CN118117164A

CN118117164A - Electrolyte and electrochemical device

Info

Publication number: CN118117164A
Application number: CN202211471783.2A
Authority: CN
Inventors: 管明明; 曹文鹏
Original assignee: Shanghai Jusheng Technology Co Ltd
Current assignee: Shanghai Jusheng Technology Co Ltd
Priority date: 2022-11-23
Filing date: 2022-11-23
Publication date: 2024-05-31

Abstract

The present invention relates to an electrolyte and an electrochemical device. The electrolyte is characterized by comprising electrolyte salt, a solvent and an additive, wherein the molecular structure of the additive is as follows: Wherein R ₁ is selected from unsubstituted or substituted C1-C4 alkyl; r ₂ is selected from C, B or P; r ₃ and R ₄ are independently selected from unsubstituted or substituted C1-C2 alkyl. The electrolyte contains the additive shown in the formula (I), so that the thermal safety of the battery core can be improved, the initial efficiency of the battery is improved, and the DCR is reduced. Analysis shows that in the preparation process of the battery cell, the additive forms a relatively stable interface film on the surface layer of the active material, the interface film has excellent lithium stability and thermal stability, has no obvious influence on the rate performance of the battery cell full temperature threshold, and can improve the threshold temperature of thermal runaway of the battery cell and obviously improve the safety performance of the battery cell in the battery cell thermal abuse scene and the electric abuse scene.

Description

Electrolyte and electrochemical device

Technical Field

The present invention relates to the technical field of electrochemical devices, and in particular, to an electrolyte and an electrochemical device.

Background

The lithium ion battery has the characteristics of higher energy density, extremely low self-discharge rate, longer service life and the like, and is the most potential energy utilization mode in the fields of energy power and the like. Manufacturers currently strive to increase the energy density and fast charge capacity of the cells, however, this can lead to a significant decrease in the safety threshold window of the cells, especially for electrical and thermal safety.

Currently, in a liquid cell system, a main strategy for improving the thermal safety and the electrical safety of a cell is to use primary particle anode materials such as single crystals and the like and flame-retardant electrolyte. The primary particle monocrystal with large particle size is used for replacing a polycrystalline structure, so that the reactivity of the positive electrode material and the electrolyte can be reduced, and the thermal runaway threshold temperature of the positive electrode material is improved; however, primary particles with large particle diameters deteriorate the initial impedance of the battery cell, affect the power characteristics of the battery cell, and in addition, the sintering process of the single crystal material is complex and has high cost. The flame-retardant electrolyte is mainly prepared by adding phosphorus-containing ester-based organic matters into the electrolyte, and the flame-retardant electrolyte has a good capturing effect on combustible free radicals, but can seriously deteriorate the self conductivity of the electrolyte, and the flame-retardant additive has a great influence on the uniformity of electrode interface intervals, so that the thermal safety threshold temperature of the battery core is deteriorated in the later period of the service life of the battery core.

Disclosure of Invention

Based on this, it is necessary to provide an electrolyte and an electrochemical device to improve the thermal safety of the battery cell.

An electrolyte comprising an electrolyte salt, a solvent and an additive, the additive having a molecular structure as shown in formula (i);

wherein R ₁ is selected from unsubstituted or substituted C1-C4 alkyl;

R ₂ is selected from C, B or P;

R ₃ and R ₄ are independently selected from unsubstituted or substituted C1-C2 alkyl.

In one embodiment, in the substituted C1-C4 alkyl group, the substituent is selected from the group consisting of C2-C3 alkenyl, C2-C3 alkynyl, fluorine atom or cyano.

In one embodiment, in the substituted C1-C2 alkyl group, the substituent is selected from the group consisting of C2-C3 alkenyl, C2-C3 alkynyl, an oxygen-containing group, or a fluorine atom.

In one embodiment, the molecular structure of the additive is one of the following formulas (1-1) to (1-7):

In one embodiment, the mass fraction of the additive is 0.01% -10%.

In one embodiment, the electrolyte salt is 5-23% by mass.

In one embodiment, the solvent is 67% to 94.99% by mass.

In one embodiment, the electrolyte salt is at least one of a lithium salt and a sodium salt;

The solvent is at least one selected from carbonate, carboxylate, nitrile solvent, sulfone solvent and ether solvent.

An electrochemical device comprising a housing, and a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte of any of the above embodiments disposed in the housing.

In one embodiment, the positive plate has a porous structure, and the porosity of the positive plate is epsilon, and the mass fraction of the additive in the electrolyte is Wa, so epsilon/Wa is between 0.001 and 50.

In one embodiment, ε/Wa is between 10 and 30.

Compared with the traditional scheme, the electrolyte and the electrochemical device have the following beneficial effects:

The electrolyte contains the additive shown in the formula (I), so that the thermal safety of the battery core can be improved, the initial efficiency of the battery is improved, and the DCR is reduced. Analysis shows that in the preparation process of the battery cell, the additive forms a relatively stable interface film on the surface layer of the active material, the interface film has excellent lithium stability and thermal stability, has no obvious influence on the rate performance of the battery cell full temperature threshold, and can improve the threshold temperature of thermal runaway of the battery cell and obviously improve the safety performance of the battery cell in the battery cell thermal abuse scene and the electric abuse scene.

The electrochemical device contains the electrolyte solution, and thus can achieve a corresponding technical effect.

Detailed Description

The present invention will be described more fully hereinafter in order to facilitate an understanding of the present invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and/or" as used herein includes any and all combinations of one or more of the associated listed items.

The invention provides an electrolyte.

The electrolyte of one embodiment comprises an electrolyte salt, a solvent, and an additive. Wherein the additive has a molecular structure shown in a formula (I);

Wherein R ₁ is selected from unsubstituted or substituted C1-C4 alkyl.

R ₂ is selected from C, B or P.

R ₁ is selected from unsubstituted or substituted C1-C4 alkyl. The unsubstituted C1-C4 alkyl group may be specifically methyl, ethyl, propyl, butyl, etc. In the substituted C1-C4 alkyl group, the substituent is, for example, a C2-C3 alkenyl group, a C2-C3 alkynyl group, a fluorine atom, a cyano group or the like. The existence of the R ₁ group can obviously reduce the electron cloud density on O, reduce the reaction activation energy barrier of the O, and further improve the film forming effect; and the molecular rigidity can be obviously reduced, and the free radical capturing capability of the unit mass additive in thermal runaway is improved. Among the substituents, the C2-C3 alkenyl is beneficial to the synergistic film formation of the anode and the cathode, improves the safety performance, and simultaneously improves the electrical performance, wherein the C2-C3 alkynyl is basically similar to the C2-C3 alkenyl substituent, the fluorine atom also has stronger free radical capturing capability, the cell safety threshold can be synergistically improved with a heterocyclic structure, the cyano can be cooperatively coordinated with an anode oxidation site, the high-temperature storage performance of the cell is improved, and the heat generation rate of the cell is reduced.

R ₂ is selected from C, B or P. Wherein, B and P both contain lone pair electrons, which can capture active Lewis acid, improve the safety performance and improve the cycle stability of the battery.

R ₃ and R ₄ are independently selected from unsubstituted or substituted C1-C2 alkyl. In the substituted C1-C2 alkyl group, the substituent is selected from a C2-C3 alkenyl group, a C2-C3 alkynyl group, a cyano group or a fluorine atom. Among the substituents, the C2-C3 alkenyl and the C2-C3 alkyne have lower reduction potential, can obviously reduce the consumption of electrolyte, improve the safety performance, and simultaneously improve the electrochemical performance, the cyano group can coordinate with the positive electrode, inhibit the degradation of the positive electrode material, and the fluorine atom can form an interface film with high inorganic component content to improve the safety performance.

In one example, the molecular structure of the additive is one of the following formulas (1-1) to (1-7).

In one example, the mass fraction of the additive is 0.01% -10%. Further, in one example, the mass fraction of the additive is 0.1% to 10%. Further, in one example, the mass fraction of the additive is 1% to 3%.

In one example, the electrolyte salt is present in the electrolyte in an amount of 5% to 23% by mass. Further, in one example, the electrolyte salt is 9% to 16% by mass.

In one example, the electrolyte salt is at least one of a lithium salt and a sodium salt.

The lithium salt may be LiPF₆、LiBF₄、LiClO₄、LiAsF₆、LiBOB、LiDFOB、LiFSI、LiTFSI、LiPO₂F₂、LiTFOP、LiN(SO₂RF)₂, liN (SO ₂F)(SO₂ RF), or the like, for example. Wherein RF represents perfluoroalkyl C _nF_2n+1, and n is an integer of 1 to 10. Preferably, the lithium salt comprises at least LiPF ₆.

The sodium salt may be NaPF₆、NaBF₄、NaClO₄、NaAsF₆、NaCF₃SO₃、NaN(CF₃SO₂)₂、NaN(C₂F₅SO₂)₂ or NaN (FSO ₂)₂, etc.).

In one example, the mass fraction of solvent is 67% to 94.99%. Further, in one example, the mass fraction of solvent is 67% to 94.9%.

The solvent is preferably an organic solvent such as carbonate, carboxylate, nitrile, sulfone, ether solvents, or the like. Wherein the carbonate comprises at least one of cyclic carbonate and halogenated derivatives thereof, and chain carboxylic acid ester and halogenated derivatives thereof.

In one example, the solvent is selected from one or more of ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylethyl carbonate, methyl formate, ethyl acetate, propyl propionate, ethyl propionate, gamma-butyrolactone, and tetrahydrofuran.

The electrolyte contains an additive shown as a formula (I), and experiments show that the additive can improve the initial efficiency of a battery, improve the thermal safety and reduce the Direct Current Resistance (DCR).

In the preparation process of the battery cell, the additive forms a relatively stable interface film on the surface layer of the active material, the interface film has excellent lithium stability and thermal stability, has no obvious influence on the multiplying power performance of the full temperature threshold of the battery cell, and can improve the threshold temperature of 32 ℃ of thermal runaway of the battery cell and obviously improve the safety performance of the battery cell in the thermal abusing scene and the electrical abusing scene of the battery cell.

Further, the invention also provides an electrochemical device.

An electrochemical device of an embodiment includes a housing, a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte disposed in the housing.

The electrochemical device may be, but is not limited to, a lithium ion battery, a sodium ion battery, a zinc ion battery, or a supercapacitor, etc.

In one example, the positive electrode sheet is porous.

The porosity of the positive plate is epsilon, the mass fraction of the additive in the electrolyte is Wa, and the ratio epsilon/Wa of the porosity of the positive plate to the mass fraction of the additive is preferably between 0.001 and 50. Further, epsilon/Wa is between 10 and 30.

The porosity of the positive electrode and the content of the additive are in the range, so that the additive can be adsorbed on the surface of the positive electrode active material in situ with high coverage after the electrolyte is injected. When the battery cell reaches the critical reaction temperature, the additive adsorbed on the surface of the active substance of the positive electrode can perform in-situ blocking reaction, the blocking of electrons and ions is realized at the positive electrode side instantly, the activation energy barrier of the electrochemical reaction of the positive electrode is obviously improved, the internal large current path is blocked, and the continuous heat generation and the further temperature rise of the battery cell are inhibited. When the ratio epsilon/Wa is lower than 0.1, the in-situ adsorption of the additive molecules on the surface of the active material of the positive electrode can be caused to have poor coverage, so that when the battery core reaches the critical temperature, a local large-current inner loop still exists, and the battery core still has local obvious temperature rise, thereby causing the overall thermal runaway of the pole piece caused by local thermal runaway. If the content of the additive is too high, the viscosity of the electrolyte is obviously reduced, and the electrochemical performance of the battery cell, especially the low-temperature performance, is seriously affected by the high content of the additive.

Hereinafter, an electrochemical device will be described in more detail by taking a lithium ion battery as an example, but the present invention is not limited thereto.

In a lithium ion battery, a positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material, and may further contain a binder, a conductive agent, and the like.

The positive electrode current collector is a supporting layer capable of conducting electricity and not reacting with other components in the electrochemical device, for example, aluminum foil may be selected as the positive electrode current collector.

The positive electrode active material is selected from substances capable of desorbing lithium ions. For example, the positive electrode active material may be selected from LiMnO₂、LiMn₂O₄LiNi_1-xCo_yO₂、LiCo_1-xMn_xO₂、LiNi_1-xMn_xO₂(0＜x＜1)、Li(Ni_xCo_yMn_z)O₄(0＜x＜1,0＜y＜1,0＜z＜1,0＜x+y+z＜1)、LiMn_2-aNi_aO₄、LiMn_2-aCo_aO₄(0＜a＜2)、LiMPO₄(M, which may be selected from one or more of Co, ni, fe, mn and V), spinel material LiMn ₂O₄, layered material lithium cobaltate (LiCoO ₂), lithium nickelate (LiNiO ₂)、Li_aNi_xA_yB_(1-x-y)O₂ (0.95.ltoreq.a.ltoreq.1, A and B may be independently selected from one of Co, mn and Al, and A and B are different, 0 < x < 1,0 < y < 1,0 < x+y < 1), and the like. The positive electrode active material may include at least one of sulfide, selenide, and halide.

In one example, the surface of the positive electrode active material also has a coating layer, or is mixed with a material having a coating layer. In one example, the coating layer includes at least one selected from an oxide, hydroxide, oxyhydroxide, oxycarbonate, and hydroxycarbonate of the coating element. The coating element comprises a mixture of one or more of Mg, al, co, K, na, ca, si, ti, V, sn, ge, ga, B, as, zr. The compound forming the coating layer may be crystalline or amorphous.

In one example, the positive electrode active material layer further includes a positive electrode binder and a positive electrode conductive agent. The positive electrode binder is used to improve the binding properties of the positive electrode active material particles to each other and to the current collector. The positive electrode binder is, for example, at least one of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, an ethyleneoxy-containing polymer, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber of acrylic acid (ester), epoxy resin, and nylon. The positive electrode conductive agent is used to provide conductivity to the electrode, and may include any conductive material as long as it does not chemically react with the positive electrode active material. The positive electrode conductive agent is, for example, at least one of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, metal powder, metal fiber, and polyphenylene derivative. Wherein the metal in the metal powder and the metal fiber comprises at least one of copper, nickel, aluminum and silver.

The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The anode active material layer contains an anode active material.

In some embodiments, the negative electrode current collector is a supporting layer that is capable of conducting electricity and that does not react with other components in the electrochemical device, for example, a copper foil may be used as the negative electrode current collector.

The negative electrode active material is a substance capable of reversibly intercalating and deintercalating active ions, or a substance capable of reversibly doping and deintercalating active ions. In one example, the negative electrode active material includes at least one of lithium metal, a lithium metal alloy, a carbon material, and a silicon-based material. The lithium metal alloy comprises an alloy of lithium and a metal selected from Na, K, rb, cs, fr, be, mg, ca, sr, si, sb, in, zn, ba, ra, ge, al, sn. In one example, the carbon material is selected from at least one of crystalline carbon and amorphous carbon. The crystalline carbon is, for example, natural graphite or artificial graphite. The crystalline carbon is amorphous, plate-shaped, sheet-shaped, spherical or fibrous in shape. In one example, the crystalline carbon is a low crystalline carbon or a high crystalline carbon. The low crystalline carbon includes at least one of soft carbon and hard carbon. The high crystalline carbon comprises at least one of natural graphite, crystalline graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microbeads, mesophase pitch, and high temperature calcined carbon. The high temperature calcined carbon is petroleum or coke derived from coal tar pitch. The amorphous carbon is at least one of soft carbon, hard carbon, mesophase pitch carbonized product and fired coke. In one example, the anode active material includes a transition metal oxide. In one example, the anode active material includes at least one of Si, siO _x (0 < x < 2), si/C composite, si-Q alloy, sn, snO _Z, sn-C composite, and Sn-R alloy. Wherein Q is at least one selected from alkali metal, alkaline earth metal, group 13 to group 16 elements, transition element and rare earth element, and Q is not Si. R is at least one selected from alkali metal, alkaline earth metal, group 13 to group 16 elements, transition element and rare earth element, and R is not Sn. Wherein Q and R comprise at least one of Mg、Ca、Sr、Ba、Sc、Y、Ti、Zr、Hf、Rf、V、Nb、Ta、Db、Cr、Mo、W、Sg、Tc、Re、Bh、Fe、Pb、Ru、Os、Hs、Rh、Ir、Pd、Pt、Cu、Ag、Au、Zn、Cd、B、Al、Ga、Sn、In、Tl、Ge、P、As、Sb、Bi、S、Se、Te and Po.

In one example, the anode active material layer further includes an anode binder and an anode conductive agent. The negative electrode binder is, for example, at least one of vinylidene fluoride-hexafluoropropylene copolymer (PVDF-Co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, an ethyleneoxy-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber of acrylic acid (ester), epoxy resin, and nylon. The negative electrode conductive agent is used to provide conductivity to the electrode, and is, for example, one or more of a carbon-based material, a metal-based material, and a conductive polymer. Wherein the carbon-based material is at least one of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber, for example. The metal-based material is, for example, at least one of metal powder of copper, nickel, aluminum, silver, etc., and metal fiber. The conductive polymer is, for example, a polyphenylene derivative.

In the preparation process of the anode slurry, a solvent is generally added, an anode active material is added with a binder, and a conductive material and a thickener are added as needed, and then dissolved or dispersed in the solvent to prepare the anode slurry. The solvent is volatilized during the drying process. The solvent is, for example, water and the thickener is, for example, sodium carboxymethylcellulose.

The mixing ratio of the negative electrode active material, the binder, and the thickener in the negative electrode active material layer is not particularly limited in the present invention, and the content may be optimized according to the performance expectation.

The separator may be a single-layer separator or a multi-layer separator. In one example, the separator includes a substrate of at least one of Polyethylene (PE), ethylene-propylene copolymer, polypropylene (PP), ethylene-butene copolymer, ethylene-hexene copolymer, and ethylene-methacrylate copolymer. In one example, the separator employs a microporous membrane of the polyolefin type.

In one example, the separator further includes a coating formed on the substrate. The coating layer includes at least one of an organic coating layer and an inorganic coating layer. Wherein the organic coating is selected from at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyimide, acrylonitrile-butadiene copolymer, acrylonitrile-styrene-butadiene copolymer, polymethyl methacrylate, polymethyl acrylate, polyethyl acrylate, acrylic acid-styrene copolymer, polydimethylsiloxane, sodium polyacrylate and sodium carboxymethyl cellulose. The inorganic coating is selected from at least one of SiO ₂、Al₂O₃、CaO、TiO₂、ZnO₂、MgO、ZrO₂ and SnO ₂.

The equipment using the above electrochemical device may be, for example, but not limited to, an automobile, a motorcycle, a skateboard, an airplane, a passenger car, a motor, a standby power supply, a large-sized storage battery for home use, a lithium ion capacitor, a computer, a cellular phone, an electronic book, a facsimile machine, a copier, a flash lamp, a television, a VR/AR, an energy storage power station, a marine vehicle, an air vehicle, and the like. Wherein the air vehicle comprises an air vehicle within the atmosphere and an air vehicle outside the atmosphere.

The invention will be further illustrated with reference to specific examples.

Using the lithium ion batteries of the same configuration as in examples 1 to 14 and comparative examples 1 to 4, lithium ion batteries were produced in the following manner.

Preparation of a positive plate:

The positive electrode active material LiNi _0.8Co_0.1Mn_0.1O₂, a binder PVDF and a conductive agent Super-P are dissolved in NMP (N-methylpyrrolidone) according to the weight fraction of 96:2:2, so as to obtain positive electrode slurry. The positive electrode slurry was coated on an aluminum foil, and the obtained membrane was subjected to porosity custom drying, and the porosity of the positive electrode sheet was as shown in table 1. The positive plate is subjected to the procedures of rolling, slitting, cutting, slitting and the like, and then is dried for 8 hours at 90 ℃ in vacuum, and the obtained positive plate is subjected to tab welding.

Preparing a negative plate:

Mixing negative electrode active material artificial graphite, conductive agent Super-P, thickener sodium carboxymethylcellulose (CMC) and binder styrene-butadiene rubber according to the mass ratio of 96.2:2:0.8:1, adding deionized water, and obtaining negative electrode slurry with the solid content of 54wt% under the action of a vacuum stirrer.

And uniformly coating the negative electrode slurry on a negative electrode current collector copper foil. And (3) drying the coated copper foil at a high temperature, cold pressing, cutting, slitting and drying for 12 hours under the vacuum condition at 120 ℃ to obtain the negative plate.

Preparation of the separator:

A 9 μm thick polyethylene-based film was selected, with a base film porosity of 55%. A slurry is applied to the base film, the slurry comprising boehmite and Al ₂O₃ in a mass ratio of 7:3, and a PVDF binder. After drying, a coating layer having a thickness of 3 μm was formed on the base film to obtain a separator.

Preparation of electrolyte:

in a dry argon glove box, solvents (including EC, DEC, EMC, mass ratio 3:2:5), additives and lithium salts were mixed. Specifically, a solvent is added firstly, then an additive is added, lithium salt is added after dissolution and full stirring, and the electrolyte is obtained after uniform mixing. Wherein the additives represented by the above-mentioned formula 1-1 were added to the electrolytes of examples 1 to 7, the additives represented by the above-mentioned formula 1-2 were added to the electrolytes of examples 8 to 9, the additives represented by the above-mentioned formula 1-3 were added to the electrolyte of example 10, the additives represented by the above-mentioned formula 1-4 were added to the electrolyte of example 11, the additives represented by the above-mentioned formula 1-5 were added to the electrolyte of example 12, the additives represented by the above-mentioned formula 1-6 were added to the electrolyte of example 13, and the additives represented by the above-mentioned formula 1-7 were added to the electrolyte of example 14. The additive content is shown in table 1. The electrolytes in comparative examples 1 to 4 differ from examples 1 to 11 in that the above-mentioned additives were not contained.

Preparation of a lithium ion battery:

and stacking the positive plate, the diaphragm and the negative plate in sequence, enabling the diaphragm to be positioned between the positive plate and the negative plate, playing an isolating role, and then winding to obtain the bare cell. After welding the electrode lugs, placing the obtained bare cell in an aluminum plastic film of an outer package, injecting the prepared electrolyte into the dried bare cell, vacuum packaging, standing, forming (charging 3.3V with a constant current of 0.02C and then charging 3.6V with a constant current of 0.1C), shaping, testing the capacity and other procedures to obtain the target cell.

The lithium ion batteries of examples 1 to 14 and comparative examples 1 to 4 were subjected to performance tests, and the test methods were specifically as follows.

Cell coulombic efficiency:

The battery cell is charged to 3.3V by constant current of 0.02C, then is charged to 3.6V by constant current of 0.1C, is charged to 4.25V by constant current of 0.2C, is charged to 0.05C by constant voltage of 4.25V, the total charge capacity C _{Capacity of} is obtained by recording, the battery cell is kept stand for 15min, the discharge capacity D _{Capacity of} of the battery cell is recorded after 0.2C is discharged to 3.0V, and the first effect is calculated by D _{Capacity of}/C_{Capacity of}.

Lithium ion battery thermal abuse test:

Before testing, the battery cell is charged to 4.25V at a constant current of 0.5C and is kept at a constant voltage of 4.25V to 0.05V, then the battery cell is placed in an oven, the surface temperature and voltage information of the battery cell are recorded, the heating rate of the oven is 5 ℃/min, the oven is heated to 130 ℃ and is kept at a constant temperature for 1 hour, and the voltage and temperature information are continuously monitored.

Lithium ion battery DCR test:

After the lithium ion battery is placed in a constant temperature box at 25 ℃ for 2 hours, the constant current charge is carried out at 0.5C to 4.2V, the constant voltage charge is carried out at 0.05C, the lithium ion battery is stood for 5min, the constant current discharge at 0.1C is carried out at 3.0V, and the capacity C ₂ is recorded. The end voltage U ₀,1C₂ was discharged for 1s at a constant current of 0.2C ₂ for 2.5h, and the end voltage U ₁ was recorded.

The calculation formula of the DC internal resistance DCR is as follows: dcr= (U ₀-U₁)/(0.9C₂).

Table 1 results of cell tests for different experimental groups

As can be seen from the first effect data in comparative examples 1 to 4, the charging first effect tends to increase and then decrease as the porosity of the positive electrode sheet increases from 10% to 60%. The analytical reasons are that the electrolyte wettability is enhanced due to the increase of the porosity, so that the film forming efficiency is enhanced in the first charging process. When the porosity is above 30%, the side reaction active sites increase, so that the charging initial effect tends to be reduced.

Compared with comparative examples 1 to 4, the addition of the additives to the electrolyte in examples 1 to 14 can improve the initial efficiency of the battery, and can reach 85 to 94% as a whole. As can be seen from a comparison of examples 1 to 7, as the content of the additive in the electrolyte increases from 0.01% to 10%, the charging initial effect tends to increase and then decrease. The analysis reason is that the film forming efficiency in the formation process is obviously improved along with the increase of the content of the additive, and the reduction potential of the additive is higher, so that the loss of active lithium can be effectively avoided. When the content of the additive is higher than 2%, the proportion of the additive and the porosity is higher, so that the local electrohydrodynamic force is seriously reduced, the polarization of the electrolyte is greatly influenced, the film forming component is changed, and the first effect is reduced. When the content of the additive is 1% -3% and the ratio epsilon/Wa is 10-30, the initial effect of the battery can reach more than 87% as a whole. When the content of the additive is 1-2%, and the ratio epsilon/Wa is 15-30, the initial effect of the battery can reach 88-94% as a whole, which is obviously superior to the situation without the additive. Compared with other groups, the ratio epsilon/Wa of the porosity of the positive plate to the mass content of the electrolyte in the embodiment 11 is 21.3, and the initial effect of the battery cell can be remarkably improved.

As can be seen from the data of the heat box passing rate in comparative examples 1 to 4, when an external heat source exists, when the porosity of the pole piece is higher, the self-generated heat temperature rise of the battery cell is obviously increased along with the increase of the electrochemical reaction area, so that the thermal safety of the battery cell is deteriorated.

Examples 1 to 14 can improve the thermal safety of the battery by adding an additive to the electrolyte as compared with comparative examples 1 to 4. As can be seen from the data of examples 1 to 7, as the ratio epsilon/Wa decreases, the thermal safety of the cell increases first and then tends to be balanced, and increasing the relative content of the additive in the electrode can form an interfacial film excellent in thermal stability, which can significantly suppress self-heat generation of the cell, as the ratio epsilon/Wa decreases to 10, the local film forming environment tends to be balanced, and excessive components rather self-decompose heat generation, resulting in slight deterioration in safety performance.

Example 11 shows that additives 1-4 have the best thermal stability, and can significantly improve the thermal safety test passing rate of the battery cell when the ratio epsilon/Wa is 21.3.

As can be seen from the DCR data in comparative examples 1 to 4, as the porosity of the electrode sheet increases, the impedance of the cell gradually decreases, mainly due to the decrease in the activation energy of the infiltration between the electrolyte and the electrode.

Examples 1 to 14 can reduce the DCR of the battery by adding an additive to the electrolyte as compared with comparative examples 1 to 4. As can be seen from the data of examples 1-7, as the ratio epsilon/Wa gradually decreases, the impedance of the cell tends to decrease first and then increase, and the early decrease is mainly due to the fact that the film-forming component of the additive is rich in fast ion conductors, has lower charge transfer impedance, and as the local concentration increases, the conductivity of the electrolyte itself is deteriorated, so that the overall impedance increases. When the content of the additive is 1-3%, the ratio epsilon/Wa is 10-30, the DCR of the battery is lower and is in the range of 11-15.

The technical features of the above-described embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above-described embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.

The above examples illustrate only a few embodiments of the invention, which are described in detail and are not to be construed as limiting the scope of the invention. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the invention, which are all within the scope of the invention. Accordingly, the scope of protection of the present invention is to be determined by the appended claims.

Claims

1. An electrolyte solution is characterized by comprising electrolyte salt, a solvent and an additive, wherein the additive has a molecular structure shown as a formula (I);

wherein R ₁ is selected from unsubstituted or substituted C1-C4 alkyl;

R ₂ is selected from C, B or P;

2. The electrolyte according to claim 1, wherein in the substituted C1 to C4 alkyl group, the substituent is selected from the group consisting of C2 to C3 alkenyl group, C2 to C3 alkynyl group, fluorine atom and cyano group.

3. The electrolyte according to claim 1, wherein in the substituted C1-C2 alkyl group, the substituent is selected from the group consisting of C2-C3 alkenyl group, C2-C3 alkynyl group, oxygen-containing group and fluorine atom.

4. The electrolyte according to claim 1, wherein the molecular structure of the additive is one of the following formulas (1-1) to (1-7):

5. The electrolyte of claim 1, wherein the additive is present in an amount of 0.01% to 10% by mass.

6. The electrolyte according to any one of claims 1 to 5, wherein the mass fraction of the electrolyte salt is 5% to 23%.

7. The electrolyte of claim 6, wherein the solvent comprises 67% to 94.99% by mass.

8. The electrolyte of any one of claims 1 to 5, wherein the electrolyte salt is at least one of a lithium salt and a sodium salt;

9. An electrochemical device comprising a case, a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte according to any one of claims 1 to 8, which are provided in the case.

10. The electrochemical device according to claim 9, wherein the positive electrode sheet has a porous structure, and wherein epsilon/Wa is 0.001 to 50 when epsilon is a porosity of the positive electrode sheet and Wa is a mass fraction of the additive in the electrolyte.

11. The electrochemical device of claim 10 wherein epsilon/Wa is between 10 and 30.