of two or more of the above.

[0395] For example, a material having a particularly excellent viscosity adjusting effect may be used in combination with another material. For example, while rubber materials and the like have excellent adhesive strength and / or elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such a case, it is preferable to mix the material with a material having a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material having a particularly excellent viscosity adjusting effect. In addition, as a water-soluble polymer having a particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, and diacetylcellulose, cellulose derivatives such as regenerated cellulose, starch, etc. may be used.

[0396] In addition, the solubility of cellulose derivatives such as carboxymethylcellulose is increased by converting them into salts such as sodium salt and ammonium salt of carboxymethylcellulose, and they are more likely to exhibit their effect as viscosity adjusters. The increased solubility can also increase the dispersibility of the active material and other components when preparing an electrode slurry. In this specification, the cellulose and cellulose derivatives used as the binder for the electrode include their salts.

[0397] Water-soluble polymers stabilize the viscosity by dissolving in water, and can stably disperse active materials and other materials combined as binders, such as styrene-butadiene rubber, in an aqueous solution. In addition, since they have functional groups, they are expected to be easily and stably adsorbed onto the surface of active materials. In addition, many cellulose derivatives, such as carboxymethyl cellulose, have functional groups such as hydroxyl groups and carboxyl groups, and since they have functional groups, the polymers interact with each other and are expected to widely cover the surface of the active material.

[0398] When the binder covers the active material surface or contacts the surface to form a film, it is expected to function as a passive film and suppress the decomposition of the electrolyte. Here, the passive film is a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress the decomposition of the electrolyte at the battery reaction potential. Moreover, it is more preferable that the passive film suppresses electrical conductivity and can conduct lithium ions.

[0399] [Negative electrode] The negative electrode includes the negative electrode active material layer and the negative electrode current collector as described above. The negative electrode active material layer may include a conductive material and a binder.

[0400] [Negative electrode active material] As the negative electrode active material, for example, an alloy material and / or a carbon material can be used.

[0401] As the negative electrode active material, an element capable of performing a charge / discharge reaction by alloying / dealloying reaction with lithium can be used. In the present specification and the like, an element capable of performing a charge / discharge reaction by alloying / dealloying reaction with lithium, and a compound containing the element, etc., may be referred to as an alloy material. For example, as the alloy material, a material containing one or more selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a large charge / discharge capacity compared to carbon, and silicon in particular has a high theoretical capacity of 4200mAh / g. For this reason, it is preferable to use a negative electrode active material containing silicon. For example, negative electrode active materials containing silicon include SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn.

[0402] In this specification, SiO refers to, for example, silicon monoxide. Alternatively, SiO x It can also be expressed as follows. Here, x preferably has a value close to 1. For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less. Alternatively, x is preferably 0.2 or more and 1.2 or less. Alternatively, x is preferably 0.3 or more and 1.5 or less. Silicon monoxide and silicon can be mixed as the negative electrode active material, and even in this case, oxygen as the negative electrode active material particles should satisfy the above range of x.

[0403] As the carbon material, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, or carbon black may be used.

[0404] Graphite includes artificial graphite and natural graphite. Artificial graphite is synthesized from raw materials at 2000°C to 3000°C, and examples of such include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. The shape of the artificial graphite is preferably spherical, and spherical artificial graphite is sometimes called spheroidized artificial graphite. Artificial graphite may be used in the form of secondary particles, or may be used in the form of fine particles by pulverization. Artificial graphite may be used in a state in which large and small particle sizes are mixed. MCMB is spheroidized artificial graphite, and the particle size can be 3 μm to 30 μm, preferably 7 μm to 12 μm. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and the surface area is 0.5 m 2 / g or more 3m 2 / g or less, preferably 0.7m 2 / g or more 1.8m 2 / g. Examples of natural graphite include flake graphite and spheroidized graphite. Spheroidized natural graphite can have a particle size of 5 μm or more and 100 μm or less, preferably 8 μm or more and 20 μm or less. Furthermore, the spheroidized natural graphite can have a surface area of ​​3 m 2 / g or more 10m 2 / g or less, preferably 5.1m 2 / g or more 6.8m 2 / g. The spheroidized natural graphite may have voids inside the particles. The spheroidized natural graphite may have a coating layer, and the crystallinity of the coating layer may be lower than the crystallinity of the interior. The coating layer is preferably thin, for example, 20 nm or less, preferably 10 nm or less.

[0405] When lithium ions are inserted into graphite (when lithium-graphite intercalation compounds are formed), the graphite exhibits a low potential similar to that of lithium metal (0.05 V to 0.3 V vs. Li / Li + ) This allows lithium ion secondary batteries to exhibit high operating voltages. Furthermore, graphite is preferred because it has the advantages of relatively high discharge capacity per unit volume, relatively small volume expansion, low cost, and high safety compared to lithium metal.

[0406] In addition, titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O 12 ), lithium graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten dioxide (WO2), molybdenum dioxide (MoO2), and other oxides can be used.

[0407] In addition, the negative electrode active material is Li3N-type nitride of lithium and transition metals. 3-x M x N (M=Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 N has a large discharge capacity (900mAh / g, 1890mAh / cm 3 ) and is preferred.

[0408] When a nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, and therefore it is preferable to combine it with a material that does not contain lithium ions, such as V2O5 or Cr3O8, as the positive electrode active material. Even when a material that contains lithium ions is used as the positive electrode active material, the nitride of lithium and a transition metal can be used as the negative electrode active material by desorbing the lithium ions contained in the positive electrode active material in advance.

[0409] In addition, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS 0.89 It also occurs in sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N and Ge3N4, phosphides such as NiP2, FeP2 and CoP3, and fluorine compounds such as FeF3 and BiF3.

[0410] As the conductive material and binder that can be contained in the negative electrode active material layer, materials similar to the conductive material and binder that can be contained in the positive electrode active material layer can be used.

[0411] [Negative electrode current collector] The negative electrode current collector may be made of the same material as the positive electrode current collector, but it is preferable that the negative electrode current collector is made of a material that does not form an alloy with carrier ions such as lithium.

[0412] [Separator] The secondary battery preferably has a separator. The separator may be made of, for example, cellulose-containing fibers such as paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers using nylon (polyamide), polyimide, vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane. The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0413] The separator may have a multi-layer structure. For example, an organic material film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine material, a polyamide material, or a mixture of these. As the ceramic material, for example, aluminum oxide particles or silicon oxide particles may be used. As the fluorine material, for example, PVDF or polytetrafluoroethylene may be used. As the polyamide material, for example, nylon or aramid (meta-aramid or para-aramid) may be used.

[0414] Coating with ceramic materials improves oxidation resistance, suppressing the deterioration of the separator during high-voltage charging and discharging, and improving the reliability of the secondary battery. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide-based materials, especially aramid, improves heat resistance, improving the safety of the secondary battery.

[0415] For example, both sides of a polypropylene film may be coated with a mixture of aluminum oxide and aramid.Alternatively, the surface of the polypropylene film that contacts the positive electrode may be coated with a mixture of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0416] By using a separator with a multi-layer structure, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the secondary battery can be increased.

[0417] [Electrolyte] A secondary battery has an electrolyte. In this specification and the like, the electrolyte includes an organic solvent that is liquid at 25° C., a solid electrolyte, and an electrolyte containing both an organic solvent that is liquid at 25° C. and a solid electrolyte (semi-solid electrolyte). Note that an organic solvent that is liquid at 25° C. may be called an electrolyte solution. An electrolyte solution other than the ionic liquid described in the above embodiment will be described.

[0418] <Liquid organic solvent> It is preferable to use an aprotic organic solvent as the organic solvent that is liquid at 25°C in the electrolyte. For example, one or more selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate (EP), propyl propionate (PP), methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone can be used. Two or more organic solvents may be called mixed solvents.

[0419] <Lithium salt> Examples of the lithium salt dissolved in the organic solvent include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B 10 Cl 10 , Li2B 12 Cl 12 One or more selected from LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2) and LiN(C2F5SO2)2 can be used.

[0420] <Additives> The organic solvent may contain an additive. The additive can suppress the reactive decomposition of the electrolyte that may occur on the positive electrode surface or the negative electrode surface when the secondary battery is operated at high voltage and / or high temperature. For example, vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), and lithium bis(oxalate)borate (LiBOB) can be used as the additive. LiBOB is particularly preferred because it is easy to form a good coating. VC or FEC is preferred because it can form a good coating on the negative electrode during charging and discharging, improving the cycle characteristics.

[0421] As an additive, a dinitrile compound containing succinonitrile, glutaronitrile, adiponitrile (ADN), ethylene glycol bis(propionitrile) ether (EGBE), or the like may be used. The dinitrile compound is preferable because the nitrile group is oriented toward the positive electrode and the negative electrode, inhibiting the oxidative decomposition of the organic solvent, and therefore can improve the voltage resistance. Furthermore, when a current collector having copper is used for the negative electrode, the dinitrile compound is preferable because it can prevent the dissolution of copper during overdischarge. Considering the use of the secondary battery at a high voltage, it is preferable to add a nitrile compound.

[0422] Fluorobenzene may be added to the organic solvent. The concentration of the additive may be, for example, 0.1 wt% to 5 wt% based on the total electrolyte. PS or EGBE are preferable because they form a good coating on the positive electrode during charging and discharging, improving cycle characteristics. FB is preferable because it improves the wettability of the organic solvent to the positive and negative electrodes.

[0423] The additive may be one or more of the above-mentioned materials.

[0424] <Semi-solid material> The organic solvent does not need to be liquid at room temperature, and a semi-solid material called a polymer gel electrolyte may be used as the organic solvent. By using a polymer gel electrolyte, safety against leakage and the like is improved. In addition, the battery cell can be made thinner and lighter.

[0425] Examples of the polymer that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel.

[0426] Examples of the polymer that can be used include polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer that is formed may have a porous shape.

[0427] <Examples of organic solvents suitable for low temperatures> An example of an organic solvent preferable for low temperature use will be described below. The organic solvent preferable for low temperature use includes ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). When the total content of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is 100 vol%, an organic solvent having a volume ratio of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate of x:y:100−x−y (where 5≤x≤35 and 0<y<65) can be used. More specifically, an organic solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC = 30:35:35 can be used. The above volume ratio may be the volume ratio before mixing the electrolytes, and the outside air may be at room temperature (typically 25° C.) when mixing the electrolytes.

[0428] EC is a cyclic carbonate and has a high relative dielectric constant, and thus has an effect of promoting the dissociation of lithium salts. On the other hand, EC has a high viscosity and a high freezing point (melting point) of 38° C. Therefore, when EC alone is used as an organic solvent, it is difficult to use it in a low temperature environment. Therefore, the organic solvent specifically described as one aspect of the present invention does not contain EC alone, but further contains EMC and DMC. EMC is a chain carbonate and has an effect of lowering the viscosity of the electrolyte solution, and its freezing point is −54° C. Further, DMC is also a chain carbonate and has an effect of lowering the viscosity of the electrolyte solution, and its freezing point is −43° C. An electrolyte produced using an organic solvent obtained by mixing EC, EMC, and DMC having such physical properties so that the volume ratio at 25° C. is x:y:100−x−y (where 5≤x≤35 and 0<y<65) with the total content of these three organic solvents being 100 vol% has a characteristic that the freezing point is −40° C. or lower.

[0429] General electrolytes used in battery cells freeze at around -40°C, making it difficult to create batteries that can be charged and discharged at -40°C. The electrolytes described above as organic solvents for low-temperature electrolytes have freeze points below -40°C, making it possible to realize battery cells that can be charged and discharged even in extremely low-temperature environments such as -40°C.

[0430] Additionally, the lithium salt dissolved in the organic solvent preferred for low temperature use can be selected from the lithium salts mentioned above.

[0431] The additives contained in the organic solvent suitable for low temperature use can be selected from the additives mentioned above.

[0432] [Exterior body] The exterior body of the secondary battery is preferably one described in the above embodiment. If the secondary battery can be freely curved, the exterior body may have a metal film. As the metal film, a metal film containing aluminum, stainless steel, platinum, copper, nickel, or an alloy of these metals can be used. The metal film has a thickness of 1 μm to 50 μm, preferably 10 μm to 30 μm.

[0433] (Embodiment 6) In this embodiment, an example of a vehicle including a secondary battery of one embodiment of the present invention will be described.

[0434] The automobile 2001 shown in FIG. 14A is an electric automobile that uses an electric motor as a power source for running. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor and an engine as a power source for running. The automobile 2001 shown in FIG. 14A has a battery pack 2200, and the battery pack 2200 has a secondary battery module in which a plurality of secondary batteries are connected in series and parallel. It is preferable that the automobile 2001 shown in FIG. 14A further has a charge control device that is electrically connected to the secondary battery module. When the secondary battery of one embodiment of the present invention is used as the secondary battery of the automobile 2001 shown in FIG. 14A, it becomes possible to bend the battery pack 2200, which is preferable because the battery pack 2200 can be freely arranged.

[0435] 14B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The secondary battery module of the transport vehicle 2002 has, for example, a four-cell unit including secondary batteries with a nominal voltage of 3.0 V to 5.0 V, and has a maximum voltage of 170 V, in which 48 cells are connected in series. In addition, except for the number of secondary batteries constituting the secondary battery module of the battery pack 2201, the transport vehicle 2002 has the same functions as those in FIG. 14A, and therefore the description thereof will be omitted. If the secondary battery of one embodiment of the present invention is used as the secondary battery of the transport vehicle 2002 shown in FIG. 14B, the vehicle can be bent, and the battery pack 2201 can be arranged freely, which is preferable.

[0436] 14C shows, as an example, a large-sized transport vehicle 2003 having an electrically controlled motor. The secondary battery module of the transport vehicle 2003 has, for example, a maximum voltage of 600 V, in which one hundred or more secondary batteries with a nominal voltage of 3.0 V to 5.0 V are connected in series and parallel. In addition, except for the number of secondary batteries constituting the secondary battery module of the battery pack 2202, the secondary battery module has the same functions as those of FIG. 14A, and therefore description thereof will be omitted. When the secondary battery of one embodiment of the present invention is used as the secondary battery of the transport vehicle 2003 shown in FIG. 14C, it becomes possible to bend the vehicle, and therefore it is preferable that the battery pack 2202 can be freely arranged.

[0437] FIG. 14D shows an aircraft 2004 having an engine that burns fuel as an example. The aircraft 2004 shown in FIG. 14D has wheels for takeoff and landing, and can be considered a part of a transportation vehicle. The aircraft 2004 has a battery pack 2203 including a secondary battery module and a charge control device, in which a plurality of secondary batteries are connected in series and parallel to form a secondary battery module. The secondary battery module of the aircraft 2004 has a maximum voltage of 32 V, for example, with eight 4 V secondary batteries connected in series. The secondary battery module of the battery pack 2203 has the same functions as those of FIG. 14A, except for the number of secondary batteries constituting the secondary battery module of the battery pack 2203, and therefore description thereof will be omitted. If the secondary battery of one embodiment of the present invention is used for the secondary battery of the aircraft 2004 shown in FIG. 14D, the aircraft 2004 can be bent, and the battery pack 2203 can be freely arranged, which is preferable.

[0438] FIG. 14E illustrates an artificial satellite 2005 including a secondary battery 2204 as an example. Since the artificial satellite 2005 is used in outer space at extremely low temperatures, it is preferable that the artificial satellite 2005 includes the secondary battery 2204 of one embodiment of the present invention, which has excellent high-temperature characteristics and / or low-temperature resistance. A secondary battery module may be formed by connecting a plurality of secondary batteries 2204 in series and parallel. It is more preferable that the secondary battery 2204 is mounted inside the artificial satellite 2005 while being covered with a heat-insulating member. When the secondary battery of one embodiment of the present invention is used as the secondary battery 2204 of the artificial satellite 2005 illustrated in FIG. 14E, it is preferable that the secondary battery can be bent and the battery pack 2203 can be freely arranged.

[0439] An electric bicycle 8700 shown in FIG. 15A includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable. FIG. 15B shows the power storage device 8702 removed from the bicycle. The power storage device 8702 includes a secondary battery 8701, a display unit 8703 that displays the remaining battery level, and a control circuit 8704 that can control charging of the secondary battery 8701 or detect an abnormality. When the secondary battery of one embodiment of the present invention is used for the secondary battery 8701, the battery can be bent, which is preferable because the secondary battery 8701 can be freely arranged.

[0440] A scooter 8600 shown in FIG. 15C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. The power storage device 8602 can be stored in an under-seat storage 8604 in the scooter 8600 shown in FIG. 15C. When a secondary battery of one embodiment of the present invention is used as the secondary battery included in the power storage device 8602, the power storage device 8602 can be bent, which is preferable because the secondary battery included in the power storage device 8602 can be freely arranged. Such a power storage device 8602 can be stored in the under-seat storage 8604.

[0441] (Embodiment 7) In this embodiment, an example of mounting a secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a secondary battery include television devices (also called televisions or television receivers), monitors for computers, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game machines, portable information terminals, audio playback devices, and large game machines such as pachinko machines. Examples of portable information terminals include notebook personal computers, tablet terminals, e-book terminals, and mobile phones.

[0442] 16A shows an example of a mobile phone. The mobile phone 2100 includes a display portion 2102 built into a housing 2101, an operation button 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that the mobile phone 2100 includes a secondary battery 2107. When the secondary battery of one embodiment of the present invention is used for the secondary battery 2107, the battery can be bent, which is preferable because the secondary battery 2107 can be freely arranged.

[0443] The mobile phone 2100 is capable of executing a variety of applications, such as mobile phone calls, e-mail, document browsing and creation, music playback, Internet communications, and computer games.

[0444] The operation button 2103 can have various functions, such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / cancellation, power saving mode activation / cancellation, etc. For example, the function of the operation button 2103 can be freely set by an operating system built into the mobile phone 2100.

[0445] The mobile phone 2100 is also capable of performing short-range wireless communication according to a communication standard. For example, the mobile phone 2100 can communicate with a wireless headset to enable hands-free calling.

[0446] The mobile phone 2100 also includes an external connection port 2104, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0447] Moreover, the mobile phone 2100 preferably has a sensor. As the sensor, for example, a fingerprint sensor, a pulse sensor, a human body sensor such as a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted.

[0448] 16B illustrates an unmanned aerial vehicle 2300 having a plurality of rotors 2302. The unmanned aerial vehicle 2300 is also called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. When the secondary battery of one embodiment of the present invention is used as the secondary battery 2301 mounted on the unmanned aerial vehicle 2300, the unmanned aerial vehicle 2300 can be bent, which is preferable because the secondary battery 2301 can be freely arranged.

[0449] A robot 6400 shown in FIG. 16(C) includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a moving mechanism 6408, and a computing device.

[0450] The microphone 6402 has a function of detecting the user's voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0451] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may be a removable information terminal, and by installing it in a fixed position on the robot 6400, charging and data transfer are possible.

[0452] The upper camera 6403 and the lower camera 6406 have a function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of an obstacle in the moving direction when the robot 6400 moves forward using the moving mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0453] The robot 6400 includes a secondary battery 6409 and a semiconductor device or an electronic component. When the secondary battery of one embodiment of the present invention is used as the secondary battery 6409 mounted on the robot 6400, the robot 6400 can be bent, which is preferable because the secondary battery 6409 can be freely arranged.

[0454] 16D includes a display unit 6302 disposed on the top surface of a housing 6301, a plurality of cameras 6303 disposed on the side surface, a brush 6304, an operation button 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 includes tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port disposed on the bottom surface.

[0455] The cleaning robot 6300 can analyze an image captured by the camera 6303 and determine the presence or absence of an obstacle such as a wall, furniture, or a step. When an object that may become entangled in the brush 6304, such as a wire, is detected by image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. When the secondary battery of one embodiment of the present invention is used as the secondary battery 6306 included in the cleaning robot 6300, the secondary battery 6306 can be bent and can be freely arranged, which is preferable.

[0456] Fig. 17(A) shows an example of a wearable device. Wearable devices use secondary batteries as a power source. In addition, when a user uses the device in daily life or outdoors, in order to improve splash-proof, water-resistant, or dust-proof performance, there is a demand for wearable devices that can be charged wirelessly as well as by wire with an exposed connector.

[0457] 17A, the secondary battery according to one embodiment of the present invention can be used, which is preferable because the device can be bent and the secondary battery 6306 can be freely arranged. The glasses-type device 4000 includes a frame 4000a and a display unit 4000b. The secondary battery can be mounted on the temples of the curved frame 4000a, and the glasses-type device 4000 can be lightweight, well-balanced, and has a long continuous use time.

[0458] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a headset type device 4001. The headset type device 4001 has at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. When the secondary battery according to one embodiment of the present invention is used as the secondary battery in the flexible pipe 4001b or the earphone unit 4001c, the flexible pipe 4001b or the earphone unit 4001c can be bent, which is preferable because the secondary battery can be freely arranged.

[0459] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. When the secondary battery according to one embodiment of the present invention is used as the secondary battery 4002b, the device can be bent, which is preferable because the secondary battery 4002b can be freely arranged.

[0460] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. When the secondary battery according to one embodiment of the present invention is used as the secondary battery 4003b, the device can be bent, which is preferable because the secondary battery 4003b can be freely arranged.

[0461] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the belt type device 4006. The belt type device 4006 has a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted in an internal region of the belt portion 4006a. When the secondary battery according to one embodiment of the present invention is used as the secondary battery, the device can be bent, which is preferable because the secondary battery can be freely arranged.

[0462] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided on the display portion 4005a or the belt portion 4005b. When the secondary battery according to one embodiment of the present invention is used as the secondary battery, the device can be bent, which is preferable because the secondary battery can be freely arranged.

[0463] The display unit 4005a can display not only the time, but also various other information such as incoming e-mails or phone calls.

[0464] In addition, since the wristwatch type device 4005 is a wearable device that is directly wrapped around the arm, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's amount of exercise and health can be accumulated to manage the user's health.

[0465] FIG. 17(B) shows a perspective view of the wristwatch type device 4005 removed from the wrist.

[0466] 17C shows a side view of the wristwatch type device 4005. Fig. 17C shows a state in which a secondary battery 913 is built in an internal region. When a secondary battery according to one embodiment of the present invention is used as the secondary battery 913, the device can be bent, which is preferable because the secondary battery can be freely arranged.

[0467] This embodiment mode can be used in appropriate combination with other embodiment modes or examples. EXAMPLES

[0468] In this example, a secondary battery according to one embodiment of the present invention was manufactured. Sample A is a secondary battery that is not provided with a protective material, and Sample B is a secondary battery that is provided with a protective material. The manufacturing conditions of Sample A and Sample B will be described.

[0469] <Preparation of positive electrode> Lithium cobalt oxide containing magnesium, nickel, and aluminum was prepared as the positive electrode active material of Sample A and Sample B, acetylene black (AB) was prepared as the conductive material, and polyvinylidene fluoride (PVDF) was prepared as the binder. PVDF was prepared by dissolving it in N-methyl-2-pyrrolidone (NMP) as a solvent at a weight ratio of 5%. Next, the positive electrode active material:AB:PVDF=95:3:2 (weight ratio) was mixed to prepare a slurry, and the slurry was applied to an aluminum positive electrode current collector. The thickness of the positive electrode current collector was set to 20 μm. After the slurry was applied to the positive electrode current collector, the solvent was volatilized.

[0470] Thereafter, in order to increase the density of the positive electrode active material layer on the positive electrode current collector, a pressing process may be performed using a roll press. The pressing process in this embodiment was performed under a linear pressure of 210 kN / m. The upper and lower rolls of the roll press were both set at 120°C.

[0471] By the above steps, a positive electrode was obtained. The amount of the positive electrode active material carried in Sample A and Sample B was 10 mg / cm. 2 More than 12mg / cm 2 The amount of the positive electrode active material carried was adjusted to the following range: 2 The area of ​​the positive electrode (hereinafter also referred to as size or dimensions) was 125 mm × 88 mm.

[0472] <Preparation of negative electrode> The negative electrode active material layer of Sample A and Sample B was prepared using artificial graphite as the negative electrode active material, VGCF-H as the conductive assistant, SBR as the binder, and CMC as the thickener. The mixing ratio (by weight) of the artificial graphite, VGCF-H (manufactured by Resonac), CMC, and SBR was 96:1:1:2. These were mixed to prepare a slurry, which was then applied to a copper negative electrode current collector. The thickness of the negative electrode current collector was 18 μm. No pressing process was performed after application.

[0473] By the above steps, a negative electrode was obtained. The amount of the negative electrode active material carried in Sample A and Sample B was 7.5 mg / cm 2 More than 9.5mg / cm 2 The amount of the negative electrode active material carried was adjusted to the following range, and in this example, the amount was 8 mg / cm 2 The area of ​​the negative electrode was 128 mm × 91 mm. The area of ​​the negative electrode was larger than the area of ​​the positive electrode.

[0474] <separator> Polyimide was used for the separators of Sample A and Sample B. The area of ​​the separator was 128 mm x 94 mm. The area of ​​the separator was made larger than the area of ​​the negative electrode. The length of one side of the separator was made longer than the length of one side of the negative electrode to adjust the area of ​​the separator. The separator was processed into a bag shape to house the positive electrode.

[0475] <Electrolyte> For the electrolyte of Sample A and Sample B, a solution was prepared in which LiFSI, a lithium salt, was dissolved in an ionic liquid to a concentration of 2.15 mol / L. Specifically, an ionic liquid having EMI as a cation and FSI as an anion was used. Since it was found that the amount of electrolyte injected for the secondary battery is preferably 10 mL or more and 20 mL or less, in this embodiment, the amount of electrolyte injected was 20 mL, and the laminate cell was sealed under reduced pressure. The reduced pressure state was confirmed by the pressure in the reaction chamber being -95 kPa or less. First, the electrolyte was injected into the laminate cell, and then the pressure was reduced three times at -95 kPa using a degassing sealer that can reduce the pressure to -95 kPa. Then, the cell was sealed using a thermocompression sealer.

[0476] <Exterior body> Techbarrier HX (manufactured by Mitsubishi Chemical Corporation) was used for the exterior bodies of Sample A and Sample B. Techbarrier HX is a film substrate containing PET resin with a silica film vapor-deposited onto it, and has high gas barrier properties. The silica film has high chemical stability, so it can suppress contact with the electrolyte and side reactions with the electrolyte. Techbarrier HX has a film thickness of 12 μm and a Young's modulus of 220 MPa. Therefore, the rigidity was calculated to be 2.64 MPa mm. The daily water vapor transmission rate of Techbarrier HX was 0.08 g / m 2 (Under an environment of 40°C and 90% humidity).

[0477] <Battery stack> In Sample A and Sample B, where the positive electrodes overlap, the positive electrode current collectors were made to face each other and the positive electrodes were wrapped in a separator. Where the negative electrodes overlap in the laminate, the negative electrode current collectors were made to face each other. The negative electrodes were positioned on the outermost layer of the laminate.

[0478] <Protective material> In sample B, a film-like member having a polyimide resin similar to that of the separator was used as the protective material. After the laminate was housed in the exterior body, a sheet-like polyimide was used as the protective material, and the protective material was placed on the outside of the negative electrode current collector with tweezers.

[0479] <Appearance of laminated cell> After that, the above-mentioned electrolyte was poured into the exterior body. A form in which the exterior body can be seen is called a laminated cell, and in this specification, the laminated cell is included in the lithium-ion secondary battery. In this embodiment, since graphite was used for the negative electrode, the laminated cell can be called a full cell. Figure 18(A) shows the appearance of sample A, and Figure 18(B) shows the appearance of sample B. In neither sample A nor sample B was electrolyte leakage or major scratches confirmed. [Explanation of symbols]

[0480] 10 Secondary battery 21 First lead electrode 22 Second lead electrode 23 Exterior body 24 Arrow 40a Protective material 40b Protective material 40 Protective Materials 41 Negative electrode 42 Separator 43 Positive electrode 45 Electrolyte 48 Adhesive area 50 Positive electrode current collector 51 Cathode active material layer 53 Negative electrode current collector 54 Negative electrode active material layer

Claims

1. It comprises a positive electrode, a negative electrode, an outer casing housing the positive electrode and the negative electrode, and a protective material positioned between the outer casing and the negative electrode. The rigidity of the outer casing is less than the rigidity of the current collector of the positive electrode. The protective material has a sheet-like member impregnated with an electrolyte. Secondary battery.

2. A device comprising a positive electrode, a negative electrode, an outer casing housing the positive electrode and the negative electrode, and a protective material positioned between the outer casing and the positive electrode, The rigidity of the outer casing is less than the rigidity of the current collector of the negative electrode. The protective material has a sheet-like member impregnated with an electrolyte. Secondary battery.

3. In Claim 1 or Claim 2, The protective material has a sheet-like polyimide impregnated with the electrolyte, Secondary battery.

4. In Claim 1 or Claim 2, The protective material is fixed to the exterior body. Secondary battery.

5. In Claim 1 or Claim 2, The secondary battery has a separator, and the separator has polyimide. Secondary battery.

Images