An electrolyte and battery for improving the ultra-low temperature performance of lithium iron phosphate batteries

Abstract

This invention belongs to the field of batteries, specifically relating to an electrolyte and battery for improving the ultra-low temperature performance of lithium iron phosphate batteries. By adding specific film-forming additives and ultra-low temperature functional additives to the electrolyte, the low-temperature liquid state is maintained through the synergistic effect of fluoroethylene carbonate (FEC), 1-ethyl-3-methylimidazolium difluorophosphate, and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, thereby reducing the electrolyte viscosity. At the same time, it can form a weakly solvated sheath layer with lithium ions, significantly reducing the energy barrier required for lithium ions to desolvate at the negative electrode interface, reducing the SEI interface impedance, thereby promoting lithium ion transport and significantly improving the ultra-low temperature performance of lithium iron phosphate batteries.

Description

Technical Field

[0001] This invention belongs to the field of batteries, specifically relating to an electrolyte and battery for improving the ultra-low temperature performance of lithium iron phosphate batteries. Background Technology

[0002] Lithium iron phosphate (LFP) batteries have advantages such as long cycle life, high safety, and low cost, and are widely used in new energy vehicles and other fields. However, at low temperatures, especially below -20°C, the performance of LFP batteries degrades sharply, including a sharp decrease in capacity, charging difficulties (due to the risk of lithium plating), and a drop in power. The root causes can be attributed to three kinetic barriers: First, low temperatures increase electrolyte viscosity, slowing the migration rate of lithium ions and reducing the bulk ionic conductivity of the electrolyte. Second, at low temperatures, the electrolyte decomposes to form LiF and Li₂CO₃ passivation layers, leading to increased interfacial film impedance and a significant increase in the energy barrier for the Li⁺ desolvation-intercalation process, resulting in a surge in electrode / electrolyte interfacial impedance. Third, solid-phase diffusion within the positive and negative electrode materials slows down. Although the electrolyte does not directly determine solid-phase diffusion, it indirectly exacerbates electrode polarization by influencing interfacial kinetics. Existing technologies mainly improve low-temperature performance through carbon coating and metal doping. For example, CN2018103105713 discloses a modified lithium iron phosphate lithium-ion battery, which uses at least one of the specific doping elements titanium, aluminum, yttrium or lanthanum for doping, and at the same time uses at least one of the specific coating materials graphene, glucose or polyvinyl alcohol for coating to jointly modify the lithium iron phosphate material, effectively improving the conductivity of the lithium iron phosphate material at low temperatures, enabling it to maintain a high discharge capacity at ultra-low temperatures of -40℃ and to charge and discharge normally at low temperatures of -20℃.

[0003] While cathode materials, anode materials, and current collectors all face limitations at low temperatures, the electrolyte, as the "blood" of lithium-ion transport, experiences the most significant impact on the low-temperature performance of lithium-ion batteries due to issues such as a sharp increase in viscosity, a sharp decrease in ion diffusion efficiency, and a significant increase in charge transfer impedance at low temperatures. Therefore, it becomes the most direct and critical factor in improving the ultra-low temperature performance of lithium iron phosphate batteries. The low-temperature performance degradation of lithium iron phosphate batteries reflects the goal of electrolyte design: to maximize bulk ion conduction and interfacial charge transfer kinetics at low temperatures while maintaining a wide electrochemical window, high thermal stability, and good interfacial characteristics. Summary of the Invention

[0004] The purpose of this invention is to provide an electrolyte and battery that improve the ultra-low temperature performance of lithium iron phosphate batteries. By adding specific film-forming additives and ultra-low temperature functional additives to the electrolyte, the low-temperature liquid state is maintained through the synergistic effect of fluoroethylene carbonate (FEC), 1-ethyl-3-methylimidazolium difluorophosphate, and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, thereby reducing the electrolyte viscosity. At the same time, it can form a weakly solvated sheath layer with lithium ions, significantly reducing the energy barrier required for lithium ions to desolvate at the negative electrode interface, reducing the SEI interface impedance, thereby promoting lithium ion transport and significantly improving the ultra-low temperature performance of lithium iron phosphate batteries.

[0005] An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries comprises 45%-90% by mass solvent, 5%-25% by mass lithium salt, 2%-5% by mass film-forming additive, and 0.5%-4.5% by mass ultra-low temperature functional additive.

[0006] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium dioxaborate (LiBOB), lithium difluorooxaborate (LiODFB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorophosphate (LiPO2F2), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0007] More preferably, the lithium salt is selected from lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium difluorophosphate (LiPO2F2).

[0008] More preferably, the mass ratio of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium difluorophosphate (LiPO2F2) is 1:0.3-1.2:0.3-1.2.

[0009] Preferably, the ultra-low temperature functional additive is 1-ethyl-3-methylimidazolium difluorophosphate and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

[0010] More preferably, the mass ratio of 1-ethyl-3-methylimidazolium difluorophosphate to 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt is 1:3-5.

[0011] Preferably, the film-forming additive is one or more of ethylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene sulfate (DTD).

[0012] More preferably, the film-forming additive is fluoroethylene carbonate (FEC).

[0013] Preferably, the solvent is a combination of cyclic carbonate and chain carbonate, with a mass ratio of cyclic carbonate to chain carbonate of 5:95-40:60. Cyclic carbonate has the characteristics of high dielectric constant, high viscosity, and high solubility for lithium salts, while chain carbonate has the characteristics of low dielectric constant, low viscosity, and can effectively reduce electrolyte viscosity. By achieving this ratio, the increase in electrolyte viscosity can be suppressed, and the degree of electrolyte dissociation can be improved. Therefore, the conductivity of the electrolyte and the solubility of the electrolyte, which are related to the charge and discharge characteristics of the battery, can be improved. Accordingly, an electrolyte with excellent conductivity at low temperatures can be formed, thereby improving the battery load characteristics at low temperatures.

[0014] More preferably, the mass ratio of cyclic carbonate to chain carbonate is 10:90-30:70, and particularly preferably, the mass ratio of cyclic carbonate to chain carbonate is 15:85-35:65.

[0015] More preferably, the cyclic carbonate is one or more of ethylene carbonate, propylene carbonate, and butyl carbonate.

[0016] More preferably, the chain carbonate is one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, dipentyl carbonate, dihexyl carbonate, ethyl octyl carbonate, and dioctyl carbonate.

[0017] The lithium-ion battery of the present invention comprises a positive electrode, a negative electrode, a separator, and the electrolyte described above.

[0018] The positive electrode comprises a positive electrode active material, a positive electrode current collector, a conductive agent, a binder, and a solvent. The positive electrode active material is a modified lithium iron phosphate material. The conductive agent is one or more of graphene, carbon nanotubes, carbon black, and flake graphite. The binder is one or more of SBR, PVDF, and NBR.

[0019] The negative electrode comprises a negative electrode active material, a negative electrode current collector, a conductive agent, a dispersant, a binder, and a solvent. The negative electrode active material is graphite. The conductive agent is one or more of graphene, carbon nanotubes, carbon black, and flake graphite. The binder is one or more of SBR, PVDF, and NBR.

[0020] The diaphragm is a porous membrane, preferably one or more of the following: polyolefin porous membrane, polyimide porous membrane, polyvinylidene fluoride porous membrane, and polyester porous membrane.

[0021] The thickness of the membrane is 1 μm-45 μm, preferably 5 μm-30 μm. The porosity of the membrane is 20%-95%, preferably 48%-70%. The average pore size of the membrane is 0.02 μm-0.5 μm, preferably 0.1 μm-0.2 μm.

[0022] The method for preparing the lithium-ion battery of the present invention includes the following steps:

[0023] S1. Mix the positive electrode active material, conductive agent, binder and solvent evenly to form a positive electrode slurry, coat it on the surface of the positive electrode current collector, dry, roll and die cut to form a positive electrode sheet;

[0024] S2. The negative electrode active material, conductive agent, dispersant, binder and solvent are made into a negative electrode slurry, which is coated on the surface of the negative electrode current collector copper foil, and then dried, rolled and die-cut to form a negative electrode sheet.

[0025] S3. Weigh each component according to the formula, mix and stir evenly to form an electrolyte;

[0026] S4. The positive electrode sheet obtained in step S1 and the negative electrode sheet obtained in step S2 are stacked to form a battery cell. Positive and negative electrode tabs are ultrasonically welded, and the battery is encapsulated. The battery is then manufactured by baking, injecting the electrolyte obtained in step S3, forming, and capacity testing.

[0027] The beneficial effects of this invention are as follows:

[0028] 1. The electrolyte of the present invention uses fluoroethylene carbonate (FEC), 1-ethyl-3-methylimidazolium difluorophosphate, and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The three components have a synergistic effect. On the one hand, they have a low melting point, which can maintain a low-temperature liquid state and reduce the viscosity of the electrolyte. On the other hand, the ionic liquid can form a weak solvation sheath with lithium ions, which can significantly reduce the energy barrier required for lithium ions to desolvate at the negative electrode interface. At the same time, it can also suppress the SEI interface impedance, thereby promoting the transport of lithium ions. The synergistic effect significantly improves the ultra-low temperature performance of lithium iron phosphate batteries.

[0029] 2. The electrolyte of the present invention uses fluoroethylene carbonate (FEC), which, compared with ethylene sulfate (DTD) and ethylene carbonate (VC), can exert a better synergistic effect with 1-ethyl-3-methylimidazolium difluorophosphate and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, better promotes the formation of a stable interfacial passivation layer in the electrolyte during charging and discharging, reduces side reactions, improves interfacial stability, and improves the ultra-low temperature performance of lithium iron phosphate batteries. Detailed Implementation

[0030] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] The present invention will be further explained below with reference to specific embodiments.

[0032] Example 1

[0033] An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries comprises 15% by mass propylene carbonate, 60% by mass dipentyl carbonate, 10% by mass lithium hexafluorophosphate (LiPF6), 6% by mass lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 5% by mass lithium difluorophosphate (LiPO2F2), 2% by mass fluoroethylene carbonate (FEC), 0.5% by mass 1-ethyl-3-methylimidazolium difluorophosphate, and 1.5% by mass 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The components are weighed according to the formula and mixed thoroughly to form the electrolyte.

[0034] Comparative Example 1

[0035] An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries is prepared according to the method of Example 1, the only difference being that 1-ethyl-3-methylimidazolium difluorophosphate in Example 1 is replaced with 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

[0036] Comparative Example 2

[0037] An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries is prepared according to the method of Example 1, the only difference being that the 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt in Example 1 is replaced with 1-ethyl-3-methylimidazolium difluorophosphate.

[0038] Comparative Example 3

[0039] An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries is prepared according to the method of Example 1, except that the 2% by mass of fluoroethylene carbonate (FEC) in Example 1 is replaced with 0.5% by mass of 1-ethyl-3-methylimidazolium difluorophosphate and 1.5% by mass of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

[0040] Comparative Example 4: No 2F-3F

[0041] An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries is prepared according to the method of Example 1, except that the 0.5% by mass of 1-ethyl-3-methylimidazolium difluorophosphate and 1.5% by mass of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt in Example 1 are replaced with 2% by mass of fluoroethylene carbonate (FEC).

[0042] Example 2

[0043] An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries is prepared according to the method of Example 1, except that the fluoroethylene carbonate (FEC) in Example 1 is replaced with ethylene sulfate (DTD).

[0044] Example 3

[0045] An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries is prepared according to the method of Example 1, except that the fluoroethylene carbonate (FEC) in Example 1 is replaced with ethylene carbonate (VC).

[0046] Performance testing

[0047] The electrolytes obtained in Examples 1-3 and Comparative Example 3 were used to prepare batteries according to the method described in this invention, and performance tests were performed. The battery preparation method of this invention includes:

[0048] S1. Weigh 95 parts by mass of positive electrode active material lithium iron phosphate WX-F18, 3 parts by mass of conductive agent graphene, and 2 parts by mass of binder PVDF. Add solvent NMP and mix evenly to form a slurry with a viscosity of 10000 mPa•s and a solid content of 55wt%. Coat the surface of the positive electrode current collector aluminum foil, dry, roll, and die-cut to form a positive electrode sheet.

[0049] S2. Weigh 93 parts by mass of artificial graphite (negative electrode active material), 3 parts by mass of conductive carbon black (conductive agent), 1 part by mass of CMC (dispersant), and 2 parts by mass of SBR (binder). Add NMP solvent and mix evenly to form a negative electrode slurry with a viscosity of 5000 mPa•s and a solid content of 50 wt%. Coat the negative electrode current collector copper foil surface and dry, roll, and die-cut to form a negative electrode sheet.

[0050] S3. Weigh each component according to the formulations of Examples 1-3 and Comparative Example 3, and mix and stir evenly to form an electrolyte;

[0051] S4. The positive electrode sheet obtained in step S1, the negative electrode sheet obtained in step S2, and the porous PP separator (the separator thickness is 18 μm, the porosity is 50%, and the average pore diameter is 0.1 μm) are stacked to form a battery cell. The positive and negative electrode tabs are ultrasonically welded, the casing is encapsulated, and the battery is made by baking, injecting the electrolyte obtained in step S3, forming, and capacity testing.

[0052] The lithium-ion batteries in Examples 1-3 and Comparative Examples 1-3 were subjected to low-temperature electrochemical performance tests. The charge-discharge cycle tests at 1C / 1C were measured at -20℃, -40℃, and -60℃. The discharge capacity was recorded, and the capacity retention rate was recorded after 300 cycles. The results are shown in Table 1 below.

[0053] Table 1 Electrolyte performance for improving the ultra-low temperature performance of lithium iron phosphate batteries

[0054]

[0055] The comparison between Example 1 and Comparative Examples 1, 2, 3, and 4 shows that the battery formed by the electrolyte of Example 1 exhibits superior discharge capacity and capacity retention after 300 cycles at 1C rate under conditions of -20°C, -40°C, and -60°C. This may be because the electrolyte of Example 1 simultaneously contains fluoroethylene carbonate (FEC), 1-ethyl-3-methylimidazolium difluorophosphate, and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. On the one hand, it has a low melting point, which can maintain a low-temperature liquid state and reduce the viscosity of the electrolyte. On the other hand, the ionic liquid can form a weak solvation sheath with lithium ions, significantly reducing the energy barrier required for lithium ions to desolvate at the negative electrode interface. It can also reduce the SEI interface impedance, thereby promoting lithium ion transport. The three have a synergistic effect, improving the ultra-low temperature performance of lithium iron phosphate batteries.

[0056] A comparison of Examples 1, 2, and 3 shows that the battery formed by the electrolyte in Example 1 exhibits superior discharge capacity and capacity retention after 300 cycles at 1C rate under conditions of -20°C, -40°C, and -60°C. This may be because the electrolyte in Example 1 uses fluoroethylene carbonate (FEC), which, compared to ethylene sulfate (DTD) used in Example 2 and ethylene carbonate (VC) used in Example 3, contains fluorine. FEC can synergistically work with 1-ethyl-3-methylimidazolium difluorophosphate and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, better promoting the formation of a stable interfacial passivation layer during charge and discharge, reducing side reactions, improving interfacial stability, and enhancing the ultra-low temperature performance of lithium iron phosphate batteries.

[0057] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries, characterized in that, It comprises 45%-90% by mass solvent, 5%-25% by mass lithium salt, 2%-5% by mass film-forming additive, and 0.5%-4.5% by mass ultra-low temperature functional additive; the film-forming additive is fluoroethylene carbonate (FEC); the ultra-low temperature functional additive is 1-ethyl-3-methylimidazolium difluorophosphate and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt in a mass ratio of 1:3-5; the solvent is a combination of cyclic carbonate and chain carbonate.

2. The electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries according to claim 1, characterized in that, The lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium dioxaborate (LiBOB), lithium difluorooxaborate (LiODFB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorophosphate (LiPO2F2), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

3. The electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries according to claim 2, characterized in that, The lithium salt is selected from lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonylimide) (LiTFSI), and lithium difluorophosphate (LiPO2F2).

4. The method for preparing the electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries according to claim 3, characterized in that, The mass ratio of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium difluorophosphate (LiPO2F2) is 1:0.3-1.2:0.3-1.

2.

5. The electrolyte for improving the ultra-low temperature performance of lithium iron phosphate batteries according to claim 1, characterized in that, The solvent is a combination of cyclic carbonates and chain carbonates, with a mass ratio of cyclic carbonate to chain carbonate of 5:95-40:

60.

6. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1-5 for improving the ultra-low temperature performance of lithium iron phosphate batteries.

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