Lithium iron phosphate battery cell and battery

Abstract

The application provides a lithium iron phosphate battery cell and a battery. The lithium iron phosphate battery cell comprises a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a shell. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active coating arranged on at least one surface of the positive electrode current collector. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating arranged on at least one surface of the negative electrode current collector. The electrolyte comprises lithium bisfluorosulfonylimide. The battery cell satisfies the following relationship: 5≤(a / 20+d) / (b+c)≤9.3. a is the membrane resistance of the positive electrode sheet after coating; b is the compaction density of the positive electrode sheet; c is the mass percentage of vinylene carbonate in the positive electrode slurry; and d is the mass percentage of lithium bisfluorosulfonylimide in the nonaqueous electrolyte. The composition of the lithium ion battery cell is optimized, so that the low-temperature discharge performance of the battery is improved, and the high-temperature cycle performance is also considered.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery, and particularly relates to a lithium iron phosphate battery cell and a battery. BACKGROUND

[0002] With the increasing demand for new energy devices, lithium ion secondary batteries have been widely used in many fields due to their own advantages, and have become the current new energy devices that are concerned. Among various systems of lithium ion batteries, lithium iron phosphate batteries have the largest share in the entire lithium ion battery market due to their strong safety and good structural stability, and are widely used in many scenes with high requirements for battery performance, such as electric vehicles, energy storage systems, etc.

[0003] The lithium iron phosphate battery is composed of a positive lithium iron phosphate material, a negative electrode, a polymer separator and an electrolyte, and is packaged by a metal shell. The positive electrode and the negative electrode of the battery are connected with aluminum foil and copper foil respectively. During charging, lithium ions are deintercalated from the positive electrode, embedded into the negative electrode through the electrolyte; during discharging, lithium ions are deintercalated from the negative electrode, embedded into the positive electrode through the electrolyte. Since the lithium iron phosphate material itself has low conductivity, the charging and discharging performance of the lithium iron phosphate battery will be greatly affected in a low temperature environment, which will lead to a decrease in discharge capacity and efficiency. In order to improve the poor low temperature performance of the lithium iron phosphate battery, a coating strategy is usually used to improve the lithium iron phosphate battery. The coating strategy aims to form a coating layer on the surface of the lithium iron phosphate material to improve the low temperature discharge performance of the battery and optimize the use performance of the battery in a low temperature environment.

[0004] Although the existing coating strategy is an optimization scheme for improving the low temperature performance of the lithium iron phosphate battery to some extent, the non-in-situ preparation process has obvious defects. On the one hand, the coating strategy cannot achieve comprehensive and close wrapping of the lithium iron phosphate, so that the improvement degree of the low temperature discharge performance is very limited, which is difficult to meet the demand for the low temperature performance of the battery in actual application scenarios. On the other hand, the non-in-situ preparation process is easy to introduce other surface defect sites on the surface of the lithium iron phosphate, and these sites will further promote the decomposition reaction of the electrolyte on the surface thereof, thereby causing rapid capacity decay of the battery and degrading the high temperature cycle performance of the battery. Therefore, the lithium iron phosphate battery in the related art cannot meet the urgent demand for wide-temperature-range lithium iron phosphate batteries in the market. SUMMARY

[0005] The present application provides a lithium iron phosphate battery cell and a battery to solve the problem that the lithium iron phosphate battery in the prior art cannot meet the low temperature discharge performance and the high temperature cycle performance.

[0006] In a first aspect, the application provides a lithium iron phosphate battery cell, the cell comprising a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a shell; the positive electrode sheet comprising a positive electrode current collector and a positive electrode active coating layer disposed on at least one surface of the positive electrode current collector; the negative electrode sheet comprising a negative electrode current collector and a negative electrode active coating layer disposed on at least one surface of the negative electrode current collector; the electrolyte comprising lithium bisfluorosulfonylimide;

[0007] wherein the cell satisfies the following relationship:

[0008] 5≤(a / 20+d) / (b+c)≤9.3;

[0009] a is the membrane resistance of the positive electrode sheet after coating;

[0010] b is the tap density of the positive electrode sheet;

[0011] c is the mass percentage of vinylene carbonate in the positive electrode slurry;

[0012] d is the mass percentage of lithium bisfluorosulfonylimide in the non-aqueous electrolyte.

[0013] The application adjusts the composition of the positive electrode sheet in the lithium iron phosphate battery cell, uses lithium iron phosphate as the positive electrode active material, controls the addition amount of vinylene carbonate (VC) in the positive electrode slurry, the membrane resistance and the tap density of the positive electrode sheet, adds an appropriate amount of lithium bisfluorosulfonylimide (LiFSI) in the non-aqueous electrolyte, and makes the battery composition satisfy specific requirements, so as to fully exert the synergistic effect between lithium bisfluorosulfonylimide (LiFSI) and vinylene carbonate (VC) and the positive electrode active material, so that the cell can improve the low-temperature discharge performance of the battery while considering the high-temperature cycle performance.

[0014] In some possible implementation manners, the positive electrode active coating layer comprises a positive electrode active material, a conductive agent, a dispersing agent and a binder, and the mass ratio of the positive electrode active material, the conductive agent, the dispersing agent and the binder is 96.4-c:1.4:c:2.2, and the dispersing agent is vinylene carbonate.

[0015] In some possible implementation manners, the positive electrode active material is a lithium iron phosphate (LiFePO4) active material coated with a carbon coating layer.

[0016] In some possible implementation manners, the membrane resistance a of the positive electrode sheet after coating is 100-400 mΩ.

[0017] In some possible implementation manners, the tap density b of the positive electrode sheet is 2.4-2.7 g / cm 3 .

[0018] In some possible implementation manners, the mass percentage content c of the vinylene carbonate in the positive electrode slurry is 0.1-0.5 wt%.

[0019] In some possible implementation manners, the mass percentage content d of the lithium bisfluorosulfonylimide in the non-aqueous electrolyte is 3-7 wt%.

[0020] In some possible implementation manners, the method for preparing the cell of the lithium iron phosphate battery comprises the following steps.

[0021] The binder is mixed with the positive electrode active material to obtain mixed powder;

[0022] The conductive agent, N-methyl-2-pyrrolidone (NMP) and the dispersant are mixed to obtain a mixed solution; wherein the mass ratio of the positive electrode active material, the conductive agent, the dispersant and the binder is 96.4-c:1.4:c:2.2; the dispersant is vinylene carbonate;

[0023] The mixed solution is uniformly stirred with the mixed powder to obtain a positive electrode slurry;

[0024] The positive electrode slurry is coated on the positive electrode current collector to obtain a positive electrode tab;

[0025] A negative electrode slurry and an electrolyte are prepared;

[0026] The negative electrode slurry is coated on the negative electrode current collector to obtain a negative electrode tab;

[0027] The positive electrode tab and the negative electrode tab are assembled, and the electrolyte is injected and vacuum packaged to obtain the cell of the lithium iron phosphate battery.

[0028] In some possible implementation manners, coating the positive electrode slurry on the positive electrode current collector to obtain a positive electrode tab comprises:

[0029] The positive electrode slurry is coated on the positive electrode current collector to obtain an electrode tab, and the electrode tab is subjected to resistance detection using a sheet resistance meter;

[0030] When the resistance of the electrode tab is less than or equal to a preset resistance value, the electrode tab is dried, rolled, die-cut or slitted to obtain a positive electrode tab.

[0031] In a second aspect, the present application provides a lithium iron phosphate battery, which comprises the cell of the lithium iron phosphate battery according to the first aspect.

[0032] By optimizing the positive electrode composition of the lithium iron phosphate battery cell and the electrolyte composition, the capacity decay of the lithium iron phosphate battery at low temperature is effectively delayed, the cycle life of the battery is improved, the internal resistance of the battery is reduced, and the low temperature performance of the battery is improved, so that a wide temperature range lithium iron phosphate battery which can balance the low temperature discharge performance and high temperature cycle performance is obtained.

[0033] From the above, the application provides a lithium iron phosphate battery cell and battery. The lithium iron phosphate battery cell comprises a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a shell; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active coating layer arranged on at least one surface of the positive electrode current collector; the negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating layer arranged on at least one surface of the negative electrode current collector; the electrolyte comprises lithium bisfluorosulfonylimide; wherein the cell satisfies the following relationship: 5≤(a / 20+d) / (b+c)≤9.3; a is the membrane resistance of the positive electrode sheet after coating; b is the tap density of the positive electrode sheet; c is the mass percentage of vinylene carbonate in the positive electrode slurry; d is the mass percentage of lithium bisfluorosulfonylimide in the non-aqueous electrolyte. By optimizing the lithium iron phosphate battery cell, using lithium iron phosphate as the positive electrode active material, controlling the vinylene carbonate addition amount in the positive electrode slurry, the positive electrode sheet membrane resistance and the tap density, adding an appropriate amount of lithium bisfluorosulfonylimide in the non-aqueous electrolyte, and making the battery composition meet the specific requirements, the synergistic effect between lithium bisfluorosulfonylimide and the positive electrode active material can be fully utilized, so that the lithium iron phosphate battery cell can improve the low temperature discharge performance of the battery while balancing the high temperature cycle performance. DETAILED DESCRIPTION

[0034] The embodiments described in the following examples do not represent all the implementations consistent with the present application. They are merely examples of systems and methods consistent with some aspects of the present application as detailed in the claims.

[0035] With the increasing demand for new energy devices, lithium ion secondary batteries have been widely used in many fields due to their own advantages, and have become the current new energy devices that attract much attention. Among various systems of lithium ion batteries, lithium iron phosphate batteries have the largest share in the entire lithium ion battery market due to their strong safety and good structural stability, and are widely used in many scenes with high requirements for battery performance, such as electric vehicles, energy storage systems, etc.

[0036] The lithium iron phosphate battery is composed of a positive lithium iron phosphate material, a negative electrode, a polymer separator and an electrolyte, and is packaged by a metal shell. The positive and negative electrodes of the battery are connected with aluminum foil and copper foil respectively. During charging, lithium ions are deintercalated from the positive electrode, embedded into the negative electrode through the electrolyte; during discharging, lithium ions are deintercalated from the negative electrode, embedded into the positive electrode through the electrolyte. The lithium iron phosphate material has poor electrical conductivity. In a low-temperature environment, the migration rate of lithium ions is significantly reduced, the chemical reaction rate in the battery is slowed down, and the charge and discharge performance of the battery is greatly reduced. For example, at-20℃, the discharge capacity of the battery may be only 30%-40% of that at normal temperature. In addition, the crystal structure of the lithium iron phosphate material has high stability at low temperature, making it difficult for lithium ions to be embedded and de-embedded, further affecting the performance of the battery. In order to improve the poor low-temperature performance of the lithium iron phosphate battery, a coating strategy is usually used to improve the lithium iron phosphate battery. The coating strategy aims to form a coating layer on the surface of the lithium iron phosphate material to improve the low-temperature discharge performance of the battery and optimize the performance of the battery in a low-temperature environment.

[0037] Although the existing coating strategy is an optimization scheme for improving the low-temperature performance of the lithium iron phosphate battery to some extent, the non-in-situ preparation process has obvious defects. On the one hand, the coating strategy cannot achieve comprehensive and close wrapping of the lithium iron phosphate, resulting in limited improvement of the low-temperature discharge performance, which is difficult to meet the demand for the low-temperature performance of the battery in actual application scenarios. On the other hand, the non-in-situ preparation process is easy to introduce other surface defect sites on the surface of the lithium iron phosphate, and these sites will further promote the decomposition reaction of the electrolyte on the surface thereof, thereby causing rapid capacity decay of the battery and degrading the high-temperature cycle performance of the battery. Therefore, the lithium iron phosphate battery in the related art cannot balance the low-temperature discharge performance and the high-temperature cycle performance, and cannot meet the urgent demand for wide-temperature-range lithium iron phosphate batteries in the market.

[0038] Therefore, in view of the problem that the lithium iron phosphate battery in the related art cannot balance the low-temperature discharge performance and the high-temperature cycle performance, the present application provides a battery cell and a battery of a lithium iron phosphate battery. The positive electrode sheet in the battery cell of the lithium iron phosphate battery is adjusted, the lithium iron phosphate is used as the positive active material, the addition amount of vinylene carbonate (VC) in the positive slurry, the membrane sheet resistance of the positive electrode sheet and the compaction density are controlled, an appropriate amount of lithium bisfluorosulfonylimide (LiFSI) is added in the non-aqueous electrolyte, and the battery composition meets specific requirements, which can fully play the synergistic effect between lithium bisfluorosulfonylimide (LiFSI) and vinylene carbonate (VC) and the positive active material, so that the battery cell can balance the low-temperature discharge performance and the high-temperature cycle performance while improving the low-temperature discharge performance of the battery.

[0039] In some embodiments, the application provides a lithium iron phosphate battery cell, the cell comprising a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a shell; the positive electrode sheet comprising a positive electrode current collector and a positive electrode active coating layer disposed on at least one surface of the positive electrode current collector; the negative electrode sheet comprising a negative electrode current collector and a negative electrode active coating layer disposed on at least one surface of the negative electrode current collector; the electrolyte comprising vinylene carbonate;

[0040] wherein the cell satisfies the following relationship:

[0041] 5≤(a / 20+d) / (b+c)≤9.3;

[0042] a is the membrane resistance of the positive electrode sheet after coating;

[0043] b is the compaction density of the positive electrode sheet;

[0044] c is the mass percentage content of vinylene carbonate in the positive electrode slurry;

[0045] d is the mass percentage content of lithium bisfluorosulfonylimide in the non-aqueous electrolyte.

[0046] The application controls the design of the positive electrode sheet and the addition amount of vinylene carbonate (VC) to obtain higher cycle life and service life of the lithium iron phosphate battery cell. Specifically, the application discloses a positive electrode sheet design of a lithium iron phosphate battery cell, which selects a lower compaction density to make the structure of the positive electrode sheet material more complete and ensure a higher liquid retention amount. By reducing the membrane resistance of the positive electrode sheet, the low-temperature discharge capacity of the cell can be improved. Too high a content of vinylene carbonate (VC) in the positive electrode slurry will affect the cycle life, so the content of vinylene carbonate (VC) should also be controlled within a reasonable range. By controlling the addition amount of vinylene carbonate (VC) in the positive electrode slurry, the membrane resistance of the positive electrode sheet and the compaction density, and adding an appropriate amount of lithium bisfluorosulfonylimide (LiFSI) in the non-aqueous electrolyte, the cell satisfies the specific relationship requirements, reduces the cell film formation impedance, and thus improves the high and low temperature performance of the cell.

[0047] In some embodiments, the positive electrode active coating layer comprises a positive electrode active material, a conductive agent, a dispersing agent, and a binder, and the mass ratio of the positive electrode active material, the conductive agent, the dispersing agent, and the binder is 96.4-c:1.4:c:2.2. The conductive agent comprises one or more of Ketjen black, mesocarbon microbeads, activated carbon, graphite, conductive carbon black, acetylene black, carbon fiber, carbon nanotube, graphene, etc. The binder comprises one or more of vinylidene fluoride, hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, hexafluorobutadiene, hexafluoroisobutene, trifluoroethylene, chlorotrifluoroethylene, and tetrafluoroethylene. The dispersing agent is vinylene carbonate. The vinylene carbonate can effectively reduce the agglomeration phenomenon between the positive electrode active material particles, so that the active material is more uniformly dispersed in the coating layer. The addition of the vinylene carbonate is conducive to increasing the contact area of the active material and the electrolyte, improving the embedding and de-embedding efficiency of lithium ions in the charging and discharging process, and thus improving the charging and discharging performance and rate performance of the battery.

[0048] During the first charging and discharging process of the battery, the vinylene carbonate will decompose on the surface of the electrode to form a layer of stable solid electrolyte interface (SEI) film. The solid electrolyte interface (SEI) film has good ionic conductivity and electronic insulation, which can effectively prevent further reaction between the electrolyte and the electrode material, reduce the occurrence of side reactions, and improve the coulombic efficiency and cycle life of the battery. The formed solid electrolyte interface (SEI) film helps to reduce the impedance of the electrode surface, especially in low temperature environment, which can improve the migration rate of lithium ions and reduce the polarization phenomenon, thereby improving the low temperature discharge performance of the battery, so that the battery can still maintain good capacity output and rate performance under low temperature conditions.

[0049] In some embodiments, the positive electrode active material is a lithium iron phosphate (LiFePO4) active material coated with a carbon coating layer.

[0050] Lithium iron phosphate itself has low electronic conductivity, while carbon has good conductivity. The presence of the carbon coating layer provides a good channel for electron transport, greatly improving the overall electronic conductivity of the material. The carbon coating layer can optimize the properties of the surface of the lithium iron phosphate particles, reduce the diffusion resistance of lithium ions on the surface of the particles, and also inhibit the growth of lithium iron phosphate grains to some extent, shorten the diffusion path of lithium ions, and thus improve the diffusion coefficient of lithium ions. The carbon coating layer can protect the lithium iron phosphate active material from direct contact with the electrolyte and prevent side reactions, reducing the loss of active material. During the charging and discharging process, the carbon coating layer can alleviate the volume change of lithium iron phosphate caused by lithium ion intercalation and deintercalation, maintaining the stability of the electrode structure. Carbon material itself has certain thermal stability, and after being coated on the surface of lithium iron phosphate, it can improve the overall thermal stability of the positive electrode material to some extent, and also prevent lithium iron phosphate from reacting adversely with the electrolyte at high temperatures. In some embodiments, the thickness of the carbon coating layer is 2-20 nm.

[0051] By using the carbon-coated lithium iron phosphate active material as the positive electrode active material, the battery can have faster electron transfer between the positive electrode active material and the positive electrode current collector during charging and discharging, thereby improving the rate performance of the battery and allowing the battery to charge and discharge at high current density, meeting the demand for high-power output such as electric vehicles. Moreover, the carbon-coated lithium iron phosphate active material can also enhance the safety and performance stability of the battery in high-temperature environments, reduce the risk of battery bulging, burning, and other safety hazards caused by high temperatures, and expand the working temperature range of the battery, allowing it to work more stably under different environmental temperature conditions.

[0052] In some embodiments, the positive electrode tab has a post-coating sheet resistance a of 100-400 mΩ. In a low-temperature environment, the ion diffusion rate and electron conduction rate of the lithium iron phosphate battery cell itself will be greatly reduced. If the post-coating sheet resistance of the positive electrode tab is high, it will further hinder the transmission of electrons and lithium ions. It is difficult for electrons to conduct smoothly from the positive electrode active material to the current collector, and the embedding and de-embedding process of lithium ions also becomes more difficult, which significantly reduces the amount of active material involved in the discharge reaction, resulting in a sharp decline in the discharge capacity of the battery cell. In a low-temperature environment, it cannot release the amount of electricity as at room temperature, which seriously affects the use time and endurance of the battery in a low-temperature environment. When the sheet resistance is low, although the low-temperature environment still inhibits the transmission of electrons and lithium ions, the relatively small sheet resistance can alleviate this hindrance to some extent. Electrons and lithium ions can move relatively smoothly, allowing more positive electrode active material to participate in the discharge reaction, thereby retaining a higher discharge capacity compared to the case of high sheet resistance, improving the available power and endurance of the battery cell in a low-temperature environment. The post-coating sheet resistance a of the positive electrode tab can be 218 mΩ, 259 mΩ, 272 mΩ, 298 mΩ, 300 mΩ, 308 mΩ, 311 mΩ, 312 mΩ, 313 mΩ, 318 mΩ, 319 mΩ, 323 mΩ, 327 mΩ, 346 mΩ, 362 mΩ, 385 mΩ, or 400 mΩ.

[0053] In some embodiments, the positive electrode tab has a compaction density b of 2.4-2.7 g / cm 3 . The higher the compaction density of the positive electrode tab, the more positive electrode active material content. By setting the positive electrode tab to have a certain compaction density, the positive electrode tab has a certain energy and a certain void to facilitate the de-embedding of active ions and improve ion movement rate. The compaction density b of the positive electrode tab can be 2.4 g / cm 3 , 2.5 g / cm 3 , 2.6 g / cm 3 , or 2.7 g / cm 3 .

[0054] In the present application, the important film-forming additive vinylene carbonate (VC) is mixed in the positive electrode material in advance, fully contacts with the lithium iron phosphate material, and due to the similar solubility principle, the electrolyte can promote the infiltration of the positive electrode material during liquid injection. Moreover, the dispersion of the film-forming additive in the different thickness layers of the positive electrode material is ensured, which is beneficial to the comprehensive coverage of the positive electrode material at different thicknesses on the pole piece. In the initial stage of the formation of the film of the battery, a protective film can be quickly formed on the surface of the lithium iron phosphate material, which not only increases the contact area between the lithium iron phosphate materials and the current collector, but also avoids the oxidation and decomposition of the components with high film-forming resistance in the electrolyte at this place, thereby reducing the initial impedance and impedance growth rate of the battery, and improving the low-temperature discharge performance and high-temperature cycle performance of the battery.

[0055] In some embodiments, the mass percentage content c of the vinylene carbonate in the positive electrode slurry is 0.1-0.5wt%. Specifically, the mass percentage content c of the vinylene carbonate in the positive electrode slurry can be 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, or 0.5wt%.

[0056] The fluorine ion in lithium fluorosulfonylimide (LiFSI) has strong electron-withdrawing properties, which weakens the coordination between the anion and the cation of the lithium salt, and the activity of lithium ions is very strong, the conductivity, thermal stability and electrochemical stability are high, and no side reactions occur basically. At the same time, it can also inhibit the swelling effect. In the present application, an appropriate amount of lithium bisfluorosulfonylimide (LiFSI) is added to the non-aqueous electrolyte of the battery cell of the lithium iron phosphate battery, which can significantly improve the conductivity of the electrolyte, thereby improving the low-temperature discharge performance of the battery cell. In addition, lithium bisfluorosulfonylimide can also further modify the positive and negative film components, reduce the film-forming impedance of the battery cell, and improve the high and low temperature performance of the battery cell.

[0057] In some embodiments, the mass percentage content d of the lithium bisfluorosulfonylimide in the non-aqueous electrolyte is 3-7wt%. Specifically, the mass percentage content d of the lithium bisfluorosulfonylimide in the non-aqueous electrolyte can be 3wt%, 4wt%, 5wt%, 6wt%, or 7wt%.

[0058] In some embodiments, the negative active coating layer comprises a negative active material, a conductive agent, a dispersing agent, and a binder, and in the present application, the negative active coating layer further comprises styrene butadiene rubber (SBR), and the mass ratio of the negative active material, the conductive agent, the dispersing agent, the styrene butadiene rubber, and the binder is 96.4:1.0:0.3:0.5:1.8. The conductive agent comprises one or more of Ketjen black, mesocarbon microbeads, activated carbon, graphite, conductive carbon black, acetylene black, carbon fiber, carbon nanotube, graphene, etc.; the dispersing agent comprises one or more of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphated starch, casein. The binder comprises one or more of polyvinylidene fluoride, hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, hexafluorobutadiene, hexafluoroisobutene, trifluoroethylene, chlorotrifluoroethylene, and tetrafluoroethylene.

[0059] In some embodiments, the separator can be any material suitable for use as a separator in a lithium ion battery in the art, for example, can be a combination of one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fiber, etc.

[0060] In some embodiments, the electrolyte can be any material suitable for use as an electrolyte in a lithium ion battery in the art, for example, can be a combination of a plurality of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), lithium bisfluorosulfonylimide (LiFSI), lithium hexafluorophosphate (LiPF6), etc. In some embodiments, the material of the shell is stainless steel or aluminum plastic film.

[0061] In some embodiments, the method for preparing the cell of the lithium iron phosphate battery comprises:

[0062] Mixing the binder with the positive active material to obtain a mixed powder;

[0063] Mixing the conductive agent, the N-methyl-2-pyrrolidone solvent, and the dispersing agent to obtain a mixed solution; wherein the mass ratio of the positive active material, the conductive agent, the dispersing agent, and the binder is 96.4-c:1.4:c:2.2; the dispersing agent is vinylene carbonate;

[0064] Stirring the mixed solution and the mixed powder uniformly to obtain a positive electrode slurry;

[0065] Coating the positive electrode slurry on the positive electrode current collector to obtain a positive electrode tab;

[0066] Preparing a negative electrode slurry and an electrolyte;

[0067] coating the negative electrode slurry on a negative electrode current collector to obtain a negative electrode sheet;

[0068] assembling the positive electrode sheet and the negative electrode sheet, and vacuum packaging after injecting an electrolyte to obtain a cell of a lithium iron phosphate battery.

[0069] In some embodiments, coating the positive electrode slurry on a positive electrode current collector to obtain a positive electrode sheet comprises:

[0070] coating the positive electrode slurry on a positive electrode current collector to obtain an electrode sheet, and performing resistance detection on the electrode sheet using a sheet resistance meter;

[0071] when the resistance of the electrode sheet is less than or equal to a preset resistance value, drying, rolling, die cutting, or slitting the electrode sheet to obtain a positive electrode sheet;

[0072] when the resistance of the electrode sheet is greater than the preset resistance value, adjusting the proportions of the positive electrode active material, the conductive agent, the dispersant, and the binder, and re-preparing a positive electrode slurry.

[0073] In order to enable the cell of the lithium iron phosphate battery to balance the low-temperature discharge performance and the high-temperature cycle performance, the sheet resistance after coating of the positive electrode sheet is limited, and the sheet resistance after coating of the positive electrode sheet is 100-400 mΩ. The electrode sheet with a sheet resistance after coating less than or equal to a preset resistance value is selected as the positive electrode sheet, so as to improve the low-temperature discharge capacity of the cell.

[0074] In some embodiments, the present application provides a lithium iron phosphate battery, which comprises the cell of the lithium iron phosphate battery in the above-mentioned embodiments.

[0075] The present application optimizes the positive electrode composition and the electrolyte composition of the cell of the lithium iron phosphate battery, effectively delays the capacity decay of the lithium iron phosphate battery at low temperature, improves the cycle life of the battery, reduces the internal resistance of the battery, and improves the low-temperature performance of the battery, so as to obtain a wide-temperature-range lithium iron phosphate battery capable of balancing the low-temperature discharge performance and the high-temperature cycle performance.

[0076] Embodiment 1:

[0077] 1) Preparation of a positive electrode sheet

[0078] Step 1: mixing the binder PVDF and the positive electrode active material powder by stirring to obtain a mixed powder.

[0079] Step 2: mixing the solvent N-methyl-2-pyrrolidone (NMP), the conductive agent carbon nanotube (CNT), and the vinylene carbonate (VC) compound by stirring to obtain a mixed liquid.

[0080] Step 3: Add the mixed liquid to the mixed powder, disperse at high speed, stir uniformly, and obtain the positive electrode slurry, the mass ratio of the positive electrode active material, the conductive agent carbon nanotube (CNT), the vinylene carbonate (VC), and the positive electrode binder = 96.4-c: 1.4: c: 2.2; the positive electrode active material is a lithium iron phosphate (LiFePO4) active material coated with a carbon coating layer.

[0081] Step 4: Uniformly coat the prepared positive electrode slurry on the positive electrode current collector (such as aluminum foil), and then detect the resistance of the electrode sheet using a film resistance meter.

[0082] Step 5: Obtain the positive electrode sheet through drying, rolling, die cutting, or striping.

[0083] 2) Preparation of negative electrode sheet

[0084] Step 1: Weigh the negative electrode sheet ingredients in the ratio of graphite: conductive carbon (superP): carboxymethyl cellulose (CMC): styrene butadiene rubber (SBR): binder = 96.4: 1.0: 0.3: 0.5: 1.8 by mass ratio.

[0085] Step 2: First, stir and pre-mix the graphite, conductive carbon (superP), and carboxymethyl cellulose (CMC), then add pure water and stir uniformly.

[0086] Step 3: Add the binder to the mixed slurry, mix thoroughly, then add the styrene butadiene rubber (SBR), and finally obtain the negative electrode slurry.

[0087] Step 4: Uniformly coat the prepared negative electrode slurry on the copper foil, and obtain the negative electrode sheet through drying, rolling, die cutting, or striping.

[0088] 3) Preparation of non-aqueous electrolyte

[0089] Mix ethylene carbonate (EC), diethyl carbonate (DMC), and methyl ethyl carbonate (EMC) in a mass ratio of ethylene carbonate: diethyl carbonate: methyl ethyl carbonate = 1: 1: 1, add a certain mass percentage of lithium bisfluorosulfonylimide (LiFSI), and then add lithium hexafluorophosphate (LiPF6) to a molar concentration of 0.9 mol / L.

[0090] 4) Preparation of lithium ion cell

[0091] Assemble the above prepared positive electrode sheet and the above negative electrode sheet into a laminated soft-pack cell.

[0092] 5) Liquid injection and formation of the cell

[0093] The electrolyte prepared above was injected into the battery cell in an environment where the dew point was controlled below -40°C, vacuum sealed, and left still for 72 h. Then the first charge conventional formation was carried out in the following steps: 0.05C constant current charging for 180 min, 0.1C constant current charging for 120 min, 0.2C constant current charging for 120 min, secondary vacuum sealing, and then further full charging (100% SOC) at a current of 0.2C, and after 72 h of normal temperature storage, full discharging (0% SOC) at a current of 0.2C.

[0094] Example 2:

[0095] The preparation method of the lithium iron phosphate battery in this example was the same as that in Example 1, but the positive electrode sheet film resistance was 346 mΩ, the positive electrode sheet compaction density was 2.55 g / cm 3 , the vinylene carbonate accounted for 0.2 wt%, and the lithium bisfluorosulfonylimide accounted for 4 wt%.

[0096] Example 3:

[0097] The preparation method of the lithium iron phosphate battery in this example was the same as that in Example 1, but the positive electrode sheet film resistance was 385 mΩ, the positive electrode sheet compaction density was 2.65 g / cm 3 , the vinylene carbonate accounted for 0.2 wt%, and the lithium bisfluorosulfonylimide accounted for 3 wt%.

[0098] Example 4:

[0099] The preparation method of the lithium iron phosphate battery in this example was the same as that in Example 1, but the positive electrode sheet film resistance was 311 mΩ, the positive electrode sheet compaction density was 2.5 g / cm 3 , the vinylene carbonate accounted for 0.2 wt%, and the lithium bisfluorosulfonylimide accounted for 3 wt%.

[0100] Example 5:

[0101] The preparation method of the lithium iron phosphate battery in this example was the same as that in Example 1, but the positive electrode sheet film resistance was 400 mΩ, the positive electrode sheet compaction density was 2.6 g / cm 3 , the vinylene carbonate accounted for 0.2 wt%, and the lithium bisfluorosulfonylimide accounted for 6 wt%.

[0102] Example 6:

[0103] The preparation method of the lithium iron phosphate battery in this example was the same as that in Example 1, but the positive electrode sheet film resistance was 362 mΩ, the positive electrode sheet compaction density was 2.6 g / cm 3 , the vinylene carbonate accounted for 0.2 wt%, and the lithium bisfluorosulfonylimide accounted for 6 wt%.

[0104] Example 7:

[0105] The preparation method of the lithium iron phosphate battery of the present embodiment is the same as that of embodiment 1, but the membrane sheet resistance of the positive electrode sheet is 298 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.1 wt%, and the lithium bisfluorosulfonylimide accounts for 4 wt%.

[0106] Embodiment 8:

[0107] The preparation method of the lithium iron phosphate battery of the present embodiment is the same as that of embodiment 1, but the membrane sheet resistance of the positive electrode sheet is 218 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.2 wt%, and the lithium bisfluorosulfonylimide accounts for 4 wt%.

[0108] Embodiment 9:

[0109] The preparation method of the lithium iron phosphate battery of the present embodiment is the same as that of embodiment 1, but the membrane sheet resistance of the positive electrode sheet is 259 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.2 wt%, and the lithium bisfluorosulfonylimide accounts for 4 wt%.

[0110] Embodiment 10:

[0111] The preparation method of the lithium iron phosphate battery of the present embodiment is the same as that of embodiment 1, but the membrane sheet resistance of the positive electrode sheet is 312 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.2 wt%, and the lithium bisfluorosulfonylimide accounts for 4 wt%.

[0112] Embodiment 11:

[0113] The preparation method of the lithium iron phosphate battery of the present embodiment is the same as that of embodiment 1, but the membrane sheet resistance of the positive electrode sheet is 272 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.1 wt%, and the lithium bisfluorosulfonylimide accounts for 3 wt%.

[0114] Embodiment 12:

[0115] The preparation method of the lithium iron phosphate battery of the present embodiment is the same as that of embodiment 1, but the membrane sheet resistance of the positive electrode sheet is 272 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.1 wt%, and the lithium bisfluorosulfonylimide accounts for 4 wt%.

[0116] Embodiment 13:

[0117] The preparation method of the lithium iron phosphate battery of the present example is the same as that of example 1, but the membrane sheet resistance of the positive electrode sheet is 272 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.1wt%, and the mass ratio of lithium bisfluorosulfonylimide is 5wt%.

[0118] Example 14:

[0119] The preparation method of the lithium iron phosphate battery of the present example is the same as that of example 1, but the membrane sheet resistance of the positive electrode sheet is 272 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.1wt%, and the mass ratio of lithium bisfluorosulfonylimide is 7wt%.

[0120] Example 15:

[0121] The preparation method of the lithium iron phosphate battery of the present example is the same as that of example 1, but the membrane sheet resistance of the positive electrode sheet is 313 mΩ, and the compaction density of the positive electrode sheet is 2.4 g / cm 3 , the vinylene carbonate accounts for 0.1wt%, and the mass ratio of lithium bisfluorosulfonylimide is 4wt%.

[0122] Example 16:

[0123] The preparation method of the lithium iron phosphate battery of the present example is the same as that of example 1, but the membrane sheet resistance of the positive electrode sheet is 308 mΩ, and the compaction density of the positive electrode sheet is 2.5 g / cm 3 , the vinylene carbonate accounts for 0.1wt%, and the mass ratio of lithium bisfluorosulfonylimide is 4wt%.

[0124] Example 17:

[0125] The preparation method of the lithium iron phosphate battery of the present example is the same as that of example 1, but the membrane sheet resistance of the positive electrode sheet is 319 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.1wt%, and the mass ratio of lithium bisfluorosulfonylimide is 4wt%.

[0126] Example 18:

[0127] The preparation method of the lithium iron phosphate battery of the present example is the same as that of example 1, but the membrane sheet resistance of the positive electrode sheet is 327 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm 3 , the vinylene carbonate accounts for 0.1wt%, and the mass ratio of lithium bisfluorosulfonylimide is 4wt%.

[0128] Example 19:

[0129] The preparation method of the lithium iron phosphate battery of the present example is the same as that of example 1, but the membrane sheet resistance of the positive electrode sheet is 318 mΩ, and the compaction density of the positive electrode sheet is 2.6 g / cm3 0.2wt% of vinylene carbonate and 4wt% of lithium bisfluorosulfonylimide.

[0130] Comparative Example 1

[0131] The lithium iron phosphate battery of this comparative example was prepared in the same way as in Example 1, but the positive electrode sheet film resistance was 285 mΩ and the positive electrode sheet compaction density was 2.5 g / cm 3 0wt% of vinylene carbonate and 3wt% of lithium bisfluorosulfonylimide.

[0132] Comparative Example 2

[0133] The lithium iron phosphate battery of this comparative example was prepared in the same way as in Example 1, but the positive electrode sheet film resistance was 364 mΩ and the positive electrode sheet compaction density was 2.5 g / cm 3 0.5wt% of vinylene carbonate and 3wt% of lithium bisfluorosulfonylimide.

[0134] Comparative Example 3

[0135] The lithium iron phosphate battery of this comparative example was prepared in the same way as in Example 1, but the positive electrode sheet film resistance was 500 mΩ and the positive electrode sheet compaction density was 2.5 g / cm 3 0.2wt% of vinylene carbonate and 3wt% of lithium bisfluorosulfonylimide.

[0136] Comparative Example 4

[0137] The lithium iron phosphate battery of this comparative example was prepared in the same way as in Example 1, but the positive electrode sheet film resistance was 500 mΩ and the positive electrode sheet compaction density was 2.2 g / cm 3 0wt% of vinylene carbonate and 10wt% of lithium bisfluorosulfonylimide.

[0138] Comparative Example 5

[0139] The lithium iron phosphate battery of this comparative example was prepared in the same way as in Example 1, but the positive electrode sheet film resistance was 450 mΩ and the positive electrode sheet compaction density was 2.3 g / cm 3 0.2wt% of vinylene carbonate and 5wt% of lithium bisfluorosulfonylimide.

[0140] Comparative Example 6

[0141] The lithium iron phosphate battery of this comparative example was prepared in the same way as in Example 1, but the positive electrode sheet film resistance was 272 mΩ and the positive electrode sheet compaction density was 2.6 g / cm 3 0.1wt% of vinylene carbonate and 0wt% of lithium bisfluorosulfonylimide.

[0142] Comparative Example 7

[0143] The lithium iron phosphate battery of the present comparative example was prepared in the same manner as Example 1, but the positive electrode sheet had a sheet resistance of 272 mΩ and a compaction density of 2.6 g / cm 3 , the vinylene carbonate accounted for 0.1wt%, and the lithium bisfluorosulfonylimide accounted for 10wt%

[0144] Comparative Example 8

[0145] The lithium iron phosphate battery of the present comparative example was prepared in the same manner as Example 1, but the positive electrode sheet had a sheet resistance of 311 mΩ and a compaction density of 2.5 g / cm 3 , the vinylene carbonate accounted for 0.2wt%, and the lithium bisfluorosulfonylimide accounted for 3wt%, and the lithium bisfluorosulfonylimide (LiTFSI) was used instead of lithium bisfluorosulfonylimide in the present comparative example.

[0146] Comparative Example 9

[0147] The lithium iron phosphate battery of the present comparative example was prepared in the same manner as Example 1, but the positive electrode sheet had a sheet resistance of 311 mΩ and a compaction density of 2.5 g / cm 3 , the vinylene carbonate accounted for 0.2wt%, and the lithium bisfluorosulfonylimide accounted for 3wt%, and the lithium bisfluorosulfonylimide (LiTFSI) was used instead of lithium bisfluorosulfonylimide in the present comparative example.

[0148] Table 1 records the formula values corresponding to Examples 1-19 and Comparative Examples 1-9.

[0149] Table 1

[0150]

[0151]

[0152] The charging performance test was performed on the above Examples 1-19 and Comparative Examples 1-9.

[0153] 1. Sheet resistance test

[0154] The sheet resistance of the prepared positive electrode sheet was tested using an electrode resistance tester to obtain the sheet resistance.

[0155] 2. DCIR test at 25°C

[0156] The lithium ion battery is placed in a constant temperature environment at 25°C, and charged to a cut-off voltage of 3.65V at a current of 0.5C, then charged at a constant voltage until the current drops to 0.05C, then discharged at a current of 1 / 3C to 2.0V, and the discharge capacity is recorded. Then the battery is fully charged again, and discharged at a current of 0.5C to 50% SOC of the capacity, and left for 30 minutes, then discharged at a current of 2C for 10s, and the DCIR of the cell is calculated.

[0157] The DCIR of the cell is calculated as follows:

[0158] DCIR = (voltage at the end of 30 minutes of standing - voltage at the end of 2C constant current discharge) / 2C discharge current.

[0159] 3. Low temperature discharge test at -20°C

[0160] The lithium ion battery is placed in a constant temperature environment at 25°C, and charged to a cut-off voltage of 3.65V at a current of 0.5C, then charged at a constant voltage until the current drops to 0.05C, then discharged at a current of 1 / 3C to 2.0V, and the discharge capacity is recorded. Then the battery is fully charged again, and discharged at a current of 0.5C to 50% SOC of the capacity, and left for 30 minutes, then discharged at a current of 2C for 10s, and the DCIR of the cell is calculated.

[0161] The capacity retention rate of the cycle is calculated as follows:

[0162] The low temperature discharge capacity ratio (%) = -20°C low temperature discharge capacity / normal temperature discharge capacity x 100%.

[0163] 4. High temperature cycle performance test

[0164] The lithium ion battery is placed in a constant temperature environment at 45°C, and charged to 3.65V at a current of 1C, then charged at a constant voltage until the current drops to 0.05C, and the internal resistance of the battery is tested, then discharged at a current of 1C to 2.0V, and this cycle is repeated 1000 times. The discharge capacity of the first time and the last time, and the internal resistance of the battery when fully charged the first time and the internal resistance of the battery after fully charged the last time are recorded.

[0165] The capacity retention rate and the internal resistance growth rate of the cycle are calculated as follows:

[0166] Capacity retention rate (%) = last discharge capacity / first discharge capacity x 100%;

[0167] Internal resistance growth rate (%) = (internal resistance of the battery after fully charged the last time - internal resistance of the battery when fully charged the first time) / internal resistance of the battery when fully charged the first time x 100%.

[0168] The results of the charging performance test of Examples 1-19 and Comparative Examples 1-9 are shown in Table 2.

[0169] Table 2

[0170]

[0171]

[0172] Through Examples 1-7, the formula value is within the limited range, each parameter is within the limit, the performance fluctuation is small, and the appropriate matching of each parameter can obtain the best matching. Example 2 has low cell impedance and good high and low temperature performance. Compared with Comparative Examples 4 and 5, some or all parameters exceed the limited range, which will cause the decline of the performance of the battery cell due to the inherent connection. Compared with Comparative Examples 8 and 9, the use of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and fluoroethylene carbonate (FEC) instead of lithium bis(fluorosulfonyl)imide (LiFSI) and vinylene carbonate (VC), the former will cause the cell impedance to decrease, but will seriously deteriorate the high temperature cycle performance, and the latter will cause the impedance to increase significantly, and the high and low temperature performance will be seriously deteriorated.

[0173] Through Examples 8-10, the formula value is within the limited range, each parameter is within the limit, the performance fluctuation is small, but it can be found that the membrane resistance is reduced, which can improve the low temperature discharge capacity of the battery cell. Compared with Comparative Example 3, the formula value is greater than the limited range, each performance is obviously deteriorated, and the membrane resistance is seriously exceeded, which causes the cell impedance to increase and the low temperature discharge capacity to deteriorate.

[0174] Through Examples 11-14, the formula value is within the limited range, each parameter is within the limit, the performance fluctuation is small, and with the increase of lithium bis(fluorosulfonyl)imide (LiFSI) content, the impedance decreases and the low temperature discharge performance improves, but the high temperature cycle capacity decreases. Compared with Comparative Examples 6 and 7, too low lithium bis(fluorosulfonyl)imide (LiFSI) will cause the cell impedance to rise and deteriorate the low temperature discharge, while too high lithium bis(fluorosulfonyl)imide (LiFSI) will also cause the cell impedance to rise and deteriorate the high temperature cycle performance.

[0175] Through Examples 15-17, the formula value is within the limited range, each parameter is within the limit, the performance fluctuation is small, but the increase of the positive electrode compaction density will cause the cell impedance to decrease, optimize the low temperature performance, but the high temperature cycle performance does not change linearly.

[0176] By implementing examples 18 and 19, the formula value is within the limited range, each parameter is within the limit, the performance fluctuation is small, and appropriately increasing the content of vinyl carbonate (VC) will cause the impedance to increase, the low temperature performance to decrease, and the high temperature cycle capability to be optimized. Compared with comparative examples 1 and 2, excessive reduction and increase of the vinyl carbonate (VC) addition amount will both cause the high temperature performance to decrease significantly.

[0177] It can be known from the above examples that the application provides a battery cell and a battery of a lithium iron phosphate battery. The battery cell of the lithium iron phosphate battery comprises a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a shell. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active coating layer arranged on at least one surface of the positive electrode current collector. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating layer arranged on at least one surface of the negative electrode current collector. The electrolyte comprises lithium bisfluorosulfonylimide. The battery cell satisfies the following relationship: 5≤(a / 20+d) / (b+c)≤9.3; a is the membrane resistance of the positive electrode sheet after coating; b is the compacted density of the positive electrode sheet; c is the mass percentage content of vinyl carbonate in the positive electrode slurry; and d is the mass percentage content of lithium bisfluorosulfonylimide in the non-aqueous electrolyte. By optimizing the composition of the lithium ion battery cell, taking lithium iron phosphate as the positive electrode active material, controlling the addition amount of vinyl carbonate in the positive electrode slurry, the membrane resistance and the compacted density of the positive electrode sheet, adding an appropriate amount of lithium bisfluorosulfonylimide in the non-aqueous electrolyte, and making the battery composition satisfy specific requirements, the synergistic effect of lithium bisfluorosulfonylimide and lithium bisfluorosulfonylimide and the positive electrode active material can be fully exerted, so that the low temperature discharge performance of the battery is improved while the high temperature cycle performance is considered.

[0178] The similar parts among the examples provided by the application can be referred to each other, and the specific embodiments provided above are only a few examples under the general concept of the application, and do not limit the protection scope of the application. For those skilled in the art, any other embodiments extended according to the application scheme without creative labor are within the protection scope of the application.

Claims

1. A lithium iron phosphate battery cell, characterized by, The battery cell comprises a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a shell; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active coating arranged on at least one surface of the positive electrode current collector; the negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating arranged on at least one surface of the negative electrode current collector; and the electrolyte comprises lithium bisfluorosulfonylimide. The battery cell satisfies the following relationship: 5≤(a / 20+d) / (b+c)≤9.3; a is the membrane resistance of the positive electrode sheet after coating; b is the compacted density of the positive electrode sheet; c is the mass percentage of vinylene carbonate in the positive electrode slurry; d is the mass percentage of lithium bisfluorosulfonylimide in the non-aqueous electrolyte; The positive electrode tab has a post-coating membrane sheet resistance a of 100-400 mΩ; the positive electrode tab has a compaction density b of 2.4-2.7 g / cm 3 ; the mass percentage content c of the vinylene carbonate in the positive electrode slurry is 0.1-0.5 wt%; and the mass percentage content d of the lithium bisfluorosulfonylimide in the non-aqueous electrolyte is 3-7 wt%.

2. The lithium iron phosphate battery cell of claim 1, wherein, The positive electrode active coating comprises a positive electrode active material, a conductive agent, a dispersing agent and a binder, and the mass ratio of the positive electrode active material, the conductive agent, the dispersing agent and the binder is 96.4-c:1.4:c:2.2; and the dispersing agent is vinylene carbonate.

3. The lithium iron phosphate battery cell of claim 2, wherein, The positive electrode active material is a LiFePO4 active material coated with a carbon coating layer.

4. The lithium iron phosphate battery cell of claim 1, wherein, The preparation method of the battery cell of the lithium iron phosphate battery comprises: mixing a binder with a positive electrode active material to obtain a mixed powder; mixing a conductive agent, an N-methyl-2-pyrrolidone solvent and a dispersing agent to obtain a mixed liquid; wherein the mass ratio of the positive electrode active material, the conductive agent, the dispersing agent and the binder is 96.4-c:1.4:c:2.2; and the dispersing agent is vinylene carbonate; stirring the mixed liquid and the mixed powder uniformly to obtain a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector to obtain a positive electrode sheet; preparing a negative electrode slurry and an electrolyte; coating the negative electrode slurry on a negative electrode current collector to obtain a negative electrode sheet; assembling the positive electrode sheet and the negative electrode sheet, injecting the electrolyte and vacuum packaging to obtain the battery cell of the lithium iron phosphate battery.

5. The lithium iron phosphate battery cell of claim 4, wherein, The coating of the positive electrode slurry on the positive electrode current collector to obtain the positive electrode sheet comprises: coating the positive electrode slurry on the positive electrode current collector to obtain an electrode sheet, and detecting the resistance of the electrode sheet using a membrane resistance meter; when the resistance of the electrode sheet is less than or equal to a preset resistance value, drying, rolling, die cutting or slitting the electrode sheet to obtain the positive electrode sheet.

6. A lithium iron phosphate battery, comprising the battery cell of any one of claims 1-5.

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