Lithium ion battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion battery. The lithium ion battery comprises a negative electrode plate and electrolyte, the negative electrode plate comprises a negative electrode current collector and a negative electrode active layer located on the surface of one side or two sides of the negative electrode current collector, the negative electrode active layer comprises a silicon material, and the weight content of the silicon material in the negative electrode active layer is 7%-60%; the electrolyte comprises ethyl difluoroacetate and fluorobutyronitrile, the fluorine atom substitution number of the fluorobutyronitrile is 1-4, based on the total weight of the electrolyte, the weight content of the ethyl difluoroacetate is 20%-60%, the weight content of the fluorobutyronitrile is 2%-20%, and the ratio of the weight of the fluorobutyronitrile to the weight of the ethyl difluoroacetate is 0.08-0.8. According to the lithium ion battery disclosed by the invention, the electrolyte can continuously repair the damaged SEI film, and meanwhile, the dynamic performance of the electrolyte is improved, so that the battery has relatively high fast charging performance and cycling stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a lithium ion battery. BACKGROUND

[0002] Energy density and fast charging are the mainstream development direction of consumer electronics. In order to improve the energy density of lithium ion batteries, silicon-doped negative electrodes have become a major direction because the lithium storage capacity of silicon is high, and the lithium storage capacity of pure silicon is almost 10 times that of commonly used graphite negative electrodes.

[0003] However, the high intrinsic expansion and contraction rate of silicon material often damages the solid electrolyte (SEI) film on the surface of the silicon negative electrode, leading to excessive consumption of electrolyte and excessive expansion of the battery. Therefore, optimizing the electrolyte of the lithium ion battery and improving the SEI on the surface of the silicon negative electrode are crucial to improving the cycle stability of the silicon negative electrode. Therefore, how to balance the fast charging performance and cycle stability is the main challenge of the current silicon negative electrode lithium ion battery. SUMMARY

[0004] In order to solve the problem of poor cycle stability of the silicon negative electrode lithium ion battery in the prior art, the present application provides a lithium ion battery. The lithium ion battery (hereinafter referred to as the battery) can continuously repair the damaged SEI film of the electrolyte, and at the same time improve the kinetic performance of the electrolyte, so that the battery has high fast charging performance and cycle stability.

[0005] In order to achieve the above-mentioned purpose, the present application provides a lithium ion battery, wherein the lithium ion battery comprises a negative electrode sheet and an electrolyte, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer located on one side or both sides of the negative electrode current collector, the negative electrode active layer comprises a silicon material, and the weight content of the silicon material in the negative electrode active layer is 7%-60%; the electrolyte comprises ethyl difluoroacetate and fluorobutyronitrile, the number of fluorine atom substitutions of the fluorobutyronitrile is 1-4, the weight content of the ethyl difluoroacetate is 20%-60% based on the total weight of the electrolyte, the weight content of the fluorobutyronitrile is 2%-20%, and the ratio of the weight of the fluorobutyronitrile to the weight of the ethyl difluoroacetate is 0.08-0.8.

[0006] Compared with the prior art, the present application has at least the following advantages:

[0007] The content of the silicon material in the lithium ion battery of the present application in the negative electrode active material is relatively high, and for the battery of the high-silicon negative electrode system, in order to improve the problem that the consumption of the film former in the electrolyte is relatively fast, the lithium ion battery of the present application simultaneously adds ethyl difluoroacetate and fluorobutyronitrile in the electrolyte, by simultaneously controlling the weight content of the fluorobutyronitrile and the ethyl difluoroacetate and the weight ratio of the two, the ability of the battery to supplement the electrolyte can be improved, the SEI film damaged by the volume expansion and shrinkage of the silicon material in the cycle process can be continuously repaired, and at the same time, the kinetic performance of the electrolyte can be improved, the transmission rate of lithium ions can be improved, so that the electrolyte has good film forming ability, high kinetic performance and high high-voltage stability, and the cycle stability and fast charging performance of the battery are improved.

[0008] Other features and advantages of the present application will be described in detail in the following specific embodiments.

[0009] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as exactly that endpoint. Any values that fall within common DETAILED DESCRIPTION

[0010] The specific embodiments of the present application are described in detail below. It should be understood that the specific embodiments described herein are merely intended to illustrate and explain the present application, and are not intended to limit the present application. In this document, the data range includes the endpoints unless otherwise specified.

[0011] It should be noted that the "first", "second" and the like numerical representation in the present disclosure are only used to distinguish different substances or use methods, and do not represent the difference in order.

[0012] The present application provides a lithium ion battery, wherein the lithium ion battery comprises a negative electrode sheet and an electrolyte, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer on one side or both sides of the negative electrode current collector, the negative electrode active layer comprises a silicon material, the weight content of the silicon material in the negative electrode active layer is 7%-60% (for example, 7%, 10%, 12%, 15%, 17%, 20%, 22%, 25%, 27%, 30%, 32%, 35%, 37%, 40%, 42%, 45%, 47%, 50%, 52%, 55%, 57% or 60%); the electrolyte comprises ethyl difluoroacetate (DFEA) and fluorobutyronitrile, the fluorobutyronitrile has 1-4 (for example, 1, 2, 3 or 4) fluorine atom substitutions, the weight content of the ethyl difluoroacetate is 20%-60% (for example, 20%, 23%, 25%, 27%, 30%, 32%, 35%, 37%, 40%, 42%, 45%, 47%, 50%, 52%, 55%, 57% or 60%) and the weight content of the fluorobutyronitrile is 2%-20% (for example, 2%, 5%, 7%, 10%, 12%, 15%, 17% or 20%) based on the total weight of the electrolyte, and the ratio of the weight of the fluorobutyronitrile to the weight of the ethyl difluoroacetate is 0.08-0.8 (for example, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75 or 0.8).

[0013] In the negative electrode sheet system with a high silicon content, the silicon material has a large volume expansion which easily destroys the SEI film on the surface of the negative electrode sheet, so that the continuously destroyed and generated SEI film needs to consume a large amount of electrolyte film forming agent to prevent the silicon material from further expanding and cracking, resulting in a too fast consumption rate of the electrolyte, and the too fast consumption of the electrolyte seriously affects the cycle stability of the lithium ion battery.

[0014] For the negative electrode sheet system with high content of silicon material, ethyl difluoroacetate is added to the electrolyte. Ethyl difluoroacetate can continuously repair the SEI film damaged during the cycle process, and ethyl difluoroacetate can also maintain a certain kinetic advantage. At the same time, fluoro butyronitrile is added to the electrolyte, which can not only improve the wettability and flowability of the electrolyte, and improve the problem of too fast consumption rate of the electrolyte caused by the expansion of silicon material, but also has good high pressure resistance. Fluoro butyronitrile can also inhibit the decomposition of solvents (such as ethylene carbonate, fluoroethylene carbonate, propylene carbonate, etc.) in the electrolyte on the positive electrode side, reduce the consumption of the electrolyte, and further improve the cycle stability of the battery. Moreover, the conductivity of fluoro butyronitrile is high, which can further improve the kinetic performance of the electrolyte and improve the fast charging performance of the battery. By controlling the weight ratio of fluoro butyronitrile to ethyl difluoroacetate in the electrolyte within the above range, the liquid supplementing ability of the high-silicon negative electrode system battery can be improved, the SEI film damaged during the cycle process can be continuously repaired, and the kinetic performance of the electrolyte can also be improved, so that the battery has high fast charging performance and high cycle stability. Moreover, by controlling the weight ratio of fluoro butyronitrile to ethyl difluoroacetate within the above range, the weak hydrogen bond or dipole-dipole interaction between fluoro butyronitrile and ethyl difluoroacetate can be formed through-CN···H-C- or-CN···F-C-. This intermolecular force can regulate Li + Solvation sheath (reducing free solvent molecule competition coordination, improving Li + migration number and inhibiting PF6 - decomposition) to further enhance the transmission efficiency of lithium ions, and the dynamic hydrogen bond network formed by the weak hydrogen bond or dipole-dipole interaction between fluoro butyronitrile and ethyl difluoroacetate can generate a more uniform interface film (SEI film) when the surface of the negative electrode sheet is decomposed, which can give the SEI film self-repairing ability, and also can optimize Li + deposition uniformity, reduce the formation of dead lithium, thereby effectively improving the fast charging performance and cycle stability of the battery. When the weight ratio of fluoro butyronitrile to ethyl difluoroacetate is less than 0.08, the overall kinetics of the electrolyte is poor, which can easily deteriorate the charging window of the lithium ion battery; when the weight ratio of fluoro butyronitrile to ethyl difluoroacetate is higher than 0.8, the negative electrode interface side reaction caused by fluoro butyronitrile is more, which can easily deteriorate the cycle performance of the lithium ion battery.

[0015] When the weight content of ethyl difluoroacetate in the electrolyte is less than 20%, the addition amount of ethyl difluoroacetate is too small, and the repair effect on the negative electrode sheet is weak, which is difficult to effectively improve the cycle stability of the battery. When the weight content of ethyl difluoroacetate in the electrolyte is higher than 60%, the viscosity of the electrolyte is large, the kinetic performance is poor, and lithium precipitation of the battery is easily caused.

[0016] When the weight content of fluorobutyronitrile in the electrolyte is less than 2%, the amount of fluorobutyronitrile added is too low, and the kinetic improvement of the electrolyte is weak, so it is difficult to effectively improve the fast-charging performance of the battery. When the weight content of fluorobutyronitrile in the electrolyte is higher than 20%, too much fluorobutyronitrile can easily damage the SEI film of the negative electrode, resulting in too fast attenuation of the capacity retention rate of the battery.

[0017] In the present application, by controlling the weight content of ethyl difluoroacetate and fluorobutyronitrile in the electrolyte and the weight ratio of the two, the battery has higher fast-charging performance and higher cycle stability compared with the prior art. In order to further improve the effect, one or more of the technical features can be further optimized.

[0018] In the present application, the ethyl difluoroacetate refers to ethyl acetate with two fluorine atoms substituted. The fluorine can be substituted at any position of ethyl acetate. For example, ethyl acetate includes a methyl group provided by acetic acid and an ethyl group provided by ethanol. The two fluorine atoms can substitute two hydrogens in the methyl group, or two hydrogens in the ethyl group, or one fluorine atom substitutes a hydrogen in the methyl group and one fluorine atom substitutes a hydrogen in the ethyl group.

[0019] In an example, the weight content of the ethyl difluoroacetate in the electrolyte is 30%-50%.

[0020] In an example, the weight content of the fluorobutyronitrile in the electrolyte is 4%-15%.

[0021] In an example, the weight ratio of the fluorobutyronitrile to the ethyl difluoroacetate is 0.1-0.6.

[0022] In the present application, the weight content of ethyl difluoroacetate in the electrolyte and the weight content of fluorobutyronitrile in the electrolyte can be tested by the following method: using pure ethyl difluoroacetate and fluorobutyronitrile as a reference by gas chromatography (GC) calibration, then, using GC to determine the sample electrolyte extracted from the battery, using solvent volatility difference for separation, hydrogen flame ionization detector (FID) quantification, and comparing the data of pure substances, the weight content of ethyl difluoroacetate and fluorobutyronitrile in the electrolyte can be obtained respectively.

[0023] In an example, the ethyl difluoroacetate includes one or more of the following structures:

[0024] In an example, the ethyl difluoroacetate is

[0025] In an example, the fluorobutyronitrile includes one or more of the following structures:

[0026]

[0027]

[0028] In an example, the silicon material comprises one or more of nanosilicon, microsilicon, amorphous silicon, silicon-oxygen material, and silicon-carbon material.

[0029] In an example, the silicon material comprises silicon element and doping element, and the doping element comprises one or more of Al, Cu, Fe, Ni, Mo, Mg, Ti, Mn, Ca, Na, and Ge. Since the above-mentioned doping element can form alloy with the silicon element in the silicon material, thereby improving the intrinsic conductivity of the silicon material and enhancing the kinetic performance of the silicon negative electrode.

[0030] The powder resistivity of the silicon material comprising the doping element can be 0.1 Ω·cm-800 Ω·cm (for example, 0.1 Ω·cm, 1 Ω·cm, 5 Ω·cm, 10 Ω·cm, 30 Ω·cm, 50 Ω·cm, 80 Ω·cm, 100 Ω·cm, 150 Ω·cm, 200 Ω·cm, 250 Ω·cm, 300 Ω·cm, 350 Ω·cm, 400 Ω·cm, 450 Ω·cm, 500 Ω·cm, 550 Ω·cm, 600 Ω·cm, 650 Ω·cm, 700 Ω·cm, 750 Ω·cm, or 800 Ω·cm).

[0031] The conductivity of the silicon material can be characterized by the powder resistivity, and the specific test method of the powder resistivity is as follows: the four-probe method is used for testing, such as using LORESTA-GX MCP-T700 powder resistivity instrument, and the test conditions are as follows: the sampling amount is 2.5 g, the test environment temperature is 25±2℃, and the resistivity under the pressure of 63.66 MPa is selected.

[0032] In an example, the weight content of the doping element in the negative electrode active layer is 0.001%-1% (for example, 0.001%, 0.003%, 0.005%, 0.008%, 0.01%, 0.03%, 0.05%, 0.08%, 0.1%, 0.3%, 0.5%, 0.8%, or 1%). When the weight content of the doping element in the negative electrode active layer is less than 0.001%, the doping amount is too low, and the improvement of the conductivity and cycle stability of the battery is limited. When the weight content of the doping element in the negative electrode active layer is higher than 1%, the doping amount is too much, which is easy to affect the lithium storage capacity of the silicon material and reduce the energy density of the battery.

[0033] It can be understood that when the doping element is one kind, the weight content of the doping element in the negative electrode active layer refers to the weight content of the doping element in the negative electrode active layer; when the doping element is multiple kinds, the weight content of the doping atom in the negative electrode active layer refers to the sum of the weight contents of the multiple kinds of doping elements in the negative electrode active layer.

[0034] In an example, the weight content of the doping element in the negative electrode active layer is 0.005%-0.5%.

[0035] In the embodiments of the present application, the doping method of the doping element can be selected from ion implantation, diffusion, chemical vapor deposition, solution method, and laser method.

[0036] In an example, the lithium ion battery simultaneously satisfies the following relationships: 0.5≤F / S≤15 (for example, 0.5, 1, 3, 5, 8, 10, 13, or 15), 0.001%≤(W / S)×100%≤1% (for example, 0.001%, 0.003%, 0.005%, 0.008%, 0.01%, 0.03%, 0.05%, 0.08%, 0.1%, 0.3%, 0.5%, 0.8%, or 1%), wherein F is the weight content of the ethyl difluoroacetate in the electrolyte, in %, S is the weight content of the silicon element in the negative electrode active layer, in %, and W is the weight content of the doping element in the negative electrode active layer, in %.

[0037] It has been found through research that the doping element in the silicon material is used in combination with the ethyl difluoroacetate and the fluorobutyronitrile in the electrolyte, which can simultaneously improve the internal and external lithium ion migration dynamics of the silicon material, thereby further improving the kinetic performance of the silicon material. Moreover, the doping site of the doping element in the silicon material is relatively stable due to the presence of the doping element, and is not easy to participate in the energy storage reaction (lithium deintercalation reaction), so as to serve as an inert site to improve the stability of the silicon material, thereby improving the cycle stability of the battery.

[0038] In view of this, the lithium ion battery is controlled to meet the following relationships simultaneously: 0.5≤F / S≤15, 0.01%≤(W / S)×100%≤1%, so that the intrinsic conductivity of the silicon material is improved, the lithium ion migration rate inside and outside the silicon material is improved, the kinetic performance of the silicon material is further improved through the synergy of electrons and ions, the rate performance and cycle stability of the battery are improved, the structure of the silicon material is stabilized, the cycle stability of the battery is improved, the fluorobutyronitrile in the electrolyte can reduce the polarization of the electrolyte, the doping elements in the silicon material can reduce the polarization of the silicon material, so as to further improve the kinetic performance of the battery, improve the rate performance of the battery, and through the synergistic cooperation of the ethyl difluoroacetate and the fluorobutyronitrile in the electrolyte, the SEI film damaged by the volume expansion of the silicon material can be continuously repaired, so as to improve the fast charging performance and cycle stability of the battery.

[0039] In an example, the lithium ion battery meets the following relationships simultaneously: 0.8≤F / S≤10, 0.01%≤(W / S)×100%≤0.5%.

[0040] In the present application, the weight content of the silicon element in the negative electrode active layer and the weight content of the doping element in the negative electrode active layer can be obtained by inductively coupled plasma mass spectrometry.

[0041] In an example, the weight content of the silicon element in the negative electrode active layer is 3%-50% (for example, 3%, 5%, 8%, 10%, 13%, 15%, 18%, 20%, 23%, 25%, 30%, 40% or 50%).

[0042] According to a specific embodiment, F is 20-60, S is 3-50, W is 0.001-1, and the lithium ion battery meets the following relationships simultaneously: 0.5≤F / S≤15, 0.001%≤(W / S)×100%≤1%.

[0043] According to a specific embodiment, F is 30-50, S is 5-40, W is 0.005-0.5, and the lithium ion battery meets the following relationships simultaneously: 0.8≤F / S≤10, 0.01%≤(W / S)×100%≤0.5%.

[0044] In an example, the negative electrode active layer further comprises a carbon material.

[0045] In an example, the carbon material comprises one or more of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon and soft carbon.

[0046] In one example, the ratio of the weight of the silicon material to the weight of the carbon material is (7%-60%):(40%-93%) (e.g., 7%:93%, 8%:92%, 10%:90%, 12%:88%, 15%:85%, 18%:82%, 20%:80%, 22%:78%, 25%:75%, 28%:72%, 30%:70%, 33%:67%, 35%:65%, 38%:62%, 40%:60%, 43%:57%, 45%:55%, 48%:52%, 50%:50%, 53%:47%, 55%:45%, 58%:42%, or 60%:40%). It can be understood that the weight content of the silicon material can be selected in the range of 7%-60% and the weight content of the carbon material can be selected in the range of 40% based on the sum of the weight of the silicon material and the weight of the carbon material, but it should be noted that the sum of the weight content of the silicon material and the weight content of the carbon material is equal to 100%.

[0047] In one example, the sphericity of the silicon material is 0.6-1 (e.g., 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1).

[0048] In one example, the sphericity of the silicon material is 0.8-0.99.

[0049] In the present application, the sphericity of the silicon material can be tested by the following method: taking a particle image of the silicon material on the surface of the negative active layer by a scanning electron microscope (SEM), using image processing software to draw the smallest rectangle that completely surrounds one particle on the obtained scanning image, i.e., drawing a rectangle whose edges of the silicon material particle are connected to the four edges of the rectangle, measuring the longest side and the shortest side of the rectangle, and calculating the sphericity = shortest side / longest side.

[0050] In one example, the lithium ion battery satisfies the following relationship: 0.02≤B / Q≤0.2 (e.g., 0.02, 0.04, 0.08, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or 0.2), where B is the weight content of the fluorobutyronitrile in the electrolyte, in %, and Q is the sphericity of the silicon material.

[0051] When the total amount of fluorobutyronitrile in the electrolyte is high, part of the fluorobutyronitrile neither adsorbs near the positive electrode sheet nor forms a weak hydrogen bond or dipole-dipole interaction with ethyl difluoroacetate. These free fluorobutyronitriles can be reduced to form a film at the unstable part (such as the tip or the corner) of the negative electrode, thereby causing lithium ion self-discharge and resulting in a large K value of the battery. When the weight content of fluorobutyronitrile in the electrolyte is high, the silicon material with low sphericity has more tips and / or corners. The tips and corners not only cause the diaphragm to be pierced, resulting in increased self-discharge, but also make the excessive fluorobutyronitrile more likely to react to form a film at the unstable tips or corners, thereby further increasing self-discharge. Therefore, it is necessary to improve the sphericity of the silicon material, reduce the pressure of the silicon material on the diaphragm, and reduce the film formation reaction of fluorobutyronitrile at the negative electrode, thereby improving the self-discharge of the battery. When the weight content of fluorobutyronitrile in the electrolyte is low, the sphericity of the silicon material can be appropriately reduced to reduce the risk of the tips and / or corners of the silicon material piercing the diaphragm and improve the self-discharge of the battery. Therefore, by controlling the lithium ion battery to satisfy the above relationship, the fluorobutyronitrile in the electrolyte and the silicon material can achieve a high matching degree, the sphericity of the silicon material can be improved, the local tips of the silicon material can be reduced, the pressure on the diaphragm can be reduced, the reaction of fluorobutyronitrile can be inhibited, and the risk of self-discharge caused by the silicon material pressing the diaphragm can be reduced, thereby effectively improving the degree of self-discharge of the battery and reducing the K value of the battery.

[0052] In an example, the lithium ion battery satisfies the following relationship: 0.04≤B / Q≤0.15.

[0053] According to a specific embodiment, B is 2%-20% and Q is 0.6-1, and the lithium ion battery satisfies the following relationship: 0.02≤B / Q≤0.2.

[0054] According to a specific embodiment, B is 5%-15% and Q is 0.8-0.99, and the lithium ion battery satisfies the following relationship: 0.04≤B / Q≤0.15.

[0055] In an example, the compaction density of the negative electrode sheet is 1.5g / cm 3 -2g / cm 3 (For example, 1.5g / cm 3 , 1.6g / cm 3 , 1.63g / cm 3 , 1.65g / cm 3 , 1.68g / cm 3 , 1.7g / cm 3 , 1.73g / cm 3 , 1.75g / cm 3 , 1.78g / cm 31.8 g / cm 3 1.9 g / cm 3 or 2 g / cm 3 ). The fluorobutyronitrile in the electrolyte can reduce the viscosity of the electrolyte, improve the infiltration effect of the electrolyte on the electrode sheet, and control the compaction density of the negative electrode sheet in the above range, so that the battery has both high energy density and good cycle performance. When the compaction density of the negative electrode sheet is less than 1.6 g / cm 3 , the compaction density of the negative electrode sheet is low, the energy density of the battery is low, or, under the same energy density, more silicon material needs to be doped in the negative active layer, which is not conducive to the cycle performance of the lithium ion battery. When the compaction density of the negative electrode sheet is higher than 1.8 g / cm 3 , the kinetic performance of the negative electrode sheet is poor, which is easy to cause lithium precipitation of the battery. Even if more fluorobutyronitrile is added in the electrolyte, the cycle stability of the battery will be deteriorated and the cycle life of the battery will be shortened due to the lack of liquid storage space of the negative electrode and the poor liquid retention.

[0056] In an example, the negative active layer includes a negative electrode material, and the negative electrode material includes a silicon material and a carbon material.

[0057] In an example, the negative active layer further includes a negative electrode conductive agent and a negative electrode binder.

[0058] In an example, the negative electrode conductive agent includes one or more of conductive carbon black, acetylene black, ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, and metal powder.

[0059] In an example, the negative electrode binder includes one or more of styrene butadiene rubber emulsion, polytetrafluoroethylene emulsion, sodium carboxymethyl cellulose, sodium alginate, polyvinyl alcohol, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, carboxymethyl chitosan, polymethyl methacrylate, and polyvinylpyrrolidone.

[0060] In an example, the weight content of the negative electrode material is 92%-99.6% (e.g., 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.6%) based on the total weight of the negative active layer, the weight content of the negative electrode conductive agent is 0.2%-4% (e.g., 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4%), and the weight content of the negative electrode binder is 0.2%-4% (e.g., 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4%).

[0061] In an example, the electrolyte further comprises a fluorosulfonamide compound. Due to the high-temperature performance of ethyl difluoroacetate and fluorobutyronitrile in general, at high temperatures, lithium-ion batteries are at risk of catching fire due to too fast heat and gas production. After adding an appropriate amount of fluorosulfonamide compound to the electrolyte, due to the ability of fluorosulfonamide compounds to quickly form a film, when the SEI film is damaged at high temperatures, the SEI film can be quickly repaired, while taking away part of the energy but not producing a large amount of gas, inhibiting the consumption of ethyl difluoroacetate in the electrolyte, making the energy release of the lithium-ion battery more moderate, thereby improving the thermal box performance of the battery.

[0062] In an example, the fluorosulfonamide compound has a structure represented by formula (III)

[0063]

[0064] wherein R4, R5 can be the same or different, each independently selected from a fluorine-substituted or unsubstituted C1-C3 alkyl group.

[0065] In the present application, the meaning of "fluorine-substituted or unsubstituted" is, for example, "fluorine-substituted or unsubstituted C1-C3 alkyl group", which means that the alkyl group can be substituted with fluorine or can be unsubstituted by any substituent, when the alkyl group is substituted with fluorine, the alkyl group can have one H substituted with fluorine, can have multiple H substituted with fluorine, or can have all H substituted with fluorine.

[0066] R4 and R5 can be directly connected to N.

[0067] N can form a ring with R4, R5 or can not form a ring. R4, R5 can form a ring, for example, formula (III-9), formula (III-10).

[0068] R4, R5 can not form a ring, for example, formula (III-1), formula (III-2), formula (III-3), formula (III-4), formula (III-5), formula (III-6), formula (III-7), formula (III-8).

[0069] C1-C3 alkyl group, for example, selected from methyl, ethyl, n-propyl, isopropyl

[0070] In an example, the fluorosulfonamide compound comprises one or more of the structures represented by the following

[0071] In an example, the fluorosulfamide compound has a weight content of 2% to 25% (for example, 2%, 5%, 7%, 10%, 12%, 15%, 17%, 20%, 22%, or 25%) in the electrolyte. When the weight content of the fluorosulfamide compound in the electrolyte is less than 2%, the amount of the fluorosulfamide compound added is small, and the inhibitory effect on the gas generation and heat generation rate is small, making it difficult to improve the heat box pass rate. When the weight content of the fluorosulfamide compound in the electrolyte is greater than 25%, the amount of the fluorosulfamide compound added is high, and because the viscosity of the fluorosulfamide compound is large, the kinetics of lithium ions and the cycle performance are affected.

[0072] In an example, the electrolyte further includes fluorinated ethylene carbonate (FEC) and 1,3-propane sultone (PS). FEC is easy to form a film, and can form an SEI film component rich in LiF and lithium alkyl carbonate composite on the surface of the silicon negative electrode, improving the cycle stability of the silicon negative electrode; 1,3-propane sultone can improve the thermal stability of the SEI, maintain the stability of the SEI at high temperatures, and block the direct contact of the positive and negative electrode materials with the solvent, thereby inhibiting the decomposition of the electrolyte to generate gas.

[0073] In an example, the fluorinated ethylene carbonate has a weight content of 5% to 25% (for example, 5%, 8%, 10%, 13%, 15%, 18%, 20%, 23%, or 25%) in the electrolyte.

[0074] In an example, the fluorinated ethylene carbonate has a weight content of 8% to 18% in the electrolyte.

[0075] In an example, the 1,3-propane sultone has a weight content of 0.2% to 5% (for example, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5%) in the electrolyte.

[0076] In an example, the ratio of the weight of the fluorinated ethylene carbonate to the weight of the 1,3-propane sultone is 3 to 30. Controlling the ratio of the weight of the fluorinated ethylene carbonate to the weight of the 1,3-propane sultone in the above range can achieve a higher matching degree of the fluorinated ethylene carbonate (FEC) and the 1,3-propane sultone, so as to improve the thermal stability of the generated SEI film rich in LiF and lithium alkyl carbonate composite, and further improve the cycle and high-temperature storage performance of the battery.

[0077] In an example, the electrolyte further includes a lithium salt, an organic solvent, and an additive.

[0078] In an example, the lithium salt comprises one or more of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiPO2F2), lithium hexafluoroantimonate (LiSbF6), lithium hexafluoroarsenate (LiAsF6), lithium hexafluorozirconate (LiZrF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethylsulfonylimide (LiTFSI), lithium trifluoromethylsulfonate, lithium trifluoromethylsulfinat, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methide, lithium difluoro oxalato borate (LiDFOB), lithium difluorobisoxalato phosphate, lithium tetrafluoroborate (LiBF4), lithium bis(difluorophosphoryl) difluoroborate, and lithium tetrakis(difluorophosphoryl) borate.

[0079] In an example, the organic solvent comprises one or more of a carboxylic acid ester and / or a carbonate ester, such as ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, methyl propionate, ethyl propionate, propyl propionate, isopropyl propionate, methyl butyrate, ethyl butyrate, methyl isobutyrate, ethyl isobutyrate, vinyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and various fluorinated compounds of these compounds.

[0080] In an example, the additive comprises one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), succinonitrile (SN), adiponitrile (AND), glutaronitrile (GN), hexanetritrile (HTCN), ethylene glycol bis(propionitrile) ether (DENE), glycerol trinitrile (TCP), 2,4,6-tricyano-l,3,5-triazine, tetrafluorotetracyanoquinodimethane, terephthalonitrile, malononitrile, nonanenitrile, heptanenitrile, acetylmalononitrile, t-butylmalononitrile, tetrafluoroisophthalonitrile, 2-phenylmalononitrile, 2-ethylmalononitrile, 3-methylglutaronitrile, 2-methylmalononitrile, 2-benzoylcyclopropanedinitrile, 1,2,3-trinitropropane, penta-methyltricyanopropane, m-trifluoromethylbenzonitrile, 2,3,4-trifluorobenzonitrile, 1,3,5-cyclohexanetricarbonitrile, tetra-vinylsilane (TVS), tri-vinylmethylsilane, tri-vinyl-ethylsilane, tri-(trimethylsilyl)borate (TMSB), hexamethyldisilazane (HMDS), fluorobenzene (FB), triphenyl phosphite (TPPi), and pentafluoroethoxy cyclotriphosphazene (PFPN).

[0081] In an example, the lithium salt has a weight content of 12.5-25% (e.g., 12.5%, 15%, 18%, 20%, 23%, or 25%) based on the total weight of the electrolyte, the organic solvent has a weight content of 15-50% (e.g., 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%) based on the total weight of the electrolyte, and the additive has a weight content of 1-10% (e.g., 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%) based on the total weight of the electrolyte.

[0082] In an example, the lithium ion battery further includes a cathode sheet.

[0083] In an example, the cathode sheet includes a cathode current collector and a cathode active layer on one or both sides of the cathode current collector.

[0084] In an example, the cathode active layer includes a cathode material, a cathode conductive agent, and a cathode binder.

[0085] In an example, the cathode material includes a coated and / or doped lithium cobalt oxide-based layered oxide material.

[0086] In an example, the cathode conductive agent includes one or more of conductive carbon black, acetylene black, ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, and metal powder.

[0087] In an example, the cathode binder includes one or more of styrene butadiene rubber emulsion, polytetrafluoroethylene emulsion, sodium carboxymethyl cellulose, sodium alginate, polyvinyl alcohol, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, carboxylated chitosan, polymethyl methacrylate, polyvinylpyrrolidone.

[0088] In an example, the cathode material has a weight content of 92-99.6% (e.g., 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.6%) based on the total weight of the cathode active layer, the cathode conductive agent has a weight content of 0.2-4% (e.g., 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4%) based on the total weight of the cathode active layer, and the cathode binder has a weight content of 0.2-4% (e.g., 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4%) based on the total weight of the cathode active layer.

[0089] In an example, the lithium ion battery is a lithium ion secondary battery.

[0090] In an example, the lithium ion battery has a voltage test window of 3.0-4.53 V.

[0091] The application will be described in detail below through examples. The examples described in the application are only part of the examples of the application, not all examples. Based on the examples in the application, all other examples obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the application.

[0092] The following examples are used to illustrate the lithium ion battery of the application.

[0093] The group of examples I is used to illustrate the effect of not adding fluorosulfonyl amide compounds in the electrolyte.

[0094] Example I-1

[0095] (1) Electrolyte

[0096] In an argon-filled glove box (H2O <0.1 ppm, O2 <0.1 ppm), ethylene carbonate (EC), fluorinated ethylene carbonate (FEC), difluoroethyl acetate (DFEA, formula 1-1), fluorobutyronitrile (formula II-1) were mixed uniformly, then lithium hexafluorophosphate (LiPF6) and lithium bis-trifluoromethylsulfonylimide (LiTFSI) were quickly added, after dissolution, hexanetricarbonitrile (HTCN), 1,3-propane sultone (PS) was added. After stirring uniformly, the electrolyte was obtained after passing the detection of moisture, free acid, color, etc. In the electrolyte, the weight percentage of ethylene carbonate was 10%, the weight percentage of fluorinated ethylene carbonate was 10%, the weight percentage of difluoroethyl acetate was 40%, the weight percentage of fluorobutyronitrile was 8%, the weight percentage of lithium hexafluorophosphate was 12%, the weight percentage of lithium bis-trifluoromethylsulfonylimide was 5%, the weight percentage of hexanetricarbonitrile was 3%, the weight percentage of 1,3-propane sultone was 2%, and the weight percentage of propyl propionate was 10%. The weight ratio of fluorobutyronitrile to difluoroethyl acetate was 0.2, and the weight ratio of fluorinated ethylene carbonate to 1,3-propane sultone was 5.

[0097] (2) Negative electrode sheet

[0098] The negative active material (consisting of 83 wt% graphite + 17 wt% silicon-carbon material uniformly mixed), styrene-butadiene rubber (SBR), lithium polyacrylate, conductive carbon black (SP), and carbon nanotubes (CNTs) are mixed uniformly in a mass ratio of 97:1:0.5:1:0.5, and then an appropriate amount of deionized water is added step by step to obtain a negative electrode slurry under the action of a vacuum stirrer; the negative electrode slurry is uniformly coated on both sides of the negative electrode current collector (copper foil) by a coating machine to form a negative electrode active layer; the coated copper foil is dried, and then subjected to processes such as rolling and slitting to obtain a negative electrode sheet. The silicon-carbon material used has Al doping, and the doping amount is 0.02%. The powder resistivity of the silicon-carbon material is about 400 Ω·cm. The sphericity of the silicon-carbon material used is 0.9. The tap density of the negative electrode sheet is 1.7 g / cm 3 . The weight content of silicon element S in the negative electrode active layer is 10%.

[0099] (3) Positive electrode sheet

[0100] The commercial lithium cobalt oxide material, polyvinylidene fluoride (PVDF), conductive carbon black (SP), and carbon nanotubes (CNTs) are dry-mixed in a mass ratio of 97:1.5:1:0.5. Then, an appropriate amount of N-methyl pyrrolidone (NMP) is gradually added under the action of a vacuum stirrer to mix into a uniform positive electrode slurry. Then, the positive electrode slurry is uniformly coated on both surfaces of an aluminum foil using a coating machine to form a positive electrode active layer; the coated aluminum foil is dried, and then subjected to processes such as rolling and slitting to obtain a positive electrode sheet.

[0101] (4) Lithium ion battery

[0102] The negative electrode sheet prepared in step (2), a commercial separator (a conventional PE-based separator), and the positive electrode sheet prepared in step (3) are stacked in the order of negative electrode sheet, separator, and positive electrode sheet, so that the separator completely separates the positive and negative electrodes and the negative electrode active layer completely covers the positive electrode active layer, and then a roll core with a certain thickness and width is prepared by winding. Subsequently, the roll core is packaged using an aluminum plastic film and injected with the electrolyte prepared in step (1), and after processes such as vacuum packaging, aging, formation, secondary sealing, and sorting, a soft-pack lithium ion battery with a certain specification is obtained. Among them, F / S = 40 / 10 = 4, (W / S) x 100% = (0.02 / 10) x 100% = 0.2%, and B / Q = 8% / 0.9 = 0.089. The electrolyte is extracted from the soft-pack lithium ion battery after formation, and the weight content of ethyl difluoroacetate in the electrolyte is measured to be 41.2%, and the weight content of fluorobutyronitrile is 7.83%.

[0103] Example I-2 group

[0104] This group of examples is used to illustrate the influence when the structure of ethyl difluoroacetate is changed.

[0105] Example I-2a

[0106] Example I-2a was carried out as in Example I-1, except that the ethyl difluoroacetate had the structure of formula (I-2).

[0107] Example I-2b

[0108] Example I-2b was carried out as in Example I-1, except that the ethyl difluoroacetate had the structure of formula (I-3).

[0109] Example I-3 group

[0110] This group of examples is intended to illustrate the effect of changing the structure of the fluorobutyronitrile.

[0111] Example I-3a

[0112] Example I-3a was carried out as in Example I-1, except that the fluorobutyronitrile had the structure of formula (II-2).

[0113] Example I-3b

[0114] Example I-3b was carried out as in Example I-1, except that the fluorobutyronitrile had the structure of formula (II-3).

[0115] Example I-4 group

[0116] This group of examples is intended to illustrate the effect of changing the weight content B of fluorobutyronitrile in the electrolyte.

[0117] This group of examples was carried out as in Example I-1, except that the weight content B of fluorobutyronitrile in the electrolyte was changed, see Table 1-1.

[0118] Example I-5 group

[0119] This group of examples is intended to illustrate the effect of changing the sphericity Q of the silicon material.

[0120] This group of examples was carried out as in Example I-1, except that the sphericity Q of the silicon material was changed, see Table 1-1.

[0121] Example I-6

[0122] Example I-6 was carried out as in Example I-1, except that B and Q were changed, see Table 1-1.

[0123] Table 1-1

[0124]

[0125] Example I-7 group

[0126] This group of examples is intended to illustrate the effect when the weight content F of ethyl difluoroacetate in the electrolyte is varied.

[0127] This group of examples is carried out analogously to example I-1, except that the weight content F of ethyl difluoroacetate in the electrolyte is varied, see Table 1-2.

[0128] Example I-8 group

[0129] This group of examples is intended to illustrate the effect when the weight ratio of fluorobutyronitrile to ethyl difluoroacetate is varied.

[0130] This group of examples is carried out analogously to example I-1, except that the weight ratio of fluorobutyronitrile to ethyl difluoroacetate is varied, see Table 1-2.

[0131] Example I-9 group

[0132] This group of examples is intended to illustrate the effect when F / S and / or (W / S) x 100% is varied.

[0133] This group of examples is carried out analogously to example I-1, except that F / S and / or (W / S) x 100% is varied, see Table 1-2.

[0134] Table 1-2

[0135]

[0136]

[0137] * means as in example I-1.

[0138] Comparative example I-1

[0139] Carried out analogously to example I-1, except that the weight ratio of fluorobutyronitrile to ethyl difluoroacetate is below 0.08, see Table 1-3.

[0140] Comparative example I-2

[0141] Carried out analogously to example I-1, except that the weight ratio of fluorobutyronitrile to ethyl difluoroacetate is above 0.8, see Table 1-3.

[0142] Comparative example I-3

[0143] Carried out analogously to example I-1, except that the weight content of ethyl difluoroacetate in the electrolyte is below 20%, see Table 1-3.

[0144] Comparative example I-4

[0145] The procedure of Example I-1 was followed except that the weight content of ethyl difluoroacetate in the electrolyte was higher than 260%, see Table 1-3.

[0146] Comparative Example I-5

[0147] The procedure of Example I-1 was followed except that no ethyl difluoroacetate was added to the electrolyte.

[0148] Comparative Example I-6

[0149] The procedure of Example I-1 was followed except that no fluorobutyronitrile was added to the electrolyte.

[0150] Comparative Example I-7

[0151] The procedure of Example I-1 was followed except that the weight content of fluorobutyronitrile in the electrolyte was higher than 20%, see Table 1-3.

[0152] Table 1-3

[0153]

[0154] * means the same as Example I-1.

[0155] Test Example I Group

[0156] The lithium ion batteries prepared in Example I Group and Comparative Example I Group were respectively subjected to the following tests.

[0157] (1) Fast charging performance test

[0158] In a thermostat at 25°C, the battery was charged at an initial rate of 3C to 4.53V, then charged at constant voltage to 0.05C as a charging process, and discharged at a rate of 0.7C to 3.0V. The battery was cycled for 20 cycles until it was fully charged (100% SOC), at which time the battery was disassembled and the lithium precipitation of the negative electrode sheet was observed. If there was gray or silver white lithium precipitation, the result was indicated as “lithium precipitation”. Otherwise, it was “no lithium precipitation”.

[0159] (2) Cycle performance test

[0160] In a thermostat at 25°C, the battery was charged at an initial rate of 3C to 4.25V, then charged at 1.5C to the upper limit voltage (4.53V), and then charged at constant voltage to 0.05C as a charging process. After the charging process, the battery was rested for 10 min, and then discharged at a rate of 0.7C to 3.0V as a discharging process. One charging and discharging process and one discharging process constituted a cycle. The above cycle process was cycled for 800 cycles, and the maximum discharge capacity of the discharging process of the first 3 cycles was the initial discharge capacity Q1, and the discharge capacity of the discharging process of the 800th cycle was Q2. The cycle capacity retention rate = (Q2 / Q1) x 100%.

[0161] (3) K value test

[0162] The open circuit potential of the lithium ion battery on the sorting lower platform was measured to obtain V1, and the open circuit potential of the battery was measured again after standing for 24 h to obtain V2. The difference between V1 and V2 divided by the standing time gave the K value of the battery.

[0163] The results obtained are recorded in Tables 1-4.

[0164] Table 1-4

[0165]

[0166]

[0167] “Test cannot be completed” means that the self-discharge of the lithium ion battery is too severe to complete the fast charging performance test and the cycle performance test.

[0168] From Table 1-4, it can be seen that by comparing the comparative examples and the examples, it can be seen that the lithium precipitation of the examples is significantly improved, the cycle capacity retention rate is significantly improved, and the K value is reduced, indicating that by controlling the weight content of ethyl difluoroacetate and fluorobutyronitrile in the electrolyte and the weight ratio of the two, the fast charging performance is improved, and the cycle stability is improved.

[0169] Example II group is used to illustrate the effect of adding fluorosulfonamide compounds to the electrolyte.

[0170] Example II-1

[0171] Reference is made to Example I-1, except that the propyl propionate in the electrolyte is replaced by the same weight part of the fluorosulfonamide compound.

[0172] Example II-2 group

[0173] This group of examples is used to illustrate the influence when the structure of the fluorosulfonamide compound is changed.

[0174] Example II-2a

[0175] Reference is made to Example II-1, except that the fluorosulfonamide compound is of the structure shown in formula (III-2).

[0176] Example II-2b

[0177] Reference is made to Example II-1, except that the fluorosulfonamide compound is of the structure shown in formula (III-3).

[0178] Example II-3 group

[0179] This group of examples is used to illustrate the effects produced when the weight content of the fluorosulfonamide compound in the electrolyte is changed.

[0180] Example II-3a

[0181] Example II-3a was conducted with the exception that the weight content of the fluorosulfonamide compound in the electrolyte was 2%.

[0182] Example II-3b

[0183] Example II-3a was conducted with the exception that the weight content of the fluorosulfonamide compound in the electrolyte was 25%.

[0184] Example II-3c

[0185] Example II-3a was conducted with the exception that the weight content of the fluorosulfonamide compound in the electrolyte was 30%.

[0186] Example II-4 group

[0187] This group of examples is used to illustrate the effects produced when the ratio of the weight of the fluorinated ethylene carbonate to the weight of the 1,3-propane sultone is changed.

[0188] This group of examples was conducted with reference to Example II-1 with the exception that the ratio of the weight of the fluorinated ethylene carbonate to the weight of the 1,3-propane sultone was changed. See Table 2-1 for details.

[0189] Table 2-1

[0190]

[0191] Example II-5

[0192] (1) The electrolyte was conducted with reference to Example II-1 with the exception that the difluoroacetic acid ethyl ester was of the structure shown in formula (I-2), the fluorobutyronitrile was of the structure shown in formula (II-2), the fluorosulfonamide compound was of the structure shown in formula (III-3), the weight content of the difluoroacetic acid ethyl ester in the electrolyte was 34%, the weight content of the fluorobutyronitrile in the electrolyte was 10%, the weight content of the fluorosulfonamide compound in the electrolyte was 14%, the weight proportion of the ethylene carbonate was 10%, the weight proportion of the fluorinated ethylene carbonate was 10%, the weight proportion of the lithium hexafluorophosphate was 12%, the weight proportion of the lithium bistrifluoromethylsulfonylimide was 5%, the weight proportion of the hexanetricarboxylic acid was 3%, the weight proportion of the 1,3-propane sultone was 2%, and the weight ratio of the fluorobutyronitrile to the difluoroacetic acid ethyl ester was 0.29.

[0193] (2) The negative electrode sheet was prepared according to the reference example II-1, except that the weight content of silicon element in the negative electrode active layer was 32.4%, the weight content of doping element in the negative electrode active layer was 0.1%, the sphericity Q of the silicon material was 0.86, and the compacted density of the negative electrode sheet was 1.68 g / cm3. 3 .

[0194] (3) The positive electrode sheet was prepared according to the reference example II-1.

[0195] (4) The lithium ion battery was prepared according to the reference example II-1, wherein F / S = 35 / 32.4 = 1.08, (W / S) x 100% = (0.1 / 32.4) x 100% = 0.3086%, and B / Q = 10% / 0.86 = 0.116.

[0196] Test group II

[0197] The lithium ion batteries prepared in the example group II were respectively tested as follows:

[0198] (1) Hot box test

[0199] In an environment of 25°C, the battery was charged to a full state of charge (100% SOC) at a constant current and constant voltage of 0.5C, and then the battery in the full state of charge was placed in a constant temperature oven of 20±5°C, followed by heating to a set temperature °C at a rate of 5°C / min and maintaining the temperature for 60 min. If the battery did not catch fire and / or explode within 60 min, it was considered to pass the hot box test. If the battery caught fire and / or exploded, it was considered to fail the hot box test. A total of 10 battery samples were tested for each example, and the highest temperature at which all 10 batteries passed the hot box test was taken as the test result.

[0200] (2) Cycle performance test, see the performance test method in the example group I for details.

[0201] (3) High temperature storage performance test

[0202] In an environment of 25°C, the battery was charged to a full state of charge (100% SOC) at a constant current and constant voltage of 0.5C, and then the battery in the full state of charge was placed in a constant temperature oven of 20±5°C, followed by heating to a set temperature °C at a rate of 5°C / min and maintaining the temperature for 60 min. If the battery did not catch fire and / or explode within 60 min, it was considered to pass the hot box test. If the battery caught fire and / or exploded, it was considered to fail the hot box test. A total of 10 battery samples were tested for each example, and the highest temperature at which all 10 batteries passed the hot box test was taken as the test result.

[0203] The results obtained are shown in Table 2-2.

[0204] Table 2-2

[0205]

[0206] The preferred embodiments of the present application are described in detail above, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including that each technical feature is combined in any other suitable manner. These simple modifications and combinations should also be considered as disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A lithium-ion battery, characterized by, The lithium ion battery comprises a negative electrode sheet and an electrolyte, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer located on one side or both sides of the negative electrode current collector, the negative electrode active layer comprises a silicon material, the weight content of the silicon material in the negative electrode active layer is 7%-60%; the electrolyte comprises ethyl difluoroacetate and fluorobutyronitrile, the fluorobutyronitrile has a fluorine atom substitution number of 1-4, the weight content of the ethyl difluoroacetate is 20%-60% and the weight content of the fluorobutyronitrile is 2%-20% based on the total weight of the electrolyte, and the ratio of the weight of the fluorobutyronitrile to the weight of the ethyl difluoroacetate is 0.08-0.

8.

2. The lithium-ion battery of claim 1, wherein, The compacted density of the negative electrode sheet is 1.5 g / cm 3 -2 g / cm 3 ; And / or, the ethyl difluoroacetate comprises one or more of the following structures: And / or, the fluorobutyronitrile comprises one or more of the following structures:

3. The lithium-ion battery of claim 1, wherein, The silicon material comprises a silicon element and a doping element, the doping element comprises one or more of Al, Cu, Fe, Ni, Mo, Mg, Ti, Mn, Ca, Na and Ge; And / or, the silicon material comprises one or more of nano-silicon, micro-silicon, amorphous silicon, silicon-oxygen material and silicon-carbon material.

4. The lithium-ion battery of claim 3, wherein, The lithium ion battery simultaneously satisfies the following relationships: 0.5≤F / S≤15 and 0.001%≤(W / S)×100%≤1%, wherein F is the weight content of the ethyl difluoroacetate in the electrolyte, S is the weight content of the silicon element in the negative electrode active layer, and W is the weight content of the doping element in the negative electrode active layer; Preferably, the weight content of the silicon element in the negative electrode active layer is 3%-50%; Preferably, the weight content of the doping element in the negative electrode active layer is 0.001%-1%, preferably 0.005%-0.5%; Preferably, the lithium ion battery simultaneously satisfies the following relationships: 0.8≤F / S≤10 and 0.01%≤(W / S)×100%≤0.5%.

5. The lithium-ion battery of claim 1, wherein, The lithium ion battery satisfies the following relationship: 0.02≤B / Q≤0.2, wherein B is the weight content of the fluorobutyronitrile in the electrolyte and Q is the sphericity of the silicon material. And / or, the sphericity of the silicon material is 0.6-1.

6. The lithium-ion battery of claim 5, wherein, The lithium ion battery satisfies the following relationship: 0.04≤B / Q≤0.

15. And / or, the weight content of the fluorobutyronitrile in the electrolyte is 4%-15%. And / or, the sphericity of the silicon material is 0.8-0.

99.

7. The lithium-ion battery of any one of claims 1-6, wherein, The electrolyte further comprises a fluorosulfonamide compound having a structure shown in formula (III) wherein R4, R5 are each independently selected from a fluorine-substituted or unsubstituted C1-C3 alkyl, R4 and R5 are directly connected to N, N is ringed or not ringed with R4, R5.

8. The lithium-ion battery of claim 7, wherein, The weight content of the fluorosulfamide compound in the electrolyte is 2%-25%; And / or, the fluorosulfamide compound comprises one or more of the following structures:

9. The lithium ion battery according to claim 1, wherein The electrolyte further comprises fluorinated ethylene carbonate and 1,3-propane sultone, and the ratio of the weight of the fluorinated ethylene carbonate to the weight of the 1,3-propane sultone is 3-30. Preferably, the weight content of the fluorinated ethylene carbonate in the electrolyte is 5%-25%. Preferably, the weight content of the 1,3-propane sultone in the electrolyte is 0.2%-5%.

10. An electrochemical device comprising the lithium ion battery of any one of claims 1-9, wherein, The negative electrode active layer further comprises a carbon material, and the carbon material comprises one or more of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon and soft carbon.

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