An electrolyte and its preparation method and application

Abstract

The present invention provides an electrolyte and a preparation method and application thereof. The electrolyte includes a lithium salt, an organic solvent and an additive, wherein the additive includes a fluorine-containing compound and fluoroethylene carbonate as shown in Formula 1, wherein 0≤n≤6, and R1, R2 and R3 are independently hydrogen atoms or fluorine atoms. The present invention partially replaces fluoroethylene carbonate with a fluorine-containing compound, reduces the amount of fluoroethylene carbonate, can mitigate the negative impact caused by the thermal instability of fluoroethylene carbonate, and improves the high temperature stability of the battery while ensuring the cycle performance.

Description

Technical Field

[0001] The invention belongs to the field of lithium ion batteries and relates to an electrolyte and a preparation method and application thereof. Background Art

[0002] Silicon has a theoretical specific capacity of up to 4200mAh / g and is a very promising negative electrode material for lithium-ion batteries. Silicon expands and contracts by more than 300% when fully embedded in lithium, causing the SEI film to rupture, regenerate, and rupture. This accelerates electrolyte consumption, causing material particles to pulverize and fall off, and deteriorating its electrical properties. In order to alleviate this situation, a large amount of fluoroethylene carbonate is often added to the electrolyte. However, fluoroethylene carbonate is thermally unstable and reacts with lithium hexafluorophosphate and the like at high temperatures, resulting in increased electrolyte acidity, high gas production, and deterioration of high-temperature storage.

[0003] A lot of research has been done to mitigate the negative impact of thermal instability of fluoroethylene carbonate and solve the problem of poor high-temperature storage performance of silicon negative electrodes.

[0004] CN105098245A discloses a lithium ion battery electrolyte containing fluoroethylene carbonate and a lithium ion battery. The additive includes fluoroethylene carbonate, and a cyclic phosphorus compound is used as a co-additive to effectively inhibit high-temperature gas generation caused by fluoroethylene carbonate under high temperature conditions.

[0005] CN109346762A discloses an electrolyte acting on a silicon-carbon negative electrode material and a lithium ion battery containing the electrolyte. The additives include fluoroethylene carbonate and triethyl phosphonoacetate, so that triethyl phosphonoacetate forms an SEI film on the electrode surface before fluoroethylene carbonate, effectively reducing the amount of fluoroethylene carbonate, and the SEI film has good thermal stability to ensure that the structure of the electrode material is protected during high-temperature cycling. However, the high-temperature cycling performance improved by the above patent is limited, and the thermal stability of the battery needs to be further improved.

[0006] How to improve the thermal stability of silicon negative electrode materials during high-temperature cycling and enhance the electrochemical performance of batteries is an important research direction in this field. Summary of the invention

[0007] In view of the shortcomings of the prior art, the object of the present invention is to provide an electrolyte and a preparation method and application thereof to solve the problem of poor high-temperature storage performance of silicon negative electrode caused by fluoroethylene carbonate.

[0008] To achieve this object, the present invention adopts the following technical solutions:

[0009] One of the purposes of the present invention is to provide an electrolyte, the electrolyte comprising a lithium salt, an organic solvent and an additive, the additive comprising a fluorine-containing compound as shown in Formula 1 and fluoroethylene carbonate,

[0010]

[0011] Wherein, 0≤n≤6, R1, R2 and R3 are independently hydrogen atoms or fluorine atoms, wherein the value of n can be 0, 1, 2, 3, 4, 5 or 6, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0012] The electrolyte in the present invention mitigates the negative impact caused by the thermal instability of fluoroethylene carbonate, and solves the problem of poor high-temperature storage performance of silicon negative electrode. The present invention partially replaces fluoroethylene carbonate with fluorine-containing compounds, reduces the amount of fluoroethylene carbonate, can mitigate the negative impact caused by the thermal instability of fluoroethylene carbonate, and improves the high-temperature stability of the battery while ensuring the cycle performance.

[0013] In the present invention, the -CF3 group at the end of the fluorine-containing compound breaks and participates in the negative electrode film formation, forming a thin SEI film rich in LiF on the silicon surface. During the process of lithium insertion and removal from the silicon negative electrode, the SEI film can adapt to its deformation and improve the high temperature stability of the electrolyte.

[0014] As a preferred technical solution of the present invention, the fluorine-containing compound includes any one of 1,2-bis(trifluoromethoxy)ethane, 1-trifluoroethylene glycol dimethyl ether or di(1-trifluoroethylene glycol) dimethyl ether, or a combination of at least two of them, wherein typical but non-limiting examples of the combination include: a combination of 1,2-bis(trifluoromethoxy)ethane and 1-trifluoroethylene glycol dimethyl ether, a combination of 1,2-bis(trifluoromethoxy)ethane and di(1-trifluoroethylene glycol) dimethyl ether, or a combination of 1-trifluoroethylene glycol dimethyl ether and di(1-trifluoroethylene glycol) dimethyl ether, etc.

[0015] Preferably, in the electrolyte, the mass fraction of the fluorine-containing compound is 0.1 to 10%, wherein the mass fraction can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable, preferably 2 to 6%.

[0016] As a preferred technical solution of the present invention, the additives also include vinylene carbonate, fluoroethylene carbonate and lithium salt additives.

[0017] Preferably, in the electrolyte, the mass fraction of vinylene carbonate is 0.1-5%, wherein the mass fraction can be 0.1%, 0.5%, 1%, 1.5%, 2%, 3%, 4% or 5%, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable, preferably 0.2-2%.

[0018] In the present invention, vinylene carbonate forms a film at the negative electrode. If the amount added is too little, a dense SEI film cannot be formed. If the amount added is too large, the impedance will be increased and the gas generation will be serious.

[0019] Preferably, in the electrolyte, the mass fraction of the fluoroethylene carbonate is 0 to 15%, wherein the mass fraction can be 0%, 2%, 4%, 6%, 8%, 10%, 12%, 14% or 15%, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable, preferably 1 to 10%.

[0020] In the present invention, fluoroethylene carbonate forms a film at the negative electrode, which can slow down the expansion and contraction of silicon materials and cause damage to the SEI film. Too much additive will deteriorate the high-temperature storage performance, and too little additive will deteriorate the cycle performance.

[0021] Preferably, in the electrolyte, the mass fraction of the lithium salt additive is 0.5-1%, wherein the mass fraction can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0022] As a preferred technical solution of the present invention, the organic solvent includes at least three of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate.

[0023] Preferably, the volume ratio of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate is (20-40):(0-20):(0-20):(5-20):(30-50), wherein the volume ratio can be 20:0:20:20:40, 30:10:10:10:40, 30:20:0:20:30, 40:0:10:20:30 or 40:0:0:20:40, but is not limited to the listed values, and other unlisted values ​​within the numerical range are equally applicable.

[0024] As a preferred technical solution of the present invention, the lithium salt includes lithium hexafluorophosphate or a combination of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

[0025] Preferably, in the electrolyte, the concentration of the lithium salt is 1.0 to 1.3 mol / L, wherein the concentration may be 1.0 mol / L, 1.2 mol / L or 1.3 mol / L, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0026] A second object of the present invention is to provide a method for preparing the electrolyte as described in the first object, the preparation method comprising:

[0027] The electrolyte is obtained by sequentially adding an additive and a lithium salt to an organic solvent and then mixing them.

[0028] As a preferred technical solution of the present invention, the atmosphere for the mixing is an argon atmosphere.

[0029] Preferably, the temperature for the mixing is 5 to 10 °C. The temperature can be 5 °C, 6 °C, 7 °C, 8 °C, 9 °C or 10 °C, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0030] A second object of the present invention is to provide a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte as described in the first object.

[0031] As a preferred technical solution of the present invention, the active material of the positive electrode sheet includes Li(Ni x Co y Mn z )O2, where x ≥ 0.8, 0 < y ≤ 0.2, 0 < z ≤ 0.2 and x + y + z = 1. The value of x can be 0.8, 0.9 or 0.95, etc., the value of y can be 0.01, 0.05, 0.1, 0.15 or 0.2, etc., and the value of z can be 0.01, 0.05, 0.1, 0.15 or 0.2, etc. But it is not limited to the listed values. Other unlisted values within the above numerical ranges are equally applicable.

[0032] As a preferred technical solution of the present invention, the active material of the negative electrode sheet includes graphite and silicon-carbon material.

[0033] In the present invention, the fluorine-containing additive forms a dense SEI film rich in LiF on the negative electrode, which can adapt to the deformation of the material during the insertion and extraction process of the silicon negative electrode. The silicon negative electrode will lead to a high dependence on fluoroethylene carbonate. The combination of the fluorine-containing additive and fluoroethylene carbonate can reduce the dosage of fluoroethylene carbonate and improve the high-temperature stability of the battery at the same time.

[0034] The numerical ranges described in the present invention include not only the above-listed point values, but also any point values between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] The present invention adopts a fluorine-containing additive in combination with fluoroethylene carbonate for a silicon negative electrode battery, thereby improving the cycle performance and high-temperature stability of the battery. The capacity retention rate of the battery can reach more than 82% after 400 cycles at 25°C, the volume expansion rate can be reduced to less than 9.2% after being stored at 60°C for 90 days, the capacity retention rate can be as high as more than 86%, and the capacity recovery rate can be as high as more than 90%. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0038] Example 1

[0039] This embodiment provides an electrolyte, the electrolyte comprising an organic solvent, a lithium salt and an additive, the additive being vinylene carbonate, fluoroethylene carbonate, a lithium salt additive and a fluorine-containing compound, the lithium salt additive being lithium difluorophosphate, the fluorine-containing compound being 1,2-bis(trifluoromethoxy)ethane as shown in Formula 2,

[0040]

[0041] In the electrolyte, the mass fraction of vinylene carbonate is 0.5%, the mass fraction of fluoroethylene carbonate is 4%, the mass fraction of 1,2-bis(trifluoromethoxy)ethane is 4%, and the mass fraction of lithium difluorophosphate is 1.0%.

[0042] The lithium salt is lithium hexafluorophosphate, and the concentration in the electrolyte is 1.0 mol / L.

[0043] The organic solvent is ethylene carbonate, diethyl carbonate and ethyl methyl carbonate, wherein, based on the total volume of the organic solvent being 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of diethyl carbonate is 20%, and the volume fraction of ethyl methyl carbonate is 50%.

[0044] This embodiment also provides a method for preparing the above electrolyte, the preparation method comprising:

[0045] In an argon atmosphere, a formulated amount of vinylene carbonate, fluoroethylene carbonate, a lithium salt additive and a fluorine-containing compound are added to an organic solvent, and then an electrolyte salt is added, and the mixture is stirred and mixed at a temperature of 10° C. to obtain the electrolyte solution.

[0046] Example 2

[0047] This embodiment provides an electrolyte, the electrolyte comprising an organic solvent, a lithium salt and an additive, the additive being vinylene carbonate, fluoroethylene carbonate, a lithium salt additive and a fluorine-containing compound, the lithium salt additive being lithium difluorophosphate, the fluorine-containing compound being 1,2-bis(trifluoromethoxy)ethane as shown in Formula 2,

[0048]

[0049] In the electrolyte, the mass fraction of vinylene carbonate is 0.2%, the mass fraction of fluoroethylene carbonate is 3%, the mass fraction of 1,2-bis(trifluoromethoxy)ethane is 5%, and the mass fraction of lithium difluorophosphate is 0.5%.

[0050] The lithium salt is lithium hexafluorophosphate, and the concentration in the electrolyte is 1.2 mol / L.

[0051] The organic solvent is ethylene carbonate, diethyl carbonate and ethyl methyl carbonate, wherein, based on the total volume of the organic solvent being 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of diethyl carbonate is 20%, and the volume fraction of ethyl methyl carbonate is 50%.

[0052] This embodiment also provides a method for preparing the above electrolyte, the preparation method comprising:

[0053] In an argon atmosphere, a formulated amount of vinylene carbonate, fluoroethylene carbonate, a lithium salt additive and a fluorine-containing compound are added to an organic solvent, and then an electrolyte salt is added, and the mixture is stirred and mixed at a temperature of 5° C. to obtain the electrolyte solution.

[0054] Example 3

[0055] This embodiment provides an electrolyte, the electrolyte comprising an organic solvent, a lithium salt and an additive, the additive being vinylene carbonate, fluoroethylene carbonate, a lithium salt additive and a fluorine-containing compound, the lithium salt additive being lithium difluorophosphate, the fluorine-containing compound being 1,2-bis(trifluoromethoxy)ethane as shown in Formula 2,

[0056]

[0057] In the electrolyte, the mass fraction of vinylene carbonate is 1%, the mass fraction of fluoroethylene carbonate is 5%, the mass fraction of 1,2-bis(trifluoromethoxy)ethane is 3%, and the mass fraction of lithium difluorophosphate is 0.8%.

[0058] The lithium salt is lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide in a mass ratio of 1:1, and the concentration in the electrolyte is 1.3 mol / L.

[0059] The organic solvent is ethylene carbonate, diethyl carbonate and ethyl methyl carbonate, wherein, based on the total volume of the organic solvent being 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of diethyl carbonate is 20%, and the volume fraction of ethyl methyl carbonate is 50%.

[0060] This embodiment also provides a method for preparing the above electrolyte, the preparation method comprising:

[0061] In an argon atmosphere, a formulated amount of vinylene carbonate, fluoroethylene carbonate, a lithium salt additive and a fluorine-containing compound are added to an organic solvent, and then an electrolyte salt is added, and the mixture is stirred and mixed at a temperature of 10° C. to obtain the electrolyte solution.

[0062] Example 4

[0063] In this embodiment, except that the fluorine-containing compound shown in Formula 2 is replaced by 1-trifluoroethylene glycol dimethyl ether shown in Formula 3, other conditions are the same as those in Embodiment 1.

[0064]

[0065] Example 5

[0066] In this embodiment, except that the fluorine-containing compound shown in Formula 2 is replaced by di(1-trifluoroethylene glycol) dimethyl ether shown in Formula 4, other conditions are the same as those in Embodiment 1.

[0067]

[0068] Example 6

[0069] Except that vinylene carbonate is not added, other conditions in this embodiment are the same as those in embodiment 1.

[0070] Example 7

[0071] Except that fluoroethylene carbonate is not added, other conditions in this embodiment are the same as those in embodiment 1.

[0072] Example 8

[0073] Except that lithium difluorophosphate is not added, other conditions in this embodiment are the same as those in embodiment 1.

[0074] Example 9

[0075] In this embodiment, except that the mass fraction of 1,2-bis(trifluoromethoxy)ethane is replaced with 10%, other conditions are the same as those in Embodiment 1.

[0076] Example 10

[0077] In this embodiment, except that the stirring and mixing temperature in the preparation of the electrolyte is changed from 10° C. to 25° C., other conditions are the same as those in Embodiment 1.

[0078] Comparative Example 1

[0079] In this comparative example, except that 1,2-bis(trifluoromethoxy)ethane was not added and the content of fluoroethylene carbonate was adjusted to 8%, other conditions were the same as those of Example 1.

[0080] The electrolytes prepared in Examples 1-10 and Comparative Example 1 were assembled into lithium-ion batteries, wherein the positive electrode active material of the lithium-ion battery was a ternary material Li(Ni x Co y Mn z )O2(x+y+z=1), NCM811 is selected in the embodiment and the comparative example, that is, x=8, y / z=1. The negative electrode active material is a composite material of graphite and silicon carbon.

[0081] The electrochemical performance of the lithium-ion batteries prepared in the above-mentioned Examples 1-10 and Comparative Example 1 was tested, and the testing method included: preparing a slurry of the negative electrode material (graphite, silicon carbon), the conductive agent acetylene black and the binder CMC and SBR in a mass percentage of 94:1:2:3 and coating it on the copper foil current collector, vacuum drying, and preparing a negative electrode plate; preparing a slurry of the positive electrode material NCM811, the conductive agent acetylene black and the binder PVDF in a mass ratio of 94:3:3 and coating it on the aluminum foil current collector, vacuum drying, and preparing a positive electrode plate. The positive electrode plate, the negative electrode plate, the Celgard2400 separator and the electrolyte prepared in the examples or comparative examples were assembled into a soft-pack battery, and the electrochemical test was carried out using a Xinwei charge and discharge test cabinet.

[0082] 1. The test method for the cycle performance of lithium-ion batteries is:

[0083] At 25°C, the lithium-ion battery was charged to 100% SOC at 0.5.0C (nominal capacity) constant current and constant voltage, left for 30 minutes, and then discharged to 0% SOC at 1C constant current. The above is one cycle. The lithium-ion battery was charged and discharged 500 times at 25°C under the above conditions.

[0084] The capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity at the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0085] 2. The high temperature storage performance test method of lithium-ion batteries is:

[0086] At 25°C, the lithium-ion battery was charged at a constant current of 1C to a voltage of 4.2V, and then charged at a constant voltage of 4.2V to a current of 0.05C. The volume of the tested lithium-ion battery was V0, and the initial capacity was C0. The lithium-ion battery was then placed in a constant temperature box at 60°C and stored for 90 days, and the volume of the tested lithium-ion battery was taken out and recorded as V1, the capacity remained C1, and the capacity recovered to C2.

[0087] The volume expansion ratio (%) of a lithium-ion battery after storage at 60°C for 90 days = (Vn-V0) / V0×100%.

[0088] The capacity retention rate (%) of the lithium-ion battery after storage at 60°C for 90 days = (C1 / C0)×100%, and the capacity recovery rate (%) of the lithium-ion battery after storage at 60°C for 90 days = (C2 / C0)×100%.

[0089] The test results of the lithium ion batteries corresponding to Examples 1-10 and Comparative Example 1 are shown in Table 1.

[0090] Table 1

[0091]

[0092] It can be seen from the above table that the lithium ion batteries of Examples 1-5 all have good cycle performance.

[0093] Comparison between Example 1 and Examples 6-8 shows that although vinylene carbonate and lithium difluorophosphate have little effect on the battery cycle performance, they can improve the battery high temperature storage performance. Fluorinated vinyl carbonate is a necessary additive for silicon negative electrode system batteries to ensure battery cycle performance.

[0094] By comparing Example 9 with Example 1, it can be seen that if the amount of 1,2-bis(trifluoromethoxy)ethane added is too much, the cycle performance of the battery is not significantly improved, and the raw materials are wasted.

[0095] By comparing Example 10 with Example 1, it can be seen that as the stirring and mixing temperature increases, the cycle performance and storage performance of the battery decrease.

[0096] By comparing Example 1 with Comparative Example 1, it can be seen that the fluorine-containing additive of the battery is used in combination with fluoroethylene carbonate, which improves the high-temperature stability of the electrolyte, improves the suppression of storage gas generation, and improves high-temperature performance while ensuring the cycle performance.

[0097] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An electrolyte for a silicon negative electrode battery, characterized in that: The electrolyte comprises a lithium salt, an organic solvent and an additive, wherein the additive comprises a fluorine-containing compound, vinylene carbonate, a lithium salt additive and fluoroethylene carbonate; The fluorine-containing compound is di(1-trifluoroethylene glycol) dimethyl ether; The lithium salt additive is lithium difluorophosphate; In the electrolyte, the mass fraction of the fluorine-containing compound is 2-6%; In the electrolyte, the mass fraction of vinylene carbonate is 0.2-2%; In the electrolyte, the mass fraction of the fluoroethylene carbonate is 1-10%.

2. The electrolyte according to claim 1, characterized in that In the electrolyte, the mass fraction of the lithium salt additive is 0.5-1%.

3. The electrolyte according to claim 1, characterized in that The organic solvent includes at least three of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate.

4. The electrolyte according to claim 3, characterized in that The volume ratio of the ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate is (20-40):(0-20):(0-20):(5-20):(30-50).

5. The electrolyte according to claim 1, characterized in that The lithium salt includes lithium hexafluorophosphate or a combination of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

6. The electrolyte according to claim 1, characterized in that In the electrolyte, the concentration of the lithium salt is 1.0-1.3 mol / L.

7. A method for preparing an electrolyte according to any one of claims 1 to 6, characterized in that: The preparation method comprises: The additive and the lithium salt are sequentially added to the organic solvent and then mixed to obtain the electrolyte.

8. The preparation method according to claim 7, characterized in that: The mixed atmosphere is an argon atmosphere.

9. The preparation method according to claim 7, characterized in that: The mixing temperature is 5-10°C.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to any one of claims 1 to 6.

11. The lithium ion battery according to claim 10, characterized in that: The active material of the positive electrode sheet includes Li(Ni x Co y Mn z )O2, where x≥0.8, 0<y≤0.2, 0<z≤0.2 and x + y + z = 1.

12. The lithium ion battery according to claim 10, characterized in that: The active material of the negative electrode sheet includes a composite material of graphite and silicon-carbon.

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