Electrolyte for lithium ion secondary battery and lithium ion secondary battery comprising same

Abstract

The present invention relates to an electrolyte for a lithium ion secondary battery. The electrolyte comprises a solvent, a lithium salt and an additive, wherein the solvent is a fluorinated solvent; and the additive comprises triallyl isocyanurate and lithium bisoxalatodifluorophosphate. The present invention further relates to a lithium ion secondary battery comprising the electrolyte and an electric device comprising the lithium ion secondary battery.

Description

Electrolyte for lithium ion secondary battery and lithium ion secondary battery comprising same TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to an electrolyte for lithium ion secondary battery comprising a solvent, a lithium salt and an additive, a lithium ion secondary battery comprising the same and an electric device comprising the lithium ion secondary battery. BACKGROUND

[0002] In recent years, the new energy vehicle market has developed rapidly, driving the explosive growth of the market value of the power battery market mainly composed of secondary alkali metal ion batteries. However, the energy density of the current commercial secondary alkali metal ion battery has approached its theoretical limit. In order to fundamentally solve the range anxiety of electric vehicle consumers and improve the energy density of the battery, increasing the voltage of the battery is an intuitive and feasible method. However, as the voltage of the battery increases, higher requirements are placed on the high-pressure resistance of the electrolyte in the battery.

[0003] Due to the high electron-withdrawing ability of fluorine atoms, fluorinated solvents (for example, fluorinated ethyl methyl carbonate (FEMC)) can better improve the oxidation stability of conventional carbonates, enabling them to be widely used in high-voltage systems as important solvent components in high-voltage battery systems, thereby significantly improving the high-pressure resistance of lithium battery electrolytes. However, at high temperatures, secondary batteries containing fluorinated solvents often have a series of problems such as high-temperature gas generation, high-temperature capacity loss, and increased direct current resistance (DCR), thereby affecting the performance of the battery.

[0004] CN106816629A discloses a high-voltage electrolyte and a lithium ion battery, which solves the problems of insufficient room temperature cycle performance and safety performance of lithium ion batteries at voltages of 4.85V and above. CN117790907A discloses a high-voltage electrolyte containing an electron-deficient ligand and its application in sodium ion batteries. By introducing an electron-deficient ligand, the electrophilic property of the electron-deficient ligand is used to coordinate with anions, reducing the generation of corrosive acids and inhibiting the oxidative decomposition of the electrolyte at high voltage. At the same time, the electron-deficient ligand participates in the formation of a positive electrode interfacial film rich in inorganic components to improve the electrochemical stability of the electrode / electrolyte interface at high voltage.

[0005] SUMMARY

[0006] Currently, lithium ion secondary batteries prepared using electrolytes containing fluorinated solvents have the problems of high direct current resistance, low high-temperature cycle stability and low high-temperature storage capacity retention rate of the battery in a high-voltage environment.

[0007] To solve the above problems in the prior art, the present application provides an electrolyte for a lithium ion secondary battery, which has a low direct current resistance and maintains excellent high voltage stability and high temperature stability.

[0008] In one aspect, the present application provides an electrolyte for a lithium ion secondary battery, which comprises a solvent, a lithium salt, and an additive. In one embodiment, the solvent is a fluoro-solvent. In one embodiment, the fluoro-solvent comprises a fluoro-carboxylate, a fluoro-carbonate, and a fluoro-ether. In one embodiment, the additive comprises triallyl isocyanurate and lithium difluorophosphate bis(oxalato).

[0009] In one embodiment, the fluoro-solvent comprises at least two of a fluoro-carboxylate, a fluoro-carbonate, and a fluoro-ether. In a preferred embodiment, the fluoro-solvent comprises a fluoro-carboxylate, a fluoro-carbonate, and a fluoro-ether.

[0010] In one embodiment, the content of triallyl isocyanurate is 0.01-1% by weight, preferably 0.3-0.8% by weight, and more preferably 0.45-0.55% by weight, based on the total weight of the electrolyte. In another embodiment, the content of lithium difluorophosphate bis(oxalato) is 0.01-1% by weight, preferably 0.3-0.8% by weight, and more preferably 0.45-0.55% by weight, based on the total weight of the electrolyte.

[0011] In another aspect, the present application provides a lithium ion secondary battery comprising the electrolyte for a lithium ion secondary battery of the present application.

[0012] In yet another aspect, the present application provides an electric device comprising the lithium ion secondary battery of the present application. DETAILED DESCRIPTION

[0013] The present application will be further described in detail below. Such description is for illustrative purposes only and is not intended to limit the present application. Other advantages and effects of the present application will be readily understood from the contents of the present specification by those skilled in the art. The present application can also be carried out or applied by other different specific embodiments. Various modifications and changes can be made thereto by those skilled in the art without departing from the spirit of the present application.

[0014] General Definitions and Terminology

[0015] All publications, patent applications, patents and other references mentioned herein, if any, are incorporated by reference in their entirety.

[0016] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. If there is a conflict between the definitions provided in this document and those provided in the patents, applications, and other references mentioned herein, the definitions provided in this document control.

[0017] All percentages, parts, ratios, etc., are by weight, unless otherwise indicated.

[0018] When describing values or ranges of values, the disclosure is understood to include the recited specific value or range of values unless otherwise indicated. When a range, preferably a range, or a preferred upper value and a preferred lower value are stated, it is understood that any range encompassing the recited range or preferred values is specifically disclosed, regardless of whether that range is specifically recited. Unless otherwise indicated, the numerical values listed in the specification and claims are intended to be approximations. Additionally, the numerical values are not intended to be limited to the precision of the figures with which they are expressed. For example, "4-25" encompasses 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, and 25, as well as any sub-range encompassing any two of those values, such as 4-22, 6-22, 8-20, and 10-20, etc.

[0019] The terms "comprising," "including," "having," "containing," or "involving," and any variations thereof, are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. Those skilled in the art will understand that a recited term such as "comprising" encompasses the meaning of "consisting of." The expression "consisting of" excludes any element, step, or ingredient not specified. The expression "consisting essentially of" means that the range of values is limited to the specified elements, steps, or ingredients, plus optional elements, steps, or ingredients that do not materially affect the basic and novel characteristics of the claimed subject matter. It is understood that the expression "comprising" encompasses the expressions "consisting essentially of" and "consisting of."

[0020] The term "selected from" means one or more elements from the group listed thereafter, independently selected, and can include combinations of two or more elements.

[0021] The terms "optional" or "optionally," as used herein, mean that the subsequently described event or circumstance can or can not occur, and that the description includes instances where the event or circumstance occurs and instances where it does not.

[0022] The terms "one or more" or "at least one," as used herein, mean one, two, three, four, five, six, seven, eight, nine, or more.

[0023] The terms "combination thereof and "mixture thereof, unless otherwise indicated, mean a multi-component mixture of the elements recited, such as two, three, four, and up to the maximum possible multi-component mixture.

[0024] The term "chain ester" refers to an ester compound having a straight chain structure, examples of which include, but are not limited to, methyl formate, ethyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, methyl ethyl carbonate, methyl propyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and the like.

[0025] The term "cyclic ester" refers to an ester compound having a cyclic structure, examples of which include, but are not limited to, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), 1,2-butylene carbonate, 2,3-butylene carbonate, vinylene carbonate (VEC), trans or cis 4,5-difluoro-1,3-dioxolan-2-one (both collectively referred to as "DFEC"), 4-ethynyl-1,3-dioxolan-2-one (EEC).

[0026] The term "electrolyte" is a medium used by a battery, which can provide ions for the normal operation of the battery.

[0027] The term "electrolyte salt" refers to an ionic salt, which is at least partially soluble in the solvent of the electrolyte composition and at least partially dissociates into ions in the solvent of the electrolyte composition to form a conductive electrolyte composition. Lithium salts are preferably used as the electrolyte salt.

[0028] The term "anode" refers to an electrode in a secondary battery, in which oxidation occurs during discharge and reduction occurs during charge.

[0029] The term "cathode" refers to an electrode in a secondary battery, in which reduction occurs during discharge and oxidation occurs during charge.

[0030] The term "lithium ion secondary battery" refers to a rechargeable battery type in which lithium ions move from the anode to the cathode during discharge and move from the cathode to the anode during charge.

[0031] In the present invention, "high voltage" or "high voltage" is a voltage of 4.5 V or more.

[0032] The term "formation" refers to a process of performing a small current charge and discharge on a battery after obtaining the battery during the battery production process. The formation process is advantageous for stabilizing the electrical properties of the battery.

[0033] The term "SEI" or "SEI film" refers to a solid electrolyte interphase film, which is formed on the solid-liquid interface between the electrode material and the electrolyte during the first charge-discharge process of the battery. It is a passivation layer covering the surface of the electrode material, which can effectively protect the contact interface between the negative electrode and the electrolyte, reduce the side reactions of the electrolyte, and ensure stable cycling of the battery.

[0034] The term "CEI" or "CEI film" refers to a cathode electrolyte interface film (CEI), which is generated on the surface of the positive electrode and is a solid electrolyte interface film through which ions can pass but electrons cannot pass. It is located between the positive electrode material and the electrolyte, plays a role in isolating and stabilizing the interface, can effectively protect the contact interface between the positive electrode and the electrolyte, reduce the side reactions of the electrolyte, and ensure stable cycling of the battery.

[0035] The term "DCR" refers to direct current resistance, also known as battery internal resistance. In lithium batteries, DCR refers to the internal resistance generated during discharge of the battery. It affects the discharge efficiency of the battery and the power output capability of the battery, and can characterize the kinetic performance of the battery, which is one of the important indicators for the long cycle of the battery.

[0036] The term "electric vehicle" refers to a vehicle powered by electricity. Examples of electric vehicles include, but are not limited to, electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc.

[0037] The term "electric two-wheeler" refers to a two-wheeled vehicle powered by electricity. Examples of electric two-wheelers include, but are not limited to, E-bikes and E-scooters.

[0038] Electrolyte of the present application

[0039] The electrolyte plays an important role in the battery, not only can transfer ions, maintain the stability of the battery, prevent the corrosion of the battery plate, but also can help to dissipate heat and ensure the safety performance of the battery. Therefore, the quality and performance of the battery electrolyte have a very important influence on the working efficiency and life of the battery.

[0040] In one aspect, the present application provides an electrolyte comprising a solvent, a lithium salt and an additive.

[0041] Solvent

[0042] The solvent in the electrolyte has the definition generally understood in the art. In the present application, "solvent" refers to a liquid used to dissociate lithium salt and as a lithium ion transport medium.

[0043] Fluorinated solvent

[0044] Fluorinated solvent refers to a non-aqueous organic solvent with fluorine atom substitution. Due to the high electron-withdrawing ability of fluorine atom, the oxidation stability of fluorinated solvent can be improved. The use of fluorinated solvent in electrolyte can improve the high-pressure resistance of electrolyte, which is beneficial to the preparation of high-voltage secondary battery.

[0045] Compared with aqueous solvent, fluorinated solvent has a wider electrochemical window, which can realize higher potential and energy density; it is not easy to be oxidized or reduced, has better shape stability, and the electrolyte structure of fluorinated solvent is more stable during the charging and discharging process of the battery, and is not easy to deform or decompose; it has higher ion conductivity, which can improve the output power of the battery; it has higher energy storage efficiency and charging efficiency, which can improve the energy storage and release capacity of the battery, and prolong the service life of the battery.

[0046] Fluorinated solvent has high-pressure stability, and by using fluorinated solvent, the overall viscosity of electrolyte can be reduced to achieve the effect of dilution, so that the electrolyte remains stable under high pressure. The fluorine atom in the fluorinated solvent can also be reduced on the negative side in the secondary battery containing it to form fluorine-containing SEI, further enhancing the stability of the battery.

[0047] In some embodiments, the electrolyte of the present application is selected from fluorinated solvent, i.e., the solvent system of the electrolyte of the present application consists of fluorinated solvent. Fluorine atom has oxidation stability due to its strong electron-withdrawing effect, and the above properties of fluorine atom in fluorinated solvent ensure the oxidation stability of the secondary battery assembled by the electrolyte of the present application under high voltage.

[0048] In some embodiments, the fluorinated solvent used in the electrolyte of the present application comprises at least two of fluorinated carboxylic acid ester, fluorinated carbonate and fluorinated ether. In a preferred embodiment, the fluorinated solvent used in the electrolyte of the present application comprises fluorinated carboxylic acid ester, fluorinated carbonate and fluorinated ether.

[0049] In some embodiments, the fluorinated solvent comprises a fluorinated carboxylic acid ester. In some embodiments, the fluorinated carboxylic acid ester includes, but is not limited to, one or more of 2,2-difluoroethyl acetate (DFEA, CAS No.: 1550-44-3), 2-fluoro-l-ethanol acetate (CAS No.: 462-26-0), 2,2,2-trifluoroethyl acetate (CAS No.: 406-95-1), 2,2-difluoroethyl propionate (CAS No.: 1133129-90-4), 2,2,2-trifluoroethyl propionate (CAS No.: 82259-34-5), 2,2,2-trifluoroethyl butyrate (CAS No.: 371-27-7), 2,2-difluoroethyl butyrate (CAS No.: 1309602-59-2). In a preferred embodiment, the fluorinated carboxylic acid ester includes 2,2-difluoroethyl acetate. The fluorinated carboxylic acid ester helps to stabilize the interface of the electrolyte on the positive electrode side.

[0050] In some embodiments, the fluorinated solvent comprises a fluorinated carbonate. In some embodiments, the fluorinated carbonate includes, but is not limited to, one or more of fluoroethylene carbonate (FEC, CAS No.: 114435-02-8), difluoroethylene carbonate (DFEC, CAS No.: 311810-76-1), fluoropropylene carbonate (FPC, also known as 3-fluoropropylene carbonate, CAS No.: 127213-73-4), difluoropropylene carbonate (DFPC, also known as 3,3-difluoropropylene carbonate, CAS No.: 186098-91-9), trifluoropropylene carbonate (TFPC, also known as 3,3,3-trifluoropropylene carbonate, CAS No.: 167951-80-6), methyl trifluoroethyl carbonate (FEMC, CAS No.: 156783-95-8), ethyl trifluoroethyl carbonate (CAS No.: 156783-96-9), di(2,2,2-trifluoroethyl) carbonate (CAS No.: 1513-87-7). In a preferred embodiment, the fluorinated carbonate comprises fluoroethylene carbonate. The fluorinated carbonate helps to improve the low-temperature and high-temperature characteristics of a battery comprising the electrolyte, and to improve the load characteristics after high-temperature charge retention.

[0051] In some embodiments, the fluorinated solvent comprises a fluorinated ether. In some embodiments, the fluorinated ether includes, but is not limited to, one or more of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (herein referred to as D2, CAS No.: 16627-68-2), 1,1,2,2-tetrafluoroethyl methyl ether (CAS No.: 425-88-7), 1,1,2,2-tetrafluoroethyl ethyl ether (CAS No.: 512-51-6), 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (CAS No.: 406-78-0), 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether (CAS No.: 16627-71-7), 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether (CAS No.: 993-95-3). In a preferred embodiment, the fluorinated ether comprises 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. The fluorinated ether helps to improve the compatibility of the electrolyte with the electrode material, improve the flame retardant effect of the electrolyte, and improve the safety performance of the electrolyte, providing an important guarantee for the efficient and stable operation of the battery.

[0052] To achieve good physical properties, the fluorinated solvents can be used in combination to achieve better results. For example, compared to a battery comprising a single fluorinated solvent, a battery comprising multiple fluorinated solvents has better high-temperature stability, as evidenced by the battery having a lower initial direct current resistance, a higher number of high-temperature cycles, and a higher high-temperature storage capacity retention rate.

[0053] Therefore, in an embodiment, the fluorinated solvent comprises a combination of a fluorinated carboxylic acid ester, a fluorinated carbonate, and a fluorinated ether. The selection of each of the fluorinated carboxylic acid ester, the fluorinated carbonate, and the fluorinated ether is as described above. In a preferred embodiment, the fluorinated solvent is a combination of a fluorinated carboxylic acid ester, a fluorinated carbonate, and a fluorinated ether. In a further preferred embodiment, the fluorinated solvent comprises 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0054] The content of the components of different fluorinated solvents in the electrolyte affects the performance of the battery. For example, too high a content of fluorinated carbonate increases the viscosity of the electrolyte, reduces the efficiency of charge movement in the battery, and reduces the efficiency of the battery. Too low a content of fluorinated carbonate reduces the conductivity of the battery and reduces the efficiency of the battery.

[0055] In some embodiments, the content of the fluorinated solvent used in the present application is 75-96% by weight, preferably 78-94% by weight, and more preferably 80-92% by weight, based on the total weight of the electrolyte.

[0056] As described above, for the battery comprising the electrolyte of the present application, a suitable weight ratio between different components in the solvent helps to achieve good battery performance. In some embodiments, the weight ratio of the fluoro-carboxylic acid ester and the fluoro-carbonate is 6:1 to 2:1. In a preferred embodiment, the weight ratio of the fluoro-carboxylic acid ester and the fluoro-carbonate is 4:1. In some embodiments, the weight ratio of the fluoro-ether and the fluoro-carbonate is 5:1 to 1:1. In a preferred embodiment, the weight ratio of the fluoro-ether and the fluoro-carbonate is 3:1.

[0057] In further embodiments, the weight ratio of the fluoro-carboxylic acid ester, the fluoro-ether and the fluoro-carbonate is 6-2:5-1:1.5-0.5, preferably 4.8-2.4:3.2-2.4:1.2-0.8, more preferably 4:3:1. In other embodiments, the weight ratio of the 2,2-difluoroethyl acetate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and the fluoro-ethylene carbonate is 6-2:5-1:1.5-0.5, preferably 4.8-2.4:3.2-2.4:1.2-0.8, more preferably 4:3:1. By selecting a suitable content, more excellent effects can be achieved, for example, the battery comprising the fluoro-solvents has high temperature stability, so that the battery has lower initial direct current resistance, higher high temperature cycle number, higher high temperature storage capacity retention rate.

[0058] Electrolyte salt

[0059] Electrolyte salt refers to an ionic salt, which is at least partially soluble in the fluoro-solvent and at least partially dissociates into ions in the fluoro-solvent to form a conductive electrolyte composition.

[0060] In some embodiments, in the electrolyte of the present application, a lithium salt is used as the electrolyte salt.

[0061] Lithium salt

[0062] Lithium salt refers to a salt having lithium as the cation. During the operation of the battery, the lithium ion of the electrolyte lithium salt plays a role in transporting ions between the positive and negative electrodes of the battery, which is crucial for the performance of the battery. In addition, the type, purity, content of the electrolyte lithium salt in the battery and the combination of the lithium salt with other substances can affect the capacity, charge and discharge performance, life and safety of the battery to varying degrees.

[0063] In some embodiments, the lithium salt used in the present application includes, but is not limited to, one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium tetrachloroborate (LiBCl4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium metaaluminate (LiAlO2), lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI), lithium hexafluoroantimonate (LiSbF6), lithium difluorophosphate (LiPO2F2), lithium monofluorophosphate (Li2PO3F), lithium difluoro oxalate borate (LiODFB), lithium difluoro di-oxalate phosphate (LiODFP), lithium difluoro oxalate borate (LiBOB), lithium bis-trifluoromethanesulfonylimide (LiTFSI). In a preferred embodiment, the lithium salt used in the present application is lithium hexafluorophosphate (LiPF6). The lithium salt used in the present application generally has good ionization ability and is easily ionized in fluorinated solvents, thus obtaining a large amount of free lithium ions, and therefore the lithium salt helps to provide sufficient lithium ions for the electrolyte and improve the conductivity of the electrolyte.

[0064] However, excessive content of lithium salt can increase the viscosity of the electrolyte, reduce the charge movement efficiency in the battery containing the electrolyte, and thus reduce the efficiency of the battery.

[0065] Therefore, in some embodiments, the content of lithium salt is 4-25 wt% of the total weight of the electrolyte of the present application, preferably 6-22 wt%, and more preferably 8-20 wt%. For example, the content of lithium salt is 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt% or 25 wt% of the total weight of the electrolyte of the present application.

[0066] Additives

[0067] Additives of the present application

[0068] The inventors found that the battery assembled using the electrolyte with fluorinated solvent still showed an undesirable decrease in performance at high temperature. Without being bound by any theory, it is possible that the fluorine atom in the fluorinated solvent is easily broken at high temperature, and the fluorine atom reacts with the active hydrogen in the electrolyte to generate hydrofluoric acid, which further corrodes the basic substances in the SEI and CEI, possibly leading to interface damage and continuous occurrence of side reactions, and eventually possibly showing a series of problems such as battery high-temperature gas generation, high-temperature capacity loss and DCR increase, which cause the battery performance to deteriorate.

[0069] However, the inventors surprisingly found that by further using a combination of triallyl isocyanurate (TAIC) and lithium difluorobis(oxalato)phosphate (LiODFP), it is possible to make a battery comprising the same as an electrolyte solvent to have excellent high-temperature stability, for example, to achieve unexpected effects such as lower initial direct current resistance, higher high-temperature cycle count, higher high-temperature storage capacity retention rate, etc.

[0070] Therefore, in an embodiment, the additive of the present application comprises a combination of triallyl isocyanurate and lithium difluorobis(oxalato)phosphate.

[0071] As an example, lithium difluorobis(oxalato)phosphate (LiODFP) has the following structure:

[0072] Lithium difluorobis(oxalato)phosphate not only can be reduced on the negative electrode graphite to generate a stable SEI film to protect the negative electrode interface, but also can be decomposed on the positive electrode side in preference to common carbonate solvents, thereby participating in the formation of a stable CEI film. The abundant inorganic elements F and P generated by its decomposition can effectively inhibit the decomposition of the electrolyte on the positive and negative electrode surfaces and reduce the overflow of transition metal ions from the positive electrode side.

[0073] In some embodiments, the content of lithium difluorobis(oxalato)phosphate is 0.01-1 wt% of the total weight of the electrolyte of the present application, preferably 0.3-0.8 wt%, more preferably 0.45-0.55 wt%, and most preferably 0.5 wt%. For example, the content of lithium difluorobis(oxalato)phosphate is 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, 0.5 wt%, 0.55 wt%, 0.6 wt%, 0.65 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt% or 1 wt% of the total weight of the electrolyte of the present application.

[0074] As an example, triallyl isocyanurate (TAIC) has the following structure:

[0075] Triallyl isocyanurate has an unsaturated double bond in its structure, which tends to preferentially oxidize on the positive electrode side, and the CEI film formed by its decomposition is not easy to be swelled by carbonate due to its low porosity. However, since the thermal stability of the six-membered ring amide is poor, the CEI film is prone to thermal dissolution and partial structure rupture at high temperatures, thereby causing continuous side reactions at the interface at high temperatures, which manifests as a comprehensive decline in the high-temperature performance of the battery.

[0076] The content of the additive in the electrolyte has an important influence on the high-temperature stability of the battery containing it. For example, a battery containing too high or too low a content of triallyl isocyanurate or lithium difluorobis(oxalato)phosphate will result in an increased initial direct current resistance, a reduced number of high-temperature cycle laps, and a reduced high-temperature storage capacity retention rate.

[0077] Therefore, in some embodiments, the content of triallyl isocyanurate is 0.01-1 wt%, preferably 0.3-0.8 wt%, more preferably 0.45-0.55 wt%, and most preferably 0.5 wt% of the total weight of the electrolyte of the present application. For example, the content of triallyl isocyanurate is 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, 0.5 wt%, 0.55 wt%, 0.6 wt%, 0.65 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt% or 1 wt% of the total weight of the electrolyte of the present application.

[0078] In some embodiments, the additive contains triallyl isocyanurate and lithium difluorobis(oxalato)phosphate, wherein the content of the additive is 0.01-2 wt%, preferably 0.45-1.6 wt%, and more preferably 1 wt% of the total weight of the electrolyte of the present application. In some embodiments, the content of triallyl isocyanurate is not more than 1 wt% of the total weight of the electrolyte of the present application. In other embodiments, the weight ratio of lithium difluorobis(oxalato)phosphate to triallyl isocyanurate is 100:1-1:100, preferably 1:0.5-50, and more preferably 1:1.

[0079] In other embodiments, the additive includes triallyl isocyanurate and lithium difluorobis(oxalato)phosphate, wherein the content of lithium difluorobis(oxalato)phosphate is 0.01-1 wt%, preferably 0.3-0.8 wt%, more preferably 0.45-0.55 wt%, and most preferably 0.5 wt% of the total weight of the electrolyte of the present application; and the content of triallyl isocyanurate is 0.01-1 wt%, preferably 0.3-0.8 wt%, more preferably 0.45-0.55 wt%, and most preferably 0.5 wt% of the total weight of the electrolyte of the present application.

[0080] In another preferred embodiment, the additives include triallyl isocyanurate and lithium difluorobis(oxalato)phosphate, wherein the lithium difluorobis(oxalato)phosphate is present in an amount of 0.5 wt% of the total weight of the electrolyte of the present application and the triallyl isocyanurate is present in an amount of 0.5 wt% of the total weight of the electrolyte of the present application. In a more preferred embodiment, the additives are a combination of triallyl isocyanurate and lithium difluorobis(oxalato)phosphate, wherein the lithium difluorobis(oxalato)phosphate is present in an amount of 0.5 wt% of the total weight of the electrolyte of the present application and the triallyl isocyanurate is present in an amount of 0.5 wt% of the total weight of the electrolyte of the present application.

[0081] The electrolyte of the present application can further include additional additives to modify desired properties.

[0082] For example, the electrolyte of the present application can further include an additive to improve high voltage stability. Such additives commonly used in the art include, for example, boron-based compounds, organic phosphorus compounds, fluorine-containing compounds, organic silicon compounds, etc. The additives can form a good interface on the positive electrode side, effectively improve the oxidation resistance of the electrolyte, and improve the high voltage resistance of the electrolyte, thereby effectively avoiding the battery failure caused by solvent decomposition due to high voltage.

[0083] For another example, the additional additives can include an additive to improve film formation. Such additives can reduce the impedance of the SEI film, help to build a stable SEI film, and improve the rate capability of the battery, while maintaining high capacity and energy density under high-rate charging and discharging conditions. In some embodiments, such additives include, but are not limited to, one or more of vinylene carbonate (VC), 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, tris(trimethylsilyl) phosphate, triallyl phosphate, tris(trimethylsilyl) borate, ethylene sulfate, and methanedimethanesulfonate (MMDS), and in particular, vinylene carbonate.

[0084] In one embodiment, the additional additives are present in an amount of 0-10 wt% of the total weight of the electrolyte of the present application. In a preferred embodiment, the additional additives are present in an amount of 3-6 wt% of the total weight of the electrolyte of the present application. In a more preferred embodiment, the additional additives are present in an amount of 4.5-6 wt% of the total weight of the electrolyte of the present application. Suitable amounts of additional additives are beneficial to improve the performance of the battery, such as the high temperature stability and film formation stability of the battery. For example, the additional additives can effectively improve the high temperature cycling performance and high temperature storage performance of the battery containing the fluorinated electrolyte.

[0085] Lithium ion secondary battery

[0086] In another aspect, the present application provides a lithium ion secondary battery comprising the electrolyte for a lithium ion secondary battery of the present application.

[0087] In still another aspect, the present application provides a lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator, and the electrolyte of the present application. In the present application, the constituent components other than the electrolyte of the present application, such as the positive electrode, the negative electrode, and the separator, can be not particularly limited.

[0088] In the present application, the structure and shape of the secondary battery are not particularly limited. For example, the structure of the secondary battery can be a case in which a prepared jelly-roll type electrode group or a stacked type electrode group is packaged into a case. Among them, the jelly-roll type electrode group can be obtained by winding the entire structure in which a positive electrode sheet, a separator, and a negative electrode sheet are stacked into a flat spiral shape. The stacked type electrode group can be obtained by stacking a positive electrode sheet, a separator, and a negative electrode sheet. The shape of the lithium ion secondary battery can be a square type, a cylindrical type, a stacked type, a coin type, a button type, a paper type, and the like.

[0089] The lithium ion secondary battery of the present application can be prepared by a conventional method. As an example, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled in order, alternately combined with a positive electrode, a separator, a negative electrode, and a separator, and subjected to formation to obtain a soft-pack battery.

[0090] Positive electrode

[0091] The positive electrode refers to an electrode into which electrons flow from an external circuit at the time of discharge, and which has a higher potential. The positive electrode is not particularly limited in the present application. As an example, the positive electrode can comprise a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector.

[0092] The material constituting the positive electrode current collector is not particularly limited in the present application. The positive electrode current collector used in the present application can be a strip-shaped material made of a foil, a perforated foil, a mesh, and the like, which is processed from a metal such as aluminum, titanium, stainless steel, and the like, or an alloy. In an embodiment, the positive electrode current collector used in the secondary battery of the present application is an aluminum foil. The thickness of the positive electrode current collector is not particularly limited, and can be, for example, 1 to 100 μm.

[0093] The positive electrode active material layer is formed on the surface of the positive electrode current collector, and the shape and area thereof are not particularly limited. The positive electrode active material layer can be formed on one side or both sides of the positive electrode current collector. The thickness of the positive electrode active material layer is not particularly limited, and can be, for example, 1 to 100 μm.

[0094] The positive electrode active material layer comprises a positive electrode active material. The positive electrode active material can be a composite metal oxide comprising at least one of cobalt, manganese, and nickel, and lithium. In some embodiments, the positive electrode active material is a mixture of the above composite metal oxides.

[0095] In some embodiments, the lithium complex metal oxide used in the present application is Li a Ni x Mn y O 4-z M z , wherein 0.90≤a≤1.10, 0.4≤x≤0.6, 1.4≤y≤1.6, 0≤z≤0.1, and element M is one or more of Cl, Br, I, S, Se, Te or F.

[0096] In addition, in order to improve the safety or cycle characteristics at overcharge or to enable use at a charging potential of 4.5 V or more, a part of the lithium complex metal oxide can also be substituted with other elements. For example, a part of manganese or nickel can be substituted with at least one or more elements of Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, Cu, Bi, Mo, La, etc., or a part of O can be substituted with S or F, or a compound containing these other elements can be coated.

[0097] The positive electrode can be prepared by the following exemplary method:

[0098] The carbon source is pre-sintered under a nitrogen atmosphere to obtain component B;

[0099] Component B is mixed with component A so that component B is coated on the surface of at least part of component A, and the weight of component B is 1.0-3.0% of the weight of component A, and secondary sintering treatment is performed to obtain the positive electrode material.

[0100] wherein the chemical formula of component A is Li a Ni x Mn y O 4-z M z , wherein 0.90≤a≤1.10, 0.4≤x≤0.6, 1.4≤y≤1.6, 0≤z≤0.1, and element M is one or more of Cl, Br, I, S, Se, Te or F; and the carbon source is one or more of fructose, polyethylene glycol, galactose, polyvinylpyrrolidone or tannic acid.

[0101] The positive electrode material prepared above, polyvinylidene fluoride as a binder and Super P as a conductive agent are mixed in a weight ratio of 98:1:1, N-methylpyrrolidone (NMP) is added, and stirring is performed under the action of a vacuum stirrer until the system becomes homogeneous and transparent to obtain a positive electrode slurry; the positive electrode slurry is uniformly coated on an aluminum foil; the aluminum foil is transferred to an oven for drying after being air-dried at room temperature, and then cold-pressed and cut to obtain a positive electrode (polar piece).

[0102] Negative electrode

[0103] The negative electrode refers to an electrode from which electrons flow out to the external circuit during discharging, and which has a lower potential. As an example, the negative electrode of the present application can include a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector.

[0104] The material of the negative electrode current collector is not particularly limited in the present application, and for example, a metal can be used. The metal used in the present application can be selected from, but is not limited to, one or more of aluminum, nickel, iron, stainless steel, titanium, copper, and the like. When the negative electrode current collector is selected from a combination of two or more metals, it can refer to the use of an alloy form, or can refer to the use of a cladding layer, a plating layer, or the like. In one embodiment, the negative electrode current collector used in the secondary battery of the present application is a copper foil. The thickness of the negative electrode current collector is not particularly limited, and for example, it can be 1 to 100 μm.

[0105] The active material of the negative electrode can be a carbon material capable of intercalating and deintercalating lithium metal or lithium alloy, which includes, but is not limited to, soft carbon, hard carbon having a (002) plane distance of 0.37 nm or more, graphite having a (002) plane distance of 0.34 nm or less, and the like. In addition, the negative electrode active material can also be tin (elemental), tin compounds, silicon (elemental), silicon oxide compounds, and silicon-carbon composites, and combinations thereof.

[0106] In some embodiments, the negative electrode active material used in the lithium ion secondary battery of the present application includes silicon element, silicon compound, artificial graphite, natural graphite, or silicon-oxygen composite artificial graphite. In a preferred embodiment, the negative electrode active material used in the lithium ion secondary battery of the present application includes artificial graphite, natural graphite, or silicon-oxygen composite artificial graphite. In a more preferred embodiment, the negative electrode active material used in the lithium ion secondary battery of the present application is artificial graphite.

[0107] The negative electrode can be prepared by the following exemplary method:

[0108] The artificial graphite is mixed as the negative electrode active material, Super P is mixed as the conductive agent, sodium carboxymethyl cellulose (CMC-Na) is mixed as the thickening agent, and styrene butadiene rubber (SBR) is mixed as the binder, at a weight ratio of 96:1:1:2, deionized water is added, and a negative electrode slurry is obtained under the action of a vacuum stirrer; the negative electrode slurry is uniformly coated on a negative electrode current collector copper foil; the copper foil is transferred to an oven for drying after being air-dried at room temperature, and then is subjected to cold pressing and slitting to obtain a negative electrode (polar piece).

[0109] Separator

[0110] The separator is a thin film located between the positive electrode and the negative electrode. The main function of the separator is to separate the positive and negative electrodes of the battery to prevent direct contact between them, which can cause a short circuit. At the same time, it is also necessary to allow the ions in the electrolyte to pass freely between the positive and negative electrodes to maintain the normal operation of the battery. Therefore, the separator needs to have appropriate ion permeability and electronic insulation.

[0111] In some embodiments, the separator includes, but is not limited to, a polypropylene (PP) separator, a polyethylene (PE) separator, a PP / PE / PP separator (three-layer structure, with polyethylene in the middle and polypropylene on both sides), a polyethylene separator coated with a ceramic material on the surface (PE-coated ceramic separator), and a polyethylene separator coated with a boehmite material on the surface (PE-coated boehmite separator). In a preferred embodiment, the separator is a polypropylene separator. In a preferred embodiment, the separator is a polypropylene film with a thickness of 12 μm.

[0112] Preparation of a secondary battery

[0113] A lithium ion secondary battery can be prepared by the following exemplary method: using a polypropylene film (PP) as a separator, stacking the above-prepared positive electrode, separator, and negative electrode in order, with the separator between the positive electrode and negative electrode to serve as a separator. Then, wrapping the battery with an aluminum plastic film, transferring it to a vacuum oven for drying, injecting the above-prepared electrolyte, and then sealing it, performing electrolytic formation, and finally preparing a lithium ion battery.

[0114] Performance of a secondary battery

[0115] The secondary battery of the present application can be tested by the following method.

[0116] (1) Secondary battery 25°C direct current resistance (DCR) test

[0117] At a specified temperature, when the battery is discharged at a 1C current to 50% SOC (state of charge, reflecting the remaining capacity of the battery), the current is increased to 4C and maintained for 30 s, and the difference between the updated stable voltage and the original platform voltage is detected. The ratio of the value to the 4C current value is the direct current resistance of the battery. The DCR test result after the first full charge of the battery is the initial DCR of the battery.

[0118] (2) Secondary battery high temperature (60°C) storage capacity retention rate test

[0119] After the secondary battery is fully charged, it is placed in a constant temperature oven at 60°C for 15 days, and after sufficient cooling, it is discharged at a rate of 0.3C to the cut-off voltage. The percentage of its capacity relative to the initial discharge capacity is compared.

[0120] (3) High temperature (45°C) cycle test

[0121] The battery is cycled at 45°C, and its capacity retention rate is measured. The voltage range is 3.4V-4.85V, and the charge and discharge rate is 1C / 1C. The capacity of each cycle is recorded. When the battery capacity reaches 80% of the first cycle capacity, the test is stopped, and the actual number of cycles is recorded.

[0122] In some embodiments, the secondary battery of the present application has a 45°C high-temperature cycle number of 270 or more. In other embodiments, the secondary battery of the present application has a 45°C high-temperature cycle number of 380 or more. In some embodiments, the secondary battery of the present application has a 45°C high-temperature cycle number of 380-440. In a preferred embodiment, the secondary battery of the present application has a 45°C high-temperature cycle number of 382-438. In a more preferred embodiment, the secondary battery of the present application has a 45°C high-temperature cycle number of 384-435.

[0123] In some embodiments, the secondary battery of the present application has an initial direct current resistance (DCR) of 150 mΩ or less. In other embodiments, the secondary battery of the present application has an initial direct current resistance (DCR) of 102 mΩ or less. In some embodiments, the secondary battery of the present application has an initial direct current resistance (DCR) of 55-102 mΩ. In a preferred embodiment, the secondary battery of the present application has an initial direct current resistance (DCR) of 60-101 mΩ.

[0124] In some embodiments, the secondary battery of the present application has a 60°C high-temperature storage capacity retention rate of 70% or more. In other embodiments, the secondary battery of the present application has a 60°C high-temperature storage capacity retention rate of 86% or more. In some embodiments, the secondary battery of the present application has a 60°C high-temperature storage capacity retention rate of 86-99%. In a preferred embodiment, the secondary battery of the present application has a 60°C high-temperature storage capacity retention rate of 87-95%. In a more preferred embodiment, the secondary battery of the present application has a 60°C high-temperature storage capacity retention rate of 88-94%.

[0125] Electric device

[0126] In one aspect, the present application provides an electric device, characterized in that the electric device comprises the lithium ion secondary battery of the present application. In a preferred embodiment, the electric device comprises an electric vehicle, an electric two-wheeled vehicle, and a power storage system. In a more preferred embodiment, the electric device is an electric vehicle, which comprises an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV).

[0127] Use

[0128] In another aspect, the present application also relates to the use of the electrolyte of the present application for the preparation of a lithium ion secondary battery. The electrolyte of the present application has excellent high-voltage stability and high-temperature stability, which is beneficial for the stable use of the battery at a higher voltage, for example, can tolerate a voltage of 4.5 V, 4.8 V, or 5 V. Therefore, the electrolyte of the present application is particularly suitable for high-voltage lithium ion secondary batteries. Advantages

[0129] The electrolyte provided by the present application has each component of the mixed solvent being a fluorinated solvent, and the stability of the electrode and electrolyte interface at high temperature is achieved by using the combination of the additives lithium difluorobisoxalate phosphate and triallyl isocyanurate, thereby significantly improving the high-temperature stability of the high-voltage battery. In addition, with the use of suitable solvents and their proportions and suitable additives of the present application, a stable protective film can be formed at the interface between the electrode sheet and the electrolyte, thereby enabling the nickel-manganese lithium 5V high-voltage system battery to have a lower normal temperature DCR, better negative electrode reduction stability, and excellent high-temperature performance. The secondary battery containing the electrolyte of the present application can have the advantages of low initial direct current resistance, high high-temperature cycle number, and high high-temperature storage capacity retention rate.

[0130] Embodiment

[0131] It should be noted that the following examples are merely examples for clearly illustrating the technical solutions of the present application, and are not intended to limit the present application. Based on the above description, those skilled in the art can make other different forms of changes or modifications, which are not required to be exhaustively enumerated here, and the obvious changes or modifications derived therefrom are still within the protection scope of the present application.

[0132] Unless otherwise specified, the instruments and reagent materials used in this paper are commercially available or can be prepared by conventional methods in the art.

[0133] Preparation Example

[0134] Preparation of the positive electrode of the secondary battery

[0135] The positive electrode of the secondary battery of the present application can be prepared by the following method

[0136] The carbon source is pre-sintered under a nitrogen atmosphere to obtain component B;

[0137] Component B is mixed with component A so that component B is coated on the surface of at least part of component A, and the weight of component B is 1.0wt%-3.0wt% of the weight of component A, and then secondary sintering treatment is performed to obtain the positive electrode material.

[0138] The chemical formula of component A is Li 0.98 Ni 0.45 Mn 1.55 O4, and the carbon source is one or more of fructose, polyethylene glycol, galactose, polyvinylpyrrolidone, or tannic acid; the weight of component B is 1.0wt%-3.0wt% of the mass of component A.

[0139] The positive electrode material prepared above, polyvinylidene fluoride as a binder and Super P as a conductive agent are mixed in a weight ratio of 98:1:1, N-methyl pyrrolidone (NMP) is added, and stirring is performed under the action of a vacuum stirrer until the system becomes homogeneous and transparent to obtain a positive electrode slurry; the positive electrode slurry is uniformly coated on an aluminum foil; the aluminum foil is dried at room temperature, then transferred to an oven for drying, and then subjected to cold pressing and slitting to obtain a positive electrode (pole piece).

[0140] Preparation of a negative electrode of a secondary battery

[0141] The negative electrode of the secondary battery of the present application can be prepared by the following method.

[0142] The artificial graphite is mixed with Super P as a conductive agent, sodium carboxymethyl cellulose (CMC-Na) as a thickening agent and styrene butadiene rubber (SBR) as a binder in a weight ratio of 96:1:1:2, deionized water is added, and a negative electrode slurry is obtained under the action of a vacuum stirrer; the negative electrode slurry is uniformly coated on a negative electrode current collector copper foil; the copper foil is dried at room temperature, then transferred to an oven for drying, and then subjected to cold pressing and slitting to obtain a negative electrode (pole piece).

[0143] Preparation of an electrolyte of a secondary battery

[0144] The electrolyte of the secondary battery of the present application can be prepared by the following method.

[0145] In an argon atmosphere glove box with a water content <10 ppm, the solvent is quantitatively configured according to the composition of the electrolyte shown in Table 1, a sufficient amount of lithium salt LiPF6 is added to make its content in the electrolyte 13.0% by weight, and other ingredients are added, mixed uniformly to obtain an electrolyte. In Table 1, the content of each ingredient other than the solvent is the weight percentage calculated based on the total weight of the electrolyte, and the content of each component in the solvent is expressed in weight parts.

[0146] Table 1. Electrolyte composition table of examples and comparative examples of different groups

[0147] Note: DFEA: 2,2-difluoroethyl acetate; FEC: fluoroethylene carbonate; D2: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether; EC: ethylene carbonate; DMC: dimethyl carbonate; TAIC: triallyl isocyanurate; LiODFP: lithium difluorobisoxalate phosphate; VC: vinylene carbonate; DTD: vinyl sulfate.

[0148] Preparation of a secondary battery

[0149] The secondary battery of the present application can be prepared by the following method.

[0150] A lithium ion battery with a capacity of 1 Ah was prepared by using the above prepared positive electrode, a polypropylene film (PP) with a thickness of 12 μm as a separator, laminating the positive electrode, the separator and the negative electrode in this order so that the separator was interposed between the positive electrode and the negative electrode to play a role of separation, then wrapping the battery with an aluminum plastic film, drying the battery in a vacuum oven at 120°C, injecting 3.0 g / Ah of the above prepared electrolyte, sealing the battery, and then performing electrolytic solution formation.

[0151] Secondary batteries of Examples 1 to 10 and Comparative Examples 1 to 8 were prepared according to the above method.

[0152] Example 1

[0153] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6 was added so that the content thereof in the electrolyte was 13.0% by weight. 0.01% of triallyl isocyanurate (TAIC) was added based on the total weight of the electrolyte to obtain the electrolyte. A secondary battery was prepared according to the above method.

[0154] Example 2

[0155] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6 was added so that the content thereof in the electrolyte was 13.0% by weight. 0.5% of triallyl isocyanurate (TAIC) was added based on the total weight of the electrolyte to obtain the electrolyte. A secondary battery was prepared according to the above method.

[0156] Example 3

[0157] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6 was added so that the content thereof in the electrolyte was 13.0% by weight. 1% of triallyl isocyanurate (TAIC) was added based on the total weight of the electrolyte to obtain the electrolyte. A secondary battery was prepared according to the above method.

[0158] Example 4

[0159] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6was added, which was sufficiently dried, so that its content in the electrolyte was 13.0% by weight. Based on the total weight of the electrolyte, 1.2% of triallyl isocyanurate (TAIC) was added to obtain the electrolyte. A secondary battery was prepared according to the method described above.

[0160] Example 5

[0161] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6was added, which was sufficiently dried, so that its content in the electrolyte was 13.0% by weight. Based on the total weight of the electrolyte, the following components were added respectively: 0.5% of triallyl isocyanurate (TAIC) and 0.01% of lithium difluorobis(oxalato)phosphate (LiODFP) to obtain the electrolyte. A secondary battery was prepared according to the method described above.

[0162] Example 6

[0163] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6was added, which was sufficiently dried, so that its content in the electrolyte was 13.0% by weight. Based on the total weight of the electrolyte, the following components were added respectively: 0.5% of triallyl isocyanurate (TAIC) and 0.5% of lithium difluorobis(oxalato)phosphate (LiODFP) to obtain the electrolyte. A secondary battery was prepared according to the method described above.

[0164] Example 7

[0165] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6was added, which was sufficiently dried, so that its content in the electrolyte was 13.0% by weight. Based on the total weight of the electrolyte, the following components were added respectively: 0.5% of triallyl isocyanurate (TAIC) and 1% of lithium difluorobis(oxalato)phosphate (LiODFP) to obtain the electrolyte. A secondary battery was prepared according to the method described above.

[0166] Example 8

[0167] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA) and fluoroethylene carbonate (FEC) were mixed in a weight ratio of 6:2. An appropriate amount of lithium salt LiPF6was added, which was sufficiently dried, so that its content in the electrolyte was 13.0% by weight. The following components were added based on the total weight of the electrolyte: 0.5% of triallyl isocyanurate (TAIC) and 0.5% of lithium difluorobis(oxalato)phosphate (LiODFP), respectively, to obtain the electrolyte. A secondary battery was prepared according to the above-described method.

[0168] Example 9

[0169] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA) and fluoroethylene carbonate (FEC) were mixed in a weight ratio of 6:2. An appropriate amount of lithium salt LiPF6was added, which was sufficiently dried, so that its content in the electrolyte was 13.0% by weight. The following components were added based on the total weight of the electrolyte: 0.5% of triallyl isocyanurate (TAIC) and 0.5% of lithium difluorobis(oxalato)phosphate (LiODFP), respectively, to obtain the electrolyte. A secondary battery was prepared according to the above-described method.

[0170] Example 10

[0171] In a dry argon atmosphere, fluoroethylene carbonate (FEC) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 2:6. An appropriate amount of lithium salt LiPF6was added, which was sufficiently dried, so that its content in the electrolyte was 13.0% by weight. The following components were added based on the total weight of the electrolyte: 0.5% of triallyl isocyanurate (TAIC) and 0.5% of lithium difluorobis(oxalato)phosphate (LiODFP), respectively, to obtain the electrolyte. A secondary battery was prepared according to the above-described method.

[0172] Comparative Example 1

[0173] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6was added, which was sufficiently dried, so that its content in the electrolyte was 13.0% by weight, to obtain the electrolyte. A secondary battery was prepared according to the above-described method.

[0174] Comparative Example 2

[0175] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6 was added to make the content thereof in the electrolyte 13.0% by weight, which was sufficiently dried. Based on the total weight of the electrolyte, 0.5% of vinyl carbonate (VC) was added to obtain an electrolyte. A secondary battery was prepared according to the above-described method.

[0176] Comparative Example 3

[0177] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6 was added to make the content thereof in the electrolyte 13.0% by weight, which was sufficiently dried. Based on the total weight of the electrolyte, 0.5% of vinyl carbonate (VC) and 0.5% of vinyl sulfide (DTD) were added, respectively, to obtain an electrolyte. A secondary battery was prepared according to the above-described method.

[0178] Comparative Example 4

[0179] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6 was added to make the content thereof in the electrolyte 13.0% by weight, which was sufficiently dried. Based on the total weight of the electrolyte, 0.5% of vinyl carbonate (VC) and 0.5% of vinyl sulfide (DTD) were added, respectively, to obtain an electrolyte. A secondary battery was prepared according to the above-described method.

[0180] Comparative Example 5

[0181] In a dry argon atmosphere, 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2) were mixed in a weight ratio of 4:1:3. An appropriate amount of lithium salt LiPF6 was added to make the content thereof in the electrolyte 13.0% by weight, which was sufficiently dried. Based on the total weight of the electrolyte, 0.5% of vinyl carbonate (VC) and 0.5% of vinyl sulfide (DTD) were added, respectively, to obtain an electrolyte. A secondary battery was prepared according to the above-described method.

[0182] Comparative Example 6

[0183] Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a weight ratio of 5:3 in a dry argon atmosphere. An appropriate amount of lithium salt LiPF6 was added to make its content in the electrolyte 13.0% by weight. The following components were added based on the total weight of the electrolyte: 0.5% of triallyl isocyanurate (TAIC) and 0.5% of lithium difluorobis(oxalato) phosphate (LiODFP) to obtain the electrolyte. A secondary battery was prepared according to the above method.

[0184] Comparative Example 7

[0185] Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a weight ratio of 5:3 in a dry argon atmosphere. An appropriate amount of lithium salt LiPF6 was added to make its content in the electrolyte 13.0% by weight. The following components were added based on the total weight of the electrolyte: 0.5% of triallyl isocyanurate (TAIC) and 0.5% of lithium difluorobis(oxalato) phosphate (LiODFP) to obtain the electrolyte. A secondary battery was prepared according to the above method.

[0186] Comparative Example 8

[0187] Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a weight ratio of 5:3 in a dry argon atmosphere. An appropriate amount of lithium salt LiPF6 was added to make its content in the electrolyte 13.0% by weight. The following components were added based on the total weight of the electrolyte: 0.5% of triallyl isocyanurate (TAIC) and 0.5% of lithium difluorobis(oxalato) phosphate (LiODFP) to obtain the electrolyte. A secondary battery was prepared according to the above method.

[0188] Performance test

[0189] The secondary battery of the present application can be tested by the following method.

[0190] (1) DC resistance (DCR) test of lithium ion battery at 25℃

[0191] At a specified temperature, when the battery is discharged at 1C current to 50% SOC (state of charge, reflecting the remaining capacity of the battery), the current is increased to 4C and maintained for 30s, the difference between the updated stable voltage and the original platform voltage is detected, and the ratio of the value to the 4C current value is the DC resistance of the battery. The DCR test result after the first full charge of the battery is the initial DCR of the battery.

[0192] (2) High temperature (60℃) storage capacity retention rate of lithium ion battery

[0193] After the lithium ion battery was fully charged, it was stored in a constant temperature oven at 60°C for 15 days. After cooling, it was discharged at a rate of 0.3C to the cut-off voltage, and its capacity percentage relative to the initial discharge capacity was compared.

[0194] (3) High temperature (45°C) cycle test of lithium ion battery

[0195] The battery was cycled at 45°C with a charge-discharge voltage range of 3.4V-4.85V and a charge-discharge rate of 1C / 1C, and the discharge capacity of each cycle was recorded. The test ended when the battery capacity remained at 80% SOC (State of Charge, reflecting the remaining capacity of the battery), and the actual number of cycles reached was recorded.

[0196] Performance test results

[0197] Table 2 shows the results of the cycle test, DCR test, and high temperature storage capacity retention of the secondary batteries of Examples 1-10 and Comparative Examples 1-8.

[0198] Table 2. Performance test results of examples and comparative examples in different groups

[0199] As shown in Table 2, according to the results of Examples 1-4, when the solvent components and proportions are the same, using TAIC alone as an additive, as the content of TAIC increases from 0.01% to 1%, the DCR of the secondary battery decreases, the high temperature storage capacity retention increases, and the high temperature cycle number increases, indicating that TAIC as an additive can maintain the thermodynamic stability of the secondary battery and reduce the reaction at the solid-liquid interface; when the content of TAIC is more than 1%, compared to the secondary battery with a content of 1%, although it still maintains a high high temperature storage capacity retention, its DCR increases significantly, and the high temperature cycle number also decreases significantly, indicating that due to the excessive thickness of the film, the initial impedance increases, the dynamics of the battery system decreases, and thus the high temperature cycle performance deteriorates. This indicates that although TAIC can maintain the thermodynamic stability of the secondary battery and reduce the reaction at the solid-liquid interface, too high a content of TAIC can cause the high temperature cycle performance of the secondary battery system to deteriorate. When the content of TAIC is 0.01%-1%, the secondary battery has good thermodynamic stability.

[0200] According to the results of Example 2 and Examples 5-7, when the solvent components and the ratio are the same, compared with Example 2, when the combination of TAIC and LiODFP is used as the additive, the DCR of the secondary battery is significantly reduced, the high-temperature storage capacity retention rate is significantly improved, and the high-temperature cycle number is also significantly increased, indicating that the combination of TAIC and LiODFP has a synergistic effect on the thermodynamic stability of the secondary battery, which can improve the high-temperature storage capacity retention rate and the high-temperature cycle number of the secondary battery. According to the results of Examples 5-7, when the combination of TAIC and LiODFP is used as the additive, the content of TAIC is kept constant, and the content of LiODFP is increased from 0.01% to 0.5%, the DCR of the secondary battery is reduced, the high-temperature storage capacity retention rate is improved, and the high-temperature cycle number is increased, but when the content of LiODFP is increased from 0.5% to 1%, the DCR of the secondary battery is increased, the high-temperature storage capacity retention rate is reduced, and the high-temperature cycle number is reduced. Although LiODFP can participate in the film formation reaction in the battery and help to further assist the formation of a stable interface passivation layer, when its content exceeds a certain limit, it can cause the thickness of the SEI to grow excessively, resulting in an increase in the DCR of the battery, which in turn affects the high-temperature stability of the battery, manifested as a decrease in the high-temperature storage capacity retention rate and a decrease in the high-temperature cycle life. When the content of LiODFP is 0.01%-1%, the secondary battery exhibits good high-temperature stability, and when the content of LiODFP is 0.5%, the secondary battery has more excellent high-temperature stability.

[0201] According to the results of Example 6, Examples 8-10, and Comparative Examples 4-6, when the additive is the same, compared with the combination of DFEA, FEC, and D2 in a weight ratio of 4:1:3 used in the solvent of the secondary battery of Example 6, when the ratio of DFEA, FEC, and D2 in the solvent is adjusted in Examples 8-10 and Comparative Examples 4-5, the DCR of the secondary battery is significantly increased, the high-temperature storage capacity retention rate is significantly reduced, and the high-temperature cycle number is also significantly reduced, indicating that the ratio of DFEA, FEC, and D2 in the solvent of the present application has an important influence on the thermodynamic stability of the secondary battery. Compared with the combination of one or two components of DFEA, FEC, and D2, the solvent in the electrolyte of the present application needs to include at least three components of DFEA, FEC, and D2, and has a suitable ratio, in order to obtain better thermodynamic stability. In Comparative Example 6, EC and DMC are used to replace DFEA, FEC, and D2 as the solvent of the electrolyte, and the prepared secondary battery fails to be used, indicating that not all solvents are suitable for the electrolyte of the present application. In the electrolyte of the present application, the appropriate ratio of DFEA, FEC, and D2 is used as the solvent of the electrolyte, which interacts with other components (such as additives) in the electrolyte of the present application, achieving the excellent thermodynamic stability of the secondary battery of the present application.

[0202] Based on the results of Examples 6, Comparative Examples 1-3, and Comparative Examples 7-8, it is evident that when the solvent composition and proportions are the same, compared to the electrolyte of Example 6 which used a combination of 0.5% TAIC and 0.5% LiODFP as additives, in Comparative Examples 1-3, the DCR of the secondary battery was significantly increased, the high-temperature storage capacity retention was significantly reduced, and the number of high-temperature cycles was significantly decreased, respectively, when no additives were used, when other additives were used alone (0.5% VC), and when a combination of two other additives (0.5% VC and 0.5% DTD) was used. In Comparative Examples 7-8, replacing one of the TAIC and LiODFP combinations with other additives also resulted in a significant increase in DCR, a decrease in high-temperature storage capacity retention, and a reduction in the number of high-temperature cycles. This indicates that the additives of the present invention, using a suitable combination of TAIC and LiODFP in appropriate proportions, interact with other components (e.g., solvents) in the electrolyte of the present invention to achieve the excellent thermodynamic stability of the secondary battery of the present invention.

[0203] In summary, the electrolyte of this invention, comprising DFEA, FEC, and D2 as solvent components and TAIC and LiODFP as additives, achieves the thermodynamic stability of the secondary battery of this invention. Specifically, when the weight ratio of DFEA, FEC, and D2 is 4:1:3, and the additive content is 0.5% TAIC and 0.5% LiODFP based on the total weight of the electrolyte, the thermodynamic stability of the secondary battery of this invention is achieved, exhibiting characteristics such as low DCR, high high-temperature storage capacity retention, and high high-temperature cycle count.

[0204] The above description is merely a specific embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent transformations made using the present invention, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. An electrolyte for a lithium ion secondary battery, comprising a solvent, a lithium salt, and an additive, characterized in that, the solvent is a fluoro-solvent; the additive comprises triallyl isocyanurate and lithium difluorobis(oxalato) phosphate.

2. The electrolyte for a lithium ion secondary battery according to claim 1, characterized in that, the fluoro-solvent comprises at least two of a fluoro-carboxylate, a fluoro-carbonate, and a fluoro-ether.

3. The electrolyte for a lithium ion secondary battery according to claim 1, characterized in that, the fluoro-solvent comprises a fluoro-carboxylate, a fluoro-carbonate, and a fluoro-ether.

4. The electrolyte for a lithium ion secondary battery according to claim 2, characterized in that, the fluoro-carboxylate comprises one or more of 2,2-difluoroethyl acetate, 2-fluoro-1-ethanol acetate, 2,2,2-trifluoroethyl acetate, 2,2-difluoroethyl propionate, 2,2,2-trifluoroethyl propionate, 2,2,2-trifluoroethyl butyrate, 2,2-difluoroethyl butyrate; and / or the fluoro-carbonate comprises one or more of fluoroethylene carbonate, difluoroethylene carbonate, fluoropropylene carbonate, difluoropropylene carbonate, trifluoropropylene carbonate, methyltrifluoroethyl carbonate, ethyltrifluoroethyl carbonate, bis(2,2,2-trifluoroethyl) carbonate; and / or the fluoro-ether comprises one or more of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl methyl ether, 1,1,2,2-tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether.

5. The electrolyte for a lithium ion secondary battery according to claim 2, characterized in that, the fluoro-carboxylate comprises 2,2-difluoroethyl acetate; and / or the fluoro-carbonate comprises fluoroethylene carbonate; and / or the fluoro-ether comprises 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

6. The electrolyte for a lithium ion secondary battery according to claim 2, characterized in that, a weight ratio of the fluoro-carboxylate and the fluoro-carbonate is 6:1-2:1; and / or a weight ratio of the fluoro-ether and the fluoro-carbonate is 5:1-1:1; and / or a weight ratio of the fluoro-carboxylate, fluoro-ether, and fluoro-carbonate is 6-2:5-1:1.5-0.

5.

7. The electrolyte for a lithium ion secondary battery according to claim 1, characterized in that, the lithium salt comprises one or more of LiPF6, LiBF4, LiBCl4, LiAsF6, LiClO4, LiAlO2, LiF, LiCl, LiBr, LiI, LiSbF6, LiPO2F2, Li2PO3F, LiODFB, LiODFP, LiBOB, LiTFSI.

8. The electrolyte for a lithium ion secondary battery according to claim 1, characterized in that, a content of the lithium salt is 8-20% by weight of a total weight of the electrolyte.

9. The electrolyte for a lithium ion secondary battery according to claim 1, characterized in that, the content of the triallyl isocyanurate is 0.45 to 0.55% by weight based on the total weight of the electrolyte; and / or the content of the lithium difluorobis(oxalato)phosphate is 0.45 to 0.55% by weight based on the total weight of the electrolyte.

10. The electrolyte for a lithium ion secondary battery according to claim 1, characterized in that, the additive further comprises one or more of vinylene carbonate, 1,3-propane sultone, 1,3- propene sultone, 1,4-butane sultone, tris(trimethylsilyl) phosphate, trisallyl phosphate, tris(trimethylsilyl) borate, ethylene sulfate, and methylene methane disulfonate.

11. A lithium ion secondary battery comprising the electrolyte for a lithium ion secondary battery according to any one of claims 1 to 10.

12. The lithium ion secondary battery according to claim 11, characterized in that, the lithium ion secondary battery further comprises a positive electrode, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer comprising a positive electrode active material formed on the surface of the positive electrode current collector; The positive electrode active material comprises Li a Ni x Mn y O 4-z M z wherein 0.90≤a≤1.10, 0.4≤x≤0.6, 1.4≤y≤1.6, 0≤z≤0.1, and element M is one or more of Cl, Br, I, S, Se, Te, or F.

13. An electric power tool comprising the lithium ion secondary battery according to claim 11 or 12.

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