Electrochemical device and electronic equipment

Abstract

The application discloses a negative electrode sheet, an electrochemical device and an electronic equipment. The electrochemical device comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode active material, the negative electrode active material comprises a silicon-based material, the silicon-based material is obtained by arranging a surface coating layer on the surface of silicon particles, the surface coating layer comprises a soluble polymer, the soluble polymer is soluble in an electrolyte, and the mass concentration of the soluble polymer in the electrolyte is 0.2 wt% to 6 wt% after the electrochemical device is injected into the electrolyte. In the electrochemical device, the surface of the negative electrode active material silicon particles is coated with a surface coating layer, the surface coating layer contains a soluble polymer, the soluble polymer is soluble in an electrolyte, and after dissolving, a gap is reserved between the silicon particles and surrounding particles, which is beneficial to improving the thickness expansion rate of the electrode sheet and improving the cycle performance and rate performance of the lithium ion battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, and particularly relates to an electrochemical device and an electronic device. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy density, high working voltage, and small environmental pollution, and have been widely applied to various small portable devices such as mobile phones, digital cameras, notebook computers and unmanned aerial vehicles. Among the non-carbon negative electrode active materials, the theoretical specific capacity of silicon material can reach 4200 mAh / g, which is 10 times higher than the current commercial graphite negative electrode. The use of silicon negative electrode can greatly improve the energy density of lithium ion batteries. However, the relatively low compaction density of silicon material causes a very significant thickness rebound of the silicon electrode sheet during processing, resulting in unsatisfactory cycle performance and rate performance of lithium ion batteries containing silicon negative electrodes. SUMMARY

[0003] Therefore, the present application provides an electrochemical device and an electronic device. The electrochemical device provided by the present application is obtained by setting a surface coating layer on the surface of silicon particles. After the silicon particles containing the surface coating layer contact with the electrolyte, the surface coating layer will dissolve, so that the silicon particles have a larger swelling space, thereby improving the thickness rebound problem of the silicon system electrode sheet during processing.

[0004] In a first aspect, the present application provides a negative electrode sheet. The negative electrode sheet comprises a negative electrode active material. The negative electrode active material comprises a silicon-based material, which is obtained by setting a surface coating layer on the surface of silicon particles. The surface coating layer comprises a soluble polymer, which is configured to be soluble in the electrolyte. After the surface coating layer dissolves, the silicon particles have a larger gap with the surrounding particles. During the charging and discharging process, the swelling of the silicon particles has a smaller extrusion effect on the surrounding particles, thereby improving the swelling of the whole electrode sheet to a greater extent, and further improving the swelling problem during the cycle and the rate performance of the battery.

[0005] In some embodiments, after the electrochemical device is injected into the electrolyte, the soluble polymer will dissolve in the electrolyte. The mass ratio of the soluble polymer to the silicon particles is 0.05-1.2:1. When the mass ratio of the soluble polymer to the silicon particles meets the above range, it is beneficial to improve the thickness swelling rate of the electrode sheet and improve the cycle performance of the lithium ion battery.

[0006] In some embodiments, further, the mass ratio of the soluble polymer to the silicon particles is 0.3-0.5:1. When the mass ratio of the soluble polymer to the silicon particles meets the above range, it is beneficial to further improve the thickness swelling rate of the electrode sheet and improve the cycle performance of the lithium ion battery.

[0007] In some embodiments, the surface coating layer further comprises carbon nanotubes; and a mass ratio of the carbon nanotubes to the silicon particles is 0.005-0.1:1. When the mass ratio of the carbon nanotubes to the silicon particles meets the above range, it is beneficial to further improve the thickness expansion rate of the electrode sheet and improve the cycle performance of the lithium ion battery.

[0008] In a second aspect, the present application provides an electrochemical device, which comprises a negative electrode sheet, the negative electrode sheet comprising a negative electrode active material, the negative electrode active material comprising a silicon-based material, the silicon-based material being obtained by disposing a surface coating layer on the surface of silicon particles, the surface coating layer comprising a soluble polymer, the soluble polymer being soluble in an electrolyte, and the silicon particles after the surface coating layer dissolves having a large gap with surrounding particles, and the expansion of the silicon particles has a small extrusion effect on the surrounding particles in the charging and discharging process, thus greatly improving the expansion of the whole electrode sheet, and further improving the expansion problem in the cycle process and improving the rate performance of the battery.

[0009] In some embodiments, after the electrochemical device is injected into the electrolyte, the soluble polymer will be dissolved in the electrolyte, and further, the mass concentration of the soluble polymer in the electrolyte is 0.2wt% to 6wt%. When the mass concentration of the soluble polymer in the electrolyte meets the above range, it is beneficial to improve the thickness expansion rate of the electrode sheet and improve the high-temperature cycle performance of the lithium ion battery. Preferably, the mass concentration of the soluble polymer in the electrolyte is 1.3wt% to 1.5wt%.

[0010] In some embodiments, the electrolyte comprises linear ester and cyclic ester, and a mass ratio of the linear ester to the cyclic ester is (1-4):(4-1). At this time, the ionic conductivity of the electrolyte is suitable, which is beneficial to dissolve the soluble polymer in the surface coating layer on the one hand, and can better be free in the pores of the negative electrode sheet on the other hand, which is beneficial to improve the rate performance of the lithium ion battery. Preferably, the mass ratio of the linear ester to the cyclic ester is (1-3):1.

[0011] In some embodiments, the soluble polymer comprises an alcohol-soluble resin, and the alcohol-soluble resin is selected from at least one of alcohol-soluble polyamide resin, alcohol-soluble acrylic resin or alcohol-soluble polyurethane resin. Preferably, the alcohol-soluble resin comprises alcohol-soluble polyurethane resin.

[0012] In some embodiments, the molecular weight of the alcohol-soluble resin is 8*10 3 to 1.5*10 5The alcohol-soluble resin has a molecular weight within the above range, which is more conducive to achieving a proper mass concentration of the alcohol-soluble resin in the electrolyte, so that the surface of the silicon particles has a greater free expansion space, and the thickness expansion rate of the electrode plate is reduced.

[0013] In some embodiments, the mass ratio of the soluble polymer to the silicon particles is 0.05-1.2:1. Preferably, the mass ratio of the soluble polymer to the silicon particles is 0.3-0.5:1. At this time, the thickness expansion rate of the negative electrode plate is more improved.

[0014] In some embodiments, the mass ratio of the carbon nanotubes to the silicon particles is 0.005:1-0.1:1. Preferably, the mass ratio of the carbon nanotubes to the silicon particles is 0.01-0.05:1.

[0015] In some embodiments, the carbon nanotubes include single-walled carbon nanotubes, and the aspect ratio of the single-walled carbon nanotubes is 1*10 4 to 4*10 4 . Preferably, the aspect ratio of the single-walled carbon nanotubes is 2.5*10 4 to 3*10 4 .

[0016] In some embodiments, the dissolution amount of the soluble polymer in a specific solution is greater than 80%, and the specific solution includes linear esters and cyclic esters, and the mass ratio of the linear esters to the cyclic esters is 3:1. The dissolution amount refers to the mass of the soluble polymer dissolved in the solution after the soluble polymer is soaked in the specific solution at 60°C.

[0017] The test method of the dissolution amount includes: preparing a film with the soluble polymer and weighing m0, soaking the film prepared from the polymer in the specific electrolyte at 60°C until the weight of the film no longer changes, taking out the soaked film and weighing after drying, which is recorded as mass m1, and the dissolution amount Co=(m0-m1) / m0.

[0018] In some embodiments, the porosity of the negative electrode plate is 25%-48%. Preferably, the porosity of the negative electrode plate is 25%-33%.

[0019] In some embodiments, the contact angle of the negative electrode plate to the electrolyte is 0°-45°.

[0020] In some embodiments, the negative electrode active material further contains graphite, and the mass ratio of the silicon particles to the graphite is 1:23-29:1.

[0021] In some embodiments, the thickness of the surface coating layer is 0.5-10 μm. Preferably, the thickness of the surface coating layer is 2-3 μm.

[0022] It should be noted that the thickness of the surface coating layer changes with the change of the mass ratio m1 / m2 of the soluble polymer and the silicon particles, specifically, m1 is the mass fraction of the soluble polymer, and m2 is the mass fraction of the silicon particles. When the mass fraction m1 of the soluble polymer is reduced (i.e., the mass ratio is reduced), the thickness of the surface coating layer is also reduced. When the mass fraction m1 of the soluble polymer is increased (i.e., the mass ratio is increased), the thickness of the surface coating layer is also increased.

[0023] In a third aspect, the present application provides an electronic device comprising the above-mentioned electrochemical device.

[0024] The technical scheme provided by some embodiments of the present application has at least the following beneficial effects: the present application provides an electrochemical device, the surface of the silicon particles of the negative electrode active material in the electrochemical device is coated with a surface coating layer, the surface coating layer contains a soluble polymer, the soluble polymer is soluble in the electrolyte, and after dissolution, a gap is reserved between the silicon particles and the surrounding particles. At this time, on the one hand, during the charging and discharging process, the expansion of the silicon particles has less extrusion effect on the surrounding particles, which is beneficial to improve the thickness expansion rate of the electrode sheet, and thus improve the cycle performance of the lithium ion battery, especially the high-temperature cycle performance. On the other hand, it is beneficial for the electrolyte to better dissociate in the pores of the negative electrode sheet, thereby improving the rate performance of the lithium ion battery. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not to limit the present application.

[0026] At present, the improvement of low compaction density for high-capacity silicon-based materials mainly focuses on the following aspects: 1) improving particle size or morphology, such as nano-SiO, nano-SiC, etc. Nano-materials can alleviate the volume effect to some extent, have better packing effect and better compaction density; 2) using new high-adhesion adhesives. However, there are two common problems in using SiO, SiC and other nano-composite materials. The specific capacity of Si composite material is lower than that of pure Si, and the actual reversible specific capacity is lower. For example, the specific capacity of SiO negative electrode material is 2400 mAh / g, and the actual reversible specific capacity is about 1500 mAh / g. Another common problem of silicon composite material is that the electrode is pulverized during the cycle process, which leads to the failure of electronic and ionic conduction path, resulting in poor cycle performance. Moreover, due to the complex process of composite materials, the preparation cost of nano-structured Si materials is still very high. When the size of Si is reduced from bulk to nanoscale, the processing cost will at least double.

[0027] To solve the problems of silicon material thickness rebound in the processing process, silicon system energy density cannot be fully played, and poor cycle performance, etc., the application provides a negative electrode sheet and an electrochemical device comprising the same, which can solve the thickness rebound of the silicon system electrode sheet in the processing process.

[0028] Electrochemical device

[0029] The electrochemical device comprises a negative electrode sheet, a positive electrode sheet, a separator, and an electrolyte.

[0030] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material, the negative electrode active material comprises a silicon-based material, the silicon-based material is obtained by arranging a surface coating layer on the surface of a silicon particle, the surface coating layer comprises a soluble polymer, and the mass concentration of the soluble polymer in the electrolyte is 0.2wt% to 6wt% after the electrochemical device is injected into the electrolyte.

[0031] Exemplarily, the mass concentration of the soluble polymer in the electrolyte is 0.2wt%, 0.5wt%, 0.8wt%, 1wt%, 1.3wt%, 1.5wt%, 1.8wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 6wt%, or a range formed by any two of the above values after the electrochemical device is injected into the electrolyte.

[0032] In some embodiments, the soluble polymer comprises an alcohol-soluble resin selected from at least one of an alcohol-soluble polyamide resin, an alcohol-soluble acrylic resin, or an alcohol-soluble polyurethane resin.

[0033] Specifically, in some examples, the alcohol-soluble resin comprises any one of an alcohol-soluble polyamide resin, an alcohol-soluble acrylic resin, or an alcohol-soluble polyurethane resin; in other examples, the alcohol-soluble resin is selected from at least two of an alcohol-soluble polyamide resin, an alcohol-soluble acrylic resin, or an alcohol-soluble polyurethane resin; and in other examples, the alcohol-soluble resin comprises an alcohol-soluble polyamide resin, an alcohol-soluble acrylic resin, and an alcohol-soluble polyurethane resin.

[0034] In some embodiments, the molecular weight of the alcohol-soluble resin is 8*10 3 to 1.5*10 5 . Exemplarily, the molecular weight of the alcohol-soluble resin is 8*10 3 , 9*10 3 , 1*104 , 3*10 4 , 5*10 4 , 8*10 4 , 1*10 5 , 1.5*10 5 , 2*10 4 , 2.5*10 4 , 3*10 4 , 3.5*10 4 , 4*10 4 , or a range between any two of the foregoing values.

[0035] In some embodiments, the mass ratio of the soluble polymer to the silicon particles is 0.05-1.2:1. Illustratively, the mass ratio of the soluble polymer to the silicon particles is 0.05:1, 0.08:1, 0.1:1, 0.3:1, 0.35:1, 0.4:1, 0.5:1, 0.8:1, 1:1, 1.2:1, or a range between any two of the foregoing values.

[0036] In some embodiments, the mass ratio of the carbon nanotubes to the silicon particles is 0.005-0.1:1. Illustratively, the mass ratio of the carbon nanotubes to the silicon particles is 0.005:1, 0.008:1, 0.01:1, 0.02:1, 0.03:1, 0.05:1, 0.08:1, 0.1:1, or a range between any two of the foregoing values.

[0037] In some embodiments, the carbon nanotubes comprise single-walled carbon nanotubes having an aspect ratio of 1*10 4 to 4*10 4 . Illustratively, the single-walled carbon nanotubes have an aspect ratio of 1*10 4 , 1.5*10 4 , 2*10 4 , 2.5*10 4 , 3*10 4 , 3.5*10 4 , 4*10 4 , or a range between any two of the foregoing values.

[0038] In some embodiments, the negative electrode tab has a porosity of 25% to 48%. Illustratively, the negative electrode tab has a porosity of 25%, 28%, 30%, 33%, 35%, 38%, 40%, 45%, 48%, or a range between any two of the foregoing values.

[0039] In some embodiments, the surface coating layer has a thickness of 0.5 pm to 10 pm. Illustratively, the surface coating layer has a thickness of 0.5 pm, 0.8 pm, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm, 8 pm, 9 pm, 10 pm, or a range between any two of the foregoing values.

[0040] In some embodiments, the negative active material layer further comprises a binder. Illustratively, the binder comprises one or more of styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), water-based acrylic resin, or carboxymethyl cellulose (CMC).

[0041] The negative current collector can use a metal foil or a porous metal plate or the like material, for example, a foil or a porous plate of a metal such as copper, nickel, titanium, or iron or an alloy thereof, such as a copper foil.

[0042] The negative electrode sheet can be prepared according to conventional methods in the art. Illustratively, the negative active material and optionally a conductive agent and a binder are dispersed in a solvent, which can be at least one of water, ethanol, acetone, butanone, dimethylformamide, N-methylpyrrolidone, diethylformamide, dimethyl sulfoxide, or tetrahydrofuran, to form a uniform negative electrode slurry, the negative electrode slurry is coated on a negative electrode current collector, and the negative electrode sheet is obtained through processes such as drying and cold pressing. Illustratively, the oven temperature is set to 60°C to 110°C, the current collector coated with the slurry is dried in the oven, and an electrode material coating with excellent adhesion and smooth surface is obtained, and the other side of the current collector is coated in the same manner.

[0043] Method for preparing silicon-based material

[0044] Illustratively: carbon nanotubes and silicon particles are mixed in a mass ratio of 0.005 to 0.1:1, a disperser is used to disperse at 300 rpm / min to 500 rpm / min for 10 to 20 minutes, a soluble polymer is then added, the mass ratio of the soluble polymer to the silicon particles is 0.05 to 1.2:1, and water is added for mixing. After mixing, the solid content of the mixed solution is 50% to 60%, a disperser is then used to disperse at 800 rpm / min to 100 rpm / min for 2 to 3 hours, water is then added to dilute the solid content of the mixed solution to 10% to 20%, and a disperser is then used to disperse at 800 rpm / min to 100 rpm / min for 2 to 3 hours. The mixed solution is then added to a cyclone separation tower, the temperature of the cyclone separation tower is set to 115°C to 125°C, and the solid powder is collected.

[0045] The electrolyte includes an organic solvent, an electrolyte lithium salt, and an additive. The organic solvent includes at least two of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), or diethyl sulfone (ESE).

[0046] In some embodiments, in the organic solvent, the mass ratio of linear ester and cyclic ester is (1-4):(4-1).

[0047] The electrolyte lithium salt includes one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonylimide), LiTFSI (lithium bis-trifluoromethanesulfonylimide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalato)borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluoro(oxalato)phosphate), or LiTFOP (lithium tetrafluoro(oxalato)phosphate).

[0048] The additive includes one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), succinonitrile (SN), adiponitrile (AND), 1,3-propanesultone (PST), tris(trimethylsilyl)phosphate (TMSP), or tris(trimethylsilyl)borate (TMSB).

[0049] Others

[0050] The positive electrode tab includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material in the positive electrode active material layer can be selected from one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and compounds obtained by adding other transition metals or non-transition metals to the above compounds.

[0051] Exemplarily, the positive electrode current collector can use a metal foil or a porous metal plate, etc. material, for example, a foil or a porous plate of a metal such as aluminum, copper, nickel, titanium, or iron, or an alloy thereof, such as an Al (aluminum) foil.

[0052] The positive electrode sheet can be prepared according to a conventional method in the art.

[0053] The separator film is not particularly limited and can be any known porous separator film having electrochemical stability and chemical stability, such as glass fiber, nonwoven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF) single layer or multilayer film.

[0054] The electrochemical device can be prepared according to a conventional method in the art. For example, the above-described positive electrode sheet, separator film, and negative electrode sheet are stacked in order with the separator film between the positive electrode sheet and the negative electrode sheet to function as a separator, thereby obtaining an electrode assembly, which can be wound to obtain an electrode assembly; and the electrode assembly is placed in a packaging case, an electrolyte solution is injected, and the case is sealed, thereby obtaining an electrochemical device.

[0055] The electrochemical device of the present application can include any device in which an electrochemical reaction occurs, and specific examples thereof include all kinds of primary or secondary batteries. 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.

[0056] Electronic device

[0057] The electronic device of the present application includes any one of the above-described electrochemical devices of the present application. The electronic device of the present application can be used for, 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, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, an illuminating appliance, a toy, a game machine, a clock, an electric tool, a flash, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0058] Hereinafter, embodiments of the present application will be described more specifically by citing examples and comparative examples. Unless otherwise stated, the parts, percentages, and ratios listed below are based on weight, and the raw materials used are commercially available or synthesized according to a conventional method.

[0059] Example 1-1

[0060] Preparation of silicon-based material

[0061] Carbon nanotubes and silicon particles were mixed at a mass ratio of 0.01:1, dispersed at 300 rpm / min for 10 min using a disperser, and then polyamide resin (molecular weight: 8*103 ~ 1.5 * 10 5 ), the mass ratio of the polyamide resin and the silicon particles is 0.35:1, and water is added for mixing. After mixing, the solid content of the mixed solution is 50%, and the mixed solution is dispersed at 800 rpm / min for 2 h using a disperser. Then, water is added to dilute the solid content of the mixed solution to 10%, and the mixed solution is dispersed at 800 rpm / min for 2 h using a disperser. Subsequently, the mixed solution is added to a cyclone separation tower through a peristaltic pump, and the temperature of the cyclone separation tower is set to 120°C. The solid powder is collected, and the silicon-based material is obtained.

[0062] Examples 1-2 to 1-17

[0063] Different from Example 1-1, some parameters and values of the parameters are adjusted in the preparation process of the silicon-based material, and the specific values are shown in Table 1. The rest is the same.

[0064] Comparative Example 1

[0065] Different from Example 1-1, the silicon-based material, i.e., the silicon particles with a silicon content of 10%, is not coated with a coating layer on the surface.

[0066] Comparative Examples 2 to 5

[0067] Different from Example 1-1, some parameters and values of the parameters are adjusted in the preparation process of the silicon-based material, and the specific values are shown in Table 1. The rest is the same.

[0068] Preparation of the negative electrode sheet

[0069] The prepared silicon-based material, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), and thickening agent sodium methyl cellulose (CMC) are uniformly mixed in a deionized water solvent according to a mass ratio of 95:2:2:1, and are coated on one side surface of a 9 μm Cu foil and dried. The above steps are repeated on the other side surface of the Cu foil to obtain a negative electrode sheet coated with a negative electrode active layer on both sides. After the coating is completed, the negative electrode sheet is dried, cold-pressed, and then cut into a sheet with a specification of 74 mm x 800 mm for use.

[0070] Preparation of the positive electrode sheet

[0071] The positive electrode active material, i.e., lithium cobalt oxide active substance LiCoO2, conductive carbon black Super-P, and binder PVDF are fully stirred in an N-methyl pyrrolidone (NMP) solvent system according to a weight ratio of 97.6:1.3:1.1 by using a vacuum stirrer to obtain a positive electrode slurry. The positive electrode slurry is coated on both surfaces of a 9 μm Al foil substrate, and the coating weight is 280 mg. After the coating is completed, the positive electrode sheet is dried, cold-pressed, and then cut into a sheet with a specification of 74 mm x 800 mm for use.

[0072] Preparation of electrolyte

[0073] In a glove box filled with dry argon, organic solvents ethylene carbonate (EC), diethyl carbonate (DEC), propyl propionate (PP) and adiponitrile (ADN) were mixed in a mass ratio of 1:2:1:0.15, then lithium hexafluorophosphate (LiPF6) was added to the above organic solvents to dissolve and mix uniformly, to obtain an electrolyte with a LiPF6 concentration of 1.15 mol / L.

[0074] Preparation of separator film

[0075] A porous polyethylene film with a thickness of 16 μm was used as the separator film.

[0076] Preparation of lithium ion battery

[0077] The positive electrode sheet, the separator film and the negative electrode sheet prepared above were sequentially stacked in order, and then wound to obtain an electrode assembly. After welding the tabs, the electrode assembly was loaded into an aluminum plastic film, dried in a vacuum oven at 80°C for 12 hours to remove water, then the above prepared electrolyte was injected, vacuum packaged, rested, formed (0.02C constant current charging to 3.5V, then 0.1C constant current charging to 3.9V), capacity, shaping and other processes to obtain a lithium ion battery. In this application, if not otherwise specified, the preparation can be carried out by referring to the conventional technical means in the art.

[0078] Test method:

[0079] (1) Test method of mass concentration

[0080] The lithium salt concentration C1 in the electrolyte was calculated by inductively coupled plasma (ICP) test. Part of the electrolyte was weighed as m1, dried in a 120°C oven for 8h, and the mass of the remaining residual solid was weighed as m2. The mass concentration of the soluble polymer in the electrolyte was m wt% = (m2-C1*m1) / m1.

[0081] (2) Test method of thickness expansion rate

[0082] The negative electrode sheet expansion rate was obtained by disassembling the anode sheet of the battery; the fresh battery after formation was charged to 3.91V, and the corresponding battery was disassembled to obtain the negative electrode sheet. The average thickness of the negative electrode sheet was tested by using a micrometer to obtain the thickness T1. The cycled battery was charged to the full charge voltage at 1C, and then the full charge voltage was maintained until the current was 0.05C. The corresponding battery was disassembled, and the average thickness of the negative electrode sheet was tested by using a micrometer to obtain the thickness T2. The negative electrode sheet expansion rate was THK = T2-T1 / T1.

[0083] (3) Test method of high temperature cycle performance

[0084] The lithium ion battery was charged at 45 °C at a constant current of 1C rate to a full charge voltage of 4.5 V, then charged at a constant voltage of full charge voltage to a current of 0.05C, and then discharged at a constant current of 1C rate to 3.0 V. This was one charge-discharge cycle, and the discharge capacity of the first cycle was recorded. The lithium ion battery was cycled 400 times according to the above method, and the capacity retention rate of the 400th cycle was calculated. The capacity retention rate % = discharge capacity of the cycle / discharge capacity of the first cycle.

[0085] (4) Test method of rate performance

[0086] The lithium ion battery was charged at 25 °C at a constant current of 0.2C rate to a full charge voltage, then charged at a constant voltage of full charge voltage to a current of 0.02C, and then discharged at a constant current of 0.2C rate to 3.0 V. The discharge capacity at 0.2C rate was recorded.

[0087] The lithium ion battery was charged at 25 °C at a constant current of 0.5C rate to a full charge voltage, then charged at a constant voltage of full charge voltage to a current of 0.02C, and then discharged at a constant current of 2C rate to 3.0 V. The discharge capacity at 2C rate was recorded.

[0088] 2C discharge capacity retention rate (%) = 2C rate discharge capacity / 0.2C rate discharge capacity x 100%.

[0089] Table 1

[0090]

[0091]

[0092] In combination with Table 1, by comparing Example 1-1 and Comparative Example 1, it can be seen that the silicon particles in Comparative Example 1 were not coated, and the thickness expansion rate of the negative electrode sheet containing the silicon particles without a surface coating layer was as high as 16.0%, and the discharge capacity retention rate at 2C was only 64.5%. In Example 1-1, the silicon particles were coated with the surface coating layer, and when the silicon-based material in Example 1-1 was used as the negative electrode active material to prepare a lithium ion battery, the thickness expansion rate of the negative electrode sheet was as low as 9.1%, which was about 7% lower than that of Comparative Example 1, and the discharge capacity retention rate at 2C was as high as 79.3%, which was about 15% higher than that of Comparative Example 1.

[0093] Compared with Example 1-1, Example 1-2 and Example 1-3 select other suitable alcohol-soluble resins as the soluble polymer to prepare the surface coating layer for coating the silicon particles, and the lithium ion battery prepared has similar or better technical effects to Example 1-1. In particular, the alcohol-soluble resin used in Example 1-3 is a polyurethane resin, and the thickness expansion rate of the negative electrode sheet of the lithium ion battery prepared in Example 1-3 is only 8.2%, and the 2C discharge capacity retention rate is increased to 82%.

[0094] Compared with Example 1-3, Example 1-4, Example 1-5 and Example 1-6 further adjust the mass ratio of the alcohol-soluble polymer to the silicon particles. It can be seen that when the mass ratio decreases (the alcohol-soluble polymer fraction is small), the thickness of the surface coating layer decreases, which causes the mass concentration of the alcohol-soluble polymer in the electrolyte to be out of the appropriate range, and the thickness expansion rate of the negative electrode sheet of the lithium ion battery prepared is significantly increased to 8.5%, and the 2C discharge capacity retention rate is reduced to 79.5%. Similar results are obtained in Example 1-9, and when the mass ratio increases (the alcohol-soluble polymer fraction is large), the thickness of the surface coating layer increases, which also causes the mass concentration of the alcohol-soluble polymer in the electrolyte to be out of the appropriate range, and Example 1-9 also has similar results.

[0095] Compared with Example 1-3, it can be seen that the surface coating layer contains an appropriate amount of single-walled carbon nanotubes, which can further improve the thickness expansion rate of the negative electrode sheet. Compared with Example 1-3, it can be seen that when the aspect ratio of the single-walled carbon nanotubes is adjusted to an appropriate range, it is more beneficial to reduce the thickness expansion rate of the negative electrode sheet and improve the rate performance of the lithium ion battery.

[0096] Examples 2-1 to 2-11 in Table 2 are all adjusted on the basis of Example 1-11, and specific reference can be made to Table 2.

[0097] Table 2

[0098]

[0099] It can be seen from Table 2 that when the mass ratio of linear ester and cyclic ester in the electrolyte is in an appropriate range, it is beneficial to improve the thickness expansion rate of the negative electrode sheet, and the porosity of the negative electrode sheet is appropriate, which can also better improve the thickness rebound problem of the silicon system electrode sheet during processing.

[0100] The above only describes preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A negative electrode sheet, characterized by, The negative electrode sheet comprises a negative electrode active material; The negative electrode active material comprises a silicon-based material, and the silicon-based material is obtained by disposing a surface coating layer on the surface of silicon particles; The surface coating layer comprises a soluble polymer, and the soluble polymer is soluble in the electrolyte; The mass ratio of the soluble polymer to the silicon particles is (0.3-0.5):1; The porosity of the negative electrode sheet is 25%-40%.

2. The negative electrode sheet according to claim 1, characterized by The surface coating layer further comprises carbon nanotubes; The mass ratio of the carbon nanotubes to the silicon particles is (0.005-0.1):

1.

3. An electrochemical device, characterized by, The electrochemical device comprises a negative electrode sheet; The negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material, and the silicon-based material is obtained by disposing a surface coating layer on the surface of silicon particles; and the porosity of the negative electrode sheet is 25%-40%; The surface coating layer comprises a soluble polymer, and the soluble polymer is soluble in the electrolyte; After the electrochemical device is injected into the electrolyte, the mass concentration of the soluble polymer in the electrolyte is 0.2wt%-6wt%.

4. The electrochemical device of claim 3, wherein After the electrochemical device is injected into the electrolyte, the mass concentration of the soluble polymer in the electrolyte is 1.3wt%-1.5wt%.

5. The electrochemical device of claim 3, wherein The surface coating layer further comprises carbon nanotubes; The electrochemical device satisfies at least one of the following conditions: (1) the mass ratio of the soluble polymer to the silicon particles is (0.05-1.2):1; (2) the mass ratio of the carbon nanotubes to the silicon particles is (0.005-0.1):1; (3) the carbon nanotubes include single-walled carbon nanotubes having an aspect ratio of 1*10 4 to 4*10 4 .

6. The electrochemical device according to claim 5, wherein The electrochemical device satisfies at least one of the following conditions: (1) the mass ratio of the soluble polymer to the silicon particles is (0.3-0.5):1; (2) the mass ratio of the carbon nanotubes to the silicon particles is (0.01-0.05):1; (3) the carbon nanotubes include single-walled carbon nanotubes having an aspect ratio of 2.5*10 4 to 3*10 4 .

7. The electrochemical device of claim 3, wherein The soluble polymer comprises an alcohol-soluble resin, and at least one of the following conditions is satisfied: (1) the alcohol-soluble resin has a molecular weight of 8*10 3 to 1.5*10 5 ; (2) the alcohol-soluble resin is selected from at least one of an alcohol-soluble polyamide resin, an alcohol-soluble acrylic resin or an alcohol-soluble polyurethane resin.

8. The electrochemical device of claim 3, wherein The soluble polymer has a dissolution amount in a specific solution of >80%; The specific solution comprises linear ester and cyclic ester, and the mass ratio of the linear ester to the cyclic ester is 3:

1.

9. The electrochemical device of claim 3, wherein, The electrolyte comprises linear ester and cyclic ester, and the mass ratio of the linear ester to the cyclic ester is (1-4):(4-1).

10. The electrochemical device of claim 3, wherein At least one of the following conditions is satisfied: (1) the porosity of the negative electrode sheet is 25%-33%; (2) the electrolyte comprises linear ester and cyclic ester, and the mass ratio of the linear ester to the cyclic ester is (1-3):

1.

11. The electrochemical device of claim 3, wherein, The thickness of the surface coating layer is 0.5μm-10μm.

12. The electrochemical device of claim 3, wherein, The negative electrode active material further comprises graphite, and the mass ratio of the silicon particles to the graphite is 1:(23-29):

1.

13. An electronic device, comprising: The electronic device comprises the electrochemical device according to any one of claims 3-12.

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