Electrolyte for lithium-ion secondary battery, and lithium-ion secondary battery comprising electrolyte

Abstract

An electrolyte for a lithium-ion secondary battery, comprising: a lithium salt, a solvent and an additive, wherein the solvent is a fluorinated solvent; and the additive comprises diethyl(thiophen-2-ylmethyl)phosphonate, and on the basis of the total weight of the electrolyte, the content of diethyl(thiophen-2-ylmethyl)phosphonate is 0.01-1 wt%. Also provided are a lithium-ion secondary battery comprising the electrolyte and an electric device. By reasonably matching components of an electrolyte, the electrolyte having excellent high-pressure stability and high-temperature stability can be prepared, thereby facilitating preparing a secondary battery having better electrochemical performance.

Description

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

[0001] The present application belongs to the field of lithium ion secondary batteries, and particularly relates to an electrolyte for a lithium ion secondary battery, a lithium ion secondary battery comprising the electrolyte, 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 dominated by 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 buyers, improving the energy density of the current battery, improving the voltage of the battery is a direct and feasible method. However, as the voltage of the battery increases, higher requirements are placed on the high-pressure resistance of the electrolyte.

[0003] Fluorinated solvents (such as fluorinated carbonates, fluorinated carboxylates, etc.) can be well used as main solvents in high-voltage system batteries due to the high electron-withdrawing ability of fluorine atoms, which can improve the oxidation stability of the solvent, significantly improve the high-pressure resistance of the lithium battery electrolyte, and thus serve as an important solvent component in high-voltage battery systems.

[0004] CN115411365A discloses a non-aqueous electrolyte comprising a carboxylic acid fluorinated alcohol ester compound. Due to the use of the carboxylic acid fluorinated alcohol ester compound, the ionic conductivity of the non-aqueous electrolyte is significantly improved and the viscosity is reduced. When the non-aqueous electrolyte is used in a battery, the energy density of the battery can be improved.

[0005] SUMMARY

[0006] At present, the lithium ion secondary battery assembled by using the electrolyte with fluorinated solvents still has high direct current impedance in a high-voltage environment, and the high-temperature cycle stability and high-temperature storage capacity retention rate are still not satisfactory.

[0007] In order to solve the above-mentioned deficiencies in the prior art, the present application provides an electrolyte for a lithium ion secondary battery, which has excellent high-voltage stability and high-temperature stability by reasonable collocation of its components.

[0008] In one aspect, the present application relates to an electrolyte for a lithium ion secondary battery, comprising: a lithium salt, a solvent and an additive; wherein the solvent is a fluorinated solvent, and the additive comprises: 2-(thiophenylmethyl) diethyl phosphonate; the content of 2-(thiophenylmethyl) diethyl phosphonate is 0.01-1% by weight based on the total weight of the electrolyte.

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

[0010] In one embodiment, the electrolyte of the present application further comprises: triallyl isocyanurate; the content of the triallyl isocyanurate is 0.01 to 1% by weight, based on the total weight of the electrolyte.

[0011] In another aspect, the present application also relates to 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 relates to an electric device comprising the lithium ion secondary battery of the present application. DETAILED DESCRIPTION

[0013] General Definitions and Terminology

[0014] All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety unless otherwise indicated.

[0015] 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 usages of terms in this specification and those of the prior art or in the prior description and definitions, the definitions provided herein control.

[0016] All percentages, parts, ratios, etc. are by weight unless otherwise indicated.

[0017] When a number, concentration or other value or parameter is given as a range, preferred range or a range of preferred ranges, it is intended to include every narrower range that falls within the broader range. Moreover, unless otherwise specified, where a range of values is provided, it is intended to include all values and / or subranges between the upper and lower limit of that range. The disclosure of a range includes each individual value, and / or sub-range within that range. The ranges are intended to encompass the particular value and / or sub-range, unless the context indicates otherwise. For example, a range of "1 to 8" is intended to encompass individual values of 1, 2, 3, 4, 5, 6, 7, and 8, as well as sub-ranges such as 2-6, 3-5, 2-4, 2-3, 4-5, 4-6, 5-6, 5-7, and 6-7, etc. All individual values and sub-ranges are included and disclosed as being within the scope of the disclosure.

[0018] 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 the terms "comprising" and "including," when used in the following description and claims, are intended to be construed in a non-limiting sense. The term "consisting of" excludes any element, step, or ingredient not specified. The term "consisting essentially of" means a composition that has the specified elements, steps, or ingredients, in addition to those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter. It will be understood that the term "comprising" encompasses the terms "consisting of" and "consisting essentially of."

[0019] 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.

[0020] When describing a numerical value or range end point herein, it is understood that the disclosure includes the recited particular value or end point.

[0021] The terms "one or more" or "at least one," as used herein, mean one, two, three, four, five, six, seven, eight, nine, or more.

[0022] The terms "combination" and "in combination," as used herein, mean a multi-component mixture of the recited elements, e.g., two, three, four, and up to the maximum possible multi-component mixture, unless otherwise indicated.

[0023] In addition, where a component or composition is not specifically marked as being "a" or "at least one," it is understood that the number of components or compositions appearing (or present) is not limited. Thus, it is to be interpreted as including one or at least one, and the singular form of the component or composition also includes the plural, unless the number is expressly indicated as singular.

[0024] 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 from the cathode to the anode during charging.

[0025] The term "anode" refers to an electrode in a secondary battery in which oxidation occurs during discharge and reduction occurs during charging.

[0026] The term "cathode" refers to an electrode in a secondary battery in which reduction occurs during discharge and oxidation occurs during charging.

[0027] The term "DCR" refers to Direct Current Resistance, also known as battery internal resistance. In lithium batteries, DCR refers to the internal resistance generated by the battery during discharging. It affects the discharge efficiency of the battery and the power output capability of the battery, can characterize the kinetic performance of the battery, and is one of the important indicators for the long cycle of the battery.

[0028] The term "SEI" or "SEI film" refers to a solid electrolyte interface 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 can effectively protect the contact interface between the negative electrode and the electrolyte, reduce the side reaction of the electrolyte, and ensure the stable cycle of the battery.

[0029] 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 reaction of the electrolyte, and ensure the stable cycle of the battery.

[0030] The term "high voltage" or "high voltage" in the present application can be a voltage of 4.5V or more.

[0031] The term "formation" refers to a process of charging and discharging the battery at a small current after the battery is prepared in the battery preparation process. The formation treatment is beneficial to stabilize the electrical performance of the battery.

[0032] 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), and the like.

[0033] 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.

[0034] Electrolyte

[0035] In one aspect, the present application relates to an electrolyte for a lithium ion secondary battery, comprising: a lithium salt, a solvent, and an additive.

[0036] The electrolyte plays an important role in the battery, can transmit ions, maintain the stable performance of the battery, and also help dissipate heat to ensure the safety performance of the battery. Therefore, the quality and performance of the battery electrolyte have important influence on the working efficiency and life of the battery.

[0037] Solvent

[0038] The solvent in the electrolyte has the definition commonly understood in the art. Herein, "solvent" refers to a liquid used to dissociate lithium salts and as a lithium ion transport medium.

[0039] Herein, the fluorinated solvent refers to a non-aqueous organic solvent having fluorine atoms substituted. Due to the high electron-withdrawing ability of fluorine atoms, the oxidation stability of the fluorinated solvent can be improved. The use of fluorinated solvents in electrolytes can improve the high-pressure resistance of the electrolyte, which is beneficial for the preparation of high-voltage secondary batteries.

[0040] In one embodiment, the solvent of the electrolyte of the present application is a fluorinated solvent, i.e., the solvent system of the electrolyte of the present application consists of fluorinated solvents. The use of fluorinated solvents with oxidation stability due to the strong electron-withdrawing effect of fluorine atoms ensures the oxidation stability of the secondary battery assembled from the electrolyte at high voltage.

[0041] In one embodiment, the fluorinated solvent comprises two or three of fluorinated carboxylic acid ester, fluorinated carbonate and fluorinated ether. In a preferred embodiment, the fluorinated solvent comprises fluorinated carboxylic acid ester, fluorinated carbonate and fluorinated ether.

[0042] The fluorinated carboxylic acid ester useful in the present application includes, but is not limited to, 2,2-difluoroethyl acetate (DFEA, CAS No. 1550-44-3), 2-fluoro-1-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), or any combination thereof. Preferably, the fluorinated carboxylic acid ester of the present application can comprise 2,2-difluoroethyl acetate, and optionally comprise one or more of the above fluorinated carboxylic acid esters. In a preferred embodiment, the fluorinated carboxylic acid ester of the present application is 2,2-difluoroethyl acetate.

[0043] Fluoro carbonates useful in the present application include, but are not limited to, 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), or any combination thereof. Preferably, the fluoro carbonates of the present application can comprise fluoroethylene carbonate, and optionally one or more of the above-mentioned fluoro carbonates. In a preferred embodiment, the fluoro carbonates of the present application are fluoroethylene carbonate.

[0044] The fluoro carboxylates and fluoro carbonates of the present application are advantageous in forming a thin and uniform stable interfacial phase, which can preferentially decompose to form a stable interfacial film during charging. The fluoro carboxylates, fluoro carbonates of the present application can form a thin and uniform stable interfacial film on the surface of the negative electrode, thereby inhibiting decomposition of the electrolyte. Also, the fluoro carbonates of the present application can help to improve the oxidative stability of the electrolyte while forming a thin and uniform SEI film, and improve the high voltage resistance performance of the battery.

[0045] Fluoro ethers useful in the present application include, but are not limited to, 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), or any combination thereof. Preferably, the fluoro carbonates of the present application can comprise 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and optionally one or more of the above-mentioned fluoro ethers. In a preferred embodiment, the fluoro ethers of the present application are 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0046] The fluorinated ether of the present application can play the role of diluent in the fluorinated solvent system. The suitable fluorinated ether improves the compatibility of the electrolyte with the electrode material, maintains the good high-voltage resistance of the electrolyte, reduces the overall macroscopic concentration of the electrolyte, and also helps to form a stable SEI film, which is beneficial to improve the high and low temperature cycle performance, storage performance and gas production inhibition of the secondary battery assembled by the electrolyte.

[0047] The solvent system formed by the suitable proportion of various fluorinated solvents can better adapt to other components (such as additives, lithium salt) of the electrolyte, which is helpful to further improve the high-voltage stability and high-temperature stability of the electrolyte. Moreover, the electrolyte of the present application can maximize the optimization of the cycle stability of the battery under the condition of maintaining the high-temperature stability of the battery.

[0048] In one embodiment, the weight ratio of fluorinated carboxylic acid ester to fluorinated carbonate is 6-2, preferably 5-3, more preferably 4, such as 6, 5, 4, 3, 2, etc. When the proportion of fluorinated carboxylic acid ester increases and the proportion of fluorinated carbonate decreases, it may cause the increase of DCR of the battery, and the possible reason is that the decomposition of fluorinated carboxylic acid ester (such as DFEA) at the interface produces high-temperature unstable by-products, and the unstable SEI may eventually lead to the decline of the high-temperature performance of the battery. Too high proportion of fluorinated carbonate may increase the viscosity of the electrolyte, reduce the charge transfer efficiency in the battery, and too low content of fluorinated carbonate is not conducive to the conductivity of the battery. In another embodiment, the weight ratio of fluorinated ether to fluorinated carbonate is 5-1, preferably 4-2, more preferably 3, such as 5, 4, 3, 2, 1, etc. The suitable proportion of fluorinated ether can maintain the good high-voltage resistance of the electrolyte and reduce the overall macroscopic concentration of the electrolyte.

[0049] In yet another embodiment, the weight ratio of fluorinated carboxylic acid ester to fluorinated carbonate is 6-2, preferably 5-3, more preferably 4, such as 6, 5, 4, 3, 2, etc., and the weight ratio of fluorinated ether to fluorinated carbonate is 5-1, preferably 4-2, more preferably 3, such as 5, 4, 3, 2, 1, etc.

[0050] In one embodiment, the weight ratio of fluoro-carboxylic acid ester, fluoro-carbonate and fluoro-ether is 6-2: 1.5-0.5: 5-1, preferably 4: 1: 3. The fluoro-solvent, which is a combination of fluoro-carboxylic acid ester (e.g. DFEA), fluoro-carbonate (e.g. FEC) and fluoro-ether (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether) in the preferred weight ratio, can not only impart suitable viscosity and ionic conductivity to the electrolyte, but also ensure the high temperature and high pressure resistance of the electrolyte. The battery assembled by the electrolyte can optimize the cycle stability of the battery to the greatest extent while maintaining its high temperature stability, for example: the battery has lower initial DC resistance, higher high temperature cycle number, higher high temperature storage capacity retention rate, etc.

[0051] In one embodiment, the content of the fluoro-solvent is 75-92% by weight based on the total weight of the electrolyte. The appropriate content of the fluoro-solvent helps to fully exert the function of the electrolyte and maintain the performance of the battery.

[0052] The fluoro-solvent of the specific composition of the present application has a small viscosity and a high ionic conductivity, which is conducive to the bulk diffusion migration of lithium ions. In addition, the preferred fluoro-solvent of the present application generally has a higher solvation energy, making the desolvation process of lithium ions more likely to occur, thereby facilitating the large rate charge and discharge of the battery at high voltage.

[0053] The inventors of the present application found that the battery assembled by the electrolyte using the fluoro-solvent still showed an undesirable decrease in performance at high temperature, which may be caused by the fluoro-solvent. Without being bound by any theory, the possible reason for the fluoro-solvent causing the performance degradation of the battery at high temperature is that the fluorine atom in the fluoro-solvent is prone to break at high temperature, and the reaction between the fluorine atom and the active hydrogen in the electrolyte generates hydrofluoric acid, which may corrode the basic substances in the SEI and CEI, may cause the interface to be damaged and the side reactions to continue to occur, and eventually may exhibit a series of problems that cause the performance of the battery to degrade, such as high-temperature gas production, high-temperature capacity loss and DCR increase. In the present application, it is found that the fluoro-solvent of the specific composition of the present application, especially the preferred fluoro-solvent, can significantly improve the high temperature stability of the electrolyte by using the additive (e.g. diethyl 2-(thiophenyl) phosphonate, triallyl isocyanurate) of the present application.

[0054] Additive

[0055] The electrolyte of the present application can contain an additive, such as a thiophene additive, which can better participate in the formation of a stable SEI, greatly improving the stability of the electrolyte in high-temperature cycling, thereby comprehensively improving the high-temperature performance of the battery. The thiophene additive useful in the present application can be diethyl 2-(thiophenylmethyl) phosphonate (also referred to herein as DTYP), the thiophene moiety in the molecular structure of which can preferentially undergo oxidation on the positive electrode side, thereby forming a stable CEI, protecting the electrolyte on the positive electrode side from further oxidation side reactions. When the electrolyte contains a lithium salt that can produce PF5 ions (such as LiPF6), the phosphonate structure contained in diethyl 2-(thiophenylmethyl) phosphonate can also capture free PF5 ions in the electrolyte, thereby inhibiting the increase in electrolyte acidity, reducing the problems of gas production and DCR growth at high temperature of the battery, etc.

[0056] A suitable content of diethyl 2-(thiophenylmethyl) phosphonate in the electrolyte helps to improve the high-temperature resistance of the secondary battery assembled from the electrolyte. The content of diethyl 2-(thiophenylmethyl) phosphonate can be 0.01-1% by weight, preferably 0.2-0.4% by weight, more preferably 0.3% by weight, based on the total weight of the electrolyte, such as 0.01% by weight, 0.05% by weight, 0.1% by weight, 0.2% by weight, 0.3% by weight, 0.4% by weight, 0.5% by weight, 0.6% by weight, 0.7% by weight, 0.8% by weight, 0.9% by weight, 1% by weight, etc. Too high a content of diethyl 2-(thiophenylmethyl) phosphonate can cause degradation of the high-temperature performance of the battery.

[0057] The additive in the electrolyte of the present application can also contain a nitrogen-containing heterocyclic additive, which helps to further assist in the formation of a stable interface passivation layer, and is beneficial to improving the stability of the electrolyte. The nitrogen-containing heterocyclic additive useful in the present application can be triallyl isocyanurate (also referred to herein as TAIC). The structure of triallyl isocyanurate has unsaturated double bonds, which tend to preferentially undergo oxidation on the positive electrode side, and the CEI formed by its decomposition is not easily swelled by fluorocarbonate due to its low porosity.

[0058] However, the applicants found that the thermal stability of the six-membered cyclic amide in triallyl isocyanurate is poor, which can lead to the SEI being prone to thermal dissolution and partial structure rupture at high temperatures, thus leading to continuous side reactions at the interface at high temperatures, resulting in a comprehensive decline in the high-temperature performance of the battery. The present inventors unexpectedly found that by using a combination of triallyl isocyanurate and diethyl 2-(thiophenyl) phosphonate, and by using specific contents and ratios of the two, and by using the combination in the fluorinated solvent of the present application, the adverse effects can be reduced or avoided, thereby effectively avoiding the decline in the high-temperature performance of the battery, so that the battery has both high-voltage stability and high-temperature stability. In one embodiment, the content of triallyl isocyanurate is 0.01-1 wt%, preferably 0.4-0.6 wt%, more preferably 0.5 wt%, for example 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, etc., based on the total weight of the electrolyte. In one embodiment, the weight ratio of diethyl 2-(thiophenyl) phosphonate and the triallyl isocyanurate is 100:1-1:100, preferably 50:1-1:10, more preferably 1:1-1:10, most preferably 3:5, for example 100:1, 50:1, 10:1, 5:1, 2:1, 1:1, 9:10, 4:5, 7:10, 3:5, 1:2, 2:5, 3:10, 1:5, 1:10, etc. Too much triallyl isocyanurate can lead to a decline in the high-temperature performance of the battery due to the poor thermal stability of the six-membered cyclic amide thereof, and too much addition can lead to excessive growth of the thickness of the SEI, which can lead to an increase in the DCR of the battery, a loss of high-temperature capacity, and a reduction in cycle life.

[0059] Other types of additives can also be included in the electrolyte of the present application to alter the desired properties. In one embodiment, the additive of the present application also includes an additive for improving the film-forming effect on the negative electrode, which helps to improve the high-temperature performance of the battery and improve the cycle stability of the battery. In one embodiment, such an additive can include one or more of vinylene carbonate, 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, tris(trimethylsilyl) phosphate, triallyl phosphate, tris(trimethylsilyl) borate, ethylene sulfate, and methanediyl dimethanesulfonate. In a preferred embodiment, such an additive can be vinylene carbonate.

[0060] Lithium salt

[0061] A lithium salt refers to a salt having lithium as a cation. During the operation of a battery, lithium ions of an electrolyte lithium salt function to transport ions between the positive and negative electrodes of the battery, and are important for the performance of the battery. In addition, the type, purity, and 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.

[0062] The lithium salt useful herein includes, but is not limited to, one or more of LiPF6, LiBF4, LiBCl4, LiAsF6, LiClO4, LiAlO2, LiF, LiCl, LiBr, LiI, LiSbF6, LiPO2F2, Li2PO3F, LiODFB, LiODFP, LiBOB, LiTFSI.

[0063] In one embodiment, the content of the lithium salt is 8-20% by weight, for example, 8% by weight, 9% by weight, 10% by weight, 11% by weight, 12% by weight, 13% by weight, 14% by weight, 15% by weight, 16% by weight, 17% by weight, 18% by weight, 19% by weight, 20% by weight, etc., based on the total weight of the electrolyte. Too much content of the lithium salt can increase the viscosity of the electrolyte, reduce the efficiency of charge movement in the battery containing the electrolyte, and thus reduce the efficiency of the battery.

[0064] Lithium ion secondary battery

[0065] In another aspect, the present application provides a lithium ion secondary battery comprising the electrolyte for lithium ion secondary batteries of the present application.

[0066] The lithium ion secondary battery of the present application can comprise: the electrolyte for lithium ion secondary batteries of the present application, a positive electrode, and a negative electrode. The positive electrode and the negative electrode can be stacked into an integrated body through a separator (if necessary), and then added to the electrolyte.

[0067] In the present application, the structure and shape of the secondary battery are not limited in any way. Specifically, for example, the structure of the secondary battery can be a wound or stacked electrode plate group packaged in a housing. The wound electrode plate group can be obtained by winding the whole structure of the positive electrode plate, the separator, and the negative electrode plate into a flat spiral shape. The stacked electrode plate group can be obtained by stacking the positive electrode plate, the separator, and the negative electrode plate. For example, the shape of the lithium ion secondary battery can be listed as, for example, a square type, a cylindrical type, a stacked type, a coin type, a button type, a paper type, and the like.

[0068] The lithium ion secondary battery of the present application can be prepared in a conventional manner. As an example, the positive electrode, the separator, the negative electrode, and the electrolyte can be assembled in order, alternately combined with the positive electrode, the separator, the negative electrode, and the separator, and then subjected to formation to obtain a soft-pack battery.

[0069] In one embodiment, the lithium ion secondary battery of the present application can be, for example, a lithium nickel manganese oxide 5V high voltage system battery.

[0070] Negative electrode

[0071] 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. The negative electrode refers to an electrode from which electrons flow out from the external circuit during discharge, and which has a lower potential.

[0072] The material constituting the negative electrode current collector is not particularly limited in the present application, and for example, a metal can be used. Specifically, the metal optionally used herein includes, but is not limited to, aluminum, nickel, iron, stainless steel, titanium, copper, or a combination thereof, and the like. In one specific embodiment, the negative electrode current collector is a copper foil. The size of the negative electrode current collector can be adjusted according to the actual use of the battery.

[0073] The negative electrode active material layer is formed on the surface of the negative electrode current collector, and the shape and area thereof are not particularly limited. The negative electrode active material layer can cover one side or both sides of the surface of the negative electrode current collector.

[0074] The negative electrode active material layer can include a negative electrode active material. The negative electrode active material is generally a material capable of intercalating or deintercalating lithium ions. The negative electrode active material that can be used herein can be a carbon material, such as graphite. The graphite described herein can include one or more of natural graphite, artificial graphite, modified graphite, silicon-oxygen composite artificial graphite.

[0075] If necessary, the negative electrode active material layer can further include a negative electrode additive (e.g., a conductive agent, a thickening agent, a binder, and the like) to impart suitable properties to the negative electrode.

[0076] Positive electrode

[0077] The positive electrode refers to an electrode into which electrons flow from the external circuit during discharge, and which has a higher potential. The positive electrode is not particularly limited herein. As an example, the positive electrode can include: a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector.

[0078] The material constituting the positive electrode current collector is not particularly limited in the present application. The positive electrode current collector used herein can be a strip-shaped material made of a foil, an open-foam foil, a mesh, and the like, processed from a metal such as aluminum, titanium, stainless steel, and the like, or an alloy thereof.

[0079] 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 cover one side or both sides of the surface of the positive electrode current collector.

[0080] The positive electrode active material layer contains a positive electrode active material. The positive electrode active material is generally a material capable of reversibly intercalating and deintercalating lithium ions. In the present application, the positive electrode active material contains a mixture of a carbon material and one or more lithium complex metal oxides. The lithium complex metal oxide herein refers to a complex metal oxide composed of at least one of cobalt, manganese and nickel and lithium.

[0081] In a preferred embodiment, the carbon material of the positive electrode active material can be obtained by pre-sintering a carbon source under a nitrogen atmosphere to obtain the carbon material in the positive electrode active material, wherein the carbon source is one or more of fructose, polyethylene glycol, galactose, polyvinylpyrrolidone or tannic acid.

[0082] In a preferred embodiment, the lithium complex metal oxide of the positive electrode active material can have a chemical formula of 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.

[0083] The carbon material and the lithium complex metal oxide are mixed and subjected to secondary sintering to obtain the positive electrode active material. The carbon material can coat part or all of the surface of the lithium complex metal oxide. In a preferred embodiment, the weight of the carbon material in the positive electrode active material is 1.0-3.0% of the weight of the metal oxide.

[0084] Separator

[0085] The separator has the function of holding the electrolyte to ensure ionic conductivity between the positive electrode and the negative electrode and as a partition wall between the positive electrode and the negative electrode. The separator can be a porous sheet separator formed of a polymer and / or fibers, a non-woven fabric separator, etc. It should be understood that the separator is not an essential element in the present application, for example, in the structure of a lithium ion secondary battery in which the positive electrode sheet and the negative electrode sheet are not required to be in direct contact.

[0086] The separator of the porous sheet formed of a polymer and / or a fiber can use a microporous substance (microporous film). Specifically, it can be a microporous substance (microporous film) separator formed of polyethylene (PE), polypropylene (PP), and the like polyolefin; a laminate obtained by laminating multiple layers of the above polyolefin (for example, a laminate having a 3-layer structure of PP / PE / PP, and the like), polyimide, aramid, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and the like hydrocarbon-based resin, glass fiber, and the like. The thickness of the separator of the porous sheet is not particularly limited. The nonwoven fabric separator can use, alone or in combination, cotton, rayon, acetate, nylon, polyester, polyolefin (such as PP, PE, and the like), polyimide, aramid, and the like known substances. In one preferred embodiment, the separator is a polypropylene film having a thickness of 12 μm.

[0087] Performance of lithium ion secondary battery

[0088] The power performance of the lithium ion secondary battery can be evaluated using DCR. The smaller the DCR value of the battery, the better the power performance of the battery. The direct current resistance (DCR) of the lithium ion secondary battery can be tested in the following manner: at a specified temperature, discharge the battery at 1C current to 50% SOC (State of Charge, reflecting the remaining capacity of the battery), increase the current to 4C, and maintain for 30 seconds, detect the difference between the updated stable voltage and the original platform voltage, and the ratio of the value to the 4C current value is the direct current resistance of the battery. The DCR test result of the battery after the first full charge is the initial DCR of the battery. In one embodiment, the direct current resistance (DCR) of the lithium ion secondary battery of the present application at 25°C is 200 mΩ or less, preferably 160 mΩ or less, and more preferably 100 mΩ or less.

[0089] The 60°C storage capacity retention rate of the lithium ion secondary battery can be tested in the following manner: after the lithium ion 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, and the percentage of its capacity relative to the initial discharge capacity is compared. The 60°C storage capacity retention rate of the lithium ion secondary battery of the present application is 60% or more, preferably 80% or more, and more preferably 90% or more.

[0090] The high temperature cycle stability of the lithium ion secondary battery can be tested in the following manner: the battery is cycled in a specified high temperature environment, with the charge / discharge cut-off voltage range being 3.4-4.85V, and the charge / discharge rate being 1C / 1C, and the discharge capacity of each cycle is recorded. The test is ended when the battery capacity is reduced to 80% SOC (State of Charge, reflecting the remaining capacity of the battery), and the actual number of cycles reached is recorded. For lithium nickel manganese oxide / graphite batteries, the charge / discharge cut-off voltage is 3.4-4.85V.

[0091] Use

[0092] In another aspect, the present application also relates to the use of the electrolyte of the present application for preparing a lithium ion secondary battery. The electrolyte of the present application has excellent high-voltage stability and high-temperature stability, which is beneficial to improve the use voltage of the battery, for example, can withstand a voltage of 4.5V, 4.8V or even 5V. Therefore, the electrolyte of the present application is particularly suitable for high-voltage lithium ion secondary batteries.

[0093] Electric device

[0094] In yet another aspect, the present application also relates to an electric device comprising the lithium ion secondary battery of the present application. The electric device includes, but is not limited to, one or more of an electric vehicle, an electric two-wheeled vehicle, an electric power storage system. The electric vehicle includes, but is not limited to, one or more of a plug-in hybrid electric vehicle, a hybrid electric vehicle. Advantages

[0095] In the present application, an electrolyte with high-voltage stability and high-temperature stability is obtained by reasonable design of each component of the electrolyte for lithium ion secondary batteries. In the electrolyte using a specific fluorinated solvent system, a specific additive system is used, which can form a stable protective film at the interface between the electrode and the electrolyte. Not only can better power performance be obtained, but also the stability of the electrode and electrolyte interface at high temperature can be ensured, thereby significantly improving the high-temperature stability of the high-voltage battery. The secondary battery assembled by the electrolyte of the present application has the advantages of low initial direct current resistance, high high-temperature cycle number and high high-temperature storage capacity retention rate.

[0096] Examples

[0097] The scheme of the present application will be further described in detail below in combination with specific examples.

[0098] It should be noted that the following examples are only examples for clearly illustrating the technical scheme of the present application, and are not a limitation on the present application. Those skilled in the art can make other different forms of changes or variations on the basis of the above description, which are not required or impossible to exhaust all the embodiments, and the obvious changes or variations derived therefrom are still within the protection scope of the present application. 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.

[0099] Preparation

[0100] 1. Preparation of positive electrode sheet

[0101] Preparation of lithium nickel-manganese oxide positive electrode:

[0102] The carbon source is one or more of fructose, polyethylene glycol, galactose, polyvinylpyrrolidone or tannic acid, and the carbon source is pre-sintered under a nitrogen atmosphere to obtain component B; and the component B is mixed with component A and subjected to secondary sintering to obtain the positive electrode material.

[0103] The chemical formula of component A is Li 0.98 Ni 0.45 Mn 1.55 O4; component B is in the form of a film coated on at least part of the surface of component A, the components of component B include carbon, and the weight of component B is 1.0wt%-3.0wt% of the mass of component A.

[0104] The positive electrode active 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 is 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 (polar piece).

[0105] 2. Preparation of a negative electrode polar piece

[0106] Artificial graphite is used as a negative electrode active material, Super P is used as a conductive agent, carboxymethyl cellulose sodium (CMC-Na) is used as a thickening agent, and styrene butadiene rubber (SBR) is used as a binder, and the mixture is mixed in a mass 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 (polar piece).

[0107] 3. Preparation of an electrolyte

[0108] In an argon atmosphere glove box with a water content of <10 ppm, the solvent components are mixed in a certain proportion to form a solvent system, wherein the weight ratio and type of each solvent component are shown in Table 1 below. A sufficient amount of LiPF6 is added to the solvent system, wherein the content of LiPF6 is 13wt% based on the total weight of the electrolyte. Other components are added to the solvent system in the following Table 1, and the mixture is uniformly mixed to obtain the electrolyte samples of the examples and comparative examples of the present application. In Table 1 below, the amount of each solvent component in the solvent is in parts by weight. The content of other components other than the solvent is the weight percentage calculated based on the total weight of the electrolyte.

[0109] 4. Preparation of a separator

[0110] The separator is a polypropylene film (PP) with a thickness of 12μm.

[0111] 5. Preparation of lithium ion secondary battery

[0112] The above prepared positive electrode, separator and negative electrode were laminated in sequence, with the separator between the positive and negative electrodes to play a role of isolation. Then, the battery was wrapped with an aluminum plastic film, dried in a vacuum oven at 120°C, injected with 3.0 g / Ah of the electrolyte of the examples and comparative examples of the present application, sealed, and subjected to electrolyte formation to finally prepare a soft-pack battery (i.e., lithium ion battery) having a capacity of 1 Ah.

[0113] Table 1 *: DFEA is 2,2-difluoroethyl acetate; FEC is fluoroethylene carbonate; D2 is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether; DMC is dimethyl carbonate; EC is ethylene carbonate **: DTYP is diethyl 2-(thiophenylmethyl)phosphonate; TAIC is triallyl isocyanurate (TAIC); VC is vinylene carbonate; DTD is vinylene sulfate

[0114] Test

[0115] The lithium ion secondary battery of the present application can be tested by the following method, and the test results are shown in Table 2.

[0116] (1) 25°C direct current resistance (DCR) test of lithium ion battery

[0117] At a temperature of 25°C, when the battery is discharged at a current of 1C to 50% SOC (State of Charge), 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 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) Capacity retention rate of lithium ion battery at high temperature storage (60°C)

[0119] After the lithium ion battery is fully charged, it is stored 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, and the percentage of its capacity relative to the initial discharge capacity is compared.

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

[0121] The battery was cycled at 45°C with a charge / discharge voltage range of 3.4-4.85V, a charge / discharge rate of 1C / 1C, and the discharge capacity of each cycle was recorded. The test was ended when the battery capacity remained at 80% SOC (State of Charge, reflecting the remaining capacity of the battery), and the actual cycle number reached was recorded. For the lithium nickel manganese oxide / graphite battery, the charge / discharge cut-off voltage was 3.4-4.85V.

[0122] Table 2

[0123] Table 2 shows the results of the cycle test, direct current resistance (DCR) test, and high-temperature storage capacity retention rate of the corresponding secondary battery samples obtained by assembling the electrolyte samples of Examples 1-10 and Comparative Examples 1-8.

[0124] From Examples 1-3, it can be seen that when the content of the DTYP additive is 0.3% by weight, the overall performance of the battery is best. When the content of DTYP increases from 0.01% by weight to 0.3% by weight, the high-temperature storage capacity recovery rate of the battery increases significantly. This indicates that the DTYP additive can maintain good thermodynamic stability under high-temperature conditions. According to Example 4, when the amount of DTYP is more than 1% by weight, the high-temperature cycle performance of the battery deteriorates to some extent.

[0125] From Examples 5-7, it can be seen that the combination of DTYP and TAIC can improve the high-temperature storage capacity retention rate and high-temperature cycle number of the corresponding battery for the fluorinated solvent system composed of DFEA, FEC, and D2.

[0126] From Examples 8-10, it can be seen that when the additive contains a combination of DTYP and TAIC, changes in the composition of the fluorinated solvent can cause significant changes in the performance of the battery. By comparing Examples 5-7 with Examples 8-10, it can be seen that the performance of the corresponding battery prepared using an electrolyte with a DFEA-FEC-D2 ternary solvent system is better than that of a binary solvent system.

[0127] From Comparative Examples 1-3, it can be seen that when no additive is used or only one other additive is used, the performance of the battery is not good, and when the conventional additives VC and DTD are used together, the lithium nickel manganese oxide high-voltage system has a large DCR, cannot be cycled at room temperature, and the capacity cannot be developed after high-temperature storage. This further confirms that the combination of DTYP and TAIC can ensure the normal operation of the high-voltage system.

[0128] As can be seen from Comparative Examples 4-5, when only DFEA or D2 is used as the electrolyte solvent component, the battery exhibits a very high initial impedance. Due to the excessively low ionic conductivity and lower lithium salt solubility of DFEA and D2, they cannot be used as a single solvent for electrolyte configuration.

[0129] As can be seen from Comparative Examples 6-7, when DTYP or TAIC is used in combination with other conventional additives, the battery performance is poor.

[0130] As can be seen from Comparative Example 8, when a conventional carbonate is used as the solvent, the battery cannot be normally cycled due to the inability to withstand the excessively high voltage of the system.

[0131] The above only describes specific embodiments of the present application and does not limit the patent scope of the present application, and any equivalent transformation or direct or indirect application in other related technical fields is also included in the patent protection scope of the present application.

Claims

1. An electrolyte for a lithium ion secondary battery, comprising: a lithium salt, a solvent, and an additive; characterized in that, the solvent is a fluoro-solvent; the additive comprises diethyl 2-(thiophenyl) phosphonate, and the content of the diethyl 2-(thiophenyl) phosphonate is 0.01 to 1% by weight based on the total weight of the electrolyte.

2. The electrolyte for a lithium ion secondary battery according to claim 1, characterized in that, the content of the diethyl 2-(thiophenyl) phosphonate is 0.2 to 0.4% by weight based on the total weight of the electrolyte.

3. 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-carboxylic acid ester, a fluoro-carbonic acid ester, and a fluoro-ether.

4. The electrolyte for a lithium ion secondary battery according to claim 1, characterized in that, the fluoro-solvent comprises a fluoro-carboxylic acid ester, a fluoro-carbonic acid ester, and a fluoro-ether.

5. The electrolyte for a lithium ion secondary battery according to claim 3, characterized in that, the fluoro-carboxylic acid ester 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-carbonic acid ester comprises one or more of fluoro-ethylene carbonate, bis-fluoro-ethylene carbonate, fluoro-propylene carbonate, bis-fluoro-propylene carbonate, tri-fluoro-propylene carbonate, methyl-trifluoroethyl carbonate, ethyl-trifluoroethyl carbonate, di(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.

6. The electrolyte for a lithium ion secondary battery according to claim 3, characterized in that, the fluoro-carboxylic acid ester comprises 2,2-difluoroethyl acetate; and / or the fluoro-carbonic acid ester comprises fluoro-ethylene carbonate; and / or the fluoro-ether comprises 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

7. The electrolyte for a lithium ion secondary battery according to claim 3, characterized in that, the weight ratio of the fluoro-carboxylic acid ester to the fluoro-carbonic acid ester is 6:1 to 2:1; and / or the weight ratio of the fluoro-ether to the fluoro-carbonic acid ester is 5:1 to 1:1, and / or; the weight ratio of the fluoro-carboxylic acid ester, the fluoro-carbonic acid ester, and the fluoro-ether is 6 to 2:1.5 to 0.5:5 to 1.

8. The electrolyte for a lithium ion secondary battery according to any one of claims 1 to 7, characterized in that, the additive further comprises triallyl isocyanurate, the content of the triallyl isocyanurate is 0.01 to 1% by weight based on the total weight of the electrolyte.

9. 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, triallyl phosphate, tris(trimethylsilyl) borate, ethylene sulfate, and methylene methane disulfonate.

10. 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, and LiTFSI.

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

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

13. The lithium ion secondary battery according to claim 12, 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.

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

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