CN112805864B - Electrolyte, electrochemical device, and electronic device - Google Patents

Abstract

An electrolyte, an electrochemical device and an electronic device, wherein the electrolyte comprises a compound of formula I:

Wherein, R1 and R2 are 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, 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, B are each independently selected from imidazole, pyridine, piperidine, One of the quaternary ammonium salt cations; X is selected from one of hexafluorophosphate, bistrifluoromethanesulfonate, bisfluoromethanesulfonate, tetrafluoroborate, bisoxalate borate, tetrafluorooxalate borate kind. 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

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

Background Art

Electrolyte is an important component of electrochemical devices (such as batteries). Electrolyte can be divided into organic liquid electrolyte, ionic liquid electrolyte, inorganic liquid electrolyte, solid polymer electrolyte, inorganic solid electrolyte and mixed electrolyte, 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 battery's specific capacity, charge and discharge efficiency, cycle stability, rate performance, operating temperature range and safety performance.

Summary of the invention

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

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

wherein R1 and R2 are each independently selected from an alkyl group having 1 to 11 carbon atoms, a substituted alkyl group having 1 to 11 carbon atoms, an alkenyl group having 2 to 11 carbon atoms, and a substituted alkenyl group having 2 to 11 carbon atoms, and 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 imidazolium cation, pyridinium cation, piperidinium cation and quaternary ammonium salt cation;

X is selected from the group consisting of hexafluorophosphate, bistrifluoromethanesulfonate, bisfluoromethanesulfonate, tetrafluoroborate, bisoxalatoborate, and tetrafluorooxalatoborate.

In the above electrolyte, the compound of formula I comprises 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 also includes: at least one of lithium difluorophosphate, polynitrile compounds or cyclic ether compounds.

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

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

(b) the percentage of the polynitrile compound to 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, wherein 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 the group consisting of hydrogen, cyano, -(CH2) _a -CN, -(CH ₂ ) _b -O-(CH ₂ ) _c -CN, -(CH ₂ ) _d -(CH＝CH)-CN, an alkyl group having 1-5 carbon atoms, and an alkoxycarbonyl group having 2-5 carbon atoms, and at least two of R ₂₁ , R ₂₂ , R ₂₃ , and R ₂₄ are cyano-containing groups, a, b, and d are each independently selected from integers 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 present application also provides an electrochemical device, comprising:

A positive electrode, a negative electrode, a separator and any of the above-mentioned electrolytes.

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

The present application also proposes an electronic device, comprising any of the electrochemical devices described above.

The electrolyte provided in the embodiments of the present application introduces a compound of formula I containing an -R1-O-R2- group into the electrolyte. This compound can improve the mobility of lithium ions, dissolve more lithium salts, improve electrical conductivity, and reduce interfacial impedance. Therefore, it can reduce the impedance of electrochemical devices (for example, lithium-ion batteries), improve the rate performance and cycle performance of electrochemical devices, thereby solving the cycle problem and high-rate charging problem of electrochemical devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features, advantages and aspects of the embodiments of the present application will become more apparent with reference to the following detailed description in conjunction with the accompanying drawings. Throughout the accompanying drawings, the same or similar reference numerals represent the same or similar elements. It should be understood that the drawings are schematic and that components and elements are not necessarily drawn to scale.

FIG1 is the structural formula of the compound of Formula 1 according to an embodiment of the present application.

DETAILED DESCRIPTION

The embodiments of the present application will be described in more detail below with reference to the accompanying drawings. Although certain embodiments of the present application are shown in the accompanying 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 described herein. Instead, these embodiments are provided to provide a more thorough and complete understanding of the present application. It should be understood that the drawings and embodiments of the present application are only for exemplary purposes and are not intended to limit the scope of protection of the present application.

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

The solution provided by 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, non-aqueous ionic liquids, liquid organic salts, etc. It is generally believed that it is a liquid composed of cations and anions, and is an organic salt that appears as a liquid at or near room temperature. However, when existing ionic liquids are used in electrolyte systems, the rate charge and discharge performance of electrochemical devices, such as lithium-ion batteries, will be reduced. This is because the diffusion coefficient of ionic liquid cations is much larger than that of lithium ions. When the battery is charged and discharged, the diffusion rate of cations is faster than that of lithium ions. Fast-migrating cations will adhere to the negative electrode and further embed into the negative electrode to form a blocking layer, preventing the insertion and removal of lithium ions.

From the above content, it can be seen that since the diffusion coefficient of cations in ionic liquids is greater than that of lithium ions, a blocking layer is formed. The existence of the blocking layer will lead to an increase in the internal resistance of the battery, which will in turn 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 described below by taking a lithium ion battery as an example. Please refer to FIG1 . In the embodiment of the present application, an electrolyte is proposed, including a compound of formula I:

wherein R1 and R2 are each independently selected from an alkyl group having 1 to 11 carbon atoms, a substituted alkyl group having 1 to 11 carbon atoms, an alkenyl group having 2 to 11 carbon atoms, or a substituted alkenyl group having 2 to 11 carbon atoms, and 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 imidazolium cation, pyridinium cation, piperidinium cation or quaternary ammonium salt cation;

X is selected from one of hexafluorophosphate, bistrifluoromethanesulfonate, bisfluoromethanesulfonate, tetrafluoroborate, bisoxalatoborate or tetrafluorooxalatoborate.

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

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%. When the content of the compound of formula I is within this range, the lithium ion mobility can be significantly improved, and the deterioration of lithium ion transmission caused by excessive content of the compound of formula I can be avoided. Therefore, it is necessary to control the content of the compound of formula I in the electrolyte.

In some embodiments of the present application, the electrolyte further includes: at least one of lithium difluorophosphate, polynitrile compounds or cyclic ether compounds. The compound of formula I and lithium difluorophosphate can act together to preferentially undergo redox reactions at the positive and negative electrodes of the battery to generate a protective film rich in LiF, thereby enhancing 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 can act 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 oxidized components in the electrolyte from the surface of the positive electrode, greatly reducing the oxidation effect of the positive electrode surface of the charged lithium-ion battery on the electrolyte, thereby improving the cycle performance and high-temperature storage performance of the lithium-ion battery. The compound of formula I and the cyclic ether compound can act together to improve the high-temperature cycle performance and high-temperature storage performance of lithium-ion batteries.

In some embodiments of the present application, the electrolyte satisfies 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 for improving 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 mass of the polynitrile compound accounts for 0.5% to 10% of the total mass of the electrolyte;

When the content of polynitrile compounds exceeds 10%, the effect of improving high temperature cycle performance decreases. This is because a 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 percentage in the electrolyte to 0.5%-10%.

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

When the mass percentage of cyclic ether compounds in the electrolyte exceeds 2%, the high temperature cycle performance and 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, resulting in accelerated cycle capacity decay, 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 mass percentage of lithium difluorophosphate to the total mass of the electrolyte is D, wherein C+D<11%, 0.5≤C/D≤10.

When C+D≥11%, the excessive addition of the compound of formula I and lithium difluorophosphate in the electrolyte will affect the transmission of lithium ions, which will lead to the deterioration of the performance of the lithium ion battery. When C/D<0.5, the addition of the compound of formula I is too small to improve the performance of lithium ions.

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 the group consisting of hydrogen, cyano, -(CH2) _a -CN, -(CH ₂ ) _b -O-(CH ₂ ) _c -CN, -(CH ₂ ) _d -(CH＝CH)-CN, an alkyl group having 1-5 carbon atoms, and an alkoxycarbonyl group having 2-5 carbon atoms, and at least two of R ₂₁ , R ₂₂ , R ₂₃ , and R ₂₄ are cyano-containing groups, a, b, and d are each independently selected from integers 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 compounds shown below;

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, which 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 at least one of fluorine, boron or phosphorus.

In some optional embodiments, the lithium salt includes at least one of lithium hexafluorophosphate LiPF ₆ , lithium bis(trifluoromethanesulfonyl)imide LiN(CF ₃ SO ₂ ) ₂ (abbreviated as LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO ₂ F) ₂ ) (abbreviated as LiFSI), lithium bis(oxalatoborate) LiB(C ₂ O ₄ ) ₂ (abbreviated as LiBOB), lithium tetrafluorophosphate oxalate (LiPF ₄ C ₂ O ₂ ), lithium difluorooxalatoborate 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 present application, the concentration of lithium salt is 0.5mol/L-1.5mol/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 high, which also affects the rate of the entire lithium-ion battery system. Optionally, the concentration of lithium salt is 0.8mol/L-1.3mol/L.

In some embodiments of the present application, the electrolyte includes a non-aqueous organic solvent, wherein the non-aqueous organic solvent includes one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate or propyl butyrate, or a combination of two or more in any proportion.

The present application also provides an electrochemical device, comprising: 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 electrochemical device comprises a positive electrode current collector and a positive electrode active material disposed on the positive electrode current collector. The specific types of the positive electrode active material are not subject to specific restrictions and can be selected according to needs.

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 manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium iron manganese phosphate, lithium vanadium phosphate, lithium vanadium phosphate, lithium iron phosphate, lithium titanate, and lithium-rich manganese-based materials.

Specifically, the chemical formula of lithium cobalt oxide may be as shown in Chemical Formula 1:

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

Wherein M1 represents at least one 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 silicon (Si), and the values of x, a, b and c are respectively in 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 manganese oxide or lithium nickel cobalt aluminum oxide may be as shown in Chemical Formula 2:

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

Wherein M2 represents at least one selected from cobalt (Co), 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), zirconium (Zr) and silicon (Si), and the values of y, d, e and f are respectively 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 shown in Chemical Formula 3:

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

Wherein M3 represents at least one selected from cobalt (Co), nickel (Ni), 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) and tungsten (W), and the values of z, g and h are respectively in the following ranges: 0.8≤z≤1.2, 0≤g<1.0 and -0.2≤h≤0.2.

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

In some embodiments, the separator includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide or aramid. For example, the polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene or ultra-high molecular weight polyethylene. In particular, polyethylene and polypropylene 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, which is disposed on at least one surface of the isolation membrane, _and the porous layer includes inorganic particles and a binder, wherein the inorganic particles are selected from at least one of aluminum oxide ( _Al2O3 ), silicon oxide ( _SiO2 ), magnesium oxide ( _MgO ), titanium oxide ( _TiO2 ), hafnium dioxide ( _HfO2 ), tin oxide (SnO2), cerium dioxide ( _CeO2 ), _nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide ( _ZrO2 ), yttrium oxide ( _Y2O3 ), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethylcellulose, polyvinylpyrrolidone, 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 wetting performance of the isolation membrane, and enhance the adhesion between the isolation membrane and the electrode.

The present application also proposes an electronic device, including any of the above electrochemical devices. The electronic device of the present application is not particularly limited, and it can be used for any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium ion capacitor, etc. For example, the electronic device includes a mobile phone containing a lithium ion battery.

In order to better illustrate the beneficial effects of the electrolyte proposed in the embodiments of the present application, the following will be described in combination with Examples 1-53 and Comparative Examples 1-4. The only difference between Examples 1-53 and Comparative Examples 1-4 is that different electrolytes are used. In Examples 1-53 and Comparative Examples 1-4, performance tests will be performed on lithium-ion batteries using different electrolytes to illustrate the influence of the electrolyte 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), and propylene carbonate (abbreviated as PC) are mixed uniformly in a mass ratio of 3:4:3 to form a non-aqueous solvent, and then fully dried lithium salt LiPF ₆ is dissolved in the non-aqueous solvent, and the concentration of LiPF ₆ is 1 mol/L to prepare the basic electrolyte in the embodiment. The electrolyte used in each embodiment and comparative example is obtained by adding at least one of the compound of formula I shown below, a polynitrile compound, a cyclic ether compound, LiPO ₂ F ₂ or cobalt ions to the basic electrolyte.

Compounds of formula I:

Polynitrile compounds:

Cyclic ether compounds:

Battery preparation

1) Preparation of positive electrode: lithium cobalt oxide, acetylene black, and polyvinylidene fluoride (abbreviated as PVDF) are fully stirred and mixed in a proper amount of N-methylpyrrolidone (abbreviated as NMP) solvent at a weight ratio of 96:2:2 to form a uniform positive electrode slurry; the slurry is coated on the positive electrode current collector Al foil, dried and cold pressed to obtain a positive electrode active material layer, and then cut into pieces and welded to the pole ears to obtain the positive electrode.

2) Preparation of negative electrode: Graphite, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are fully stirred and mixed in a proper amount of deionized water solvent at a weight ratio of 97:2:1 to form a uniform negative electrode slurry; the slurry is coated on the negative electrode current collector Cu foil, dried, and cold pressed to obtain a negative electrode active material layer, and then cut into pieces and welded to the pole ears to obtain the negative electrode.

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

4) Preparation of lithium-ion batteries: stack the positive electrode separator and the negative electrode in order, so that the separator is between the positive and negative electrodes to play a role of isolation, then wind them up and place them in an outer packaging foil, inject the prepared electrolyte into the dried battery, and complete the preparation of the lithium-ion battery after vacuum packaging, standing, forming, shaping and other processes.

High temperature cycle test

Place the lithium-ion battery in a 45℃ constant temperature box and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature; charge it at a constant current of 0.7C to 4.45V, and charge it at a constant voltage to a current of 0.05C; then discharge it at 0.7C to 3.0V, and use this step capacity as the benchmark, record it as the first cycle discharge capacity; repeat this step 300 times, record the discharge capacity of the 300th cycle, and calculate the capacity retention rate.

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

Lithium-ion battery high temperature storage performance test

The lithium-ion battery was discharged at 0.5C to 3.0V at 25°C, then charged at 0.7C to 4.45V, and then charged at a constant voltage of 0.05C at 4.45V. The thickness of the battery was measured and recorded with a micrometer and recorded as _H11 . The battery was placed in an oven at 85°C and kept at a constant voltage of 4.45V for 16 hours. After 16 hours, the thickness of the lithium-ion battery was measured and recorded with a micrometer and recorded as _H12 . The thickness expansion rate was calculated. Thickness expansion rate = ( _H12 - _H11 )/ _H11 × 100%

Lithium-ion battery DC resistance (DCR) test at 0°C

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

20% SOC DC resistance = (V ₁ -V ₂ )/1C

Ratio test

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

Cycle impedance test

The lithium-ion battery was charged to 4.45V at 0.7C at 45°C, charged to 0.05C at a constant voltage at 4.45V, and then discharged to 3.0V at a constant current of 1.0C. The battery was cycled for 300 cycles under this condition. A resistivity meter was used to monitor the impedance change at 100% SOC during the cycling of the lithium-ion battery, and the cyclic impedance of 300 cycles was recorded.

Examples 1-16 and Comparative Examples 1-4

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

Table 1

Table 2

By comparing Examples 1-7 with 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, the 20% SOC impedance of Examples 1-7 is significantly lower than that of Comparative Example 1, and 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, the high-rate discharge performance of the lithium-ion battery can be improved, the impedance of the lithium-ion battery can be reduced, and the cycle performance can be improved. This is because the compound represented by Formula I has a _-CH2 -O- _CH2- group, and the compound represented by Formula I having the group helps to improve the mobility of lithium ions. Therefore, the electrolyte having the compound represented by Formula I can dissolve more lithium salts, enhance 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 Examples 1 and 2 that the capacity retention rate of Comparative Example 2 after 300 cycles at 45°C is significantly lower than that of Comparative Example 1. This is because too much compound of formula 1 is added to Comparative Example 2. When there is too much compound of formula I in the electrolyte, the cycle performance of the lithium ion battery will be reduced. It can be seen from the performance test results of Comparative Examples 1 and 3 that when there is too little compound of formula I in the electrolyte, 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%.

From the performance test results of Comparative Examples 1 and 4, it can be seen that the test results of 20% SOC DC impedance and capacity retention rate after 300 cycles at 45°C of Comparative Example 4 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 the lithium ion battery can be improved and the impedance of the lithium ion battery 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 redox reactions at the positive and negative electrode interfaces to generate 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.

It can be seen from the performance test results of Examples 2 and 8 that adding the compound of Formula I and LiPO ₂ F ₂ to the electrolyte can further reduce the DC impedance of the lithium ion battery and improve the cycle performance of the lithium ion battery. The same conclusion can be drawn from the performance test results of Examples 13-16 and Comparative Example 4, that is, adding the compound of Formula I and LiPO ₂ F ₂ , utilizing the synergistic effect of the two, can further improve the cycle and reduce the impedance.

It can be seen from the performance test results of Examples 8-12 that with the increase of the LiPO ₂ F ₂ content in the electrolyte, the impedance of the lithium ion battery first decreases and then increases, and the capacity retention of the lithium ion battery after 300 cycles at 45°C first increases and then decreases. It can be seen that the higher the content of LiPO ₂ F ₂ in the electrolyte, the better. Therefore, in some embodiments of the present application, the percentage of LiPO ₂ F ₂ in the total mass of the electrolyte is limited to less than 1% to prevent excessive LiPO ₂ F ₂ from causing performance degradation of the lithium ion battery.

Examples 17-33

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

Table 3

Table 4

By comparing the performance test results of Examples 13, 17-32, it can be seen that by adding a polynitrile compound to the electrolyte, the thickness expansion rate of the lithium-ion battery at 85°C-16h can be significantly reduced, and the capacity retention rate after 300 cycles at 45°C can be improved, that is, the high-temperature storage performance and high-temperature cycle performance of the lithium-ion battery can be improved by adding a polynitrile compound. This is because nitrile compounds can complex with transition metals on the surface of the positive electrode, reduce the dissolution of transition metals, and reduce the contact between the electrolyte and the positive electrode interface, further reducing 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 kinetic performance of the lithium-ion battery.

From embodiment 20, embodiment 29 and embodiment 30, it can be seen that different polynitrile compounds have different improvement degrees to the cycle performance and storage performance of lithium ion battery, and this is because the organic molecules containing nitrile functional groups of different structures will produce different isolation effects to electrolyte and positive electrode surface.Along with the increase of the number of nitrile functional groups in organic molecules, the isolation effect it plays is more remarkable.Meanwhile, the size of the organic molecules containing nitrile functional groups has an optimum value, and the molecule is too small, and the isolation space formed is limited, and the easily oxidized components in the electrolyte can not be separated from the positive electrode surface effectively, and the molecule is too large, and the easily oxidized components in the electrolyte can contact with the positive electrode surface through the gap of the organic molecules containing nitrile functional groups, and still can not play a good isolation effect.

Examples 34-47

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

Table 5

Table 6

From the performance test results of Example 22 and Examples 37-45, it can be seen that when a mass fraction of 0.1%-2% of the cyclic ether compound 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 is oxidized on the surface of the positive electrode to generate an organic lithium salt, which is relatively stable, thereby enhancing the stability of the solid electrolyte interface (SEI) film, alleviating the oxidative decomposition of the electrolyte of the lithium ion battery on the electrode surface during the high temperature process, 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 and 47 with those of Examples 37-45, it can be seen that when the amount of the cyclic ether compound added exceeds 2%, the high temperature cycle performance and 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, thereby accelerating the attenuation of the lithium ion battery cycle capacity and deteriorating 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 improvement effect of the compound of Formula Ⅰ-1 used alone in combination with cyclic ether, or in combination with lithium difluorophosphate (LiPO ₂ F ₂ ) and cyclic ether is not as significant as the improvement effect of adding nitriles at the same time.

Examples 48-53

Examples 48-53 are obtained based on Example 39, wherein the electrolyte further contains cobalt ions. The electrolytes used in Examples 48-53 and the performance test results are shown in Table 7. For ease of comparison, the electrolyte parameters and performance test results of Example 39 are added to Table 7.

Table 7

By comparing the performance test results of Examples 39, 48-53, it can be seen that a small amount of cobalt ions in the electrolyte can significantly improve the cycle impedance. 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 the cycle impedance.

The electrolyte proposed in the present application can improve the rate performance and cycle performance of the lithium ion battery using the electrolyte and reduce the internal resistance 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 technical principles used. Those skilled in the art should understand that the scope of disclosure involved in the present application is not limited to the technical solution formed by a specific combination of the above technical features, but should also cover other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the above disclosed concept. For example, the above features are replaced with the technical features with similar functions disclosed in this application (but not limited to) by each other to form a technical solution.

Although the subject matter has been described in language specific to structural features and/or methodological logical actions, 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 example forms of implementing the claims.

Claims (11)

1. An electrolyte comprising a compound of formula I:

wherein R1 and R2 are each independently selected from an alkyl group having 1 to 11 carbon atoms, a substituted alkyl group having 1 to 11 carbon atoms, an alkenyl group having 2 to 11 carbon atoms, and a substituted alkenyl group having 2 to 11 carbon atoms, and 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 imidazolium cation, pyridinium cation, piperidinium cation and quaternary ammonium salt cation; X is selected from one of hexafluorophosphate, bistrifluoromethanesulfonate, bisfluoromethanesulfonate, tetrafluoroborate, bisoxalatoborate and tetrafluorooxalatoborate; The electrolyte also includes: lithium difluorophosphate; The percentage of the lithium difluorophosphate in the total mass of the electrolyte is less than 1%, 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, wherein C+D<11%, 0.5≤C/D≤10. 2. The electrolyte according to claim 1, characterized in that the compound of formula I comprises at least one of the following compounds:

3. The electrolyte according to claim 1, characterized in that the percentage of the compound of formula I to the total mass of the electrolyte is 0.01%-10%. 4. The electrolyte according to claim 1, characterized in that it also includes: at least one of a polynitrile compound or a cyclic ether compound. 5. The electrolyte according to claim 4, characterized in that the electrolyte satisfies at least one of the following conditions (a)-(b): (a) the percentage of the polynitrile compound to the total mass of the electrolyte is 0.5%-10%; (b) The percentage of the cyclic ether compound to the total mass of the electrolyte is 0.01%-2%. 6. The electrolyte according to claim 4, characterized in that 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 the group consisting of hydrogen, cyano, -(CH ₂ ) _a -CN, -(CH ₂ ) _b -O-(CH ₂ ) _c -CN, -(CH ₂ ) _d -(CH＝CH)-CN, an alkyl group having 1-5 carbon atoms, and an alkoxycarbonyl group having 2-5 carbon atoms, and at least two of R ₂₁ , R ₂₂ , R ₂₃ , and R ₂₄ are cyano-containing groups, a, b, and d are each independently selected from integers of 0-10, and c is selected from an integer of 1-5. 7. The electrolyte according to claim 4, characterized in that the polynitrile compound comprises at least one of the compounds shown below;

8. The electrolyte according to claim 4, characterized in that the cyclic ether compound comprises at least one of 1,3-dioxolane, 1,3-dioxane or 1,4-dioxane. 9. An electrochemical device, comprising: A positive electrode, a negative electrode, a separator and an electrolyte as described in any one of claims 1 to 8. 10 . The electrochemical device according to claim 9 , wherein the electrolyte further comprises cobalt ions, and the cobalt ions account for 1 ppm to 50 ppm of the total mass of the electrolyte. 11. An electronic device, characterized by comprising the electrochemical device according to any one of claims 9-10.

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