Electrolyte with high-temperature and low-temperature and high-rate performance of high-specific-energy battery cell and preparation method of electrolyte

Abstract

The invention discloses an electrolyte with high-low temperature and high rate performance of a high-specific-energy battery cell and a preparation method. The electrolyte comprises a mixed solvent, a mixed additive and a mixed electrolyte salt, the mixed solvent is a combination of non-fluorinated and / or fluorinated carbonic ester and carboxylic ester, the mixed additive comprises a low-temperature additive and a positive and negative electrode film-forming additive, and the mixed electrolyte salt comprises lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide, phosphoric acid lithium salt and boric acid lithium salt; the electrolyte is applied to the ternary high-specific-energy soft-package lithium ion battery, so that the ternary high-specific-energy soft-package lithium ion battery can work in an ultralow-temperature environment lower than-40 DEG C, and meanwhile, the high-temperature performance is not damaged. According to the electrolyte with high and low temperature and high rate performance of the high-specific-energy battery cell and the preparation method, the electrolyte is suitable for the ternary high-specific-energy lithium ion soft package battery, continuous rate discharge can be carried out at-40 DEG C or below, and good comprehensive performance such as high temperature and high rate is taken into account.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrolytes, and specifically relates to a non-aqueous electrolyte that takes into account the ultra-low temperature and high temperature and high rate performance of a ternary high-energy-density soft-pack lithium-ion battery, a preparation method, and an application in a lithium-ion battery. Background Art

[0002] With the vigorous development of the new energy industry, people have increasingly stringent requirements on the use and performance of lithium-ion batteries. At present, lithium-ion batteries have been widely used in electronic products such as mobile phones, laptops, digital cameras, and are also gradually being widely used as power batteries in electric bicycles, model airplanes, and electric vehicles, especially in the military and aerospace fields, which requires lithium-ion batteries to have both high and low temperature and high rate discharge performance.

[0003] In lithium-ion batteries, the electrolyte is a very critical component with the following main functions: 1) Ion conduction. The electrolyte provides a channel for lithium ions to move between the positive and negative electrodes, allowing lithium ions to shuttle back and forth between the electrodes during the battery's charge and discharge process; 2) Participation in electrode reactions. During the operation of the battery, the electrolyte will participate in the electrode reaction to a certain extent; for example, it can help form a stable solid electrolyte interface (SEI) film on the electrode surface; this film can protect the electrode material and prevent the electrode material from being destroyed by excessive reaction with the electrolyte. At the same time, it is also beneficial to the conduction of lithium ions, just like putting a layer of "protective clothing" on the electrode; 3) Ensure the balance of the battery's electrochemical reaction. As a medium inside the battery, the electrolyte can balance the charge between the positive and negative electrodes and ensure that the electrochemical reaction inside the battery can proceed continuously and stably.

[0004] As a carrier of ion transmission in lithium batteries, electrolyte plays a vital role in the performance of all aspects of lithium-ion batteries. However, current lithium-ion electrolytes can hardly take into account both high-energy-density battery cells' high and low-temperature performance and high-rate performance, which to some extent limits the widespread application of lithium-ion batteries.

[0005] The electrolyte of ternary high specific energy soft pack lithium-ion battery that takes into account ultra-low temperature, high temperature and high rate mainly needs to solve the following technical problems: 1. Balance between high rate performance and low temperature performance: During high rate charge and discharge, the electrode material needs to quickly embed and extract lithium ions. However, at low temperatures, the diffusion rate of lithium ions in the electrode material will drop significantly. To solve this problem, it is necessary to find or optimize the electrode material so that it can maintain high rate performance while ensuring rapid diffusion of lithium ions in low temperature environments.

[0006] 2. Stability of electrode materials at high temperatures: High-rate discharge will generate a lot of heat, and heat accumulation is more serious in high-temperature environments. This requires that the electrode material can still maintain good chemical stability and structural integrity under the dual effects of high temperature and high rate.

[0007] 3. Coordination of ion transport and low temperature performance at high rate: High rate discharge requires the electrolyte to conduct ions quickly, while low temperature will increase the viscosity of the electrolyte and reduce the ionic conductivity. It is necessary to develop a new electrolyte system and add special additives to improve the ionic conductivity of the electrolyte at low temperature while meeting the requirements of high rate discharge for ion transport. For example, use low viscosity, high ionic conductivity organic solvents and add additives that help reduce the activation energy of ion migration.

[0008] 4. Ensure the stability of the electrolyte at high temperature and high rate: When charging and discharging at high rates and in a high temperature environment, the electrolyte is prone to decomposition and oxidation-reduction reactions, which can cause battery performance degradation and safety issues. Therefore, it is necessary to look for electrolytes with high boiling points, high flash points, and good thermal stability, and ensure that the electrolyte has good compatibility with the electrode material under high rate working conditions to avoid side reactions.

[0009] 5. Relationship between internal resistance change and temperature at high rate: High rate charge and discharge will increase the internal resistance of the battery, and temperature change will also affect the internal resistance. It is a key issue to reduce the internal resistance of the battery at high rate while taking into account high and low temperatures.

[0010] Therefore, it is of great significance to develop electrolytes and lithium-ion batteries that can withstand both high and low temperatures while achieving high-rate discharge performance. Summary of the invention

[0011] Purpose of the invention: In order to overcome the above shortcomings, the purpose of the present invention is to provide an electrolyte and a preparation method that take into account both the high and low temperature and high rate performance of high-energy-density batteries. The electrolyte has a reasonable design and is suitable for ternary high-energy-density lithium-ion soft-pack batteries. It can be continuously discharged at a rate of -40°C or below, and has good high-temperature, high-rate and other comprehensive performances, and has broad application prospects.

[0012] The objective of the present invention is achieved through the following technical solutions: An electrolyte that takes into account both high-low temperature and high-rate performance of a high-energy-density battery cell, the electrolyte comprising the following raw materials in percentage by mass: 70-90% of a mixed solvent, wherein the mixed solvent is a combination of non-fluorinated and / or fluorinated carbonates and carboxylates; Mixed additives 0-10%, the mixed additives include low temperature additives, positive and negative electrode film-forming additives; 10-30% of mixed electrolyte salt, wherein the mixed electrolyte salt includes lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, phosphoric acid lithium salt, and boric acid lithium salt.

[0013] Preferably, the carbonate is one or more of a cyclic carbonate, a linear carbonate, a fluorinated cyclic carbonate, and a fluorinated linear carbonate. Preferably, the carboxylate is one or more of a linear carboxylate, a fluorinated carboxylate (more preferably a fluorinated linear carboxylate).

[0014] Preferably, the low-temperature additive is one of tris(trimethylsilyl) phosphate and fluoroethylene carbonate, or a combination of two of them; the positive and negative electrode film-forming additives are one of vinyl sulfate, vinylene carbonate, 1,3-propane sultone, tripropynyl phosphate, and citraconic anhydride, or a combination of more of them.

[0015] Preferably, the mixed additive also includes a water-removing and acid-suppressing additive.

[0016] More preferably, the water-removing and acid-inhibiting additive is one or a combination of p-toluenesulfonyl isocyanate, trimethylsilyl imidazole, trimethylsilyl oxazolidinone, dimethyldiphenoxysilane, and N-acetylcaprolactam.

[0017] Preferably, the phosphate lithium salt is one or more of lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium phosphate, etc. The borate lithium salt is one or more of lithium tetrafluoroborate, lithium bisoxalate borate, lithium difluorooxalate borate.

[0018] The present invention improves the lithium ion conductivity by increasing the electrolyte salt content, constructs a stable SEI film with high ionic conductivity on the surface of the silicon negative electrode by adding a multi-component additive, improves the compatibility of the mixed solvent with the carbon-based negative electrode, accelerates the interface reaction kinetics, and thus takes into account the ultra-low temperature and high temperature, high rate performance of the lithium ion battery.

[0019] Furthermore, in the above-mentioned electrolyte that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, the mixed solvent includes a linear carbonate with a freezing point below 4.6°C and a linear carboxylic acid ester with a freezing point below -73.5°C.

[0020] The present invention uses linear carbonate with a freezing point lower than 4.6°C and linear carboxylic acid ester with a freezing point lower than -73.5°C as solvents to replace or partially replace carbonate with a high freezing point as solvent, thereby reducing the viscosity of the electrolyte.

[0021] Furthermore, in the above-mentioned electrolyte that takes into account both high and low temperature performance and high rate performance of the high specific energy battery cell, the mixed solvent includes fluorinated carboxylic acid ester.

[0022] The present invention also adjusts the Li by adjusting the ratio of the fluorinated carboxylic acid ester.+ The desolvation process improves the antioxidant properties of the electrolyte and optimizes the stability of SEI.

[0023] Preferably, the amount of the fluorinated carboxylic acid ester added is 5% of the total mass of the electrolyte.

[0024] Furthermore, the above-mentioned electrolyte that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, the mixed solvent includes the following raw materials in percentage by mass: 0-30% cyclic carbonate, 20-70% linear carbonate, 6-13% fluorinated carbonate, 10-40% linear carboxylate, and 0-30% fluorinated carboxylate.

[0025] Furthermore, the above-mentioned electrolyte that takes into account both high and low temperature and high rate performance of high specific energy battery cells, the mixed additive includes the following raw materials in mass percentage: tris(trimethylsilyl) phosphate 0-1%, vinyl sulfate 1-2%, tripropynyl phosphate 0-0.2%, citraconic anhydride 0-0.2%, vinylene carbonate 0-1%, 1,3-propane sultone 0.5%-2%, and p-toluenesulfonyl isocyanate 0-0.5%.

[0026] Furthermore, the electrolyte that takes into account both high-low temperature and high-rate performance of the high-energy-density battery cell, the mixed electrolyte salt comprises: Lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, with a total mass molar ratio of 1-1.6M; The mixed electrolyte salt also includes the following raw materials in percentage by mass: Lithium difluorophosphate 0.5-1%, lithium tetrafluoroborate 0-0.5%, lithium difluorobisoxalate phosphate 0-0.2%, lithium difluorooxalate borate 0-0.5%.

[0027] The present invention also relates to a method for preparing the electrolyte that takes into account both the high and low temperature performance and high rate performance of the high energy-density battery cell, comprising the following contents: in an inert atmosphere glove box with a water content and an oxygen content both lower than 1 ppm, carbonates and carboxylates are mixed to form a mixed solvent; phosphate lithium salts, positive and negative electrode film-forming additives, and lithium bis(fluorosulfonyl)imide are added to the mixed solvent to form a mixed solution; the solution is frozen at -20°C for 1-3 hours, and lithium hexafluorophosphate is quickly added to the mixed solution; the solution is stirred evenly, and the physical and chemical properties are tested, wherein the physical and chemical properties include water content and free acid; after the physical and chemical properties are qualified, a low-temperature additive and a boric acid lithium salt are added, and the solution is stirred evenly to obtain the electrolyte that takes into account both the high and low temperature performance and high rate performance of the high energy-density battery cell.

[0028] Preferably, after the physical and chemical properties are qualified, in addition to adding low-temperature additives and boric acid lithium salts, water-removing and acid-inhibiting additives can also be added.

[0029] The present invention also relates to the application of the electrolyte that takes into account both the high and low temperature performance and high rate performance of high specific energy battery cells. The electrolyte that takes into account both the high and low temperature performance and high rate performance of high specific energy battery cells is applied to lithium ion batteries.

[0030] Furthermore, the application of the above-mentioned electrolyte that takes into account both high and low temperature and high rate performance of high-energy-density batteries, the electrolyte is applied to ternary high-energy-density soft-pack lithium-ion batteries, and can be continuously discharged at -40°C and below, so that the ternary high-energy-density soft-pack lithium-ion batteries can operate in an ultra-low temperature environment below -40°C without compromising high temperature performance.

[0031] Furthermore, the above-mentioned application of the electrolyte that takes into account both the high and low temperature performance and high rate performance of the high-energy-density battery cell, the ternary high-energy-density soft-pack lithium-ion battery is a battery system in which the negative electrode material system has a silicon content of more than 20% and the positive electrode material system has a nickel content of more than 92%.

[0032] Compared with the prior art, the present invention has the following beneficial effects: (1) The electrolyte disclosed in the present invention has both high-low temperature and high-rate performance of high-energy-density battery cells, and is reasonably designed. It uses linear carbonates with a freezing point below 4.6°C and linear carboxylates with a freezing point below -73.5°C as solvents to replace or partially replace carbonate solvents with high freezing points, thereby reducing the viscosity of the electrolyte. The Li + Optimize the desolvation process, improve the antioxidant properties of the electrolyte, and take into account the stability of SEI; improve the lithium ion conductivity by increasing the electrolyte salt content, add multi-component additives to build a stable SEI film with high ion conductivity on the surface of the silicon negative electrode, improve the compatibility of the mixed solvent and the carbon-based negative electrode, and accelerate the interface reaction kinetics, thereby taking into account the ultra-low temperature and high temperature, high rate performance of lithium-ion batteries; (2) The present invention proposes a method for preparing an electrolyte that takes into account both high-low temperature and high-rate performance of a high-energy-density battery cell. The preparation method is simple and highly flexible, and has application prospects for large-scale industrial production. (3) The application of the electrolyte proposed in the present invention that takes into account both the high and low temperature performance and high rate performance of high specific energy battery cells is suitable for ternary high specific energy lithium-ion soft-pack batteries, which can be continuously discharged at a rate of -40°C or below, and has good comprehensive performance such as high temperature and high rate. DETAILED DESCRIPTION

[0033] The following examples 1-5 and comparative examples 1-2 are combined with specific experimental data to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of the present invention. The following Examples 1-5 provide an electrolyte suitable for a ternary high-energy-density soft-pack lithium-ion battery and a preparation method.

[0034] Example 1 The electrolyte of Example 1 includes: a mixed solvent, a mixed electrolyte salt, and a mixed additive. This Example 1 is applied to a 1.5Ah soft-pack lithium-ion battery whose negative electrode material system has a silicon content of more than 20% and whose positive electrode material system has a nickel content of more than 92%.

[0035] The preparation method of the electrolyte suitable for the ternary high specific energy soft-pack lithium ion battery of the present embodiment 1 is as follows: in an inert atmosphere glove box with a water content and an oxygen content both lower than 1 ppm, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), and ethyl propionate (EP) are mixed in a mass ratio of 12:13:12.5:12.5:50 to prepare a mixed solvent accounting for 77.2% of the total mass of the electrolyte; then, 0.8% of the total mass of the electrolyte is added to (lithium difluorophosphate) LiPO2F2, 1% of the total mass of the electrolyte, and 1.5% of the total mass of the electrolyte. The electrolyte was prepared by mixing DTD, VC, and LiFSI, accounting for 0.5% of the total mass of the electrolyte, and then freezing the electrolyte at -20°C for 2h, and then quickly adding lithium hexafluorophosphate (LiPF6) accounting for 11% of the total mass of the electrolyte to the mixed solution; after stirring evenly, testing the physical and chemical properties such as water content and free acid; when the physical and chemical properties such as water content and free acid were qualified, adding TMSP, accounting for 0.5% of the total mass of the electrolyte, and LiODFB, accounting for 0.5% of the total mass of the electrolyte, and stirring evenly, the electrolyte of Example 1 was obtained.

[0036] Example 2 The electrolyte of Example 2 includes: a mixed solvent, a mixed electrolyte salt, and a mixed additive. This Example 2 is applied to a 1.5Ah soft-pack lithium-ion battery whose negative electrode material system has a silicon content of more than 20% and whose positive electrode material system has a nickel content of more than 92%.

[0037] The preparation method of the electrolyte suitable for the ternary high specific energy soft-pack lithium ion battery of the present embodiment 2 is as follows: in an inert atmosphere glove box with a water content and an oxygen content both lower than 1 ppm, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), ethyl propionate (EP), and ethyl trifluoroacetate (ETFA) are mixed in a mass ratio of 12:13:12.5:12.5:45:5 to prepare a mixed solvent accounting for 77.2% of the total mass of the electrolyte; and then (difluorolithium phosphate) LiPO2F2 accounting for 0.8% of the total mass of the electrolyte, (1,3-propane sultone) PS accounting for 1% of the total mass of the electrolyte, and (difluorolithium phosphate) LiPO2F2 accounting for 0.8% of the total mass of the electrolyte, (1,3-propane sultone) PS accounting for 1% of the total mass of the electrolyte, and (difluorolithium phosphate) LiPO2F2 accounting for 0.8% of the total mass of the electrolyte, and (difluorolithium phosphate) PS accounting for 0.8% of the total mass of the electrolyte are added. 1.5% (vinyl sulfate) DTD, 0.5% (vinyl carbonate) VC, and 7% (lithium bis(fluorosulfonyl)imide) LiFSI of the total mass of the electrolyte are prepared to obtain a mixed solution; frozen at -20°C for 2h, and lithium hexafluorophosphate (LiPF6) accounting for 11% of the total mass of the electrolyte is quickly added to the above mixed solution; after stirring evenly, the physical and chemical properties such as water content and free acid are tested; when the physical and chemical properties such as water content and free acid are qualified, 0.5% (tri(trimethylsilyl) phosphate) TMSP of the total mass of the electrolyte and 0.5% (lithium difluorooxalatoborate) LiODFB of the total mass of the electrolyte are added, and after stirring evenly, the electrolyte of Example 2 is obtained.

[0038] Example 3 The electrolyte of Example 3 includes: a mixed solvent, a mixed electrolyte salt, and a mixed additive. This Example 3 is applied to a 1.5Ah soft-pack lithium-ion battery whose negative electrode material system has a silicon content of more than 20% and whose positive electrode material system has a nickel content of more than 92%.

[0039] The preparation method of the electrolyte suitable for the ternary high specific energy soft-pack lithium ion battery of this embodiment 3 is as follows: in an inert atmosphere glove box with a water content and an oxygen content both lower than 1 ppm, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), ethyl propionate (EP), and ethyl trifluoroacetate (ETFA) are mixed in a mass ratio of 12:13:12.5:12.5:40:10 to prepare a mixed solvent accounting for 77.2% of the total mass of the electrolyte; then, (lithium difluorophosphate) LiPO2F2 accounting for 0.8% of the total mass of the electrolyte, (1,3-propane sultone) PS accounting for 1% of the total mass of the electrolyte, and (1,3-propane sultone) PS accounting for 1% of the total mass of the electrolyte are added. 1.5% (vinyl sulfate) DTD, 0.5% (vinyl carbonate) VC, and 7% (lithium bis(fluorosulfonyl)imide) LiFSI of the total mass of the electrolyte were added to obtain a mixed solution; the mixture was frozen at -20°C for 2h, and lithium hexafluorophosphate (LiPF6) accounting for 11% of the total mass of the electrolyte was quickly added to the mixed solution; after stirring evenly, the physical and chemical properties such as water content and free acid were tested; when the physical and chemical properties such as water content and free acid were qualified, 0.5% (tri(trimethylsilyl) phosphate) TMSP of the total mass of the electrolyte and 0.5% (lithium difluorooxalate borate) LiODFB of the total mass of the electrolyte were added, and after stirring evenly, the electrolyte of Example 3 was obtained.

[0040] Example 4 The electrolyte of Example 4 includes: a mixed solvent, a mixed electrolyte salt, and a mixed additive. This Example 4 is applied to a 1.5Ah soft-pack lithium-ion battery whose negative electrode material system has a silicon content of more than 20% and whose positive electrode material system has a nickel content of more than 92%.

[0041] The preparation method of the electrolyte suitable for the ternary high specific energy soft-pack lithium ion battery of this embodiment 4 is as follows: in an inert atmosphere glove box with a water content and an oxygen content both lower than 1 ppm, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), ethyl propionate (EP), and ethyl trifluoroacetate (ETFA) are mixed in a mass ratio of 12:13:12.5:12.5:35:15 to prepare a mixed solvent accounting for 77.2% of the total mass of the electrolyte; then, (lithium difluorophosphate) LiPO2F2 accounting for 0.8% of the total mass of the electrolyte, (1,3-propane sultone) PS accounting for 1% of the total mass of the electrolyte, and (1,3-propane sultone) PS accounting for 1% of the total mass of the electrolyte are added. 1.5% (vinyl sulfate) DTD, 0.5% (vinyl carbonate) VC, and 7% (lithium bis(fluorosulfonyl)imide) LiFSI of the total mass of the electrolyte were added to prepare a mixed solution; the mixture was frozen at -20°C for 2h, and lithium hexafluorophosphate (LiPF6) accounting for 11% of the total mass of the electrolyte was quickly added to the mixed solution; after stirring evenly, the physical and chemical properties such as water content and free acid were tested; when the physical and chemical properties such as water content and free acid were qualified, 0.5% (tri(trimethylsilyl) phosphate) TMSP of the total mass of the electrolyte and 0.5% (lithium difluorooxalate borate) LiODFB of the total mass of the electrolyte were added, and after stirring evenly, the electrolyte of Example 4 was obtained.

[0042] Example 5 The electrolyte of Example 5 includes: a mixed solvent, a mixed electrolyte salt, and a mixed additive. This Example 5 is applied to a 1.5Ah soft-pack lithium-ion battery whose negative electrode material system has a silicon content of more than 20% and whose positive electrode material system has a nickel content of more than 92%.

[0043] The preparation method of the electrolyte suitable for the ternary high specific energy soft-pack lithium ion battery of this embodiment 5 is as follows: in an inert atmosphere glove box with a water content and an oxygen content both lower than 1 ppm, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), ethyl propionate (EP), and ethyl trifluoroacetate (ETFA) are mixed in a mass ratio of 12:13:12.5:12.5:20:30 to prepare a mixed solvent accounting for 77.2% of the total mass of the electrolyte; then, (lithium difluorophosphate) LiPO2F2 accounting for 0.8% of the total mass of the electrolyte, (1,3-propane sultone) PS accounting for 1% of the total mass of the electrolyte, and (1,3-propane sultone) PS accounting for 1% of the total mass of the electrolyte are added. 1.5% (vinyl sulfate) DTD, 0.5% (vinyl carbonate) VC, and 7% (lithium bis(fluorosulfonyl)imide) LiFSI of the total mass of the electrolyte were added to prepare a mixed solution; the mixture was frozen at -20°C for 2h, and lithium hexafluorophosphate (LiPF6) accounting for 11% of the total mass of the electrolyte was quickly added to the mixed solution; after stirring evenly, the physical and chemical properties such as water content and free acid were tested; when the physical and chemical properties such as water content and free acid were qualified, 0.5% (tri(trimethylsilyl) phosphate) TMSP of the total mass of the electrolyte and 0.5% (lithium difluorooxalatoborate) LiODFB of the total mass of the electrolyte were added, and after stirring evenly, the electrolyte of Example 5 was obtained.

[0044] The following comparative examples 1-2 provide an electrolyte suitable for a ternary high-energy-density soft-pack lithium-ion battery.

[0045] Comparative Example 1 The electrolyte of Comparative Example 1 is a commercial electrolyte purchased from the market, which is composed of a mixed organic solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) accounting for 71.2% of the total mass of the electrolyte in a mass ratio of 20:50:30, and then mixed with lithium hexafluorophosphate (LiPF6) accounting for 8.75% of the total mass of the electrolyte, lithium difluorophosphate (LiPO2F2) accounting for 0.8% of the total mass of the electrolyte, 1,3-propane sultone (PS) accounting for 1% of the total mass of the electrolyte, vinyl sulfate (DTD) accounting for 1% of the total mass of the electrolyte, fluoroethylene carbonate (FEC) accounting for 10% of the total mass of the electrolyte, lithium bis(fluorosulfonyl)imide (LiFSI) accounting for 6.75% of the total mass of the electrolyte, and lithium difluorooxalatoborate (LiODFB) accounting for 0.5% of the total mass of the electrolyte.

[0046] This comparative example 1 is applied to a 1.5Ah soft-pack lithium-ion battery in which the negative electrode material system has a silicon content exceeding 20% ​​and the positive electrode material system has a nickel content exceeding 92%.

[0047] Comparative Example 2 The electrolyte of Comparative Example 2 is the same as the electrolyte of Comparative Example 1.

[0048] This comparative example 2 is applied to a 20Ah soft-pack lithium-ion battery in which the negative electrode material system has a silicon content exceeding 20% ​​and the positive electrode material system has a nickel content exceeding 92%.

[0049] Effect verification The electrochemical performance of the electrolytes of Examples 1-5 and Comparative Examples 1-2 was evaluated as follows: Take 60 soft-pack lithium-ion batteries of the same specification and capacity of 1.5Ah to be injected, wherein the positive electrode main material of the soft-pack lithium-ion battery is Ni93 ternary material, the negative electrode main material is artificial graphite and silicon oxide material, and the positive electrode and the negative electrode also contain auxiliary materials such as conductive agent and binder. Inject the same amount of electrolyte of Examples 1-5 and Comparative Example 1 into each of the above 10 soft-pack lithium-ion batteries to prepare finished batteries.

[0050] At the same time, 10 soft-pack lithium-ion batteries of the same specification and capacity of 20Ah to be filled with liquid are taken, wherein the positive electrode main material of the soft-pack lithium-ion battery is Ni93 ternary material, the negative electrode main material is artificial graphite and silicon oxide material, and the positive electrode and the negative electrode also contain auxiliary materials such as conductive agent and binder. A specified amount of electrolyte of comparative example 2 is injected into the above 10 soft-pack lithium-ion batteries respectively to make finished batteries.

[0051] The electrochemical properties of the above batteries were tested under the same conditions. The test items included: battery capacity and initial efficiency test, room temperature AC impedance test, low temperature discharge test, rate charge test, rate discharge test and 55°C storage test.

[0052] 1) Battery capacity and first efficiency test: After the battery is formed, it is aged at 45℃ for 24h, then fully charged at 0.33C constant current and constant voltage, with the upper limit voltage of charging being 4.25V and the cut-off current being 0.05C. After being left alone for 10min, it is discharged at 0.33C constant current to 2.5V, and cycled for 2 times. The first discharge capacity and first efficiency are calculated. The test results are shown in Table 1.

[0053] 2) Room temperature AC impedance test: The battery was fully charged at 0.33C constant current and constant voltage, with the upper limit voltage of charging being 4.25V and the cut-off current being 0.05C. After being left for 10 minutes, it was discharged at 0.33C constant current to 50% SOC and left for 30 minutes. The battery AC impedance was tested with a 1000KHZ AC internal resistance tester. The test results are shown in Table 2.

[0054] 3) -40℃ 2C DCR test: The battery is fully charged at 0.33C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, and after 10 minutes of storage, it is discharged at 0.33C constant current to 50% SOC, stored for 30 minutes, and discharged at 2C constant current for 10S. The -40℃ 2C DCR is calculated. The test results are shown in Table 2.

[0055] 4) Low temperature discharge test: First, at 25°C, use a current of 0.33C / 1C and a voltage range of 2.5-4.25V to set the battery capacity. Then fully charge it, place it in a 20°C constant temperature box for 7 hours, and then discharge it at a constant current of 1C to 2.0V to obtain the -20°C discharge capacity. The same method was used to test the 40°C discharge capacity. The test results are shown in Table 3.

[0056] 5) Rate charging test: at 25°C, fully charged with 0.33C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, and it is left for 10 minutes. Then, the battery is discharged with 0.33C constant current to 2.5V, and it is left for 10 minutes, and it is cycled for 2 cycles; fully charged with 1C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, and it is left for 10 minutes. Then, the battery is discharged with 0.33C constant current to 2.5V, and it is left for 10 minutes, and it is cycled for 2 cycles. The test results are shown in Table 4.

[0057] 6) Rate discharge test: At 25°C, fully charge at 0.33C constant current and constant voltage, the upper limit voltage of charge is 4.25V, the cut-off current is 0.05C, leave for 10min, discharge at 0.33C constant current to 2.5V, leave for 10min, cycle 2 times; fully charge at 0.33C constant current and constant voltage, the upper limit voltage of charge is 4.25V, the cut-off current is 0.05C, leave for 10min, discharge at 1C constant current to 2.5V, leave for 10min, cycle 2 cycles; fully charged at 0.33C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, leave for 10min, discharge at 2C constant current to 2.5V, leave for 10min, cycle 2 cycles; fully charged at 0.33C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, leave for 10min, discharge at 3C constant current to 2.5V, leave for 10min, cycle 2 cycles; the test results are shown in Table 4.

[0058] 7) AC impedance test after storage at 55°C: After storage, the battery was taken out and cooled at room temperature for 2 hours. The battery AC impedance was tested using a 1000KHZ AC internal resistance tester. The test results are shown in Table 5.

[0059] 8) 55℃ storage test: At 25℃, fully charge with 1C constant current and constant voltage, the upper limit voltage of charge is 4.25V, the cut-off current is 0.05C, leave for 10min, discharge with 1C constant current to 2.5V, leave for 10min, and cycle 3 times; after that, put it in a 55℃ air drying oven for 7 days, take it out and cool it to room temperature; then at 25℃, discharge with 1C constant current to 2.5V, leave for 10min, fully charge with 1C constant current and constant voltage, the upper limit voltage of charge is 4.25V, the cut-off current is 0.05C, leave for 10min, and cycle 3 times, calculate the discharge capacity retention rate and discharge capacity recovery rate, and the test results are shown in Table 5.

[0060] Table 1 serial number First effect% Capacity (Ah) Comparative Example 1 82.7 1.50 Comparative Example 2 83.4 20.0 Example 1 82.7 1.51 Example 2 82.2 1.50 Example 3 81.8 1.50 Example 4 82.0 1.52 Example 5 81.0 1.50 Table 2 serial number Normal temperature 50% SOC AC impedance (mΩ) -40℃ 2C DCR (mΩ) Comparative Example 1 18.54 671.29 Comparative Example 2 1.90 53.47 Example 1 17.86 578.40 Example 2 18.38 595.60 Example 3 19.35 639.07 Example 4 22.07 764.35 Example 5 38.03 859.85 Table 3 Table 4 Table 5 From the electrical performance test data in Table 1-5, we can see that: (1) Comparison of the electrolytes of Examples 1-5 shows that the addition of fluorinated carboxylic acid ester has no significant adverse effect on the capacity and initial efficiency of the battery. Increasing the amount of fluorinated carboxylic acid ester increases the AC impedance and -40°C DCR of the battery, reduces the low-temperature discharge capacity retention rate and rate charge and discharge performance, and improves the high-temperature storage performance.

[0061] (2) Comparison of the electrolytes of Comparative Examples 1-2 shows that the battery capacity has a great influence on the low-temperature discharge capacity and high-temperature storage performance. The larger the capacity, the better the low-temperature performance, and the worse the high-temperature impedance and capacity retention rate.

[0062] (3) Comparison of the electrolytes of Examples 1-5 shows that increasing the content of fluorinated carboxylate to 5% of the entire solvent system can take into account the low-temperature, rate and high-temperature performances. Further increasing the content of fluorinated carboxylate ETFA to 10%-30% of the entire solvent system significantly reduces the low-temperature and rate performances, while improving the high-temperature performance. This indicates that the content of fluorinated carboxylate ETFA has an important influence on the battery performance, and the optimal addition ratio is 5% of the entire solvent system.

[0063] In summary, the present invention provides a ternary high specific energy soft-pack lithium ion battery electrolyte and a preparation method thereof that takes ultra-low temperature, high temperature and high rate into consideration. Compared with the prior art, the present invention has the following beneficial effects: 1. High ion conductivity 1) At low temperatures: The electrolyte of the present invention uses a high content of a new type of mixed electrolyte salt, including bis(fluorosulfonyl)imide lithium salt, phosphate lithium salt, and boric acid lithium salt, which has a high dissociation characteristic, can promote the rapid and stable migration of the electrolyte at low temperatures, and make the battery have good charge and discharge performance at low temperatures.

[0064] 2) At high temperatures: By optimizing the electrolyte composition, the boiling point and flash point of the electrolyte can be increased, the viscosity at high temperatures can be reduced, and the resistance to ion migration caused by increased temperature can be reduced, thereby improving the ion conduction efficiency of the battery at high temperatures and achieving high-rate charge and discharge cycles.

[0065] 2. Stable electrode interface 1) At low temperatures: The electrolyte of the present invention can make the passivation layer (SEI film) formed on the electrode surface more stable and have lower impedance at low temperatures. For example, ethyl trifluoroacetate electrolyte can accelerate the deposition rate of lithium ions on the electrode surface, while ensuring the stability of the SEI film at low temperatures, preventing it from excessively thickening and causing a decrease in lithium ion conductivity, thereby ensuring the low temperature performance of the battery.

[0066] 2) At high temperatures: The use of additives can enhance the stability of the electrode interface, form a good interface contact with the high-nickel positive electrode surface, promote the insertion and extraction of lithium ions, enable the silicon negative electrode to form a dense SEI film, slow down the decomposition and destruction of the SEI film, and inhibit expansion, thereby improving the battery's cycle performance and storage safety performance at high temperatures. 3. Good solvation structure and dynamics 1) At low temperatures: The dielectric constant of fluorinated carboxylates is lower than that of commercial carbonate solvents, such as EC and PC. When the solvent is sufficient to dissolve lithium salts, the interaction between the solvent and lithium ions is greatly reduced, the desolvation energy is reduced, and the rapid desolvation kinetics of lithium ions are promoted, so that the battery can maintain good ion transport performance at low temperatures and achieve high-rate discharge.

[0067] 2) At high temperature: The special solvation structure of this electrolyte can improve the thermal stability of the electrolyte, and can remain stable in a high temperature environment, providing a guarantee for the battery to operate under high temperature and high rate conditions.

[0068] The present invention has many specific application paths, and the above is only a preferred embodiment of the present invention. It should be pointed out that the above embodiments are only used to illustrate the present invention, and are not used to limit the protection scope of the present invention. For those of ordinary skill in the art, several improvements can be made without departing from the principle of the present invention, and these improvements should also be regarded as the protection scope of the present invention.

Claims

1. An electrolyte that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The electrolyte comprises the following raw materials in percentage by weight: 70-90% of a mixed solvent, wherein the mixed solvent is a combination of non-fluorinated and / or fluorinated carbonates and carboxylates; Mixed additives 0-10%, the mixed additives include low temperature additives, positive and negative electrode film-forming additives; 10-30% of mixed electrolyte salt, wherein the mixed electrolyte salt includes lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, phosphoric acid lithium salt, and boric acid lithium salt.

2. The electrolyte according to claim 1 that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The mixed solvent includes a linear carbonate ester having a freezing point lower than 4.6°C and a linear carboxylic acid ester having a freezing point lower than -73.5°C.

3. The electrolyte according to claim 1 that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The mixed solvent includes a fluorinated carboxylic acid ester.

4. The electrolyte according to claim 1 that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The mixed solvent comprises the following raw materials in percentage by mass: 0-30% of cyclic carbonate, 20-70% of linear carbonate, 6-13% of fluorinated carbonate, 10-40% of linear carboxylate, and 0-30% of fluorinated carboxylate.

5. The electrolyte according to claim 1 that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The mixed additive comprises the following raw materials in percentage by mass: 0-1% tri(trimethylsilyl) phosphate, 1-2% vinyl sulfate, 0-0.2% tripropynyl phosphate, 0-0.2% citraconic anhydride, 0-1% vinylene carbonate, 0.5%-2% 1,3-propane sultone, and 0-0.5% p-toluenesulfonyl isocyanate.

6. The electrolyte according to claim 1 that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The mixed electrolyte salt comprises: Lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, with a total mass molar ratio of 1-1.6M; The mixed electrolyte salt also includes the following raw materials in percentage by mass: Lithium difluorophosphate 0.5-1%, lithium tetrafluoroborate 0-0.5%, lithium difluorobisoxalate phosphate 0-0.2%, lithium difluorooxalate borate 0-0.5%.

7. The method for preparing an electrolyte having both high-low temperature and high-rate performances for a high-energy-density battery cell according to any one of claims 1 to 6, characterized in that: The method comprises the following contents: in an inert atmosphere glove box with a water content and an oxygen content both lower than 1 ppm, carbonate esters and carboxylates are mixed to prepare a mixed solvent; phosphate lithium salts, positive and negative electrode film-forming additives, and lithium bis(fluorosulfonyl)imide are added to the mixed solvent to prepare a mixed solution; the solution is frozen at -20°C for 1-3 hours, and lithium hexafluorophosphate is quickly added to the mixed solution; the solution is stirred evenly, and the physical and chemical properties are tested, wherein the physical and chemical properties include water content and free acid; after the physical and chemical properties are qualified, low-temperature additives and boric acid lithium salts are added, and the solution is stirred evenly to prepare an electrolyte having both high and low temperature performance and high rate performance of a high specific energy battery cell.

8. The electrolyte according to any one of claims 1 to 6 that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The electrolyte is applied to lithium ion batteries.

9. The electrolyte according to claim 8 that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The electrolyte is applied to a ternary high-energy-density soft-pack lithium-ion battery, and can be discharged at a continuous rate at -40°C or below, so that the ternary high-energy-density soft-pack lithium-ion battery can operate in an ultra-low temperature environment below -40°C without compromising high-temperature performance.

10. The electrolyte according to claim 9 that takes into account both high and low temperature performance and high rate performance of high specific energy battery cells, characterized in that: The ternary high-energy-density soft-pack lithium-ion battery is a battery system in which the negative electrode material system has a silicon content exceeding 20% ​​and the positive electrode material system has a nickel content exceeding 92%.

Images