Fluorodiethyl phosphinate, flame-retardant electrolyte and lithium ion secondary battery

Abstract

The invention discloses fluorodiethyl phosphinate, a flame-retardant electrolyte and a lithium ion secondary battery, and belongs to the technical field of batteries. The fluorodiethyl phosphinate is used as an additive, a flame-retardant electrolyte or a solid electrolyte is provided, and the flame-retardant electrolyte or the solid electrolyte is further used for preparing an electrochemical energy storage device, especially a lithium ion secondary battery. An electrolyte / electrode interface layer generated by oxidation of the flame-retardant electrolyte has the advantages of uniformity, stability, high ion transmission capacity, high mechanical strength and the like, electrolyte decomposition and irreversible phase change of a cathode structure are inhibited, and the fast charging property and the safety of the lithium ion secondary battery are greatly improved; or the high-voltage cycle stability and safety of the lithium ion secondary battery are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and particularly relates to fluorinated diethyl phosphinate, a flame-retardant electrolyte, and a lithium-ion secondary battery. Background Art

[0002] Lithium-ion batteries are widely used in portable electronic devices due to their high specific capacity and excellent cycling performance, and also have broader prospects in electric and hybrid vehicles, aerospace, and smart grids. However, the long charging process of lithium-ion batteries limits the development of mobile phones, electric vehicles, etc. To solve this problem, it is crucial to develop lithium-ion batteries with fast charging capabilities. However, side effects during the fast charging process, such as lithium plating, solid electrolyte interface growth, mechanical degradation, and heat generation, will accelerate the degradation of battery performance, resulting in a decrease in capacity and power performance, and may even cause safety problems. Flame-retardant additives can reduce the flammability of conventional electrolytes, but due to their high viscosity, interfacial incompatibility, or anodic electrochemical instability, they often show a trade-off between flame-retardant performance and battery performance, harmfully affecting the Coulomb efficiency and performance.

[0003] Currently, there have been many literature reports on using phosphinates as film-forming additives, flame-retardant additives, or flame-retardant solvents in electrolytes to improve the high-temperature characteristics and safety performance of lithium / sodium-ion secondary batteries. The prior art with the Chinese patent document publication number CN102916223A discloses a non-aqueous electrolyte that contains at least one organic phosphorus compound such as phosphine oxide, phosphonate, and phosphinate. In this prior art, it is mentioned that the additive phosphorus compound can be at least one organic phosphate compound selected from the group consisting of phosphine oxide, phosphonate, and phosphinate, and the organic phosphate compound includes a substituent having one or more unsaturated bonds, and the substituent has a carbon atom connected to phosphorus. This document believes that when a phosphorus compound is included in the electrolyte, during the electrode reaction, specifically during the initial charging or subsequent charging, a coating derived from the phosphorus compound forms on the surface of at least one of the positive electrode and the negative electrode. This coating can improve battery characteristics such as high-temperature storage characteristics and high-temperature cycling characteristics. This coating derived from the phosphorus compound is a solid electrolyte interface. The prior art with the Chinese patent document publication number CN1685556A discloses that an electrolyte containing phosphinate can prevent the electrolyte from decomposing during high trickle charging or high-temperature storage, resulting in a decrease in performance and gas evolution, causing battery deformation or rupture, thereby improving the high-temperature stability of secondary batteries.A diethyl ethylphosphinate is disclosed as an additive added to the electrolyte in its examples, and the high-temperature storage and high-temperature trickle charging performance of the lithium secondary battery are improved, but the fast charging ability of the battery is not involved in this prior art.

[0004] Prior art published in Chinese Patent Publication No. CN108017669A discloses a method for preparing a class of phosphinates, specifically trifluoroethanol dibutylphosphinate prepared in Example 1. The method also investigates the performance of trifluoroethanol dibutylphosphinate when added to an electrolyte and used in a battery. However, this prior art does not address the fast-charging capability of the battery.

[0005] Although the prior art attempts to achieve flame retardancy, improve battery performance in high-temperature environments, or inhibit decomposition of the electrolyte at high temperatures by adding alkyl phosphinates as additives to the electrolyte, none of these prior arts achieves both flame retardancy and fast charging effects by adding alkyl phosphinates. Simultaneously achieving both flame retardancy and fast charging remains an important issue that needs to be urgently addressed in this field. Summary of the Invention

[0006] 1. Problem to be solved

[0007] Based on this, the first purpose of the present invention is to provide a new fluorodialkyl phosphinate, which is added to an electrolyte or solid electrolyte and applied to an electrochemical energy storage device, especially a lithium-ion secondary battery, and can simultaneously achieve flame retardancy and fast charging effects, at least partially solving the problems described in the background technology.

[0008] The second object of the present invention is to provide a flame-retardant electrolyte or solid electrolyte containing a certain amount of a new fluorodialkylphosphinate, which is used in electrochemical energy storage devices, especially lithium-ion secondary batteries, and can simultaneously achieve flame retardancy and fast charging effects.

[0009] The third object of the present invention is to provide an electrochemical energy storage device, especially a lithium-ion secondary battery, whose electrolyte or solid electrolyte contains a certain amount of a new fluorodialkylphosphinate. The electrochemical energy storage device, especially the lithium-ion secondary battery, can simultaneously achieve safety (i.e., flame retardant performance) and fast charging effect at high temperatures.

[0010] 2. Technical solution

[0011] The present invention proposes a new fluorodiethylphosphinate, using the fluorodiethylphosphinate as an additive to provide a flame-retardant electrolyte or solid electrolyte, and further using the flame-retardant electrolyte or solid electrolyte to prepare an electrochemical energy storage device, especially a lithium-ion secondary battery. The electrolyte / electrode interface layer generated by oxidation of the flame-retardant electrolyte has the advantages of being uniform, stable, having strong ion transmission capacity, and having high mechanical strength, inhibiting electrolyte decomposition and irreversible phase change of the cathode structure, greatly improving the fast charging and safety of the lithium-ion secondary battery, or improving the high-voltage cycle stability and safety of the lithium-ion secondary battery.

[0012] The technical solutions of the present invention are as follows:

[0013] [Compound]

[0014] The first aspect of the present invention provides a series of compounds having the structure of formula I:

[0015]

[0016] Wherein, R 1 is selected from one of pentafluorophenyl, trifluorophenyl, difluoromethyl, trifluoromethyl, difluoroethyl, trifluoroethyl, tetrafluoropropyl, pentafluoropropyl, hexafluoroisopropyl.

[0017] As a preference of the compound according to any embodiment of the first aspect of the present invention, wherein, R 1 is selected from one of difluoromethyl, trifluoromethyl, difluoroethyl, trifluoroethyl, tetrafluoropropyl.

[0018] As a preference of the compound according to any embodiment of the first aspect of the present invention, the compound has a structural formula selected from the following:

[0019]

[0020] It should be noted that when the compounds of formula II to formula IV are added to the electrolyte, while improving the flame retardancy performance, they can also meet the requirements of fast charging or improving the high-voltage cycle stability of lithium-ion secondary batteries.

[0021] [Use of the compound as a flame retardant additive]

[0022] The second aspect of the present invention provides the use of one or more of the compounds according to any embodiment of the first aspect of the present invention as a flame retardant additive in a flame retardant electrolyte or a solid electrolyte.

[0023] [Flame retardant electrolyte]

[0024] The third aspect of the present invention provides a flame retardant electrolyte comprising one or more of the compounds according to any embodiment of the first aspect.

[0025] In the flame-retardant electrolyte described in the third aspect of the present invention, the addition of tetrafluoropropyl diethylphosphinate, trifluoroethyl diethylphosphinate, and difluoroethyl diethylphosphinate significantly improves the flame-retardant effect compared to the electrolyte without addition, further enhancing the safety of the lithium-ion secondary battery during operation at high temperatures. When the content of the flame-retardant additive in the electrolyte is between 0 wt% and 5 wt% (greater than 0 wt%), the flame-retardant effects of difluoroethyl diethylphosphinate and trifluoroethyl diethylphosphinate are the best; when the content of the flame-retardant additive in the electrolyte is between 10 wt% and 20 wt%, the flame-retardant effect of trifluoroethyl diethylphosphinate is the best. Therefore, considering from the perspective of the flame-retardant effect, the most preferred compound of the present invention added to the electrolyte is trifluoroethyl diethylphosphinate.

[0026] As a preference of the flame-retardant electrolyte according to any embodiment of the third aspect of the present invention, the flame-retardant electrolyte comprises:

[0027]

[0028] When the flame-retardant electrolyte described in the third aspect of the present invention is applied to a lithium-ion secondary battery, a lithium-ion secondary battery prepared from an electrolyte added with 1% - 3% of tetrafluoropropyl diethylphosphinate, trifluoroethyl diethylphosphinate, and difluoroethyl diethylphosphinate has a good protective effect on lithium metal for the formed negative solid electrolyte interface film (SEI) compared to a lithium-ion secondary battery prepared from an electrolyte without adding these compounds. The overpotential during the cycling process is lower than that of the base electrolyte, indicating that the additive fluorinated diethylphosphinate can reduce the polarization in the lithium symmetric battery, enabling lithium ions to electroplate / strip better and inhibiting the growth of lithium dendrites, further enhancing the cycle life of the lithium-ion secondary battery. At the same addition amount, the cycle life: tetrafluoropropyl diethylphosphinate > trifluoroethyl diethylphosphinate > difluoroethyl diethylphosphinate. Therefore, considering from the perspective of the cycle life, the most preferred compound of the present invention added to the electrolyte is tetrafluoropropyl diethylphosphinate.

[0029] As a preference of the flame-retardant electrolyte according to any embodiment of the third aspect of the present invention, the flame-retardant electrolyte comprises:

[0030]

[0031] The flame-retardant electrolyte described in the third aspect of the present invention is applied to a lithium-ion secondary battery. A lithium-ion secondary battery prepared with an electrolyte added with tetrafluoropropyl diethylphosphinate, trifluoroethyl diethylphosphinate, and difluoroethyl diethylphosphinate, compared with a lithium-ion secondary battery prepared with an electrolyte without adding these compounds, at 25 °C, in the voltage range of 3.0 to 4.3 V, cycles 300 times at 0.5C or 200 times at 1C. Trifluoroethyl diethylphosphinate can effectively increase the capacity retention rate of the battery and improve the cycling performance of the lithium metal battery. Therefore, considering from the perspective of cycling performance, the compound of the present invention added to the flame-retardant electrolyte is most preferably trifluoroethyl diethylphosphinate.

[0032] The flame-retardant electrolyte described in the third aspect of the present invention is applied to a lithium-ion secondary battery. A lithium-ion secondary battery prepared with an electrolyte added with trifluoroethyl diethylphosphinate, compared with a lithium-ion secondary battery prepared with an electrolyte without adding this compound, at 25 °C, in the voltage range of 3.0 to 4.5 V, cycles at 0.5C. After 200 cycles, the capacity retention rate is significantly improved, and it can effectively increase the high-voltage cycling performance of the lithium-ion battery. Therefore, considering from the perspective of high-voltage cycling performance, the compound of the present invention added to the flame-retardant electrolyte is most preferably trifluoroethyl diethylphosphinate.

[0033] The flame-retardant electrolyte described in the third aspect of the present invention is applied to a lithium-ion secondary battery. A lithium-ion secondary battery prepared with an electrolyte added with trifluoroethyl diethylphosphinate, compared with a lithium-ion secondary battery prepared with an electrolyte without adding this compound, has improved high-voltage rate performance: The test results show that when the cut-off voltage is 4.5 V, at 5C and higher rates, the discharge specific capacity of the lithium-ion secondary battery prepared with an electrolyte containing 1 wt% of trifluoroethyl diethylphosphinate is significantly higher than that of the base electrolyte. Under 5C rate charge-discharge cycling, in some embodiments, the electrolyte containing 1 wt% of trifluoroethyl diethylphosphinate increases the discharge specific capacity from 101.7 mAh / g to 146.8 mAh / g. When charging and discharging at a super-high rate of 10C, the electrolyte containing 1 wt% of trifluoroethyl diethylphosphinate increases the discharge specific capacity from 36 mAh / g to 68.2 mAh / g. Compared with the initial discharge specific capacity at 0.1C, the capacity recovery rate of the base electrolyte is 92.6%, and the capacity recovery rate of the electrolyte containing 1 wt% of trifluoroethyl diethylphosphinate is 97.3%. Trifluoroethyl diethylphosphinate effectively increases the discharge specific capacity and capacity recovery rate of the lithium-ion battery under high voltage, and effectively improves the rate performance of the lithium-ion battery under high voltage. Therefore, considering from the perspective of high-voltage rate performance, the compound of the present invention added to the flame-retardant electrolyte is most preferably trifluoroethyl diethylphosphinate.

[0034] Furthermore, the flame-retardant electrolyte described in the third aspect of the present invention, when applied to a lithium-ion secondary battery, can achieve fast charging at a voltage level higher than that in the prior art (4.5 V). And the data in the examples show that, for example, when trifluoroethyl diethylphosphinate is added as an additive to the electrolyte at 1 wt%, the conversion rate (Coulombic efficiency) of the lithium-ion secondary battery operating at 4.5 V reaches more than 99%.

[0035] When the flame-retardant electrolyte described in the third aspect of the present invention is applied to a lithium-ion secondary battery, the lithium-ion secondary battery prepared with the electrolyte added with trifluoroethyl diethylphosphinate has improved capacity retention rate and capacity recovery rate after high-temperature storage compared with the lithium-ion secondary battery prepared with the electrolyte without adding this compound, and can effectively improve the high-temperature storage performance of the lithium-ion battery.

[0036] As a preference for the flame-retardant electrolyte of any implementation manner of the third aspect of the present invention, the flame-retardant electrolyte comprises:

[0037]

[0038] As described in the present invention, when the flame-retardant electrolyte described in the third aspect of the present invention is applied to a lithium-ion secondary battery, the lithium-ion secondary battery prepared with the electrolyte added with tetrafluoropropyl diethylphosphinate, trifluoroethyl diethylphosphinate, and difluoroethyl diethylphosphinate can effectively improve the discharge specific capacity of the lithium-ion secondary battery and the fast charging performance of the lithium-ion secondary battery in the voltage range of 3.0 - 4.3 V compared with the lithium-ion secondary battery prepared with the electrolyte without adding these compounds. In particular, the discharge specific capacity of the electrolyte with difluoroethyl diethylphosphinate at 10C is much higher than that of the blank electrolyte without addition, and the improvement of the fast charging performance of the lithium-ion secondary battery is the most significant, and the damage to the battery is also the smallest. At the same time, in the voltage range of 3.0 - 4.3 V, after 500 cycles at 10C, the addition of difluoroethyl diethylphosphinate significantly improves both the discharge specific capacity and the capacity retention rate, effectively improving the cycle stability of the lithium-ion battery under fast charging. Therefore, from the perspective of fast charging performance, the compound of the present invention added to the flame-retardant electrolyte is most preferably difluoroethyl diethylphosphinate.

[0039] As a preference for the flame-retardant electrolyte of any implementation manner of the third aspect of the present invention, the flame-retardant electrolyte further comprises:

[0040] Aprotic organic solvents; and

[0041] Lithium salts.

[0042] As a preference for the flame-retardant electrolyte of any implementation manner of the third aspect of the present invention, the concentration of the lithium salt is 0.5 - 2 M.

[0043] As a preference of the flame retardant electrolyte according to any embodiment of the third aspect of the present invention, the compounds selected from Formula I, Formula II, Formula III, and Formula IV are present in the flame retardant electrolyte at a mass concentration of 0.5 wt% to 20 wt%, preferably at a mass concentration of 0.5 wt% to 3 wt% in the flame retardant electrolyte.

[0044] Preferably, the addition amount of the compounds selected from Formula I, Formula II, Formula III, and Formula IV is:

[0045] 0.5 wt% - 18 wt%; 0.5 wt% - 15 wt%; 0.5 wt% - 12 wt%; 0.5 wt% - 10 wt%; 0.5 wt% - 8 wt%; 0.5 wt% - 5 wt%; 0.5 wt% - 4 wt%; 0.5 wt% - 3 wt%; 0.5 wt% - 2 wt%; 0.5 wt% - 1 wt%;

[0046] 1 wt% - 20 wt%; 1 wt% - 18 wt%; 1 wt% - 15 wt%; 1 wt% - 12 wt%; 1 wt% - 10 wt%; 1 wt% - 8 wt%; 1 wt% - 5 wt%; 1 wt% - 4 wt%; 1 wt% - 3 wt%; 1 wt% - 2 wt%;

[0047] 2 wt% - 20 wt%; 2 wt% - 18 wt%; 2 wt% - 15 wt%; 2 wt% - 12 wt%; 2 wt% - 10 wt%; 2 wt% - 8 wt%; 2 wt% - 5 wt%; 2 wt% - 4 wt%; 2 wt% - 3 wt%;

[0048] 3 wt% - 20 wt%; 3 wt% - 18 wt%; 3 wt% - 15 wt%; 3 wt% - 12 wt%; 3 wt% - 10 wt%; 3 wt% - 8 wt%; 3 wt% - 5 wt%; 3 wt% - 4 wt%;

[0049] 4 wt% - 20 wt%; 4 wt% - 18 wt%; 4 wt% - 15 wt%; 4 wt% - 12 wt%; 4 wt% - 10 wt%; 4 wt% - 8 wt%; 4 wt% - 5 wt%;

[0050] 5 wt% - 20 wt%; 5 wt% - 18 wt%; 5 wt% - 15 wt%; 5 wt% - 12 wt%; 5 wt% - 10 wt%; 5 wt% - 8 wt%;

[0051] 8 wt% - 20 wt%; 8 wt% - 18 wt%; 8 wt% - 15 wt%; 8 wt% - 12 wt%; 8 wt% - 10 wt%;

[0052] 10 wt% to 20 wt%; 10 wt% to 18 wt%; 10 wt% to 15 wt%; 10 wt% to 12 wt%;

[0053] 12 wt% to 20 wt%; 12 wt% to 18 wt%; 12 wt% to 15 wt%;

[0054] 15 wt% to 20 wt%; 15 wt% to 18 wt%;

[0055] 18 wt% to 20 wt%.

[0056] From the perspective of improving the flame retardancy effect, the addition amount of the above compound as an additive is preferably at a relatively high level within the above range in the proportion of the electrolyte, for example, preferably 5 wt% to 20 wt%, more preferably 8 wt% to 20 wt%; but from the perspective of maintaining the cycle performance and rate performance of the battery, the addition amount of the above compound as an additive is preferably at a relatively low level within the above range in the proportion of the electrolyte, for example, preferably 0.5 wt% to 3 wt%, further preferably 1 wt% ± 0.2 wt%, and most preferably 1 wt% ± 0.1 wt%.

[0057] Preferably, in some examples, the addition amounts of the compounds selected from Formula I, Formula II, Formula III, and Formula IV are 1.0 wt%, 2.0 wt%, 3.0 wt%, and 5.0 wt% of the total mass of the electrolyte.

[0058] Preferably, in some examples, the addition amounts of the compounds selected from Formula I, Formula II, Formula III, and Formula IV are 1.0 wt%, 2.0 wt%, 3.0 wt%, and 5.0 wt% of the total mass of the conventional electrolyte (not the total mass of the electrolyte) without adding such additives.

[0059] As a preference for the flame-retardant electrolyte according to any embodiment of the third aspect of the present invention, the aprotic organic solvent is present in the flame-retardant electrolyte at a mass concentration of 60% to 90%.

[0060] As a preference for the flame-retardant electrolyte according to any embodiment of the third aspect of the present invention,

[0061] the aprotic organic solvent is selected from one or more combinations of carbonates, carboxylates, ethers, and sulfones;

[0062] Preferably, the carbonate solvents include one or more combinations of ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and methyl propyl carbonate;

[0063] Preferably, the carboxylic acid solvent includes one or a combination of more than one of γ-lactone, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, ethyl fluoroacetate, methyl fluoropropionate, ethyl fluoropropionate, and propyl fluoropropionate;

[0064] Preferably, the ether solvent includes one or a combination of more than one of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-dimethoxyethane, dimethoxypropane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol dibutyl ether, tetraglycine, crown ether, and cryptand;

[0065] Preferably, the sulfone solvent includes one or a combination of more than one of sulfolane, fluorosulfolane, methyl ethyl sulfone, methyl propyl sulfone, methyl isopropyl sulfone, fluorinated methyl ethyl sulfone, fluorinated methyl propyl sulfone, and fluorinated methyl isopropyl sulfone.

[0066] As a preference for the flame-retardant electrolyte of any implementation manner of the third aspect of the present invention,

[0067] The lithium salt is selected from one or a combination of more than one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium hexafluoroarsenate, anhydrous lithium perchlorate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalato)phosphate, lithium difluoro bis(oxalato)phosphate, lithium monooxalato bis(fluoroborate), lithium difluorophosphate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(malonato)borate, lithium bis(difluoromalonato)borate, lithium malonato oxalato borate, lithium (difluoromalonato oxalato)borate, and lithium tris(difluoromalonato)phosphate.

[0068] [Flame-Retardant Solid Electrolyte]

[0069] The fourth aspect of the present invention provides a flame-retardant solid electrolyte, which contains one or more of the compounds described in any implementation manner of the first aspect.

[0070] In the flame-retardant solid electrolyte described in the fourth aspect of the present invention, based on the addition of tetrafluoropropyl diethylphosphinate, trifluoroethyl diethylphosphinate, and difluoroethyl diethylphosphinate with flame-retardant properties, compared with the solid electrolyte without addition, it is beneficial to improve the flame-retardant effect.

[0071] As a preference for the flame-retardant solid electrolyte of any implementation manner of the fourth aspect of the present invention, the flame-retardant solid electrolyte contains:

[0072]

[0073] The flame-retardant solid electrolyte described in the fourth aspect of the present invention is applied to a lithium-ion secondary battery. Compared with a lithium-ion secondary battery prepared from a solid electrolyte without adding these compounds, it is beneficial to form a cathode electrolyte interface (CEI) film, which has a good protective effect on lithium metal. The overpotential during cycling is lower than that of the base electrolyte, which can reduce the polarization in a lithium symmetric battery, enable better plating / stripping of lithium ions, inhibit the growth of lithium dendrites, and further improve the cycle life of the lithium-ion secondary battery. Considering from the perspective of cycle life, the compound of the present invention added to the flame-retardant solid electrolyte is most preferably tetrafluoropropyl diethylphosphinate.

[0074] As a preference for the solid electrolyte of any embodiment of the fourth aspect of the present invention, the solid electrolyte contains:

[0075]

[0076] The flame-retardant solid electrolyte described in the fourth aspect of the present invention is applied to a lithium-ion secondary battery, which is beneficial to improving the cycle performance of a lithium metal battery. Considering from the perspective of cycle performance, the compound of the present invention added to the flame-retardant solid electrolyte is preferably trifluoroethyl diethylphosphinate.

[0077] The flame-retardant solid electrolyte described in the fourth aspect of the present invention is applied to a lithium-ion secondary battery, which is beneficial to increasing the high-voltage cycle performance of a lithium-ion battery. Considering from the perspective of high-voltage cycle performance, the compound of the present invention added to the flame-retardant solid electrolyte is preferably trifluoroethyl diethylphosphinate.

[0078] The flame-retardant solid electrolyte described in the fourth aspect of the present invention is applied to a lithium-ion secondary battery, which is beneficial to improving the rate performance of a lithium-ion battery under high voltage. Considering from the perspective of high-voltage rate performance, the compound of the present invention added to the flame-retardant solid electrolyte is preferably trifluoroethyl diethylphosphinate.

[0079] Furthermore, the flame-retardant solid electrolyte described in the fourth aspect of the present invention, when applied to a lithium-ion secondary battery, is beneficial to achieving fast charging at a voltage level higher than that in the prior art (4.5V).

[0080] The flame-retardant solid electrolyte described in the fourth aspect of the present invention is applied to a lithium-ion secondary battery, which is beneficial to improving the high-temperature storage performance of a lithium-ion battery.

[0081] As a preference for the flame-retardant solid electrolyte of any embodiment of the fourth aspect of the present invention, the flame-retardant solid electrolyte contains:

[0082]

[0083] When the flame-retardant solid electrolyte described in the fourth aspect of the present invention is applied to a lithium-ion secondary battery, it is beneficial to improve the discharge specific capacity of the lithium-ion secondary battery and the fast charging performance of the lithium-ion secondary battery, and reduce the damage to the battery. At the same time, the capacity retention rate can be significantly increased, and the cycle stability of the lithium-ion battery under fast charging can be improved. From the perspective of fast charging performance, the compound of the present invention added to the flame-retardant solid electrolyte is preferably difluoroethyl diethylphosphinate.

[0084] As a preference of the solid electrolyte according to any one of the embodiments of the fourth aspect of the present invention, the solid electrolyte further comprises:

[0085] a polymer matrix; and

[0086] a conductive lithium salt.

[0087] As a preference of the solid electrolyte according to any one of the embodiments of the fourth aspect of the present invention, the polymer matrix is selected from one or a mixture of more than one of the following substances: polyethylene oxide, polyethylene glycol, poly(ethylene carbonate), poly(propylene carbonate), polyacrylonitrile, polymethyl methacrylate or poly(vinylidene fluoride); and / or

[0088] The conductive lithium salt is selected from one or a mixture of more than one of the following substances: lithium hexafluorophosphate, lithium difluorophosphate, lithium tris(pentafluoroethyl)trifluorophosphate, lithium tetrafluoroxalate phosphate, lithium trioxalate phosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluorooxalate borate, lithium pentafluoroethyltrifluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide or lithium trifluoromethanesulfonate.

[0089] As a preference of the solid electrolyte according to any one of the embodiments of the fourth aspect of the present invention, the solid electrolyte is in the form of a film with a thickness of 1 μm to 100 μm.

[0090] [Electrochemical energy storage device]

[0091] The fifth aspect of the present invention provides an electrochemical energy storage device, comprising:

[0092] a cathode;

[0093] an anode;

[0094] a separator; and

[0095] the flame-retardant electrolyte according to any one of the embodiments of the third aspect of the present invention.

[0096] The fifth aspect of the present invention provides another electrochemical energy storage device, comprising:

[0097] a cathode;

[0098] an anode; and

[0099] The flame-retardant solid electrolyte according to any one of the embodiments of the fourth aspect of the present invention.

[0100] [Lithium-ion secondary battery]

[0101] The sixth aspect of the present invention provides a lithium-ion secondary battery, comprising:

[0102] A cathode;

[0103] An anode;

[0104] A separator; and

[0105] The flame-retardant electrolyte according to any one of the embodiments of the third aspect of the present invention.

[0106] Due to the addition of the compound of formula I in the electrolyte, the above-mentioned lithium-ion secondary battery at least partially has excellent high-voltage cycle stability, fast charging performance and safety stability.

[0107] As a preference of the lithium-ion secondary battery of the sixth aspect of the present invention, the flame-retardant electrolyte contains the following compounds:

[0108]

[0109] As described above, the flame-retardant electrolyte added with the compound of formula II is applied to the lithium-ion secondary battery, improving the cycle life of the lithium-ion secondary battery.

[0110] As a preference of the lithium-ion secondary battery of the sixth aspect of the present invention, the flame-retardant electrolyte contains the following compounds:

[0111]

[0112] As described above, the flame-retardant electrolyte added with the compound of formula III is applied to the lithium-ion secondary battery, which can effectively increase the capacity retention rate of the battery and improve the cycle performance, high-voltage cycle performance, high-voltage rate performance and high-temperature storage performance of the battery.

[0113] As a preference of the lithium-ion secondary battery of the sixth aspect of the present invention, the flame-retardant electrolyte contains the following compounds:

[0114]

[0115] As described above, the flame-retardant electrolyte added with the compound of formula IV is applied to the lithium-ion secondary battery, significantly improving both the discharge specific capacity and the capacity retention rate, and effectively improving the cycle stability of the lithium-ion battery under fast charging.

[0116] The sixth aspect of the present invention provides another lithium-ion secondary battery, comprising:

[0117] Cathode;

[0118] Anode; and

[0119] The flame - retardant solid electrolyte according to any embodiment of the fourth aspect of the present invention.

[0120] [Preparation method of compound]

[0121] The present invention discloses a method for preparing compounds of Formula II and Formula III in the seventh aspect, comprising the following steps:

[0122] Adding thionyl chloride to diethylphosphinic acid under the protection of inert gas to form diethylphosphinic acid chloride;

[0123] Adding fluoroalcohol to diethylphosphinic acid chloride to form diethylphosphinic acid fluoroester.

[0124] Among them, the step of adding thionyl chloride is preferably carried out by slowly dropping it under stirring at room temperature, and after the dropping is completed and stabilized, the temperature is raised to a slightly reflux state, and the reaction duration is about 1 - 3 h.

[0125] Among them, the step of adding fluoroalcohol is preferably carried out by slowly dropping it under stirring at room temperature, and after the dropping is completed, the temperature is raised to reflux, and the reaction duration is about 4 - 8 h.

[0126] The present invention discloses another method for preparing a compound of Formula IV in the seventh aspect, comprising the following steps:

[0127] Mixing sodium diethylphosphinate and fluoro - chloroalkane at low temperature and heating and reacting in an autoclave to obtain diethylphosphinic acid fluoroester.

[0128] Among them, the temperature range of the low temperature is preferably 0 - 10 °C; the temperature range of heating in the autoclave is preferably 100 - 200 °C; the heating reaction duration is preferably 4 - 8 h.

[0129] 3. Beneficial effects

[0130] Compared with the prior art, the beneficial effects of the present invention are:

[0131] (1) The compound of Formula I provided by the present invention can be used as a flame - retardant additive in conventional electrolytes or solid electrolytes, effectively improving the safety of lithium - ion batteries;

[0132] (2) While the compound of Formula IV provided by the present invention is used as a flame - retardant additive for conventional electrolytes or solid electrolytes, not only the discharge specific capacity and capacity retention rate are significantly improved under normal - pressure fast charging, effectively improving the cycle stability of lithium - ion batteries under fast charging, but also the damage to the battery is minimized, and it hardly affects the discharge specific capacity and capacity retention rate under slow charging again;

[0133] (3) While the compound of Formula III provided by the present invention serves as a flame retardant additive for conventional electrolytes or solid electrolytes, it not only has good cycle stability under normal pressure, but also has excellent high-voltage cycle stability, high-voltage rate performance, high-voltage conversion rate, and high-temperature storage performance;

[0134] (4) While the compound of Formula II provided by the present invention serves as a flame retardant additive for conventional electrolytes or solid electrolytes, it can significantly improve the cycle life of lithium-ion batteries;

[0135] (5) The additive provided by the present invention contains a P=O bond, and the lone pair electrons of the oxygen atom can play the role of a Lewis base, effectively scavenging the strong Lewis acids PF5 and PF3O derived from LiPF6, avoiding their corrosion and degradation of the surface of the positive electrode material, better maintaining the structural stability of the positive electrode material, and further leading to an improvement in the cycle stability performance of the battery;

[0136] The additive provided by the present invention is rich in C-F bonds, reducing the LUMO and HOMO values of the additive, contributing to the reduction of the additive to form a stable, LiF-rich solid electrolyte interphase layer (SEI) on the negative electrode, significantly inhibiting the growth of lithium dendrites, reducing the decomposition of the electrolyte, and further leading to an improvement in the cycle stability performance of the battery;

[0137] The additive provided by the present invention participates in the formation of a thin, C-F-rich cathode electrolyte interface layer (CEI), improving the stable cycling of the cathode at high voltages. Description of the Drawings

[0138] Figure 1 1H NMR characterization diagram of the compound of Formula II prepared in Preparation Example 1 of the present invention; 1 H NMR characterization diagram;

[0139] Figure 2 1H NMR characterization diagram of the compound of Formula III prepared in Preparation Example 2 of the present invention; 1 1HNMR characterization diagram;

[0140] Figure 3 1H NMR characterization diagram of the compound of Formula IV prepared in Preparation Example 3 of the present invention; 1 H NMR characterization diagram;

[0141] Figure 4 Self-extinguishing time test diagram of electrolyte formulations with different addition ratios of fluorinated diethyl phosphite;

[0142] Figure 5 Cycling performance diagram of lithium symmetric batteries using the electrolyte formulations prepared in Comparative Example 1 and Examples 1 to 4 as electrolytes;

[0143] Figure 6 Cycling performance diagram of lithium symmetric batteries using the electrolyte formulations prepared in Comparative Example 1 and Examples 5 to 8 as electrolytes;

[0144] Figure 7 Cycling performance graphs of lithium symmetric batteries using the electrolyte formulations prepared in Comparative Example 1 and Examples 9-12 as electrolytes;

[0145] Figure 8 Discharge specific capacity performance graphs of NCM811||Li half-cells using the electrolyte formulations prepared in Comparative Example 1 and Examples 1, 5, 9 as electrolytes (25 °C, voltage range: 3.0 - 4.3 V, 0.5C cycling for 300 cycles after 0.1C activation for 5 cycles);

[0146] Figure 9 Discharge specific capacity performance graphs of NCM811||Li half-cells using the electrolyte formulations prepared in Comparative Example 1 and Examples 1, 5, 9 as electrolytes (25 °C, voltage range: 3.0 - 4.3 V, 1C cycling for 200 cycles after 0.1C activation for 5 cycles);

[0147] Figure 10 Discharge specific capacity performance graphs of NCM811||Li half-cells using the electrolyte formulations prepared in Comparative Example 1 and Example 5 as electrolytes (25 °C, voltage range: 3.0 - 4.5 V, 0.5C cycling for 200 cycles after 0.1C activation for 5 cycles);

[0148] Figure 11 Discharge specific capacity performance graphs of NCM811||Li half-cells using the electrolyte formulations prepared in Comparative Example 1 and Example 9 as electrolytes (25 °C, voltage range: 3.0 - 4.3 V, 10C cycling for 500 cycles after 0.1C activation for 5 cycles);

[0149] Figure 12 Rate performance graphs of NCM811||Li half-cells using the electrolyte formulations prepared in Comparative Example 1 and Examples 1, 5, 9 as electrolytes (25 °C, voltage range: 3.0 - 4.3 V, cycling at 0.1C, 0.2C, 0.5C, 1C, 2C, 3C, 5C, 10C, 0.1C for 5 cycles respectively);

[0150] Figure 13 Rate performance graphs of NCM811||Li half-cells using the electrolyte formulations prepared in Comparative Example 1 and Example 5 as electrolytes (25 °C, voltage range: 3.0 - 4.5 V, cycling at 0.1C, 0.2C, 0.5C, 1C, 2C, 3C, 5C, 10C, 0.1C for 5 cycles respectively);

[0151] Figure 14 :

[0152] (a) TEM image (50 nm) of the CEI film layer of the NCM811||Li half-cell after 200 cycles with the electrolyte formulation prepared in Comparative Example 1 as the electrolyte;

[0153] (b) TEM image (20 nm) of the CEI film layer of the NCM811||Li half-cell after 200 cycles with the electrolyte formulation prepared in Comparative Example 1 as the electrolyte;

[0154] (c) TEM image (50 nm) of the CEI film layer of the NCM811||Li half-cell after 200 cycles with the electrolyte formulation prepared in Example 5 as the electrolyte;

[0155] (d) TEM image (20 nm) of the CEI film layer of the NCM811||Li half-cell after 200 cycles with the electrolyte formulation prepared in Example 5 as the electrolyte. Detailed Description of the Invention

[0156] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this invention belongs; the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0157] For those not specified in the examples, they are carried out under conventional conditions or conditions recommended by the manufacturer. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0158] As used herein, the term "about" is used to provide flexibility and imprecision associated with a given term, measurement, or value. Those skilled in the art can readily determine the degree of flexibility for a particular variable.

[0159] Concentrations, amounts, and other numerical data may be presented herein in a range format. It should be understood that such range formats are used merely for convenience and brevity and should be interpreted flexibly to include not only the explicitly recited values as the limits of the range, but also all individual values or sub-ranges subsumed within the stated range as if each value and sub-range were explicitly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly recited limits of 1 to about 4.5, but also the individual numbers (such as 2, 3, 4) and sub-ranges (such as 1 to 3, 2 to 4, etc.). The same principle applies to ranges that only recite one numerical value, such as "less than about 4.5", which should be interpreted to include all of the above values and ranges. In addition, this interpretation should apply regardless of the breadth of the range or feature being described.

[0160] The present invention will be further described below in conjunction with specific examples.

[0161] Preparation materials:

[0162] The basic electrolyte formulation (electrolyte, 1M LiPF6 EC / EMC = 3:7 vol%) and polypropylene separator were purchased from Dongguan Kelude Innovation Technology Co., Ltd., the lithium sheet was purchased from Tianjin Zhongneng Lithium Industry Co., Ltd., the NCM811 sheet was purchased from Shenzhen Huaqing New Materials Technology Co., Ltd., diethylphosphinic acid and sodium diethylphosphinate were purchased from Lanzhou Ruipu Technology Co., Ltd., dichloromethane, thionyl chloride, 2,2,3,3-tetrafluoropropanol, trifluoroethanol, sodium carbonate solution, N,N-dimethylformamide (DMF), and 1,1-difluoro-2-chloroethane were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0163] Preparation Example 1

[0164] In a two-necked flask, 100 g of diethylphosphinic acid (0.82 mol, 1 eq) was dissolved in 100 ml of dichloromethane. Under nitrogen protection, 292.7 g of thionyl chloride (2.46 mol, 3 eq) was slowly added dropwise with stirring at room temperature. After the addition was completed, the mixture was continuously stirred and heated to a slight reflux of dichloromethane (60 °C). The tail gases SO2 and HCl were absorbed with a sodium carbonate solution until no tail gas was released (about 2 h). Then, the excess dichloromethane and thionyl chloride were distilled off under reduced pressure to obtain diethylphosphinic acid chloride.

[0165] 200 ml of dichloromethane was added to the above diethylphosphinic acid chloride. Under nitrogen protection, 162.4 g of 2,2,3,3-tetrafluoropropanol (1.23 mol, 1.5 eq) was added dropwise with stirring at room temperature. After the addition was completed, the mixture was continuously stirred and heated to a reflux of dichloromethane (55 °C). The tail gas was absorbed with a sodium carbonate solution. After the reaction was completed in 5 h, the excess 2,2,3,3-tetrafluoropropanol and dichloromethane were distilled off under reduced pressure. After rectification and drying, diethylphosphinic acid 2,2,3,3-tetrafluoropropyl ester (as shown in Formula II) was obtained, and its 1 1H NMR characterization is as Figure 1 shown.

[0166]

[0167] Preparation Example 2

[0168] In a two-necked flask, 100 g of diethylphosphinic acid (0.82 mol, 1 eq) was dissolved in 100 ml of dichloromethane. Under nitrogen protection, 292.7 g of thionyl chloride (2.46 mol, 3 eq) was slowly added dropwise with stirring at room temperature. After the addition was completed, the mixture was continuously stirred and heated to a slight reflux of dichloromethane (60 °C). The tail gases SO2 and HCl were absorbed with a sodium carbonate solution until no tail gas was released (about 2 h). Then, the excess dichloromethane and thionyl chloride were distilled off under reduced pressure to obtain diethylphosphinic acid chloride.

[0169] 200 mL of dichloromethane was added to the above-mentioned diethylphosphinic acid chloride. Under nitrogen protection and stirring at room temperature, 123.0 g of trifluoroethanol (1.23 mol, 1.5 eq) was added dropwise. After the addition was completed, stirring was continued and the temperature was raised to the reflux temperature of dichloromethane (55 °C). The tail gas was absorbed with sodium carbonate solution. After 5 h of reaction, the excess trifluoroethanol and dichloromethane were distilled off under reduced pressure. After rectification and drying, trifluoroethyl diethylphosphinate (as shown in Formula III) was obtained. Its 1 HNMR characterization was as Figure 2 shown.

[0170]

[0171] Preparation Example 3

[0172] 400 mL of N,N-dimethylformamide (DMF, 4 times the volume of 1,1-difluoro-2-chloroethane) and 187.2 g of sodium diethylphosphinate (1.3 mol, 1.3 eq) were added to a 1 L autoclave. Then, 100.5 g of 1,1-difluoro-2-chloroethane (1 mol, 1 eq) was quickly added at 6 °C. The temperature was raised and maintained at 150 °C for 6 h. Then, the autoclave was cooled to room temperature, and the solvent was removed by distillation under reduced pressure. After rectification and drying, difluoroethyl diethylphosphinate (as shown in Formula IV) was obtained. Its 1 HNMR characterization was as Figure 3 shown.

[0173]

[0174] Preparation Example 4

[0175] This preparation example was for the preparation of an electrolyte formulation. The preparation process was carried out in a dry argon-filled glove box (moisture < 0.1 ppm, oxygen content < 0.1 ppm).

[0176] The various fluoro-substituted diethylphosphinates prepared in Preparation Examples 1 to 3 were used as flame retardant additives and added to the basic electrolyte formulation at 1 wt%, 2 wt%, 3 wt%, and 5 wt% of the electrolyte formulation, respectively, to obtain the electrolyte formulations shown in Examples 1 to 12 in Table 1. At the same time, the basic electrolyte formulation without additives was used as a control group (Comparative Example 1).

[0177] In the above basic electrolyte formulation, the volume ratio of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) was 3:7, and lithium hexafluorophosphate (LiPF6) was used as the Li ion conducting salt, and its concentration was 1 M.

[0178] All the above electrolyte components were stirred and mixed in a glass vial for 24 hours to ensure that all solids were completely dissolved.

[0179] Table 1 Electrolyte formulations with different addition ratios of fluorinated diethyl phosphinates

[0180]

[0181]

[0182] Test Example 1

[0183] This test example is for the flammability test of flame-retardant electrolytes with different addition ratios of fluorinated diethyl phosphinates

[0184] In this test example, the preparation process of the electrolyte formulations was carried out in a dry argon-filled glove box

[0185] All kinds of fluorinated diethyl phosphinates prepared in Preparation Examples 1-3 were used as additive materials and added to the basic electrolyte formulation at 0 wt%, 1 wt%, 3 wt%, 5 wt%, 10 wt%, and 20 wt% of the mass concentration of the electrolyte formulation to obtain the flame-retardant electrolytes shown in Table 2

[0186] In the above basic electrolyte formulation, the volume ratio of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) was 3:7, and lithium hexafluorophosphate (LiPF6) was used as the Li-ion conducting salt with a concentration of 1 M

[0187] Specifically, 0.1 g of the above flame-retardant electrolyte was respectively sucked and dropped on quartz cotton balls to make them fully infiltrated. The cotton balls were ignited in a closed container for 3 s uniformly, and the time from removing the ignition device to the cotton ball extinguishing was recorded to calculate SET (s). The test results are shown in Table 2 and Figure 4 as follows

[0188] Table 2 Self-extinguishing time test of flame-retardant additives with different addition ratios

[0189]

[0190] From Table 2 and Figure 4 it can be seen that the addition of fluorinated diethyl phosphinates can sharply reduce the flammability of the electrolyte. The curve of trifluoroethyl diethyl phosphinate decreases relatively evenly, and when the additive content is 10-20%, its flame-retardant effect is the best among the three. The curve of tetrafluoropropyl diethyl phosphinate shows a trend of first decreasing slowly and then rapidly, while the curve of difluoroethyl diethyl phosphinate is opposite to it. The above data and charts show that when the electrolyte additive content is between 0%-5%, the flame-retardant effects of difluoroethyl diethyl phosphinate and trifluoroethyl diethyl phosphinate are the best; when the electrolyte additive content is between 10%-20%, the flame-retardant effect of trifluoroethyl diethyl phosphinate is the best

[0191] Test Example 2

[0192] This test example is to test the various performances of the lithium symmetric battery using the electrolyte formulation prepared in Preparation Example 4 as the electrolyte in a button battery.

[0193] Specifically, for the lithium symmetric battery, a CR2032 type button battery is used. The battery assembly process is completed in an argon glove box with H2O / O2 content < 0.1 ppm. The electrolyte is selected from the electrolyte formulation prepared in Preparation Example 4, the separator is selected from a polypropylene separator (Celgard 2500, with a thickness of 25 μm and a diameter of 18 mm), and the lithium sheet has a diameter of 16 mm.

[0194] The battery assembly process is as follows: stack the negative electrode case → negative electrode plate (lithium sheet) → inject 40 μl of electrolyte → place the separator → inject another 40 μl of electrolyte → positive electrode plate (lithium sheet) → gasket → spring piece → positive electrode case. Use a button battery sealer to perform the final encapsulation at a pressure of 0.5 MPa.

[0195] 2.1 Cycle life test of lithium symmetric battery

[0196] The assembled lithium symmetric battery is subjected to a cycle life test using a BlueTEC test system. The test parameters are: deposition capacity density is 0.5 mAh / cm 2 , and the current density is 1 mA / cm 2 . The test results are shown in Table 3.

[0197] Table 3 Results of cycle life test of lithium symmetric batteries containing electrolyte formulations of Comparative Example 1 and Examples 1 - 12

[0198] Number Cycle life (h) Comparative Example 1 235 Example 1 415 Example 2 389 Example 3 373 Example 4 215 Example 5 375 Example 6 359 Example 7 301 Example 8 250 Example 9 314 Example 10 287 Example 11 286 Example 12 146

[0199] 2.2 Cycle performance test of lithium symmetric battery

[0200] The assembled lithium symmetric battery is subjected to a cycle performance test using a BlueTEC test system. The test parameters are: deposition capacity density is 0.5 mAh / cm 2 , and the current density is 1 mA / cm 2 . The test results are as Figures 5 - 7 shown.

[0201] From the cycle test results of the lithium symmetric battery (Table 2 and Appendix Figures 5 - 7 ), it can be seen that the optimal addition ratio of the three additives is 1%. The lithium symmetric batteries can be stably cycled for 415 h, 375 h, and 314 h respectively, and all have a low overpotential, indicating that this electrolyte system can effectively improve the lithium ion migration rate, and the deposition / stripping process of lithium metal has good reversibility. When the additive concentration is relatively high, the cycle stability of the lithium symmetric battery is poor because as the additive concentration increases, the electrolyte conductivity decreases, the impedance increases, the polarization becomes larger, and the cycle life is reduced.

[0202] Test Example 3

[0203] This test example is to test various properties of NCM811||Li half-cells using the electrolyte formulations prepared in Comparative Example 1 and Examples 1, 5, and 9 as electrolytes.

[0204] Specifically, the NCM811||Li half-cell utilizes CR2032 button cells. The battery assembly process was completed in an argon glove box with an H2O / O2 content of less than 0.1 ppm. The electrolyte was selected from the electrolyte formulations prepared in Comparative Example 1 and Examples 1, 5, and 9. The separator was a polypropylene separator (Celgard 2500, 25 μm thick and 18 mm in diameter). The NCM811 sheet had a diameter of 12 mm, and the lithium sheet had a diameter of 16 mm.

[0205] The battery assembly process is as follows: stack the negative electrode shell, negative electrode sheet (lithium sheet), inject 40μl of electrolyte, place the separator, inject another 40μl of electrolyte, positive electrode sheet (NCM811 sheet), gasket, spring, and positive electrode shell. The final seal is performed using a button cell sealer at a pressure of 0.5MPa.

[0206] 3.1 NCM811||Li half-cell cycle test

[0207] a) At 25°C, the voltage range is 3.0-4.3V. The NCM811||Li half-cell using the electrolyte formulations prepared in Comparative Example 1 and Examples 1, 5, and 9 as the electrolyte was tested for its discharge specific capacity after 5 cycles of activation at 0.1C and 300 cycles of cycling at 0.5C. The discharge specific capacity performance is shown in Tables 4 and Figure 8 shown.

[0208] b) At 25°C, the voltage range is 3.0-4.3V. The NCM811||Li half-cell using the electrolyte formulations prepared in Comparative Example 1 and Examples 1, 5, and 9 as the electrolyte was activated at 0.1C for 5 cycles and then cycled at 1C for 200 cycles to test its discharge specific capacity. The discharge specific capacity performance is shown in Table 4 and Figure 9 shown.

[0209] Table 4 Cycling performance test results of NCM811||Li half-cells containing electrolyte formulations of Comparative Example 1 and Examples 1, 5, and 9

[0210]

[0211] From the cycling performance test results of NCM811||Li half-cell (Table 4 and Appendix Figure 8 、 9 ) It can be seen that 1% trifluoroethyl diethylphosphinate (Example 5) can effectively increase the cycle performance of lithium metal batteries.

[0212] 3.2 Measurement of the Thickness of the CEI Film Layer after Cycling of the NCM811||Li Half-Cell

[0213] At 25 °C, in the voltage range of 3.0 - 4.3 V, after the NCM811||Li half-cell using the electrolyte formulations prepared in Comparative Example 1 and Example 5 as the electrolyte was activated at 0.1C for 5 cycles and then cycled at 0.5C for 200 cycles, the thickness of its CEI film layer was measured using a transmission electron microscope (TEM). The measurement results are shown in the TEM images as Figure 14 shown.

[0214] From Figure 14 (a) and Figure 14 (b), it can be seen that the thickness of the CEI film formed on the surface of NCM811 particles by the conventional electrolyte is about 18.89 nm and is uneven. This is because during the cycling process, the continuous deintercalation / insertion of lithium ions leads to the continuous shrinkage / swelling of the NCM811 lattice volume, and the unstable CEI layer continuously breaks, resulting in irreversible phase transformation of the layered structure, grain cracks, and even particle fragmentation, shortening the life of the lithium battery; from Figure 14 (c) and Figure 14 (d), it can be seen that the thickness of the CEI film formed on the surface of NCM811 particles by the electrolyte containing 1.0 wt% trifluoroethyl diethylphosphinate is about 7.49 nm and is continuous and uniform, which can prevent the corrosion of the NCM811 cathode by HF and is beneficial to the stability of the internal structure of NCM811 particles. In summary, the electrolyte containing 1.0 wt% trifluoroethyl diethylphosphinate forms a thinner film than the conventional electrolyte, further leading to an improvement in the stable cycling performance of the cathode at high voltage. In the presence of C-F bonds, the conductivity of the CEI film is better, and the deposits from the decomposition of the electrolyte will decrease, further leading to an improvement in the rate performance.

[0215] 3.3 High-Voltage Cycling Performance Test of the NCM811||Li Half-Cell

[0216] At 25 °C, in the voltage range of 3.0 - 4.5 V, after the NCM811||Li half-cell using the electrolyte formulations prepared in Comparative Example 1 and Example 5 as the electrolyte was activated at 0.1C for 5 cycles and then cycled at 0.5C for 200 cycles, its discharge specific capacity was tested. The discharge specific capacity performance is shown in Table 5 and Figure 10 shown.

[0217] Table 5 Test Results of the High-Voltage Cycling Performance of the NCM811||Li Half-Cell Containing the Electrolyte Formulations of Comparative Example 1 and Example 5

[0218]

[0219] From the test results of the high-voltage cycling performance of the NCM811||Li half-cell (Table 5 andFigure 10 ) It can be seen that 1% trifluoroethyl diethylphosphinate (Example 5) can increase the discharge specific capacity from 65.4 mAh / g to 136.1 mAh / g, and the capacity retention rate from 39.42% to 69.36%, greatly increasing the cycle stability of the lithium-ion battery under high voltage and effectively improving the high-voltage cycle performance of the lithium-ion battery.

[0220] 3.4 NCM811||Li Half-cell Fast Charging Performance Test

[0221] At 25 °C, in the voltage range of 3.0 - 4.3 V, after the NCM811||Li half-cell using the electrolyte formulations prepared in Comparative Example 1 and Example 9 as the electrolyte was activated at 0.1C for 5 cycles, it was cycled at 10C for 500 cycles to test its discharge specific capacity. The discharge specific capacity performance is as Figure 11 shown.

[0222] From Figure 11 the data in, it can be seen that 1% difluoroethyl diethylphosphinate can increase the discharge specific capacity from 50.4 mAh / g to 91.9 mAh / g, and the capacity retention rate from 35.07% to 60.03%, effectively improving the cycle stability of the lithium-ion battery under fast charging and improving the fast charging performance of the lithium-metal battery.

[0223] 3.5 NCM811||Li Half-cell Rate Performance Test

[0224] 3.5.1 NCM811||Li Half-cell Atmospheric Pressure Rate Performance Test

[0225] In the voltage range of 3.0 - 4.3 V, for the NCM811||Li half-cells with electrolytes selected from the electrolyte formulations of Comparative Example 1 and Examples 1, 5, and 9, they were cycled at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 0.1C for 5 cycles respectively to test their rate performance. The test results are as Figure 12 shown.

[0226] From Figure 12 the data in, it can be seen that at a cut-off voltage of 4.3 V and charging and discharging at different rates, 1% tetrafluoropropyl diethylphosphinate (Example 1) and 1% difluoroethyl diethylphosphinate (Example 9) can effectively increase the discharge specific capacity of the lithium-metal battery. In particular, the electrolyte of 1% difluoroethyl diethylphosphinate (Example 9) still has 106 mAh / g at 10C, which is much higher than the blank electrolyte, and can effectively improve the fast charging performance of the lithium-ion battery.

[0227] 3.5.2 NCM811||Li Half-cell High Voltage Rate Performance Test

[0228] The voltage range is 3.0 - 4.5V. The NCM811||Li half-cells with electrolytes selected from Comparative Example 1 and Example 5 are cycled 5 times at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 0.1C respectively to test their rate performance. The test results are as Figure 13 shown.

[0229] From Figure 13 the data in it, it can be seen that at a cut-off voltage of 4.5V and charging and discharging at different rates, 1% trifluoroethyl diethylphosphinate (Example 5) effectively improves the discharge specific capacity and capacity recovery rate of the lithium-ion battery under high voltage, and effectively improves the rate performance of the lithium-ion battery for fast charging under high voltage.

[0230] 3.6 High-temperature storage performance test of NCM811||Li half-cell

[0231] The NCM811||Li half-cells with electrolytes selected from Comparative Example 1 and Example 5 are charged at a constant current and constant voltage of 1C to 4.5V at 25°C, cut off at 0.05C, and left standing for 1h. Then, they are discharged at a constant current of 1C to 2.5V, and this discharge specific capacity is the initial capacity C0;

[0232] At 25°C, they are charged at a constant current and constant voltage of 1C to 4.5V, cut off at 0.05C, and then the battery is transferred to a high-temperature test cabinet and stored at 60°C for 28 days. After the storage is completed, the test battery is taken out, left standing at room temperature for 10h, and then discharged at a constant current of 1C to 2.5V, and the discharge specific capacity C1 is recorded;

[0233] After the above test battery is left standing for 2h, it is charged at a constant current and constant voltage of 1C to 4.5V, cut off at 0.05C, left standing for 1h, and then discharged at a constant current of 1C to 2.5V, and the discharge specific capacity C2 is recorded.

[0234] The test results of its high-temperature storage performance are shown in Table 6.

[0235] Capacity retention rate (%) = C1 / C0 * 100%

[0236] Capacity recovery rate (%) = C2 / C0 * 100%

[0237] Table 6 Test results of high-temperature storage performance of NCM811 / Li half-cells assembled from Comparative Example 1 and Example 5

[0238]

[0239]

[0240] From the data in Table 6, it can be seen that 1% trifluoroethyl diethylphosphinate (Example 5) improves both the capacity retention rate and the capacity recovery rate of the lithium-ion battery after high-temperature storage, and can effectively improve the high-temperature storage performance of the lithium-ion battery.

[0241] As can be seen from the above embodiments:

[0242] (1) Compound I provided by the present invention, especially Compound II, Compound III and Compound IV, can be used as flame retardant additives for conventional electrolytes, and can effectively improve the safety of lithium-ion batteries.

[0243] (2) The additive provided by the present invention contains P=O bonds, and the lone pair electrons of oxygen atoms can play the role of Lewis base, effectively scavenging the strong Lewis acids PF5 and PF3O derived from LiPF6, avoiding their corrosion and degradation of the surface of the cathode material, thereby avoiding the dissolution of transition metals in the cathode material. Because the dissolution of transition metal manganese is accompanied by the loss of oxygen in the material, resulting in irreversible changes in the structure of the cathode material, thus having an irreversible impact on the battery capacity. Moreover, the precipitated oxygen has strong oxidizing properties and will react with the electrolyte. After the manganese in the cathode material is dissolved, it can undergo a reduction reaction on the surface of the anode and deposit on the surface of the anode, becoming part of the SEI film (such as MnF, MnCO3, etc.). The deposited manganese will block the migration of lithium ions in the SEI film, resulting in an increase in battery impedance. If metal ions are reduced to metals and exist in the SEI film, since metals are good electronic conductors, this will exacerbate the decomposition of the electrolyte. Therefore, the additive provided by the present invention can better maintain the structural stability of the cathode material and further improve the battery cycle stability.

[0244] (3) Compound I provided by the present invention, especially Compound II, Compound III and Compound IV, as additives added to the electrolyte or solid electrolyte, can effectively improve the safety and stability of lithium-ion secondary batteries; in particular, the addition of Compound IV can effectively improve the fast charging performance of lithium-ion secondary batteries, and the conversion rate (Coulomb efficiency) is above 99% at high voltages of 4.3V and 4.5V; Compound III can effectively improve the high-voltage cycle stability of lithium-ion secondary batteries.

[0245] Finally, it should be noted that the above examples are only descriptive and do not limit the present invention in any way; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A compound, characterized in that, Has the structure of formula I: Among them, R 1 is selected from one of pentafluorophenyl, trifluorophenyl, difluoromethyl, trifluoromethyl, difluoroethyl, trifluoroethyl, tetrafluoropropyl, pentafluoropropyl, and hexafluoroisopropyl.

2. The compound according to claim 1, characterized in that, The compound has a structural formula selected from the following:

3. Use of one or more of the compounds according to claim 1 or 2 as a flame retardant additive in a flame retardant electrolyte or a solid electrolyte.

4. Flame-retardant electrolyte, characterized in that, Contains one or more of the compounds according to claim 1 or 2.

5. The flame-retardant electrolyte according to claim 4, wherein, Further comprises: An aprotic organic solvent; and A lithium salt.

6. The flame retardant electrolyte according to claim 5, characterized in that The concentration of the lithium salt is 0.5 to 2 M; The compounds selected from formula I, formula II, formula III, and formula IV in the electrolyte are present in the flame retardant electrolyte at a mass concentration of 0.5% to 20%, preferably at a mass concentration of 0.5% to 3% in the flame retardant electrolyte; The aprotic organic solvent is present in the flame retardant electrolyte at a mass concentration of 60% to 90%.

7. The flame retardant electrolyte according to claim 6, characterized in that The aprotic organic solvent is selected from one or more combinations of carbonates, carboxylates, ethers, and sulfones; Preferably, the carbonate solvents include one or more combinations of ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and methyl propyl carbonate; Preferably, the carboxylate solvents include one or more combinations of γ-lactone, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, fluoroethyl acetate, fluoromethyl propionate, fluoroethyl propionate, and fluoropropyl propionate; Preferably, the ether solvents include one or more combinations of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-dimethoxyethane, dimethoxypropane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol dibutyl ether, tetraethylene glycol, crown ethers, and cryptands; Preferably, the sulfone solvents include one or more combinations of sulfolane, fluorosulfolane, methyl ethyl sulfone, methyl propyl sulfone, methyl isopropyl sulfone, fluorinated methyl ethyl sulfone, fluorinated methyl propyl sulfone, and fluorinated methyl isopropyl sulfone; and / or The lithium salt is selected from one or more combinations of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium hexafluoroarsenate, anhydrous lithium perchlorate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalato)phosphate, lithium difluoro bis(oxalato)phosphate, lithium monooxalato bis(fluoroborate), lithium difluorophosphate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(malonato)borate, lithium bis(difluoromalonato)borate, lithium malonato oxalato borate, (lithium difluoromalonato oxalato)borate, and lithium tris(difluoromalonato)phosphate.

8. Flame-retardant solid electrolyte, characterized in that, Contains one or more of the compounds according to claim 1 or 2.

9. The flame-retardant solid electrolyte according to claim 8, wherein Further comprises: A polymer matrix; and A conductive lithium salt.

10. The flame-retardant solid electrolyte according to claim 9, characterized in that, The polymer matrix is selected from one or more mixtures of the following substances: polyethylene oxide, polyethylene glycol, poly(ethylene carbonate), poly(propylene carbonate), polyacrylonitrile, polymethyl methacrylate, or poly(vinylidene fluoride); and / or The conductive lithium salt is selected from one or more mixtures of the following substances: lithium hexafluorophosphate, lithium difluorophosphate, lithium tris(pentafluoroethyl)trifluorophosphate, lithium tetrafluoroxalate phosphate, lithium trioxalate phosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium pentafluoroethyltrifluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, or lithium trifluoromethanesulfonate.

11. Electrochemical energy storage device, characterized in that, Comprising: A cathode; An anode; A separator; And The flame-retardant electrolyte according to any one of claims 4 to 7.

12. A lithium ion secondary battery, characterized in that, Comprising: A cathode; An anode; A separator; And The flame-retardant electrolyte according to any one of claims 4 to 7.

13. A lithium ion secondary battery, characterized in that, Containing: A cathode; An anode; and The flame-retardant solid electrolyte according to any one of claims 8 to 10.

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