High-performance low-temperature electrolyte and application thereof in lithium ion battery

Abstract

The invention belongs to the technical field of low-temperature lithium ion batteries, and discloses a high-performance low-temperature electrolyte and application thereof in a lithium ion battery, the electrolyte comprises 7-8 parts by mass of a main solvent, 1-2 parts by mass of a lithium salt, 1-2 parts by mass of a cosolvent and 0.1-0.2 part by mass of an ion additive; the cosolvent is cyclic fluorocarbonate, and the ion additive is silver salt; the cosolvent and the ion additive are combined to enable the electrolyte to form a stable SEI membrane structure on the negative electrode, the main solvent is chain carboxylic ester, the cosolvent is cyclic fluorocarbonate, and the ion additive is silver salt. The electrolyte provided by the invention has stable charge-discharge cycle performance and relatively high capacity retention ratio under high-rate cycle and low-temperature conditions, the high-rate cycle is 2C, 5C and 10C, and the low temperature is-10 DEG C. According to the invention, the compatibility of the linear carboxylic ester solvent and the carbon-based negative electrode is effectively improved, and the interface reaction kinetics is accelerated, so that the low-temperature performance of the lithium ion battery is remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of low-temperature lithium-ion battery technology, and in particular to a high-performance low-temperature electrolyte and its application in lithium-ion batteries. BACKGROUND

[0002] With the continuous rise of global energy demand, the urgency of developing new clean energy is highlighted. As an advanced energy storage technology, lithium-ion batteries have been widely used in consumer electronics and electric vehicles due to their high energy density, long cycle life, and environmental friendliness. In recent years, their application in extreme environments such as aerospace and polar exploration has also been increasingly valued, which puts higher requirements on the low-temperature performance of batteries. However, in low-temperature environments, lithium-ion batteries generally face problems such as significant capacity decline, poor rate performance, prolonged charging time, and lithium precipitation. For example, currently commercial batteries can usually only release about 30% of the room temperature capacity at -20℃, making it difficult to meet the needs of special application scenarios. Therefore, it is urgent to improve the low-temperature performance of lithium-ion batteries to expand their application potential in special fields.

[0003] Studies have shown that the decline in battery low-temperature performance involves multiple factors, including: 1) reduced electrolyte conductivity leading to slow lithium ion diffusion; 2) significantly increased interfacial charge transfer resistance (R); 3) slower solid-phase diffusion kinetics of lithium ions in electrode materials. As a key component of the battery, the electrolyte not only affects ion migration but also participates in the formation of the solid-state electrolyte interface (SEI) film, which is crucial for low-temperature performance.

[0004] For electrolytes, to meet the excellent electrochemical performance of batteries at low temperatures, the following points must be met: high ionic conductivity, wide electrochemical stability window, low desolvation energy, and formation of low-impedance SEI film. Although linear carboxylic esters have extremely low freezing points (usually <-75℃), their reduction stability is insufficient, and they easily decompose in higher potentials (>1.0 V vs. Li / Li + ) to form thick and high-impedance SEI films, leading to poor compatibility with carbon-based anodes, which severely limits their application in low-temperature lithium-ion batteries. Therefore, developing linear carboxylic ester-based high-performance low-temperature electrolytes compatible with carbon-based anodes is an important research direction for improving the low-temperature performance of batteries. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a high-performance low-temperature electrolyte and its application in lithium ion batteries. The electrolyte aims to solve the compatibility problem of carbon-based negative electrode materials, thereby improving the low-temperature performance of the battery. Specifically, the present application uses a linear carboxylate with a lower freezing point to replace the carbonate solvent with a higher freezing point in the prior art to enhance the conductivity of the electrolyte. At the same time, by introducing specific electrolyte salts and film-forming ion additives, a SEI film with high ionic conductivity and stable structure is constructed on the negative electrode surface. This strategy effectively improves the compatibility of linear carboxylate solvents with carbon-based negative electrodes, accelerates the interface reaction kinetics, and thus significantly improves the low-temperature performance of lithium ion batteries.

[0006] According to a first aspect of the present application, a high-performance low-temperature electrolyte is provided, which comprises 8-9 parts by mass of a main solvent, 1-2 parts by mass of a lithium salt, 1-2 parts by mass of a co-solvent, and 0.1-0.2 parts by mass of an ionic additive. The electrolyte has stable charge-discharge cycle performance under high-rate cycling, and the high-rate cycling is 2C, 5C, and 10C. The electrolyte has a high capacity retention rate under low-temperature cycling, and the low temperature is -10℃.

[0007] Preferably, the main solvent is a linear carboxylate, the co-solvent is a cyclic fluorinated carbonate, and the ionic additive is a silver salt.

[0008] Preferably, the linear carboxylate is ethyl acetate, the cyclic fluorinated carbonate is fluoroethylene carbonate, and the silver salt is silver hexafluorophosphate.

[0009] Preferably, the lithium salt is lithium hexafluorophosphate.

[0010] According to another aspect of the present application, a preparation method of any of the high-performance low-temperature electrolytes is provided. The main solvent and the co-solvent are mixed, and then the lithium salt and the ionic additive are dissolved to obtain the high-performance low-temperature electrolyte. The mass fraction of the main solvent is 8-9, the mass fraction of the lithium salt is 1-2, the mass fraction of the co-solvent is 1-2, and the mass fraction of the ionic additive is 0.1-0.2.

[0011] According to another aspect of the present application, any of the high-performance low-temperature electrolytes is used for graphite negative electrode button lithium batteries.

[0012] According to another aspect of the present application, a graphite negative electrode button lithium battery is provided, which comprises any of the high-performance low-temperature electrolytes.

[0013] The positive electrode of the graphite negative electrode button lithium battery is a graphite negative electrode material, the negative electrode is a lithium sheet, and the positive and negative electrodes are separated by a polyethylene separator. The high-performance low-temperature electrolyte is used as the electrolyte of the graphite negative electrode button lithium battery.

[0014] Overall, compared with the prior art, the above technical solutions conceived by the present application mainly have the following technical advantages: (1) The electrolyte of the present application has a wide electrochemical stability window and low desolvation energy; the reason is that: the electrolyte of the present application adds a co-solvent with a lower LUMO energy level and an ion additive that promotes the decomposition of the co-solvent in the linear carboxylic acid ester solvent, which can preferentially decompose on the surface of graphite to form a dense SEI film with high ionic conductivity, and inhibit the continuous decomposition of the electrolyte in contact with the electrode; at the same time, the use of co-solvent can also improve the oxidation resistance of the electrolyte, thereby widening the electrochemical window. In addition, due to the lower dielectric constant of linear carboxylic acid ester solvent compared to commercial carbonate solvents such as EC, PC, the dielectric constant is reduced, and under the condition of sufficient dissolution of lithium salt, the interaction between the solvent and lithium ion is greatly reduced, and the desolvation energy is reduced.

[0015] (2) The new high-performance low-temperature electrolyte is first dissolved in the main solvent ethyl acetate to obtain a lithium hexafluorophosphate solution, and then the co-solvent ethylene carbonate fluoride is added to the lithium hexafluorophosphate solution to construct a stable solvation structure. The preparation method of the present application is simple and easy to control, and the whole process needs to be prepared in a glove box, and after preparation is completed, it can be used immediately, is not flammable, and can be stored at room temperature.

[0016] (3) The present application creatively solves the compatibility problem of linear carboxylic acid ester and carbon-based negative electrode, because the introduced co-solvent and ion additive can preferentially decompose in the solvent to form a stable interface, inhibit the reduction of the solvent linear carboxylic acid ester, and then overcome the problem of poor reduction stability of linear carboxylic acid ester.

[0017] (4) The present application mixes a new type of silver ion additive and co-solvent in the linear carboxylic acid ester solvent, and constructs a stable SEI film with higher inorganic content, low impedance and high ionic conductivity on the surface of the negative electrode, improves the compatibility of linear carboxylic acid ester solvent with carbon negative electrode, and speeds up the interface reaction kinetics. The high-performance low-temperature electrolyte has a significant improvement effect on the low-temperature charge / discharge performance of lithium ion battery and the high-rate cycle at room temperature.

[0018] (5) The present application preferably prepares a new type of low-temperature electrolyte solution using fluoroethylene carbonate and silver salt as co-solvent and ion additive, ethyl acetate as main solvent, and lithium hexafluorophosphate as lithium salt. The electrolyte solution uses a linear carboxylic acid ester solvent with a low freezing point to replace the traditional carbonate solvent, which reduces the freezing point and viscosity of the electrolyte, improves the ion migration kinetics in the electrolyte at low temperature, and has a good protective effect on the graphite negative electrode. It can form a relatively dense SEI film on the graphite negative electrode. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The SEI scanning electron microscope image of the graphite electrode sheet after 100 cycles of the low-temperature electrolyte of Comparative Example 2, a is a 10 μm electron microscope field, and b is a 1 μm electron microscope field; Figure 2 The SEI scanning electron microscope image of the graphite electrode sheet after 100 cycles of the low-temperature electrolyte of Example 1, a is a 10 μm electron microscope field, and b is a 1 μm electron microscope field; Figure 3 The percentage capacity retention rate graph of the lithium ion half-battery (graphite||Li) assembled using the electrolyte prepared in Example 1, Comparative Example 1 and Comparative Example 2 under different rates; Figure 4 The charge-discharge curve at different temperatures, a is the lithium ion half-battery (graphite||Li) assembled using the electrolyte prepared in Comparative Example 2, and b is the lithium ion half-battery (graphite||Li) assembled using the electrolyte prepared in Example 1 of the application; Figure 5 The long cycle performance graph of the lithium ion half-battery (graphite||Li) assembled using the electrolyte prepared in Example 1, Comparative Example 1 and Comparative Example 2 under 5C high rate. DETAILED DESCRIPTION

[0020] The specific embodiments of the application are described in detail below, but it should be understood that the scope of protection of the application is not limited by the specific embodiments. Based on the examples in the application, all other examples obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the application. The test methods described in the embodiments of the application are conventional methods unless otherwise specified.

[0021] Example 1 A preparation method of a high-performance low-temperature electrolyte, comprising the following steps: In a glove box, ethyl acetate and fluoroethylene carbonate are mixed in a volume ratio of 90:10, then lithium hexafluorophosphate and silver hexafluorophosphate are added in a concentration of 1 mol / L and a molar ratio of 9:1, stirred for 12 h, and mixed uniformly to obtain the electrolyte.

[0022] Example 2 A preparation method of a high-performance low-temperature electrolyte, comprising the following steps: In a glove box, ethyl acetate and fluoroethylene carbonate are mixed in a volume ratio of 90:10, then lithium hexafluorophosphate and silver hexafluorophosphate are added in a concentration of 1 mol / L and a molar ratio of 9:1, stirred for 12 h, and mixed uniformly to obtain the electrolyte.

[0023] The high-performance low-temperature electrolyte prepared in this embodiment 2 has a similar solvation structure to that of embodiment 1, and the cycle efficiency and specific capacity of the electrochemical test are close to the cycle test results of embodiment 1.

[0024] Embodiment 3 A preparation method of a high-performance low-temperature electrolyte includes the following steps: In a glove box, ethyl acetate and fluoroethylene carbonate are mixed in a volume ratio of 95:5, then lithium hexafluorophosphate and silver hexafluorophosphate are added in a molar ratio of 9:1 at a concentration of 1 mol / L, stirred for 12 h, and uniformly mixed to obtain the electrolyte.

[0025] The high-performance low-temperature electrolyte prepared in this embodiment 4 has a similar solvation structure to that of embodiment 1, and the cycle efficiency and specific capacity of the electrochemical test are close to the cycle test results of embodiment 1.

[0026] Embodiment 4 A preparation method of a high-performance low-temperature electrolyte includes the following steps: In a glove box, ethyl acetate and fluoroethylene carbonate are mixed in a volume ratio of 85:15, then lithium hexafluorophosphate and silver hexafluorophosphate are added in a molar ratio of 9:1 at a concentration of 1 mol / L, stirred for 12 h, and uniformly mixed to obtain the electrolyte.

[0027] The high-performance low-temperature electrolyte prepared in this embodiment 4 has a similar solvation structure to that of embodiment 1, and the cycle efficiency and specific capacity of the electrochemical test are close to the cycle test results of embodiment 1.

[0028] Embodiment 5 A preparation method of a high-performance low-temperature electrolyte includes the following steps: In a glove box, ethyl acetate and fluoroethylene carbonate are mixed in a volume ratio of 90:10, then lithium hexafluorophosphate and silver hexafluorophosphate are added in a molar ratio of 95:5 at a concentration of 1 mol / L, stirred for 12 h, and uniformly mixed to obtain the electrolyte.

[0029] The high-performance low-temperature electrolyte prepared in this embodiment 5 has a similar solvation structure to that of embodiment 1, and the cycle efficiency and specific capacity of the electrochemical test are close to the cycle test results of embodiment 1.

[0030] Comparative example 1 A preparation method of an electrolyte includes the following steps: In a glove box, ethylene carbonate and methyl ethyl carbonate are mixed in a volume ratio of 3:7, then lithium hexafluorophosphate is added at a concentration of 1 mol / L, stirred for 12 h, and uniformly mixed to obtain the electrolyte. Comparative example 1 is one of the currently commercial electrolytes.

[0031] Comparative Example 2 A preparation method of electrolyte, comprising the following steps: Ethyl acetate and fluoroethylene carbonate were mixed in a volume ratio of 90:10, then lithium hexafluorophosphate was added at a concentration of 1 mol / L, stirred for 12 h, and mixed uniformly to obtain the electrolyte.

[0032] Test results and discussion The electrolyte was assembled in a lithium ion half-cell and electrochemical performance test was carried out: taking graphite electrode as working electrode (wherein the mass ratio of graphite powder, conductive agent and binder was 8:1:1), taking lithium sheet as counter electrode, adding 70 μL of electrolyte of Examples 1-5 and Comparative Examples 1 and 2, and assembling into a button cell.

[0033] The test condition was constant current charge and discharge, and the charge and discharge voltage range of Examples 1, Comparative Examples 1 and 2 was 0.005 V-1 V: the assembled battery was cycled at room temperature at a current density of 0.1 C, 0.2 C, 0.5 C, 2 C, 5 C and 10 C for 5 cycles respectively; the low temperature charge capacity was tested: the assembled battery was cycled at room temperature at a rate of 0.1 C for 5 cycles, then was placed at different temperatures for 0.1 C charge and discharge to test the low temperature charge and discharge performance: Table 1: Test results of rate cycle charge and discharge performance

[0034] From the test results of the half-cell of Examples 1 and Comparative Examples 1 and 2 in Table 1, it can be seen that the graphite half-cell using the electrolyte of the application in the examples showed much better performance than the graphite half-cell using the conventional electrolyte in the comparative examples, especially at an ultra-high rate of 5 C, the discharge capacity of Example 1 was basically maintained at 70% of the 0.1 C capacity, while the discharge capacity of Comparative Examples 1 and 2 was only 19% and 25% of the 0.1 C capacity.

[0035] Table 2: Test results of low temperature charge and discharge performance

[0036] From the test results of the half-cell of Examples in Table 2, it can be seen that the graphite half-cell using the electrolyte of the application had excellent low temperature charge and discharge performance, and the discharge capacity retention rate reached 75% under the condition of charge and discharge at-10℃, which provided a reference for better work of lithium ion batteries in low temperature environment.

[0037] Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalent technologies, the present application also intends to include these modifications and variations.

Claims

1. A high-performance low-temperature electrolyte, characterized in that, The electrolyte comprises 8-9 parts by mass of a main solvent, 1-2 parts by mass of a lithium salt, 1-2 parts by mass of a co-solvent, and 0.1-0.2 parts by mass of an ionic additive; the electrolyte exhibits stable charge-discharge cycle performance under high-rate cycling, wherein the high-rate cycling is 2C, 5C, and 10C; the electrolyte has high capacity retention under low-temperature cycling, wherein the low-temperature is -10℃.

2. The high-performance low-temperature electrolyte according to claim 1, characterized in that, The main solvent is a chain carboxylic acid ester, the co-solvent is a cyclic fluorocarbonate, and the ionic additive is a silver salt.

3. The high-performance low-temperature electrolyte according to claim 2, characterized in that, The chain carboxylic acid ester is ethyl acetate; the cyclic fluorocarbonate is fluoroethylene carbonate; and the silver salt is silver hexafluorophosphate.

4. The high-performance low-temperature electrolyte according to claim 3, characterized in that, The lithium salt is lithium hexafluorophosphate.

5. A method for preparing a high-performance low-temperature electrolyte, characterized in that, The main solvent and co-solvent are mixed, and then lithium salt and ionic additives are added to dissolve them to obtain the high-performance low-temperature electrolyte; the main solvent has a mass fraction of 8-9, the lithium salt has a mass fraction of 1-2, the co-solvent has a mass fraction of 1-2, and the ionic additive has a mass fraction of 0.1-0.

2.

6. The method for preparing the high-performance low-temperature electrolyte according to claim 5, characterized in that, The main solvent is a chain carboxylic acid ester, the co-solvent is a cyclic fluorocarbonate, and the ionic additive is a silver salt.

7. The method for preparing the high-performance low-temperature electrolyte according to claim 6, characterized in that, The chain carboxylic acid ester is ethyl acetate; the cyclic fluorocarbonate is fluoroethylene carbonate; and the silver salt is silver hexafluorophosphate.

8. The method for preparing the high-performance low-temperature electrolyte according to claim 7, characterized in that, The lithium salt is lithium hexafluorophosphate.

9. A graphite negative electrode coin cell lithium battery, characterized in that, Includes the high-performance low-temperature electrolyte as described in any one of claims 1-4.

10. The graphite negative electrode coin cell lithium battery according to claim 9, characterized in that, The positive electrode of the graphite negative electrode coin cell lithium battery is a graphite negative electrode material, the negative electrode is a lithium sheet, and a polyethylene separator is placed between the positive and negative electrodes. The high-performance low-temperature electrolyte is used as the electrolyte for the graphite negative electrode coin cell lithium battery.

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