Crosslinking agent for electrolyte, electrolyte composition containing the same, and lithium ion battery

Abstract

A crosslinking agent having a structure of formula (I) for an electrolyte, an electrolyte composition containing the crosslinking agent, and a lithium-ion battery, wherein M, R, and X are as defined in the specification. By means of the crosslinking agent having the structure of formula (I), not only the mechanical strength, heat resistance, ionic conductivity, and electrochemical stability of the prepared electrolyte composition are improved, but also the long-term charge-discharge cycle stability of the lithium-ion battery is improved, having extremely high industrial application value.

Description

Technical Field

[0001] The present invention relates to additives and electrolytes in the field of electrochemistry, and particularly to a crosslinking agent for a solid electrolyte of a secondary battery, an electrolyte composition containing the crosslinking agent, and a lithium ion battery. Background Art

[0002] With the development of the consumer electronics field, the application of secondary batteries has become increasingly popular. Among them, lithium ion batteries are the most common. Because lithium ion batteries have advantages such as high energy density, long service life, high working voltage, stable discharge, and no memory effect, they can be widely used in fields such as portable electronic devices, military, electric vehicles, and aerospace.

[0003] Currently, commercially available lithium ion battery electrolytes still use volatile and flammable solvents. Although this liquid electrolyte has high ionic conductivity at room temperature, after long-term use, there is a risk of solvent leakage in the liquid electrolyte. Once an abnormal situation occurs in the lithium ion battery, it is easy to cause the liquid electrolyte to overheat, and it is even more likely to cause dangerous situations such as spontaneous combustion and explosion, which cannot be underestimated.

[0004] To solve the above safety problems, although there is a technology that changes to a solid polymer electrolyte, its mechanical strength is insufficient, and it is easily punctured by unevenly stacked lithium metal on the electrode surface, leading to micro-short circuits. Moreover, the mobility of the polymer chains of the electrolyte polymer is poor, which also limits its ionic conductivity.

[0005] Even if there is a current technology that introduces a crosslinking agent (commonly such as polyethylene glycol diacrylate) into the electrolyte, although it improves its mechanical strength, in an environment of high current density, due to charge imbalance near the lithium metal electrode, it is easy to cause an out-of-control lithium ion reduction phenomenon, and there is a problem of the formation of lithium dendrites, which affects the performance of the battery.

[0006] In view of this, it is necessary to propose an electrolyte with both high safety and high performance to meet the actual needs of current applications. Summary of the Invention

[0007] To solve the problems existing in the above-known technologies, the present invention provides a crosslinking agent having a structure of formula (I) for an electrolyte:

[0008]

[0009] Wherein, M is selected from a monovalent imidazole ion, a triazole ion, a pyridine ion, a substituted or unsubstituted phosphonium ion, or a substituted or unsubstituted ammonium ion;

[0010] R is a C 1-12 linear alkylene group, an ethyleneoxy group or a polyethoxy group, a phenylene group or a polyphenylene group; and

[0011] X is a monovalent anion containing a halogen atom, an anion containing a carboxylate group, or a thiocyanate ion.

[0012] In a specific embodiment of the present invention, M is selected from one of the following groups:

[0013] and wherein, * represents the position where M is connected to the structure of formula (I).

[0014] In another specific embodiment of the present invention, M is a monovalent imidazole ion and X is a monovalent anion containing a halogen atom; wherein, the anion containing a halogen atom is selected from chloride ion, bromide ion, tetrafluoroborate ion (BF4 - ), hexafluorophosphate ion (PF6 - ), bis(trifluoromethylsulfonyl)imide anion (TFSI - ), and trifluoromethanesulfonate ion (CF3SO3 - ).

[0015] In other specific embodiments of the present invention, R is a straight-chain butylene group, a straight-chain octylene group, or a straight-chain dodecylene group.

[0016] In a specific embodiment of the present invention, the crosslinking agent having the structure of formula (I) is represented by the following compounds (I-1) to (I-3),

[0017]

[0018] The present invention also provides an electrolyte composition comprising a polymer crosslinked by the above crosslinking agent, wherein the polymer is obtained by reacting a reactive monomer having an alkenyl or mercapto group with an initiator, and based on the total weight of the electrolyte composition, the content of the moiety of the crosslinking agent having the structure of formula (I) in the crosslinked polymer is 1 to 25% by weight.

[0019] In a specific embodiment of the present invention, the electrolyte composition further includes an additive and an electrolyzable lithium salt.

[0020] In a specific embodiment of the present invention, the crosslinking density of the electrolyte composition is 5 to 25%.

[0021] In a specific embodiment of the present invention, the thermal cracking temperature of the electrolyte composition is 100 to 282 °C.

[0022] In a specific embodiment of the present invention, the stress of the electrolyte composition at a strain of -40% is 0.029 to 0.064 megapascals (MPa).

[0023] In a specific embodiment of the present invention, the conductivity of the electrolyte composition at room temperature is 1.17x10 -4 to 1.52x10 -4 Siemens / cm (S / cm).

[0024] The present invention also provides a method for preparing an electrolyte composition, comprising: providing a reactive oligomer having an alkenyl or mercapto group; and subjecting the reactive oligomer, the crosslinking agent and the initiator of the present invention to a free radical polymerization reaction in the presence of an additive and an electrolyzable lithium salt to obtain the electrolyte composition.

[0025] In a specific embodiment of the present invention, the weight ratio of the crosslinking agent to the reactive oligomer is 5:95 to 25:75.

[0026] In a specific embodiment of the present invention, the initiator is a thermal initiator and is selected from one of the group consisting of azobisisobutyronitrile (AIBN) and azobis(isobutyramidine) hydrochloride (AIBA); wherein, the temperature of the free radical polymerization reaction is 55 to 80 °C, and the reaction time is 6 to 24 hours.

[0027] In another specific embodiment of the present invention, the initiator is a photoinitiator and is selected from one of the group consisting of 2,2-dimethoxy-2-phenylacetophenone (DMPA), 2-hydroxy-2-methylpropiophenone and 1-hydroxycyclohexyl phenyl ketone; wherein, the light source wavelength range of the free radical polymerization reaction is 350 to 400 nm, and the reaction time is 5 to 10 minutes.

[0028] In a specific embodiment of the present invention, the electrolyzable lithium salt is selected from at least one of the group consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF6), lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiODFB), lithium bis(fluorosulfonyl)imide salt (LiFSI), lithium difluorophosphate (LiPO2F2) and lithium tetrafluoro(oxalato)phosphate (LiFOP).

[0029] In a specific embodiment of the present invention, the additive is selected from at least one of the group consisting of polyethylene glycol dimethyl ether (PEGDME), succinonitrile (SN) and ionic liquid, wherein the ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI) or 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI).

[0030] The present invention also provides a lithium ion battery, comprising: a positive electrode; a negative electrode; and the electrolyte composition of the present invention.

[0031] In a specific embodiment of the present invention, the positive electrode is lithium iron phosphate (LiFePO4).

[0032] In another specific embodiment of the present invention, the negative electrode is lithium metal.

[0033] In a specific embodiment of the present invention, the charge-discharge capacity of the lithium-ion battery at 60 °C within a voltage range of 2.5 to 4.0 volts at a discharge rate of 1 to 2C is greater than 160 mAh / g.

[0034] In another specific embodiment of the present invention, the charge-discharge capacity of the lithium-ion battery at 25 °C within a voltage range of 2.5 to 4.0 volts at a discharge rate of 0.2 to 0.3C is greater than 158 mAh / g.

[0035] In a specific embodiment of the present invention, under the conditions of 60 °C and a discharge rate of 0.2 to 0.5C, after 100 charge-discharge cycles, the capacitance of the lithium-ion battery is more than 90% of the initial capacitance.

[0036] In another specific embodiment of the present invention, under the conditions of 25 °C and a discharge rate of 0.2C, after 150 charge-discharge cycles, the capacitance of the lithium-ion battery is more than 90% of the initial capacitance.

[0037] By adding a crosslinking agent having the structure of formula (I), the ionic groups of the crosslinking agent are uniformly distributed in the structure of the electrolyte composition prepared by the present invention. In addition to improving the ionic conductivity of the electrolyte composition, it also serves as a local reservoir for anions. In an environment of high current density, it is used to alleviate the charge imbalance near the lithium metal electrode and delay the formation of lithium dendrites.

[0038] On the other hand, through the dense crosslinked structure provided by the crosslinking agent having the structure of formula (I), the mechanical strength and heat resistance of the prepared electrolyte composition are improved to maintain the smooth surface of the lithium metal electrode and avoid the problem of micro short circuit caused by puncture, significantly improving the electrochemical stability and long-term charge-discharge cycle stability of the lithium-ion battery, and extending its service life, having extremely high industrial application value and market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The embodiments of the present invention are illustrated by exemplary reference drawings:

[0040] Figure 1 It is the TGA weight change curve of the electrolyte composition of the examples and comparative examples of the present invention;

[0041] Figures 2A to 2B It is the stress-strain curve of the DMA compression test of the electrolyte composition of the examples and comparative examples of the present invention;

[0042] Figure 3 Graph of the change of the ionic conductivity (σ) of the electrolyte compositions of the embodiments and comparative examples of the present invention with temperature (T);

[0043] Figures 4A to 4B Linear sweep voltammogram of the electrolyte compositions of the embodiments and comparative examples of the present invention;

[0044] Figure 5A Graph of the capacitance of the lithium-ion battery of the test example of the present invention at 25 °C and different charge-discharge rates;

[0045] Figure 5B Graph of the capacitance of the lithium-ion battery of the test example of the present invention at 60 °C and different charge-discharge rates;

[0046] Figure 5C Graph of the capacitance of the lithium-ion battery of the comparative test example at 25 °C and different charge-discharge rates;

[0047] Figure 5D Graph of the capacitance of the lithium-ion battery of the comparative test example at 60 °C and different charge-discharge rates;

[0048] Figure 6A Graph of the long-term charge-discharge cycle stability of the lithium-ion battery of the test example of the present invention at 25 °C and a discharge rate of 0.2C;

[0049] Figure 6B Graph of the long-term charge-discharge cycle stability of the lithium-ion battery of the test example of the present invention at 60 °C and a discharge rate of 0.2C;

[0050] Figure 6C Graph of the long-term charge-discharge cycle stability of the lithium-ion battery of the test example of the present invention at 60 °C and a discharge rate of 0.5C;

[0051] Figure 6D Graph of the long-term charge-discharge cycle stability of the lithium-ion battery of the comparative test example at 60 °C and a discharge rate of 0.2C; and

[0052] Figure 7 Schematic structural diagram of the lithium-ion battery of the present invention.

[0053] Symbol Explanation

[0054] 1 Lithium-ion battery

[0055] 10 Lower cover

[0056] 11 Upper cover

[0057] 12 Positive circular electrode sheet

[0058] 13 Electrolyte film

[0059] 14 Negative electrode plate

[0060] 15 Stainless steel sheet

[0061] 16 Spring piece. Detailed implementation manners

[0062] The following describes the implementation manners of the present invention through specific specific examples. Those skilled in the art can easily understand the advantages and effects of the present invention from the content described in this specification. The present invention can also be implemented or applied through other different implementation manners. All details in this specification can also be given different modifications and changes based on different viewpoints and applications without departing from the spirit disclosed by the present invention. In addition, all ranges and values herein are inclusive and combinable. Any numerical value or point within the ranges described herein, for example, any integer, can be used as the minimum or maximum value to derive sub-ranges, etc.

[0063] According to the present invention, a crosslinking agent having the structure of formula (I) for an electrolyte is provided:

[0064]

[0065] Wherein, M is selected from a monovalent imidazole ion, a triazole ion, a pyridine ion, a substituted or unsubstituted phosphonium ion or a substituted or unsubstituted ammonium ion;

[0066] R is C 1-12 A straight-chain alkylene group, an ethyleneoxy group or a polyethoxy group, a phenylene group or a polyphenylene group; and

[0067] X is a monovalent halogen atom-containing anion, a carboxylate-containing anion, a thiocyanate ion.

[0068] In the text, "substituted" in the expression "substituted or unsubstituted" means that one or more hydrogen atoms in a certain functional group are replaced by another atom or group (i.e., a substituent), and the substituent can be selected from C 1-20 A straight-chain or branched-chain alkyl group.

[0069] In the text, the "polyethoxy" is a polymer group having an ethoxy group as a repeating unit and a polymerization degree ranging from 1 to 230.

[0070] In the text, the "polyphenylene" is a polymer group having a phenyl group as a repeating unit and a polymerization degree ranging from 1 to 3.

[0071] In the text, the "halogen atom" is an element of fluorine, chlorine, bromine or iodine.

[0072] In the text, the "carboxylate-containing anion" refers to, for example, but not limited to, acetate, propionate, butyrate, isobutyrate, or derivatives of any of the above groups.

[0073] In the structure of the crosslinking agent of the present invention, since both ends thereof have olefin groups, compared with those having a single-terminal olefin group, the resulting electrolyte has significant mechanical strength.

[0074] In the structure of the crosslinking agent of the present invention, in addition to improving the ionic conductivity of the resulting electrolyte, the cationic group M also serves as a local reservoir for anions, which is used to alleviate the charge imbalance near the lithium metal electrode and delay the formation of lithium dendrites in an environment of high current density, thereby improving the electrochemical stability of the resulting electrolyte. For example, the structure and linkage of the M can be selected from one of the following groups:

[0075] and

[0076] wherein, * represents the position where the M is connected to the structure of formula (I). In a specific embodiment, M is preferably an imidazolium ion.

[0077] In the structure of the crosslinking agent of the present invention, the spacer group R functions to adjust the hardness and softness of the molecular structure of its crosslinked polymer. If R is selected as a C 1-12 straight-chain alkylene group, phenylene group or polyphenylene group, a crosslinked polymer with relatively rigid properties can be prepared; if R is selected as an oxygen-containing segment, such as ethyleneoxy or polyethoxy, a crosslinked polymer with relatively soft properties can be prepared. In a specific embodiment, R is a straight-chain butylene group, a straight-chain octylene group or a straight-chain dodecylene group.

[0078] In the structure of the crosslinking agent of the present invention, the anion group X functions to adjust the hydrophilicity and hydrophobicity of the resulting electrolyte. If X is selected as a chloride ion or a bromide ion, a more hydrophilic electrolyte can be prepared; if X is selected as a bis(trifluoromethylsulfonyl)imide anion (TFSI - ), a more hydrophobic electrolyte can be prepared.

[0079] In a preferred specific embodiment, M is a monovalent imidazolium ion and X is a monovalent halogen atom-containing anion.

[0080] In the text, the "halogen atom-containing anion" is selected from chloride ion, bromide ion, tetrafluoroborate ion (BF4 - ), hexafluorophosphate ion (PF6 - ), bis(trifluoromethylsulfonyl)imide anion (TFSI - ) and trifluoromethanesulfonate ion (CF3SO3 - ).

[0081] In another preferred specific embodiment, the crosslinking agent having the structure of formula (I) of the present invention is shown as the following compounds (I-1) to (I-3),

[0082]

[0083]

[0084] Regarding the method for preparing the crosslinking agents of the above structures (I-1) to (I-3), it includes: in the presence of a solvent, pre-reacting N-vinylimidazole and a dihaloalkane to obtain a pre-reactant; and subjecting the pre-reactant to a substitution reaction with a salt containing a halogen atom, a carboxylate group or a thiocyanate group to form a crosslinking agent having the above structure.

[0085] In a specific embodiment, the preparation method of the crosslinking agent of the above structure (I-1) is to use ethyl acetate as a solvent and pre-react N-vinylimidazole and 1,4-dihalobutane at a temperature of 60 to 80 °C; then, using water as a solvent, subjecting the pre-reaction product to a substitution reaction with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to obtain a crosslinking agent having the structure of formula (I-1).

[0086] In addition, the present invention also provides an electrolyte composition for use in a battery system and as an ion transfer medium, wherein the electrolyte composition contains a polymer crosslinked by the above crosslinking agent, wherein the polymer is obtained by reacting a reactive oligomer selected from the group consisting of methoxypolyethylene glycol methacrylate (PEGME) and polyethylene glycol diacrylate (PEGDA), an initiator and the above crosslinking agent; in an embodiment, the content of the crosslinked polymer containing the crosslinking agent of formula (I) is 1 to 25% by weight based on the total weight of the electrolyte composition.

[0087] In a specific embodiment, the electrolyte composition of the present invention further includes an additive and an electrolyzable lithium salt. In an embodiment, the weight ratio of the crosslinked polymer, the additive and the electrolyzable lithium salt is 5:4:3, and the initiator is 1 wt% of the total weight of the crosslinked polymer.

[0088] In the text, the "polymer" refers to a polymer formed by a polymerization reaction of a reactive oligomer selected from the group consisting of methoxypolyethylene glycol methacrylate (PEGME) and polyethylene glycol diacrylate (PEGDA), an initiator and the above crosslinking agent; in an embodiment, the content of the polymer is 20 to 60% by weight based on the total weight of the electrolyte composition.

[0089] In the text, the function of the "additive" is not only to endow the electrolyte itself with sufficient ionic conductivity, but also to improve and reduce the interfacial impedance between the electrolyte and the electrode, so as to enhance the overall stability of the battery. In practical applications, different types of additives can be selected according to different requirements, such as polyethylene glycol dimethyl ether (PEGDME), succinonitrile (SN), or ionic liquids; among them, the ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) or 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-TFSI).

[0090] Under the condition of the same composition ratio, the electrolyte with succinonitrile has good ionic conductivity; the electrolyte with ionic liquid has good thermal stability and chemical stability; the electrolyte with PEGDME has the characteristics of good thermal stability and high ionic conductivity within the allowable range of each property performance.

[0091] In a specific embodiment, the additive can be used alone or in combination of two or more. When used in combination, its combination and ratio are adjusted according to the effect as appropriate.

[0092] In an embodiment, the total content of the additive is 20 to 50% by weight based on the total weight of the electrolyte composition. In a preferred embodiment, the total content of the additive is 33% by weight based on the total weight of the electrolyte composition.

[0093] In the text, the "electrolyzable lithium salt" can be selected from one of the group consisting of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF6), lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiODFB), lithium bis(fluorosulfonyl)imide salt (LiFSI), lithium difluorophosphate (LiPO2F2), and lithium tetrafluoro(oxalato)phosphate (LiFOP); in an embodiment, the content of the electrolyzable lithium salt is 10 to 40% by weight based on the total weight of the electrolyte composition. In a preferred embodiment, the total content of the electrolyzable lithium salt is 25% by weight based on the total weight of the electrolyte composition.

[0094] In a preferred specific embodiment, the reactive oligomer is polyethylene glycol methyl ether methacrylate, the additive is polyethylene glycol dimethyl ether, and the electrolyzable lithium salt is lithium bis(trifluoromethylsulfonyl)imide.

[0095] Next, the preparation method of the electrolyte composition of the present invention is further described, including: providing a reactive oligomer having an alkenyl or mercapto group; and in the presence of an additive and an electrolyzable lithium salt, subjecting the reactive oligomer, an initiator, and the crosslinking agent having the structure of formula (I) to a free radical polymerization reaction to obtain the electrolyte composition.

[0096] The "reactive oligomer" has functional groups of alkenyl or mercapto groups, so it can undergo a copolymerization reaction with the crosslinking agent having the structure of formula (I) of the present invention in the presence of an initiator.

[0097] When preparing the electrolyte composition, the molecular weight of the reactive oligomer is selected considering the mutual solubility between the reactive oligomer and the crosslinking agent. The condition for suitability is that they have good mutual solubility. For example, when poly(ethylene glycol) methyl ether methacrylate (PEGME) is used as the reactive oligomer, a molecular weight less than 1000 g / mol is selected to avoid chain segment crystallization caused by too high a molecular weight, which affects its ion transport property. In one embodiment, the molecular weight of the poly(ethylene glycol) methyl ether methacrylate (PEGME) is 500 g / mol.

[0098] In a specific embodiment, the weight ratio of the crosslinking agent to the reactive oligomer is 5:95 to 25:75, that is, the crosslinking density of the electrolyte in the prepared electrolyte composition is 5 to 25%, and among them, a crosslinking density of 20% is preferred.

[0099] The "initiator" can be a thermal initiator or a photoinitiator. Among them, the thermal initiator can be selected from one of the group consisting of azobisisobutyronitrile (AIBN) and azobis(2-methylpropionamidine) dihydrochloride (AIBA); moreover, the photoinitiator can be selected from one of the group consisting of 2,2-dimethoxy-2-phenylacetophenone (DMPA), 2-hydroxy-2-methylpropiophenone, and 1-hydroxycyclohexyl phenyl ketone.

[0100] When the radical polymerization reaction is initiated by a thermal initiator, the temperature of the radical polymerization reaction is 55 to 80 °C, and its reaction time is 6 to 24 hours. In a specific embodiment, the thermal initiator is azobisisobutyronitrile (AIBN), the temperature of the radical polymerization reaction is 55 °C, and its reaction time is 6 hours.

[0101] When the radical polymerization reaction is initiated by a photoinitiator, the light source wavelength range of the radical polymerization reaction is 350 to 400 nm, and its reaction time is 5 to 10 minutes. In a specific embodiment, the photoinitiator is 2,2-dimethoxy-2-phenylacetophenone (DMPA), the light source wavelength range of the radical polymerization reaction is 350 nm, and its reaction time is 5 minutes.

[0102] The electrolyte composition of the present invention forms a dense cross-linked structure through a cross-linking agent having the structure of formula (I). Therefore, its initial thermal cracking temperature is 100 to 282 °C, showing excellent thermal stability in a high-temperature environment. In addition, the thermal cracking temperature of this electrolyte composition varies depending on different additives. For example, when using succinonitrile (SN) as an additive, its initial thermal cracking temperature is about 100 °C; when using additives of the polyethylene glycol type, its initial thermal cracking temperature is about 200 °C. Especially when using dimethyl ether of polyethylene glycol (PEGDME), the thermal cracking temperature range is between 255 and 282 °C; when using ionic liquid as an additive, its initial thermal cracking temperature reaches above 300 °C.

[0103] In addition, this dense cross-linked structure also endows the electrolyte composition with good mechanical strength, that is, the stress of the electrolyte composition is 0.029 to 0.064 megapascals (MPa) when the strain is -40%, so as to maintain the smooth surface of the lithium metal electrode and avoid the problem of micro short circuit caused by puncture, significantly improving the electrochemical stability of the lithium-ion battery.

[0104] On the other hand, by introducing multiple ionic groups through the cross-linking agent of the present invention and making the ionic groups uniformly distributed in the structure of the prepared electrolyte composition, the electrolyte composition can exhibit a conductivity of 1.17x10 -4 to 1.52x10 -4 Siemens / cm (S / cm) under the condition of high cross-linking density, showing the effect that the cross-linking agent endows it with high ionic conductivity.

[0105] Furthermore, the present invention also provides a lithium-ion battery, including: a positive electrode; a negative electrode; and the above-mentioned electrolyte composition.

[0106] In the text, the "positive electrode" can be selected from one of the group consisting of lithium iron phosphate, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganite (LiMnO2), lithium manganese oxide (Li x Mn2O4, 0 ≤ X ≤ 3), and lithium nickel cobalt manganese oxide (NCM), but not limited thereto.

[0107] In the text, the "negative electrode" can be selected from one of the group consisting of lithium metal, graphite-based carbon materials, and silicon-based materials, but not limited thereto.

[0108] In terms of the application of the lithium-ion battery system, by adding a cross-linking agent having the structure of formula (I) to the electrolyte composition, it can be seen that the lithium-ion battery has a charge-discharge capacity greater than 160 mAh / g within the voltage range of 2.5 to 4.0 volts at a discharge rate of 1 to 2C at 60 °C, showing the effect that the cross-linking agent endows it with high charge-discharge capacity.

[0109] In a specific embodiment, the lithium-ion battery has a charge-discharge capacity of 158 mAh / g at a discharge rate of 0.3C within a voltage range of 2.5 to 4.0 volts at 25°C.

[0110] In another specific embodiment, the lithium-ion battery has a charge-discharge capacity of 165 mAh / g at a discharge rate of 1C within a voltage range of 2.5 to 4.0 volts at 60°C.

[0111] In terms of capacitance, when the electrolyte composition has this cross-linked structure, under the conditions of 60°C and a discharge rate of 0.2 to 0.5C, after 100 charge-discharge cycles, its capacitance is more than 90% of the initial capacitance. Obviously, even at a high operating temperature, the electrolyte composition of the present invention can inhibit the formation of lithium dendrites and stably perform high charge-discharge rate electrical cycles for a long time, showing excellent electrochemical stability and durability.

[0112] In a specific embodiment, the lithium-ion battery has a capacitance of 94% of the initial capacitance after 150 charge-discharge cycles under the conditions of 25°C and a discharge rate of 0.2C.

[0113] In another specific embodiment, the lithium-ion battery has a capacitance of 92% of the initial capacitance after 180 charge-discharge cycles under the conditions of 60°C and a discharge rate of 0.2C.

[0114] In yet another specific embodiment, the lithium-ion battery has a capacitance of 95% of the initial capacitance after 100 charge-discharge cycles under the conditions of 60°C and a discharge rate of 0.5C.

[0115] The following further elaborates on the present invention through specific examples, but the scope of the present invention is not limited by the examples.

[0116] Preparation Example 1: Preparation of Crosslinking Agent (I-1)

[0117] Place N-vinylimidazole (10 g, 2.2 equivalents), 1,4-diiodobutane (15 g, 1 equivalent), and ethyl acetate (75 mL) in a reaction kettle, and reflux at a temperature of 77°C and normal pressure for a pre-reaction; after the reaction is completed, extract with ethyl acetate and water, and collect the aqueous layer part of the extract, which contains a pre-reaction product.

[0118] Next, dropwise add an aqueous solution containing lithium bis(trifluoromethanesulfonyl)imide (33.5 g, 1.1 equivalents) to the extract of the aqueous layer part containing the pre-reaction product, and place it in a reaction kettle, and stir at room temperature and normal pressure for 3 hours for a displacement reaction; after the reaction is completed, a white solid powder can be obtained.

[0119] The above-mentioned white solid powder was subjected to the following purification procedure: The white solid powder was dissolved in ethyl acetate (150 mL), and the residual lithium iodide was washed away with brine. Then, activated carbon and anhydrous sodium sulfate were added and stirred for 30 minutes to remove water and impurities. After filtration and vacuum drying, a crosslinking agent in the form of a white powder having the structure of formula (I-1) was obtained.

[0120]

[0121] Preparation Example 2: Preparation of Crosslinking Agent (I-2)

[0122] The preparation method was the same as that of Preparation Example 1, but 1,4-diiodobutane was changed to 1,8-diiodooctane (17.7 g, 1 equivalent). After reaction and purification, a crosslinking agent having the structure of formula (I-2) was obtained.

[0123]

[0124] Preparation Example 3: Preparation of Crosslinking Agent (I-3)

[0125] The preparation method was the same as that of Preparation Example 1, but 1,4-diiodobutane was changed to 1,12-dibromododecane (15.9 g, 1 equivalent). After reaction and purification, a crosslinking agent having the structure of formula (I-3) was obtained.

[0126] Comparative Example: Preparation of Electrolyte Composition

[0127] Using 2 g of polyethylene glycol methyl ether methacrylate (molecular weight 500 g / mol) as the reactive oligomer, 0.02 g of azobisisobutyronitrile (AIBN) as the initiator, 1.6 g of polyethylene glycol dimethyl ether (PEGDME) as the additive, and 1.2 g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the lithium salt, the above components were mixed to form a clear and transparent solution. And at a temperature of 55 °C and normal pressure, the precursor solution was quantitatively taken with a dropper onto the surface of lithium metal, and a free radical polymerization reaction was carried out for 6 hours to form an electrolyte film with a thickness of 200 to 300 microns. After the reaction was completed, an electrolyte composition was obtained.

[0128] Example 1: Preparation of Electrolyte Composition

[0129] Using 1.8 g of polyethylene glycol methyl ether methacrylate (molecular weight 500 g / mol) as the reactive oligomer, 0.02 g of azobisisobutyronitrile (AIBN) as the initiator, 0.2 g of the crosslinking agent of Preparation Example 1 above, 1.6 g of polyethylene glycol dimethyl ether (PEGDME) as the additive, and 1.2 g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the lithium salt, the above components were mixed to form a clear and transparent precursor solution; and at a temperature of 55 °C and normal pressure, the precursor solution was quantitatively taken with a dropper onto the surface of lithium metal, and a free radical polymerization reaction was carried out for 6 hours to form an electrolyte film with a thickness of 200 to 300 microns. Among them, the weight ratio of the crosslinked polymer, additive, and lithium salt was set to 5:4:3, and the initiator was set to 1 wt% of the total weight of the polymer; after the reaction was completed, an electrolyte composition was obtained.

[0130] Examples 2 to 6: Preparation of electrolyte composition

[0131] The preparation method was the same as that of Example 1, but as shown in Table 1, the types and amounts of the reactive oligomer and the crosslinking agent of the above Preparation Example were changed. After the free radical polymerization reaction, an electrolyte composition was obtained.

[0132] Table 1

[0133]

[0134] The electrolyte composition products of the above Comparative Examples and Examples were analyzed according to the following test methods and recorded in Figures 1 to 4B :

[0135] (1) Thermal cracking experiment evaluation

[0136] Measurement was carried out using a thermogravimetric analyzer (TGA, Perkin Elmer, TGA 4000). Under normal pressure and in a nitrogen atmosphere, with a programmed heating rate of 10 °C / min, the change in weight loss of the prepared electrolyte composition with temperature was measured. The temperature at which the weight decreased to 5% of the initial weight was the thermal cracking temperature.

[0137] (2) Dynamic mechanical experiment evaluation

[0138] The prepared electrolyte composition was made into a cylindrical sample with a diameter of 10 mm and a height of 5 mm, and measurement was carried out using dynamic mechanical analysis (DMA, TA Instruments, ARES G2) at a compression rate of 0.01 mm / s and in the range of strain from 0 to -40% to obtain a stress-strain curve.

[0139] (3) Ion conductivity experiment evaluation

[0140] Using an electrochemical impedance analyzer (CH Instruments, 6116E), under the set condition of an amplitude of 10 M, and with a frequency scanning range between 10 and 100,000 Hz (Hz), measure with a two-electrode stainless steel electrode, and take the lowest point of the impedance spectrum as the resistance value (R b ) of the prepared electrolyte composition, and substitute it into the following formula to estimate its ionic conductivity (conductivity, σ):

[0141]

[0142] where L is the film thickness (cm) of the electrolyte composition film, and A is the area (cm 2 ) of the stainless steel electrode.

[0143] Regarding Figure 3 the variation graph of ionic conductivity (σ) with temperature (T), record the ionic conductivity at each temperature within the range of 25 to 80 °C.

[0144] (4) Linear sweep voltammetry test

[0145] Using an electrochemical impedance analyzer (CH Instruments, 6116E), with stainless steel as the working electrode, lithium metal as the auxiliary electrode, and lithium metal as the reference electrode, under the set condition of a potential sweep rate of 5 mV / S, and with a voltage scanning range between 0 and 6 volts, cyclically test the electrochemical stability of the prepared electrolyte composition with a two-electrode stainless steel electrode.

[0146] From Figure 1 the results of the TGA weight change curve, it can be seen that compared with the comparative example without a cross-linking agent, for the electrolyte compositions of Examples 1 to 4 of the present invention, as the addition amount of the cross-linking agent increases, their cracking temperatures range from 255 °C to 282 °C, which is higher than 227 °C of the comparative example, obviously indicating that the introduction of the cross-linking agent greatly improves the thermal stability of the electrolyte composition.

[0147] From Figure 2A and 2B the results of the stress-strain curves of the DMA compression test, it can be seen that compared with the comparative example without a cross-linking agent, for the electrolyte compositions of Examples 1 to 6 of the present invention, due to the introduction of the cross-linking agent, cross-linked points are generated in the polymer structure, greatly increasing their molecular weights, and thus significantly increasing their mechanical strength; especially for Examples 5 to 6, because the carbon chains of the R groups of the cross-linking agent are longer, their mechanical strength is also significantly improved.

[0148] Furthermore, from Figure 3As a result of the change in ionic conductivity, compared with the comparative examples without a crosslinking agent, the ionic conductivity of the electrolyte compositions of Examples 1 to 3 of the present invention shows a slight increase with the increase in the addition amount of the crosslinking agent; compared with the electrolytes of the prior art and the electrolyte composition of Example 4, due to the too high crosslinking density and the too tight molecular structure, the ionic conductivity is limited. Therefore, the electrolyte composition of Example 3 of the present invention can exhibit the best ionic conductivity when having both an extremely high charge density and an ideal crosslinking density.

[0149] Moreover, from Figure 4A and 4B the results of the linear sweep voltammogram, compared with the comparative examples without a crosslinking agent, the initial oxidation voltage of the electrolyte compositions of Examples 1 to 6 of the present invention increases from 4.7 V to 5.3 V with the increase in the addition amount of the crosslinking agent, indicating that the introduction of the crosslinking agent significantly improves the electrochemical stability of the electrolyte composition.

[0150] Test Example: Preparation of Lithium-Ion Battery

[0151] Preparation of the positive electrode material: First, weigh the amounts of lithium iron phosphate (LiFePO4), sulfur / carbon black (Super P), and polyvinylidene fluoride (PVDF) (weight ratio is 80:10:10). Place 0.1 g of PVDF and 2.5 g of N-methylpyrrolidone (NMP) solvent into a degassing machine and stir for 15 minutes first, then add 0.1 g of Super P and stir for 15 minutes. Finally, add the remaining 0.8 g of LiFePO4 and continue stirring for 30 minutes (total stirring time is 60 minutes) to obtain a paste-like slurry.

[0152] Using an automatic coater, under the conditions of a blade thickness of 150 μm and a coating speed of 300 mm / s, uniformly coat the slurry on the aluminum foil to obtain a pole piece, and place this pole piece in a vacuum oven at 100 °C for 24 hours to remove the NMP solvent. After drying is completed, place the pole piece in a rolling press and compact it (the rolling thickness is set to 0.04 mm) to make the thickness of the pole piece uniform and denser. Finally, cut the pole piece into a circle with a cutter with a diameter of 13 mm, which is the positive circular pole piece.

[0153] Assembly of the lithium-ion battery: According to Figure 7The structure of the lithium-ion battery 1 shown in the figure was assembled. The positive circular electrode sheet 12 prepared above was placed in the center of a lower cover 10. Then, using lithium metal (UBIQ) as the negative electrode sheet 14, the electrolyte composition of Example 3 prepared above was applied to the surface of the negative electrode to form an electrolyte film 13 with a thickness of about 200 to 300 micrometers, such that the electrolyte film 13 was interposed between the positive circular electrode sheet 12 and the negative electrode sheet 14. A stainless steel sheet 15, a spring sheet 16, and an upper cover 11 were sequentially arranged above the negative electrode sheet 14, and the assembly was pressed three times under a pressure of 1000 psi using a sealing machine to obtain a button-type lithium-ion battery.

[0154] The manufactured product of the lithium-ion battery assembled above was analyzed for charge-discharge capacity and long-term charge-discharge cycle stability according to the following method:

[0155] Using a charge-discharge tester (Jiayou Technology, BAT-750B), under the set condition of 25 °C, at charge-discharge rates of 0.1C, 0.2C, 0.3C, 0.5C, and 1C, based on the mass of the positive electrode material, the battery capacity and charge-discharge current of the lithium-ion battery in the voltage range of 2.5 to 4.0V were measured to obtain charge-discharge curves, which were recorded in Figure 5A .

[0156] Moreover, under the set condition of 60 °C, at charge-discharge rates of 0.1C, 0.2C, 0.3C, 0.5C, 1C, 2C, and 3C, based on the mass of the positive electrode material, the battery capacity and charge-discharge current of the lithium-ion battery in the voltage range of 2.5 to 4.0V were measured to obtain charge-discharge curves, which were recorded in Figure 5B .

[0157] Then, under the set conditions of 25 °C and 60 °C respectively, the charge-discharge was repeated multiple times at a charge-discharge rate of 0.2C, the capacitance of the lithium-ion battery was measured for each cycle, and the results were recorded in Figure 6A and 6B .

[0158] Moreover, under the set condition of 60 °C, the charge-discharge was repeated multiple times at a charge-discharge rate of 0.5C, the capacitance of the lithium-ion battery was measured for each cycle, and the results were recorded in Figure 6C .

[0159] Comparative test example: Preparation of lithium-ion battery

[0160] The preparation method was the same as that of Test Example 1, but the electrolyte composition of Example 3 was changed to the electrolyte composition of the comparative example, and under the set conditions of 25 and 60 °C, the measurement was carried out according to the above test method for charge-discharge capacity, and the results were recorded in Figure 5C and 5D .

[0161] Next, under the set condition of 60 °C, measure according to the above test method for long-term charge-discharge cycle stability at a charge-discharge rate of 0.2C, and record the results in Figure 6D .

[0162] From Figure 5A and 5C of the charge-discharge curve results, it can be seen that compared with the lithium-ion battery of the comparative test example whose charge-discharge capacity has declined to 124 mAh / g at a charge-discharge rate of 0.5C, the lithium-ion battery of the test example of the present invention still has a charge-discharge capacity higher than 140 mAh / g at a charge-discharge rate of 0.5C. Since the electrolyte in the lithium-ion battery of the test example of the present invention has the dense cross-linked structure provided by the cross-linking agent of Example 3, it can still have a charge-discharge capacity of nearly 100 mAh / g at a charge-discharge rate of 1C. Obviously, the prepared lithium-ion battery can effectively enhance the capacity performance under high-speed charge and discharge due to the use of the cross-linking agent of the present invention in the electrolyte.

[0163] From Figure 5B and 5D of the charge-discharge curve results, it can be seen that compared with the lithium-ion battery of the comparative test example whose charge-discharge capacity has declined to 145 mAh / g at a charge-discharge rate of 2C, the lithium-ion battery of the test example of the present invention still has a charge-discharge capacity higher than 160 mAh / g at a charge-discharge rate of 2C. Obviously, the prepared lithium-ion battery can also effectively enhance the capacity performance under high-temperature and high-speed charge and discharge due to the use of the cross-linking agent of the present invention in the electrolyte.

[0164] From Figure 6B and 6D of the long-term charge-discharge cycle stability (60 °C, charge-discharge rate of 0.2C) results, it can be seen that after the lithium-ion battery of the comparative test example is repeatedly charged and discharged 100 times, it can only maintain 82.4% of the initial capacity; compared with the lithium-ion battery of the comparative test example, since the electrolyte in the lithium-ion battery of the test example of the present invention has the dense cross-linked structure provided by the cross-linking agent of Example 3, it can be repeatedly charged and discharged up to 180 times and still maintain 92% of the initial capacity.

[0165] In addition, from Figure 6A and 6C it can be known that the lithium-ion battery of the test example of the present invention, under the conditions of 25 °C and a discharge rate of 0.2C, after 150 charge-discharge cycles, its capacity is 94% of the initial capacity. And under the conditions of 60 °C and a discharge rate of 0.5C, after 100 charge-discharge cycles, its capacity is 95% of the initial capacity. Obviously, the prepared lithium-ion battery can effectively inhibit the generation of lithium dendrites and improve the physical properties of the electrolyte due to the use of the cross-linking agent of the present invention in the electrolyte, thereby strengthening the cycle stability of the lithium-ion battery and extending its service life.

[0166] In summary, by adding the cross-linking agent with the structure of formula (I), the ionic groups of the cross-linking agent are uniformly distributed in the structure of the prepared electrolyte composition. In addition to improving the ionic conductivity of the electrolyte composition, it also serves as a local reservoir for anions. In an environment with a high current density, it is used to alleviate the charge imbalance near the lithium metal electrode and delay the formation of lithium dendrites.

[0167] On the other hand, the dense cross-linked structure provided by the cross-linking agent with the structure of formula (I) improves the mechanical strength and heat resistance of the prepared electrolyte composition, maintains the smooth surface of the lithium metal electrode, and avoids the problem of micro-short circuit caused by puncture, significantly improving the electrochemical stability and long-term charge-discharge cycle stability of the lithium-ion battery, and extending its service life, having extremely high industrial application value and market prospects.

[0168] The above embodiments are only illustrative and not intended to limit the present invention. Any person skilled in the art can modify and change the above embodiments without departing from the spirit and scope of the present invention. Therefore, the scope of the protection of the present invention is defined by the appended claims of the present invention, and should be covered by the disclosed technical content as long as it does not affect the effects and implementation purposes of the present invention.

Claims

1. A crosslinking agent having the structure of formula (I) for an electrolyte: Wherein, M is selected from a monovalent imidazole ion, triazole ion, pyridine ion, substituted or unsubstituted phosphonium ion or substituted or unsubstituted ammonium ion; R is ethoxy, polyethoxy, phenylene or polyphenylene; and X is a monovalent halogen atom-containing anion, carboxylate-containing anion, thiocyanate ion.

2. The crosslinking agent according to claim 1, characterized in that, The M is selected from one of the following groups: Wherein, * represents the position where the M is structurally connected to the formula (I).

3. The crosslinking agent according to claim 1, characterized in that, The M is a monovalent imidazole ion and X is a monovalent halogen atom-containing anion.

4. The crosslinking agent according to claim 1, characterized in that, The halogen atom-containing anion is selected from chloride ion, bromide ion, tetrafluoroborate ion, hexafluorophosphate ion, bis(trifluoromethylsulfonyl)imide anion and trifluoromethanesulfonate ion.

5. An electrolyte composition comprising a polymer crosslinked with the crosslinking agent according to claim 1, wherein, The polymer is obtained by reacting a reactive monomer having an alkenyl or mercapto group with an initiator, and based on the total weight of the electrolyte composition, the content of the crosslinking agent having the structure of formula (I) in the crosslinked polymer is 1 to 25% by weight.

6. The electrolyte composition according to claim 5, further comprising an additive and an electrolyzable lithium salt.

7. The electrolyte composition according to claim 5, characterized in that, The thermal cracking temperature of the electrolyte composition is 100 to 282 °C.

8. The electrolyte composition according to claim 5, characterized in that, The stress of the electrolyte composition at a strain of -40% is 0.029 to 0.064 MPa.

9. The electrolyte composition according to claim 5, characterized in that, The conductivity of the electrolyte composition is 1.17x10 -4 to 1.52x10 -4 Siemens / cm.

10. A method for preparing an electrolyte composition, comprising: Providing a reactive oligomer having an alkenyl or mercapto group; and In the presence of an additive and an electrolyzable lithium salt, subjecting the reactive oligomer, the crosslinking agent according to claim 1 and an initiator to a radical polymerization reaction to obtain the electrolyte composition.

11. The method according to claim 10, characterized in that, The weight ratio of the crosslinking agent to the reactive oligomer is 5:95 to 25:

75.

12. The method according to claim 10, characterized in that, The initiator is a thermal initiator and is selected from one of the group consisting of azobisisobutyronitrile and azobis(isobutyramidine) hydrochloride.

13. The method according to claim 12, characterized in that, The temperature of the radical polymerization reaction is 55 to 80 °C and the reaction time is 6 to 24 hours.

14. The method according to claim 10, characterized in that, The initiator is a photoinitiator and is selected from one of the group consisting of 2,2-dimethoxy-2-phenylacetophenone, 2-hydroxy-2-methylpropiophenone and 1-hydroxycyclohexyl phenyl ketone.

15. The method according to claim 14, characterized in that, The light source wavelength range of the radical polymerization reaction is 350 to 400 nm and the reaction time is 5 to 10 minutes.

16. The method according to claim 10, characterized in that, The electrolyzable lithium salt is selected from at least one of the group consisting of lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide salt, lithium difluorophosphate and lithium tetrafluoro(oxalato)phosphate.

17. The method according to claim 10, wherein, The additive is selected from at least one of the group consisting of polyethylene glycol dimethyl ether, succinonitrile and ionic liquid.

18. A lithium-ion battery, comprising: A positive electrode; A negative electrode; and The electrolyte composition according to claim 5.

19. The lithium-ion battery according to claim 18, wherein, The positive electrode is lithium iron phosphate.

20. The lithium-ion battery according to claim 18, wherein, The negative electrode is lithium metal.

21. The lithium-ion battery according to claim 18, wherein, The lithium ion battery has a charge-discharge capacity of greater than 160 mAh / g at a discharge rate of 1 to 2C within a voltage range of 2.5 to 4.0 volts at 60 °C.

22. The lithium-ion battery according to claim 18, wherein, The lithium ion battery has a capacitance of more than 90% of the initial capacitance after 100 charge-discharge cycles under the conditions of 60 °C and a discharge rate of 0.2 to 0.5C.

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