Lithium manganese iron phosphate battery

Abstract

The invention provides a lithium manganese iron phosphate battery. The battery comprises a positive pole piece, a negative pole piece and a non-aqueous electrolyte, the positive pole piece comprises a positive active material and a positive active coating arranged on at least one surface of the positive active material; the positive electrode active material comprises lithium manganese iron phosphate LiMnxFe1-xPO4 coated with a carbon coating layer, and x is more than or equal to 0.5 and less than or equal to 0.8; the negative pole piece comprises a negative active material and a negative active coating arranged on at least one surface of the negative active material; the battery satisfies the following relational expressions: 4.5 < = (a / 20 + d) / (b + c) < = 12.8; 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, the low-temperature discharge performance of the battery is improved, and meanwhile, the high-temperature cycle performance is considered.

Description

Technical Field

[0001] This application relates to the technical field of lithium batteries, and particularly to a lithium iron manganese phosphate battery. Background Art

[0002] As a new energy device with great potential, lithium-ion secondary batteries are booming at an astonishing speed, and their application fields are also constantly expanding. Among various types of lithium-ion batteries, lithium iron manganese phosphate batteries stand out in the lithium-ion battery market and occupy the largest share due to their outstanding safety and structural stability, becoming the preferred battery type in many fields. Their olivine structure endows them with good thermal stability, and in extreme situations such as high temperature or overcharging, they are not prone to dangerous situations such as thermal runaway, greatly improving the safety during battery use. This stability makes lithium iron manganese phosphate batteries have irreplaceable advantages in application scenarios with extremely high safety requirements, such as electric vehicles and energy storage power stations.

[0003] Lithium iron manganese phosphate itself has low conductivity, and its poor low-temperature performance becomes a huge disadvantage in its practical application. When the ambient temperature decreases, the chemical reaction rate inside the battery slows down, and ion migration is blocked, resulting in a significant decrease in the discharge capacity and charge-discharge efficiency of the battery. To improve the low-temperature performance of lithium iron manganese phosphate batteries, various strategies have been adopted currently. For example, the coating and doping strategies aim to improve the electronic conductivity of the material and promote ion transport by coating a layer of material with good conductivity, such as carbon material, on the surface of lithium iron manganese phosphate material, or doping some metal ions, such as magnesium ions, aluminum ions, etc. Changing the particle size of the material is to shorten the diffusion path of lithium ions inside the material by reducing the particle size of the material, thereby improving the performance of the battery at low temperature.

[0004] Although the existing coating strategies are, to a certain extent, optimization schemes for improving the low-temperature performance of lithium iron manganese phosphate batteries, the non-in-situ preparation process has obvious defects. The existing non-in-situ preparation process, that is, the method of performing coating and other treatments after material synthesis, due to process conditions limitations, cannot achieve a comprehensive and tight wrapping of lithium iron manganese phosphate. Incomplete coating in some areas greatly reduces the improvement effect of low-temperature discharge performance. More seriously, this preparation method introduces other defect sites on the material surface, and these defect sites become the active centers for electrolyte decomposition, further promoting the decomposition of the electrolyte on its surface. The decomposition of the electrolyte not only consumes active substances, leading to a rapid decline in battery capacity, but also generates a large amount of gas and by-products, seriously deteriorating the high-temperature cycle performance of the battery. Therefore, it is difficult for lithium iron manganese phosphate batteries in related technologies to balance low-temperature discharge performance and high-temperature cycle performance, and cannot meet the urgent market demand for lithium iron manganese phosphate batteries with a wide temperature range. Summary of the Invention

[0005] The present application provides a lithium iron manganese phosphate battery to solve the problem in the prior art that it is difficult for a lithium iron manganese phosphate battery to balance low-temperature discharge performance and high-temperature cycle performance.

[0006] The present application provides a lithium iron manganese phosphate battery, which includes a positive electrode sheet, a negative electrode sheet, and a non-aqueous electrolyte; the positive electrode sheet includes a positive electrode active material and a positive electrode active coating provided on at least one surface of the positive electrode active material; the positive electrode active material includes lithium iron manganese phosphate (LiMn x Fe 1-x PO4) coated with a carbon coating layer, where 0.5 ≤ x ≤ 0.8;

[0007] The negative electrode sheet includes a negative electrode active material and a negative electrode active coating provided on at least one surface of the negative electrode active material; the non-aqueous electrolyte includes lithium bis(fluorosulfonyl)imide;

[0008] Wherein the battery satisfies the following relational expression:

[0009] 4.5 ≤ (a / 20 + d) / (b + c) ≤ 12.8;

[0010] a is the film resistance of the positive electrode sheet after coating;

[0011] b is the compaction density of the positive electrode sheet;

[0012] c is the mass percentage of vinylene carbonate in the positive electrode slurry;

[0013] d is the mass percentage of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte.

[0014] The present application adjusts the composition of the positive electrode sheet in the battery cell of the lithium iron manganese phosphate battery. Using lithium iron manganese phosphate as the positive electrode active material, it controls the addition amount of vinylene carbonate (VC) in the positive electrode slurry, the film resistance of the positive electrode sheet, and the compaction density. Appropriate lithium bis(fluorosulfonyl)imide (LiFSI) is added to the non-aqueous electrolyte, and the battery composition meets specific requirements, which can give full play to the synergistic effect between lithium bis(fluorosulfonyl)imide (LiFSI) and vinylene carbonate (VC) and the positive electrode active material, improving the low-temperature discharge performance of the battery while taking into account the high-temperature cycle performance.

[0015] In some possible implementation manners, the positive electrode active coating includes a positive electrode active substance, a conductive agent, a dispersant, and a binder. The mass percentages of the conductive agent, the dispersant, and the binder are 1-2:c:1-2. The sum of the mass percentages of the positive electrode active substance and the dispersant ranges from 95 to 97, and the mass percentage of the positive electrode active substance is (95-97)-c; the dispersant is vinylene carbonate.

[0016] In some possible implementation manners, the negative electrode active coating includes a negative electrode active material, a conductive agent, a thickening agent, an additive, and a binder. The mass percentages of the negative electrode active material, the conductive agent, the thickening agent, the additive, and the binder are 95 to 97: 1 to 2: 0.1 to 0.5: 0.1 to 0.8: 1.5 to 2, and the additive is styrene-butadiene rubber.

[0017] In some possible implementation manners, the film resistance a of the positive electrode sheet after coating is 200 to 500 mΩ.

[0018] In some possible implementation manners, the compaction density b of the positive electrode sheet is 2.4 to 2.7 g / cm 3 。

[0019] In some possible implementation manners, the mass percentage content c of vinylene carbonate in the positive electrode slurry is 0.1 to 0.5 wt%.

[0020] In some possible implementation manners, the mass percentage content d of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte is 3 to 7 wt%.

[0021] In some possible implementation manners, the conductive agent includes one or more of Ketjen black, mesophase carbon microspheres, activated carbon, graphite, conductive carbon black, acetylene black, carbon fiber, carbon nanotube, graphene; the thickening agent includes one or more of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein; the binder includes one or more of polyvinylidene fluoride, hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, hexafluorobutadiene, hexafluoroisobutene, trifluoroethylene, trifluorochloroethylene, and tetrafluoroethylene.

[0022] In some possible implementation manners, the battery further includes a separator and a housing. The separator is used to separate the positive electrode sheet and the negative electrode sheet, and the housing is used to install and encapsulate the positive electrode sheet, the negative electrode sheet, the separator, and the non-aqueous electrolyte; wherein the non-aqueous electrolyte includes ethylene carbonate, diethyl carbonate, ethyl methyl carbonate, and lithium bis(fluorosulfonyl)imide; the mass percentages of ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate are 1 to 2: 1 to 2: 1 to 2.

[0023] In some possible implementation manners, the preparation method of the lithium iron phosphate manganese battery includes:

[0024] Weigh positive electrode active materials, conductive agents, dispersants, and binders in a preset ratio and mix them to obtain a positive electrode slurry; wherein, the mass percentages of the positive electrode active materials, conductive agents, dispersants, and binders in the preset ratio are 95 to 97: 1 to 2: 0.1 to 0.5: 1 to 2; the dispersant is vinylene carbonate;

[0025] Coat the positive electrode paste on the positive electrode active material to obtain a positive electrode plate;

[0026] Prepare a negative electrode paste and a non-aqueous electrolyte; the negative electrode paste includes a negative electrode active material, a conductive agent, a thickening agent, an additive, and a binder, and the mass percentages of the negative electrode active material, the conductive agent, the thickening agent, the additive, and the binder are 95-97:1-2:0.1-0.5:0.1-0.8:1.5-2, and the additive is styrene-butadiene rubber;

[0027] Coat the negative electrode paste on the negative electrode active material to obtain a negative electrode plate;

[0028] Assemble the positive electrode plate and the negative electrode plate, and inject the non-aqueous electrolyte and vacuum package to obtain a lithium iron manganese phosphate battery.

[0029] By optimizing the positive electrode composition and the non-aqueous electrolyte composition of the lithium iron manganese phosphate battery, the capacity decay of the lithium iron manganese 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 as to obtain a wide-temperature lithium iron manganese phosphate battery that can balance the low-temperature discharge performance and the high-temperature cycle performance.

[0030] As can be seen from the above content, a lithium iron manganese phosphate battery of the present application includes a positive electrode plate, a negative electrode plate, and a non-aqueous electrolyte; the positive electrode plate includes a positive electrode active material and a positive electrode active coating provided on at least one surface of the positive electrode active material; the positive electrode active material includes lithium iron manganese phosphate (LiMn x Fe 1-x PO4) coated with a carbon coating layer, where 0.5≤x≤0.8; the negative electrode plate includes a negative electrode active material and a negative electrode active coating provided on at least one surface of the negative electrode active material; the non-aqueous electrolyte includes lithium bis(fluorosulfonyl)imide; where the battery satisfies the following relationship: 4.5≤(a / 20 + d) / (b + c)≤12.8; a is the film resistance of the positive electrode plate after coating; b is the tap density of the positive electrode plate; c is the mass percentage of vinylene carbonate in the positive electrode paste; d is the mass percentage of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte. By optimizing the composition of the lithium ion battery, using lithium iron manganese phosphate as the positive electrode active material, controlling the addition amount of VC in the positive electrode paste, the film resistance and the tap density of the positive electrode plate, adding an appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI) in the non-aqueous electrolyte, and making the battery composition meet specific requirements, the synergistic effect between lithium bis(fluorosulfonyl)imide (LiFSI) and vinylene carbonate (VC) and the positive electrode active material can be fully exerted, so that the battery can balance the low-temperature discharge performance and the high-temperature cycle performance while improving the low-temperature discharge performance of the battery. Detailed Embodiments

[0031] The embodiments described in the following examples do not represent all embodiments consistent with this application. They are merely examples of systems and methods consistent with some aspects of this application as detailed in the claims.

[0032] As a highly potential new energy device, lithium-ion secondary batteries are booming at an astonishing speed, and their application fields are also constantly expanding. Among various types of lithium-ion batteries, lithium iron manganese phosphate batteries stand out in the lithium-ion battery market and occupy the largest share, becoming the preferred battery type in many fields, due to their outstanding safety and structural stability. Their olivine structure endows them with good thermal stability, and in extreme situations such as high temperature or overcharging, they are not prone to dangerous conditions such as thermal runaway, greatly improving the safety during battery use. This stability gives lithium iron manganese phosphate batteries irreplaceable advantages in application scenarios with extremely high safety requirements, such as electric vehicles and energy storage power stations.

[0033] Lithium iron manganese phosphate itself has low conductivity, and its poor low-temperature performance becomes a huge disadvantage in practical applications. When the ambient temperature decreases, the chemical reaction rate inside the battery slows down, and ion migration is hindered, resulting in a significant decrease in the discharge capacity and charge-discharge efficiency of the battery. To improve the low-temperature performance of lithium iron manganese phosphate batteries, various strategies have been adopted currently. For example, the coating and doping strategies aim to improve the electronic conductivity of the material and promote ion transport by coating a material with good conductivity, such as carbon material, on the surface of the lithium iron manganese phosphate material, or doping some metal ions, such as magnesium ions, aluminum ions, etc. Changing the particle size of the material is to shorten the diffusion path of lithium ions inside the material by reducing the particle size of the material, thereby improving the performance of the battery at low temperatures.

[0034] Although the existing coating strategies are, to a certain extent, optimization schemes for improving the low-temperature performance of lithium iron manganese phosphate batteries, there are obvious defects in this non-in-situ preparation process. The existing non-in-situ preparation process, that is, the method of performing coating and other treatments after material synthesis, due to process conditions limitations, cannot achieve a comprehensive and tight wrapping of lithium iron manganese phosphate. Incomplete coating in some areas greatly reduces the improvement effect of low-temperature discharge performance. More seriously, this preparation method introduces other defect sites on the material surface, and these defect sites become active centers for electrolyte decomposition, further promoting the decomposition of the electrolyte on its surface. The decomposition of the electrolyte not only consumes active substances, leading to a rapid decay of the battery capacity, but also generates a large amount of gas and by-products, seriously deteriorating the high-temperature cycle performance of the battery. Therefore, it is difficult for lithium iron manganese phosphate batteries in related technologies to balance low-temperature discharge performance and high-temperature cycle performance, and they cannot meet the urgent market demand for lithium iron manganese phosphate batteries with a wide temperature range.

[0035] Based on this, in view of the problem that the lithium iron manganese phosphate battery in the related art is difficult to balance the low-temperature discharge performance and the high-temperature cycle performance, the present application provides a lithium iron manganese phosphate battery.

[0036] In some embodiments, the present application provides a lithium iron manganese phosphate battery, which includes a positive electrode sheet, a negative electrode sheet, and a non-aqueous electrolyte; the positive electrode sheet includes a positive electrode active material and a positive electrode active coating provided on at least one surface of the positive electrode active material; the positive electrode active material includes lithium iron manganese phosphate (LiMn x Fe 1-x PO4) coated with a carbon coating layer, where 0.5 ≤ x ≤ 0.8;

[0037] The negative electrode sheet includes a negative electrode active material and a negative electrode active coating provided on at least one surface of the negative electrode active material; the non-aqueous electrolyte includes lithium bis(fluorosulfonyl)imide (LiFSI);

[0038] Wherein the battery satisfies the following relationship:

[0039] 4.5 ≤ (a / 20 + d) / (b + c) ≤ 12.8;

[0040] a is the film resistance of the positive electrode sheet after coating;

[0041] b is the compaction density of the positive electrode sheet;

[0042] c is the mass percentage content of vinylene carbonate in the positive electrode slurry;

[0043] d is the mass percentage content of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte.

[0044] The present application controls the design of the positive electrode sheet and the addition amount of vinylene carbonate (VC), so that the lithium iron manganese phosphate battery obtains higher cycle life and service life. Specifically, the present application discloses a design of the positive electrode sheet of a lithium iron manganese 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 amount. By reducing the film resistance of the positive electrode sheet, the low-temperature discharge ability of the battery cell can be improved. If the content of vinylene carbonate (VC) in the positive electrode slurry is too high, it will affect the cycle life. Therefore, the vinylene carbonate (VC) should also be controlled within a reasonable range. By controlling the addition amount of vinylene carbonate (VC), the film resistance, and the compaction density of the positive electrode slurry, and adding an appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI) to the non-aqueous electrolyte, 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.

[0045] By controlling the ratio of the above formula to be between 4.5 and 12.8, it helps to balance the internal resistance of the battery and the distribution and characteristics of the internal substances. The lower diaphragm resistance a of the positive electrode sheet after coating, combined with the appropriate mass percentage d of lithium bis(fluorosulfonyl)imide (LiFSI), can significantly improve the overall conductivity of the battery. When this ratio is within a reasonable range, it can effectively reduce the energy loss during the charge and discharge process of the battery and improve the charge and discharge efficiency of the battery. For example, in practical applications, it can reduce the charging time, and at the same time, when discharging, the battery can output a more stable and strong current, improving the performance of the device.

[0046] In some embodiments, the positive electrode active coating includes a positive electrode active material, a conductive agent, a dispersant, and a binder. The mass percentages of the positive electrode active material, the conductive agent, the dispersant, and the binder are (95 - 97)-c:1 - 2:c:1 - 2, where the sum of the mass percentages of the positive electrode active material and vinylene carbonate VC is greater than or equal to 95 and less than or equal to 97. 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 nanotube, graphene, etc.; the binder includes one or more of vinylidene fluoride, hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, hexafluorobutadiene, hexafluoro isobutene, trifluoroethylene, trifluorochloroethylene, and tetrafluoroethylene; the dispersant is vinylene carbonate (VC). Vinylene carbonate (VC) can effectively reduce the agglomeration phenomenon between the positive electrode active material particles, making the active material more evenly dispersed in the coating. The addition of vinylene carbonate (VC) is beneficial to increasing the contact area between the active material and the non-aqueous electrolyte, improving the insertion and extraction efficiency of lithium ions during the charge and discharge process, and thus enhancing the charge and discharge performance and rate performance of the battery.

[0047] During the first charge and discharge process of the battery, vinylene carbonate (VC) 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, can effectively prevent the further reaction between the non-aqueous electrolyte and the electrode material, reduce the occurrence of side reactions, and improve the Coulomb efficiency and cycle life of the battery. The formed solid electrolyte interface (SEI) film helps to reduce the impedance on the electrode surface. Especially in a low-temperature environment, it can improve the migration rate of lithium ions, reduce the polarization phenomenon, thereby improving the low-temperature discharge performance of the battery, enabling the battery to still maintain good capacity output and rate performance under low-temperature conditions.

[0048] In some embodiments, the positive electrode active material is lithium manganese iron phosphate active material (LiMn x Fe 1-x PO4) coated with a carbon coating layer, where 0.5 ≤ x ≤ 0.8.

[0049] Lithium iron manganese phosphate itself has a relatively low electronic conductivity, while carbon has good electrical conductivity. The presence of the carbon coating provides a good channel for electron transport, greatly improving the overall electronic conductivity of the material. The carbon coating can optimize the properties of the lithium iron manganese phosphate particle surface, reduce the diffusion resistance of lithium ions on the particle surface, and can also inhibit the growth of lithium iron manganese phosphate grains to a certain extent, shortening the diffusion path of lithium ions, thereby increasing the diffusion coefficient of lithium ions. The carbon coating can play a role in protecting the lithium iron manganese phosphate active material, preventing it from directly contacting the electrolyte to cause side reactions and reducing the loss of active material; and during the charge and discharge process, it can relieve the volume change caused by the insertion and extraction of lithium ions in lithium iron manganese phosphate, maintaining the stability of the electrode structure. The carbon material itself has a certain thermal stability. After being coated on the surface of lithium iron manganese phosphate, it can improve the overall thermal stability of the positive electrode material to a certain extent and can also prevent adverse reactions between lithium iron manganese phosphate and the electrolyte at high temperatures. Among them, in some embodiments, the thickness of the carbon coating is 2 - 20 nm.

[0050] By using the lithium iron manganese phosphate active material coated with a carbon coating as the positive electrode active material, during the charge and discharge process of the battery, electrons can be transferred more quickly between the positive electrode active material and the positive electrode active material, thereby improving the rate performance of the battery, enabling the battery to be charged and discharged at a high current density, and meeting the requirements for high power output such as in electric vehicles. Moreover, the lithium iron manganese phosphate active material coated with a carbon coating can also enhance the safety and performance stability of the battery in a high-temperature environment, reduce safety hazards such as battery swelling and combustion caused by high temperature, broaden the working temperature range of the battery, and enable it to work more stably under different ambient temperature conditions.

[0051] The electrochemical potential of manganese element is higher than that of iron element. Appropriately increasing the proportion of manganese element can increase the overall electrochemical potential of the lithium iron manganese phosphate material, thereby increasing the output voltage of the battery. When the volume and mass of the battery remain unchanged, the increase in output voltage means that the battery can store and release more electrical energy, thereby effectively increasing the energy density of the battery and meeting the requirements for high energy density of the battery in electronic devices and the like. When the proportion of Mn element in the lithium iron manganese phosphate active material is in the range of 0.5 ≤ x ≤ 0.8, the crystal structure of the material will be more stable. During the charge and discharge process of the battery, it can better resist the distortion and damage of the structure, reduce the loss of active material and the increase in battery internal resistance caused by structural changes, and contribute to improving the cycle performance and service life of the battery. The above proportion range helps to optimize the electronic structure of the lithium iron manganese phosphate material, make the electron transport inside the material smoother, and increase the electronic conductivity of the material. Good electronic conductivity can reduce the polarization phenomenon during the charge and discharge process of the battery, improve the charge and discharge efficiency of the battery, reduce energy loss, and is also beneficial to improving the rate performance of the battery, enabling the battery to still maintain good performance under high-current charge and discharge conditions.

[0052] In some embodiments, the film resistance a of the positive electrode tab after coating is 200 to 500 mΩ. In a low-temperature environment, the ion diffusion rate and electron conduction rate of the lithium iron phosphate manganese battery itself will decrease significantly. If the film resistance of the positive electrode tab after coating is high, it will further hinder the transmission of electrons and lithium ions. It is difficult for electrons to be smoothly conducted from the positive electrode active material to the active material, and the process of lithium ion insertion and extraction also becomes more difficult. This significantly reduces the amount of active material participating in the discharge reaction, resulting in a sharp drop in the discharge capacity of the battery cell. At low temperatures, the battery cannot release the normal amount of electricity at room temperature, seriously affecting the battery's usage time and endurance in low-temperature scenarios. When the film resistance is low, although the low-temperature environment still inhibits the transmission of electrons and lithium ions, the relatively small film resistance can alleviate this hindrance to a certain extent. Electrons and lithium ions can move relatively smoothly, enabling more positive electrode active materials to participate in the discharge reaction. Therefore, compared with the case of high film resistance, a higher discharge capacity can be retained, 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 tab after coating can be 320 mΩ, 362 mΩ, 371 mΩ, 373 mΩ, 374 mΩ, 375 mΩ, 399 mΩ, 403 mΩ, 409 mΩ, 410 mΩ, 412 mΩ, 416 mΩ, 421 mΩ, 420 mΩ, 430 mΩ, 444 mΩ, 464 mΩ, 487 mΩ, 498 mΩ.

[0053] In some embodiments, the compaction density b of the positive electrode tab is 2.4 to 2.7 g / cm 3 . The higher the compaction density of the positive electrode tab, the more positive electrode active material it contains. Setting the positive electrode tab to have a certain compaction density not only gives the positive electrode tab a certain amount of energy but also provides a certain amount of voids for the positive electrode tab, facilitating the insertion and extraction of active ions and increasing the ion movement rate. The compaction density b of the positive electrode tab can be 2.4 g / cm 3 , 2.5 g / cm 3 , 2.55 g / cm 3 , 2.6 g / cm 3 , 2.65 g / cm 3 , 2.7 g / cm 3 .

[0054] The compaction density b of the positive electrode plate has a significant impact on the battery performance. A reasonable compaction density can optimize the packing state of the electrode material and improve the volumetric energy density of the electrode. If the compaction density is too high, it may lead to poor contact between material particles and increase the resistance; if it is too low, it will waste space and reduce the overall energy storage capacity of the battery. The relational expression, in cooperation with other parameters, ensures that b is within the range that can optimize the battery performance, guarantees a good ion transport channel between the positive active material and the non-aqueous electrolyte, and maintains the stable operation of the battery.

[0055] In this application, the important film-forming additive vinylene carbonate (VC) is premixed into the positive electrode material in advance, making full contact with the lithium iron phosphate manganese material. During the injection of the electrolyte, due to the principle of "like dissolves like", it can promote the wetting of the non-aqueous electrolyte to the positive electrode material, and ensure the dispersion of the film-forming additive in different thickness layers of the positive electrode material, which is beneficial to its complete coverage of the positive electrode material with different thicknesses on the electrode plate. At the initial stage of cell film formation, it can quickly form a protective film on the surface of the lithium iron phosphate manganese material, which not only increases the contact area between the lithium iron phosphate manganese materials and between the lithium iron phosphate manganese materials and the active material, but also avoids the oxidation and decomposition of the components with high film-forming impedance in the non-aqueous electrolyte at this place, reducing the initial impedance and impedance growth rate of the cell, thereby improving the low-temperature discharge performance and high-temperature cycle performance of the battery.

[0056] In some embodiments, the mass percentage content c of the vinylene carbonate (VC) in the positive electrode slurry is 0.1 - 0.5 wt%. Specifically, the mass percentage content c of the vinylene carbonate (VC) in the positive electrode slurry can be 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%.

[0057] The mass percentage content c of the vinylene carbonate (VC) in the positive electrode slurry acts as a film-forming additive in the battery. An appropriate amount of vinylene carbonate (VC) can form a stable solid electrolyte interface film (SEI film) on the positive electrode surface. When the relational expression defines a reasonable ratio of c, this film can effectively prevent the further decomposition of the non-aqueous electrolyte, protect the positive active material, and improve the cycle stability of the battery. After multiple charge and discharge cycles, the attenuation rate of the battery capacity slows down, thereby extending the service life of the battery.

[0058] The fluoride ions in lithium bis(fluorosulfonyl)imide (LiFSI) have strong electron-withdrawing properties, which weaken the coordination between the anions and cations of the lithium salt. The lithium ions have strong mobility, high thermal stability and electrochemical stability, and basically do not undergo side reactions. At the same time, it can also inhibit the swelling effect. In this application, adding an appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI) to the non-aqueous electrolyte of the lithium iron manganese phosphate battery can significantly improve the conductivity of the non-aqueous electrolyte, thereby improving the low-temperature discharge performance of the battery cell. In addition, lithium bis(fluorosulfonyl)imide (LiFSI) can also further modify the positive and negative electrode film components, reduce the film-forming impedance of the battery cell, and thus improve the high and low-temperature performance of the battery cell.

[0059] In some embodiments, the mass percentage content d of the lithium bis(fluorosulfonyl)imide (LiFSI) in the non-aqueous electrolyte is 3-7 wt%. Specifically, the mass percentage content d of the lithium bis(fluorosulfonyl)imide (LiFSI) in the non-aqueous electrolyte can be 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%. The mass percentage content d of the lithium bis(fluorosulfonyl)imide (LiFSI) in the non-aqueous electrolyte not only affects the conductivity of the non-aqueous electrolyte, but also plays an important role in the high and low-temperature performance of the battery. A suitable d value, combined with other parameters in the relationship formula, can improve the ion transport ability of the battery in a low-temperature environment and alleviate the problem of poor low-temperature performance of the lithium iron manganese phosphate battery itself. At low temperatures, the battery can maintain a relatively high discharge capacity and charge-discharge efficiency, reducing the performance degradation caused by low temperatures. At the same time, in a high-temperature environment, it also helps to maintain the internal chemical balance of the battery, avoid excessive reactions of the non-aqueous electrolyte caused by temperature rise, and improve the high-temperature stability of the battery.

[0060] In some embodiments, the negative electrode active coating includes a negative electrode active material, a conductive agent, a thickening agent, and a binder. In this application, the negative electrode active coating further includes styrene-butadiene rubber (SBR). The mass percentages of the negative electrode active material, the conductive agent, the thickening agent, the styrene-butadiene rubber, and the binder are 95-97:1-2:0.1-0.5:0.1-0.8:1.5-2. 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 thickening agent includes one or more of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein. The binder includes one or more of polyvinylidene fluoride, hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, hexafluorobutadiene, hexafluoroisobutene, trifluoroethylene, trifluorochloroethylene, and tetrafluoroethylene.

[0061] In some embodiments, the battery further includes a separator membrane and a housing. The separator membrane is used to separate the positive electrode sheet and the negative electrode sheet, and the housing is used to install and encapsulate the positive electrode sheet, the negative electrode sheet, the separator membrane, and the non-aqueous electrolyte. The separator membrane can be various materials suitable for lithium-ion battery separator membranes in the art. For example, it includes, but is not limited to, one or a combination of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers, etc. In some embodiments, the material of the housing is stainless steel or aluminum plastic film.

[0062] In some embodiments, the non-aqueous electrolyte can be various materials suitable for lithium-ion battery electrolytes in the art. For example, it includes, but is not limited to, a combination of ethylene carbonate (EC), diethyl carbonate (DMC), ethyl methyl carbonate (EMC), lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), etc. In some embodiments, the mass percentage of ethylene carbonate (EC), diethyl carbonate (DMC), and ethyl methyl carbonate (EMC) is 1-2:1-2:1-2.

[0063] In some embodiments, the method for preparing the lithium iron manganese phosphate battery in the above embodiments provided by the present application includes:

[0064] Weigh a preset ratio of positive electrode active material, conductive agent, dispersant, and binder and mix them to obtain a positive electrode paste; wherein, the mass percentage of the preset ratio of positive electrode active material, conductive agent, dispersant, and binder is 95-97:1-2:0.1-0.5:1-2; the dispersant is vinylene carbonate;

[0065] Coat the positive electrode paste on the positive electrode active material to obtain a positive electrode sheet;

[0066] Prepare a negative electrode paste and a non-aqueous electrolyte; the negative electrode paste includes a negative electrode active material, a conductive agent, a thickening agent, an additive, and a binder. The mass percentage of the negative electrode active material, conductive agent, thickening agent, additive, and binder is 95-97:1-2:0.1-0.5:0.1-0.8:1.5-2, and the additive is styrene-butadiene rubber;

[0067] Coat the negative electrode paste on the negative electrode active material to obtain a negative electrode sheet;

[0068] Assemble the positive electrode sheet and the negative electrode sheet, and inject the non-aqueous electrolyte and vacuum encapsulate to obtain a lithium iron manganese phosphate battery.

[0069] In order to enable the lithium iron manganese phosphate battery to take into account both low-temperature discharge performance and high-temperature cycle performance, it is necessary to limit the film resistance of the positive electrode sheet after coating. Specifically, the film resistance of the positive electrode sheet after coating is 200-500 mΩ. The electrode sheet with a film less than or equal to the preset resistance value after coating is selected as the positive electrode sheet, thereby improving the low-temperature discharge ability of the battery cell.

[0070] In this application, by optimizing the composition of the positive electrode and the components of the non-aqueous electrolyte of the lithium iron manganese phosphate battery, the capacity decay of the lithium iron manganese 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, thereby obtaining a wide-temperature lithium iron manganese phosphate battery that can take into account both low-temperature discharge performance and high-temperature cycle performance.

[0071] Example 1:

[0072] 1) Preparation of the positive electrode sheet

[0073] Step 1: Stir and mix the binder polyvinylidene fluoride (PVDF) and the positive electrode active material lithium iron manganese phosphate (LMFP) powder to obtain a mixed powder.

[0074] Step 2: Stir and mix the solvent N-methyl-2-pyrrolidone (NMP), the conductive agent carbon nanotubes (CNT), and the vinylene carbonate (VC) compound to obtain a mixed liquid.

[0075] Step 3: Add the mixed liquid to the mixed powder, disperse it at high speed, and stir evenly to obtain a positive electrode slurry. The mass percentage of the positive electrode active material, the conductive agent carbon nanotubes (CNT), the vinylene carbonate (VC), and the positive electrode binder polyvinylidene fluoride (PVDF) = 96.4 - c: 1.4: c: 2.2; the positive electrode active material is lithium iron manganese phosphate (LiMn 0.6 Fe 0.4 PO4) active material, c = 0.1%.

[0076] Step 4: Uniformly coat the prepared positive electrode slurry on the positive electrode active material (such as aluminum foil), and then use a film resistance meter to detect the resistance of the electrode sheet. The film resistance is 403 mΩ, and the compaction density is 2.5 g / cm 3 .

[0077] Step 5: Obtain the positive electrode sheet through drying, rolling, die-cutting or slitting.

[0078] 2) Preparation of the negative electrode sheet

[0079] Step 1: Weigh the following substances for the negative electrode sheet according to 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 percentage.

[0080] Step 2: First, stir and premix graphite, conductive carbon (superP), and carboxymethyl cellulose (CMC), and then add pure water and stir well.

[0081] Step 3: Add the binder to the mixed slurry, mix well, and then add styrene-butadiene rubber (SBR) to finally obtain the negative electrode slurry.

[0082] Step 4: Coat the prepared negative electrode slurry evenly on the copper foil, and obtain the negative electrode sheet through drying, rolling, die-cutting or slitting.

[0083] 3) Preparation of non-aqueous electrolyte

[0084] Mix ethylene carbonate (EC), diethyl carbonate (DMC), and ethyl methyl carbonate (EMC) according to the mass percentage of ethylene carbonate (EC): diethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 1:1:1, add 3% lithium bis(fluorosulfonyl)imide (LiFSI), and then add lithium hexafluorophosphate (LiPF6) until the molar concentration reaches 0.9 mol / L. The value of the above formula is 8.90.

[0085] 4) Preparation of lithium-ion battery cell

[0086] Assemble the prepared positive electrode sheet and the above-mentioned negative electrode sheet into a laminated soft-pack battery cell.

[0087] 5) Injection and formation of the battery cell

[0088] In an environment with the dew point controlled below -40°C, inject the above-prepared non-aqueous electrolyte into the battery cell, perform vacuum packaging, and let it stand for 72 h. Then, perform the conventional formation for the first charge according to the following steps: constant current charge at 0.05C for 180 min, constant current charge at 0.1C for 120 min, constant current charge at 0.2C for 120 min, perform secondary vacuum sealing, and then further perform full charge (100% SOC) at a current of 0.2C. After standing at room temperature for 72 h, perform full discharge (0% SOC) at a current of 0.2C.

[0089] Example 2:

[0090] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the film resistance of the positive electrode sheet is 444 mΩ, the compaction density of the positive electrode sheet is 2.55 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0091] Example 3:

[0092] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 487 mΩ, and the tap density of the positive electrode sheet is 2.65 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 3 wt%.

[0093] Example 4:

[0094] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 412 mΩ, and the tap density of the positive electrode sheet is 2.5 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 3 wt%.

[0095] Example 5:

[0096] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 498 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 6 wt%.

[0097] Example 6:

[0098] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 464 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 6 wt%.

[0099] Example 7:

[0100] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 399 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0101] Example 8:

[0102] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 320 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0103] Example 9:

[0104] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 362 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0105] Example 10:

[0106] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 410 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0107] Example 11:

[0108] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 371 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 3 wt%.

[0109] Example 12:

[0110] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 375 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0111] Example 13:

[0112] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 374 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 5 wt%.

[0113] Example 14:

[0114] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 373 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 7 wt%.

[0115] Example 15:

[0116] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 416 mΩ, and the tap density of the positive electrode sheet is 2.4 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0117] Example 16:

[0118] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 409 mΩ, and the tap density of the positive electrode sheet is 2.5 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0119] Example 17:

[0120] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 421 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0121] Example 18:

[0122] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 430 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0123] Example 19:

[0124] The preparation method of the lithium iron manganese phosphate battery in this example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 420 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 , the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 4 wt%.

[0125] Comparative Example 1

[0126] The preparation method of the lithium iron manganese phosphate battery in this comparative example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 389 mΩ, and the tap density of the positive electrode sheet is 2.5 g / cm 3 , the proportion of vinylene carbonate is 0 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 3 wt%

[0127] Comparative Example 2

[0128] The preparation method of the lithium iron manganese phosphate battery in this comparative example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 467 mΩ, and the tap density of the positive electrode sheet is 2.5 g / cm3 Among them, the proportion of vinylene carbonate is 0.5 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 3 wt%

[0129] Comparative Example 3

[0130] The preparation method of the lithium iron manganese phosphate battery in this comparative example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 596 mΩ, and the tap density of the positive electrode sheet is 2.5 g / cm 3 Among them, the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 3 wt%

[0131] Comparative Example 4

[0132] The preparation method of the lithium iron manganese phosphate battery in this comparative example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 598 mΩ, and the tap density of the positive electrode sheet is 2.2 g / cm 3 Among them, the proportion of vinylene carbonate is 0 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 10 wt%

[0133] Comparative Example 5

[0134] The preparation method of the lithium iron manganese phosphate battery in this comparative example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 553 mΩ, and the tap density of the positive electrode sheet is 2.3 g / cm 3 Among them, the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 5 wt%

[0135] Comparative Example 6

[0136] The preparation method of the lithium iron manganese phosphate battery in this comparative example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 374 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 Among them, the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 0 wt%

[0137] Comparative Example 7

[0138] The preparation method of the lithium iron manganese phosphate battery in this comparative example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 373 mΩ, and the tap density of the positive electrode sheet is 2.6 g / cm 3 Among them, the proportion of vinylene carbonate is 0.1 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 10 wt%

[0139] Comparative Example 8

[0140] The preparation method of the lithium iron manganese phosphate battery in this comparative example is the same as that in Example 1, but the sheet resistance of the positive electrode sheet is 412 mΩ, and the tap density of the positive electrode sheet is 2.5 g / cm 3, the proportion of vinylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(trifluoromethanesulfonyl)imide is 3 wt%. In this comparative example, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is used to replace lithium bis(fluorosulfonyl)imide.

[0141] Comparative Example 9

[0142] The preparation method of the lithium iron phosphate manganese battery in this comparative example is the same as that in Example 1, but the film resistance of the positive electrode sheet is 412 mΩ, and the tap density of the positive electrode sheet is 2.5 g / cm 3 , the proportion of fluoroethylene carbonate is 0.2 wt%, and the mass proportion of lithium bis(fluorosulfonyl)imide is 3 wt%. In this comparative example, fluoroethylene carbonate (FEC) is used to replace vinylene carbonate.

[0143] Among them, Table 1 records the corresponding formula values of Examples 1-19 and Comparative Examples 1-9.

[0144] Table 1

[0145]

[0146]

[0147] The charging performance of the above Examples 1-19 and Comparative Examples 1-9 was tested.

[0148] 1. Film resistance test

[0149] An electrode sheet resistance tester was used to test the resistance of the fabricated positive electrode sheet to obtain the film resistance.

[0150] 2. DCIR test at 25°C

[0151] The lithium-ion battery was placed in a constant-temperature environment at 25°C and charged at a constant current of 0.5C to the cut-off voltage of 3.65V, then charged at a constant voltage until the current dropped to 0.05C, and then discharged at a constant current of 1 / 3C to 2.0V. The discharge capacity was recorded. Subsequently, the battery was charged to full again, discharged at a constant current of 0.5C to 50% SOC of the capacity, and left standing for 30 min. Then, it was discharged at a constant current of 2C for 10 s, and the DCIR of the battery cell was calculated.

[0152] The DCIR of the battery cell was calculated according to the following formula:

[0153] DCIR = (voltage after 30 min of standing - voltage after 2C constant-current discharge) / 2C discharge current.

[0154] 3. Low-temperature discharge test at -20°C

[0155] Place the lithium-ion battery in a constant-temperature environment of 25°C and charge it at a constant current of 0.5C until the cut-off voltage of 3.65V is reached. Then, charge it at a constant voltage until the current drops to 0.05C. Subsequently, discharge it at a constant current of 1 / 3C until 2.0V is reached, and record the room-temperature discharge capacity. Repeat the charging step to fully charge the lithium battery. Then, keep it in a constant-temperature environment of -20°C and let it stand for 6 hours. After that, discharge it at a constant current of 1 / 3C until 2.0V is reached, and record the low-temperature discharge capacity at -20°C. Calculate the proportion of the low-temperature discharge capacity.

[0156] Calculate the capacity retention rate of the cycle according to the following formula:

[0157] Proportion of low-temperature discharge capacity (%) = low-temperature discharge capacity at -20°C / room-temperature discharge capacity × 100%.

[0158] 4. High-temperature cycle performance test

[0159] Place the lithium-ion battery in a constant-temperature environment of 45°C and charge it at a constant current of 1C until 3.65V is reached. 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 at a constant current of 1C until 2.0V is reached. Repeat this cycle 1000 times, and record the discharge capacity of the first time and the last time, 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 it is fully charged for the last time.

[0160] Calculate the capacity retention rate and internal resistance growth rate of the cycle according to the following formula:

[0161] Capacity retention rate (%) = discharge capacity of the last time / discharge capacity of the first time × 100%;

[0162] Internal resistance growth rate (%) = (internal resistance of the battery after it is fully charged for the last time - internal resistance of the battery when it is fully charged for the first time) / internal resistance of the battery when it is fully charged for the first time × 100%.

[0163] The results of the charging performance tests for Examples 1-19 and Comparative Examples 1-9 are shown in Table 2.

[0164] Table 2

[0165]

[0166]

[0167] Through Examples 1 to 7, the formula values are within the defined ranges, and each parameter is within the defined limits, with small performance fluctuations. Appropriate matching of each parameter can obtain the best combination in Example 2, which has low cell impedance and good high and low temperature performance. For Comparative Examples 4 and 5, when some or all of the parameters a, b, c, and d in the formula exceed the defined range of the formula, the cell performance will decline due to their internal relationship. For Comparative Examples 8 and 9, when lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and fluoroethylene carbonate (FEC) are used to replace lithium bis(fluorosulfonyl)imide (LiFSI) and vinylene carbonate (VC), the former will cause the cell impedance to decrease but seriously deteriorate the high temperature cycle performance, and the latter will cause a significant increase in impedance and serious deterioration of the high and low temperature performance.

[0168] Through Examples 8 to 10, by adjusting the diaphragm resistance value, the aim is to explore the influence of the diaphragm resistance range on the battery performance. The formula values of each example are within the defined ranges, and each parameter is within the defined limits, with small performance fluctuations. However, it can be found that reducing the diaphragm resistance can improve the low temperature discharge capacity of the cell. Compared with Comparative Example 3, the formula value is greater than the defined range, and each performance is significantly deteriorated, and the diaphragm resistance seriously exceeds the upper limit, resulting in an increase in cell impedance and deterioration of the low temperature discharge capacity.

[0169] Through Examples 11 to 14, by adjusting the addition amount of lithium bis(fluorosulfonyl)imide (LiFSI), the aim is to explore the influence of the addition amount of lithium bis(fluorosulfonyl)imide (LiFSI) on the battery performance. The formula values of each example are within the defined ranges, and each parameter is within the defined limits, with small performance fluctuations. As the content of lithium bis(fluorosulfonyl)imide (LiFSI) increases, the impedance decreases and the low temperature discharge performance improves, but the high temperature cycle ability decreases. Compared with Comparative Examples 6 and 7, too low an addition amount of lithium bis(fluorosulfonyl)imide (LiFSI) will cause the cell impedance to rise and deteriorate the low temperature discharge, while too high an addition amount of lithium bis(fluorosulfonyl)imide (LiFSI) will also cause the cell impedance to rise and deteriorate the high temperature cycle performance.

[0170] Through the analysis of Examples 15 to 17, by adjusting the compaction density, the aim is to explore the influence of the compaction density range on the battery performance. The formula values of each example are within the defined ranges, and each parameter is within the defined limits, with small performance fluctuations. However, an increase in the positive electrode compaction density will cause the cell impedance to decrease, optimize the low temperature performance, but the influence on the high temperature cycle performance does not change linearly.

[0171] Through Examples 18 and 19, by adjusting the addition amount of vinylene carbonate (VC), the aim is to explore the influence of the content of vinylene carbonate (VC) on the battery performance. The formula values of each example are within the specified range, and each parameter is within the specified range, with small performance fluctuations. Appropriately increasing the content of vinylene carbonate (VC) will lead to an increase in impedance and a decrease in low-temperature performance, while optimizing the high-temperature cycle ability. Compared with Comparative Examples 1 and 2, excessive reduction or increase in the addition amount of vinylene carbonate (VC) will result in a significant decrease in high-temperature performance.

[0172] As can be seen from the above examples, a lithium iron manganese phosphate battery of the present application includes a positive electrode sheet, a negative electrode sheet, and a non-aqueous electrolyte; the positive electrode sheet includes a positive electrode active material and a positive electrode active coating provided on at least one surface of the positive electrode active material; the positive electrode active material includes lithium iron manganese phosphate LiMn x Fe 1-x PO4, where 0.5 ≤ x ≤ 0.8; the negative electrode sheet includes a negative electrode active material and a negative electrode active coating provided on at least one surface of the negative electrode active material; the non-aqueous electrolyte includes lithium bis(fluorosulfonyl)imide; where the battery satisfies the following relationship: 4.5 ≤ (a / 20 + d) / (b + c) ≤ 12.8; a is the film resistance of the positive electrode sheet after coating; b is the compaction density of the positive electrode sheet; c is the mass percentage content of vinylene carbonate in the positive electrode slurry; d is the mass percentage content of lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte. By optimizing the composition of the lithium-ion battery, using lithium iron manganese phosphate as the positive electrode active material, controlling the addition amount of vinylene carbonate (VC), the film resistance of the positive electrode sheet, and the compaction density, adding an appropriate amount of lithium bis(fluorosulfonyl)imide (LiFSI) to the non-aqueous electrolyte, and making the battery composition meet specific requirements, the synergistic effect between lithium bis(fluorosulfonyl)imide (LiFSI) and vinylene carbonate (VC) and the positive electrode active material can be fully exerted, enabling the battery to take into account the high-temperature cycle performance while improving the low-temperature discharge performance of the battery.

[0173] For the similar parts between the examples provided in the present application, reference can be made to each other. The specific embodiments provided above are only several examples under the general concept of the present application and do not constitute a limitation on the protection scope of the present application. For those skilled in the art, any other implementation manner extended based on the solution of the present application without creative efforts belongs to the protection scope of the present application.

Claims

1. A lithium manganese iron phosphate battery, characterized in that: The battery comprises a positive electrode sheet, a negative electrode sheet and a non-aqueous electrolyte; the positive electrode sheet comprises a positive electrode active material and a positive electrode active coating arranged on at least one surface of the positive electrode active material; the positive electrode active material comprises lithium manganese iron phosphate LiMn coated with a carbon coating layer x Fe 1-x PO4, where 0.5≤x≤0.8; The negative electrode plate comprises a negative electrode active material and a negative electrode active coating disposed on at least one surface of the negative electrode active material; the non-aqueous electrolyte comprises lithium bis(fluorosulfonyl)imide; The battery satisfies the following relationship: 4.5≤(a / 20+d) / (b+c)≤12.8; 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 manganese iron phosphate battery 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 percentage of the conductive agent, the dispersant and the binder is 1-2:c:1-2, and the mass percentage and value of the positive electrode active material and the dispersant range from 95 to 97; the dispersant is vinylene carbonate.

3. The lithium manganese iron phosphate battery according to claim 2, characterized in that: The negative electrode active coating comprises a negative electrode active material, a conductive agent, a thickener, an additive and a binder, wherein the mass percentages of the negative electrode active material, the conductive agent, the thickener, the additive and the binder are 95-97:1-2:0.1-0.5:0.1-0.8:1.5-2, and the additive is styrene-butadiene rubber.

4. The lithium manganese iron phosphate battery according to claim 3, characterized in that: The film resistance a of the positive electrode sheet after coating is 200 to 500 mΩ.

5. The lithium manganese iron phosphate battery 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 manganese iron phosphate battery 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 manganese iron phosphate battery 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 manganese iron phosphate battery according to claim 3, characterized in that: 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, and graphene; the thickener 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, chlorotrifluoroethylene, and tetrafluoroethylene.

9. The lithium manganese iron phosphate battery according to claim 1, characterized in that: The battery also includes a separator and a shell, wherein the separator is used to separate the positive electrode plate from the negative electrode plate, and the shell is used to install and package the positive electrode plate, the negative electrode plate, the separator and the non-aqueous electrolyte; wherein the non-aqueous electrolyte includes ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and lithium bis(fluorosulfonyl)imide; the mass percentage of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate is 1-2:1-2:1-2.

10. The lithium manganese iron phosphate battery according to claim 1, characterized in that: The preparation method of the lithium manganese iron phosphate battery comprises: Weighing a preset ratio of positive electrode active material, conductive agent, dispersant and binder and mixing them to obtain positive electrode slurry; wherein the mass percentage of the preset ratio of positive electrode active material, conductive agent, dispersant and binder is 95-97:1-2:0.1-0.5:1-2; the dispersant is vinylene carbonate; Applying the positive electrode slurry on the positive electrode active material to obtain a positive electrode sheet; A negative electrode slurry and a non-aqueous electrolyte are prepared; the negative electrode slurry comprises a negative electrode active material, a conductive agent, a thickener, an additive and a binder, the mass percentages of the negative electrode active material, the conductive agent, the thickener, the additive and the binder are 95-97:1-2:0.1-0.5:0.1-0.8:1.5-2, and the additive is styrene-butadiene rubber; Applying the negative electrode slurry on the negative electrode active material to obtain a negative electrode sheet; The positive electrode sheet and the negative electrode sheet are assembled, and a non-aqueous electrolyte is injected and vacuum-sealed to obtain a lithium manganese iron phosphate battery.

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