Positive electrode, preparation method and battery

Abstract

The invention discloses a positive electrode, a preparation method and a battery. The positive electrode comprises a positive electrode active material and a binder A, the binder A is prepared from 4, 4 '-(hexafluoroisopropylidene) diphthalic anhydride and 3, 5-diaminobenzonitrile through a reaction, and the molecular structure of the binder A contains characteristic structures such as an imide benzene ring, a fluoromethyl group and a nitrile group. The positive electrode aims at solving the problems that in the prior art, when a lithium ion battery positive electrode binder (such as PVDF) is applied to a high-nickel system, the high-temperature and high-pressure resistance performance is insufficient, and the cycling stability is poor, by adopting the positive electrode, the direct current internal resistance (DCR) performance of the battery can be remarkably improved, and the service life of the battery is prolonged. The cycling stability and the capacity retention ratio of the battery under normal-temperature and high-temperature conditions are greatly improved, and the overall safety of the battery is expected to be enhanced.

Description

Technical Field

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

[0002] Lithium-ion batteries, with their numerous advantages, including high specific capacity, excellent cycle performance, and compact size, have been widely used in the electronics field and have become one of the best choices for electric vehicle power batteries. Binders, as a crucial component of positive and negative electrode materials, primarily ensure uniformity and safety during slurry preparation, acting as a bonding bridge between active material particles and the current collector. During the cycling of lithium-ion batteries, the active material undergoes structural changes as lithium ions are intercalated and deintercalated, necessitating a stabilizing effect.

[0003] Currently, PVDF (polyvinylidene fluoride) is the mainstream cathode binder due to its electrochemical stability and efficient bonding properties. However, amid the rapid development of lithium batteries, PVDF has experienced supply shortages and rapidly rising prices, necessitating the search for a novel alternative cathode binder. Currently, PVDF modification focuses on two main approaches: altering the polymer's molecular weight to improve bonding properties and modifying the comonomer type of vinylidene fluoride, such as polyvinylidene fluoride-tetrafluoroethylene-ethylene terpolymers, to enhance PVDF's mechanical properties and viscoelasticity. Even so, traditional PVDF binders operate based on a physical bonding mechanism. Due to loosening and weakened bonding to the cathode active material, as well as a lack of thermal stability under aggressive conditions, their tolerance to high pressure and high temperature is limited. Given the demand for high energy density, high-nickel cathode materials urgently need to replace traditional PVDF binders with new functional binders to improve the tolerance of high-nickel materials under high temperature and pressure.

[0004] Therefore, the industry urgently needs a better way to solve the above problems. Summary of the Invention

[0005] In view of this, the present invention utilizes a specially designed fluorinated polyimide binder to significantly enhance the electrochemical performance, thermal stability, and potential safety of lithium-ion batteries (especially those using high-nickel cathode active materials). The technical solution of the present invention is as follows: In one aspect, the present invention provides a positive electrode, comprising a positive electrode layer, wherein the positive electrode layer comprises 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 binder A includes a compound shown in the following structural formula: , Among them, n=150~600.

[0006] In one embodiment, in the positive electrode layer, the binder A accounts for 1.5% to 2% by mass.

[0007] In one embodiment, the weight average molecular weight of the binder A is 100,000D-300,000D.

[0008] In one embodiment, 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 Co 0.025 Mn 0.025 O2、LiNi 0.98 Co 0.01 Mn 0.01 At least one of O2.

[0009] In one embodiment, in the positive electrode layer, the mass proportion of the positive electrode active material is 96% to 97%.

[0010] In one embodiment, the positive electrode layer further includes a positive electrode conductive agent, and the positive electrode conductive agent includes at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon nanofibers, and graphene.

[0011] Another aspect of the present invention provides a method for preparing a positive electrode, comprising the following steps: S1. Dissolve 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and 3,5-diaminobenzonitrile in N,N-dimethylacetamide and mix under a nitrogen atmosphere; S2, heating and continuing the reaction; S3, adding acetic anhydride and pyridine to react; S4, adding deionized water dropwise to the reaction solution to obtain a polymer precipitate, which was washed and dried to obtain a binder A; S5. Mix the positive electrode active material, binder A, and positive electrode conductive agent according to a mass ratio, add a solvent to prepare a positive electrode slurry, and apply it on the positive electrode current collector to form a positive electrode.

[0012] In one embodiment, in step S1, the molar ratio of 4,4-(hexafluoroisopropylidene)diphthalic anhydride to 3,5-diaminobenzonitrile is 1:(0.5-2).

[0013] In one embodiment, in step S3, the molar ratio of acetic anhydride to pyridine is 1:(0.2-1).

[0014] The present invention also discloses a battery comprising the above-mentioned positive electrode, negative electrode and separator.

[0015] The advantages of the present invention are as follows: The present invention uses binder A 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.

[0016] Binder A includes an imide benzene ring, which effectively improves thermal stability. The functional group -CF3 provides the polymer with greater resistance to electrochemical oxidation and thermal stability, effectively preventing Binder A from being oxidized by high-nickel cathode materials. Furthermore, the presence of the -CN nitrile group effectively forms anchor points with the surface of the metal oxide particles through chemical bonds. This anchor point facilitates the formation of a more stable CEI, allowing the cathode material particles to remain relatively intact during repeated charge and discharge cycles, thereby improving battery cycle performance. The highly stable chemical network provides a stable channel for ion transport, resulting in lower battery internal resistance. DETAILED DESCRIPTION

[0017] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0018] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by technicians in the technical field to which the present invention belongs; the terms used in the specific embodiments are only for the purpose of describing specific embodiments and are not intended to limit the present invention; the terms "including" and "having" in the description and claims of the present invention and any variations thereof are intended to cover non-exclusive inclusions.

[0019] In the description of the specific embodiments of the present invention, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the technical features indicated. In the description of the embodiments of the present invention, the meaning of "plurality" is more than two, unless otherwise specifically defined.

[0020] References to "embodiments" in this disclosure mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the disclosure. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0021] In the description of the embodiments of the present invention, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exists simultaneously, and B exists alone. In addition, the character " / " in this invention generally indicates that the associated objects are in an "or" relationship.

[0022] It should be noted that, for the convenience of description, in the following embodiments, all identical technical features are marked with the same symbols.

[0023] High-nickel cathode materials (such as NCM811, NCA, and NCM9 series) are key materials for next-generation high-energy-density lithium-ion batteries due to their high specific capacity (over 200 mAh / g) and high energy density. However, their large-scale application still faces a series of technical challenges and problems. In addition to the inherent susceptibility of high-nickel cathode materials to lattice oxygen release, transition metal dissolution, and subsequent structural collapse during long-term cycling, high-nickel cathode materials are highly oxidizing, which can easily lead to the decomposition of polymer binders, resulting in the breakage and shedding of cathode material particles, degrading the battery's cycling performance. Furthermore, after bonding failure, the connections between active materials and between active materials and current collectors are not tight enough, which can hinder electron transport and increase the DCR.

[0024] Therefore, the present invention proposes a positive electrode, comprising 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.

[0025] The binder A includes a compound shown in the following structural formula: , Among them, n=150~600.

[0026] Binder A includes an imide benzene ring, which effectively improves thermal stability. The functional group -CF3 provides the polymer with greater resistance to electrochemical oxidation and thermal stability, effectively preventing Binder A from being oxidized by high-nickel cathode materials. Furthermore, the presence of the -CN nitrile group effectively forms anchor points with the surface of the metal oxide particles through chemical bonds. This anchor point facilitates the formation of a more stable CEI, allowing the cathode material particles to remain relatively intact during repeated charge and discharge cycles, thereby improving battery cycle performance. The highly stable chemical network provides a stable channel for ion transport, resulting in a lower DCR.

[0027] In some specific embodiments, the positive electrode includes a positive electrode layer, the positive electrode layer including the positive electrode active material and a binder, and the binder in the positive electrode layer has a mass ratio of 1.5% to 2%. For example, it can be 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2.0%. If the mass ratio of the binder is too high, on the one hand, it is not conducive to improving the energy density of the battery. On the other hand, as the mass ratio increases, the binder occupies the pores, causing the electrolyte infiltration to decrease and the internal resistance of the battery to increase. If the mass ratio of the binder is too low, the bonding strength is insufficient, and the transmission of ions is affected.

[0028] Compared with the prior art, the adhesive A in the present application has stronger bonding ability and can achieve good bonding with a smaller addition amount.

[0029] In some specific embodiments, the weight-average molecular weight of the binder A is 100,000D to 300,000D. For example, it can be 100,000D, 108,000D, 162,000D, 200,000D, 216,000D, 270,000D, or 300,000D. Binders with higher molecular weights can better bond active materials and conductive agents together, forming a continuous and stable conductive network that facilitates electron conduction and reduces battery internal resistance. At the same time, the larger the molecular weight of the positive electrode binder, the stronger the adhesion and the greater the peel strength of the electrode sheet. However, an increase in the binder molecular weight will result in a decrease in the binder's toughness and poor deformation capacity.

[0030] 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 Co 0.025 Mn 0.025 O2、LiNi 0.98Co 0.01 Mn 0.01 At least one of O2.

[0031] In some specific embodiments, the mass proportion of the positive electrode active material in the positive electrode layer is 96% to 97%, for example, 96%, 96.1%, 96.2%, 96.3%, 96.4%, 96.5%, 96.6%, 96.7%, 96.9%, 96.9%, or 97%.

[0032] In some specific embodiments, the positive electrode layer further includes a positive electrode conductive agent, and the positive electrode conductive agent includes at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon nanofibers, and graphene.

[0033] In some specific embodiments, the positive electrode includes a positive electrode current collector. For example, the positive electrode current collector may be aluminum (Al), but is not limited thereto.

[0034] The present invention also provides a method for preparing the positive electrode as described above, comprising the following steps: S100, preparing adhesive A; S200, preparing a positive electrode.

[0035] In some specific embodiments, the preparation process of binder A is as follows: S1. Dissolve 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and 3,5-diaminobenzonitrile in N,N-dimethylacetamide and mix under a nitrogen atmosphere; S2, heating and continuing the reaction; S3, adding acetic anhydride and pyridine to react; S4. Deionized water is added dropwise to the reaction solution to obtain a polymer precipitate, which is then washed and dried to obtain a binder A.

[0036] In some specific embodiments, in step S1, the molar ratio of 4,4-(hexafluoroisopropylidene)diphthalic anhydride to 3,5-diaminobenzonitrile is 1:(0.5-2).

[0037] In some specific embodiments, in step S1, the specific step of mixing is stirring at a temperature of 0-10° C. for 5-10 h.

[0038] In some specific embodiments, in step S2, the temperature raising step is to raise the temperature to 20-25°C.

[0039] In some specific embodiments, in step S2, the reaction step can be specifically stirred for 15-24 hours.

[0040] In some specific embodiments, in step S3, the molar ratio of acetic anhydride to pyridine is 1:(0.2-1).

[0041] In some specific embodiments, step S3 further includes a temperature raising step, wherein the temperature is raised to 80-120°C.

[0042] In some specific embodiments, in step S3, the reaction time is 20-30 h.

[0043] In some specific embodiments, in step S3, a cooling step is further included before adding deionized water, and the reaction solution is cooled to room temperature.

[0044] In some specific embodiments, in step S3, the washing and drying steps are specifically filtering the precipitated polymer and washing it with a mixture of water and ethanol, and vacuum drying it at 150°C to obtain the binder A.

[0045] In some specific embodiments, step S200 includes the following steps: 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 mixture on the positive electrode current collector to form a positive electrode.

[0046] The present invention also provides a battery comprising the positive electrode of any one of the above embodiments.

[0047] The battery also includes a negative electrode and a separator.

[0048] In some specific embodiments, the negative electrode includes a current collector and a negative electrode active material layer disposed on the current collector.

[0049] In the present invention, the specific types of negative electrode active materials are not subject to specific restrictions and can be selected according to needs. Specifically, the negative electrode active material is selected from natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structured lithiated TiO2-Li4Ti5O 12 , Li-Al alloy. Non-limiting examples of carbon materials include crystalline carbon, amorphous carbon, and mixtures thereof. Crystalline carbon can be amorphous or flake-shaped, platelet-shaped, spherical, or fibrous natural graphite or artificial graphite. Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbide, calcined coke, etc.

[0050] In some specific embodiments, metal elements and metal compounds can also be selected as negative electrode active materials, such as compounds containing metals or metalloids such as Li, Ag, Al, Bi, Cu, Ga, Ge, In, Ni, Pb, Sb, Si, Sn, Sr, and Zn.

[0051] In some specific embodiments, the mass proportion of the negative electrode active material contained in the negative electrode active material layer may be 80%-99%, for example, 80%, 85%, 90%, 95%, 97%, 99%, etc., preferably 95%-97%.

[0052] In some specific embodiments, the negative electrode active material layer may include a binder; the binder improves the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector.

[0053] In some specific embodiments, non-limiting examples of binders include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like.

[0054] In some specific embodiments, the negative electrode active material layer can be formed by coating a negative electrode slurry on a negative electrode current collector, followed by drying and other operations. The negative electrode slurry includes at least a negative electrode active material and a negative electrode binder. When an aqueous solvent is used as the liquid medium for forming the negative electrode slurry, a thickener is preferably used for slurrying. The thickener is generally used to adjust the viscosity of the slurry.

[0055] In some specific 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, phosphorylated starch, casein, and salts thereof.

[0056] In some specific embodiments, the mass proportion of the thickener in the negative electrode slurry can be 0.1%-5%, for example, 0.1%, 0.2%, 0.5%, 0.6%, 1%, 2%, 3%, 4%, 5%, etc., preferably 0.5%-3%, and more preferably 0.6%-2%.

[0057] In some specific embodiments, the negative electrode active material layer includes a conductive material, thereby making the electrode conductive. The conductive material may include any conductive material as long as it does not cause chemical changes. Non-limiting examples of conductive materials 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.

[0058] In some specific embodiments, the negative electrode current collector can be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0059] In some specific embodiments, the battery of the present invention is provided with a separator between the positive electrode and the negative electrode to prevent short circuit. The present invention has no particular limitation on the material and shape of the separator used in the battery, which can be any technology disclosed in the prior art.

[0060] In some specific embodiments, the separator includes a polymer or an inorganic substance formed of a material that is stable to the electrolyte of the present application.

[0061] In some specific embodiments, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, film or composite film with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide.

[0062] In some specific embodiments, the battery according to the present invention further comprises an electrolyte.

[0063] In some specific embodiments, the electrolyte includes a lithium salt and a solvent.

[0064] In some specific embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments of the present invention, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalatoborate) LiB(C2O4)2 (LiBOB), or lithium difluorooxalatoborate LiBF2(C2O4) (LiDFOB).

[0065] In some specific embodiments, the solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl 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), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0066] In some specific embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature performance.

[0067] In some specific embodiments, the additives include at least one of 1,3-propane sultone, 1,4-butane sultone, propenyl-1,3-sultone, vinyl sulfate, 4-methyl vinyl sulfate, propenyl sulfate, saturated phosphate compounds and unsaturated phosphate compounds, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, tris(triethylsilane) borate, succinonitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrinitrile, adiponitrile, pimelonitrile, suberonitrile, azelaic acid dinitrile, and sebaconitrile, and a compound comprising the following chemical formula: .

[0068] Among them, R 41 、R 42 、R 43 Each independently selected from C1-C5 saturated hydrocarbon group, unsaturated hydrocarbon group, halogenated hydrocarbon group, -Si(C m H 2m+1 )3, m is a natural number from 1 to 3, and R 41 、R 42 、R 43 At least one of them is an unsaturated hydrocarbon group.

[0069] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them.

[0070] Example 1: This embodiment provides a positive electrode: including positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2, binder A, binder A includes the following structure: ;

[0071] n=370; the weight average molecular weight of binder A is 200,000D.

[0072] The positive electrode includes a positive electrode layer, and the mass proportion of the binder A in the positive electrode layer is 1.8%.

[0073] 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.

[0074] The positive electrode conductive agent consists of conductive carbon black and multi-walled carbon nanotubes.

[0075] Preparation method of positive electrode: 1) Preparation of binder A: S1. Dissolve 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and 3,5-diaminobenzonitrile in N,N-dimethylacetamide at a molar ratio of 1:1, and stir at 0°C for 5 hours under a nitrogen atmosphere; S2, the temperature was raised to 24 ° C, and the reaction was stirred for 17 hours; S3, adding acetic anhydride and pyridine in a ratio of 1:1 to the reaction solution, and heating the mixture to 80° C. and continuing to stir. After reacting for 24 hours, the resulting polymer solution was cooled to room temperature; S4. Distilled water is added dropwise to the polymer solution to allow sedimentation to obtain a polymer precipitate, which is filtered, washed with a mixed solution of water and ethanol, and dried to obtain a binder A. The volume ratio of water to ethanol is 2:1, and the drying step is vacuum drying at 150°C.

[0076] 2) Preparation of positive electrode: ① The positive electrode active material, positive electrode binder, and positive electrode conductive agent are mixed evenly in a mass ratio of 96.2:1.8:2, added to N-methylpyrrolidone solvent to form a slurry, coated on the positive electrode collector, and dried to form a positive electrode sheet.

[0077] 2. Preparation of negative electrode The negative electrode active material graphite, the negative electrode binder styrene-butadiene rubber latex, the negative electrode conductive agent conductive carbon black, and the dispersant sodium carboxymethyl cellulose are mixed evenly in a mass ratio of 92:3:3:2 to form a slurry, which is coated on the negative electrode collector and dried to form a negative electrode sheet.

[0078] The positive electrode and negative electrode are assembled to form a battery cell.

[0079] Example 2: This example differs from Example 1 in that the binder A in the positive electrode layer accounts for 1.5% by mass. The remaining technical features are the same.

[0080] Example 3: This example differs from Example 1 in that the binder A in the positive electrode layer accounts for 2% by mass. The remaining technical features are the same.

[0081] Example 4: This example differs from Example 1 in that, in this example, the weight average molecular weight of the binder A is 100,000 D. The remaining technical features are the same.

[0082] Example 5: This example differs from Example 1 in that, in this example, the weight average molecular weight of the binder A is 300,000 D. The remaining technical features are the same.

[0083] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that polyvinylidene fluoride (PVDF) is used as the positive electrode binder in Comparative Example 1. The remaining technical features are the same.

[0084] The batteries of Examples 1 to 5 were tested as follows, and the test results are recorded in Table 1 below.

[0085] Cyclic performance test: At 25°C, charge with a constant current of 1C to a charge cut-off voltage of 4.25V; let it stand for 30s, and discharge with a constant current of 1C to a discharge cut-off voltage of 2.8V; this is one cycle. Repeat the charge and discharge cycle 1500 times. The capacity retention rate = 1500th discharge capacity / first discharge capacity.

[0086] Battery internal resistance test: 1) Discharge the fully charged battery to 50% SOC; 2) Let it sit for 30 minutes; 3) Tested at three temperatures: 25°C, 0°C, and -20°C; 4) Let the battery sit for 10 seconds and record the initial voltage V0 and current I0.

[0087] Apply discharge pulse: discharge at -2C current for 10 seconds; Record the voltage V1 and current I1 at the end of discharge.

[0088] Calculate discharge DCR: DCR discharge = (V0 − V1) / (I1 − I0).

[0089]

[0090] As shown in Table 1, compared with Examples 1-5 and Comparative Example 1, the DCR data of Binder A are significantly improved compared to conventional PVDF. The cycle capacity retention rate in Table 1 shows that the advantages of this binder are more evident in high-temperature and long-cycle conditions, which is due to the uniqueness 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 the -CN nitrile group can effectively form chemical bonds with the surface of the metal oxide particles, thereby inhibiting the degradation of the high-nickel cathode particles in the electrolyte. The highly stable chemical network provides a stable channel for ion transport, resulting in a lower DCR and better long-cycle performance.

[0091] Comparing Examples 1 to 3, the DCR data of binder A with different addition amounts in the positive electrode layer showed a trend of first decreasing and then slightly increasing. The cycle capacity retention rate found that with the increase in the addition amount, the cycle performance gradually improved. This was mainly attributed to the increase in the addition amount of fluorinated polyimide, which made the bonding between the positive electrode particles and between the particles and the foil tighter, which was manifested as a decrease in DCR. However, as the addition amount further increased, the binder occupied the pores, which reduced the electrolyte infiltration and led to an increase in DCR. In the later stage of the cycle, since the bonding strength of Example 3 with a high addition amount can be better maintained, the capacity retention rate is significantly improved compared with Example 2 with a low addition amount.

[0092] Comparing Example 1 with Examples 4-5, the DCR decreases with increasing molecular weight, and the cycle capacity retention rate shows that the cycle performance gradually improves with increasing molecular weight. The possible reason is that when the molecular weight of the binder is low, the bonding network formed in the electrode is not strong enough, and the connection between the active materials and between the active materials and the current collector is not tight enough, which will hinder electron transport and increase the DCR. In the later stage of the cycle, the bonding strength of the high molecular weight examples can be better maintained and is not easy to break and fall off, so the capacity retention rate is significantly improved compared to the basic group.

[0093] Other embodiments of the present invention will readily occur to those skilled in the art after considering the specification and practicing the disclosure disclosed herein. The present invention is intended to cover any variations, uses, or adaptations of the present invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein.

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 binder A includes a compound shown in the following structural formula: ， Among them, n=150~600.

2. A positive electrode according to claim 1, characterized in that In the positive electrode layer, the binder A accounts for 1.5% to 2% by mass.

3. A positive electrode according to claim 1, characterized in that The weight average molecular weight of the binder A is 100,000D to 300,000D.

4. A 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.

5. A positive electrode according to claim 2, characterized in that: In the positive electrode layer, the mass proportion of the positive electrode active material is 96% to 97%.

6. A positive electrode according to claim 2, characterized in that: The positive electrode layer further includes a positive electrode conductive agent, and the positive electrode conductive agent includes at least one of conductive carbon black, conductive graphite, carbon nanotubes, carbon nanofibers, and graphene.

7. A method for preparing a positive electrode, for preparing the positive electrode according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: S1, dissolving 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and 3,5-diaminobenzonitrile in N,N-dimethylacetamide and mixing them under a nitrogen atmosphere; S2, heating and continuing the reaction; S3, adding acetic anhydride and pyridine to react; S4, dripping deionized water into the reaction solution, settling to obtain a polymer precipitate, washing and drying to obtain a binder A; S5, mixing a positive electrode active material, the binder A, and a positive electrode conductive agent according to a mass ratio, adding a solvent to prepare a positive electrode slurry, and coating the slurry on a positive electrode current collector to form a positive electrode.

8. The method for preparing a positive electrode according to claim 7, wherein: In the step S1, the molar ratio of the 4,4-(hexafluoroisopropylidene)diphthalic anhydride to the 3,5-diaminobenzonitrile is 1:(0.5-2).

9. The method for preparing a positive electrode according to claim 7, wherein: In the step S3, the molar ratio of the acetic anhydride to the pyridine is 1:(0.2-1).

10. A battery comprising a negative electrode and a separator, characterized in that: Also includes the positive electrode according to any one of claims 1 to 9.

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