Electrochemical device and electronic device

Abstract

The present application provides an electrochemical device and an electronic device. The electrochemical device comprises a negative electrode sheet; the negative electrode sheet comprises a negative electrode current collector, a first negative electrode active material layer and a second negative electrode active material layer; the first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer. The first negative electrode active material layer comprises a first silicon-based material, the second negative electrode active material layer comprises a second silicon-based material, Dv10 of the first silicon-based material is 3 μm to 10 μm, Dv10 of the second silicon-based material is 1 μm to 4 μm, and Dv10 refers to the particle size corresponding to the point where the volume distribution of particles reaches 10%, starting from small particles. According to the embodiments of the present application, the lithium precipitation problem and the cyclic swelling ratio problem of the negative electrode sheet of the electrochemical device can be ameliorated, and the cyclic capacity retention ratio of the electrochemical device can be increased.

Description

Electrochemical devices and electronic devices Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular, to electrochemical devices and electronic devices. Background Art

[0002] With the widespread application of electrochemical devices (e.g., lithium-ion batteries) in various electronic products, users have placed increasingly higher demands on the rate performance and cycle performance of electrochemical devices. Therefore, further improvements are urgently needed to meet people's increasing usage needs.

[0003] Summary of the Invention

[0004] An embodiment of the present application provides an electrochemical device, comprising a negative electrode plate, the negative electrode plate comprising a negative electrode current collector, a first negative electrode active material layer, and a second negative electrode active material layer, the first negative electrode active material layer being located between the negative electrode current collector and the second negative electrode active material layer; the first negative electrode active material layer comprising a first silicon-based material, the second negative electrode active material layer comprising a second silicon-based material, the Dv10 of the first silicon-based material being 3 μm to 10 μm, and the Dv10 of the second silicon-based material being 1 μm to 4 μm. Dv10 refers to the particle size corresponding to the 10% volume distribution of particles, starting from the smallest particles. Therefore, Dv10 can reflect the level of small-sized particles in the material. A smaller Dv10 indicates a smaller particle size of the small-sized particles in the material, and also indicates a greater number of small-sized particles in the material.

[0005] In some embodiments, the Dv10 of the first silicon-based material is 3 μm to 8 μm, and the Dv10 of the second silicon-based material is 1 μm to 2 μm.

[0006] In some embodiments, based on the total mass of the first negative electrode active material layer, the mass content of the first silicon-based material in the first negative electrode active material layer is 5% to 20%; based on the total mass of the second negative electrode active material layer, the mass content of the second silicon-based material in the second negative electrode active material layer is 10% to 40%; the mass content of the second silicon-based material in the second negative electrode active material layer is at least 5% greater than the mass content of the first silicon-based material in the first negative electrode active material layer.

[0007] In some embodiments, the first silicon-based material in the first negative electrode active material layer has a mass content of 5% to 15% based on the total mass of the first negative electrode active material layer; the second silicon-based material in the second negative electrode active material layer has a mass content of 10% to 30% based on the total mass of the second negative electrode active material layer. In some embodiments, the silicon content in the first negative electrode active material layer is 2% to 10% based on the total mass of the first negative electrode active material layer; the silicon content in the second negative electrode active material layer is 4% to 18% based on the total mass of the second negative electrode active material layer; and the silicon content in the second negative electrode active material layer is at least 2% greater than the silicon content in the first negative electrode active material layer. In some embodiments, the silicon content in the first negative electrode active material layer is 2% to 6.6% based on the total mass of the first negative electrode active material layer; the silicon content in the second negative electrode active material layer is 4% to 13.2% based on the total mass of the second negative electrode active material layer.

[0008] In some embodiments, the first negative electrode active material layer further includes a first conductive agent, and the second negative electrode active material layer further includes a second conductive agent; based on the total mass of the first negative electrode active material layer, the mass content of the first conductive agent in the first negative electrode active material layer is 0.05% to 2%; based on the total mass of the second negative electrode active material layer, the mass content of the second conductive agent in the second negative electrode active material layer is 0.2% to 1%; the mass content of the second conductive agent in the second negative electrode active material layer is greater than or equal to the mass content of the first conductive agent in the first negative electrode active material layer. In some embodiments, the mass content of the second conductive agent in the second negative electrode active material layer is 0.2% to 0.4% based on the total mass of the second negative electrode active material layer.

[0009] In some embodiments, the first silicon-based material and the second silicon-based material each independently include at least one of silicon oxide, silicon carbon or pure silicon; the first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, carbon nanotubes, metal particles or carbon fibers.

[0010] An embodiment of the present application further provides an electronic device comprising the above-mentioned electrochemical device.

[0011] The present application improves the lithium plating problem and cycle expansion rate of the negative electrode sheet of the electrochemical device and improves the cycle capacity retention rate of the electrochemical device by making the Dv10 of the first silicon-based material 3μm to 10μm and the Dv10 of the second silicon-based material 1μm to 4μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 illustrates a cross-sectional view of a negative electrode sheet taken along a width direction according to some embodiments. DETAILED DESCRIPTION

[0013] The following examples may enable those skilled in the art to more fully understand the present application, but are not intended to limit the present application in any way.

[0014] Embodiments of the present application provide an electrochemical device comprising a negative electrode sheet. In some embodiments, the negative electrode sheet comprises a negative electrode current collector 110, a first negative electrode active material layer 111, and a second negative electrode active material layer 112, wherein the first negative electrode active material layer 111 is located between the negative electrode current collector 110 and the second negative electrode active material layer 112. In some embodiments, the first negative electrode active material layer 111 and the second negative electrode active material layer 112 may be present on one side of the negative electrode current collector 110 (as shown in FIG. 1 ), or may be present on both sides of the negative electrode current collector 110 (not shown).

[0015] In some embodiments, the first negative electrode active material layer 111 includes a first negative electrode active material, which includes a first silicon-based material. In some embodiments, the second negative electrode active material layer 112 includes a second negative electrode active material, which includes a second silicon-based material. In some embodiments, the Dv10 of the first silicon-based material is 3 μm to 10 μm, and the Dv10 of the second silicon-based material is 1 μm to 4 μm. Dv10 refers to the particle size at which the volume distribution of particles reaches 10%, starting from the smallest particle size. The size of Dv10 reflects the level of small particles within the material.

[0016] In some embodiments, if the Dv10 of the first silicon-based material is too small, the side reaction area increases due to the excessive proportion of the first silicon-based material particles with small particle size, which will deteriorate the cycle performance and increase the consumption of the electrolyte during the cycle; if the Dv10 of the first silicon-based material is too large and the Dv10 of the second silicon-based material is relatively large, it indicates that the subdivision content of the first silicon-based material is too low, and the particles are mainly large in size, resulting in a large diffusion distance within the lithium ion particles, which will adversely affect the overall rate performance of the electrochemical device and cause lithium deposition in different parts of the electrochemical device. In some embodiments, if the Dv10 of the second silicon-based material is too small, the side reaction area increases due to the excessive proportion of the second silicon-based material particles with small particle size, which will deteriorate the cycle performance and increase the consumption of the electrolyte during the cycle; if the Dv10 of the second silicon-based material is too large, it will adversely affect the rate performance of the electrochemical device. In addition, since the Dv10 of the first silicon-based material is at least 2 μm larger than the Dv10 of the second silicon-based material, the relatively large Dv10 of the first silicon-based material is beneficial to improving the energy density of the negative electrode. Generally, the second negative electrode active material layer 112 is located on the surface of the negative electrode. The relatively small Dv10 of the second silicon-based material is beneficial to improving the rate performance of the electrochemical device and reducing lithium plating. This is because the lithium insertion reaction of the silicon-based material is an alloying reaction, and the bottleneck lies in the SEI film impedance and the solid-phase diffusion coefficient of the material particles. When Dv10 is small, the proportion of small particles in the silicon-based material is high, which is beneficial to increasing the reaction area, reducing the diffusion distance of lithium ions within the particles, and improving the lithium plating phenomenon.

[0017] In some embodiments, the Dv10 of the first silicon-based material is 3 μm to 8 μm, and the Dv10 of the second silicon-based material is 1 μm to 2 μm. In this case, the electrochemical device has excellent rate performance, no lithium plating occurs, high cycle capacity retention, and relatively small cycle expansion rate.

[0018] In some embodiments, the mass content of the first silicon-based material in the first negative electrode active material layer 111 is 5% to 20% based on the total mass of the first negative electrode active material layer 111; the mass content of the second silicon-based material in the second negative electrode active material layer 112 is 10% to 40% based on the total mass of the second negative electrode active material layer 112. Generally, the higher the mass content of the first silicon-based material in the first negative electrode active material layer 111 or the second silicon-based material in the second negative electrode active material layer 112, the thinner the corresponding coating layer can be while maintaining the same capacity, thereby reducing lithium plating. However, as the mass content of the silicon-based material in the corresponding coating layer increases, the cycling capacity retention and cycling expansion rate of the electrochemical device deteriorate. In some embodiments, the mass content of the second silicon-based material in the second negative electrode active material layer 112 is at least 5% greater than the mass content of the first silicon-based material in the first negative electrode active material layer 111. Thus, the second negative electrode active material layer 112 with a higher mass content of the second silicon-based material can be thinner while maintaining the same capacity, thereby reducing lithium plating.

[0019] In some embodiments, the mass content of the first silicon-based material in the first negative electrode active material layer 111 is 5% to 15% based on the total mass of the first negative electrode active material layer 111; and the mass content of the second silicon-based material in the second negative electrode active material layer 112 is 10% to 30% based on the total mass of the second negative electrode active material layer 112. In this case, the electrochemical device has excellent rate performance, no lithium plating occurs, a relatively high cycle capacity retention rate, and a relatively low cycle expansion rate.

[0020] In some embodiments, the silicon content of the first negative electrode active material layer 111 is 2% to 10% by mass based on the total mass of the first negative electrode active material layer 111; and the silicon content of the second negative electrode active material layer 112 is 4% to 18% by mass based on the total mass of the second negative electrode active material layer 112. Generally, the higher the silicon content of the first negative electrode active material layer 111 or the second negative electrode active material layer 112, the thinner the coating layer can be while maintaining the same capacity, thereby reducing lithium plating. However, as the silicon content of the coating layer increases, the cycling capacity retention and cycling expansion rate of the electrochemical device deteriorate. In some embodiments, the silicon content of the second negative electrode active material layer 112 is at least 2% greater than the silicon content of the first negative electrode active material layer 111. Thus, the second negative electrode active material layer 112 with a higher silicon content can be thinner while maintaining the same capacity, thereby reducing lithium plating.

[0021] In some embodiments, the silicon content of the first negative electrode active material layer 111 is 2% to 6.6% by mass based on the total mass of the first negative electrode active material layer 111; and the silicon content of the second negative electrode active material layer 112 is 4% to 13.2% by mass based on the total mass of the second negative electrode active material layer 112. In this case, the electrochemical device has excellent rate performance, no lithium plating occurs, a relatively high cycle capacity retention rate, and a relatively low cycle expansion rate.

[0022] In some embodiments, the first negative electrode active material layer 111 further includes a first conductive agent, and the second negative electrode active material layer 112 further includes a second conductive agent. In some embodiments, the mass content of the first conductive agent in the first negative electrode active material layer 111 is 0.05% to 2% based on the total mass of the first negative electrode active material layer 111; the mass content of the second conductive agent in the second negative electrode active material layer 112 is 0.2% to 1% based on the total mass of the second negative electrode active material layer 112, and the mass content of the second conductive agent in the second negative electrode active material layer 112 is greater than or equal to the mass content of the first conductive agent in the first negative electrode active material layer 111. Ensuring that the mass content of the second conductive agent in the second negative electrode active material layer 112 is greater than or equal to the mass content of the first conductive agent in the first negative electrode active material layer 111 helps improve the cyclic expansion rate of the electrochemical device. In some embodiments, the mass content of the second conductive agent in the second negative electrode active material layer 112 is 0.2% to 0.4% based on the total mass of the second negative electrode active material layer 112. At this time, the rate performance of the electrochemical device is better, lithium plating does not occur, the cycle capacity retention rate is relatively high, and the cycle expansion rate is relatively small.

[0023] In some embodiments, the thickness of the first negative electrode active material layer 111 is 40 μm to 180 μm. In some embodiments, the thickness of the first negative electrode active material layer 111 is 40 μm to 100 μm. When the thickness of the first negative electrode active material layer 111 is too thick, it will affect the lithium ion conduction distance and the charging capacity; when the thickness of the first negative electrode active material layer 111 is too thin, the energy density of the electrochemical device is reduced. In some embodiments, the thickness of the second negative electrode active material layer 112 is 20 μm to 50 μm. In some embodiments, the thickness of the second negative electrode active material layer 112 is 20 μm to 40 μm. When the thickness of the second negative electrode active material layer 112 is too thick, it will affect the lithium ion conduction distance and the charging capacity; when the thickness of the second negative electrode active material layer 112 is too thin, the energy density of the electrochemical device is reduced.

[0024] In some embodiments, the negative electrode current collector 110 may be made of at least one of copper foil, nickel foil, or a carbon-based current collector. In some embodiments, the first silicon-based material and the second silicon-based material each independently include at least one of silicon oxide, silicon carbon, or pure silicon. In some embodiments, the first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, carbon nanotubes, metal particles, or carbon fibers. In some embodiments, the first negative electrode active material layer 111 and the second negative electrode active material layer 112 may each independently include graphite, such as artificial graphite, natural graphite, etc.

[0025] In some embodiments, the first negative electrode active material layer 111 further includes a first binder, and the weight content of the first binder in the first negative electrode active material layer 111 is 1% to 6%. In some embodiments, the second negative electrode active material layer 112 further includes a second binder, and the weight content of the second binder in the second negative electrode active material layer 112 is 0.5% to 5%. In some embodiments, the weight content of the second binder in the second negative electrode active material layer 112 is 1% to 4%. In some embodiments, the weight content of the second binder in the second negative electrode active material layer 112 is greater than the weight content of the first binder in the first negative electrode active material layer 111. When the weight content of the first binder in the first negative electrode active material layer 111 or the weight content of the second binder in the second negative electrode active material layer 112 is too low, the charging capacity is improved, but there is a risk of negative electrode sheet delamination. When the weight content of the first binder in the first negative electrode active material layer 111 or the weight content of the second binder in the second negative electrode active material layer 112 is too high, the charging capacity is degraded, resulting in lithium plating during high-rate charge and discharge. In some embodiments, the first binder and the second binder each independently include at least one of styrene-butadiene rubber (SBR), lithium polyacrylate, sodium polyacrylate, polyurethane, polyvinyl alcohol, or polyvinylidene fluoride (PVDF).

[0026] In some embodiments, the mass ratio of the first negative electrode active material, the binder, and the conductive agent in the first negative electrode active material layer 111 may be (90 to 98): (1 to 5): (1 to 5). In some embodiments, the mass ratio of the second negative electrode active material, the binder, and the conductive agent in the second negative electrode active material layer 112 may be (90 to 98): (1 to 5): (1 to 5). It should be understood that the above-described component ratios of the first negative electrode active material layer 111 and the second negative electrode active material layer 112 are merely examples, and any other suitable mass ratios may be employed.

[0027] In some embodiments, the electrochemical device may further include a positive electrode sheet and a separator, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet to perform isolation.

[0028] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer, with the positive electrode active material layer being located on one or both sides of the positive electrode current collector. In some embodiments, the positive electrode current collector can be made of aluminum foil, although other common positive electrode current collectors in the art can also be used. In some embodiments, the positive electrode current collector can have a thickness of 5 μm to 30 μm.

[0029] In some embodiments, the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder. In some embodiments, the positive electrode active material may include at least one of lithium cobaltate, lithium iron phosphate, lithium aluminate, lithium manganate, or lithium nickel cobalt manganate. In some embodiments, the conductive agent in the positive electrode active material layer may include at least one of conductive carbon black, lamellar graphite, graphene, or carbon nanotubes. In some embodiments, the binder in the positive electrode active material layer may include at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, a styrene-acrylate copolymer, a styrene-butadiene copolymer, a polyamide, a polyacrylonitrile, a polyacrylate, a polyacrylic acid, a polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, a polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. In some embodiments, the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer is (90-99): (0.5-5): (0.5-5), but this is only an example and any other suitable mass ratio may be used.

[0030] In some embodiments, the separator 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. Polyethylene and polypropylene are particularly effective in preventing short circuits and can improve battery stability through the shutdown effect. In some embodiments, the separator has a thickness ranging from approximately 3 μm to 20 μm.

[0031] In some embodiments, the surface of the isolation membrane may further include a porous layer, disposed on at least one surface of the isolation membrane. The porous layer includes inorganic particles and a binder, wherein the inorganic particles are selected from at least one of aluminum oxide (Al2O3), 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 isolation membrane have a diameter ranging from approximately 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 and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The porous layer on the surface of the separator can improve the separator's heat resistance, oxidation resistance, and electrolyte wettability, and enhance the adhesion between the separator and the electrode.

[0032] In some embodiments, the electrochemical device includes a lithium ion battery, but the present application is not limited thereto. In some embodiments, the electrochemical device further includes an electrolyte, and the electrolyte includes at least one of a fluoroether, a fluoroethylene carbonate, or an ether nitrile. In some embodiments, the electrolyte further includes a lithium salt, and the lithium salt includes lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, and the concentration of the lithium salt is 1 mol / L to 2 mol / L. In some embodiments, the electrolyte may further include a non-aqueous solvent. The non-aqueous solvent may be a carbonate compound, a carboxylate compound, an ether compound, other organic solvents, or a combination thereof.

[0033] The carbonate compound may be a linear carbonate compound, a cyclic carbonate compound, a fluorinated carbonate compound, or a combination thereof.

[0034] Examples of chain carbonate compounds are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), and combinations thereof. Examples of cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylethylene carbonate (VEC), or combinations thereof. Examples of fluorocarbonate compounds are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0035] Examples of the carboxylic acid ester compound are methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, methyl formate, or a combination thereof.

[0036] Examples of the ether compound are dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or a combination thereof.

[0037] Examples of other organic solvents are dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphoric acid esters or combinations thereof.

[0038] The embodiment of the present application also provides an electronic device including the above-mentioned electrochemical device. The electronic device of the embodiment 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 laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a drone, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium ion capacitor, etc.

[0039] Some specific embodiments and comparative examples are listed below to better illustrate the present application, wherein a lithium-ion battery is used as an example.

[0040] Comparative Example 1

[0041] Preparation of the negative electrode sheet: The negative electrode active material silicon carbon (Dv10 of 12μm), artificial graphite, binder styrene butadiene rubber (SBR), and conductive carbon black were dissolved in deionized water in a weight ratio of 5:93.3:1.5:0.2 to form a first slurry. A 6μm-thick copper foil was used as the negative electrode current collector. The first slurry was applied to both sides of the negative electrode current collector to form a first negative electrode active material layer. The negative electrode active material silicon carbon (Dv10 of 3μm), artificial graphite, binder styrene butadiene rubber (SBR), and conductive carbon black were dissolved in deionized water in a weight ratio of 10:86.5:3:0.5 to form a second slurry. The second slurry was applied to the first negative electrode active material layer to form a second negative electrode active material layer. After drying, cold pressing, and slitting, the negative electrode sheet was obtained. The thickness of the first negative electrode active material layer was 50μm, and the thickness of the second negative electrode active material layer was 50μm.

[0042] Positive electrode sheet preparation: The positive electrode active material, lithium cobalt oxide, the conductive agent, conductive carbon black, and the binder, polyvinylidene fluoride (PVDF), were dissolved in an N-methylpyrrolidone (NMP) solution at a weight ratio of 94:3:3 to form a positive electrode slurry. An 8μm-thick aluminum foil was used as the positive electrode current collector. The positive electrode slurry was coated on both sides of the positive electrode current collector to a thickness of 80μm. After drying, cold pressing, and slitting, the positive electrode sheets were obtained.

[0043] Preparation of the isolation membrane: The isolation membrane substrate is 8 μm thick polyethylene (PE), and a 2 μm alumina ceramic layer is coated on both sides of the isolation membrane substrate. Finally, a 2.5 mg / 1540.25 mm thick alumina ceramic layer is coated on both sides of the ceramic layer. 2 The binder polyvinylidene fluoride (PVDF) is dried.

[0044] Preparation of electrolyte: In an environment with a water content of less than 10 ppm, lithium hexafluorophosphate and a non-aqueous organic solvent (ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC): propyl propionate (PP): vinylene carbonate (VC) = 20:30:20:28:2, mass percentage) are prepared into an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0045] Preparation of a lithium-ion battery: The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide insulation. The electrodes are then wound to form an electrode assembly. The electrode assembly is then placed in an aluminum-plastic film package. After dehydration at 80°C, the electrolyte is injected and the battery is packaged. The battery undergoes a series of processes, including formation, degassing, and shaping, to produce a lithium-ion battery.

[0046] The parameters of Comparative Examples 2-5 and Examples 1 to 19 differ from those of Comparative Example 1 in some parameters in the preparation of the negative electrode sheet. Please see Table 1 and Table 2 for specific differences.

[0047] The test methods for various parameters of this application are described below.

[0048] Particle size distribution test:

[0049] Add 0.02g of powder sample (the powder is a silicon-based material, which can be purchased during the production process or separated from the negative electrode) to a 50ml clean beaker, add about 20ml of deionized water, and then add a few drops of 1% surfactant to completely disperse the powder in the water. Ultrasonicate in a 120W ultrasonic cleaner for 5 minutes, and measure the particle size distribution using a MasterSizer 2000. The volume-based particle size distribution curve is obtained using a laser scattering particle size analyzer, and the Dv10 of the sample is calculated. Cycling performance test:

[0050] The battery was allowed to stand for 30 minutes in an environment of 25±3°C, and was charged with a constant current of 3C (1C is the rated capacity of the battery) as the maximum current until the battery voltage reached the upper limit cut-off voltage. The battery was then switched to constant voltage charging, and charging was stopped when the current reached 0.05C. The battery was allowed to stand for 30 minutes; the battery was discharged to 3.0V at a current of 0.5C and allowed to stand for 30 minutes. The same charge and discharge process was followed for 500 cycles, and the discharge capacity of each cycle was divided by the initial discharge capacity as the capacity retention rate.

[0051] Cyclic expansion rate test:

[0052] The thickness of a fresh lithium-ion battery at half charge (50% state of charge (SOC)) is measured using a flat plate thickness gauge (600g load). At 500 cycles, the battery is fully charged (100% SOC). The thickness of the battery is measured again using the flat plate thickness gauge. Comparing this thickness with the initial half-charge thickness provides the cyclic expansion rate of the fully charged battery.

[0053] Lithium deposition test:

[0054] At 25°C, charge the lithium-ion battery to the upper cutoff voltage at a constant current of 3C. Then charge it to a current of 0.05C at a constant voltage of the upper cutoff voltage, let it stand for 2 minutes, and then discharge it to 3.0V at a constant current of 0.5C, let it stand for 2 minutes. This is considered one cycle. After repeating 10 cycles, disassemble the lithium-ion battery to obtain the electrode assembly. Spread the electrode assembly flat. If any part of the negative electrode sheet is larger than 2mm 2 Lithium deposition in the area is determined to be lithium deposition on the negative electrode sheet, where the edge area is the area where the negative electrode sheet extends beyond the positive electrode (generally, the area from the end of the negative electrode sheet to the middle of the negative electrode sheet is within the range of 0.2mm to 1.5mm), and the main area is the area on the negative electrode sheet where the negative electrode sheet overlaps with the positive electrode sheet.

[0055] Silicon content test method:

[0056] Test of Si element content: scrape the negative electrode to obtain 0.10g powder, put the powder into a nickel crucible, add 1.2g to 1.5g KOH and cover the crucible, slowly heat the muffle furnace to 400℃, and the heating program is to heat from room temperature to 300℃ for 2h, and from 300℃ to 400℃ for 2h, start cooling, end the digestion program, and take out the crucible; wait for the crucible after burning to cool to about 100℃, take it out and place it in a beaker, slowly add 30ml boiling water, soak for 1h, pick up the crucible with tweezers and clean it, control the volume to 50ml, filter it into a 400ml beaker, pay attention to control the solution volume during the transfer process, and after the filtration is completed, add 20ml concentrated nitric acid to neutralize the solution at one time to make the solution acidic. After the test solution is cooled to room temperature, add solid KCl to saturation under constant stirring, and add 2g in excess, and add 10mL fluorine After adding potassium chloride solution, a white precipitate appears after adding potassium fluoride. It is aged for 15 minutes and filtered with medium-speed quantitative filter paper. The beaker and the precipitate are washed with 8 mL of potassium chloride solution each time for a total of three times. The filter paper is removed and returned to the original beaker. 20 mL of potassium chloride ethanol solution and 10 drops of phenolphthalein are added. The residual acid is neutralized with a standard sodium hydroxide solution. The filter paper is carefully stirred and the wall of the cup is scrubbed until the solution turns light red. The paper pulp is broken up as much as possible with a glass rod to complete the reaction. 200 mL of neutralized boiling water is added to the cup (after boiling, 10 drops of phenolphthalein are added and neutralized with a standard sodium hydroxide solution until it turns slightly red. The volume consumed in the titration is recorded. The silicon content is calculated according to the following formula:

[0057] ωsi=(V-V0)×c×0.0072 / m×100%, where: c is the concentration of sodium hydroxide standard solution, mol / L; V is the volume of sodium hydroxide standard solution consumed in titration, V0 is the volume of sodium hydroxide standard solution consumed in blank; 0.0072 is the molar mass of 1 / 4Si; m is the mass of the sample in g;

[0058] When measuring the silicon content in the second negative electrode active material layer, the powder within a thickness of 10 μm near the surface of the negative electrode sheet should be scraped off with a scraper; when measuring the silicon content in the first negative electrode active material layer, the powder within a thickness of 10 μm near the surface of the negative electrode collector should be scraped off with a scraper.

[0059] Furthermore, for different silicon materials, the above method can be applied to test the silicon element and obtain the mass content of the silicon-based material.

[0060] Tables 1 and 2 show various parameters and evaluation results of Examples 1 to 19 and Comparative Examples 1 to 5.

[0061] Table 1

[0062] Table 2

[0063] By comparing Examples 1 to 7 with Comparative Examples 1 to 5, it can be seen that when the Dv10 of the first silicon-based material is 3μm to 10μm and the Dv10 of the second silicon-based material is 1μm to 4μm, the lithium plating of the lithium-ion battery is improved, and its cycle capacity retention rate and cycle expansion rate are also within an acceptable range (since the cycle and expansion of the lithium-ion battery containing silicon-based materials in the negative electrode plate are lower than those of the lithium-ion battery of pure graphite, the acceptable level is also relatively low). As the Dv10 of the first silicon-based material and / or the Dv10 of the second silicon-based material increases, the difficulty of lithium ion embedding increases, and lithium plating is prone to occur. When the Dv10 of the first silicon-based material is 3μm to 8μm and the Dv10 of the second silicon-based material is 1μm to 3μm, the lithium plating situation can be significantly improved at this time, and the cycle performance is also better. Dv10 can indicate the level of small-sized particles in the material. The smaller the Dv10, the smaller the particle size of the small-sized particles in the material, or the larger the Dv10, the larger the particles of the material, and the smaller the proportion of small particles. The Dv10 of the first silicon-based material and the second silicon-based material is within this range, indicating that the first layer of silicon-based material has relatively few small-sized particles, and the second silicon-based material has relatively more small-sized particles. Since the second silicon-based material is on the surface of the negative electrode, the contact area between the small-sized silicon-based material and the electrolyte is relatively larger, which is more conducive to the insertion and extraction of lithium ions, thereby improving lithium precipitation. The first silicon-based material is distributed in the lower layer of the negative electrode, and its small-sized particles are relatively few, which can balance the reaction rate between the overall negative electrode and the electrolyte, thereby improving the cycle retention rate. By comparing Examples 2 to 4, it can be seen that when the Dv10 of the second silicon-based material is the same, as the Dv10 of the first silicon-based material increases, the cycle capacity retention rate of the lithium-ion battery increases and the cycle expansion rate decreases.

[0064] Comparison of Examples 8 to 13 shows that increasing the mass content of the first silicon-based material in the first negative electrode active material layer or the second silicon-based material in the second negative electrode active material layer decreases the cycle capacity retention rate of the lithium-ion battery and increases the cycle expansion rate. Since silicon-based materials have a large specific capacity but are prone to expansion during cycling, to balance volumetric energy density, the amount of silicon-based material can be appropriately increased while reducing the thickness of the first negative electrode active material layer or the second negative electrode active material layer, thereby improving lithium plating.

[0065] Comparison of Examples 14 to 17 shows that as the mass content of the second conductive agent in the second negative electrode active material layer increases, the cycle capacity retention of the lithium-ion battery first increases and then decreases, while the cycle expansion first remains unchanged and then increases. Comparison of Examples 18 and 19 shows that as the mass content of the first conductive agent in the first negative electrode active material layer increases, the cycle capacity retention rate of the lithium-ion battery decreases and the cycle expansion rate increases.

[0066] The above description is merely a preferred embodiment of the present application and an illustration of the technical principles employed. Those skilled in the art should understand that the scope of disclosure in this application is not limited to technical solutions formed by a specific combination of the above-mentioned technical features, but also encompasses other technical solutions formed by any combination of the above-mentioned technical features or their equivalents. For example, a technical solution formed by replacing the above-mentioned features with technical features with similar functions disclosed in this application.

Claims

1. An electrochemical device, characterized in that: Including the negative electrode plate, The negative electrode sheet comprises a negative electrode current collector, a first negative electrode active material layer and a second negative electrode active material layer, wherein the first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer; The first negative electrode active material layer includes a first silicon-based material, and the second negative electrode active material layer includes a second silicon-based material. The Dv10 of the first silicon-based material is 3μm to 10μm, and the Dv10 of the second silicon-based material is 1μm to 4μm, and the Dv10 of the first silicon-based material is greater than the Dv10 of the second silicon-based material. Dv10 refers to the particle size corresponding to the volume distribution of the particles, starting from the small particles, when the volume distribution reaches 10%.

2. The electrochemical device according to claim 1, characterized in that The Dv10 of the first silicon-based material is 3 μm to 8 μm, and the Dv10 of the second silicon-based material is 1 μm to 2 μm.

3. The electrochemical device according to claim 1, characterized in that Based on the total mass of the first negative electrode active material layer, the mass content of the first silicon-based material in the first negative electrode active material layer is 5% to 20%; The mass content of the second silicon-based material in the second negative electrode active material layer is 10% to 40% based on the total mass of the second negative electrode active material layer; A mass content of the second silicon-based material in the second negative electrode active material layer is at least 5% greater than a mass content of the first silicon-based material in the first negative electrode active material layer.

4. The electrochemical device according to claim 3, characterized in that Based on the total mass of the first negative electrode active material layer, the mass content of the first silicon-based material in the first negative electrode active material layer is 5% to 15%; The mass content of the second silicon-based material in the second negative electrode active material layer is 10% to 30% based on the total mass of the second negative electrode active material layer.

5. The electrochemical device according to claim 1, characterized in that Based on the total mass of the first negative electrode active material layer, the mass content of silicon in the first negative electrode active material layer is 2% to 10%; Based on the total mass of the second negative electrode active material layer, the mass content of silicon in the second negative electrode active material layer is 4% to 18%; The mass content of silicon in the second negative electrode active material layer is at least 2% greater than the mass content of silicon in the first negative electrode active material layer.

6. The electrochemical device according to claim 5, characterized in that Based on the total mass of the first negative electrode active material layer, the mass content of silicon in the first negative electrode active material layer is 2% to 6.6%; The mass content of silicon in the second negative electrode active material layer is 4% to 13.2% based on the total mass of the second negative electrode active material layer.

7. The electrochemical device according to claim 1, characterized in that The first negative electrode active material layer further includes a first conductive agent, and the second negative electrode active material layer further includes a second conductive agent; The mass content of the first conductive agent in the first negative electrode active material layer is 0.05% to 2% based on the total mass of the first negative electrode active material layer; The mass content of the second conductive agent in the second negative electrode active material layer is 0.2% to 1% based on the total mass of the second negative electrode active material layer; The mass content of the second conductive agent in the second negative electrode active material layer is greater than or equal to the mass content of the first conductive agent in the first negative electrode active material layer.

8. The electrochemical device according to claim 7, characterized in that The second conductive agent in the second negative electrode active material layer has a content of 0.2% to 0.4% by mass based on the total mass of the second negative electrode active material layer.

9. The electrochemical device according to claim 7, characterized in that: The first silicon-based material and the second silicon-based material each independently include at least one of silicon oxygen, silicon carbon or pure silicon; The first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, carbon nanotubes, metal particles or carbon fibers.

10. An electronic device comprising the electrochemical device according to any one of claims 1 to 9.

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