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

Abstract

The present invention improves cycle characteristics. This positive electrode comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer includes: a positive electrode active material sintered body in which particles of a positive electrode active material are partially bonded to each other and which has pores; and a coating layer which covers at least a portion of the surface of the positive electrode active material sintered body in the pores and is formed of a conductor.

Description

Positive electrode, secondary battery, and method for manufacturing the positive electrode

[0001] The present invention relates to a positive electrode, a secondary battery, and a method for producing a positive electrode.

[0002] Patent Document 1 discloses a positive electrode using a sintered body of a positive electrode active material.

[0003] Patent No. 6496435

[0004] However, the positive electrode disclosed in Patent Document 1 has the possibility of deteriorating cycle characteristics due to a decrease in electrical conductivity between particles of the positive electrode active material.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to improve cycle characteristics.

[0006] A positive electrode according to one aspect of the present invention includes a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector, the positive electrode active material layer including a positive electrode active material sintered body in which particles of the positive electrode active material are partially bonded to each other and which has pores, and a coating layer made of a conductor and which coats at least a portion of the positive electrode active material sintered body within the pores.

[0007] A secondary battery according to an aspect of the present invention includes the positive electrode according to an aspect of the present invention, a negative electrode, and an electrolyte.

[0008] A method for manufacturing a positive electrode according to one aspect of the present invention includes the steps of: firing a material containing a positive electrode active material to produce a positive electrode active material sintered body; and forming a coating layer made of a conductor on the positive electrode active material sintered body by atomic layer deposition.

[0009] According to the present invention, cycle characteristics can be improved.

[0010] FIG. 1 is a cross-sectional view showing an example of a secondary battery according to an embodiment. FIG. 2 is an enlarged cross-sectional view showing a portion of a cross section of the electrode body according to FIG. 1. FIG. 3 is a schematic cross-sectional view showing a positive electrode according to an embodiment. FIG. 4 is a cutaway view showing a different example of a secondary battery according to an embodiment. FIG. 5 is a schematic cross-sectional view taken along line V-V in FIG. 4. FIG. 6 is a schematic cross-sectional view showing a positive electrode according to a comparative example.

[0011] Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited to this embodiment.

[0012] (Secondary Battery) Fig. 1 is a cross-sectional view showing an example of a secondary battery according to an embodiment. The secondary battery 1 shown in Fig. 1 is a laminated lithium-ion secondary battery. As shown in Fig. 1, the secondary battery 1 includes a battery element 20, an exterior member 30, and an adhesive 32.

[0013] The battery element 20 is provided inside an exterior member 30. As shown in FIG. 1 , the battery element 20 includes an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal drawn from a positive electrode 210 (described later) to the outside of the exterior member 30. That is, the positive electrode lead 21 is a terminal that serves as a positive electrode of the secondary battery 1. In FIG. 1 , the positive electrode lead 21 is provided on an end surface of the electrode body 200. The negative electrode lead 22 is a terminal drawn from the inside of a negative electrode 220 (described later) to the outside of the exterior member 30. That is, the negative electrode lead 22 is a terminal that serves as a negative electrode of the secondary battery 1. In FIG. 1 , the negative electrode lead 22 is provided on an end surface of the electrode body 200. Details of the electrode body 200 will be described later.

[0014] The exterior member 30 is a case in which the battery element 20 is housed. The exterior member 30 includes two exterior sheets 30a and 30b. The exterior sheets 30a and 30b each include an insulating layer, a metal layer, and an outermost layer. In the example of FIG. 1 , the exterior sheet 30a has a recess 31. As a result, the battery element 20 is housed in the exterior member 30 by housing the battery element 20 in the recess 31 and bonding the peripheral edges of the exterior sheets 30a and 30b.

[0015] The exterior sheets 30a, 30b are constructed by laminating an insulating layer, a metal layer, and an outermost layer in this order from the inside, i.e., the side where the battery element 20 is provided, and then bonding them together by lamination or other processing. The insulating layers of the exterior sheets 30a, 30b are made of a resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a polyolefin resin containing ethylene or propylene as a monomer. This allows the exterior sheets 30a, 30b to reduce the moisture permeability of the secondary battery 1 and improve its airtightness. The metal layers of the exterior sheets 30a, 30b are made of a metal plate or foil material such as aluminum, stainless steel, nickel, or iron. The outermost layer may be made of any material, but is preferably made of the same resin as the insulating layer or a material with high resistance to tearing, punctures, etc., such as nylon.

[0016] The adhesive 32 is a member for making the exterior member 30 airtight. The adhesive 32 is provided between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the adhesive 32 preferably has adhesion to the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metal material, the adhesive 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. This allows the adhesive 32 to seal the gap between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the interior of the exterior member 30 airtight.

[0017] Fig. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode assembly in Fig. 1. More specifically, Fig. 2 is a cross-sectional view showing a portion of one layer of a positive electrode 210 and one layer of a negative electrode 220 of the electrode assembly 200. As shown in Fig. 2, the electrode assembly 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode assembly 200 has a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 interposed therebetween. The positive electrode 210 and the negative electrode 220 included in the electrode assembly 200 are layered members for the charge / discharge reaction of the secondary battery according to the embodiment.

[0018] Fig. 3 is a schematic cross-sectional view showing a positive electrode according to an embodiment. As shown in Fig. 2 and Fig. 3, the positive electrode 210 includes a positive electrode current collector 211 and a positive electrode active material layer 212. In the example of Fig. 2, in the positive electrode 210, the positive electrode current collector 211 is stacked between the positive electrode active material layers 212.

[0019] The positive electrode current collector 211 is a conductor, and may be, for example, aluminum foil. In the example of Fig. 1, the positive electrode current collector 211 has a rectangular shape in plan view in the thickness direction, with protrusions on the positive electrode lead 21 side. The protrusions of the positive electrode current collector 211 are connected to the positive electrode lead 21.

[0020] The positive electrode active material layer 212 is a layer containing a positive electrode active material. As shown in Fig. 3, the positive electrode active material layer 212 includes a positive electrode active material sintered body 213 containing a positive electrode active material and a coating layer 214 made of a conductor.

[0021] The positive electrode active material sintered body 213 is a sintered body. In the present disclosure, being a sintered body refers to having a structure in which a plurality of particles are partially bonded. In the embodiment, in the positive electrode active material sintered body 213, particles of the positive electrode active material are partially bonded to each other. This can improve the rate characteristics of the secondary battery 1. Whether the positive electrode active material sintered body 213 is a sintered body, i.e., whether it has a structure in which a plurality of particles are partially bonded, can be determined by observing a cross section of the positive electrode active material layer 212 using TEM (Transmission Electron Microscope)-EDX (Energy Dispersive X-ray Spectroscopy). More specifically, if there are at least two partially bonded particles of the positive electrode active material in a TEM observation image of a cross section of the positive electrode active material sintered body 213, it can be said that the positive electrode active material sintered body 213 is a sintered body of the positive electrode active material, i.e., has a structure in which a plurality of particles of the positive electrode active material are partially bonded. In the example of FIG. 3 , some particles of the positive electrode active material are not bonded to other particles of the positive electrode active material, because the particles of the positive electrode active material are bonded to other particles of the positive electrode active material at positions other than the cross section shown in FIG. 3 .

[0022] The positive electrode active material contained in the positive electrode active material sintered body 213 is preferably a lithium composite oxide containing at least one element selected from Ni, Co, Al, and Mn. This increases the capacity of the secondary battery 1. The lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing composite oxide has, for example, a layered rock salt type or a spinel type crystal structure. Specific examples of the lithium-containing composite oxide include LiNiO ２ , LiCoO ２ , LiCo ０．９８ Al ０．０１ Mg ０．０１ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２ , Li １．２ Mn ０．５２ Co ０．１７５ Ni ０．１ O ２ , Li １．１５ (Mn ０．６５ Ni ０．２２ Co ０．１３ ) O ２ , LiMn ２ O ４ etc. The positive electrode active material preferably contains Ni, and more preferably is a so-called NCA-based positive electrode active material containing Co, Ni, and Mn. This allows for increased capacity while reducing costs. The composition of the positive electrode active material can be identified by quantitative analysis using EDX mapping of a cross section of the positive electrode active material layer 212 using TEM-EDX. Note that the positive electrode active material sintered body 213 is not limited to being made of a positive electrode active material, and may further contain, for example, a plasticizer, a binder, a conductive agent, and a dispersant.

[0023] The positive electrode active material sintered body 213 has pores 213a. In the present disclosure, the pores of the positive electrode active material sintered body 213 refer to spaces surrounded by particles of the positive electrode active material of the positive electrode active material sintered body 213. The presence or absence of pores in the positive electrode active material sintered body 213 can be determined by observing a cross section of the positive electrode active material layer 212 with a TEM. If a region surrounded by particles of the positive electrode active material of the positive electrode active material sintered body 213 is present in a TEM image of the cross section of the positive electrode active material layer 212, it can be said that the positive electrode active material sintered body 213 has pores 213a.

[0024] The average pore diameter of the positive electrode active material sintered body 213 is preferably 10 μm or less, which allows for a high energy density. The average pore diameter of the positive electrode active material sintered body 213 can be measured with a mercury porosimeter for the positive electrode active material layer.

[0025] The coating layer 214 is a layer that coats at least a portion of the surface of the positive electrode active material sintered body 213. The coating layer 214 is made of a conductor. In the present disclosure, being made of a conductor means that the electrical resistivity at room temperature (20° C.) is 1.0×10 －６ This means that the positive electrode active material is made of a material with a resistivity of Ω·m or less. This improves the electronic conductivity between the positive electrode active material particles, thereby preventing the deterioration of the crystal structure of the positive electrode active material and improving the cycle characteristics.

[0026] The coating layer 214 is made of Ru, RuO ２ , Pt, Pd, Rh, and TiN, which allows the coating layer 214 to be formed satisfactorily by atomic layer deposition (ALD), which will be described later, and also increases the electrical conductivity of the coating layer 214, thereby improving the electronic conductivity between the positive electrode active material particles.

[0027] The presence and composition of the coating layer 214 can be measured by SEM (Scanning Electron Microscope)-EDX on a cross section of the positive electrode active material layer 212. First, the area containing the positive electrode active material layer 212 is identified using the SEM, and EDX mapping of the positive electrode active material layer 212 for that area is obtained. Here, EDX mapping is performed on elements that are not contained in the positive electrode active material sintered body 213 but are contained in the coating layer 214, such as Ru, Pt, Pd, Rh, and Ti. If the elements contained in the coating layer 214 are detected at levels above the detection limit in the obtained EDX mapping, it can be said that the coating layer 214 is present. Furthermore, EDX mapping of other elements, such as O and N, is performed on the same area, and the composition of the coating layer 214 can be determined by comparing this with the area occupied by the coating layer 214.

[0028] 3, coating layer 214 covers at least a portion of the surface of positive electrode active material sintered body 213 within pores 213a. This means that the surfaces of a larger number of positive electrode active material particles are coated with coating layer 214, thereby further improving cycle characteristics.

[0029] 3 , the coating layer 214 is preferably in contact with the positive electrode current collector 211. This electrically connects the surfaces of the positive electrode active material particles and the surface 211 a of the positive electrode current collector 211 on the side of the positive electrode active material layer 212 by the coating layer 214, thereby improving the electronic conductivity between the positive electrode active material particles and the positive electrode current collector 211, suppressing deterioration of the crystalline structure of the positive electrode active material, and improving the cycle characteristics.

[0030] 3 , the coating layer 214 preferably exists continuously from one principal surface to the other principal surface of the positive electrode active material layer 212. In the embodiment, the one principal surface of the positive electrode active material layer 212 corresponds to the surface of the positive electrode active material layer 212 facing the positive electrode current collector 211, and the other principal surface of the positive electrode active material layer 212 corresponds to the surface of the positive electrode active material layer 212 opposite to the positive electrode current collector 211. This means that a conductive path exists that passes through the coating layer 214 from the surface 211 a on the positive electrode active material layer 212 side to the surface of the positive electrode 210 on the negative electrode 220 side. This can improve the electronic conductivity between the positive electrode active material particles and the positive electrode current collector 211, suppress deterioration of the crystalline structure of the positive electrode active material, and improve the cycle characteristics.

[0031] The distribution of the coating layer 214 can be measured by SEM-EDX on a cross section of the positive electrode active material layer 212. The measurement method will be described in detail below. First, an area extending from one main surface of the positive electrode active material layer 212 to the other main surface is identified using an SEM, and EDX mapping of the positive electrode active material layer 212 for that area is obtained. Here, the EDX mapping is performed on elements that are not contained in the positive electrode active material sintered body 213 but are contained in the coating layer 214, such as Ru, Pt, Pd, Rh, and Ti. If the elements contained in the coating layer 214 are detected at levels above the detection limit from one main surface to the other main surface in the obtained EDX mapping, it can be said that "the coating layer 214 is present from one main surface to the other main surface."

[0032] The coating layer 214 preferably covers 40% or more by area of ​​the surface of the positive electrode active material sintered body 213, and more preferably covers 50% or more by area. That is, the area of ​​the coating layer 214 relative to the surface area of ​​the positive electrode active material sintered body 213 is preferably 40% or more, and more preferably 50% or more. Here, the surface of the positive electrode active material sintered body 213 refers to the surface of the positive electrode active material particles of the positive electrode active material sintered body 213 that is not bonded to other positive electrode active material particles, and includes the surface of the positive electrode active material particles of the positive electrode active material sintered body 213 located within the pores 213a. This provides sufficient electrical conductivity to the surface of the positive electrode active material sintered body 213, thereby improving cycle characteristics. In the following description, the area of ​​the coating layer 214 relative to the surface area of ​​the positive electrode active material sintered body 213 may be referred to as the coverage rate of the coating layer 214.

[0033] The coverage of the coating layer 214 can be measured by SEM-EDX on a cross section of the positive electrode active material layer 212. The measurement method is described in detail below. First, a range extending from one principal surface of the positive electrode active material layer 212 to the other principal surface is identified using an SEM, and EDX mapping of the positive electrode active material layer 212 for that range is obtained. As with the method described above, EDX mapping is performed on elements that are not contained in the positive electrode active material sintered body 213 but are contained in the coating layer 214, such as Ru, Pt, Pd, Rh, and Ti. In the obtained EDX mapping, the presence of the coating layer 214 can be determined by detecting elements contained in the coating layer 214 at levels above the detection limit. The distribution of the coating layer 214 can be identified using the above method, and the coverage can be calculated by dividing the square of the contour length of the surface of the coating layer 214 facing the positive electrode active material sintered body 213 in that region by the square of the contour length of the surface of the positive electrode active material sintered body 213.

[0034] The thickness of the coating layer 214 is preferably 20 nm or less. By setting the thickness within this range, it is possible to prevent the coating layer 214 from interfering with the desorption and insertion of lithium ions into the positive electrode active material layer 212, thereby improving output performance. The thickness of the coating layer 214 is more preferably 0.4 nm or more. By setting the thickness within this range, sufficient electrical conductivity can be imparted to the surface of the positive electrode active material sintered body 213, thereby improving the cycle retention rate.

[0035] The thickness of the coating layer 214 can be measured by TEM-EDX on a cross section of the positive electrode active material layer 212. The measurement method is described in detail below. First, a range extending from one principal surface of the positive electrode active material layer 212 to the other principal surface is identified using a TEM, and EDX mapping of the positive electrode active material layer 212 for that range is obtained. As with the method described above, EDX mapping is performed on elements that are not contained in the positive electrode active material sintered body 213 but are contained in the coating layer 214, such as Ru, Pt, Pd, Rh, and Ti. In the obtained EDX mapping, the coating layer 214 can be said to be present at locations where elements contained in the coating layer 214 are detected at levels above the detection limit. The distribution of the coating layer 214 can be identified using the above method, and the arithmetic average of the thicknesses of the coating layer 214 in that region can be calculated as the thickness of the coating layer 214.

[0036] The negative electrode 220 includes a negative electrode current collector 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode current collector 221 is laminated between the negative electrode active material layers 222.

[0037] The negative electrode current collector 221 is a conductor, and for example, copper foil or the like can be used. In the example of Fig. 1 , the shape of the negative electrode current collector 221 is a rectangular sheet having protrusions on the negative electrode lead 22 side when viewed in a plan view in the thickness direction. The protrusions of the negative electrode current collector 221 are connected to the negative electrode lead 22.

[0038] The negative electrode active material layer 222 is a layer containing a negative electrode active material. The negative electrode active material layer 222 is not limited to being made of only a negative electrode active material, and may contain, for example, a conductive agent and a binder.

[0039] The negative electrode active material refers to a reducing agent that can absorb and desorb charge carriers of the secondary battery 1 through charge and discharge reactions, such as a carbon material, a metal, a semimetal, a silicon alloy or compound, or a tin (Sn) alloy or compound.

[0040] The negative electrode active material containing silicon includes, for example, elemental silicon, silicon alloys, and silicon compounds. Examples of silicon alloys that can be used as the negative electrode active material include those containing at least one element selected from the group consisting of tin (Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr) as the second constituent element other than silicon. Examples of silicon compounds that can be used as the negative electrode active material include silicon oxide (SiO ｘ Examples of the negative electrode active material include those containing oxygen (O) or carbon (C), such as silicon carbide (SiC), and may contain the second constituent element described above in addition to silicon. The negative electrode active material may also be doped with Li. ｘ In this case, it is preferable that Li is pre-doped by doping with Li in the negative electrode production process. ｘ The negative electrode active material may be a composite of Si and other materials such as carbon, or a composite of a Si alloy and other materials such as carbon. In this case, the irreversible capacity can be reduced. In addition, it is preferable that the particle surfaces of the negative electrode active material are partially or entirely coated with carbon. This can improve the electronic conductivity of the particle surfaces of the negative electrode active material.

[0041] Examples of carbon materials that can be used as the negative electrode active material include MCMB (MesoCarbon MicroBeads), artificial graphite, natural graphite, non-graphitizable carbon, and graphitizable carbon. More specifically, examples of materials that can be used as the negative electrode active material include pyrolytic carbons, cokes, glassy carbon fiber, fired organic polymer compounds, activated carbon, and carbon blacks. Examples of cokes include pitch coke, needle coke, and petroleum coke. Here, fired organic polymer compounds are produced by firing a polymer compound such as a phenolic resin or a furan resin at an appropriate temperature and carbonizing it.

[0042] The negative electrode active material is not limited to those listed above, and may contain other negative electrode active materials, such as metals, semimetal alloys or compounds, and tin (Sn) alloys or compounds, which can absorb and release lithium. Examples of metals and semimetals that can be used as negative electrode active materials include tin (Sn), lead (Pb), aluminum (Al), indium (In), zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, germanium, tin, and lead are preferred. Tin is more preferred because it has a high ability to absorb and release lithium and can achieve a high energy density.

[0043] Examples of tin alloys that can be used as the negative electrode active material include those containing at least one of nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin. Examples of tin compounds that can be used as the negative electrode active material include those containing oxygen or carbon, and may contain the above-mentioned second constituent element in addition to tin.

[0044] The negative electrode active material layer 222 is not limited to containing only a negative electrode active material and a binder. For example, the negative electrode active material layer 222 may further contain a negative electrode conductive agent. The negative electrode conductive agent includes at least one of a carbon material, a metal material, and a conductive polymer compound. Specific examples of carbon materials used as the negative electrode conductive agent include particulate carbon materials such as carbon black, acetylene black, and ketjen black, and fibrous carbon materials such as carbon nanotubes. Examples of carbon nanotubes include single-wall carbon nanotubes (SWCNTs). This improves the electronic conductivity of the particle surfaces of the negative electrode active material. The mass ratio of the negative electrode conductive agent to the negative electrode active material layer 222 is preferably 5% or less, more preferably 2% or less. This improves the paintability of the negative electrode slurry.

[0045] The separator 230 is a film that insulates the positive electrode 210 from the negative electrode 220. The separator 230 is provided between the main surface of the positive electrode 210 and the main surface of the negative electrode 220 so that the positive electrode 210 and the negative electrode 220 do not come into direct contact with each other. In the example of Fig. 1 , the shape of the separator 230 is a rectangular sheet when viewed in plan in the thickness direction.

[0046] The separator 230 is preferably made of a material that is electrically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and is insulating. The separator 230 can be made of, for example, a polymer nonwoven fabric, a porous film, or a layer of glass or ceramic fibers. The separator 230 is more preferably made of a porous polyolefin film. This improves battery safety by preventing short circuits and providing a shutdown effect.

[0047] The electrolyte solution is impregnated into the separator 230. In the example of Fig. 1, the electrolyte solution fills the space inside the exterior member 30. The electrolyte solution is a non-aqueous electrolyte solution containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0048] The electrolyte salt is, for example, lithium perchlorate (LiClO ４ ), lithium hexafluorophosphate (LiPF ６), lithium tetrafluoroborate (LiBF ４ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO ２ CF ３ ) ２ ), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO ２ C ２ F ５ ) ２ ), lithium hexafluoroarsenate (LiAsF ６ ) and other lithium salts.

[0049] Examples of the solvent include lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

[0050] The electrolytic solution may further contain an additive such as a fluorinated carboxylic acid ester, a sulfonic acid ester, a sulfonic acid anhydride, or a carboxylic acid anhydride.

[0051] Although the battery according to the embodiment has been described above, the secondary battery according to the embodiment is not limited to that shown in Fig. 1. Other examples will be described below with reference to the drawings, but the same components as those in Figs. 1 and 2 will be denoted by reference numerals and will not be described again.

[0052] Fig. 4 is a cutaway view showing a different example of the secondary battery according to the embodiment. Fig. 5 is a schematic cross-sectional view taken along line VV in Fig. 4. The secondary battery 1A shown in Figs. 4 and 5 differs from the example shown in Fig. 1 in that the electrode body 200A has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center.

[0053] The battery element 20A is provided inside the exterior member 30. As shown in FIG. 5 , the battery element 20A includes an electrode body 200A, a positive electrode lead 21A, a negative electrode lead 22A, and a protective material 23. The positive electrode lead 21A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the positive electrode lead 21A is provided near the center of the battery element 20A. The negative electrode lead 22A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the negative electrode lead 22A is provided near the center of the battery element 20A. The protective material 23 is a member that protects the outside of the battery element 20A. The protective material 23 is provided so as to be wrapped around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

[0054] 5 , the electrode assembly 200A is a laminate for charge / discharge reactions of the secondary battery according to the embodiment. The electrode assembly 200A includes a positive electrode 210A including a positive electrode current collector 211A and a positive electrode active material layer 212A, a negative electrode 220A including a negative electrode current collector 221A and a negative electrode active material layer 222A, and a separator 230A. The electrode assembly 200A has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center, and is laminated in the following order from the outside, i.e., from the protective material 23 side: the negative electrode current collector 221A, the negative electrode active material layer 222A, the separator 230A, the positive electrode active material layer 212A, the positive electrode current collector 211A, the positive electrode active material layer 212A, the separator 230A, and the negative electrode active material layer 222A. In the electrode body 200A, no layers other than the negative electrode current collector 221A, separator 230A, and positive electrode current collector 211A are provided near the positive electrode lead 21A and the negative electrode lead 22A. With this structure, the positive electrode current collector 211A is connected to the positive electrode lead 21A, and the negative electrode current collector 221A is connected to the negative electrode lead 22A.

[0055] As described above, the positive electrode 210 according to this embodiment includes a positive electrode current collector 211 and a positive electrode active material layer 212 provided on the positive electrode current collector 211. The positive electrode active material layer 212 includes a positive electrode active material sintered body 213 in which particles of the positive electrode active material are partially bonded to each other and which has pores, and a coating layer 214 made of a conductor and which coats at least a portion of the surface of the positive electrode active material sintered body within the pores. This improves the rate characteristics while improving the electrical conductivity between the particles of the positive electrode active material due to the coating layer 214, thereby improving the cycle characteristics.

[0056] In a preferred embodiment, the positive electrode active material is a lithium composite oxide containing at least one element selected from the group consisting of Ni, Co, Al, and Mn, which can increase the capacity of the battery.

[0057] In a preferred embodiment, the coating layer 214 is made of Ru or RuO. ２ , Pt, Pd, Rh, and TiN, whereby cycle characteristics can be further improved.

[0058] In a preferred embodiment, the coating layer 214 is in contact with the positive electrode current collector 211. This can further improve the cycle characteristics.

[0059] In a desirable embodiment, the coating layer 214 is present continuously from one principal surface to the other principal surface of the positive electrode active material layer 212. This provides electrical continuity from one principal surface of the positive electrode active material layer 212 to the other principal surface through the coating layer 214, thereby improving electrical conductivity and further improving cycle characteristics.

[0060] In a desirable embodiment, the coating layer 214 covers 40 area % or more of the surface of the positive electrode active material sintered body 213. This makes it possible to impart sufficient electrical conductivity to the surface of the positive electrode active material sintered body 213, thereby further improving the cycle characteristics.

[0061] In a more desirable embodiment, the coating layer 214 covers 50 area % or more of the surface of the positive electrode active material sintered body 213. This makes it possible to impart sufficient electrical conductivity to the surface of the positive electrode active material sintered body 213, thereby further improving the cycle characteristics.

[0062] In a desirable embodiment, the thickness of the coating layer 214 is 20 nm or less. This can prevent the coating layer 214 from interfering with the desorption and insertion of lithium ions into the positive electrode active material layer 212, thereby improving output performance.

[0063] In a desirable embodiment, the average pore size of the positive electrode active material sintered body 213 is 10 μm or less, which allows the energy density to be increased.

[0064] As described above, the secondary battery 1 according to the embodiment includes the positive electrode 210 according to the embodiment, the negative electrode 220, and the electrolyte. This allows the coating layer 214 to improve the electrical conductivity between particles of the positive electrode active material, thereby improving cycle characteristics.

[0065] A method for manufacturing the positive electrode 210 according to the embodiment will be described below. Note that the method for manufacturing the positive electrode 210 described below is an example and is not limited to this. The method for manufacturing the positive electrode 210 according to the embodiment includes a step of producing a positive electrode active material sintered body 213, a step of forming a coating layer 214, and a step of stacking a positive electrode current collector 211.

[0066] In the process of producing the positive electrode active material sintered body 213, a material containing the positive electrode active material is fired to produce the positive electrode active material sintered body 213. Specifically, a positive electrode mixture paste containing particles of the positive electrode active material is applied to a resin green sheet, dried and pressed, and then the resin film is removed and the resulting sheet is fired. The firing temperature is adjusted appropriately depending on the type of positive electrode active material and can be, for example, 400°C or higher and 1000°C or lower. This allows the positive electrode active material particles to be partially bonded to each other, forming a positive electrode active material sintered body having pores. Note that the method of producing the positive electrode active material sintered body 213 is not limited to this. For example, the positive electrode active material sintered body 213 may be produced by firing a compact obtained by compressing a mixed powder of raw materials of the positive electrode active material into a plate shape.

[0067] In the step of forming the coating layer 214, the coating layer 214 made of a conductor is formed on the positive electrode active material sintered body 213 to produce the positive electrode active material layer 212. By performing the step of forming the coating layer 214 after the step of producing the positive electrode active material sintered body 213, the coating layer 214 can be formed on the positive electrode active material sintered body 213 without being burned off by firing. Here, the coating layer 214 is formed by atomic layer deposition (ALD). In the embodiment, the coating layer 214 is formed by thermal ALD using a precursor. The precursor is, for example, an organic compound containing a metal element that constitutes the coating layer 214, such as Ru, Ti, Pt, Pd, or Rh. To form the coating layer 214 containing an oxide or nitride, a cycle of introducing a precursor containing a metal element, performing ALD, and exhausting the gas, and then introducing oxygen gas or nitrogen gas into the ALD apparatus, performing ALD, and exhausting the gas, can be repeated.

[0068] FIG. 6 is a schematic cross-sectional view showing a positive electrode according to a comparative example. When the coating layer 214X is formed by sputtering in the coating layer formation process, the target atoms or molecules travel in a straight line and come into contact with the positive electrode active material sintered body 213. Therefore, as shown in FIG. 6, the target atoms or molecules cannot enter the pores 213a of the positive electrode active material sintered body 213, and therefore the coating layer 214X is not formed on the surface of the positive electrode active material sintered body 213 within the pores 213a, and the coating layer 214X is formed only on the main surface of the positive electrode active material sintered body 213. In this case, the electrical conductivity between particles of the positive electrode active material cannot be sufficiently improved, which may result in a deterioration in cycle characteristics. Similarly, when the sol-gel method is used in the process of forming the coating layer, the sol containing the coating material does not penetrate into the pores 213a of the positive electrode active material sintered body 213, so that the coating layer 214X is not formed on the surface of the positive electrode active material sintered body 213 within the pores 213a, and the coating layer 214X is formed only on the main surface of the positive electrode active material sintered body 213.

[0069] On the other hand, when the coating layer 214 is formed by ALD in the coating layer formation process, as in the positive electrode manufacturing method according to the present disclosure, the precursor containing the coating element can penetrate as a gas into the pores 213a of the positive electrode active material sintered body 213. Furthermore, in ALD, the coating layer 214 is formed due to the self-regulating nature of the precursor molecules. That is, the formation of a coating layer 214 is suppressed in areas where a coating layer 214 has already been formed, making it easier for the coating layer 214 to form in areas where a coating layer 214 has not yet been formed. This is because the bonding of subsequent precursor molecules is inhibited on the surface where precursor molecules have bonded by ALD. As a result, the coating layer 214 grows along the surface direction of the positive electrode active material sintered body 213. Therefore, in this case, as shown in FIG. 3 , the coating layer 214 is also formed on the surface of the positive electrode active material sintered body 213 within the pores 213a, including those with diameters of 10 μm or less, thereby improving the electrical conductivity between the positive electrode active material particles and improving the cycle characteristics.

[0070] In the step of laminating the positive electrode current collector 211, the positive electrode current collector 211 is laminated on the prepared positive electrode active material layer 212. By laminating the positive electrode current collector 211 after the step of preparing the positive electrode active material sintered body 213, the positive electrode 210 can be prepared without melting the positive electrode current collector 211 during firing of the positive electrode active material. Here, the lamination of the positive electrode current collector 211 is performed, for example, by laminating a metal foil used as the positive electrode current collector 211 on the positive electrode active material layer 212, but is not limited thereto, and the positive electrode current collector 211 may be laminated by a method such as sputtering, for example.

[0071] As described above, the method for manufacturing the positive electrode 210 according to the embodiment includes a step of producing the positive electrode active material sintered body 213 by firing a material containing a positive electrode active material, and a step of forming the coating layer 214 made of a conductor by atomic layer deposition on the positive electrode active material sintered body 213. This allows the coating layer 214 to be formed on the surface of the positive electrode active material sintered body 213 inside the pores 213a, thereby improving the electrical conductivity between particles of the positive electrode active material and improving the cycle characteristics.

[0072] EXAMPLES Examples will be described below, but the present invention is not limited to these examples.

[0073] Table 1 shows examples and comparative examples.

[0074]

[0075] Example 1 The positive electrode according to Example 1 was fabricated by the following method. ０．８0 Co ０．１5 Al ０．05 O ２ A positive electrode mixture was prepared by mixing 97.1% by mass of ZnO, 2.5% by mass of polyvinyl butyral (PVB) as a binder, and 0.4% by mass of dibutyl phthalate as an additive. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry, and then the prepared positive electrode mixture slurry was uniformly applied to a green sheet made of polyethylene terephthalate (PET). The resulting coating was dried at 90°C for 30 minutes and then punched and pressed into a circle with a diameter of 16.5 mm. The green sheet was then peeled off, sandwiched between zirconia setters, and fired at 670°C for 1.5 hours in an oxygen atmosphere. This resulted in a sheet-shaped positive electrode active material sintered body. A coating layer made of Ru was formed on the prepared positive electrode active material sintered body using thermal ALD under the following conditions. Here, the cycle in the ALD process refers to a cycle in which a precursor is introduced into an ALD apparatus, ALD is performed, and then the gas is exhausted. ALD apparatus: SAVANNAH S200 (manufactured by Cambridge NanoTech Inc.) Precursor: TRuST (manufactured by Tanaka Precious Metals) Film formation temperature: 210°C Number of cycles: 60 cycles Then, a carbon-coated aluminum foil (manufactured by MTI) was laminated on the positive electrode active material sintered body on which the coating layer was formed, to produce the positive electrode according to Example 1. A positive electrode lead was attached to the exposed portion of the positive electrode current collector layer of the produced positive electrode.

[0076] <<SEM-EDX Measurement>> The thickness and coverage of the coating layer were measured for a cross section of the positive electrode according to Example 1 by the following method. First, a certain region on the surface of the positive electrode active material sintered body was identified by SEM, and the thickness and coverage of the coating portion for that region were measured by the above-described method based on EDX mapping of the constituent element (Ru) of the coating portion. The thickness and coverage of the coating portion were the values ​​shown in Table 1.

[0077] <Charge / Discharge Test> A half cell was fabricated using the positive electrode according to Example 1, and a charge / discharge test was performed. The counter electrode for the half cell was metallic lithium, and the separator was a microporous polyethylene film (F20BHE, manufactured by Toray Battery Separator Film Co., Ltd.). The electrolyte for the half cell was a solvent mixture of ethyl carbonate (EC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FE) in a mass ratio of 30:60:10, and LiPF as an electrolyte salt. ６ The half cell was fabricated by laminating a separator and a counter electrode in this order on the positive electrode of Example 1, and then impregnating the separator with 14 μL of the prepared electrolyte solution.

[0078] In the charge-discharge test, the fabricated half cell was subjected to 100 charge-discharge cycles under the following conditions: Charging method: CCCV, Charging rate: 0.1 C, Charging control voltage: 4.25 V, Charging cut-off current: 0.01 C, Discharging method: CC, Discharging rate: 0.1 C, Discharging cut-off voltage: 2.0 V.

[0079] (Example 2) In Example 2, a positive electrode was prepared in the same manner as the positive electrode of Example 1, except that the composition of the positive electrode active material and the firing temperature of the positive electrode mixture were changed to those shown in Table 1, and measurements and tests were performed.

[0080] Example 3 In Example 3, a positive electrode was prepared in the same manner as the positive electrode of Example 1, and measurements and tests were carried out, except that the composition of the positive electrode active material, the baking temperature of the positive electrode mixture, the material of the coating portion, and the thickness of the coating portion were changed to those shown in Table 1. In the step of forming the coating portion of Example 3, ALD was carried out by changing the ALD cycle to one in which TRuST (made by Tanaka Precious Metals) was introduced as a first precursor and exhausted, and then oxygen generated by an ozone generator was introduced as a second precursor and exhausted.

[0081] Example 4 In Example 4, a positive electrode was prepared in the same manner as the positive electrode of Example 1, and measurements and tests were carried out, except that the composition of the positive electrode active material, the baking temperature of the positive electrode mixture, the material of the coating portion, and the thickness of the coating portion were changed to those shown in Table 1. In the step of forming the coating portion of Example 4, ALD was carried out by changing the ALD cycle to one in which a titanium-containing precursor (TDMAT, manufactured by Aldrich) was introduced as a first precursor and evacuated, and then nitrogen gas was introduced as a second precursor and evacuated.

[0082] (Example 5) In Example 5, a positive electrode was prepared in the same manner as the positive electrode of Example 1, except that the composition of the positive electrode active material and the thickness of the coating portion were changed to those shown in Table 1, and measurements and tests were performed.

[0083] Example 6 In Example 6, a positive electrode was prepared in the same manner as the positive electrode of Example 1, and measurements and tests were carried out, except that the composition of the positive electrode active material, the baking temperature of the positive electrode mixture, the material of the coating portion, and the thickness of the coating portion were changed to those shown in Table 1. In the step of forming the coating portion of Example 6, ALD was carried out by changing the ALD cycle to one in which TRuST (made by Tanaka Precious Metals) was introduced as a first precursor and exhausted, and then oxygen generated by an ozone generator was introduced as a second precursor and exhausted.

[0084] Example 7 In Example 7, a positive electrode was prepared in the same manner as the positive electrode of Example 1, except that the composition of the positive electrode active material and the thickness of the coating portion were changed to those shown in Table 1, and measurements and tests were performed.

[0085] (Example 8) In Example 8, a positive electrode was prepared in the same manner as the positive electrode of Example 1, except that the composition of the positive electrode active material and the thickness of the coating portion were changed to those shown in Table 1, and measurements and tests were performed.

[0086] Example 9 In Example 9, a positive electrode was prepared in the same manner as the positive electrode of Example 1, except that the composition of the positive electrode active material, the material of the coating portion, and the thickness of the coating portion were changed to those shown in Table 1. In the step of forming the coating portion of Example 9, the precursor used in ALD was changed to a Pt-containing precursor ((MeCp)PtMe3, manufactured by Aldrich).

[0087] (Comparative Example 1) In Comparative Example 1, a positive electrode was prepared in the same manner as the positive electrode of Example 1, except that the composition of the positive electrode active material was changed to that shown in Table 1 and a positive electrode was prepared without forming a coating portion, and measurements and tests were performed.

[0088] (Comparative Example 2) In Comparative Example 2, a positive electrode was produced in the same manner as the positive electrode of Example 1, except that the composition of the positive electrode active material and the baking temperature of the positive electrode mixture were changed to those shown in Table 1 and the coating portion was formed by sputtering, and measurements and tests were carried out. In the step of forming the coating portion of Comparative Example 2, sputtering was carried out under the following conditions: Target: Ru (manufactured by Kojundo Chemical Laboratory Co., Ltd.) Film formation atmosphere: Ar Power supply type: DC (direct current)

[0089] Comparative Example 3 In Comparative Example 3, the composition of the positive electrode active material and the firing temperature of the positive electrode mixture were changed to those shown in Table 1, and a coating portion made of TiN was formed by a sol-gel method. Except for this, a positive electrode was fabricated in the same manner as the positive electrode of Example 1, and measurements and tests were carried out. In the step of forming the coating portion of Comparative Example 2, TiN having a particle size of 20 nm, ammonia water (NH ４ A colloidal liquid obtained by mixing hydroxypropyltrimonium chloride (OH) and ethanol was applied to the surface of the sintered body of the positive electrode active material, and the coated portion was formed by drying at 160° C. for 2 hours.

[0090] As shown in Table 1, in Examples 1 to 9 in which a covering portion was formed, the cycle retention rate was improved compared to Comparative Example 1 in which a covering portion was not formed. This shows that the cycle characteristics can be improved by forming a covering portion.

[0091] As shown in Table 1, in Examples 1 to 9 in which the coverage was 40 area % or more, the cycle characteristics were improved compared to Comparative Examples 2 and 3 in which the coverage was less than 40 area %. This shows that the cycle characteristics can be improved by setting the coverage to 40 area % or more.

[0092] As shown in Table 1, in Examples 1 to 8 in which the coverage was 50 area % or more, the cycle characteristics were improved compared to Example 9 and Comparative Examples 2 and 3 in which the coverage was less than 50 area %. This shows that the cycle characteristics can be improved by setting the coverage to 50 area % or more.

[0093] As shown in Table 1, Examples 1 to 9, in which the coating portion was formed by ALD, had improved cycle retention rates compared to Comparative Example 1, in which no coating portion was formed, and Comparative Examples 2 and 3, in which the coating portion was formed by a method other than ALD. This indicates that forming the coating portion by ALD can improve cycle characteristics. Here, Examples 1 to 9, in which the coating portion was formed by ALD, had improved coverage rates compared to Comparative Examples 2 and 3, in which the coating portion was formed by a method other than ALD. This result is thought to be due to the fact that, when the coating portion was formed by a method other than ALD, the coating portion was formed only on the main surface of the positive electrode active material sintered body opposite the positive electrode current collector, and no coating portion was formed on the surface of the positive electrode active material sintered body within the pores. In contrast, when the coating portion was formed by ALD, the coating portion was also formed on the surface of the positive electrode active material sintered body within the pores, thereby improving electrical conductivity between the positive electrode active material particles.

[0094] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit thereof, and equivalents thereof are also included in the present invention.

[0095] The present invention may also be related to the following aspects.

[0096] (1) A positive electrode comprising: a positive electrode current collector; and a positive electrode active material layer provided on the positive electrode current collector, wherein the positive electrode active material layer includes: a positive electrode active material sintered body in which particles of the positive electrode active material are partially bonded to each other and which has pores; and a coating layer made of a conductor and coating at least a portion of the surface of the positive electrode active material sintered body within the pores. (2) The positive electrode according to (1), wherein the positive electrode active material is a lithium composite oxide containing at least one selected from Ni, Co, Al, and Mn. (3) The coating layer is made of a material selected from the group consisting of Ru, RuO ２ , Pt, Pd, Rh, and TiN. (4) The positive electrode according to any one of (1) to (3), wherein the coating layer is in contact with the positive electrode current collector. (5) The positive electrode according to any one of (1) to (4), wherein the coating layer is present continuously from one main surface of the positive electrode active material layer to the other main surface. (6) The positive electrode according to any one of (1) to (5), wherein the coating layer covers 40 area% or more of the surface of the positive electrode active material sintered body. (7) The positive electrode according to (6), wherein the coating layer covers 50 area% or more of the surface of the positive electrode active material sintered body. (8) The positive electrode according to any one of (1) to (7), wherein the coating layer has a thickness of 20 nm or less. (9) The positive electrode according to any one of (1) to (8), wherein the average pore diameter of the positive electrode active material sintered body is 10 μm or less. (10) A secondary battery comprising the positive electrode according to any one of (1) to (9), a negative electrode, and an electrolyte. (11) A method for producing a positive electrode, comprising: a step of firing a material containing a positive electrode active material to produce a positive electrode active material sintered body; and a step of forming a coating layer made of a conductor on the positive electrode active material sintered body by atomic layer deposition.

[0097] REFERENCE SIGNS LIST 1, 1A Secondary battery 20, 20A Battery element 21, 21A Positive electrode lead 22, 22A Negative electrode lead 23 Protective material 30 Exterior member 30a, 30b Exterior sheet 31 Recess 32 Adhesive material 200, 200A Electrode body 210, 210A Positive electrode 211, 211A Positive electrode current collector 212, 212A Positive electrode active material layer 213 Positive electrode active material sintered body 214 Coating layer 220, 220A Negative electrode 221, 221A Negative electrode current collector 222, 222A Negative electrode active material layer 230, 230A Separator

Claims

1. A positive electrode comprising: a positive electrode current collector; and a positive electrode active material layer provided on the positive electrode current collector, wherein the positive electrode active material layer includes: a positive electrode active material sintered body in which particles of the positive electrode active material are partially bonded to each other and which has pores; and a coating layer made of a conductor and which coats at least a portion of the surface of the positive electrode active material sintered body within the pores.

2. The positive electrode according to claim 1, wherein the positive electrode active material is a lithium composite oxide containing at least one element selected from the group consisting of Ni, Co, Al, and Mn.

3. The coating layer is made of Ru, RuO ２ 3. The positive electrode according to claim 1, further comprising at least one selected from the group consisting of Pt, Pd, Rh and TiN.

4. The positive electrode according to any one of claims 1 to 3, wherein the coating layer is in contact with the positive electrode current collector.

5. The positive electrode according to any one of claims 1 to 4, wherein the coating layer is present continuously from one main surface of the positive electrode active material layer to the other main surface.

6. The positive electrode according to any one of claims 1 to 5, wherein the coating layer covers 40 area % or more of the surface of the positive electrode active material sintered body.

7. The positive electrode according to claim 6, wherein the coating layer covers 50% or more by area of ​​the surface of the positive electrode active material sintered body.

8. The positive electrode according to any one of claims 1 to 7, wherein the coating layer has a thickness of 20 nm or less.

9. The positive electrode according to any one of claims 1 to 8, wherein the average pore size of the positive electrode active material sintered body is 10 μm or less.

10. A secondary battery comprising the positive electrode according to any one of claims 1 to 9, a negative electrode, and an electrolyte.

11. A method for manufacturing a positive electrode, comprising: a step of firing a material containing a positive electrode active material to prepare a positive electrode active material sintered body; and a step of forming a coating layer made of a conductor on the positive electrode active material sintered body by atomic layer deposition.

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