Secondary battery electrolyte, method for preparing the same, secondary battery, and electronic device

Abstract

The secondary battery electrolyte provided by the embodiment of the present application comprises an electrolyte salt, an organic solvent and a flame retardant, wherein the flame retardant comprises pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene and trifluorocyclotriphosphazene. The electrolyte simultaneously adds pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene and trifluorocyclotriphosphazene as the flame retardant, endows the electrolyte with excellent flame retardant capacity, and makes the battery have high safety performance and good electrochemical performance. The embodiment of the present application further provides a secondary battery with the above secondary battery electrolyte and an electronic device.

Description

Technical Field

[0001] This application relates to the field of battery electrolyte technology, and in particular to secondary battery electrolytes and their preparation methods, secondary batteries, and electronic devices. Background Technology

[0002] The widespread use of rechargeable batteries in consumer electronics (such as mobile phones and tablets) and electric vehicles has led to higher demands on their energy density and safety performance. Currently, the electrolytes in rechargeable batteries are mainly non-aqueous organic electrolytes (commonly carbonate or carboxylic acid ester electrolytes). When batteries overheat under conditions such as overcharging or internal short circuits, the volatile and flammable nature of their electrolytes can easily cause combustion and explosion accidents.

[0003] The mainstream strategy for improving the safety of secondary battery electrolytes is to add flame retardant additives to conventional electrolytes. While many types of flame retardant additives have been reported, such as phosphate esters and halogenated compounds, most of them have poor compatibility with the positive and negative electrode materials. Even when using phosphazene flame retardants with good compatibility with the positive and negative electrode materials, a fairly high addition level is still required to achieve a good flame retardant effect in order to ensure the safety of high-energy-density batteries. However, this can impair the battery's rate performance, low-temperature cycling performance, and other electrochemical properties. Summary of the Invention

[0004] Therefore, this application provides a secondary battery electrolyte. By simultaneously adding three flame retardants—pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene—to the electrolyte, the battery can possess both high safety performance and good electrochemical performance.

[0005] Specifically, the first aspect of this application provides a secondary battery electrolyte, including a lithium salt, an organic solvent, and a flame retardant, wherein the flame retardant includes pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene.

[0006] The secondary battery electrolyte provided in the first aspect of this application, by simultaneously adding three flame retardants—pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene—to the electrolyte, can impart flame retardant capabilities to the electrolyte in both the electrolyte volatilization range and the flammable gas generation range under the synergistic effect of the three flame retardants. Moreover, the addition of a small amount of each of them gives the electrolyte excellent flame retardant effect, and avoids the problem of deterioration of battery electrochemical performance caused by the addition of a large amount of phosphazene flame retardants. This allows the battery to have both high safety performance and good electrochemical performance.

[0007] In the embodiments of this application, the chemical structural formula of the pentafluorocyclic triphosphazene is shown in formula (I), the chemical structural formula of the tetrafluorocyclic triphosphazene is shown in formula (II), and the chemical structural formula of the trifluorocyclic triphosphazene is shown in formula (III).

[0008]

[0009] R1, R2, R3, R4, R5, and R6 are independently selected from any one of alkoxy, haloalkoxy, aryloxy, haloaryloxy, alkyl, haloalkyl, alkenyl, haloalkenyl, alkenyloxy, haloalkenyloxy, aryl, haloaryl, epoxy, haloepoxy, phosphate ester, substituted phosphate ester, carbonate, substituted carbonate, sulfonate, substituted sulfonate, alkyl ketone, haloalkylketone, alkoxysiloxy, haloalkoxysiloxy, substituted imide, substituted sulfonimide, and nitrile.

[0010] In the embodiments of this application, the halogen in the haloalkoxy, haloaryloxy, haloalkyl, haloalkenyl, haloalkenoxy, haloaryl, haloepoxy, and haloalkylketone groups is fluorine; the phosphate ester group is an alkyl phosphate ester group, and the substituted phosphate ester group is a fluoroalkyl phosphate ester group; the carbonate group is an alkyl carbonate ester group, and the substituted carbonate group is a fluoroalkyl carbonate ester group; the sulfonate group is an alkyl sulfonate ester group, and the substituted sulfonate ester group is a fluoroalkyl sulfonate ester group; the alkoxysiloxy group is a trialkoxysiloxy group, and the haloalkoxysiloxy group is a fluorotrialkoxysiloxy group.

[0011] In this embodiment, the alkoxy, haloalkoxy, alkyl, and haloalkyl groups have 1-10 carbon atoms; the alkenyl, haloalkenyl, alkenoxy, haloalkenoxy, epoxy, and haloepoxy groups have 2-10 carbon atoms; the aryl, haloaryl, aryloxy, and haloaryloxy groups have 6-15 carbon atoms; and the phosphate ester group, substituted phosphate ester group, carbonate group, substituted carbonate group, sulfonate group, and substituted sulfonate group have 1-10 carbon atoms. A smaller number of carbon atoms is beneficial for controlling the molecular weight of each flame retardant, thereby better controlling the viscosity of the electrolyte and ensuring the solubility of the flame retardant in the electrolyte.

[0012] In this embodiment, the total mass percentage of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene in the electrolyte is 0.1%-20%. The low total amount of these flame retardants effectively improves the flame retardant properties of the electrolyte without significantly affecting its ionic conductivity and viscosity.

[0013] In some embodiments, the pentafluorocyclotriphosphazene has a mass percentage of 1%-10% in the secondary battery electrolyte, the tetrafluorocyclotriphosphazene has a mass percentage of 1%-5% in the secondary battery electrolyte, and the trifluorocyclotriphosphazene has a mass percentage of 1%-5% in the secondary battery electrolyte. In this case, the electrolyte better balances good flame retardant properties and electrochemical performance.

[0014] In some embodiments of the present application, the mass percentage contents of the pentafluorocyclotriphosphazene, the tetrafluorocyclotriphosphazene, and the trifluorocyclotriphosphazene in the secondary battery electrolyte are independently within the range of 1% - 5%. At this time, the flame retardant performance of the above electrolyte is relatively good, and the high-voltage cycling performance, high-temperature storage performance, and rate performance of the battery using this electrolyte are all relatively good. Furthermore, the safety performance and electrochemical performance are better balanced.

[0015] In an embodiment of the present application, the average fluorine substitution degree DS of the flame retardant satisfies the following relationship:

[0016] DS = 5×ω1 + 4×ω2 + 3×ω3, where ω1, ω2, and ω3 respectively represent the mole percentages of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene in the flame retardant; wherein, the value range of DS is: 3 < DS < 5. At this time, the electrolyte contains appropriate proportions of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene at the same time.

[0017] In some embodiments of the present application, ω3 is less than or equal to 50%. At this time, the molar proportion of trifluorocyclotriphosphazene in the flame retardant will not be too high to significantly increase the viscosity of the electrolyte, and it can ensure that the above flame retardant can play a good flame retardant effect in time during the evaporation stage of the electrolyte solvent.

[0018] In an embodiment of the present application, the organic solvent includes linear esters and cyclic esters.

[0019] In some embodiments of the present application, the linear esters include linear carbonates and / or linear carboxylates, and the cyclic esters include cyclic carbonates and / or cyclic carboxylates.

[0020] In an embodiment of the present application, the cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone; the linear carbonate includes but is not limited to one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate; the linear carboxylate includes one or more of methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate; the cyclic carboxylate includes one or more of α-ethyrolactone, β-propiolactone, γ-butyrolactone, δ-valerolactone, caprolactone.

[0021] In some embodiments of the present application, the mass ratio between the pentafluorocyclotriphosphazene and the tetrafluorocyclotriphosphazene is close to the mass ratio between the linear ester and the cyclic ester. This can facilitate the flame retardant to maximize the flame retardant ability of the electrolyte.

[0022] In this embodiment, the electrolyte further includes other additives, including fluoroethylene carbonate, difluoroethylene carbonate, trifluoromethyl ethylene carbonate, vinylene carbonate, ethylene ethylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, vinyl sulfite, propylene sulfite, butene sulfite, 4-methyl vinyl sulfite, dimethyl sulfite, diethyl sulfite, vinyl sulfate, 4-methyl vinyl sulfate, 4-propane... The following are some of the following: vinyl sulfate, 1,4-butanediol sulfate, 4-fluorophenyl acetate, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, methylene disulfonate, N,N-thionyl diimidazole, succinate, adiponitrile, 1,3,6-hexanetrionitrile, 1,3,5-pentanetricarbonyl nitrile, 1,2-bis(2-cyanoethoxy)ethane, ethylene glycol dipropionitrile ether, fluoroether D2, N,N-dimethyltrifluoroacetamide, biphenyl and benzene derivatives.

[0023] In this embodiment of the application, the other additives have a mass percentage content of 0.2%-15% in the electrolyte.

[0024] In some embodiments of this application, the electrolyte salt is a lithium salt, which includes one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium difluorophosphate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(perfluoroethylsulfonyl)imide, lithium trifluoromethylsulfonate, and lithium perfluorobutylsulfonate.

[0025] In this embodiment of the application, the molar concentration of the lithium salt in the secondary battery electrolyte is 0.01 mol / L-2.0 mol / L.

[0026] The second aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the secondary battery electrolyte described in the first aspect of this application.

[0027] The secondary battery provided in this application embodiment has excellent safety performance and electrochemical performance because the specific flame retardant mentioned above is added to its electrolyte, and the flame retardant has good compatibility with the positive and negative electrode materials, and the flame retardant can have excellent flame retardant effect when the amount used is low.

[0028] A third aspect of this application provides an electronic device that includes the secondary battery described in the first aspect of this application.

[0029] This electronic device, powered by the secondary battery provided in the embodiments of this application, can improve the user experience and market competitiveness of the product. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the structure of the secondary battery provided in the embodiments of this application.

[0031] Figure 2 This is a schematic diagram illustrating the mechanism of action of the flame retardant provided in the embodiments of this application.

[0032] Figure 3 This is a schematic diagram of the structure of an electronic device provided in an embodiment of this application.

[0033] Figure 4 This is another structural schematic diagram of the electronic device provided in the embodiments of this application. Detailed Implementation

[0034] The embodiments of this application are described below with reference to the accompanying drawings.

[0035] like Figure 1 As shown, Figure 1 This is a schematic diagram of the structure of a secondary battery 100 provided in an embodiment of this application. Specifically, the secondary battery 100 can be a lithium secondary battery. The lithium secondary battery includes a positive electrode 101, a negative electrode 102, a separator 103, an electrolyte 104, and corresponding connecting components and circuits. The positive electrode 101 and negative electrode 102 can intercalate and deintercalate active metal ions (for lithium secondary batteries, the active metal ions are lithium ions) to achieve energy storage and release: under the drive of an external battery, lithium ions are extracted from the positive electrode and migrate to the negative electrode, realizing battery charging; when an external electrical load is connected, lithium ions are extracted from the negative electrode and migrate back to the positive electrode, initiating the discharge process. The separator 103 separates the positive and negative electrodes, preventing internal short circuits. The electrolyte 104 is the medium for lithium ion transport between the positive and negative electrodes, playing a crucial role in the electrochemical and safety performance of the battery. The electrolyte 104 is mainly composed of lithium salts, non-aqueous organic solvents (typically carbonate or carboxylic acid ester solvents), and additives. With the development of high-energy-density batteries, when batteries are subjected to continuous overcharging, overheating, external short circuits, or internal short circuits caused by external damage (such as squeezing, puncture, or collision), their electrolytes may be prone to volatilization and combustion, which can easily lead to battery thermal runaway and safety problems.

[0036] To reduce the flammability of electrolytes and improve the safety performance of secondary batteries, the industry adds flame retardants to conventional electrolytes. Among them, phosphazene flame retardants have attracted attention due to their good compatibility with positive and negative electrode materials and their good flame retardant properties. However, to ensure the safety of high-energy-density batteries, a relatively high addition amount (e.g., ≥20 wt.%) is required to achieve a good flame retardant effect. However, the large-scale addition of a single type of phosphazene flame retardant can impair the electrochemical performance of the battery. To solve this problem, the industry has focused on designing and optimizing the structure of phosphazene molecules (e.g., introducing fluorine-containing groups, alkoxy groups, etc.) to improve their flame retardant efficiency and reduce the damage to battery electrical performance caused by large amounts of additives. However, these methods have not been able to effectively improve the flame retardant effect of the electrolyte. In addition, the industry has combined phosphazene flame retardants with other non-phosphazene flame retardants or non-flammable co-solvents to reduce the amount of phosphazene flame retardants used. However, non-phosphazene flame retardants not only have poor flame retardant effects but also affect the electrochemical performance of the battery; while non-flammable co-solvents greatly impair the ionic conductivity of the electrolyte. These compounding strategies have failed to significantly improve the flame retardant effect of the electrolyte. In view of this, this application provides a secondary battery electrolyte that enables the battery to have both high safety performance and good electrochemical performance.

[0037] Specifically, the secondary battery electrolyte provided in this application includes lithium salt, organic solvent and flame retardant, wherein the flame retardant includes pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene and trifluorocyclotriphosphazene.

[0038] The secondary battery electrolyte provided in this application embodiment achieves effective flame retardancy across the entire range for all components and their decomposition products of a conventional electrolyte by simultaneously adding three types of fluorinated cyclic triphosphazenes as flame retardants. Specifically, the electrolyte solvent generally contains linear and cyclic esters, which have different volatility ranges. Fluorinated cyclic triphosphazenes with different degrees of fluorine substitution also have different volatility characteristics, such as... Figure 2As shown (the vertical axis represents the volatilization ratio of linear and cyclic esters in the electrolyte with increasing temperature, and the generation ratio of flammable gases in the electrolyte), the volatilization characteristic curve of pentafluorocyclic triphosphazene is close to that of linear esters in the electrolyte. It can co-volatilize with linear esters to inhibit the combustion of electrolyte vapors and achieve flame retardancy for linear esters. The volatilization characteristic curve of tetrafluorocyclic triphosphazene is close to that of cyclic esters in the electrolyte. It can co-volatilize with most cyclic esters and achieve flame retardancy for cyclic esters. During the thermal runaway of a secondary battery, the electrolyte may also undergo irreversible oxidative decomposition to generate a large amount of flammable gases (such as H2, CO, etc.). Trifluorocyclic triphosphazene can effectively cover the generation temperature range of flammable gases during battery thermal runaway, improving its flame retardancy against flammable gases. Therefore, through the synergistic effect of three types of fluorinated triphosphazenes with different degrees of fluorine substitution, the effective flame retardant capability of the flame retardant can be broadened to cover both the electrolyte volatilization range and the flammable gas generation range. This avoids the problem of using a single (pentafluoro)triphosphazene, which causes the flame retardant to volatilize in the early stages of battery thermal runaway and fails to provide long-term effective flame retardant protection for the electrolyte. Moreover, the addition of a small amount of the above-mentioned flame retardant can give the electrolyte excellent flame retardant effects, while avoiding the problem of deteriorated battery electrochemical performance caused by adding large amounts of phosphazene flame retardants. This allows the battery to possess both high safety performance and good electrochemical performance.

[0039] In addition to their ability to co-volatilize with electrolyte solvents / flammable gases, the aforementioned pentafluorocyclic triphosphazenes, tetrafluorocyclic triphosphazenes, and trifluorocyclic triphosphazenes also possess positive electrode film-forming properties. These properties can inhibit electrolyte oxidation at the positive electrode, suppress electrolyte decomposition to generate active O and active H, and reduce the flammability of the electrolyte. Furthermore, they can capture active free radicals such as H and OH generated during electrolyte decomposition, inhibiting combustion. Moreover, their positive electrode film-forming ability can improve the high-voltage capability of the electrolyte, inhibit electrolyte decomposition at high voltages, suppress positive electrode damage at high potentials, improve the battery's resistance to high voltage and high temperature, and enhance high-voltage cycle performance.

[0040] Therefore, the synergistic effect of three types of fluorinated triphosphazenes with different degrees of fluorine substitution can effectively improve the flame retardancy of secondary battery electrolytes and enhance the safety performance of secondary batteries, while maintaining excellent electrochemical performance such as rate performance and cycle characteristics.

[0041] In the embodiments of this application, the chemical structural formula of the pentafluorocyclic triphosphazene can be represented by the following formula (I), the chemical structural formula of the tetrafluorocyclic triphosphazene can be represented by the following formula (II), and the chemical structural formula of the trifluorocyclic triphosphazene can be represented by the following formula (III):

[0042]

[0043] R1, R2, R3, R4, R5, and R6 are independently selected from any one of alkoxy, haloalkoxy, aryloxy, haloaryloxy, alkyl, haloalkyl, alkenyl, haloalkenyl, alkenyloxy, haloalkenyloxy, aryl, haloaryl, epoxy, haloepoxy, phosphate ester, substituted phosphate ester, carbonate, substituted carbonate, sulfonate, substituted sulfonate, alkyl ketone, haloalkylketone, alkoxysiloxy, haloalkoxysiloxy, substituted imide, substituted sulfonimide, and nitrile.

[0044] In this application, the number of carbon atoms in alkoxy, haloalkoxy, alkyl, and haloalkyl groups involved in R1, R2, R3, R4, R5, and R6 can be 1-10, and more preferably 1-6, preferably 1-4; the number of carbon atoms in alkenyl, haloalkenyl, alkenoxy, haloalkenoxy, epoxy, and haloepoxy groups can be 2-10, and more preferably 2-6; the number of carbon atoms in aryl, haloaryl, aryloxy, and haloaryloxy groups is 6-15, and more preferably 6-10, with specific carbon atom numbers such as 6, 7, 8, 9, and 10. A smaller number of carbon atoms is beneficial for controlling the molecular weight of each fluorocyclic triphosphazene, thereby better controlling the viscosity of the electrolyte and the solubility of the aforementioned flame retardant in the electrolyte. In the embodiments of this application, the halogen in halogenated alkyl, halogenated alkoxy, halogenated alkenyl, halogenated alkenyloxy, halogenated aryl, halogenated aryloxy, and halogenated epoxy groups can be fluorine, chlorine, bromine, or iodine, and more particularly, fluorine, which can improve the film-forming ability of each flame retardant. The halogenation can be fully halogenated or partially halogenated. Alkyl, halogenated alkyl, alkenyl, halogenated alkenyl, alkenyloxy, halogenated alkenyloxy, alkoxy, and halogenated alkoxy groups can be straight-chain or branched.

[0045] In this application, both phosphate ester groups and substituted phosphate ester groups can be represented as -OP(=O)(OR)2. When R is a hydrocarbon group, it is a phosphate ester group; when R includes a substituted hydrocarbon group, it is a substituted phosphate ester group. Substituted or unsubstituted phosphate ester groups form a PO bond with a phosphorus atom in the six-membered ring structure of cyclotriphosphazene through an oxygen atom. The hydrocarbon group includes alkyl, alkenyl, aryl, etc.; the substituted hydrocarbon group can include haloalkyl groups (including haloalkyl, haloalkenyl, haloaryl, etc.), hydroxyl groups (i.e., hydrocarbon groups substituted by oxygen atoms, including alkoxy, alkenyloxy, aryloxy), halohydroxyl groups (including haloalkoxy, haloalkenyloxy, haloaryloxy, etc.), etc.

[0046] Similarly, both carbonate groups and substituted carbonate groups can be represented as -OC(=O)-T. When T is a hydrocarbon group, it is a carbonate group; when T includes a substituted hydrocarbon group, it is a substituted carbonate group. Hydrocarbon groups include alkyl, alkenyl, aryl, etc.; substituted hydrocarbon groups can include haloalkyl groups (including haloalkyl, haloalkenyl, haloaryl, etc.), alkyloxy groups (including alkoxy, alkenoxy, aryloxy), haloalkyloxy groups (including haloalkoxy, haloalkenoxy, haloaryloxy, etc.), etc.

[0047] Similarly, both sulfonate groups and substituted sulfonate groups can be represented as -OS(=O)2-Q. When Q is a hydrocarbon group, it is a sulfonate group; when Q includes a substituted hydrocarbon group, it is a substituted sulfonate group. The hydrocarbon group includes alkyl, alkenyl, and aryl groups; the substituted hydrocarbon group can include haloalkyl groups, alkyloxy groups, haloalkyloxy groups, etc.

[0048] In the embodiments of this application, the halogen-containing substituents (such as halogenated hydrocarbon groups or halogenated hydrocarbon oxy groups) in the substituted phosphate ester group, substituted carbonate group, and substituted sulfonate group can be fluorine, chlorine, bromine, or iodine, preferably fluorine, and the halogenation can be fully halogenated or partially halogenated. Alkyl, haloalkyl, alkoxy, haloalkoxy, alkenyl, haloalkenyl, alkenoxy, and haloalkenoxy can be straight-chain or branched.

[0049] In some embodiments of this application, the phosphate ester group is an alkyl phosphate ester group (i.e., R is alkyl), and the substituted phosphate ester group is a fluoroalkyl phosphate ester group (i.e., R is fluoroalkyl). The carbonate group is an alkyl carbonate ester group (i.e., T is alkyl), and the substituted carbonate group is a fluoroalkyl carbonate ester group (i.e., T is fluoroalkyl). The sulfonate group is an alkyl sulfonate ester group (i.e., Q is alkyl), and the substituted sulfonate ester group is a fluoroalkyl sulfonate ester group (i.e., Q is fluoroalkyl).

[0050] In this application, the alkoxysiloxy group can be represented by the general formula -O-Si(R7)(R8)(R9), and at least one of R7, R8, and R9 is an alkoxy group. The alkoxysiloxy group forms a PO bond with a phosphorus atom in the six-membered ring structure of cyclotriphosphazene through its own oxygen atom. This alkoxysiloxy group can be a monoalkoxysiloxy group (one of R7, R8, and R9 is an alkoxy group, and the other two are alkyl or aryl groups, etc.), a dialkoxysiloxy group (two of R7, R8, and R9 are alkoxy groups, and the other is an alkyl or aryl group, etc.), or a trialkoxysiloxy group (all of R7, R8, and R9 are alkoxy groups). In some embodiments of this application, R7, R8, and R9 are all alkoxy groups. That is, the alkoxysiloxy group is a trialkoxysiloxy group. The number of carbon atoms in R7, R8, and R9 can be the same or different, and their number of carbon atoms is independently 1-10, further, the number of carbon atoms can be 1-6, preferably 1-4. In some embodiments, R7, R8, and R9 may all be alkoxy groups having 1-6 carbon atoms. The haloalkoxysiloxy group may also be represented by the general formula -O-Si(R7)(R8)(R9), and at least one of R7, R8, and R9 may be a haloalkoxy group or a non-halogenated alkoxy group, and at least one of R7, R8, and R9 may be a halogroup (specifically, it may be a haloalkoxy, haloalkyl, or haloaryl group, etc.). For example, R7, R8, and R9 may all be haloalkoxy groups; or one of R7, R8, and R9 may be a haloalkoxy group, and the other two may be independently selected from alkyl, haloalkyl, and alkoxy groups; or two of R7, R8, and R9 may be haloalkoxy groups, and the remaining one may be an alkyl, haloalkyl, or alkoxy group; or one of R7, R8, and R9 may be an alkoxy group, and the other two may be haloalkyl, or a combination of haloalkyl and alkyl; or two of R7, R8, and R9 may be alkoxy groups, and the remaining one may be a haloalkyl group. In some embodiments, the haloalkoxysiloxy group is a trihaloalkoxysiloxy group, and more particularly a trifluoroalkoxysiloxy group.

[0051] In this application, both alkyl ketone groups and haloalkyl ketone groups can be represented by the general formula -R 10 -C(=O)-R 11 This indicates that when it is an alkyl ketone group, R 10 It is an alkylene group, R 11 It is an alkyl group; when it is a haloalkylketone group, R 10 R is an alkylene group and / or a haloalkylene group. 11 It is an alkyl and / or haloalkyl group, and R 10 and R 11 At least one of them contains a halogen. The halogen can be fluorine, chlorine, bromine, or iodine, preferably fluorine. The alkyl halide or alkyl halide can be fully or partially halogenated. In some embodiments, R 10 and R 11These are alkylene groups and alkyl groups having 1-10 carbon atoms, further having 1-4 carbon atoms. In other embodiments, R... 10 and R 11 These are fluoroalkylene and fluoroalkyl groups with 1-10 carbon atoms, and further, their carbon number is 1-4.

[0052] In this application, the substituted imide group may be represented as -NH-C(=O)-X, wherein X may be selected from any one of alkyl, haloalkyl, alkoxy, haloalkoxy, alkenyl, haloalkenyl, alkenoxy, haloalkenoxy, aryl, haloaryl, aryloxy, and haloaryloxy.

[0053] In this application, the substituted sulfonylimide group may be represented as -NH-S(=O)2-Z, wherein Z may be selected from any one of alkyl, haloalkyl, alkoxy, haloalkoxy, alkenyl, haloalkenyl, alkenoxy, haloalkenoxy, aryl, haloaryl, aryloxy, and haloaryloxy.

[0054] The nitrile group (-CN) can be directly attached to the phosphorus atom in the six-membered ring structure of cyclotriphosphazene, or it can be attached to the phosphorus atom in the six-membered ring structure of cyclotriphosphazene through an alkylene group with 1-6 carbon atoms.

[0055] In some embodiments of this application, R1, R2, R3, R4, R5, and R6 are independently selected from alkoxy, fluoroalkoxy, aryloxy, fluoroaryloxy, alkyl phosphate, fluoroalkyl phosphate, alkyl carbonate, fluoroalkyl carbonate, alkyl sulfonate, fluoroalkyl sulfonate, trialkoxysiloxy, fluorotrikoxysiloxy, alkyl ketone, fluoroalkyl ketone, and nitrile. In some embodiments, R2 and R3 are the same. R4, R5, and R6 can all be the same.

[0056] In some specific embodiments, the pentafluorocyclotriphosphazene may be one or more of the following: methoxypentafluorophosphazene as shown in formula (i1), ethoxypentafluorophosphazene as shown in formula (i2), trifluoroethoxypentafluorocyclotriphosphazene as shown in formula (i3), perfluorobutoxypentafluorocyclotriphosphazene as shown in formula (i4), phenoxypentafluorophosphazene as shown in formula (i5), diethyl phosphate-based pentafluorophosphazene as shown in formula (i6), ethyl carbonate-based pentafluorophosphazene as shown in formula (i7), methyl sulfonate-based pentafluorophosphazene as shown in formula (i8), ethyl sulfonate-based pentafluorocyclotriphosphazene as shown in formula (i9), trimethoxysiloxypentafluorophosphazene as shown in formula (i10), ethyl ketone methyl pentafluorophosphazene as shown in formula (i11), and acetonitrile-based pentafluorophosphazene as shown in formula (i12), but is not limited thereto.

[0057]

[0058] In some specific embodiments, tetrafluorocyclotriphosphazene can be one or more of the following: dimethoxytetrafluorocyclotriphosphazene, diethoxytetrafluorocyclotriphosphazene, ditrifluoroethoxytetrafluorocyclotriphosphazene, diperfluorobutoxytetrafluorocyclotriphosphazene, diphenoxytetrafluorocyclotriphosphazene, diethyl bisphosphate tetrafluorocyclotriphosphazene, diethyl carbonate tetrafluorocyclotriphosphazene, dimethyl sulfonate tetrafluorocyclotriphosphazene, diethyl sulfonate tetrafluorocyclotriphosphazene, ditrimethoxysiloxytetrafluorocyclotriphosphazene, diethyl ketone methyltetrafluorocyclotriphosphazene, and diacetonitrile tetrafluorocyclotriphosphazene.

[0059] In some specific embodiments, trifluorocyclotriphosphazene can specifically be trimethoxytrifluorocyclotriphosphazene as shown in formula (ⅲ1), triethoxytrifluorocyclotriphosphazene as shown in formula (ⅲ2), tritrifluoroethoxytrifluorocyclotriphosphazene as shown in formula (ⅲ3), triperfluorobutoxytrifluorocyclotriphosphazene as shown in formula (ⅲ4), triphenoxytrifluorocyclotriphosphazene as shown in formula (ⅲ5), and diethyltriphosphate-based trifluorocyclotriphosphazene as shown in formula (ⅲ6), etc. The following are one or more of the following: triethyl carbonate trifluorocyclotriphosphazene as shown in formula (ⅲ7), trimethyl sulfonate trifluorocyclotriphosphazene as shown in formula (ⅲ8), triethyl sulfonate trifluorocyclotriphosphazene as shown in formula (ⅲ9), tritrimethoxysiloxy trifluorocyclotriphosphazene as shown in formula (ⅲ10), triethyl ketone methyl trifluorocyclotriphosphazene as shown in formula (ⅲ11), and triacetonitrile trifluorocyclotriphosphazene as shown in formula (ⅲ12), but not limited to these.

[0060]

[0061]

[0062] In this embodiment of the application, the aforementioned trifluorocyclic triphosphazene can be prepared by a two-step substitution-fluorination method. When R4, R5, and R6 in the trifluorocyclic triphosphazene shown in formula (III) are all the same (e.g., all are R'), the preparation process of the trifluorocyclic triphosphazene is illustrated below. The specific preparation process is shown in the following formula:

[0063]

[0064] Taking the triethoxytrifluorocyclotriphosphazene shown in formula (iii2) as an example, its specific preparation process is as follows:

[0065] (1) Dissolve 1 mol of hexachlorocyclotriphosphazene in 300 mL of hexane, add 3 mol of sodium ethoxide (R'=-OCH2CH3), reflux the mixture at 50 °C for 12 hours, and then distill the resulting mixed solution to obtain triethoxytrichlorocyclotriphosphazene.

[0066] (2) Dissolve 1 mol of triethoxytrichlorocyclotriphosphazene in 300 mL of acetonitrile, add 3 mol of sodium fluoride, and carry out a condensation reflux reaction at 50 °C for 12 hours. Then, distill and separate the resulting mixed solution to collect triethoxytrifluorocyclotriphosphazene. After dehydration treatment, it is sealed and stored for later use under dry conditions.

[0067] In the embodiments of the present application, the mass percentage content of the flame retardant in the electrolyte is 0.1% - 20%. That is, the total mass percentage content of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene in the electrolyte is 0.1% - 20%. Specifically, this total mass percentage content can be 1%, 3%, 5%, 8%, 10%, 12%, or 15%, etc. The addition of a lower content of the flame retardant facilitates effectively improving the flame retardant ability of the electrolyte, while ensuring that the ionic conductivity of the electrolyte will not be excessively reduced and the viscosity will not be too high to affect the battery performance. This avoids the problem of deterioration of the battery's electrochemical performance caused by the addition of a large amount of phosphazene flame retardant, enabling the battery to have both high safety performance and good electrochemical performance. In some embodiments, this total mass percentage content can be 2% - 12%, and further can be 5% - 10%. At this time, the electrolyte can better balance good flame retardant performance and electrochemical performance.

[0068] In some embodiments of the present application, the mass percentage content of pentafluorocyclotriphosphazene in the secondary battery electrolyte is 1% - 10%, and further can be 1% - 5%; the mass percentage content of tetrafluorocyclotriphosphazene in the secondary battery electrolyte is 1% - 5%, for example, it can be 1%, 2%, 3%, 4%, or 5%, etc.; the mass percentage content of trifluorocyclotriphosphazene in the secondary battery electrolyte is 1% - 5%. The mass percentage contents of these three in the electrolyte can be equal or unequal. Controlling the mass percentage contents of these three within the above ranges can enable them to fully exert their synergistic effects, making the battery with the electrolyte have excellent high-voltage cycling performance, high-temperature storage performance, and good rate performance while having excellent safety performance, and making the comprehensive performance of the battery at a relatively optimal level.

[0069] In the embodiments of the present application, in the flame retardant system composed of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene, the average fluorine substitution degree DS of the flame retardant satisfies the following relational expression: DS = 5×ω1 + 4×ω2 + 3×ω3, where ω1, ω2, and ω3 respectively represent the mole percentages of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene in the flame retardant; among them, the value range of DS is: 3 < DS < 5. This represents that the flame retardant of the present application contains appropriate proportions of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene at the same time, which can better improve the flame retardant effect of the electrolyte.

[0070] In some embodiments of this application, the molar percentage (i.e., ω3) of the trifluorocyclic triphosphazene in the total molar percentage of the pentafluorocyclic triphosphazene, tetrafluorocyclic triphosphazene, and trifluorocyclic triphosphazene is less than or equal to 50%. The molar percentage of trifluorocyclic triphosphazene in the flame retardant is not too high, ensuring that the viscosity of the electrolyte is not too high and that the flame retardant can effectively retard the electrolyte during the solvent evaporation stage.

[0071] In this embodiment of the application, depending on the different secondary battery system, the electrolyte salt can be lithium salt, sodium salt, potassium salt, magnesium salt, zinc salt, aluminum salt, etc. Specifically, lithium salt, sodium salt, and potassium salt can be one or more of MClO4, MBF4, MPF6, MAsF6, MPF2O2, MB(C2O4)2(MBOB), MBF2C2O4(MDFOB), M[(FSO2)2N], M[(CF3SO2)2N], M(C2F5SO2)2N, MCF3SO3, and MC4F9SO3, wherein M is Li, Na, or K. Similarly, magnesium salt, zinc salt, and aluminum salt can also be salts formed by magnesium ions, zinc ions, aluminum ions, and anions from the above-mentioned lithium salt, sodium salt, and potassium salt.

[0072] In some embodiments of this application, the secondary battery is a lithium secondary battery, and the electrolyte salt in its electrolyte is a lithium salt, specifically including but not limited to one or more of lithium perchlorate LiClO4, lithium tetrafluoroborate LiBF4, lithium hexafluorophosphate LiPF6, lithium hexafluoroarsenate LiAsF6, lithium difluorophosphate LiPF2O2, lithium bis(oxalato)borate LiB(C2O4)2 (LiBOB), lithium difluorooxalato)borate LiBF2C2O4 (LiDFOB), lithium bis(fluorosulfonyl)imide Li[(FSO2)2N, LiFSI], lithium bis(trifluoromethylsulfonyl)imide (Li(CF3SO2)2N, LiTFSI), lithium bis(perfluoroethyl)imide (Li(C2F5SO2)2N), lithium trifluoromethylsulfonate LiCF3SO3, and lithium perfluorobutylsulfonate (LiC4F9SO3). In some embodiments of this application, the molar concentration of lithium salt in the electrolyte can be 0.01 mol / L-2.0 mol / L, and more specifically 0.1 mol / L-1.5 mol / L.

[0073] In this embodiment, the organic solvent in the electrolyte typically includes linear esters and cyclic esters. The esters may include at least one of carbonates and carboxylic acid esters. Specifically, the linear esters include at least one of linear carbonates and linear carboxylic acid esters, and the cyclic esters include at least one of cyclic carbonates and cyclic carboxylic acid esters. This facilitates the complete dissolution of the aforementioned flame retardant and electrolyte salt.

[0074] Specifically, cyclic carbonates may include, but are not limited to, one or more of ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); linear carbonates may include, but are not limited to, one or more of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC). The carboxylic acid ester solvent may include linear carboxylic acid esters, specifically including, but not limited to, one or more of methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), butyl acetate, methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), and butyl propionate (BP). Cyclic carboxylic acid esters may include, but are not limited to, one or more of lactones or their derivatives, such as α-hydantoin (CAS: 42879-41-4), β-propiolactone, γ-butyrolactone, δ-valerolactone, and caprolactone. In some embodiments, the organic solvent is a mixed solvent composed of linear and cyclic carbonates.

[0075] The aforementioned organic solvents can be mixed in any proportion. In some embodiments of this application, the mass ratio (denoted as a) between the pentafluorocyclotriphosphazene and tetrafluorocyclotriphosphazene is close to the mass ratio (denoted as b) between the linear ester and the cyclic ester. In some embodiments, the deviation (denoted as k) between a and b is within 40%, i.e., k = |ab| / b, k ≤ 40%. Optionally, k ≤ 35%, or k ≤ 30%, or k ≤ 25%, or even k = 0. As mentioned above, the volatility characteristics of pentafluorocyclotriphosphazene and linear esters are similar, and the volatility characteristics of tetrafluorocyclotriphosphazene and cyclic esters are similar. In this case, the aforementioned flame retardant containing such a mass ratio of pentafluorocyclotriphosphazene and tetrafluorocyclotriphosphazene can maximize the flame retardant ability of the electrolyte and effectively improve safety.

[0076] In this embodiment, in addition to the flame retardant mentioned above, other additives may be added to the secondary battery electrolyte according to different performance requirements. Specifically, other additives may include one or more of the following: film-forming additives, high-voltage additives, and overcharge protection additives.

[0077] The film-forming additives may include, but are not limited to, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFEC), vinylene carbonate (VC), ethylene ethylene carbonate (VEC), 1,3-propanesulfonate lactone (1,3-PS), 1,4-butanesulfonate lactone, vinyl sulfite (ES), propylene sulfite (TMS), butenyl sulfite (BS), 4-methyl vinyl sulfite (MeES), and dimethyl sulfite (DS). The additives include, but are not limited to, one or more of the following: diethyl sulfite, vinyl sulfate (DTD), 4-methyl vinyl sulfate (MeDTD), 4-propyl vinyl sulfate (PEGLST), 1,4-butanediol sulfate, 4-fluorophenyl acetate (FPA), tris(trimethylsilane) phosphate (TMSP), tris(trimethylsilane) borate (TMSB), methanedisulfonate methylene ester (MMDS), N,N-thionyl diimidazole (SDI), and N,N-dimethyltrifluoroacetamide (DTA). DTA can form a film on the electrode surface to improve the low-temperature performance of the battery. High-voltage additives may include, but are not limited to, succinate (SN), adiponitrile (ADN), 1,3,6-hexanetrionitrile (HTCN), 1,3,5-pentanetricarbonyl nitrile (PTCN), 1,2-bis(2-cyanoethoxy)ethane (DENE), ethylene glycol dipropionitrile ether (DENE), and fluoroether D2. Overcharge prevention additives may include, for example, biphenyl and benzene derivatives. In some embodiments of this application, other additives include only fluorocarbonates, vinylene carbonate (VC), and high-voltage additives.

[0078] In this embodiment, the total mass percentage of other additives in the electrolyte can be 0.2%-15%, and more preferably 0.5%-10%.

[0079] The secondary battery electrolyte provided in this invention, by simultaneously adding three types of fluorinated triphosphazenes with different degrees of fluorine substitution as flame retardants, can effectively improve the flame retardancy of the electrolyte through their synergistic effect, thereby significantly improving the safety performance of the secondary battery. At the same time, they have good compatibility with positive and negative electrode materials, do not degrade the rate performance, low-temperature cycle performance and other electrochemical performance of the secondary battery, and also improve the battery's resistance to high voltage and high temperature, thus having a broader application prospect.

[0080] Accordingly, embodiments of this application also provide a method for preparing the above-mentioned secondary battery electrolyte, including the following steps:

[0081] In an inert or sealed environment (such as a glove box filled with argon), the thoroughly dried electrolyte salt and flame retardant (including pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene and trifluorocyclotriphosphazene) are added to an organic solvent and mixed evenly to obtain the secondary battery electrolyte.

[0082] The specific selection of raw materials such as electrolyte salts, non-aqueous organic solvents, flame retardants, and other additives has been described above and will not be repeated here. When the electrolyte also includes other additives, they can be added together with the flame retardant. Alternatively, the electrolyte salt and flame retardant can be added to the organic solvent simultaneously, or the flame retardant and other additives can be added to the organic solvent first, mixed evenly, and then the fully dried lithium salt can be added and mixed evenly again to obtain the secondary battery electrolyte. Each operation in the above preparation method can be implemented according to existing conventional electrolyte preparation processes.

[0083] The preparation method of secondary battery electrolyte provided in this application embodiment is simple and suitable for industrial production.

[0084] This application also provides a secondary battery, the structural schematic diagram of which is shown below. Figure 1 As shown, the secondary battery includes a positive electrode 101, a negative electrode 102, a separator 103, and an electrolyte 104, wherein the electrolyte 104 is the secondary battery electrolyte provided in the embodiments of this application. The specific structure of the secondary battery is as described above and will not be repeated here.

[0085] The secondary battery provided in this application embodiment has excellent safety performance and electrochemical performance because the specific flame retardant mentioned above is added to its electrolyte, and the flame retardant has good compatibility with the positive and negative electrode materials, and the flame retardant can have excellent flame retardant effect when the amount used is low.

[0086] In this application embodiment, the secondary battery can be, in addition to the lithium secondary battery mentioned above, a potassium secondary battery, a sodium secondary battery, a magnesium secondary battery, a zinc secondary battery, an aluminum secondary battery, etc. The secondary battery provided in this application embodiment can be used in end consumer products, such as mobile phones, tablets, power banks, laptops, digital cameras, and other wearable or portable electronic devices, as well as drones, automobiles, and other products, to improve product performance.

[0087] In this embodiment, the positive electrode 101 includes a positive electrode active material capable of reversibly inserting / deintercalating metal ions (lithium ions, sodium ions, potassium ions, magnesium ions, zinc ions, aluminum ions, etc.). This application does not impose any special limitations on the selection of the positive electrode active material; any positive electrode active material conventionally used in existing secondary batteries can be used. Taking a lithium secondary battery as an example, the positive electrode active material can be, but is not limited to, lithium cobalt oxide (LiCoO2), lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium manganese phosphate, lithium cobalt phosphate, etc. Each positive electrode active material can be undoped or doped and modified.

[0088] In this embodiment, the negative electrode 102 includes a negative electrode active material capable of reversibly inserting / deintercalating metal ions (lithium ions, sodium ions, potassium ions, magnesium ions, zinc ions, aluminum ions, etc.). This application does not impose any special limitations on the selection of the negative electrode active material; any negative electrode active material conventionally used in existing secondary batteries can be used. Taking a lithium secondary battery as an example, the negative electrode active material can be one or more of, but not limited to, lithium titanate, lithium metal, lithium alloys, carbon-based materials, silicon-based materials, and tin-based materials. Among these, carbon-based materials may include graphite (such as natural graphite and artificial graphite) and non-graphitized carbon (soft carbon, hard carbon, etc.); silicon-based materials may include one or more of elemental silicon, silicon-based alloys, silicon oxides, and silicon-carbon composite materials; and tin-based materials may include one or more of elemental tin and tin alloys.

[0089] In the embodiments of this application, the diaphragm can be a conventional diaphragm, including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP, and triple-layer PP / PE / PP diaphragms.

[0090] This application also provides an electronic device equipped with the aforementioned secondary battery. This electronic device can be various consumer electronics products, such as mobile phones, tablets, power banks, laptops, notebook computers, and other wearable or portable electronic devices, televisions, DVD players, video recorders, camcorders, radios, tape recorders, stereo systems, record players, laser disc players, home office equipment, and home electronic health devices. It can also be automobiles, energy storage devices, and other electronic products.

[0091] In some implementations, see Figure 3 This application provides an electronic device 300, which includes a housing 301 and electronic components (not shown) and a battery 302 housed within the housing 301. The battery 302 supplies power to the electronic device 300 and includes the secondary battery described above in this application embodiment. In some embodiments, the housing 301 may include a front cover assembled on the front side of the terminal and a rear cover assembled on the rear side, and the battery 302 may be fixed inside the rear cover. Figure 3 The electronic device 300 shown is typically a small, portable electronic device, such as a mobile phone.

[0092] In other implementations, see Figure 4This application provides an electronic device 400, which can be various movable devices used for loading, transportation, assembly, disassembly, security, etc., and can be various types of vehicles. Specifically, the electronic device 400 may include a vehicle body 401, a moving component 402, and a drive component. The drive component includes a motor 403 and a battery system 404 for supplying power to the motor 403. The battery system 404 includes the secondary battery provided in this application embodiment. The moving component 402 may be wheels; the battery system 404 may be a battery pack of the secondary battery 100, housed at the bottom of the vehicle body and electrically connected to the motor 403. The battery system 404 supplies power to the motor 403, which provides power to drive the moving component 402 of the electronic device 400 to move.

[0093] The technical solution of this application will be further described below through specific embodiments.

[0094] Example 1

[0095] Preparation of electrolyte: In a glove box filled with dry argon, dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylene carbonate (EC) were mixed uniformly in a mass ratio of 1:1:1 to form an organic solvent. Additives were added to the organic solvent, and after thorough stirring, fully dried lithium salt LiPF6 was added and mixed uniformly to obtain the lithium secondary battery electrolyte. The concentration of LiPF6 in the electrolyte was 1 mol / L. The additives contained in the electrolyte in the following mass percentages: 3 wt% methoxypentafluorocyclotriphosphazene, 2 wt% dimethoxytetrafluorocyclotriphosphazene, 5 wt% trimethoxytrifluorocyclotriphosphazene, 1 wt% ethylene carbonate (VC), 5 wt% fluoroethylene carbonate (FEC), and 2 wt% hexanetrionitrile (HTCN).

[0096] Manufacturing of lithium secondary batteries:

[0097] Preparation of positive electrode sheet: Lithium cobalt oxide (LiCoO2), conductive agent Super P, and binder polyvinylidene fluoride (PVDF) are added to N-methylpyrrolidone (NMP) in a mass ratio of 96:2:2. The positive electrode slurry is obtained by vacuum stirring. The positive electrode slurry is coated on an aluminum foil current collector, dried, rolled, and sliced ​​to obtain the positive electrode sheet. After vacuum drying at 85°C, it is transferred to a glove box for later use.

[0098] Preparation of negative electrode sheet: The negative electrode active material - graphite, conductive agent Super P, binder sodium carboxymethyl cellulose (CMC) and binder styrene-butadiene rubber (SBR) are added to deionized water in a mass ratio of 95:2:1:2 and stirred under vacuum to obtain a negative electrode slurry; the negative electrode slurry is coated on a copper foil current collector, dried, rolled and sliced ​​to obtain a negative electrode sheet, which is then vacuum dried at 120℃ and transferred to a glove box for later use.

[0099] The positive electrode sheet, commercial ceramic-coated PE separator, and negative electrode sheet prepared above are stacked in sequence to form a square battery cell. Aluminum-plastic film is used as the encapsulation material for the commercial ceramic coating. The electrolyte prepared in Example 1 above is injected into the battery cell. After encapsulation, standing, formation, and capacity testing, a soft-pack lithium secondary battery is produced.

[0100] Example 2

[0101] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 3 wt% ethoxypentafluorocyclotriphosphazene, 3 wt% diethoxytetrafluorocyclotriphosphazene, 4 wt% triethoxytrifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0102] Lithium secondary batteries were prepared using the electrolyte prepared in Example 2, and the preparation method was the same as in Example 1.

[0103] Example 3

[0104] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte are contained in the following components in the electrolyte with the following mass percentages: 4 wt% phenoxypentafluorocyclotriphosphazene, 3 wt% diphenoxytetrafluorocyclotriphosphazene, 3 wt% triphenoxytrifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0105] Lithium secondary batteries were prepared using the electrolyte prepared in Example 3, and the preparation method was the same as in Example 1.

[0106] Example 4

[0107] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 4 wt% ethyl ketone methyl pentafluorocyclotriphosphazene, 3 wt% diethyl ketone methyl tetrafluorocyclotriphosphazene, 3 wt% triethyl ketone methyl trifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0108] Lithium secondary batteries were prepared using the electrolyte prepared in Example 4, and the preparation method was the same as in Example 1.

[0109] Example 5

[0110] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 4 wt% diethyl phosphate-based pentafluorocyclotriphosphazene, 3 wt% diethyl bisphosphate-based tetrafluorocyclotriphosphazene, 3 wt% diethyl triphosphate-based trifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0111] Lithium secondary batteries were prepared using the electrolyte prepared in Example 5, and the preparation method was the same as in Example 1.

[0112] Example 6

[0113] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 4 wt% ethyl carbonate-based pentafluorocyclotriphosphazene, 3 wt% diethyl carbonate-based tetrafluorocyclotriphosphazene, 3 wt% triethyl carbonate-based trifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0114] A lithium secondary battery was prepared using the electrolyte prepared in Example 6, and the preparation method was the same as in Example 1.

[0115] Example 7

[0116] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 4 wt% of methyl sulfonate pentafluorocyclotriphosphazene, 3 wt% of dimethyl sulfonate tetrafluorocyclotriphosphazene, 3 wt% of trimethyl sulfonate trifluorocyclotriphosphazene, 1 wt% of VC, 5 wt% of FEC and 2 wt% of HTCN.

[0117] Lithium secondary batteries were prepared using the electrolyte prepared in Example 7, and the preparation method was the same as in Example 1.

[0118] Example 8

[0119] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 4 wt% of trimethoxysiloxypentafluorocyclotriphosphazene, 3 wt% of bistrimethoxysiloxytetrafluorocyclotriphosphazene, 3 wt% of tritrimethoxysiloxytrifluorocyclotriphosphazene, 1 wt% of VC, 5 wt% of FEC and 2 wt% of HTCN.

[0120] A lithium secondary battery was prepared using the electrolyte prepared in Example 8, and the preparation method was the same as in Example 1.

[0121] Example 9

[0122] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 4 wt% acetonitrile-based pentafluorocyclotriphosphazene, 3 wt% bisacetonitrile-based tetrafluorocyclotriphosphazene, 3 wt% triacetonitrile-based trifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0123] A lithium secondary battery was prepared using the electrolyte prepared in Example 9, and the preparation method was the same as in Example 1.

[0124] Example 10

[0125] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 2 wt% ethoxypentafluorocyclotriphosphazene, 2 wt% diphenoxytetrafluorocyclotriphosphazene, 1 wt% triacetonitrile-based trifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0126] A lithium secondary battery was prepared using the electrolyte prepared in Example 10, and the preparation method was the same as in Example 1.

[0127] Example 11

[0128] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 2 wt% ethoxypentafluorocyclotriphosphazene, 1 wt% diphenoxytetrafluorocyclotriphosphazene, 2 wt% triacetonitrile-based trifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0129] A lithium secondary battery was prepared using the electrolyte prepared in Example 11, and the preparation method was the same as in Example 1.

[0130] Example 12

[0131] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte include the following components in the electrolyte with the following mass percentages: 2 wt% diethyl phosphate-based pentafluorocyclotriphosphazene, 1 wt% bismethoxytetrafluorocyclotriphosphazene, 2 wt% triacetonitrile-based trifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0132] Lithium secondary batteries were prepared using the electrolyte prepared in Example 12, and the preparation method was the same as in Example 1.

[0133] Example 13

[0134] The preparation method of the electrolyte is basically the same as that in Example 1, except that the additives used in the electrolyte are contained in the following components in the electrolyte with the following mass percentages: 3 wt% phenoxypentafluorocyclotriphosphazene, 1 wt% diethyl ketone methyltetrafluorocyclotriphosphazene, 1 wt% triethyl carbonate trifluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0135] A lithium secondary battery was prepared using the electrolyte prepared in Example 13, and the preparation method was the same as in Example 1.

[0136] To highlight the beneficial effects of the embodiments of this application, the following comparative examples are provided.

[0137] Comparative Example 1

[0138] The preparation method of the electrolyte in Comparative Example 1 is basically the same as that in Example 1, except that the additives used in the electrolyte are only those components in which the following mass percentages are present: 1 wt% VC, 5 wt% FEC, and 2 wt% HTCN. That is, this electrolyte does not contain flame retardants.

[0139] A lithium secondary battery was prepared using the electrolyte prepared in Comparative Example 1, and the preparation method was the same as in Example 1.

[0140] Comparative Example 2

[0141] The preparation method of the electrolyte in Comparative Example 2 is basically the same as that in Example 2, except that the additives used in the electrolyte are only the components in which the following mass percentages are contained: 10 wt% ethoxypentafluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0142] A lithium secondary battery was prepared using the electrolyte prepared in Comparative Example 2, and the preparation method was the same as in Example 1.

[0143] Comparative Example 3

[0144] The preparation method of the electrolyte in Comparative Example 3 is basically the same as that in Example 2, except that the additives used in the electrolyte are only the components in which the following mass percentages are contained: 10 wt% diethoxytetrafluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0145] A lithium secondary battery was prepared using the electrolyte prepared in Comparative Example 3, and the preparation method was the same as in Example 1.

[0146] Comparative Example 4

[0147] The preparation method of the electrolyte in Comparative Example 4 is basically the same as that in Example 2, except that the additives used in the electrolyte are only the components in which the following mass percentages are contained: 10 wt% of triethoxytrifluorocyclotriphosphazene, 1 wt% of VC, 5 wt% of FEC and 2 wt% of HTCN.

[0148] A lithium secondary battery was prepared using the electrolyte prepared in Comparative Example 4, and the preparation method was the same as in Example 1.

[0149] Comparative Example 5

[0150] The preparation method of the electrolyte in Comparative Example 5 is basically the same as that in Example 2, except that the additives used in the electrolyte are only the components in which the following mass percentages are contained: 5 wt% ethoxypentafluorocyclotriphosphazene, 5 wt% diethoxytetrafluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0151] A lithium secondary battery was prepared using the electrolyte prepared in Comparative Example 5, and the preparation method was the same as in Example 1.

[0152] Comparative Example 6

[0153] The preparation method of the electrolyte in Comparative Example 6 is basically the same as that in Example 3, except that the additives used in the electrolyte are only the components in which the following mass percentages are contained: 10 wt% phenoxypentafluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0154] A lithium secondary battery was prepared using the electrolyte prepared in Comparative Example 6, and the preparation method was the same as in Example 1.

[0155] Comparative Example 7

[0156] The preparation method of the electrolyte in Comparative Example 7 is basically the same as that in Example 3, except that the additives used in the electrolyte are only the components in which the following mass percentages are contained: 5% phenoxypentafluorocyclotriphosphazene, 5% diphenoxytetrafluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0157] A lithium secondary battery was prepared using the electrolyte prepared in Comparative Example 7, and the preparation method was the same as in Example 1.

[0158] Comparative Example 8

[0159] The preparation method of the electrolyte in Comparative Example 8 is basically the same as that in Example 2, except that the additives used in the electrolyte are only the components in which the following mass percentages are contained: 30 wt% ethoxypentafluorocyclotriphosphazene, 1 wt% VC, 5 wt% FEC and 2 wt% HTCN.

[0160] A lithium secondary battery was prepared using the electrolyte prepared in Comparative Example 8, and the preparation method was the same as in Example 1.

[0161] The electrolytes and lithium secondary batteries of Examples 1-12 and Comparative Examples 1-7 of this application were subjected to the following performance tests, and the results are summarized in Table 1 below.

[0162] 1. Electrolyte self-extinguishing performance test

[0163] 5g of electrolyte was placed in a 5.0ml crucible, and the electrolyte was quickly ignited using an ignition device. The time from when the ignition device was removed until the flame extinguished automatically was recorded. The extinguishing time per unit mass of electrolyte is the self-extinguishing time (SET) of the electrolyte. The SET test was performed 5 times for each electrolyte sample, and the average value was taken. In addition, the combustion characteristics of each electrolyte under continuous exposure to the flame were recorded.

[0164] 2. Performance testing of lithium secondary batteries

[0165] 2.1 Cyclic Performance Test

[0166] Under ambient temperature conditions of 25℃±3℃, the lithium secondary battery was subjected to charge-discharge cycle tests at a charge-discharge rate of 0.7 / 0.7C. The voltage range of the graphite / LiCoO2 battery was 3.0-4.5V, and the capacity retention rate after 300 cycles was recorded.

[0167] 2.2 Ratio Performance Test

[0168] Under ambient temperature conditions of 25℃±2℃, the lithium secondary battery was charged and discharged at 0.2 / 0.2C and 0.2 / 2.0C rates, respectively. The voltage range of the graphite / LiCoO2 battery was 3.0-4.5V. The capacity retention rate at 2C rate was recorded as (2C discharge capacity / 0.2C discharge capacity*100%).

[0169] 2.3 High-Temperature Storage Performance Test

[0170] Under ambient temperature of 25℃±2℃, the battery was charged and discharged once at 0.2C / 0.2C, and the capacity of this charge was recorded as the initial capacity. The battery was then fully charged again at 0.2C. After charging, the battery was left to stand at 70℃ for 72 hours, and then left to stand in open circuit at room temperature for 2 hours. It was then discharged at a constant current of 0.2C until the termination voltage was reached, and this was recorded as the remaining capacity. The voltage range of the graphite / LiCoO2 battery is 3.0-4.5V. The remaining capacity retention rate (remaining capacity / initial capacity*100%) was recorded.

[0171] Table 1 Summary of test data for each embodiment and comparative example

[0172]

[0173]

[0174] Table 1 shows that the electrolyte using pentafluorocyclic triphosphazene, tetrafluorocyclic triphosphazene, and trifluorocyclic triphosphazene as flame retardants is non-flammable upon ignition, and no fire occurs during the entire heating process until the electrolyte completely evaporates. This is mainly attributed to the fact that pentafluorocyclic triphosphazene can effectively cover the volatilization range of linear ester solvents in the electrolyte, tetrafluorocyclic triphosphazene can effectively cover the volatilization range of cyclic ester solvents, and trifluorocyclic triphosphazene can effectively cover the generation range of flammable gases from electrolyte decomposition, thus achieving a wider range of flame retardancy. In addition, the fluorocyclic triphosphazene flame retardant itself has high antioxidant capacity and the ability to form a composite interface film on the positive electrode surface, effectively inhibiting the damage of the positive electrode at high potentials, thereby improving the cycle stability of the lithium secondary battery. Therefore, the battery cycle performance of Examples 1-9 is also better than that of Comparative Example 1. In contrast, the electrolyte of Comparative Example 1, which does not contain flame retardants, can be immediately ignited by a flame and continues to burn, resulting in poor battery safety performance.

[0175] Furthermore, a comparison of Example 2 with Comparative Example 2, and Example 3 with Comparative Example 6, shows that when the total amount of flame retardant added is equal, the electrolyte does not initially ignite when a single pentafluorocyclic triphosphazene is added as a flame retardant. However, combustion occurs after 60 seconds of continuous ignition. This indicates that a single pentafluorocyclic triphosphazene can only cover the volatilization range of the linear esters in the electrolyte to provide flame retardancy. Comparative Examples 3-4 show that when the total amount of flame retardant added is equal, the electrolyte ignites initially when a single tetrafluorocyclic triphosphazene or trifluorocyclic triphosphazene is added as an electrolyte. This may be because a single tetrafluorocyclic triphosphazene or trifluorocyclic triphosphazene cannot cover the early volatilization range of the linear esters, thus the electrolyte ignites initially and then extinguishes. As can be seen from Comparative Examples 5 and 7, when the total amount of flame retardant added is equal, the electrolyte does not ignite initially when both pentafluorocyclotriphosphazene and tetrafluorocyclotriphosphazene are added as flame retardants, and the duration of non-combustion is longer than when using pentafluorocyclotriphosphazene alone. However, in the later stages of continuous ignition, the electrolyte still ignites, which may be because the flame retardant cannot cover the generation range of flammable gases produced by the electrolyte in the later stages. The electrolyte using pentafluorocyclotriphosphazene as a single flame retardant only shows flame retardant performance similar to Examples 1-9 of this application when the amount of pentafluorocyclotriphosphazene added is large (Comparative Example 8), but the rate performance and cycle performance of the battery in Comparative Example 8 are significantly degraded.

[0176] Furthermore, the electrolytes of Examples 10-12 of this application still exhibited good flame-retardant properties even when the total amount of the flame retardant system composed of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene was reduced to 5% of the electrolyte. Additionally, a comparison between Examples 10 and 11 shows that, with the same composition and total content of the flame retardant system, the electrolyte exhibited superior flame-retardant properties when the mass ratio of pentafluorocyclotriphosphazene to tetrafluorocyclotriphosphazene in the electrolyte was the same as the mass ratio of linear ester to cyclic ester in the electrolyte solvent (Example 11).

Claims

1. A secondary battery electrolyte, characterized in that, The mixture includes an electrolyte salt, an organic solvent, and a flame retardant. The organic solvent comprises linear and cyclic esters, and the flame retardant comprises pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene, but excludes cyclic phosphate anhydrides. Furthermore, the mass ratio 'a' between the pentafluorocyclotriphosphazene and tetrafluorocyclotriphosphazene is close to the mass ratio 'b' between the linear and cyclic esters; 'a' and 'b' being close means |ab| / b ≤ 40%. The chemical structural formula of the pentafluorocyclic triphosphazene is shown in formula (I), the chemical structural formula of the tetrafluorocyclic triphosphazene is shown in formula (II), and the chemical structural formula of the trifluorocyclic triphosphazene is shown in formula (III). （I） （Ⅱ） （Ⅲ） Wherein, R1, R2, R3, R4, R5, and R6 are independently selected from any one of alkoxy, haloalkoxy, aryloxy, haloaryloxy, alkyl, haloalkyl, alkenyl, haloalkenyl, alkenyloxy, haloalkenyloxy, aryl, haloaryl, epoxy, haloepoxy, phosphate ester, substituted phosphate ester, carbonate, substituted carbonate, sulfonate, substituted sulfonate, alkyl ketone, haloalkyl ketone, alkoxysiloxy, haloalkoxysiloxy, substituted imide, substituted sulfonimide, and nitrile; wherein, the alkoxy, haloalkoxy, alkyl, haloalkyl, ... The alkyl group has 1-10 carbon atoms; the alkenyl, haloalkenyl, alkenoxy, haloalkenoxy, epoxy, and haloepoxy groups have 2-10 carbon atoms; the aryl, haloaryl, aryloxy, and haloaryloxy groups have 6-15 carbon atoms; the phosphate ester group, substituted phosphate ester group, carbonate group, substituted carbonate group, sulfonate group, and substituted sulfonate group have 1-10 carbon atoms; the alkoxy group in the alkoxysiloxy group and the haloalkoxy group in the haloalkoxysiloxy group each have 1-10 carbon atoms; the alkyl ketone group and the haloalkyl ketone group each have the following general formula: , where R 10 and R 11 The number of carbon atoms is 1-10; the substituted imide group is represented as... The substituted sulfonylimide group is represented as X and Z are independently selected from any one of alkyl, haloalkyl, alkoxy, haloalkoxy, alkenyl, haloalkenyl, alkenoxy, haloalkenoxy, aryl, haloaryl, aryloxy, and haloaryloxy.

2. The secondary battery electrolyte as described in claim 1, characterized in that, The total mass percentage of the pentafluorocyclic triphosphazene, tetrafluorocyclic triphosphazene and trifluorocyclic triphosphazene in the electrolyte is 0.1%-20%.

3. The secondary battery electrolyte as described in claim 2, characterized in that, The total mass percentage of the pentafluorocyclic triphosphazene, tetrafluorocyclic triphosphazene and trifluorocyclic triphosphazene in the electrolyte is 5%-20%.

4. The secondary battery electrolyte as described in claim 1, characterized in that, The pentafluorocyclic triphosphazene has a mass percentage of 1%-10% in the secondary battery electrolyte, the tetrafluorocyclic triphosphazene has a mass percentage of 1%-5% in the secondary battery electrolyte, and the trifluorocyclic triphosphazene has a mass percentage of 1%-5% in the secondary battery electrolyte.

5. The secondary battery electrolyte according to any one of claims 1-4, characterized in that, The average degree of fluorine substitution (DS) of the flame retardant satisfies the following relationship: DS = 5 × ω1 + 4 × ω2 + 3 × ω3, where ω1, ω2, and ω3 represent the molar percentages of pentafluorocyclotriphosphazene, tetrafluorocyclotriphosphazene, and trifluorocyclotriphosphazene in the flame retardant, respectively. The value range of DS is: 3 <DS<5。 6. The secondary battery electrolyte as described in claim 5, characterized in that, The ω3 is less than or equal to 50%.

7. The secondary battery electrolyte as described in claim 1, characterized in that, The halogens in the haloalkoxy, haloaryloxy, haloalkyl, haloalkenyl, haloalkenoxy, haloaryl, haloepoxy, and haloalkylketone groups are all fluorine. The phosphate ester group is an alkyl phosphate ester group, and the substituted phosphate ester group is a fluoroalkyl phosphate ester group; the carbonate group is an alkyl carbonate ester group, and the substituted carbonate group is a fluoroalkyl carbonate ester group; the sulfonate group is a alkyl sulfonic acid ester group, and the substituted sulfonate group is a fluoroalkyl sulfonic acid ester group. The alkoxysiloxy group is a trialkoxysiloxy group, and the haloalkoxysiloxy group is a fluorotrialkoxysiloxy group.

8. The secondary battery electrolyte as described in claim 1, characterized in that, The linear ester includes at least one of linear carbonates and linear carboxylic acid esters, and the cyclic ester includes at least one of cyclic carbonates and cyclic carboxylic acid esters.

9. The secondary battery electrolyte as described in claim 8, characterized in that, The cyclic carbonates include one or more of ethylene carbonate, propylene carbonate, and butylene carbonate; the linear carbonates include one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dipropyl carbonate; the linear carboxylic acid esters include one or more of methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate; and the cyclic carboxylic acid esters include one or more of α-hydantoin, β-propiolactone, γ-butyrolactone, δ-valerolactone, and caprolactone.

10. The secondary battery electrolyte as described in claim 1, characterized in that, The electrolyte also includes other additives, including fluoroethylene carbonate, difluoroethylene carbonate, trifluoromethyl ethylene carbonate, vinylene carbonate, ethylene ethylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, vinyl sulfite, propylene sulfite, butene sulfite, 4-methyl vinyl sulfite, dimethyl sulfite, diethyl sulfite, vinyl sulfate, 4-methyl vinyl sulfate, and 4-propyl ethyl sulfate. The following are some of the following: olefin ester, 1,4-butanediol sulfate, 4-fluorophenyl acetate, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, methane disulfonate, N,N-thionyl diimidazole, succinate, adiponitrile, 1,3,6-hexanetrionitrile, 1,3,5-pentanetricarbonyl nitrile, 1,2-bis(2-cyanoethoxy)ethane, ethylene glycol dipropionitrile ether, fluoroether D2, N,N-dimethyltrifluoroacetamide, biphenyl and benzene derivatives.

11. The secondary battery electrolyte as described in claim 10, characterized in that, The other additives are present in the electrolyte at a mass percentage of 0.2%-15%.

12. The secondary battery electrolyte as described in claim 1, characterized in that, The electrolyte salt is a lithium salt, which includes one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium difluorophosphate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(perfluoroethylsulfonyl)imide, lithium trifluoromethylsulfonate, and lithium perfluorobutylsulfonate.

13. The secondary battery electrolyte as described in claim 12, characterized in that, The molar concentration of the lithium salt in the secondary battery electrolyte is 0.01 mol / L - 2.0 mol / L.

14. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is a secondary battery electrolyte as described in any one of claims 1-13.

15. An electronic device, characterized in that, Includes the secondary battery as described in claim 14.

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