A positive electrode, a production method, and a battery

Abstract

The application discloses a positive electrode, a preparation method and a battery, and the positive electrode comprises a positive electrode active material and a binder A. The binder A is prepared by the reaction of 4,4'- (hexafluoroisopropylidene) diphenyl dicarboxylic anhydride and 3,5-diaminobenzonitrile, and contains characteristic structures such as imide benzene ring, fluoromethyl and nitrile group in the molecular structure. The positive electrode aims to solve the problems that the lithium ion battery positive electrode binder (such as PVDF) in the prior art has insufficient high-temperature and high-pressure resistance and poor cycle stability when applied to a high-nickel system, and by adopting the positive electrode, the direct current internal resistance (DCR) performance of the battery can be significantly improved, the cycle stability and capacity retention rate of the battery under normal temperature and high-temperature conditions can be greatly improved, and the overall safety of the battery can be expected to be enhanced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid-state lithium ion batteries, in particular to a positive electrode, a preparation method and a battery. BACKGROUND

[0002] Lithium ion batteries have many advantages such as large specific capacity, good cycle performance, small size, etc., and have been widely used in the electronic field, becoming one of the best choices for electric vehicle power batteries. As one of the important components of positive and negative electrode materials, the main function of the binder is to ensure the uniformity and safety performance during slurry preparation, and to act as a bonding bridge between active material particles and current collectors. During the cycle process of lithium ion batteries, the active material will undergo certain structural changes with the deintercalation of lithium ions, so the binder is required to play a stabilizing role.

[0003] PVDF (polyvinylidene fluoride) is currently the mainstream positive electrode binder, which benefits from its electrochemical stability and high adhesion performance. However, under the background of rapid development of lithium batteries, PVDF has experienced a shortage of supply and a rapid rise in price, which requires a new type of positive electrode binder to replace it. Currently, the modification of PVDF mainly has the following two directions: changing the molecular weight of the polymer to improve the adhesion performance, and changing the copolymer monomer type of polyvinylidene fluoride, such as polyvinylidene fluoride-tetrafluoroethylene-ethylene terpolymer, to enhance the mechanical properties and viscoelasticity of PVDF. Even so, traditional PVDF binders work based on a physical bonding mechanism, and therefore have limited tolerance to high pressure and high temperature due to the loosening of positive active materials and the weakening of the binding force, as well as the lack of thermal stability under aggressive conditions. Under the background of high energy density demand, high-nickel positive electrode materials urgently need new functional binders to replace traditional PVDF binders to improve their tolerance to high temperature and high pressure.

[0004] Therefore, there is an urgent need in the industry for a better way to solve the above problems. SUMMARY

[0005] Therefore, the present application significantly improves the electrochemical performance, thermal stability and potential safety of lithium ion batteries (especially those using high-nickel positive active materials) by using a specially designed fluorinated polyimide binder. The technical solution of the present application is as follows:

[0006] In one aspect, the present application provides a positive electrode, which comprises a positive electrode layer, the positive electrode layer comprising a positive electrode active material and a binder A;

[0007] The positive electrode active material is a high-nickel positive material LiNi a Co b Mn c O2, a≥0.8, a+b+c=1;

[0008] The binder A comprises a compound shown in the following structural formula:

[0009] ,

[0010] Wherein, n=150~600.

[0011] In an embodiment, in the positive electrode layer, the mass ratio of the binder A is 1.5%~2%.

[0012] In an embodiment, the weight average molecular weight of the binder A is 100000D~300000D.

[0013] In an embodiment, the positive electrode active material is at least one of LiNi 0.8 Co 0.1 Mn 0.1 O2、LiNi 0.9 Co 0.05 Mn 0.05 O2、LiNi 0.92 Co 0.04 Mn 0.04 O2、LiNi 0.95 Co 0.025 Mn 0.025 O2、LiNi 0.98 Co 0.01 Mn 0.01 O2.

[0014] In an embodiment, in the positive electrode layer, the mass ratio of the positive electrode active material is 96%~97%.

[0015] In an embodiment, the positive electrode layer further comprises a positive electrode conductive agent, and the positive electrode conductive agent comprises at least one of conductive carbon black, conductive graphite, carbon nanotube, carbon nanofiber and graphene.

[0016] Another aspect of the present application provides a preparation method of a positive electrode, comprising the following steps:

[0017] S1, dissolving 4,4'-(hexafluoroisopropylidene) diphenyl dicarboxylic anhydride and 3,5-diaminobenzonitrile in N,N-dimethylacetamide, and mixing under nitrogen atmosphere;

[0018] S2, warming and continuing to react;

[0019] S3, adding acetic anhydride and pyridine, and reacting;

[0020] S4, dropping the reaction solution into deionized water, and obtaining polymer precipitate by sedimentation, and obtaining the binder A by washing and drying;

[0021] S5, mixing the positive electrode active material, the binder A and the positive electrode conductive agent according to a mass ratio, adding a solvent to prepare a positive electrode slurry, and coating the positive electrode slurry on a positive electrode current collector to form a positive electrode.

[0022] In an embodiment, the molar ratio of 4,4-(hexafluoroisopropylidene)diphthalic anhydride to 3,5-diaminobenzonitrile in step S1 is 1: (0.5-2).

[0023] In an embodiment, the molar ratio of acetic anhydride to pyridine in step S3 is 1: (0.2-1).

[0024] The application also discloses a battery comprising the positive electrode, the negative electrode and the separator.

[0025] The application has the following advantages: the binder A is used as the binder of the high-nickel positive electrode, which can effectively improve the cycle performance of the battery and reduce the internal resistance of the battery.

[0026] The binder A comprises an imide benzene ring, which can effectively improve the thermal stability, and the functional group -CF3 provides greater electrochemical oxidation resistance and thermal stability for the polymer, which can effectively prevent the binder A from being oxidized by the high-nickel positive electrode material, and further, the presence of the -CN nitrile group can form an effective combination with the surface of the metal oxide particles through a chemical bond to form an anchor point, which is easy to form a more stable CEI relying on the anchor point, so that the positive electrode material particles remain relatively complete in repeated charge and discharge cycles, so as to improve the cycle performance of the battery. The highly stable chemical network provides a stable channel for ion transmission, and brings a lower battery internal resistance. DETAILED DESCRIPTION

[0027] The technical solutions of the application will be described clearly and completely below by combining the embodiments of the application. Obviously, the described embodiments are only some of the embodiments of the application, but not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used in the detailed description section of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application; the terms "including" and "comprising" and any variations thereof herein are intended to cover a non-exclusive inclusion.

[0029] In the description of the embodiments of the present application, the technical terms "first", "second", and the like are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is more than two, unless otherwise explicitly and specifically limited.

[0030] In the present application, the phrase "embodiments" means that the specific features, structures or properties described in conjunction with the embodiments can be included in at least one embodiment of the present application. The appearance of this phrase at various places in the specification does not necessarily refer to the same embodiment, nor is it independent or alternative to other embodiments. It is explicitly and implicitly understood by those skilled in the art that the described embodiments of the present application can be combined with other embodiments.

[0031] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in the present application generally represents an "or" relationship between the associated objects before and after it.

[0032] It should be noted that, for ease of description, in the following embodiments, all the same technical features are marked with the same symbol.

[0033] High-nickel cathode materials (such as NCM811, NCA, NCM9 series, etc.) are key materials for the next generation of high-energy-density lithium-ion batteries due to their high specific capacity (which can exceed 200 mAh / g) and high energy density. However, its large-scale application still faces a series of technical challenges and problems. In addition to the problem that high-nickel cathode materials are prone to lattice oxygen release, transition metal dissolution and structural collapse during long cycle, high-nickel cathode materials have strong oxidizing properties, which can easily cause the decomposition of the polymer binder, and then cause the problem of particle breakage and shedding of the cathode material, the cycle performance of the battery is deteriorated, on the other hand, after the failure of the binder, the connection between the active materials and the current collector is not tight enough, which will hinder the electron transport, thereby increasing the DCR.

[0034] Therefore, the present application provides a cathode, which comprises a cathode active material, a binder A; the cathode active material is a high-nickel cathode material LiNi a Co b Mn c O2, a≥0.8, a+b+c=1.

[0035] The binder A comprises a compound represented by the following structural formula:

[0036] wherein n = 150-600.

[0037] The binder A includes an imide benzene ring, which can effectively improve the thermal stability, and the functional group -CF3 provides greater resistance to electrochemical oxidation and thermal stability for the polymer, which can effectively prevent the binder A from being oxidized by the high-nickel positive electrode material. Further, the presence of the -CN nitrile group can form an effective bond with the surface of the metal oxide particles through chemical bonds to form anchor points, which are easy to form more stable CEI, so that the positive electrode material particles remain relatively complete in repeated charge and discharge cycles to improve the battery cycle performance. The highly stable chemical network provides a stable channel for ion transport, resulting in lower DCR.

[0038] In some specific embodiments, the positive electrode includes a positive electrode layer, the positive electrode layer includes the positive electrode active material and the binder, and the mass ratio of the binder in the positive electrode layer is 1.5%-2%. Exemplarily, it can be 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%. If the mass ratio of the binder is too high, on the one hand, it is not conducive to the improvement of the energy density of the battery, and on the other hand, as the mass ratio increases, the binder occupies the pores, which reduces the infiltration of the electrolyte and increases the internal resistance of the battery. If the mass ratio of the binder is too low, the bonding strength is not enough, and the ion transmission is affected.

[0039] Compared with the prior art, the binder A in the present application has stronger bonding ability and can achieve good bonding with less addition amount.

[0040] In some specific embodiments, the weight average molecular weight of the binder A is 100000D-300000D. Exemplarily, it can be 100000D, 108000D, 162000D, 200000D, 216000D, 270000D, or 300000D. A binder with a higher molecular weight can better bond the active material and the conductive agent together to form a continuous and stable conductive network, which is conducive to the conduction of electrons and reduces the internal resistance of the battery. At the same time, the greater the molecular weight of the positive electrode binder, the stronger the bonding, and the greater the peel strength of the pole piece. However, as the molecular weight of the binder increases, the toughness of the binder decreases and the deformation ability is poor.

[0041] In some specific embodiments, the positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.92 Co 0.04 Mn 0.04 O2, LiNi 0.95 Co0.025 Mn 0.025 O2, LiNi 0.98 Co 0.01 Mn 0.01 at least one of O2, LiNi

[0042] In some specific embodiments, the mass percentage of the positive electrode active material in the positive electrode layer is 96% to 97%. Exemplarily, it can be 96%, 96.1%, 96.2%, 96.3%, 96.4%, 96.5%, 96.6%, 96.7%, 96.9%, 96.9%, or 97%.

[0043] In some specific embodiments, the positive electrode layer further comprises a positive electrode conductive agent, and the positive electrode conductive agent comprises at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon nanofibers, and graphene.

[0044] In some specific embodiments, the positive electrode comprises a positive electrode current collector, which can exemplarily be aluminum (Al), but is not limited thereto.

[0045] The present application also provides a preparation method of the positive electrode as described above, comprising the following steps:

[0046] S100, preparing a binder A;

[0047] S200, preparing a positive electrode.

[0048] In some specific embodiments, the preparation process of the binder A is as follows:

[0049] S1, dissolving 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and 3,5-diaminobenzonitrile in N,N-dimethylacetamide, and mixing under a nitrogen atmosphere;

[0050] S2, warming and continuing the reaction;

[0051] S3, adding acetic anhydride and pyridine, and performing the reaction;

[0052] S4, dropping the reaction solution into deionized water, and obtaining a polymer precipitate by sedimentation, and obtaining the binder A by washing and drying.

[0053] In some specific embodiments, in step S1, the molar ratio of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride to 3,5-diaminobenzonitrile is 1: (0.5 to 2).

[0054] In some specific embodiments, in step S1, the specific mixing step is stirring at a temperature of 0 to 10°C for 5 to 10 h.

[0055] In some specific implementations, in step S2, the heating step is to heat the temperature to 20~25°C.

[0056] In some specific implementations, in step S2, the reaction step can specifically be a stirred reaction for 15-24 hours.

[0057] In some specific embodiments, in step S3, the molar ratio of acetic anhydride to pyridine is 1:(0.2~1).

[0058] In some specific implementations, step S3 further includes a heating step, where the temperature rises to 80~120°C.

[0059] In some specific implementations, the reaction time in step S3 is 20-30 h.

[0060] In some specific embodiments, step S3 includes a cooling step before adding deionized water, cooling the reaction solution to room temperature.

[0061] In some specific embodiments, in step S3, the washing and drying steps specifically involve filtering the precipitated polymer and washing it with a mixture of water and ethanol, and then vacuum drying it at 150°C to obtain binder A.

[0062] In some specific embodiments, step S200 includes the following steps: mixing the positive electrode active material, binder A, and positive electrode conductive agent according to the mass ratio, adding solvent, preparing a positive electrode slurry, and coating it on the positive electrode current collector to form a positive electrode.

[0063] The present invention also provides a battery comprising a positive electrode according to any of the above embodiments.

[0064] A battery also includes a negative electrode and a separator.

[0065] In some specific embodiments, the negative electrode includes a current collector and a layer of negative electrode active material disposed on the current collector.

[0066] In this invention, the specific type of negative electrode active material is not specifically limited and can be selected according to requirements. Specifically, the negative electrode active material is selected from natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 One or more of Li-Al alloys. Non-limiting examples of carbon materials include crystalline carbon, amorphous carbon, and mixtures thereof. Crystalline carbon can be amorphous or flake-shaped, small flake-shaped, spherical, or fibrous natural or artificial graphite. Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbides, calcined coke, etc.

[0067] In some embodiments, elemental metals and metal-based compounds can also be selected as the negative active material, such as compounds containing Li, Ag, Al, Bi, Cu, Ga, Ge, In, Ni, Pb, Sb, Si, Sn, Sr, Zn, and the like.

[0068] In some embodiments, the mass percentage of the negative active material contained in the negative active material layer can be 80-99%, such as 80%, 85%, 90%, 95%, 97%, 99%, and the like, preferably 95-97%.

[0069] In some embodiments, the negative active material layer can include a binder; the binder improves the binding between the negative active material particles and the binding between the negative active material and the current collector.

[0070] In some embodiments, non-limiting examples of the binder include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like.

[0071] In some embodiments, the negative active material layer can be obtained by coating a negative electrode slurry on the negative current collector and then performing drying and the like, the negative electrode slurry including at least the negative active material and the negative binder. When an aqueous solvent is used as the liquid medium for forming the negative electrode slurry, it is preferable to use a viscosity enhancer for slurry formation, which is generally used to adjust the viscosity of the slurry.

[0072] In some embodiments, the aforementioned viscosity enhancer can be one or more of the following: carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphated starch, casein and its salts, and the like.

[0073] In some embodiments, the mass percentage of the viscosity enhancer in the negative electrode slurry can be 0.1-5%, such as 0.1%, 0.2%, 0.5%, 0.6%, 1%, 2%, 3%, 4%, 5%, and the like, preferably 0.5-3%, and further preferably 0.6-2%.

[0074] In some embodiments, the negative active material layer includes a conductive material to make the electrode electrically conductive. The conductive material can include any electrically conductive material that does not cause chemical changes. Non-limiting examples of the conductive material include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, etc.), metal-based materials (e.g., metal powder, metal fiber, etc., such as copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0075] In some embodiments, the negative current collector can be selected from a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with an electrically conductive metal, and combinations thereof.

[0076] In some embodiments, the battery of the present application includes a separator between the positive electrode and the negative electrode to prevent short circuiting. The material and shape of the separator used in the battery of the present application are not particularly limited and can be any of the techniques disclosed in the prior art.

[0077] In some embodiments, the separator includes a polymer or an inorganic material formed of a material stable to the electrolyte of the present application, etc.

[0078] In some embodiments, the separator can include a base material layer and a surface treatment layer. The base material layer is a non-woven fabric, a film, or a composite film having a porous structure, and the material of the base material layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide.

[0079] In some embodiments, the battery of the present application further includes an electrolyte.

[0080] In some embodiments, the electrolyte includes a lithium salt and a solvent.

[0081] In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments of the present application, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis-trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalato)borate LiB(C2O4)2 (LiBOB), or lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB).

[0082] In some embodiments, the solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0083] In some embodiments, the electrolyte solution can further include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and an additive that improves certain properties of the battery, such as an additive that improves overcharge performance of the battery, an additive that improves high-temperature performance of the battery, an additive that improves low-temperature performance of the battery, and the like.

[0084] In some embodiments, the additive includes at least one of 1,3-propane sultone, 1,4-butane sultone, propenyl-1,3-sulfonic acid lactone, ethylene sulfate, 4-methyl ethylene sulfate, propylene sulfate, saturated phosphorus ester compounds and unsaturated phosphorus ester compounds, tris(trimethylsilyl) phosphate, tris(trimethylsilyl) borate, tris(triethylsilyl) borate, butanedinitrile, pentanedinitrile, ethylene glycol bis(propionitrile) ether, hexanetetracarboxylic acid, hexanedinitrile, heptanedinitrile, octanedinitrile, nonanedinitrile, decanedinitrile, and a compound including the following formula: .

[0085] wherein R 41 , R 42 , R 43 are each independently selected from a saturated hydrocarbon group, an unsaturated hydrocarbon group, a halogenated hydrocarbon group, -Si(C m H 2m+1 )3, m is a natural number of 1 to 3, and R 41 , R 42 , R 43 at least one of which is an unsaturated hydrocarbon group.

[0086] The concept and the resulting technical effects of the present application will be described below in conjunction with examples in a clear and complete manner, so as to fully understand the purpose, features and effects of the present application. It is obvious that the described examples are only a part of the embodiments of the present application, rather than all the embodiments.

[0087] Embodiment 1: This embodiment provides a positive electrode: comprising a positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2, a binder A, the binder A comprising the following structure:

[0088] ;

[0089] n=370; the weight average molecular weight of the binder A is 200000D.

[0090] The positive electrode comprises a positive electrode layer, and the mass fraction of the binder A in the positive electrode layer is 1.8%.

[0091] In the positive electrode layer, the mass ratio of the positive electrode active material, the binder A, and the positive electrode conductive agent is 96.2:1.8:2.

[0092] The positive electrode conductive agent is composed of conductive carbon black and multi-walled carbon nanotubes.

[0093] The preparation method of the positive electrode:

[0094] 1) Preparation of the binder A:

[0095] S1, 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and 3,5-diaminobenzonitrile are dissolved in N,N-dimethylacetamide at a molar ratio of 1:1, stirred at 0°C under nitrogen atmosphere for 5 hours;

[0096] S2, the temperature is increased to 24°C, and the reaction is stirred for 17h;

[0097] S3, acetic anhydride and pyridine are added to the reaction solution at a ratio of 1:1, and the mixture is heated to 80°C and continues to be stirred, and after the reaction for 24 hours, the obtained polymer solution is cooled to room temperature;

[0098] S4, distilled water is added dropwise to the polymer solution to precipitate, and the polymer precipitate is filtered, washed with a mixed solution of water and ethanol, and dried to obtain the binder A. The volume ratio of water and ethanol is 2:1, and the drying step is vacuum drying at 150°C.

[0099] 2) Preparation of the positive electrode:

[0100] ①The positive electrode active material, the positive electrode binder, and the positive electrode conductive agent are mixed uniformly at a mass ratio of 96.2:1.8:2, added into N-methylpyrrolidone solvent to form a slurry, coated on the positive electrode current collector, and dried to form a positive electrode sheet.

[0101] II. Preparation of the negative electrode

[0102] The negative active material graphite, the negative binder styrene-butadiene rubber emulsion, the negative conductive agent conductive carbon black, and the dispersant sodium carboxymethyl cellulose are mixed uniformly according to a mass ratio of 92:3:3:2 to form a slurry, which is coated on a negative current collector and dried to form a negative electrode sheet.

[0103] The above positive electrode and negative electrode are assembled to form a battery cell.

[0104] Example 2: The difference between this example and Example 1 is that, in this example, the mass ratio of the binder A in the positive electrode layer is 1.5%. The remaining technical features are the same.

[0105] Example 3: The difference between this example and Example 1 is that, in this example, the mass ratio of the binder A in the positive electrode layer is 2%. The remaining technical features are the same.

[0106] Example 4: The difference between this example and Example 1 is that, in this example, the weight average molecular weight of the binder A is 100000D. The remaining technical features are the same.

[0107] Example 5: The difference between this example and Example 1 is that, in this example, the weight average molecular weight of the binder A is 300000D. The remaining technical features are the same.

[0108] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that, in Comparative Example 1, polyvinylidene fluoride (PVDF) is used as the positive electrode binder. The remaining technical features are the same.

[0109] The batteries of Examples 1-5 are tested as follows, and the test results are recorded in Table 1 below.

[0110] Cycle performance test:

[0111] At 25℃, charge to the charge cut-off voltage 4.25V at a charge current of 1C, stand for 30s, discharge to the discharge cut-off voltage 2.8V at a discharge current of 1C, which is 1 cycle, repeat the charge-discharge cycle for 1500 times, capacity retention rate = the 1500th discharge capacity / first discharge capacity.

[0112] Battery internal resistance test:

[0113] 1) Discharge the full battery to 50% SOC;

[0114] 2) Stand for 30 minutes;

[0115] 3) Test at three temperature conditions of 25℃, 0℃, and -20℃, respectively;

[0116] 4) Stand the battery for 10s, record the initial voltage V0 and current I0.

[0117] Apply discharging pulse: discharge at -2C current for 10 seconds;

[0118] Record the voltage V1 and current I1 at the end of discharging.

[0119] Calculate discharging DCR: DCR discharging = (V0-V1) / (I1-I0).

[0120]

[0121] From Table 1, it can be seen that the DCR data of the binder A in Comparative Examples 1-5 and Comparative Example 1 are obviously improved compared with the conventional PVDF. The cycle capacity retention rate in Table 1 shows that the binder has more advantages in high-temperature long cycle, which is due to the particularity of the polymer structure. The imide benzene ring and the functional group -CF3 provide the polymer with greater resistance to electrochemical oxidation and thermal stability. The presence of -CN nitrile group can form effective binding with the surface of metal oxide particles through chemical bonds, so as to inhibit the degradation of high-nickel positive electrode particles in the electrolyte. The highly stable chemical network provides a stable channel for ion transmission, resulting in lower DCR and better long cycle performance.

[0122] Comparative Examples 1-3, the DCR data of the binder A with different addition amounts in the positive electrode layer show a trend of first decreasing and then slightly increasing. The cycle capacity retention rate shows that the cycle performance gradually improves with the increase of the addition amount. The main reason is that the increase of the addition amount of fluorinated polyimide makes the adhesion between the positive electrode particles and the particles-foil more closely, which is manifested as the decrease of DCR. However, with the further increase of the addition amount, the binder occupies the pores, which makes the electrolyte infiltration decrease, resulting in the increase of DCR. In the later cycle stage, the adhesion strength of Example 3 with high addition amount can be well maintained, so the capacity retention rate is obviously improved compared with Example 2 with low addition amount.

[0123] Comparative Example 1 and Examples 4-5, the DCR decreases with the increase of the molecular weight. The cycle capacity retention rate shows that the cycle performance gradually improves with the increase of the molecular weight. The possible reason is that when the molecular weight of the binder is low, the adhesion network formed in the electrode is not firm enough, and the connection between the active substances and the current collector is not close enough, which will hinder the electron transmission, thus increasing the DCR. In the later cycle stage, the adhesion strength of the high molecular weight example can be well maintained, and it is not easy to break off, so the capacity retention rate is obviously improved compared with the basic group.

[0124] Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the features disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the application being indicated by the following claims.

Claims

1. A positive electrode, characterized in that, The positive electrode layer includes a positive electrode active material and a binder A. The positive electrode active material is a high-nickel positive electrode material LiNi. a Co b Mn c O2, a≥0.8, a+b+c=1; The adhesive A comprises a compound with the following structural formula: Wherein, n=150~600; the weight-average molecular weight of the adhesive A is 100000D~300000D.

2. The positive electrode according to claim 1, characterized in that, In the positive electrode layer, the mass percentage of binder A is 1.5% to 2%.

3. The positive electrode according to claim 1, characterized in that, The positive electrode active material includes LiNi. 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.92 Co 0.04 Mn 0.04 O2, LiNi 0.95 Co 0.025 Mn 0.025 O2, LiNi 0.98 Co 0.01 Mn 0.01 At least one of O2.

4. A positive electrode according to claim 1, characterized in that, In the positive electrode layer, the mass percentage of the positive electrode active material is 96% to 97%.

5. A positive electrode according to claim 1, characterized in that, The positive electrode layer further includes a positive electrode conductive agent, which includes at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon nanofibers, and graphene.

6. A method for preparing a positive electrode, used to prepare the positive electrode as described in any one of claims 1-5, characterized in that, The method includes the following steps: S1. Dissolve 4,4'-(hexafluoroisopropylidene) phthalic anhydride and 3,5-diaminobenzonitrile in N,N-dimethylacetamide and mix under a nitrogen atmosphere; S2, Increase the temperature and continue the reaction; S3. Add acetic anhydride and pyridine to carry out the reaction; S4. Deionized water is added dropwise to the reaction solution, and the polymer precipitate is obtained by sedimentation. After washing and drying, binder A is obtained. S5. Mix the positive electrode active material, binder A, and positive electrode conductive agent according to the mass ratio, add solvent, prepare positive electrode slurry, and coat it on the positive electrode current collector to form a positive electrode.

7. The method for preparing a positive electrode according to claim 6, characterized in that, In step S1, the molar ratio of 4,4-(hexafluoroisopropylidene) phthalic anhydride and 3,5-diaminobenzonitrile is 1:(0.5~2).

8. The method for preparing a positive electrode according to claim 6, characterized in that, In step S3, the molar ratio of acetic anhydride to pyridine is 1:(0.2~1).

9. A battery comprising a negative electrode and a separator, characterized in that, It also includes the positive electrode as described in any one of claims 1 to 5, or, The positive electrode prepared by the method according to any one of claims 6-8.

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