CN112151864B

CN112151864B - Electrolyte solution, and electrochemical device and electronic device comprising same

Info

Publication number: CN112151864B
Application number: CN202011093519.0A
Authority: CN
Inventors: 刘建; 刘建禹; 管明明; 郑建明
Original assignee: Ningde Amperex Technology Ltd
Current assignee: Ningde Amperex Technology Ltd
Priority date: 2020-10-13
Filing date: 2020-10-13
Publication date: 2021-11-30
Anticipated expiration: 2040-10-13
Also published as: CN112151864A

Abstract

The present application relates to an electricityAn electrolyte comprising a compound of formula I:

wherein R is₁Selected from substituted or unsubstituted C_1‑10Chain alkylene, substituted or unsubstituted C_3‑10Cycloalkylene, substituted or unsubstituted C_1‑10Chain carbonate group, substituted or unsubstituted C_6‑12Aryl or C_3‑12At least one of heterocyclic groups; the heteroatom in the heterocyclyl group is selected from S, O or N; wherein when R is₁When the structure is a cyclic structure, the cyano group of the compound in the formula I is positioned at the ortho position of the boron atom; when substituted, the substituents are selected from fluorine atoms, cyano groups, C_1‑3Alkyl group of (1). The present application also provides an electrochemical device and an electronic device comprising the electrolyte.

Description

Electrolyte solution, and electrochemical device and electronic device comprising same

Technical Field

The present disclosure relates to the field of energy storage technologies, and particularly to an electrolyte, and an electrochemical device and an electronic device including the same.

Background

Electrochemical devices (e.g., lithium ion batteries) have the advantages of environmental friendliness, high energy density, high operating voltage, and long cycle life, and thus are now popular and favored as energy storage devices for electrical energy. With the development of technology, the requirements of energy density and working voltage of batteries are higher and higher, and the conventional electrolyte cannot meet the working requirements under high voltage, so that the development of electrolyte resistant to oxidation under high voltage is urgent.

Disclosure of Invention

The present application solves at least one of the problems occurring in the related art by providing an electrolyte. In particular, the electrolyte provided herein can significantly improve cycle performance and high-temperature storage performance of an electrochemical device. The present application also relates to an electrochemical device and an electronic device comprising such an electrolyte.

The present application provides an electrolyte comprising a compound of formula I:

wherein R is₁Selected from substituted or unsubstituted C_1-10Chain alkylene, substituted or unsubstituted C_3-10Cycloalkylene, substituted or unsubstituted C_1-10Chain carbonate group, substituted or unsubstituted C_6-12Aryl or C_3-12At least one of heterocyclic groups; the heteroatom in the heterocyclyl group is selected from S, O or N;

wherein when R is₁When the structure is a cyclic structure, the cyano group of the compound in the formula I is positioned at the ortho position of the boron atom;

when substituted, the substituents are selected from fluorine atoms, cyano groups, C_1-3An alkyl group.

In some embodiments, the compound of formula I in the electrolyte comprises:

at least one of formula I-8.

In some embodiments, the compound of formula I is present in an amount of 0.1% to 5% based on the total weight of the electrolyte.

In some embodiments, the electrolyte further comprises at least one additive selected from the group consisting of:

(1) lithium difluorophosphate, the content of lithium difluorophosphate being less than or equal to 1.5 percent based on the total weight of the electrolyte;

(2) a fluoro carbonate compound comprising at least one of a compound of formula II or a compound of formula III in an amount of 2% to 40% based on the total weight of the electrolyte

Wherein R is₂、R₃、R₄、R₅、R₆And R₇Each independently selected from substituted or unsubstituted C_1-10Chain alkyl or substituted or unsubstituted C_3-10At least one of cyclic alkyl, wherein, when substituted, the substituent is selected from fluorine atom, cyano, sulfuryl, C_1-3At least one of alkyl groups; and R is₂And R₃At least one of which contains a fluorine atom, R₄、R₅、R₆And R₇At least one of which contains fluorine atoms;

wherein n in the compound of formula III is 0 to 3.

In some embodiments, the compound of formula II comprises at least one of the following compounds:

the compound of formula III includes at least one of the following compounds:

an electrochemical device includes a positive electrode including a positive electrode current collector and a positive electrode active material layer, a negative electrode including a negative electrode current collector and a negative electrode active material layer, and an electrolyte according to the present application.

In some embodiments, the anode active material layer in the electrochemical device of the present application has a diffraction peak at 24 ° to 26 ° as measured by X-ray diffraction (XRD), wherein the diffraction peak intensity ≦ 15000.

In some embodiments, a protective layer comprising at least one of lithium iron phosphate, lithium manganese iron phosphate, or conductive carbon is on the positive current collector of the electrochemical device.

In some embodiments, the protective layer on the positive current collector has a thickness of 0.5 to 7 microns.

The present application also provides an electronic device comprising an electrochemical device according to the present application.

Additional aspects and advantages of embodiments of the present application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of embodiments of the present application.

Drawings

Drawings necessary for describing embodiments of the present application or the prior art will be briefly described below in order to describe the embodiments of the present application. It is to be understood that the drawings in the following description are only some of the embodiments of the present application. It will be apparent to those skilled in the art that other embodiments of the drawings can be obtained from the structures illustrated in these drawings without the need for inventive work.

Fig. 1 shows the X-ray diffraction test results of the negative electrode obtained in the case of using the electrolyte containing the compound of formula I and not containing the compound of formula I.

Detailed Description

Embodiments of the present application will be described in detail below. Throughout the specification, the same or similar components and components having the same or similar functions are denoted by like reference numerals. The embodiments described herein with respect to the figures are illustrative in nature, are diagrammatic in nature, and are used to provide a basic understanding of the present application. The embodiments of the present application should not be construed as limiting the present application.

As used herein, the term "about" is used to describe and illustrate minor variations. When used in conjunction with an event or circumstance, the terms can refer to instances where the event or circumstance occurs precisely as well as instances where the event or circumstance occurs in close proximity. For example, when used in conjunction with numerical values, the term can refer to a range of variation that is less than or equal to ± 10% of the stated numerical value, such as less than or equal to ± 5%, less than or equal to ± 4%, less than or equal to ± 3%, less than or equal to ± 2%, less than or equal to ± 1%, less than or equal to ± 0.5%, less than or equal to ± 0.1%, or less than or equal to ± 0.05%. For example, two numerical values are "about" the same if the difference between the two numerical values is less than or equal to ± 10% (e.g., less than or equal to ± 5%, less than or equal to ± 4%, less than or equal to ± 3%, less than or equal to ± 2%, less than or equal to ± 1%, less than or equal to ± 0.5%, less than or equal to ± 0.1%, or less than or equal to ± 0.05%) of the mean of the values.

Amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity, and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited.

In the detailed description and claims, a list of items connected by the terms "one of," "one of," or other similar terms may mean any one of the listed items. For example, if items a and B are listed, the phrase "one of a and B" means a alone or B alone. In another example, if items A, B and C are listed, the phrase "one of A, B and C" means only a; only B; or only C. Item a may comprise a single element or multiple elements. Item B may comprise a single element or multiple elements. Item C may comprise a single element or multiple elements.

In the detailed description and claims, a list of items linked by the term "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items a and B are listed, the phrase "at least one of a and B" means a only; only B; or A and B. In another example, if items A, B and C are listed, the phrase "at least one of A, B and C" means a only; or only B; only C; a and B (excluding C); a and C (excluding B); b and C (excluding A); or A, B and C. Item a may comprise a single element or multiple elements. Item B may comprise a single element or multiple elements. Item C may comprise a single element or multiple elements.

The following definitions are used in this application (unless explicitly stated otherwise):

for simplicity, the "Cn-m" group is meant to have from "n" to "m" carbonsA group of atoms, wherein "n" and "m" are integers. For example, "C_1-10Alkyl "refers to an alkyl group having 1 to 10 carbon atoms.

The term "alkyl" is intended to be a straight chain saturated hydrocarbon structure having from 1 to 20 carbon atoms. "alkyl" is also contemplated to be a branched or cyclic hydrocarbon structure having from 3 to 20 carbon atoms. For example, the alkyl group may be an alkyl group of 1 to 20 carbon atoms, an alkyl group of 1 to 10 carbon atoms, an alkyl group of 1 to 5 carbon atoms, an alkyl group of 5 to 20 carbon atoms, an alkyl group of 5 to 15 carbon atoms, or an alkyl group of 5 to 10 carbon atoms. When an alkyl group having a particular carbon number is specified, all geometric isomers having that carbon number are intended to be encompassed; thus, for example, "butyl" is meant to include n-butyl, sec-butyl, isobutyl, tert-butyl, and cyclobutyl; "propyl" includes n-propyl, isopropyl and cyclopropyl. Examples of alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, cyclobutyl, n-pentyl, isopentyl, neopentyl, cyclopentyl, methylcyclopentyl, ethylcyclopentyl, n-hexyl, isohexyl, cyclohexyl, n-heptyl, octyl, cyclopropyl, cyclobutyl, norbornyl, and the like. In addition, the alkyl group may be optionally substituted.

The term "alkenyl" refers to a monovalent unsaturated hydrocarbon group that can be straight or branched chain and has at least one and typically 1,2, or 3 carbon-carbon double bonds. Unless otherwise defined, the alkenyl group typically contains 2 to 20 carbon atoms, and may be, for example, an alkenyl group of 2 to 20 carbon atoms, an alkenyl group of 6 to 20 carbon atoms, an alkenyl group of 2 to 10 carbon atoms, or an alkenyl group of 2 to 6 carbon atoms. Representative alkenyl groups include, by way of example, ethenyl, n-propenyl, isopropenyl, n-but-2-enyl, but-3-enyl, n-hex-3-enyl, and the like. In addition, the alkenyl group may be optionally substituted.

The term "cycloalkyl" encompasses cyclic alkyl groups. The cycloalkyl group may be a cycloalkyl group of 2 to 20 carbon atoms, a cycloalkyl group of 6 to 20 carbon atoms, a cycloalkyl group of 2 to 10 carbon atoms, a cycloalkyl group of 2 to 6 carbon atoms. For example, cycloalkyl groups can be cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and the like. In addition, cycloalkyl groups may be optionally substituted.

The term "alkylene" means a divalent saturated alkyl group that may be straight chain or branched. Unless otherwise defined, the alkylene group typically contains 1 to 10, 1 to 6, 1 to 4, or 2 to 4 carbon atoms and includes, for example, C_2-3Alkylene and C_2-6An alkylene group. Representative alkylene groups include, for example, methylene, ethane-1, 2-diyl ("ethylene"), propane-1, 2-diyl, propane-1, 3-diyl, butane-1, 4-diyl, pentane-1, 5-diyl, and the like.

The term "alkenylene" means a bifunctional group obtained by removing one hydrogen atom from an alkenyl group as defined above. Preferred alkenylene groups include, but are not limited to, -CH ═ CH-, -C (CH)₃)＝CH-、-CH＝CHCH₂-and the like.

The term "cycloalkylene" means a bifunctional radical obtained by removing one hydrogen atom from a cycloalkyl radical as defined above. For example cyclopropylene, cyclobutylene, cyclopentylene, cyclohexylene, cycloheptylene or cyclooctylene.

The term "aryl" means a monovalent aromatic hydrocarbon having a single ring (e.g., phenyl) or a fused ring. Fused ring systems include those that are fully unsaturated (e.g., naphthalene) as well as those that are partially unsaturated (e.g., 1,2,3, 4-tetrahydronaphthalene). Unless otherwise defined, the aryl group typically contains 6 to 26, 6 to 20, 6 to 15, 6 to 12, or 6 to 10 carbon ring atoms and includes, for example, C_6-10And (4) an aryl group. Representative aryl groups include, for example, phenyl, methylphenyl, propylphenyl, isopropylphenyl, benzyl, and naphthalen-1-yl, naphthalen-2-yl, phenyl, naphthyl, pyridyl, thienyl, oxadiazolyl, imidazolyl, thiazolyl, furyl, pyrrolyl, phenoxy, naphthoxy, pyridyloxy, thienyloxy, oxadiazoyloxy, imidazolyloxy, thiazolyloxy, furanyloxy, pyrrolyloxy, and the like.

The term "heterocycle" or "heterocyclyl" means a substituted or unsubstituted 5 to 8 membered mono-or bicyclic non-aromatic hydrocarbon in which 1 to 3 carbon atoms are replaced by a heteroatom selected from nitrogen, oxygen or sulfur atoms. Examples include pyrrolidin-2-yl; pyrrolidin-3-yl; a piperidinyl group; morpholin-4-yl, and the like, which groups may be substituted subsequently. "heteroatom" means an atom selected from N, O and S.

As used herein, the term "halogen" may be F, Cl, Br or I.

As used herein, the term "cyano" encompasses organic species containing an organic group-CN.

When the above substituents are substituted, the substituents may be selected from the group consisting of: halogen, alkyl, alkenyl, aryl and heteroaryl.

First, electrolyte

1. A compound of formula I

In some embodiments, the compound of formula I in the electrolyte comprises:

at least one of formula I-8.

To address the problems in the prior art, the present application will now provideThe compound of formula I is introduced into an electrolyte. The electrolyte comprising the compound of formula I has the following overall advantages: (1) can stabilize LiCoO₂The intermediate oxygen free radical stabilizes the structure of the anode material, delays the structural collapse and the crystal structure change of the anode material under the high-voltage environment, and plays a role in improving the high-temperature storage performance of the electrochemical device; (2) can effectively complex LiCoO₂The dissolution of cobalt ions in the positive electrode material in a high lithium removal state is reduced, the structural collapse and the crystal structure change of the positive electrode material in the high lithium removal state are delayed, and the high-temperature storage performance of the electrochemical device is further improved; (3) the high steric hindrance can be formed on the interface of the positive electrode, the electrolyte is effectively prevented from contacting with the active material of the positive electrode under the high-voltage environment, and further oxidative decomposition of the electrolyte is prevented, so that the service life and the oxidation resistance of the electrolyte are improved, byproducts generated by decomposition of the electrolyte are reduced, and the thickness expansion rate of the electrochemical device in the circulating process is reduced; (4) the compound of formula I forms a film on the surface of an active material in preference to a solvent, so that the electrolyte containing the compound of formula I has obvious advantages for high-temperature circulation and high-temperature storage of a high-voltage system. In summary, the electrolyte of the present application is advantageous for improving the high-temperature storage performance and cycle performance of an electrochemical device operating at high voltage and having high energy density.

In some embodiments, the compound of formula I is present in an amount of about 0.1% to about 5%, for example, the compound of formula I may be present in an amount of about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 1%, about 2%, about 3%, about 4%, about 5%, or a range between any two of the foregoing values, based on the total weight of the electrolyte. When the compound of formula I is within the above range, the high-temperature storage performance and the cycle performance of the electrochemical device can be more effectively improved.

2. Lithium difluorophosphate

In some embodiments, the electrolyte further comprises lithium difluorophosphate. The lithium difluorophosphate can be present in an amount of 1.5% or less based on the total weight of the electrolyte; for example, the lithium difluorophosphate can be present in an amount of about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.49%, about 0.5%, about 1%, about 1.5%, or a range between any of the foregoing and zero.

3. Fluorocarbonate compound

In some embodiments, the electrolyte further comprises a fluoro carbonate compound comprising at least one of a compound of formula II or a compound of formula III

Wherein R is₂、R₃、R₄、R₅、R₆And R₇Each independently selected from substituted or unsubstituted C_1-10Chain alkyl or substituted or unsubstituted C_3-10At least one of cyclic alkyl, wherein, when substituted, the substituent is selected from fluorine atom, cyano, sulfuryl, C_1-3At least one of alkyl groups; and R is₂And R₃At least one of which contains a fluorine atom, R₄、R₅、R₆And R₇At least one of which contains fluorine atoms; wherein n is 0 to 3 in the compound of formula III, for example, n may be 0, 1,2 or 3.

the compound of formula III includes at least one of the following compounds:

in some embodiments, the amount of the fluoro carbonate compound may be about 2% to about 40% based on the total weight of the electrolyte, for example, the amount of the compound of formula II or III may be about 2%, about 3%, about 4%, about 5%, about 6%, about 7%, about 8%, about 9%, about 10%, about 15%, about 20%, about 25%, about 30%, about 35%, about 40%, or a range between any two of the above values.

In this application, the oxidation resistance of electrolyte has further been improved in the addition of fluoro carbonic ester compound, has reduced the oxidation of electrolyte at the positive pole surface to reduce the consumption of electrolyte, use with this application formula I compound combination, not only protect the interface of pole piece, also improved the stability of electrolyte self simultaneously.

The compound of formula I used in combination with the cyclic fluoro carbonate compound (compound of formula III) can further improve the cycle performance and high-temperature storage performance of the battery because the SEI formed at the negative electrode of the mixed electrolyte is more dense and stable, on the one hand, prevents the contact of the solvent with the active material, and reduces the decomposition and consumption of the solvent; on the other hand, certain solvents or additives are prevented from being embedded into the graphite layer, resulting in exfoliation of the graphite. Thereby improving the cycle stability of the battery. When the content of the fluoro-carbonate is within the range defined in the present application, the increase in viscosity of the electrolyte and the resulting increase in polarization resistance for lithium ion transport in the electrolyte can be effectively avoided; in addition, a decrease in the conductivity of the electrolytic solution due to fluorine atoms can also be avoided.

3. Organic solvent and lithium salt compound

In some embodiments, the electrolyte of the present application may further comprise one or more of the following organic solvents: ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate ether and dimethyl carbonate.

In some embodiments, the electrode fluids of the present application may also include one or more lithium salt compounds of: lithium hexafluorophosphate, lithium bistrifluoromethylsulphonylimide, lithium bis (fluorosulphonyl) imide, lithium 4, 5-dicyano-2-trifluoromethylimidazolium, lithium perchlorate, lithium bis (1, 1-trifluoromethyloxalate) borate, lithium bis (1-trifluoromethyloxalate) borate, lithium difluoro (1, 1-trifluoromethyloxalate) borate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium bisoxalato borate, lithium fluoromalonic acid difluoroborate and lithium bis (fluoromalonic acid) borate.

In some embodiments, the concentration of the lithium salt compound is in the range of 0.8 to 3mol/L, such as in the range of 0.8 to 2.5mol/L, in the range of 0.8 to 2mol/L, in the range of 1 to 2mol/L, 0.5 to 1.5mol/L, 0.8 to 1.3mol/L, 0.5 to 1.2 mol/L.

Two, electrochemical device

1. Positive electrode

In some embodiments, a protective layer comprising at least one of lithium iron phosphate particles, lithium manganese iron phosphate particles, or conductive carbon is on the positive current collector of the electrochemical device.

In some embodiments, the protective layer on the positive current collector has a thickness of about 0.5 microns to about 7 microns. In some embodiments, the protective layer can have a thickness of, for example, about 0.5 microns, about 1 micron, about 2 microns, about 3 microns, about 4 microns, about 5 microns, about 6 microns, about 7 microns, or a range between any two of the foregoing.

In the electrochemical device of the present application, it is advantageous to provide a protective layer on the positive electrode current collector. On one hand, the addition of the protective layer on the current collector can increase the adhesion of the active material to the current collector and prevent the active material from stripping during the winding process. On the other hand, the addition of the protective layer can reduce the interface impedance between the active material and the current collector, and is beneficial to the rapid transmission of electrons from the surface of the positive electrode material to the current collector, thereby accelerating the transmission of lithium ions from the inside of the positive electrode material, being beneficial to the insertion and extraction of the lithium ions, and further improving the cycle performance of the electrochemical device.

In some embodiments, the electrochemical device according to the present application, after being formed, was subjected to infrared testing at 2240cm for the positive electrode thereof^-1To 2280cm^-1Has obvious absorption vibration peak. The absorption peak results from the compound of formula I contained in the electrolyte of the present application.

2. Negative electrode

In the present application, the kind of the anode active material is not limited, and the anode material used may be selected from carbonaceous materials or metal compounds capable of intercalating and deintercalating metal ions, lithium metal, lithium alloys, and the like. Among these, carbonaceous materials, particularly graphite or graphite surface-coated with amorphous carbon are more preferable. In addition, as the negative electrode active material, a metal compound capable of inserting and extracting a metal ion may also be used. Such a metal compound may be selected from compounds containing metals such as Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, and the like. Such metals may be used in any form such as simple substance, oxide, and alloy with lithium. Among these, a negative electrode active material having at least one atom of the group consisting of a Si (silicon) atom, a Sn (tin) atom, and a Pb (lead) atom is preferable.

The negative electrode active material of at least one atom selected from Si atoms, Sn atoms, and Pb atoms, and a metal simple substance of any one metal element among Si, Sn, and Pb; an alloy composed of 2 or more metal elements among Si, Sn, and Pb; an alloy composed of 1 or 2 or more metal elements among Si, Sn, and Pb and 1 or 2 or more other metal elements; and a compound containing 1 or 2 or more metal elements among Si, Sn and Pb.

Any of these negative electrode materials may be used alone, or 2 or more of them may be used in any combination and ratio.

In some embodiments, the anode active material layer in the electrochemical device of the present application has a diffraction peak at 24 ° to 26 ° and has a diffraction peak intensity ≦ 15000, measured by X-ray diffraction (XRD).

3. Isolation film

In the application, the types of the isolating membranes used for the electrochemical device are not limited, the isolating membranes can be selected from polyethylene membranes, polypropylene membranes, polyvinylidene fluoride membranes and multilayer composite membranes of the polyethylene membranes, the polypropylene membranes and the polyvinylidene fluoride membranes, and meanwhile, inorganic or organic coatings can be coated on the surfaces of isolating membrane substrates according to actual requirements so as to enhance the hardness of batteries or improve the adhesion of the isolating membranes to positive and negative electrode interfaces.

Electronic device

The type of the electronic device of the present application is not particularly limited. In some embodiments, the electronic device of the present application may include devices for, but is not limited to, notebook computers, pen-input computers, mobile computers, electronic book players, cellular phones, portable facsimile machines, portable copiers, portable printers, headphones, video recorders, liquid crystal televisions, hand-held cleaners, portable CDs, mini-discs, transceivers, electronic organizers, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, mopeds, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large batteries, lithium ion capacitors, and the like.

Examples

The present application is further illustrated below with reference to examples. It is specifically stated that the following examples are intended to be illustrative of the present application only and are not intended to limit the scope of the present application.

1. Preparation method

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

(1) preparation of electrolyte

In an argon atmosphere glove box with the water content of less than 10ppm, Ethylene Carbonate (EC), Propylene Carbonate (PC) and diethyl carbonate (DEC) are uniformly mixed according to the mass ratio of 1:1:1, and lithium hexafluorophosphate (LiPF) is added₆) Dissolving and stirring uniformly to form a basic electrolyte, wherein LiPF₆The concentration of (2) is 1.15 mol/L. In this base electrolyte, other additives were added according to the amounts and kinds provided in the following tables, respectively, to obtain electrolytes of respective examples and comparative examples.

(2) Preparation of positive electrode

The positive electrode active material lithium cobaltate (LiCoO)₂) Mixing conductive agent carbon (Super p) and adhesive polyvinylidene fluoride according to a weight ratio of 95:2:3, adding N-methyl pyrrolidone (NMP), stirring under the action of a vacuum stirrer until a system forms uniform positive electrode slurry, and uniformly coating the positive electrode slurry on a positive electrode current collector aluminum foil which does not contain conductive carbon or contains conductive carbon, wherein the thickness of the coating of the conductive carbon on the current collector coated with the conductive carbon is 5 microns; drying at 85 ℃, then carrying out cold pressing, sheet cutting, slitting and tab welding, and then drying at 85 ℃ for 4h under vacuum condition to obtain the anode.

(3) Preparation of negative electrode

Fully stirring and mixing the negative active material graphite, a binder Styrene Butadiene Rubber (SBR) and a thickener sodium carboxymethyl cellulose (CMC) in a proper amount of deionized water according to a weight ratio of 95:2:3 to form uniform negative slurry; coating the slurry on a copper foil of a negative current collector, drying in an oven at 85 ℃, and performing cold pressing, cutting, slitting and tab welding to obtain the negative electrode.

(4) Preparation of isolating film

A Polyethylene (PE) film was used as the separator.

(5) Preparation of lithium ion battery

And sequentially stacking the anode, the isolating membrane and the cathode to enable the isolating membrane to be positioned between the anode and the cathode to play an isolating role, then winding, placing the wound membrane into an aluminum foil bag, sealing the edge of the aluminum foil bag, placing the aluminum foil bag into a vacuum oven to remove moisture, then injecting the electrolyte prepared according to each embodiment and comparative example into the battery after vacuum drying, and carrying out the processes of vacuum packaging, standing, forming and the like to finish the preparation of the lithium ion battery.

2. Test method

(1) Method for testing high-temperature storage performance of lithium ion battery

After the lithium ion battery was left to stand at 25 ℃ for 30 minutes, and then charged to 4.53V at a constant current at a rate of 0.5C, and then charged to 0.05C at a constant voltage at 4.53V, left to stand for 5 minutes, the thickness of the battery was measured, and after being stored at 85 ℃ for 8 hours or at 60 ℃ for 24 days, the thickness expansion rate of the battery was calculated by the following formula:

the battery thickness expansion ratio is [ (thickness after storage-thickness before storage) ÷ thickness before storage ] × 100%.

(2) Method for testing high-temperature capacity retention rate of lithium ion battery

At 45 ℃, the lithium ion battery is charged to 4.53V at a constant current of 0.5C, then charged at a constant voltage to a current of 0.05C, and then discharged to 3.0V at a constant current of 0.5C, this time the first cycle. The lithium ion battery is cycled for a plurality of times according to the above conditions. And (3) repeatedly carrying out charge-discharge cycles with the first discharge capacity as 100% until the discharge capacity is attenuated to 80% of the initial discharge capacity, stopping testing, and recording the number of cycles as an index for judging the cycle performance of the lithium ion battery.

(3) Testing method for testing surface of pole piece

And (3) disassembling the battery after the forming procedure is finished in an argon atmosphere glove box with the water content of less than 0.1ppm, taking part of the negative electrode, washing the taken negative electrode by using DMC (dimethyl carbonate), cleaning lithium salt remained on the surface, and drying to prepare a sample. And (3) testing and characterizing the crystal structure of the negative electrode material by using an X-ray diffractometer, and analyzing the change and difference of the crystal structure of the negative electrode material.

3. Test results

(1) Effect of Compounds of formula I on Battery Performance

The results of the battery performance tests obtained with different amounts of the compound of formula I are provided in table 1.

TABLE 1

As shown in table 1, the incorporation of a certain amount of the compound of formula I into the electrolyte can improve not only high-temperature cycle but also high-temperature storage performance. This is mainly because the boron ions contained in formula I can stabilize oxygen radicals in LCO, effectively suppressing structural collapse and crystal structure change of the positive electrode material. When the content of the compound of formula I is in the range of 0.1 to 5% by weight, it is possible to avoid causing deterioration of high-temperature cycle properties and adverse effects on high-temperature storage properties, and if an excessive amount of the compound of formula I, for example, more than 5% by weight, is added, it causes an increase in viscosity of the electrolyte and at the same time, decreases the conductivity of the electrolyte. In addition, when the content of the compound of the formula I is within the range defined in the application (0.1% to 5%), a compact protective film is easily formed on the surfaces of the positive electrode and the negative electrode, the interface impedance of the pole piece is prevented from increasing, the influence on the lithium ion extraction capacity of the battery is reduced, and the attenuation of the cycle performance of the battery is avoided.

(2) Influence on the negative electrode in the case of using an electrolyte containing and not containing the compound of formula I

And respectively carrying out X-ray diffraction test on the negative pole pieces of the batteries using the electrolyte containing the compound shown in the formula I and the electrolyte not containing the compound shown in the formula I, wherein the negative pole pieces used in the test are formed and the batteries are disassembled after being fully discharged. As can be seen from the test results shown in FIG. 1, the diffraction peak of the negative electrode sheet using the electrolyte containing the compound of formula I is significantly reduced at 24-26 deg.. Under a 4.53V high voltage system, the cycle at 45 ℃ can be improved by 40 to 50 turns compared with the comparative example 1; under a high-voltage system of 4.53V, the thickness expansion rate can be reduced by 5% compared with comparative example 1 when the film is stored at 85 ℃ for 8h, and the thickness expansion rate can be reduced by 1% compared with comparative example 1 when the film is stored at 60 ℃ for 24 days.

The diffraction peak shows that the compound in the formula I forms an effective protective film on the negative electrode, the protective film can reduce the further contact between the negative electrode and the electrolyte, prevent the electrolyte from being reduced on the surface of the negative electrode, reduce the consumption of active lithium and effectively improve the high-temperature storage performance of the battery. The interface impedance of lithium ions inserted into and taken out of the graphite layer is reduced, and the residue of the lithium ions in the negative electrode is reduced. The stable SEI is formed, and the damage of the transition metal dissolved in the positive electrode to the negative electrode structure is effectively inhibited, so that the cycling stability of the battery is improved.

(3) Effect of the Compound of formula I in combination with varying amounts of lithium difluorophosphate on Battery Performance

Examples of the use of the compound of formula I in combination with varying amounts of lithium difluorophosphate, wherein the amounts of the compound of formula I-1 and lithium difluorophosphate are in weight percent based on the total weight of the electrolyte, and the results of their performance testing are provided in table 2.

TABLE 2

As shown in table 2 above, the additional inclusion of lithium difluorophosphate in the electrolyte according to the present application can further improve the high-temperature cycle performance and the high-temperature storage performance of the battery, mainly because the lithium difluorophosphate is included in the electrolyte in an amount effective to suppress the decomposition of lithium hexafluorophosphate, reducing the decomposition of the lithium salt. An excessively high content of lithium difluorophosphate may result in a high acidity of the electrolyte, thereby causing a decrease in the stability of the protective film on the surface of the active material, and thus, a decrease in the cycle performance and high-temperature storage performance of the battery. When the content of lithium difluorophosphate is within the range of the present application, the above-mentioned adverse effects can be avoided, and a balanced improvement in high-temperature storage performance and cycle performance can be achieved.

(3) Effect of Compounds of formula I and fluoro-carbonate Compounds having the Structure of formula II on Battery Performance

Examples of the use of a compound of formula I in combination with a fluoro carbonate compound having the structure of formula II, wherein the contents of the compound of formula I-1 and the fluoro carbonate are in wt% based on the total weight of the electrolyte, and the test results thereof are provided in Table 3.

TABLE 3

As shown in table 3, the addition of the fluoro carbonate compound having the structure of formula II in the electrolyte can further improve the high-temperature cycle performance and the high-temperature storage performance of the battery, which is mainly that the linear fluoro carbonate is beneficial to improving the oxidation resistance of the solvent, reducing the oxidation of the electrolyte on the surface of the positive electrode, and reducing the consumption of the electrolyte. When the content of the fluoro-carbonate is within the range defined in the present application, the increase in viscosity of the electrolyte and the resulting increase in polarization resistance for lithium ion transport in the electrolyte can be effectively avoided; in addition, a decrease in the conductivity of the electrolytic solution due to fluorine atoms can also be avoided.

(4) Effect of Compounds of formula I and fluoro-carbonate Compounds having the Structure of formula III on Battery Performance

Examples of the use of a compound of formula I in combination with a fluoro carbonate compound having the structure of formula III, wherein the contents of the compound of formula I-1 and the fluoro carbonate are in weight% based on the total weight of the electrolyte, and the results of the tests thereof are provided in Table 4.

TABLE 4

As shown in table 4, the compound of formula I in combination with the cyclic fluoro carbonate compound provides the electrolyte with a higher dielectric constant and good conductivity, improves the oxidation resistance of the solvent in the electrolyte, improves the stability of the solvent, reduces the consumption of the electrolyte during the circulation process, and can effectively improve the cycle performance and high-temperature storage performance of the battery. When the content of the fluoro-carbonate is within the range defined in the present application, the increase in viscosity of the electrolyte and the resulting increase in polarization resistance for lithium ion transport in the electrolyte can be effectively avoided; in addition, a decrease in the conductivity of the electrolytic solution due to fluorine atoms can also be avoided.

(5) Influence of protective layer on positive current collector on battery performance

The above table 1 to table 4 use the positive electrode current collectors without the protective layer thereon, and the following table 5 provides examples in which the positive electrode current collectors have the protective layer, wherein the protective layer has a thickness ranging from 0.5 μm to 7 μm, the protective layers of examples 5-1 to 5-5 include conductive carbon, and the protective layers of examples 5-6 include lithium iron phosphate particles, and specific comparative data are as follows.

TABLE 5

As shown in table 5, the batteries of examples 5-1 to 5-5 had a conductive carbon layer on the positive electrode current collector while the electrolyte included the compound of formula I-1. By comparison, it can be found that having a protective layer on the positive electrode current collector can further improve the cycle performance and high-temperature storage performance of the battery. On one hand, the protective layer is added on the current collector, so that the adhesion of the active material and the current collector can be improved, and the active material is prevented from stripping in the winding process; on the other hand, the addition of the protective layer can reduce the interface impedance between the active material and the current collector, and is beneficial to the rapid transmission of electrons from the surface of the positive electrode material to the current collector, thereby accelerating the transmission of lithium ions from the inside of the positive electrode material, being beneficial to the insertion and extraction of the lithium ions, and further improving the cycle performance of the battery.

(6) Effect of the Compound of formula I in combination with various Compounds on Battery Performance

Examples of the use of a compound of formula I in combination with at least one of lithium difluorophosphate, a fluoro carbonate compound having a structure of formula II, and a fluoro carbonate compound having a structure of formula III, wherein the contents of the compound of formula I-1 and the fluoro carbonate are in wt% based on the total weight of the electrolyte, and the test results thereof are provided in Table 6.

TABLE 6

As shown in table 6, the electrolyte according to the present application may include the compound of formula I in combination with at least one of lithium difluorophosphate, a fluoro carbonate having a structure of formula II, and a fluoro carbonate compound having a structure of formula III, in order to further improve cycle performance and high temperature storage performance of the battery. From the above test results, it can be seen that the compound of formula I used in combination with any one of lithium difluorophosphate, a fluoro carbonate compound having a structure of formula II, and a fluoro carbonate compound having a structure of formula III exhibits superior cycle performance and high temperature storage performance, as compared to the use of the compound of formula I alone. As shown in examples 6 to 6, when all kinds of compounds were used together, the performance of the battery could be still significantly improved as compared with examples 1 to 1 and 1 to 2.

Reference throughout this specification to "some embodiments," "one embodiment," "another example," "an example," "a specific example," or "some examples" means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present application. Thus, throughout the specification, descriptions appear, for example: "in some embodiments," "in an embodiment," "in one embodiment," "in another example," "in one example," "in a particular example," or "by example," which do not necessarily refer to the same embodiment or example in this application. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

Although illustrative embodiments have been illustrated and described, it will be appreciated by those skilled in the art that the above embodiments are not to be construed as limiting the application and that changes, substitutions and alterations can be made to the embodiments without departing from the spirit, principles and scope of the application.

Claims

1. An electrolyte comprising a compound of formula I:

wherein the compound of formula I comprises

At least one of (1).

2. The electrolyte of claim 1, wherein the compound of formula I is present in an amount of 0.1% to 5% based on the total weight of the electrolyte.

3. The electrolyte of claim 1, further comprising at least one additive selected from the group consisting of:

Wherein

The compound of formula II includes at least one of the following compounds:

the compound of formula III includes at least one of the following compounds:

4. an electrochemical device comprising a positive electrode, a negative electrode, and the electrolyte of any one of claims 1 to 3, wherein the positive electrode comprises a positive electrode current collector and a positive electrode active material layer, and the negative electrode comprises a negative electrode current collector and a negative electrode active material layer.

5. The electrochemical device according to claim 4, wherein the anode active material layer has a diffraction peak at 24 ° to 26 ° by X-ray diffraction (XRD) test, wherein the diffraction peak intensity is ≦ 15000.

6. The electrochemical device according to claim 5, wherein a protective layer is provided on the positive electrode current collector, and the protective layer comprises at least one of lithium iron phosphate, lithium iron manganese phosphate, or conductive carbon.

7. The electrochemical device of claim 6, wherein the protective layer has a thickness of 0.5 to 7 microns.

8. An electronic device comprising the electrochemical device of any one of claims 4-7.