Electrolyte, preparation method thereof and high-nickel lithium ion battery containing same

Abstract

This invention provides an electrolyte, its preparation method, and a high-nickel lithium-ion battery containing the same. The electrolyte comprises a lithium salt, an organic solvent, and additives; the additives include nitrile additives, and the lithium salt includes lithium fluorosulfonyl difluorophosphate. This invention uses lithium fluorosulfonyl difluorophosphate as the main electrolyte lithium salt, while simultaneously adding a lithium salt, lithium fluorosulfonyl difluorophosphate, and a hydrocarbon nitrile additive. By adjusting the mixing ratio of lithium fluorosulfonyl difluorophosphate and the content of the hydrocarbon nitrile additive, not only is the corrosion of lithium fluorosulfonyl difluorophosphate effectively inhibited, but the high-temperature storage characteristics and rate performance are also significantly improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more particularly to an electrolyte, a method for preparing the electrolyte, and a high-nickel lithium-ion battery containing the electrolyte. Background Technology

[0002] Currently, lithium hexafluorophosphate (LiPF6) is the primary electrolyte used as the main salt in the market. However, due to its poor thermal stability, LiPF6 decomposes at high temperatures (around 60°C), leading to capacity decay. Furthermore, the presence of trace amounts of water (greater than 10 ppm) generates HF, increasing electrolyte acidity and causing the dissolution of transition metal ions, resulting in severe system deterioration and poor high-temperature performance. Existing electrolyte solute salts include lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiODFB), lithium difluorooxalatoborate (LiBOB), and lithium difluorophosphate (LiPO2F2), but these all suffer from low conductivity, low solubility, environmental pollution, and safety risks.

[0003] CN112010894A discloses a thiophosphate compound, a non-aqueous lithium-ion battery electrolyte containing the thiophosphate compound, and a lithium-ion battery. The thiophosphate compound can improve the flame retardancy of the battery, but it will produce side reactions and reduce the rate performance of the battery.

[0004] CN112652816A discloses an electrolyte that balances low-temperature fast charging performance and high-temperature performance, as well as its preparation method and application. The film-forming additives include fluoroethylene carbonate and / or vinylene carbonate, and the functional additives include sulfur-containing additives and nitrile additives. However, the selected nitrile additives cannot provide high-rate performance for the battery, and the overall formulation does not have a beneficial effect on improving the battery's rate performance and electrochemical performance.

[0005] Therefore, how to prepare a lithium-ion battery with low high-temperature storage gas generation, high capacity retention and high rate performance is an important research direction in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide an electrolyte, a method for preparing the electrolyte, and a high-nickel lithium-ion battery containing the electrolyte.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] One of the objectives of this invention is to provide an electrolyte comprising lithium salt, organic solvent and additives.

[0009] The additives include nitrile additives, and the structures of the nitrile additives are shown in Formula 1:

[0010] N≡CRC≡N where R is a substituted or unsubstituted C1-C5 alkylene group, and the number of nitrile substituents in the branched chain is ≤1, wherein the number of nitrile substituents can be 0 or 1.

[0011] The nitrile additives may specifically be, for example, 1,3,6-hexanetrionitrile, adiponitrile, or tert-butylmalononitrile.

[0012] The lithium salts include lithium difluorophosphate fluorosulfonylimide and lithium difluorosulfonylimide.

[0013] This invention adds a lithium salt to the electrolyte: lithium fluorosulfonyl difluorophosphate, with the structure shown in Formula 2: Simultaneously, a hydrocarbon nitrile additive is added, wherein R is a saturated or unsaturated alkane chain with 1 to 5 carbon atoms, and the branched chain may contain at most one nitrile functional group. In this invention, the nitrile functional group in the additive A structure can be oxidized and reduced, participating in the formation of positive and negative electrode films, effectively improving the high-temperature cycling and storage performance of the ternary high-voltage system.

[0014] As a preferred embodiment of the present invention, the lithium salt further includes lithium bis(fluorosulfonyl)imide.

[0015] Preferably, the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte is 0.9 to 1.2 mol / L, wherein the concentration may be 0.9 mol / L, 1.0 mol / L, 1.1 mol / L or 1.2 mol / L, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0016] Preferably, the concentration of lithium fluorosulfonyl difluorophosphate in the electrolyte is 0.1 to 0.3 mol / L, wherein the concentration may be 0.1 mol / L, 0.2 mol / L or 0.3 mol / L, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0017] In this invention, the mixing ratio of lithium bis(fluorosulfonyl)imide and lithium difluorophosphate fluorosulfonylimide not only effectively inhibits the corrosion of lithium bis(fluorosulfonyl)imide, but also significantly improves its high-temperature storage characteristics and rate performance.

[0018] As a preferred embodiment of the present invention, the nitrile additive accounts for 0.2% to 1.0% of the electrolyte by mass fraction. The mass fraction can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1.0%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0019] This invention uses lithium difluorosulfonylimide as the main electrolyte lithium salt, and adds a novel lithium salt, lithium difluorophosphate fluorosulfonylimide, and hydrocarbon nitrile additives. By adjusting the mixing ratio of lithium difluorosulfonylimide and lithium difluorophosphate fluorosulfonylimide and the content of hydrocarbon nitrile additives, not only is the corrosion of lithium difluorosulfonylimide effectively inhibited, but its high-temperature storage characteristics and rate performance are also significantly improved.

[0020] As a preferred embodiment of the present invention, the additives further include carbonate additives and sulfur-containing additives.

[0021] Preferably, the carbonate additives include vinylene carbonate and / or fluoroethylene carbonate.

[0022] Preferably, the sulfur-containing additive includes any one or a combination of at least two of propanesulfonate lactone (PS), propanesulfonate lactone (PST), or vinyl sulfate (DTD), wherein typical but non-limiting examples of the combination include combinations of PS and PST, combinations of PST and DTD, or combinations of PS and DTD, etc.

[0023] Preferably, the lithium salt additive includes any one or a combination of at least two of lithium difluorophosphate, lithium bis(oxalato)borate, or lithium difluorobis(oxalato)phosphate, wherein typical but non-limiting examples of the combination include: a combination of difluorophosphate and lithium bis(oxalato)borate, a combination of lithium bis(oxalato)borate and lithium difluorobis(oxalato)phosphate, or a combination of lithium difluorophosphate and lithium difluorobis(oxalato)phosphate, etc.

[0024] As a preferred embodiment of the present invention, the carbonate additive accounts for 0.2% to 1.0% of the electrolyte by mass fraction. The mass fraction can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1.0%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the sulfur-containing additive accounts for 0.5% to 2.0% of the electrolyte by mass fraction. The mass fraction can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2.0%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] Preferably, the lithium salt additive accounts for 0.5% to 1% of the electrolyte by mass fraction, 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 this range are also applicable.

[0027] In this invention, carbonate additives form a film on the negative electrode, but if the amount added is too small, a dense SEI film cannot be formed, and if the amount added is too large, the impedance will be too high and gas will be generated at high temperature. Sulfur-containing additives assist in film formation on both the positive and negative electrodes and have high thermal stability. If the amount added is too small, a dense SEI film cannot be formed on the negative electrode, and if the amount added is too large, over-film formation will occur, resulting in a decrease in cycle performance.

[0028] As a preferred embodiment of the present invention, the organic solvent includes any two or any combination of any three of ethylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate. Typical but non-limiting examples of such combinations include: combinations of ethylene carbonate and dimethyl carbonate, combinations of dimethyl carbonate and diethyl carbonate, combinations of diethyl carbonate and methyl ethyl carbonate, or combinations of ethylene carbonate, dimethyl carbonate, and diethyl carbonate.

[0029] Preferably, the volume ratio of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate is (20-40):(0-20):(0-20):(30-50), wherein the volume ratio can be 20:0:20:30, 20:0:10:30, 20:0:0:30, 20:0:20:50, 20:0:10:50, 20:0:0:50, 20:20:20:30, 20 The range of values ​​is 20:10:30, 20:20:0:30, 20:10:20:30, 20:10:10:30, 20:10:0:30, 30:0:20:30, 30:0:10:30, 30:0:0:30, 40:0:20:30, 40:0:10:30, or 40:0:0:30, but it is not limited to the listed values. Other unlisted values ​​within this range also apply.

[0030] A second objective of this invention is to provide a method for preparing an electrolyte as described in one objective, the method comprising:

[0031] In an inert atmosphere, carbonate additives, sulfur-containing additives, nitrile additives, and lithium salt additives are added sequentially to an organic solvent, and finally lithium salt is added and mixed at low temperature to obtain the electrolyte.

[0032] As a preferred embodiment of the present invention, the inert atmosphere includes an argon atmosphere.

[0033] Preferably, the temperature of the low-temperature mixing is 8-12 °C. The temperature can be 8 °C, 9 °C, 10 °C, 11 °C or 12 °C, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0034] The third object of the present invention is to provide a high-nickel lithium-ion battery, and the lithium-ion battery includes the electrolyte as described in the first object;

[0035] The lithium-ion battery further includes a positive electrode, a negative electrode and a separator.

[0036] As a preferred technical solution of the present invention, the active material of the positive electrode includes Li(Ni

[0041] , , Co y Mn z )O2, where 0.8 ≤ x < 0.9, 0 < y ≤ 0.1, 0 < z ≤ 0.1 and x + y + z = 1. The value of x can be 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88 or 0.89, etc. The value of y can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1, etc. The value of z can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0037] Preferably, the active material of the negative electrode includes graphite.

[0038] The numerical ranges described in the present invention not only include the above-mentioned listed point values, but also include any point values between the above-mentioned 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.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] The lithium-ion battery prepared by the present invention has low gas generation during high-temperature storage. When stored at 60 °C for 90 days, the volume expansion rate can be as low as below 10.2%, the capacity retention rate can reach above 89.4%, the capacity recovery rate can reach above 91.5%, and can reach 100% at 25 °C / 1C / 1C, can reach above 62.5% at 25 °C / 3C / 1C, and can reach above 51.6% at 25 °C / 5C / 1C. The HF content can be as low as 106.3 ppm after 90 days of storage. Specific Embodiments

[0041] The technical solutions of the present invention will be further described below through specific embodiments.

[0042] Example 1

[0043] This embodiment provides an electrolyte and its preparation method:

[0044] Electrolyte: The electrolyte is composed of organic solvent, lithium salt and additives, wherein the additives are carbonate additives, sulfur-containing additives, nitrile additives and lithium salt additives.

[0045] The carbonate additive is fluoroethylene carbonate, with a mass fraction of 0.3% in the electrolyte;

[0046] The sulfur-containing additive is 1,3-propanesulfonic acid lactone, with a mass fraction of 1.0% in the electrolyte;

[0047] The nitrile additive is 1,3,6-hexanetrionitrile, with a mass fraction of 1% in the electrolyte;

[0048] The lithium salt additive is lithium difluorophosphate, which accounts for 0.7% of the electrolyte by mass.

[0049] The lithium salt is a mixed salt of lithium bis(fluorosulfonyl)imide and lithium difluorophosphate fluorosulfonylimide, with a concentration of 1.2 mol / L in the electrolyte, and the contents of the two electrolyte salts are 1 mol / L and 0.2 mol / L, respectively.

[0050] The organic solvent is composed of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate. Based on the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of diethyl carbonate is 20%, and the volume fraction of methyl ethyl carbonate is 50%.

[0051] Electrolyte preparation method: Under an argon atmosphere, the prescribed amounts of carbonate additives, lithium salt additives, sulfur-containing additives, and nitrile additives are added to an organic solvent, followed by the addition of electrolyte salts. The mixture is stirred and mixed at 8°C to obtain the electrolyte.

[0052] Example 2

[0053] This embodiment provides an electrolyte and its preparation method:

[0054] Electrolyte: The electrolyte consists of an organic solvent, an electrolyte salt, and additives. The additives include carbonate additives, lithium salt additives, sulfur-containing additives, and nitrile additives.

[0055] The carbonate additive is vinylene carbonate, with a mass fraction of 0.2% in the electrolyte;

[0056] The sulfur-containing additive is PST at a mass fraction of 0.5% in the electrolyte;

[0057] The nitrile additive is adiponitrile, with a mass fraction of 0.5% in the electrolyte;

[0058] The lithium salt additive is lithium bis(oxalato)borate, which accounts for 0.5% of the electrolyte by mass.

[0059] The lithium salt is a mixed salt of lithium bis(fluorosulfonyl)imide and lithium difluorophosphate fluorosulfonylimide, with a concentration of 1.0 mol / L in the electrolyte, and the contents of the two electrolyte salts are 0.9 mol / L and 0.1 mol / L, respectively.

[0060] The organic solvent is composed of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate. Based on the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 40%, the volume fraction of diethyl carbonate is 20%, and the volume fraction of methyl ethyl carbonate is 40%.

[0061] Electrolyte preparation method: Under an argon atmosphere, add the prescribed amounts of carbonate additives, lithium salt additives, sulfur-containing additives, and nitrile additives to an organic solvent, then add the electrolyte salt, and stir and mix at 10°C to obtain the electrolyte.

[0062] Example 3

[0063] This embodiment provides an electrolyte and its preparation method:

[0064] Electrolyte: The electrolyte consists of an organic solvent, an electrolyte salt, and additives. The additives include carbonate additives, lithium salt additives, sulfur-containing additives, and nitrile additives.

[0065] The carbonate additive is fluoroethylene carbonate, with a mass fraction of 1.0% in the electrolyte;

[0066] The sulfur-containing additive is DTD at a mass fraction of 2.0% in the electrolyte;

[0067] The nitrile additive is tert-butylmalononitrile, with a mass fraction of 0.2% in the electrolyte;

[0068] The lithium salt additive, lithium difluorobis(oxalato) phosphate, accounts for 1% of the mass fraction of the electrolyte.

[0069] The lithium salt is a mixed salt of lithium bis(fluorosulfonyl)imide and lithium difluorophosphate fluorosulfonylimide, with a concentration of 1.5 mol / L in the electrolyte, and the contents of the two electrolyte salts are 1.2 mol / L and 0.3 mol / L, respectively.

[0070] The organic solvent is composed of ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate. Based on the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of dimethyl carbonate is 20%, and the volume fraction of methyl ethyl carbonate is 50%.

[0071] Electrolyte preparation method: Under an argon atmosphere, the prescribed amounts of carbonate additives, lithium salt additives, sulfur-containing additives, and nitrile additives are added to an organic solvent, followed by the addition of electrolyte salts. The mixture is stirred and mixed at 12°C to obtain the electrolyte.

[0072] Example 4

[0073] In this embodiment, all conditions are the same as in Example 1, except that the mass fraction of fluoroethylene carbonate in the electrolyte is replaced with 0.1% instead of 0.3% by mass.

[0074] Example 5

[0075] In this embodiment, all conditions are the same as in Example 1, except that the mass fraction of fluoroethylene carbonate in the electrolyte is replaced with 1.2% instead of 0.3% in the electrolyte.

[0076] Example 6

[0077] In this embodiment, all conditions are the same as in Example 1, except that the mass fraction of 1,3-propanesulfonic acid lactone in the electrolyte is replaced with 0.3% by 1.0% by mass fraction.

[0078] Example 7

[0079] In this embodiment, all conditions are the same as in Example 1, except that the mass fraction of 1,3-propanesulfonic acid lactone in the electrolyte is replaced with 2.2% by replacing 1.0% by replacing ...

[0080] Example 8

[0081] In this embodiment, all conditions are the same as in Example 1, except that the mass fraction of nitrile additive in the electrolyte is replaced with 0.1% instead of 1.0% by mass.

[0082] Example 9

[0083] In this embodiment, all conditions are the same as in Example 1, except that the mass fraction of nitrile additive in the electrolyte is replaced with 1.2% instead of 1.0%.

[0084] Example 10

[0085] In this embodiment, except that no carbonate additives are added, the remaining solution except for lithium salt and additives is made up with organic solvents, so that the total amount of solution in this embodiment is the same as in Example 1, and all other conditions are the same as in Example 1.

[0086] Example 11

[0087] In this embodiment, except that no sulfur-containing additives are added, the remaining solution except for lithium salts and additives is made up with organic solvents, so that the total amount of solution in this embodiment is the same as in Example 1, and all other conditions are the same as in Example 1.

[0088] Example 12

[0089] In this embodiment, except that no lithium salt additive is added, the remaining solution excluding lithium salt and additive is made up with an organic solvent, so that the total amount of solution in this embodiment is the same as in Example 1, and all other conditions are the same as in Example 1.

[0090] Example 13

[0091] In this embodiment, the only differences are that the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte is replaced with 0.8 mol / L and the concentration of lithium difluorophosphate fluorosulfonylimide in the electrolyte is replaced with 0.4 mol / L; all other conditions are the same as in Example 1.

[0092] Example 14

[0093] In this embodiment, the only differences are that the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte is replaced with 1.15 mol / L and the concentration of lithium difluorophosphate fluorosulfonylimide in the electrolyte is replaced with 0.05 mol / L; all other conditions are the same as in Example 1.

[0094] Comparative Example 1

[0095] The conditions for this comparative example were the same as in Example 1, except that lithium fluorosulfonyl difluorophosphate was not added and the concentration of lithium fluorosulfonyl difluorophosphate was replaced with 1.2 mol / L.

[0096] Comparative Example 2

[0097] Except for the absence of nitrile additives, the remaining solution in this comparative example was made up with organic solvents to make up the remaining solution excluding lithium salt and additives, so that the total amount of solution in this comparative example was the same as in Example 1, and all other conditions were the same as in Example 1.

[0098] The electrolytes from Examples 1-14 and Comparative Examples 1-2 were assembled into batteries, and their performance was tested using lithium-ion batteries. The test results are shown in Table 1.

[0099] The specific preparation method of the lithium-ion battery used in the test includes: preparing a slurry of graphite (negative electrode material), acetylene black (conductive agent), and CMC and SBR (binder) in a mass ratio of 96:1:1:2, coating it onto a copper foil current collector, and vacuum drying to obtain the negative electrode sheet; preparing a slurry of NCM811 (positive electrode material), acetylene black (conductive agent), and PVDF (binder) in a mass ratio of 98:1:1, coating it onto an aluminum foil current collector, and vacuum drying to obtain the positive electrode sheet. The positive electrode sheet, negative electrode sheet, Celgard 2400 separator, and electrolyte prepared in the examples or comparative examples are assembled into a pouch battery. Electrochemical tests are performed using a Xinwei charge-discharge test cabinet, and the HF content of the electrolyte is determined using an ice-water titration method.

[0100] (1) Electrolyte HF content test:

[0101] The electrolyte was stored at 60°C, and the HF content at 0d and 90d was tested by ice-water titration, and recorded as HF-0d and HF-90d respectively.

[0102] (2) Rate discharge performance test of lithium-ion batteries:

[0103] At 25°C, the lithium-ion battery is charged at a constant current of 1C (nominal capacity) to a voltage of 4.25V, then charged at a constant voltage of 4.25V until the current is ≤0.05C. After resting for 10 minutes, it is discharged at a constant current of 1C / 3C / 5C to a cutoff voltage of 2.8V.

[0104] Capacity retention rate (%) of lithium-ion battery after discharge at different rates = (discharge capacity at different rates / 1C discharge capacity) × 100%.

[0105] (3) High-temperature storage performance test of lithium-ion batteries:

[0106] At 25℃, the lithium-ion battery was charged at a constant current of 1C to a voltage of 4.25V, and then charged at a constant voltage of 4.25V to a current of 0.05C. The volume of the lithium-ion battery was measured as V0, and the initial capacity was measured as C0. After that, the lithium-ion battery was placed in a constant temperature chamber at 60℃ and stored for 90 days. The volume of the lithium-ion battery was measured and recorded as V1, the capacity was kept as C1, and the capacity was restored to C2.

[0107] The volume expansion rate (%) of a lithium-ion battery after storage at 60°C for 90 days is calculated as (V1-V0) / V0×100%.

[0108] The capacity retention rate (%) of a lithium-ion battery after 90 days of storage at 60°C is (C1 / C0) × 100%, and the capacity recovery rate (%) of a lithium-ion battery after 90 days of storage at 60°C is (C2 / C0) × 100%.

[0109] Table 1

[0110]

[0111] As can be seen from the table above, the additive content in Examples 1-3 is within the preferred range of this invention, exhibiting superior high-temperature gas generation during storage, high-temperature volume expansion rate, high-temperature capacity retention rate, and high-temperature capacity recovery rate, as well as good rate performance at room temperature. In Example 4, compared to Example 1, the content of fluoroethylene carbonate was reduced, resulting in decreased high-temperature and room-temperature performance, and a decrease in rate performance. Insufficient addition prevents the formation of a dense SEI film. In Example 5, the content of fluoroethylene carbonate was increased, leading to increased high-temperature gas generation during storage. In Example 6, the content of the sulfur-containing additive 1,3-propanesulfonic acid lactone was reduced, resulting in decreased rate performance. This is because the sulfur-containing additive assists in film formation at both the positive and negative electrodes and has high thermal stability; insufficient addition prevents the formation of a dense SI film at the negative electrode. EI film; In Example 7, the content of additive 1,3-propanesulfonate lactone was increased, and excessive addition led to excessive film formation, resulting in a decrease in the rate performance of the battery; In Example 8, the content of nitrile additives was reduced, and in Example 9, the content of nitrile additives was increased, and the electrochemical performance of the battery was reduced compared with Example 1; In Examples 10 to 12, carbonate additives, sulfur-containing additives, and lithium salt additives were not added independently, and the rate performance and cycle performance of the battery were reduced to a certain extent; In Examples 13-14, the amount of lithium difluorophosphate fluorosulfonylimide was replaced with a value outside the preferred range of the present invention, and the high-temperature storage performance and rate performance of the battery were reduced because the mixing ratio of lithium difluorosulfonylimide and lithium difluorophosphate fluorosulfonylimide can effectively inhibit the corrosion of lithium difluorosulfonylimide.

[0112] Comparative Example 1 did not contain lithium fluorosulfonyl difluorophosphate. Compared with Example 1, the high-temperature storage performance and rate performance of the battery in Comparative Example 1 decreased significantly. Comparative Example 2 did not contain nitrile additives. Compared with Example 1, the high-temperature storage performance and rate performance of the battery in Comparative Example 2 also decreased significantly.

[0113] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are 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 within the protection scope of the present invention.

Claims

1. An electrolyte, characterized in that, The electrolyte includes lithium salt, organic solvent, and additives; The additives include nitrile additives, and the structures of the nitrile additives are shown in Formula 1: Formula 1, wherein R is a substituted or unsubstituted C1~C5 alkylene group, and the number of nitrile substituents in the branched chain is ≤1; The lithium salt includes lithium difluorophosphate fluorosulfonylimide and lithium difluorosulfonylimide; The structural formula of the lithium difluorophosphate fluorosulfonylimide is shown in Formula 2: Equation 2; The concentration of lithium difluorosulfonyl imide in the electrolyte is 0.9~1.2 mol / L, and the concentration of lithium difluorophosphate fluorosulfonyl imide in the electrolyte is 0.1~0.3 mol / L; The nitrile additives account for 0.2% to 1.0% of the electrolyte by mass fraction.

2. The electrolyte according to claim 1, characterized in that, The additives also include carbonate additives, sulfur-containing additives, and lithium salt additives.

3. The electrolyte according to claim 2, characterized in that, The carbonate additives include vinylene carbonate and / or fluoroethylene carbonate.

4. The electrolyte according to claim 2, characterized in that, The sulfur-containing additive includes any one or a combination of at least two of propanesulfonate lactone, propylene sulfonate lactone, or vinyl sulfate.

5. The electrolyte according to claim 2, characterized in that, The lithium salt additive includes any one or a combination of at least two of lithium difluorophosphate, lithium bis(oxalato)borate, or lithium difluorobis(oxalato)phosphate.

6. The electrolyte according to claim 2, characterized in that, The carbonate additives account for 0.2% to 1.0% of the electrolyte by mass fraction.

7. The electrolyte according to claim 2, characterized in that, The sulfur-containing additive accounts for 0.5 to 2.0% of the mass fraction of the electrolyte.

8. The electrolyte according to claim 2, characterized in that, The lithium salt additive accounts for 0.5-1% of the electrolyte by mass fraction.

9. The electrolyte according to claim 1, characterized in that, The organic solvent includes any two or any combination of any three of ethylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

10. The electrolyte according to claim 9, characterized in that, The volume ratio of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate is (20~40):(0~20):(0~20):(30~50).

11. A method for preparing an electrolyte as described in any one of claims 1-10, characterized in that, The preparation method includes: In an inert atmosphere, carbonate additives, sulfur-containing additives, nitrile additives, and lithium salt additives are added sequentially to an organic solvent, and finally lithium salt is added and mixed at low temperature to obtain the electrolyte.

12. The preparation method according to claim 11, characterized in that, The inert atmosphere includes an argon atmosphere.

13. The preparation method according to claim 11, characterized in that, The temperature for the low-temperature mixing is 8~12℃.

14. A high-nickel lithium-ion battery, characterized in that, The lithium-ion battery includes the electrolyte as described in any one of claims 1-10; The lithium-ion battery also includes a positive electrode, a negative electrode, and a separator.

15. The lithium-ion battery according to claim 14, characterized in that, The active material of the positive electrode includes Li(Ni x Co y Mn z )O2, where 0.8≤x<0.9, 0<y≤0.1, 0<z≤0.1 and x+y+z=1.

16. The lithium-ion battery according to claim 14, characterized in that, The active material of the negative electrode includes graphite.

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