Electrochemical devices and electronic devices

Abstract

The application provides an electrochemical device and an electronic device. The electrochemical device comprises a negative electrode sheet and an electrolyte, the negative electrode sheet comprises a negative electrode active material layer, the negative electrode active material layer comprises silicon-based material particles and a binder, the negative electrode sheet satisfies: (B+C) / 7A<(B+C) / 1.8, wherein A represents the mass percentage loss rate of the negative electrode active material layer of the negative electrode sheet when heated to 480 DEG C under an Ar atmosphere; B represents the relative percentage content fluctuation value of silicon elements in the silicon-based material particles; C represents the content of silicon elements in the negative electrode sheet in mass percentage, wherein 1.5%≤A≤18%, 1%≤C≤20%, the electrolyte comprises fluoroethylene carbonate, and the content X of fluoroethylene carbonate in mass percentage and the content C of silicon elements in the negative electrode sheet in mass percentage satisfy: X≥C. By making the negative electrode sheet satisfy (B+C) / 7A<(B+C) / 1.8, the cycle performance, rate performance and expansion performance of the electrochemical device can be significantly improved.

Description

[0001] This application is a divisional application of application No. 202180031196.9, titled "Electrochemical device and electronic device", with a filing date of December 24, 2021. TECHNICAL FIELD

[0002] The present application relates to the field of electrochemical energy storage, and in particular to an electrochemical device and an electronic device. BACKGROUND

[0003] With the development and progress of electrochemical devices (e.g., lithium ion batteries), there are increasingly high requirements for the cycle performance and energy density of the electrochemical devices. At present, in terms of improving the energy density of the electrochemical devices, the use of silicon-based materials in the negative electrode sheet is the current trend. However, in the process of lithium deintercalation of the silicon-based material, there is a large volume expansion and shrinkage, forming a large amount of new solid electrolyte interphase film (SEI), consuming the limited lithium ions and electrolyte in the electrochemical device, significantly increasing the impedance of the electrochemical device, and hindering the industrial large-scale application of the silicon-based material. SUMMARY

[0004] Some embodiments of the present application provide an electrochemical device, the electrochemical device comprising a negative electrode sheet and an electrolyte, the negative electrode sheet comprising a negative electrode active material layer, the negative electrode active material layer comprising silicon-based material particles and a binder, the negative electrode sheet satisfying: (B+C) / 7<A<(B+C) / 1.8, wherein A represents the mass percentage loss rate of the negative electrode active material layer of the negative electrode sheet when heated to 480°C under an Ar atmosphere; B represents the relative percentage content fluctuation value of silicon elements within the silicon-based material particles; C represents the content of silicon elements in the negative electrode sheet in mass percentage, wherein 1.5%≤A≤18%, 1%≤C≤20%, the electrolyte comprising fluoroethylene carbonate, and the content X of the fluoroethylene carbonate in mass percentage and the content C of the silicon elements in the negative electrode sheet in mass percentage satisfy: X≥C. By making the negative electrode sheet satisfy (B+C) / 7<A<(B+C) / 1.8, the cycle performance, rate performance and expansion performance of the electrochemical device can be significantly improved.

[0005] In some embodiments, the loss rate A of the negative electrode active material layer of the negative electrode sheet when heated to 480°C under an Ar atmosphere satisfies: 1.5%≤A≤18%. If the value of A is too large, it indicates that there is too much binder in the negative electrode active material layer, which will inhibit the transmission of lithium ions, increase the polarization of the electrochemical device, and deteriorate the rate performance of the electrochemical device; if the value of A is too small, it indicates that there is too little binder in the negative electrode active material layer, which is not conducive to alleviating the volume expansion of the silicon-based material during the cycle process, and deteriorating the cycle performance of the electrochemical device.

[0006] In some embodiments, the relative percentage fluctuation value B of the silicon element within the silicon-based material particles satisfies: B < 16%. If B is too large, it indicates poor uniformity of the silicon element, which is not conducive to improving the cycling performance and expansion performance of the electrochemical device. In some embodiments, B ≤ 10%.

[0007] In some embodiments, the silicon content (C) in the negative electrode sheet, expressed as a mass percentage, satisfies the following conditions: 1% ≤ C ≤ 20%. If the value of C is too high, the silicon-based material will expand excessively during cycling, hindering the improvement of the cycling performance of the electrochemical device. If the value of C is too low, it will not be conducive to increasing the energy density of the electrochemical device. In some embodiments, 1% ≤ C ≤ 10%.

[0008] In some embodiments, the porosity P of the negative electrode sheet and the mass percentage C of the silicon element in the negative electrode sheet satisfy: P ≥ 15 × C 1 / 4 When the silicon content C in the negative electrode is larger in mass percentage, more pores are needed to alleviate the expansion of the silicon-based material. 1 / 4 When the volume expansion of silicon-based materials is reduced, it can effectively alleviate the volume expansion of silicon-based materials.

[0009] In some embodiments, the porosity P of the negative electrode sheet satisfies the following: 18% ≤ P ≤ 40%. When the porosity of the negative electrode sheet is too low, the electrolyte cannot fully penetrate the sheet, increasing the transmission distance of lithium ions, deteriorating the kinetic performance of the electrochemical device, and hindering the expansion of the silicon-based material during cycling. When the porosity of the negative electrode sheet is too high, it is not conducive to improving the energy density of the electrochemical device.

[0010] In some embodiments, the content X of fluoroethylene carbonate in mass percentage and the content C of silicon in the negative electrode sheet in mass percentage satisfy: X ≥ C. When the FEC content in the electrolyte is greater than or equal to the silicon content in the negative electrode sheet, the electrochemical device has better cycling performance, expansion performance, and rate performance.

[0011] In some embodiments, the content X of fluoroethylene carbonate, expressed as a percentage by mass, satisfies the following: 2% ≤ X ≤ 20%. When the value of X is too small, the effect of fluoroethylene carbonate on improving the cycling performance of the electrochemical device is relatively limited. When the value of X is too large, the FEC content is too high. Excessive FEC can reduce the mobility of lithium ions in the electrolyte, affecting the rate performance of the electrochemical device. In some embodiments, 2% ≤ X ≤ 10%.

[0012] In some embodiments, the silicon-based material particles include silicon and carbon. By embedding the silicon-based particles in the carbon matrix, direct contact between the silicon-based particles and the electrolyte is avoided, and the repeated formation of SEI during cycling is reduced, thereby reducing reversible lithium loss.

[0013] Embodiments of the present application also provide an electronic device comprising the above-mentioned electrochemical device.

[0014] Embodiments of the present application can limit the expansion of the silicon-based material within the range that the electrode sheet can withstand by making the negative electrode sheet satisfy (B+C) / 7<A<(B+C) / 1.8 from the perspective of the sheet layer, significantly improving the expansion deformation performance of the electrochemical device; from the perspective of the material layer, reducing the lithium intercalation expansion of the silicon-based material, reducing the repeated generation and destruction of SEI on the surface of the silicon-based particles, and significantly improving the cycle performance and rate performance of the electrochemical device. DETAILED DESCRIPTION

[0015] The following examples can enable those skilled in the art to more fully understand the present application, but in no way limit the present application.

[0016] Some embodiments of the present application provide an electrochemical device, the electrochemical device comprising a negative electrode sheet and an electrolyte. In some embodiments, the negative electrode sheet comprises a negative electrode active material layer, the negative electrode active material layer comprising silicon-based material particles and a binder. In some embodiments, the binder can bond the silicon-based material particles together, reducing the expansion and contraction degree of the silicon-based material particles during the cycle process.

[0017] In some embodiments, the negative electrode sheet satisfies: (B+C) / 7 < A < (B+C) / 1.8, where A represents the weight loss rate of the negative electrode active material layer of the negative electrode sheet in mass percent when heated to 480°C under an Ar atmosphere; B represents the relative percentage content fluctuation value of silicon elements within the silicon-based material particles; and C represents the content of silicon elements in the negative electrode sheet in mass percent. In some embodiments, the value of A can reflect the content of the binder to some extent, the larger A is, the more the amount of the binder in the negative electrode active material layer is, and the smaller A is, the less the amount of the binder in the negative electrode active material layer is. In order to reduce the deformation of the electrochemical device during the cycle process, there can be enough binder in the negative electrode sheet, but too much binder will inhibit the lithium ion transmission, increase the polarization of the electrochemical device, and deteriorate the rate performance of the electrochemical device. In some embodiments, the relative percentage content fluctuation value of silicon elements within the silicon-based material particles represents the uniformity of the distribution of silicon elements, the larger the fluctuation value is, the worse the uniformity is. The uniformity of the distribution of silicon elements has a significant correlation with the cycle performance and the swelling performance of the electrochemical device, the higher the uniformity is, the more conducive to reducing the stress generated by the swelling of the lithium ion battery, and the better the cycle performance and the swelling performance of the electrochemical device are. In some embodiments, generally, the content of silicon elements in the negative electrode sheet in mass percent increases, and the energy density of the electrochemical device can be correspondingly improved. By making the negative electrode sheet satisfy (B+C) / 7 < A < (B+C) / 1.8, the cycle performance, the rate performance, and the swelling performance of the electrochemical device can be significantly improved.

[0018] In some embodiments, the weight loss rate A of the negative electrode active material layer of the negative electrode sheet when heated to 480°C under an Ar atmosphere satisfies: 1.5% ≤ A ≤ 18%. If the value of A is too large, it indicates that there is too much binder in the negative electrode active material layer, which will inhibit the lithium ion transmission, increase the polarization of the electrochemical device, and deteriorate the rate performance of the electrochemical device; if the value of A is too small, it indicates that there is too little binder in the negative electrode active material layer, which is not conducive to alleviating the volume expansion of the silicon-based material during the cycle process and deteriorating the cycle performance of the electrochemical device. In some embodiments, A can be 1.5%, 5%, 8%, 12%, 15%, 18%, or any other suitable value.

[0019] In some embodiments, the relative percentage content fluctuation value B of silicon elements within the silicon-based material particles satisfies: B < 16%. If B is too large, it indicates that the uniformity of silicon elements is poor, which is not conducive to the improvement of the cycle performance and the swelling performance of the electrochemical device. In some embodiments, B can be 15%, 10%, 8%, 5%, or a smaller value. Preferably, B ≤ 10%.

[0020] In some embodiments, the content C of silicon element in the negative electrode tab in mass percentage satisfies: 1%≤C≤20%. In some embodiments, if the value of C is too large, the expansion of the silicon-based material during the cycle process is too large, which is not conducive to the improvement of the cycle performance of the electrochemical device; if the value of C is too small, it is not conducive to the improvement of the energy density of the electrochemical device. In some embodiments, C can be 1%, 5%, 10%, 15%, 20% or any other suitable value. Preferably, 1%≤C≤10%.

[0021] In some embodiments, the porosity P of the negative electrode tab and the mass percentage content C of silicon element in the negative electrode tab satisfy: P≥15×C 1 / 4 The volume of the silicon-based material particles expands by about 300% after lithium intercalation at room temperature, and the huge volume expansion effect easily leads to problems such as negative electrode tab demolding and powder falling, and reserving certain porosity in the negative electrode tab can effectively alleviate the volume expansion of the silicon-based material. When the content C of silicon element in the negative electrode tab in mass percentage is larger, more porosity is needed to alleviate the expansion of the silicon-based material. When P≥15×C 1 / 4 , the volume expansion of the silicon-based material can be effectively alleviated.

[0022] In some embodiments, the porosity P of the negative electrode tab satisfies: 18%≤P≤40%. When the porosity of the negative electrode tab is too low, the electrolyte is difficult to fully infiltrate, increasing the transmission distance of lithium ions, deteriorating the kinetic performance of the electrochemical device, and also not conducive to alleviating the expansion of the silicon-based material during the cycle process. When the porosity of the negative electrode tab is too large, it is not conducive to the improvement of the energy density and kinetic performance of the electrochemical device.

[0023] In some embodiments, the electrolyte is reduced at the negative electrode tab, generating an SEI layer on the surface of the negative electrode active material, stabilizing the interface and delaying the continuous consumption of reversible lithium. If the SEI is too thin, it is easy to break during the expansion of the negative electrode active material particles, generating a new interface and deteriorating the cycle performance of the electrochemical device; if the SEI is too thick, it will reduce the rate of charge transfer and deteriorate the rate performance of the electrochemical device. In some embodiments, fluoroethylene carbonate is included in the electrolyte, and the content X of fluoroethylene carbonate in the electrolyte in mass percentage and the content C of silicon element in the negative electrode tab in mass percentage satisfy: X≥C. Fluoroethylene carbonate (FEC) is an important film-forming additive in the electrolyte, and the SEI generated by its decomposition during the cycle process isolates the negative electrode active material and the electrolyte from further contact, reduces the consumption of lithium ions, and has an improvement effect on the cycle performance. When the content of FEC in the electrolyte is greater than or equal to the content of silicon element in the negative electrode tab, the cycle performance, expansion performance and rate performance of the electrochemical device are better.

[0024] In some embodiments, the content X of fluoroethylene carbonate in terms of mass percentage satisfies: 2%≤X≤20%. When the value of X is too small, the improvement effect of fluoroethylene carbonate on the cycle performance of the electrochemical device is relatively limited; when the value of X is too large, it indicates that the FEC content is too high, and too much FEC will reduce the migration rate of lithium ions in the electrolyte, affecting the rate performance of the electrochemical device. In some embodiments, X can be 2%, 5%, 10%, 12%, 15%, 20% or other suitable values. Preferably, 2%≤X≤10%.

[0025] In some embodiments, the silicon-based material particles include silicon elements and carbon elements, for example, silicon-carbon compounds or composites, for example, SiC. By embedding the silicon-based particles in the carbon matrix, direct contact between the silicon-based particles and the electrolyte is avoided, and the repeated generation of SEI during the cycle is reduced, thereby reducing the loss of reversible lithium.

[0026] In some embodiments, the preparation method of the silicon-based material can adopt the following preparation method: placing the porous carbon matrix in a rotary furnace, purging the furnace tube with nitrogen at room temperature for 20 to 40 minutes, and then increasing the temperature of the porous carbon matrix sample to 450°C to 500°C. The nitrogen flow rate is adjusted so that the residence time of the gas in the rotary furnace is at least 90 seconds, and the flow rate is maintained for about 30 minutes. Then, the gas supply is switched from nitrogen to a mixed gas containing silicon-containing gas and nitrogen (the volume fraction of the silicon-containing gas in the mixed gas is 5% to 30%). After deposition at a gas flow rate of 200 sccm to 400 sccm for 8 to 16 hours, the rotary furnace is continuously purged with nitrogen to blow out the silicon-containing gas from the furnace, and the rotary furnace is purged with nitrogen for 30 minutes. Then, the nitrogen in the rotary furnace is gradually converted to air from a compressed air source by switching the gas flow from nitrogen to air within 1 to 2 hours to obtain the silicon-based material.

[0027] In some embodiments, the porous carbon matrix can be selected from at least one of hard carbon, soft carbon, and graphite. For example, the aforementioned hard carbon can include resin carbon, carbon black, organic polymer pyrolytic carbon, and combinations thereof. The aforementioned soft carbon can include carbon fibers, carbon microspheres, and combinations thereof. The particle size of the porous carbon matrix is not limited as long as the purpose of the present application can be achieved. For example, the particle size of the porous carbon matrix ranges from 3 μm < Dv50 < 15 μm.

[0028] The weight loss rate A of the negative electrode active material layer of the negative electrode tab when heated to 480°C under an Ar atmosphere is related to the content of the binder in the negative electrode active material layer, for example, the more the amount of the binder in the negative electrode active material layer, the larger A is, and the less the amount of the binder in the negative electrode active material layer, the smaller A is. Based on this, A can be adjusted by adjusting the amount of the binder.

[0029] The relative percentage content fluctuation value B of silicon element in the silicon-based material particles is related to the uniformity of the pore distribution and the pore size in the carbon matrix, for example, the more uniform the pore distribution in the carbon matrix, the smaller the B. Based on this, B can be adjusted by adjusting the pore distribution and the pore size.

[0030] The mass percentage content C of silicon element in the negative electrode tab is related to the addition amount of the silicon-based material in the negative electrode active material layer, wherein the silicon content deposited in the silicon-based material can be adjusted by adjusting the deposition temperature, the deposition time, and the concentration of the silicon-containing gas, for example, C generally increases with the increase of the deposition temperature, C generally increases with the increase of the deposition time, and C generally increases with the increase of the concentration of the silicon-containing gas. Based on this, the mass content C of silicon element in the negative electrode material layer can be adjusted.

[0031] The porosity of the negative electrode tab generally decreases with the increase of the compaction density of the negative electrode tab, based on which the compaction density of the negative electrode tab can be adjusted by adjusting the cold pressing pressure of the negative electrode tab, so as to adjust the porosity of the negative electrode tab.

[0032] In some embodiments, the binder in the negative electrode active material layer can include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyvinyl pyrrolidone, polyaniline, polyimide, polyamide-imide, polysiloxane, butadiene-styrene rubber, epoxy resin, polyester resin, polyurethane resin, or polyfluorene.

[0033] In some embodiments, the electrochemical device can include an electrode assembly including a positive electrode tab, a negative electrode tab, and a separator disposed between the positive electrode tab and the negative electrode tab. In some embodiments, the negative electrode tab further includes a negative current collector. In some embodiments, the negative electrode active material layer can be located on one side or both sides of the negative current collector. In some embodiments, the negative electrode active material layer can further include a conductive agent. In some embodiments, the conductive agent in the negative electrode active material layer can include at least one of nano-conductive carbon black, carbon nanotubes, carbon fibers, flake graphite, graphene, or ketjen black. In some embodiments, the negative electrode active material layer can further include a negative electrode active material graphite, i.e., the negative electrode active material layer can include a silicon-based material and graphite as negative electrode active materials. In some embodiments, the mass ratio of the negative electrode active material, the conductive agent, and the binder in the negative electrode active material layer can be (78 to 98.5):(0.1 to 10):(0.1 to 10). It should be understood that the above is only an example, and any other suitable materials and mass ratios can be used. In some embodiments, the negative current collector can include at least one of a copper foil, a nickel foil, or a carbon-based current collector.

[0034] In some embodiments, the positive electrode tab includes a positive current collector and a positive active material layer disposed on the positive current collector, which can include a positive active material. In some embodiments, the positive active material includes at least one of lithium cobaltate, lithium iron phosphate, lithium manganese iron phosphate, sodium iron phosphate, lithium vanadium phosphate, sodium vanadium phosphate, lithium vanadium oxyphosphate, sodium vanadium oxyphosphate, lithium vanadate, lithium manganate, lithium nickelate, lithium nickel cobalt manganate, lithium-rich manganese-based material, or lithium nickel cobalt aluminum phosphate. In some embodiments, the positive active material layer can further include a conductive agent. In some embodiments, the conductive agent in the positive active material layer can include at least one of conductive carbon black, ketjen black, exfoliated graphite, graphene, carbon nanotube, or carbon fiber. In some embodiments, the positive active material layer can further include a binder, which can include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyvinylpyrrolidone, polyaniline, polyimide, polyamide-imide, polysiloxane, butadiene-styrene rubber, epoxy resin, polyester resin, polyurethane resin, or polyfluorene. In some embodiments, the mass ratio of the positive active material, the conductive agent, and the binder in the positive active material layer can be (80 to 99):(0.1 to 10):(0.1 to 10). In some embodiments, the thickness of the positive active material layer can be 10 to 500 µm. It should be understood that the above is merely an example, and the positive active material layer can employ any other suitable material, thickness, and mass ratio.

[0035] In some embodiments, the positive current collector can employ an Al foil, although other current collectors commonly used in the art can also be employed. In some embodiments, the thickness of the positive current collector can be 1 to 50 µm. In some embodiments, the positive active material layer can be coated only on a partial area of the positive current collector.

[0036] In some embodiments, the separator film includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene includes at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. In particular, polyethylene and polypropylene have a good effect on preventing short circuits and can improve the stability of the battery through the shutdown effect. In some embodiments, the thickness of the separator film is in the range of about 3 to 20 µm.

[0037] In some embodiments, the surface of the separator film can further include a porous layer disposed on at least one surface of the separator film, the porous layer including inorganic particles and a binder, the inorganic particles being selected from at least one of aluminum oxide (AI2O3), silicon oxide (SiO2), magnesium oxide (MgO), titanium oxide (TiO2), hafnium dioxide (HfO2), tin oxide (SnO2), cerium dioxide (CeO2), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO2), yttrium oxide (Y2O3), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, the pores of the separator film have a diameter in a range of about 0.01 μm to 1 μm. The binder of the porous layer is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The porous layer of the surface of the separator film can improve the heat resistance, oxidation resistance, and electrolyte impregnation properties of the separator film, and enhance the adhesion between the separator film and the electrode sheet.

[0038] In some embodiments, the electrolyte further includes a lithium salt, which can include at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB, or lithium difluoroborate. Preferably, the lithium salt includes LiPF6.

[0039] In some embodiments, the electrolyte can further include a non-aqueous solvent. The non-aqueous solvent can be a carbonate compound, a carboxylic ester compound, an ether compound, other organic solvents, or a combination thereof. The carbonate compound can be a chain carbonate compound, a cyclic carbonate compound, a fluorinated carbonate compound, or a combination thereof. Examples of the chain carbonate compound are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and a combination thereof. Examples of the cyclic carbonate compound are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), or a combination thereof. Examples of the fluorinated carbonate compound are 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, trifluoromethyl ethylene carbonate, or a combination thereof. Examples of the carboxylic ester compound are methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, methylvaleronolactone, hexanolactone, methyl formate, or a combination thereof. Examples of the ether compound are dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or a combination thereof. Examples of the other organic solvents are dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, or a combination thereof. The content of the non-aqueous solvent is not particularly limited as long as the purpose of the present application can be achieved, for example, the mass percentage content of the above-mentioned other non-aqueous solvents is 67% to 86%, for example, 67%, 67.5%, 70%, 75%, 80%, 83%, 85%, 85.5%, 86%, or any range therebetween.

[0040] In some embodiments, the electrochemical device includes a lithium ion battery, but the present application is not limited thereto.

[0041] In some embodiments of the present application, the electrode assembly of the electrochemical device is a jelly-roll electrode assembly, a stacked electrode assembly, or a folded electrode assembly. In some embodiments, the positive electrode sheet and / or the negative electrode sheet of the electrochemical device can be a multi-layer structure formed by rolling or stacking, or a single-layer structure in which a single layer of positive electrode, a separator film, and a single layer of negative electrode are stacked.

[0042] In some embodiments of the present application, taking a lithium ion battery as an example, the positive electrode sheet, the separator and the negative electrode sheet are sequentially wound or stacked into an electrode assembly, and then packaged in, for example, an aluminum plastic film, injected with electrolyte, formed, packaged, and thus a lithium ion battery is prepared. Then, the prepared lithium ion battery is subjected to performance test.

[0043] Those skilled in the art will understand that the preparation method of the above-described electrochemical device (e.g., lithium ion battery) is only an embodiment. Other methods commonly used in the art can be employed without departing from the content disclosed in the present application.

[0044] Embodiments of the present application also provide an electronic device comprising the above-described electrochemical device. The electronic device of embodiments of the present application is not particularly limited, and it can be any electronic device known in the prior art. In some embodiments, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, an illuminating appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0045] Some specific examples and comparative examples are listed below to better illustrate the present application, taking a lithium ion battery as an example.

[0046] Comparative Example 1-1

[0047] Preparation of the positive electrode sheet: the positive electrode active material lithium cobaltate, conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:1.4:1.6, and N-methyl pyrrolidone (NMP) was added as a solvent and stirred uniformly. The slurry (solid content of 72 wt%) was uniformly coated on the positive electrode current collector aluminum foil with a coating thickness of 80 μm, dried at 85°C, and then subjected to cold pressing, sheet cutting, and cutting, and dried at 85°C under vacuum for 4 hours to obtain the positive electrode sheet.

[0048] Preparation of silicon-based composite material: The porous carbon matrix with Dv50 of 10 pm is placed in a rotary furnace, the furnace tube is purged with nitrogen for 30 minutes at room temperature, and then the heating temperature of the porous carbon sample is increased to 450°C. The nitrogen flow rate is adjusted so that the gas residence time in the rotary furnace is at least 90 seconds, and maintained at this flow rate for 30 minutes. The gas supply is then switched from nitrogen to a mixed gas containing silicon-containing gas (such as silane) and nitrogen, wherein the volume ratio of silicon-containing gas to nitrogen in the mixed gas is 5:95. After 8 hours of deposition at a gas flow rate of 200 sccm, the rotary furnace is continuously purged with nitrogen to blow out the silicon-containing gas from the furnace, and then the rotary furnace is purged with nitrogen for 30 minutes. The nitrogen in the rotary furnace is then gradually converted to air from a compressed air source by switching the gas flow from nitrogen to air over a period of 2 hours, obtaining a silicon-based composite material, i.e. silicon-based particles. It is determined that the relative percentage content of silicon element in the silicon-based material particles fluctuates by B of 10%.

[0049] Preparation of negative electrode sheet: The silicon-based material prepared above, artificial graphite, binder polyacrylic acid and sodium carboxymethyl cellulose (CMC) are dissolved in deionized water in a ratio of 5.7:91.8:1:1.5 by weight to form a negative electrode slurry (solid content of 40 wt%). A 10 pm thick copper foil is used as the negative electrode current collector, and the negative electrode slurry is coated on the current collector of the negative electrode with a coating thickness of 50 pm, dried at 85°C, and then subjected to cold pressing, sheet cutting and slitting, and dried at 120°C under vacuum for 12 hours to obtain a negative electrode sheet.

[0050] Preparation of separator film: The separator film is a 7 pm thick polyethylene (PE).

[0051] Preparation of electrolyte: In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) are mixed in a mass ratio of EC:PC:DEC = 1:1:1, dissolved and stirred thoroughly, then lithium salt LiPF6 is added, mixed uniformly to obtain an electrolyte, wherein the mass percentage content of LiPF6 is 12.5%.

[0052] Preparation of lithium ion battery: The positive electrode sheet, the separator film and the negative electrode sheet are sequentially stacked in order with the separator film between the positive electrode and the negative electrode to play a separating role, and wound to obtain an electrode assembly. The electrode assembly is placed in an outer packaging aluminum plastic film, and after removing the water at 80°C, the above electrolyte is injected and packaged, and after formation, degassing, edge cutting and other process procedures, a lithium ion battery is obtained.

[0053] Other examples and comparative examples are based on the steps of Comparative Example 1-1 with parameter changes, and the specific changed parameters are described in the following table.

[0054] Test methods of various parameters of the present application are described below.

[0055] 1. Porosity test of negative electrode tab: The porosity of the negative electrode tab is tested by gas displacement method: More than 50 pieces of electrode tabs with radius d are punched by the same mold, the thickness h of each electrode tab is measured respectively, and then the electrode tabs are loaded into the sample cup of the true density tester (AccuPyc II 1340), and the electrode tabs are filled with He in a closed sample chamber, so that the true volume V of the electrode tab is measured, and finally the porosity P of the electrode tab is obtained by the following formula: P = (1-V / πd 2 ×50×h)×100%.

[0056] 2. Test of silicon element content in negative electrode tab: The negative electrode tab is placed in a vacuum oven at 100°C for 24h, and the mass M1 of the active material layer scraped off the negative electrode tab is measured, then the scraped active material layer is heat-treated at 800°C in a continuous air atmosphere to remove carbonaceous material, and the mass M2 of the remaining material is measured, and finally the silicon element content C in the electrode tab is obtained by the following formula: C = 0.467×M2 / M1.

[0057] 3. Test of relative percentage content fluctuation value of silicon element in silicon-based material particles: The electrode tab is placed in a vacuum oven at 100°C for 24h, and the silicon-based material particles in the electrode tab are processed into 50-100nm thin slices under a protective atmosphere using focused ion beam (FIB), and then the relative percentage content of silicon atoms in the silicon-based particles is tested using X-ray energy dispersive spectrometer (EDS) line scanning in the projection electron microscope (TEM) equipment, the line scanning position is selected at any position inside the silicon-based particles, and the fluctuation value of the silicon element is the difference between the highest value and the lowest value of the relative percentage content of silicon atoms in the entire line scanning.

[0058] 4. Test of fluoroethylene carbonate (FEC) content in electrolyte: The electrochemical device is discharged to 0% state of charge (SOC) and then centrifuged, and the liquid obtained after centrifugation is tested by GC-MS to detect the percentage of FEC component.

[0059] 5. Test of mass percentage weight loss rate of negative active material layer when heated to 480°C in Ar atmosphere: The mass A1 of the active material layer scraped off the negative electrode tab is measured, and then the scraped active material layer is heated to 480°C in Ar atmosphere, and the mass A2 is measured, and the weight loss rate A = (A1-A2) / A1.

[0060] 6. Cycle performance test: the test temperature is 25°C and -10°C respectively, constant current charge to 4.4V at 0.7C, constant voltage charge to 0.025C, rest for 5 minutes, then discharge to 3.0V at 0.5C. Take the capacity obtained at this time as the initial capacity, carry out cycle test at 0.7C charge / 0.5C discharge, and the capacity at 25°C / -10°C after 400 cycles is the actual capacity, the capacity retention rate = actual capacity / initial capacity.

[0061] 7. Rate performance test: at 25°C, discharge to 3.0V at 0.2C, rest for 5 min, charge to 4.45V at 0.5C, constant voltage charge to 0.05C, then rest for 5 minutes, adjust the discharge rate, discharge test at 0.2C and 2.0C, respectively, get the discharge capacity, divide the capacity obtained at 2C rate by the capacity obtained at 0.2C, get the rate performance.

[0062] 8. Deformation rate test: use a screw micrometer to test the thickness h1 of fresh lithium ion battery at half charge, cycle to 400 cycles, the lithium ion battery is in full charge state, then use a screw micrometer to test the thickness h2 of the lithium ion battery at this time, the deformation rate of the lithium ion battery = (h2-h1) / h1.

[0063] It should be understood that the above-mentioned parameters of the lithium ion battery are tested by the technology known to those skilled in the art, which will not be described here, and the test method is not limited to the method described in the present application, and other suitable test methods can also be used.

[0064] Comparative Examples 1-1 to 1-10 and Examples 1-1 to 1-15:

[0065] The preparation methods of Comparative Examples 1-2 to 1-10 and Examples 1-1 to 1-15 are the same as that of Comparative Example 1-1, the only difference is that the values of A, B and / or C are adjusted to be different.

[0066] Table 1:

[0067]

[0068] As can be seen by comparing Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-2, when the silicon element content C in the negative electrode sheet and the uniformity B of the distribution of the silicon element in the silicon-based material particles are constant, as the weight loss rate A increases, the cycle capacity retention of the lithium ion battery first increases and then decreases, the deformation rate of the lithium ion battery first decreases and then increases, and the rate performance of the lithium ion battery first increases and then decreases. This is because the higher the A value, the stronger the adhesion of the negative electrode sheet, the higher the stability of the SEI on the surface of the silicon-based material particles, and the better the protection of the silicon-based material during the cycle process; but when A is too high, the cycle capacity retention, deformation rate and rate performance of the lithium ion battery are affected, because the content of the binder and SEI is too high, which will inhibit the transmission of lithium ions and increase the polarization of the lithium ion battery. The same conclusion can be obtained by comparing Comparative Examples 1-3 to 1-4 and Examples 1-4 to 1-6, or by comparing Comparative Examples 1-5 to 1-6 and Examples 1-7 to 1-9, or by comparing Comparative Examples 1-7 to 1-8 and Examples 1-10 to 1-12, or by comparing Comparative Examples 1-9 to 1-10 and Examples 1-13 to 1-15.

[0069] In addition, the higher the silicon element content in the negative electrode sheet and the lower the uniformity of the distribution of the silicon element in the silicon-based material, the more binder and SEI are needed, and when it satisfies the inequality: (B+C) / 7

[0070] Examples 2-1 to 2-10:

[0071] The preparation method of Examples 2-1 to 2-10 is the same as that of Example 1-2, except that the values of the porosity P of the negative electrode sheet and / or the silicon element content C in the negative electrode sheet are adjusted.

[0072] Table 2:

[0073]

[0074] The silicon-based material particles expand by about 300% in volume after lithium intercalation at room temperature, and the huge volume effect easily leads to problems such as negative active material layer release and powder falling, and reserving a certain porosity in the negative electrode sheet can effectively alleviate the volume expansion of the silicon-based material. When the porosity of the negative electrode sheet is too low, the electrolyte is difficult to fully infiltrate the negative electrode sheet, increasing the transmission distance of lithium ions and worsening the kinetic performance of the lithium ion battery. As can be seen by comparing Example 1-2 and Examples 2-1 to 2-10, when the porosity P of the negative electrode sheet and the silicon element content C in the negative electrode sheet satisfy the following relationship: P > 15xC 1 / 4 , the cycle capacity retention and deformation rate of the lithium ion battery at room temperature and low temperature are improved.

[0075] Examples 3-1 to 3-5:

[0076] The preparation method of Examples 3-1 to 3-5 is the same as that of Example 2-6, but a certain amount of fluoroethylene carbonate is added to the electrolyte, and the only difference is that the content X of fluoroethylene carbonate in the electrolyte and / or the value of the content C of silicon element in the negative electrode sheet are different.

[0077] Table 3:

[0078]

[0079] In Table 3, " / " means that the corresponding preparation parameter does not exist.

[0080] Fluoroethylene carbonate (FEC) is an important film-forming additive in the electrolyte, which decomposes to produce SEI during the cycle process, isolates the further contact between the negative active material and the electrolyte, reduces the consumption of lithium ions, and plays an important role in the cycle performance of lithium ion batteries. By comparing Example 1-1, Example 3-1 to Example 3-5, it can be seen that after adding FEC to the electrolyte, when the FEC content in the electrolyte is greater than or equal to the content of silicon element in the negative electrode sheet, the cycle capacity retention rate and the deformation rate of the lithium ion battery at room temperature and low temperature are improved. However, the FEC content cannot be too high, because the addition of too much FEC will reduce the migration rate of lithium ions in the electrolyte, affect the rate performance, and increase the deformation rate of the lithium ion battery, and the capacity retention rate at low temperature is also reduced.

[0081] The above description is only the preferred embodiments of the present application and the explanation of the applied technical principles. It should be understood by those skilled in the art that the disclosed range in the present application is not limited to the technical solutions formed by the specific combination of the above technical features, and should also cover other technical solutions formed by any combination of the above technical features or their equivalent features. For example, the technical solutions formed by replacing the above features with the technical features disclosed in the present application which have similar functions.

Claims

1. An electrochemical device, wherein, Comprising: a negative electrode tab comprising a negative electrode active material layer comprising silicon-based material particles and a binder, the negative electrode tab satisfying: (B+C) / 7 wherein A represents a mass percentage loss rate of the negative electrode active material layer of the negative electrode tab when heated to 480 DEG C under an Ar atmosphere; B represents a relative percentage content fluctuation value of silicon elements within the silicon-based material particles; and C represents a content of silicon elements in the negative electrode tab in mass percentage; the mass percentage loss rate A of the negative electrode active material layer of the negative electrode tab when heated to 480 DEG C under an Ar atmosphere satisfies: 1.5%≤A≤18%; the content C of silicon elements in the negative electrode tab in mass percentage satisfies: 1%≤C≤10%; the electrochemical device further comprises an electrolyte, the electrolyte comprising fluoroethylene carbonate, and a content X of the fluoroethylene carbonate in mass percentage and the content C of silicon elements in the negative electrode tab in mass percentage satisfy: X≥C.

2. The electrochemical device of claim 1, wherein, the relative percentage content fluctuation value B of silicon elements within the silicon-based material particles satisfies: B<16%.

3. The electrochemical device of claim 1, wherein, The porosity P of the negative electrode sheet and the mass percentage content C of the silicon element in the negative electrode sheet satisfy: P > 15 x C 1 / 4 .

4. The electrochemical device of claim 3, wherein, the porosity P of the negative electrode tab satisfies: 18%≤P≤40%.

5. The electrochemical device according to any one of claims 1 to 4, wherein, the content X of the fluoroethylene carbonate in mass percentage satisfies: 2%≤X≤20%.

6. The electrochemical device according to any one of claims 1 to 4, wherein, B≤10%。 7. The electrochemical device of claim 5, wherein, 2%≤X≤10%。 8. The electrochemical device according to any one of claims 1 to 4, wherein, the silicon-based material particles comprise silicon elements and carbon elements.

9. An electronic device, wherein, comprising the electrochemical device according to any one of claims 1 to 8.

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