Silicon-carbon negative electrode material and application thereof

Abstract

The invention discloses a silicon-carbon negative electrode material and application thereof, and belongs to the technical field of batteries, the silicon-carbon negative electrode material is provided with a coating layer on the surface, and meanwhile, the order degree of the material and the balance of the overall material interface stability are regulated and controlled; according to the present invention, the influence caused by the silicon material interface expansion / shrinkage effect when the silicon material is applied to the secondary battery can be effectively reduced, and the side reaction degree with the electrolyte is reduced, such that the initial DCR is low, and the ideal capacity retention rate during the cycle process is provided.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a silicon-carbon negative electrode material and its application. Background Art

[0002] Due to their high theoretical specific capacity (the theoretical capacity of elemental silicon is as high as 4200mAh / g), silicon-based materials, when compounded with highly conductive carbon materials to form silicon-carbon materials, should theoretically exhibit ideal electrochemical performance when used as negative electrode materials for secondary batteries. However, due to the significant interfacial expansion and contraction of silicon during cycling, and the susceptibility of the overall material to side reactions with the electrolyte, resulting in a high initial DCR, the capacity retention rate of secondary batteries during cycling is poor, making it difficult to achieve ideal application results. Summary of the Invention

[0003] The purpose of the present application is to overcome the shortcomings of the existing technology and provide a silicon-carbon negative electrode material. By providing a coating layer on the surface of the material and regulating the balance between the order of the material and the overall material interface stability, the impact of the silicon material interface expansion / contraction effect when it is used in a secondary battery can be effectively reduced, and the degree of side reaction with the electrolyte is reduced, so that the initial DCR is low and the capacity retention rate is ideal during the cycle.

[0004] To achieve the above-mentioned object, in a first aspect of the present application, the present application provides a silicon-carbon negative electrode material, comprising a carbon matrix and silicon particles, wherein the carbon matrix contains silicon particles;

[0005] A carbon coating layer is provided on the surface of the silicon-carbon negative electrode material;

[0006] The silicon-carbon negative electrode material satisfies: T / O=1.25-2.45;

[0007] Wherein T=15×(1-α) / γ, (1-α) is the coating integrity of the carbon coating layer on the surface of the silicon-carbon negative electrode material, and γnm is the average thickness of the carbon coating layer;

[0008] O is the degree of ordering of the silicon-carbon negative electrode material.

[0009] In a second aspect of the present application, the present application further provides a secondary battery comprising a negative electrode plate, wherein the negative electrode plate comprises the silicon-carbon negative electrode material.

[0010] In a third aspect of the present application, the present application further provides an electrical device, comprising the secondary battery, wherein the secondary battery is used as a power supply in the electrical device.

[0011] The beneficial effects of this application are:

[0012] The present application provides a silicon-carbon negative electrode material. By providing a coating layer on the surface of the material and regulating the balance between the order of the material and the overall material interface stability, the impact of the silicon material interface expansion / contraction effect when it is used in a secondary battery can be effectively reduced, and the degree of side reaction with the electrolyte is reduced, thereby having a low initial DCR and an ideal capacity retention rate during the cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a schematic diagram of the silicon-carbon composite material described in this application under an ultra-high magnification transmission electron microscope. DETAILED DESCRIPTION

[0014] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0015] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0016] In this application, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0017] The present application is further described below with specific examples:

[0018] A silicon-carbon negative electrode material comprises a carbon matrix and silicon particles, wherein the carbon matrix contains silicon particles;

[0019] A carbon coating layer is provided on the surface of the silicon-carbon negative electrode material;

[0020] The silicon-carbon negative electrode material satisfies: T / O=1.25-2.45;

[0021] Wherein T=15×(1-α) / γ, (1-α) is the coating integrity of the carbon coating layer on the surface of the silicon-carbon negative electrode material, and γnm is the average thickness of the carbon coating layer;

[0022] O is the degree of ordering of the silicon-carbon negative electrode material.

[0023] In order to overcome the defects of low capacity retention and high DCR of silicon-carbon negative electrode materials when applied to secondary battery cycles, in the technical solution of the present application, in a composite material with a carbon matrix and silicon particles as the main body, the silicon particles are first arranged (partially or completely) in the carbon matrix, and a carbon coating layer is arranged on the surface. The carbon matrix and the coating layer are used to control the influence of the interface expansion / contraction effect of the silicon particles during the cycle, and the interface stability of the overall material and the order of the material are regulated. The present application solution represents the interface stability of the material by the ratio of the coating integrity of the carbon coating layer on the surface of the silicon-carbon negative electrode material and the average thickness of the carbon coating layer. When the stability is too high, the dynamics of the overall material is affected, which is not conducive to the rapid conduction of lithium ions. However, if the stability is too low, it will On the other hand, the order degree of the material needs to match the interface stability. If the order degree is too low, it will not only affect the conductivity of the overall material, but also the probability of side reactions after the material is in contact with and infiltrated with the electrolyte will increase, and the stability of the material structure will also be poor. If the order degree is too high, the kinetic performance of the overall material will deteriorate, which is not conducive to the efficiency of lithium ion deintercalation and extraction. Therefore, it is necessary to regulate both within a limited range. Based on the setting of material structure and parameters, the negative electrode silicon-carbon material has ideal structural stability when applied to secondary batteries, a low probability of side reactions with the electrolyte, and thus a low initial DCR, improved kinetic performance, high capacity retention rate during cycling, long service life, and excellent comprehensive electrochemical performance.

[0024] In some embodiments, the T / O is in the range of one or any two of 1.25, 1.5, 1.55, 1.6, 1.7, 1.75, 1.8, 1.85, 1.89, 2, 2.25, 2.3, 2.45, 2.45.

[0025] More preferably, the T / O=1.55-1.89.

[0026] In the present application, the interface stability of the overall material and the degree of order of the material jointly affect the structural stability, ion / electron conduction efficiency and activity of the reaction with the electrolyte of the silicon-carbon negative electrode material during the charge and discharge cycle. When the ratio of the two is preferably within the above range, the silicon and carbon materials in the overall material can play a better synergistic role, while taking into account the kinetics and chemical stability, achieving further improvement in ion / electron conduction efficiency, and can exhibit lower initial DCR and better cycle capacity retention when applied to secondary batteries.

[0027] In some embodiments, T=0.9-2.7.

[0028] Further preferably, T is in the range of one or any two of 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, and 2.7.

[0029] More preferably, T=1.4-1.8.

[0030] The interfacial stability of the silicon-carbon negative electrode material is related to the kinetic properties of the overall material during the charge and discharge process and the reaction activity of the material with the electrolyte. When the ratio of the coating integrity of the silicon-carbon negative electrode material to the average thickness of the coating layer is preferably within the above range, the interfacial stability of the material is more adapted to the charge and discharge performance of the material. While taking into account the expansion / contraction effect of the stabilizing silicon material interface, better kinetic performance can be achieved, and the reaction activity of the material with the electrolyte can be controlled at a lower level, and the comprehensive electrochemical performance of the product is better.

[0031] In some specific embodiments, the (1-α)=90-100%.

[0032] Further preferably, the (1-α)=90%, 91%, 92%, 93%, 94%, 95%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 100% or any two of the range values.

[0033] More preferably, the (1-α)=96-98.5%.

[0034] The coating integrity of the carbon coating layer on the surface of the silicon-carbon negative electrode material described in the present application is related to the coating degree of the carbon coating layer and the carbon matrix on the silicon particles, the pores between the material particles, and the exposure degree of the silicon particles. When the coating integrity of the carbon coating layer is preferably within the above range, the side reaction activity of the silicon-carbon negative electrode material and the electrolyte can be maintained at a lower level, thereby improving the cycle stability of the secondary battery. At the same time, the diffusion distance of lithium ions during transmission and diffusion is shortened, so the product can achieve better kinetic transmission performance and better overall performance.

[0035] It should be noted that, in the present application, the coating integrity of the carbon coating layer on the surface of the silicon-carbon negative electrode material can be tested and confirmed by, but not limited to, the drainage method. The specific method is as follows: If the silicon-carbon negative electrode material is not in a secondary battery, directly test the silicon-carbon negative electrode material, prepare two aluminum-plastic bags with a length of 18 cm and a width of 12 cm and mark them, use a 500 mL beaker to fix the aluminum-plastic bag in the center of the balance, weigh 30 g of the sample to be tested and pour it into the aluminum-plastic bag at one time, then weigh 45 mL of deionized water and add it to the aluminum-plastic bag. After completing the above operations, put the aluminum-plastic bag containing the mixture of the sample to be tested and deionized water into the aluminum-plastic bag. The bag was allowed to stand for 30 minutes. After the air in the bag was exhausted, the aluminum-plastic bag was transferred to a hot press for sealing. The sealed aluminum-plastic bag was shaken to mix the sample to be tested and deionized water evenly (standard for judging uniform mixing: no lumps in the bag). The sealed aluminum-plastic bag was placed in a water displacement measuring device to measure the volume. This volume was taken as the initial volume V0. After the volume was tested, it was placed in a 60°C oven and allowed to stand for 48 hours. After that, it was taken out and the volume was measured after the temperature cooled to room temperature. This volume was taken as the 48-hour volume V1. The 48-hour volume change α=100%×(V1-V0) / V0 was calculated to obtain (1-α).

[0036] It should be noted that in the present application, the coating integrity of the carbon coating layer of the silicon-carbon negative electrode material can be regulated by, but not limited to, the composition of the silicon source gas raw material during the preparation of the material, the flow rate of the gas raw material, the inflow time, the temperature and time of the silicon deposition stage, etc.

[0037] In some embodiments, the γ is 5 to 18 nm.

[0038] Further preferably, the γ is in the range of one or any two of 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, and 18 nm.

[0039] More preferably, the γ is 8 to 10 nm.

[0040] In the present application, the thickness of the carbon coating layer on the surface of the silicon-carbon negative electrode material will also affect the degree of side reaction between the overall material and the electrolyte, as well as the transmission efficiency of lithium ions. When the thickness of the carbon coating layer is preferably within the above range, it can not only ensure a shorter lithium ion transmission path, but also will not cause excessive pores in the overall material or excessive exposure of silicon particles, thereby increasing the degree of side reaction between the negative electrode material and the electrolyte. When the overall material is used in a secondary battery, the initial DCR is low and the cycle life is long.

[0041] It should be noted that the thickness of the carbon coating layer of the silicon-carbon negative electrode material described in the present application can be confirmed by but not limited to the following method. The specific method is: the silicon-carbon negative electrode material is tested, and the cross-section of the sample to be tested is polished using an argon ion beam. Four samples to be tested are grouped together, and the polished samples are dispersed using anhydrous ethanol as a dispersion liquid for solution dispersion-dropping method to directly prepare TEM samples. The prepared samples are placed on the sample stage and started to be photographed. The shooting position is the edge of the carbon coating layer of the silicon-carbon negative electrode material and at least four positions are selected. The coating thickness is calculated according to the ruler, and the final result is averaged by combining 16 groups of data.

[0042] In some embodiments, O=0.7-1.1.

[0043] More preferably, O is in the range of one or any two of 0.7, 0.75, 0.8, 0.85, 0.87, 0.9, 0.92, 0.95, 0.98, 1, 1.05, and 1.1.

[0044] More preferably, O=0.87-0.95.

[0045] More preferably, the O=I D / I G , where I D The Raman spectrum of silicon-carbon negative electrode material is between 1520 and 1590 cm -1 The intensity at I G The Raman spectrum of silicon-carbon negative electrode material is between 1300 and 1380 cm -1 The strength of the place.

[0046] In the present application, the degree of order of the silicon-carbon negative electrode material is related to the degree of structural rearrangement of the amorphous carbon in the material. The higher the degree of order, the greater the degree to which the carbon atoms in the material are transformed from a disordered structural phase in two-dimensional space to an ordered graphite phase in three-dimensional space, and the higher the integrity of the crystal structure, thereby improving the arrangement quality of the overall carbon material and having a high packing density, which can preferably inhibit the side reactions between the material and the electrolyte. However, the efficiency of lithium ions during conduction is relatively reduced, and the kinetic performance is low. When the degree of order of the silicon-carbon negative electrode material is preferably within the above range, the silicon-carbon negative electrode material can take into account both reaction stability and kinetic performance, so that it can exhibit better cycle performance when applied to secondary batteries.

[0047] It should be noted that, in the present application, the degree of ordering of the silicon-carbon negative electrode material can be confirmed by Raman testing: the sample to be tested is placed in a Raman spectrometer for testing, and the test parameters are: the laser wavelength is 532nm, and the test range wave number is 50-2000cm -1 , take 1300~1380cm -1The scattering peak intensity of the D band at the position I D , at 1520~1590cm -1 The position of the scattering peak I of the G band D , through I D / I G The degree of ordering of silicon-carbon negative electrode materials can be obtained by calculation using the formula.

[0048] It should be noted that the degree of ordering of the carbon coating layer of the silicon-carbon negative electrode material in the present application can be regulated by, but not limited to, conventional conditions such as the composition of the carbon source raw material gas during the preparation of the material, the flow rate of the gas raw material, and the time of coating the carbon layer.

[0049] In some embodiments, the carbon matrix includes at least one of porous hard carbon, porous soft carbon, carbon molecular sieve, carbon gel, carbon fiber, and porous graphite carbon.

[0050] It should be noted that in the present application, there is no special limitation on the source of the carbon matrix. In addition to the above types, it can also be selected from but not limited to one or two of biomass materials and resin materials as raw materials to prepare the product.

[0051] In some embodiments, the specific surface area of ​​the silicon-carbon negative electrode material is 1 to 4 m 2 / g.

[0052] In some embodiments, the average particle size D50 of the silicon-carbon negative electrode material is 6 to 10 μm.

[0053] In some embodiments, the mass ratio of silicon to carbon in the silicon-carbon negative electrode material is (47-50): (49-52). It should be noted that in the present application, the mass content of silicon and carbon in the silicon-carbon negative electrode material can be confirmed by, but not limited to, the following methods:

[0054] In an air environment, weigh the sample to be tested and place it in a weighed crucible. Then use a box-type resistance furnace to raise the temperature to 500°C at a heating rate of 5-10°C / min and keep it warm for 2 hours to remove volatile substances and organic components; then continue to heat it to 600°C and keep it warm for 1 hour, and then continue to heat it to 1000°C and keep it warm for 1 hour. Weigh the remaining material and calculate the mass content of silicon in the sample using the following formula. Then you can confirm the mass of silicon in the sample. The formula includes the sample weighing mass m0, the initial crucible mass m1, the total mass of the crucible and product after the insulation is completed and the weight is constant m2, the molar mass of Si is 28.09g / mol, and the molar mass of SiO2 is 60.09g / mol. The formula is:

[0055]

[0056] Subsequently, the remaining material is directly tested by the CS analyzer to confirm the mass ratio of carbon elements in the sample. Finally, the mass ratio of carbon elements in the sample can be confirmed by calculation.

[0057] In some embodiments, the powder resistivity of the silicon-carbon negative electrode material is ≤10Ω˙cm.

[0058] Further preferably, the powder resistivity of the silicon-carbon negative electrode material is 1 to 10 Ω˙cm.

[0059] The completeness of the carbon coating layer on the surface of the silicon-carbon negative electrode material, the coating thickness of the carbon coating layer, and the degree of order of the material will affect the conductivity of the overall material, and thus affect the resistivity of the material. When the powder resistivity of the silicon-carbon negative electrode material is preferably within the above range, the material can further optimize the overall conductivity, improve the kinetic properties of the material, and achieve better electrochemical performance.

[0060] It should be noted that in the present application, the powder resistivity of the silicon-carbon negative electrode material can be confirmed by, but is not limited to, the following method: testing using a powder four-probe resistivity tester. Place the sample in the tester's sample chamber, turn the handwheel counterclockwise, and press down the upper electrode. When the upper electrode is pressed against the table, the sample begins to be compressed, and the pressure P shown in the lower left digital display window begins to rise significantly. During the test, the sample is pressurized in sections (2 MPa as a gradient), and data is manually collected every 2 MPa until the pressure reaches 20 MPa. After recording the data, stop measuring and calculate the powder resistivity of the sample based on the data.

[0061] In some embodiments, the silicon-carbon negative electrode material can be prepared by, but is not limited to, the following preparation method:

[0062] The carbon substrate is heated and pretreated to a first set temperature, then kept warm and a protective atmosphere and a silicon source are introduced to perform a silicon deposition reaction; the temperature is then adjusted to a second set temperature, kept warm, a carbon source is introduced to perform a carbon coating reaction, the temperature is lowered, the substrate is crushed, sieved, and impurities are removed to obtain the silicon-carbon negative electrode material.

[0063] Specifically, the carbon matrix includes but is not limited to at least one of porous hard carbon, porous soft carbon, carbon molecular sieve, carbon gel, carbon fiber, and porous graphite carbon.

[0064] Specifically, the heating pretreatment is carried out in a protective atmosphere;

[0065] More specifically, the protective atmosphere may be nitrogen; and the flow rate of the protective atmosphere is 8 to 12 L / min.

[0066] Specifically, the heating rate during the heating pretreatment is 8 to 12° C. / min, and the treatment time of the heating pretreatment is 45 min to 1 h.

[0067] More specifically, the heating pretreatment can be, but is not limited to, performed in a rotary kiln, and the rotation speed of the rotary kiln is 0.5-1.5 rpm.

[0068] Specifically, the silicon source is a gaseous silicon source, which includes but is not limited to at least one of monosilane, disilane, trichlorosilane, dichlorosilane, and monochlorosilane.

[0069] More specifically, the volume ratio of the introduced protective atmosphere to the silicon source is 1:(1-6).

[0070] More specifically, the flow rate of the mixed gas of the protective atmosphere and the silicon source is 0.2 to 0.6 L / min.

[0071] Specifically, the first set temperature is 460-520°C.

[0072] Specifically, the carbon source includes but is not limited to at least one of methane, ethane, methylene, acetylene, carbon dioxide, and propyne.

[0073] More specifically, the carbon source includes acetylene and carbon dioxide, and the volume ratio of acetylene to carbon dioxide is (0.5-2):1.

[0074] More specifically, the flow rate of the carbon source is 1 to 3 L / min, and the carbon coating time is 14 to 22 hours.

[0075] More specifically, the second set temperature is 480-540°C.

[0076] In the present application, there is no limitation on the preparation method of the silicon-carbon negative electrode material. Those skilled in the art can prepare the silicon-carbon negative electrode material by the above method or other methods according to conventional technical means.

[0077] In some embodiments, the present application also provides a secondary battery, including a negative electrode plate, wherein the negative electrode plate includes the silicon-carbon negative electrode material described in the present application.

[0078] In some embodiments, the negative electrode sheet includes a current collector and a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-carbon negative electrode material.

[0079] More preferably, the negative electrode active material further includes graphite, and the graphite includes at least one of artificial graphite and natural graphite.

[0080] Further preferably, the mass ratio of the graphite to the silicon-carbon negative electrode material is (70-99):(1-30).

[0081] More preferably, the negative electrode active material layer includes a negative electrode active material, a binder, and a conductive agent.

[0082] More preferably, in the negative electrode active material layer, the mass ratio of the negative electrode active material, the conductive agent, the thickener, and the binder is (94-97):(1-2):(1-2):(1-2).

[0083] In some embodiments, the secondary battery further includes a positive electrode sheet.

[0084] In some embodiments, the positive electrode sheet includes a current collector and a positive active material layer.

[0085] More preferably, the positive electrode active material layer includes a positive electrode active material.

[0086] In some embodiments, the positive electrode active material includes but is not limited to at least one of lithium nickel cobalt manganese oxide and lithium iron phosphate.

[0087] More preferably, the structural formula of the lithium nickel cobalt manganese oxide is LiNi a Co b Mn c O2, where 0<a<1; 0<b<1; 0<c<1; a+b+c=1.

[0088] Further preferably, the lithium nickel cobalt manganese oxide may further include a doping element M, wherein the doping element M includes at least one of Zr, Al, Sr, W, Y, Nb, Sb, and Mg, and the doping concentration is 500 to 3000 ppm.

[0089] More preferably, the positive electrode active material layer further includes a binder and a conductive agent.

[0090] More preferably, in the positive electrode active material layer, the mass ratio of the positive electrode active material, the conductive agent, and the binder is (94-97.5): (0.8-2): (1.5-4).

[0091] It should be noted that the conductive agent in the positive electrode active material layer and the negative electrode active material layer is used to provide conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Exemplary conductive agents in the positive electrode active material layer include, but are not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes. Carbon fibers include, for example, carbon nanofibers; and carbon blacks include, for example, SP (Super P, hereinafter the same), acetylene black, and Ketjen black.

[0092] The binder and thickener in the positive electrode active material layer and the negative electrode active material layer are used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Any binder can be used without particular limitation, as long as it has suitable binding properties and does not significantly cause adverse chemical changes in the battery. Exemplary, the binder in the positive electrode active material layer includes but is not limited to fluorinated polyolefin binders, fluorinated polyolefin binders include but are not limited to polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers or their modified (for example, carboxylic acid, acrylic acid, acrylonitrile, etc.) derivatives, etc., the thickener in the negative electrode active material layer includes but is not limited to carboxymethyl cellulose or its salt, and the binder includes but is not limited to styrene-butadiene rubber (SBR).

[0093] In some embodiments, the secondary battery further includes an electrolyte including a solvent and a lithium salt.

[0094] For example, in some embodiments, the solvent includes at least one of a carbonate solvent, a carboxylate solvent, an ether solvent, a sulfone solvent, a nitrile solvent, and a phosphate solvent.

[0095] Exemplarily, the carbonate solvent includes but is not limited to at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); the carboxylate solvent includes but is not limited to at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; the ether solvent includes at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; the sulfone solvent includes at least one of methyl sulfone and dimethyl sulfoxide; the nitrile solvent includes at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrinitrile; the phosphate solvent includes at least one of trimethyl triphosphate and triethyl phosphate.

[0096] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium trifluoromethanesulfonate, lithium bisfluoromethanesulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium difluorobisoxalatophosphate, lithium tetrafluorooxalatophosphate, and the like.

[0097] In some embodiments, the concentration of the lithium salt in the electrolyte is 0.5 to 2.5 mol / L.

[0098] In addition, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives capable of improving certain battery properties, such as additives that improve high-temperature battery performance, additives that improve overcharge performance, and additives that improve low-temperature battery performance.

[0099] The battery may also include a separator positioned between the positive electrode sheet and the negative electrode sheet to separate the positive electrode sheet and prevent short circuits between the positive electrode sheet and the negative electrode sheet. The separator may be made of any suitable battery-isolation material known in the art. Exemplarily, the separator includes, but is not limited to, at least one of polypropylene and polyethylene.

[0100] In some embodiments, the positive electrode sheet includes a current collector and a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material.

[0101] The present invention is further described below with reference to specific examples, which are not to be construed as limiting the scope of the present invention.

[0102] Example 1

[0103] A secondary battery, the preparation method comprising the following steps:

[0104] (1) Preparation of silicon-carbon negative electrode material: porous hard carbon is placed in a rotary furnace with a rotation speed of 1 rpm and nitrogen gas at a flow rate of 10 L / min is introduced. The temperature is raised to 480°C at a rate of 10°C / min for heating pretreatment for 1 hour. A mixed gas I of nitrogen and monosilane is introduced to carry out silicon deposition reaction. After the deposition is completed, the introduction of monosilane is stopped and only nitrogen is retained. The temperature is raised to 500°C at a rate of 10°C / min and kept for 1 hour. Subsequently, a mixed gas II of acetylene and carbon dioxide is introduced to carry out carbon layer deposition reaction (also known as carbon coating reaction). The gas is turned off and the temperature is lowered. The obtained block material is sieved and demagnetized to obtain the silicon-carbon negative electrode material. The silicon-carbon negative electrode material is placed under an ultra-high magnification transmission electron microscope for observation. Figure 1 As shown by the arrow in the middle, it can be seen that the carbon matrix and the outer layer of the silicon particles are provided with an obvious carbon coating layer.

[0105] The preparation and product parameters of the silicon-carbon negative electrode material are shown in Tables 1 to 3;

[0106] (2) Preparation of negative electrode sheet: artificial graphite, silicon-carbon negative electrode material, conductive agent acetylene black, conductive agent single-walled carbon nanotubes, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are dispersed in water according to a mass ratio of 86:10:0.95:0.05:1.2:1.8, and vacuum stirred to prepare a slurry. The slurry is then coated on both sides of the current collector copper foil, dried, cold pressed, and cut to obtain the negative electrode sheet. The compacted density of the negative electrode sheet is 1.6 g / cm3 ;

[0107] (3) Preparation of positive electrode sheet: The positive electrode active material lithium nickel cobalt manganese oxide LiNi 0.8 Co 0.1 Mn 0.1 O2, conductive agent SP and binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone in a mass ratio of 96:2:2, and vacuum stirred to prepare a slurry, which is then coated on both sides of the current collector aluminum foil. After drying, cold pressing and slitting, the positive electrode sheet is obtained; the compaction density of the positive electrode sheet is 3.55g / cm 3 ;

[0108] (3) Preparation of electrolyte: EC, EMC, and DEC were prepared into a solvent at a volume ratio of 1:1:1, and then lithium hexafluorophosphate was added to prepare a concentration of 1 mol / L to obtain the electrolyte;

[0109] (4) The positive electrode sheet, the separator (commercially available PE separator), and the negative electrode sheet are stacked or wound in sequence to form a battery cell, the battery cell is placed in an outer packaging shell, and after drying, the electrolyte is injected. After vacuum packaging, standing, forming, and constant capacity, the secondary battery is obtained.

[0110] Examples 2 to 25

[0111] A secondary battery is different from Example 1 only in that the parameter conditions during the preparation of the secondary battery are different, as shown in Tables 1 to 3.

[0112] Among them, the coating integrity, coating layer thickness, ordering degree, specific surface area, pore size distribution, resistance, etc. of the silicon-carbon negative electrode material are regulated by the process parameters during the preparation of the silicon-carbon negative electrode material described in Table 1.

[0113] Examples 26 to 35

[0114] A secondary battery differs from Example 1 only in that the parameters and conditions during preparation of the secondary battery are different, and the positive electrode active material is lithium iron phosphate.

[0115] Comparative Examples 1-2

[0116] A secondary battery is different from Example 1 only in that the parameter conditions during the preparation of the secondary battery are different, as shown in Tables 1 to 3.

[0117] Comparative Example 3

[0118] A secondary battery differs from Example 1 only in that the parameters and conditions during preparation of the secondary battery are different, and the positive electrode active material is lithium iron phosphate, as shown in Tables 1 to 3.

[0119] Among them, the methods for confirming the coating integrity, coating layer thickness, and ordering degree of the silicon-carbon negative electrode material in Table 3 are as described above.

[0120] Table 1

[0121]

[0122]

[0123]

[0124] Table 2

[0125]

[0126]

[0127] Table 3

[0128]

[0129]

[0130]

[0131] Effect Examples

[0132] The secondary batteries obtained in each embodiment and comparative example were tested as follows:

[0133] (1) Cyclic performance test: The secondary battery was left to stand for 24 h, then charged to 4.1 V at 0.02 C, then charged to 4.25 V at 0.33 C, and then charged to 0.05 C at a constant voltage. After standing for 10 minutes, discharge at a constant current of 0.33C to 2.5V to complete formation, then discharge and charge at a constant current rate of 0.33C to 4.25V, and then charge at a constant voltage to 0.05C. After standing for 10 minutes, discharge at a constant current rate of 0.33C to 2.5V. This is considered one cycle to constant capacity. After cycling and constant capacity twice, the secondary battery is cycled as follows according to the constant capacity: charge at a constant current rate of 0.33C to 4.25V, and then charge at a constant voltage to 0.05C and cut off. After standing for 10 minutes, discharge at a constant current rate of 0.33C to 2.5V. The discharge capacity under this cycle is recorded as Q1. Cycle in this way for several times until the test discharge capacity Qn is less than 80% of Q1, stop cycling, and record the number of cycles n.

[0134] (2) DCR test: Place the secondary battery in a fixture and apply a force of 3000N. Then charge it to 4.25V at a constant current of 0.33C at room temperature (25°C), let it rest for 30min, and discharge it to 2.5V at a constant current of 0.33C. Let it rest for 30min. Repeat the cycle for 3 times. Record the third discharge capacity Cs as the basis for load adjustment. Adjust the capacity to 50% SOC and let it rest in a temperature box for 2h until thermal equilibrium. The voltage at the end of the resting period is U0. Then, perform a 1C pulse discharge for 18s at a current rate of 1C. Record the voltage at the end of discharge as U1.

[0135] The DCR of the secondary battery is calculated according to DCR=|(U1-U0) / I|, where I is the discharge current at 18s.

[0136] The test results are shown in Table 4.

[0137] Table 4

[0138]

[0139]

[0140] According to Table 5, we can see that:

[0141] (1) The silicon-carbon negative electrode material used in the secondary battery described in the present application can effectively reduce the impact of the interface expansion / contraction effect of the silicon material in the material because a carbon coating layer is provided on the surface of the material, and the coating integrity, thickness and order of the carbon coating layer are synchronously adjusted. In addition, the side reaction degree after contact and infiltration with the electrolyte is low. When the corresponding positive electrode material is a ternary material of nickel cobalt manganese oxide, the secondary battery can maintain 80% of its capacity for more than 1200 times during the cycle performance, and the DCR value in the DCR test can be controlled within 42mΩ, with excellent comprehensive performance. When the positive electrode material is lithium iron phosphate, the cycle number of the secondary battery can reach more than 1100 times, and the DCR value can be controlled within 39mΩ, which also shows a good level. In comparison, although the secondary batteries described in Comparative Examples 1 and 2 also use similar silicon-carbon negative electrode materials as negative electrode active materials, since the coating integrity, thickness and ordering degree of the carbon coating layer are not controlled within the specified range of the present application scheme, the electrochemical performance level of the secondary batteries cannot reach the level of the secondary batteries of the embodiments under the same positive electrode active material system; similarly, Comparative Example 3 also has similar technical effects.

[0142] (2) According to Examples 1 to 35, it can be clearly seen that the interface stability of the overall silicon-carbon negative electrode material and the proportional relationship of the ordering degree of the material T / O jointly affect the electrochemical performance of the secondary battery. In particular, when the T / O ratio is preferably in the range of 1.55 to 1.89, the kinetic performance, chemical stability and electrode stability of the silicon-carbon negative electrode material are all better, the DCR value of the secondary battery is lower and the cycle stability is better. The corresponding cycle number of the ternary secondary battery can be increased to more than 1,600 times, and the DCR value can be reduced to within 32 mΩ. The lithium iron phosphate secondary battery also has a similar improvement effect. On the other hand, after optimizing the ratio of the two, the performance of the secondary battery will be further improved after separately optimizing the interface stability of the silicon-carbon negative electrode material and the degree of order of the carbon coating layer. The best effect is achieved when T is preferably in the range of 1.4 to 1.8, and / or O is preferably in the range of 0.87 to 0.95. The maximum number of cycles of the ternary and lithium iron phosphate secondary batteries reaches about 1,800 times, and the DCR value can be controlled at a minimum of about 27 mΩ.

Claims

1. A silicon-carbon negative electrode material, characterized in that: It comprises a carbon matrix and silicon particles, wherein the carbon matrix contains silicon particles; A carbon coating layer is provided on the surface of the silicon-carbon negative electrode material; The silicon-carbon negative electrode material satisfies: T / O=1.25-2.45; Wherein T=15×(1-α) / γ, (1-α) is the coating integrity of the carbon coating layer on the surface of the silicon-carbon negative electrode material, and γnm is the average thickness of the carbon coating layer; O is the degree of ordering of the silicon-carbon negative electrode material.

2. The silicon-carbon negative electrode material according to claim 1, wherein: The T / O=1.55~1.

89.

3. The silicon-carbon negative electrode material according to claim 1, wherein: The T=0.9~2.

7.

4. The silicon-carbon negative electrode material according to claim 1, wherein The (1-α)=90-100%, and / or the γ=5-18 nm.

5. The silicon-carbon negative electrode material according to claim 1, wherein: Said O=0.7~1.

1.

6. The silicon-carbon negative electrode material according to claim 1, wherein: The carbon matrix includes at least one of porous hard carbon, porous soft carbon, carbon molecular sieve, carbon gel, carbon fiber, and porous graphite carbon.

7. The silicon-carbon negative electrode material according to claim 1, wherein: The specific surface area of ​​the silicon-carbon negative electrode material is 1 to 4 m 2 / g.

8. The silicon-carbon negative electrode material according to claim 1, wherein: The average particle size D50 of the silicon-carbon negative electrode material is 6 to 10 μm.

9. The silicon-carbon negative electrode material according to claim 1, wherein: In the silicon-carbon negative electrode material, the mass ratio of silicon element to carbon element is (47-50): (49-52).

10. The silicon-carbon negative electrode material according to claim 1, wherein: The powder resistivity of the silicon-carbon negative electrode material is 1 to 10 Ω·cm.

11. A secondary battery, characterized in that: It comprises a positive electrode plate, and the positive electrode plate comprises the silicon-carbon negative electrode material according to any one of claims 1 to 10.

12. An electrical device, characterized in that: The secondary battery according to claim 11 is used as a power supply in an electrical device.

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