Electrolyte and secondary battery

Abstract

The application provides an electrolyte and a secondary battery, and belongs to the technical field of secondary batteries. The electrolyte comprises a lithium salt, a solvent, a first additive and a second additive. The first additive comprises a compound shown in formula I, and the second additive comprises methanedimethanesulfonate. The electrolyte provided by the application can adjust the acid-base degree of the electrolyte by using the tertiary amino group in the compound shown in formula I, so that the chemical environment of the electrolyte is more stable. In addition, the electrolyte can initiate a copolymerization film forming reaction between the MMDS free radical fragments and the compound shown in formula I on the surface of the positive electrode by using the characteristics of the methylene and methylenediamine sulfonic acid free radicals formed by the breaking of MMDS under high voltage, so that a polymer film with high mechanical strength and chemical stability is formed on the surface of the positive electrode, and the side reaction gas generation between the positive electrode and the electrolyte is effectively reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to an electrolyte and a secondary battery. BACKGROUND

[0002] Lithium ion batteries are widely used in consumer electronics, electric vehicles and energy storage fields due to their long cycle life, high operating voltage, low self-discharge rate, no memory effect and environmental friendliness.

[0003] Currently, the upper limit of the operating voltage of the positive electrode material (such as ternary material or lithium iron phosphate material) of high-capacity lithium ion batteries is gradually increasing and approaching the material limit after development. The electronic impedance of the positive electrode material increases and the polarization increases at high voltage (> 3.80V), and the lattice collapse caused by uneven delithiation easily occurs during the charging and discharging process, which causes the side reaction of the positive electrode and the electrolyte and the increase of gas production, thereby deteriorating the storage performance and cycle life of the battery.

[0004] Therefore, it is necessary to design an electrolyte and a secondary battery to solve the above technical problems. SUMMARY

[0005] In view of the above shortcomings of the prior art, the present application provides an electrolyte and a secondary battery to improve the technical problem that the existing electrolyte is prone to side reaction with the high-potential positive electrode at high temperature, thereby deteriorating the gas production of the battery.

[0006] To achieve the above object and other related objects, the present application provides an electrolyte, which comprises a lithium salt, a solvent, a first additive and a second additive; wherein the first additive comprises a compound represented by Formula I, and the second additive comprises methanedisulfonate.

[0007]

[0008] R1, R2, R3, R4 in Formula I are each independently selected from a substituent group having 1-6 carbon atoms, 0-4 unsaturation and 0-3 heteroatoms, and the heteroatoms are selected from any one of nitrogen atom, sulfur atom, oxygen atom, boron atom and phosphorus atom.

[0009] In an example of the present application, the mass content of the first additive in the electrolyte is 0.05%-3%, and the mass content of the second additive is 0.1%-0.8%.

[0010] In an example of the present application, the mass content of the first additive in the electrolyte is 0.1%-0.5%.

[0011] In an example of the present application, R1, R2, R3, R4 in the formula I are each independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino or heterocycle, and the heterocycle is selected from any one of pyridine, pyrrole, thiophene, thiazole, furan.

[0012] In an example of the present application, the compound of formula I includes at least one of the following compounds:

[0013]

[0014] In an example of the present application, the electrolyte further includes a fourth additive selected from at least one of fluoroethylene carbonate, propenyl-1,3-sultone, vinylidene carbonate and tetra-vinylsilane.

[0015] In an example of the present application, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methylsulfonate and lithium trifluoromethylsulfonate; and the mass content of the lithium salt in the electrolyte is 12% to 17%.

[0016] In an example of the present application, the solvent includes a carbonate and a carboxylic acid ester; the carbonate is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate and fluoroethylene carbonate, and the carboxylic acid ester is selected from at least one of ethyl formate, ethyl acetate, propyl acetate and ethyl propionate; the mass content of the solvent in the electrolyte is 70% to 85%, and the mass content of the carboxylic acid ester in the electrolyte is 30% or more.

[0017] The present application also provides a secondary battery including a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte of any one of the examples.

[0018] In an example of the present application, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes a lithium iron phosphate material; and the solvent of the electrolyte includes a carboxylic acid ester, and the mass content of the carboxylic acid ester in the electrolyte is 30% or more.

[0019] The electrolyte provided by the application introduces the compound shown in formula I and methanedisulfonic acid methylene ester (MMDS) as additives, on the one hand, the tertiary amino group in the compound of formula I is used to adjust the acid-base degree in the electrolyte, so that the chemical environment of the electrolyte is more stable; on the other hand, the characteristics of MMDS breaking to form methylene and methylene disulfonic acid radicals under high voltage are used to initiate the copolymerization film reaction of the MMDS radical fragments and the compound of formula I on the surface of the positive electrode, the related groups are introduced in the direction of the polymer film formed on the positive electrode interface, the mechanical strength and chemical stability of the CEI film on the positive electrode interface are improved, the positive electrode is well protected in the charging and discharging process, the side reaction gas generated between the positive electrode and the electrolyte is effectively reduced, the irreversible consumption of lithium ions in the battery is reduced, and the storage and cycle life of the battery are improved. DETAILED DESCRIPTION

[0020] The present application also can be embodied in a different way or applied, and the details in the specification can be modified or changed based on different views and applications without departing from the spirit of the present application. It should be noted that the following examples and features in the examples can be combined with each other without conflict. It should also be understood that the terms used in the examples of the present application are for describing specific embodiments, not for limiting the protection scope of the present application. The test methods in the following examples are not specified, and are usually carried out under conventional conditions or under the conditions recommended by the manufacturers.

[0021] For simplicity, only some numerical ranges are explicitly disclosed herein, and each point or individual value between the range endpoints is included in the range. Thus, each point or individual value can be combined with any other point or individual value or with other lower or upper limits to form a range not explicitly recited.

[0022] Herein, "a plurality of", "a plurality of", "a plurality of" and the like, unless specifically limited, refer to more than two or equal to two in number. For example, "one or more" means one or more than two. "Further", "further", "in particular" and the like are used for descriptive purposes, indicating differences in content, but should not be understood as limiting the scope of protection of the present application.

[0023] Unless otherwise specified, "%" herein refers to mass percent.

[0024] In this paper, substituent refers to the atom or group of atoms substituted on the main chain or ring in an organic compound. They can replace a hydrogen atom or other atoms in the molecule, affecting the chemical and physical properties of the molecule. The type and location of the substituent have an important influence on the reactivity, polarity, solubility and other characteristics of the molecule.

[0025] Unsaturation, also known as hydrogen deficiency index or cyclic double bond index, is a quantitative indicator of the unsaturation degree of an organic compound molecule. If m < 2n + 2, the hydrocarbon with the molecular formula CnHm and the derivative of the hydrocarbon with the molecular formula CnHmOx has a certain unsaturation Ω, that is, compared with the open-chain alkane with the same number of carbon atoms, for every 2 hydrogen atoms reduced, the unsaturation of the organic compound increases by 1.

[0026] Number of heteroatoms: In organic chemistry, non-carbon atoms are collectively referred to as heteroatoms, the most common heteroatoms are nitrogen atoms, sulfur atoms and oxygen atoms.

[0027] The first aspect of the present application provides an electrolyte, which introduces a compound represented by formula I and methanedisulfonic acid methylene ester (MMDS) as an additive to adjust the acidity and alkalinity of the electrolyte, avoid the increase of electrolyte acidity at high temperature, and improve the chemical stability of the electrolyte; and can also polymerize on the surface of the high-potential positive electrode to form a polymer film with high mechanical strength and high stability, thereby effectively protecting the positive electrode interface and avoiding the collapse of the positive electrode interface at high voltage to react with the electrolyte to produce gas, thereby improving the storage and cycle life of the battery.

[0028] The above-mentioned electrolyte includes a lithium salt, a solvent, a first additive and a second additive. The solvent is used as the main part of the electrolyte to dissolve the lithium salt and the additive. After the lithium salt is dissolved in the organic solvent, a large number of active lithium ions can be released, so that the electrolyte has good electrical conductivity. The additive is an auxiliary substance of the electrolyte, which can be selected according to actual needs, such as a film-forming additive, a flame-retardant additive, a high-voltage additive, etc.

[0029] In the present application, the first additive includes a compound represented by formula I, and the second additive includes methanedisulfonic acid methylene ester (MMDS).

[0030] In the present application, the first additive includes a compound represented by formula I, and the second additive includes methanedisulfonic acid methylene ester (MMDS).

[0031]

[0032] R1, R2, R3, R4 in formula I are each independently selected from substituents with carbon atom number of 1-6, unsaturation degree of 0-4 and heteroatom number of 0-3. That is, the types of R1, R2, R3, R4 substituents in formula I are independent of each other, and can be completely identical substituents, or partially identical substituents, for example, R1 and R2 are identical or R1, R2 and R3 are identical, etc., or can be completely different substituents. The carbon atom number in the above substituents can be any value in 1-6, for example, 1, 3, 5 or 6, etc., and the unsaturation degree can be any value in 0-4, for example, 0, 1, 3 or 4, etc. The heteroatom is a non-carbon atom, and the heteroatom is selected from any one of nitrogen (N) atom, sulfur (S) atom, oxygen (O) atom, boron (B) atom, phosphorus (P) atom, for example, the heteroatom can be a nitrogen atom, or a sulfur atom, or an oxygen atom, but is not limited thereto. The heteroatom number can be any value in 0-3. For example, 0, 1, 2 or 3, etc.

[0033] The first additive is a compound represented by formula I, which contains a tertiary amino group (R3N, R represents an alkyl group, and the tertiary amino group is formed by combining a trimethyl group with an amino group (NH2)). The tertiary amino group exhibits basicity due to the lone pair of electrons on the nitrogen atom, and can chemically react with acidic substances in the electrolyte (such as acidic substances generated by decomposition of lithium salt), thereby adjusting the pH of the electrolyte and making the chemical environment of the electrolyte more stable. Furthermore, the compound represented by formula I also contains an unsaturated double bond, which can be opened under the electric field and oxidizing environment on the electrode surface, and copolymerize with other components in the electrolyte (such as solvent molecules, other additives, etc.), thereby forming a polymer film.

[0034] The second additive MMDS, as a sulfur-based additive, is broken to form methylene and methylenedisulfonic acid radicals under the high voltage of the cathode interface, and a polymer film is formed on the surface of the cathode by the polymerization reaction of the radicals and the compound of formula I. Due to the polymerization reaction of MMDS and the compound of formula I, functional groups such as amine groups and methylene groups are introduced into the polymer film formed on the surface of the cathode, thereby effectively improving the mechanical strength and chemical stability of the polymer film on the surface of the cathode. Among them, the amine group will introduce hydrogen bonds into the polymer film when participating in the polymerization reaction, so as to form a hydrogen bond network in the polymer film. These hydrogen bond networks not only can enhance the cohesion and mechanical strength of the polymer film, but also help the polymer film to maintain integrity at high temperatures; the methylene group is a flexible alkyl chain, which can increase the steric hindrance between polymer chains in the polymer film, reduce the close packing between the chains, thereby improving the flexibility and mechanical strength of the polymer film, and the methylene group is relatively stable at high temperatures, which can improve the thermal stability of the polymer film; the methylenedisulfonic acid group can introduce additional sulfonic acid groups into the polymer film in the polymerization reaction. The strong ionic interaction between the sulfonic acid group and lithium ions can effectively enhance the mechanical strength of the polymer film, and the sulfonic acid group can further improve the thermal stability of the polymer film due to its good heat resistance.

[0035] The polymer film formed by the polymerization of the compound of formula I and MMDS covers the surface of the cathode, and based on its good mechanical strength and chemical stability, it can better resist the volume change and mechanical stress generated during the charging and discharging process of the battery, and effectively protects the surface of the cathode. The side reactions between the cathode and the electrolyte due to contact are effectively reduced, the irreversible consumption of active lithium ions in the battery is reduced, and the gas production of the battery is reduced. In addition, the polymerization reaction of MMDS and the compound of formula I also slows down the increase in impedance caused by the reduction of excess lithium nitride on the surface of the cathode by introducing tertiary amino groups into the polymer film on the surface of the cathode.

[0036] In some embodiments, the first additive has a mass content in the electrolyte of any value in the range of 0.05% to 3%, such as 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 0.7%, 1%, 1.5%, 2%, 2.5%, or 3%. Alternatively, the first additive has a mass content in the electrolyte of any value in the range of 0.1% to 0.5%, such as 0.1%, 0.2%, 0.3%, 0.4%, or 0.5%. The inventors have found that when the content of the first additive in the electrolyte is limited to the above range, it can promote the formation of a solid electrolyte interphase film (CEI) at the cathode interface that is stable in properties and has an appropriate thickness, thereby balancing the mechanical strength and chemical stability of the CEI film while not excessively increasing the battery impedance. When the mass content of the first additive in the electrolyte is too low, a CEI film of sufficient thickness cannot be formed at the cathode interface, and the protection of the cathode is poor. When the content of the first additive in the electrolyte is too high, a CEI film of excessive thickness is formed at the cathode interface, which affects the ionic conductivity of the cathode and negatively impacts the cycle performance of the battery.

[0037] In some embodiments, the second additive has a mass content in the electrolyte of any value in the range of 0.1% to 0.8%, such as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, or 0.8%. The inventors have found that when the second additive is limited to the above range, it can effectively enhance the structural strength of the CEI film formed at the cathode interface without excessively increasing the thickness of the CEI film. When the content of the second additive MMDS is higher than 0.8%, the thickness of the formed CEI film is too thick, which is not conducive to the kinetics and interface stability of the battery, thereby affecting the cycle performance of the battery.

[0038] In some embodiments, R1, R2, R3, and R4 in the compound of Formula I are each independently selected from an alkyl group, an alkenyl group, an alkynyl group, a carbonyl group, an ester group, an amino group, or a heterocycle. The heterocycle refers to a ring formed by atoms other than carbon atoms, and these non-carbon atoms are heteroatoms. As an example, the heterocycle includes but is not limited to any one of pyridine, pyrrole, thiophene, thiazole, furan, etc. That is, R1, R2, R3, and R4 can each be selected from any one of an alkyl group, an alkenyl group, an alkynyl group, a carbonyl group, an ester group, an amino group, and a heterocycle, and can all be the same, partially the same, or all different. As an example, R1, R2, R3, and R4 can all be alkyl groups; or R1 and R2 are alkyl groups, R3 is an alkenyl group, and R4 is an alkynyl group; or R1 is an alkyl group, R2 is an ester group, R3 is an amino group, and R4 is an alkenyl group; etc.

[0039] In some embodiments, the compound of Formula I is selected from at least one of Compound I-1, Compound I-2, Compound I-3, and Compound I-4. Alternatively, the compound of Formula I is selected from Compound I-1. The chemical formulas of Compound I-1, Compound I-2, Compound I-3, and Compound I-4 are shown as follows:

[0040]

[0041] In some embodiments, the electrolyte further comprises a third additive selected from at least one of fluoroethylene carbonate (FEC), propargyl-1,3-sultone (PST), vinylene carbonate (VC), and tetravinylsilane (TVSi). The third additive helps to form a stable solid electrolyte interphase (SEI) film at the negative electrode interface, which can further improve the film-forming effect of the electrolyte at the negative electrode interface, enhance the protection effect of the negative electrode interface, and improve the charge-discharge cycle performance of the battery.

[0042] In some embodiments, the third additive is selected from one or a combination of two of FEC, PST, VC, and TVSi, and the mass content of each type of additive in the third additive in the electrolyte is any value in the range of 0.05% to 3%, such as 0.05%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2%, 2.3%, 2.5%, 2.7%, or 3%. When the mass content of each type of additive in the third additive in the electrolyte is in the above range, the SEI film at the negative electrode interface can be promoted without excessively increasing the impedance of the battery. When the content of each type of additive in the third additive in the electrolyte is too low, the SEI film at the negative electrode interface may not be sufficient, making it difficult to effectively protect the negative electrode, thereby negatively affecting the cycle capacity of the battery. When the content of each type of additive in the third additive in the electrolyte is too high, the SEI film at the negative electrode interface may have too high an impedance, negatively affecting the cycle performance of the battery.

[0043] In addition, it should be noted that the lithium salt in the electrolyte can be selected from conventional lithium salts in the art. As an example, the lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis(trifluoromethyl)sulfonylimide (LiTFSI), lithium acetate (CH3COOLi), lithium methylsulfonate (CH3SO3Li), and lithium trifluoromethylsulfonate (CF3SO3Li). Preferably, the lithium salt is selected from lithium hexafluorophosphate (LiPF6) which has good overall performance, or lithium hexafluorophosphate (LiPF6) is used as the main component, and other lithium salts are added in appropriate amounts to take advantage of their performance and improve the performance of the electrolyte. In some embodiments, the mass percentage of lithium salt in the electrolyte is any value in the range of 12% to 17%, for example, it can be 12%, 13%, 14%, 15%, 16%, or 17%. For example, in one example, the lithium salt includes lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonylimide (LiFSI), and the mass ratio of lithium hexafluorophosphate (LiPF6) to lithium bisfluorosulfonylimide (LiFSI) in the electrolyte is (1-14):(1-16).

[0044] The solvent in the electrolyte can be selected from conventional solvents suitable for lithium ion batteries in the art. The solvent includes at least one of a carbonate and a carboxylate, wherein the carbonate is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (EMC), and fluoroethylene carbonate (FEC), and the carboxylate is selected from one or more of ethyl formate (EF), ethyl acetate (EA), propyl acetate (PA), and ethyl propionate (EP). That is, the solvent can be selected from any one of the above-listed solvents, or a combination of any two or two, as an example, the solvent is a combination of EF and EC, a combination of EC, DMC, and EMC, or a combination of EC, DMC, PC, and EA, etc. In some embodiments, the solvent includes at least a carbonate and a carboxylate, and the mass content of the solvent in the electrolyte is generally 70% to 85%, for example, 70%, 75%, 80%, 81%, 83%, or 85%, etc.

[0045] The gas production problem of the positive electrode material with a general layered structure or spinel structure at high temperature is serious, and the structure problem of the olivine structure positive electrode material is not serious in the battery. However, the inventors found that when the olivine structure positive electrode material (such as lithium iron phosphate material) is matched with a high content of carboxylate solvent, the side reaction between the two produces more gas, so that the battery assembled with lithium iron phosphate also faces a serious gas production problem. In view of the above problem, the electrolyte provided by the application forms a CEI film on the surface of the positive electrode by the first additive compound of formula I and the second additive MMDS to isolate the contact reaction of the positive electrode and the electrolyte; at the same time, the carbonyl group of the carboxylate can also be introduced into the formed CEI film when the film is polymerized, to form a site for lithium ion combination in the CEI film, thereby reducing the ion conduction impedance of the CEI film, thereby effectively improving the gas production problem of the high carboxylate electrolyte battery.

[0046] In addition, in some embodiments, the solvent can also include at least one of an ether and a nitrile, wherein the ether is selected from ethylene glycol dimethyl ether and / or diethylene glycol diethyl ether, and the nitrile includes one or more of acetonitrile, propionitrile, butyronitrile, and valeronitrile.

[0047] The electrolyte of the application can be prepared according to the conventional preparation method, for example, the electrolyte is prepared in a glove box with an argon or nitrogen content of 99.999%, an actual oxygen content of 0.1 ppm, and a moisture content of less than 10 ppm. In the glove box, the solvent is mixed uniformly according to the set proportion, and then the fully dried lithium salt and the additive are added to the above-mentioned solvent and mixed uniformly to prepare the electrolyte. The content of each component in the electrolyte is the weight percentage calculated based on the total weight of the electrolyte.

[0048] The application also provides a secondary battery, which comprises a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte of any one of the above embodiments. During the charging and discharging process of the battery, lithium ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet, the separator is arranged between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and the electrolyte plays a role of conducting lithium ions between the positive electrode sheet and the negative electrode sheet. It should be noted that the structure type of the above-mentioned secondary battery can not be limited, and the secondary battery can be any conventional structure of the secondary battery on the market, such as a soft package battery, a square shell battery or a cylindrical battery.

[0049] In addition, the secondary battery of the present application can be used in the form of a single battery, a battery module or a battery pack for an electronic device to provide power for the same. The electronic device includes, but is not limited to, a mobile phone, a tablet, a notebook computer, an electric toy, an electric vehicle, a new energy vehicle, a ship, a spacecraft, etc. Among them, the electric toy can include a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy and an electric plane toy, etc. The spacecraft can include an airplane, a rocket, a space shuttle and a spacecraft, etc. The new energy vehicle can be a pure electric vehicle, a hybrid electric vehicle or a range extended electric vehicle, etc.

[0050] The composition and preparation method of the secondary battery are described in detail as follows:

[0051] The positive electrode tab includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode current collector is, for example, a foil formed after surface treatment of nickel, titanium, aluminum, nickel, silver, stainless steel or carbon, etc. In addition to the foil, the positive electrode current collector can also be used in any one or a combination of multiple forms such as a film, a mesh, a porous material, a foam or a non-woven fabric, etc. The thickness of the positive electrode current collector is, for example, 8 μm-15 μm. In an embodiment, the positive electrode current collector is, for example, an aluminum foil, and the thickness of the aluminum foil is, for example, 13 μm. The positive electrode current collector has two opposite surfaces in the thickness direction thereof, and the positive electrode active material layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent and a positive electrode binder, which are not specifically limited herein and can be selected by those skilled in the art according to actual needs.

[0052] The positive active material can be selected from any material suitable for use in a lithium ion battery, i.e. a compound that reversibly intercalates and deintercalates lithium ions. As an example, the positive active material can be selected from a ternary material, a lithium-containing phosphate, a spinel material, wherein the ternary material includes lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide doped with metal ions, lithium nickel cobalt aluminum oxide doped with metal ions, etc.; the lithium-containing phosphate includes lithium manganese iron phosphate, lithium iron phosphate, lithium manganese phosphate, etc. The binder of the positive electrode is, for example, selected from any one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene, or polymerized styrene butadiene rubber (SBR), etc. The positive electrode conductive agent is, for example, selected from one or a combination of two or more in any proportion of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc.

[0053] The negative electrode tab includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative current collector can be made of a material having good electrical conductivity and mechanical strength, such as a copper foil. The negative current collector has two opposite surfaces in the thickness direction thereof, and the negative active material layer is disposed on either one or both of the two opposite surfaces of the negative current collector. The negative active material layer includes a negative active material, a negative conductive agent, a negative binder, and a thickening agent, and the specific types of the negative active material, the negative conductive agent, and the negative binder are not specifically limited herein and can be selected from materials known in the art that can be used in lithium ion batteries.

[0054] The negative active material is selected from a compound capable of intercalating and deintercalating lithium ions, and as an example, the negative active material is selected from one or a combination of two or more of a carbon-based material and a silicon-based material, wherein the carbon-based material includes, for example, soft carbon, hard carbon, artificial graphite, natural graphite, etc., and the silicon-based material includes, for example, elemental silicon, a silicon oxide compound, a silicon carbon compound, etc. The negative conductive agent is selected from one or a combination of two or more in any proportion of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc. The negative binder is selected from any one or a combination of several in any proportion of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene butadiene rubber (SBR); and the thickening agent is selected from sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0055] The separator is selected from conventional types in the art, for example, a polyethylene film (PE), a polypropylene film (PP), a glass fiber film, a polyethylene film, or a composite film, etc. The thickness of the separator is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL, and the porosity is 30% to 50%.

[0056] The battery is assembled according to a conventional method, for example: after preparation, the negative electrode sheet, the separator, and the positive electrode sheet are sequentially stacked and wound, so that the separator is between the positive electrode and the negative electrode to play a role of isolation. Then, the electrode assembly formed by winding or stacking is packaged into a shell, transferred to a vacuum oven for drying at 120°C, injected with the prepared electrolyte 3.0 g / Ah, and then sealed, electrolyzed, and finally prepared into a lithium ion battery with a capacity of 1 Ah.

[0057] In some embodiments, the secondary battery uses a positive electrode active material with a high upper limit of working voltage (such as > 3.80 V), such as a high-nickel ternary material or a lithium iron phosphate material. Although the use of the above positive electrode active material can improve the capacity and fast-charging performance of the battery, it is prone to lattice collapse caused by uneven lithium stripping during charging and discharging, and the number of side reactions between the electrolyte and the active material increases, resulting in loss of active material. The use of the electrolyte in the above embodiments in the secondary battery can effectively protect the positive electrode interface, and further improve the cycle life of the battery compared to the prior art. In an example, the positive electrode active material in the secondary battery is selected to be a lithium iron phosphate material, and the solvent of the electrolyte includes a carboxylate with a mass content higher than 30%. The lithium iron phosphate material not only has the problem of unstable kinetic performance of high-voltage positive electrode materials, but also will have a side reaction with the high-content carboxylate in the electrolyte at high temperature environments (> 45°C), increasing the acidity of the electrolyte and the gas production of the battery. The use of the electrolyte in the above embodiments in the secondary battery can effectively adjust the acidity and alkalinity of the electrolyte by using the first additive to improve the chemical stability of the electrolyte, and can form a CEI film on the surface of the positive electrode with the second additive to effectively protect the positive electrode interface without excessively affecting the ion exchange capacity of the positive electrode interface, thereby effectively reducing the gas production of the battery.

[0058] The technical solutions of the present application are described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art.

[0059] Example 1

[0060] The embodiment provides an electrolyte, which comprises a lithium salt, a solvent, a first additive, a second additive and a third additive. The solvent is a combination of ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC) and ethyl acetate (EA), and the volume ratio of the four is EC:DMC:PC:EA=2.5:1:0.5:6; the lithium salt is a combination of lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonylimide (LiFSI), and the mass content of the lithium salt in the electrolyte is 15%, the mass content of LiPF6 is 9%, and the mass content of LiFSI is 6%; the first additive is a compound shown in formula I-1, the mass content of the first additive in the electrolyte is 0.3%, the second additive is methanedisulfonate (MMDS), the mass content of the second additive in the electrolyte is 0.5%, and the third additive comprises vinylidene carbonate (VC) and fluoroethylene carbonate (FEC), the mass content of VC in the electrolyte is 3%, and the mass content of FEC in the electrolyte is 0.5%.

[0061] The preparation process of the electrolyte is as follows: in an argon atmosphere glove box with a water content of <10 ppm, first, EC, DMC, PC and EA are mixed according to a volume ratio of 2.5:1:0.5:6 to form a solvent; then, the lithium salt LiPF6 and LiFSI and the additives are added to the solvent, and the non-aqueous electrolyte is obtained after uniform mixing. The added amount of LiPF6 and LiFSI is 9% of the total weight of the electrolyte, the added amount of LiFSI is 6% of the total weight of the electrolyte, the added amount of the first additive, the compound of formula I-1, is 0.3% of the total weight of the electrolyte, the added amount of the second additive, the compound of MMDS, is 0.5% of the total weight of the electrolyte, the added amount of VC in the third additive is 3% of the total weight of the electrolyte, and the added amount of FEC in the third additive is 0.5% of the total weight of the electrolyte.

[0062] Embodiment 2

[0063] The embodiment provides an electrolyte with the same system as that of embodiment 1, and the difference between the embodiment and embodiment 1 is that the mass content of the first additive in the electrolyte is adjusted to 0.05%, and the mass content of the second additive in the electrolyte is adjusted to 0.3%.

[0064] Embodiment 3

[0065] The embodiment provides an electrolyte with the same system as that of embodiment 1, and the difference between the embodiment and embodiment 1 is that the mass content of the first additive in the electrolyte is adjusted to 3%, and the mass content of the second additive in the electrolyte is adjusted to 0.3%.

[0066] Embodiment 4

[0067] This example provides an electrolyte of the same system as Example 1, the difference between this example and Example 1 is that the mass content of the second additive in the electrolyte is adjusted to 0.1%.

[0068] Example 5

[0069] This example provides an electrolyte of the same system as Example 1, the difference between this example and Example 1 is that the mass content of the second additive in the electrolyte is adjusted to 0.8%.

[0070] Example 6

[0071] This example provides an electrolyte of the same system as Example 1, the difference between this example and Example 1 is that the compound shown as formula I-2 is used as the first additive.

[0072] Example 7

[0073] This example provides an electrolyte of the same system as Example 1, the difference between this example and Example 1 is that the compound shown as formula I-3 is used as the first additive.

[0074] Example 8

[0075] This example provides an electrolyte of the same system as Example 1, the difference between this example and Example 1 is that the compound shown as formula I-4 is used as the first additive.

[0076] Comparative Example 1

[0077] This comparative example provides an electrolyte of the same system as Example 1, the difference between this comparative example and Example 1 is that the first additive compound of formula I and the second additive MMDS are not added to the electrolyte.

[0078] Comparative Example 2

[0079] This comparative example provides an electrolyte of the same system as Example 1, the difference between this comparative example and Example 1 is that the second additive MMDS is not added to the electrolyte.

[0080] Comparative Example 3

[0081] This comparative example provides an electrolyte of the same system as Example 1, the difference between this comparative example and Example 1 is that the first additive compound of formula I is not added to the electrolyte.

[0082] The electrolytes prepared in Examples 1 to 8 and Comparative Examples 1 to 3 are respectively assembled in lithium ion secondary batteries, and the lithium ion batteries are tested to verify the efficacy of the present application. The preparation process of the lithium ion secondary battery is as follows:

[0083] Preparation of the positive electrode sheet: the positive electrode active material LiFePO4, the positive electrode binder polyvinylidene fluoride and the positive electrode conductive agent Super P were mixed in a mass ratio of 97:1:2, a solvent N-methyl pyrrolidone was added, and stirring was performed under the action of a vacuum stirrer until a uniform transparent state was obtained, to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on an aluminum foil, the aluminum foil was dried at room temperature, then transferred to an oven for drying, and then subjected to cold pressing and slitting to obtain the positive electrode sheet.

[0084] Preparation of the negative electrode sheet: the negative electrode active material artificial graphite, the negative electrode conductive agent Super P, the negative electrode thickening agent sodium carboxymethyl cellulose (CMC-Na) and the negative electrode binder styrene-butadiene rubber (SBR) were mixed in a mass ratio of 96:1:1:2, deionized water was added, and stirring was performed under the action of a vacuum stirrer until a uniform state was obtained to obtain a negative electrode slurry; the negative electrode slurry was uniformly coated on a negative electrode current collector copper foil, dried at room temperature, then transferred to an oven for drying, and then subjected to cold pressing, slitting and other processes to obtain the negative electrode sheet.

[0085] Selection of the separator: a polypropylene film with a thickness of 12 μm was used as the separator.

[0086] Preparation of the battery: the positive electrode sheet, the separator and the negative electrode sheet were placed in sequence, the separator was placed between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and then wound to form a bare cell. Then, an aluminum plastic film was wrapped, transferred to a vacuum oven for drying at 120°C, sealed after injecting the electrolyte prepared above at 3.0 g / Ah, and subjected to electrolyte formation to obtain a soft-pack lithium ion battery with a capacity of 1 Ah.

[0087] The lithium ion batteries assembled with the electrolytes in Examples 1 to 8 and Comparative Examples 1 to 3 were subjected to performance tests, wherein the electrolyte parameters and test results of Examples 1 to 8 and Comparative Examples 1 to 3 are shown in Table 1, and the test methods are as follows:

[0088] The battery gas production performance test included: testing the initial volume V1 of the battery after being fully charged, then storing in a 60°C high-temperature cabinet, charging every 7 days and testing the volume, testing the volume of the battery as V2 after 28 days, and the gas production growth rate was (V2-V1) / V1.

[0089] Table 1: Performance test results of the batteries assembled with the electrolytes in Examples 1 to 8 and Comparative Examples 1 to 3

[0090]

[0091] The contents in Table 1 are all mass percentages of the components relative to the electrolyte.

[0092] By comparing the test results of Examples 1 to 8 and Comparative Example 1, it can be known that, by using the compound of Formula I and MMDS as additives in the electrolyte, the acidification of the electrolyte at high temperature can be inhibited, the chemical stability of the electrolyte can be improved, and the CEI film with mechanical strength and thermal stability can be formed at the positive electrode interface, thereby effectively inhibiting the gas production of the battery compared with Comparative Example 1.

[0093] By comparing the test results of Example 1 and Comparative Examples 2 to 3, it can be known that, compared with Comparative Example 2 using only the compound of Formula I as an additive or Comparative Example 3 using only MMDS as an additive, the present application introduces appropriate amounts of the compound of Formula I and MMDS as additives in the electrolyte, which can introduce amine groups and methylene groups and other functional groups in the CEI film formed at the positive electrode interface, thereby further improving the mechanical strength and thermal stability of the CEI film, improving the protection effect of the CEI film on the positive electrode, and further reducing the gas production of the positive electrode and the electrolyte under the same dosage.

[0094] As can be seen from the test results of Examples 1, 6, 7, and 8, compounds I-1, I-2, I-3, and I-4 can achieve the effect of improving the high-temperature gas production of the battery when used as additives with MMDS because they have the same structure and functional groups as the compound of Formula I. Therefore, the use of different types of R1, R2, R3, and R4 groups in the compound of Formula I has little effect on the conductivity and high-temperature cycle performance of the battery.

[0095] The above examples are only illustrative of the principles and effects of the present application, and are not intended to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical ideas disclosed by the present application should be covered by the claims of the present application.

Claims

1. An electrolyte, characterized by, The electrolyte comprises a lithium salt, a solvent, a first additive and a second additive; The first additive comprises a compound shown in Formula I: Formula I; R1, R2, R3, R4 in Formula I are each independently selected from a substituent with 1-6 carbon atoms, 0-4 unsaturation and 0-3 heteroatoms, the heteroatoms being selected from any one of nitrogen atom, sulfur atom, oxygen atom, boron atom and phosphorus atom; The second additive comprises methylene methanedisulfonate; The mass content of the first additive is 0.05%-3%, and the mass content of the second additive is 0.1%-0.8%.

2. The electrolyte according to claim 1, characterized in that, The mass content of the first additive in the electrolyte is 0.1%-0.5%.

3. The electrolyte of claim 1, wherein R1, R2, R3, R4 in Formula I are each independently selected from an alkyl group, an alkenyl group, an alkyne group, a carbonyl group, an ester group, an amino group or a heterocyclic ring, the heterocyclic ring being selected from any one of pyridine, pyrrole, thiophene and thiazole.

4. The electrolyte according to claim 1 or 3, characterized in that, The compound shown in Formula I comprises at least one of the following compounds: I-1； I-2； I-3； I-4。 5. The electrolyte of claim 1, wherein The electrolyte further comprises a third additive selected from at least one of fluoroethylene carbonate, propenyl-1,3-sultone, vinylidene carbonate and tetra-vinylsilane.

6. The electrolyte of claim 1, wherein, The lithium salt comprises at least one of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methylsulfonate and lithium trifluoromethylsulfonate; the mass content of the lithium salt in the electrolyte is 12%-17%.

7. The electrolyte of claim 1, wherein The solvent comprises a carbonate and a carboxylic acid ester; the carbonate is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate and fluoroethylene carbonate, and the carboxylic acid ester is selected from at least one of ethyl formate, ethyl acetate, propyl acetate and ethyl propionate; the mass content of the solvent in the electrolyte is 70%-85%, and the mass content of the carboxylic acid ester in the electrolyte is above 30%.

8. A secondary battery characterized by comprising: The electrolyte comprises a lithium salt, a solvent, a first additive and a second additive; 9. The secondary battery according to claim 8, characterized by The positive electrode active material comprises a lithium iron phosphate material; the solvent of the electrolyte comprises a carboxylic acid ester, and the mass content of the carboxylic acid ester in the electrolyte is above 30%.

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