Electrolyte and lithium ion battery

Abstract

The invention relates to the field of battery materials, and discloses an electrolyte and a lithium ion battery. The electrolyte provided by the invention comprises a compound A and a compound B, the compound A accounts for x% of the total mass of the electrolyte, x is more than 0.1 and less than or equal to 5, the compound B accounts for y% of the total mass of the electrolyte, and y is more than or equal to 5 and less than or equal to 60. The invention also provides a lithium ion battery containing the electrolyte. Through mutual cooperation of the compound A and the compound B added in the electrolyte, the cycle life and thermal runaway safety of the battery under high voltage can be remarkably improved.

Description

Technical Field

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

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

[0003] To develop higher voltage batteries, it is necessary to solve the problem of battery material stability. The factors affecting battery stability include but are not limited to the following two aspects:

[0004] First, as the voltage increases, especially when the battery is at a high voltage of 4.5V or above, the chemical reaction of the battery's internal materials becomes more intense, which may cause structural damage to the electrode materials, leading to battery performance degradation and even safety issues. Second, the electrolyte is easily oxidized or decomposed under high voltage conditions, producing gas, which not only reduces the battery's cycle life, but may also cause safety hazards such as battery swelling and leakage. Summary of the invention

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

[0006] The present application provides an electrolyte, comprising compound A and 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, and y satisfies: 5≤y≤60;

[0007] The structure of the compound A is shown in Formula 1 below:

[0008] Formula 1: In formula 1, n = 0 or 1, R 1 , R 2 , R 3 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 Formula 2 below:

[0010] Formula 2: In formula 2, R 4 is selected from substituted or unsubstituted C1-C10 alkyl, substituted or unsubstituted C1-C10 alkoxy; R 5Selected from substituted or unsubstituted C1-C10 alkoxy groups, substituted or unsubstituted C1-C10 carbonate groups, substituted or unsubstituted C2-C10 carbonate groups containing alkoxy groups, and substituted or unsubstituted C1-C10 carboxylate groups.

[0011] Further, the compound A comprises at least one of the following structural formulas 1-1 to 1-8:

[0012]

[0013] And / or, the compound B comprises at least one of the following structural formulas 2-1 to 2-8:

[0014]

[0015] Furthermore, the electrolyte also includes a fluorine-containing compound C,

[0016] Preferably, the fluorine-containing compound C includes at least one of fluorocarbonate, fluorocarboxylate and fluoroether.

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

[0018] More preferably, the fluorine-containing compound C includes at least one of fluoroethylene carbonate (FEC), methyl trifluoroethyl carbonate (FEMC), fluorodiethyl 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), and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE).

[0019] Furthermore, x and q satisfy: 3<x+q≤20.

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

[0021] The present invention also provides a lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the electrolyte is the above-mentioned electrolyte.

[0022] Furthermore, the positive electrode sheet includes a positive electrode active material, the positive electrode active material includes lithium cobalt oxide, and the lithium cobalt oxide is doped with sodium element.

[0023] Furthermore, the doping amount of the sodium element accounts for z‰ of the total amount of the active material of the positive electrode material, and z satisfies: 0.1<z≤5.

[0024] Furthermore, the negative electrode sheet includes a negative electrode active layer, the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a silicon-carbon composite material, wherein the weight proportion 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] Furthermore, the positive electrode sheet and / or the negative electrode sheet are provided with a plurality of recess structures, and the recesses meet at least one of the following conditions:

[0027] 1) The distance between two adjacent recesses is d1 mm, and d1 satisfies: 0.5≤d1≤10;

[0028] 2) The depth of the concave portion is d2 μm, and d2 satisfies: 5 ≤ d2 ≤ 60;

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

[0030] The present invention also provides an electrochemical device, comprising the above-mentioned lithium-ion battery.

[0031] The technical solution of this application has the following advantages:

[0032] The present application provides an electrolyte, comprising compound A and 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, and y satisfies: 5≤y≤60. Among them, compound B has a higher oxidation potential, which can improve the overall oxidation resistance of the electrolyte and reduce the oxidative decomposition of the electrolyte under high voltage; however, when the content of compound B is high, the viscosity of the electrolyte will be significantly increased. The present invention further adds compound A with higher stability, which can preferentially form a dense SEI film containing components with higher chemical stability (such as alkyl sulfonate lithium) on the electrode surface, so that the electrode is not easy to react with the acidic substances produced by the reaction and decomposition of the electrolyte, inhibit gas production, and effectively improve the stability of the electrode. At the same time, the addition of compound A can also improve the problem of excessive viscosity of the electrolyte caused by the large amount of compound B. Therefore, the present invention can effectively and synergistically improve the stability of the electrolyte itself and the electrode material under high voltage through the mutual cooperation between compound A and compound B, thereby significantly improving the cycle life and thermal runaway safety of the battery under high voltage.

[0033] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the subsequent description, or explained through the implementation of the embodiments of the present application. DETAILED DESCRIPTION

[0034] The following examples are provided for a better understanding of the present application, but are not limited to the best implementation mode described, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the protection scope of the present application.

[0035] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0036] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0037] For current lithium-ion batteries, the battery's gram capacity can be significantly increased by increasing the battery's upper voltage limit, meeting users' demand for high energy density (ED) batteries. However, when the battery voltage reaches above 4.5V, the electrolyte is easily oxidized and decomposed under the action of high voltage, and reacts with the electrode material, which will not only produce a large amount of gas and heat, posing a threat to the battery's thermal safety, but its byproducts will further damage the positive and negative electrode materials, causing the battery's cycle performance to decline.

[0038] The present 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, and y satisfies: 5≤y≤60;

[0039] The structure of the compound A is shown in Formula 1 below:

[0040] Formula 1: In formula 1, n = 0 or 1, R 1 , R 2 , R 3 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 Formula 2 below:

[0042] Formula 2: In formula 2, R 4 is selected from substituted or unsubstituted C1-C10 alkyl, substituted or unsubstituted C1-C10 alkoxy; R 5Selected from substituted or unsubstituted C1-C10 alkoxy groups, substituted or unsubstituted C1-C10 carbonate groups, substituted or unsubstituted C2-C10 carbonate groups containing alkoxy groups, and substituted or unsubstituted C1-C10 carboxylate groups.

[0043] The present invention uses an electrolyte that is more resistant to high voltage and an electrode protection additive to cooperate with each other, that is, through the cooperation of the compound A and the compound B with the above-mentioned specific content and composition, a series of problems such as the electrolyte being easily oxidized and decomposed under high voltage, resulting in a decrease in the electrolyte cycle performance, and aggravated side reactions between the electrolyte and other components of the battery are improved. It can withstand higher high voltages above 4.5V, 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 invention has a higher oxidation potential, improves the overall oxidation resistance of the electrolyte, and reduces the oxidative decomposition of the electrolyte under high voltage. However, when the content of compound B is too much, the viscosity of the electrolyte will be significantly increased. By further adding compound A with higher stability, it can preferentially form a dense SEI film containing components with higher chemical stability (such as lithium alkyl sulfonate) on the electrode surface, so that the electrode is not easy to react with the acidic substances produced by the decomposition of the electrolyte, 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 of specific composition and content to the electrolyte to cooperate with each other, the effect of significantly improving the cycle life of the battery under high voltage and the safety problem of thermal runaway can be achieved.

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

[0045] In an optional embodiment, the compound A comprises at least one of the following structural formulas 1-1 to 1-8:

[0046]

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

[0048]

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

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

[0051] Compound A forms an SEI film mainly composed of organic components on the electrode surface. During the charge and discharge process, the SEI film of organic components has poor mechanical properties, especially when the volume rapidly expands and contracts during the charge and discharge process of the negative electrode, it is difficult to withstand the mechanical stress caused by the volume expansion and contraction of the negative electrode material, resulting in film rupture, thereby affecting the battery cycle performance. The fluorine-containing compound C described in the present invention can decompose on the surface of the negative electrode to produce fluorine-containing free radicals, forming an SEI film mainly composed of inorganic LiF components, which is intertwined with the SEI film of organic components formed by compound A to form an SEI film intertwined with organic components and inorganic components, thereby improving the mechanical properties of the electrode surface protective film, being able to better adapt to the expansion of the electrode, and improving the cycle stability of the battery.

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

[0053] In an optional embodiment, the fluorine-containing compound C includes at least one of fluorocarbonate, fluorocarboxylate, and fluoroether;

[0054] Preferably, the fluorine-containing compound C includes at least one of fluoroethylene carbonate (FEC), methyl trifluoroethyl carbonate (FEMC), fluorodiethyl 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), and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE).

[0055] In an optional embodiment, x and q satisfy: 3<x+q≤20. When the total amount of compound A and fluorine-containing compound C in the electrolyte is too much, the film thickness will be thicker, which will increase the interface impedance, increase the resistance to lithium ion insertion and extraction, and fail to improve the cycle performance; when the total amount of compound A and fluorine-containing compound C in the electrolyte is too little, the effect of forming a SEI film with high mechanical strength and density cannot be achieved, affecting the stability of the pole piece and the cycle performance. The present invention achieves the effect of further improving the cycle life by controlling the content of x+q.

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

[0057] In an optional embodiment, the electrolyte further includes a lithium salt, and the lithium salt accounts for a% of the total mass of the electrolyte, where a satisfies: 10≤a≤28.

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

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

[0060] In an optional embodiment, the positive electrode sheet includes a positive electrode active material, the positive electrode active material includes lithium cobalt oxide, and the lithium cobalt oxide is doped with sodium element; the doping amount of the sodium element accounts for z‰ of the total amount of the positive electrode active material, and z satisfies: 0.1<z≤5.

[0061] In the positive electrode active material of the present invention, sodium ions can occupy positions originally belonging to other ions, such as transition metal ions, by sodium ion doping. The occupation of the sodium ions causes other ions to migrate and need to overcome additional potential barriers, thereby increasing the energy required for the dissolution of other ions and further improving the stability of the positive electrode material under high voltage. Since sodium ion doping can inhibit the dissolution of transition metal ions, reduce the side reactions between the electrode material and the electrolyte, and further reduce the generation of gas and heat, the cycle performance and thermal safety performance of the battery under high voltage can be further improved.

[0062] As an example, the mass proportion z‰ of the doping amount of the sodium element in the positive electrode active material can be 0.1‰, 0.2‰, 0.5‰, 1‰, 2‰, 3‰, 4‰, 5‰ or within the range of any two of the above values.

[0063] In an optional embodiment, a plurality of recess structures are provided on the positive electrode sheet and / or the negative electrode sheet, the spacing 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 above-mentioned concave structure can be prepared by laser wire-punching, laser drilling, laser embossing, physical embossing and other preparation methods. For example, when the laser wire-punching method is adopted, fine line processing can be achieved by precisely controlling the energy and moving path of the laser; when the laser drilling method is adopted, the high energy density characteristics of the laser are used to quickly form small holes of specific size and depth on various materials; when the laser embossing method is adopted, the laser is used to create a specific pattern or texture on the surface of the material; when physical embossing is adopted, a special mold is used to apply high pressure using traditional machinery, so that the surface is deformed according to the shape of the mold, thereby forming the desired pattern or texture. That is, the present invention can prepare a concave structure with precisely controlled length, width and depth through laser wire-punching, laser drilling, laser embossing, physical embossing and other preparation methods.

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

[0066] The present invention can be provided with a recessed structure composed of several recessed parts on the positive electrode sheet, can also be provided with a recessed structure composed of several recessed parts on the negative electrode sheet, and can also be provided with recessed structures on both the positive electrode sheet and the negative electrode sheet at the same time; the provision of the recessed structure in the present invention not only increases the contact area between the electrode interface and the electrolyte, but also makes it easier for the electrolyte to infiltrate the interior of the electrode material, thereby increasing the embedding efficiency of lithium ions, and can significantly improve the transmission kinetics of lithium ions, and improve the degradation of the fast charging performance of the battery caused by the high viscosity of compound B. Among them, when the spacing d1 mm between two adjacent recesses in the recessed structure is too large and the depth d2μm of the recess is too shallow, the improvement of the infiltration effect of the electrode material electrolyte on the electrode material is limited; when the spacing d1 mm between two adjacent recesses is too small or the depth d2μm of the recess is too deep, it is easy to cause the negative electrode material to fall off and the structure to be damaged during the charge and discharge process, affecting the fast charging and cycle performance; the present invention further improves the rate performance and cycle performance of the battery by optimizing the spacing d1 mm between two adjacent recesses in the recessed structure and the depth d2μm of the recess.

[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 the range formed by 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 the range formed by 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 the range formed by any two of the above values.

[0068] In an optional embodiment, the negative electrode sheet includes a negative electrode active layer, the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a silicon-carbon composite material, wherein the weight proportion 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 invention contains a silicon-carbon composite material, which can add more lithium storage sites to the negative electrode material, and lithium ions can be embedded in the negative electrode material more quickly, thereby improving the fast charging performance of the battery. At the same time, the negative electrode sheet of the present invention can improve the degradation of the cycle performance caused by the expansion of the silicon-carbon composite material by controlling the silicon content and porosity. Specifically, if the silicon content in the negative electrode active material of the present invention is too large, the expansion is too large, which affects the stability of the SEI membrane structure and the cycle performance; if the silicon content is too small, the energy density cannot be improved. If the porosity of the negative electrode sheet of the present invention is too large, the spherical structure of the raw material will be broken, while if it is too small, the porous structure cannot play a role. The present invention can promote the penetration of the electrode liquid and increase the diffusion rate of lithium ions by setting the porosity within the above range, thereby improving the cycle and rate performance of the battery. By controlling the silicon content and porosity, the present invention can improve the degradation of the cycle performance caused by the expansion of the silicon-carbon composite material, and further improve the fast charging performance and cycle performance of the battery.

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

[0071] The porosity p% of the negative electrode sheet can be tested by the organic solvent infiltration method. Specifically, the test liquid is filled into the pores of the battery cell electrode sheet to ensure that the gas in the pores of the battery cell electrode sheet is fully replaced by the test liquid. The test liquid on the battery cell electrode sheet is then separated into solid and liquid and the liquid volume is measured to infer the porosity.

[0072] The porosity p% of the negative electrode sheet can also be tested using a specific surface area analyzer (BET instrument), which includes: (1) cutting a small piece (about 0.1-0.5 g) from the negative electrode sheet, trying to select an area without wrinkles or damage; then peeling the active layer from the current collector (copper foil) and grinding it into powder, and then drying it at 60°C in vacuum for 12 hours to remove residual electrolyte or impurities to obtain a sample. (2) Put the sample into the sample tube of the BET instrument and place it in a vacuum or inert gas (such as N 2 ) environment to heat and degas to remove adsorbed moisture and gas; then, at liquid nitrogen temperature (-196°C), the adsorption isotherm of the material is determined by nitrogen adsorption method; the specific surface area of ​​the obtained adsorption-desorption isotherm is calculated by BET equation, 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 invention. (3) The porosity is calculated by the pore volume measured by BET instrument and the apparent volume of the electrode sheet: Porosity (%) = (pore volume × material density) / electrode sheet volume × 100%; wherein the pore volume is the total pore volume in the BET adsorption data (usually from the adsorption isotherm at relative pressure P / P 0 The adsorption capacity when the electrode is close to 1), the volume of the electrode sheet is measured by geometric dimensions (area × thickness), and the material density is the theoretical density of the active material (such as graphite: 2.26 g / cm 3 ) or compacted density (which can be obtained by separate testing using existing known means).

[0073] In an optional embodiment, the battery of the present invention further comprises an aluminum-plastic film, wherein the aluminum-plastic film comprises a stacked nylon layer, an aluminum layer and a polypropylene layer (polypropylene layer), wherein the thickness of the polypropylene layer is mμm, and m satisfies 30≤m≤100.

[0074] The battery of the present invention can improve the upper limit voltage of the battery by increasing the thickness of the polypropylene layer in the aluminum-plastic film, thereby improving the sealing performance of the aluminum-plastic film, thereby preventing water vapor and oxygen outside the battery from entering the battery, preventing them from reacting with the electrolyte and electrode materials inside the battery, and improving the cycle and thermal safety performance of the battery under high voltage. The present invention can further improve the battery safety performance without reducing the battery energy density by setting the thickness of the polypropylene layer within the above range.

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

[0076] The electrolyte of the present invention also includes commonly used organic solvents other than the above-disclosed substances, for example, one or more of ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), ethylene carbonate (EC), propyl propionate (PP), propylene oxide (EP), etc.

[0077] In an optional embodiment, the lithium salt B includes lithium hexafluorophosphate (LiPF 6 ), at least one of lithium difluorophosphate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium difluorobis(oxalatophosphate), lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(pentafluoroethylsulfonyl)imide, tris(trifluoromethylsulfonyl)methyllithium and lithium bis(trifluoromethylsulfonyl)imide.

[0078] The positive electrode sheet of the present invention comprises a current collector and a positive electrode active material layer arranged on the surface of the current collector, the positive electrode active material layer comprises 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 comprises lithium cobalt oxide, and the lithium cobalt oxide is doped with sodium.

[0079] The present invention does not particularly limit the conductive agent in the positive electrode sheet, which can be selected from the conductive agents commonly 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, and graphene;

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

[0081] The negative electrode sheet of the present invention comprises 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 comprises a negative electrode active material, the negative electrode active material can be a negative electrode active material for lithium ion batteries known in the art, the negative electrode active material comprises a silicon-carbon composite material, the present invention does not particularly limit the types of the conductive agent and the binder in the negative electrode sheet, and the selection range can refer to the types of the conductive agent and the binder in the positive electrode sheet, which will not be repeated here.

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

[0083] The present application is further described in detail below in conjunction with specific examples, which should not be construed as limiting the scope of protection claimed in the present application. In all examples and comparative examples of the present application, the unit wt% represents the mass percentage content.

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

[0085] A lithium ion battery, the preparation process of which is as follows:

[0086] 1) Preparation of positive electrode

[0087] The positive electrode active material, polyvinylidene fluoride (PVDF), conductive carbon black (SP, super P) and carbon nanotubes (CNT) are mixed in a mass ratio of 96:2:1.5:0.5, N-methylpyrrolidone (NMP) is added, and the mixture is stirred under the action of a vacuum mixer until the mixed system becomes a positive electrode active slurry with uniform fluidity; the positive electrode active slurry is evenly coated on both surfaces of the aluminum foil; the coated aluminum foil is dried, then rolled, cut, and laser-treated to form a concave structure composed of a number of concave parts, wherein the concave parts in the concave structure are matrix grooves with a groove width of 100 μm, a groove depth of 30 μm, and a groove spacing of 2 mm. The negative electrode sheet after laser treatment is cleaned and sheeted to obtain the desired positive electrode sheet. The positive electrode active material is lithium cobalt oxide, and the lithium cobalt oxide is doped with sodium element, and the doping amount of the sodium element accounts for 2‰ of the total mass of the positive electrode active material.

[0088] 2) Negative electrode preparation

[0089] The negative electrode active material artificial graphite and silicon-carbon composite material are mixed with sodium carboxymethyl cellulose (CMC-Na), styrene-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 is added, and the negative electrode active slurry is obtained under the action of a vacuum mixer; the negative electrode active slurry is evenly coated on the two surfaces of the copper foil; the coated copper foil is dried at room temperature, then transferred to an 80°C oven for drying for 10 hours, and then cold pressed and cut, and the cut negative electrode sheet is laser-set on the entire surface of the negative electrode sheet to form a concave structure composed of several concave parts, where the concave parts in the concave structure are matrix grooves with a groove width of 100μm, a groove depth of 30μm, and a groove spacing of 2mm. The negative electrode sheet after laser treatment is cleaned and sliced ​​to obtain a negative electrode sheet with a porosity of 30%. The silicon content in the silicon-carbon composite material accounts for 66.7%, so silicon accounts for 20% of the total weight of the negative electrode active layer.

[0090] 3) Preparation of electrolyte

[0091] In a glove box filled with argon (H 2 O<0.1ppm, O 2<0.1ppm), EC / PC / PP / EP are mixed in a mass ratio of 10:10:60:20 to obtain a mixed organic solvent. Take the mixed organic solvent, then add compound B (the compound shown in formula 2-1) accounting for y% of the total mass of the electrolyte and mix evenly, then quickly add fully dried lithium salt (LiPF6) accounting for 16% of the total mass of the electrolyte, add compound A (the compound shown in formula 1-1) accounting for x% of the total mass of the electrolyte after dissolution, and then add fluorine-containing compound C based on q% of the total mass of the electrolyte, and the specific addition amount is as described in Table 1 below.

[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 Embodiment 11 1 30 2 3 Example 12 1 30 5 6 Embodiment 13 1 30 10 11 Embodiment 14 1 30 19 20 Embodiment 15 1 30 0 1 Example 16 1 30 20 21 Embodiment 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 evenly, the desired electrolyte is obtained after passing the moisture and free acid tests. The fluorine-containing compound C in the above-mentioned embodiments 1-17 all includes 15% of fluoroethylene carbonate (FEC), wherein the fluorine-containing compound C in embodiments 14, 16 and 17 includes the remaining amount of 2,2-difluoroethyl acetate (DFEA) in addition to fluoroethylene carbonate (FEC), for example: embodiment 14 contains 15% of fluoroethylene carbonate (FEC) and 4% of 2,2-difluoroethyl acetate (DFEA), embodiment 16 contains 15% of fluoroethylene carbonate (FEC) and 5% of 2,2-difluoroethyl acetate (DFEA), and embodiment 17 contains 15% of fluoroethylene carbonate (FEC) and 8% of 2,2-difluoroethyl acetate (DFEA).

[0095] 4) Preparation of lithium-ion batteries

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

[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 below.

[0099] Table 2

[0100]

[0101] The other parameter conditions in the above embodiments 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 differs from Example 1 in that the doping amount z‰ of the sodium element in the positive electrode active material, the silicon content n% in the negative electrode active material layer, the parameter settings (d1, d2) of the concave 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 electrode active layer of the above embodiment, the total weight of the negative electrode active material is ensured to remain unchanged, and the silicon content ratio n% is adjusted by the mass ratio of artificial graphite and silicon-carbon composite material. For example, if the silicon content ratio is 1.5%, artificial graphite and silicon-carbon composite material with a mass ratio of 92.2:2.3 can be used as the negative electrode active material, and so on; other parameter conditions are exactly the same as those in Example 1.

[0108] Experimental example

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

[0110] 1. 3C cycle performance test

[0111] The battery obtained in the embodiment and the comparative example was discharged to 3.0V at 0.5C at 25°C. Then it was charged at 3C constant current to 4.55V, then charged at 4.55V to 0.05C at constant voltage, left to stand for 5 minutes, and then discharged at 3C constant current to 3.0V, which was a charge and discharge cycle. The discharge capacity of the first week of the test was x mAh, and the discharge capacity of the Nth week was y mAh; the capacity of the Nth week was divided by the capacity of the first week to obtain the cycle capacity retention rate R=y / x of the Nth week, and the capacity retention rate of the battery was recorded at 500T of the cycle.

[0112] 2. 5C cycle performance test

[0113] The battery obtained in the embodiment and the comparative example was discharged to 3.0V at 0.5C at 25°C. Then it was charged at 5C constant current to 4.55V, then charged at 4.55V to 0.05C constant voltage, left to stand for 5min, and then discharged at 5C constant current to 3.0V. This is one charge and discharge cycle. The discharge capacity of the first week of the test is x mAh, and the discharge capacity of the Nth week is y mAh; the capacity of the Nth week is divided by the capacity of the first week to obtain the cycle capacity retention rate R=y / x of the Nth week, and the capacity retention rate of the battery is recorded at 500T of the cycle.

[0114] 3. Hot box test

[0115] At room temperature, the battery obtained in the embodiment and comparative example was charged to 4.55V at 1C constant current, left to stand for 60 minutes, and the appearance was checked and photographed. Then the temperature was raised to 132℃±2℃ at a rate of 3℃ / min±2℃ / min and maintained for 60 minutes. The sample was observed, and it was recorded as passing the test if there was no leakage, no smoke, no fire, and no explosion. Ten samples were tested for each embodiment or comparative example, and the pass rate of the hot box performance test was recorded.

[0116] 4. Lithium precipitation degree assessment

[0117] The battery after 500T of 3C cycle was fully charged, and then the lithium deposition on the surface of the negative electrode was observed. The lithium deposition was divided into: A-no lithium deposition, B-slight lithium deposition (lithium deposition on the top, bottom, and crease), and C-severe lithium deposition.

[0118] The above test results of the embodiments of the present invention and the comparative examples are shown in Tables 4 to 6 below.

[0119] Table 4

[0120]

[0121]

[0122] Table 5

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

[0124] Table 6

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

[0126] It can be seen from Tables 4-6 above that by controlling the content of compound A and compound B in the electrolyte to meet the range value requirements of the present invention, the stability of the electrolyte itself and the electrode material under high voltage can be effectively improved through the mutual coordination of compound A and compound B, thereby significantly improving the cycle life and thermal runaway safety issues of the battery under high voltage. Among them, it can be seen from the data in Table 4 that by further reducing the content of compound A, compound B, fluorine-containing compound C and 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 electrode active material layer, and increasing the porosity of the negative electrode sheet, the lithium precipitation can be further reduced.

[0127] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection created by this application.

Claims

1. An electrolyte, characterized in that: Comprising compound A and 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; The structure of the compound A is shown in Formula 1 below: Formula 1: In Formula 1, n=0 or 1, R1, R2, and R3 are independently selected from halogen, halogen-substituted or unsubstituted C1-C10 alkyl, halogen-substituted or unsubstituted C2-C6 alkenyl, and halogen-substituted or unsubstituted C2-C6 alkynyl; The structure of the compound B is shown in Formula 2 below: Formula 2: In Formula 2, R4 is selected from a substituted or unsubstituted C1-C10 alkyl group, a substituted or unsubstituted C1-C10 alkoxy group; R5 is selected from a substituted or unsubstituted C1-C10 alkoxy group, a substituted or unsubstituted C1-C10 carbonate group, a substituted or unsubstituted C2-C10 alkoxy-containing carbonate group, and a substituted or unsubstituted C1-C10 carboxylate group.

2. The electrolyte according to claim 1, characterized in that The compound A comprises at least one of the following structural formulas 1-1 to 1-8: And / or, the compound B comprises at least one of the following structural formulas 2-1 to 2-8:

3. The electrolyte according to claim 1 or 2, characterized in that It also includes a fluorine-containing compound C, wherein the fluorine-containing compound C includes at least one of fluorinated carbonate, fluorinated carboxylic acid ester, and fluorinated ether.

4. The electrolyte according to claim 3, characterized in that The fluorine-containing compound C includes at least one of fluoroethylene carbonate, methyl trifluoroethyl carbonate, fluorodiethyl carbonate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, ethyl 2,2,2-trifluoroacetate, ethyl 2,2-difluoroacetate, and 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, and q satisfies: 2≤q≤20.

5. The electrolyte according to claim 4, characterized in that The x and q satisfy: 3<x+q≤20.

6. A lithium ion battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 5.

7. The lithium-ion battery according to claim 6, characterized in that: The positive electrode sheet includes a positive electrode active material, the positive electrode active material includes lithium cobalt oxide, and the lithium cobalt oxide is doped with sodium element; preferably, the doping amount of the sodium element accounts for z‰ of the total amount of the positive electrode active material, and z satisfies: 0.1<z≤5.

8. The lithium-ion battery according to claim 6, characterized in that: The negative electrode sheet includes a negative electrode active layer, the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a silicon-carbon composite material, wherein the weight proportion 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.

9. The lithium-ion battery according to claim 6, characterized in that: The positive electrode sheet and / or the negative electrode sheet are provided with a plurality of recesses, and the recesses meet at least one of the following conditions: 1) The distance between two adjacent recesses is d1 mm, and d1 satisfies: 0.5≤d1≤10; 2) The depth of the concave portion is d2 μm, and d2 satisfies: 5 ≤ d2 ≤ 60; 3) The size of the concave portion is d3 μm, and d3 satisfies: 30≤d3≤170.

10. An electrochemical device, characterized in that: A lithium ion battery comprising any one of claims 6 to 9.

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