CN117039157A

CN117039157A - Electrolyte for lithium ion secondary battery and lithium ion secondary battery

Info

Publication number: CN117039157A
Application number: CN202311078826.5A
Authority: CN
Inventors: 刘佛林; 熊亚丽; 施开赢; 王群峰; 陶亮
Original assignee: Huai'an Junsheng New Energy Technology Co ltd; Sinochem Yangzhou Lithium Battery Technology Co Ltd
Current assignee: Huai'an Junsheng New Energy Technology Co ltd; Sinochem Yangzhou Lithium Battery Technology Co Ltd
Priority date: 2023-08-25
Filing date: 2023-08-25
Publication date: 2023-11-10

Abstract

The invention discloses an electrolyte for a lithium ion secondary battery and the lithium ion secondary battery containing the electrolyte, wherein the electrolyte contains a solvent, lithium salt, a pyrocarbonate compound and a fluorobenzene compound, and the mass percentage of additives in the electrolyte is as follows: k1 is more than or equal to 0.2% and less than or equal to 1.2%, k2 is more than or equal to 1% and less than or equal to 5%, k1/k2 is more than or equal to 0.1 and less than or equal to 0.3, wherein k1 represents the mass percent of pyrocarbonate compounds in the electrolyte, and k2 represents the mass percent of fluorobenzene compounds in the electrolyte. The electrolyte provided by the invention can be used for constructing the SEI film with low impedance and high stability, and effectively solves the problems of poor dynamic performance and short cycle life of the battery.

Description

Electrolyte for lithium ion secondary battery and lithium ion secondary battery

Technical Field

The invention belongs to the field of electrolyte, and relates to electrolyte for a lithium ion secondary battery and the lithium ion secondary battery.

Background

The lithium ion battery has the characteristics of high voltage platform, high specific energy density, long cycle life and the like, is widely applied to various fields of mobile phones, automobiles, power stations and the like, and is particularly strong in demand along with the vigorous development of new energy automobile industry. Compared with soft package and square battery, the cylindrical battery has the advantages of simple process, high standardization, good consistency, less whole-package surface structural members, simple grouping and high grouping efficiency. The 46-series large cylindrical battery with the diameter of 46mm has the advantages of energy density, power density and economy, and is the large cylindrical battery with the optimal diameter size under the current production technology level. At present, various vehicle enterprises at home and abroad have publicly announced that large cylindrical batteries are to be used or considered.

In the first charging process of the lithium ion battery, the electrolyte reacts with the negative electrode to form a solid electrolyte interface film, namely SEI film. The SEI film can prevent the electrolyte and the negative electrode from further reacting while well transmitting ions, and has the characteristics of thinner thickness, excellent stability and the like, and the thinner thickness can reduce interface impedance, reduce battery impedance and improve dynamic performance; the excellent stability can effectively resist deformation caused by lithium deintercalation of the negative electrode in the battery cycle process, inhibit the rupture repair process of the SEI film, reduce the loss of active lithium and improve the battery cycle performance. Because large cylindrical batteries have higher requirements on rate performance and cycle life, constructing a good SEI film is a key to improving battery performance and meeting market demands.

Currently, the main approach to build SEI films is to add suitable film forming additives to the electrolyte. The film forming additive is subjected to oxidation-reduction reaction under the charge and discharge potential of the battery, and is deposited on the surface of the negative electrode to form an SEI film, and the type and the dosage of the film forming additive have decisive effect on the property of the SEI film. The pyrocarbonate compound is used as an electrolyte additive, can release carbon dioxide, can form a compact and stable SEI film on the surface of a negative electrode, and can improve the dynamic performance and the cycle performance of a battery to a certain extent, but the improvement effect is still not ideal.

Accordingly, there is a need in the art for an electrolyte for lithium ion secondary batteries that can construct a low-resistance, high-stability SEI film and effectively solve the problems of poor battery dynamic performance and short cycle life.

Disclosure of Invention

In order to solve the problems, the invention provides an electrolyte for a lithium ion secondary battery, which contains a pyrocarbonate compound and a fluorobenzene compound, wherein the mass percentage of additives in the electrolyte is as follows: k1 is more than or equal to 0.2% and less than or equal to 1.2%, k2 is more than or equal to 1% and less than or equal to 5%, k1/k2 is more than or equal to 0.1 and less than or equal to 0.3, wherein k1 represents the mass percent of pyrocarbonate compounds in the electrolyte, and k2 represents the mass percent of fluorobenzene compounds in the electrolyte. The electrolyte provided by the invention can be used for constructing an SEI film with low impedance and high stability, effectively solves the problems of poor dynamic performance and short cycle life of a battery, and is particularly suitable for a large-cylinder lithium ion secondary battery.

Specifically, one aspect of the present invention provides an electrolyte for a lithium ion secondary battery, the electrolyte containing a solvent, a lithium salt, a pyrocarbonate compound and a fluorobenzene compound, wherein the mass percentage of additives in the electrolyte is as follows: k1 is more than or equal to 0.2% and less than or equal to 1.2%, k2 is more than or equal to 1% and less than or equal to 5%, k1/k2 is more than or equal to 0.1 and less than or equal to 0.3, wherein k1 represents the mass percent of pyrocarbonate compounds in the electrolyte, and k2 represents the mass percent of fluorobenzene compounds in the electrolyte.

In one or more embodiments, the pyrocarbonate compound is a compound of formula I:

wherein R is ₁ And R is ₂ Each independently selected from C1-C6 alkyl.

In one or more embodiments, the pyrocarbonate compound is selected from one or more of the compounds of formula I-1, formula I-2, formula I-3, and formula I-4:

in one or more embodiments, the fluorobenzene compound is a compound of the formula II:

wherein n is an integer of 1 to 3.

In one or more embodiments, the fluorobenzene compound is selected from one or more of the compounds of formula II-1, formula II-2 and formula II-3:

in one or more embodiments, 0.3% k1 is 0.9%.

In one or more embodiments, 2% or less than or equal to k 2% or less than or equal to 5%.

In one or more embodiments, 0.1.ltoreq.k1/k2.ltoreq.0.25.

In one or more embodiments, the electrolyte further includes a third additive selected from one or more of vinylene carbonate, 1, 3-propane sultone, vinyl sulfate, propene sultone, methylene methane disulfonate, tris (trimethylsilyl) borate, tris (trimethylsilyl) phosphate, adiponitrile, maleic anhydride, triallyl isocyanate, and trimethylsilyl diethylamine.

In one or more embodiments, the third additive comprises 0.5% to 15% by mass of the electrolyte.

In one or more embodiments, the solvent includes a carbonate-based solvent selected from one or more of a cyclic carbonate selected from one or more of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, and butylene carbonate, and a chain carbonate selected from one or more of dimethyl carbonate, diethyl carbonate, and methylethyl carbonate.

In one or more embodiments, the carbonate-based solvent comprises at least one cyclic carbonate and at least one chain carbonate.

In one or more embodiments, the mass ratio of the cyclic carbonate to the chain carbonate is from 1:4 to 2:3.

In one or more embodiments, the lithium salt is selected from one or more of lithium bis (oxalato) borate, lithium bis (trifluoromethanesulfonyl) imide, lithium bis (pentafluoroethylsulfonyl) imide, lithium acetate, lithium trifluoroacetate, lithium fluoroalkylphosphate, lithium hexafluorophosphate, lithium difluorooxalato borate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, and lithium fluoride.

In one or more embodiments, the molar concentration of the lithium salt in the electrolyte is 0.5 to 2mol/L.

Another aspect of the present invention provides an electrochemical device or lithium ion secondary battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte as described in any of the embodiments herein.

In one or more embodiments, the positive electrode active material in the positive electrode sheet is selected from one or more of lithium cobaltate, lithium manganate, lithium nickel cobalt aluminate, lithium nickel cobalt manganese aluminate, lithium iron phosphate, and lithium manganese iron phosphate.

In one or more embodiments, the negative electrode active material in the negative electrode is selected from lithium metal, structured lithium metal, natural graphite, artificial graphite, mesophase carbon spheres, hard carbon, soft carbon, silicon and silicon-carbon composites, li-Sn alloys, li-Sn-O alloys, spinel structured lithiated TiO ₂ -Li ₄ Ti ₅ O ₁₂ And one or more of Li-Al alloys.

In one or more embodiments, the lithium ion secondary battery is in the form of a large cylindrical battery.

In one or more embodiments, the large cylindrical battery gauge is 46mm diameter by 60mm height, 46mm diameter by 90mm height, or 46mm diameter by 120mm height.

Detailed Description

So that those skilled in the art can appreciate the features and effects of the present invention, a general description and definition of the terms and expressions set forth in the specification and claims follows. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs, and in the event of a conflict, the present specification shall control.

The theory or mechanism described and disclosed herein, whether right or wrong, is not meant to limit the scope of the invention in any way, i.e., the present disclosure may be practiced without limitation to any particular theory or mechanism.

Herein, "comprising," "including," "containing," and similar terms are intended to cover the meaning of "consisting essentially of … …" and "consisting of … …," e.g., "a consisting essentially of B and C" and "a consisting of B and C" should be considered to have been disclosed herein when "a comprises B and C" is disclosed herein.

In this document, all features such as values, amounts, and concentrations that are defined as ranges of values or percentages are for brevity and convenience only. Accordingly, the description of a numerical range or percentage range should be considered to cover and specifically disclose all possible sub-ranges and individual values (including integers and fractions) within the range.

Herein, unless otherwise specified, percentages refer to mass percentages, and proportions refer to mass ratios.

Herein, when embodiments or examples are described, it should be understood that they are not intended to limit the invention to these embodiments or examples. On the contrary, all alternatives, modifications, and equivalents of the methods and materials described herein are intended to be included within the scope of the invention as defined by the appended claims.

In this context, not all possible combinations of the individual technical features in the individual embodiments or examples are described in order to simplify the description. Accordingly, as long as there is no contradiction between the combinations of these technical features, any combination of the technical features in the respective embodiments or examples is possible, and all possible combinations should be considered as being within the scope of the present specification.

The pyrocarbonate compound is easy to be decomposed into carbon dioxide CO under heating due to the existence of the unique pyrocarbonate group ₂ And carbonate compounds. CO ₂ Is an excellent inorganic film forming additive, and the SEI film formed by the excellent inorganic film forming additive has thin film thickness, high LiF content and low interface impedance, and meanwhile, the SEI film is compact, strong in toughness and excellent in stability. And the byproduct carbonate compound belongs to a common solvent of the electrolyte, and has no side effect on the battery performance.

The invention has found that although the large cylindrical battery shell has certain mechanical strength and is provided with a pressure release valve, the gaseous CO generated by the pyrocarbonate compound can not be generated ₂ Causing flatulence but excessive CO ₂ The electrolyte is accumulated in the battery, so that the contact area between the electrolyte and the pole piece is reduced, an ion transmission path from the electrolyte to the pole piece is blocked, a similar air resistance effect is formed, the internal resistance of the battery is deteriorated, and adverse contact between the electrolyte and the pole piece is easy to cause side reactions such as lithium precipitation, and the cycle performance of the battery is deteriorated.

According to the invention, the fluorobenzene compound has a smaller contact angle with the battery pole piece, so that the surface tension of the electrolyte can be reduced, the wettability of the pole piece is improved, the electrolyte can fully and uniformly infiltrate the pole piece in the battery, and the problems of reduced contact area between the electrolyte and the pole piece and obstruction of an ion transmission passage caused by gas production of pyrocarbonate are solved.

Therefore, the invention combines the pyrocarbonate compound and the fluorobenzene compound as electrolyte additives to achieve a coordination effect, can construct a good SEI film and achieve unexpected synergy in improving the dynamics and the cycle performance of the battery.

The electrolyte comprises a solvent, an additive and a lithium salt, wherein the additive comprises a pyrocarbonate compound serving as a first additive and a fluorobenzene compound serving as a second additive. In some embodiments, the electrolyte of the present invention consists of a solvent, an additive, and a lithium salt.

The pyrocarbonate compounds suitable for use in the present invention may be of formula I:

wherein R is ₁ And R is ₂ Each independently selected from C1-C6 alkyl.

In some embodiments, in a compound of formula I, R ₁ And R is ₂ Each independently selected from C1-C3 alkyl groups such as methyl, ethyl, n-propyl, isopropyl.

In some preferred embodiments, the pyrocarbonate compounds used in the electrolytes of the present invention are selected from one or more of the compounds of formula I-1, formula I-2, formula I-3, and formula I-4:

the research of the invention discovers that the pyrocarbonate compounds shown in the formula I, such as the compound shown in the formula I-1, the compound shown in the formula I-2, the compound shown in the formula I-3 and the compound shown in the formula I-4, belong to homologs with similar C atoms, have similar physicochemical properties, can achieve a coordination effect when being used together with fluorobenzene compounds as electrolyte additives, can construct a good SEI film, and can achieve unexpected synergistic effect on improving the dynamics and the cycle performance of a battery.

In some preferred embodiments, the pyrocarbonate compounds used in the electrolytes of the present invention are compounds of formula I-1. Compared with other pyrocarbonate compounds, the compound of the formula I-1 and the fluorobenzene compound can lead the battery to obtain better dynamic performance and cycle performance after being matched for use.

Fluorobenzene compounds suitable for use in the present invention can be of the formula II:

wherein n is 1, 2 or 3.

When n is 2 or 3, the position of the F atom on the benzene ring in the compound of formula II is not particularly limited.

In some preferred embodiments, the fluorobenzene compound used in the electrolyte of the present invention is selected from one or more of the compounds of formula II-1, formula II-2 and formula II-3:

the research of the invention discovers that fluorobenzene compounds shown in the formula II, such as a formula II-1 compound, a formula II-2 compound and a formula II-3 compound, belong to fluorobenzene with similar structures, have similar physicochemical properties, can achieve a coordination effect when being used together with a pyrocarbonate compound as an electrolyte additive, can construct a good SEI film, and can achieve unexpected synergistic effect on improving the dynamics and the cycle performance of a battery.

In some preferred embodiments, the fluorobenzene compound used in the electrolyte of the present invention is a compound of the formula II-1. Compared with other fluorobenzene compounds, the compound of the formula II-1 and the pyrocarbonate compound can lead the battery to obtain better dynamic performance and cycle performance after being matched for use.

In some preferred embodiments, the pyrocarbonate compounds used in the electrolytes of the present invention are compounds of formula I-1 and the fluorobenzene compounds are compounds of formula II-1.

One of the key points of the electrolyte with obviously improved dynamic performance and cycle performance is that the contents of the pyrocarbonate compounds and the fluorobenzene compounds in the electrolyte are strictly controlled so as to meet the following requirements: k1 is more than or equal to 0.2% and less than or equal to 1.2%, k2 is more than or equal to 1% and less than or equal to 5%, k1/k2 is more than or equal to 0.1 and less than or equal to 0.3, wherein k1 represents the mass fraction of pyrocarbonate compounds in the electrolyte, and k2 represents the mass fraction of fluorobenzene compounds in the electrolyte. The ranges of k1, k2 and k1/k2 are controlled within the range, so that the functions of decomposing the pyrocarbonate compound to generate carbon dioxide to form an SEI film with stability, compactness, strong toughness, low interface impedance, thin thickness and high LiF content can be simultaneously exerted, and the fluorobenzene compound can improve the electrolyte wettability of the pole piece and the contact area of the electrolyte and the pole piece and the ion transmission flux, so that the battery has excellent dynamic performance and cycle performance.

In some embodiments, k1 is 0.2%, 0.24%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2% or within a range of any two of the foregoing values. When k1 is more than or equal to 0.2% and less than or equal to 1.2%, the coke acid ester compound and the fluorobenzene compound are used as electrolyte additives in combination, so that a coordination effect can be achieved, a good SEI film is constructed, and the dynamics and the cycle performance of the battery are effectively improved. In some preferred embodiments, 0.3% k1 0.9% can result in better improvements in the dynamic and cycling performance of the cell.

In some embodiments, k2 is 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or within a range of any two of the foregoing values. When k2 is more than or equal to 1% and less than or equal to 5%, the fluorobenzene compound and the pyrocarbonate compound are used as electrolyte additives in combination, so that a coordination effect can be achieved, a good SEI film is constructed, and the dynamics and the cycle performance of the battery are effectively improved. In some preferred embodiments, 2% or less than or equal to k 2% or less than or equal to 5% can result in better improvements in the dynamic and cycling performance of the battery.

In some embodiments, k1/k2 is 0.1, 0.15, 0.17, 0.2, 0.25, 0.3, or within a range of any two of the foregoing values. When k1/k2 is more than or equal to 0.1 and less than or equal to 0.3, the fluorobenzene compound and the pyrocarbonate compound are used as electrolyte additives in combination, so that a coordination effect can be achieved, a good SEI film is constructed, and the dynamics and the cycle performance of a battery are effectively improved. In some preferred embodiments, 0.1.ltoreq.k1/k2.ltoreq.0.25, which enables better improvements in the dynamic and cyclic performance of the battery.

The electrolyte of the present invention may optionally or optionally contain, as a third additive, an electrolyte additive other than the pyrocarbonate compound or the fluorobenzene compound in addition to the first additive and the second additive, as required. The third additive suitable for use in the present invention includes, but is not limited to, one or more selected from the group consisting of vinylene carbonate, 1, 3-propane sultone, vinyl sulfate, propenesulfonic acid lactone, methylene methane disulfonate, tris (trimethylsilyl) borate, tris (trimethylsilyl) phosphate, adiponitrile, maleic anhydride, triallyl isocyanate, and trimethylsilyldiethylamine. The appropriate third additive may be selected according to the performance requirements of the electrolyte. In some embodiments, the electrolyte of the present invention contains a third additive that contains or consists of vinylene carbonate and vinyl sulfate. The vinylene carbonate is used as an organic film forming additive and an overcharge protection additive, has good high-low temperature performance and anti-flatulence function, and can improve the capacity and the cycle life of the battery. The vinyl sulfate can inhibit the reduction of the initial capacity of the battery, increase the initial discharge capacity, reduce the expansion of the battery after high-temperature placement, and improve the charge-discharge performance and the cycle times of the battery.

In the present invention, the addition amount of the third additive may be selected as needed. The total mass fraction of the third additive in the electrolyte is typically 0.5% to 15%, for example 1%, 2%, 3%, 5%, 10%. When the third additive comprises vinylene carbonate, the mass fraction of vinylene carbonate in the electrolyte may be 0.2% -2%, for example 1±0.5%; when the third additive comprises vinyl sulfate, the mass fraction of vinyl sulfate in the electrolyte may be 0.2% -2%, for example 1±0.5%.

The solvent in the electrolyte of the present invention may include a carbonate-based solvent. The carbonate-based solvent may comprise 50%, 60%, 70%, 80%, 90%, 100% or a range of any two of the foregoing, by mass of the solvent in the electrolyte. In some embodiments, the solvent in the electrolyte of the present invention is a carbonate-based solvent. Useful carbonate solvents include, but are not limited to, one or more selected from the group consisting of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate. Wherein ethylene carbonate, fluoroethylene carbonate, propylene carbonate and butylene carbonate belong to cyclic carbonates, and dimethyl carbonate, diethyl carbonate and methylethyl carbonate belong to chain carbonates. In a preferred embodiment, the carbonate-based solvent used in the present invention comprises at least one cyclic carbonate and at least one chain carbonate; preferably, the mass ratio of cyclic carbonate to chain carbonate is 1:4 to 2:3. In some embodiments, the carbonate-based solvent comprises Ethylene Carbonate (EC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and Ethyl Methyl Carbonate (EMC), and the mass ratio of EC: FEC: DMC: EMC may be (15.+ -.2): 10.+ -.2): 30.+ -. 5): 45.+ -. 5.

In some embodiments, other solvents than the carbonate-based solvent, such as a carboxylate-based solvent, an ether-based solvent, or other aprotic solvents, may be added to the electrolyte as desired.

The electrolyte of the present invention includes a lithium salt. The lithium salt may be selected from one or more of an organic lithium salt and an inorganic lithium salt. Useful lithium salts include, but are not limited to, one or more selected from the group consisting of lithium bis (oxalato) borate, lithium bis (trifluoromethanesulfonyl) imide, lithium bis (pentafluoroethylsulfonyl) imide, lithium acetate, lithium trifluoroacetate, lithium fluoroalkylphosphate, lithium hexafluorophosphate, lithium difluorooxalato borate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, and lithium fluoride. In some embodiments, the lithium salt used in the present invention is lithium hexafluorophosphate. The concentration of the lithium salt in the electrolyte may be 0.5 to 2mol/L, for example 1mol/L, 1.2mol/L, 1.5mol/L.

When the electrolyte is prepared, the solvent can be uniformly mixed in a dry environment protected by inert gas, and then the lithium salt and the additive are added to be uniformly mixed, so that the electrolyte is obtained.

The invention also includes electrochemical devices, such as lithium ion secondary batteries (abbreviated as lithium ion batteries), containing the electrolyte of the invention. The electrolyte of the invention is particularly suitable for large cylindrical batteries in the form of lithium ion batteries. The invention uses the combination of two electrolyte additives of the pyrocarbonate compound and the fluorobenzene compound to construct a proper SEI film through coordination, and can improve the dynamics and the cycle performance of a large cylindrical battery; wherein, the pyrocarbonate compound brings gas production problem when participating in forming SEI film, so that the pyrocarbonate compound is more suitable for large-column batteries with such hard shells; in addition, the fluorobenzene compound can solve the problem of poor electrolyte wettability in a large cylindrical battery to a certain extent, so that the electrolyte additive combination and the electrolyte are suitable for the large cylindrical battery.

The electrochemical device of the present invention includes a positive electrode, a negative electrode, and the electrolyte of the present invention. The lithium ion battery comprises a positive electrode, a negative electrode, a diaphragm and the electrolyte, wherein the diaphragm is arranged between the positive electrode and the negative electrode. The sheet-like positive electrode and negative electrode are called a positive electrode sheet and a negative electrode sheet, respectively.

The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode current collector may be aluminum foil, copper foil, titanium foil, nickel foil, iron foil, zinc foil, or the like. The positive electrode active material layer includes a positive electrode active material (also referred to as positive electrode material). The positive electrode active material may include a compound capable of reversibly intercalating and deintercalating lithium ions. The positive electrode active material may include a composite oxide including lithium and at least one selected from cobalt, manganese, nickel, and iron. Examples of the positive electrode active material include binary positive electrode materials (lithium cobalt oxide, lithium manganate), ternary positive electrode materials (for example, lithium nickel cobalt manganate, lithium nickel cobalt aluminate), quaternary positive electrode materials (lithium nickel cobalt manganese aluminate), lithium iron phosphate, lithium manganese iron phosphate, and the like. In some embodiments, the positive electrode active material is lithium nickel cobalt manganate, such as high nickel lithium nickel cobalt manganate. As used herein, "high nickel" means that the amount of the nickel element in the positive electrode active material is not less than 80%, for example not less than 90%, of the total amount of the metal elements other than lithium. Examples of high nickel cobalt lithium manganates include NCM811, liNi _0.8 Co _0.1 Mn _0.1 O ₂ 。

The positive electrode active material layer may further include one or both selected from a conductive agent and a binder. The conductive agent is used to improve the conductivity of the electrode. Conductive agents that may be used for the positive electrode include carbon black (e.g., conductive carbon black), carbon fiber, acetylene black, conductive graphite, graphene, carbon Nanotubes (CNT), carbon microspheres, and the like. The binder of the positive electrode is used to improve the binding property of the positive electrode active material particles to each other and to the current collector. The binder that can be used for the positive electrode includes polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyvinyl alcohol, polyolefin, styrene-butadiene rubber, fluorinated rubber, polyurethane, sodium alginate, and the like. In some embodiments, the conductive agent in the positive electrode active material layer is conductive carbon black and carbon nanotubes, and the binder is PVDF.

The content ratio of each component in the positive electrode active material layer may be conventional. For example, in the positive electrode active material layer, the mass fraction of the positive electrode active material may be 90% to 98%, the mass fraction of the conductive agent may be 1% to 5%, and the mass fraction of the binder may be 1% to 5%.

The positive electrode active material layer is obtained by coating positive electrode slurry containing components of the positive electrode active material layer and a solvent on a positive electrode current collector, drying and pressing. The solvent of the positive electrode slurry may be N-methylpyrrolidone (NMP).

The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode current collector may be copper foil or the like. The anode active material layer includes an anode active material (also called anode material). The anode active material may include a material capable of reversibly intercalating/deintercalating lithium ions, lithium metal, a lithium metal alloy, a material capable of doping/deintercalating lithium, or a transition metal oxide. Examples of the negative electrode active material include lithium metal, structured lithium metal, natural graphite, artificial graphite, mesophase carbon spheres, hard carbon, soft carbon, silicon-carbon composite (Si/C composite material), li-Sn alloy, li-Sn-O alloy, spinel structured lithiated TiO ₂ -Li ₄ Ti ₅ O ₁₂ Li-Al alloy, and the like. In some embodiments, the negative electrode active material is a silicon-carbon composite.

The anode active material layer may further include one or both selected from a conductive agent and a binder. The conductive agent is used to improve the conductivity of the electrode. Conductive agents that may be used for the negative electrode include carbon black (e.g., conductive carbon black), acetylene black, carbon nanotubes, carbon nanowires, carbon microspheres, carbon fibers, graphene, and the like. The binder of the anode is used to improve the binding property of the anode active material particles with each other and with the current collector. The binder that can be used for the negative electrode includes polyvinylidene fluoride, polytetrafluoroethylene, acrylonitrile copolymer, polyacrylic acid (PAA), polybutyl acrylate, polyacrylonitrile, styrene Butadiene Rubber (SBR), and the like. The anode active material layer may further contain a thickener such as sodium carboxymethyl cellulose (CMC-Na). In some embodiments, the conductive agent in the negative electrode active material layer is conductive carbon black, the binder is SBR, and the thickener is CMC-Na.

The mass ratio of each component in the anode active material layer may be conventional. For example, in the anode active material layer, the mass fraction of the anode active material may be 90% to 98%, the mass fraction of the conductive agent may be 0.5% to 2%, the mass fraction of the binder may be 0.5% to 4%, and the mass fraction of the thickener may be 1% to 4%.

The negative electrode active material layer is obtained by coating a negative electrode slurry containing components of the negative electrode active material layer and a solvent onto a negative electrode current collector, drying and pressing. The solvent of the anode slurry may be water.

The separator may be a polymer porous separator, an inorganic porous separator, or a polymer-inorganic composite porous separator. The polymeric porous separator includes a single layer polymeric porous separator and a multi-layer polymeric porous separator. In some embodiments, the separator is a polyethylene separator.

Laminating and/or winding the positive plate, the negative plate and the diaphragm to enable the diaphragm to be positioned between the positive plate and the negative plate, packaging the diaphragm in a shell, and drying, injecting liquid (injecting electrolyte), sealing, standing, forming and shaping the diaphragm to obtain the electrochemical device, such as a lithium ion battery. The form of the lithium ion battery of the present invention is not particularly limited, and may be a cylindrical lithium ion battery, a soft-pack lithium ion battery, an aluminum-shell lithium ion battery, or the like, preferably a large cylindrical lithium ion battery. Specifications of large cylindrical lithium ion batteries include 4680 (diameter 46mm×height 60 mm), 4690 (diameter 46mm×height 90 mm), 46120 (diameter 46mm×height 120 mm), and the like.

The present invention also includes an electronic device comprising the electrochemical device of the present invention. The electrochemical device of the present invention is suitable for various fields of electronic devices. The use of the electrochemical device of the present invention is not particularly limited, and it may be used for any use known in the art. In some embodiments, the electrochemical device of the present invention may be used in, but is not limited to, the following electronic devices: notebook computers, pen-input computers, mobile computers, electronic book players, portable telephones, portable facsimile machines, portable copiers, portable printers, headsets, video recorders, liquid crystal televisions, hand-held cleaners, portable CD players, mini compact discs, transceivers, electronic notebooks, calculators, memory cards, portable audio recorders, radios, stand-by power supplies, motors, automobiles, motorcycles, mopeds, bicycles, lighting fixtures, toys, game machines, watches, electric tools, flash lamps, cameras, household large-sized storage batteries, unmanned aerial vehicles, lithium ion capacitors, and the like.

The invention has the following advantages: the invention provides electrolyte which can be used for lithium ion batteries, in particular to large-cylinder lithium ion batteries, and the electrolyte additive is a pyrocarbonate compound and a fluorobenzene compound, and the dosage and the proportion of the two additives are controlled to achieve a coordination effect, so that an SEI film with low impedance, high stability and excellent performance is constructed, and the dynamic performance and the cycle performance of the battery are obviously improved.

The invention will be illustrated by way of specific examples. It should be understood that these examples are illustrative only and are not intended to limit the scope of the invention. The methods, reagents and materials used in the examples are those conventional in the art unless otherwise indicated. The starting compounds in the examples are all commercially available.

Examples 1 to 5 and comparative examples 1 to 9

The lithium ion batteries in the examples and comparative examples were each prepared as follows:

(1) Electrolyte preparation

Ethylene carbonate was fed into an argon glove box with a water content of < 10ppmUniformly mixing Ester (EC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC) according to the weight ratio of EC to FEC to DMC to EMC=15:10:30:45, and adding LiPF ₆ Uniformly stirring a pyrocarbonate compound, a fluorobenzene compound, ethylene carbonate (VC) and ethylene sulfate (PS) to form an electrolyte, wherein LiPF ₆ The concentration of (3) is 1.2mol/L, the mass fraction of the pyrocarbonate compound in the electrolyte is k1, the mass fraction of the fluorobenzene compound in the electrolyte is k2, k1 and k2 in each example and comparative example are shown in Table 1, the mass fraction of the vinylene carbonate in the electrolyte is 1%, the mass fraction of the vinyl sulfate in the electrolyte is 1%, the pyrocarbonate compound in examples 1-5, comparative examples 1 and comparative examples 3-8 is a compound of formula I-1, and the fluorobenzene compound in examples 1-5 and comparative examples 2-8 is a compound of formula II-1.

(2) Preparation of positive plate

Fully stirring and mixing positive electrode active substances lithium nickel cobalt manganese oxide (NCM 811), conductive carbon black (Super P), carbon Nano Tubes (CNT) and a binder polyvinylidene fluoride (PVDF) in an N-methylpyrrolidone (NMP) solvent according to a weight ratio of 95:1.5:0.5:3 to form uniform positive electrode slurry; and (3) coating the slurry on an aluminum foil of the positive electrode current collector, and drying, cold pressing and cutting to obtain the positive electrode plate.

(3) Preparation of negative plate

Fully stirring and mixing a negative electrode active material silicon-carbon composite (SiO mass fraction is 6%), conductive carbon black (Super P), styrene-butadiene rubber (SBR) serving as a binder and sodium carboxymethyl cellulose (CMC-Na) serving as a thickener in a deionized water solvent according to a weight ratio of 95.7:1:1.5:1.8 to form uniform negative electrode slurry; and (3) coating the slurry on a copper foil of a negative current collector, and drying, cold pressing and cutting to obtain a negative plate.

(4) The diaphragm is made of Polyethylene (PE).

(5) Lithium ion battery preparation

Sequentially stacking the positive plate, the diaphragm and the negative plate, winding the diaphragm between the positive plate and the negative plate, shaping the electrode lugs, loading the positive plate, the diaphragm and the negative plate into a stainless steel shell with the specification of 46120 (diameter 46mm multiplied by height 120 mm), adding a cover plate, rolling, sealing and welding, injecting the prepared electrolyte into a dried battery to complete assembly, and standing, forming and shaping the battery to complete the preparation of the large-cylinder lithium ion battery.

Test example 1: DC discharge impedance (DCR) test at 25 DEG C

Standing the lithium ion battery in a constant temperature box at 25 ℃ for 30 minutes to keep the lithium ion battery constant temperature; discharging to 2.8V at constant current of 1C, and standing for 30 minutes; and then charging to 4.2V by using a constant current of 1C, charging to a constant voltage of 0.05C, and standing for 30 minutes to obtain the actual capacity. Discharging for 15min with constant current of 2C (calculated by the actual capacity obtained in one step above the capacity), and recording the voltage at the moment as U1; and then discharging for 10 seconds with a constant current of 2C (the capacity is calculated by the nominal capacity of the lithium ion battery), recording the voltage at the moment as U2, and calculating the corresponding direct current impedance of the lithium ion battery in a 50% SOC state, wherein the result is shown in Table 1.

Dcr= (U1-U2)/2C was discharged at 25 ℃.

Test example 2: test of cycle Performance at 35℃

Placing the lithium ion battery in a 35 ℃ incubator, and standing for 30 minutes to keep the lithium ion battery constant; discharging to 2.8V at constant current of 0.5C, and standing for 30 minutes; constant current charging is carried out to 4.2V at 0.5C, and constant voltage charging is carried out to the current of 0.05C; standing for 30 minutes; then, the discharge was carried out at 0.5C to 2.8V, and the discharge capacity was recorded as the 1 st cycle discharge capacity based on the capacity of this step. Activating the battery for 50 circles per cycle, discharging to 2.8V at a constant current of 0.1C, and standing for 30 minutes; charging to 4.2V at 0.1C constant current, and discharging to 2.8V at 0.1C. The battery was cycled for 300 cycles in the above steps, the discharge capacity at cycle 300 was recorded, and the capacity retention was calculated, and the results are shown in table 1.

Capacity retention (%) =discharge capacity at 300 th cycle/discharge capacity at 1 st cycle×100% after 300 cycles at 35 ℃

Table 1: examples and comparative examples additive mass fractions and corresponding battery performance data

As is clear from the results of the performance tests of comparative examples 1-2 and examples 1-5 in Table 1, when the pyrocarbonate compound and the fluorobenzene compound were used in combination and the amounts of the pyrocarbonate compound and the fluorobenzene compound used satisfied the following conditions: k1 is more than or equal to 0.2% and less than or equal to 1.2%, k2 is more than or equal to 1% and less than or equal to 5%, k1/k2 is more than or equal to 0.3, and the discharge DCR at 25 ℃ and the circulation capacity retention rate at 35 ℃ are better than the single addition performance, so that better dynamics and circulation performance improvement effect are obtained due to the coordination effect of the discharge DCR at 25 ℃. When the amounts of the pyrocarbonate compound and the fluorobenzene compound further satisfy one or more of the following conditions: k1 is more than or equal to 0.3% and less than or equal to 0.9%, k2 is more than or equal to 2% and less than or equal to 5%, k1/k2 is more than or equal to 0.1 and less than or equal to 0.25, and better dynamic performance and cycle performance improvement effects can be obtained.

As is clear from the results of the performance tests of comparative examples 3 to 4 and examples 1 to 5 in Table 1, when the pyrocarbonate compound is out of the range (0.2% -1.2%) set in the present invention, both the 25℃discharge DCR and the 35℃cycle capacity retention rate are deteriorated. Because CO generated by decomposition is generated when the content of the pyrocarbonate compound is too low ₂ The amount is too small, so that a stable SEI film is not formed on the negative electrode, side reactions at the negative electrode in the formation stage are aggravated, the SEI film is thickened, the DCR is discharged at 25 ℃ is worsened, and the dynamic performance is worsened; unstable SEI films are easily broken during cycling, deteriorating the cycling capacity retention at 35℃and the cycling performance. When the content of the pyrocarbonate compound is too high, CO generated by decomposition is decomposed ₂ Excessive and excessive CO ₂ The electrolyte is accumulated in the battery, so that the contact area between the electrolyte and the pole piece is reduced, the ion transmission path from the electrolyte to the pole piece is blocked, a similar air resistance effect is formed, the DCR (direct current collector) is discharged at 25 ℃ and the dynamic performance is deteriorated; poor contact between the electrolyte and the pole piece easily causes side reactions such as lithium precipitation, and the like, thereby deteriorating the cycle capacity retention rate at 35 ℃ and the cycle performance.

As is clear from the results of the performance tests of comparative examples 5 to 6 and examples 1 to 5 in Table 1, when the fluorobenzene compound is out of the setting range (1% -5%) of the present invention, both the 25℃discharge DCR and the 35℃cycle capacity retention rate are deteriorated. Because CO generated by decomposition of pyrocarbonate compounds when the content of fluorobenzene compounds is too low ₂ In the obvious 'air resistance effect', the fluorobenzene compound is changedThe effect of infiltration between the pole piece and the electrolyte is insufficient to relieve the negative effect brought by the above, and the DCR discharge at 25 ℃ is deteriorated, so that the dynamic performance is deteriorated; meanwhile, the electrolyte and the pole piece ion transmission path are not smooth, side reactions such as lithium precipitation and the like are easy to occur, the 35 ℃ cycle capacity retention rate is deteriorated, and the cycle performance is deteriorated. When the content of fluorobenzene compound is too high, the conductivity of the electrolyte is obviously increased, the discharge DCR at 25 ℃ is deteriorated, and the dynamic performance is deteriorated; while lower conductivity would decrease Li ⁺ Migration rate, which results in lithium precipitation during charge and discharge of the battery, worsens the cycle capacity retention at 35 ℃ and worsens cycle performance.

As can be seen from the results of the performance tests of comparative examples 7 to 8 and examples 1 to 5 in Table 1, when the ratio k1/k2 of the mass fraction k1 of the pyrocarbonate compound to the mass fraction k2 of the fluorobenzene compound exceeds the set range (0.1 to 0.3) of the present invention, both the discharge DCR at 25℃and the cycle capacity retention at 35℃are deteriorated. When the k1/k2 value is higher than the preferable range, the effect is similar to that when the content of fluorobenzene compound is too low, and the effect and the reason are influenced as described above; when the k1/k2 value is lower than the preferable range, the effect is similar to that when the fluorobenzene compound content is too high, and the effect and the reason are affected as described above.

From the results of the performance tests of example 1, comparative examples 1-2 and comparative example 9 in table 1, it is apparent that the dynamic performance and cycle performance of the batteries of comparative example 1 and comparative example 2 are slightly improved as compared to comparative example 9 when only one of the pyrocarbonate compound and the fluorobenzene compound is used, whereas the dynamic performance and cycle performance of the battery of example 1 are significantly improved as compared to comparative example 9 when the pyrocarbonate compound and the fluorobenzene compound are used at the same time, which means that the pyrocarbonate compound and the fluorobenzene compound have unexpected synergistic effects as electrolyte additives in improving the dynamic performance and cycle performance of the lithium ion battery.

While the invention has been described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. The electrolyte for the lithium ion secondary battery is characterized by comprising a solvent, lithium salt, a pyrocarbonate compound and a fluorobenzene compound, wherein the mass percentage of additives in the electrolyte is as follows: k1 is more than or equal to 0.2% and less than or equal to 1.2%, k2 is more than or equal to 1% and less than or equal to 5%, k1/k2 is more than or equal to 0.1 and less than or equal to 0.3, wherein k1 represents the mass percent of pyrocarbonate compounds in the electrolyte, and k2 represents the mass percent of fluorobenzene compounds in the electrolyte;

the pyrocarbonate compound is a compound of formula I:

wherein R is ₁ And R is ₂ Each independently selected from C1-C6 alkyl;

the fluorobenzene compound is a compound shown in a formula II:

wherein n is an integer of 1 to 3.

2. The electrolyte of claim 1, wherein the pyrocarbonate compound is selected from one or more of the group consisting of a compound of formula I-1, a compound of formula I-2, a compound of formula I-3, and a compound of formula I-4:

3. the electrolyte of claim 1 wherein the fluorobenzene compound is selected from one or more of the compounds of formula II-1, formula II-2 and formula II-3:

4. the electrolyte of claim 1, wherein the electrolyte has one or more of the following characteristics:

0.3％≤k1≤0.9％；

2％≤k2≤5％；

0.1≤k1/k2≤0.25。

5. the electrolyte of claim 1, further comprising a third additive selected from one or more of vinylene carbonate, 1, 3-propane sultone, vinyl sulfate, propenolactone, methane disulfonic acid methylester, tris (trimethylsilyl) borate, tris (trimethylsilyl) phosphate, adiponitrile, maleic anhydride, triallyl isocyanate, and trimethylsilyl diethylamine; preferably, the third additive accounts for 0.5-15% of the electrolyte by mass.

6. The electrolyte of claim 1, wherein the solvent comprises a carbonate solvent selected from one or more of a cyclic carbonate selected from one or more of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, and butylene carbonate, and a chain carbonate selected from one or more of dimethyl carbonate, diethyl carbonate, and methylethyl carbonate; preferably, the carbonate-based solvent comprises at least one cyclic carbonate and at least one chain carbonate; preferably, the mass ratio of the cyclic carbonate to the chain carbonate is 1:4 to 2:3.

7. The electrolyte of claim 1, wherein the lithium salt is selected from one or more of lithium bis (oxalato) borate, lithium bis (trifluoromethanesulfonyl) imide, lithium bis (pentafluoroethylsulfonyl) imide, lithium acetate, lithium trifluoroacetate, lithium fluoroalkylphosphate, lithium hexafluorophosphate, lithium difluorooxalato borate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, and lithium fluoride; preferably, the molar concentration of the lithium salt in the electrolyte is 0.5 to 2mol/L.

8. An electrochemical device or a lithium ion secondary battery, characterized in that the electrochemical device or the lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet and the electrolyte as defined in any one of claims 1 to 7.

9. The lithium-ion secondary battery according to claim 8, wherein,

the positive electrode active material in the positive electrode sheet is selected from one or more of lithium cobaltate, lithium manganate, lithium nickel cobalt aluminate, lithium nickel cobalt manganese aluminate, lithium iron phosphate and lithium manganese iron phosphate;

the negative electrode active material in the negative electrode is selected from lithium metal, structured lithium metal, natural graphite, artificial graphite, mesophase carbon spheres, hard carbon, soft carbon, silicon-carbon composite, li-Sn alloy, li-Sn-O alloy and spinel structured lithiated TiO ₂ -Li ₄ Ti ₅ O ₁₂ And one or more of Li-Al alloys.

10. The lithium-ion secondary battery according to claim 8, wherein the lithium-ion secondary battery is in the form of a large cylindrical battery.