An electrochemical device

Abstract

The application provides an electrochemical device, which satisfies 4xA+C+10xD>=6xB+405; wherein A is the percentage of the mass of a halogen-substituted carbonate compound in the total mass of an electrolyte, and the unit is wt%; B is the percentage of the mass of silicon in the total mass of a negative electrode active material, and the unit is wt%; C is the tensile strength of a negative electrode current collector, and the unit is MPa; and D is the roughness of the negative electrode current collector, and the unit is mu m. The electrochemical device can obviously inhibit the volume expansion of a silicon-based negative electrode, improve the upper limit of the silicon content in the negative electrode active material in the electrochemical device, maintain the capacity of the electrochemical device in the case of high silicon content, and improve the structural stability and safety of the electrochemical device.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical devices, and specifically relates to an electrochemical device, in particular to an electrochemical device containing a silicon-based negative electrode active material. Background Art

[0002] Lithium-ion batteries have become an indispensable part of modern life. With their high efficiency, environmental friendliness, and safety, they offer the most promising solution for meeting society's diverse energy needs. A lithium battery's specific capacity—the amount of energy stored per unit volume or mass—is a key performance metric. A high specific capacity means a lithium battery can store more energy per unit volume or mass, providing devices with longer battery life and greater energy density. This is crucial for modern devices that require long-term operation and efficient energy utilization.

[0003] In lithium-ion batteries, the anode is the primary site for lithium ion insertion and directly determines the battery's specific capacity. Carbon-based materials remain the preferred anode material for commercial lithium-ion batteries, particularly graphite, which is widely used due to its excellent conductivity and crystallinity. Among non-carbon materials, silicon-based materials have garnered significant attention due to their high specific capacity (theoretically ten times that of graphite), high safety, and low intercalation and deintercalation potential.

[0004] Despite theoretical advantages over graphite, silicon-based materials face numerous challenges in practical industrialization, the most significant of which is the volume expansion of silicon-based materials. This volume expansion of the anode material can alter the anode structure, disrupting contact between the electrode material and the current collector, severely impacting battery cycle performance and capacity retention. Furthermore, increased internal stress can alter the battery's overall morphology, damage certain components, and even lead to the precipitation of metallic lithium, posing serious safety concerns. Summary of the Invention

[0005] In order to suppress the volume expansion of silicon-based negative electrodes during cycling, which damages the internal structure of electrochemical devices, resulting in reduced capacity retention of electrochemical devices and affecting the structural stability and safety of electrochemical devices, the present invention provides an electrochemical device. The electrochemical device increases the upper limit of the negative electrode expansion stress through physical and chemical effects, thereby significantly suppressing the volume expansion of the silicon-based negative electrode. This allows the electrochemical device to maintain a high capacity retention rate when having a negative electrode active material with a higher silicon content, and also improves the structural stability and safety of the electrochemical device.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] An electrochemical device, comprising a positive electrode, a negative electrode and an electrolyte; the negative electrode comprises a negative electrode active material layer and a negative electrode current collector, the negative electrode active material layer is arranged on at least one side surface of the negative electrode current collector; the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material comprises a silicon-based negative electrode material; the electrolyte comprises an organic solvent, a lithium salt and an additive, the additive comprises a halogen-substituted carbonate compound;

[0008] The electrochemical device satisfies:

[0009] 4x A+C+10xD>6xB+405;

[0010] wherein A is the percentage of the mass of the halogen-substituted carbonate compound in the total mass of the electrolyte, in wt%; B is the percentage of the mass of the silicon element in the total mass of the negative electrode active material, in wt%; C is the tensile strength of the negative electrode current collector, in MPa; and D is the roughness of the negative electrode current collector, in μm.

[0011] According to the embodiments of the present application, it is found that, with the increasing demand for the capacity of the electrochemical device, the content of the silicon element in the negative electrode active material in the electrochemical device is also increasing, which makes the volume expansion problem of the negative electrode particularly serious. The electrochemical device of the present application comprises a halogen-substituted carbonate compound and a copper foil, and satisfies 4x A+C+10xD>6xB+405, at this time, the combined effect of the halogen-substituted carbonate compound and the copper foil can be fully played, the inhibition effect on the volume expansion of the silicon-based negative electrode is realized from the two aspects of chemical action and physical action, the electrochemical device of the present application can ensure that the silicon element in the negative electrode active material is at a higher content, and the electrochemical device effectively inhibits the volume expansion of the silicon-based negative electrode and improves the cycle capacity retention rate of the electrochemical device under room temperature and high temperature conditions, and the safety performance of the electrochemical device is also improved.

[0012] Specifically, the halogen-substituted carbonate compound can generate polymers and lithium halide through a decomposition reaction in the electrolyte, and at the same time participate in the formation of the SEI film on the surface of the silicon-based negative electrode, and can adjust the structure and composition of the SEI film; the polymers participate in the formation of the SEI film on the surface of the silicon-based negative electrode, so that the flexibility of the obtained SEI film is improved, the lithium halide participates in the formation of the SEI film on the surface of the silicon-based negative electrode, so that the mechanical strength of the obtained SEI film is improved, and the combination of the two can effectively inhibit the volume expansion of the silicon-based negative electrode from the aspect of chemical action.

[0013] Specifically, the negative current collector is a high tensile strength copper foil, which has good stability and moderate roughness, can form a good interaction force with the negative active material layer, and avoid separation of the negative active material layer and the negative current collector. Meanwhile, the copper foil can bear part of the expansion stress of the negative active material due to its high tensile strength, limit the negative expansion from the physical action, and achieve the effect of inhibiting the negative expansion.

[0014] It is found that when the halogen-substituted carbonate compound and the high tensile strength copper foil satisfy 4xA+C+10xD≥6xB+405, the SEI film can remain stable within a certain range, not only the composition of the SEI film is stable, but also the SEI film is not prone to chemical and electrochemical reactions in the battery, and the high tensile strength copper foil also makes the SEI film not prone to physical rupture, so that the whole battery can work stably for a long time, and the capacity retention rate can be maintained at a high level in the later cycle stage, without obvious decrease.

[0015] In addition, the high-strength SEI film and the copper foil in the battery also play a key role in the battery expansion. With the increasing of the cycle number of the battery, both the gas production in the battery and the expansion of the silicon negative electrode will cause the volume expansion of the whole battery. The overall strength and toughness of the battery of the application will be greatly improved, and within a certain range, the stress imparted by the battery expansion is less than the force required to maintain the stability of the battery, so the battery expansion is greatly inhibited and can be maintained at a certain level in the later cycle stage.

[0016] These stable and strong protective forces not only inhibit the battery expansion, but also when the battery is in a relatively active environment, such as under high temperature conditions, the chemical reaction in the battery is more intense, which can cause various safety problems of the battery. The battery of the application has more strain remaining under such extreme conditions, can exist stably for a longer time in a more extreme environment, and thus can avoid some safety problems and improve the safety performance of the battery.

[0017] In summary, the combination of the halogen-substituted carbonate compound and the high tensile strength copper foil can improve the structural stability of the electrochemical device, inhibit the volume expansion of the electrochemical device under room temperature and high temperature conditions, improve the cycle capacity retention rate of the electrochemical device under room temperature and high temperature conditions, and improve the safety performance of the electrochemical device.

[0018] According to the embodiment of the application, when the electrochemical device satisfies 4xA+C+10xD<6xB+405, the improvement effect on the volume expansion of the silicon-based negative electrode is poor, which affects the stable operation of the electrochemical device and has obvious deteriorating effect on the cycle capacity retention rate and safety performance of the electrochemical device.

[0019] According to an embodiment of the present application, the halogen-substituted carbonate compound is a halogen-substituted carbonate-containing compound.

[0020] According to an embodiment of the present application, the halogen-substituted carbonate compound is selected from at least one of the compounds having the structural formula shown in Formula I:

[0021]

[0022] In Formula I, R1, R2, R3, R4 are the same or different, and are independently selected from hydrogen, halogen, alkyl, alkenyl or alkynyl, which are unsubstituted or optionally substituted with one, two or more Ra; each Ra is the same or different, and is independently selected from halogen or C

[0023] In Formula I, R1, R2, R3, R4 are the same or different, and are independently selected from hydrogen, halogen, C 1-12 alkyl, C 2-12 alkenyl or C 2-12 alkynyl, which are unsubstituted or optionally substituted with one, two or more Ra; each Ra is the same or different, and is independently selected from halogen or C 1-12 alkyl, and at least one of R1, R2, R3, R4 contains halogen.

[0024] In Formula I, R1, R2, R3, R4 are the same or different, and are independently selected from hydrogen, halogen, C 1-6 alkyl, C 2-6 alkenyl or C 2-6 alkynyl, which are unsubstituted or optionally substituted with one, two or more Ra; each Ra is the same or different, and is independently selected from halogen or C 1-6 alkyl, and at least one of R1, R2, R3, R4 contains halogen.

[0025] In Formula I, R1, R2, R3, R4 are the same or different, and are independently selected from hydrogen, halogen, C 1-3 alkyl, C 2-3 alkenyl or C 2-3 alkynyl, which are unsubstituted or optionally substituted with one, two or more Ra; each Ra is the same or different, and is independently selected from halogen or C 1-3 alkyl, and at least one of R1, R2, R3, R4 contains halogen.

[0026] According to an embodiment of the present application, the halogen-substituted carbonate compound includes at least one of compounds having structures shown in Formulae II-V:

[0027]

[0028] According to an embodiment of the present application, the halogen-substituted carbonate compound can be obtained by purchasing commercially or prepared by methods known in the art.

[0029] According to an embodiment of the present application, the percentage A of the mass of the halogen-substituted carbonate compound in the total mass of the electrolyte is 8wt%-25wt%, for example, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt% or 25wt%.

[0030] According to an embodiment of the present application, when the percentage A of the mass of the halogen-substituted carbonate compound in the total mass of the electrolyte is 8wt%-25wt%, the halogen-substituted carbonate compound in the electrolyte can undergo a decomposition reaction to generate a polymer and lithium halide, and participate in the formation of the SEI film on the surface of the silicon-based negative electrode, so as to adjust the structure and composition of the SEI film; wherein the polymer participates in the formation of the SEI film on the surface of the silicon-based negative electrode, so as to improve the flexibility of the obtained SEI film, and the lithium halide participates in the formation of the SEI film on the surface of the silicon-based negative electrode, so as to improve the mechanical strength of the obtained SEI film, and the combination of the two can effectively inhibit the volume expansion of the silicon-based negative electrode from the perspective of chemical action. At the same time, the obtained SEI film has high mechanical strength, good stability and certain self-repairing ability, which can effectively inhibit the side reaction between the electrolyte and the electrode active material, reduce the self-discharge and capacity decay of the electrochemical device, and improve the stability of the electrochemical device. The halogen-substituted carbonate compound can also increase the protection effect on copper foil and increase the migration rate of lithium ions, reduce the impedance of the electrochemical device, and significantly improve the charge-discharge performance of the electrochemical device at high rate. The improvement of the stability of the electrochemical device can also reduce the risk of thermal runaway and combustion of the electrochemical device, and improve the safety performance.

[0031] When the percentage A of the mass of the halogen-substituted carbonate compound in the total mass of the electrolyte is greater than 25 wt%, the content of the halogen-substituted carbonate compound is too high, which on one hand leads to the deposition of excessive lithium halide in the SEI film, hinders the embedding and extraction of lithium ions in the negative electrode, and further increases the internal resistance of the battery, reduces the charge and discharge efficiency, and affects the cycle life and energy density of the battery, and on the other hand, the excessive halogen-substituted carbonate compound can also produce a large amount of halogen-containing acidic substances in the electrolyte, which not only destroys the copper foil and the protective film, but also greatly increases the risk of gas production; when the percentage A of the mass of the halogen-substituted carbonate compound in the total mass of the electrolyte is less than 8 wt%, the content of the halogen-substituted carbonate compound is too low, which may not form a complete or strong SEI film, and the interface between the electrolyte and the negative active material is unstable and cannot fully function.

[0032] According to an embodiment of the present application, the percentage B of the mass of the silicon element in the total mass of the negative active material is ≤25 wt%, preferably 2 wt%-20 wt%, for example 2 wt%, 3 wt%, 5 wt%, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 18 wt%, 20 wt%, 22 wt% or 25 wt%. When the percentage B of the mass of the silicon element in the total mass of the negative active material is ≤25 wt%, the volume expansion of the silicon-based negative electrode in the obtained electrochemical device can be well inhibited, and the combination effect of the halogen-substituted carbonate compound and the copper foil can still be fully played after the negative active material is assembled into an electrochemical device, thereby achieving the inhibition effect on the volume expansion of the silicon-based negative electrode from both chemical and physical aspects. When the percentage B of the mass of the silicon element in the total mass of the negative active material is >25 wt%, the expansion of the silicon-based negative electrode is too severe, and the combination of the halogen-substituted carbonate compound and the copper foil gradually weakens the improvement effect on the performance of the silicon-based negative electrode.

[0033] According to an embodiment of the present application, the silicon-based negative electrode material comprises at least one of silicon element, silicon alloy, silicon-oxygen negative electrode material (SiOx (0 < x < 2)) and silicon-carbon negative electrode material.

[0034] According to embodiments of the present application, the silicon-carbon negative electrode material comprises particles formed by mixing silicon or partially oxidized silicon with amorphous or crystalline carbon, the carbon material generally has a more stable structure and a smaller volume change compared to silicon, when silicon expands during charging and discharging, the carbon material can act as a buffer layer to absorb part of the volume change of silicon, thereby slowing down the overall expansion of the negative electrode; and / or, the silicon-carbon negative electrode material comprises particles formed by filling (including partial filling or complete filling) silicon or partially oxidized silicon in the pores of porous amorphous carbon or porous crystalline carbon, the porous carbon structure provides a spatial restriction for silicon, when silicon expands, it is restricted by the surrounding carbon structure, which helps to prevent the breakage of silicon particles and the shedding of silicon particles from the electrode, and the filling of silicon in the pores of carbon can disperse the stress generated during charging and discharging to some extent, thereby reducing the overall expansion of the negative electrode.

[0035] According to embodiments of the present application, the average pore size of the porous amorphous carbon is 1 nm-15 nm, for example, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 12 nm or 15 nm.

[0036] According to embodiments of the present application, the Dv50 of the silicon-carbon negative electrode material is 2 μm-14 μm, for example, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm or 14 μm. When the Dv50 of the silicon-carbon negative electrode material is within this range, the particle size and particle size distribution of the obtained silicon-carbon negative electrode material are suitable, such a silicon-carbon negative electrode material has a shorter lithium ion diffusion path and smaller stress accumulation, which helps to alleviate its own volume expansion. When the Dv50 of the silicon-carbon negative electrode material is <2 μm, the particle size of the obtained silicon-carbon negative electrode material is too small, causing excessive side reactions between the negative electrode active material and the electrolyte, and the side reactions between the silicon-carbon negative electrode material and the electrolyte are more intense, generating more gas and by-products, which will exacerbate the volume expansion of the negative electrode. When the Dv50 of the silicon-carbon negative electrode material is >14 μm, the particle size of the obtained silicon-carbon negative electrode material is too large, increasing the lithium ion diffusion path inside the silicon-carbon negative electrode material, which will deteriorate the cycle performance and rate performance of the electrochemical device, and at the same time, increase the stress accumulation of the silicon-carbon negative electrode material, exacerbating the problem of volume expansion of the silicon-carbon negative electrode material.

[0037] According to embodiments of the present application, the specific surface area of the silicon-carbon negative electrode material is ≤18 m 2 / g, for example, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9m 2 / g, 10m 2 / g, 12m 2 / g, 14m 2 / g, 15m 2 / g, 16m 2 / g, or 18m 2 / g. When the specific surface area of the silicon-carbon negative electrode material is ≤18m 2 / g, the silicon-carbon negative electrode material has more pores and defects, which provides more space for the volume expansion of silicon, to some extent, can buffer the volume change of silicon, and reduce the expansion degree of the negative electrode; at the same time, it can also avoid the violent reaction between the silicon-carbon negative electrode material and the electrolyte, and slow down the volume expansion of the negative electrode. When the specific surface area of the silicon-carbon negative electrode material is >18m 2 / g, the silicon-carbon negative electrode material has more contact area with the electrolyte, which leads to more violent reaction between the silicon-carbon negative electrode material and the electrolyte, and produces more gas and by-products, which will exacerbate the volume expansion of the negative electrode. At the same time, the silicon-carbon negative electrode material with larger specific surface area usually has more pores and defects, which will also lead to the destruction of the negative electrode structure and the rapid decay of the capacity.

[0038] According to the embodiment of the application, the negative electrode active material further comprises a carbon-based negative electrode material.

[0039] According to the embodiment of the application, the carbon-based negative electrode material comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, and soft carbon.

[0040] According to the embodiment of the application, the negative electrode current collector is a copper foil. Illustratively, the material of the copper foil is metal copper or alloy copper, and the alloy copper is, for example, alloy copper doped with alloying elements such as magnesium, silver, chromium, and the mass content of copper elements in the alloy copper is ≥99%. The higher the content of copper in the copper foil, the higher the tensile strength of the obtained copper foil, and the introduction of alloying elements in the copper foil can prevent the oxidation of copper and avoid weakening the performance of the copper foil.

[0041] According to the embodiment of the application, the negative electrode current collector can be obtained by purchasing through commercial channels, or can be prepared by a method known in the art.

[0042] According to an embodiment of the present application, the tensile strength C of the negative current collector is > 400 MPa, preferably 450 MPa-800 MPa, and exemplarily, the tensile strength of the negative current collector is 450 MPa, 500 MPa, 550 MPa, 600 MPa, 650 MPa, 700 MPa, 750 MPa or 800 MPa. When the tensile strength C of the negative current collector is > 400 MPa, the negative current collector can bear part of the expansion stress of the negative active material, limit the expansion of the negative electrode from the perspective of physical action, so as to achieve the effect of inhibiting the expansion of the negative electrode. When the tensile strength C of the negative current collector is ≤ 400 MPa, the tensile strength of the negative current collector is too small to inhibit the volume expansion of the silicon-based negative electrode, and the expansion of the negative electrode cannot be limited from the perspective of physical action. When the tensile strength C of the negative current collector is > 800 MPa, although the tensile strength of the negative current collector is large, the inhibitory effect on the volume expansion of the silicon-based negative electrode is not significantly increased, and the preparation cost of the negative current collector is significantly increased.

[0043] According to an embodiment of the present application, the thickness of the negative current collector is 4 μm-8 μm, and exemplarily, the thickness of the negative current collector is 4 μm, 5 μm, 6 μm, 7 μm or 8 μm. Generally, the thicker the negative current collector, the higher the tensile strength, the thicker negative current collector has a larger cross-sectional area and more material to resist external forces, and exhibits higher tensile strength, thereby facilitating the inhibition of the volume expansion of the negative electrode. When the thickness of the negative current collector is 4 μm-8 μm, a suitable tensile strength can be obtained, and the negative current collector can bear part of the expansion stress of the negative active material, limit the expansion of the negative electrode from the perspective of physical action, so as to achieve the effect of inhibiting the expansion of the negative electrode. When the thickness of the negative current collector is > 8 μm, it will increase the internal resistance of the electrochemical device, reduce the energy density, discharge performance and reaction rate of the electrochemical device, and also increase the preparation cost of the electrochemical device. When the thickness of the negative current collector is < 4 μm, it cannot provide sufficient cross-sectional area and material to resist external forces, thereby exhibiting lower tensile strength, which is insufficient to inhibit the volume expansion of the negative electrode. Therefore, the thickness of the negative current collector is preferably 4 μm-8 μm.

[0044] According to an embodiment of the present application, the roughness D of the negative current collector is 0.1-2.5 μm. For example, the roughness of the negative current collector is 0.1 μm, 0.2 μm, 0.5 μm, 0.7 μm, 1.0 μm, 1.3 μm, 1.5 μm, 1.8 μm or 2.0 μm. Higher roughness can increase the mechanical interlocking force between the negative current collector and the negative active material layer, which can increase the adhesion between them to some extent, so that the negative active material is less likely to fall off or separate from the negative current collector during charging and discharging, increasing the stability of the negative structure. When the roughness D of the negative current collector is 0.1-2.5 μm, it is more appropriate. When the roughness D of the negative current collector is > 2.5 μm, too high roughness can lead to uneven distribution of electrolyte on the surface of the negative current collector, increasing the contact resistance, thereby affecting the charging and discharging performance of the electrochemical device, and can also exacerbate the volume expansion of the negative electrode, leading to the destruction of the negative structure and capacity decay. When the roughness D of the negative current collector is < 0.1 μm, too low roughness cannot provide mechanical interlocking force between the negative active material layer, so that the negative active material is easily separated from the negative current collector during charging and discharging, greatly reducing the stability of the negative structure.

[0045] According to an embodiment of the present application, the negative active material layer further comprises a conductive agent and a binder.

[0046] According to an embodiment of the present application, the mass percentage of each component in the negative active material layer is: 80-99.8 wt% of negative active material, 0.1-10 wt% of conductive agent, 0.1-10 wt% of binder.

[0047] Preferably, the mass percentage of each component in the negative active material layer is: 90-99.6 wt% of negative active material, 0.2-5 wt% of conductive agent, 0.2-5 wt% of binder.

[0048] According to an embodiment of the present application, the lithium salt comprises one or more of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiPO2F2), lithium difluoro oxalate borate (LiDFOB), lithium bisfluorosulfonylimide (LiTFSI), lithium bis-trifluoromethylsulfonylimide, lithium difluorobisoxalate phosphate, lithium tetrafluoroborate, lithium bisoxalate borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methide or lithium bis(trifluoromethylsulfonyl)imide.

[0049] Preferably, the electrolyte lithium salt comprises one or more of lithium difluoro oxalate borate (LiDFOB), lithium bisfluorosulfonylimide (LiTFSI) and lithium difluorobisoxalate phosphate.

[0050] The research finds that lithium salts such as lithium difluoro(oxalato)borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiTFSI) and lithium difluorophosphate bis(oxalate) have good film-forming ability, can be combined with the halogen-substituted carbonate compound of the application and the high-tensile-strength negative electrode current collector to form a SEI film with improved fluorine content on the surface of the negative electrode, the improvement of the fluorine content can improve the physical strength of the SEI film on the negative electrode interface, avoid the destruction of the negative electrode structure caused by the deformation of the negative electrode active material within a certain range, and improve the oxidation resistance of the SEI film formed on the surface of the negative electrode, thereby inhibiting the destruction of the SEI film formed on the surface of the negative electrode from the chemical aspect and further improving the effect of inhibiting the volume expansion of the negative electrode.

[0051] According to the embodiment of the application, the mass percentage of one or more of lithium difluoro(oxalato)borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiTFSI) and lithium difluorophosphate bis(oxalate) in the total mass of the electrolyte is 0.5-8wt%; when the mass percentage of one or more of lithium difluoro(oxalato)borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiTFSI) and lithium difluorophosphate bis(oxalate) in the total mass of the electrolyte is within this range, the cost, cycle performance and inhibition of negative electrode expansion of the electrochemical device can be considered.

[0052] According to the embodiment of the application, the organic solvent is selected from fluorinated or non-fluorinated carbonates and / or carboxylic acid esters, the carbonates are selected from one or more of the following solvents: propylene carbonate (PC), vinyl carbonate (EC), dimethyl carbonate, diethyl carbonate (DEC), methyl ethyl carbonate; the carboxylic acid esters are selected from one or more of the following solvents: ethyl acetate (EA), propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isoamyl acetate, propyl propionate (PP), ethyl propionate (EP), methyl propionate (MP), methyl butyrate, ethyl n-butyrate.

[0053] According to the embodiment of the application, the electrochemical device is a battery, for example, a lithium ion battery, such as a lithium ion secondary battery.

[0054] According to the embodiment of the application, the electrolyte is a non-aqueous electrolyte.

[0055] According to the embodiment of the application, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer coated on one side or both sides of the positive electrode current collector, and the positive electrode active material layer comprises a positive electrode active material, a conductive agent and a binder.

[0056] According to an embodiment of the present application, the mass percentage of each component in the positive electrode active material layer is: 80-99.8wt% of positive electrode active material, 0.1-10wt% of conductive agent, and 0.1-10wt% of binder.

[0057] Preferably, the mass percentage of each component in the positive electrode active material layer is: 90-99.6wt% of positive electrode active material, 0.2-5wt% of conductive agent, and 0.2-5wt% of binder.

[0058] According to an embodiment of the present application, the conductive agent includes at least one of conductive carbon black, acetylene black, ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, and metal powder.

[0059] According to an embodiment of the present application, the binder includes at least one of sodium carboxymethyl cellulose, styrene butadiene latex, polytetrafluoroethylene, and polyethylene oxide.

[0060] According to an embodiment of the present application, the positive electrode active material includes one or more of transition metal lithium oxide, polyanion compound, and sulfide. The transition metal lithium oxide has a chemical formula of Li 1+x Ni y Co z M k O2, wherein -0.1≤x≤1; 0≤y≤1, 0≤z≤1, and 0≤k≤1; wherein M is one or more of Mg, Zn, Ga, Ba, Al, Fe, Cr, Sn, V, Mn, Sc, Ti, Nb, Mo, Zr; the polyanion compound has a chemical formula of LiMX, wherein M is a metal element such as Fe, Mn, Co, Ni, etc.; and X is an anion group such as PO4 3- , SiO4 4- , BO3 3- , etc.; and the sulfide includes but is not limited to lithium sulfide Li2S.

[0061] According to an embodiment of the present application, the electrochemical device further includes a separator film.

[0062] According to an embodiment of the present application, the additive further includes nitrile compound, sulfur-containing additive, and fluoro compound.

[0063] According to an embodiment of the present application, the nitrile compound includes at least one of 1,3,6-hexanetricarbonitrile (HTCN), adiponitrile (ADN), succinonitrile (SN), and ethylene glycol bis(propionitrile) ether (DENE). The nitrile compound can coordinate with the positive electrode, improve the stability of the positive electrode interface, and improve the high-voltage performance of the electrochemical device.

[0064] According to an embodiment of the present application, the electrochemical device satisfies:

[0065] F≤6.5H-20;

[0066] wherein F is the mass percentage of the nitrile compound in the total mass of the electrolyte, in wt%; and H is the thickness of the negative current collector, in μm.

[0067] It is found that when the electrochemical device satisfies F≤6.5H-20, the nitrile compound can be matched with the positive electrode, so that the electrochemical device has higher positive electrode film forming performance and improved electrical performance at high voltage, and has little effect on the film forming performance of the negative electrode, ensuring that the electrochemical device still has good cycle capacity retention rate and safety performance under the condition of high voltage and high silicon content. When the electrochemical device satisfies F>6.5H-20, the content of the nitrile compound is too high, and the negative electrode film forming ability of the nitrile compound is poor, which will cause the deterioration of the negative electrode volume expansion performance; or the copper foil is too thin to resist the volume expansion of the silicon negative electrode, which will cause the deterioration of the negative electrode volume expansion performance.

[0068] According to the embodiments of the present application, the mass percentage of the nitrile compound in the total mass of the electrolyte is 2-8 wt%, and exemplarily, the mass percentage of the nitrile compound in the total mass of the electrolyte is 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt% or 8 wt%.

[0069] According to the embodiments of the present application, the fluorinated compound includes but is not limited to methyl trifluoroethyl carbonate (FEMC), fluorinated 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, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE). The fluorinated compound itself has good oxidation resistance, strong stability to the positive electrode, and can also form a film on the negative electrode, reducing the negative electrode interface side reaction.

[0070] According to the embodiments of the present application, the mass percentage of the fluorinated compound in the total mass of the electrolyte is 2-8 wt%, and exemplarily, the mass percentage of the fluorinated compound in the total mass of the electrolyte is 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt% or 8 wt%.

[0071] According to an embodiment of the present invention, the sulfur-containing additive includes, but is not limited to, at least one of thiosulfates, sulfates, thioethers, disulfides, thiols, thiophenols, thioureas, thioamides, thiocarboxylic acids and their salts, thiophosphoric acids and their salts, organic sulfide polymers, and sulfur-containing organic small molecules. The sulfur-containing additive can play a variety of roles in the electrolyte, such as increasing electrical conductivity, stabilizing electrode materials, and improving battery performance.

[0072] According to an embodiment of the present invention, the mass percentage of the sulfur-containing additive to the total mass of the electrolyte is 2-8wt%. Exemplarily, the mass percentage of the sulfur-containing additive to the total mass of the electrolyte is 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt% or 8wt%.

[0073] Beneficial effects of the present invention:

[0074] The present invention provides an electrochemical device that can significantly suppress the volume expansion of a silicon-based negative electrode, increase the upper limit of the silicon content in the negative electrode active material in the electrochemical device, maintain the capacity of the electrochemical device even at a high silicon content, and improve the structural stability and safety of the electrochemical device. DETAILED DESCRIPTION

[0075] The present invention will be described in further detail below with reference to specific examples. It should be understood that the following examples are merely illustrative and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are encompassed within the scope of protection that the present invention is intended to protect.

[0076] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the reagents, materials, etc. used in the following examples are all commercially available unless otherwise specified.

[0077] The lithium-ion batteries of the examples and comparative examples shown in Table 1 below were prepared using the following method:

[0078] 1) Preparation of positive electrode sheet

[0079] The positive electrode active materials lithium cobalt oxide (LiCoO2), polyvinylidene fluoride (PVDF), SP (superP) 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 an aluminum foil; the coated aluminum foil is dried, and then rolled and cut to obtain the required positive electrode sheet.

[0080] 2) Negative electrode preparation

[0081] The negative active material (artificial graphite and silicon-carbon negative material, wherein the mass percentage of silicon element in the total mass of the negative active material is Bwt%, and the particle size of the silicon-carbon negative material is 5 μm, and the specific surface area is 5 m 2 The negative active material (artificial graphite and silicon-carbon negative material, wherein the mass percentage of silicon element in the total mass of the negative active material is Bwt%, and the particle size of the silicon-carbon negative material is 5 μm, and the specific surface area is 5 m

[0082] 3) Preparation of the separator

[0083] A polyethylene separator with a particle size of 7-9 μm was selected.

[0084] 4) Preparation of the electrolyte

[0085] In an argon-filled glove box (H2O<0.1 ppm, O2<0.1 ppm), EC / PC / DEC / PP were mixed uniformly at a mass ratio of 15:25:20:40. Then, 14wt% of fully dried lithium hexafluorophosphate (LiPF6) based on the total mass of the electrolyte was quickly added, and after dissolution, Awt% of the compound shown in Formula I (the specific selection and addition amount are shown in Table 1) was added based on the total mass of the electrolyte. Then, 2wt% of HTCN and 1wt% of ADN based on the total mass of the electrolyte were added. After stirring uniformly, the obtained electrolyte was subjected to water and free acid detection to obtain the required electrolyte.

[0086] 5) Preparation of the lithium ion battery

[0087] The positive electrode sheet of step 1), the negative electrode sheet of step 2), and the separator of step 3) 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, and the electrolyte of step 4) was injected into the outer packaging. 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 3.0-4.5V.

[0088] The lithium ion batteries obtained in the examples and comparative examples were subjected to performance tests, respectively.

[0089] 25℃ cycle performance test

[0090] The batteries prepared in the examples and comparative examples were subjected to charge-discharge cycling at 25°C within the charge-discharge cut-off voltage range at a rate of 1C for 500 cycles, the discharge capacity of the first week was tested to be x1 mAh, and the thickness of the lithium ion battery was y1 cm; the discharge capacity of the 500th week was x2 mAh, and the thickness of the lithium ion battery after the 500th week was y2 cm. The capacity of the 500th week was divided by the capacity of the first week to obtain the cycle capacity retention rate R1 of the 500th week = x2 / x1; the thickness of the lithium ion battery after the 500th week was subtracted from the thickness of the first week, and the thickness of the battery after the 500th week was divided by the thickness of the first week to obtain the thickness expansion rate H1 of the battery after the 500th week = (y2-y1) / y1.

[0091] 45°C cycle performance test

[0092] The batteries prepared in the examples and comparative examples were subjected to charge-discharge cycling at 45°C within the charge-discharge cut-off voltage range at a rate of 1C for 500 cycles, the discharge capacity of the first week was tested to be x3 mAh, and the thickness of the lithium ion battery was y3 cm; the discharge capacity of the 500th week was x4 mAh, and the thickness of the lithium ion battery after the 500th week was y4 cm. The capacity of the 500th week was divided by the capacity of the first week to obtain the cycle capacity retention rate R2 of the 500th week = x4 / x3; the thickness of the lithium ion battery after the 500th week was subtracted from the thickness of the first week, and the thickness of the battery after the 500th week was divided by the thickness of the first week to obtain the thickness expansion rate H2 of the battery after the 500th week = (y4-y3) / y3.

[0093] 75°C cycle safety test

[0094] The batteries prepared in the examples and comparative examples were subjected to charge-discharge cycling at 75°C within the charge-discharge cut-off voltage range at a rate of 1C for 20 cycles, and whether the battery caught fire was observed and recorded.

[0095] Tensile strength test of negative electrode current collector:

[0096] Two samples were taken from the negative electrode current collector roll width in the horizontal and vertical directions, and cut into strips with a length of 200±0.5 mm and a width of 15±0.25 mm. The distance between the clamping heads of the testing machine was 125±0.1 mm, and the gauge length was 50 mm. The tensile test was carried out at a speed of 50 mm / min, and the tensile strength was obtained from the electronic universal testing machine.

[0097] Roughness test of negative electrode current collector:

[0098] The negative electrode current collector was fixed on the measurement table to ensure that the sample surface was flat and free of impurities. The scanning parameters were set in the microscope software, including the scanning range and point spacing, etc., to ensure that accurate roughness data was obtained. The microscope was started to scan, and after the scanning was completed, the data was processed using the microscope software, including noise reduction, smoothing, etc., to obtain the accurate roughness value of the negative electrode current collector.

[0099] Table 1: Composition of electrolyte in the battery of examples and comparative examples and characterization test results of copper foil

[0100]

[0101]

[0102] Table 2: Performance test results of the battery of examples and comparative examples

[0103]

[0104]

[0105] As can be seen from the above Table 2, when the electrochemical device satisfies 4xA+C+10xD≥6xB+405, the combined effect of the halogen-substituted carbonate compound and the copper foil can be fully exerted at this time, and the inhibition effect on the volume expansion of the silicon-based negative electrode is realized from two aspects of chemical action and physical action. Compared with the inhibition effect of the halogen-substituted carbonate compound or the copper foil on the volume expansion of the silicon-based negative electrode alone, the electrochemical device of the present application can ensure that the silicon element in the negative active material is at a higher content, and the electrochemical device can still operate stably and safely, and the cycle capacity retention rate of the electrochemical device is also improved under the combined effect of the halogen-substituted carbonate compound and the copper foil.

[0106] The lithium ion battery of the example shown in the following Table 3 is prepared by the following method:

[0107] Other operations are the same as those in Example 1.6, except that the thickness of the copper foil and the amount of the added nitrile compound in the electrolyte are different, and the specific values are shown in Table 3.

[0108] Table 3: Composition of electrolyte in the battery of examples and comparative examples and characterization test results of copper foil

[0109]

[0110] Table 4: Performance test results of the battery of examples and comparative examples

[0111]

[0112] As can be seen from Table 4, when the electrochemical device satisfies F≤6.5H-20, the nitrile compound can be matched with the positive electrode, so that the electrochemical device has high positive electrode film-forming performance while improving the electrical performance of the electrochemical device at high voltage, and has little effect on the film-forming performance of the negative electrode, ensuring that the electrochemical device still has good cycle capacity retention and safety performance under conditions of high voltage and silicon content. When the electrochemical device satisfies F>6.5H-20, due to the excessive content of the nitrile compound and the poor negative electrode film-forming ability of the nitrile compound, the negative electrode volume expansion performance deteriorates; or because the copper foil thickness is too thin, it cannot resist the volume expansion of the silicon negative electrode, resulting in deterioration of the negative electrode volume expansion performance.

[0113] The lithium-ion batteries of the embodiments shown in Table 5 below were prepared using the following method:

[0114] Other operations are the same as in Example 1.6, except that:

[0115] The particle size and specific surface area of ​​silicon-carbon negative electrode materials are different, as shown in Table 5.

[0116] Table 5 Characterization test results of silicon-carbon negative electrode materials in batteries of Examples and Comparative Examples

[0117]

[0118]

[0119] Table 6 Performance test results of batteries of Examples and Comparative Examples

[0120]

[0121] As can be seen from Table 6, when the Dv50 of the silicon-carbon negative electrode material is 2μm-14μm, the particle size and particle size distribution of the silicon-carbon negative electrode material are appropriate. Such a silicon-carbon negative electrode material has a shorter lithium ion diffusion path and smaller stress accumulation, which helps to reduce its own volume expansion. When the Dv50 of the silicon-carbon negative electrode material is less than 2μm, the particle size of the obtained silicon-carbon negative electrode material is too small, resulting in excessive side reactions between the negative electrode active material and the electrolyte, and the side reactions between the silicon-carbon negative electrode material and the electrolyte are more intense, producing more gas and byproducts, which will aggravate the volume expansion of the negative electrode. When the Dv50 of the silicon-carbon negative electrode material is greater than 14μm, the particle size of the obtained silicon-carbon negative electrode material is too large, which increases the lithium ion diffusion path inside the silicon-carbon negative electrode material, deteriorates the cycle performance and rate performance of the electrochemical device, and increases the stress accumulation of the silicon-carbon negative electrode material, aggravating the problem of volume expansion of the silicon-carbon negative electrode material.

[0122] The specific surface area of ​​the silicon-carbon negative electrode material is ≤18m 2When the specific surface area of the silicon-carbon negative electrode material is greater than 18 m 2 When the specific surface area of the silicon-carbon negative electrode material is greater than 18 m

[0123] The above describes the embodiments of the present application. However, the present application is not limited to the above-described embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An electrochemical device, the electrochemical device comprising a positive electrode, a negative electrode and an electrolyte; the negative electrode comprises a negative electrode active material layer and a negative electrode current collector, the negative electrode active material layer being disposed on at least one surface of the negative electrode current collector; the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material comprising a silicon-based negative electrode material; the electrolyte comprises an organic solvent, a lithium salt and an additive, the additive comprising a halogen-substituted carbonate compound; The electrochemical device satisfies: 4×A + C + 10×D ≥ 6×B + 405; in, A is the percentage by mass of the halogen-substituted carbonate compound in the total mass of the electrolyte, in wt%; B is the percentage by mass of silicon element in the negative electrode active material in the total mass of the negative electrode active material, in wt%; C is the tensile strength of the negative electrode current collector, in MPa; D is the roughness of the negative electrode current collector, in µm; The percentage by mass A of the halogen-substituted carbonate compound in the total mass of the electrolyte is 8 wt% - 25 wt%; the percentage by mass B of silicon element in the negative electrode active material in the total mass of the negative electrode active material is ≤ 25 wt%; the tensile strength C of the negative electrode current collector is 450 MPa - 800 MPa; the roughness D of the negative electrode current collector is 0.1 - 2.5 µm.

2. The electrochemical device according to claim 1, wherein The halogen-substituted carbonate compound is selected from at least one of the compounds having the structural formula shown in Formula I: Formula I In Formula I, R1, R2, R3, R4 are the same or different, and are independently selected from hydrogen, halogen, an alkyl group, an alkenyl group or an alkynyl group which is unsubstituted or optionally substituted by one, two or more Ra; each Ra is the same or different, and is independently selected from halogen or an alkyl group, and at least one of the groups R1, R2, R3, R4 contains halogen.

3. The electrochemical device according to claim 2, wherein In formula I, R1, R2, R3, and R4 are the same or different and are independently selected from hydrogen, halogen, C which is unsubstituted or optionally substituted by one, two or more Ra. 1-12 Alkyl, C 2-12 Alkenyl or C 2-12 Alkynyl; each Ra is the same or different and is independently selected from halogen or C 1-12 Alkyl, and at least one of the R1, R2, R3, and R4 groups contains halogen.

4. The electrochemical device according to claim 3, wherein In formula I, R1, R2, R3, and R4 are the same or different and are independently selected from hydrogen, halogen, C which is unsubstituted or optionally substituted by one, two or more Ra. 1-6 Alkyl, C 2-6 Alkenyl or C 2-6 Alkynyl; each Ra is the same or different and is independently selected from halogen or C 1-6 Alkyl, and at least one of the R1, R2, R3, and R4 groups contains halogen.

5. The electrochemical device according to claim 4, wherein In formula I, R1, R2, R3, and R4 are the same or different and are independently selected from hydrogen, halogen, C which is unsubstituted or optionally substituted by one, two or more Ra. 1-3 Alkyl, C 2-3 Alkenyl or C 2-3 Alkynyl; each Ra is the same or different and is independently selected from halogen or C 1-3 Alkyl, and at least one of the R1, R2, R3, and R4 groups contains halogen.

6. The electrochemical device according to claim 5, wherein The halogen-substituted carbonate compound comprises at least one of the compounds having the structures shown in Formula II to Formula V: Formula II Formula III Formula IV Formula V.

7. The electrochemical device according to claim 1, wherein The silicon-based negative electrode material comprises at least one of silicon单质, a silicon alloy, a silicon oxide negative electrode material SiOx, where 0 < x < 2, and a silicon-carbon negative electrode material.

8. The electrochemical device according to any one of claims 1 to 7, wherein: The additive further comprises a nitrile compound, the nitrile compound comprising at least one of 1,3,6-hexanetricarbonitrile (HTCN), adiponitrile (ADN), succinonitrile (SN) and ethylene glycol bis(propionitrile) ether (DENE).

9. The electrochemical device according to claim 8, wherein The electrochemical device satisfies: F ≤ 6.5H - 20; wherein, F is the percentage by mass of the nitrile compound in the total mass of the electrolyte, in wt%; H is the thickness of the negative electrode current collector, in µm.

10. The electrochemical device according to claim 9, wherein The percentage by mass of the nitrile compound in the total mass of the electrolyte is 2 - 8 wt%; and / or, the thickness of the negative electrode current collector is 4 µm - 8 µm.

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