Electrolyte and lithium ion battery

Abstract

The application relates to the field of battery materials, and discloses an electrolyte and a lithium ion battery. The electrolyte provided by the application comprises a compound A and a compound B; the compound A accounts for x% of the total mass of the electrolyte, and x satisfies 0.1 < x <= 5; the compound B accounts for y% of the total mass of the electrolyte, and y satisfies 5 <= y <= 60. The application further provides a lithium ion battery containing the electrolyte. Through the mutual cooperation of the compound A and the compound B added in the electrolyte, the cycle life and the thermal runaway safety problem of the battery under high voltage can be significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of battery materials, in particular to an electrolyte and a lithium ion battery. BACKGROUND

[0002] High-voltage batteries can provide higher energy density and power output, that is, they have the advantage of storing more electrical energy and releasing more electrical power in a shorter time under the same volume or weight; therefore, in order to meet the demand for efficient and reliable energy storage, it is urgent to develop higher voltage batteries.

[0003] However, developing higher voltage batteries requires solving the stability problem of battery materials, and the factors affecting the stability of the battery include but are not limited to the following two aspects:

[0004] First, as the voltage increases, especially at high voltages above 4.5V, the chemical reactions of the materials inside the battery become more intense, which can lead to the destruction of the structure of the electrode material, and further cause the performance degradation of the battery and even safety problems. Secondly, the electrolyte is easily oxidized or decomposed under high voltage environment, producing gas, which not only reduces the cycle life of the battery, but also may cause battery swelling, liquid leakage and other safety hazards. SUMMARY

[0005] The present application provides a lithium ion battery, which aims to improve the stability of the electrolyte under high voltage environment to some extent.

[0006] The electrolyte provided by the present application comprises compound A and compound B; the compound A accounts for x% of the total mass of the electrolyte, x satisfies: 0.1

[0007] The structure of the compound A is shown in the following formula 1,

[0008] Formula 1: In formula 1, n=0 or 1, R1, R2, R3 are independently selected from halogen, halogen-substituted or unsubstituted C1-C10 alkyl, halogen-substituted or unsubstituted C2-C6 alkenyl, halogen-substituted or unsubstituted C2-C6 alkynyl;

[0009] The structure of the compound B is shown in the following formula 2,

[0010] Formula 2: In formula 2, R4 is selected from substituted or unsubstituted C1-C10 alkyl, substituted or unsubstituted C1-C10 alkoxy; R5 is selected from substituted or unsubstituted C1-C10 alkoxy, substituted or unsubstituted C1-C10 carbonate, substituted or unsubstituted C2-C10 alkoxy-containing carbonate, substituted or unsubstituted C1-C10 carboxylate.

[0011] Further, the compound A includes at least one of the following formula 1-1 to formula 1-8:

[0012]

[0013] Further, the compound B includes at least one of the following formula 2-1 to formula 2-8:

[0014]

[0015] Further, the electrolyte further includes a fluorine-containing compound C,

[0016] Preferably, the fluorine-containing compound C includes at least one of fluoro-carbonate, fluoro-carboxylate, fluoro-ether.

[0017] Preferably, the fluorine-containing compound C accounts for q% of the total mass of the electrolyte, q satisfies: 2≤q≤20.

[0018] More preferably, the fluorine-containing compound C includes at least one of fluoro-ethylene carbonate (FEC), methyl trifluoroethyl carbonate (FEMC), fluoro-diethyl carbonate (FDEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), 2,2,2-trifluoroethyl acetate (FEA), 2,2-difluoroethyl acetate (DFEA), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE).

[0019] Further, x and q satisfy: 3

[0020] The electrolyte further includes a lithium salt, and the lithium salt accounts for a% of the total mass of the electrolyte, a satisfies: 10≤a≤28.

[0021] The application further provides a lithium ion battery, including a positive electrode sheet, a negative electrode sheet and an electrolyte, the electrolyte being the electrolyte described above.

[0022] Further, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes lithium cobaltate, and the lithium cobaltate is doped with sodium elements.

[0023] Further, the doping amount of the sodium elements accounts for z‰ of the total amount of the positive electrode active material, and z satisfies: 0.1

[0024] Further, the negative electrode sheet comprises a negative electrode active layer, the negative electrode active layer comprises a negative electrode active material, the negative electrode active material comprises a silicon-carbon composite material, wherein the weight percentage of silicon in the negative electrode active layer is n%, and n satisfies 1.5≤n≤40.

[0025] And / or, the porosity of the negative electrode sheet is p%, and p satisfies 5≤p≤45.

[0026] Further, a plurality of recess structures are arranged on the positive electrode sheet and / or the negative electrode sheet, and the recesses satisfy at least one of the following conditions:

[0027] 1) The spacing between two adjacent recesses is d1 mm, and d1 satisfies 0.5≤d1≤10.

[0028] 2) The depth of the recess is d2 μm, and d2 satisfies 5≤d2≤60.

[0029] 3) The size of the recess is d3 μm, and d3 satisfies 30≤d3≤170.

[0030] The application further provides an electrochemical device comprising the lithium ion battery.

[0031] The technical scheme has the following advantages:

[0032] The electrolyte provided by the application comprises compound A and compound B; the compound A accounts for x% of the total mass of the electrolyte, and x satisfies 0.1

[0033] Additional aspects and advantages of the embodiments of the application will be described in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following description or can be learned from the practice of the embodiments of the application. DETAILED DESCRIPTION

[0034] The following examples are provided to better enable those skilled in the art to further understand the application, and are not intended to limit the content and scope of the application. The content and scope of the application are not limited to the best mode contemplated by the inventors, and any product that is the same or similar to the application derived from the disclosure of the application or the combination of the application with other prior art features falls within the scope of the application.

[0035] The specific experimental steps or conditions not mentioned in the examples can be carried out according to the conventional experimental steps described in the literature in the art or the operation or conditions. The reagents or instruments not mentioned by the manufacturer are conventional reagent products that can be obtained by purchase.

[0036] In addition, the technical features involved in the different embodiments of the application described below can be combined with each other as long as there is no conflict between them.

[0037] For the current lithium ion battery, by increasing the upper limit voltage of the battery, the gram capacity of the battery can be significantly increased to meet the use demand of users for high energy density (ED) battery. However, when the battery voltage reaches 4.5V or more, the electrolyte is easily oxidized and decomposed under the action of high voltage, and reacts with the electrode material. Not only will a large amount of gas and heat be generated, which will threaten the thermal safety of the battery, but the by-products will further damage the positive and negative electrode materials, causing the cycle performance of the battery to decrease.

[0038] The application provides an electrolyte, comprising a compound A and a compound B; the compound A accounts for x% of the total mass of the electrolyte, x satisfies: 0.1 < x ≤ 5, and the compound B accounts for y% of the total mass of the electrolyte, y satisfies: 5 ≤ y ≤ 60;

[0039] The structure of the compound A is shown in the following formula 1,

[0040] Formula 1: In formula 1, n = 0 or 1, R1, R2, R3 are independently selected from halogen, halogen-substituted or unsubstituted C1-C10 alkyl, halogen-substituted or unsubstituted C2-C6 alkenyl, halogen-substituted or unsubstituted C2-C6 alkynyl;

[0041] The structure of the compound B is shown in the following formula 2,

[0042] Formula 2: In formula 2, R4 is selected from substituted or unsubstituted C1-C10 alkyl, substituted or unsubstituted C1-C10 alkoxy; R5 is selected from substituted or unsubstituted C1-C10 alkoxy, substituted or unsubstituted C1-C10 carbonate, substituted or unsubstituted C2-C10 alkoxy-containing carbonate, substituted or unsubstituted C1-C10 carboxylate.

[0043] The present application uses electrolyte and electrode protection additives that are more resistant to high voltage, i.e., through the mutual cooperation of the above-mentioned specific content and composition of compound A and compound B, the problems of electrolyte oxidation and decomposition under high voltage, leading to decreased electrolyte cycle performance, intensified side reactions between electrolyte and other components of the battery, and other problems are improved. It can withstand higher high voltage of 4.5V or above, and under high voltage conditions, it can still maintain good cycle performance and improve safety problems caused by thermal runaway, effectively improving the cycle life of the battery. Specifically, the compound B in the present application has a high oxidation potential, which improves the overall oxidation resistance of the electrolyte and reduces the oxidation and decomposition of the electrolyte under high voltage. However, too much compound B will significantly increase the viscosity of the electrolyte. By further adding compound A with high stability, it can preferentially form a dense SEI film containing high chemical stability components (such as lithium alkyl sulfonate) on the electrode surface, so that the electrode is less likely to react with acidic substances produced by electrolyte decomposition, inhibiting gas production and effectively improving the stability of the electrode material. Therefore, the amount of compound B can be reduced to prevent the viscosity from being too high and affecting the cycle performance of the battery. Therefore, by adding compound A and compound B with specific composition and content in the electrolyte, the cycle life and thermal runaway safety problems of the battery under high voltage can be significantly improved.

[0044] As an example, the mass percentage x% of compound A in the electrolyte can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or within a range composed of any two of the above values; the mass percentage y% of compound B in the electrolyte can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or within a range composed of any two of the above values.

[0045] In an alternative embodiment, the compound A includes at least one of the following formula 1-1 to formula 1-8:

[0046]

[0047] And / or, the compound B includes at least one of the following formula 2-1 to formula 2-8:

[0048]

[0049] The compound A of the above formula 1-1 to formula 1-8 and the compound B of the above formula 2-1 to formula 2-8 are all existing compounds that can be directly purchased.

[0050] In an alternative embodiment, the electrolyte further includes a fluorine-containing compound C, the fluorine-containing compound C accounts for q% of the total mass of the electrolyte, q satisfies: 2≤q≤20.

[0051] Compound A forms an SEI film mainly composed of organic components on the electrode surface. During the charging and discharging process, the SEI film mainly composed of organic components has poor mechanical properties, especially when the volume of the negative electrode material expands and shrinks rapidly during the charging and discharging process, it is difficult to withstand the mechanical stress caused by the volume expansion and shrinkage of the negative electrode material, resulting in film rupture, thereby affecting the cycle performance of the battery. The fluorine-containing compound C in the present application can decompose to produce fluorine-containing free radicals on the surface of the negative electrode, forming an SEI film mainly composed of inorganic LiF components. The SEI film formed by the fluorine-containing compound C and the SEI film formed by compound A are interwoven with each other to form an SEI film in which organic components and inorganic components are interwoven, thereby improving the mechanical properties of the protective film on the electrode surface and better adapting to the expansion of the electrode, thereby improving the cycle stability of the battery.

[0052] As an example, the mass percentage q of the fluorine-containing compound C in the electrolyte can be 2%, 3%, 5%, 7%, 9%, 10%, 12%, 14%, 16%, 18%, 20%, or within a range formed by any two of the above values.

[0053] In an alternative embodiment, the fluorine-containing compound C includes at least one of fluoro carbonate, fluoro carboxylate, fluoro ether.

[0054] Preferably, the fluorine-containing compound C includes at least one of fluoro ethylene carbonate (FEC), methyl trifluoroethyl carbonate (FEMC), fluoro diethyl carbonate (FDEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), 2,2,2-trifluoroethyl acetate (FEA), 2,2-difluoroethyl acetate (DFEA), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE).

[0055] In an alternative embodiment, x and q satisfy 3 < x + q ≤ 20. When the total amount of compound A and the fluorine-containing compound C in the electrolyte is too much, the film thickness will be thicker, which will increase the interfacial impedance, increase the resistance of lithium ion insertion and extraction, and cannot improve the cycle performance. When the total amount of compound A and the fluorine-containing compound C in the electrolyte is too little, the effect of forming an SEI film with high mechanical strength and density cannot be achieved, which affects the stability of the electrode sheet and the cycle performance. The present application controls the content of x + q to further improve the cycle life.

[0056] As an example, the value of x + q can be 5, 7, 9, 10, 12, 14, 16, 18, 20, or within a range formed by any two of the above values.

[0057] In an alternative embodiment, the electrolyte further comprises a lithium salt, and the lithium salt accounts for a percentage a of the total mass of the electrolyte, a satisfying 10≤a≤28.

[0058] For example, the percentage a of the mass of the lithium salt in the electrolyte can be 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, or within a range defined by any two of the above values.

[0059] The present application provides a lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the electrolyte is the electrolyte described above. By applying the electrolyte described above to the battery, the cycle life and thermal runaway safety of the battery at high voltage can be significantly improved compared to other electrolytes.

[0060] In an alternative embodiment, the positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprises lithium cobaltate, and the lithium cobaltate is doped with sodium elements; the doping amount of the sodium elements accounts for a percentage z of the total mass of the positive electrode active material, z satisfying 0.1

[0061] In the positive electrode active material of the present application, the sodium ions can occupy the positions originally occupied by other ions, such as transition metal ions, by doping with sodium elements. The occupation of the sodium ions by the sodium elements requires additional potential barriers to be overcome for the migration of other ions, thus increasing the energy required for the dissolution of other ions and further improving the stability of the positive electrode material at high voltage. Since the doping of sodium elements can inhibit the dissolution of transition metal ions and reduce the side reactions between the electrode material and the electrolyte, the generation of gas and heat is further reduced, thus further improving the cycle performance and thermal safety performance of the battery at high voltage.

[0062] For example, the percentage z of the mass of the sodium elements in the positive electrode active material can be 0.1‰, 0.2‰, 0.5‰, 1‰, 2‰, 3‰, 4‰, 5‰, or within a range defined by any two of the above values.

[0063] In an alternative embodiment, the positive electrode sheet or / and the negative electrode sheet is provided with a plurality of recess structures, the distance between two adjacent recess structures is d1 mm, the depth of the recess structure is d2 μm, and the size of the recess is d3 μm; wherein d1 satisfies 0.5≤d1≤10, or / and d2 satisfies 5≤d2≤60, or / and d3 satisfies 30≤d3≤170.

[0064] The recess structure described above can be prepared by a preparation method such as laser wire printing, laser drilling, laser embossing, and physical embossing. For example, when the laser wire printing method is used, fine line processing can be achieved by accurately controlling the energy and movement path of the laser; when the laser drilling method is used, small holes of a specific size and depth can be quickly formed on various materials by using the high energy density characteristics of the laser; when the laser embossing method is used, a specific pattern or texture can be created on the surface of the material by using the laser; and when physical embossing is used, a special mold is used to apply high pressure by using traditional machinery, so that the surface is deformed according to the shape of the mold, thereby forming the required pattern or texture. That is, the recess structure with accurately controlled length, width, and depth can be prepared by the preparation method such as laser wire printing, laser drilling, laser embossing, and physical embossing.

[0065] When the recess structure is a circular hole, the size d3 of the recess refers to the diameter of the circular hole; when the recess structure is a linear structure, the size d3 of the recess refers to the width of the linear structure; when the recess structure is other regular or irregular shapes, the size d3 of the recess refers to the distance between the farthest vertices on the edge; and the spacing d1 between the recess structures refers to the shortest distance between the edges of adjacent two recess structures.

[0066] The present application can be provided with a plurality of recesses on the positive electrode sheet to form a recess structure, or a plurality of recesses on the negative electrode sheet to form a recess structure, or a recess structure on both the positive electrode sheet and the negative electrode sheet. The provision of the recess structure in the present application not only increases the contact area between the electrode interface and the electrolyte, but also makes the electrolyte more easily infiltrate into the electrode material, improves the lithium ion insertion efficiency, and can significantly improve the lithium ion transport dynamics and improve the battery fast charging performance caused by the large viscosity of the compound B. When the spacing d1 mm between adjacent two recesses in the recess structure is too large and the depth d2 μm of the recess is too shallow, the electrode material electrolyte has limited improvement in the infiltration effect on the electrode material; when the spacing d1 mm between adjacent two recesses is too small or the depth d2 μm of the recess is too deep, the negative electrode material is prone to powdering and structural damage during the charging and discharging process, which affects the fast charging and cycle performance; and the present application further improves the rate performance and cycle performance of the battery by optimizing the spacing d1 mm between adjacent two recesses and the depth d2 μm of the recess in the recess structure.

[0067] As an example, the spacing d1 between two adjacent recesses can be 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or within a range consisting of any two of the above values; the depth d2 of the recess can be 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60 or within a range consisting of any two of the above values, and the size d3 of the recess can be 30, 35, 40, 45, 50, 55, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170 or within a range consisting of any two of the above values.

[0068] In an optional embodiment, the negative electrode sheet comprises a negative electrode active layer, the negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-carbon composite material, wherein the weight percentage of silicon in the negative electrode active layer is n%, and n satisfies 1.5≤n≤40; and / or the porosity of the negative electrode sheet is p%, and p satisfies 5≤p≤45.

[0069] The negative electrode active material of the present application contains a silicon-carbon composite material, which can increase the lithium storage sites of the negative electrode material, enable lithium ions to be embedded in the negative electrode material more quickly, and improve the fast charging performance of the battery. In addition, by controlling the silicon content and the porosity of the negative electrode sheet, the present application can improve the deterioration of the cycle performance caused by the expansion of the silicon-carbon composite material. Specifically, if the silicon content in the negative electrode active material is too high, the expansion will be too large, which will affect the stability of the SEI film structure and the cycle performance; if the silicon content is too low, the energy density cannot be improved. If the porosity of the negative electrode sheet is too high, the spherical structure of the raw material will be broken, and if the porosity is too low, the porous structure cannot play a role. By setting the porosity within the above range, the present application can promote the penetration of the electrode liquid, improve the diffusion rate of lithium ions, and thus improve the cycle and rate performance of the battery. By controlling the silicon content and the porosity, the present application can improve the deterioration of the cycle performance caused by the expansion of the silicon-carbon composite material, and further improve the fast charging performance and the cycle performance of the battery.

[0070] As an example, the mass percentage n% of the silicon content in the positive electrode active material layer can be 1.5%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or within a range consisting of any two of the above values; and the porosity p% of the negative electrode sheet can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or within a range consisting of any two of the above values.

[0071] The porosity p% of the negative electrode sheet can be detected by an organic solvent infiltration method, specifically: by filling the pores of the electrode sheet of the battery with a test liquid, ensuring that the gas in the pores of the final electrode sheet of the battery is fully replaced by the test liquid, then performing solid-liquid separation on the test liquid on the electrode sheet and measuring the liquid volume, thereby calculating the porosity.

[0072] The porosity p% of the negative electrode sheet can also be detected by a specific surface area analyzer (BET instrument), specifically including: (1) cutting a small piece (about 0.1-0.5g) from the negative electrode sheet, and trying to select an area without wrinkles and damage; then peeling off the active layer from the current collector (copper foil) and grinding into powder, and obtaining the sample after removing residual electrolyte or impurities by vacuum drying at 60℃ for 12h.(2) Put the sample into the sample tube of the BET instrument, heat and degas in a vacuum or inert gas (such as N2) environment to remove adsorbed water and gas; then measure the adsorption isotherm of the material by nitrogen adsorption method at liquid nitrogen temperature (-196℃); the obtained adsorption-desorption isotherm is calculated by BET equation to calculate the specific surface area, and the pore size distribution and pore volume are analyzed by BJH (Barrett-Joyner-Halenda) or DFT (Density Functional Theory) model, the above analysis process is prior art, and will not be repeated in the present application.(3) Calculate the porosity by the pore volume measured by the BET instrument and the apparent volume of the electrode sheet: porosity (%) = (pore volume x material density) / electrode sheet volume x 100%; wherein the pore volume is the total pore volume in the BET adsorption data (usually from the adsorption amount at relative pressure P / P0 close to 1 in the adsorption isotherm), the electrode sheet volume is measured by geometric size (area x thickness), and the material density is the theoretical density of the active material (such as graphite: 2.26g / cm 3 ) or the compaction density (which can be tested separately by known means).

[0073] In an alternative embodiment, the battery of the present application further comprises an aluminum plastic film, which comprises a nylon layer, an aluminum layer and a polypropylene layer (polypropylene layer) arranged in layers, and the thickness of the polypropylene layer is m μm, and m satisfies 30≤m≤100.

[0074] The battery of the present application can improve the sealing performance of the aluminum plastic film by increasing the thickness of the polypropylene layer in the aluminum plastic film, thereby preventing the entry of water vapor and oxygen from the outside of the battery into the inside of the battery, preventing reaction with the electrolyte and electrode materials in the inside of the battery, and improving the cycle and thermal safety performance of the battery at high voltage. By setting the thickness of the polypropylene layer within the above range, the safety performance of the battery can be further improved without reducing the energy density of the battery.

[0075] As an example, the polypropylene layer thickness m can be 30, 40, 50, 60, 70, 80, 90, 100 or within a range between any two of the above values.

[0076] The electrolyte of the present application also includes a conventional organic solvent other than the above disclosed substance, for example: one or more of methyl ethyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), ethylene carbonate (EC), propyl propionate (PP), propylene oxide (EP), etc.

[0077] In an alternative embodiment, the lithium salt B includes at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate, lithium difluoro oxalate borate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethylsulfonylimide, lithium difluorobisoxalate phosphate, lithium tetrafluoroborate, lithium bisoxalate borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methide, and lithium bis(trifluoromethylsulfonyl)imide.

[0078] The positive electrode sheet of the present application includes a current collector and a positive electrode active material layer disposed on the surface of the current collector, the positive electrode active material layer includes 80% to 99.8% of a positive electrode active material, 0.1% to 10% of a conductive agent, and 0.1% to 10% of a binder; preferably, the positive electrode active material includes lithium cobaltate, and the lithium cobaltate is doped with sodium elements.

[0079] The conductive agent in the positive electrode sheet of the present application is not particularly limited, and can be selected from the conductive agents conventionally used in the art, including but not limited to one or more of acetylene black, conductive carbon black, ketjen black, conductive graphite, carbon nanotubes, conductive carbon fibers, graphene.

[0080] The binder in the positive electrode sheet of the present application is not particularly limited, and can be selected from the binders conventionally used in the art, including but not limited to one or more of polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene butadiene rubber, polyethylene oxide.

[0081] The negative electrode sheet of the present application includes a current collector and a negative electrode active layer disposed on at least one side surface of the current collector, the negative electrode active layer includes a negative electrode active material, which can be any of the negative electrode active materials known in the art for lithium ion batteries, and the negative electrode active material includes a silicon-carbon composite material. The types of conductive agent and binder in the negative electrode sheet of the present application are not particularly limited, and the selection range can refer to the types of conductive agent and binder in the positive electrode sheet, which will not be described here.

[0082] The material and shape of the separator used in the lithium ion battery of the present application are not particularly limited, and can include any of the techniques disclosed in the prior art.

[0083] The application will be further described in detail below in connection with specific examples, which are not to be understood as limiting the scope of the application as claimed. In all examples and comparative examples of the present application, the unit wt% represents the mass percentage.

[0084] Examples 1-17 and Comparative Examples 1-5

[0085] A lithium ion battery was prepared as follows:

[0086] 1) Preparation of positive electrode sheet

[0087] The positive electrode active material, polyvinylidene fluoride (PVDF), conductive carbon black (SP, super P) and carbon nanotubes (CNT) were mixed in a mass ratio of 96:2:1.5:0.5, N-methyl pyrrolidone (NMP) was added, and stirring was carried out under the action of a vacuum stirrer until the mixed system became a positive electrode active slurry with uniform fluidity; the positive electrode active slurry was uniformly coated on both surfaces of an aluminum foil; the coated aluminum foil was dried, then rolled, cut, and laser treated to form a recess structure composed of a plurality of recesses, where the recesses in the recess structure were matrix grooves with a groove width of 100 μm, a groove depth of 30 μm, and a groove pitch of 2 mm; the laser-treated negative electrode sheet was cleaned and sheeted to obtain the desired positive electrode sheet. The positive electrode active material was lithium cobaltate, and sodium elements were doped in the lithium cobaltate, with the doping amount of the sodium elements accounting for 2 ‰ of the total mass of the positive electrode material active material.

[0088] 2) Preparation of negative electrode sheet

[0089] The negative electrode active material artificial graphite and silicon-carbon composite material were mixed with carboxymethyl cellulose sodium (CMC-Na), butadiene rubber, conductive carbon black (SP) and single-walled carbon nanotubes (SWCNTs) in a mass ratio of 64.5:30:2.5:1.5:1:0.5, deionized water was added, and a negative electrode active slurry was obtained under the action of a vacuum stirrer; the negative electrode active slurry was uniformly coated on both surfaces of a copper foil; the coated copper foil was dried at room temperature, then transferred to a 80°C oven for drying for 10 h, then cold-pressed and cut, and a plurality of recesses were set on the entire surface of the cut negative electrode sheet by laser to form a recess structure, where the recesses in the recess structure were matrix grooves with a groove width of 100 μm, a groove depth of 30 μm, and a groove pitch of 2 mm; the laser-treated negative electrode sheet was cleaned and sheeted to obtain a negative electrode sheet with a porosity of 30%. The content of silicon in the silicon-carbon composite material accounted for 66.7%, so silicon accounted for 20% of the total weight of the negative electrode active layer.

[0090] 3) Preparation of electrolyte

[0091] In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), EC / PC / PP / EP was mixed uniformly to obtain a mixed organic solvent according to a mass ratio of 10:10:60:20. The mixed organic solvent was taken, then compound B (compound shown in formula 2-1) accounting for y% of the total mass of the electrolyte was added and mixed uniformly, then 16% of the total mass of the electrolyte of a fully dried lithium salt (LiPF6) was quickly added, after dissolution, x% of the total mass of the electrolyte of compound A (compound shown in formula 1-1) was added, and then q% of the total mass of the electrolyte of fluorine-containing compound C was added, and the specific amount is shown in Table 1.

[0092] Table 1

[0093] x(％) y(％) q(％) x + q Example 1 1 30 15 16 Example 2 0.1 30 15 15.1 Example 3 2 30 15 17 Example 4 3 30 15 18 Example 5 5 30 15 20 Example 6 1 5 15 16 Example 7 1 10 15 16 Example 8 1 20 15 16 Example 9 1 40 15 16 Example 10 1 60 15 16 Example 11 1 30 2 3 Example 12 1 30 5 6 Example 13 1 30 10 11 Example 14 1 30 19 20 Example 15 1 30 0 1 Example 16 1 30 20 21 Example 17 1 30 23 24 Comparative Example 1 0 30 10 10 Comparative Example 2 1 0 10 11 Comparative Example 3 7 30 10 17 Comparative Example 4 1 3 10 11 Comparative Example 5 1 70 10 11

[0094] After stirring uniformly, the desired electrolyte was obtained after passing the water and free acid detection. The fluorine-containing compound C in the above-mentioned examples 1-17 all includes 15% of fluoroethylene carbonate (FEC), wherein, in addition to fluoroethylene carbonate (FEC), the fluorine-containing compound C in example 14, example 16 and example 17 also includes the remaining amount of 2,2-difluoroethyl acetate (DFEA), for example: example 14 contains 15% of fluoroethylene carbonate (FEC) and 4% of 2,2-difluoroethyl acetate (DFEA), example 16 contains 15% of fluoroethylene carbonate (FEC) and 5% of 2,2-difluoroethyl acetate (DFEA), and example 17 contains 15% of fluoroethylene carbonate (FEC) and 8% of 2,2-difluoroethyl acetate (DFEA).

[0095] 4) Preparation of lithium ion battery

[0096] The positive electrode sheet of step 1), the negative electrode sheet of step 2) and the commercially available separator were stacked in the order of positive electrode sheet, separator and negative electrode sheet, and then wound to obtain a battery cell; the battery cell was placed in an outer packaging aluminum foil, the outer packaging aluminum foil comprising a nylon layer, an aluminum layer and a polypropylene layer stacked, the thickness of the polypropylene layer in the outer packaging aluminum foil was 70 μm, the electrolyte of step 3) was injected into the outer packaging, and after vacuum packaging, standing, formation, shaping, sorting and other processes, a lithium ion battery was obtained. The battery of the present application has a charge-discharge range of 3.0-4.55 V.

[0097] Examples 18-25

[0098] A lithium ion battery, which is different from example 1 in that the types of compound A, compound B, fluorine-containing compound C and lithium salt are different, and the specific settings are shown in Table 2.

[0099] Table 2

[0100]

[0101] Other parameter conditions in the above examples are exactly the same as those in Example 1, the mass ratio of FEC to FDEC in Example 26 is 1:1, and the mass ratio of FEC to FEA in Example 27 is 3:1.

[0102] Examples 26-44

[0103] A lithium ion battery, which is different from Example 1 in that the doping amount z‰ of sodium element of the positive active material, the content n% of silicon in the negative active material layer, the parameter settings (d1, d2) of the recess structure, and the porosity p% are different, and the specific settings are shown in Table 3 below.

[0104] Table 3

[0105]

[0106]

[0107] In the negative active layer of the above examples, the total weight of the negative active material is kept unchanged, and the content n% of silicon is adjusted by the mass ratio of artificial graphite and silicon-carbon composite material, for example: the content of 1.5% of silicon can use artificial graphite and silicon-carbon composite material with a mass ratio of 92.2:2.3 as negative active material, and so on; other parameter conditions are exactly the same as those in Example 1.

[0108] Experimental examples

[0109] The lithium ion batteries obtained in the examples and comparative examples were respectively subjected to 3C cycle performance test, 5C cycle performance test, heat box test and lithium precipitation degree evaluation.

[0110] 1. 3C cycle performance test

[0111] The batteries obtained in the examples and comparative examples were discharged to 3.0V at 25℃ with a current of 0.5C. Then charged to a voltage of 4.55V at 3C constant current, and then charged to a current of 0.05C at 4.55V constant voltage, and then discharged to a voltage of 3.0V at 3C constant current, which was one charge-discharge cycle. The discharge capacity of the first week was counted as x mAh, and the discharge capacity of the Nth week was counted as y mAh; the capacity of the Nth week divided by the capacity of the first week, the cycle capacity retention rate R of the Nth week was obtained, and the capacity retention rate of the battery at 500T cycles was recorded.

[0112] 2. 5C cycle performance test

[0113] The batteries obtained in the examples and comparative examples were discharged at 0.5C to 3.0V at 25°C. Then charged at 5C to 4.55V, and then charged at 4.55V to 0.05C, and then discharged at 5C to 3.0V, which was one cycle. The discharge capacity of the first week was x mAh, and the discharge capacity of the Nth week was y mAh; the capacity of the Nth week divided by the capacity of the first week, the cycle capacity retention rate of the Nth week R = y / x, and the capacity retention rate of the battery at 500T was recorded.

[0114] 3. Hot box test

[0115] The batteries obtained in the examples and comparative examples were charged at 1C to 4.55V at room temperature, and then rested for 60 minutes. The appearance was checked and photographed. Then heated to 132°C at a rate of 3°C / min ± 2°C / min and held for 60 minutes. The sample was observed, and if there was no leakage, no smoke, no fire and no explosion, it was recorded as passing the test. Ten samples were tested for each example or comparative example, and the pass rate of the hot box performance test was recorded.

[0116] 4. Lithium precipitation evaluation

[0117] The batteries after 3C cycling for 500T were fully charged, and then the negative electrode sheet surface was observed for lithium precipitation. The lithium precipitation was divided into: A-no lithium precipitation, B-mild lithium precipitation (lithium precipitation at the top, bottom and crease), and C-severe lithium precipitation.

[0118] The above test results of the examples and comparative examples of the present application are shown in Tables 4-6.

[0119] Table 4

[0120]

[0121]

[0122] Table 5

[0123] 3C cycle 500T capacity retention 5C cycle 500T capacity retention Hot box test pass rate Lithiation condition Example 18 89.4％ 86.6％ 10 / 10 No lithium precipitation Example 19 91.1％ 87.3％ 10 / 10 No lithium precipitation Example 20 89.3％ 86.8％ 10 / 10 No lithium precipitation Example 21 89.6％ 87.0％ 10 / 10 No lithium precipitation Example 22 90.4％ 87.2％ 10 / 10 No lithium precipitation Example 23 89.9％ 86.6％ 10 / 10 No lithium precipitation Example 24 89.5％ 86.1％ 10 / 10 No lithium precipitation Example 25 89.3％ 85.4％ 9 / 10 No lithium precipitation

[0124] Table 6

[0125] 3C cycle 500T capacity retention 5C cycle 500T capacity retention Hot box test pass rate Lithiation condition Example 26 89.6％ 86.5％ 10 / 10 No lithium precipitation Example 27 90.5％ 87.1％ 10 / 10 No lithium precipitation Example 28 88.1％ 84.3％ 10 / 10 No lithium precipitation Example 29 89.7％ 86.2％ 10 / 10 No lithium precipitation Example 30 88.7％ 85.5％ 9 / 10 Light lithium precipitation Example 31 85.2％ 82.4％ 9 / 10 Light lithium precipitation Example 32 85.9％ 83.3％ 8 / 10 Light lithium precipitation Example 33 86.3％ 83.8％ 8 / 10 Light lithium precipitation Example 34 90.7％ 87.1％ 10 / 10 No lithium precipitation Example 35 91.2％ 87.6％ 10 / 10 No lithium precipitation Example 36 91.7％ 88.2％ 10 / 10 No lithium precipitation Example 37 90.3％ 87.4％ 10 / 10 No lithium precipitation Example 38 88.6％ 85.9％ 8 / 10 No lithium precipitation Example 39 86.2％ 83.1％ 6 / 10 Light lithium precipitation Example 40 88.6％ 84.5％ 10 / 10 Light lithium precipitation Example 41 90.1％ 85.9％ 10 / 10 No lithium precipitation Example 42 89.8％ 86.0％ 9 / 10 No lithium precipitation Example 43 85.1％ 81.7％ 7 / 10 No lithium precipitation Example 44 85.4％ 82.5％ 10 / 10 Light lithium precipitation

[0126] It can be seen from the above Tables 4-6 that by controlling the content of the compound A and the compound B in the electrolyte to meet the value requirements in the range of the present application, the stability of the electrolyte itself and the electrode material at high voltage can be effectively improved through the mutual synergistic cooperation of the compound A and the compound B, thereby significantly improving the cycle life and thermal runaway safety of the battery at high voltage. Among them, it can be seen from the data in Table 4 that by further reducing the content of the compound A, the compound B, the fluorine-containing compound C and the lithium salt, the occurrence of lithium precipitation can be reduced; it can be seen from the data in Table 6 that by further optimizing the parameter conditions of the concave structure, reducing the content of the silicon-carbon composite material in the negative active material layer and increasing the porosity of the negative plate, the occurrence of lithium precipitation can be further reduced.

[0127] Obviously, the above embodiments are only examples for clearly illustrating but not limiting the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all the embodiments need not and cannot be exhausted. The changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. An electrolyte, characterized by, The electrolyte comprises compound A and compound B; the compound A accounts for x% of the total mass of the electrolyte, x satisfies: 0.1 The structure of the compound A is shown in the following formula 1, Formula 1: In Formula 1, n = 0 or 1, and R1, R2, R3are each independently selected from the group consisting of halogen, halogen-substituted or unsubstituted C1-C10alkyl, halogen-substituted or unsubstituted C2-C6alkenyl, and halogen-substituted or unsubstituted C2-C6alkynyl. The structure of the compound B is shown in the following formula 2, Formula 2: , in Formula 2, R4 is selected from substituted or unsubstituted C1-C10 alkyl, substituted or unsubstituted C1-C10 alkoxy; R5 is selected from substituted or unsubstituted C1-C10 alkoxy, substituted or unsubstituted C1-C10 carbonate, substituted or unsubstituted C2-C10 alkoxy-containing carbonate, substituted or unsubstituted C1-C10 carboxylate; Further comprising a fluorine-containing compound C, the fluorine-containing compound C comprises at least one of fluorinated carbonate, fluorinated carboxylate, fluorinated ether.

2. The electrolyte according to claim 1, characterized in that, The compound A comprises at least one of the following structural formulae: ； And / or, the compound B comprises at least one of the following formulae 2-1 to 2-8: 。 3. The electrolyte of claim 1, wherein The fluorine-containing compound C comprises at least one of fluorinated ethylene carbonate, methyl trifluoroethyl carbonate, diethyl fluorinated carbonate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 2,2,2-trifluoroethyl acetate, 2,2-difluoroethyl acetate, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether; And / or, the fluorine-containing compound C accounts for q% of the total mass of the electrolyte, q satisfies: 2 4. The electrolyte according to claim 3, characterized in that, The x and q satisfy: 3 5. A lithium ion battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, characterized by The electrolyte is the electrolyte of any one of claims 1-4.

6. The lithium-ion battery of claim 5, wherein, The positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprises lithium cobaltate, and the lithium cobaltate is doped with sodium elements.

7. The lithium-ion battery of claim 6, wherein, The doping amount of the sodium elements accounts for z‰ of the total amount of the positive electrode active material, z satisfies: 0.1 8. The lithium-ion battery of claim 6, wherein, The negative electrode sheet comprises a negative electrode active layer, the negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-carbon composite material, wherein the weight percentage of silicon in the negative electrode active layer is n%, n satisfies: 1.5 And / or, the porosity of the negative electrode sheet is p%, p satisfies: 5 9. The lithium-ion battery of claim 6, wherein, The positive electrode sheet and / or the negative electrode sheet are provided with a plurality of recesses, the recesses satisfy at least one of the following conditions: 1) The distance between two adjacent recesses is d1 mm, d1 satisfies: 0.5 2) The depth of the recess is d2 μm, d2 satisfies: 5 3) The size of the recess is d3 μm, d3 satisfies: 30 10. An electrochemical device, characterized by, The lithium ion battery comprises the lithium ion battery of any one of claims 6-9.

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