Method for improving high-rate low-temperature cycle performance of ternary lithium battery by using tris (trimethylsilyl) phosphate as electrolyte additive

Abstract

The invention provides a method for improving the high-rate low-temperature cycle performance of a ternary nickel cobalt lithium manganate battery by taking tris (trimethylsilyl) phosphate as an electrolyte additive, belongs to the field of electrolyte additive application, and aims to improve the rapid charging performance and the cycle life of the battery under a low-temperature condition. According to the main scheme, 0.5-2 wt% of tris (trimethylsilyl) phosphate is added into a substrate electrolyte of 1M LiPF6-EC / DEC to serve as an electrolyte additive, wherein the volume ratio of EC to DEC is 1: 1. After the tri (trimethylsilyl) phosphate is added, the capacity retention ratio of the lithium / nickel cobalt lithium manganate half battery can still be kept at 72.77% after 300 cycles at the current density of 5C, and is increased by nearly 31.15% compared with 41.62% of a traditional electrolyte system; and meanwhile, a good improvement effect is achieved in a total battery system (lithium titanate / nickel cobalt lithium manganate). Therefore, the tris (trimethylsilyl) phosphate additive designed in a customized mode can achieve the fast charging performance at the ultralow temperature, and an obvious effect on promoting comprehensive popularization of lithium battery automobiles in China is achieved.

Description

Technical Field

[0001] The present invention relates to the field of electrolyte additives, and in particular to a method for using tris(trimethylsilyl)phosphate as an electrolyte additive to improve the high rate and low temperature cycle performance of a ternary (lithium nickel cobalt manganese oxide) lithium battery. Background Art

[0002] With the progress of the energy revolution, the application of traditional fossil energy has been greatly challenged due to its storage capacity and environmental friendliness. It is urgent to develop new and environmentally friendly new energy. As a national strategic industry, my country's new energy vehicle industry has developed rapidly and is at the world's leading level. Among the power batteries of new energy vehicles, lithium-ion batteries (LIB) have been increasingly valued for their high energy density, good cycle performance, no memory effect, low self-discharge rate, and no pollution. They are widely used in new energy electric vehicles, especially as one of the main power batteries for electric vehicles. At present, my country's electric vehicle industry is developing rapidly, so it is even more necessary to develop batteries with higher capacity, higher operating voltage, and longer cycle life.

[0003] Lithium-ion batteries have developed rapidly in recent years, especially the development of positive and negative electrode materials is extremely rapid, while the development of electrolytes has stagnated in comparison. As one of the main components of LIB, the performance of the electrolyte largely determines the performance of the battery. For example, the interface between the electrolyte and the positive and negative electrode materials, the resistance of the electrolyte and other factors largely determine the performance of the battery. Therefore, a suitable electrolyte system is an important part of the battery. At present, the electrolyte of domestic LIB is generally a carbonate organic mixed solvent, such as propylene carbonate (PC) and ethylene carbonate (EC), which have high polarity and large dielectric constant, but due to the strong intermolecular force, the viscosity is relatively large. DMC (dimethyl carbonate) and DEC (diethyl carbonate) have low polarity and low dielectric constant.

[0004] The viscosity is low, but the dielectric constant is small. Therefore, commercial electrolytes are mostly mixed with the two, taking advantage of each to achieve the effect of high dielectric constant and low viscosity. Ethylene carbonate (EC) is widely used due to its film-forming properties. Therefore, commercial electrolytes are mainly EC-based binary or ternary mixed solvents, such as EC+DMC, EC+DEC, EC+DMC+EMC and other conventional carbonate electrolytes. However, the performance of lithium batteries in traditional electrolyte systems will be greatly attenuated at low temperatures and during fast charging, which seriously limits the further popularization of lithium batteries in the entire industry. Summary of the invention

[0005] In view of the current difficulties of existing lithium batteries, the present invention provides a new method for using tris(trimethylsilyl)phosphate (TMSP) as an electrolyte additive. The molecule has a lower HOMO orbit and is oxidized before the electrolyte components during the electrochemical charging process. The phosphate undergoes a self-polymerization reaction on the surface of the positive electrode material to form a stable positive electrode-electrolyte interface layer film, thereby inhibiting the interface decomposition between the liquid electrolyte and the ternary nickel cobalt manganese oxide positive electrode material, and improving the high-rate and low-temperature cycle performance of the ternary lithium battery.

[0006] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0007] First, the present invention provides an electrolyte containing tris(trimethylsilyl)phosphate, comprising: adding 0.5-2wt% tris(trimethylsilyl)phosphate as an electrolyte additive to a 1MLiPF6-EC / DEC base electrolyte, wherein the volume ratio of EC to DEC is 1:1.

[0008] The present invention also provides a method for preparing an electrolyte containing tris(trimethylsilyl)phosphate, comprising the following steps:

[0009] Step (1) preparing a base electrolyte in an environment with an oxygen content of less than 0.01ppm and a water content of less than 0.01ppm, wherein the main ratio is 1M LiPF6-EC / DEC, and the specific steps are taking equal volumes of EC and DEC and magnetically stirring to completely dissolve EC, and after stirring, standing to allow the EC / DEC solution to completely cool, and after cooling, adding 1M LiPF6 in multiple times, and then stirring at a low speed of 200-800rpm to evenly disperse LiPF6;

[0010] Step (2) on the basis of the prepared base electrolyte, 0.5-5wt% tris(trimethylsilyl) phosphate is added dropwise to the base electrolyte, followed by magnetic stirring for uniformity, after which 1g of 4A molecular sieve and fresh metal lithium sheet are added thereto, followed by standing to obtain an electrolyte solution with 0.5-2wt% tris(trimethylsilyl) phosphate;

[0011] In the above scheme, after the stirring in step (1) is completed, the EC / DEC solution is allowed to stand for 3 hours to completely cool down, and the standing time in step (2) is 1 week.

[0012] In the above scheme, the magnetic stirring time is 10 h.

[0013] The present invention also provides a ternary nickel-cobalt-manganese-oxide lithium battery, wherein the battery assembly sequence is: a positive electrode shell, a positive electrode sheet, an electrolyte containing 0.5-2wt% tris(trimethylsilyl)phosphate, a PP separator, a negative electrode sheet, and a negative electrode shell, wherein the electrolyte containing 0.5-2wt% tris(trimethylsilyl)phosphate is obtained by adding 0.5-2wt% tris(trimethylsilyl)phosphate as an electrolyte additive to a 1M LiPF6-EC / DEC base electrolyte.

[0014] The present invention also provides a method for improving the high-rate low-temperature cycle performance of a ternary nickel-cobalt-manganese-oxide lithium battery by using tris(trimethylsilyl)phosphate as an electrolyte additive, specifically, adding 0.5-2wt% of tris(trimethylsilyl)phosphate as an electrolyte additive to a 1M LiPF6-EC / DEC base electrolyte, wherein the volume ratio of EC to DEC is 1:1.

[0015] Because the present invention adopts the above technical solution, it has the following beneficial effects:

[0016] (1) The invention effectively utilizes tris(trimethylsilyl)phosphate as an electrolyte additive to improve the high-rate cycling performance of the ternary nickel-cobalt-manganese-oxide lithium battery system. Compared with the traditional electrolyte, the capacity retention rate after 300 cycles is increased by nearly 31.15%;

[0017] (2) The invention uses tri(trimethylsilyl) phosphate as an electrolyte additive, which also has obvious effects on the cycling performance under ultra-low temperature (-20°C) and high rate (5C). Compared with traditional electrolytes, the capacity retention rate after 120 cycles is increased by nearly 69%.

[0018] (3) The electrolyte additive provided by the present invention can be directly applied to the Li-nickel cobalt manganese oxide NCM811 battery system. After adding tris(trimethylsilyl) phosphate, the capacity retention rate of the Li / NCM811 half-cell can still be maintained at 72.77% after 300 cycles at a current density of 5C, which is nearly 31.15% higher than the 41.62% of the traditional electrolyte system. In addition, it has also been proven that tris(trimethylsilyl) phosphate has good compatibility with lithium titanate negative electrodes. At the same time, tris(trimethylsilyl) phosphate also has a good improvement effect in the full battery system (LTO / NCM811), with a capacity retention rate of 69.46% after 300 cycles at a current density of 5C, which is 28.71% higher than the 40.75% of the traditional electrolyte system. Finally, the full battery performance of the battery with the addition of tris(trimethylsilyl) phosphate and the traditional electrolyte system at a current density of 5C was tested at -20°C. It was found that the capacity retention rate after 120 cycles with the addition of tri(trimethylsilyl) phosphate was 77%, while the retention rate of traditional electrolyte was only 8%. Therefore, the custom-designed tri(trimethylsilyl) phosphate additive can achieve fast charging performance at ultra-low temperatures, which plays an important role in promoting the full popularization of lithium battery vehicles in my country. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Comparison of the cycle stability (5C, 300 cycles) of the Li / nickel cobalt manganese oxide NCM811 half-cell tested in Example 1 in different electrolyte systems (conventional electrolyte, containing 0.2%, 0.5%, and 1wt% tris(trimethylsilyl) phosphate, respectively).

[0020] Figure 2 The rate performance comparison of the Li / nickel cobalt manganese oxide NCM81 1 half-cell tested in Example 2 in a traditional electrolyte and an electrolyte containing 0.5 wt % tris(trimethylsilyl) phosphate.

[0021] Figure 3 The cycling performance (5C, 300 cycles) of the lithium titanate LTO / nickel cobalt manganese oxide NCM81 1 full battery tested in Example 3 in a traditional electrolyte and an electrolyte containing 0.5 wt% tris(trimethylsilyl) phosphate.

[0022] Figure 4 The cycling performance of the traditional electrolyte and the electrolyte containing 0.5 wt% tris(trimethylsilyl) phosphate at -20°C and 5°C tested in Example 4. Specific embodiments

[0023] Example 1

[0024] First, the cycling performance at high rate (5C) was discussed in detail, comparing different additive ratios. Based on the base electrolyte, the addition of TMSP (tris(trimethylsilyl) phosphate) in multiple ratios was discussed ( Figure 1 ), including 0.2wt%, 0.5wt%, 1wt%, and 2wt%. At the beginning of the cycle, the capacities of Baseline, 0.2% TMSP, 0.5% TMSP, 1% TMSP, and 2% TMSP were 13.04mAh / g, 155.50mAh / g, 164.98mAh / g, 158.07mAh / g, and 163.89mAh / g, respectively; after 150 cycles, they were 70.36mAh / g, 116.802mAh / g, 139.39mAh / g, 138.71mAh / g, and 120.60mAh / g, and after 300 cycles, the capacities were 47.05mAh / g, 97.75mAh / g, 120.06mAh / g, 112.48mAh / g, and 104.43mAh / g, respectively. Therefore, the capacity retention rates after 300 cycles are 41.62%, 62.86%, 72.77%, 71.15% and 63.71% respectively. Therefore, it can be seen that the performance of adding 0.5% TMSP is the best, followed by 1% TMSP. Therefore, it can be concluded that when only 0.2% TMSP is added, it is too little to help the positive electrode form a stable and dense CEI protective film. With the increase of the additive content, TMSP really plays an effective role in regulating and protecting CEI, but with the excessive addition of TMSP (2%), it has a negative effect on CEI, forming a relatively thick CEI, which is not conducive to the long-term cycle stability of the battery.

[0025] Example 2

[0026] Next, the rate performance of the nickel cobalt manganese oxide NCM81 1 / Li half-cell was compared with that after adding 0.5% TMSP. Figure 2It can be seen that the capacity of the baseline is 181.35mAh / g at a rate of 0.5C, while the capacity after adding 0.5% TMSP is 202.07mAh / g, which is 20.72mAh / g higher. As the current density continues to increase, at 1C, the capacity after adding 0.5% TMSP can still be maintained at 194.55mAh / g, while the baseline is only 165.56mAh / g. At high current densities of 2C, 5C, and 10C, the capacity of 0.5% TMSP is 40.28mAh / g, 40.34mAh / g, and 57.481mAh / g higher than the baseline capacity, respectively. After returning to the current density of 0.5C, 0.5% TMSP can still return to 200.84mAh / g after addition. Therefore, not only at low current density or high current density, the addition of 0.5% TMSP has better electrochemical rate performance. Especially at the extreme high current density, the performance is still very outstanding. The excellent rate performance is mainly due to the fact that after the addition of TMSP, it participates in the construction of the CEI membrane and forms a stable and dense CEI membrane interface.

[0027] Example 3

[0028] Cycling 300 times at a high current density of 5C (LTO / nickel cobalt manganese oxide NCM811), the baseline cycle capacity is 144.426mAh / g at the beginning of the cycle, while the initial cycle capacity of 0.5% TMSP is 166.847mAh / g, which is 22.421mAh / g higher than the baseline capacity. After 300 cycles, the effect of TMSP addition gradually becomes prominent, with a capacity of 115.89mAh / g, and a capacity retention rate of 69.46% after 300 cycles at 5C. Compared with 0.5% TMSP, the capacity of the baseline is only 58.867mAh / g, and the capacity retention rate is 40.75%, such as Figure 3 .

[0029] Example 4

[0030] To explore whether TMSP molecules can improve the cycling performance of full batteries (LTO / nickel cobalt manganese oxide NCM811) at low temperature (-20°C) and ultra-fast charge and discharge rate (5C), the capacity of baseline and TMSP at the initial stage of the cycle is 39.20mAh / g and 81.63mAh / g, respectively. Figure 4 It can be seen that TMSP has excellent initial capacity output, while baseline has only very low output capacity. After 120 cycles, the capacity retention rates of baseline and TMSP are 8% and 77%, respectively.

Claims

1. An electrolyte containing tris(trimethylsilyl)phosphate, characterized in that: include: 0.5-2wt% of tris(trimethylsilyl) phosphate was added as an electrolyte additive to a 1M LiPF6-EC / DEC base electrolyte, wherein the volume ratio of EC to DEC was 1:

1.

2. A method for preparing an electrolyte containing tris(trimethylsilyl)phosphate as claimed in claim 1, characterized in that: The steps include: Step (1) preparing a base electrolyte in an environment with an oxygen content of less than 0.01 ppm and a water content of less than 0.01 ppm, wherein the main ratio is 1M LiPF6-EC / DEC, and the specific steps are taking equal volumes of EC and DEC and magnetically stirring to completely dissolve EC, and after stirring, standing to allow the EC / DEC solution to completely cool, and after cooling, adding 1M LiPF6 in multiple times, and then stirring at a low speed of 200-800 rpm to evenly disperse LiPF6; Step (2) on the basis of the prepared base electrolyte, 0.5-2wt% tris(trimethylsilyl) phosphate is added dropwise to the base electrolyte, followed by magnetic stirring to make it uniform, after the stirring is completed, 1g 4A molecular sieve and fresh metal lithium sheet are added thereto, and then allowed to stand to obtain an electrolyte with 0.5-2wt% tris(trimethylsilyl) phosphate; 3. The method for preparing an electrolyte containing tris(trimethylsilyl)phosphate according to claim 2, characterized in that: After the stirring in step (1) is completed, the EC / DEC solution is allowed to stand for 3 hours to allow it to cool completely. The standing time in step (2) is 1 week.

4. The method for preparing an electrolyte containing tris(trimethylsilyl)phosphate according to claim 2, characterized in that: The magnetic stirring time was 10 h.

5. A ternary nickel-cobalt-manganese-oxide lithium battery, characterized in that: The battery assembly order is: positive electrode shell, nickel cobalt lithium manganese oxide positive electrode sheet, electrolyte with 0.5-2wt% tris(trimethylsilyl) phosphate added, PP separator, negative electrode sheet, negative electrode shell, wherein the electrolyte with 0.5-2wt% tris(trimethylsilyl) phosphate is obtained by adding 0.5-2wt% tris(trimethylsilyl) phosphate as an electrolyte additive to the base electrolyte of 1M LiPF6-EC / DEC.

6. A ternary nickel-cobalt-manganese-oxide lithium battery according to claim 1, characterized in that: The amount of the electrolyte containing 0.5-2wt% tris(trimethylsilyl)phosphate added is 0.5wt%.

7. A method for improving the high-rate and low-temperature cycle performance of a ternary lithium battery by using tris(trimethylsilyl)phosphate as an electrolyte additive, characterized in that: 0.5-2wt% of tris(trimethylsilyl) phosphate was added as an electrolyte additive to a 1M LiPF6-EC / DEC base electrolyte, wherein the volume ratio of EC to DEC was 1:1.

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