Negative electrode sheet, and electrochemical device and electronic device comprising the same

Abstract

The application relates to a negative electrode sheet, an electrochemical device and an electronic device. The negative electrode sheet comprises a current collector, a first active material layer and a second active material layer between the current collector and the first active material layer, wherein the first active material layer comprises first silicon-based material particles, the second active material layer comprises second silicon-based material particles, the mass percentage content of lithium in the first silicon-based material particles is A%, the mass percentage content of lithium in the second silicon-based material particles is B%, and A > B. The application improves the expansion of the silicon material in the negative electrode sheet to the current collector, alleviates the expansion stress of the electrode sheet, avoids the interface problem caused by the deformation of the current collector, thereby improving the electrode sheet delamination after the cycle, the deformation of the electrode assembly, the cycle expansion, and the effect of improving the energy density and the cycle performance and reducing the cycle expansion.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of energy storage, and in particular to a negative electrode sheet, an electrochemical device comprising the negative electrode sheet, and an electronic device. BACKGROUND

[0002] Silicon-based materials have a theoretical specific capacity of up to 4200 mAh / g, and are a promising negative active material for the next generation of electrochemical devices. However, silicon-based materials have a volume expansion of about 300% during charging and discharging, which generates a large mechanical stress, and during the cycle process, the active material layer is prone to peeling off from the current collector due to lateral expansion, i.e. delamination, or causing the current collector to wrinkle or even tear, which prevents the transmission of electrons during charging and discharging of the electrochemical device, resulting in failure of the entire electrochemical device. At the same time, due to the wrinkles of the current collector, the electrode assembly deforms, which affects the cycle expansion and safety performance of the electrochemical device, and restricts the application of silicon materials.

[0003] Generally, to solve the problem of wrinkles or tearing of the current collector, the silicon content in the active material needs to be reduced, the amount of binder in the electrode sheet needs to be increased, and the thickness of the current collector needs to be increased to improve the problem of wrinkles of the current collector and deformation of the electrochemical device. However, whether the silicon content is reduced, the amount of binder is increased, or the thickness of the current collector is increased, the proportion of non-active materials in the electrochemical device will increase, resulting in a decrease in energy density, reducing the energy density improvement advantage brought by silicon materials, and restricting the further application of silicon materials. SUMMARY

[0004] To address the deficiencies of the prior art, the present application provides a negative electrode sheet, which adjusts the lithium supplement mass distribution of silicon-based materials in the active material layer, which is beneficial to improve the expansion of silicon-based materials on the current collector, relieve the expansion stress of the electrode sheet, avoid interface problems caused by deformation of the current collector, and thus improve the delamination of the electrode sheet after cycling, deformation of the electrode assembly, cycle expansion, energy density improvement, cycle performance improvement, and cycle expansion balance. The present application also relates to an electrochemical device comprising such a negative electrode sheet.

[0005] A first aspect of the present application provides a negative electrode sheet, comprising a current collector, a first active material layer, and a second active material layer between the current collector and the first active material layer, wherein the first active material layer comprises first silicon-based material particles, the second active material layer comprises second silicon-based material particles, the mass percentage content of lithium in the first silicon-based material particles is A%, the mass percentage content of lithium in the second silicon-based material particles is B%, and A > B.

[0006] According to some embodiments of the present application, 8 ≤ A ≤ 15, and 0 ≤ B ≤ 3. According to some embodiments of the application, A / B ≥ 5.

[0007] According to some embodiments of the present application, the first silicon-based material particles include one or more of Li2SiO3, Li4SiO4, or Li2Si2O5.

[0008] According to some embodiments of the present application, the surface of the first silicon-based material particles has M x SiO y or one or more of carbon, wherein M includes at least one of Mg, Al, Zn, Ca, Ba, or B, 0 < x < 3, and 0.5 < y < 6.

[0009] According to some embodiments of the present application, the second silicon-based material particles include SiO x wherein 0.6 ≤ x ≤ 1.5.

[0010] According to some embodiments of the present application, the second silicon-based material particles satisfy 0 < I2 / I1 ≤ 1, wherein I1 represents the highest intensity of a peak at 2θ of 28.0° to 29.5° in an X-ray diffraction pattern of the second silicon-based material particles, and I2 represents the highest intensity of a peak at 2θ of 20.0° to 21.5° in the X-ray diffraction pattern of the second silicon-based material particles.

[0011] According to some embodiments of the present application, the average particle diameter Dv50 of the first silicon-based material particles is greater than the average particle diameter Dv50 of the second silicon-based material particles, the average particle diameter Dv50 of the first silicon-based material particles satisfies 5 μm ≤ Dv50 ≤ 25 μm, and the average particle diameter Dv50 of the second silicon-based material particles satisfies 1 μm ≤ Dv50 ≤ 8 μm.

[0012] According to some embodiments of the present application, the porosity of the first active material layer is K1%, the porosity of the second active material layer is K2%, K1 > K2, 25 < K1 < 60, and 23 < K2 < 55.

[0013] According to some embodiments of the present application, the first active material layer and the second active material layer each independently further include at least one of artificial graphite, natural graphite, mesocarbon microbeads, lithium titanate, soft carbon, or hard carbon.

[0014] A second aspect of the present application provides an electrochemical device including the negative electrode sheet of the first aspect.

[0015] A third aspect of the present application provides an electronic device including the electrochemical device of the second aspect.

[0016] The application adjusts the mass distribution of lithium in the silicon-based material in the active material layer, so that the content of lithium in the silicon-based material in the active material layer close to the current collector is lower than that in the active material layer far from the current collector, reduces the expansion of the active material layer close to the current collector to the current collector, avoids the interface problem caused by the deformation of the current collector, thereby improving the electrode sheet demolding after cycling, the deformation of the electrode assembly, improving the cycle expansion, achieving the effect of improving the energy density, improving the cycle performance and balancing the cycle expansion. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The structure schematic diagram of the negative electrode sheet of one embodiment of the application is shown, wherein 1 is the first active material layer, 2 is the second active material layer, and 3 is the current collector.

[0018] Figure 2 The structure schematic diagram of the negative electrode sheet of another embodiment of the application is shown, wherein 1 is the first active material layer, 2 is the second active material layer, and 3 is the current collector.

[0019] Figure 3 The cycle expansion comparison diagram of Example 1 and Comparative Example 1 is shown. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical scheme and advantages of the application clearer, the technical scheme of the application will be described clearly and completely in combination with the embodiments below. Obviously, the described embodiments are part of the embodiments of the application, rather than all the embodiments. The embodiments described herein are illustrative in nature and are used to provide a basic understanding of the application. The embodiments of the application should not be interpreted as limiting the application. All other embodiments obtained by those skilled in the art based on the technical scheme provided by the application and the embodiments given are within the scope of protection of the application.

[0021] Unless otherwise specified, the terms used in the application have the commonly understood meanings understood by those skilled in the art. Unless otherwise specified, the values of each parameter mentioned in the application can be measured by various measurement methods commonly used in the art (for example, the test can be performed according to the method given in the embodiments of the application).

[0022] The term "about" is used to describe and account for small variations. When utilized in connection with an event or circumstance, the term can refer to instances where the event or circumstance occurs exactly, as well as instances where the event or circumstance occurs with close approximation. For example, when used in connection with a numerical value, the term can refer to a range of variation less than or equal to ±10% of the numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. Additionally, quantities, ratios and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be taken as a literal disclosure of all individual values falling within the range. All individual values used and / or

[0023] A list of items joined by "at least one of" or "one or more of" can mean any combination of the listed terms. For example, if a list of items includes A, B, and C, then "at least one of A, B, and C" or "one or more of A, B, and C" can mean A alone; B alone; C alone; A and B together; A and C together; B and C together; or A, B, and C together. A, B, and C can include individual components or a group of components. B can include individual components or a group of components. C can include individual components or a group of components.

[0024] Compared with silicon material, the theoretical specific capacity of SiOx material is about 2000 mAh / g. Because of the mixed structure of silicon and oxide, the volume effect of silicon during charging and discharging can be effectively alleviated, and the cycle stability of the material is improved, so that commercial application is more easily realized. Specifically, during the first lithium intercalation process, the irreversible phase of lithium silicate and Li2O can be used as a buffer matrix to resist volume change during lithiation / delithiation. Therefore, by adjusting the oxygen content of the SiOx material, the proportion of the buffer phase can be changed, thereby improving the expansion characteristics and enhancing the cycle performance of SiOx. However, with the increase of oxygen content, the irreversible phase of lithium silicate and Li2O will increase, resulting in a decrease in the first storage efficiency, thereby reducing the energy density advantage. In order to solve the problem of low first efficiency of SiOx material, various Li compensation strategies have been developed to compensate for the Li consumed in the negative electrode during the first charge, obtaining high first efficiency silicon material, and further solving the energy density problem. However, since pre-lithiation is an exothermic reaction and often requires high-temperature sintering, the silicon grains of the silicon material obtained by pre-lithiation grow in high temperature, resulting in that although the first efficiency of the silicon material after pre-lithiation is improved, the cycle performance and expansion characteristics are deteriorated. In order to solve the above technical problems, the application provides a negative electrode sheet, and an electrochemical device and an electronic device comprising the same.

[0025] I. Negative electrode sheet

[0026] The negative electrode sheet provided by the application comprises a current collector, a first active material layer, and a second active material layer located between the current collector and the first active material layer. The first active material layer comprises first silicon-based material particles, and the second active material layer comprises second silicon-based material particles. The mass percentage content of lithium in the first silicon-based material particles is A%, and the mass percentage content of lithium in the second silicon-based material particles is B%, wherein A > B. By adjusting the mass distribution of lithium in the silicon-based material in the active material layer, the content of lithium in the silicon-based material in the active material layer close to the current collector is lower than that in the active material layer away from the current collector, the expansion of the active material layer close to the current collector to the current collector is reduced, the interface problem caused by the deformation of the current collector is avoided, and the cycle expansion is improved, thereby achieving the effects of energy density improvement, cycle performance improvement, and cycle expansion balance.

[0027] As shown in Figure 1 Some embodiments of the application provide a negative electrode sheet, which comprises a current collector 3 and an active material layer. The active material layer comprises a first active material layer 1 and a second active material layer 2. It should be understood that, although Figure 1 the active material layer is shown as being located on one side of the current collector in Figure 2The current collector is shown located on both sides of the current collector. In some embodiments, the current collector of the negative electrode tab can include at least one of a copper foil, an aluminum foil, a nickel foil, or a carbon-based current collector.

[0028] According to some embodiments of the present application, 8≤A≤15. In some embodiments of the present application, A is 8.5, 9.0, 9.5, 10, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, or any value between them. If the mass percentage of lithium in the first silicon-based material particles is too high, a higher temperature is required in the pre-lithiation process, which leads to the growth of the silicon grain size, which is not conducive to cycling and reduces the expansion.

[0029] According to some embodiments of the present application, 0≤B≤3. In some embodiments, B is 0, 0.1, 0.3, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, 1.7, 1.9, 2.0, 2.3, 2.5, 2.7, 2.9, or any value between them. If the mass percentage of lithium in the second silicon-based material particles is too high, the pre-lithiation process will generate heat due to the high degree of pre-lithiation, which leads to the growth of the silicon grain size, increases the volume expansion in cycling, and worsens the expansion and deformation of the current collector.

[0030] According to some embodiments of the present application, A / B≥5. In some embodiments, A / B is 5, 7, 10, 15, 20, or 30. By controlling the appropriate A / B value in the present application, the energy density and cycle life, as well as the cycle expansion performance, can be balanced. If the A / B ratio is too low, the proportion of active lithium participating in the cycle of the overall tab after formation will decrease, affecting the energy density of the electrochemical device, and the energy density advantage of the silicon system cannot be realized.

[0031] According to some embodiments of the present application, the first silicon-based material particles include one or more of Li2SiO3, Li4SiO4, or Li2Si2O5. According to some embodiments of the present application, the surface of the first silicon-based material particles has M x SiO y or carbon, wherein M includes at least one of Mg, Al, Zn, Ca, Ba, or B, 0 i2 O5, etc. The first silicon-based material particles of the present application have high first efficiency and can significantly improve the energy density. In some embodiments of the present application, the first silicon-based material particles include pre-lithiated silicon oxide, and the pre-lithiation method can use conventional methods in the art.

[0032] According to some embodiments of the present application, the second silicon-based material particles include SiO xwherein 0.6≤x≤1.5. In some embodiments, the second silicon-based material particles comprise nano-Si grains, SiO, SiO2, or any combination thereof.

[0033] According to some embodiments of the present application, the second silicon-based material particles satisfy: 0<I2 / I1≤1, wherein I1 represents the highest intensity of the peak of the X-ray diffraction pattern of the second silicon-based material particles at 2θ of 28.0° to 29.5°, and I2 represents the highest intensity of the peak of the X-ray diffraction pattern of the second silicon-based material particles at 2θ of 20.0° to 21.5°. In some embodiments, I2 / I1 is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or any value therebetween. When I2 / I1 is greater than 1, the size of the nano-silicon grains inside the silicon anode material is large, which leads to excessive expansion of the anode and is not conducive to the cycle performance.

[0034] According to some embodiments of the present application, the average particle size Dv50 of the first silicon-based material particles is greater than the average particle size Dv50 of the second silicon-based material particles. When the Dv50 of the silicon-based material particles in the first active material layer is greater than the Dv50 of the silicon-based material particles in the second active material layer, the expansion force on the current collector is small due to the small size and low cycle expansion of the bottom layer of silicon, and thus the tearing rate of the electrode sheet after cycling is significantly reduced. In some embodiments, the average particle size Dv50 of the first silicon-based material particles satisfies 5 μm≤Dv50≤25 μm, such as 7 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 15 μm, 17 μm, 19 μm, 20 μm, 23 μm, etc. In some embodiments, the average particle size Dv50 of the second silicon-based material particles satisfies 1 μm≤Dv50≤8 μm, such as 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, etc. In the present application, Dv50 is the particle diameter at which 50% of the cumulative volume is obtained based on the volume reference distribution tested by a laser scattering particle size analyzer.

[0035] According to some embodiments of the present application, the porosity of the first active material layer is K1%, the porosity of the second active material layer is K2%, and K1>K2. When the porosity of the first active material layer is less than the porosity of the second active material layer, lithium ions are difficult to diffuse, which leads to lithium precipitation after cycling and significant capacity deterioration. When the porosity of the first active material layer is greater than the porosity of the second active material layer, the cycle performance and kinetic performance of the lithium ion battery can be improved, and large porosity is conducive to electrolyte infiltration and improvement of lithium ion diffusion, thereby further reducing the risk of negative electrode lithium precipitation after cycling and avoiding cycle decay due to lithium precipitation. In some embodiments, 25 K1 K2

[0036] According to some embodiments of the present application, the first active material layer and the second active material layer each independently further include at least one of artificial graphite, natural graphite, mesocarbon microbeads, lithium titanate, soft carbon, or hard carbon.

[0037] According to some embodiments of the present application, the first active material layer has a thickness of 0.2 μm to 200 μm, and the second active material layer has a thickness of 0.2 μm to 200 μm. If the thickness of the first active material layer or the second active material layer is too small, sufficient active material cannot be maintained, affecting the energy density. On the other hand, if the thickness of the first active material layer or the second active material layer is too large, the liquid phase diffusion of the electrolyte in the active material layer close to the current collector (i.e., the first active material layer) is not favorable.

[0038] According to some embodiments of the present application, the mass percentage content of silicon element in the first active material layer is 0.1% to 40%. According to some embodiments of the present application, the mass percentage content of silicon element in the second active material layer is 0.1% to 20%.

[0039] According to some embodiments of the present application, the mass percentage content of silicon element in the first active material layer is greater than the mass percentage content of silicon element in the second active material layer. The mass percentage content of silicon element in the first active material layer being greater than the mass percentage content of silicon element in the second active material layer can effectively reduce the stress on the current collector caused by the extension of silicon-based material particles, reducing the phenomenon of active material layer peeling, electrode assembly deformation, and current collector tearing.

[0040] According to some embodiments of the present application, the first active material layer and the second active material layer each independently further include a conductive agent and / or a binder. In some embodiments, the conductive agent includes at least one of conductive carbon black, acetylene black, carbon nanotube, ketjen black, conductive graphite, or graphene. In some embodiments, the conductive agent has a mass percentage of 0.5% to 10% of the active material layer. In some embodiments, the binder includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylic acid salt, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, or styrene butadiene rubber. In some embodiments, the binder has a mass percentage of 0.5% to 10% of the active material layer. The negative electrode sheet of the present application can be prepared by a method known in the art. Generally, the negative active material and optional conductive agent (such as carbon material such as carbon black and metal particles), binder (such as SBR), other optional additives (such as PTC thermistor material), and the like are mixed together and dispersed in a solvent (such as deionized water), stirred uniformly, and then uniformly coated on a negative current collector to obtain a negative electrode containing a negative active material layer after drying. A metal foil or a porous metal plate, and the like can be used as the negative current collector.

[0041] According to some embodiments of the present application, the thickness of the negative electrode sheet of the present application is 3 μm to 500 μm.

[0042] II. Electrochemical device

[0043] The electrochemical device of the present application includes any device that generates an electrochemical reaction, and specific examples thereof include primary batteries, secondary batteries of all kinds. In particular, the electrochemical device is a lithium secondary battery, including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery. In some embodiments, the electrochemical device of the present application includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0044] 1. Negative electrode

[0045] The negative electrode in the electrochemical device of the present application includes the negative electrode sheet of the first aspect.

[0046] 2. Positive electrode

[0047] The materials, constitution, and manufacturing method thereof that can be used for the positive electrode in the embodiments of the present application include any technology disclosed in the prior art.

[0048] According to some embodiments of the present application, the cathode includes a current collector and a cathode active material layer on the current collector. According to some embodiments of the present application, the cathode active material includes, but is not limited to, lithium cobalt oxide (LiCo02), lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide, lithium iron phosphate (LiFeP04), or lithium manganese oxide (LiMn204).

[0049] According to some embodiments of the present application, the cathode active material layer further includes a binder and, optionally, a conductive material. The binder improves the cohesion between the cathode active material particles and also improves the cohesion between the cathode active material and the current collector. In some embodiments, the binder includes polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon, etc.

[0050] According to some embodiments of the present application, the conductive material includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from carbon black, acetylene black, ketjen black, carbon fiber, carbon nanotube, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0051] According to some embodiments of the present application, the current collector can include, but is not limited to, aluminum.

[0052] 3. Electrolyte

[0053] The electrolyte that can be used in the embodiments of the present application can be an electrolyte known in the art.

[0054] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and an additive. The organic solvent of the electrolyte according to the present application can be any organic solvent known in the art that can be used as a solvent of an electrolyte. The electrolyte used in the electrolyte according to the present application is not limited and can be any electrolyte known in the art. The additive of the electrolyte according to the present application can be any additive known in the art that can be used as an additive of an electrolyte.

[0055] In some embodiments, the organic solvent includes, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate.

[0056] In some embodiments, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis-trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalato)borate LiB(C2O4)2 (LiBOB), or lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB).

[0057] In some embodiments, the concentration of the lithium salt in the electrolyte is about 0.5 mol / L to 3 mol / L, about 0.5 mol / L to 2 mol / L, or about 0.8 mol / L to 1.5 mol / L.

[0058] 4. Separator film

[0059] The material and shape of the separator film used in the electrochemical device of the present application are not particularly limited and can be any of the techniques disclosed in the prior art. In some embodiments, the separator film includes a polymer or inorganic substance formed of a material stable to the electrolyte of the present application, and the like.

[0060] For example, the separator film can include a base material layer and a surface treatment layer. The base material layer is a nonwoven fabric, a film, or a composite film having a porous structure, and the material of the base material layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

[0061] The surface treatment layer is provided on at least one surface of the base material layer and can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance.

[0062] The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinylalkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0063] The polymer layer includes a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinylalkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0064] III. Electronic device

[0065] The present application further provides an electronic device comprising the electrochemical device of the second aspect of the present application.

[0066] The electronic device or apparatus of the present application is not particularly limited. In some embodiments, the electronic device of the present application includes, 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, a portable copier, a portable printer, a 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 audio player, a radio, a backup power supply, a drone, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a home-use large storage battery, and a lithium-ion capacitor, etc. The technical solutions of the present application are exemplarily described below through specific embodiments:

[0067] Embodiment 1

[0068] 1. Preparation of the negative electrode tab

[0069] The current collector is a copper foil with a thickness of 10 μm; the first active material is pre-lithiated silicon oxide and graphite, wherein the pre-lithiated silicon oxide includes one or more of Li2SiO3, Li4SiO4, or Li2Si2O5; the second active material is SiO 1.2 and artificial graphite and optionally pre-lithiated SiO 1.2 ; the conductive agent is acetylene black; and the binder is styrene-butadiene rubber and sodium carboxymethyl cellulose. The second active material (15% SiO 1.2 and optionally pre-lithiated SiO 1.2 + 85% graphite), acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose are mixed in a weight ratio of 96:1:1.5:1.5 and then dispersed in deionized water to form a slurry. After uniform stirring, the slurry is coated on the copper foil, dried, and a second active material layer is formed. The thickness of the second active material layer is 25 μm, and the mass percentage of silicon in the second active material layer is 5%. The first active material (15% pre-lithiated silicon oxide + 85% graphite), acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose are mixed in a weight ratio of 96:1:1.5:1.5 and then dispersed in deionized water to form a slurry. After uniform stirring, the slurry is coated on the second active material layer, dried, and a first active material layer is formed. The thickness of the first active material layer is 80 μm, and the mass percentage of silicon in the first active material layer is 15%. After cold pressing and slitting, a negative electrode tab is obtained.

[0070] 2. Preparation of the positive electrode tab

[0071] After the positive active material LiCoO2, acetylene black, and the binder polyvinylidene fluoride (PVDF) are mixed uniformly in a mass ratio of 94:3:3 in an N-methylpyrrolidone solvent system with sufficient stirring, the mixture is coated on an aluminum foil, and then dried and cold-pressed to obtain a positive electrode sheet.

[0072] 3. Preparation of the separator film

[0073] Polyvinylidene fluoride is dissolved in water to form a uniform slurry by mechanical stirring, and the slurry is coated on both sides of the porous substrate (polyethylene) which has been coated with ceramic coating on both sides. After drying, a separator film is formed.

[0074] 4. Preparation of the electrolyte

[0075] 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, weight ratio) are prepared in a weight ratio of 8:92 to form an electrolyte with a lithium salt concentration of 1 mol / L.

[0076] 5. Preparation of the lithium ion battery

[0077] 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 sheet and the negative electrode sheet to serve as a separator, and the electrode assembly is obtained by winding. The electrode assembly is placed in an outer packaging aluminum plastic film, and after removing the water at 80°C, the above-mentioned electrolyte is injected and packaged, and after formation, degassing, shaping, and other process procedures, a lithium ion battery is obtained.

[0078] In Examples 2-15 and Comparative Example 1, the positive electrode sheet, the separator film, the electrolyte, and the lithium ion battery are prepared in the same way as in Example 1, except that the preparation of the negative electrode sheet is different, and the differences in the parameters are shown in the corresponding tables below.

[0079] Method for measuring performance parameters of examples and comparative examples

[0080] Sheet porosity test: the fully charged lithium ion battery is subjected to full discharge operation, 0.1C discharge to 3.0V, and after standing for 5min, 0.1C discharge to 3.0V is repeated three times to obtain a fully discharged lithium ion battery. After disassembly, a fixed mold is used to punch the sheet into a 10mm or 14mm diameter disc, and the sheet thickness is measured. The sheet porosity is measured according to GB / T24586.

[0081] XRD test: 1.0-2.0 g of the sample was weighed into the groove of a glass sample holder and compacted and smoothed with a glass bottle, and tested according to JJS K 0131-1996 "General methods for X-ray diffractometer analysis" using an X-ray diffractometer. The test voltage was set to 40 KV, the current was 30 mA, the scanning angle range was 10° to 85°, the scanning step was 0.0167°, and the time set for each step was 0.24 s, and the X-ray diffraction pattern was obtained. From the figure, the 2 theta belonging to 28.4° highest intensity value I2 was obtained, and the highest intensity I1 belonging to 21.0°, and the I2 / I1 ratio was calculated.

[0082] Particle size distribution test: 0.02 g of the powder sample was added to a 50 ml clean beaker, about 20 ml of deionized water was added, and a few drops of 1% surfactant were added to make the powder completely dispersed in the water, and ultrasonic cleaning was performed in a 120 W ultrasonic cleaner for 5 min, and the particle size distribution was tested using a MasterSizer 2000. The volume-based particle size distribution curve was obtained by laser scattering particle size instrument test, and the Dv50 of the sample was calculated.

[0083] Capacity retention rate test: the lithium ion battery was charged and discharged at 25°C under constant temperature conditions using a new Wei machine to test its cycle performance; the charging process was 1.0C charging to the upper limit cutoff voltage, and then discharging to 3.0V at a rate of 0.5C, and the ratio of the discharge capacity after 500 cycles to the discharge capacity of the first cycle was taken as the capacity retention rate.

[0084] Cycle expansion rate test: the thickness of the fresh lithium ion battery at half charge (50% Soc) was tested using a flat plate thickness gauge (load of 600g). When cycled to 500 cycles, the lithium ion battery was in a fully charged state (100% Soc), and the thickness of the lithium ion battery at this time was tested again using a flat plate thickness gauge (600g load), and compared with the initial half-charge thickness, and the cycle expansion rate of the fully charged lithium ion battery at this time was obtained.

[0085] Lithium precipitation disassembly test after cycling: the lithium ion battery was charged to the upper limit cutoff voltage at 25°C with a constant current of 1C. Then charged at a constant voltage of the upper limit cutoff voltage to a current of 0.05C, and stood for 2 min, and then discharged to 3.0V at a constant current of 1.5C, and stood for 2 min, and this was one cycle. After 500 cycles, the lithium ion battery was disassembled to obtain the electrode assembly. The electrode assembly was laid flat, and if lithium precipitation was found in any area of the negative electrode sheet greater than 2 mm 2 , the negative electrode sheet was determined to have lithium precipitation.

[0086] Negative active material layer peeling off / current collector creasing rate: at 25℃, the lithium ion battery was charged to the upper limit voltage with a constant current of 0.5C, then charged to a current of 0.05C with a constant voltage, rested for 2min, then discharged to 3.0V with a constant current of 0.5C, rested for 2min, and this was one cycle. After 100 cycles, the lithium ion battery was disassembled to obtain an electrode assembly, the electrode assembly was spread out, and if it was found that the active material layer of the negative electrode sheet was peeled off from the current collector at any place greater than 2mm 2 , the active material layer of the negative electrode sheet was determined to be peeled off. If it was found that the current collector was creased at any place greater than 1mm 2 , the negative current collector was determined to be creased. As long as one of the above peeling and tearing was met, it was recorded as the negative active material layer peeling off / current collector creasing deformation.

[0087] Test results of each example and comparative example

[0088] Table 1 shows the effect of the mass percentage content of lithium element in the first silicon-based material particles A% and the mass percentage content of lithium element in the second silicon-based material particles B% on the performance of the lithium ion battery. In examples 1 to 5 and comparative example 1, the average particle size D V 50 of the first silicon-based material particles in the first active material layer was 15nm, and the average particle size D V 50 of the second silicon-based material particles in the second active material layer was 5nm; the porosity of the first active material layer was 42.1%, and the porosity of the second active material layer was 38.1%; the mass percentage content of silicon element in the first active material layer was 15%, and the mass percentage content of silicon element in the second active material layer was 5%; the I2 / I1 of the second silicon-based material particles was 0.2.

[0089] Table 1

[0090]

[0091] From the comparison of examples 1-5 and comparative example 1, it can be seen that when the mass percentage content of lithium element in the first silicon-based material particles is higher than that in the second silicon-based material particles, the cycle capacity retention rate and the lithium ion battery expansion rate are significantly improved, and the probability of active material layer peeling off or current collector creasing after cycling is significantly reduced.

[0092] From the comparison of examples 1-5, it can be found that when the mass percentage of lithium element in the second silicon-based material particles increases, the cycle capacity retention rate decreases, and the lithium ion battery expansion rate and the probability of active material layer peeling off / current collector creasing increase.

[0093] Table 2 shows the influence of the percentage content of silicon element D1% in the first active material layer, the percentage content of silicon element D2% in the second active material layer, and the second silicon-based material particles I2 / I1 on the performance of the lithium ion battery. The lithium ion batteries of Examples 6-10 are prepared and the remaining parameters are consistent with Example 1.

[0094] Table 2

[0095]

[0096] By comparing Example 1 with Examples 6-10, it can be found that as I2 / I1 increases, the cycle retention rate decreases, and therefore it is necessary to select a silicon oxide with 0

[0097] By comparing Examples 6-10, it can be found that by using the technical solution, even if the upper layer silicon content increases, there is still no active material layer separation / current collector wrinkling phenomenon after cycling, effectively improving the problem.

[0098] Table 3 shows the influence of the first active material layer electrode porosity K1% and the second active material layer electrode porosity K2% on the performance of the lithium ion battery. The lithium ion batteries of Examples 11-15 are prepared and the remaining parameters are consistent with Example 1, and different electrode porosities are obtained by adjusting the process.

[0099] Table 3

[0100]

[0101] By comparing Examples 11-15, it can be found that when the first active material layer electrode porosity is greater than the second active material layer electrode porosity, the cycle performance and kinetic performance of the lithium ion battery can be improved, and large porosity is beneficial to electrolyte infiltration and improvement of lithium ion diffusion, thereby reducing the risk of negative electrode lithium precipitation after cycling and avoiding cycle decay due to lithium precipitation.

[0102] Table 4 shows the influence of the average particle size D V 50 of the first silicon-based material particles and the average particle size D V 50 of the second silicon-based material particles on the performance of the lithium ion battery. The lithium ion batteries of Examples 16-20 are prepared and the remaining parameters are consistent with Example 1, and Examples 16-20 are obtained by adjusting the D V 50 of different materials.

[0103] Table 4

[0104]

[0105] By comparing Examples 16-20, it can be found that when the average particle size D V 50 of the first silicon-based material particles is greater than the average particle size DV 50 At this time, due to the small size of the underlying silicon, the low cycle expansion, and the small expansion force on the current collector, the tearing rate of the pole piece after cycling is significantly reduced.

[0106] While the illustrative embodiments have been demonstrated and described, it will be understood by those skilled in the art that the above-described embodiments are not to be construed as limiting, and that changes, alternatives, and modifications can be suggested by the application, and are to be within the spirit and scope of the application.

Claims

1. A negative electrode sheet, comprising a current collector, a first active material layer, and a second active material layer between the current collector and the first active material layer, wherein the first active material layer comprising first silicon-based material particles, the second active material layer comprising second silicon-based material particles, a mass percentage content of lithium in the first silicon-based material particles being A%, a mass percentage content of lithium in the second silicon-based material particles being B%, wherein 8≤A≤15, 0 2. The negative electrode sheet according to claim 1, wherein the first silicon-based material particles comprising one or more of Li 2 SiO 3, Li 4 SiO 4, or Li 2 Si 2 O 5.

3. The negative electrode sheet according to claim 2, wherein The surface of the first silicon-based material particles has M x SiO y or one or more of carbon, wherein M comprises at least one of Mg, Al, Zn, Ca, Ba, or B, 0 < x < 3, 0.5 < y < 6.

4. The negative electrode sheet according to claim 1, wherein The second silicon-based material particles comprise SiO x wherein 0.6≤x≤1.

5.

5. The negative electrode sheet according to claim 4, wherein the second silicon-based material particles satisfying: 0 6. The negative electrode sheet according to claim 1, wherein an average particle size Dv50 of the first silicon-based material particles being greater than an average particle size Dv50 of the second silicon-based material particles, the average particle size Dv50 of the first silicon-based material particles satisfying: 5μm≤Dv50≤25μm, and the average particle size Dv50 of the second silicon-based material particles satisfying: 1μm≤Dv50≤8μm.

7. The negative electrode sheet according to claim 1, wherein the first active material layer and the second active material layer each independently further comprising at least one of artificial graphite, natural graphite, mesocarbon microbeads, lithium titanate, soft carbon, or hard carbon. 8.An electrochemical device comprising the negative electrode sheet according to any one of claims 1 to 7. 9.An electronic device comprising the electrochemical device according to claim 8.

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