WO2021223093A1 - Electrolyte, electrochemical device and electronic equipment - Google Patents

Abstract

Disclosed are an electrolyte, an electrochemical device and an electronic equipment. The electrolyte comprises a compound of formula I: wherein R1 and R2 are each independently selected from one of an alkyl group having 1-11 carbon atoms, a substituted alkyl group having 1-11 carbon atoms, an alkenyl group having 2-11 carbon atoms, and a substituted alkenyl group having 2-11 carbon atoms, and when substituted, a substituent is selected from at least one of fluorine, methyl, and cyano; A and B are each independently selected from one of imidazole, pyridine, piperidine, and quaternary ammonium salt cations; and X is selected from one of hexafluorophosphate, bistrifluoromethylsulfonate, bisfluoromethylsulfonate, tetrafluoroborate, bis(oxalate)borate, and tetrafluoro(oxalate)borate. The electrolyte can reduce the internal resistance of the electrochemical device and improve the cycle performance and rate performance of the electrochemical device.

Description

Electrolyte, electrochemical device and electronic device Technical field

This application relates to the field of electrochemistry, in particular to an electrolyte, an electrochemical device and an electronic device.

Background technique

Electrolyte is an important part of electrochemical devices (such as batteries). Electrolytes can be divided into organic liquid electrolytes, ionic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, inorganic solid electrolytes and hybrid electrolytes, etc. . The electrolyte plays a role in transferring charge between the positive and negative electrodes of the battery, and plays a vital role in the specific capacity, charge and discharge efficiency, cycle stability, rate performance, operating temperature range and safety performance of the battery.

Summary of the invention

In view of the above shortcomings of the prior art, the purpose of this application is to reduce the internal resistance of the electrochemical device to improve the cycle performance and rate performance of the electrochemical device.

This application provides an electrolyte, including a compound of formula I:

Wherein, R1 and R2 are each independently selected from alkyl groups with 1-11 carbon atoms, substituted alkyl groups with 1-11 carbon atoms, alkenyl groups with 2-11 carbon atoms, and One of the substituted alkenyl groups between 2-11. When substituted, the substituent is selected from at least one of fluorine, methyl, and cyano;

A and B are each independently selected from one of imidazole cations, pyridine cations, piperidine cations, and quaternary ammonium salt cations;

X is selected from one of hexafluorophosphate, bistrifluoromethanesulfonate, difluoromethanesulfonate, tetrafluoroborate, bisoxalate borate, and tetrafluorooxalate borate.

In the above electrolyte, the compound of formula I includes at least one of the following compounds:

In the above electrolyte, the compound of formula I accounts for 0.01%-10% of the total mass of the electrolyte.

The above electrolyte solution further includes at least one of lithium difluorophosphate, a polynitrile compound, or a cyclic ether compound.

In the above electrolyte, the electrolyte satisfies at least one of the following conditions (a)-(d):

(a) The percentage of the lithium difluorophosphate to the total mass of the electrolyte is less than 1%;

(b) The percentage of the polynitrile compound in the total mass of the electrolyte is 0.5%-10%;

(c) The percentage of the cyclic ether compound to the total mass of the electrolyte is 0.01%-2%;

(d) The percentage of the compound of formula I in the total mass of the electrolyte is C, and the percentage of the lithium difluorophosphate in the total mass of the electrolyte is D, where C+D<11%, 0.5≤C /D≤10.

In the above electrolyte, the polynitrile compound includes at least one of the following compounds,

Wherein, R ₂₁ , R ₂₂ , R ₂₃ , and R ₂₄ are each independently selected from hydrogen, cyano, -(CH2) _a -CN, -(CH ₂ ) _b -O-(CH ₂ ) _c -CN, -( CH ₂ ) _d -(CH=CH)-CN, one of alkyl groups having 1-5 carbon atoms, alkoxycarbonyl groups having 2-5 carbon atoms, and R ₂₁ , R ₂₂ , At least two of R ₂₃ and R ₂₄ are cyano group-containing groups, a, b and d are each independently selected from an integer of 0-10, and c is selected from an integer of 1-5.

In the above electrolyte, the polynitrile compound includes at least one of the following compounds;

In the above electrolyte, the cyclic ether compound includes at least one of 1,3-dioxolane, 1,3-dioxane, or 1,4-dioxane.

The application also provides an electrochemical device, including:

Positive electrode, negative electrode, separator and any one of the above-mentioned electrolytes.

In the above electrochemical device, the electrolyte solution further contains cobalt ions, and the cobalt ion accounts for 1 ppm-50 ppm of the total mass of the electrolyte solution.

This application also proposes an electronic device, including the electrochemical device described in any one of the above.

In the electrolyte provided by the examples of the application, the compound of formula I containing -R1-O-R2- group is introduced into the electrolyte. This compound can increase the mobility of lithium ions, can dissolve more lithium salts, and can improve Conductivity and lower interface impedance can reduce the impedance of electrochemical devices (such as lithium ion batteries) and improve the rate performance and cycle performance of electrochemical devices, thereby solving the cycling problems and high rate charging problems of electrochemical devices.

Description of the drawings

With reference to the drawings and the following specific implementations, the above and other features, advantages, and aspects of the embodiments of the present application will become more apparent. Throughout the drawings, the same or similar reference signs indicate the same or similar elements. It should be understood that the drawings are schematic and elements and elements are not necessarily drawn to scale.

Fig. 1 is the structural formula of the compound of formula 1 in an example of the present application.

Detailed ways

Hereinafter, embodiments of the present application will be described in more detail with reference to the accompanying drawings. Although some embodiments of the present application are shown in the drawings, it should be understood that the present application can be implemented in various forms and should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided for Have a more thorough and complete understanding of this application. It should be understood that the drawings and embodiments of the present application are only used for exemplary purposes, and are not used to limit the protection scope of the present application.

The following examples may enable those skilled in the art to understand the application more comprehensively, but do not limit the application in any way.

The solutions provided in the embodiments of the present application will be described in detail below.

Ionic liquids are also called room temperature ionic liquids or room temperature molten salts, and are also called non-aqueous ionic liquids, liquid organic salts, and the like. It is generally considered to be a liquid composed of cations and anions, which are organic salts that are liquid at or near room temperature. However, when the existing ionic liquid is used in the electrolyte system, the rate charge and discharge performance of electrochemical devices, such as lithium ion batteries, will decrease. During charge and discharge, cations diffuse faster than lithium ions, and fast-migrating cations will adhere to the negative electrode and be further embedded in the negative electrode to form a blocking layer to prevent the insertion and extraction of lithium ions.

From the above content, the ionic liquid has a higher diffusion coefficient of cations than lithium ions, forming a blocking layer. The presence of the blocking layer will increase the internal resistance of the battery, which will affect the rate performance and cycle performance of the lithium ion battery.

In order to reduce the internal resistance of the electrochemical device and improve the cycle performance and rate performance of the electrochemical device, the electrochemical device is a lithium ion battery as an example for description. Please refer to FIG. 1. An electrolyte is proposed in an embodiment of this application. Including compounds of formula I:

Wherein, R1 and R2 are each independently selected from alkyl groups with 1-11 carbon atoms, substituted alkyl groups with 1-11 carbon atoms, alkenyl groups with 2-11 carbon atoms, or carbon atoms One of the substituted alkenyl groups between 2-11. When substituted, the substituent is selected from at least one of fluorine, methyl or cyano;

A and B are each independently selected from one of imidazole cation, pyridine cation, piperidine cation or quaternary ammonium salt cation;

X is selected from one of hexafluorophosphate, bistrifluoromethanesulfonate, difluoromethanesulfonate, tetrafluoroborate, bisoxalate borate or tetrafluorooxalate borate.

In this embodiment, a compound of formula I containing a -R1-O-R2- group is introduced into the electrolyte. This group can increase the mobility of lithium ions and can dissolve more lithium salts. Therefore, for The lithium ion battery using the electrolyte proposed in this embodiment can increase the conductivity and reduce the interface impedance, thereby reducing the internal resistance of the lithium ion battery, improving the rate performance and cycle performance of the lithium ion, and can solve the problem of the lithium ion battery. Cycle problems and high-rate charging problems.

In some embodiments of the present application, the compound of formula I includes at least one of the following compounds:

In some embodiments of the present application, the percentage of the compound of formula I in the total mass of the electrolyte is 0.01%-10%. The content of the compound represented by formula I within this range can significantly improve the mobility of lithium ions, and can avoid the deterioration of lithium ion transmission due to the excessively high content of the compound represented by formula I. Therefore, it is necessary to control the electrolyte shown in formula I The content of the compound.

In some embodiments of the present application, the electrolyte further includes: at least one of lithium difluorophosphate, a polynitrile compound, or a cyclic ether compound. The compound of formula I and lithium difluorophosphate work together to give priority to the oxidation-reduction reaction at the positive and negative electrodes of the battery to form a LiF-rich protective film, which enhances the stability of the solid electrolyte interface film, thereby improving the cycle performance of the lithium-ion battery . The compound of formula I and the polynitrile compound work together to further form an organic protective layer on the surface of the positive electrode. The organic molecules on the surface of the positive electrode can well separate the easily oxidizable components in the electrolyte from the surface of the positive electrode, greatly reducing the state of charge. The positive surface of the lithium-ion battery has an oxidation effect on the electrolyte, thereby improving the cycle performance and high-temperature storage performance of the lithium-ion battery. The combined action of the compound of formula I and the cyclic ether compound can improve the high-temperature cycle performance and high-temperature storage performance of the lithium ion battery.

In some embodiments of the present application, the electrolyte meets at least one of the following conditions (a)-(d):

(a) The mass of lithium difluorophosphate accounts for less than 1% of the total mass of the electrolyte;

Lithium difluorophosphate is beneficial to improve the cycle performance of lithium-ion batteries, but when its content is too high, it will have a deteriorating effect, so its content needs to be controlled.

(b) The percentage of the mass of the polynitrile compound to the total mass of the electrolyte is 0.5%-10%;

When the polynitrile compound content exceeds 10%, the high temperature cycle performance improvement effect is reduced. This is because the high content of polynitrile compounds increases the viscosity of the electrolyte and deteriorates the dynamic performance of the battery. Therefore, it is necessary to control its content in the electrolyte. The percentage is 0.5%-10%.

(c) The percentage of the mass of the cyclic ether compound to the total mass of the electrolyte is 0.01%-2%;

When the mass percentage of the cyclic ether compound in the electrolyte exceeds 2%, the high-temperature cycle performance and the high-rate discharge performance of the lithium ion battery are reduced. This is because when the cyclic ether content is high, the impedance of the lithium-ion battery increases, which leads to accelerated cycle capacity attenuation, which deteriorates the cycle performance and high-rate discharge performance of the lithium-ion battery.

(d) The mass percentage of the compound of formula I to the total mass of the electrolyte is C, and the percentage of lithium difluorophosphate to the total mass of the electrolyte is D, where C+D<11%, 0.5≤C/D≤10.

When C+D is greater than or equal to 11%, the addition of the compound of formula I and lithium difluorophosphate in the electrolyte will affect the transmission of lithium ions, and on the contrary, the performance of the lithium ion battery will deteriorate. When C/D<0.5, the added amount of the compound of formula I is not enough to improve the performance of lithium ion.

In some embodiments of the present application, the polynitrile compound includes at least one of the following compounds,

Wherein, R ₂₁ , R ₂₂ , R ₂₃ , and R ₂₄ are each independently selected from hydrogen, cyano, -(CH2) _a -CN, -(CH ₂ ) _b -O-(CH ₂ ) _c -CN, -( CH ₂ ) _d -(CH=CH)-CN, one of alkyl groups having 1-5 carbon atoms, alkoxycarbonyl groups having 2-5 carbon atoms, and R ₂₁ , R ₂₂ , At least two of R ₂₃ and R ₂₄ are cyano group-containing groups, a, b and d are each independently selected from an integer of 0-10, and c is selected from an integer of 1-5.

In some embodiments of the present application, the polynitrile compound includes at least one of the following compounds;

In some embodiments of the present application, the cyclic ether compound includes at least one of 1,3-dioxolane, 1,4-dioxane, or 1,3-dioxane.

In some embodiments of the present application, the electrolyte contains a lithium salt, and the lithium salt may be at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments of the present application, the lithium salt contains fluorine element and boron. At least one of element or phosphorus element.

In some optional embodiments, the lithium salt includes lithium hexafluorophosphate LiPF ₆ , lithium bistrifluoromethanesulfonimide LiN(CF ₃ SO ₂ ) ₂ (abbreviated as LiTFSI), lithium bis(fluorosulfonyl)imide Li( N(SO ₂ F) ₂ ) (LiFSI in abbreviation), LiB(C ₂ O ₄ ) ₂ (LiBOB in abbreviation), lithium tetrafluorophosphate oxalate (LiPF ₄ C ₂ O ₂ ), difluoro oxalate borate At least one of lithium LiBF ₂ (C ₂ O ₄ ) (abbreviated as LiDFOB) or lithium hexafluorocesium oxide (LiCsF ₆ ). Optionally, the lithium salt is lithium hexafluorophosphate LiPF ₆ .

In some embodiments of the application, the concentration of the lithium salt is 0.5 mol/L-1.5 mol/L. If the concentration of lithium salt is too low, the conductivity of the electrolyte is low, which will affect the rate and cycle performance of the entire lithium-ion battery system; if the concentration of lithium salt is too high, the viscosity of the electrolyte is too large, which also affects the rate of the entire lithium-ion battery system. Optionally, the concentration of the lithium salt is 0.8 mol/L-1.3 mol/L.

In some embodiments of the present application, the electrolyte includes a non-aqueous organic solvent, wherein the non-aqueous organic solvent includes ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyl Lactone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate or butyl One or two or more of the propyl acids are combined in an arbitrary ratio.

The present application also proposes an electrochemical device, including: a positive electrode, a negative electrode, a separator, and any one of the above electrolytes.

In some embodiments of the present application, the electrolyte in the electrochemical device further contains cobalt ions, and the cobalt ions account for 1 ppm-50 ppm of the total mass of the electrolyte.

The positive electrode of the above-mentioned electrochemical device includes a positive electrode current collector and a positive electrode active material provided on the positive electrode current collector. The specific types of positive electrode active materials are not subject to specific restrictions, and can be selected according to requirements.

In some embodiments, the positive electrode active material includes a positive electrode material capable of absorbing and releasing lithium (Li). Examples of positive electrode materials capable of absorbing/releasing lithium (Li) may include lithium cobalt oxide, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, lithium manganate, lithium iron manganese phosphate, lithium vanadium phosphate, lithium vanadyl phosphate, and phosphoric acid. Lithium iron, lithium titanate and lithium-rich manganese-based materials.

Specifically, the chemical formula of lithium cobalt oxide can be as chemical formula 1:

Li _x Co _a M1 _b O _2-c Chemical formula 1

Wherein M1 represents selected from nickel (Ni), manganese (Mn), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), Copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), tungsten (W), yttrium (Y), lanthanum (La), zirconium (Zr) and For at least one of silicon (Si), the values of x, a, b, and c are within the following ranges: 0.8≤x≤1.2, 0.8≤a≤1, 0≤b≤0.2, -0.1≤c≤0.2.

The chemical formula of lithium nickel cobalt manganate or lithium nickel cobalt aluminate can be as chemical formula 2:

Li _y Ni _d M2 _e O _2-f Chemical formula 2

Wherein M2 represents selected from cobalt (Co), manganese (Mn), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), At least one of copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), tungsten (W), zirconium (Zr), and silicon (Si), The values of y, d, e, and f are in the following ranges: 0.8≤y≤1.2, 0.3≤d≤0.98, 0.02≤e≤0.7, -0.1≤f≤0.2.

The chemical formula of lithium manganate can be as chemical formula 3:

Li _z Mn _2-g M3 _g O _4-h Chemical formula 3

Wherein M3 represents selected from cobalt (Co), nickel (Ni), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), At least one of copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr) and tungsten (W), with z, g and h values in the following ranges respectively Inner: 0.8≤z≤1.2, 0≤g<1.0 and -0.2≤h≤0.2.

A conductive agent or a binder may be added to the positive electrode of the above electrochemical device. In some embodiments of the present application, the positive electrode further includes a carbon material. The carbon material may include conductive carbon black, graphite, graphene, carbon nanotubes, and carbon fibers. Or at least one of carbon black. The binder may include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, styrene-acrylate copolymer, styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, poly At least one of acrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene .

In some embodiments, the isolation film includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. Especially polyethylene and polypropylene, they have a good effect on preventing short circuits, and can improve the stability of the battery through the shutdown effect.

In some embodiments, the surface of the isolation membrane may further include a porous layer, the porous layer is disposed on at least one surface of the isolation membrane, the porous layer includes inorganic particles and a binder, and the inorganic particles are selected from alumina (Al ₂ O ₃ ), Silicon oxide (SiO ₂ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ), hafnium dioxide (HfO ₂ ), tin oxide (SnO ₂ ), ceria (CeO ₂ ), nickel oxide (NiO), oxide Zinc (ZnO), calcium oxide (CaO), zirconium oxide (ZrO ₂ ), yttrium oxide (Y ₂ O ₃ ), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or sulfuric acid At least one of barium. The binder is selected from polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyethylene pyrrole At least one of alkanone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The porous layer on the surface of the isolation membrane can improve the heat resistance, oxidation resistance and electrolyte infiltration performance of the isolation membrane, and enhance the adhesion between the isolation membrane and the pole piece.

This application also proposes an electronic device, including any one of the electrochemical devices described above. The electronic device of the present application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, electronic devices may include, but are not limited to, notebook computers, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, headsets, Video recorders, LCD TVs, portable cleaners, portable CD players, mini discs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, assisted bicycles, bicycles, Lighting equipment, toys, game consoles, clocks, power tools, flashlights, cameras, large household storage batteries and lithium-ion capacitors, etc. For example, electronic devices include cell phones that contain lithium-ion batteries.

In order to better illustrate the beneficial effects of the electrolyte solution proposed in the embodiments of the present application, the following will be described in conjunction with Examples 1-53 and Comparative Examples 1-4. The difference between Examples 1-53 and Comparative Examples 1-4 is only The electrolytes used are different. In Examples 1-53 and Comparative Examples 1-4, performance tests of lithium-ion batteries using different electrolytes will be performed to illustrate the effect of electrolytes on the performance of lithium-ion batteries.

Preparation of electrolyte

In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (abbreviated as EC), diethyl carbonate (abbreviated as DEC), propylene carbonate (abbreviated as PC), according to the quality of 3:4:3 It becomes a non-aqueous solvent by mixing uniformly, and then the fully dried lithium salt LiPF _{6 is} dissolved in the above-mentioned non-aqueous solvent, _{the concentration of LiPF 6} is 1 mol/L, and the basic electrolyte in the embodiment is prepared. The electrolyte used in each embodiment and comparative example is obtained by adding at least one of the _{following formula I compound, polynitrile compound, cyclic ether compound, LiPO 2} F _{2 or cobalt ion to the basic electrolyte.}

Compound of formula I:

Polynitrile compounds:

Cyclic ether compound:

Battery preparation

1) Preparation of the positive electrode: Mix lithium cobaltate, acetylene black, and polyvinylidene fluoride (PVDF in abbreviation) in an appropriate amount of N-methylpyrrolidone (NMP in abbreviation) solvent at a weight ratio of 96:2:2. , To form a uniform positive electrode slurry; coating the slurry on the positive electrode current collector Al foil, drying and cold pressing to obtain the positive electrode active material layer, and then cutting and welding the tabs to obtain the positive electrode.

2) Preparation of negative electrode: Mix graphite, styrene-butadiene rubber (abbreviated as SBR), sodium carboxymethyl cellulose (abbreviated as CMC) in an appropriate amount of deionized water solvent according to the weight ratio of 97:2:1 to make It forms a uniform negative electrode slurry; this slurry is coated on the negative electrode current collector Cu foil, dried and cold pressed to obtain the negative electrode active material layer, and then cuts and welds the tabs to obtain the negative electrode.

3) Isolation membrane: PE porous polymer film is used as the isolation membrane.

4) Preparation of lithium ion battery: Lay the positive electrode separator film and the negative electrode in order, so that the separator film is located between the positive electrode and the negative electrode for isolation, and then wound and placed in the outer packaging foil. The electrolyte is injected into the dried battery, and the preparation of the lithium-ion battery is completed after vacuum packaging, standing, forming, and shaping.

High temperature cycle test

Place the lithium ion battery in a thermostat at 45°C and let it stand for 30 minutes to make the lithium ion battery reach a constant temperature; charge at 0.7C constant current to 4.45V, constant voltage charge to a current of 0.05C; then discharge at 0.7C to 3.0 V, the capacity of this step is used as the reference, and it is recorded as the discharge capacity of the first cycle; this step is cycled for 300 cycles, the discharge capacity of the 300th cycle is recorded, and the capacity retention rate is calculated.

Capacity retention rate after 300 cycles (%)=discharge capacity at the 300th cycle/discharge capacity at the first cycle×100%

High temperature storage performance test of lithium ion battery

Discharge the lithium-ion battery at 25°C at 0.5C to 3.0V, charge at 0.7C to 4.45V, and charge at constant voltage to 0.05C at 4.45V, test with a micrometer and record the thickness of the battery as H ₁₁ , and place it In an oven at 85℃, keep the constant pressure at 4.45V for 16 hours. After 16 hours, test and record the thickness of the lithium ion battery with a micrometer, record it as H ₁₂ , calculate the thickness expansion rate, the thickness expansion rate = (H ₁₂ -H ₁₁ )/H ₁₁ ×100%

DC Impedance (DCR) test of Li-ion battery at 0℃

Put the lithium ion battery in a 0℃ high and low temperature box for 4 hours to make the lithium ion battery reach a constant temperature; charge at a constant current of 0.1C to 4.45V, charge at a constant voltage to a current of 0.05C, and stand for 10 minutes; C discharge at a constant current to 3.4V and let it stand for 5 minutes to obtain the actual capacity in this step. Charge the lithium-ion battery at a constant current of 0.1C to 4.45V at 0°C, charge at a constant voltage to a current of 0.05C, and let stand for 10 minutes; discharge at a constant current of 0.1C for 8h (calculated from the actual capacity obtained in the previous step), Record the voltage at this time as V ₁ ; discharge at a constant current of 1C for 1 s (the capacity is calculated as the nominal capacity of the lithium ion battery), record the voltage at this time as V ₂ , and calculate the corresponding DC impedance of the lithium ion battery in the 20% SOC state.

20% SOC DC impedance = (V ₁ -V ₂ )/1C

Magnification test

The lithium-ion battery was charged to 4.45V at a constant current of 0.5C/constant voltage at 25°C, allowed to stand for 10 minutes, and discharged at a constant current of 0.5C to a cut-off voltage of 3.0V, and the discharge capacity Q1 was recorded. Charge at a constant current of 0.5C/constant voltage to 4.45V at 25°C, stand for 10min, discharge at a constant current of 2C to a cut-off voltage of 3.0V, and record the discharge capacity Q2. Divide Q2 by Q1 to get 2C discharge efficiency.

Loop impedance test

Charge the lithium-ion battery at 45°C at 0.7C to 4.45V, charge at a constant voltage of 4.45V to 0.05C, and then discharge at a constant current of 1.0C to 3.0V, cycle 300 times under this condition, and use a resistivity meter , Monitor the impedance change of the lithium-ion battery at 100% SOC during cycling, and record the cycle impedance of 300 cycles.

Examples 1-16 and Comparative Examples 1-4

In Examples 1-16 and Comparative Examples 1-4, the electrolyte used was obtained by adding one or more compounds to the basic electrolyte as shown in Table 1. For Example 1-16 and Comparative Example 1- The performance test results of the lithium-ion battery in 4 are shown in Table 2.

Table 1

Table 2

Comparing Examples 1-7 and Comparative Example 1, it can be seen that the 2C discharge efficiency of Examples 1-7 is significantly higher than that of Comparative Example 1, and the 20% SOC impedance of Examples 1-7 is significantly lower than that of Comparative Example 1. 20% SOC resistance, the capacity retention rate of Examples 1-7 after 300 cycles at 45°C is also significantly higher than that of Comparative Example 1. That is, by adding the compound represented by formula I to the electrolyte, it can be improved The high-rate discharge performance of lithium-ion batteries, the lower the impedance of lithium-ion batteries, and the improvement of cycle performance are because the compound of formula I has a -CH ₂ -O-CH ₂ -group, and the compound of formula I with this group helps The lithium ion mobility is improved. Therefore, the electrolyte with the compound of formula I can dissolve more lithium salt, strengthen the transport effect of cations, and improve the conductivity, thereby reducing the impedance of the lithium ion battery and improving the cycle performance and rate performance.

It can be seen from the performance test results of Comparative Example 1 and Comparative Example 2 that the capacity retention rate of Comparative Example 2 after 300 cycles of 45°C cycling is significantly lower than that of Comparative Example 1. This is because too much was added to Comparative Example 2. The compound of formula 1, when the compound of formula I in the electrolyte is too much, it will cause the cycle performance of the lithium ion battery to decrease. From the performance test results of Comparative Examples 1 and 3, it can be seen that when the compound of formula I in the electrolyte is too small The performance improvement of the lithium ion battery is not obvious. Therefore, in some embodiments of the present application, the percentage of the compound of formula I in the total mass of the electrolyte is limited to 0.01%-10%.

It can be seen from the performance test results of Comparative Example 1 and Comparative Example 4 that the test results of the 20% SOC DC impedance of Comparative Example 4 and the capacity retention rate after 300 cycles of 45°C cycling are better than those of Comparative Example 1. It can be seen that, By adding LiPO ₂ F _{2 to the} electrolyte, the cycle performance of lithium-ion batteries can be improved and the impedance of lithium-ion batteries can be reduced. This is because LiPO ₂ F ₂ has a lower oxidation potential and a higher reduction potential, so LiPO ₂ F ₂ can preferentially undergo oxidation-reduction reactions at the interface between the positive and negative electrodes to form a LiF-rich protective film, which enhances the stability of the solid electrolyte interface film, thereby achieving the effect of improving the high-temperature cycle performance of the lithium-ion battery.

From the performance test results of Example 2 and Example 8, it can be seen that adding LiPO ₂ F ₂ while adding the compound of formula I to the electrolyte can further reduce the DC impedance of the lithium ion battery and improve the cycle performance of the lithium ion battery From the performance test results of Examples 13-16 and Comparative Example 4, the same conclusion can also be drawn, that is, adding the compound of formula I and LiPO ₂ F ₂ to use the synergistic effect of the two to further improve circulation and reduce impedance Effect.

It can be seen from the performance test results of Examples 8-12 that as the _{content of LiPO 2} F _{2 in the} electrolyte increases, the impedance of the lithium-ion battery first decreases and then increases, and the capacity retention of the lithium-ion battery after 300 cycles of 45°C cycles First increase and then decrease, it can be seen that _{the content of LiPO 2} F ₂ in the electrolyte is not as high as possible. Therefore, in some embodiments of this application, _{the percentage of LiPO 2} F _{2 in} the total mass of the electrolyte is limited to less than 1% to prevent excessive Too much LiPO ₂ F ₂ leads to deterioration of lithium ion battery performance.

Example 17-33

The electrolyte used in Examples 17-33 is obtained by adding at least one compound to the basic electrolyte according to Table 3, and the performance test results of Examples 17-33 are shown in Table 4. For the convenience of comparison, In Table 3 and Table 4, the electrolyte parameters and performance test results of Example 13 are added.

table 3

Table 4

By comparing the performance test results of Examples 13 and 17-32, it can be seen that by adding polynitrile compounds in the electrolyte, the thickness expansion rate of the lithium ion battery at 85°C-16h can be significantly reduced, and the thickness expansion rate of the lithium-ion battery can be increased after 300 cycles of 45°C cycling Capacity retention rate, that is, the high-temperature storage performance and high-temperature cycle performance of lithium-ion batteries can be improved by adding polynitrile compounds. This is because nitrile compounds can complex with transition metals on the surface of the positive electrode, reducing the dissolution of transition metals, and at the same time reducing The contact between the electrolyte and the positive electrode interface further reduces the side reactions of the electrolyte at high temperatures, thereby improving high-temperature cycling and high-temperature storage performance.

It can be seen from the performance test results of Example 33 that when the content of polynitrile compounds exceeds 10%, the high temperature cycle performance deteriorates. This is because the high content of nitrile compounds increases the viscosity of the electrolyte and deteriorates the lithium ion battery. Dynamic performance.

From Example 20, Example 29, and Example 30, it can be seen that different polynitrile compounds have different degrees of improvement on the cycle performance and storage performance of lithium-ion batteries. It will produce a different isolation effect from the surface of the positive electrode. As the number of nitrile functional groups in an organic molecule increases, the isolation effect is more pronounced. At the same time, the size of organic molecules containing nitrile functional groups has an optimal value. If the molecules are too small, the isolation space formed is limited, and the easily oxidizable components in the electrolyte cannot be effectively separated from the surface of the positive electrode. If the molecules are too large, the electrolyte The easily oxidizable components in the nitrile functional group can contact the surface of the positive electrode through the gap of the organic molecule containing the nitrile functional group, but still cannot achieve a good isolation effect.

Examples 34-47

The electrolyte used in Examples 34-47 is obtained by adding one or more compounds to the basic electrolyte according to Table 5, and the performance test results of Examples 34-47 are shown in Table 6, for the convenience of comparison. In Table 5 and Table 6, the electrolyte parameters and performance test results of Example 22 are added.

table 5

Table 6

From the performance test results of Example 22 and Examples 37-45, it can be seen that when the cyclic ether compound with a mass fraction of 0.1%-2% is added at the same time, the high-temperature cycle performance and high-temperature storage performance of the lithium ion battery are significantly improved. This is because the cyclic ether has a low oxidation potential and oxidizes on the surface of the positive electrode to generate an organic lithium salt. The organic lithium salt is relatively stable, thereby enhancing the stability of the solid electrolyte interface (SEI) membrane and alleviating the lithium ion battery during high temperature processes. Oxidation and decomposition of the electrolyte solution on the electrode surface, thereby achieving the effect of improving the high-temperature storage performance and high-temperature cycle performance of the lithium-ion battery.

Comparing the performance test results of Examples 46, 47 and Examples 37-45, it can be seen that when the addition amount of the cyclic ether compound exceeds 2%, the high-temperature cycle performance and the high-rate discharge performance of the lithium ion battery are reduced. This is because when the content of the cyclic ether compound is too high, the impedance of the lithium-ion battery will increase, which will lead to the acceleration of the cycle capacity of the lithium-ion battery, and deteriorate the cycle performance and high-rate discharge performance of the lithium-ion battery.

By comparing the performance test results of Examples 34-36, it can be seen that the compound of formula I-1 used in combination with cyclic ether alone or in combination with lithium difluorophosphate (LiPO ₂ F ₂ ) and cyclic ether has both improved effects. Not as good as adding nitriles at the same time.

Examples 48-53

Examples 48-53 are based on Example 39, and the electrolyte further contains cobalt ions. The electrolyte used in Examples 48-53 and the performance test results are shown in Table 7. For the convenience of comparison, in Table 7 Add the electrolyte parameters and performance test results of Example 39.

Table 7

Comparing the performance test results of Examples 39 and 48-53, it can be seen that a small amount of cobalt ions in the electrolyte can significantly improve the cycle resistance. This is mainly because a small amount of cobalt ions can enhance the conductivity of the electrolyte, thereby improving the effect of improving the growth of cycle impedance.

The electrolyte proposed in the present application can improve the rate performance, cycle performance and reduce internal resistance of the lithium ion battery using the electrolyte by adding the compound represented by formula I.

The above description is only a preferred embodiment of the present application and an explanation of the applied technical principles. Those skilled in the art should understand that the scope of disclosure involved in this application is not limited to the technical solutions formed by the specific combination of the above technical features, and should also cover the above technical features or technical solutions without departing from the above disclosed concept. Other technical solutions formed by arbitrarily combining the equivalent features. For example, the above-mentioned features and the technical features disclosed in this application (but not limited to) with similar functions are mutually replaced to form a technical solution.

Although the subject matter has been described in language specific to structural features and/or logical actions of the method, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or actions described above. On the contrary, the specific features and actions described above are merely exemplary forms of implementing the claims.

Claims (11)

An electrolyte, characterized in that it comprises a compound of formula I:

Wherein, R1 and R2 are each independently selected from alkyl groups with 1-11 carbon atoms, substituted alkyl groups with 1-11 carbon atoms, alkenyl groups with 2-11 carbon atoms, and One of the substituted alkenyl groups between 2-11. When substituted, the substituent is selected from at least one of fluorine, methyl, and cyano; A and B are each independently selected from one of imidazole cations, pyridine cations, piperidine cations, and quaternary ammonium salt cations; X is selected from one of hexafluorophosphate, bistrifluoromethanesulfonate, difluoromethanesulfonate, tetrafluoroborate, bisoxalate borate, and tetrafluorooxalate borate. The electrolyte according to claim 1, wherein the compound of formula I comprises at least one of the following compounds:

The electrolyte according to claim 1, wherein the percentage of the compound of formula I in the total mass of the electrolyte is 0.01%-10%. The electrolyte according to claim 1, further comprising: at least one of lithium difluorophosphate, a polynitrile compound, or a cyclic ether compound. The electrolyte according to claim 4, wherein the electrolyte satisfies at least one of the following conditions (a)-(d): (a) The percentage of the lithium difluorophosphate to the total mass of the electrolyte is less than 1%; (b) The percentage of the polynitrile compound in the total mass of the electrolyte is 0.5%-10%; (c) The percentage of the cyclic ether compound to the total mass of the electrolyte is 0.01%-2%; (d) The percentage of the compound of formula I in the total mass of the electrolyte is C, and the percentage of the lithium difluorophosphate in the total mass of the electrolyte is D, where C+D<11%, 0.5≤C /D≤10. The electrolyte according to claim 4, wherein the polynitrile compound comprises at least one of the following compounds,

Wherein, R ₂₁ , R ₂₂ , R ₂₃ , and R ₂₄ are each independently selected from hydrogen, cyano, -(CH2) _a -CN, -(CH ₂ ) _b -O-(CH ₂ ) _c -CN, -( CH ₂ ) _d -(CH=CH)-CN, one of alkyl groups having 1-5 carbon atoms, alkoxycarbonyl groups having 2-5 carbon atoms, and R ₂₁ , R ₂₂ , At least two of R ₂₃ and R ₂₄ are cyano group-containing groups, a, b and d are each independently selected from an integer of 0-10, and c is selected from an integer of 1-5. The electrolyte according to claim 4, wherein the polynitrile compound comprises at least one of the following compounds;

The electrolyte according to claim 4, wherein the cyclic ether compound comprises at least one of 1,3-dioxolane, 1,3-dioxane, or 1,4-dioxane . An electrochemical device, characterized by comprising: The positive electrode, the negative electrode, the separator and the electrolyte according to any one of claims 1-8. 9. The electrochemical device according to claim 9, wherein the electrolyte further contains cobalt ions, and the cobalt ions account for 1 ppm-50 ppm of the total mass of the electrolyte. An electronic device, characterized by comprising the electrochemical device according to any one of claims 9-10.

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