Electrolyte and preparation method therefor, secondary battery, and power-consuming device

Abstract

The present invention relates to an electrolyte and a preparation method therefor, a secondary battery, and a power-consuming device. The electrolyte comprises an electrolyte salt, a first solvent and a composite flame retardant, wherein the composite flame retardant comprises a flame retardant and a gel polymer, the flame retardant is present in the internal pores of the gel polymer, the electrolyte salt is dissolved in the first solvent, and the composite flame retardant is dispersed in the first solvent. When the electrolyte is used in a secondary battery, it is beneficial to prolonging the service life of the battery and reducing the damage caused by thermal runaway of the battery.

Description

Electrolyte and preparation method thereof, secondary battery and power-consuming device

[0001] Related applications

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on October 24, 2023, with application number 202311385280.8, entitled “Electrolyte and its preparation method, secondary battery and electrical device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0003] The present application relates to the field of new energy technology, and in particular to an electrolyte and a preparation method thereof, a secondary battery and an electrical device. Background Art

[0004] The statements herein merely provide background information related to the present application and do not necessarily constitute prior art.

[0005] In recent years, the application of secondary batteries, such as lithium-ion batteries, has become increasingly widespread. They are widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. The electrolyte is a crucial component of lithium-ion batteries. Currently, commercially available electrolytes for power batteries are flammable, exacerbating safety risks when thermal runaway occurs. With the rapid development of lithium-ion batteries, minimizing the damage caused by thermal runaway has become increasingly important.

[0006] Summary of the Invention

[0007] The present application provides an electrolyte solution capable of reducing the degree of damage caused by thermal runaway of a battery, a preparation method thereof, a secondary battery, and an electrical device.

[0008] In order to achieve the above-mentioned objectives, the first aspect of the present application provides an electrolyte, comprising an electrolyte salt, a first solvent and a composite flame retardant, wherein the composite flame retardant comprises a flame retardant and a gel polymer, the flame retardant is located in the internal pores of the gel polymer, the electrolyte salt is dissolved in the first solvent, and the composite flame retardant is dispersed in the first solvent.

[0009] Therefore, in the electrolyte of the present application, the flame retardant and the gel polymer are added to the electrolyte in the form of a composite flame retardant. The composite flame retardant is dispersed as a whole in the first solvent of the electrolyte, and the flame retardant in the composite flame retardant is located in the internal pores of the gel polymer. This can inhibit the flame retardant from directly contacting the surface of the electrode and inhibit the flame retardant from causing side reactions on the surface of the electrode. At the same time, when the battery temperature is too high, the gel polymer of the composite flame retardant in the electrolyte can be further cured at high temperature to form a cross-linked cured polymer to block conduction and play a preliminary role in preventing thermal runaway, especially in preventing thermal runaway during the overcharge stage. As the temperature continues to rise, the battery cell fails and burns. The flame retardant in the composite flame retardant absorbs the free radicals generated during the combustion process, thereby blocking the continued reaction of the combustion and playing a further role in flame retardancy and preventing thermal runaway. Such multi-faceted synergy is conducive to extending the battery life and reducing the degree of damage caused by thermal runaway.

[0010] In some embodiments of the present application, the electrolyte satisfies at least one of the following conditions:

[0011] (1) In the electrolyte, the mass content of the composite flame retardant is 1% to 10%;

[0012] (2) In the composite flame retardant, the mass content of the flame retardant is 30% to 70%;

[0013] (3) In the electrolyte solution, the concentration of the electrolyte salt is 0.5 mol / L to 1.5 mol / L.

[0014] In some embodiments of the present application, the electrolyte satisfies at least one of the following conditions:

[0015] (1) In the electrolyte, the mass content of the composite flame retardant is 2% to 6%;

[0016] (2) In the components of the composite flame retardant excluding the solvent, the mass content of the flame retardant is 40% to 60%;

[0017] (3) In the electrolyte solution, the concentration of the electrolyte salt is 0.8 mol / L to 1.2 mol / L.

[0018] In some embodiments of the present application, the electrolyte satisfies at least one of the following conditions:

[0019] (1) The flame retardant includes a phosphate flame retardant,

[0020] (2) The components of the gel polymer include a main polymer and a second solvent;

[0021] (3) The dynamic viscosity of the electrolyte is between 3.5 cp and 5 cp.

[0022] In some embodiments of the present application, the electrolyte satisfies at least one of the following conditions:

[0023] (1) The flame retardant comprises at least one of a halogenated phosphate, a halogenated phosphite and a halogenated phosphonate;

[0024] (2) the host polymer has a carbonyl group;

[0025] (3) The second solvent includes a carbonate solvent.

[0026] In some embodiments of the present application, the electrolyte satisfies at least one of the following conditions:

[0027] (1) The flame retardant comprises at least one of triethyl 2-fluoro-2-phosphoryl acetate and bis(2,2,2-trifluoroethyl)methyl phosphate;

[0028] (2) The host polymer includes an ester polymer.

[0029] In some embodiments of the present application, the electrolyte satisfies at least one of the following conditions:

[0030] (1) The electrolyte salt includes at least one of an electrolyte lithium salt and an electrolyte sodium salt;

[0031] (2) the first solvent includes at least one of an ester solvent, an ether solvent, and a sulfone solvent;

[0032] (3) The ionic conductivity of the electrolyte is 4 mS / cm to 10 mS / cm.

[0033] A second aspect of the present application further provides a method for preparing any of the above-mentioned electrolytes, comprising the following steps:

[0034] Mixing the flame retardant and the raw materials for preparing the gel polymer and performing a curing reaction to obtain the composite flame retardant;

[0035] The composite flame retardant, the electrolyte salt, and the first solvent are stirred and mixed to form the electrolyte solution.

[0036] In some embodiments of the present application, the stirring step is controlled so that the dynamic viscosity of the electrolyte is controlled within a range of 3.5 to 5 cp.

[0037] In some embodiments of the present application, the raw materials for preparing the gel polymer include polymer monomers and initiators.

[0038] In some embodiments of the present application, the preparation method satisfies at least one of the following conditions:

[0039] (1) The polymer monomer includes an ester monomer;

[0040] (2) The mass ratio of the polymer monomer to the flame retardant is (1-5):1;

[0041] (3) The mass ratio of the polymer monomer to the initiator is (10-20):1.

[0042] A second aspect of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte as described above, wherein the electrolyte is disposed between the positive electrode sheet and the negative electrode sheet.

[0043] In some embodiments of the present application, the negative electrode plate includes a carbon-based negative electrode active material.

[0044] In some embodiments of the present application, the carbon-based negative electrode active material includes at least one of artificial graphite, natural graphite, soft carbon and hard carbon.

[0045] A third aspect of the present application provides an electrical device comprising any of the secondary batteries described above.

[0046] The electric device of the present application includes the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery.

[0047] The details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the conventional technology, the following briefly introduces the drawings required for use in the embodiments or the conventional technology descriptions. Obviously, the drawings described below are merely embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the disclosed drawings without any creative work.

[0049] FIG1 is a schematic diagram of a battery cell according to an embodiment of the present application.

[0050] FIG. 2 is an exploded view of the battery cell according to one embodiment of the present application shown in FIG. 1 .

[0051] FIG3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0052] FIG4 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0053] FIG5 is an exploded view of the battery pack shown in FIG4 according to an embodiment of the present application.

[0054] FIG6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0055] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 battery cell; 51 housing; 52 electrode assembly; 53 cover plate; 6 electrical device. DETAILED DESCRIPTION

[0056] Below, some embodiments of the electrolyte and its preparation method, secondary battery and electrical device of the present application are described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0057] " scope " disclosed in the present application can be limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and any end value can be included or not included independently, and can be arbitrarily combined, that is, any lower limit can form a scope with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 listed, and if the maximum range value 3,4 and 5 are also listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is merely an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to listing the parameter as, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and so on. For example, when a parameter is expressed as an integer selected from "2-10", this is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0058] In this application, "a plurality of" or "a plurality of" refers to a number greater than or equal to 2 unless otherwise specified. For example, "one or more" means one or more than or equal to two.

[0059] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0060] References to "embodiments" herein mean that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment or implementation of the present application. The appearance of such phrases in various locations in the specification does not necessarily refer to the same embodiment, nor does it necessarily refer to independent or alternative embodiments that are mutually exclusive with other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments. References to "implementations" herein have a similar understanding.

[0061] It will be appreciated by those skilled in the art that, in the methods of various embodiments or examples, the order in which the steps are written does not imply a strict order of execution and does not constitute any limitation on the implementation process, and the detailed order of execution of each step should be determined by its function and possible inherent logic. Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0062] In this application, in the open technical features or technical solutions described with words such as "contain", "include", and "include", unless otherwise specified, additional members other than the listed members are not excluded, and it can be regarded as providing both closed features or solutions consisting of the listed members and open features or solutions that also include additional members in addition to the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or may not include additional members. It can be regarded as providing both the feature or solution of "A consists of a1, a2, and a3" and the feature or solution of "A includes not only a1, a2, and a3, but also other members". In this application, unless otherwise specified, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0063] In this application, the terms "optionally," "optional," and "optional" are optional and refer to either option being present or absent. If a technical solution contains multiple "options," each option is considered independent unless otherwise specified and there are no conflicts or constraints.

[0064] The secondary battery and the electric device of the present application will be described below with reference to the accompanying drawings as appropriate.

[0065] Typically, a secondary battery consists of a positive electrode, a negative electrode, and an electrolyte. The electrolyte is located between the positive and negative electrodes. During the battery's charge and discharge processes, active ions are inserted and removed from the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes.

[0066] electrolyte

[0067] The electrolyte is located between the positive electrode and the negative electrode, and plays the role of conducting ions between the positive electrode and the negative electrode.

[0068] One embodiment of the present application provides an electrolyte, including an electrolyte salt, a first solvent and a composite flame retardant, wherein the composite flame retardant includes a flame retardant and a gel polymer, the flame retardant is located in the internal pores of the gel polymer, the electrolyte salt is dissolved in the first solvent, and the composite flame retardant is dispersed in the first solvent.

[0069] In order to achieve a flame retardant effect, traditional technology is to directly add and disperse flame retardants into the electrolyte. This method is simple and easy, but during the operation of the battery, the flame retardant components in the prepared electrolyte will migrate to the surface of the electrode due to the action of the electric field. Due to the high reducing property of the electrode surface, side reactions will occur. For example, phosphate flame retardants are prone to gas production, which causes the battery cell to swell, thereby affecting the long-term life of the battery.

[0070] It's worth noting that the gel polymer in the composite flame retardant comprises a polymer and a second solvent filled within the polymer. The gel polymer is swollen in the electrolyte. The polymer in the gel polymer has a network structure, providing a certain degree of physical support and structural stability. When the flame retardant is dispersed within the gel polymer, it is confined within the internal pores of the gel polymer. The polymer framework inhibits the free diffusion of the flame retardant, thereby preventing it from diffusing from the liquid electrolyte to the electrode surface and causing electrochemical side reactions.

[0071] Without wishing to be bound by any theory, the electrolyte of the present application adds a flame retardant and a gel polymer to the electrolyte in the form of a composite flame retardant. The composite flame retardant is dispersed as a whole in the first solvent of the electrolyte, and the flame retardant in the composite flame retardant is located in the internal pores of the gel polymer. This can inhibit the flame retardant from directly contacting the surface of the electrode and inhibiting the problem of side reactions of the flame retardant on the surface of the electrode. At the same time, when the battery temperature is too high, the gel polymer of the composite flame retardant in the electrolyte can further solidify at high temperature to form a cross-linked solidified polymer, which blocks conduction and plays a preliminary role in preventing thermal runaway, especially in the overcharge stage. As the temperature continues to rise, the battery cell fails and burns. The flame retardant in the composite flame retardant absorbs the free radicals generated during the combustion process, thereby blocking the continued reaction of combustion and playing a further role in flame retardancy and preventing thermal runaway. Such multi-faceted synergy is conducive to extending the battery life and reducing the damage caused by thermal runaway.

[0072] In some embodiments, the first solvent includes at least one of an ester solvent, an ether solvent, and a sulfone solvent.

[0073] As an example, the ester solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), γ-butyrolactone (BL), 1,3-propane sultone (1,3-PS), methyl formate, methyl acetate, ethyl acetate (EA), propyl acetate, methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB) and ethyl butyrate (EB).

[0074] As an example, the ether solvent may include at least one of ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TRGDME), tetraethylene glycol dimethyl ether (TEGDME), and 1,3-dioxolane (DOL).

[0075] As an example, the sulfone solvent includes one or more of dimethyl sulfoxide (DMSO), sulfolane, methyl ethyl sulfone, and diethyl sulfone.

[0076] In some embodiments, the mass content of the composite flame retardant in the electrolyte is 1% to 10%, optionally 2% to 6%, or optionally 3% to 5%. As an example, the mass content of the composite flame retardant can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%. Controlling the composite flame retardant content in the electrolyte within this range can achieve both good flame retardancy and good ion transmission.

[0077] In some embodiments, the mass content of the flame retardant in the composite flame retardant components other than the solvent is 30% to 70%, and optionally 40% to 60%. As an example, the mass content of the flame retardant in the composite flame retardant can be 30%, 40%, 50%, 60%, or 70%. The components other than the solvent in the composite flame retardant include the flame retardant and the main polymer in the gel polymer, but do not include the solvent.

[0078] The mass content of the composite flame retardant in the electrolyte and the mass content of the flame retardant in the composite flame retardant can be determined by conventional organic analysis methods such as nuclear magnetic resonance, liquid chromatography, gas chromatography-mass spectrometry, and plasma chromatography.

[0079] The gel polymer in the composite flame retardant provides a framework for the flame retardant, binds the flame retardant molecules, and prevents the flame retardant from diffusing to the surface of the electrode to cause electrochemical side reactions. Due to the limited internal space of the polymer and the need to take into account the life of the battery cell, the mass content of the flame retardant in the composite flame retardant is further controlled within the above range to take into account good flame retardant performance and battery cell life.

[0080] In some embodiments, the flame retardant includes a phosphate flame retardant, such as a halogenated phosphate flame retardant. Furthermore, the flame retardant includes at least one of a halogenated phosphate, a halogenated phosphite, and a halogenated phosphonate. Alternatively, the flame retardant includes at least one of triethyl 2-fluoro-2-phosphorylacetate and bis(2,2,2-trifluoroethyl)methyl phosphate. Flame retardants containing both halogen and phosphorus flame retardants exhibit greater flame retardancy.

[0081] In some embodiments, the components of the gel polymer include a main polymer and a second solvent, wherein the main polymer is a cross-linked polymer.

[0082] Optionally, the host polymer has a carbonyl group (C=O); the carbonyl group can have good compatibility with the electrolyte salt in the electrolyte, ensuring that the electrolyte can maintain high ionic conductivity and good interfacial contact, so that the gel polymer can form a stable interface at the negative electrode interface, promote uniform lithium deposition at the negative electrode and inhibit the growth of lithium dendrites. It is understood that the host polymer includes but is not limited to polymers with carbonyl groups, and can also be polymers without carbonyl groups, such as polyolefins such as polydivinylbenzene.

[0083] Optionally, the host polymer comprises an ester polymer; more optionally, the host polymer comprises a carboxylate polymer; more optionally, the polymer comprises at least one of polyethylene glycol dimethacrylate, polypentaerythritol tetraacrylate and ethylene glycol dimethacrylate-pentaerythritol tetraacrylate copolymer.

[0084] Optionally, the second solvent comprises a carbonate solvent.

[0085] In some embodiments, the concentration of the electrolyte salt in the electrolyte is 0.5 mol / L to 1.5 mol / L, and may be 0.8 mol / L to 1.5 mol / L or 0.8 mol / L to 1.2 mol / L. As an example, the concentration of the electrolyte salt in the electrolyte may be 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, or 1.5 mol / L.

[0086] In some embodiments, the electrolyte salt includes at least one of an electrolyte lithium salt and an electrolyte sodium salt.

[0087] In some embodiments, the electrolyte lithium salt in the electrolyte salt may also include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO2F2), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorodioxalatophosphate (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0088] In some embodiments, the electrolyte sodium salt in the electrolyte salt may also include one or more of sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6), sodium hexafluoroarsenate (NaAsF6), sodium trifluoroacetate (CF3COONa), sodium tetraphenylborate (NaB(C6H5)4), sodium trifluoromethanesulfonate (NaSO3CF3), sodium bis(fluorosulfonyl)imide (Na[(FSO2)2N]) or sodium bis(trifluoromethylsulfonyl)imide (Na[(CF3SO2)2N]).

[0089] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0090] In some embodiments, the additives in the electrolyte may include, but are not limited to, one or more of difluoroethylene carbonate (DFEC), trifluoromethylethylene carbonate (TFPC), etc. In some embodiments, the dynamic viscosity of the electrolyte is between 3.5cp and 5cp, such as 3.5cp, 4cp, 4.5cp, and 5cp; it can be optionally 3.5cp to 4cp. The dynamic viscosity of the electrolyte is related to the degree of cross-linking of the gel polymer, and the degree of cross-linking of the gel polymer can reflect the degree of formation of the gel network structure. Generally speaking, when other conditions remain unchanged, the higher the degree of cross-linking of the gel polymer, the greater the dynamic viscosity of the electrolyte. The dynamic viscosity of the electrolyte is controlled within this range. On the one hand, the gel polymer has a better gel network structure, and on the other hand, the electrolyte as a whole has better fluidity.

[0091] In some embodiments, the ionic conductivity of the electrolyte is 4 mS / cm to 10 mS / cm. Such an electrolyte has good ion transport capability.

[0092] As an example, ionic conductivity can be measured by a conductivity meter.

[0093] Another embodiment of the present application further provides a method for preparing the above-mentioned electrolyte. The preparation method comprises the following steps S10 to S20:

[0094] S10, mixing the flame retardant and the raw materials for preparing the gel polymer and causing a curing reaction to occur to obtain a composite flame retardant.

[0095] In some embodiments, the raw materials for preparing the gel polymer include polymer monomers and an initiator. Furthermore, the raw materials also include the second solvent mentioned above.

[0096] Optionally, the polymer monomer includes a comonomer, and the comonomer includes but is not limited to an ester monomer and an olefin monomer.

[0097] Optionally, the comonomer includes at least one of methyl methacrylate, methyl acrylate, ethyl methacrylate, ethyl acrylate, n-butyl methacrylate, butyl acrylate, n-octyl methacrylate, n-octyl acrylate, vinyl acetate, vinylene carbonate, acrylonitrile, vinyl ethylene carbonate, triethylene glycol dimethacrylate, diethylene glycol dimethacrylate, ethylene glycol dimethacrylate, allyl methacrylate, divinylbenzene, polyvinyl alcohol, hexafluoropropylene, trichloroethylene, tetrafluoroethylene, chlorotrifluoroethylene and styrene.

[0098] Optionally, the initiator for initiating polymerization of the comonomer to form a polymer includes at least one of a persulfate initiator, an azo initiator and an organic peroxide initiator; more optionally, the azo initiator includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile and dimethyl azobisisobutyrate.

[0099] Optionally, the mass ratio of the polymer monomer to the flame retardant is (1-5):1, for example, 1:1, 1:2, 1:3, 1:4, 1:5.

[0100] Optionally, the mass ratio of the polymer monomer to the initiator is (10-20):1, for example, 10:1, 15:1, or 20:1.

[0101] Optionally, the solid content (by weight) of the resulting solution obtained by mixing the polymer monomer, flame retardant, initiator, and second solvent is 2% to 20%, for example, 2%, 5%, 8%, 10%, 12%, 15%, 18%, or 20%. The solid content refers to the weight of the polymer monomer, flame retardant, and initiator in the resulting solution. The resulting composite flame retardant, after curing, is in a gel state, filled with a large amount of the second solvent.

[0102] In some embodiments, the curing reaction conditions in S10 include curing at a temperature of 70°C to 120°C for 10 to 60 minutes. The temperature for heat curing can be 70°C, 75°C, 80°C, 85°C, 90°C, 100°C, 110°C, or 120°C, optionally 90 to 100°C, or a range consisting of any two of the above values. The heat curing time can be 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, or 60 minutes, optionally 20 to 40 minutes, or a range consisting of any two of the above values.

[0103] It is understood that in some examples, all the products obtained in step S10 can be directly put into step S20.

[0104] S20, stirring and mixing the composite flame retardant, the electrolyte salt, and the first solvent to form an electrolyte solution.

[0105] The preparation method of the above-mentioned electrolyte comprises the following steps: firstly mixing the flame retardant and the raw materials for preparing the gel polymer and causing a curing reaction to obtain a composite flame retardant; then mixing the composite flame retardant with the electrolyte salt and the first solvent under stirring conditions, and controlling the electrolyte to have good fluidity through stirring.

[0106] In some embodiments, the stirring step is controlled so that the dynamic viscosity of the electrolyte is controlled within a range of 3.5 to 5 cp.

[0107] Positive electrode

[0108] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector.

[0109] As a non-limiting example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0110] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be obtained by forming a metal material on a polymer material substrate. Non-limiting examples of the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. Non-limiting examples of the polymer material substrate may include one or more of substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0111] In some embodiments, the positive electrode film layer may include a positive electrode active material layer. The positive electrode active material layer contains positive electrode active materials.

[0112] The positive electrode active material may be a positive electrode active material for a battery that is well known in the art. As a non-limiting example, the positive electrode active material may include one or more of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide (such as LiCoO2), lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, nickel cobalt manganese ternary positive electrode materials (or lithium nickel cobalt manganese oxide), lithium nickel cobalt aluminum oxide, and its modified compounds.

[0113] Non-limiting examples of olivine-structured lithium-containing phosphates may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0114] Non-limiting examples of lithium cobalt oxides may include LiCoO2; non-limiting examples of lithium nickel oxides may include LiNiO2; non-limiting examples of lithium manganese oxides may include LiMnO2, LiMn2O4, etc.; non-limiting examples of lithium nickel cobalt aluminum oxides may include LiNi 0.80 Co 0.15 Al 0.05 O2.

[0115] Non-limiting examples of nickel-cobalt-manganese ternary positive electrode materials (also known as lithium nickel-cobalt-manganese oxide) may include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ) etc. It is understandable that the nickel-cobalt-manganese ternary positive electrode material may also be doped with other metal elements.

[0116] It is understandable that the battery will be accompanied by lithium (Li) deintercalation and consumption during the charge and discharge process, and the content of Li in the positive electrode active material is different when the battery is discharged to different states. In the list of positive electrode active materials in this application, unless otherwise specified, the Li content is the initial state of the material. The positive electrode active material is applied to the positive electrode plate in the battery system, and after the charge and discharge cycle, the Li content in the positive electrode active material contained in the plate will usually change. Among them, the Li content can be measured by molar content, but is not limited to this. Regarding "the Li content is the initial state of the material", the initial state of the material refers to the state before the material is added to the positive electrode slurry. It is understandable that new materials obtained by appropriate modification on the basis of the listed positive electrode active materials are also within the scope of positive electrode active materials. The aforementioned appropriate modification refers to acceptable modification methods for positive electrode active materials, and non-limiting examples include coating modification.

[0117] In the examples of positive electrode active materials in this application, the oxygen (O) content is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual O content will fluctuate. The O content can be measured by molar content, but is not limited to this.

[0118] In some embodiments, the positive electrode active material may further include a sodium ion active material.

[0119] As an example, the sodium ion active material may include one or more of the following materials: sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds. However, the present application is not limited to these materials, and other conventionally known materials that can be used as sodium ion battery positive electrode active materials may also be used.

[0120] As an optional technical solution of the present application, in the sodium transition metal oxide, the transition metal may include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Non-limiting examples of sodium transition metal oxides may be Na x MO2, wherein M may include one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, and 0<x≤1.

[0121] As an optional technical solution of the present application, the polyanionic compound can be a compound having sodium ions, transition metal ions and tetrahedral (YO4) n- A class of compounds with anionic units. Transition metals may include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may be one or more of P, S, and Si; n represents (YO4) n- valence.

[0122] Polyanionic compounds can also be sodium ions, transition metal ions, tetrahedral (YO4) n- A class of compounds containing anion units and halogen anions. Transition metals may include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may be one or more of P, S, and Si, and n represents (YO4) n- valence state; the halogen can be one or more of F, Cl and Br.

[0123] Polyanionic compounds can also be sodium ions, tetrahedral (YO4) n- Anion unit, polyhedron unit (ZO y ) m+ and an optional halogen anion. Y can be one or more of P, S and Si, and n represents (YO4) n- valence state; Z represents a transition metal, which may include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce, and m represents (ZO y ) m+ valence state; the halogen can be one or more of F, Cl and Br.

[0124] Polyanionic compounds may include NaFePO4, Na3V2(PO4)3 (sodium vanadium phosphate, abbreviated as NVP), Na4Fe3(PO4)2(P2O7), NaM'PO4F and Na3(VO y )2(PO4)2F 3-2y (0≤y≤1) wherein M′ in NaM′PO4F may include one or more of V, Fe, Mn and Ni.

[0125] Prussian blue compounds can be sodium ions, transition metal ions and cyanide ions (CN - ). The transition metal may include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Non-limiting examples of Prussian blue compounds may include Na a Me b Me' c (CN)6, wherein Me and Me' can each independently be one or more of Ni, Cu, Fe, Mn, Co and Zn, 0<a≤2, 0<b<1, 0<c<1.

[0126] In some embodiments, the positive electrode active material layer may further optionally include a binder. As non-limiting examples, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0127] In some embodiments, the positive electrode active material layer may further include a conductive agent. As non-limiting examples, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0128] In some embodiments, in the positive electrode active material layer, the mass content of the positive electrode active material is 75% to 98%.

[0129] In some embodiments, in the positive electrode active material layer, the mass content of the conductive agent is 0.1% to 15%.

[0130] In some embodiments, in the positive electrode active material layer, the binder has a mass content of 0.5% to 15%.

[0131] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent to form a positive electrode slurry; the positive electrode slurry is coated on at least one side of the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0132] The type of solvent can be selected from but not limited to any one of the aforementioned embodiments, such as N-methylpyrrolidone (NMP). The surface of the positive electrode collector coated with the positive electrode slurry can be on a single surface of the positive electrode collector or on both surfaces of the positive electrode collector. The surface of the positive electrode collector coated with the positive electrode slurry can be on a single surface of the positive electrode collector or on both surfaces of the positive electrode collector. The solid content of the positive electrode slurry can be 40wt% to 80wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5000mPa·s to 25000mPa·s. When coating the positive electrode slurry, the coating unit surface density on a dry weight basis (excluding the solvent) can be 15mg / cm 2 ~35mg / cm 2 The compaction density of the positive electrode can be 3.0g / cm 3 ~3.6g / cm 3 , optional 3.3g / cm 3 ~3.5g / cm 3 .

[0133] Negative electrode

[0134] Generally, a negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector.

[0135] As a non-limiting example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two facing surfaces of the negative electrode current collector.

[0136] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be obtained by forming a metal material on a polymer material substrate. In the negative electrode current collector, non-limiting examples of the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. In the negative electrode current collector, non-limiting examples of the polymer material substrate may include one or more of substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0137] Furthermore, the negative electrode film layer may include a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material.

[0138] In some of the embodiments, the negative electrode active material may adopt the negative electrode active material for batteries that is well known in the art. As a non-limiting example, the negative electrode active material may include one or more of the following materials: carbon-based negative electrode active materials such as artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. Silicon-based materials may include one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0139] Optionally, the negative electrode active material includes a carbon-based negative electrode active material, and the carbon-based negative electrode active material includes at least one of artificial graphite, natural graphite, soft carbon and hard carbon. The above-mentioned electrolyte is particularly suitable for negative electrode sheets containing carbon-based negative electrode active materials. This is because the traditional technology of directly adding flame retardants to the electrolyte has poor compatibility with the negative electrode sheets, especially the carbon-based negative electrode active materials, due to the high viscosity of the flame retardants. Therefore, although the flame retardants can improve the flame retardancy of the electrolyte after addition, they will reduce the ionic conductivity of the electrolyte and greatly shorten the battery cycle life. Therefore, the electrolyte of the present application first fixes the flame retardant in the form of a polymer within the framework of the formed polymer, which can inhibit the excessive reaction of the single molecule flame retardant with the components in the electrolyte to a certain extent, thereby extending its service life.

[0140] In some embodiments, the negative electrode active material layer may further include a binder. The binder may include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0141] In some embodiments, the negative electrode active material layer may further include a conductive agent, which may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0142] In some embodiments, the negative electrode active material layer may optionally further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0143] In some embodiments, the mass content of the positive electrode active material in the negative electrode film layer is 75% to 98%.

[0144] In some embodiments, the mass content of the conductive agent in the negative electrode film layer is 0.1% to 15%.

[0145] In some embodiments, the mass content of the binder in the negative electrode film layer is 0.5% to 15%.

[0146] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (a non-limiting example of the solvent is deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on at least one surface of the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0147] The negative electrode slurry may be applied to a single surface of the negative electrode current collector or to both surfaces of the negative electrode current collector. The negative electrode slurry may have a solid content of 40 wt % to 60 wt %. The viscosity of the negative electrode slurry at room temperature may be adjusted to 2000 mPa·s to 10000 mPa·s.

[0148] Isolation film

[0149] The isolation membrane is set between the positive electrode and the negative electrode, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0150] In some embodiments, the material of the separator may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0151] In some embodiments, the thickness of the isolation film is 6 μm to 40 μm, and optionally 12 μm to 20 μm.

[0152] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0153] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0154] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft shell, such as a pouch-type soft shell. The material of the soft shell can be plastic. Further, non-limiting examples of plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0155] A secondary battery includes at least one battery cell. A secondary battery may include one or more battery cells. In one example, a secondary battery may also be a battery cell.

[0156] In this application, unless otherwise specified, a "battery cell" refers to a basic unit that can achieve the mutual conversion of chemical energy and electrical energy. Further, generally speaking, it includes at least a positive electrode plate, a negative electrode plate, and an electrolyte. During the battery's charge and discharge process, active ions are embedded in and out of the positive and negative electrode plates. The electrolyte plays the role of conducting active ions between the positive and negative electrode plates.

[0157] The present application has no particular limitation on the shape of the battery cell, which can be cylindrical, square, or any other shape. For example, FIG1 shows a battery cell 5 with a square structure as an example.

[0158] In some embodiments, referring to Figure 2, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to actual needs.

[0159] The secondary battery may be a battery cell 5 , a battery module 4 or a battery pack 1 .

[0160] A battery module includes at least one battery cell. The number of battery cells contained in a battery module can be one or more, and those skilled in the art can select an appropriate number based on the application and capacity of the battery module.

[0161] FIG3 shows an example battery module 4. Referring to FIG3 , within the battery module 4, multiple battery cells 5 may be arranged sequentially along the length of the battery module 4. Of course, they may also be arranged in any other manner. Furthermore, the multiple battery cells 5 may be secured together using fasteners.

[0162] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0163] In some embodiments, the battery modules may be assembled into a battery pack. The battery pack may contain one or more battery modules. Those skilled in the art may select an appropriate number based on the application and capacity of the battery pack.

[0164] Figures 4 and 5 illustrate an example battery pack 1. Referring to Figures 4 and 5 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0165] In addition, the present application also provides an electrical device, which includes the secondary battery provided in the present application. The secondary battery can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device can include, but is not limited to, mobile devices, electric vehicles, electric trains, ships, satellites, energy storage systems, etc. Among them, mobile devices can be, for example, mobile phones, laptops, etc.; electric vehicles can be, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited to these.

[0166] As an electrical device, a secondary battery can be selected according to its usage requirements.

[0167] Figure 6 shows an example of an electric device 6. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the electric device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module may be used.

[0168] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0169] The following are specific examples.

[0170] Below, the embodiment of the present application is described. The embodiment described below is exemplary, is only used to explain the present application, and is not to be construed as limiting the present application. Where the technology or conditions are not specified in the embodiment, the technology or conditions described in the literature in this area or the product instructions are used. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be obtained commercially.

[0171] Example 1

[0172] 1) Preparation of positive electrode sheet

[0173] The positive electrode active material NCM 333 , conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) are mixed uniformly in an appropriate amount of solvent N-methylpyrrolidone (NMP) in a mass ratio of 96.2:2.7:1.1 to obtain a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector aluminum foil, and through drying, cold pressing, slitting, cutting and other processes, a positive electrode active material layer with a thickness of 140 μm is formed to obtain a positive electrode sheet.

[0174] 2) Preparation of negative electrode sheet

[0175] The negative electrode active material artificial graphite, the conductive agent carbon black (Super P), the binder styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) were mixed uniformly in an appropriate amount of solvent deionized water at a mass ratio of 96.4:0.7:1.8:1.1 to obtain a negative electrode slurry; the negative electrode slurry was coated on the negative electrode current collector copper foil, and through the processes of drying, cold pressing, slitting and cutting, a negative electrode active material layer with a thickness of 110 μm was formed to obtain a negative electrode sheet.

[0176] 3) Isolation film

[0177] A 12μm thick polypropylene isolation film was selected.

[0178] 4) Preparation of electrolyte

[0179] 4.1 Flame retardant selection: The flame retardant is triethyl 2-fluoro-2-phosphoryl acetate; the polymerization monomers are ethylene glycol dimethacrylate and pentaerythritol tetraacrylate; the initiator is 2,2-azobisisobutyronitrile;

[0180] 4.2 Preparation of composite flame retardant (polymerization gelation of flame retardant): The flame retardant 2-fluoro-2-phosphoryl acetate triethyl ester, the polymer monomer ethylene glycol dimethacrylate, the pentaerythritol tetraacrylate and the initiator azobisisobutyronitrile in a mass ratio of 2:1:1:0.1 were dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (mass ratio 1:1) to prepare a solution in which the first four components (flame retardant, polymer monomer and initiator) accounted for 8% by mass. After stirring at room temperature until dissolved, the solution was heated at 60°C for 12 hours to gelate to obtain a gel composite flame retardant.

[0181] 4.3 Preparation of electrolyte: Continue to add the mixed solvent (EC / EMC) in the same amount and mass ratio as in 4.2 to the above-mentioned gel composite flame retardant, and then add lithium hexafluorophosphate (LiPF6) to prepare an electrolyte with a LiPF6 concentration of 1.1 mol / L. Stir and adjust the dynamic viscosity of the mixed solution to 4 cP to obtain the finished electrolyte.

[0182] 5) Preparation of secondary batteries

[0183] The positive electrode sheet, separator, and negative electrode sheet are stacked and wound in sequence to obtain an electrode assembly; the electrode assembly is placed in an outer package, dried, and then injected with electrolyte. After vacuum packaging, standing, formation, shaping and other processes, a secondary battery is obtained.

[0184] The secondary batteries of Examples 2 to 18 and the secondary battery of Example 1 were prepared in a similar manner, with the difference being that different electrolytes were used. See Table 1 for specific differences.

[0185] Comparative Example 1

[0186] It is basically the same as Example 1, except that the preparation steps of the electrolyte are different. The step of pre-polymerization and gelation of the flame retardant in 4.2 is omitted. The flame retardant 2-fluoro-2-phosphoryl acetate triethyl, the polymerization monomer ethylene glycol dimethacrylate, pentaerythritol tetraacrylate and the initiator azobisisobutyronitrile with a mass ratio of 2:1:1:0.1 are directly dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (mass ratio 1:1), and lithium hexafluorophosphate (LiPF6) is continuously added to prepare an electrolyte with a LiPF6 concentration of 1.1 mol / L, and the finished electrolyte is obtained after stirring.

[0187] Accordingly, in the preparation process of the secondary battery, after the electrolyte is injected, it is heated to 80° C. and kept warm for 24 hours so that the finished electrolyte is completely solidified in situ.

[0188] Comparative Example 2

[0189] The method is basically the same as Example 1, except that the preparation steps of the composite flame retardant are different, specifically as follows: the flame retardant triethyl 2-fluoro-2-phosphoryl acetate, polyethylene glycol dimethacrylate and water are mixed to obtain a mixed solution, wherein the mass ratio of the flame retardant to polyethyl methacrylate in the mixed solution is 1:1; the mixed solution is then spray-granulated and dried to obtain core-shell structure composite flame retardant particles in which the flame retardant is coated with a polyethyl methacrylate shell.

[0190] Comparative Example 3

[0191] The method is basically the same as Example 1, except that the preparation steps of the composite flame retardant are different. Specifically, the polymerized monomer ethylene glycol dimethacrylate in Example 1 is replaced by styrene of equal mass.

[0192] The above description of the various embodiments tends to emphasize the differences between the various embodiments. The same or similar aspects can be referenced with each other and will not be repeated herein for the sake of brevity.

[0193] The following is a performance test.

[0194] (1) Flame retardant performance test:

[0195] In the first step, the gel solidifies and blocks the internal conductive path. The effect can be evaluated by overcharge abuse effects:

[0196] Perform an overcharge abuse test on the battery cell: First, fully charge the battery cell to 100% SOC, then overcharge at a 1 / 3C charge rate until the battery cell cannot be charged further or thermal runaway occurs. Record the corresponding short-circuit SOC or failure when charging cannot be continued as the evaluation standard. The lower the short-circuit SOC, the better.

[0197] The second step is to evaluate the flame retardant's effectiveness through the hot box abuse effect:

[0198] Perform a hot box abuse test on the battery cell: First, fully charge the battery cell to 100% SOC, heat the battery cell at a temperature rise rate of 5℃ / min, and after heating to 55℃, keep the temperature for 30 minutes at each 5℃ increase until the battery cell thermal runaway occurs. Record the state of thermal runaway, such as whether there is open flame or the severity of the combustion (such as explosion). In this application, the severity of combustion is divided into the following categories from low to high: smoke without open flame, open flame extinguished quickly, and open flame extinguished slowly.

[0199] (2) Cyclic performance test:

[0200] The battery was charged at 25°C at a constant current of 0.5C to 4.25V, then charged at a constant voltage of 4.25V until the current dropped to 0.05C, and then discharged at a constant current of 0.5C to 2.8V to obtain the first cycle discharge capacity (Cd1). This charge and discharge process was repeated until the nth cycle, and the discharge capacity of the battery after n cycles was obtained, which was recorded as Cdn.

[0201] Capacity retention rate = discharge capacity after n cycles (Cdn) / first cycle discharge capacity (Cd1). According to this formula, the number of cycles n corresponding to a capacity retention rate of 80% is obtained.

[0202] Some parameters and performance test results of the above embodiments and comparative examples are shown in Table 1 below.

[0203] In Table 1, the mass content of the composite flame retardant refers to the mass content of the composite flame retardant in the electrolyte, which can be represented by the mass of the raw materials used to prepare the composite flame retardant to represent the mass of the composite flame retardant, thereby obtaining the mass content of the composite flame retardant in the electrolyte.

[0204] Table 1

[0205] It should be noted that the number of cycles n corresponding to 80% in Table 1 is the result of rounding off the units digit to an integer multiple of 10.

[0206] It can be seen from Table 1 that the flame retardant in the electrolyte in the injection state in Comparative Example 1 is not confined in the network structure, which will trigger side reactions and lead to reduced cycle performance. The time required for in-situ curing after injection is prolonged and the low fluidity of the electrolyte after curing will also reduce the cycle performance. In addition, the battery in Comparative Example 1 has a high short-circuit SOC corresponding to the overcharge test, indicating that its ability to prevent thermal runaway in the early stage is low.

[0207] Comparative Example 2 uses a core-shell composite flame retardant that lacks the gel polymer used in the examples. All 10 test samples failed in the overcharge test, demonstrating that it failed to prevent thermal runaway during overcharge, as demonstrated in the examples. Consequently, Comparative Example 2 exhibited greater combustion severity in the hot box abuse test than Comparative Examples 1 and 3.

[0208] Comparative Example 3 uses a linear polymer instead of the gel polymer in the examples of the present application. The results show that its effect in preventing thermal runaway during the overcharge stage is not as good as the gel polymer. In the overcharge abuse test of its battery cells, some of them failed, with 4 failures out of 10 test samples and a failure probability of 40%.

[0209] The electrolyte used in each embodiment adds a flame retardant and a gel polymer to the electrolyte in the form of a composite flame retardant. When the battery temperature is too high, the gel polymer of the composite flame retardant in the electrolyte can be further cured at high temperature to form a cross-linked cured polymer, which blocks conduction and plays a preliminary role in preventing thermal runaway, especially in the overcharge stage. As the temperature continues to rise, the battery cell fails and burns. The flame retardant in the composite flame retardant absorbs the free radicals generated during the combustion process, thereby blocking the continuous combustion reaction and playing a further role in flame retardancy and preventing thermal runaway. Such multi-faceted synergy is conducive to extending the battery life and reducing the degree of damage caused by thermal runaway.

[0210] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0211] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. An electrolyte, comprising an electrolyte salt, a first solvent and a composite flame retardant, wherein the composite flame retardant comprises a flame retardant and a gel polymer, the flame retardant is located in the internal pores of the gel polymer, the electrolyte salt is dissolved in the first solvent, and the composite flame retardant is dispersed in the first solvent.

2. The electrolyte according to claim 1, wherein The electrolyte satisfies at least one of the following conditions: (1) In the electrolyte, the mass content of the composite flame retardant is 1% to 10%; (2) In the composite flame retardant, the mass content of the flame retardant is 30% to 70%; (3) In the electrolytic solution, the concentration of the electrolyte salt is 0.5 mol / L to 1.5 mol / L.

3. The electrolyte according to claim 2, wherein The electrolyte satisfies at least one of the following conditions: (1) In the electrolyte, the mass content of the composite flame retardant is 2% to 6%; (2) In the components of the composite flame retardant other than the solvent, the mass content of the flame retardant is 40% to 60%; (3) In the electrolyte solution, the concentration of the electrolyte salt is 0.8 mol / L to 1.2 mol / L.

4. The electrolyte according to any one of claims 1 to 3, wherein The electrolyte satisfies at least one of the following conditions: (1) The flame retardant includes a phosphate flame retardant; (2) The components of the gel polymer include a main polymer and a second solvent; (3) The dynamic viscosity of the electrolyte is between 3.5 cp and 5 cp.

5. The electrolyte according to claim 4, wherein The electrolyte satisfies at least one of the following conditions: (1) The flame retardant comprises at least one of halogenated phosphates, halogenated phosphites and halogenated phosphonates; (2) the host polymer has a carbonyl group; (3) The second solvent includes a carbonate solvent.

6. The electrolyte according to claim 5, wherein The electrolyte satisfies at least one of the following conditions: (1) The flame retardant comprises at least one of triethyl 2-fluoro-2-phosphoryl acetate and bis(2,2,2-trifluoroethyl)methyl phosphate; (2) The host polymer includes an ester polymer.

7. The electrolyte according to any one of claims 1 to 6, wherein The electrolyte satisfies at least one of the following conditions: (1) The electrolyte salt includes at least one of an electrolyte lithium salt and an electrolyte sodium salt; (2) the first solvent includes at least one of an ester solvent, an ether solvent and a sulfone solvent; (3) The ionic conductivity of the electrolyte is 4 mS / cm to 10 mS / cm.

8. The method for preparing the electrolyte according to any one of claims 1 to 7, comprising the following steps: The flame retardant and the raw material for preparing the gel polymer are mixed and subjected to a curing reaction to obtain the composite flame retardant; The composite flame retardant, the electrolyte salt and the first solvent are stirred and mixed to form the electrolyte solution.

9. The method for preparing an electrolyte according to claim 8, wherein: The stirring step is controlled so that the dynamic viscosity of the electrolyte is controlled within a range of 3.5 cp to 5 cp.

10. The method for preparing an electrolyte according to any one of claims 8 to 9, wherein: The raw materials for preparing the gel polymer include polymer monomers and initiators.

11. The method for preparing an electrolyte according to claim 10, wherein: The preparation method satisfies at least one of the following conditions: (1) The polymer monomer includes an ester monomer; (2) The mass ratio of the polymer monomer to the flame retardant is (1-5):1; (3) The mass ratio of the polymer monomer to the initiator is (10-20):

1.

12. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet and the electrolyte according to any one of claims 1 to 7, wherein the electrolyte is disposed between the positive electrode sheet and the negative electrode sheet.

13. The secondary battery according to claim 12, wherein: The negative electrode plate includes a carbon-based negative electrode active material.

14. The secondary battery according to claim 13, wherein The carbon-based negative electrode active material includes at least one of artificial graphite, natural graphite, soft carbon and hard carbon.

15. An electrical device comprising the secondary battery according to any one of claims 12 to 14.

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