Electrolyte and secondary battery

Abstract

The invention provides an electrolyte and a secondary battery, and belongs to the technical field of secondary batteries, and the electrolyte comprises a lithium salt and a solvent. Wherein the lithium salt comprises lithium bis (fluorosulfonyl) imide; the solvent is prepared from carbonic ester and 2, 2-difluoroethyl acetate; the mass content of the lithium bis (fluorosulfonyl) imide in the lithium salt is 30%-80%, and the mass content of the 2, 2-difluoroethyl acetate in the electrolyte is 20%-70%. According to the electrolyte disclosed by the invention, on the premise that a flame-retardant additive is not added, a proper amount of lithium bis (fluorosulfonyl) imide and the 2, 2-difluoroethyl acetate solvent are introduced to achieve a synergistic effect, so that the thermal safety performance of the battery is remarkably improved while the electrical performance of the battery is considered.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and more particularly to an electrolyte and a secondary battery. Background Technology

[0002] Currently, electric vehicles are rapidly gaining market share and popularity due to their low energy consumption and zero carbon emissions. With technological advancements, issues such as short driving range and long charging times are gradually being resolved. However, safety remains a major factor hindering the widespread adoption of electric vehicles, making improving battery safety performance under various operating conditions a key focus of technological research within the industry.

[0003] In lithium-ion secondary batteries, the electrolyte plays a crucial role in battery safety. Conventionally used organic carbonate electrolytes suffer from poor thermal stability and flammability, which are significant causes of battery thermal runaway. To mitigate the poor thermal stability of carbonate electrolytes, the industry typically adds high levels of flame-retardant additives. However, existing flame-retardant additives are generally composed of phosphate esters and phosphazene compounds, which have poor compatibility with the battery's negative electrode. While adding high levels of flame-retardant additives can improve the electrolyte's thermal stability, it can also lead to deterioration in the battery's electrochemical performance, resulting in decreased cycle capacity and rapid increase in gas production.

[0004] Therefore, it is necessary to design an electrolyte and a secondary battery to improve the above problems. Summary of the Invention

[0005] This invention provides an electrolyte and a secondary battery to improve the technical defect of poor thermal safety of the electrolyte without negatively affecting its electrochemical performance.

[0006] In a first aspect, the present invention provides an electrolyte comprising a lithium salt and a solvent. The lithium salt comprises lithium bisfluorosulfonylimide; the solvent comprises a carbonate and 2,2-difluoroethyl acetate; the lithium bisfluorosulfonylimide comprises 30% to 80% by mass in the lithium salt, and the 2,2-difluoroethyl acetate comprises 20% to 70% by mass in the electrolyte.

[0007] In one example of the present invention, the 2,2-difluoroethyl acetate in the electrolyte has a mass content of 20% to 50%.

[0008] In one example of the present invention, the carbonate includes ethylene carbonate, and the ethylene carbonate in the electrolyte has a mass content of less than or equal to 12%.

[0009] In one example of the present invention, the lithium bis(fluorosulfonyl)imide in the lithium salt has a mass content of 50% to 80%.

[0010] In one example of the present invention, the lithium salt further includes lithium hexafluorophosphate, wherein the mass content of lithium hexafluorophosphate in the lithium salt is less than or equal to 70%.

[0011] In one example of the present invention, the lithium bisfluorosulfonylimide in the electrolyte has a mass content of 5% to 12%.

[0012] In one example of the present invention, the lithium hexafluorophosphate has a mass content of 3% to 10% in the electrolyte.

[0013] In one example of the present invention, the electrolyte further includes additives, the additives including 1,3-propenesulfonate lactone and triphenyl phosphate; wherein the mass content of 1,3-propenesulfonate lactone in the electrolyte is greater than or equal to 0.1%, and the mass content of triphenyl phosphate in the electrolyte is greater than or equal to 0.1%.

[0014] In a second aspect, the present invention also provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte as described in any of the above examples.

[0015] In one example of the present invention, the positive electrode includes a positive electrode active material, which includes LiNi. x Co y Mn 1-x-y O2, where 0.6≤x<1, 0<y<0.4; the upper limit of the operating voltage of the secondary battery is less than or equal to 4.4V.

[0016] In one example of the present invention, the negative electrode includes a negative electrode active material, which includes graphite and silicon negative electrode material, wherein the mass content of the silicon negative electrode material in the negative electrode active material is less than or equal to 5%.

[0017] The electrolyte provided by this invention introduces lithium bisfluorosulfonylimide into the lithium salt and replaces part of the carbonate solvent with a suitable mass content of 2,2-difluoroethyl acetate solvent. This electrolyte utilizes the good chemical stability of lithium bisfluorosulfonylimide and 2,2-difluoroethyl acetate solvent to delay the reaction time between the lithium salt and the solvent during heat accumulation inside the battery, thereby increasing the battery's thermal runaway trigger temperature and reducing heat generation during thermal runaway. Furthermore, the electrolyte utilizes the relatively vigorous reaction rate of lithium bisfluorosulfonylimide and the low boiling point of 2,2-difluoroethyl acetate solvent to ensure rapid reaction between the solvent and lithium salt during thermal runaway, preventing heat accumulation inside the battery. Simultaneously, the vigorous reaction of the lithium salt also promotes faster ejection of the electrolyte from the battery. The ejected 2,2-difluoroethyl acetate solvent can form fluorine vapor to isolate the flammable materials of the battery from contact with air, thereby suppressing combustion reactions within the battery and further limiting the heat spread during thermal runaway.

[0018] In summary, the electrolyte provided by this invention, without the addition of flame retardant additives, effectively increases the thermal runaway trigger temperature and reduces the maximum thermal runaway temperature by introducing an appropriate amount of lithium bis(fluorosulfonyl)imide and 2,2-difluoroethyl acetate solvent, thereby significantly improving the thermal safety performance of the battery while taking into account its electrical performance. Detailed Implementation

[0019] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. It should also be understood that the terminology used in the embodiments of the present invention is for describing specific implementation schemes and not for limiting the scope of protection of the present invention. Test methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the conditions recommended by the respective manufacturers.

[0020] For simplicity, this document only explicitly discloses some numerical ranges, and each point or individual value between the endpoints of the range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit, combined with any other point or individual value, or combined with other lower or upper limits to form a range not explicitly stated. It should be noted that, in this invention, the content of each component in the electrolyte represents the mass percentage of that component relative to the solvent or electrolyte.

[0021] In a first aspect, this application provides an electrolyte comprising a lithium salt and a solvent. The lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI), and the solvent comprises carbonate and 2,2-difluoroethyl acetate (DFEA).

[0022] Lithium difluorosulfonylimide exhibits relatively good thermal stability, showing better thermal stability compared to lithium hexafluorophosphate (LiPF6). 2,2-Difluoroethyl acetate, due to the strong electron-withdrawing ability of its internal fluorine atoms, makes the solvent molecules more resistant to oxidation, thus possessing high chemical inertness. Simultaneous introduction of appropriate amounts of lithium difluorosulfonylimide and 2,2-difluoroethyl acetate into the electrolyte can effectively delay the reaction between the lithium salt and the solvent before thermal runaway occurs, and reduce the heat generated during thermal runaway, thereby increasing the thermal runaway trigger temperature and decreasing the maximum thermal runaway temperature.

[0023] Due to the strong reactivity and rapid reaction rate of lithium bisfluorosulfonylimide, the electrolyte, through the synergistic effect of lithium bisfluorosulfonylimide and 2,2-difluoroethyl acetate solvent, enables the lithium salt and solvent to react rapidly at the onset of thermal runaway, thereby effectively releasing energy, preventing heat accumulation inside the battery, and suppressing the thermal spread at the onset of thermal runaway. Furthermore, under the intense reaction of the lithium salt, the electrolyte will eject from the battery at a faster rate, while the 2,2-difluoroethyl acetate solvent in the electrolyte, based on its low boiling point, can rapidly form fluorine vapor outside the battery at the relatively low temperature at the onset of thermal runaway. This isolates the flammable materials inside the battery from contact with air, thereby suppressing the combustion reaction inside the battery, limiting the thermal spread during thermal runaway, and further reducing the maximum temperature of thermal runaway.

[0024] This electrolyte utilizes the synergistic effect of lithium bis(fluorosulfonyl)imide and 2,2-difluoroethyl acetate solvent to improve the thermal safety performance of the battery without introducing a large number of flame retardant additives. Since the electrolyte does not require the addition of many phosphorus-based flame retardant additives, only the mass content of lithium bis(fluorosulfonyl)imide and 2,2-difluoroethyl acetate solvent needs to be adjusted to maintain good kinetic performance, achieving a balanced improvement in both thermal safety and electrical performance of the battery.

[0025] The mass content of 2,2-difluoroethyl acetate in the electrolyte can be any value within the range of 20% to 70%, for example, the mass content of 2,2-difluoroethyl acetate in the electrolyte can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%. Introducing an appropriate amount of 2,2-difluoroethyl acetate solvent into the electrolyte to replace part of the carbonate solvent can improve the thermal safety performance of the battery while maintaining good electrical performance. If the mass content of 2,2-difluoroethyl acetate solvent is too low, it will not effectively improve the thermal safety performance of the battery; if the mass content of 2,2-difluoroethyl acetate solvent is too high, it will seriously affect the electrical performance of the battery, causing battery capacity decay and increased gas production.

[0026] In some embodiments, the mass content of 2,2-difluoroethyl acetate in the electrolyte is any value in the range of 20% to 50%, for example, the mass content of 2,2-difluoroethyl acetate in the electrolyte can be 20%, 23%, 25%, 27%, 30%, 33%, 35%, 37%, 40%, 43%, 45%, 47%, or 50%.

[0027] The mass content of lithium bis(fluorosulfonyl)imide in the lithium salt can be any value within the range of 30% to 80%, for example, the mass content of lithium bis(fluorosulfonyl)imide in the lithium salt can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80%. By introducing an appropriate amount of lithium bis(fluorosulfonyl)imide into the lithium salt, the thermal safety performance of the battery can be improved by combining it with 2,2-difluoroethyl acetate solvent while ensuring the conductivity of the electrolyte. If the mass content of lithium bis(fluorosulfonyl)imide in the lithium salt is too low, it cannot effectively delay the reaction between the lithium salt and the solvent, and it is difficult to promote the rapid eruption of the electrolyte to form oxygen-isolated fluorine vapor in the early stage of thermal runaway, thus making it difficult to increase the thermal runaway initiation temperature and reduce the maximum thermal runaway temperature of the battery. If the mass content of lithium bis(fluorosulfonyl)imide in the lithium salt is too high, it will excessively increase the viscosity of the electrolyte, affecting the conductivity and electrochemical stability of the electrolyte, resulting in battery capacity decay and increased gas production.

[0028] In some embodiments, the mass content of lithium bisfluorosulfonamide in the lithium salt is any value in the range of 50% to 80%, for example, the mass content of lithium bisfluorosulfonamide in the lithium salt can be 50%, 53%, 55%, 57%, 60%, 63%, 65%, 67%, 70%, 73%, 75%, 77%, or 80%.

[0029] It should be noted that the type of carbonate solvent in the electrolyte is not limited, and any one or more common carbonate solvents can be used. For example, in some embodiments, the carbonate is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). That is, the carbonate can be any one of the types listed above, such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate; the carbonate can also be a combination of any two or more types listed above, such as a combination of ethylene carbonate and dimethyl carbonate, or a combination of propylene carbonate and methyl ethyl carbonate, or a combination of ethylene carbonate, propylene carbonate, and methyl ethyl carbonate. It should be noted that when the carbonate is a composition, the proportions of the components within the composition are not limited, and they can be mixed in any proportion.

[0030] In some embodiments, the carbonate in the electrolyte includes at least ethylene carbonate, and the mass content of ethylene carbonate in the electrolyte is less than or equal to 12%. In one example, the mass content of ethylene carbonate in the electrolyte can be any value ranging from 5% to 12%, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, or 12%. Introducing an appropriate amount of ethylene carbonate solvent into the electrolyte can effectively improve the electrochemical performance of the battery without excessively affecting its thermal safety performance, and further suppress the growth of gas production in the battery.

[0031] It should be noted that lithium salts, in addition to lithium bisfluorosulfonylimide, also include other conventional lithium salt materials in the art. For example, lithium salts may also include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(oxalate borate) (LiBOB), lithium bis(oxalate borate) (LiODFB), and lithium bis(trifluoromethanesulfonylimide) (LiTFSI).

[0032] In some embodiments, the lithium salt further comprises at least lithium hexafluorophosphate, wherein the mass content of lithium hexafluorophosphate in the lithium salt is less than or equal to 70%. For example, in one example, the mass content of lithium hexafluorophosphate in the lithium salt is any value in the range of 20% to 70%, such as 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%. By introducing an appropriate amount of lithium hexafluorophosphate into the electrolyte, the conductivity and reaction kinetics of the electrolyte can be further improved, thereby enhancing the electrochemical stability of the battery, increasing the high-temperature storage life of the battery, and suppressing the growth of gas production in the battery.

[0033] Optionally, in some embodiments, the mass content of lithium bis(fluorosulfonyl)imide in the electrolyte is any value in the range of 5% to 12%, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, or 12%; and the mass content of lithium hexafluorophosphate in the electrolyte is any value in the range of 3% to 10%, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%.

[0034] In some embodiments, the electrolyte further includes additives, including 1,3-propenesulfonate lactone (PST) and triphenyl phosphate (TPP). These additives help form a stable SEI interfacial film at the negative electrode interface, further improving the film-forming effect of the electrolyte at the negative electrode interface, enhancing the protection of the negative electrode interface, improving the charge-discharge cycle life of the battery, and reducing the growth of gas production in the battery.

[0035] In some embodiments, the mass content of 1,3-propenylsulfonate lactone in the electrolyte is greater than or equal to 0.1%. For example, in one instance, the mass content of 1,3-propenylsulfonate lactone in the electrolyte is any value within the range of 0.1% to 1%, such as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%. The mass content of triphenyl phosphate in the electrolyte is greater than or equal to 0.1%. For example, in one instance, the mass content of triphenyl phosphate in the electrolyte is any value within the range of 0.1% to 1%, such as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%. When the mass content of 1,3-propenesulfonate lactone and triphenyl phosphate introduced into the electrolyte is greater than 0.1%, the cycle life of the battery can be effectively improved and the gas generation problem of the battery can be alleviated.

[0036] The electrolyte of this invention can be prepared using conventional methods. For example, the electrolyte can be prepared in a glove box with an argon or nitrogen content of 99.999%, an actual oxygen content of 0.1 ppm, and a moisture content of less than 10 ppm. In the glove box, the solvent is mixed and stirred. Then, under stirring conditions, the lithium salt is added to the mixed solvent, and stirring continues until the lithium salt is completely dissolved. Finally, additives can be added and stirred until homogeneous.

[0037] The present invention also provides a secondary battery, comprising a positive electrode, a negative electrode, a separator, and the electrolyte described in any of the above embodiments. During the charging and discharging process of the battery, lithium ions repeatedly insert and extract between the positive and negative electrodes. The separator is disposed between the positive and negative electrodes to provide isolation; the electrolyte conducts lithium ions between the positive and negative electrodes.

[0038] The composition and preparation method of secondary batteries are described in detail below:

[0039] The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive current collector is, for example, a foil formed by surface treatment of materials such as nickel, titanium, aluminum, silver, stainless steel, or carbon. Besides foil, the positive current collector can also be used in any combination of one or more forms such as film, mesh, porous, foam, or nonwoven fabric. The thickness of the positive current collector is, for example, 8 μm-15 μm. In one embodiment, the positive current collector is, for example, an aluminum foil, and the thickness of the aluminum foil is, for example, 13 μm. The positive current collector has two surfaces opposite each other in its thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector. The positive active material layer includes a positive active material, a positive conductive agent, and a positive binder. No specific limitations are placed on the positive active material, positive conductive agent, and positive binder here; those skilled in the art can select them according to actual needs.

[0040] As an example, the positive electrode active material can be selected from ternary materials, lithium phosphates, and spinel materials. Ternary materials include lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide doped with metal ions, and lithium nickel cobalt aluminum oxide doped with metal ions, etc.; lithium phosphates include lithium manganese iron phosphate, lithium iron phosphate, and lithium manganese phosphate, etc. The positive electrode binder is selected, for example, from polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). The positive electrode conductive agent is selected, for example, from one of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc., or a combination of two or more in any proportion.

[0041] The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be made of a material with good conductivity and mechanical strength, such as copper foil. The negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a thickener. No specific limitations are placed on the specific types of negative electrode active material, negative electrode conductive agent, and negative electrode binder; materials known in the art for use in lithium-ion secondary batteries can be used, and those skilled in the art can select them according to actual needs.

[0042] As an example, the negative electrode active material is selected from one or more combinations of carbon materials and silicon materials. The carbon materials include, for example, hard carbon, artificial graphite, natural graphite, etc. The silicon materials include, for example, elemental silicon, silicon oxide materials, silicon carbon materials, etc. The negative electrode conductive agent is selected from one of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc. or a composition of two or more in any proportion. The negative electrode binder is selected from any one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), or several compositions mixed in any proportion; the thickener is selected from carboxymethyl cellulose CMC-Na or CMC-Li.

[0043] The separator is selected from conventional types in the art. For example, a PE or PP porous membrane can be selected as the separator. The thickness of the separator is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL; the porosity is 30% to 50%.

[0044] The battery assembly is carried out according to the conventional method. Taking a soft-pack battery as an example, the prepared negative electrode sheet, separator, and positive electrode sheet are stacked in sequence, put into an aluminum-plastic film, and baked at 80 °C to remove water to obtain a dry battery cell. Finally, the prepared electrolyte is injected into the dry battery cell and sealed to obtain a finished lithium-ion secondary battery.

[0045] In some embodiments, the upper limit of the working voltage of the secondary battery is less than or equal to 4.4 V. The positive electrode active material in the positive electrode of the secondary battery is selected from nickel-cobalt-manganese ternary material LiNi x Co [[ID=1​​​​​​​​​​​​​​​​​​​​​​​​

[0048] Example 1

[0049] This embodiment provides a secondary battery, the composition and preparation method of which are described below.

[0050] Electrolyte preparation: In an argon atmosphere glove box with a water content of <10ppm, battery-grade ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and 2,2-difluoroethyl acetate (DFEA) were mixed in appropriate proportions to form a solvent; then 1,3-propenesulfonyl lactone (PST) and triphenyl phosphate (TPP) were added to the solvent as additives; finally, lithium hexafluorophosphate (LiPF6) and lithium difluoromethanesulfonylimide (LiFSI) were added to the solvent and mixed evenly to obtain the electrolyte. The main component contents of the prepared electrolyte are shown in Table 1. Based on the total mass of the electrolyte as 100%, the mass content of EC is 10%, the mass content of EMC is 40%, the mass content of DFEA is 30%, the mass content of LiPF6 is 10%, the mass content of LiFSI is 5%, the mass content of PST is 0.2%, and the mass content of TPP is 0.1%. The PC solvent not shown in the table is always used to make up the total mass of the electrolyte so that the total amount of electrolyte in each example and comparative example is the same.

[0051] Preparation of the positive electrode: The positive electrode active material LiNi... 0.9 Co 0.05 Mn 0.05 O2 (abbreviated as NCM in Table 1 below), positive electrode conductive agent Super P, and positive electrode binder polyvinylidene fluoride are mixed in a mass ratio of 98:1:1. N-methylpyrrolidone solvent is added, and the mixture is stirred under vacuum until it becomes homogeneous and transparent, obtaining the positive electrode slurry. The positive electrode slurry is uniformly coated onto aluminum foil, which is then air-dried at room temperature and transferred to an oven for further drying. After cold pressing and slitting, the positive electrode is obtained.

[0052] Preparation of the negative electrode: The negative electrode active material artificial graphite, the negative electrode conductive agent Super P, the negative electrode thickener sodium carboxymethyl cellulose (CMC-Na), and the negative electrode binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96:1:1:2 and stirred thoroughly in a vacuum mixer until uniform to obtain a negative electrode slurry; the negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, dried at room temperature, and then transferred to an oven for drying. The negative electrode is obtained through cold pressing, slitting and other processes.

[0053] Selection of diaphragm: A 12μm thick polypropylene membrane was used as the diaphragm.

[0054] Battery fabrication: The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. This stacking process yields a bare battery cell. The cell is then wrapped in an aluminum-plastic film, transferred to a vacuum oven, dried at 120°C, and then filled with 3.0 g / Ah of electrolyte before sealing. Electrolytic formation is then carried out to prepare a 1 Ah lithium-ion secondary battery.

[0055] Example 2

[0056] This embodiment provides a secondary battery with the same system as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass content of DEFA in the electrolyte is adjusted to 20%, and the mass content of EMC in the electrolyte is adjusted to 50%.

[0057] Example 3

[0058] This embodiment provides a secondary battery with the same system as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass content of DEFA in the electrolyte is adjusted to 70%, and the mass content of EMC in the electrolyte is adjusted to 0%.

[0059] Example 4

[0060] This embodiment provides a secondary battery with the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass content of LiPF6 in the electrolyte is adjusted to 8%, the mass content of LiFSI in the electrolyte is adjusted to 8%, and the mass content of PC in the electrolyte is adaptively adjusted to maintain the total electrolyte volume the same as in Embodiment 1.

[0061] Example 5

[0062] This embodiment provides a secondary battery with the same system as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass content of LiPF6 in the electrolyte is adjusted to 3%, and the mass content of LiFSI in the electrolyte is adjusted to 12%.

[0063] Example 6

[0064] This embodiment provides a secondary battery with the same system as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass content of EC in the electrolyte is adjusted to 0%, and the mass content of EMC in the electrolyte is adjusted to 50%.

[0065] Example 7

[0066] This embodiment provides a secondary battery with the same system as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass content of EC in the electrolyte is adjusted to 12%, and the mass content of EMC in the electrolyte is adjusted to 38%.

[0067] Example 8

[0068] This embodiment provides a secondary battery with the same system as in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass content of PST in the electrolyte is adjusted to 0.1%, the mass content of TPP in the electrolyte is adjusted to 0.05%, and the mass content of PC in the electrolyte is adaptively adjusted to maintain the total electrolyte volume the same as in Embodiment 1.

[0069] Example 9

[0070] This embodiment provides a secondary battery with the same system as that in Embodiment 1. The difference between this embodiment and Embodiment 1 is that the positive electrode active material used in the positive electrode is lithium iron phosphate (LiFePO4) (abbreviated as LFP in Table 1 below).

[0071] Comparative Example 1

[0072] This comparative example provides a secondary battery with the same system as Example 1. The difference between this comparative example and Example 1 is that the mass content of DEFA in the electrolyte is adjusted to 15%, and the mass content of EMC in the electrolyte is adjusted to 55%.

[0073] Comparative Example 2

[0074] This comparative example provides a secondary battery with the same system as Example 1. The difference between this comparative example and Example 1 is that the mass content of DEFA in the electrolyte is adjusted to 80%, the mass content of EMC in the electrolyte is adjusted to 0%, and the mass content of EC in the electrolyte is adjusted to 0%.

[0075] Comparative Example 3

[0076] This comparative example provides a secondary battery with the same system as Example 1. The difference between this comparative example and Example 1 is that the mass content of LiPF6 in the electrolyte is adjusted to 12%, and the mass content of LiFSI in the electrolyte is adjusted to 3%.

[0077] Comparative Example 4

[0078] This comparative example provides a secondary battery with the same system as Example 1. The difference between this comparative example and Example 1 is that the mass content of LiPF6 in the electrolyte is adjusted to 2%, and the mass content of LiFSI in the electrolyte is adjusted to 13%.

[0079] Comparative Example 5

[0080] This comparative example provides a secondary battery with the same system as Example 1. The difference between this comparative example and Example 1 is that the mass content of EC in the electrolyte is adjusted to 15%, and the mass content of EMC in the electrolyte is adjusted to 35%.

[0081] Comparative Example 6

[0082] This comparative example provides a secondary battery with the same system as Example 9. The difference between this comparative example and Example 9 is that the mass content of LiPF6 in the electrolyte is adjusted to 15%, and the mass content of LiFSI in the electrolyte is adjusted to 0%.

[0083] Performance tests were conducted on the secondary batteries provided in Examples 1 to 9 and Comparative Examples 1 to 6. The battery parameters for Examples 1 to 9 and Comparative Examples 1 to 6 are shown in Table 1, and the test results are shown in Table 2. The test methods are as follows:

[0084] (1) Battery thermal safety test: The battery was fully charged using a constant current and constant voltage method at a current rate of 0.33C. The battery was then placed in the test chamber of the ARC equipment, and the thermocouple was positioned at the center of the large surface of the battery. The battery was heated using the ARC test chamber in 5°C increments, and then heating was stopped to allow the cell temperature to equalize with the ambient temperature. If the battery temperature rise rate was detected to be greater than 0.02°C / min, it was determined that the battery was self-heating and entered adiabatic mode; the temperature was then increased in 5°C increments, and the temperature rise rate was monitored, repeating this process. When the battery temperature rise rate reached 1°C / min, the temperature at this point was recorded as T2, and the highest battery temperature was recorded as T3.

[0085] (2) High-Temperature Storage Performance Test of Battery: First, the battery was charged and discharged at a rate of 0.33C within a test voltage range of 2.5V (discharge cut-off voltage) to 4.25V (charge cut-off voltage) to determine its initial capacity. Then, after fully charging the battery, it was stored in a temperature chamber at a specified temperature (high temperature 60℃) for 15 days. After that, the battery was removed, and its recovery capacity was tested at a rate of 0.33C. This process was repeated for 120 days, and the ratio of the recovery capacity on the 120th day to the initial capacity was collected, which was recorded as the storage capacity recovery rate.

[0086] (3) High-Temperature Storage Gas Generation Performance Test of Battery: First, the battery was fully charged at a rate of 0.33C, and the initial volume of the battery in the fully charged state was measured. Then, the battery was stored in a temperature chamber at a specified temperature (high temperature 60℃) for 7 days. After that, the battery was removed, and the volume of the battery was measured again. The battery was then fully charged and stored again. This process was repeated until 56 days, and the ratio of the battery volume on the 56th day to the initial volume was collected and recorded as the gas generation growth rate.

[0087] Table 1: Parameters of secondary batteries prepared in Examples 1 to 9 and Comparative Examples 1 to 6.

[0088]

[0089] Table 2: Battery performance test results of Examples 1 to 9 and Comparative Examples 1 to 6.

[0090]

[0091] Comparing the test results of Examples 1 to 5 and Comparative Examples 1 to 4, it can be seen that introducing appropriate amounts of DFEA solvent and LiFSI lithium salt into the electrolyte can significantly increase the battery thermal runaway initiation temperature T2 and reduce the battery thermal runaway maximum temperature T3 while taking into account the battery's electrical performance, thereby effectively improving the battery's thermal safety performance.

[0092] Comparing the test results of Examples 1 to 5 and Comparative Examples 1 to 2, it can be seen that if the DFEA solvent content added to the electrolyte in Comparative Example 1 is too low, even if the LiFSI lithium salt in the electrolyte is at an appropriate level, it cannot effectively improve the thermal safety performance of the battery in conjunction with the DFEA solvent. If the DFEA solvent content added to the electrolyte in Comparative Example 2 is too high, it will affect the ionic conductivity of the electrolyte, affect the electrical performance of the battery, and cause a decrease in battery storage performance and an increase in gas production.

[0093] Comparing the test results of Examples 1 to 5 and Comparative Examples 3 to 4, it can be seen that if the LiFSI mass content introduced into the electrolyte in Comparative Example 3 is too low, the insufficient amount of LiFSI lithium salt introduced will not produce a sufficiently violent reaction in the early stage of battery thermal runaway, making it difficult to promote the rapid evaporation of DFEA solvent in the electrolyte to the battery periphery to isolate the combustion reaction of flammable materials in the battery. Therefore, although the thermal runaway initiation temperature T2 can be increased, it cannot effectively reduce the maximum thermal runaway temperature T3. If the LiFSI mass content added to the electrolyte in Comparative Example 4 is too high, the excessive LiFSI is prone to side reactions with the negative electrode at higher storage temperatures, affecting the battery's electrical performance, causing battery storage life degradation and increased gas production.

[0094] Comparing the test results of Examples 1, 6, 7 and Comparative Example 5, it can be seen that adding an appropriate amount of EC solvent to the electrolyte helps improve the electrochemical stability of the electrolyte, further improves the battery's storage life, and reduces the gas generation rate of the battery under high-temperature storage. However, adding too much EC solvent to the electrolyte will deteriorate the battery's thermal stability and affect the electrolyte's effect on improving the battery's thermal safety performance.

[0095] Comparing the test results of Examples 1 and 8, it can be seen that introducing TPP and PST additives into the electrolyte can further improve the electrochemical stability of the battery, thereby increasing the battery's high-temperature storage life and suppressing gas production growth under high-temperature conditions. However, if the added TPP and PST content is too low, it is difficult to significantly improve the battery's electrochemical stability, and it cannot effectively improve the battery's high-temperature storage life or reduce gas production growth.

[0096] Comparing the test results of Example 1 and Comparative Examples 1 to 4, and Example 9 and Comparative Example 6, it can be seen that the electrolyte provided in this application, when adapted to batteries containing ternary cathode materials, can effectively increase the battery's thermal runaway initiation temperature and reduce its maximum thermal runaway temperature by rapidly evaporating fluorine vapor in the early stages of battery thermal runaway, thereby significantly improving the battery's thermal safety performance. However, when this electrolyte system is adapted to batteries containing lithium iron phosphate cathode materials, the lithium iron phosphate material has higher thermal stability than ternary materials, making it less prone to crosstalk reactions with the cathode through the electrolyte. Furthermore, the LiFSI in the electrolyte reacts with the cathode, contributing to thermal runaway heat. Therefore, its effect on improving the thermal safety performance of lithium iron phosphate batteries is not as significant as its effect on improving the thermal safety performance of ternary batteries.

[0097] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. An electrolyte, characterized in that, include: Lithium salts, including lithium bis(fluorosulfonyl)imide; Solvents, said solvents including carbonates and 2,2-difluoroethyl acetate; The lithium difluorosulfonylimide has a mass content of 30% to 80% in the lithium salt, and the 2,2-difluoroethyl acetate has a mass content of 20% to 70% in the electrolyte.

2. The electrolyte according to claim 1, characterized in that, The 2,2-difluoroethyl acetate in the electrolyte contains 20% to 50% by mass.

3. The electrolyte according to claim 1, characterized in that, The carbonate includes ethylene carbonate, and the ethylene carbonate content in the electrolyte is less than or equal to 12% by mass.

4. The electrolyte according to claim 1, characterized in that, The lithium difluorosulfonylimide has a mass content of 50% to 80% in the lithium salt.

5. The electrolyte according to claim 1, characterized in that, The lithium salt further includes lithium hexafluorophosphate, wherein the mass content of lithium hexafluorophosphate in the lithium salt is less than or equal to 70%.

6. The electrolyte according to claim 5, characterized in that, The lithium difluorosulfonylimide has a mass content of 5% to 12% in the electrolyte; and / or, the lithium hexafluorophosphate has a mass content of 3% to 10% in the electrolyte.

7. The electrolyte according to claim 1, characterized in that, The electrolyte further includes additives, including 1,3-propenesulfonate lactone and triphenyl phosphate; wherein the mass content of 1,3-propenesulfonate lactone in the electrolyte is greater than or equal to 0.1%, and the mass content of triphenyl phosphate in the electrolyte is greater than or equal to 0.1%.

8. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1 to 7.

9. The secondary battery according to claim 8, characterized in that, The positive electrode includes a positive electrode active material, which includes LiNi. x Co y Mn 1-x-y O2, where 0.6≤x<1, 0<y<0.4; the upper limit of the operating voltage of the secondary battery is less than or equal to 4.4V.

10. The secondary battery according to claim 8, characterized in that, The negative electrode includes a negative electrode active material, which includes graphite and silicon negative electrode material, wherein the mass content of the silicon negative electrode material in the negative electrode material is less than or equal to 5%.

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