A lithium iron phosphate battery

Abstract

To overcome the problems of lithium dendrite and iron ion dissolution existing in the existing high-compaction lithium iron phosphate battery, the present invention provides a lithium iron phosphate battery, which includes a positive electrode, a negative electrode and a non-aqueous electrolyte. The positive electrode includes a positive electrode material layer, the compaction density of the positive electrode material layer is 2.3 to 2.8 g / cc, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes LiFePO4; the non-aqueous electrolyte includes a solvent, an electrolyte salt, vinylene carbonate and a compound shown in Structural Formula 1: A-D-B-E-C Structural Formula 1 Based on the total mass of the non-aqueous electrolyte being 100%, the addition amount of the compound shown in Structural Formula 1 is 0.01 to 5%. The lithium iron phosphate battery provided by the present invention uses the combination of vinylene carbonate and the compound shown in Structural Formula 1, which can inhibit the formation of lithium dendrites at the negative electrode interface and the Fe dissolution of the positive electrode, and finally improve the high-temperature and safety performance of the high-compaction density lithium iron phosphate battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular relates to a lithium iron phosphate battery. Background Art

[0002] Since lithium-ion batteries were put on the market in 1991, they have been widely used in mobile communications, laptops and other fields due to their advantages such as high operating voltage, long cycle life, high energy density and no memory effect. In recent years, with the rapid development and popularization of new energy vehicles, consumers have higher and higher requirements for the energy density of lithium-ion batteries. Compared with ternary batteries, lithium iron phosphate batteries have more advantages in cycle life, safety and cost, but the energy density of lithium iron phosphate is still far behind that of ternary materials. At present, the energy density of single lithium iron phosphate batteries is mainly improved by increasing the compaction density of lithium iron phosphate positive electrodes.

[0003] However, blindly increasing the compaction density of lithium iron phosphate cathode materials will first increase the dissolution of positive electrode Fe ions, and the oxidation and decomposition of the electrolyte will be aggravated, resulting in poor performance of battery high-temperature storage and cycling; secondly, it will lead to the generation of lithium dendrites at the electrode interface, causing the battery to short-circuit and catch fire, seriously affecting the safety performance of the battery; this restricts the application of high-density lithium iron phosphate cathode materials. Although cathode material manufacturers have also used various doping coatings to improve the stability of the cathode material structure, it is still a huge challenge to use the current commercial electrolyte in the high-density lithium iron phosphate cathode material system to improve high-temperature cycling, high-temperature storage and Fe ion dissolution at high temperature. Therefore, it is of great significance to develop an additive for high-density (2.3-2.8g / cc) lithium iron phosphate batteries to improve the high-temperature storage, high-temperature cycling and Fe ion dissolution of lithium iron phosphate batteries. Summary of the invention

[0004] In view of the problems of lithium dendrites and iron ion dissolution in existing high-density lithium iron phosphate batteries, the present invention provides a lithium iron phosphate battery.

[0005] The technical solution adopted by the present invention to solve the above technical problems is as follows:

[0006] The present invention provides a lithium iron phosphate battery, comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer, the compaction density of the positive electrode material layer is 2.3-2.8 g / cc, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises LiFePO4;

[0007] The non-aqueous electrolyte includes a solvent, an electrolyte salt, vinylene carbonate and a compound shown in structural formula 1:

[0008] ADBEC

[0009] Structural formula 1

[0010] wherein A, B, and C are each independently selected from a group containing a cyclic carbonate group, a cyclic sulfate group, a cyclic sulfite group, a cyclic sulfonate group, a cyclic sulfone group, a cyclic sulfoxide group, a cyclic carboxylate group, or a cyclic anhydride group;

[0011] D and E are each independently selected from a single bond, or a group containing an alkylene group, an ether bond, a sulfur-oxygen double bond, or a carbon-oxygen double bond;

[0012] Based on the total mass of the non-aqueous electrolyte being 100%, the added amount of the compound represented by the structural formula 1 is 0.01-5%.

[0013] Optionally, based on 100% of the total mass of the non-aqueous electrolyte, the amount of vinylene carbonate added is 0.01-5%, and more preferably, the amount of vinylene carbonate added is 0.1-2%.

[0014] Optionally, the positive electrode surface is detected by X-ray photoelectron spectroscopy, and when the 1s peak of carbon is obtained at 284.5 eV, a characteristic peak of S appears in the region of 162 to 174 eV.

[0015] Optionally, the charge and discharge voltage range of the lithium iron phosphate battery is 2.0 to 3.8V.

[0016] Optionally, the number of cyclic carbonate groups, cyclic sulfate groups, cyclic sulfite groups, cyclic sulfonate groups, cyclic sulfone groups, cyclic sulfoxide groups, cyclic carboxylate groups and cyclic anhydride groups contained independently by A, B and C is 1 to 5, and the total number of cyclic carbonate groups, cyclic sulfate groups, cyclic sulfite groups, cyclic sulfonate groups, cyclic sulfone groups, cyclic sulfoxide groups, cyclic carboxylate groups and cyclic anhydride groups contained by A, B and C is less than or equal to 10.

[0017] Optionally, A and C are each independently selected from the group shown in Structural Formula 2:

[0018]

[0019] wherein n is an integer selected from 0 to 4, R1 is selected from hydrogen, halogen, C1 to C5 hydrocarbon group or halogenated hydrocarbon group; R2, R3, R4, R5, R6, and R7 are each independently selected from C1 to C3 hydrocarbon group, C1 to C3 alkoxy group, oxygen atom, At least one of R2, R3, and R4 is selected from At least one of R2, R3, and R4 is selected from an oxygen atom, and at least one of R5, R6, and R7 is selected from And at least one of R5, R6 and R7 is selected from oxygen atoms.

[0020] Optionally, B is selected from the group shown in structural formula 3:

[0021]

[0022] Wherein, m is selected from an integer of 1 to 4, R8, R9, R 10 Each independently selected from a C1-C3 alkylene group, a C1-C3 alkoxy group, an oxygen atom, R8, R9, R 10 At least one selected from And R8, R9, R 10 At least one of them is selected from oxygen atoms.

[0023] Optionally, D and E are each independently selected from the group shown in structural formula 4:

[0024]

[0025] Structural formula 4

[0026] Wherein, z is selected from an integer of 0 to 4, R 11 and R 13 Each independently selected from a single bond or a C1-C5 alkylene group, R 12 Selected from single bonds,

[0027] Optionally, D and E are each independently selected from a single bond or a C1-C5 alkylene group, A, B, and C are each independently selected from a substituted or unsubstituted cyclic carbonate group, cyclic sulfate group, cyclic sulfite group, cyclic sulfonate group, cyclic sulfone group, cyclic sulfoxide group, cyclic carboxylate group or cyclic anhydride group, and when A, B or C is substituted, the substituent is selected from a halogen, an alkyl group or a haloalkyl group.

[0028] Optionally, A and C are the same as each other, A and B are the same as or different from each other, and D and E are the same as each other.

[0029] Optionally, the compound represented by structural formula 1 is selected from one or more of the following compounds:

[0030]

[0031]

[0032]

[0033]

[0034]

[0035]

[0036]

[0037] According to the lithium iron phosphate battery provided by the present invention, vinylene carbonate and the compound shown in structural formula 1 are added to the non-aqueous electrolyte, wherein the vinylene carbonate and the compound shown in structural formula 1 participate in the formation of the interface film on the surfaces of the positive electrode and the negative electrode. It is speculated that the decomposition products of vinylene carbonate and the decomposition products of the compound shown in structural formula 1 together constitute an organic-inorganic mosaic interface film, which can effectively increase the migration rate of lithium ions during the high-temperature cycle of the battery, and at the same time has a good protective effect on the electrode material, reduces the redox reaction between the electrolyte and the electrode material, inhibits the formation of lithium dendrites at the negative electrode interface and the dissolution of Fe ions at the positive electrode, and ultimately improves the high temperature and safety performance of the lithium iron phosphate battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the full XPS spectrum of the battery positive electrode sheet after battery capacity division provided in Example 5 of the present invention;

[0039] Figure 2 yes Figure 1 Fine spectrum of characteristic peaks in the

[0040] Figure 3 is a SEM image of the negative electrode of the battery provided in Example 5 of the present invention after 1000 cycles;

[0041] Figure 4 This is a SEM image of the negative electrode of the battery provided in Comparative Example 1 of the present invention after 1000 cycles. DETAILED DESCRIPTION

[0042] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0043] An embodiment of the present invention provides a lithium iron phosphate battery, comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer, the compaction density of the positive electrode material layer is 2.3 to 2.8 g / cc, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises LiFePO4;

[0044] The non-aqueous electrolyte includes a solvent, an electrolyte salt, vinylene carbonate and a compound shown in structural formula 1:

[0045] ADBEC

[0046] Structural formula 1

[0047] Wherein, A, B, and C are each independently selected from a group containing a cyclic ester group or a cyclic anhydride group, and the cyclic ester group includes one or more of a cyclic carbonate group, a cyclic sulfate group, a cyclic sulfite group, a cyclic sulfonate group, a cyclic sulfone group, a cyclic sulfoxide group, and a cyclic carboxylate group;

[0048] D and E are each independently selected from a single bond, or a group containing an alkylene group, an ether bond, a sulfur-oxygen double bond, or a carbon-oxygen double bond;

[0049] Based on the total mass of the non-aqueous electrolyte being 100%, the added amount of the compound represented by the structural formula 1 is 0.01-5%.

[0050] The inventors speculated through research that vinylene carbonate and the compound shown in structural formula 1 participate in the formation of the interface film on the surface of the positive and negative electrodes, and the decomposition products of vinylene carbonate and the decomposition products of the compound shown in structural formula 1 together constitute an organic-inorganic interfacial film, which can effectively increase the migration rate of lithium ions during the high-temperature cycle of the battery, and at the same time has a good protective effect on the electrode material, reduces the redox reaction between the electrolyte and the electrode material, inhibits the formation of lithium dendrites at the negative electrode interface and the dissolution of Fe ions at the positive electrode, and ultimately improves the high temperature and safety performance of the lithium iron phosphate battery.

[0051] Vinylene carbonate can form an organic interface of a polymer structure on the surface of the electrode, but in a high-density lithium iron phosphate system, this organic interface on the negative electrode is not conducive to the migration of lithium ions, and thus lithium dendrites will be deposited on the surface of the negative electrode. The reaction between lithium dendrites and the electrolyte causes further deterioration of battery performance and iron ion dissolution. The inventors have found through experiments that when vinylene carbonate and the compound shown in structural formula 1 are added to a non-aqueous electrolyte at the same time, a good coordination effect can be formed with a high-density lithium iron phosphate battery. After the compound shown in structural formula 1 is added, the compound shown in structural formula 1 can form an interface film containing special components on the surface of the electrode, which can improve the lithium ion conduction rate of the interface film, protect the positive and negative electrode structures, and improve the stability of the battery material under high-temperature cycles.

[0052] In a preferred embodiment, the added amount of the compound represented by structural formula 1 is 0.1-2%.

[0053] In some embodiments, the surface of the positive electrode is detected by X-ray photoelectron spectroscopy, and when the 1s peak of carbon is obtained at 284.5 eV, a characteristic peak of S appears in the region of 162 to 174 eV, such as Figure 1 and Figure 2As shown, it is shown that vinylene carbonate and the compound shown in structural formula 1 jointly form an interface film on the surface of the positive electrode. The interface film has relatively excellent organic and inorganic compositions, and the performance of high-density lithium iron phosphate batteries is significantly improved.

[0054] In some embodiments, based on the total mass of the non-aqueous electrolyte being 100%, the amount of vinylene carbonate added is 0.01-5%.

[0055] In a preferred embodiment, based on 100% of the total mass of the non-aqueous electrolyte, the amount of vinylene carbonate added is 0.1-2%.

[0056] In the lithium iron phosphate battery, vinylene carbonate and the compound shown in structural formula 1 have a good coordination relationship. When the addition amount of either vinylene carbonate or the compound shown in structural formula 1 is too low, it is difficult to produce a good coordination effect; when the addition amount of either vinylene carbonate or the compound shown in structural formula 1 is too much, not only will the film be too thick to increase the impedance, but it will also significantly increase the viscosity of the electrolyte, affecting the performance of the battery.

[0057] In some embodiments, the charge and discharge voltage range of the lithium iron phosphate battery is 2.0 to 3.8V.

[0058] In some embodiments, the number of cyclic carbonate groups, cyclic sulfate groups, cyclic sulfite groups, cyclic sulfonate groups, cyclic sulfone groups, cyclic sulfoxide groups, cyclic carboxylate groups and cyclic anhydride groups independently contained in A, B, and C is 1 to 5, and the total number of cyclic carbonate groups, cyclic sulfate groups, cyclic sulfite groups, cyclic sulfonate groups, cyclic sulfone groups, cyclic sulfoxide groups, cyclic carboxylate groups and cyclic anhydride groups of A, B, and C is less than or equal to 10.

[0059] In some embodiments, A and C are each independently selected from the group shown in Structural Formula 2:

[0060]

[0061] wherein n is an integer selected from 0 to 4, R1 is selected from hydrogen, halogen, C1 to C5 hydrocarbon group or halogenated hydrocarbon group; R2, R3, R4, R5, R6, and R7 are each independently selected from C1 to C3 hydrocarbon group, C1 to C3 alkoxy group, oxygen atom, At least one of R2, R3, and R4 is selected from At least one of R2, R3, and R4 is selected from an oxygen atom, and at least one of R5, R6, and R7 is selected from And at least one of R5, R6 and R7 is selected from oxygen atoms.

[0062] In a preferred embodiment, the combination of -R3-R2-R4- and the combination of -R7-R5-R6- are each independently selected from

[0063] In some embodiments, B is selected from the group shown in Structural Formula 3:

[0064]

[0065] Wherein, m is selected from an integer of 1 to 4, R8, R9, R 10 Each independently selected from a C1-C3 alkylene group, a C1-C3 alkoxy group, an oxygen atom, R8, R9, R 10 At least one selected from And R8, R9, R 10 At least one of them is selected from oxygen atoms.

[0066] In a preferred embodiment, -R9-R8-R 10 -The combination groups are each independently selected from

[0067] In some embodiments, D and E are each independently selected from the group shown in Structural Formula 4:

[0068]

[0069] Wherein, z is selected from an integer of 0 to 4, R 11 and R 13 Each independently selected from a single bond or a C1-C5 alkylene group, R 12 Selected from single bonds,

[0070] In some embodiments, A and C are the same as each other, A and B are the same as or different from each other, and D and E are the same as each other.

[0071] When A and C are the same as each other, and D and E are the same as each other, the compound shown in Structural Formula 1 is a symmetrical structure. Compared with an asymmetrical structure, the compound shown in Structural Formula 1 with a symmetrical structure is easier to synthesize, has a higher product yield, and is beneficial to reducing production costs.

[0072] In some embodiments, D and E are each independently selected from a single bond or a C1-C5 alkylene group, and A, B, and C are each independently selected from a substituted or unsubstituted cyclic carbonate group, a cyclic sulfate group, a cyclic sulfite group, a cyclic sulfonate group, a cyclic sulfone group, a cyclic sulfoxide group, a cyclic carboxylate group, or a cyclic anhydride group. Preferably, when A, B or C is substituted, the substituent is selected from a halogen, a hydrocarbon group, or a halogenated hydrocarbon group. More preferably, when A, B or C is substituted, the substituent is selected from a halogen, an alkyl group, or a halogenated alkyl group.

[0073] As an example, the compound represented by structural formula 1 can be selected from one or more of the following compounds:

[0074]

[0075]

[0076]

[0077]

[0078] In some embodiments, D and E are each independently selected from the group shown in Structural Formula 4:

[0079]

[0080] Wherein, z is selected from an integer of 1 to 4, R 11 and R 13 Each independently selected from a single bond or a C1-C5 alkylene group, R 12 Selected from

[0081] A, B, and C are each independently selected from a substituted or unsubstituted cyclic carbonate group, a cyclic sulfate group, a cyclic sulfite group, a cyclic sulfonate group, a cyclic sulfone group, a cyclic sulfoxide group, a cyclic carboxylate group, or a cyclic anhydride group. Preferably, when A, B or C is substituted, the substituent is selected from a halogen, a hydrocarbon group, or a halogenated hydrocarbon group. More preferably, when A, B or C is substituted, the substituent is selected from a halogen, an alkyl group, or a halogenated alkyl group.

[0082] As an example, the compound represented by structural formula 1 can be selected from one or more of the following compounds:

[0083]

[0084]

[0085]

[0086] In some embodiments, the compound represented by the structural formula 1 may also be selected from one or more of the following compounds:

[0087]

[0088] It should be noted that the above are some of the compounds claimed to be protected by the present invention, but are not limited thereto and should not be construed as limiting the present invention.

[0089] A person skilled in the art who knows the structural formula of the compound of structural formula 1 can know the preparation method of the above compound according to the common knowledge in the field of chemical synthesis. For example:

[0090] Compound 1 can be prepared by the following method:

[0091] Sorbitol, dimethyl carbonate, methanol, alkaline substance catalyst potassium hydroxide and organic solvent such as DMF are placed in a reaction container, and reacted under heating conditions for several hours, and then a certain amount of oxalic acid is added to adjust the pH to neutral. After filtering and recrystallization, intermediate product 1 can be obtained. Then, intermediate product 1, carbonate, dichlorothionyl, etc. are subjected to esterification reaction under high temperature conditions to obtain intermediate product 2, and intermediate product 2 is oxidized using an oxidant such as sodium periodate to obtain compound 1.

[0092] Compound 2 can be prepared by the following method:

[0093] After diacetone-D-mannitol, dimethyl carbonate, methanol, potassium carbonate and dioxane are reacted under heating and stirring for several hours, a certain amount of oxalic acid is added to adjust the pH of the solution to neutral, and the intermediate product 3 is obtained after filtering and concentration; an appropriate amount of pure water, carbonate, acid, etc. are added to the intermediate product 3 for hydrolysis reaction to obtain the intermediate product 4; then the intermediate product 4, dichlorothionyl and carbonate solvent are prepared under heating conditions to obtain the intermediate product 5; finally, the intermediate product 5 is oxidized using an oxidant such as sodium periodate to obtain compound 2.

[0094] In some embodiments, the solvent includes one or more of an ether solvent, a nitrile solvent, a carbonate solvent, and a carboxylate solvent.

[0095] Among them, the ether solvents include cyclic ethers or chain ethers.

[0096] Examples of cyclic ethers include one or more of 1,3-dioxolane (DOL), 1,4-dioxolane (DX), crown ethers, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), and 2-trifluoromethyltetrahydrofuran (2-CF3-THF).

[0097] Examples of the chain ether include one or more of dimethoxymethane (DMM), 1,2-dimethoxyethane (DME), and diethylene glycol dimethyl ether (TEGDME).

[0098] Examples of nitrile solvents include one or more of acetonitrile, glutaronitrile, and malononitrile.

[0099] The carbonate solvents include cyclic carbonates or chain carbonates.

[0100] Examples of the cyclic carbonate include one or more of ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), and butylene carbonate (BC).

[0101] Examples of the chain carbonate include one or more of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl n-propyl carbonate, ethyl n-propyl carbonate, and dipropyl carbonate (DPC).

[0102] The carboxylate solvents include cyclic carboxylate or chain carbonate.

[0103] Examples of the cyclic carboxylic acid ester include one or more of γ-butyrolactone, γ-valerolactone, and δ-valerolactone.

[0104] Examples of the chain carbonate include one or more of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, propyl propionate (PP), and butyl propionate.

[0105] In some embodiments, the electrolyte salt is selected from lithium salts. In a preferred embodiment, the electrolyte salt is selected from LiPF6, LiBF4, LiBOB, LiDFOB, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2CF3)3, LiN(SO2F)2, LiClO4, LiAsF6, LiSbF6, LiCF3SO3, Li2B 10 Cl 10 , a low-order aliphatic carboxylic acid lithium salt, LiAlCl4, LiDFOP, LiPO2F2, or one or more thereof. In a more preferred embodiment, the electrolyte salt is selected from one or more of LiPF6, LiPO2F2, and LiFSI.

[0106] In some embodiments, the concentration of the electrolyte salt in the non-aqueous electrolyte is 0.1 mol / L-8 mol / L. In a preferred embodiment, the concentration of the electrolyte salt in the non-aqueous electrolyte is 0.5 mol / L-4 mol / L. Specifically, the concentration of the electrolyte salt can be 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L or 4 mol / L.

[0107] In some embodiments, the non-aqueous electrolyte further includes auxiliary additives, and the auxiliary additives include at least one of fluorinated cyclic carbonate compounds, sultone compounds, aromatic additives, fluorinated anisole compounds, dicarboxylic anhydride, lithium difluorophosphate, vinyl sulfate (DTD), lithium bis(fluorosulfonyl)imide (LiFSI), methylene disulfonate, tris(trimethylsilyl)phosphate, and tris(trimethylsilyl)borate.

[0108] In some embodiments, the fluorinated cyclic carbonate compound includes one or more of fluoroethylene carbonate (FEC), 4,4-difluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4-fluoro-4-methylethylene carbonate, 4,5-difluoro-4-methylethylene carbonate, 4-fluoro-5-methylethylene carbonate, 4,4-difluoro-5-methylethylene carbonate, 4-(fluoromethyl)ethylene carbonate, 4-(difluoromethyl)ethylene carbonate, 4-(trifluoromethyl)ethylene carbonate, 4-(fluoromethyl)-4-fluoroethylene carbonate, 4-(fluoromethyl)-5-fluoroethylene carbonate, 4-fluoro-4,5-dimethylethylene carbonate, 4,5-difluoro-4,5-dimethylethylene carbonate, and 4,4-difluoro-5,5-dimethylethylene carbonate.

[0109] When the non-aqueous electrolyte contains a fluorinated cyclic carbonate compound, the content of the fluorinated cyclic carbonate compound is 0.1-30% based on the total mass of the non-aqueous electrolyte as 100%;

[0110] In some embodiments, the sultone compound is selected from one or more of 1,3-propane sultone (PS), 1,4-butane sultone (BS), and 1,3-propylene sultone (PST).

[0111] When the non-aqueous electrolyte contains a sultone compound, the mass percentage of the sultone compound is 0.1-5% based on the total mass of the non-aqueous electrolyte being 100%.

[0112] In some embodiments, the aromatic additive includes aromatic compounds such as biphenyl, alkyl biphenyl, terphenyl, partial hydrogenation of terphenyl, cyclohexylbenzene, tert-butylbenzene, tert-amylbenzene, diphenyl ether, dibenzofuran, etc.; one or more of 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene.

[0113] When the non-aqueous electrolyte contains an aromatic additive, the mass percentage of the aromatic additive is 0.1-5% based on the total mass of the non-aqueous electrolyte being 100%.

[0114] In some embodiments, the fluorine-containing anisole compound includes one or more of 2,4-difluoroanisole, 2,5-difluoroanisole, and 2,6-difluoroanisole.

[0115] When the non-aqueous electrolyte contains a fluorine-containing anisole compound, the mass percentage of the fluorine-containing anisole compound is 0.1-5% based on the total mass of the non-aqueous electrolyte being 100%.

[0116] In some embodiments, the dicarboxylic anhydride includes one or more of succinic acid, maleic acid, and phthalic acid.

[0117] When the non-aqueous electrolyte contains dicarboxylic anhydride, the mass percentage of the dicarboxylic anhydride is 0.1-5% based on the total mass of the non-aqueous electrolyte being 100%.

[0118] In some embodiments, when the non-aqueous electrolyte contains lithium difluorophosphate, the mass percentage of the lithium difluorophosphate is 0.1-2% based on the total mass of the non-aqueous electrolyte being 100%;

[0119] In some embodiments, when the non-aqueous electrolyte contains vinyl sulfate (DTD), the mass percentage of the vinyl sulfate (DTD) is 0.1-5% based on the total mass of the non-aqueous electrolyte as 100%;

[0120] In some embodiments, when the non-aqueous electrolyte contains lithium bis(fluorosulfonyl)imide (LiFSI), the mass percentage of lithium bis(fluorosulfonyl)imide (LiFSI) is 0.1-5% based on the total mass of the non-aqueous electrolyte being 100%.

[0121] In some embodiments, the auxiliary additives also include nitrogen-containing compounds such as 1-methyl-2-pyrrolidone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methylsuccinimide; hydrocarbon compounds such as heptane, octane, cycloheptane; fluorine-containing aromatic compounds such as fluorobenzene, difluorobenzene, trifluorotoluene, etc.

[0122] It should be noted that, unless otherwise specified, generally, the mass percentage range of any optional substance in the auxiliary additives in the non-aqueous electrolyte is less than 10%, preferably, the mass percentage range is 0.1-5%.

[0123] In a preferred embodiment, the auxiliary additive is selected from one or more of 1,3-propane sultone, methylene disulfonate, fluoroethylene carbonate and vinyl sulfate.

[0124] In some embodiments, the positive electrode further comprises a positive electrode current collector, and the positive electrode material layer covers the surface of the positive electrode current collector. The positive electrode current collector is selected from a metal material that can conduct electrons. Preferably, the positive electrode current collector comprises one or more of Al, Ni, and stainless steel. In a more preferred embodiment, the positive electrode current collector is selected from aluminum foil.

[0125] In some embodiments, the positive electrode material layer further includes a positive electrode binder and a positive electrode conductor, and the positive electrode active material, the positive electrode binder and the positive electrode conductor are blended to obtain the positive electrode material layer.

[0126] The positive electrode binder includes polyvinylidene fluoride, copolymers of vinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride-hexafluoropropylene, copolymers of tetrafluoroethylene-hexafluoropropylene, copolymers of tetrafluoroethylene-perfluoroalkyl vinyl ether, copolymers of ethylene-tetrafluoroethylene, copolymers of vinylidene fluoride-tetrafluoroethylene, copolymers of vinylidene fluoride-trifluoroethylene, copolymers of vinylidene fluoride-trichloroethylene, copolymers of vinylidene fluoride-fluoroethylene, copolymers of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, thermoplastic resins such as polyethylene and polypropylene; acrylic resin; and one or more of styrene butadiene rubber.

[0127] The positive electrode conductive agent includes one or more of conductive carbon black, conductive carbon balls, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene or reduced graphene oxide.

[0128] In some embodiments, the negative electrode includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes one or more of a silicon-based negative electrode, a carbon-based negative electrode, a tin-based negative electrode, and a lithium negative electrode. Among them, the silicon-based negative electrode includes one or more of silicon materials, silicon oxides, silicon-carbon composite materials, and silicon alloy materials; the carbon-based negative electrode includes one or more of graphite, hard carbon, soft carbon, graphene, and mesophase carbon microspheres; the tin-based negative electrode includes one or more of tin, tin carbon, tin oxide, and tin metal compounds; the lithium negative electrode includes one or more of metallic lithium or lithium alloys. The lithium alloy can specifically be at least one of a lithium silicon alloy, a lithium sodium alloy, a lithium potassium alloy, a lithium aluminum alloy, a lithium tin alloy, and a lithium indium alloy.

[0129] In a preferred embodiment, the negative electrode active material is selected from one or more of artificial graphite, natural graphite and silicon oxygen carbon. In some embodiments, the negative electrode further comprises a negative electrode current collector, and the negative electrode material layer covers the surface of the negative electrode current collector. The negative electrode current collector is selected from a metal material that can conduct electrons. Preferably, the negative electrode current collector includes one or more of Cu, Ni, and stainless steel. In a more preferred embodiment, the negative electrode current collector is selected from copper foil.

[0130] In some embodiments, the negative electrode material layer further includes a negative electrode binder and a negative electrode conductive agent, and the negative electrode active material, the negative electrode binder and the negative electrode conductive agent are blended to obtain the negative electrode material layer.

[0131] The negative electrode binder includes polyvinylidene fluoride, copolymers of vinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride-hexafluoropropylene, copolymers of tetrafluoroethylene-hexafluoropropylene, copolymers of tetrafluoroethylene-perfluoroalkyl vinyl ether, copolymers of ethylene-tetrafluoroethylene, copolymers of vinylidene fluoride-tetrafluoroethylene, copolymers of vinylidene fluoride-trifluoroethylene, copolymers of vinylidene fluoride-trichloroethylene, copolymers of vinylidene fluoride-fluoroethylene, copolymers of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, thermoplastic resins such as polyethylene and polypropylene; acrylic resin; and one or more of styrene butadiene rubber.

[0132] The negative electrode conductive agent includes one or more of conductive carbon black, conductive carbon balls, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene or reduced graphene oxide.

[0133] In some embodiments, the battery further includes a separator, which is located between the positive electrode and the negative electrode. The separator can be an existing conventional separator, which can be a ceramic separator, a polymer separator, a non-woven fabric, an inorganic-organic composite separator, etc., including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP and triple-layer PP / PE / PP separators.

[0134] The present invention is further described below by way of examples.

[0135] 1. Examples 1 to 29 and Comparative Examples 1 to 13

[0136] 1) Preparation of electrolyte: Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of EC:EMC=3:7, and then lithium hexafluorophosphate (LiPF6) was added to a molar concentration of 1 mol / L, and then the additives in the following tables were added respectively. The amount of the additives was calculated as a percentage of the total mass of the electrolyte.

[0137] 2) Preparation of positive electrode

[0138] The positive electrode active material lithium iron phosphate, conductive carbon black Super-P and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 93:4:3, and then dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The slurry is evenly coated on both sides of the aluminum foil, dried, rolled and vacuum dried, and welded with an ultrasonic welder to obtain a positive electrode plate. The thickness of the plate is 120-150μm, and the compaction density of the positive electrode material layer is shown in the following tables.

[0139] 3) Preparation of negative electrode plate

[0140] The negative electrode active material modified natural graphite, conductive carbon black Super-P, binder styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were mixed in a mass ratio of 94:1:2.5:2.5, and then dispersed in deionized water to obtain a negative electrode slurry. The slurry was coated on both sides of the copper foil, dried, rolled, and welded with a nickel lead wire using an ultrasonic welder to obtain a negative electrode plate. The porosity of each test group and control group after rolling is shown in Table 1.

[0141] 4) Preparation of battery cells

[0142] A polyethylene microporous membrane with a thickness of 20 μm is placed between the positive plate and the negative plate as a separator, and then the sandwich structure consisting of the positive plate, the negative plate and the separator is wound, and then the wound body is flattened and placed in an aluminum-plastic film. After the lead wires of the positive and negative electrodes are led out respectively, the aluminum-plastic film is hot-pressed and sealed to obtain a battery cell to be injected with liquid.

[0143] 5) Battery filling and formation

[0144] In a glove box with a dew point controlled below -40°C, the prepared electrolyte is injected into the battery cell through the injection hole. The amount of electrolyte should be enough to fill the gaps in the battery cell. Then the formation is carried out according to the following steps: 0.05C constant current charging for 180min, 0.1C constant current charging for 180min, leaving it for 24hrs, shaping and sealing, and then further charging it to 3.65V at 0.2C constant current, leaving it at room temperature for 24hrs, and discharging it to 2.0V at 0.2C constant current.

[0145] 2. Performance Test

[0146] The lithium ion batteries prepared in Examples 1 to 29 and Comparative Examples 1 to 13 were subjected to the following performance tests:

[0147] High temperature cycle performance test

[0148] 1) At 45°C, charge at a constant current of 1C to 3.65V, then charge at a constant voltage until the current drops to 0.1C, then discharge at a constant current of 1C to 2.0V, and cycle 1000 times. Record the discharge capacity of the first week and the discharge capacity of the 1000th time, and calculate the capacity retention rate as follows:

[0149] Capacity retention rate = (1000th discharge capacity ÷ 1st cycle discharge capacity) × 100%

[0150] 2) 3.65V fully charged 60℃ storage test:

[0151] At room temperature, the divided battery was charged to 3.65V at 0.5C, with a cut-off current of 0.02C. After standing for 5 minutes, it was discharged to 2.0V at 0.5C, and the initial capacity D1 was recorded. Then, it was charged to 3.65V at 0.5C constant current and constant voltage, with a cut-off current of 0.02C, and the thickness T1, voltage and internal resistance of the battery were tested. After the fully charged battery was stored in a 60°C constant temperature box for 30 days, the hot thickness of the battery was measured and recorded as T2. After standing at room temperature for 4 hours, the cold thickness T3, voltage and internal resistance of the battery were tested, and then it was discharged to 2.0V at 0.5C, and the retention capacity D2 was recorded. After standing for 5 minutes, it was charged to 3.65V at 0.5C constant current and constant voltage, with a cut-off current of 0.02C. After standing for 5 minutes, it was discharged to 2.0V at 0.5C, and the recovery capacity D3 was recorded.

[0152] Battery capacity retention rate (%) = D2 / D1*100%;

[0153] Battery capacity recovery rate (%) = D3 / D1*100%

[0154] 3) Iron ion dissolution test:

[0155] Disassemble the battery after cycling, take out the negative electrode and the negative electrode side diaphragm of the battery, and dissolve them in a mixed solution of water: concentrated nitric acid = 2:1 (mass ratio). After complete digestion, take 20g of the digestion solution and divide it into 50ml reagent bottles, and then use inductively coupled plasma optical emission spectrometry (ICP-OES) to test the amount of iron ions dissolved into the negative electrode and diaphragm.

[0156] 1. Fill in the test results obtained in Examples 1 to 12 and Comparative Examples 1 to 2 into Table 1.

[0157] Table 1

[0158]

[0159] From the test data of Examples 1-12 and Comparative Examples 1 and 2, it can be seen that at high compaction density (2.5 g / cm 3), compared with the separate addition of vinylene carbonate (VC) and the compound shown in structural formula 1, the simultaneous addition of vinylene carbonate (VC) and the compound shown in structural formula 1 in the electrolyte significantly improves the 45°C high-temperature cycle and 60°C high-temperature storage performance and effectively inhibits the dissolution of iron ions. That is, the additive combination provided by the present invention can significantly improve the high-temperature cycle and storage performance of high-density lithium iron phosphate batteries.

[0160] It can be seen from the test data of Examples 1-8 that in a 2% VC non-aqueous electrolyte, as the content of the compound shown in Structural Formula 1 increases, the performance first improves and then deteriorates after 1000 cycles at 45°C 1C / 1C and 30 days of storage at 60°C. When the compound shown in Structural Formula 1 is added in an amount of 0.5% in the non-aqueous electrolyte, it can have a good coordination effect with 2% VC and can effectively improve the overall performance of the battery; when the content is greater than 0.5%, its residual amount in the positive electrode sheet also increases, and an excessive amount of the compound shown in Structural Formula 1 will increase the side reactions inside the battery, increase the battery impedance, and is not conducive to further improvement of the battery performance; when the content is less than 0.5%, the interface film formed at the negative electrode is not stable enough, and the performance improvement of the lithium iron phosphate battery is not obvious.

[0161] It can be seen from the test data of Examples 5, 9-12 that, based on the compound shown in the structural formula 1 with the above-determined addition amount of 0.5%, as the addition amount of vinylene carbonate (VC) increases, the high temperature cycle performance of the lithium ion battery first improves and then decreases, especially, when the addition amount of vinylene carbonate (VC) is 2%, it has the best matching effect with the compound shown in the structural formula 1. If the addition amount of vinylene carbonate (VC) is too low, the film formation is unstable, and the improvement of the high temperature cycle performance of the lithium iron phosphate battery is not obvious. If the content of vinylene carbonate (VC) is too much, the addition of the compound shown in the structural formula 1 cannot suppress the thickening of the interface film caused by the increase in the VC content, and the performance of the lithium iron phosphate battery will deteriorate instead.

[0162] 2. Fill in Table 2 with the test results obtained in Examples 5, 13 to 16 and Comparative Examples 1, 3 to 11.

[0163] Table 2

[0164]

[0165] The test results of Examples 5, 13-16 and Comparative Examples 1, 3-11 show that adding the compound of formula 1 to the electrolyte containing vinylene carbonate (VC) has an effect on the high compaction density (2.3-2.8 g / cm 3 ) The improvement of high temperature storage and cycle performance of lithium iron phosphate batteries is obvious, while for low compaction density (1.8-2.3g / cm 3)Although the high temperature storage and cycle performance of lithium iron phosphate batteries can be improved to a certain extent, the improvement effect is not obvious. That is, the compound shown in structural formula 1 has a certain effect on high compaction density lithium iron phosphate batteries (2.3-2.8g / cm 3 ) has a more significant effect on the high-temperature cycle and storage performance. The test results show that the compound represented by structural formula 1 and VC form an interface film on the electrode surface together, which improves the migration rate of lithium ions in high-density lithium iron phosphate batteries during high-temperature cycles, reduces the redox reaction between the electrolyte solvent and the electrode material, and inhibits the formation of lithium dendrites at the negative electrode interface, ultimately improving the high-temperature cycle and high-temperature storage performance of lithium iron phosphate batteries.

[0166] 3. Fill in Table 3 with the test results obtained in Examples 5, 17 to 27 and Comparative Examples 1 to 2.

[0167] Table 3

[0168]

[0169] Comparing the test data of Examples 5, 17-26 and Comparative Examples 1-2, it can be seen that at high compaction density (2.5 g / cm 3 ) in lithium iron phosphate batteries, adding different compounds of structural formula 1 to the electrolyte containing vinylene carbonate (VC) improves the 45°C high temperature cycle and 60°C high temperature storage performance to varying degrees, and effectively inhibits the dissolution of iron ions. This shows that the compound of structural formula 1 has a good effect on the performance of the battery at high compaction density (2.5g / cm 3 )'s lithium iron phosphate battery has good compatibility with vinyl carbonate (VC).

[0170] 4. Fill in Table 4 with the test results obtained in Examples 5, 28-29 and Comparative Examples 1, 12-13.

[0171] Table 4

[0172]

[0173]

[0174] It can be seen from the test data of Examples 5, 28-29 and Comparative Examples 1, 12-13 that further adding ethylene sulfate (DTD) or methylene disulfonate (MMDS) to the non-aqueous electrolyte system of vinylene carbonate (VC) and the compound shown in Structural Formula 1 can further improve the high-temperature cycling performance and high-temperature storage performance of high-density lithium iron phosphate batteries.

[0175] At the same time, from the test data of Example 5 and Comparative Examples 12-13, it can be seen that compared with adding ethylene sulfate (DTD) or methylene disulfonate (MMDS) to the non-aqueous electrolyte of vinyl carbonate (VC), the compound shown in Structural Formula 1 in the non-aqueous electrolyte of vinyl carbonate (VC) has a more prominent improvement effect on the high-density lithium iron phosphate battery.

[0176] 5. After the lithium ion batteries obtained in Example 5 and Comparative Example 1 were cycled 1000 times, they were disassembled to obtain the negative electrode sheets, and SEM imaging was performed. The test structure obtained in Example 5 is as follows: Figure 3 As shown, the test results obtained in Comparative Example 1 are as follows Figure 4 shown.

[0177] from Figure 3 and Figure 4 It can be seen that compared with comparative example 1, when only 2.0% VC is added to the electrolyte, there are obvious lithium dendrites on the negative electrode sheet after 1000 cycles ( Figure 4 In Example 1, when 0.5% of the compound shown in structural formula 1 is added on the basis of 2.0% VC, there is almost no lithium dendrite on the negative electrode after 1000 cycles. Therefore, adding a combination of the compound shown in structural formula 1 and vinylene carbonate (VC) can significantly inhibit the formation of lithium dendrites.

[0178] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A lithium iron phosphate battery, characterized in that: The invention comprises a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer, the compaction density of the positive electrode material layer is 2.3-2.8 g / cc, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises LiFePO4; The non-aqueous electrolyte includes a solvent, an electrolyte salt, vinylene carbonate and a compound shown in structural formula 1: ADBEC Structural formula 1 The compound represented by the structural formula 1 is selected from one or more of the following compounds: Compound 20 Compound 21 Compound 22 Compound 23 Compound 24 Compound 25 Compound 26 Compound 35 Compound 36 Compound 37 Compound 38 Compound 39 Compound 40; Based on the total mass of the non-aqueous electrolyte being 100%, the added amount of the compound represented by structural formula 1 is 0.1% to 5%, and the added amount of the vinylene carbonate is 0.5% to 3%; The surface of the positive electrode is detected by X-ray photoelectron spectroscopy. When the 1s peak of carbon is obtained at 284.5 eV, a characteristic peak of S appears in the region of 162-174 eV.

2. The lithium iron phosphate battery according to claim 1, characterized in that: Based on the total mass of the non-aqueous electrolyte being 100%, the added amount of the vinylene carbonate is 0.5-2%.

3. The lithium iron phosphate battery according to claim 1, characterized in that: The charging and discharging voltage range of the lithium iron phosphate battery is 2.0~3.8V.

4. The lithium iron phosphate battery according to any one of claims 1 to 3, characterized in that: A and C are identical to each other, A and B are identical to or different from each other, and D and E are identical to each other.

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