Wide-temperature-range electrolyte of bis (fluorosulfonyl) imide salt-based solvent as well as preparation method and application of wide-temperature-range electrolyte

Abstract

The invention belongs to the technical field of lithium ion batteries, and relates to a bis (fluorosulfonyl) imide ester-based solvent wide-temperature-range electrolyte as well as a preparation method and application of the bis (fluorosulfonyl) imide ester-based solvent wide-temperature-range electrolyte. According to the invention, the solvation and interface structure of the electrolyte are regulated and controlled by introducing a new anion group (salt), reasonably selecting a multi-component solvent and optimizing the proportion of each component, so that the electrolyte with excellent low-temperature de-intercalation / intercalation performance and high-temperature stability is designed; when the composite material is applied to the lithium ion battery, the wide-temperature performance (-30 DEG C to 60 DEG C) of the battery is successfully improved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries and is a bis(fluorosulfonyl)imide salt ester-based solvent wide temperature range electrolyte and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have outstanding advantages such as high operating voltage, high specific energy, low self-discharge, long cycle life, no memory effect, fast charge and discharge, and no environmental pollution. They are green, environmentally friendly, and high-energy batteries with broad development prospects. With the continuous expansion of the application scenarios of lithium-ion batteries, new application scenarios such as aerospace, military, polar and deep space exploration require lithium-ion batteries to have a wider operating temperature range. Therefore, the development of wide-temperature range lithium-ion battery technology is imminent, and the development of an electrolyte system suitable for wide-temperature range lithium-ion batteries is the key to solving the above problems.

[0003] The working performance of liquid electrolytes is affected by many factors, among which temperature is the main external factor affecting the ion transport performance of electrolytes. Too low working temperature will lead to increased electrolyte viscosity, increased battery internal resistance, decreased battery reversible capacity, changes in electrochemical reaction paths, etc.; while too high working temperature will bring challenges to the electrolyte / electrode interface and electrode material stability, causing problems such as increased side reactions and potential safety hazards. Although solid electrolytes have certain advantages at high temperatures, their specific application requires the innovation of the existing battery production system, which brings additional costs. Therefore, the more feasible solution at this stage is to optimize the composition of liquid electrolytes to solve the problems faced by lithium-ion batteries at high and low temperatures. The ideal wide-temperature range electrolyte should have the characteristics of low viscosity, high dielectric constant, and environmental friendliness. For lithium batteries working at extreme temperatures, the electrolyte also needs to meet the requirements of low melting point, high boiling point, and high flash point to ensure the wide working temperature range and safety of the electrolyte.

[0004] While extreme temperatures pose severe challenges to the operation of lithium-ion batteries, they also bring some new possibilities. For example, at high temperatures, the rate performance of lithium-ion batteries is better than at room temperature, and low-viscosity lithium salts such as LiBOB can also be well dissolved; low-temperature environments may slow down the occurrence of side reactions. After solving the problem of lithium dendrite growth through strategies such as additives, it is expected that a thinner SEI film and CEI film structure will be formed in a low-temperature environment to reduce the consumption of electrolyte. Proper use of the characteristics of electrochemical reactions in different temperature environments provides a new approach to the optimization of lithium-ion batteries. Although impressive research work continues to be reported, the large-scale production and application of lithium-ion batteries in a wide temperature range are still full of challenges. Summary of the invention

[0005] In view of the above problems, the present invention provides a bis(fluorosulfonyl)imide salt ester-based wide temperature range electrolyte and its preparation method and application. The present invention introduces new anionic groups (salts), rationally selects multi-component solvents and optimizes the proportion of each component to regulate the electrolyte solvation and interface structure, and designs an electrolyte with excellent low-temperature desolvation / insertion performance and high-temperature stability. It is applied to lithium-ion batteries and successfully improves the wide temperature performance of the battery (-30℃~60℃).

[0006] The technical solution of the present invention is as follows:

[0007] A bisfluorosulfonyl imide salt ester-based solvent wide temperature range electrolyte consists of an electrolyte salt, a main solvent and a diluent, wherein the electrolyte salt is a bisfluorosulfonyl imide salt, and the main solvent and the diluent are ester-based solvents.

[0008] The bisfluorosulfonyl imide salt is lithium bisfluorosulfonyl imide (LiFSI); the ester-based solvent is a mixture of ethylene carbonate (EC), fluoroethylene carbonate (FEC), diethyl carbonate (DEC) and ethyl acetate (EA); EC and FEC are used as main solvents, DEC and EA are used as diluents; EC accounts for 10% to 25% of the total volume of the ester-based solvent, FEC accounts for 5% to 20% of the total volume of the ester-based solvent, DEC accounts for 15% to 40% of the total volume of the ester-based solvent, and EA accounts for 30% to 55% of the total volume of the ester-based solvent. The molar concentration of LiFSI in the ester-based solvent is 1M.

[0009] The method for preparing the above-mentioned bis(fluorosulfonyl)imide salt ester-based solvent wide temperature range electrolyte comprises the following steps:

[0010] (1) Four ester-based solvents, EC, FEC, DEC, and EA, were uniformly mixed according to a certain ratio to obtain a transparent mixed solvent A.

[0011] (2) Add LiFSI powder to the mixed solvent A and stir evenly to obtain a clear liquid B, which is a bis(fluorosulfonyl)imide salt ester-based solvent with a wide temperature range electrolyte.

[0012] The above preparation processes are all completed in a glove box with water <0.1ppm and oxygen <0.1ppm. The obtained bis(fluorosulfonyl)imide salt ester-based solvent wide temperature range electrolyte can be used as lithium-ion battery electrolyte to meet the needs of mobile devices, new energy vehicles, etc., especially in application scenarios within a wide temperature range (-30℃~60℃).

[0013] The beneficial effects of the present invention are:

[0014] (1) The present invention replaces lithium hexafluorophosphate (LiPF6), the most commonly used lithium salt in existing commercial electrolytes, with LiFSI, which has the advantages of good solubility, high ionic conductivity, low moisture sensitivity, and high stability. This is mainly because in its anion structure, the highly electronegative nitrogen is connected to the sulfur connected to the strong electron-withdrawing group (-F). This structure disperses the electron cloud density in the anion space, making it easier for anions and cations to dissociate, thereby improving the solubility and ionic conductivity of the lithium salt. The electrolyte using LiFSI has a higher fluorine content in the negative electrode SEI and better wide temperature performance.

[0015] (2) The present invention significantly improves the overall performance of the electrolyte under a wide temperature range by mixing a variety of ester-based organic solvents. EC is the most commonly used solvent in lithium-ion batteries. It has a high dielectric constant and can effectively dissolve most lithium salts. At the same time, it can form a uniform and stable SEI layer on the graphite negative electrode. However, its high melting point (34°C) makes it impossible to use it alone. It needs to be mixed with other low-viscosity chain carbonates, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), so as to accelerate the transmission of lithium ions in the electrolyte. Among them, DEC is the best choice among several auxiliary solvents in terms of low-temperature comprehensive properties such as viscosity and freezing point. Compared with EC, the electron density of the oxygen atom in the C=O bond of the FEC molecule is lower, which is similar to LI + The interaction is weaker than that of Li + -EC interaction, thus promoting Li + The desolvation process to overcome the Li + -Li in EC + The problem of slow desolvation process. Secondly, the electron-withdrawing property of fluorine atoms makes FEC less likely to donate electrons, showing better oxidation stability, overcoming the problem of easy oxidation of EC molecules on the positive electrode side, thereby improving the cycle stability of the battery. EA is known for its low freezing point (-84°C) and low viscosity, and is widely used as a low-temperature solvent for lithium-ion batteries. In this system, EA itself can provide a certain conductivity while acting as a diluent to ensure a wide liquid range and low viscosity.

[0016] (3) In summary, the present invention regulates the electrolyte solvation and interface structure by introducing new anionic groups (salts), rationally selecting multi-component solvents and optimizing the ratio of each component, and designs an electrolyte with excellent low-temperature desolvation / embedding performance and high-temperature stability. It is applied to lithium-ion batteries and successfully improves the battery's wide temperature performance (-30℃~60℃). BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1The low temperature -30°C 0.1C cycle performance diagram of a button-type lithium-ion battery in which an electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.15:0.075:0.2875:0.4875 is applied (Example 1);

[0018] Figure 2 The low temperature -50°C 0.1C cycle performance diagram of a button-type lithium-ion battery in which an electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.15:0.075:0.2875:0.4875 is applied (Example 1);

[0019] Figure 3 The low temperature -30°C 0.1C cycle performance diagram of a button-type lithium-ion battery in which an electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.15:0.1:0.325:0.425 is applied (Example 2);

[0020] Figure 4 The low temperature -50°C 0.1C cycle performance diagram of a button-type lithium-ion battery in which an electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.15:0.1:0.325:0.425 is applied (Example 2);

[0021] Figure 5 The low temperature -30°C 0.1C cycle performance diagram of a button-type lithium-ion battery in which an electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.1:0.2:0.4:0.3 is applied (Example 3);

[0022] Figure 6 The low temperature -50°C 0.1C cycle performance diagram of a button-type lithium-ion battery in which an electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.1:0.2:0.4:0.3 is applied (Example 3);

[0023] Figure 7 The low temperature -30°C 0.1C cycle performance diagram of a button-type lithium-ion battery in which an electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.25:0.05:0.15:0.55 is applied (Example 4);

[0024] Figure 8 The low temperature -50°C 0.1C cycle performance diagram of a button-type lithium-ion battery in which an electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.25:0.05:0.15:0.55 is applied (Example 4);

[0025] Fig. 9The high temperature 60°C 1C cycle performance diagram of a button-type lithium-ion battery in which a wide temperature range electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.15:0.075:0.2875:0.4875 is applied (Example 1);

[0026] Fig.10 The high temperature 60°C 1C cycle performance diagram of a button-type lithium-ion battery in which a wide temperature range electrolyte uniformly mixed with 1M LiFSI and EC:FEC:DEC:EA=0.15:0.1:0.325:0.425 is applied (Example 2);

[0027] Fig.11 The low temperature -30°C cycle and rate performance diagram of 1Ah soft-pack lithium-ion battery with 1M LiFSI and EC:FEC:DEC:EA=0.15:0.1:0.325:0.425 uniformly mixed electrolyte and commercial LB-141 electrolyte (Example 5 and Comparative Example 2);

[0028] Fig.12 The high temperature 60°C 1C cycle performance diagram of 1Ah soft-pack lithium-ion battery with 1M LiFSI and EC:FEC:DEC:EA=0.15:0.1:0.325:0.425 uniformly mixed electrolyte and commercial LB-141 electrolyte (Example 5 and Comparative Example 2);

[0029] Fig.13 This is the low temperature -30℃ 0.1C cycle performance diagram of commercial LB-141 electrolyte applied to button lithium-ion batteries (comparative sample 1);

[0030] Fig.14 This is the high temperature 60℃ 1C cycle performance diagram of commercial LB-141 electrolyte applied to button lithium-ion batteries (comparative sample 1). DETAILED DESCRIPTION

[0031] The background technology has described that lithium-ion batteries face problems such as increased electrolyte viscosity, increased battery internal resistance, and low battery reversible capacity at low temperatures; while at high temperatures, they face problems such as increased side reactions, rapid capacity decay, and serious safety hazards. In response to the above problems, the present invention designs a wide-temperature range electrolyte based on bis(fluorosulfonyl)imide salt (LiFSI) and ester-based solvents (EC, FEC, DEC, and EA).

[0032] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions. In the following examples, the above-mentioned bis(fluorosulfonyl)imide salt ester-based solvent wide temperature range electrolyte was prepared and used as a lithium-ion battery electrolyte to assemble corresponding button cells and soft-pack batteries, and a common commercial low-temperature electrolyte was used as a control.

[0033] Embodiment 1:

[0034] (1) 150 μl EC, 75 μl FEC, 287.5 μl DEC and 487.5 μl DMC solvent were uniformly mixed to obtain a transparent liquid.

[0035] (2) Weigh 0.187 g of LiFSI powder and add it to the above transparent liquid and stir to make it completely dissolved, finally obtaining a wide temperature range electrolyte.

[0036] (3) Take the pre-rolled commercial lithium titanate (LTO) pole piece to make the positive electrode, assemble it with 80μl of the above electrolyte and lithium metal negative electrode into a CR2032 button cell, and test the electrochemical performance. All the above steps are carried out in a glove box with water <0.1ppm and oxygen <0.1ppm.

[0037] Embodiment 2:

[0038] (1) 150 μl EC, 100 μl FEC, 325 μl DEC and 425 μl DMC solvent were uniformly mixed to obtain a transparent liquid.

[0039] (2) Weigh 0.187 g of LiFSI powder and add it to the above transparent liquid and stir to make it completely dissolved, finally obtaining a wide temperature range electrolyte.

[0040] (3) Take the pre-rolled commercial lithium titanate (LTO) pole piece to make the positive electrode, assemble it with 80μl of the above electrolyte and lithium metal negative electrode into a CR2032 button cell, and test the electrochemical performance. All the above steps are carried out in a glove box with water <0.1ppm and oxygen <0.1ppm.

[0041] Embodiment 3:

[0042] (1) Evenly mix 100 μl EC, 200 μl FEC, 400 μl DEC and 300 μl DMC solvent to obtain a transparent liquid.

[0043] (2) Weigh 0.187 g of LiFSI powder and add it to the above transparent liquid and stir to make it completely dissolved, finally obtaining a wide temperature range electrolyte.

[0044] (3) Take the pre-rolled commercial lithium titanate (LTO) pole piece to make the positive electrode, assemble it with 80μl of the above electrolyte and lithium metal negative electrode into a CR2032 button cell, and test the electrochemical performance. All the above steps are carried out in a glove box with water <0.1ppm and oxygen <0.1ppm.

[0045] Embodiment 4:

[0046] (1) 250 μl EC, 50 μl FEC, 150 μl DEC and 550 μl DMC solvent were uniformly mixed to obtain a transparent liquid.

[0047] (2) Weigh 0.187 g of LiFSI powder and add it to the above transparent liquid and stir to make it completely dissolved, finally obtaining a wide temperature range electrolyte.

[0048] (3) Take the pre-rolled commercial lithium titanate (LTO) pole piece to make the positive electrode, assemble it with 80μl of the above electrolyte and lithium metal negative electrode into a CR2032 button cell, and test the electrochemical performance. All the above steps are carried out in a glove box with water <0.1ppm and oxygen <0.1ppm.

[0049] Embodiment 5:

[0050] (1) 1500 μl EC, 1000 μl FEC, 3250 μl DEC and 4250 μl DMC solvent were uniformly mixed to obtain a transparent liquid.

[0051] (2) Weigh 1.87 g of LiFSI powder and add it to the above transparent liquid and stir to make it completely dissolved, finally obtaining a wide temperature range electrolyte.

[0052] (3) A 1Ah soft pack battery was assembled using a Z-type stacked lithium iron phosphate-graphite (LFP-Gr) battery cell and 5 ml of the above electrolyte to test the electrochemical performance. All the above steps were carried out in a glove box with water < 0.1 ppm and oxygen < 0.1 ppm.

[0053] Comparison examples

[0054] (1) In a glove box with water <0.1 ppm and oxygen <0.1 ppm, 80 μl of commercial electrolyte LB-141 was taken as the electrolyte, and assembled into a CR2032 button cell with a lithium titanate (LTO) positive electrode and a lithium metal negative electrode to test the electrochemical performance.

[0055] (2) In a glove box with water <0.1 ppm and oxygen <0.1 ppm, 5 ml of commercial electrolyte LB-141 was taken as the electrolyte and assembled into a 1 Ah soft pack battery with Z-type stacked lithium iron phosphate-graphite (LFP-Gr) cells to test the electrochemical performance.

[0056] Performance Testing

[0057] Embodiment 2 Figure 3 For example, Figure 3This is the low temperature cycle performance diagram of the bis(fluorosulfonyl)imide salt ester-based wide temperature range electrolyte. The battery test voltage range is between 1.0V and 2.5V, and the battery electrochemical test is carried out using the Land CT2001A system. The electrochemical performance diagram is the electrochemical performance of the LTO button battery assembled with this electrolyte after 6 cycles. Fig.13 ) and Example 1 ( Figure 1 )、Example 2( Figure 3 )、Example 3( Figure 5 ) or Example 4 ( Figure 7 ) After comparison, it can be found that the wide temperature electrolyte after changing to bis(fluorosulfonyl)imide salt and readjusting the ester-based solvent formula can significantly improve the low temperature cycle capacity of LTO button batteries. The final electrolyte formula can provide about 70% of the room temperature capacity at -30°C ( Figure 3 ), which is significantly higher than that of comparison sample 1 (30%).

[0058] Embodiment 2 Fig.10 For example, Fig.10 The figure below shows the high temperature cycle performance of the bis(fluorosulfonyl)imide ester-based wide temperature range electrolyte. The battery test voltage range is between 1.0V and 2.5V, and the battery electrochemical test is performed using the Land CT2001A system. The electrochemical performance graph shows the electrochemical performance of the LTO button battery assembled with this electrolyte after 200 cycles. Fig.14 ) and Example 1 ( Fig. 9 ) and Example 2 ( Fig.10 ) After comparison, it can be found that the wide-temperature electrolyte after changing to bisfluorosulfonyl imide salt and readjusting the ester-based solvent formula can significantly improve the high-temperature capacity retention rate of LTO button batteries. The final electrolyte formula supports the LTO button battery sample to stably cycle more than 100 times at 60°C without obvious capacity decay. After further adjustment of the solvent component ratio, it can support the LTO button battery sample to stably cycle more than 200 times at 60°C ( Fig.10 ), while the comparison example 1( Fig.14 )'s capacity decays rapidly within 25 cycles until the battery fails.

[0059] Embodiment 5 Fig.11 For example, Fig.11 This is a low temperature cycle performance diagram of a bis(fluorosulfonyl)imide salt ester-based wide temperature range electrolyte. The battery test voltage range is between 2.5V and 3.65V, and the battery electrochemical test is performed using the Land CT2001A system. The electrochemical performance diagram is the electrochemical performance of the LFP soft pack battery (the "EFDE" corresponding line in the figure) assembled with this electrolyte after 10 cycles. Comparison of Comparative Example 2 with Example 5 ( Fig.11 and Fig.12) It can be found that the bis(fluorosulfonyl)imide salt ester-based wide temperature range electrolyte can significantly improve the cycle capacity of 1Ah LFP soft-pack batteries at different rates at low temperatures and the capacity anti-fading performance at high temperatures.

[0060] Based on the above performance test results, it can be explained that the bis(fluorosulfonyl)imide salt ester-based wide temperature range electrolyte prepared by the present invention can significantly improve the performance of lithium-ion batteries of different systems under a wide temperature range.

[0061] The above specific implementation method part specifically introduces the analytical method involved in the present invention. It should be noted that the above introduction is only to help those skilled in the art better understand the method and ideas of the present invention, rather than limiting the relevant content. Without departing from the principle of the present invention, those skilled in the art may also make appropriate adjustments or modifications to the present invention, and the above adjustments and modifications shall also fall within the scope of protection of the present invention.

Claims

1. A bis(fluorosulfonyl)imide salt ester-based solvent wide temperature range electrolyte, characterized in that: The bisfluorosulfonyl imide salt ester-based solvent wide temperature range electrolyte is composed of an electrolyte salt, a main solvent and a diluent, wherein the electrolyte salt is a bisfluorosulfonyl imide salt, and the main solvent and the diluent are ester-based solvents; The bisfluorosulfonyl imide salt is lithium bisfluorosulfonyl imide LiFSI; the ester-based solvent is a mixture of four substances: ethylene carbonate EC, fluoroethylene carbonate FEC, diethyl carbonate DEC and ethyl acetate EA; wherein EC and FEC are used as main solvents, and DEC and EA are used as diluents; EC accounts for 10% to 25% of the total volume of the ester-based solvent, FEC accounts for 5% to 20% of the total volume of the ester-based solvent, DEC accounts for 15% to 40% of the total volume of the ester-based solvent, and EA accounts for 30% to 55% of the total volume of the ester-based solvent.

2. The bis(fluorosulfonyl)imide salt ester-based solvent wide temperature range electrolyte according to claim 1, characterized in that: The molar concentration of LiFSI in the ester-based solvent was 1 M.

3. The method for preparing a bisfluorosulfonyl imide salt ester-based solvent wide temperature range electrolyte according to claim 1 or 2, characterized in that: The steps include: (1) EC, FEC, DEC, and EA, four ester-based solvents, are uniformly mixed according to a certain proportion to obtain a transparent mixed solvent A; (2) Add LiFSI powder to the mixed solvent A and stir evenly to obtain a clear liquid B, which is a bis(fluorosulfonyl)imide salt ester-based solvent with a wide temperature range electrolyte.

4. The method for preparing a bis(fluorosulfonyl)imide salt ester-based solvent wide temperature range electrolyte according to claim 3, characterized in that: The preparation process was completed in a glove box with water <0.1 ppm and oxygen <0.1 ppm.

5. The bis(fluorosulfonyl)imide salt ester-based solvent wide temperature range electrolyte according to claim 1 or 2 is used as a lithium ion battery electrolyte.

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