Electrolyte, secondary battery and electronic device

Abstract

The invention provides an electrolyte, a secondary battery and an electronic device. The electrolyte comprises a compound as shown in a formula I and a compound as shown in a formula II. In the electrolyte disclosed by the invention, the compound in the formula II and the compound in the formula I have a synergistic effect, so that an electrolyte body has a good ion transmission rate under a low-temperature condition; and lithium ions in the negative pole piece and the positive pole piece under a low-temperature condition can have a good ion transmission rate. When the secondary battery provided by the invention is used in the secondary battery, the secondary battery can have good quick charge capacity at low temperature.

Description

Technical Field

[0001] The present application relates to the field of energy storage, and particularly to an electrolyte, a secondary battery, and an electronic device. Background Art

[0002] Secondary batteries such as lithium-ion batteries are widely used in portable electronic products, electric vehicles, aerospace, energy storage and other fields due to their advantages of high energy density, good cycle performance, safety, environmental protection, and no memory effect. However, the current secondary batteries have poor electrical properties at low temperatures, which greatly affects the use of secondary batteries. Summary of the Invention

[0003] The present application can provide an electrolyte, a secondary battery, and an electronic device. The electrolyte of the present application has good ion transport performance at low temperatures. When used in a secondary battery, the secondary battery can have good fast charging ability at low temperatures.

[0004] In a first aspect, the present application provides an electrolyte, which includes a compound of Formula I and a compound of Formula II. The general structural formula of the compound of Formula I is shown as Formula I below:

[0005]

[0006] Wherein R x is selected from any one of a single bond, an alkyl group having 1 to 4 carbon atoms, or an alkyl group having 1 to 4 carbon atoms substituted by R4. Each of R1 to R4 is independently selected from an unsubstituted or Ra-substituted alkyl group having 2 to 6 carbon atoms, an unsubstituted or Ra-substituted alkenyl group having 2 to 6 carbon atoms, an unsubstituted or Ra-substituted alkynyl group having 2 to 6 carbon atoms, an unsubstituted or Ra-substituted nitrogen-containing heteroaryl group having 5 to 12 carbon atoms, and an unsubstituted or Ra-substituted aryl group having 6 to 12 carbon atoms. Each substituent Ra of each group is independently selected from any one of fluorine or a fluoroalkyl group having 1 to 6 carbon atoms. The general structural formula of the compound of Formula II is shown as Formula II below:

[0007]

[0008] Wherein R5 to R 10 are each independently selected from any one of hydrogen, fluorine, an alkyl group having 1 to 3 carbon atoms, or a fluoroalkyl group having 1 to 3 carbon atoms. Based on the mass of the electrolyte, the mass content of the compound of Formula I is A%, and the mass content of the compound of Formula II is B%. 5 ≤ A ≤ 50, 0.36 ≤ A / B ≤ 5.

[0009] In the above technical solution, the inventor found that when the above electrolyte is used in a secondary battery, the compound of Formula I can decompose at the interface of the negative electrode plate of the secondary battery to form an inorganic SEI film (SEI stands for Solid Electrolyte Interface, and the SEI film is a solid electrolyte interface film) with a uniform thickness and a low impedance, which is beneficial to the rapid transfer of lithium ions at the negative electrode interface at low temperature. However, the compound of Formula I has a relatively large viscosity, which hinders the ion transfer rate of the electrolyte bulk. At the same time, the acidic substances generated by the decomposition of the compound of Formula I are likely to damage the interface of the positive electrode plate, resulting in the attenuation of the active material in the positive electrode plate. The compound of Formula II has a relatively low viscosity, which is beneficial to improving the ion transport rate of the electrolyte bulk. Moreover, the compound of Formula II also has a high affinity for the interface and can decompose to generate a passivation layer at the interface of the positive electrode plate, so as to protect the positive electrode plate from being damaged by the acidic substances decomposed by the compound of Formula I. Therefore, when the mass content A% of the compound of Formula I and the mass content B% of the compound of Formula II satisfy 5 ≤ A ≤ 50 and 0.36 ≤ A / B ≤ 5, the two act synergistically, making the electrolyte of the present application have good ion transport performance even at low temperature; when the electrolyte of the present application is used in a secondary battery, the secondary battery can have good fast charging ability at low temperature. If A is less than 5 or A / B is less than 0.36, it means that the content of the compound of Formula I is relatively small, which will lead to the hindrance of the transfer of lithium ions at the negative electrode interface at low temperature, thus affecting the fast charging ability of the secondary battery at low temperature; if A is greater than 50 or A / B is greater than 5, it means that the content of the compound of Formula II is relatively small, and it cannot play a good role in protecting the positive electrode plate, resulting in the positive electrode plate being easily damaged by the acidic substances decomposed by the compound of Formula I, which will also affect the fast charging ability of the secondary battery at low temperature.

[0010] In a possible implementation, the electrolyte satisfies at least one of the following conditions: (1) 10 ≤ A ≤ 30; (2) 5 ≤ B ≤ 40; (3) 0.9 ≤ A / B ≤ 2.

[0011] In a possible implementation, the compound of Formula I includes at least one of the following compounds:

[0012]

[0013] In a possible implementation, the compound of Formula II includes at least one of the following compounds:

[0014]

[0015] In a possible implementation, the electrolyte further includes a first component, and the first component includes at least one of fluoroethylene carbonate, vinylene carbonate, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, ethoxypentafluorocyclotriphosphazene, succinonitrile, adiponitrile, or 1,3,6-hexanetricarbonitrile; based on the mass of the electrolyte, the mass content of the first component is C%, and 0.3 ≤ C ≤ 22.

[0016] In the above technical solution, including 0.3% to 22% of the first component in the electrolyte can form an organic SEI film on the interface of the negative electrode plate of the secondary battery. On the one hand, the organic SEI film can improve the tensile properties of the SEI film, and on the other hand, it can further improve the transmission rate of lithium ions at the interface of the negative electrode plate at low temperature, so that the secondary battery has better fast charging ability at low temperature.

[0017] In a possible implementation, 1 ≤ C ≤ 15.

[0018] In a second aspect, the present application provides a secondary battery, which includes the above electrolyte. Therefore, the secondary battery provided by the present application still has good fast charging ability at low temperature.

[0019] In a possible implementation, the secondary battery further includes a negative electrode plate, and the negative electrode plate includes a negative electrode current collector, and a negative electrode active material layer is provided on at least one surface of the negative electrode current collector, and the negative electrode active material layer contains a silicon-based active material.

[0020] In the above technical solution, containing a silicon-based active material in the negative electrode plate can improve the capacity of the secondary battery.

[0021] In a possible implementation, the negative electrode active material layer includes a silicon-based active material, and the negative electrode active material layer includes a first active layer and a second active layer. The first active layer is located between the negative electrode current collector and the second active layer. Based on the mass of the first active layer, the mass content of silicon element in the first active layer is X%, and 0 ≤ X ≤ 35. Based on the mass of the second active layer, the mass content of silicon element in the second active layer is Y%, and 3.5 ≤ Y ≤ 60, and Y > X.

[0022] In a possible implementation, the thickness of the first active layer is 20 μm to 90 μm, and the thickness of the second active layer is 10 μm to 70 μm.

[0023] The inventors found that although silicon-based active materials can improve the capacity of secondary batteries, the kinetic performance of silicon-based active materials is poor, which may have an adverse effect on the fast charging performance of secondary batteries at low temperatures. In the above technical solution, the inventors also found that in the negative electrode active material layer, since the first active layer is close to the negative electrode current collector, when the silicon element content in the first active layer is small (i.e., X < Y), the ion transport performance of the negative electrode plate is better. Therefore, the capacity of the secondary battery can be improved on the basis of basically not affecting the fast charging ability of the secondary battery at low temperatures.

[0024] In a possible implementation, the silicon-based active material includes at least one of silicon-carbon material, silicon-oxygen material, or Si.

[0025] In a third aspect, the present application provides an electronic device, which includes the above secondary battery. Therefore, the electronic device provided by the present application has good use performance.

[0026] Advantages of the present application;

[0027] The present application provides an electrolyte, a secondary battery, and an electronic device. The electrolyte of the present application includes a compound of Formula I shown in Formula I and a compound of Formula II shown in Formula II. In the electrolyte of the present application, the compound of Formula II and the compound of Formula I act synergistically. On the one hand, it can make the electrolyte body have good ion transport rate under low temperature conditions; on the other hand, the compound of Formula I and the compound of Formula II can act on the interfaces of the negative electrode plate and the positive electrode plate of the secondary battery respectively, so that lithium ions can still be quickly transferred at the interfaces of the negative electrode plate and the positive electrode plate under low temperature conditions. Therefore, when the secondary battery of the present application is used in a secondary battery, the secondary battery can have good fast charging ability at low temperatures. Detailed implementation manners

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application. For those conditions not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used without indicating the manufacturer are all conventional products that can be obtained through commercial purchase.

[0029] The secondary battery in the present application includes an electrolyte, a positive electrode plate, a negative electrode plate, and a separator. In the secondary battery of the present application, the components in the electrolyte act synergistically, so that the secondary battery of the present application has good fast charging performance even under low temperature conditions. The following is a specific description of each component in the secondary battery of the present application:

[0030] Electrolyte

[0031] The electrolyte can play a role in transporting ions and electrons. The electrolyte of this application includes a compound of Formula I and a compound of Formula II. The general structural formula of the compound of Formula I is shown as Formula I below:

[0032]

[0033] Wherein R x is selected from any one of a single bond, an alkyl group having 1 to 4 carbon atoms or an alkyl group having 1 to 4 carbon atoms substituted by R4. R1 to R4 are each independently selected from an unsubstituted or Ra-substituted alkyl group having 2 to 6 carbon atoms, an unsubstituted or Ra-substituted alkenyl group having 2 to 6 carbon atoms, an unsubstituted or Ra-substituted alkynyl group having 2 to 6 carbon atoms, an unsubstituted or Ra-substituted nitrogen-containing heteroaryl group having 5 to 12 carbon atoms, and an unsubstituted or Ra-substituted aryl group having 6 to 12 carbon atoms; the substituent Ra of each group is independently selected from any one of fluorine or a fluoroalkyl group having 1 to 6 carbon atoms; the general structural formula of the compound of Formula II is shown as Formula II below:

[0034]

[0035] Wherein R5 to R 10 are each independently selected from any one of hydrogen, fluorine, an alkyl group having 1 to 3 carbon atoms or a fluoroalkyl group having 1 to 3 carbon atoms; and based on the mass of the electrolyte, the mass content of the compound of Formula I is A%, and the mass content of the compound of Formula II is B%, 5 ≤ A ≤ 50, 0.36 ≤ A / B ≤ 5.

[0036] The inventors found that in the electrolyte of the present application, the compound of Formula I can decompose at the interface of the negative electrode sheet of the secondary battery to form an inorganic SEI film (SEI stands for Solid Electrolyte Interface, and the SEI film is a solid electrolyte interface film) with a uniform thickness and a low impedance, which is beneficial to the rapid transfer of lithium ions at the negative electrode interface at low temperatures. However, the compound of Formula I has a relatively large viscosity, which results in the hindrance of the ion transfer rate in the bulk electrolyte. At the same time, the acidic substances generated by the decomposition of the compound of Formula I are likely to damage the interface of the positive electrode sheet, leading to the attenuation of the active material in the positive electrode sheet. The compound of Formula II has a lower viscosity, which is beneficial to improving the ion transport rate of the bulk electrolyte. Moreover, the compound of Formula II also has a high affinity for the interface and can decompose at the interface of the positive electrode sheet to generate a passivation layer, which can protect the positive electrode sheet from being damaged by the acidic substances decomposed by the compound of Formula I. Therefore, when 5 ≤ A ≤ 50 and 0.36 ≤ A / B ≤ 5, the compound of Formula I and the compound of Formula II can act synergistically, enabling the electrolyte to have good ion transport performance even at low temperatures. Therefore, the secondary battery of the present application also has good fast charging performance under low temperature conditions. If A is less than 5 or A / B is less than 0.36, it indicates that the content of the compound of Formula I is relatively small, which will lead to the hindrance of the transfer of lithium ions at the negative electrode interface at low temperatures, thereby affecting the fast charging ability of the secondary battery at low temperatures. If A is greater than 50 or A / B is greater than 5, it indicates that the content of the compound of Formula II is relatively small, and it cannot play a good role in protecting the positive electrode sheet, resulting in the positive electrode sheet being easily damaged by the acidic substances decomposed by the compound of Formula I, which will also affect the fast charging ability of the secondary battery at low temperatures. Preferably, in the embodiments of the present application, based on the mass of the electrolyte, the mass content of the compound of Formula I is A%, 10 ≤ A ≤ 30; for example, A can be 10, 14, 18, 20, 24, 28, 30, etc. or within the range composed of any two of the above values; the types of the compound of Formula I include but are not limited to at least one of the following compounds:

[0037]

[0038] Preferably, in the embodiments of the present application, based on the mass of the electrolyte, the mass content of the compound of Formula II is B%, 5 ≤ B ≤ 40; for example, B can be 5, 8, 10, 18, 20, 22, 28, 30, 34, 36, 40, etc. or within the range composed of any two of the above values; the types of the compound of Formula II include but are not limited to at least one of the following compounds:

[0039]

[0040] In addition, in some embodiments of the present application, in order to further improve the transport rate of lithium ions at the interface of the negative electrode sheet at low temperatures, so that the fast charging ability of the secondary battery at low temperatures is better, the electrolyte further includes a first component, and the first component includes at least one of fluoroethylene carbonate, vinylene carbonate, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, ethoxypentafluorocyclotriphosphazene, succinonitrile, adiponitrile or 1,3,6-hexanetricarbonitrile. Specifically, based on the mass of the electrolyte, the mass content of the first component is C%, 0.3 ≤ C ≤ 22; preferably 1 ≤ C ≤ 15. For example, C can be 0.3, 0.5, 0.7, 1, 4, 8, 10, 15, 16, 19, 20, 22, etc. or within the range composed of any two of the above values.

[0041] Of course, in some embodiments of the present application, other substances can also be added to the electrolyte according to actual situations, such as lithium salts, solvents, additives, etc. There are no special restrictions in this application as long as the purpose of this application can be met. Specifically, the lithium salt can be dissolved in the solvent to form an ionic conductor, which is used as a conductive medium and a lithium ion transport medium; the lithium salt includes but is not limited to at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalato)borate, lithium bis(trifluoromethylsulfonyl)imide, lithium difluoro(oxalato)borate, lithium difluorosulfonylimide, etc.

[0042] The solvent can play the role of dissolving the lithium salt and the additive. The solvent can be at least one of carbonates, carboxylates, ethers, alcohols, etc. Among them, carbonates can be divided into cyclic carbonates, linear carbonates, etc. The cyclic carbonates specifically include but are not limited to at least one of ethylene carbonate, propylene carbonate, etc.; the linear carbonates specifically include but are not limited to at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, etc.; the carboxylates include but are not limited to at least one of methyl formate, methyl acetate, methyl butyrate, ethyl propionate, propyl propionate, propyl acetate, etc.; the ethers include but are not limited to at least one of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, etc.; the alcohols include but are not limited to at least one of ethanol, ethylene glycol, glycerol, etc.

[0043] The additives include but are not limited to nitriles, sulfones, sulfoxides, fluoronitriles, fluoroesters, etc.

[0044] Negative electrode sheet

[0045] The negative electrode plate includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The components of the negative electrode active material layer include the negative electrode plate active substance. That is, in the present application, the negative electrode active material layer can be provided on one surface in the thickness direction of the negative electrode current collector, or can be provided on both surfaces in the thickness direction of the negative electrode current collector. Moreover, in the present application, the "surface of the negative electrode current collector" can be the entire area of the negative electrode current collector or a partial area of the negative electrode current collector, and there is no special limitation in the present application as long as the purpose of the present application can be achieved.

[0046] Generally, the negative electrode active material layer contains the negative electrode plate active material, and there is no special limitation on the negative electrode plate active material in the present application. Specifically, the negative electrode plate active material can include at least one of a carbon material or a silicon-based active material. More specifically, the carbon material includes but is not limited to at least one of natural graphite, artificial graphite, mesophase microcarbon balls, hard carbon or soft carbon; the silicon-based active material includes but is not limited to at least one of silicon, silicon oxide material or silicon carbon material.

[0047] Containing a silicon-based active material in the negative electrode active material layer can increase the capacity of the secondary battery. However, due to the poor kinetic performance of the silicon-based active material, it has an adverse effect on the fast charging performance of the secondary battery (especially the fast charging performance at low temperatures). However, the inventors found that when the negative electrode active material contains a silicon-based active material, when the negative electrode active material layer is layered and meets the following conditions, the capacity of the secondary battery can be improved on the basis of basically not affecting the fast charging ability of the secondary battery at low temperatures:

[0048] The first active layer is located between the negative electrode current collector and the second active layer. The thickness of the first active layer is 20 μm to 90 μm, and the thickness of the second active layer is 10 μm to 70 μm; based on the mass of the first active layer, the mass content of silicon element in the first active layer is X%, 0 ≤ X ≤ 35, based on the mass of the second active layer, the mass content of silicon element in the second active layer is Y%, 3.5 ≤ Y ≤ 60, and Y > X.

[0049] This is because in order to ensure the fast charging performance, in the negative electrode active material, the closer to the negative electrode current collector, the higher the ion transport ability is required. The above-mentioned negative electrode active material layer can be prepared by a layered coating method.

[0050] In some embodiments, the negative electrode active material layer usually further contains a negative electrode plate conductive agent. There is no special limitation on the type of the negative electrode plate conductive agent in the present application as long as the purpose of the present application can be achieved. For example, the negative electrode plate conductive agent includes but is not limited to at least one of acetylene black, Ketjen black, carbon nanotubes, carbon fibers, carbon dots or graphene, etc.

[0051] In some embodiments, the negative electrode active material layer may also contain a negative electrode binder and a thickener. The types of the negative electrode binder and the thickener are not particularly limited in this application, as long as the objectives of this application can be achieved. For example, the negative electrode binder may include, but is not limited to, at least one of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, styrene-butadiene rubber, or acrylated styrene-butadiene rubber; the thickener in the negative electrode slurry may include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

[0052] In the negative electrode tab, the material of the negative electrode current collector includes, but is not limited to, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a polymer substrate coated with a conductive metal, etc., and there is no particular limitation in this application. Among them, the conductive metal includes, but is not limited to, copper, nickel, or titanium, and the material of the polymer substrate includes, but is not limited to, at least one of polyethylene, polypropylene, ethylene-propylene copolymer, polyethylene terephthalate, poly(ethylene naphthalate), or poly(p-phenylene terephthalamide).

[0053] In addition, in this application, there is no particular limitation on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the objectives of this application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the single-sided negative electrode active material layer is 30 μm to 160 μm.

[0054] When preparing the negative electrode sheet, the components in the above-mentioned negative electrode active material layer can be dissolved or dispersed in a liquid solvent to form a negative electrode slurry, and then the negative electrode slurry is coated on the negative electrode current collector and dried, so as to form a negative electrode active material layer on the negative electrode current collector, and thus the negative electrode sheet can be obtained. When preparing the negative electrode sheet in this way, the solvent in the negative electrode slurry is not particularly limited as long as it can dissolve or disperse the above components. Specifically, the solvent in the negative electrode slurry includes any one of aqueous solvents and organic solvents. Aqueous solvents include, but are not limited to, a mixed solvent of alcohol and water or water, etc. Organic solvents include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide, etc. In addition, in some other embodiments, when using an aqueous solvent, the components of the negative electrode slurry will also include a thickener and a styrene-butadiene rubber (SBR) emulsion to slurry the negative electrode slurry, so as to adjust the viscosity of the negative electrode slurry. The types of thickeners in the negative electrode slurry include, but are not limited to, at least one of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts, etc. In addition, when preparing the negative electrode sheet, the components in the negative electrode active material layer can also be dry-mixed to form flakes, and then the obtained flakes are pressed onto the negative electrode current collector.

[0055] Positive electrode sheet

[0056] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. That is, in this application, the positive electrode active material layer can be provided on one surface in the thickness direction of the positive electrode current collector, or can be provided on both surfaces in the thickness direction of the positive electrode current collector. Moreover, in this application, the "surface of the positive electrode current collector" can be the entire area of the positive electrode current collector or a partial area of the positive electrode current collector, which is not particularly limited in this application as long as the purpose of this application can be achieved.

[0057] The components of the positive electrode active material layer include a positive electrode active substance, and the positive electrode active substance can be any substance that can reversibly intercalate and deintercalate Li + 、Na +Substances such as alkali metal ions are used to ensure the normal charge and discharge of the secondary battery. For example, the positive electrode active material includes at least one of, but is not limited to, lithium iron phosphate (LiFePO4), lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium nickel oxide, ternary materials, etc. The ternary materials include, but are not limited to, LiNi x Co y Mn z O2, LiNi x Co y Al z O2, etc. Moreover, the contents of Ni, Co, Mn, Al, etc. can be adjusted, and it is only necessary to ensure that x + y + z = 1. For example, the ternary material can be LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.88 Co 0.08 Mn 0.04 O2, LiNi 0.8 Co 0.15 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.88 Co 0.1 Mn 0.02 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.88 Co 0.1 Al 0.02 O2, etc.

[0058] In some embodiments, the components of the positive electrode active material layer further include a positive electrode conductive agent; the present application places no restrictions on the type of the positive electrode conductive agent, and any known conductive material can be used. Specifically, the positive electrode conductive agent includes at least one of, but is not limited to, carbon blacks such as acetylene black and Super-P, or amorphous carbon materials such as needle coke, or carbon nanotubes, or graphene, etc.

[0059] In some embodiments, the components of the positive electrode active material layer generally further contain a positive electrode binder. There is no particular limitation on the type of the positive electrode binder used in the manufacture of the positive electrode active material layer. In the case of the coating method, any material that can be dissolved or dispersed in the liquid medium used during electrode manufacturing is acceptable. The positive electrode binder includes, but is not limited to, any one or at least two of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber-like polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, polybutene rubber, and ethylene-propylene rubber; thermoplastic elastomer-like polymers such as styrene-butadiene-styrene block copolymer or its hydride, ethylene-propylene-diene terpolymer (EPDM), styrene-ethylene-butadiene-ethylene copolymer, and styrene-isoprene-styrene block copolymer or its hydride; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, and propylene-α-olefin copolymer; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer; and polymer compositions having ionic conductivity of alkali metal ions (especially lithium ions), etc.

[0060] In the positive electrode tab, there is no particular limitation on the type of the positive electrode current collector, and it can be made of any known material suitable for use as a positive electrode current collector. The materials of the positive electrode current collector include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum. In addition, in order to reduce the electron contact resistance between the positive electrode current collector and the positive electrode active material layer, a conductive aid or a conductive coating, etc., can be provided on the surface of the positive electrode current collector. The conductive aid includes, but is not limited to, carbon and precious metals such as gold, platinum, and silver. The conductive coating can be a mixture layer of inorganic oxides, conductive agents, and positive electrode binders.

[0061] In addition, similar to the preparation of the negative electrode tab, when preparing the positive electrode tab, it can be configured into a positive electrode slurry, and then the positive electrode slurry is coated on the positive electrode current collector and dried, so as to form a positive electrode active material layer on the positive electrode current collector, thereby obtaining the positive electrode tab; or the components in the positive electrode active material layer can be dry-mixed, made into flakes, and then the obtained flakes are pressed onto the positive electrode current collector to form a positive electrode active material layer, thereby obtaining the positive electrode tab. Specifically, the solvents in the positive electrode slurry include, but are not limited to, N-methylpyrrolidone (NMP), ethylene carbonate (EC), etc.

[0062] Separator

[0063] In order to prevent short circuit, a separator is usually provided between the positive electrode tab and the negative electrode tab. In this case, the electrolyte of the present application usually penetrates into the separator and is used.

[0064] There are no particular limitations on the material and shape of the separator membrane, as long as the effects of this application are not significantly impaired. The material of the separator membrane can be a resin, glass fiber, inorganic substance, etc. formed from a material that is stable to the electrolyte of this application. In some embodiments, the separator membrane includes a porous sheet or a non-woven fabric-like substance with excellent liquid retention properties, etc. Examples of the material of the resin or glass fiber separator membrane include, but are not limited to, polyolefin, aromatic polyamide, polyimide (PI), polyamide (PA), polytetrafluoroethylene, polyethersulfone, spandex, or aramid, etc. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The above materials of the separator membrane can be used alone or in any combination.

[0065] The separator membrane can also be a material formed by laminating the above materials. Examples include, but are not limited to, a three-layer separator membrane laminated in the order of polypropylene, polyethylene, and polypropylene, etc.

[0066] The materials of the inorganic substances include, but are not limited to, oxides such as alumina and silica, nitrides such as aluminum nitride and silicon nitride, and sulfates (e.g., barium sulfate, calcium sulfate, etc.). The forms of the inorganic substances include, but are not limited to, granular or fibrous.

[0067] The form of the separator membrane can be a thin film form, including, but not limited to, non-woven fabric, woven fabric, microporous membrane, etc. In the thin film form, the pore diameter of the separator membrane is 0.01 μm to 1 μm, and the thickness is 5 μm to 50 μm. In addition to the above independent thin film-like separator membrane, the following separator membrane can also be used: a separator membrane formed by forming a composite porous layer containing the above inorganic substance particles on the surface of the positive electrode sheet and / or the negative electrode sheet by using a resin-based adhesive. For example, a separator membrane formed by using a fluororesin as an adhesive to form a porous layer on both sides of the positive electrode sheet with 90% of the alumina particles having a particle size less than 1 μm.

[0068] The thickness of the separator membrane is arbitrary. In some embodiments, the thickness of the separator membrane is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the separator membrane is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the separator membrane is within the above range, insulation and mechanical strength can be ensured, and the rate performance and energy density of the secondary battery can also be ensured.

[0069] In the present application, the separator may include a substrate and a surface treatment layer. The substrate may be a non-woven fabric or a composite film having a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate, and the surface treatment layer may be a polymer layer, an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. For example, the inorganic layer includes inorganic particles and a binder. There is no particular limitation on the above-mentioned inorganic particles in the present application. For example, it may include at least one of alumina, silica, magnesia, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. There is no particular limitation on the above-mentioned binder in the present application. For example, it may be at least one of the foregoing binders. The polymer layer contains a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinylpyrrolidone, polyethylene ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0070] The secondary battery of the present application further includes a packaging bag for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte, as well as other components known in the art in the secondary battery. There is no limitation on the above-mentioned other components in the present application. There is no particular limitation on the packaging bag in the present application. It may be a packaging bag well-known in the art as long as it can achieve the purpose of the present application.

[0071] The present application also provides an electronic device, which includes the secondary battery in any of the foregoing embodiments. Therefore, the electronic device provided by the present application has good use performance.

[0072] There is no particular limitation on the type of the electronic device in the present application, and it may be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo earphone, a video recorder, a liquid crystal TV, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power source, a motor, an automobile, a motorcycle, a moped, a bicycle, a lighting fixture, a toy, a game machine, a clock, a power tool, a flash, a camera, a large household battery, and a lithium ion capacitor, etc.

[0073] Examples

[0074] Hereinafter, taking a lithium-ion battery as an example, examples and comparative examples are given to more specifically illustrate the implementation modes of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0075] Testing methods and equipment:

[0076] Fast charging performance test at low temperature

[0077] Take 8 lithium-ion batteries of the same sample and place them in a low-temperature environment of -5°C. Let them stand for 1 h to make the lithium-ion batteries reach a constant temperature. Then, charge and discharge the lithium-ion batteries cyclically at 3C. After the cycle is completed, disassemble the lithium battery. After comparison, determine the lithium precipitation window of the lithium-ion battery according to the lithium precipitation situation on the negative electrode plate (i.e., the anode).

[0078] The steps of cyclic charge and discharge are specifically as follows:

[0079] (1) Constant current discharge at 0.7C to 3.0V; (2) Stand for 5 minutes; (3) Constant current charge at 3C rate to 4.5V, and then constant voltage charge to 0.05C; (4) Stand for 5 minutes; (5) Repeat steps (1) to (4) 10 times.

[0080] In the test results, the higher the magnification of the lithium precipitation window, the stronger the fast charging ability at low temperature. Under the lithium precipitation window of the same magnification, the milder the lithium precipitation degree and the fewer the lithium precipitation regions, the stronger the fast charging ability at low temperature. Specifically, the ranking of the fast charging ability from poor to good is: severe lithium precipitation on the entire anode surface < serious lithium precipitation on the entire anode surface < moderate lithium precipitation on the entire anode surface < slight lithium precipitation on the entire anode surface < very slight lithium precipitation on the entire anode surface < severe lithium precipitation in a local area of the anode < moderate lithium precipitation in a local area of the anode < slight lithium precipitation in a local area of the anode < slight lithium precipitation on the anode < no lithium precipitation on the anode, corresponding to "9", "8", "7", "6", "5", "4", "3", "2", "1", "0" respectively.

[0081] Capacity test

[0082] Place the lithium-ion battery that has not undergone any tests in an environment of 45°C, perform the first charge at 0.5C, the charging cut-off voltage is 4.5V, then constant voltage charge to 0.05C, stand for 1 minute, and discharge at 0.5C to 3.0V. The discharge capacity at this time is the measured capacity, with the unit of mAh.

[0083] Test method for the content of silicon element

[0084] Along the thickness direction of the negative electrode plate, scrape the powder sample of the active material layer in the area 5 μm thick from the surface layer of the negative electrode plate, and measure the content Y of silicon element in the second active material layer by inductively coupled plasma spectrometer (ICP). Scrape the powder sample of the active material layer in the area 5 μm thick from the negative electrode current collector, and measure the content X of silicon element in the first active material layer by ICP.

[0085] Example 1-1

[0086] <Preparation of electrolyte>

[0087] In an argon atmosphere glove box with water content less than 10 ppm, mix ethylene carbonate and ethyl acetate according to a mass percentage of 1:1 to prepare a basic solvent, and then add lithium salt lithium hexafluorophosphate (LiPF6), Compound of Formula I and Compound of Formula II. Based on the total mass of the electrolyte, the mass percentage content of LiPF6 is 12.5%, and the mass contents of the Compound of Formula I and the Compound of Formula II are shown in Table 1, and the balance is the basic solvent.

[0088] For the codes in the table, please refer to the above content in this article.

[0089] <Preparation of positive electrode plate>

[0090] Mix lithium cobaltate, conductive agent conductive carbon black, and polyvinylidene fluoride (PVDF) according to a mass ratio of 95:2:3, add N-methylpyrrolidone (NMP), and stir evenly under the action of a vacuum mixer to obtain a positive electrode slurry with a solid content of 70 wt%. Coat the positive electrode slurry evenly on the upper and lower surfaces of a positive electrode current collector aluminum foil with a thickness of 9 μm, and after drying and pressing, cut it into a specified size to obtain a positive electrode plate.

[0091] <Preparation of negative electrode plate>

[0092] Mix artificial graphite, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), carbon nanotubes (CNT), and carboxymethyl cellulose (CMC) according to a mass ratio of 95.8:2.4:0.5:0.5:0.8, and then add deionized water as a solvent and stir evenly to prepare a negative electrode slurry with a solid content of 45 wt%. Coat the negative electrode slurry evenly on the upper and lower surfaces of a negative electrode current collector copper foil with a thickness of 6 μm, and after drying and pressing, cut it into a specified size to obtain a negative electrode plate.

[0093] <Separator>

[0094] Use a porous polyethylene film with a thickness of 15 μm as the separator.

[0095] <Preparation of secondary battery>

[0096] Stack the prepared positive electrode sheet, separator, negative electrode sheet, and separator in sequence, with the separator in the middle of the positive electrode sheet and the negative electrode sheet to play a role in isolation, and then wind to obtain an electrode assembly. After welding the electrode tabs, place the electrode assembly in an aluminum-plastic film packaging bag, dry it in a vacuum oven at 85°C for 12 hours to remove moisture, inject the prepared electrolyte, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, formation, shaping, and capacity testing.

[0097] Examples 1-2 to 1-33

[0098] Except for adjusting the parameters of the electrolyte according to Table 1, the rest are the same as in Example 1-1.

[0099] Comparative Examples 1 to 6

[0100] Except for adjusting the parameters of the electrolyte according to Table 1, the rest are the same as in Example 1-1.

[0101] Table 1

[0102]

[0103]

[0104]

[0105] In Table 1, it can be seen from the comparative examples and examples that when the electrolyte does not contain the compound of formula I, or the content of the compound of formula I is relatively small, the ion transport performance of the electrolyte at low temperature is poor, so the fast charging performance of the secondary battery at low temperature is poor; when the electrolyte does not contain the compound of formula II, or the content of the compound of formula II is relatively small, the positive electrode sheet cannot be well protected and is easily damaged by the acidic substances decomposed by the compound of formula I, and the fast charging ability of the secondary battery at low temperature is also poor. It can be seen from Examples 1-1 to 1-9 that when the content of the compound of formula I is in the range of 10% to 30%, the fast charging performance of the secondary battery at low temperature is better; it can be seen from Examples 1-16 to 1-27 that when the content of the compound of formula II is in the range of 5% to 40%, the fast charging performance of the secondary battery at low temperature is better. In addition, it can also be seen from Table 1 that when 0.9 ≤ A / B ≤ 2, the fast charging performance of the secondary battery at low temperature is also better.

[0106] Examples 2-1 to 2-20

[0107] Except for adjusting the parameters of the electrolyte according to Table 2, the rest are the same as in Example 1-4.

[0108] Table 2

[0109]

[0110]

[0111]

[0112] In Table 2, it can be seen from Examples 2-1 to 2-20 and Example 1-4 that when the mass content of the first component is in the range of 0.3% to 22%, the first component can form an organic SEI film on the interface of the negative electrode sheet of the secondary battery. On the one hand, the organic SEI film can improve the tensile properties of the SEI film, and on the other hand, it can further improve the transmission rate of lithium ions at the interface of the negative electrode sheet at low temperature, so that the fast charging ability of the secondary battery at low temperature is better. In particular, when the content of the first component is in the range of 1% to 15%, the fast charging ability of the secondary battery at low temperature can be further improved.

[0113] Example 3-1

[0114] Except for preparing the negative electrode sheet in the following manner, the rest is the same as Example 1-4.

[0115] <Preparation of Negative Electrode Sheet>

[0116] Mix artificial graphite, SBR, PAA, CNT, CMC, and silicon-carbon composite material (the mass ratio of silicon element in the silicon-carbon composite material is 50%), and the mass ratio of each raw material is 91.7:1:0.5:0.8:6; then add deionized water as a solvent and stir evenly to prepare a first negative electrode slurry with a solid content of 45wt%; evenly coat the first negative electrode slurry on the upper and lower surfaces of a negative electrode current collector copper foil with a thickness of 6μm, and form a first active layer after drying and pressure treatment.

[0117] Mix artificial graphite, SBR, PAA, CNT, CMC, and silicon-carbon composite material (the mass ratio of silicon element in the silicon-carbon composite material is 50%), and the mass ratio of each raw material is 57.7:1:0.5:0.8:40. Then add deionized water as a solvent and stir evenly to prepare a second negative electrode slurry with a solid content of 45wt%; evenly coat the second negative electrode slurry on the upper and lower surfaces of the first active layer, form a second active layer after drying and pressure treatment, and then cut it into a specified size to obtain the negative electrode sheet.

[0118] Examples 3-2 to 3-6

[0119] Except for adjusting the parameters of the negative electrode active material layer according to the parameters in Table 3, the rest is the same as in Example 3-1; specifically: when preparing the first active layer or the second active layer, PAA, CNT, and CMC are the same as in Example 3-1, and the mass percentage of silicon element is as shown in Table 3, and the rest is artificial graphite; therefore, in Examples 3-2 to 3-6, based on the mass of the first active layer or the second active layer, the sum of the mass contents of artificial graphite and silicon-based active material is the same as that in Example 3-2, both being 97.7%.

[0120] Table 3

[0121]

[0122] Note: In Table 3, the electrolytes in Examples 3-1 to 3-6 are exactly the same as those in Example 1-4, so the electrolytes in Examples 3-1 to 3-6 are not described in detail in Table 3.

[0123] In Table 3, from Examples 3-1 to 3-6 and Example 1-4, it can be seen that when the negative electrode active material layer contains a silicon-based active material, the capacity of the secondary battery can be improved, and when both the first active layer and the second active layer contain a silicon-based active material, the capacity improvement of the secondary battery is particularly obvious. For example, in Example 3-5, the first active layer does not contain a silicon-based active material, so although the capacity of its secondary battery is higher than that of Example 1-4, it is significantly worse than other examples. Particularly, due to the poor kinetic performance of the negative electrode active material, in the negative electrode active material layer, when the mass content of silicon element in the first active layer is small, the silicon-based active material has little influence on the low-temperature fast charging performance of the secondary battery. At this time, the secondary battery not only has good battery capacity but also can take into account the fast charging performance at low temperature; for example, in Example 3-3, since the mass content X% of the silicon-based active material in its first active layer is greater than the mass content Y% of the silicon-based active material in the second active layer, its fast charging performance at low temperature is significantly worse than other examples. In addition, it can also be seen from Table 3 that when the thickness of the second active layer is greater than that of the first active layer, the low-temperature fast charging performance of the secondary battery can be further improved, such as in Example 3-4.

[0124] It should be noted that in this article, the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method or article including a series of elements not only includes those elements but also includes other elements not explicitly listed, or further includes elements inherent to such a process, method or article.

[0125] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other, and the key points of each embodiment are the differences from other embodiments.

[0126] The above are only the embodiments of the present application and are not intended to limit the protection scope of the present application. For those skilled in the art, various changes and modifications can be made to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. An electrolyte, characterized in that: It includes a compound of formula I and a compound of formula II, wherein the general structural formula of the compound of formula I is shown in the following formula I: Where R x Any one selected from a single bond, a C1-C4 alkyl group or a C1-C4 alkyl group substituted by R4, R1-R4 are each independently selected from a C2-C6 alkyl group unsubstituted or substituted by Ra, a C2-C6 alkenyl group unsubstituted or substituted by Ra, a C2-C6 alkynyl group unsubstituted or substituted by Ra, a C5-C12 nitrogen-containing heteroaryl group unsubstituted or substituted by Ra, and a C6-C12 aryl group unsubstituted or substituted by Ra; the substituent Ra of each group is each independently selected from fluorine or a C1-C6 fluoroalkyl group; The general structural formula of the compound of formula II is shown in the following formula II: Among them, R5~R 10 Each is independently selected from any one of hydrogen, fluorine, C1-C3 alkyl or C1-C3 fluoroalkyl; Based on the mass of the electrolyte, the mass content of the compound of formula I is A%, the mass content of the compound of formula II is B%, 5≤A≤50, 0.36≤A / B≤5.

2. The electrolyte according to claim 1, characterized in that It meets at least one of the following conditions: (1)10≤A≤30； (2)5≤B≤40； (3)0.9≤A / B≤2.

3. The electrolyte according to claim 1, characterized in that The compound of formula I includes at least one of the following compounds:

4. The electrolyte according to claim 1, characterized in that The compound of formula II includes at least one of the following compounds:

5. The electrolyte according to claim 1, characterized in that The electrolyte further includes a first component, the first component including at least one of fluoroethylene carbonate, vinylene carbonate, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, ethoxypentafluorocyclotriphosphazene, succinonitrile, adiponitrile or 1,3,6-hexanetrinitrile; Based on the mass of the electrolyte, the mass content of the first component is C%, and 0.3≤C≤22.

6. The electrolyte according to claim 5, characterized in that Based on the mass of the electrolyte, 1≤C≤15.

7. A secondary battery, characterized in that: It comprises the electrolyte according to any one of claims 1 to 6.

8. The secondary battery according to claim 7, characterized in that: It also includes a negative electrode plate, which includes a negative electrode collector. A negative electrode active material layer is arranged on at least one surface of the negative electrode collector, and the negative electrode active material layer includes a silicon-based active material.

9. The secondary battery according to claim 8, characterized in that: The negative electrode active material layer includes a first active layer and a second active layer, the first active layer is located between the negative electrode current collector and the second active layer, based on the mass of the first active layer, the mass content of silicon in the first active layer is X%, 0≤X≤35, based on the mass of the second active layer, the mass content of silicon in the second active layer is Y%, 3.5≤Y≤60, Y>X.

10. The secondary battery according to claim 9, characterized in that: The thickness of the first active layer is 20 μm to 90 μm, and the thickness of the second active layer is 10 μm to 70 μm.

11. The secondary battery according to claim 8, characterized in that: The silicon-based active material includes at least one of a silicon-carbon material, a silicon-oxygen material or Si.

12. An electronic device, characterized in that: It comprises the secondary battery according to any one of claims 7 to 11.

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