Binder and method for producing same, lithium iron phosphate cathode and method for producing same, lithium ion battery

Abstract

The application provides a binder and a preparation method thereof, a lithium iron phosphate positive electrode and a preparation method thereof, and a lithium ion battery. 20 The binder is a random copolymer obtained by copolymerization of a first comonomer and a second comonomer; the first comonomer is selected from N-alkenyl-substituted pyrrolidone, and the second comonomer is a C5-C The weight percentage of the second comonomer corresponding segment in the binder is greater than or equal to 9.1 wt%. The molecular chain of the binder contains both polar segments and non-polar segments, which significantly improves the solubility of the binder and promotes the enrichment of non-polar conductive agents in the non-polar segment region, thereby improving the dispersibility of the binder and the conductive agent, improving the microstructure of the prepared positive electrode active material layer, and further improving the rate performance, fast charging performance and cycle stability of the lithium ion battery.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, specifically to a binder and its preparation method, a lithium iron phosphate cathode and its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium iron phosphate (LFP) batteries are lithium-ion batteries that use lithium iron phosphate as the positive electrode active material. With their stable chemical properties, excellent safety performance, low cost, good cycle life, and mature manufacturing processes, they have become an indispensable player in the lithium-ion battery market, widely used in electric vehicles, energy storage devices, and consumer electronics. However, LFP batteries suffer from high polarization and poor rate performance during charging, which to some extent limits their application in fast charging and high-rate discharging scenarios.

[0003] Currently, the main methods to improve the rate performance and fast charging performance of lithium iron phosphate batteries include: optimizing the particle size and morphology of lithium iron phosphate cathode materials; surface coating modification of lithium iron phosphate cathode materials; developing electrolytes with high conductivity; and reducing the tortuosity of lithium-ion transport at the electrode level.

[0004] Existing literature (publication number CN119864485A) discloses a fast-charging lithium-ion battery and its preparation method. The preparation method includes: S1, preparation of the positive electrode: granulating lithium iron phosphate to obtain granulated lithium iron phosphate, using the granulated lithium iron phosphate to prepare a positive electrode slurry; preparing the positive electrode slurry into an electrode sheet; and surface-treating at least one side of the electrode sheet using lithium titanium phosphate nanowires to obtain the positive electrode sheet; S2, preparation of the negative electrode sheet; and S3, preparation of the fast-charging lithium-ion battery. The preparation method provided in this literature can improve the fast-charging performance of lithium-ion batteries.

[0005] Existing literature (publication number CN119742324A) discloses a MoN / TiN nanotube / lithium iron phosphate composite material, its preparation, and its application. This material is obtained by mixing lithium iron phosphate cathode material and MoN / TiN nanotubes and then calcining the mixture at a temperature of 300–700 °C. This method improves the lithium-ion diffusion rate at the lithium iron phosphate interface and reduces the Li-ion diffusion rate. + The diffusion barrier increased the Li + The diffusion rate is increased, thereby improving the low-temperature fast-charging performance of lithium iron phosphate.

[0006] Existing literature (publication number CN120073066A) discloses a battery cell, a battery device, and an electrical appliance, wherein the electrolyte comprises a chain-like carboxylic acid ester, and the conductivity of the electrolyte at room temperature is 10 mS / cm to 18 mS / cm. This literature improves the fast-charging performance of the battery cell by increasing the conductivity of the electrolyte.

[0007] Existing literature (publication number CN119650699A) discloses a positive electrode slurry, a positive electrode sheet, a lithium-ion battery, and an electrical device. The positive electrode lithium replenishment agent provided in this literature includes lithium iron phosphate positive electrode material, a binder, a conductive agent, and a porous additive. Among them, the conductive agent includes conductive carbon black with a high specific surface area of ​​≥130 m². 2 The positive electrode slurry contains lithium iron phosphate particles filled with medium-pore porous additives and small-pore high-specific-surface-conductivity carbon black. This results in a positive electrode sheet with small pore spacing and a gradient pore structure that reduces the tortuosity of lithium-ion transport. This improves the electrolyte wettability of the electrode and enhances the battery's fast-charging capability, cycle life, and low-temperature discharge capability.

[0008] While the above methods have improved the rate performance and fast charging performance of lithium iron phosphate batteries to some extent, these methods are basically aimed at optimizing the lithium iron phosphate cathode material or adjusting the electrolyte formulation. They may introduce potential interface damage problems, thereby affecting the specific capacity of the lithium iron phosphate cathode material. In addition, the above methods have not effectively solved the problems of uniform distribution of conductive agent in the cathode sheet and compatibility between lithium iron phosphate cathode material and conductive agent. This limits the improvement of the fast charging performance and rate performance of lithium-ion batteries.

[0009] Therefore, a binder and its preparation method were researched and developed to improve the uniformity of the distribution of lithium iron phosphate cathode material and conductive agent in the cathode sheet, improve the lithium ion and electron transport efficiency, and thus improve the electrochemical performance of lithium-ion batteries, such as rate performance, fast charging performance and cycle performance. Summary of the Invention

[0010] The main objective of this application is to provide a binder and its preparation method, a lithium iron phosphate cathode and its preparation method, and a lithium-ion battery, in order to solve the problem of uneven distribution of lithium iron phosphate cathode material and conductive agent within the cathode sheet in the prior art, which leads to poor rate performance and fast charging performance of lithium-ion batteries.

[0011] To achieve the above objectives, this application provides an adhesive that is a random copolymer obtained by copolymerization of a first comonomer and a second comonomer; the first comonomer is selected from N-alkenyl-substituted pyrrolidone; the second comonomer is C5-C6. 20 The olefins; the weight average molecular weight of the binder is 900-1200 kDa; the weight percentage of the second comonomer segment in the binder is ≥9.1 wt%.

[0012] Furthermore, the first comonomer is selected from N-vinylpyrrolidone and / or 1-(allyl)pyrrolidone-2-one.

[0013] Furthermore, based on the weight percentage of the binder, the binder comprises 9.1 wt% to 23.1 wt% of the corresponding segments of the second comonomer and the balance being the corresponding segments of the first comonomer.

[0014] Furthermore, the second comonomer is C6~C 12 The olefin; preferably, the second comonomer is C6-C6. 10 The second comonomer is selected from one or more of the group consisting of 1-hexene, 1-heptene and 1-nonene.

[0015] Furthermore, the glass transition temperature of the adhesive is -35 to -25°C.

[0016] To achieve the above objectives, another aspect of this application provides a method for preparing the adhesive provided in this application. The method includes: step S1, in a protective gas atmosphere, causing a first comonomer and a second comonomer to undergo a copolymerization reaction in a first solvent under the action of an initiator to obtain a reaction product; step S2, subjecting the reaction product to a first drying process to obtain the adhesive.

[0017] Further, in step S1, the weight ratio of the first comonomer to the second comonomer is 1:(0.1 to 0.3).

[0018] Further, the weight ratio of the initiator to the first comonomer is (0.01 to 0.03):1; preferably, the initiator is selected from one or more of the group consisting of azobisisobutyronitrile, azobisisoheptanenitrile and dimethyl azobisisobutyrate.

[0019] Further, the weight ratio of the first solvent to the first comonomer is (1-10):1; preferably, the first solvent is selected from toluene and / or N,N-dimethylformamide.

[0020] Furthermore, in step S1, the copolymerization reaction is carried out at a temperature of 60–80°C for a time of 0.2–2 hours.

[0021] Further, step S1 includes: stirring the first comonomer and the first solvent under a protective gas atmosphere to obtain a mixture; mixing the mixture, the second comonomer and the initiator and performing a copolymerization reaction to obtain a reaction product; preferably, the stirring rate is 300-3000 rpm, the temperature is 30-50°C, and the time is 2-6 h; preferably, the protective gas is selected from nitrogen and / or an inert gas.

[0022] Furthermore, in step S2, the temperature of the first drying is 100-110°C, and the time is 12-24 hours.

[0023] Another aspect of this application provides a lithium iron phosphate cathode, including a cathode current collector and a cathode active material layer disposed on at least one side of the cathode, the cathode active material layer including lithium iron phosphate cathode material, a conductive agent and a binder, wherein the binder is the binder provided in this application.

[0024] The fourth aspect of this application also provides a method for preparing the above-mentioned lithium iron phosphate cathode provided in this application. The method includes: step N1, mixing a binder with a second solvent to obtain a slurry; step N2, mixing the slurry, a conductive agent, lithium iron phosphate cathode material with a third solvent to obtain a cathode slurry; and step N3, coating the cathode slurry onto at least one side surface of a cathode current collector, and then drying it to obtain a lithium iron phosphate cathode.

[0025] Further, in step N1, the weight ratio of the adhesive to the second solvent is 1:(10-20); preferably, the second solvent is N-methylpyrrolidone.

[0026] Furthermore, a second stirring is performed during the mixing process in step N1; preferably, the second stirring rate is 2000-4000 rpm and the time is 5-8 h.

[0027] Further, in step N2, the weight ratio of the adhesive, conductive agent, lithium iron phosphate cathode material and the third solvent is (0.3~0.4):(0.01~0.015):1:(0.05~0.5).

[0028] Furthermore, the D of lithium iron phosphate cathode material 50 The diameter is 0.8–3 μm; preferably, the D of the conductive agent is... 50 The size is 0.1–1 μm; preferably, the conductive agent is selected from one or more of the group consisting of conductive carbon black, multi-walled carbon nanotubes and single-walled carbon nanotubes.

[0029] Furthermore, the third solvent is N-methylpyrrolidone.

[0030] Furthermore, a third stirring is performed during the mixing process in step N2; preferably, the third stirring rate is 1000-3000 rpm and the time is 6-10 h.

[0031] Further, the viscosity of the positive electrode slurry is 3000–8000 mPa·s, and the solid content is 60 wt%–64 wt%; preferably, in step N3, the coating amount of the positive electrode slurry on the surface of the positive electrode current collector is 150–200 g / cm³. 2 .

[0032] Furthermore, the second drying temperature is 100–110°C, and the time is 12–24 hours.

[0033] Furthermore, the compaction density of the lithium iron phosphate cathode is 2.4–2.5 g / cm³. 3 .

[0034] The fifth aspect of this application also provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode is the lithium iron phosphate positive electrode provided in this application.

[0035] This application provides a binder, which is a random copolymer obtained by copolymerization of the first and second comonomers of the specific types described above. The first comonomer contains pyrrolidone groups, allowing the molecular chain of the random copolymer to simultaneously contain polar segments (i.e., segments corresponding to the first comonomer) and non-polar segments (i.e., segments corresponding to the second comonomer). The presence of polar segments improves the solubility of the binder, thereby enhancing its dispersibility in the cathode slurry. The presence of non-polar segments allows the equally non-polar conductive agent to accumulate in the non-polar segment regions, thus improving the dispersibility of the conductive agent in the cathode slurry. Applying this amphoteric binder to the preparation of cathode slurry promotes the uniform dispersion of lithium iron phosphate (LFP) cathode material and conductive agent, improves the microstructure of the prepared cathode active material layer, and optimizes the lithium-ion and electron transport paths, thereby effectively improving the rate performance, fast-charging performance, and cycle stability of the prepared lithium-ion battery. Furthermore, the adhesive provided in this application is chemically compatible with the high stability of lithium iron phosphate cathode materials and will not introduce additional side reaction risks. Moreover, since the areal density of lithium iron phosphate cathodes is usually high, applying the above-mentioned adhesive to lithium iron phosphate cathodes can better demonstrate the beneficial effect of the above-mentioned adhesive in improving the dispersibility of conductive agents.

[0036] Compared to other ranges, limiting the weight-average molecular weight of the binder to the above-mentioned specific range can improve the bonding performance of the binder, improve the dispersion uniformity of the binder, conductive agent and lithium iron phosphate cathode material in the cathode slurry, and also improve the processing performance of the cathode slurry. This can improve the microstructure of the prepared cathode active material layer, optimize the lithium ion and electron transport path, and thus improve the rate performance, fast charging performance and cycle stability of the prepared lithium-ion battery.

[0037] Compared to other ranges, limiting the content of the corresponding chain segment of the second comonomer within the above-mentioned specific range can ensure that the binder contains an appropriate amount of non-polar chain segments, thereby better exerting its function, improving the dispersion uniformity of the binder, conductive agent and lithium iron phosphate cathode material in the cathode slurry, improving the microstructure of the prepared cathode active material layer, forming a more uniform ion and electron transport network, and thus reducing the internal resistance and polarization of the prepared lithium-ion battery during the charging and discharging process, improving its rate performance, fast charging performance and cycle stability. Attached Figure Description

[0038] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0039] Figure 1 The surface SEM image of the positive electrode sheet prepared in Embodiment 1 of this application is shown;

[0040] Figure 2 The surface SEM image of the positive electrode sheet prepared in Comparative Example 1 of this application is shown;

[0041] Figure 3 The diagram shows a comparison of the fast-charging cycle capacity retention rates of lithium iron phosphate monocells assembled in Example 1 and Comparative Example 1 of this application. Detailed Implementation

[0042] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to the embodiments.

[0043] As described in the background section, existing lithium iron phosphate cathodes suffer from uneven distribution of the lithium iron phosphate cathode material and conductive agent, resulting in poor rate performance and fast charging performance of lithium-ion batteries. To address these technical problems, the first aspect of this application provides a binder, which is a random copolymer obtained by copolymerization of a first comonomer and a second comonomer; the first comonomer is selected from N-alkenyl-substituted pyrrolidone; the second comonomer is C5-C6. 20 The olefins; the weight average molecular weight of the binder is 900-1200 kDa; the weight percentage of the second comonomer segment in the binder is ≥9.1 wt%.

[0044] The binder provided in this application is a random copolymer obtained by copolymerization of a first comonomer and a second comonomer of the aforementioned specific type. The first comonomer contains pyrrolidone groups, which allows the molecular chain of the random copolymer to simultaneously contain polar segments (i.e., segments corresponding to the first comonomer) and non-polar segments (i.e., segments corresponding to the second comonomer). The presence of polar segments improves the solubility of the binder, thereby enhancing its dispersibility in the cathode slurry. The presence of non-polar segments allows the equally non-polar conductive agent to accumulate in the non-polar segment region, thus improving the dispersibility of the conductive agent in the cathode slurry. Applying this amphoteric binder to the preparation of cathode slurry can promote the uniform dispersion of lithium iron phosphate (LFP) cathode material and conductive agent, improve the microstructure of the prepared cathode active material layer, and optimize the lithium-ion and electron transport path, thereby effectively improving the rate performance, fast-charging performance, and cycle stability of the prepared lithium-ion battery. Furthermore, the adhesive provided in this application is chemically compatible with the high stability of lithium iron phosphate cathode materials and will not introduce additional side reaction risks. Moreover, since the areal density of lithium iron phosphate cathodes is usually high, applying the above-mentioned adhesive to lithium iron phosphate cathodes can better demonstrate the beneficial effect of the above-mentioned adhesive in improving the dispersibility of conductive agents.

[0045] Compared to other ranges, limiting the weight-average molecular weight of the binder to the above-mentioned specific range can improve the bonding performance of the binder, improve the dispersion uniformity of the binder, conductive agent and lithium iron phosphate cathode material in the cathode slurry, and also improve the processing performance of the cathode slurry. This can improve the microstructure of the prepared cathode active material layer, optimize the lithium ion and electron transport path, and thus improve the rate performance, fast charging performance and cycle stability of the prepared lithium-ion battery.

[0046] Compared to other ranges, limiting the content of the corresponding chain segment of the second comonomer within the above-mentioned specific range can ensure that the binder contains an appropriate amount of non-polar chain segments, thereby better exerting its function, improving the dispersion uniformity of the binder, conductive agent and lithium iron phosphate cathode material in the cathode slurry, improving the microstructure of the prepared cathode active material layer, forming a more uniform ion and electron transport network, and thus reducing the internal resistance and polarization of the prepared lithium-ion battery during the charging and discharging process, improving its rate performance, fast charging performance and cycle stability.

[0047] In a preferred embodiment, the binder comprises 9.1 wt% to 23.1 wt% of the corresponding segments of the second comonomer and the balance being the corresponding segments of the first comonomer, based on the weight percentage of the binder. Compared to other ranges, limiting the content of the corresponding segments of the first and second comonomers within the above range is beneficial for controlling the content of polar and non-polar segments in the binder molecular chain, thereby facilitating better performance, improving the dispersion uniformity of the binder, conductive agent, and lithium iron phosphate cathode material in the cathode slurry, and optimizing the distribution of ion and electron transport paths, thus improving the rate performance, fast charging performance, and cycle stability of the lithium-ion battery.

[0048] In order to improve the bonding performance of the binder and the mechanical properties of the lithium iron phosphate cathode made therefrom, in a preferred embodiment, the glass transition temperature (Tg) of the binder is -35 to -25°C.

[0049] It should be noted that the weight-average molecular weight of the adhesive in this application was determined by gel permeation chromatography (GPC). The gel permeation chromatograph was a Malvern Panalytical Omnisc Reveal. The test conditions were as follows: the adhesive sample concentration was 1-2 mg / mL, the injection volume was 50 μL, the flow rate of the mobile phase was 1 mL / min, the mobile phase composition was dimethyl sulfoxide (DMSO), and the column oven temperature was 30 °C.

[0050] The contents of the corresponding segments of the first and second comonomers were measured using a carbon and nitrogen elemental analyzer (model: vario EL cube). The test conditions were as follows: the outlet pressure of the helium (carrier gas) and oxygen pressure reducing valves was set to 0.1 MPa, the carrier gas flow rate was 200 mL / min, the elemental analysis control software was run, the sample was instantaneously combusted and decomposed in oxygen, and after reduction separation, it was detected by TCD. The peak area and weight were calculated by automatic integration to obtain the contents of the corresponding segments of the first and second comonomers.

[0051] The glass transition temperature (Tg) of the adhesive was determined by differential scanning calorimetry (DSC) using a TA Instruments Rcs90 instrument. The test conditions were as follows: the adhesive sample weight was 5 mg, the nitrogen flow rate was 50 mL / min, the sample was cooled to the target temperature of -50 °C, held at that temperature for 5 min, then heated from -50 °C to 50 °C at a rate of 10 °C / min to eliminate the thermal history of the sample, then cooled from 50 °C to -50 °C at a rate of 10 °C / min to quench the sample to the glassy state, and then heated from -50 °C to 50 °C at a rate of 10 °C / min to obtain the target curve. The Tg was then analyzed.

[0052] In a preferred embodiment, the first comonomer includes, but is not limited to, N-vinylpyrrolidone and / or 1-(allyl)pyrrolidone-2-one. Compared to other types, using the above-mentioned first comonomers is beneficial to improving the chemical stability, bonding performance, and solubility of the prepared binder, thereby improving the dispersion uniformity of the binder in the cathode slurry, optimizing the ion and electron transport path distribution of the prepared lithium iron phosphate cathode, and further improving the rate performance, fast charging performance, and cycle stability of the prepared lithium-ion battery.

[0053] In a preferred embodiment, the second comonomer is C6 to C6. 12 Olefins. Compared to other types, using the above-mentioned second comonomers is beneficial to improving the chemical stability and bonding performance of the prepared binder, and to improving the interaction between the non-polar segments in the binder molecular chain and the conductive agent. This, in turn, helps to improve the dispersion uniformity of the conductive agent in the positive electrode slurry, optimizes the distribution of ion and electron transport paths, and further helps to improve the rate performance, fast charging performance and cycle stability of lithium-ion batteries.

[0054] To further improve the chemical stability and bonding performance of the prepared binder, and to further enhance the interaction between the non-polar segments of the binder molecular chain and the conductive agent, thereby further improving the dispersion uniformity of the conductive agent in the positive electrode slurry and optimizing the distribution of ion and electron transport paths, preferably, the second comonomer is C6~C6. 10 The second comonomer includes, but is not limited to, one or more of the group consisting of 1-hexene, 1-heptene and 1-nonene.

[0055] To further improve the chemical stability and bonding performance of the prepared binder, and to further enhance the interaction between the non-polar segments in the binder molecular chain and the conductive agent, thereby further improving the dispersion uniformity of the conductive agent in the positive electrode slurry and optimizing the distribution of ion and electron transport paths, more preferably, the second comonomer is 1-heptene.

[0056] The second aspect of this application also provides a method for preparing the adhesive provided in this application, the method comprising: step S1, in a protective gas atmosphere, causing a first comonomer and a second comonomer to undergo a copolymerization reaction in a first solvent under the action of an initiator to obtain a reaction product; step S2, subjecting the reaction product to a first drying process to obtain the adhesive.

[0057] Using the preparation method provided in this application, the carbon-carbon double bonds in the first comonomer and the second comonomer can undergo a copolymerization reaction via a free radical polymerization mechanism to form a random copolymer comprising polar segments (i.e., the segments corresponding to the first comonomer) and non-polar segments (i.e., the segments corresponding to the second comonomer). After a first drying process, a binder is obtained. The presence of polar segments in the binder can improve its solubility, thereby improving its dispersibility in the positive electrode slurry. The presence of non-polar segments can enrich the non-polar conductive agent in the non-polar segment region, thereby improving the dispersibility of the conductive agent in the positive electrode slurry.

[0058] Applying the binder prepared by the above-described preparation method of this application to the preparation process of positive electrode slurry can improve the bonding force of each component in the positive electrode slurry, improve the peel force between the prepared positive electrode active material layer and the positive electrode current collector, and also promote the uniform dispersion of lithium iron phosphate positive electrode material and conductive agent, improve the microstructure of the prepared positive electrode active material layer, and optimize the transport path of lithium ions and electrons, thereby effectively improving the rate performance, fast charging performance and cycle stability of the prepared lithium-ion battery.

[0059] In a preferred embodiment, in step S1, the weight ratio of the first comonomer to the second comonomer is 1:(0.1 to 0.3). The weight ratio of the first comonomer to the second comonomer includes, but is not limited to, the range described above. Limiting it to this range is beneficial for improving the reaction efficiency of the copolymerization reaction, for obtaining a random copolymer with a more suitable content of polar and non-polar segments, thereby improving the adhesion and solubility of the prepared binder, for improving the dispersion uniformity of the lithium iron phosphate cathode material and conductive agent in the cathode slurry, and further for improving the microstructure of the prepared cathode active material layer, optimizing the lithium ion and electron transport path, and improving the rate performance, fast charging performance, and cycle stability of the prepared lithium-ion battery.

[0060] In a preferred embodiment, the weight ratio of the initiator to the first comonomer is (0.01 to 0.03):1. The weight ratio of the initiator to the first comonomer includes, but is not limited to, the above range. Limiting it to this range is beneficial for initiating the copolymerization reaction and for controlling the reaction rate within a more suitable range, which is beneficial for improving the yield and purity of the binder and improving the molecular chain structure of the binder.

[0061] In order to further improve the reaction efficiency of the copolymerization reaction, control the reaction rate of the copolymerization reaction to maintain it within a more suitable range, further improve the yield and purity of the binder, and obtain a binder with a more stable structure, in a preferred embodiment, the initiator includes, but is not limited to, one or more of the group consisting of azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate.

[0062] In a preferred embodiment, the weight ratio of the first solvent to the first comonomer is (1-10):1. This weight ratio includes, but is not limited to, the range described above. Limiting it to this range is beneficial for improving the dispersibility of N-vinylpyrrolidone, the second comonomer, and the initiator, thereby improving the reaction efficiency of the copolymerization reaction.

[0063] In order to further improve the dispersibility of the first comonomer, the second comonomer, and the initiator, thereby further improving the reaction efficiency of the copolymerization reaction and further improving the yield and purity of the binder, in a preferred embodiment, the first solvent includes, but is not limited to, toluene and / or N,N-dimethylformamide.

[0064] In a preferred embodiment, in step S1, the copolymerization reaction temperature is 60–80°C, and the time is 0.2–2 hours. The copolymerization reaction temperature and time include, but are not limited to, the above ranges. Limiting them to the above ranges is beneficial to improving the reaction efficiency of the copolymerization reaction, increasing the yield and purity of the binder, and also beneficial to controlling the weight-average molecular weight of the binder within a more suitable range, thereby improving the molecular chain structure of the binder.

[0065] In order to further improve the dispersibility of the reactants in the copolymerization reaction, thereby further improving the reaction efficiency of the copolymerization reaction and further improving the yield and purity of the binder, in a preferred embodiment, step S1 includes: stirring the first comonomer and the first solvent under a protective gas atmosphere to obtain a mixture; mixing the mixture, the second comonomer and the initiator and carrying out a copolymerization reaction to obtain the reaction product.

[0066] In a preferred embodiment, the first stirring rate is 300–3000 rpm, the temperature is 30–50°C, and the time is 2–6 h. The stirring rate, temperature, and time are not limited to the above ranges, and limiting them to the above ranges is beneficial to improving the dispersibility of the first comonomer, thereby improving the reaction efficiency of the copolymerization reaction.

[0067] To suppress side reactions caused by impurities such as oxygen and moisture, and to further improve the reaction efficiency of the copolymerization reaction, the purity and stability of the resulting binder are further enhanced. In a preferred embodiment, the protective gas includes, but is not limited to, nitrogen and / or an inert gas.

[0068] In a preferred embodiment, in step S2, the temperature of the first drying is 100–110°C, and the time is 12–24 hours. The temperature and time of the first drying include, but are not limited to, the above ranges. Limiting them to the above ranges is beneficial for removing solvent components from the reaction product and for improving the purity of the obtained binder.

[0069] A third aspect of this application also provides a lithium iron phosphate cathode, comprising a cathode current collector and a cathode active material layer disposed on at least one side of the current collector. The cathode active material layer comprises a lithium iron phosphate cathode material, a conductive agent, and a binder, wherein the binder is the binder provided in this application. The molecular chain of the binder provided in this application simultaneously contains polar segments (i.e., segments corresponding to the first comonomer) and non-polar segments (i.e., segments corresponding to the second comonomer). The presence of polar segments can improve the solubility of the binder, thereby improving its dispersibility in the cathode slurry. The presence of non-polar segments can enrich the non-polar conductive agent in the non-polar segment region, thereby improving the dispersibility of the conductive agent in the cathode slurry. Applying this amphoteric binder to the preparation of cathode slurry can improve the bonding force of the components in the cathode slurry, enhance the peel force between the prepared cathode active material layer and the cathode current collector, promote the uniform dispersion of lithium iron phosphate cathode material and conductive agent, improve the microstructure of the prepared cathode active material layer, and optimize the lithium ion and electron transport path, thereby effectively improving the rate performance, fast charging performance and cycle stability of the prepared lithium-ion battery.

[0070] The positive current collector used in this application may be of a type commonly used in the art. In a preferred embodiment, the positive current collector includes, but is not limited to, aluminum foil.

[0071] In a preferred embodiment, the coating surface density of the positive electrode active material layer is 150–200 g / cm³. 2 The surface density of the positive electrode active material layer coating includes, but is not limited to, the range described above. Limiting it to the range is beneficial to improving the energy density of the prepared lithium iron phosphate positive electrode, thereby improving the rate performance, fast charging performance, and cycle stability of the lithium-ion battery.

[0072] The fourth aspect of this application also provides a method for preparing the above-mentioned lithium iron phosphate cathode provided in this application. The preparation method includes: step N1, mixing a binder with a second solvent to obtain a slurry; step N2, mixing the slurry, a conductive agent, lithium iron phosphate cathode material with a third solvent to obtain a cathode slurry; and step N3, coating the cathode slurry onto at least one side surface of a cathode current collector, and then drying it to obtain a lithium iron phosphate cathode.

[0073] Using the method for preparing the lithium iron phosphate cathode provided in this application, in step N1, the binder is mixed with the second solvent to obtain a slurry of suitable viscosity, allowing it to function better in the subsequent preparation of the cathode slurry. In step N2, the slurry, conductive agent, lithium iron phosphate cathode material, and a third solvent are mixed to obtain a cathode slurry with suitable viscosity and solid content. The introduction of the slurry can improve the adhesion of each component in the cathode slurry and improve the dispersibility of the conductive agent and the lithium iron phosphate cathode material, thereby improving the microstructure of the prepared cathode active material layer, optimizing the lithium ion and electron transport path, and thus improving the rate performance, fast charging performance, and cycle stability of the prepared lithium-ion battery. In step S3, the cathode slurry is coated on at least one side of the cathode current collector, and after a second drying, a lithium iron phosphate cathode with excellent peel strength, adhesion, and flexibility can be obtained.

[0074] In a preferred embodiment, in step N1, the weight ratio of the binder to the second solvent is 1:(10-20). The weight ratio of the binder to the second solvent includes, but is not limited to, the range described above. Limiting it to this range is beneficial for improving the dispersibility of the binder, for obtaining a suitable viscosity solution, and thus for better utilizing the binding effect of the binder. Simultaneously, it is beneficial for improving the dispersion uniformity of the conductive agent and the lithium iron phosphate cathode material in subsequent preparation processes.

[0075] To further improve the dispersibility of the binder and promote the formation of a more suitable viscosity solution, in a preferred embodiment, the second solvent is N-methylpyrrolidone.

[0076] To further improve the dispersibility of the adhesive and obtain a uniform adhesive solution with a more suitable viscosity, while reducing the formation of bubbles, in a preferred embodiment, a second stirring is performed during the mixing process in step N1. Preferably, the second stirring rate is 2000-4000 rpm and the time is 5-8 hours.

[0077] In a preferred embodiment, in step N2, the weight ratio of the adhesive, conductive agent, lithium iron phosphate cathode material, and third solvent is (0.3–0.4):(0.01–0.015):1:(0.05–0.5). The weight ratio of the adhesive, conductive agent, lithium iron phosphate cathode material, and third solvent includes, but is not limited to, the above range. Limiting it to this range helps to control the flowability and viscosity of the cathode slurry within a more suitable range, improves the dispersibility of the adhesive, conductive agent, and lithium iron phosphate cathode material, and enhances the structural stability and electrochemical performance of the prepared lithium iron phosphate cathode, thereby improving the rate performance, fast charging performance, and cycle stability of the prepared lithium-ion battery.

[0078] In a preferred embodiment, the D of the lithium iron phosphate cathode material50 The diameter (D) of lithium iron phosphate cathode materials ranges from 0.8 to 3 μm. 50 Including but not limited to the above-mentioned range, limiting it to the above-mentioned range is beneficial to improving the dispersion uniformity of lithium iron phosphate cathode material, beneficial to improving lithium ion transport efficiency, and thus beneficial to improving the electrochemical performance of the prepared lithium iron phosphate cathode.

[0079] To improve the dispersion uniformity of the conductive agent, thereby further improving the conductivity of the prepared lithium iron phosphate cathode, in a preferred embodiment, the D of the conductive agent... 50 The range is 0.1–1 μm.

[0080] The conductive agent used in this application may be of commonly used types in the art. In a preferred embodiment, the conductive agent includes, but is not limited to, one or more of the group consisting of conductive carbon black, multi-walled carbon nanotubes, and single-arm carbon nanotubes. Using the above-mentioned types of conductive agents is beneficial to improving the conductivity of the prepared lithium iron phosphate cathode, optimizing the ion and electron transport paths, thereby improving the rate performance, fast charging performance, and cycle stability of the prepared lithium-ion battery.

[0081] In order to improve the dispersibility of the adhesive, conductive agent and lithium iron phosphate cathode material and obtain a cathode slurry with a more suitable viscosity, in a preferred embodiment, the third solvent is N-methylpyrrolidone.

[0082] To further improve the dispersibility of the adhesive, conductive agent, and lithium iron phosphate cathode material, and to form a uniform cathode slurry with a more suitable viscosity, in a preferred embodiment, a third stirring is performed during the mixing process in step N2; preferably, the third stirring rate is 1000-3000 rpm and the time is 6-10 h.

[0083] In a preferred embodiment, the viscosity of the cathode slurry is 3000–8000 mPa·s, and the solid content is 60 wt%–64 wt%. The viscosity and solid content of the cathode slurry include, but are not limited to, the above ranges, and limiting them to the above ranges is beneficial to improving the coating performance of the cathode slurry.

[0084] In a preferred embodiment, in step N3, the coating amount of the positive electrode slurry on the surface of the positive electrode current collector is 150–200 g / cm³. 2 The amount of positive electrode slurry coated on the surface of the positive electrode current collector includes, but is not limited to, the range mentioned above. Limiting it to the range mentioned above is beneficial to improving the continuity and uniformity of the positive electrode slurry coating, and beneficial to improving the energy density of the prepared lithium iron phosphate positive electrode, thereby improving the rate performance, fast charging performance and cycle stability of the lithium-ion battery.

[0085] In order to remove residual solvent components in the electrode to be dried, further improve the bonding strength of each component in the positive electrode active material layer, and further improve the peel strength between the positive electrode active material layer and the positive electrode current collector, in a preferred embodiment, the second drying temperature is 100-110°C and the time is 12-24 hours.

[0086] In order to improve the compaction density and dimensional consistency of the lithium iron phosphate cathode, in a preferred embodiment, step N3 further includes rolling and cutting performed sequentially after the second drying.

[0087] In a preferred embodiment, the compaction density of the lithium iron phosphate cathode is 2.4–2.5 g / cm³. 3 Compared to other ranges, limiting the compaction density of lithium iron phosphate cathodes to the above range is beneficial to improving the energy density of lithium iron phosphate cathodes, thereby improving the rate performance, fast charging performance and cycle stability of lithium-ion batteries.

[0088] It should be noted that the specific method for testing the compaction density of lithium iron phosphate cathode is as follows: a hole with a fixed diameter of 16 mm (area S = 0.201 cm²) is obtained using a punch. 2 Several circular electrode samples were rolled and weighed, and their weights were recorded. The average weight was calculated and denoted as n (in grams). The thickness of the lithium iron phosphate positive electrode (with the positive electrode active material layer) was measured and recorded using a micrometer (Mitutoyo), and the average value was calculated and denoted as t1 (in μm). The thickness of the positive electrode current collector was measured and recorded, and the average value was calculated and denoted as t2 (in μm). The compaction density d was calculated according to formula (I). (I).

[0089] A fifth aspect of this application also provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive and negative electrodes. The positive electrode is the lithium iron phosphate positive electrode provided in this application. The lithium iron phosphate positive electrode provided in this application, by introducing a binder with the specific structure described above, improves the uniformity of the distribution of the lithium iron phosphate positive electrode material and the conductive agent, improves the distribution of lithium ion and electron transport paths, and enhances the adhesion between the positive electrode active material layer and the positive electrode current collector. Using the lithium iron phosphate positive electrode provided in this application as the positive electrode of a lithium-ion battery can effectively improve the rate performance, fast charging performance, and cycle stability of the lithium-ion battery.

[0090] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0091] Example 1

[0092] A method for preparing a lithium iron phosphate cathode specifically includes the following steps:

[0093] (1) Preparation of adhesive A-1: ​​4g N-vinylpyrrolidone (NVP) was mixed with 20g of toluene and stirred at 45℃ and 1000rpm for 4h to obtain a mixture, wherein the weight ratio of NVP to toluene was 1:5. The obtained mixture, 1g of 1-heptene, and 0.06g of azobisisobutyronitrile (AIBN) were mixed and copolymerized at 70℃ under a nitrogen atmosphere (purity ≥99.5%) for 0.5h. After the reaction was completed, the mixture was naturally cooled to room temperature to obtain the reaction product, wherein the weight ratio of NVP, 1-heptene, and AIBN was 1:0.25:0.015. The product was then dried at 100℃ for 12h to obtain binder A-1. The binder A-1 has a weight-average molecular weight of 1000kDa, a Tg of -30℃, and the 1-heptene segment (-CH2-CH(CH2-CH2-CH2-CH2-CH3)-) accounts for 20wt% of binder A-1. The NVP segment (i.e., ...) accounts for 20wt% of the total content of binder A-1. The content of adhesive A-1 is 80 wt%;

[0094] (2) Mix 2g of adhesive A-1 obtained in step (1) with 32g of N-methylpyrrolidone (NMP), stir at 30℃ and 3000rpm for 6h, and obtain adhesive solution after vacuum defoaming, wherein the weight ratio of adhesive A-1 to NMP is 1:16.

[0095] (3) Take 30g of the adhesive solution obtained in step (2) and 1.2g of conductive carbon black (D 50 (0.2μm), 100g lithium iron phosphate cathode material (D) 50 A mixture of 1.5 μm conductive carbon black and 30 g NMP was stirred at 30 °C and 2500 rpm for 8 h to obtain a positive electrode slurry. The weight ratio of the slurry, conductive carbon black, lithium iron phosphate positive electrode material, and NMP was 0.3:0.012:1:0.3. The viscosity of the positive electrode slurry was 4200 mPa·s, and the solid content was 62.5 wt%.

[0096] (4) The positive electrode slurry obtained in step (3) is coated onto one side surface of a 13 μm thick aluminum foil, with a coating amount of 180 g / m. 2 The electrode sheet to be dried was obtained and dried at 100℃ for 24 hours to obtain the positive electrode active material layer. The single-sided coating surface density of the positive electrode active material layer was 165 g / m². 2 Subsequently, it was rolled under a pressure of 25T, and then cut into 4cm×5cm pieces to obtain an 80μm thick lithium iron phosphate cathode; the compaction density of this lithium iron phosphate cathode was 2.45g / cm³. 3 .

[0097] The surface SEM image of the lithium iron phosphate cathode prepared in Example 1 is shown below. Figure 1As shown, by Figure 1 It can be seen that the conductive agent in the lithium iron phosphate cathode prepared in Example 1 is uniformly dispersed and there is no agglomeration. This indicates that the use of the binder with the specific structure provided in this application can effectively improve the dispersion uniformity of the conductive agent.

[0098] Example 2

[0099] The difference from Example 1 is as follows: In step (1), the weight ratio of NVP, 1-heptene and AIBN is adjusted to 1:0.25:0.012, and the adhesive obtained in step (1) is designated as adhesive A-2; the weight average molecular weight of adhesive A-2 is 900 kDa, the Tg is -30℃, the content of the 1-heptene segment in adhesive A-2 is 20 wt%, and the content of the NVP segment in adhesive A-2 is 80 wt%; in step (2), an equal weight of adhesive A-2 is used to replace adhesive A-1 in Example 1; the remaining steps are the same as in Example 1.

[0100] Example 3

[0101] The difference from Example 1 is as follows: In step (1), the weight ratio of NVP, 1-heptene and AIBN is adjusted to 1:0.25:0.018, and the adhesive obtained in step (1) is designated as adhesive A-3; the weight average molecular weight of adhesive A-3 is 1200kDa, the Tg is -30℃, the content of the 1-heptene segment in adhesive A-3 is 20wt%, and the content of the NVP segment in adhesive A-3 is 80wt%; in step (2), an equal weight of adhesive A-3 is used to replace adhesive A-1 in Example 1; the remaining steps are the same as in Example 1.

[0102] Example 4

[0103] The difference from Example 1 is as follows: In step (1), the weight ratio of NVP, 1-heptene and AIBN is adjusted to 1:0.1:0.015, and the adhesive obtained in step (1) is designated as adhesive A-4; the weight average molecular weight of adhesive A-4 is 1000kDa, the Tg is -30℃, the weight percentage of the 1-heptene segment in adhesive A-4 is 9.1wt%, and the weight percentage of the NVP segment in adhesive A-4 is 90.9wt%; in step (2), an equal weight of adhesive A-4 is used to replace adhesive A-1 in Example 1; the remaining steps are the same as in Example 1.

[0104] Example 5

[0105] The difference from Example 1 is as follows: In step (1), the weight ratio of NVP, 1-heptene and AIBN is adjusted to 1:0.3:0.015, and the adhesive obtained in step (1) is designated as adhesive A-5; the weight average molecular weight of adhesive A-5 is 1000kDa, the Tg is -30℃, the content of the 1-heptene segment in adhesive A-5 is 23.1wt%, and the content of the NVP segment in adhesive A-5 is 76.9wt%; in step (2), an equal weight of adhesive A-5 is used to replace adhesive A-1 in Example 1; the remaining steps are the same as in Example 1.

[0106] Example 6

[0107] The difference from Example 1 is as follows: In step (1), the weight ratio of NVP, 1-heptene and AIBN is adjusted to 1:0.5:0.015, and the adhesive obtained in step (1) is designated as adhesive A-6; the weight average molecular weight of adhesive A-6 is 1000kDa, the Tg is -30℃, the content of the 1-heptene segment in adhesive A-6 is 33wt%, and the content of the NVP segment in adhesive A-6 is 67wt%; in step (2), an equal weight of adhesive A-6 is used to replace adhesive A-1 in Example 1; the remaining steps are the same as in Example 1.

[0108] Example 7

[0109] The difference from Example 1 is as follows: In step (1), 1-hexene of equal weight is used to replace 1-heptene, and the adhesive obtained in step (1) is designated as adhesive A-7; the weight average molecular weight of adhesive A-7 is 1000 kDa, the Tg is -30℃, the content of the 1-hexene corresponding segment in adhesive A-7 is 20 wt%, and the content of the NVP corresponding segment in adhesive A-7 is 80 wt%; In step (2), adhesive A-7 of equal weight is used to replace adhesive A-1 in Example 1; the remaining steps are the same as in Example 1.

[0110] Example 8

[0111] The difference from Example 1 is as follows: In step (1), 1-nonene of equal weight is used to replace 1-heptene, and the adhesive obtained in step (1) is designated as adhesive A-8; the weight average molecular weight of adhesive A-8 is 1000 kDa, the Tg is -30℃, the weight percentage of the 1-nonene segment in adhesive A-8 is 20 wt%, and the weight percentage of the NVP segment in adhesive A-8 is 80 wt%; In step (2), adhesive A-8 of equal weight is used to replace adhesive A-1 in Example 1; the remaining steps are the same as in Example 1.

[0112] Example 9

[0113] The difference from Example 1 is as follows: In step (1), an equal weight of 1-(allyl)pyrrolidone-2-one (CAS No.: 2687-97-0) is used to replace N-vinylpyrrolidone (NVP), and the adhesive obtained in step (1) is designated as adhesive A-9; the weight average molecular weight of adhesive A-9 is 1000kDa, the Tg is -25℃, the 1-heptene corresponding chain segment accounts for 20wt% of the weight of adhesive A-9, and the 1-(allyl)pyrrolidone corresponding chain segment accounts for 80wt% of the content of adhesive A-9; In step (2), an equal weight of adhesive A-9 is used to replace adhesive A-1 in Example 1; the remaining steps are the same as in Example 1.

[0114] Comparative Example 1

[0115] The difference from Example 1 is that step (1) is omitted, and in step (2) the binder A-1 in Example 1 is replaced with an equal weight of binder polyvinylidene fluoride (PVDF), and the weight-average molecular weight of PVDF is 1000kDa; the remaining steps are the same as in Example 1.

[0116] The surface SEM image of the lithium iron phosphate cathode prepared in Comparative Example 1 is shown below. Figure 2 As shown. By Figure 2 It can be seen that the conductive agent in the lithium iron phosphate cathode prepared in Comparative Example 1 exhibits agglomeration.

[0117] Comparative Example 2

[0118] The difference from Example 1 is that in step (1), adhesive D-1 is prepared by the following method: 4g of N-vinylpyrrolidone (NVP) and 20g of toluene are mixed and stirred at 45°C and 1000rpm for 4h to obtain a mixture; the obtained mixture and 1g of... 1-Heptene and 0.08 g of azobisisobutyronitrile (AIBN) were mixed and copolymerized at 75°C for 0.5 h under a nitrogen atmosphere (purity ≥ 99.5%). After the reaction was completed, the mixture was naturally cooled to room temperature to obtain the reaction product. The weight ratio of NVP, 1-heptene and AIBN was 1:0.25:0.02. The product was then dried at 100°C for 12 h to obtain binder D-1. The weight average molecular weight of binder D-1 was 1500 kDa, the Tg was -30°C, the weight percentage of the 1-heptene segment in binder D-1 was 20 wt%, and the weight percentage of the NVP segment in binder D-1 was 80 wt%. In step (2), binder D-1 of equal weight was used to replace binder A-1 in Example 1. The remaining steps were the same as in Example 1.

[0119] Comparative Example 3

[0120] The difference from Example 1 is as follows: In step (1), the weight ratio of NVP, 1-heptene and AIBN is adjusted to 1:0.05:0.015, and the adhesive obtained in step (1) is designated as adhesive D-2. The weight average molecular weight of adhesive D-2 is 1000kDa, the Tg is -30℃, the weight percentage of the 1-heptene segment in adhesive D-2 is 4.7wt%, and the weight percentage of the NVP segment in adhesive D-2 is 95.3wt%. In step (2), an equal weight of adhesive D-2 is used to replace adhesive A-1 in Example 1. The remaining steps are the same as in Example 1.

[0121] Comparative Example 4

[0122] The difference from Example 1 is that step (1) is omitted, and in step (2) the adhesive A-1 in Example 1 is replaced by an equal weight of polyvinylpyrrolidone (PVP), the weight-average molecular weight of which is 1000 kDa; the remaining steps are the same as in Example 1.

[0123] The types of binders, weight-average molecular weights, glass transition temperatures (Tg), contents of the first comonomer corresponding to the chain segment, and contents of the second comonomer corresponding to the chain segment used in all the embodiments and comparative examples of this application are shown in Table 1.

[0124] Table 1

[0125]

[0126] Application Examples

[0127] The lithium iron phosphate single-cell battery was assembled as follows: (1) The lithium iron phosphate positive electrode was prepared using all the above-mentioned embodiments and comparative examples of this application; (2) The graphite negative electrode was prepared by mixing 10g of artificial graphite, 0.1g of conductive carbon black, 0.3g of binder (including 0.18g of styrene-butadiene rubber and 0.12g of carboxymethyl cellulose) with deionized water to obtain a negative electrode slurry; the negative electrode slurry was coated on the surface of a 6μm thick copper foil, and then dried at 100°C for 12h to obtain a graphite negative electrode sheet with a thickness of 56μm; wherein the weight ratio (i.e., N / P ratio) of the negative electrode active material to the positive electrode active material was 1:1.15; the single-sided coating surface density of the graphite negative electrode was 75g / m 2(3) Assembling lithium iron phosphate single-cell batteries: The lithium iron phosphate positive electrode prepared by all the above-mentioned embodiments and comparative examples of this application is used as the positive electrode, and a (9+2) μm single-sided separator (i.e., a single-sided separator with a polyethylene film thickness of 9 μm and an alumina ceramic coating thickness of 2 μm) is used as the separator, and the graphite negative electrode prepared above is used as the negative electrode; after being assembled in sequence, the cells are baked at 90℃ and 100Pa for 24h to make the moisture content of the cell ≤200ppm; then the electrolyte is injected into the assembled cell, the electrolyte injection coefficient is 3g / Ah, the electrolyte composition is 0.8mol / L LiPF6 lithium salt, and the solvent is a mixed solvent of dimethyl carbonate (DMC) and ethylene carbonate (EC) (the volume ratio is 7:3); after the electrolyte injection is completed, the cells are subjected to high-temperature wetting, formation, aging and capacity testing in sequence to obtain lithium iron phosphate single-cell batteries.

[0128] The lithium iron phosphate single-cell batteries assembled in all the above embodiments and comparative examples of this application were subjected to multiple charge performance tests, multiple discharge performance tests, and constant current charge-to-discharge ratio tests.

[0129] The steps for the double-charge performance test are as follows: Charging was performed at rates of 0.33C, 1C, 2C, 3C, and 4C, using a constant current and constant voltage charging process. The cutoff current rate was 0.05C. After charging, a constant current discharge of 1C was performed until the voltage reached 2.5V. The charging capacity during the constant current phase was recorded, and the ratio of the constant current phase charging capacity at each rate to the constant current phase charging capacity at 0.33C was calculated. The voltage range for the double-charge performance test was 2.5–3.65V, and the temperature was 25℃. The test results are shown in Table 2.

[0130] Table 2

[0131]

[0132] As can be seen from Table 2, at a larger charging rate (>1C), the ratio of constant current charging capacity at different rates in Example 1 to constant current charging capacity at 0.33C is 1-2% higher than that in Comparative Example 1. This is because the specific binder in Example 1 improves the dispersion uniformity of the lithium iron phosphate cathode material and the conductive agent, making the ion transport path and electron transport path distribution more reasonable, thereby reducing the polarization phenomenon during charging and increasing the charging capacity in the constant current section.

[0133] The steps for the discharge performance test are as follows: Charging is performed at 1C, using a constant current and constant voltage charging process. The cutoff current is set at 0.05C. Constant current discharge is then performed at 0.33C, 1C, 2C, 3C, and 4C, and the discharge capacity is recorded. The ratio of the discharge capacity at each rate to the discharge capacity at 0.33C is calculated. The voltage range for the discharge performance test is 2.5–3.65V, and the temperature is 25℃. The test results are shown in Table 3.

[0134] Table 3

[0135]

[0136] As can be seen from Table 3, at a larger discharge rate (>1C), the ratio of discharge capacity at different discharge rates in Example 1 to discharge capacity at 0.33C is about 2% higher than that in Comparative Example 1. This is because Example 1 improved the dispersion uniformity of the conductive agent, optimized the distribution of ion transport paths and electron transport paths, and made electron transport and ion transport more matched, thereby increasing the discharge capacity.

[0137] The steps for the constant current charge ratio test are as follows: First, adjust the charge of the battery under test to 0% SOC (i.e., the battery is fully discharged). Charge the battery at 1C, 2C, 3C, and 4C rates using a constant current and constant voltage process, with a cutoff condition of 0.05C. Record the capacity values ​​of the constant current and constant voltage segments at different rates. Calculate the ratio of the capacity charged in the constant current segment to the total capacity charged, which is recorded as the constant current charge ratio. The constant current test voltage range is 2.5–3.65V, and the temperature is 25℃. The test results are shown in Table 4.

[0138] Table 4

[0139]

[0140] As can be seen from Table 4, the ratio of the constant current charging capacity to the total charging capacity in Example 1 at different rates is 1-2% higher than that in Comparative Example 1. This indicates that the binder with the specific structure provided in this application can effectively reduce the polarization effect of lithium-ion batteries during the charging process. This means that more electricity can be charged in the constant current stage, thereby improving the overall charging efficiency.

[0141] Fast-charge cycle tests were conducted on the lithium iron phosphate single-cell batteries assembled in Embodiment 1 and Comparative Example 1 of this application. The test steps were as follows: The charging process employed a 20-minute stepped fast-charge strategy across 20-80% SOC; the discharging process used a 1C constant current discharge, cycling 50 times. The discharge capacity and capacity retention rate were recorded for each cycle. The fast-charge cycle test voltage range was 2.5-3.65V, and the temperature was 25℃. The test results are as follows: Figure 3 As shown.

[0142] The fast-charging cycle capacity retention rate of lithium iron phosphate monocells assembled in Example 1 and Comparative Example 1 of this application is compared as follows: Figure 3 As shown. By Figure 3 It can be seen that the capacity retention rate of Example 1 after 50 fast charge cycles is about 1% higher than that of Comparative Example 1. This indicates that applying the above-mentioned binder provided in this application to lithium-ion batteries can improve the transport path between lithium ions and electrons, allowing for greater capacity utilization, thereby effectively improving the fast charge cycle performance of the prepared lithium-ion batteries.

[0143] As can be seen from the above description, the embodiments of this application achieve the following technical effects:

[0144] Comparing Example 1 with Comparative Examples 1 and 4, and considering the data in Tables 2, 3, and 4, it can be seen that the lithium-ion battery assembled in Example 1 exhibits superior rate performance, fast-charging performance, and cycle stability. This is because the molecular chain of the binder provided in this application contains both polar segments (i.e., the segments corresponding to the first comonomer) and non-polar segments (i.e., the segments corresponding to the second comonomer), which significantly improves the solubility and dispersibility of the binder and promotes the enrichment of non-polar conductive agents in the non-polar segment regions. This significantly improves the dispersibility of the conductive agents in the positive electrode slurry, thereby effectively improving the microstructure of the positive electrode active material layer obtained therefrom, optimizing the transport path of lithium ions and electrons, and thus effectively improving the rate performance, fast-charging performance, and cycle stability of the prepared lithium-ion battery.

[0145] Comparing Examples 1 to 3 and Comparative Example 2, and referring to the data in Tables 2, 3, and 4, it can be seen that, compared to other ranges, limiting the weight-average molecular weight of the binder to the specific range described above in this application can improve the bonding performance of the binder, improve the dispersion uniformity of the binder, conductive agent, and lithium iron phosphate cathode material in the cathode slurry, improve the processing performance of the cathode slurry, thereby significantly improving the microstructure of the obtained cathode active material layer, optimizing the lithium ion and electron transport path, and thus effectively improving the rate performance, fast charging performance, and cycle stability of the obtained lithium-ion battery.

[0146] Comparing Examples 1, 4 to 6 and Comparative Example 3, and considering the data in Tables 2, 3 and 4, it can be seen that, compared to other ranges, limiting the content of the corresponding chain segments of the first comonomer and the corresponding chain segments of the second comonomer to the specific ranges described above in this application ensures that the binder contains an appropriate amount of non-polar chain segments, thereby better exerting its function, improving the dispersion uniformity of the binder, conductive agent and lithium iron phosphate cathode material in the cathode slurry, improving the microstructure of the prepared cathode active material layer, forming a more uniform ion and electron transport network, and thus effectively reducing the internal resistance and polarization of the prepared lithium-ion battery during charging and discharging, improving its rate performance, fast charging performance and cycle stability.

[0147] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0148] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. An adhesive, characterized in that, The adhesive is a random copolymer, which is obtained by copolymerization of a first comonomer and a second comonomer; the first comonomer is selected from N-alkenyl-substituted pyrrolidone; the second comonomer is C5-C6. 20 olefins; The binder has a weight-average molecular weight of 900–1200 kDa; the second comonomer segment accounts for ≥9.1 wt% of the weight of the binder.

2. The adhesive according to claim 1, characterized in that, The first comonomer is selected from N-vinylpyrrolidone and / or 1-(allyl)pyrrolidone-2-one; Preferably, based on the weight percentage of the adhesive, the adhesive comprises 9.1 wt% to 23.1 wt% of the corresponding segments of the second comonomer and the balance being the corresponding segments of the first comonomer; Preferably, the second comonomer is C6 to C6. 12 olefins; Preferably, the second comonomer is C6 to C6. 10 The second comonomer is selected from one or more of the group consisting of 1-hexene, 1-heptene and 1-nonene; Preferably, the glass transition temperature of the adhesive is -35 to -25°C.

3. A method for preparing the adhesive according to claim 1 or 2, characterized in that, The preparation method includes: Step S1: Under a protective gas atmosphere, the first comonomer and the second comonomer undergo a copolymerization reaction in a first solvent under the action of an initiator to obtain the reaction product; Step S2: The reaction product is first dried to obtain the adhesive.

4. The method for preparing the adhesive according to claim 3, characterized in that, In step S1, the weight ratio of the first comonomer to the second comonomer is 1:(0.1 to 0.3). Preferably, the weight ratio of the initiator to the first comonomer is (0.01-0.03):1; Preferably, the initiator is selected from one or more of the group consisting of azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate; Preferably, the weight ratio of the first solvent to the first comonomer is (1-10):1; Preferably, the first solvent is selected from toluene and / or N,N-dimethylformamide.

5. The method for preparing the adhesive according to claim 3 or 4, characterized in that, In step S1, the copolymerization reaction is carried out at a temperature of 60–80°C for a time of 0.2–2 hours. Preferably, step S1 includes: stirring the first comonomer and the first solvent under the protective gas atmosphere to obtain a mixture; mixing the mixture, the second comonomer and the initiator and carrying out the copolymerization reaction to obtain the reaction product; more preferably, the stirring speed is 300-3000 rpm, the temperature is 30-50°C, and the time is 2-6 h. Preferably, the protective gas is selected from nitrogen and / or an inert gas; Preferably, in step S2, the temperature of the first drying is 100-110°C and the time is 12-24 hours.

6. A lithium iron phosphate cathode, comprising a cathode current collector and a cathode active material layer disposed on at least one surface thereon, said cathode active material layer comprising lithium iron phosphate cathode material, a conductive agent, and a binder, characterized in that, The adhesive is the adhesive as described in claim 1 or 2.

7. A method for preparing the lithium iron phosphate cathode according to claim 6, characterized in that, The preparation method includes: Step N1: Mix the adhesive with the second solvent to obtain the adhesive solution; Step N2: Mix the adhesive, conductive agent, lithium iron phosphate cathode material with a third solvent to obtain cathode slurry; Step N3: The positive electrode slurry is coated on at least one side of the positive electrode current collector, and then dried to obtain the lithium iron phosphate positive electrode.

8. The method for preparing the lithium iron phosphate cathode according to claim 7, characterized in that, In step N1, the weight ratio of the adhesive to the second solvent is 1:(10-20); preferably, the second solvent is N-methylpyrrolidone. Preferably, a second stirring is performed during the mixing process in step N1; more preferably, the stirring rate is 2000-4000 rpm and the stirring time is 5-8 hours. Preferably, in step N2, the weight ratio of the adhesive, the conductive agent, the lithium iron phosphate cathode material, and the third solvent is (0.3-0.4):(0.01-0.015):1:(0.05-0.5). Preferably, the D of the lithium iron phosphate cathode material 50 Its diameter is 0.8–3 μm; Preferably, the conductive agent has a D 50 The range is 0.1–1 μm; Preferably, the conductive agent is selected from one or more of the group consisting of conductive carbon black, multi-walled carbon nanotubes, and single-walled carbon nanotubes; Preferably, the third solvent is N-methylpyrrolidone; Preferably, a third stirring is performed during the mixing process in step N2; more preferably, the third stirring rate is 1000-3000 rpm and the time is 6-10 h.

9. The method for preparing the lithium iron phosphate cathode according to claim 7 or 8, characterized in that, The viscosity of the positive electrode slurry is 3000–8000 mPa·s, and the solid content is 60 wt%–64 wt%. Preferably, in step N3, the coating amount of the positive electrode slurry on the surface of the positive electrode current collector is 150–200 g / cm³. 2 ; Preferably, the second drying temperature is 100-110°C and the time is 12-24 hours; Preferably, the compaction density of the lithium iron phosphate cathode is 2.4–2.5 g / cm³. 3 .

10. A lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, characterized in that, The positive electrode is the lithium iron phosphate positive electrode according to claim 6.

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