Battery cell of lithium iron phosphate battery and battery

Abstract

The invention provides a battery cell of a lithium iron phosphate battery and the battery. The battery cell of the lithium iron phosphate battery comprises a positive pole piece, a negative pole piece, an isolating membrane, electrolyte and a shell, the positive pole piece comprises a positive current collector and a positive active coating arranged on at least one surface of the positive current collector; the negative pole piece comprises a negative current collector and a negative active coating arranged on at least one surface of the negative current collector; the electrolyte comprises lithium bis (fluorosulfonyl) imide; wherein the battery cell meets the following relational expression: (a / 20 + d) / (b + c) is more than or equal to 5 and less than or equal to 9.3; a is a coated diaphragm resistor of the positive pole piece; b is the compaction density of the positive pole piece; c is the mass percentage content of vinylene carbonate in the positive electrode slurry; and d is the mass percentage content of the lithium bis (fluorosulfonyl) imide in the non-aqueous electrolyte. By optimizing the composition of the lithium ion battery cell, the battery cell has high-temperature cycle performance while improving the low-temperature discharge performance of the battery.

Description

Technical Field

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

[0002] With the growing demand for new energy devices, lithium-ion secondary batteries have been widely used in many fields due to their own advantages, becoming a new energy device that has attracted much attention. Among various lithium-ion battery systems, lithium iron phosphate batteries have occupied the largest share in the entire lithium-ion battery market due to their strong safety and good structural stability. They are widely used in many scenarios such as electric vehicles and energy storage systems that have high requirements for battery performance.

[0003] Lithium iron phosphate batteries are composed of positive lithium iron phosphate materials, negative electrodes, polymer separators, and electrolytes, and are encapsulated by metal shells. The positive and negative electrodes of the battery are connected to aluminum foil and copper foil respectively. During charging, lithium ions are deintercalated from the positive electrode and embedded in the negative electrode through the electrolyte; during discharging, lithium ions are deintercalated from the negative electrode and embedded in the positive electrode through the electrolyte. Due to the low conductivity of lithium iron phosphate materials themselves, the charging and discharging performance of lithium iron phosphate batteries will be greatly affected in low temperature environments, resulting in reduced discharge capacity and efficiency. In order to improve the disadvantage of poor low-temperature performance of lithium iron phosphate batteries, coating strategies are often used to improve lithium iron phosphate batteries. This coating strategy aims to improve the low-temperature discharge performance of the battery by forming a coating layer on the surface of the lithium iron phosphate material, thereby optimizing the performance of the entire battery in low-temperature environments.

[0004] Although the existing coating strategy is to some extent an optimized solution for improving the low-temperature performance of lithium iron phosphate batteries, the non-in-situ preparation process has obvious defects. On the one hand, the coating strategy cannot achieve comprehensive and tight encapsulation of lithium iron phosphate, resulting in a very limited improvement in low-temperature discharge performance, which is difficult to meet the demand for low-temperature performance of batteries in actual application scenarios. On the other hand, the non-in-situ preparation process easily introduces other surface defect sites on the surface of lithium iron phosphate, and these sites will further promote the decomposition reaction of the electrolyte on its surface, which will lead to rapid attenuation of the battery capacity and deteriorate the high-temperature cycle performance of the battery. Therefore, it is difficult for lithium iron phosphate batteries in related technologies to take into account both low-temperature discharge performance and high-temperature cycle performance, and cannot meet the market's urgent demand for wide-temperature lithium iron phosphate batteries. Summary of the invention

[0005] The present application provides a lithium iron phosphate battery cell and a battery to solve the problem that lithium iron phosphate batteries in the prior art are difficult to achieve both low-temperature discharge performance and high-temperature cycle performance.

[0006] In a first aspect, the present application provides a battery cell of a lithium iron phosphate battery, the battery cell comprising a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a shell; the positive electrode sheet comprises a positive electrode collector and a positive electrode active coating disposed on at least one surface of the positive electrode collector; the negative electrode sheet comprises a negative electrode collector and a negative electrode active coating disposed on at least one surface of the negative electrode collector; the electrolyte comprises lithium bis(fluorosulfonyl)imide;

[0007] The battery cell satisfies the following relationship:

[0008] 5≤(a / 20+d) / (b+c)≤9.3;

[0009] a is the film resistance after coating the positive electrode;

[0010] b is the compaction 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 bis(fluorosulfonyl)imide in the non-aqueous electrolyte.

[0013] The present application adjusts the composition of the positive electrode sheet in the battery cell of the lithium iron phosphate battery, uses lithium iron phosphate as the positive electrode active material, controls the amount of vinyl carbonate (VC) added in the positive electrode slurry, the positive electrode sheet membrane resistance and the compaction density, adds an appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI) to the non-aqueous electrolyte, and makes the battery composition meet specific requirements, which can give full play to the synergistic effect between lithium bis(fluorosulfonyl)imide (LiFSI) and vinyl carbonate (VC) and the positive electrode active material, so that the battery cell can improve the low-temperature discharge performance of the battery while taking into account the high-temperature cycle performance.

[0014] In some possible implementations, the positive electrode active coating includes a positive electrode active material, a conductive agent, a dispersant and a binder, 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, and the dispersant is vinylene carbonate.

[0015] In some possible implementations, the positive electrode active material is a lithium iron phosphate (LiFePO 4 ) active material coated with a carbon coating layer.

[0016] In some possible implementations, the film resistance a of the positive electrode sheet after coating is 100 to 400 mΩ.

[0017] In some possible implementations, the compaction density b of the positive electrode sheet is 2.4 to 2.7 g / cm 3 .

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

[0019] In some possible implementations, the mass percentage d of the lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte is 3-7 wt %.

[0020] In some possible implementations, the method for preparing the lithium iron phosphate battery cell includes:

[0021] Mixing a binder with a positive electrode active material to obtain a mixed powder;

[0022] A conductive agent, N-methyl-2-pyrrolidone (NMP) and a 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; and the dispersant is vinylene carbonate;

[0023] The mixed solution and the mixed powder are stirred evenly to obtain a positive electrode slurry;

[0024] Applying the positive electrode slurry on the positive electrode current collector to obtain a positive electrode sheet;

[0025] preparing negative electrode slurry and electrolyte;

[0026] Applying the negative electrode slurry on the negative electrode current collector to obtain a negative electrode sheet;

[0027] The positive electrode sheet and the negative electrode sheet are assembled, and after injecting electrolyte and vacuum packaging, a lithium iron phosphate battery cell is obtained.

[0028] In some possible implementations, coating the positive electrode slurry on a positive electrode current collector to obtain a positive electrode sheet includes:

[0029] Applying the positive electrode slurry on the positive electrode current collector to obtain an electrode sheet, and using a film resistance meter to perform resistance detection on the electrode sheet;

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

[0031] In a second aspect, the present application provides a lithium iron phosphate battery, wherein the lithium iron phosphate battery comprises a battery cell of the lithium iron phosphate battery described in the first aspect.

[0032] By optimizing the positive electrode composition and electrolyte ingredients of the lithium iron phosphate battery cell, the capacity decay of the lithium iron phosphate battery at low temperatures can be effectively delayed, the cycle life of the battery can be increased, the internal resistance of the battery can be reduced, and the low-temperature performance of the battery can be improved, thereby obtaining a wide-temperature range lithium iron phosphate battery that can take into account both low-temperature discharge performance and high-temperature cycle performance.

[0033] From the above content, it can be seen that the present application provides a cell and battery of a lithium iron phosphate battery. The cell of the lithium iron phosphate battery includes a positive electrode plate, a negative electrode plate, a separator, an electrolyte and a shell; the positive electrode plate includes a positive electrode collector and a positive electrode active coating arranged on at least one surface of the positive electrode collector; the negative electrode plate includes a negative electrode collector and a negative electrode active coating arranged on at least one surface of the negative electrode collector; the electrolyte includes lithium bis(fluorosulfonyl)imide; wherein the cell satisfies the following relationship: 5≤(a / 20+d) / (b+c)≤9.3; a is the film resistance after the positive electrode plate is coated; b is the compaction density of the positive electrode plate; c is the mass percentage of vinylene carbonate in the positive electrode slurry; d is the mass percentage of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte. By optimizing the battery cell of lithium iron phosphate battery, using lithium iron phosphate as the positive electrode active material, controlling the amount of vinylene carbonate added in the positive electrode slurry, the positive electrode sheet resistance and the compaction density, adding an appropriate amount of lithium bis(fluorosulfonyl)imide to the non-aqueous electrolyte, and making the battery composition meet specific requirements, the synergistic effect between lithium bis(fluorosulfonyl)imide and the positive electrode active material can be fully utilized, so that the battery cell of lithium iron phosphate battery can improve the low-temperature discharge performance of the battery while taking into account the high-temperature cycle performance. DETAILED DESCRIPTION

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

[0035] With the growing demand for new energy devices, lithium-ion secondary batteries have been widely used in many fields due to their own advantages, becoming a new energy device that has attracted much attention. Among various lithium-ion battery systems, lithium iron phosphate batteries have occupied the largest share in the entire lithium-ion battery market due to their strong safety and good structural stability. They are widely used in many scenarios such as electric vehicles and energy storage systems that have high requirements for battery performance.

[0036] Lithium iron phosphate batteries are composed of positive lithium iron phosphate materials, negative electrodes, polymer separators, and electrolytes, and are encapsulated by metal shells. The positive and negative electrodes of the battery are connected to aluminum foil and copper foil respectively. When charging, lithium ions are deintercalated from the positive electrode and embedded in the negative electrode through the electrolyte; when discharging, lithium ions are deintercalated from the negative electrode and embedded in the positive electrode through the electrolyte. The conductivity of lithium iron phosphate materials is poor. In low temperature environments, the migration rate of lithium ions is significantly reduced, and the chemical reaction rate inside the battery is slowed down, resulting in a significant decrease in the charge and discharge performance of the battery. For example, at -20°C, the discharge capacity of the battery may be only 30%-40% of that at room temperature. In addition, its crystal structure is too stable at low temperatures, making it difficult for lithium ions to be embedded and removed, further affecting battery performance. In order to improve the disadvantage of poor low-temperature performance of lithium iron phosphate batteries, a coating strategy is often used to improve lithium iron phosphate batteries. This 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 entire battery in a low-temperature environment.

[0037] Although the existing coating strategy is to some extent an optimized solution for improving the low-temperature performance of lithium iron phosphate batteries, the non-in-situ preparation process has obvious defects. On the one hand, the coating strategy cannot achieve comprehensive and tight encapsulation of lithium iron phosphate, resulting in a very limited improvement in low-temperature discharge performance, which is difficult to meet the demand for low-temperature performance of batteries in actual application scenarios. On the other hand, the non-in-situ preparation process easily introduces other surface defect sites on the surface of lithium iron phosphate, and these sites will further promote the decomposition reaction of the electrolyte on its surface, which will lead to rapid attenuation of the battery capacity and deteriorate the high-temperature cycle performance of the battery. Therefore, it is difficult for lithium iron phosphate batteries in related technologies to take into account both low-temperature discharge performance and high-temperature cycle performance, and cannot meet the market's urgent demand for wide-temperature lithium iron phosphate batteries.

[0038] Based on this, in order to solve the problem that lithium iron phosphate batteries in the related art are difficult to take into account both low-temperature discharge performance and high-temperature cycle performance, the present application provides a lithium iron phosphate battery cell and a battery, adjusts the positive electrode plate in the lithium iron phosphate battery cell, uses lithium iron phosphate as the positive electrode active material, controls the amount of vinyl carbonate (VC) added in the positive electrode slurry, the positive electrode plate diaphragm resistance and the compaction density, adds an appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI) to the non-aqueous electrolyte, and makes the battery composition meet specific requirements, which can give full play to the synergistic effect between lithium bis(fluorosulfonyl)imide (LiFSI) and vinyl carbonate (VC) and the positive electrode active material, so that the battery cell can improve the low-temperature discharge performance of the battery while taking into account the high-temperature cycle performance.

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

[0040] The battery cell satisfies the following relationship:

[0041] 5≤(a / 20+d) / (b+c)≤9.3;

[0042] a is the film resistance after coating the positive electrode;

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

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

[0045] d is the mass percentage of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte.

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

[0047] In some embodiments, the positive electrode active coating includes a positive electrode active material, a conductive agent, a dispersant and a binder, and 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. Among them, the conductive agent includes one or more of Ketjen black, mesophase carbon microspheres, activated carbon, graphite, conductive carbon black, acetylene black, carbon fiber, carbon nanotubes, graphene, etc.; the binder includes one or more of vinylidene fluoride, hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutylene, hexafluorobutadiene, hexafluoroisobutylene, trifluoroethylene, trifluorochloroethylene and tetrafluoroethylene; the dispersant is vinylene carbonate. Vinylene carbonate can effectively reduce the agglomeration phenomenon between the particles of the positive electrode active material, so that the active material is more evenly dispersed in the coating. The addition of vinylene carbonate is beneficial to increase the contact area between the active material and the electrolyte, improve the insertion and deinsertion efficiency of lithium ions during the charge and discharge process, and thus improve the charge and discharge performance and rate performance of the battery.

[0048] During the first charge and discharge process of the battery, vinylene carbonate will decompose on the electrode surface to form a stable solid electrolyte interface (SEI) film. This 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 environments, which can increase the migration rate of lithium ions and reduce polarization, 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 electrical conductivity. The presence of the carbon coating provides a good channel for electron transmission, greatly improving the overall electronic conductivity of the material. The carbon coating can optimize the properties of the surface of 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 a certain extent, shorten the diffusion path of lithium ions, and thus improve the diffusion coefficient of lithium ions. The carbon coating can protect the active material of lithium iron phosphate, prevent it from directly contacting with the electrolyte to cause side reactions, and reduce the loss of active materials; and during the charge and discharge process, it can alleviate the volume change of lithium iron phosphate caused by the insertion and extraction of lithium ions, and maintain the stability of the electrode structure. The carbon material itself has a certain thermal stability. After being coated on the surface of lithium iron phosphate, it can improve the overall thermal stability of the positive electrode material to a certain extent, and can also prevent lithium iron phosphate from reacting adversely with the electrolyte at high temperatures. Among them, in some embodiments, the thickness of the carbon coating is 2 to 20 nm.

[0051] By using the lithium iron phosphate active material coated with a carbon coating layer as the positive electrode active material, electrons can be transferred more quickly between the positive electrode active material and the positive electrode current collector during the battery charging and discharging process, thereby improving the battery's rate performance, allowing the battery to be charged and discharged at a high current density, meeting the needs of high power output such as electric vehicles. In addition, the lithium iron phosphate active material coated with a carbon coating layer can also enhance the safety and performance stability of the battery in a high temperature environment, reduce safety hazards such as battery bulging and combustion caused by high temperature, and broaden the operating temperature range of the battery, so that it can work more stably under different ambient temperature conditions.

[0052] In some embodiments, the film resistance a after the positive electrode sheet is coated is 100 to 400 mΩ. In a low temperature environment, the ion diffusion rate and the electron conduction rate of the lithium iron phosphate battery cell itself will be greatly reduced. If the film resistance after the positive electrode sheet is coated is high, the transmission of electrons and lithium ions will be further hindered. It is difficult for electrons to be smoothly conducted from the positive electrode active material to the current collector, and the insertion and extraction process of lithium ions has become more difficult, which significantly reduces the number of active substances involved in the discharge reaction, resulting in a sharp drop in the discharge capacity of the battery cell. Under low temperature conditions, the amount of electricity that should be released at room temperature cannot be released, which seriously affects the battery's service life and endurance in low temperature scenarios. When the film resistance is low, although the low temperature environment still has an inhibitory effect on the transmission of electrons and lithium ions, the relatively small film resistance can alleviate this obstacle to a certain extent. Electrons and lithium ions can move relatively smoothly, allowing more positive electrode active materials to participate in the discharge reaction, thereby retaining a higher discharge capacity than in the case of high film resistance, and improving the available power and endurance performance of the battery cell in a low temperature environment. Among them, the film resistance a of the positive electrode sheet after coating can be 218mΩ, 259mΩ, 272mΩ, 298mΩ, 300mΩ, 308mΩ, 311mΩ, 312mΩ, 313mΩ, 318mΩ, 319mΩ, 323mΩ, 327mΩ, 346mΩ, 362mΩ, 385mΩ, and 400mΩ.

[0053] In some embodiments, the compaction density b of the positive electrode sheet is 2.4 to 2.7 g / cm 3 The higher the compaction density of the positive electrode sheet, the more the positive electrode active material content. Setting the positive electrode sheet to have a certain compaction density not only makes the positive electrode sheet have a certain energy, but also makes the positive electrode sheet have a certain gap to facilitate the deintercalation of active ions and improve the ion movement rate. The compaction density b of the positive electrode sheet can be 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 .

[0054] In the present application, the important film-forming additive vinylene carbonate (VC) is mixed into the positive electrode material in advance and fully contacted with the lithium iron phosphate material. During the injection, the electrolyte can promote the infiltration of the positive electrode material due to the principle of like dissolves like, and ensure the dispersion of the film-forming additive in the layers of different thicknesses of the positive electrode material, which is beneficial to the comprehensive coverage of the positive electrode materials of different thicknesses on the electrode sheet. In the early stage of film formation of the battery cell, 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 with the current collector, but also avoids the oxidative decomposition of the components with high film-forming impedance in the electrolyte, reduces the initial impedance and impedance growth rate of the battery cell, and thus improves the low-temperature discharge performance and high-temperature cycle performance of the battery.

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

[0056] The fluoride ions in lithium fluorosulfonyl imide (LiFSI) have strong electron-withdrawing properties, which weaken the coordination effect between anions and cations of lithium salts. The lithium ions are very active, and have high conductivity, thermal stability and electrochemical stability. There are basically no side reactions, and the expansion effect can also be suppressed. In this application, adding an appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI) to the non-aqueous electrolyte of the battery cell of the lithium iron phosphate battery can significantly improve the conductivity of the electrolyte, thereby improving the low-temperature discharge performance of the battery cell. In addition, lithium bis(fluorosulfonyl)imide can also further modify the positive and negative electrode membrane components, reduce the film formation impedance of the battery cell, and thus improve the high and low temperature performance of the battery cell.

[0057] In some embodiments, the mass percentage d of the lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte is 3-7wt%. Specifically, the mass percentage d of the lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte may be 3wt%, 4wt%, 5wt%, 6wt%, or 7wt%.

[0058] In some embodiments, the negative electrode active coating includes a negative electrode active material, a conductive agent, a dispersant and a binder. In the present application, the negative electrode active coating also includes styrene butadiene rubber (SBR), and the mass ratio of the negative electrode active material, the conductive agent, the dispersant, the styrene butadiene rubber and the binder is 96.4:1.0:0.3:0.5:1.8. Among them, the conductive agent includes one or more of Ketjen black, mesophase carbon microspheres, activated carbon, graphite, conductive carbon black, acetylene black, carbon fiber, carbon nanotubes, graphene, etc.; the dispersant includes one or more of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein. The binder includes one or more of polyvinylidene fluoride, hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutylene, hexafluorobutadiene, hexafluoroisobutylene, trifluoroethylene, trifluorochloroethylene and tetrafluoroethylene.

[0059] In some embodiments, the separator can be any material suitable for lithium-ion battery separators in the art, for example, it can be a combination of one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fibers, etc., but not limited to.

[0060] In some embodiments, the electrolyte can be any material suitable for lithium-ion battery electrolyte in the art, for example, it can be a combination including but not limited to ethylene carbonate (EC), diethyl carbonate (DMC), ethyl methyl carbonate (EMC), lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), etc. In some embodiments, the shell is made of stainless steel or aluminum-plastic film.

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

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

[0063] A conductive agent, an N-methyl-2-pyrrolidone solvent and a 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; and the dispersant is vinylene carbonate;

[0064] The mixed solution and the mixed powder are stirred evenly to obtain a positive electrode slurry;

[0065] Applying the positive electrode slurry on the positive electrode current collector to obtain a positive electrode sheet;

[0066] preparing negative electrode slurry and electrolyte;

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

[0068] The positive electrode sheet and the negative electrode sheet are assembled, and after injecting electrolyte and vacuum packaging, a lithium iron phosphate battery cell is obtained.

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

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

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

[0072] When the resistance of the electrode sheet is greater than a preset resistance value, the ratio of the positive electrode active material, the conductive agent, the dispersant and the binder is adjusted to prepare the positive electrode slurry again.

[0073] In order to make the battery cell of lithium iron phosphate battery have both low-temperature discharge performance and high-temperature cycle performance, it is necessary to limit the membrane resistance of the positive electrode plate after coating. The specific membrane resistance of the positive electrode plate after coating is 100-400mΩ. The electrode sheet with a membrane resistance less than or equal to the preset resistance value after coating is selected as the positive electrode plate, thereby improving the low-temperature discharge capability of the battery cell.

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

[0075] The present application optimizes the positive electrode composition and electrolyte components of the lithium iron phosphate battery cell to effectively delay the capacity decay of the lithium iron phosphate battery at low temperatures, increase the cycle life of the battery, reduce the internal resistance of the battery, and improve the low-temperature performance of the battery, thereby obtaining a wide-temperature range lithium iron phosphate battery that can take into account both low-temperature discharge performance and high-temperature cycle performance.

[0076] Embodiment 1:

[0077] 1) Preparation of positive electrode

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

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

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

[0081] Step 4: Evenly apply the prepared positive electrode slurry on the positive electrode collector (such as aluminum foil), and then use a film resistance meter to test the resistance of the electrode sheet.

[0082] Step 5: The positive electrode sheet is obtained by drying, rolling, die-cutting or slitting.

[0083] 2) Preparation of negative electrode sheet

[0084] Step 1: Weigh each substance according to the mass ratio of graphite: conductive carbon (superP): carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR): binder = 96.4:1.0:0.3:0.5:1.8 for the negative electrode sheet.

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

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

[0087] Step 4: evenly apply the prepared negative electrode slurry on the copper foil, and obtain the negative electrode sheet by drying, rolling, die-cutting or striping.

[0088] 3) Preparation of non-aqueous electrolyte

[0089] Ethylene carbonate (EC), diethyl carbonate (DMC) and ethyl methyl carbonate (EMC) are mixed in a mass ratio of ethylene carbonate: diethyl carbonate: ethyl methyl carbonate = 1:1:1, a certain mass percentage of lithium bis(fluorosulfonyl)imide (LiFSI) is added, and then lithium hexafluorophosphate (LiPF6) is added to a molar concentration of 0.9 mol / L.

[0090] 4) Lithium-ion battery cell preparation

[0091] The prepared positive electrode sheet and the negative electrode sheet are assembled into a laminated soft-pack battery cell.

[0092] 5) Battery filling and formation

[0093] In an environment where the dew point is controlled below -40°C, the prepared electrolyte is injected into the battery cell, vacuum-sealed, and left to stand for 72 hours. Then, the conventional formation of the first charge is carried out according to the following steps: 0.05C constant current charging for 180 minutes, 0.1C constant current charging for 120 minutes, 0.2C constant current charging for 120 minutes, secondary vacuum sealing, and then further full charging at a current of 0.2C (100% SOC), after being left at room temperature for 72 hours, full discharge at a current of 0.2C (0% SOC).

[0094] Embodiment 2:

[0095] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 346 mΩ, and the positive electrode sheet compaction density is 2.55 g / cm 3 , vinylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 4wt%.

[0096] Embodiment 3:

[0097] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 385 mΩ, and the positive electrode sheet compaction density is 2.65 g / cm 3 , vinylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 3wt%.

[0098] Embodiment 4:

[0099] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 311 mΩ, and the positive electrode sheet compaction density is 2.5 g / cm 3 , vinylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 3wt%.

[0100] Embodiment 5:

[0101] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 400 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm 3 , vinylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 6wt%.

[0102] Embodiment 6:

[0103] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 362 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm 3 , vinylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 6wt%.

[0104] Embodiment 7:

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

[0106] Embodiment 8:

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

[0108] Embodiment 9:

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

[0110] Embodiment 10:

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

[0112] Embodiment 11:

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

[0114] Embodiment 12:

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

[0116] Embodiment 13:

[0117] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 272 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm 3 , vinylene carbonate accounts for 0.1wt%, and lithium bis(fluorosulfonyl)imide accounts for 5wt%.

[0118] Embodiment 14:

[0119] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 272 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm 3 , vinylene carbonate accounts for 0.1wt%, and lithium bis(fluorosulfonyl)imide accounts for 7wt%.

[0120] Embodiment 15:

[0121] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 313 mΩ, and the positive electrode sheet compaction density is 2.4 g / cm 3 , vinylene carbonate accounts for 0.1wt%, and lithium bis(fluorosulfonyl)imide accounts for 4wt%.

[0122] Embodiment 16:

[0123] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 308 mΩ, and the positive electrode sheet compaction density is 2.5 g / cm 3 , vinylene carbonate accounts for 0.1wt%, and lithium bis(fluorosulfonyl)imide accounts for 4wt%.

[0124] Embodiment 17:

[0125] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 319 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm 3 , vinylene carbonate accounts for 0.1wt%, and lithium bis(fluorosulfonyl)imide accounts for 4wt%.

[0126] Embodiment 18:

[0127] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 327 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm 3 , vinylene carbonate accounts for 0.1wt%, and lithium bis(fluorosulfonyl)imide accounts for 4wt%.

[0128] Embodiment 19:

[0129] The preparation method of the lithium iron phosphate battery in this embodiment is the same as that in embodiment 1, but the positive electrode sheet resistance is 318 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm3 , vinylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 4wt%.

[0130] Comparative Example 1

[0131] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 285 mΩ, and the positive electrode sheet compaction density is 2.5 g / cm 3 , vinylene carbonate accounts for 0wt%, and lithium bis(fluorosulfonyl)imide accounts for 3wt%

[0132] Comparative Example 2

[0133] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 364 mΩ, and the positive electrode sheet compaction density is 2.5 g / cm 3 , vinylene carbonate accounts for 0.5wt%, and lithium bis(fluorosulfonyl)imide accounts for 3wt%

[0134] Comparative Example 3

[0135] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 500 mΩ, and the positive electrode sheet compaction density is 2.5 g / cm 3 , vinylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 3wt%

[0136] Comparative Example 4

[0137] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 500 mΩ, and the positive electrode sheet compaction density is 2.2 g / cm 3 , vinylene carbonate accounts for 0wt%, and lithium bis(fluorosulfonyl)imide accounts for 10wt%

[0138] Comparative Example 5

[0139] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 450mΩ, and the positive electrode sheet compaction density is 2.3g / cm 3 , vinylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 5wt%

[0140] Comparative Example 6

[0141] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 272 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm 3 , vinylene carbonate accounts for 0.1wt%, and lithium bis(fluorosulfonyl)imide accounts for 0wt%

[0142] Comparative Example 7

[0143] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 272 mΩ, and the positive electrode sheet compaction density is 2.6 g / cm 3 , vinylene carbonate accounts for 0.1wt%, and lithium bis(fluorosulfonyl)imide accounts for 10wt%

[0144] Comparative Example 8

[0145] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 311 mΩ, and the positive electrode sheet compaction density is 2.5 g / cm 3 , the proportion of vinylene carbonate is 0.2wt%, the proportion of lithium bis(trifluoromethanesulfonyl)imide is 3wt%, and this comparative example uses lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) instead of lithium bis(fluorosulfonyl)imide.

[0146] Comparative Example 9

[0147] The preparation method of the lithium iron phosphate battery in this comparative example is the same as that in Example 1, but the positive electrode sheet resistance is 311 mΩ, and the positive electrode sheet compaction density is 2.5 g / cm 3 , fluoroethylene carbonate accounts for 0.2wt%, and lithium bis(fluorosulfonyl)imide accounts for 3wt%. In this comparative example, fluoroethylene carbonate (FEC) is used instead of vinylene carbonate.

[0148] Among them, 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-mentioned Examples 1-19 and Comparative Examples 1-9.

[0153] 1. Diaphragm resistance test

[0154] The electrode sheet resistance tester is used to test the positive electrode sheet resistance and obtain the diaphragm sheet resistance.

[0155] DCIR test at 2.25℃

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

[0157] Calculate the DCIR of the battery cell as follows:

[0158] DCIR = (voltage after standing for 30 minutes - voltage after 2C constant current discharge) / 2C discharge current.

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

[0160] The lithium-ion battery was placed in a constant temperature environment of 25°C, charged at a constant current of 0.5C to a cut-off voltage of 3.65V, then charged at a constant voltage until the current drops to 0.05C, and then discharged at a constant current of 1 / 3C to 2.0V, and the room temperature discharge capacity was recorded. The charging steps were repeated to fully charge the lithium battery, and then placed in a constant temperature environment of -20°C, left to stand for 6 hours, and then discharged at a constant current of 1 / 3C to 2.0V, and the -20°C low-temperature discharge capacity was recorded, and the low-temperature discharge capacity ratio was calculated.

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

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

[0163] 4. High temperature cycle performance test

[0164] Place the lithium-ion battery in a constant temperature environment of 45°C, charge it to 3.65V at a constant current of 1C, then charge it at a constant voltage until the current drops to 0.05C, test the internal resistance of the battery, and then discharge it to 2.0V at a constant current of 1C. Repeat this cycle 1000 times, and record the first discharge capacity and the last discharge capacity, as well as the internal resistance of the battery when it is fully charged for the first time and the internal resistance of the battery after the last time it is fully charged.

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

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

[0167] Internal resistance growth rate (%) = (battery internal resistance after the last full charge - battery internal resistance after the first full charge) / battery internal resistance after the first full charge × 100%.

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

[0169] Table 2

[0170]

[0171]

[0172] Through Example 1 to Example 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, which has low battery impedance and good high and low temperature performance. Compared with Comparative Examples 4 and 5, some or all of the parameters exceed the limited range, which will cause the battery performance to decline due to its inherent connection. Compared with Comparative Examples 8 and 9, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and fluoroethylene carbonate (FEC) are used to replace lithium bis(fluorosulfonyl)imide (LiFSI) and vinyl carbonate (VC). The former will cause the battery impedance to decrease, but will seriously degrade 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 degraded.

[0173] Through Examples 8 to 10, the formula values ​​are within the specified range, the various parameters are within the specified range, the performance fluctuation is small, but it can be found that the diaphragm resistance is reduced, which can improve the low-temperature discharge capability of the battery. Compared with Comparative Example 3, the formula values ​​are greater than the specified range, the various performances are significantly degraded, and the diaphragm resistance seriously exceeds the upper limit, resulting in an increase in the battery impedance and deterioration of the low-temperature discharge capability.

[0174] Through Examples 11 to 14, the formula values ​​are within the specified range, the various parameters are within the specified range, the performance fluctuations are small, and as the content of lithium bis(fluorosulfonyl)imide (LiFSI) increases, the impedance decreases, the low-temperature discharge performance improves, but the high-temperature cycle capability 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 to 17, the formula values ​​are within the limited range, all parameters are within the limits, the performance fluctuation is small, but the increase in the positive electrode compaction density will lead to a decrease in the battery cell impedance and optimize the low-temperature performance, but the high-temperature cycle performance is not affected linearly.

[0176] Through Examples 18 and 19, the formula values ​​are within the specified range, the various parameters are within the specified range, the performance fluctuation is small, and the appropriate increase in the content of vinyl carbonate (VC) will lead to increased impedance, reduced low-temperature performance, and optimized high-temperature cycle capacity. Compared with Comparative Examples 1 and 2, excessive reduction and increase in the amount of vinyl carbonate (VC) added will lead to a significant reduction in high-temperature performance.

[0177] It can be seen from the above embodiments that the present application provides a cell and battery of a lithium iron phosphate battery. The cell of the lithium iron phosphate battery includes a positive electrode plate, a negative electrode plate, a separator, an electrolyte and a shell; the positive electrode plate includes a positive electrode collector and a positive electrode active coating disposed on at least one surface of the positive electrode collector; the negative electrode plate includes a negative electrode collector and a negative electrode active coating disposed on at least one surface of the negative electrode collector; the electrolyte includes lithium bis(fluorosulfonyl)imide; wherein the cell satisfies the following relationship: 5≤(a / 20+d) / (b+c)≤9.3; a is the film resistance after the positive electrode plate is coated; b is the compaction density of the positive electrode plate; c is the mass percentage of vinylene carbonate in the positive electrode slurry; d is the mass percentage of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte. By optimizing the composition of lithium-ion batteries, using lithium iron phosphate as the positive electrode active material, controlling the amount of vinylene carbonate added in the positive electrode slurry, the positive electrode sheet resistance and the compaction density, adding an appropriate amount of lithium bis(fluorosulfonyl)imide to the non-aqueous electrolyte, and making the battery composition meet specific requirements, the synergistic effect between lithium bis(fluorosulfonyl)imide and the positive electrode active material can be fully utilized, so that the battery can improve the low-temperature discharge performance of the battery while taking into account the high-temperature cycle performance.

[0178] Similar parts between the embodiments provided in this application can be referenced to each other. The specific implementation methods provided above are only a few examples under the general concept of this application and do not constitute a limitation on the protection scope of this application. For those skilled in the art, any other implementation methods expanded based on the scheme of this application without creative work belong to the protection scope of this application.

Claims

1. A lithium iron phosphate battery cell, characterized in that: 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 collector and a positive electrode active coating arranged on at least one surface of the positive electrode collector; the negative electrode sheet comprises a negative electrode collector and a negative electrode active coating arranged on at least one surface of the negative electrode collector; the electrolyte comprises lithium bis(fluorosulfonyl)imide; The battery cell satisfies the following relationship: 5≤(a / 20+d) / (b+c)≤9.3; a is the film resistance after coating the positive electrode; b is the compaction 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 bis(fluorosulfonyl)imide in the non-aqueous electrolyte.

2. The lithium iron phosphate battery cell according to claim 1, characterized in that: The positive electrode active coating comprises a positive electrode active material, a conductive agent, a dispersant and a binder, 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; and the dispersant is vinylene carbonate.

3. The lithium iron phosphate battery cell according to claim 2, characterized in that: The positive electrode active material is a LiFePO4 active material coated with a carbon coating layer.

4. The lithium iron phosphate battery cell according to claim 2, characterized in that: The film resistance a of the positive electrode sheet after coating is 100 to 400 mΩ.

5. The lithium iron phosphate battery cell according to claim 4, characterized in that: The compaction density b of the positive electrode sheet is 2.4 to 2.7 g / cm 3 .

6. The lithium iron phosphate battery cell according to claim 5, characterized in that: The mass percentage c of the vinylene carbonate in the positive electrode slurry is 0.1-0.5wt%.

7. The lithium iron phosphate battery cell according to claim 6, characterized in that: The mass percentage d of the lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte is 3-7 wt %.

8. The lithium iron phosphate battery cell according to claim 1, characterized in that: The method for preparing the lithium iron phosphate battery cell comprises: Mixing a binder with a positive electrode active material to obtain a mixed powder; A conductive agent, an N-methyl-2-pyrrolidone solvent and a 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; and the dispersant is vinylene carbonate; The mixed solution and the mixed powder are stirred evenly to obtain a positive electrode slurry; Applying the positive electrode slurry on the positive electrode current collector to obtain a positive electrode sheet; preparing negative electrode slurry and electrolyte; Applying the negative electrode slurry on the negative electrode current collector to obtain a negative electrode sheet; The positive electrode sheet and the negative electrode sheet are assembled, and after injecting electrolyte and vacuum packaging, a lithium iron phosphate battery cell is obtained.

9. The lithium iron phosphate battery cell according to claim 8, characterized in that: The positive electrode slurry is coated on the positive electrode current collector to obtain a positive electrode sheet, comprising: Applying the positive electrode slurry on the positive electrode current collector to obtain an electrode sheet, and using a film resistance meter to perform resistance detection on the electrode sheet; When the resistance of the electrode sheet is less than or equal to a preset resistance value, the electrode sheet is dried, rolled, die-cut or stripped to obtain a positive electrode sheet.

10. A lithium iron phosphate battery, comprising the lithium iron phosphate battery cell according to any one of claims 1 to 9.

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