Electrolyte additive composition containing a novel lithium salt and use thereof

Abstract

The application discloses a kind of electrolyte additive compositions containing novel lithium salt, and its application in the preparation of electrolyte and lithium ion secondary battery, the additive composition includes: as the first additive, at least including the structure shown in the following formula (I-1) novel lithium salt and as the second additive, the structure of the nitrile compound shown in the following formula (II): the definition of each substituent group is described in detail in the specification;The mass ratio of CN functional group in the first additive and the second additive is 0.03-4.0.The first additive accounts for 0.05-5.0wt% of the total mass of electrolyte when the additive composition is used for electrolyte preparation, and the second additive accounts for 0.1-10wt% of the total mass of electrolyte.The electrolyte prepared by using the additive composition disclosed in the application can improve the high-temperature cycle stability and high-temperature storage stability of lithium ion battery under high voltage, especially inhibit the gas expansion of battery under high-temperature working condition, and also consider low initial impedance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery electrolyte, in particular to an additive composition containing pyrosulfate boron trifluoride complex lithium salt and polynitrile compound and application thereof in electrolyte preparation, and application of the prepared electrolyte in lithium ion battery. BACKGROUND

[0002] In the past nearly three decades, consumer electronics (3C) has achieved unprecedented success, and lithium cobaltate has become the most widely used positive electrode material in 3C lithium ion batteries, and its high volume energy density is the greatest competitiveness. Improving the voltage of lithium cobaltate positive electrode and developing high-voltage electrolyte are the basic route for the development of current high-end digital battery. In early 2013, the lithium battery industry generally produced LCO battery with a charging cutoff voltage of 4.2V, and in 2019, the charging cutoff voltage of the produced LCO battery was generally 4.45V. In the next five years, it is planned to produce battery systems with a voltage of 4.52V or even higher. Although the working voltage of the positive electrode seems to increase by only 0.15-0.25V, the technical accumulation and progress required behind it are the continuous technical optimization of the positive electrode material and the electrolyte, including how to inhibit the high-temperature gas expansion, improve the cycle life, and take into account the low-temperature performance, etc., and the electrolyte plays an important role in it. Lithium cobaltate battery at a higher charging voltage, there is a transition from hexagonal phase to monoclinic phase, which leads to the following side reactions of the electrolyte: (1) the stability of the positive electrode interface is poor, the number of high oxidation active sites increases, and the electrolyte is oxidized and decomposed; (2) the positive electrode is accompanied by an oxygen evolution reaction, and oxygen participates in the charge transfer to further oxidize the electrolyte; (3) the transition metal of the positive electrode, especially cobalt, is dissolved, which catalyzes the decomposition of the negative electrode SEI film, the electrolyte repair gas is produced, and the impedance increases. In view of the above technical problems, the research on high-voltage electrolyte mainly focuses on the development of electrolyte additives and the optimization and adjustment of the formula.

[0003] In order to improve the comprehensive performance of the battery at high voltage, lithium salt compounds such as borate and phosphate are widely used in lithium ion battery electrolyte due to their excellent positive and negative electrode film forming function, which is used to improve the long cycle life of the battery.

[0004] Qingyu Dong et al. (ACS Appl. Energy Mater. 2020, 3, 695-704) disclosed that LiDFOB can stabilize the NCM811 electrode interface, reduce polarization, and improve cycle performance. However, LiDFOB containing oxalate structure is easy to produce CO2 and other gases during high-temperature long-period storage, which can easily cause soft package battery swelling and other problems, reducing the use safety and battery life.

[0005] CN110148784A disclosed by Zhuhai Guanyu Patent, using a combination of fluorinated cyclic carbonate, nitrile compounds, lithium difluorophosphate and tetra-vinyl silane additives, so that the battery has high and low temperature performance. However, the solubility of lithium difluorophosphate in conventional ester solvents is low, which brings certain difficulties to the production of electrolyte formula.

[0006] In addition, sulfuric acid ester additives (such as vinyl sulfate, DTD) and fluorinated carbonate additives (such as fluorinated ethylene carbonate, FEC) are often used in the electrolyte industry to improve the performance of the battery at high voltage, but DTD has poor thermal stability and is prone to discoloration during storage, and increases the acidity of the electrolyte; FEC in the electrolyte with lithium hexafluorophosphate as the main salt, during high-temperature storage, will produce HF, which has a negative effect on the high-voltage anode, causing the gas production to increase sharply during high-temperature storage.

[0007] Pyrosulfate boron trifluoride composite lithium salt is a new type of electrolyte additive developed by Zhejiang Chemical Research Institute Co., Ltd. Its patent CN202211583064.X discloses that the additive can improve the cycle performance, high-temperature storage performance and low-temperature performance of the battery. At the same time, patent CN202211583043.8 also discloses that pyrosulfate boron trifluoride composite lithium salt, vinyl sulfate and / or 1,3-propanesulfonic acid lactone are used in combination, or further combined with vinylene carbonate, so that the lithium ion battery in a high energy density system still has excellent cycle performance, high-temperature storage performance and gas production inhibition effect, and can inhibit the impedance growth during battery cycling, further improving the low-temperature performance. However, in a high-voltage working environment, especially when the anode working voltage is greater than or equal to 4.45V, although lithium salt additives have the advantages of reducing impedance and improving cycle, the battery system still faces the difficulty of poor high-temperature storage stability at high voltage, especially at 85°C ultra-high temperature working condition, the gas production increases after high-temperature storage of the battery. This is a technical difficulty that high-voltage battery systems need to consider both low impedance and low temperature.

[0008] Therefore, it is an important research topic to continue to develop more stable, safe and environmentally friendly pyrosulfate boron trifluoride composite lithium salt compositions and electrolyte formulations to meet the performance requirements of batteries at high voltage and 85°C ultra-high temperature. SUMMARY

[0009] The chemical names of the various compounds involved in the present application are as follows:

[0010] 1. LiDFOB, chemical name: lithium difluoro oxalate borate;

[0011] 2. LiPO2F2, chemical name: lithium difluorophosphate;

[0012] 3, PS, chemical name: 1, 3-propane sultone;

[0013] 4, TVS, chemical name: tetraethenylsilane (CAS: 1112-55-6);

[0014] 5, HTCN, chemical name: 1, 3, 6-hexanetricarbonitrile;

[0015] 6, ADN, chemical name: adiponitrile;

[0016] 7, SN, chemical name: succinonitrile;

[0017] 8, TCP, chemical name: 1, 2, 3-tris (cyanoethoxy) propane;

[0018] 9, DFEA, chemical name: 2, 2-difluoroethyl acetate (CAS: 1550-44-3);

[0019] 10, MMDS, chemical name: methyl methylene disulfonate;

[0020] 11, DTD, chemical name: ethylene sulfate.

[0021] In order to solve the above technical problems, the present application further proposes an additive composition containing pyrosulfate boron trifluoride complex lithium salt and nitrile compound and its application in electrolyte preparation and lithium ion battery, which can further improve the high-temperature cycle stability and high-temperature storage stability of lithium ion battery under high voltage on the basis of maintaining the high and low temperature performance of pyrosulfate boron trifluoride complex lithium salt, especially inhibiting the gas expansion of battery under high temperature working condition, and reducing the initial impedance of battery.

[0022] The purpose of the present application is realized by the following technical scheme:

[0023] An electrolyte additive composition containing a new lithium salt, the additive composition comprising:

[0024] The first additive at least comprises a new lithium salt with the structure shown in the following formula (I-1):

[0025]

[0026] The second additive is a nitrile compound selected from the structure shown in the following formula (II), which is a polynitrile compound:

[0027]

[0028] wherein M1 is selected from -CH, P or -P=O, n is an integer from 0 to 5; R1, R2, R3 are independently selected from a direct bond, oxygen, -R4-, -O-R4- or -R5-O-R4-, wherein R4, R5 are independently selected from C1-C5 alkylene, C2-C5 alkenylene, or C1-C5 alkylene, C2-C5 alkenylene substituted by C1-C3 alkyl or cyano;

[0029] The mass ratio of CN functional groups in the first additive and the second additive is 0.03 to 4.0; preferably, the mass ratio of CN functional groups in the first additive and the second additive is 0.08 to 1.0; more preferably, the mass ratio of CN functional groups in the first additive and the second additive is 0.1 to 0.3.

[0030] The mass ratio of CN functional groups in the first additive and the second additive of the present application refers to the ratio of the mass content of the first additive in the electrolyte to the mass content of the CN functional groups in the second additive in the electrolyte. The mass content of the first additive in the electrolyte = (mass of the first additive / total mass of the electrolyte)*100%; the mass content of the CN functional groups in the second additive in the electrolyte = (mass of the second additive*mass content of the CN functional groups in the second additive / total mass of the electrolyte)*100% = mass content of the second additive in the electrolyte*mass content of the CN functional groups in the second additive. The mass content of the CN functional groups in the second additive is calculated by the following formula:

[0031]

[0032] wherein i = 1, 2, …, n, n represents the number of different nitriles, 1≤n≤10, M i is the relative molecular mass of different nitriles, a i is the number of CN functional groups of the i-th nitrile, 26 is the relative molecular mass of CN functional groups. For example, the second additive only contains one nitrile compound, which contains 2 CN functional groups, then n is 1, the relative molecular mass of the nitrile compound is M1, and the mass content of the CN functional groups in the second additive is 52 / M1. For another example, the second additive contains two different nitrile compounds, the first nitrile compound contains 2 CN functional groups, the relative molecular mass is M1, the second nitrile compound contains 3 CN functional groups, the relative molecular mass is M2, then n is 2,

[0033]

[0034] In commercial high-voltage electrolyte formulations, the combination of nitrile compounds usually consists of 2-4 second additives, and the number of CN functional groups of different nitrile compounds is different, so the mass content of CN functional groups is more reasonable to replace the mass content of nitrile compounds to express the synergistic effect of the first additive and the second additive.

[0035] In the additive composition of the present application, the first additive can form a stable passivation film on the negative electrode interface before the second additive at the first formation, inhibit the destruction of the second additive (polycyanogen additive) to the negative electrode interface, and reduce the negative electrode impedance. In this way, it can also promote the second additive to combine with the active sites on the positive electrode surface (such as high-valence metal ions such as nickel / cobalt / manganese, etc.) by the strong coordination ability of the cyano (CN) functional group, and play a role in masking the active ions on the positive electrode surface and reducing the decomposition of the electrolyte on the electrode; and the nitrogen triple bond in the cyano (CN) functional group in the second additive has high bond energy and is not easy to be oxidized, and has strong stability on the high-voltage positive electrode. By using both the first additive and the second additive, the swelling of the lithium ion battery under high-temperature storage is inhibited, the negative electrode impedance is reduced, the kinetic performance of the battery is improved, and the long cycle stability of the lithium ion battery is ensured.

[0036] In the preparation process of the first additive of the present application, in addition to the novel lithium salt of the structure (I-1) as described above, at least one of the compounds of the following formula (I-2), (I-3), (I-4), (I-5), (I-6) will also be included:

[0037]

[0038]

[0039] and the first additive contains at least 80wt% or more of the novel lithium salt of the structure (I-1). Preferably, the first additive contains 80.0-95.0wt% of the novel lithium salt of the structure (I-1), and the rest is at least one of the compounds (I-2), (I-3), (I-4), (I-5) or (I-6).

[0040] In the second additive, M1 is selected from -CH, P or -P=O, n is an integer selected from 0-3; R1, R2, R3 are independently selected from a direct bond, oxygen, -R4-, -O-R4- or -R5-O-R4-, wherein R4, R5 are independently selected from C1-C3 alkylene, C2-C3 alkenylene, or C1-C3 alkylene, C2-C3 alkenylene substituted with C1-C3 alkyl or cyano.

[0041] Further, the second additive is selected from at least one of the nitrile compounds represented by the following structure:

[0042]

[0043] The application also provides a preparation method of the electrolyte, which comprises: adding a main lithium salt in a non-aqueous solvent to account for 8-20 wt% of the total mass of the electrolyte, and then adding the additive composition to obtain the electrolyte, so that the first additive accounts for 0.01-5.0 wt% of the total mass of the electrolyte, and the second additive accounts for 0.1-10 wt% of the total mass of the electrolyte; further preferably, the first additive accounts for 0.01-2.0 wt% of the total mass of the electrolyte, and the second additive accounts for 1-8 wt% of the total mass of the electrolyte; more preferably, the first additive accounts for 0.1-1.0 wt% of the total mass of the electrolyte, and the second additive accounts for 2-6 wt% of the total mass of the electrolyte.

[0044] The non-aqueous solvent is selected from a mixture of a cyclic carbonate and a linear carbonate and / or a linear carboxylic acid ester, the cyclic carbonate is selected from at least one of ethylene carbonate, propylene carbonate or fluorinated ethylene carbonate; the linear carbonate is selected from at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate or methyl trifluoroethyl carbonate; the linear carboxylic acid ester is selected from at least one of ethyl acetate, ethyl propionate, propyl propionate, 2,2-difluoroethyl acetate or 2,2,2-trifluoroethyl acetate, and the amount of any non-aqueous solvent accounts for 0.1-50 wt% of the total mass of the electrolyte;

[0045] The main lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bisfluorosulfonimide or lithium bis(trifluoromethylsulfonyl)imide.

[0046] In the preparation method of the electrolyte, a third additive is further added, the third additive is selected from at least one of the sulfur-oxygen double bond compounds shown in the following structure, and the amount accounts for 0.5-5.0 wt% of the total mass of the electrolyte:

[0047]

[0048] The third additive containing sulfur-oxygen double bond can form a film on the surface of the positive and negative electrodes, and the formed lithium alkyl sulfate with high valence state such as tetravalent or hexavalent can improve the oxidation resistance of the positive electrode interface, and has high lithium ion conductivity, and cooperates with the first additive to form an organic-inorganic composite SEI film, especially when the mass ratio of the CN functional groups in the first additive and the second additive is less than 1.0, the introduction of the third additive can more effectively inhibit the damage of the second additive to the negative electrode interface film, and improve the storage and cycle performance of the battery.

[0049] Preferably, the amount of the third additive accounts for 1.0-4.0 wt% of the total mass of the electrolyte.

[0050] According to the application scenarios and the battery electrochemical performance requirements of different electrolytes, in the preparation method of the electrolyte, a basic additive is further added, the basic additive is at least one selected from fluoroethylene carbonate, vinylene carbonate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluoro oxalate borate, lithium difluoro bis-oxalate phosphate, tris(trimethylsilyl) phosphate, tris(trimethylsilyl) borate, and any additive amount is 0.1-2.0 wt% of the total mass of the electrolyte.

[0051] With the increase of the cut-off working voltage of the lithium ion battery, higher challenges are put forward to the high-temperature storage performance at higher temperatures (such as 85℃). Therefore, further, in the preparation method of the electrolyte, a fourth additive is further added, the fourth additive is at least one selected from tetra-vinyl silane, 2,4,6-tris(allyloxy)-1,3,5-triazine, 1,3,5-triallyl isocyanurate, 1,3-dioxane, 1,4-dioxane, and any additive amount is 0.1-2.0 wt% of the total mass of the electrolyte, which is used for positive electrode film formation, gas production inhibition, and improvement of the high-temperature storage performance of the battery at 85℃.

[0052] The application also provides a lithium ion secondary battery, which comprises a positive electrode sheet, a negative electrode sheet and a separator, and is prepared by the following method:

[0053] The positive electrode sheet, the separator and the negative electrode sheet are wound together into a roll core, sealed with an aluminum plastic film, and then baked, after which the electrolyte prepared by any of the above preparation methods is injected into the roll core, and the lithium ion secondary battery is obtained after standing, formation, capacity distribution and aging.

[0054] The active material of the positive electrode sheet of the lithium secondary battery is selected from lithium cobaltate, lithium nickel-manganese oxide, lithium-rich manganese-based material, lithium manganate, lithium iron phosphate, lithium manganese iron phosphate or LiNi x Co y Mn z L (1-x-y-z) O2, wherein L is Al, Sr, Mg, Ti, Ca, Zr, Zn, Si, W or Fe, 0≤x≤1, 0≤y≤1, 0≤z≤1, 0.5≤x+y+z≤1, and the active material of the negative electrode sheet is selected from graphite, silicon-carbon or silicon-oxygen material.

[0055] Preferably, the active material of the positive electrode sheet at least comprises LiCoO2. That is, the active material of the positive electrode sheet is LiCoO2, or a composite material of LiCoO2 and LiNi x Co y Mn z L (1-x-y-z) O2.

[0056] The electrolyte prepared by using the additive composition of the present application is suitable for lithium ion secondary battery with a cutoff voltage of 4.35V or more, and is also suitable for lithium ion secondary battery with a cutoff voltage of 4.45V or more, and is more suitable for lithium ion secondary battery with a cutoff voltage of 4.48V or more. In the electrolyte formula of the present application, when it is used in a high-voltage environment with a cutoff voltage of 4.48V, the high-temperature cycle stability, high-temperature storage stability and low-temperature performance of the battery are still maintained.

[0057] The electrolyte additive of the present application participates in the construction of the electrode / electrolyte interface during the formation of the battery, so that in the electrolyte liquid of the lithium ion secondary battery, the first additive accounts for 0.01-2.0wt% of the total mass of the electrolyte liquid, and the second additive accounts for 0.1-6.0wt% of the total mass of the electrolyte liquid.

[0058] Further, the third additive accounts for 0.1-3.0wt% of the total mass of the electrolyte liquid, and the fourth additive accounts for 0.02-1.5wt% of the total mass of the electrolyte liquid.

[0059] Compared with the prior art, the present application has the beneficial effects of:

[0060] The additive composition of the present application, through the synergistic effect of the CN functional groups in the first additive and the second additive on the basis of specific ratio and specific content, not only inhibits the outgassing of lithium ion battery under high-temperature storage, but also reduces the negative electrode impedance, improves the kinetic performance of the battery, and ensures the long cycle stability of the lithium ion battery. Compared with the conventional lithium salt additives on the market, the inhibition effect of the additive composition on high-temperature storage outgassing is obviously better than that of the combination of nitrile compounds and LiDFOB, LiDFOP and other lithium oxalate salt additives. DETAILED DESCRIPTION

[0061] The present application will be further described below in conjunction with specific embodiments, but the present application is not limited to these specific embodiments. Those skilled in the art should realize that the present application encompasses all alternatives, improvements and equivalents within the scope of the claims.

[0062] In the examples and comparative examples of the present application, the types of the first additive include the following:

[0063] The first additive A1 contains 95wt% of compound I-1 and 5wt% of compound I-2.

[0064] The first additive A2 contains 90wt% of compound I-1, 6wt% of compound I-2 and 4wt% of compound I-3.

[0065] I. Preparation of electrolyte

[0066] In an argon-filled glove box (moisture <5 ppm, oxygen <10 ppm), ethylene carbonate (EC), propylene carbonate (PC), propyl propionate (PP), diethyl carbonate (DEC), fluoroethylene carbonate (FEC) were mixed in a mass ratio of 1.5:1.5:3:1:1 to form an organic solvent, and 14% of LiPF6 by mass was added based on the total mass of the electrolyte to form a base electrolyte M.

[0067] The first additive and the second additive were quantitatively added to the base electrolyte M described above, and mixed uniformly to form an electrolyte, wherein the mass ratio of the CN functional groups in the first additive and the second additive is denoted as a, and the mass content of the CN functional group in the second additive is denoted as CN%. The specific electrolyte formulation is shown in Table 1 below:

[0068] Table 1 High-voltage electrolyte formulation Table 1

[0069]

[0070]

[0071]

[0072] The first additive, the second additive and other additives were quantitatively added to the base electrolyte M, and mixed uniformly to form an electrolyte. The specific electrolyte formulation is shown in Table 2 below:

[0073] Table 2 High-voltage electrolyte formulation Table 2

[0074]

[0075] II. Electrochemical performance test

[0076] The electrolyte formulation used in the present application solves the problem of poor interface stability of high-voltage positive electrode electrolyte by using both the first additive and the second additive, and simultaneously improves the compatibility problem of nitrile compounds with the negative electrode interface, which is suitable for lithium cobaltate / graphite, lithium cobaltate / silicon-carbon, ternary / graphite, ternary / silicon-carbon, lithium manganese iron phosphate / graphite, etc. The present embodiment only optimizes the lithium cobaltate / graphite system for performance testing.

[0077] The lithium ion battery electrolytes of the above examples and comparative examples were respectively filled into soft-pack lithium ion batteries with a capacity of 1000 mAh. The lithium ion power batteries included positive electrode sheets, negative electrode sheets, separators, electrolytes, and battery accessories. The positive active material was LiCoO2, and the negative active material was graphite.

[0078] The preparation process is as follows: the positive electrode sheet, the separator and the negative electrode sheet are wound together into a core, and after sealing with an aluminum plastic film, the core is baked to make the electrode moisture meet the requirements, and after baking, the core is subjected to electrolyte injection, standing, formation, capacity grading and aging processes to obtain a finished soft package battery core.

[0079] The performance of the lithium ion battery is tested, including:

[0080] (1) Cycle performance test

[0081] At a specified temperature, charge at a current of 1C to a charge cut-off voltage (4.50V, same below), then charge at a constant voltage until the current drops to 0.1C, then discharge at a current of 1C to a discharge cut-off voltage (3.0V, same below), and so on for a certain number of cycles. Record the discharge capacity of the first week and the discharge capacity of the last week, and calculate the capacity retention rate of the battery cycle according to the following formula:

[0082] Capacity retention rate = discharge capacity of the last week / discharge capacity of the first week * 100%.

[0083] (2) 85°C high temperature storage performance test

[0084] Charge at a current of 1C to a charge cut-off voltage at room temperature, and charge at a constant voltage until the current drops to 0.1C, then store in a constant temperature oven at 85°C for 12h (hours). Record the volume change rate and capacity recovery rate before and after high temperature storage.

[0085] (3) Low temperature discharge capacity at -20°C

[0086] Charge at a current of 0.5C to a charge cut-off voltage at room temperature, then charge at a constant voltage until the current drops to 0.1C, then discharge at a current of 0.5C to a discharge cut-off voltage at -20°C. Record the discharge capacity at low temperature. Low temperature capacity retention rate = discharge capacity at low temperature / discharge capacity of the first week * 100%.

[0087] (3) Initial ACR impedance

[0088] At room temperature, adjust the battery capacity to 50% SOC (state of charge) with a standard charging current, and test the internal resistance (mΩ) of the fixed tab position of the capacity graded lithium battery at a frequency of 1KHz using a Japanese Denchi internal resistance meter, recorded as the initial ACR impedance (alternating current resistance).

[0089] The test results are shown in Table 3 below:

[0090] Table 3 LCO / AG-4.5V electrochemical test results (4.50V-3.0V)

[0091]

[0092]

[0093]

[0094] According to the data in Table 3 above, by comparing Example 2 and Comparative Examples 1-2, after the first additive and the second additive are used together, compared with the use of the two alone, in the high-voltage lithium cobaltate system, the gas expansion after the cell is stored at an ultra-high temperature of 85°C for 12h can be inhibited, the capacity recovery rate of the battery can be improved, the low-temperature discharge performance of the cell at-20°C can be effectively improved, and the high-temperature cycle performance at high voltage can be improved.

[0095] Further comparison of Examples 19-22 and Comparative Examples 4-7 shows that when the mass ratio of the CN functional groups in the first additive and the second additive is in the range of 0.03-4.0, and the addition amount of the first additive in the electrolyte satisfies 0.05-5.0wt%, and the second additive satisfies 0.5-10wt%, the high-temperature storage and cycle performance of the battery at high voltage is effectively improved, and the battery has a better low-temperature discharge capacity at-20°C. At the same time, when preparing the electrolyte of Comparative Example 7, when the content of the second additive SN reaches 12%, a small amount of white crystals appear in the electrolyte, which is due to the reaction of LiPF6 with SN to precipitate, further proving that the content of the second additive is recommended to be no more than 10.0wt%. Further comparison of Examples 26-29 shows that the mass ratio of the CN functional groups in the first additive and the second additive is preferably 0.1-0.3. When the above conditions cannot be met at the same time, the improvement of the high and low temperature comprehensive performance of the battery cannot be achieved.

[0096] Further comparison of Examples 8, 19 and Examples 22, 23 shows that on the basis of the appropriate amount of the first additive and the second additive, further adding a third additive containing a sulfur-oxygen double bond can form a film on the positive and negative electrode surfaces, and the lithium alkyl sulfate containing high-valent alkyl groups such as tetravalent or hexavalent formed can improve the oxidation resistance of the positive electrode interface, and has high lithium ion conductivity. The third additive and the first additive synergistically form an organic-inorganic composite SEI film. Especially when the mass ratio of the CN functional groups in the first additive and the second additive is less than 1.0, the introduction of the third additive can more effectively inhibit the damage of CN to the negative electrode interface, and improve the storage and cycle performance of the battery.

[0097] Comparison of Example 19 and Examples 30-34 shows that on the basis of the use of the first additive, the second additive and the third additive, the use of fluorine-containing solvent (DFEA) can further improve the low-temperature discharge performance of the battery, and also consider the cycle performance. However, DFEA has the risk of deteriorating the gas production at high temperature, and by further using the gas production inhibiting additive TVS, the high and low temperature performance can be effectively considered.

[0098] Comparing Example 19 and Comparative Examples 8-9, the use of the first additive, the second additive and the third additive in combination, compared to the use of LiDFOB or LiPO2F2 in combination with the second additive and the third additive, is more helpful to inhibit the gas expansion in high-temperature long-cycle storage and reduce the battery safety risk caused by the swelling of the soft package battery.

[0099] The electrolyte in the lithium cobalt oxide battery after formation (0% SOC) was taken by centrifugation, acidified with dilute nitric acid, filtered, and the filtrate was taken. The main components and content in the electrolyte were measured by gas chromatography mass spectrometry. The test results are as follows:

[0100] Table 4 Residual amount of additives before and after battery formation and storage

[0101] Formulation Set First Additive HTCN PS Example 1 - Charge Amount 0.5％ 1％ 0％ Example 1 - After Formation 0.3％ 0.8％ 0％ Example 1 - After 400 Cycles 0.03％ 0.5％ 0％ Example 19 - Charge Amount 0.5％ 3％ 3％ Example 19 - After Formation 0.4％ 2.6％ 2.2％ Example 19 - After 400 Cycles 0.25％ 2.3％ 0.7％

[0102] According to the results in Table 4 above, in the absence of the third additive PS, the first additive and the second additive in Example 1 gradually consumed during battery formation and cycling. It is speculated that the first additive and the second additive are involved in the modification of the positive and negative electrode interface, respectively, to improve the cycle stability of the high-voltage battery. Compared with Example 1, the content of the second additive is increased to 3% and the third additive PS is introduced in Example 19, and the consumption of the first additive during the cycle is reduced. It is speculated that the second additive and the third additive have a synergistic effect with the first additive, which improves the stability of the electrode interface and is more helpful to improve the cycle performance of the full battery compared with Example 1.

Claims

1. An electrolyte additive composition containing a novel lithium salt, characterized by: The additive composition comprises: a first additive, which comprises at least a novel lithium salt of the structure shown in the following formula (I-1): a second additive, which is selected from at least one nitrile compound of the structure shown in the following formula (II): In the formula, M1 is selected from -CH, P or -P=O, and n is an integer from 0 to 5; R1, R2 and R3 are independently selected from a direct bond, oxygen, -R4-, -O-R4- or -R5-O-R4-, wherein R4 and R5 are independently selected from C1-C5 alkylene, C2-C5 alkenylene, or C1-C5 alkylene or C2-C5 alkenylene substituted with C1-C3 alkyl or cyano; The mass ratio of CN functional groups in the first additive and the second additive is 0.03-4.

0.

2. The electrolyte additive composition containing a novel lithium salt according to claim 1, characterized by: The mass ratio of CN functional groups in the first additive and the second additive is 0.08-1.

0.

3. The electrolyte additive composition containing a novel lithium salt according to claim 1 or 2, characterized by: The first additive further comprises at least one of the compounds of the following formulae (I-2), (I-3), (I-4), (I-5) and (I-6): The first additive contains at least 80.0 wt% of the novel lithium salt of the structure shown in the formula (I-1).

4. The electrolyte additive composition containing a novel lithium salt according to claim 3, characterized by: The first additive contains 80.0-95.0 wt% of the novel lithium salt of the structure shown in the formula (I-1), and the rest is at least one of the compounds (I-2), (I-3), (I-4), (I-5) and (I-6).

5. The electrolyte additive composition containing a novel lithium salt according to claim 1 or 2, characterized by: In the second additive, M1 is selected from -CH, P or -P=O, and n is an integer from 0 to 3; R1, R2 and R3 are independently selected from a direct bond, oxygen, -R4-, -O-R4- or -R5-O-R4-, wherein R4 and R5 are independently selected from C1-C3 alkylene, C2-C3 alkenylene, or C1-C3 alkylene or C2-C3 alkenylene substituted with C1-C3 alkyl or cyano.

6. The electrolyte additive composition containing a novel lithium salt according to claim 5, characterized by: The second additive is selected from at least one nitrile compound of the structure shown in the following formula (II):

7. A method of formulating an electrolyte solution, characterized by: The preparation method comprises: adding a main lithium salt in a non-aqueous solvent to make the main lithium salt account for 8-20 wt% of the total mass of the electrolyte, then adding the additive composition of any one of claims 1-6 to obtain the electrolyte, and making the first additive account for 0.01-5.0 wt% of the total mass of the electrolyte and the second additive account for 0.5-10 wt% of the total mass of the electrolyte; The non-aqueous solvent is a mixture of at least one of a cyclic carbonate, a linear carbonate and a linear carboxylate, the cyclic carbonate is at least one of ethylene carbonate, propylene carbonate or fluorinated ethylene carbonate, the linear carbonate is at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate or methyl trifluoroethyl carbonate, and the linear carboxylate is at least one of ethyl acetate, ethyl propionate, propyl propionate, 2,2-difluoroethyl acetate or 2,2,2-trifluoroethyl acetate, and the amount of any non-aqueous solvent accounts for 0.1-50 wt% of the total mass of the electrolyte; The main lithium salt is at least one of lithium hexafluorophosphate, lithium bisfluorosulfonimide and lithium bis(trifluoromethylsulfonyl)imide.

8. The method of formulating an electrolyte solution of claim 7, wherein: The first additive accounts for 0.1-2.0 wt% of the total mass of the electrolyte, and the second additive accounts for 2.0-6.0 wt% of the total mass of the electrolyte.

9. The method of formulating an electrolyte solution of claim 7, wherein: A third additive is added to the electrolyte, the third additive being at least one of the sulfur-oxygen double bond compounds shown in the following structures, and the amount of the third additive accounting for 0.5-5.0 wt% of the total mass of the electrolyte:

10. The method of formulating an electrolyte solution according to any one of claims 7-9, characterized in that: A base additive is added to the electrolyte, the base additive being at least one of fluorinated ethylene carbonate, vinylene carbonate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluoro oxalato borate, lithium difluoro bis-oxalato phosphate, tris(trimethylsilyl) phosphate, and tris(trimethylsilyl) borate, and the amount of any additive accounting for 0.1-2.0 wt% of the total mass of the electrolyte.

11. The method of formulating an electrolyte solution of claim 10, wherein: A fourth additive is added to the electrolyte, the fourth additive being at least one of tetra-vinyl silane, 2,4,6-tris(allyloxy)-1,3,5-triazine, 1,3,5-triallyl isocyanurate, 1,3-dioxane, and 1,4-dioxane, and the amount of any additive accounting for 0.1-2.0 wt% of the total mass of the electrolyte.

12. A lithium-ion secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator, characterized by: The lithium ion secondary battery is prepared by the following method: winding the positive electrode sheet, the separator, and the negative electrode sheet together into a roll core, sealing with an aluminum plastic film, and then baking, after which the electrolyte prepared by the preparation method of any one of claims 7-11 is injected into the roll core, and the lithium ion secondary battery is obtained after standing, formation, capacity distribution, and aging.

13. The lithium-ion secondary battery according to claim 12, characterized by: The active material of the positive electrode tab is selected from lithium cobaltate, lithium nickel manganese acid, lithium-rich manganese-based, lithium manganate, lithium iron phosphate, lithium manganese iron phosphate or LiNi x Co y Mn z L (1-x-y-z) O2, wherein L is Al, Sr, Mg, Ti, Ca, Zr, Zn, Si, W or Fe, 0≤x≤1, 0≤y≤1, 0≤z≤1, 0.5≤x+y+z≤1, and the active material of the negative electrode tab is selected from graphite, silicon-carbon or silicon-oxygen material.

14. The lithium-ion secondary battery according to claim 13, characterized by: The active material of the positive electrode sheet comprises LiCoO2.

15. The lithium-ion secondary battery according to claim 14, characterized by: The cut-off voltage of the lithium ion secondary battery is ≥4.45 V.

16. The lithium-ion secondary battery according to claim 15, characterized by: The electrolyte additive participates in the construction of the electrode / electrolyte interface during the formation of the battery, so that in the electrolyte liquid of the lithium ion secondary battery, the first additive accounts for 0.01-2.0 wt% of the total mass of the electrolyte liquid, and the second additive accounts for 0.1-9.0 wt% of the total mass of the electrolyte liquid.

17. The lithium-ion secondary battery according to claim 16, characterized by: The third additive accounts for 0.1-4.5 wt% of the total mass of the electrolyte liquid, and the fourth additive accounts for 0.02-1.5 wt% of the total mass of the electrolyte liquid.

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