Positive electrode for secondary battery, method for producing same, and secondary battery

Abstract

This positive electrode (13) for a secondary battery is provided with a positive electrode current collector (11) and a positive electrode active material layer (12) supported on the positive electrode current collector (11). The positive electrode active material layer (12) contains a positive electrode active material and a polyvinyl alcohol modified with a phosphorus compound. A method for producing a positive electrode (13) for a secondary battery includes preparing a polymer solution containing a polyvinyl alcohol, a phosphorus compound, and a solvent, preparing a positive electrode slurry containing the polymer solution and a positive electrode active material, and applying the positive electrode slurry on a positive electrode current collector (11) to form a positive electrode active material layer.

Description

Technical Field

[0001] The present disclosure relates to a positive electrode for a secondary battery, a method for manufacturing the same, and a secondary battery. Background Art

[0002] The upper limit of the operating voltage of lithium secondary batteries today is around 4.3 V. A high operating voltage is beneficial to improving the output and energy density of lithium secondary batteries. However, when the operating voltage rises, lithium secondary batteries will deteriorate prematurely due to side reactions such as the decomposition reaction of the electrolyte.

[0003] As a technology for coping with the decomposition reaction of the electrolyte, for example, Patent Documents 1 and 2 are known. Patent Document 1 describes a coated positive electrode active material having a coating portion, the coating portion containing at least one selected from a specific phosphonic acid and a specific phosphite triester. Patent Document 2 describes adding a specific phosphorus compound and a specific phosphoric acid diester salt to a non-aqueous electrolyte.

[0004] Prior Art Literature

[0005] Patent Document 1: International Publication No. 2017 / 126276

[0006] Patent Document 2: International Publication No. 2016 / 199823 Summary of the invention

[0007] Problems to be solved by the invention

[0008] When a secondary battery is stored in a high temperature environment, the side reaction in the positive electrode of the secondary battery becomes obvious. In other words, suppressing the reduction in discharge capacity caused by storing the secondary battery at high temperature is also one of the important issues. The purpose of the present disclosure is to provide a positive electrode for a secondary battery, which can suppress the side reaction in the positive electrode when the secondary battery is stored at high temperature.

[0009] Means for solving problems

[0010] The present disclosure provides a positive electrode for a secondary battery, comprising a positive electrode collector and a positive electrode active material layer supported on the positive electrode collector, wherein the positive electrode active material layer contains a positive electrode active material and polyvinyl alcohol modified with a phosphorus compound.

[0011] Effects of the Invention

[0012] According to the present disclosure, it is possible to provide a secondary battery positive electrode capable of suppressing a side reaction in the positive electrode when the secondary battery is stored at a high temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a cross-sectional view schematically showing the structure of a secondary battery positive electrode according to the first embodiment.

[0014] Figure 2A This is a diagram showing a reaction formula between PVA and a specific phosphorus compound represented by chemical formula (3).

[0015] Figure 2B This is a diagram showing a reaction formula between PVA and a specific phosphorus compound represented by chemical formula (1).

[0016] Figure 3A This is a process diagram showing a method for producing a positive electrode for a secondary battery.

[0017] Figure 3B This is a process diagram showing another method for producing a positive electrode for a secondary battery.

[0018] Figure 4 This is a cross-sectional view schematically showing the structure of a lithium secondary battery according to the second embodiment.

[0019] Figure 5A This is an optical photograph of the Li metal foil surface of the lithium secondary battery of Example 1 and the negative electrode side surface of the separator.

[0020] Figure 5B This is an optical photograph of the Li metal foil surface of the lithium secondary battery of Comparative Example 1 and the negative electrode side surface of the separator. DETAILED DESCRIPTION

[0021] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0022] (Implementation Method 1)

[0023] Figure 1 It is a cross-sectional view showing the general structure of the positive electrode 13 for secondary batteries involved in Embodiment 1. The positive electrode 13 for secondary batteries includes a positive electrode collector 11 and a positive electrode active material layer 12. The positive electrode active material layer 12 is supported on the positive electrode collector 11. The positive electrode active material layer 12 contains a positive electrode active material and polyvinyl alcohol modified with a phosphorus compound. In this specification, polyvinyl alcohol modified with a phosphorus compound is sometimes referred to as "modified PVA". The positive electrode 13 for secondary batteries is sometimes referred to as "positive electrode 13". "Modifying polyvinyl alcohol with a phosphorus compound" means introducing a phosphorus compound into the molecular chain of polyvinyl alcohol.

[0024] The modified PVA acts as a binder in the positive electrode active material layer 12. Since the structure derived from the phosphorus compound exerts an oxidation resistance effect, the modified PVA has a higher effect of suppressing side reactions in the positive electrode 13 than PVA. As the suppressed side reactions, decomposition of the electrolyte, dissolution of metal elements from the positive electrode active material, etc. can be cited. When the side reactions are suppressed, the reduction in discharge capacity is suppressed.

[0025] The phosphorus compound may be a compound containing phosphorus and oxygen. Specifically, the phosphorus compound includes at least one selected from the group consisting of a compound represented by chemical formula (1), a compound represented by chemical formula (2), and a compound represented by chemical formula (3). The compound represented by chemical formula (1) and the compound represented by chemical formula (2) are organic phosphorus compounds having a carbon-phosphorus bond. These phosphorus compounds can easily modify PVA.

[0026]

[0027] In the chemical formula (1), R 2 Represents a direct bond with a carbon atom contained in the main chain of the modified PVA. 1 and R 3 R independently represents an alkyl group having 1 to 17 carbon atoms. 1 and R 3 Each of them may be independently an alkyl group having 5 to 17 carbon atoms.

[0028]

[0029] In the chemical formula (2), R 4 and R 5 Each of the groups independently represents a hydrogen atom, a methyl group, an ethyl group, an alkylsilyl group having 1 to 6 carbon atoms, or a direct bond to a carbon atom contained in the main chain of the modified PVA. 4 and R 5 At least one of represents a direct bond with a carbon atom contained in the main chain of the modified PVA. 4 and R 5 When both contain the above-mentioned bonds, these bonds are bonds with the main chain of the same PVA molecule, or bonds with the main chains of different PVA molecules. 6 represents an alkyl group having 1 to 17 carbon atoms. 6 The alkyl group may have 5 to 17 carbon atoms. The alkylsilyl group may have 1 to 3 carbon atoms.

[0030]

[0031] In the chemical formula (3), R 7 , R 8 and R 9 Each of the groups independently represents a hydrogen atom, a methyl group, an ethyl group, an alkylsilyl group having 1 to 6 carbon atoms, or a direct bond to a carbon atom contained in the main chain of the modified PVA. 7 , R 8 and R 9 At least one of represents a direct bond with a carbon atom contained in the main chain of the modified PVA.7 , R 8 and R 9 When two or more of the above-mentioned bonds are present, these bonds are bonds to the main chain of the same PVA molecule or bonds to the main chains of different PVA molecules. The number of carbon atoms in the alkylsilyl group may be 1 to 3.

[0032] Figure 2A : is a diagram showing the reaction formula of PVA and a specific phosphorus compound represented by chemical formula (3). Figure 2A In the reaction formula, the phosphorus compound is tris(trimethylsilyl)phosphonic acid (TMSP). The trimethylsilyl group reacts with the hydroxyl group of PVA to form trimethylsilanol. The part of the phosphorus compound other than the trimethylsilyl group is bonded to the main chain of PVA as a residue. Figure 2A The remaining two trimethylsilyl groups can also form bonds with the same or other PVA molecules.

[0033] Figure 2B : is a diagram showing a reaction formula of PVA and a specific phosphorus compound represented by chemical formula (1). Figure 2B In the reaction formula, the phosphorus compound has a group that can react with the hydroxyl group of PVA. The group that can react with the hydroxyl group of PVA is an ethyl group. The group that can react with the hydroxyl group of PVA can also be a hydroxyl group, an alkylsilyl group, etc. The ethyl group of the phosphorus compound and the hydroxyl group of PVA are separated to generate an ethanol byproduct. The remaining part of the phosphorus compound is bonded to the main chain of PVA.

[0034] according to Figure 2A and Figure 2B As can be seen from the reaction formula, the phosphorus compound represented by the chemical formula (1), the phosphorus compound represented by the chemical formula (2), and the phosphorus compound represented by the chemical formula (3) can be a residue bonded to the main chain of PVA.

[0035] The phosphorus compound represented by the chemical formula (2) and the phosphorus compound represented by the chemical formula (3) can have a plurality of bonding sites and thus can impart a crosslinked structure to the modified PVA.

[0036] In the positive electrode active material layer 12, the modified PVA can be attached to the surface of the particles of the positive electrode active material. The modified PVA can cover at least a portion of the surface of the positive electrode active material particles. According to such a structure, the direct contact between the positive electrode active material and the electrolyte is hindered, so it is easy to exert the oxidation resistance effect of the modified PVA. In the case where the positive electrode 13 is used in a solid battery, the direct contact between the positive electrode active material and the solid electrolyte will be hindered. For example, the modified PVA can be locally present on the surface of the positive electrode active material particles. However, the modified PVA can also be uniformly dispersed in the positive electrode active material layer 12.

[0037] The ratio of the phosphorus compound to the hydroxyl group contained in the modified PVA is not particularly limited. As long as it is modified by the phosphorus compound, the desired effect can be exerted. In one example, the ratio of the phosphorus compound to the hydroxyl group contained in the modified PVA is 10% or more based on the amount of substance. In this case, the oxidation resistance effect based on the phosphorus compound is more fully exerted, so the amount of modified PVA used in the positive electrode 13 can be suppressed. The upper limit of the ratio is not particularly limited. The upper limit of the ratio is, for example, 95%. The ratio can be investigated by NMR measurement.

[0038] For example, when the phosphorus compound is represented by chemical formula (1), chemical formula (2) or chemical formula (3), the ratio (M2 / M1) of the amount M2 of P atoms to the amount M1 of hydroxyl groups remaining in the modified PVA is 10% or more.

[0039] In the positive electrode 13, the positive electrode active material is not particularly limited. As the positive electrode active material, lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanion materials, fluorinated polyanion materials, transition metal sulfides, transition metal oxysulfides, transition metal oxynitrides, etc. can be used. In particular, when lithium-containing transition metal oxides or lithium-containing transition metal phosphates are used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. As lithium-containing transition metal oxides, lithium cobaltate, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, etc. can be cited. As lithium-containing transition metal phosphates, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium nickel phosphate, etc. can be cited.

[0040] Among them, the positive electrode active material may include lithium nickel oxide having a layered rock salt type crystal structure. The proportion of Ni in the metal elements other than Li contained in the lithium nickel oxide may be more than 50 atomic %. The lithium nickel oxide may also contain other transition metals. Lithium nickel oxide can be used to achieve a high operating voltage. By applying the modified PVA of this embodiment, it is possible to inhibit the dissolution of metal ions from the positive electrode active material. Since nickel is easily dissolved from the positive electrode active material, the dissolution inhibition effect brought about by the technology disclosed in the present invention is particularly expected.

[0041] Lithium nickel oxide can be represented by the following composition formula (I). Element M1 is at least one selected from V, Co and Mn. Element M2 is at least one selected from Mg, Al, Ca, Ti, Cu, Zn and Nb. Composition formula (I) satisfies 0.9≤α≤1.10, -0.05≤β≤0.05, 0.5≤x1<1, 0≤x2≤0.5 and 0<1-x1-x2≤0.5.

[0042] Li α Ni x1 M1 x2 M2(1-x1-x2) O 2+β ···(I)

[0043] The positive electrode active material layer 12 may contain other materials such as an ion conductor and a conductive aid. The positive electrode active material layer 12 may contain a binder other than modified PVA.

[0044] The positive electrode current collector 11 is a foil made of a metal material such as aluminum, aluminum alloy, stainless steel, titanium, or titanium alloy.

[0045] The positive electrode 13 can be produced by the following method. Figure 3A It is a process diagram showing a method for manufacturing the positive electrode 13 .

[0046] In step S1 , a polymer solution including PVA, a phosphorus compound, and a solvent is prepared.

[0047] As described above, the phosphorus compound contains at least one selected from the group consisting of the compound represented by the chemical formula (1), the compound represented by the chemical formula (2), and the compound represented by the chemical formula (3).

[0048] The phosphorus compound at the raw material stage is in a state before combining with PVA, and can be distinguished from the structure after combining with modified PVA. That is, in the phosphorus compound represented by the chemical formula (1), R 2 R represents a hydrogen atom, a methyl group, an ethyl group or an alkylsilyl group having 1 to 6 carbon atoms. 1 and R 3 is an alkyl group having 1 to 17 carbon atoms. In the phosphorus compound represented by the chemical formula (2), R 4 and R 5 Each R independently represents a hydrogen atom, a methyl group, an ethyl group or an alkylsilyl group having 1 to 6 carbon atoms. 6 represents an alkyl group having 1 to 17 carbon atoms. In the phosphorus compound represented by the chemical formula (3), R 7 , R 8 and R 9 Each independently represents a hydrogen atom, a methyl group, an ethyl group or an alkylsilyl group having 1 to 6 carbon atoms. When the phosphorus compound has such a structure, PVA can be modified relatively easily, and oxidation resistance can be imparted to PVA.

[0049] The phosphorus compound preferably contains a silylphosphite. Since the silyl group has high reactivity, when the phosphorus compound is a silylphosphite having a silyl group, the reaction between PVA and the phosphorus compound easily proceeds.

[0050] Specifically, among the compounds represented by the chemical formula (2), R 4 and R 5At least one of them may be an alkylsilyl group having 1 to 6 carbon atoms. 7 , R 8 and R 9 At least one of them may be an alkylsilyl group having a carbon number of 1 to 6. When the phosphorus compound has an alkylsilyl group, the reaction between PVA and the phosphorus compound proceeds easily.

[0051] The solvent of the polymer solution is not particularly limited as long as it is a solvent that can dissolve PVA and the phosphorus compound. Examples of the solvent of the polymer solution include water, N-methylpyrrolidone (NMP), and the like. PVA may be PVA having no groups other than hydroxyl and acetic acid groups, or PVA obtained by replacing hydroxyl groups with functional groups other than phosphorus compounds, or a mixture thereof. The saponification degree of PVA is also not particularly limited.

[0052] In step S2, the polymer solution is heated. Figure 2A or Figure 2B As described above, by heating, the phosphorus compound and PVA are reacted to form modified PVA. The polymer solution contains modified PVA. The heating temperature is, for example, 80° C. or higher and 150° C. or lower. The heating time is, for example, 60 minutes or higher and 50 hours or lower.

[0053] In step S3, a positive electrode slurry including a polymer solution and a positive electrode active material is prepared. Specifically, a powder of the positive electrode active material is mixed in the polymer solution and stirred. The positive electrode slurry may contain other materials such as a conductive additive.

[0054] In step S4, the positive electrode slurry is applied on the positive electrode collector 11 to form the positive electrode active material layer 12. Specifically, the positive electrode slurry is applied on the positive electrode collector 11 to form a coating. The solvent is removed from the coating to obtain the positive electrode active material layer 12. As a method for removing the solvent, a method of heating the coating can be cited.

[0055] After the above process, we get the reference Figure 1 The positive electrode 13 is described.

[0056] The polymer solution prepared in step S2 can also be used to treat the surface of the particles of the positive electrode active material with modified PVA. For example, after adding particles of the positive electrode active material to the polymer solution, the modified PVA is attached to the particles of the positive electrode active material by spray drying. Thus, particles of the positive electrode active material having at least a portion of the surface coated with modified PVA are obtained. The particles of the positive electrode active material coated with modified PVA are used to prepare the positive electrode slurry.

[0057] Alternatively, the following method may be used. The modified PVA powder is obtained by removing the solvent from the polymer solution prepared in step S2. The modified PVA powder is mixed with particles of the positive electrode active material to obtain a mixed powder. The mixed powder is heated to melt the modified PVA, and at least a portion of the surface of the positive electrode active material particles is coated with the modified PVA.

[0058] Figure 3B 1 is a process diagram showing another method for manufacturing the positive electrode 13. In step ST1, a positive electrode slurry containing a positive electrode active material, PVA, a phosphorus compound and a solvent is prepared. In step ST2, the positive electrode slurry is applied to the positive electrode collector 11 to form a coating. In step ST3, the coating is heated to form a positive electrode active material layer 12. When the coating is heated, the solvent is removed from the coating, and a reaction of combining the phosphorus compound and the PVA is carried out to generate a modified PVA.

[0059] Figure 3A The manufacturing method and Figure 3B Any of the manufacturing methods shown can effectively manufacture the positive electrode 13. Figure 3A In the manufacturing method shown, modified PVA is generated before the preparation of the positive electrode slurry. Therefore, the heating of the polymer solution in step S2 can be carried out under conditions suitable for the generation of modified PVA. Figure 3B In the production method shown, the step of removing the solvent from the coating film also serves as the step of producing the modified PVA, so the number of steps can be reduced.

[0060] (Implementation Method 2)

[0061] Figure 4 2 is a cross-sectional view schematically showing a structure of a lithium secondary battery 100 according to Embodiment 2. The lithium secondary battery 100 includes the positive electrode 13 described in Embodiment 1. By using the positive electrode 13, the characteristics of the lithium secondary battery 100 such as discharge capacity and capacity retention rate can be improved.

[0062] The lithium secondary battery 100 further includes a negative electrode 16, an electrolyte layer 17, and a housing 18 and a non-aqueous electrolyte 19. The negative electrode 16 includes a negative electrode collector 14 and a negative electrode active material layer 15. The negative electrode active material layer 15 is provided on the negative electrode collector 14. The electrolyte layer 17 is provided between the positive electrode 13 and the negative electrode 16. The electrolyte layer 17 is a separator. The positive electrode 13, the negative electrode 16, the electrolyte layer 17, and the non-aqueous electrolyte 19 are accommodated in the housing 18.

[0063] The negative electrode current collector 14 is a foil made of a metal material such as stainless steel, nickel, a nickel alloy, copper, or a copper alloy.

[0064] The negative electrode active material layer 15 contains a negative electrode active material. The negative electrode active material may be a material capable of absorbing and releasing lithium ions. Examples of the negative electrode active material include lithium titanate, graphite, silicon, silicon compounds, and NiBi alloys.

[0065] The negative electrode active material layer 15 may contain other materials such as a conductive additive, an ion conductor, and a binder.

[0066] The non-aqueous electrolyte 19 is impregnated in the positive electrode 13, the negative electrode 16, and the electrolyte layer 17. The non-aqueous electrolyte 19 may fill the inner space of the outer case 18.

[0067] The nonaqueous electrolyte 19 contains a nonaqueous solvent and a lithium salt.

[0068] As the non-aqueous solvent, cyclic carbonate, chain carbonate, cyclic ether, chain ether, nitrile, amide, etc. may be used. One kind selected from these solvents may be used alone, or two or more kinds may be used in combination.

[0069] As lithium salts, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(perfluoroethylsulfonyl)imide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, lithium difluoro(oxalate)borate, etc. can be used. One selected from these electrolyte salts can be used, or two or more can be used in combination.

[0070] The electrolyte layer 17 has lithium ion conductivity. As long as lithium ions are allowed to pass through, the material of the electrolyte layer 17 is not particularly limited. The material of the electrolyte layer 17 can be at least one selected from a solid electrolyte, a gel electrolyte, an ion exchange resin membrane such as a lithium cation exchange resin, a semipermeable membrane, and a porous membrane. If the electrolyte layer 17 is made of these materials, the safety of the lithium secondary battery 100 can be fully ensured. As solid electrolytes, sulfide solid electrolytes such as Li2S-P2S5, Li7La3Zr2O 12 As the gel electrolyte, there can be mentioned a gel electrolyte containing a fluororesin such as PVdF. As the ion exchange resin membrane, there can be mentioned a cation exchange membrane, an anion exchange membrane, etc. As the porous membrane, there can be mentioned a porous membrane made of a polyolefin resin, a porous membrane composed of a cellophane obtained by weaving glass fibers into a non-woven fabric, etc.

[0071] There is no particular limitation on the shape of the lithium secondary battery 100. The lithium secondary battery 100 may be in various shapes such as a coin shape, a cylinder shape, a square shape, a sheet shape, a button shape, a flat shape, and a stacked shape.

[0072] The application of the positive electrode 13 of the first embodiment is not limited to the lithium secondary battery 100. The positive electrode 13 can be applied to various secondary batteries such as sodium secondary batteries and magnesium secondary batteries. Various secondary batteries may be batteries using electrolyte solutions or solid batteries.

[0073] (Other embodiments)

[0074] (Note)

[0075] The following technical solutions are disclosed through the description of the above embodiments.

[0076] (Technical Solution 1)

[0077] A positive electrode for a secondary battery comprises a positive electrode collector and a positive electrode active material layer supported on the positive electrode collector, wherein the positive electrode active material layer contains a positive electrode active material and polyvinyl alcohol modified with a phosphorus compound.

[0078] According to this technical configuration, it is possible to suppress side reactions in the positive electrode of the secondary battery, and in particular, it is possible to suppress side reactions when the secondary battery is stored at high temperature.

[0079] (Technical Solution 2)

[0080] According to the secondary battery positive electrode of claim 1, the ratio of the phosphorus compound to the hydroxyl group contained in the modified polyvinyl alcohol is 10% or more on a substance basis. In this case, the oxidation resistance effect of the phosphorus compound is more fully exerted, so the amount of modified PVA used in the positive electrode can be reduced.

[0081] (Technical Solution 3)

[0082] According to the secondary battery positive electrode of claim 1 or 2, the modified polyvinyl alcohol is attached to the surface of the particles of the positive electrode active material. With such a structure, direct contact between the positive electrode active material and the electrolyte is prevented, so that the oxidation resistance effect of the modified PVA is easily exerted.

[0083] (Technical Solution 4)

[0084] According to the secondary battery positive electrode according to any one of technical solutions 1 to 3, the phosphorus compound comprises at least one selected from the compound represented by chemical formula (1), the compound represented by chemical formula (2) and the compound represented by chemical formula (3). These phosphorus compounds can easily modify PVA. By applying the modified PVA of this embodiment, it is possible to inhibit the dissolution of metal ions from the positive electrode active material.

[0085] (Technical Solution 5)

[0086] According to any one of technical solutions 1 to 4, the positive electrode for a secondary battery, the positive electrode active material comprises a lithium nickel oxide having a layered rock salt type crystal structure, and the proportion of Ni in the metal elements other than Li contained in the lithium nickel oxide is 50 atomic % or more. Since nickel is easily dissolved from the positive electrode active material, the dissolution inhibition effect brought about by the technical solution of the present disclosure is particularly expected.

[0087] (Technical Solution 6)

[0088] A secondary battery comprises the secondary battery positive electrode according to any one of claims 1 to 5, a negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0089] According to this technical configuration, the characteristics of the secondary battery, such as the discharge capacity and the capacity retention rate, can be improved.

[0090] (Technical Solution 7)

[0091] A method for manufacturing a positive electrode for a secondary battery comprises: preparing a polymer solution containing polyvinyl alcohol, a phosphorus compound and a solvent; preparing a positive electrode slurry containing the polymer solution and a positive electrode active material; and coating the positive electrode slurry on a positive electrode collector to form a positive electrode active material layer.

[0092] (Technical Solution 8)

[0093] A method for manufacturing a positive electrode for a secondary battery comprises: preparing a positive electrode slurry containing a positive electrode active material, polyvinyl alcohol, a phosphorus compound and a solvent; and coating the positive electrode slurry on a positive electrode collector to form a positive electrode active material layer.

[0094] According to the configurations of claim 7 and claim 8 , the secondary battery positive electrode of the present disclosure can be efficiently manufactured.

[0095] (Technical Solution 9)

[0096] According to the method for producing a secondary battery positive electrode according to claim 7 or 8, the phosphorus compound comprises a silyl phosphite. Since the silyl group has high reactivity, when the phosphorus compound is a silyl phosphite having a silyl group, the reaction between PVA and the phosphorus compound proceeds easily.

[0097] (Technical Solution 10)

[0098] According to the method for manufacturing a positive electrode for a secondary battery according to any one of technical solutions 7 to 9, the phosphorus compound comprises at least one selected from the group consisting of a compound represented by chemical formula (1), a compound represented by chemical formula (2), and a compound represented by chemical formula (3). Since the phosphorus compound has such a structure, PVA can be modified relatively easily and oxidation resistance can be imparted to PVA.

[0099] (Technical Solution 11)

[0100] According to the method for manufacturing a positive electrode for a secondary battery according to technical solution 10, in the compound represented by the chemical formula (2), R 4 and R 5 At least one of them is the alkylsilyl group having 1 to 6 carbon atoms, and in the compound represented by the chemical formula (3), R 7 , R 8 and R 9 At least one of them is the alkylsilyl group having 1 to 6 carbon atoms. When the phosphorus compound has an alkylsilyl group, the reaction between PVA and the phosphorus compound proceeds easily.

[0101] Example

[0102] (Example 1)

[0103] PVA (weight average molecular weight of about 254,000) was dissolved in N-methylpyrrolidone (NMP) at a concentration of 7 wt% to prepare a PVA NMP solution. Tris(trimethylsilyl)phosphonic acid (TMSP) was added to the PVA NMP solution to obtain a polymer solution. The concentration of TMSP in the polymer solution was adjusted so that the ratio of the silyl group contained in TMSP to the hydroxyl group contained in PVA was (silyl group): (hydroxyl group) = 1:4 in terms of the molar ratio. The polymer solution was stirred while heating at 90°C for 24 hours. Thus, a polymer solution containing PVA modified by a phosphorus compound was obtained.

[0104] Next, the polymer solution, lithium nickel cobalt manganese oxide (NCM) and acetylene black (AB) were mixed to prepare a positive electrode slurry. The mass ratio of these materials in the positive electrode active material layer was (NCM): (AB): (modified PVA) = 98:1:1.

[0105] Next, the positive electrode slurry was applied on an aluminum foil to form a coating film, and the coating film was dried to form a positive electrode active material layer. Thus, the positive electrode of Example 1 was obtained.

[0106] The lithium secondary battery of Example 1 was prepared using the positive electrode of Example 1, a Li metal foil as a counter electrode, a separator and an electrolyte. As a separator, a microporous membrane made of polyolefin was used. The concentration of LiPF6 in the electrolyte was 1.35 mol / L. The solvent of the electrolyte contained fluoroethylene carbonate (FEC), ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of FEC:EC:EMC:DMC=10:10:5:75.

[0107] (Example 2)

[0108] The concentration of TMSP in the polymer solution was adjusted so that the ratio of the silyl group contained in TMSP to the hydroxyl group contained in PVA was (silyl group):(hydroxyl group)=3:4 in terms of the molar ratio. The positive electrode of Example 2 was prepared in the same manner as in Example 1. The lithium secondary battery of Example 2 was prepared in the same manner as in Example 1 using the positive electrode of Example 2.

[0109] (Comparative Example 1)

[0110] The positive electrode slurry is prepared by mixing PVDF, lithium nickel cobalt manganese oxide (NCM) and acetylene black (AB). The mass ratio of these materials in the positive electrode active material layer is (NCM): (AB): (PVDF) = 98: 1: 1. The positive electrode slurry is applied on an aluminum foil to form a coating. The coating is dried to form a positive electrode active material layer. Thus, the positive electrode of Comparative Example 1 is obtained. Using the positive electrode of Comparative Example 1, the lithium secondary battery of Comparative Example 1 is prepared by the same method as Example 1.

[0111] (Comparative Example 2)

[0112] The positive electrode of Comparative Example 2 was prepared in the same manner as in Comparative Example 1 except that PVA was used instead of PVDF. The lithium secondary battery of Comparative Example 2 was prepared in the same manner as in Example 1 using the positive electrode of Comparative Example 2.

[0113] [Charge and discharge test]

[0114] The charge and discharge test of the lithium secondary battery of the embodiment and comparative example is implemented in the following order. At an ambient temperature of 25°C, the initial charge and discharge is carried out at a current value of 0.1C. Specifically, constant current charging is carried out at a current value of 0.1C until the voltage reaches 4.5V. Then, constant current discharge is carried out at a current value of 0.1C until the voltage reaches 2.5V, and the initial discharge capacity is measured. The initial discharge capacity of the lithium secondary battery of the embodiment and comparative example is shown in Table 1. The discharge capacity is the discharge capacity per unit mass of the positive electrode active material.

[0115] Next, constant current charging was performed at a current value of 0.1C until the voltage reached 4.5V. Then, the voltage was maintained at 4.5V and the lithium secondary battery was stored at an ambient temperature of 55°C for 72 hours. Next, discharge was performed at an ambient temperature of 25°C at a current value of 0.1C to measure the discharge capacity after high-temperature storage. The discharge capacity of the lithium secondary batteries of the embodiments and comparative examples after high-temperature storage is shown in Table 1.

[0116] The ratio (percentage) of the discharge capacity after high-temperature storage to the initial discharge capacity was calculated as the "recovery rate".

[0117] [Table 1]

[0118]

[0119] The recovery rate of the lithium secondary batteries of Comparative Examples 1 and 2 is low. When the lithium secondary batteries are stored at high temperatures in a charged state, the decomposition reaction of the electrolyte is most likely to occur. It is speculated that in the lithium secondary batteries of Comparative Examples 1 and 2, the decomposition reaction of the electrolyte progresses due to high-temperature storage, and the discharge capacity is greatly reduced.

[0120] In contrast, the lithium secondary batteries of Examples 1 and 2 show a high recovery rate. It is speculated that due to the presence of modified PVA, side reactions such as the decomposition reaction of the electrolyte in the positive electrode are suppressed. Furthermore, in lithium secondary batteries, along with charging and discharging, a fine interface is formed in the active material, the electrolyte enters the interface, and the capacity that is not used in the previous cycle is sometimes reflected in the next cycle. In this case, as shown in Examples 1 and 2, the discharge capacity in the subsequent cycle sometimes exceeds the discharge capacity in the previous cycle.

[0121] [Visual observation]

[0122] After the charge and discharge test, the lithium secondary batteries of Example 1 and Comparative Example 1 were disassembled and visually observed. Figure 5A This is an optical photograph of the surface of the Li metal foil 25 and the negative electrode side surface of the separator 24 of the lithium secondary battery of Example 1. Figure 5B This is an optical photograph of the surface of the Li metal foil 25 and the negative electrode side surface of the separator 24 of the lithium secondary battery of Comparative Example 1.

[0123] like Figure 5B As shown in FIG. 1 , black objects are attached to the surfaces of the separator 24 and the Li metal foil 25 of the lithium secondary battery of Comparative Example 1. Figure 5A As shown, the amount of black matter attached to the surfaces of the separator 24 and the Li metal foil 25 of the lithium secondary battery of Example 1 is significantly small. This indicates that in the lithium secondary battery of Example 1, side reactions in the positive electrode, such as the dissolution of metal ions (Ni ions, Mn ions, Co ions) from the positive electrode to the negative electrode and the decomposition of the electrolyte, are suppressed.

[0124] Industrial Availability

[0125] The technology of the present disclosure can be used for secondary batteries requiring a binder.

Claims

1. A positive electrode for a secondary battery, comprising a positive electrode collector and a positive electrode active material layer carried on the positive electrode collector, The positive electrode active material layer contains a positive electrode active material and polyvinyl alcohol modified with a phosphorus compound.

2. The secondary battery positive electrode according to claim 1, The ratio of the phosphorus compound to the hydroxyl group contained in the modified polyvinyl alcohol is 10% or more on a substance amount basis.

3. The secondary battery positive electrode according to claim 1, The modified polyvinyl alcohol adheres to the surface of the particles of the positive electrode active material.

4. The secondary battery positive electrode according to claim 1, The phosphorus compound comprises at least one selected from the group consisting of a compound represented by Chemical Formula (1), a compound represented by Chemical Formula (2), and a compound represented by Chemical Formula (3), R 2 represents a direct bond with a carbon atom contained in the main chain of the modified polyvinyl alcohol, R 1 and R 3 independently represent an alkyl group having 1 to 17 carbon atoms, R 4 and R 5 each independently represents a hydrogen atom, a methyl group, an ethyl group, an alkylsilyl group having 1 to 6 carbon atoms, or a direct bond with a carbon atom contained in the main chain of the modified polyvinyl alcohol, Selected from R 4 and R 5 At least one of represents the binding bond, R 4 and R 5 When both contain the above-mentioned bonding bonds, these bonding bonds are bonding bonds with the main chain of the same polyvinyl alcohol molecule, or bonding bonds with the main chains of different polyvinyl alcohol molecules. R 6 represents an alkyl group having 1 to 17 carbon atoms, R 7 , R 8 and R 9 each independently represents a hydrogen atom, a methyl group, an ethyl group, an alkylsilyl group having 1 to 6 carbon atoms, or a direct bond with a carbon atom contained in the main chain of the modified polyvinyl alcohol, Selected from R 7 , R 8 and R 9 At least one of represents the binding bond, Selected from R 7 , R 8 and R 9 When two or more of the above-mentioned bonds are present, these bonds are bonds to the main chain of the same polyvinyl alcohol molecule or bonds to the main chains of different polyvinyl alcohol molecules.

5. The secondary battery positive electrode according to claim 1, The positive electrode active material comprises lithium nickel oxide having a layered rock salt type crystal structure, The ratio of Ni in the metal elements other than Li contained in the lithium nickel oxide is 50 atomic % or more.

6. A secondary battery comprising: The secondary battery positive electrode according to claim 1, Negative electrode, and A separator is disposed between the positive electrode and the negative electrode.

7. A method for manufacturing a positive electrode for a secondary battery, comprising: preparing a polymer solution containing polyvinyl alcohol, a phosphorus compound and a solvent; preparing a positive electrode slurry containing the polymer solution and a positive electrode active material; as well as The positive electrode slurry is applied on a positive electrode current collector to form a positive electrode active material layer.

8. A method for manufacturing a positive electrode for a secondary battery, comprising: preparing a positive electrode slurry containing a positive electrode active material, polyvinyl alcohol, a phosphorus compound and a solvent; as well as The positive electrode slurry is applied on a positive electrode current collector to form a positive electrode active material layer.

9. The method for producing a positive electrode for a secondary battery according to claim 7, The phosphorus compound comprises silyl phosphite.

10. The method for producing a positive electrode for a secondary battery according to claim 7, The phosphorus compound comprises at least one selected from the group consisting of a compound represented by Chemical Formula (1), a compound represented by Chemical Formula (2), and a compound represented by Chemical Formula (3), R 2 represents a hydrogen atom, a methyl group, an ethyl group, or an alkylsilyl group having 1 to 6 carbon atoms, R 1 and R 3 represents an alkyl group having 1 to 17 carbon atoms, R 4 and R 5 each independently represents a hydrogen atom, a methyl group, an ethyl group, or an alkylsilyl group having 1 to 6 carbon atoms, R 6 represents an alkyl group having 1 to 17 carbon atoms, R 7 , R 8 and R 9 Each independently represents a hydrogen atom, a methyl group, an ethyl group, or an alkylsilyl group having 1 to 6 carbon atoms.

11. The method for producing a positive electrode for a secondary battery according to claim 10, In the compound represented by the chemical formula (2), R 4 and R 5 At least one of them is the alkylsilyl group having 1 to 6 carbon atoms, In the compound represented by the chemical formula (3), R 7 , R 8 and R 9 At least one of them is the alkylsilyl group having 1 to 6 carbon atoms.

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