Composite negative electrode and secondary electric equipment thereof

Abstract

A coating of the composite negative electrode comprises graphite particles and silicon carbon particles, and the ratio of the weight percentage of the graphite particles to the weight percentage of the composite negative electrode coating to the weight percentage of the silicon carbon particles to the weight percentage of the composite negative electrode coating is larger than or equal to 1.6 and smaller than or equal to 8.5; the requirements that the fluorine content of the section of the graphite particles is larger than or equal to 3 wt% and larger than or equal to 12 wt% and larger than the fluorine content of the section of the silicon carbon particles are met, the phosphorus content of the composite negative electrode coating is 0.8-3.4 wt%, the oxygen content is 6.5-18 wt%, and the oxygen dispersion distribution degree is smaller than the phosphorus dispersion distribution degree; in the 400-1200-cycle charging and discharging cycle process of the composite negative electrode, the diffusion impedance Rss growth rate / cycle is smaller than 1.2%, the direct current impedance DCR growth rate / cycle is smaller than 1.2%, and the diffusion impedance Rss growth rate / cycle is larger than the direct current impedance DCR growth rate / cycle. The problem of impedance increase in the charging and discharging process of electric equipment is solved, and the cycle life of the electric equipment is prolonged.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of negative electrode, in particular to a composite negative electrode and a secondary electrical equipment thereof. BACKGROUND

[0002] With the rapid development of lithium ion secondary electrical equipment for many years, the demand for lithium ion secondary electrical equipment with high energy density is increasing to meet various applications, including portable electronic products and electric vehicles. Gas phase deposition silicon carbon stands out with its high lithiation capacity, >70% available first coulomb efficiency and highly engineered production application. The negative electrode is obtained by compounding graphite particle material and gas phase deposition silicon carbon. The face density and thickness are reduced by more than 50%, which significantly improves the energy density of lithium ion secondary electrical equipment and has a large-scale commercial application prospect.

[0003] However, the lithium ion conducting ability of gas phase deposition silicon carbon material is far inferior to that of graphite particle material, and the negative electrode charging rebound is higher. It is a good strategy to use the good rate of graphite particle material and low expansion characteristics to make up for the deficiency of gas phase deposition silicon carbon. However, how to use the graphite particle material to design the composite negative electrode under the condition of high silicon carbon addition ratio to meet the high discharge efficiency under the charging transmission of high rate electrical equipment, resist the impedance increase in the charging and discharging process of electrical equipment and ensure the cycle life of electrical equipment is still a problem to be solved.

[0004] Therefore, it is necessary to develop a composite negative electrode coating, a composite negative electrode and a secondary electrical equipment with low battery impedance and improved cycle life of high rate electrical equipment. SUMMARY

[0005] The present application discloses a composite negative electrode and a secondary electrical equipment thereof to solve the problems in the prior art. In the present application, the graphite particle region and the silicon carbon particle region of the composite negative electrode coating both contain fluorine and phosphorus with strong electron absorption capacity. The fluorocarbon layer inside and on the surface of the graphite particle contains more fluorine, followed by the SEI film layer on the surface of the silicon carbon particle. The phosphorus is more uniformly dispersed inside the composite negative electrode, which improves the oxidation resistance and reduction potential of the composite negative electrode, and the Li + migration number inside the carbon layer of the graphite particle; it is helpful to form a good SEI film on the negative electrode; in addition, the SEI film formed by the graphite particle is thin, dense and rich in LiF, thereby reducing the charge transfer impedance, having high ionic conductivity, reducing the polarization of the electrical equipment, solving the problem of high lithium diffusion impedance of gas phase deposition silicon carbon, maintaining high charge and discharge efficiency of the composite negative electrode, and improving the rate and cycle performance of the electrical equipment.

[0006] The present application is realized by the following technical scheme:

[0007] The present application first provides a composite negative electrode, comprising a composite negative electrode coating layer;

[0008] The composite negative electrode coating layer comprises graphite particles and silicon-carbon particles, and the graphite particles and the silicon-carbon particles satisfy: 1.6≤weight percentage of graphite particles in the composite negative electrode coating layer / weight percentage of silicon-carbon particles in the composite negative electrode coating layer≤8.5;

[0009] The composite negative electrode coating layer contains fluorine, and satisfies: 12wt%≥mass percentage of fluorine in the cross section of the graphite particles in the total amount of carbon, oxygen, fluorine, silicon and phosphorus on the cross section>mass percentage of fluorine in the cross section of the silicon-carbon particles in the total amount of carbon, oxygen, fluorine, silicon and phosphorus on the cross section≥3wt%;

[0010] The content of phosphorus in the composite negative electrode coating layer is 0.8-3.4wt%, the content of oxygen is 6.5-18wt%, and the degree of dispersion of oxygen is <the degree of dispersion of phosphorus;

[0011] During 400-1200 cycles of charge and discharge of the composite negative electrode, the growth rate of diffusion impedance Rss per cycle is <1.2%, the growth rate of direct current impedance DCR per cycle is <1.2%, and the growth rate of diffusion impedance Rss per cycle is >the growth rate of direct current impedance DCR per cycle.

[0012] The DCR growth rate is calculated as follows: ① charge the secondary electrical equipment containing the composite negative electrode to 3.95V at a current of 0.5C, and stand for 300s; ② continue to discharge at a current of 0.1C for 10s, and record the voltage at the discharge cutoff; ③ discharge at a current of 1C for 1s, and record the voltage at the discharge cutoff; ④ DCR=(voltage at the discharge cutoff in step ③-voltage at the discharge cutoff in step ②) / 0.9C;

[0013] The Rss growth rate is calculated as follows: ① charge the secondary electrical equipment containing the composite negative electrode to 3.95V at a current of 0.5C, and stand for 300s; ② continue to discharge at a current of 0.1C for 1800s; ③ stand for another 3600s, and record the voltage before standing and at the cutoff; ④ Rss=(voltage before standing in step ③-voltage at the cutoff in step ③) / 0.1C;

[0014] The cross section of the graphite particles and the cross section of the silicon-carbon particles are respectively any cross section of the composite negative electrode coating layer, and the cross section area where the cross section of the graphite particles is located and the cross section area where the silicon-carbon particles are located;

[0015] The weight percentage of the graphite particles is the percentage of the weight of the graphite particles in the composite negative electrode coating layer;

[0016] The weight percentage of the silicon-carbon particles is the percentage of the weight of the silicon-carbon particles in the composite negative electrode coating layer. As a further solution, the specific surface area of the graphite particles is ≥2m2 / g, the specific surface area of the silicon-carbon particles is ≥ 2 m 2 / g;

[0017] As a further preferred solution, the specific surface area of the graphite particles is > the specific surface area of the silicon-carbon particles.

[0018] As a further solution, the Dv50 particle size of the graphite particles is < the Dv50 particle size of the silicon-carbon particles ≤ 10 μm;

[0019] As a further preferred solution, the porosity of the composite negative electrode coating is 21-65%.

[0020] As a further preferred solution, the thickness of the composite negative electrode layer is 40-245 μm and the areal density of the coating is 0.005-0.040 g / cm 2 .

[0021] In the secondary battery containing the composite negative electrode, further comprising an electrolyte, a positive electrode, and a separator.

[0022] As a further solution, the electrolyte comprises a fluorine-containing lithium salt, an electrolyte solvent, and an electrolyte additive.

[0023] As a further preferred solution, the fluorine-containing lithium salt comprises one or more of a fluorine-containing lithium phosphate salt, a fluorine-containing lithium sulfonate salt, and a fluorine-containing lithium oxalate phosphate salt.

[0024] As a further preferred solution, the concentration of the fluorine-containing lithium salt in the electrolyte is 3.2-12.5 mol / L.

[0025] As a further preferred solution, the concentration of the fluorine-containing lithium phosphate salt in the electrolyte is 2.5-9.7 mol / L, the concentration of the fluorine-containing lithium sulfonate salt in the electrolyte is 0.5-1.8 mol / L, and the concentration of the fluorine-containing lithium oxalate phosphate salt in the electrolyte is 0.2-1.0 mol / L.

[0026] As a further solution, the electrolyte additive comprises one or more of a fluorinated carbonate, a sulfonate, a borate, a tantalate, and a silicon-containing phosphate.

[0027] As a further solution, the mass fraction of the electrolyte additive in the electrolyte is 1.5-8.5 wt% in the electrolyte.

[0028] As a further preferred solution, the fluorinated carbonate comprises one or more of a fluorinated ethylene carbonate (FEC), a difluorinated ethylene carbonate (DFEC), a fluorinated propylene carbonate, a fluorinated vinylene carbonate, and a trifluoromethyl ethylene carbonate.

[0029] As a further preferred aspect, the sulfonate includes one or more of 1,3-propylene sulfite, methane disulfite, 1,3-propane sulfite, 1,4-butane sulfite.

[0030] As a further preferred aspect, the borate includes one or more of polymeric borate, tris(trimethylsilyl) borate (TMSB), tri(ethylene glycol) diborate (BEG), tris(2,2,2-trifluoroethyl) borate (TFEB).

[0031] As a further preferred aspect, the tantalate includes one or more of dimethyl tantalate, diethyl tantalate, pentanormal butyl tantalate.

[0032] As a further aspect, the fluorocarbonate, sulfonate, borate, tantalate, silicon-containing phosphate in the electrolyte additive have a mass ratio of (1.2-4.5):(0.2-2.4):(0.2-1.8):(0.1-0.8):(0.1-0.8).

[0033] As a further preferred aspect, the fluorocarbonate, sulfonate, borate, tantalate, phosphate have a mass ratio of (3.5-4.2):(0.8-1.5):(0.4-1.1):(0.1-0.5):(0.1-0.4).

[0034] The electrolyte solvent is selected from one or more of a cyclic carbonate solvent, a chain carbonate solvent.

[0035] As a further aspect, the electrolyte solvent has a mass content of 55-90wt% in the electrolyte.

[0036] As a further aspect, the composite negative electrode satisfies: 9wt%≥graphite particle cross-section fluorine content>silicon-carbon particle cross-section fluorine content≥5wt%;

[0037] The phosphorus content in the coating of the composite negative electrode is 1.5wt%-3.4wt%, the oxygen content is 8.5-15wt%, and the oxygen dispersion distribution degree is < the phosphorus dispersion distribution degree.

[0038] As a further preferred aspect, the fluorine-containing lithium salt is lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonylimide (LiFSI), and lithium difluorophosphate dioxalate (LiDFOP).

[0039] The molar ratio of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium difluorophosphate dioxalate in the lithium salt is (7.2-9.4):(0.8-1.6):(0.3-0.7).

[0040] The electrolyte additive is fluoroethylene carbonate, tris(trimethylsilyl)phosphate, 1,3- propene sultone, dimethyltantalum oxide, polymeric borate, methanediyl dimethanesulfonate;

[0041] The molar ratio of the fluoroethylene carbonate, 1,3-propene sultone, polymeric borate, dimethyltantalum oxide, tris(trimethylsilyl)phosphate is (3.5-4.2):(0.8-1.5):(0.4-1.1):(0.1-0.5):(0.1-0.4);

[0042] Preferably, the molar ratio of the fluoroethylene carbonate, 1,3-propene sultone, polymeric borate, dimethyltantalum oxide, tris(trimethylsilyl)phosphate is 4:1.35:0.6:0.3:0.2.

[0043] As a further aspect, the composite negative electrode coating comprises graphite particles, silicon-carbon particles, further comprising a negative electrode conductor material and a negative electrode binder material.

[0044] As a further aspect, the positive electrode comprises a positive electrode coating.

[0045] As a further aspect, the positive electrode coating comprises a positive electrode material, a positive electrode conductor material, a positive electrode binder material, and an electrolyte.

[0046] As a further aspect, the positive electrode material is selected from one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium nickel cobalt manganese aluminate, lithium-rich manganese-based, lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium cobalt oxide, lithium nickel manganese oxide.

[0047] As a further aspect, the positive electrode conductor material and the negative electrode conductor material are each independently selected from one or more of conductive carbon black, conductive ketjen black, conductive acetylene black, conductive graphene, single-walled carbon nanotubes, multi-walled carbon nanotubes, few-walled carbon nanotubes.

[0048] As a further aspect, the electrolyte is selected from one or more of lanthanum lithium zirconate (LLZO), lanthanum lithium tantalate (LLTO), lanthanum lithium zirconium tantalate (LLZTO), lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP), polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polymethyl methacrylate (PMMA), lithium indium chloride (Li3InCl6), lithium indium bromide (Li3InBr6), lithium yttrium chloride (LiYCl), lithium yttrium bromide (LiYBr), lithium phosphorus sulfide chloride (LiPSCl), lithium germanium phosphide (LiGePS).

[0049] As a further aspect, the positive electrode binder material and the negative electrode binder material are each independently selected from one or more of a polyacrylic acid metal salt and a polyacrylic acid ester derivative, a polystyrene and a derivative thereof, a carboxyalkyl cellulose and a derivative thereof, a polyvinylidene fluoride, a polyacrylic acid block copolymer derivative.

[0050] As a further aspect, the polyacrylic acid metal salt and the polyacrylic acid ester derivative are selected from one or more of a polyacrylic acid sodium / lithium, a polymethyl methacrylate, a polyethyl methacrylate.

[0051] As a further aspect, the polystyrene and the derivative thereof are selected from one or more of a polystyrene-butadiene, a polystyrene-butadiene-methyl / ethyl / propyl acrylate, a polystyrene-butadiene-acrylic acid-methyl / ethyl / propyl acrylate, a polystyrene-butadiene-acrylic acid sodium / lithium-methyl / ethyl / propyl acrylate-acrylonitrile.

[0052] As a further aspect, the carboxyalkyl cellulose salt and the derivative thereof are selected from one or more of a carboxymethyl cellulose, a carboxymethyl cellulose lithium, a carboxymethyl cellulose sodium.

[0053] As a further aspect, the polyacrylic acid block copolymer derivative is selected from one or more of a polyacrylic acid sodium / lithium-methyl / ethyl / propyl acrylate, a polyacrylic acid sodium / lithium-acrylonitrile, a polyacrylic acid sodium / lithium-butadiene-acrylonitrile, a polyacrylic acid sodium / lithium-methyl / ethyl / propyl acrylate-acrylonitrile-styrene.

[0054] As a further aspect, a solvent is used in the process of preparing the composite negative electrode coating or the positive electrode, and the solvent is selected from one or more of N-methyl pyrrolidone, dimethyl carbonate, diethyl carbonate, ethylene carbonate, water, ethanol.

[0055] The negative electrode binder material includes a binder material a, a binder material b, and a binder material c.

[0056] As a further preferred aspect, the binder material a is a polyacrylic acid metal salt and / or a polyacrylic acid ester derivative, the binder material b is a carboxyalkyl cellulose and a derivative thereof, and the binder material c is a polyacrylic acid block copolymer derivative.

[0057] As a further preferred aspect, the electrolyte solvent is vinylene carbonate, ethylene carbonate, propylene carbonate, methyl ethyl carbonate.

[0058] As a further preferred aspect, the mass ratio of the electrolyte solvents vinylene carbonate, ethylene carbonate, propylene carbonate, methyl ethyl carbonate is (2-10):(20-30):(3-10):(30-40).

[0059] As a further preferred aspect, the mass ratio of the binder material a in the negative electrode coating is 1.2-4.0%, the mass ratio of the binder material b in the negative electrode coating is 0.2-1.2%, and the mass ratio of the binder material c in the negative electrode coating is 0.4-1.8%.

[0060] As a further aspect, the mass ratio of the positive electrode material, the conductor material, the binder material, and the electrolyte in the positive electrode coating is (90-99.3):(0.05-6):(0.6-10):(0.02-0.8).

[0061] As a further preferred aspect, the mass ratio of the positive electrode material, the conductor material, the binder material, and the electrolyte in the positive electrode coating is 98.2:1.0:0.5:0.2.

[0062] As a further aspect, the mass ratio of the negative electrode material (graphite particles and silicon-carbon particles), the conductor material, and the negative electrode binder material in the composite negative electrode coating is (90-98.6):(0.2-10):(1.2-8).

[0063] As a further preferred aspect, the mass ratio of the negative electrode material (graphite particles and silicon-carbon particles), the conductor material, and the negative electrode binder material in the composite negative electrode coating is 95:1.4:3.6.

[0064] The application also provides a preparation method of a secondary battery, comprising:

[0065] S1: the conductor material, the graphite particles, and the silicon-carbon particles are premixed; the binder material, the conductor material, and the solvent are mixed and stirred to obtain a composite negative electrode slurry;

[0066] S2: the conductor material and the positive electrode material are premixed; the binder material, the conductor material, the solvent, and the electrolyte are mixed and kneaded to obtain a positive electrode slurry;

[0067] S3: the composite negative electrode slurry is coated on a foil, and the solvent is removed by drying, rolling, drying, and die cutting to obtain a composite negative electrode sheet;

[0068] S4: the positive electrode slurry is coated on a foil, and the solvent is removed by drying, rolling, drying, and die cutting to obtain a positive electrode sheet;

[0069] S5: the positive electrode sheet, the separator, and the composite negative electrode sheet are stacked, the tab is welded, the shell is assembled, the liquid is injected, the formation is performed, the capacity is divided, and the secondary battery is obtained.

[0070] As a further preferred place, the S1 is to pre-mix the conductor material a, graphite particles, silicon-carbon particles; add 30%~60% of the binder material a, 30%~60% of the binder material b, solvent for mixing for 90min kneading; add the remaining 70%~40% of the binder material a, the remaining 70%~40% of the binder material b, the conductor material b, solvent for mixing for 120min stirring, control the solid content at 45%; add the binder material c, solvent for 60min stirring, control the solid content at 40%, fineness ≤45μm, to obtain the composite negative electrode slurry;

[0071] As a further preferred place, the S1 is to pre-mix the conductor material a, graphite particles, silicon-carbon particles; add 30%~60% of the binder material a, 30%~60% of the binder material b, solvent for mixing for 90min kneading; add the remaining 70%~40% of the binder material a, the remaining 70%~40% of the binder material b, the conductor material b, solvent for mixing for 120min stirring, control the solid content at 45%; add the binder material c, solvent for 60min stirring, control the solid content at 40%, fineness ≤45μm, to obtain the composite negative electrode slurry.

[0072] A secondary power equipment comprising the secondary battery prepared according to the preparation method of claim 9.

[0073] The application has the characteristics and advantages that: under the synergistic effect of multiple factors, the application optimizes the electrolyte additives, the type and content of lithium salt, and combines the physical properties of the negative electrode surface, to skillfully form reasonable concentration distribution and position distribution of fluorine (F), phosphorus (P) and oxygen (O) elements, to overcome the ion migration obstruction of the graphite area and the instability of the SEI of the silicon-carbon area caused by the traditional homogeneous fluorine distribution, and the distribution characteristics significantly improve the comprehensive performance of the negative electrode. The graphite particles and silicon-carbon particles in the composite negative electrode electrode coating area both contain fluorine and phosphorus with strong electron absorption capacity, and the fluorocarbon layer inside and on the surface of the graphite particles has more distribution, followed by the surface SEI film layer of the silicon-carbon particles, and the carbon layer inside is more uniform, and the phosphorus is more uniformly dispersed inside the composite negative electrode electrode, to improve the oxidation resistance and reduction potential of the composite negative electrode electrode, and the graphite particle carbon layer inside can maintain a high Li+ migration number; it is helpful to form a good SEI film at the negative electrode; in addition, the SEI film formed by the graphite particles is thin, dense and rich in LiF, to reduce the charge transfer impedance, the high ion conductivity, reduce the polarization of the power equipment, make up for the high lithium diffusion impedance of the vapor deposited silicon-carbon, maintain the high charge and discharge efficiency of the composite negative electrode electrode, and improve the rate and cycle performance of the power equipment. BRIEF DESCRIPTION OF DRAWINGS

[0074] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings.

[0075] Figure 1 is a composite negative electrode coating section of Example 1;

[0076] Figure 1 in which a is a graphite particle section of Example 1, Figure 1 in which b is a silicon-carbon particle section of Example 1.

[0077] Figure 2 is a phosphorus distribution map of the composite negative electrode coating of Example 1.

[0078] Figure 3 is an oxygen distribution map of the composite negative electrode coating of Example 1.

[0079] Figure 4 is a fluorine-containing region of the composite negative electrode coating section of Example 1;

[0080] Figure 4 in which a is a fluorine-containing region of the graphite particle section of Example 1, Figure 1 in which b is a fluorine-containing region of the silicon-carbon particle section of Example 1. DETAILED DESCRIPTION

[0081] In order to facilitate the understanding of the present application, the following will more fully describe the present application, and embodiments of the present application are given, but the scope of the present application is not limited thereby.

[0082] The present application discloses a composite negative electrode and a secondary electrical equipment thereof, and the graphite particle region and the silicon-carbon particle region of the composite negative electrode coating both contain fluorine and phosphorus with strong electron-withdrawing ability, and the fluorocarbon layer of the graphite particle has more fluorine and phosphorus inside and on the surface, followed by the SEI film layer on the surface of the silicon-carbon particle, and the phosphorus is more uniformly dispersed inside the composite negative electrode, thereby improving the oxidation resistance and reduction potential of the composite negative electrode, and the carbon layer of the graphite particle can maintain a high Li + migration number; it is helpful to form a good SEI film on the negative electrode; in addition, the SEI film formed by the graphite particle is thin, dense and rich in LiF, thereby reducing the charge transfer impedance, having a high ionic conductivity, reducing the polarization of the electrical equipment, solving the problem of high lithium diffusion impedance of the vapor-deposited silicon-carbon, maintaining a high charge and discharge efficiency of the composite negative electrode, and improving the rate and cycle performance of the electrical equipment.

[0083] The present application is realized by the following technical solutions:

[0084] The present application first provides a composite negative electrode, comprising a composite negative electrode coating;

[0085] The composite negative electrode coating comprises graphite particles and silicon-carbon particles, and the graphite particles and the silicon-carbon particles satisfy: 1.6≤weight percentage of graphite particles in the composite negative electrode coating / weight percentage of silicon-carbon particles in the composite negative electrode coating≤8.5;

[0086] The composite negative electrode coating contains fluorine, and satisfies: 12wt%≥mass percentage of fluorine in the cross section of the graphite particles in the total mass percentage of carbon, oxygen, fluorine, silicon and phosphorus in the cross section>mass percentage of fluorine in the cross section of the silicon-carbon particles in the total mass percentage of carbon, oxygen, fluorine, silicon and phosphorus in the cross section≥3wt%;

[0087] The content of phosphorus in the composite negative electrode coating is 0.8-3.4wt%, the content of oxygen is 6.5-18wt%, and the dispersion degree of oxygen is less than the dispersion degree of phosphorus;

[0088] Specifically, the content of phosphorus refers to the mass percentage of phosphorus (P) in the total mass percentage of the main elements (including carbon (C), oxygen (O), fluorine (F), silicon (Si), and phosphorus (P)) contained in the composite negative electrode coating. Similarly, the content of oxygen refers to the mass percentage of oxygen (O) in the total mass percentage of the main elements contained in the composite negative electrode coating. The relative contents of the above-mentioned carbon (C), oxygen (O), fluorine (F), silicon (Si), and phosphorus (P) can be analyzed by energy dispersive X-ray spectroscopy (EDS) area scanning or point scanning to obtain the mass percentage data of each element;

[0089] Specifically, the uniformity of the distribution of the oxygen element and the phosphorus element, i.e., the dispersion degree of the oxygen element and the dispersion degree of the phosphorus element, can be evaluated by the following method: selecting a plurality of (for example, n, n≥3) representative different microzones on the composite negative electrode coating, and using EDS point scanning or small-area area scanning to respectively determine the mass percentage (wt%) of the oxygen element (O) and the mass percentage (wt%) of the phosphorus element (P) in each microzone. The standard deviation of the oxygen content (wt%O) data and the standard deviation of the phosphorus content (wt%P) data of the n microzones are calculated respectively. The smaller the standard deviation, the more uniform the distribution of the element in the coating (the higher the dispersion degree). According to this definition, if the standard deviation of the oxygen content (wt%O) data is greater than the standard deviation of the phosphorus content (wt%P) data, it means that the uniformity of the distribution of the phosphorus element in the coating is better than that of the oxygen element, i.e., the dispersion degree of oxygen is significantly lower than that of phosphorus;

[0090] The diffusion impedance Rss growth rate per week is less than 1.2%, the direct current impedance DCR growth rate per week is less than 1.2%, and the diffusion impedance Rss growth rate per week is greater than the direct current impedance DCR growth rate per week during 400-1200 cycles of charging and discharging.

[0091] The DCR growth rate is calculated as follows: ① charging the secondary electrical equipment containing the composite negative electrode to 3.95V at a current of 0.5C and standing for 300s; ② continuing to discharge at a current of 0.1C for 10s and recording the voltage at the discharge cutoff; ③ discharging at a current of 1C for 1s and recording the voltage at the discharge cutoff; ④ DCR=(voltage at the discharge cutoff in step ③-voltage at the discharge cutoff in step ②) / 0.9C;

[0092] The Rss growth rate is calculated as follows: ① charging the secondary electrical equipment containing the composite negative electrode to 3.95V at a current of 0.5C and standing for 300s; ② continuing to discharge at a current of 0.1C for 1800s; ③ standing for 3600s and recording the voltage before standing and at the cutoff; ④ Rss=(voltage before standing in step ③-voltage at the cutoff in step ③) / 0.1C;

[0093] The graphite particle section and the silicon-carbon particle section are the section area where the graphite particle section is located and the section area where the silicon-carbon particle is located on any section of the composite negative electrode coating;

[0094] The graphite particle weight percentage is the percentage of the weight of the graphite particle to the composite negative electrode coating;

[0095] The silicon-carbon particle weight percentage is the percentage of the weight of the silicon-carbon particle to the composite negative electrode coating.

[0096] The present application aims to improve the comprehensive performance of the negative electrode material and obtain a secondary electrical equipment with low impedance and good cycle performance. In order to meet the application requirements of high lithiation capacity and high initial coulomb efficiency, a high proportion of silicon-carbon needs to be used in the negative active material. At this time, the composite negative electrode is designed by using graphite particle material matching, which needs to meet the high discharge efficiency under the charging transmission of high-rate electrical equipment, resist the impedance increase during the charging and discharging process of the electrical equipment, and ensure the cycle life of the electrical equipment. The present application proposes a composite negative electrode design, which cooperatively introduces graphite particles and silicon-carbon particles in the coating. Under the synergistic action of multiple factors, the content, spatial position distribution and dispersion distribution degree of fluorine (F), phosphorus (P) and oxygen (O) elements in the composite negative electrode coating are precisely controlled, which overcomes the problems of ion migration obstruction in the graphite area and unstable SEI film in the silicon-carbon area caused by high oxygen dispersion distribution degree and traditional homogeneous fluorine distribution, and significantly improves the comprehensive performance of the negative electrode.

[0097] Under the optimized conditions of the present application, the fluorine element is more highly enriched in the graphite particle region, not only existing in the surface SEI film, but also deeply embedded and uniformly distributed in the graphite carbon layer. In contrast, in the silicon-carbon particle region, the fluorine element is mainly enriched in the surface SEI film layer, and the distribution concentration in the carbon layer of the silicon-carbon particle is lower than that in the graphite region. The strong electron-withdrawing property of the fluorine element synergistically improves the overall oxidation resistance stability of the composite negative electrode, and enables it to maintain a higher reduction potential in the electrochemical process. In the graphite particle region, the deeply embedded F element in the carbon layer helps to maintain a higher Li + migration number, promoting the Li + efficient diffusion in the graphite structure. This differential distribution of fluorine elements (deep embedding in graphite region > surface enrichment in silicon-carbon region) is the key to reducing the increase in battery impedance and improving the cycle performance of the battery.

[0098] The fluorine element source in the cross section of the graphite particles and the silicon-carbon particles in the application is very extensive, which provides an important source basis for the wide and high content distribution of fluorine element in the negative electrode coating. The fluorine on the cross section of the negative electrode particles in the application can be derived from fluorine-containing lithium salt, fluorine-containing electrolyte additive, and fluorine-containing bonding material. The type and amount of the fluorine-containing lithium salt, the fluorine-containing electrolyte additive, and the fluorine-containing bonding material are one of the core factors for regulating the total fluorine content (3% to 12%) on the cross section of the negative electrode particles. First, since the fluorine-containing electrolyte additive and the decomposition products formed in the charging and discharging process have stronger infiltration ability to the negative electrode particles, and the reduction potential of part of the fluorine-containing electrolyte additive is higher than the decomposition potential of the fluorine-containing lithium salt and the electrolyte solvent, the fluorine-containing SEI film is preferentially formed on the negative electrode, so the fluorine-containing electrolyte additive contributes more to the fluorine content of the composite negative electrode than the fluorine-containing lithium salt. When the content of the fluorine-containing electrolyte additive is higher, the fluorine content in the graphite particles and the silicon-carbon particles is higher. In addition, the specific types of the fluorine-containing electrolyte additive and the fluorine-containing lithium salt can affect the fluorine content in the cross section of the negative electrode particles. When the fluorine-containing electrolyte additive or the fluorine-containing lithium salt with stronger infiltration ability or more conducive to inhibiting the fluorine source loss caused by the side reaction is used, the fluorine content in the cross section of the negative electrode particles is higher. At the same time, the mass ratio between different fluorine-containing electrolyte additives and the mass ratio between different fluorine-containing lithium salts all affect the fluorine content on the negative electrode particles. For example, when the content of the lithium salt that is beneficial to the infiltration of the graphite particles is higher, the fluorine content in the graphite particles is higher. When the content of the lithium salt that is conducive to inhibiting the side reaction at the silicon-carbon interface and is beneficial to the film formation on the surface of the silicon-carbon particles is higher, the fluorine content on the silicon-carbon particles is higher. When the bonding material is a fluorine-containing bonding material, as a part of the composite negative electrode coating, it has a certain contribution to the fluorine content on the cross section of the particles. The contribution degree is related to the type and amount of the fluorine-containing bonding material. Generally, the more the number of fluorine atoms in the fluorine-containing bonding material used, the higher the amount, and the greater the contribution. In addition, when the physical characteristics of the graphite particles and the silicon-carbon particles, such as the Dv50 particle size, the specific surface area, and the porosity of the negative electrode coating, are controlled at the same time, the fluorine-containing lithium salt and the fluorine-containing electrolyte additive can better cover the surface of the particles and infiltrate into the interior of the particles under the condition of having moderate particle size and specific surface area and providing sufficient porosity. At this time, the insufficient infiltration caused by the too large particle size, the too small specific surface area, and the too few pores is avoided, thereby affecting the fluorine content on the cross section of the particles. Also, the too small particle size, the too large specific surface area, and the too many pores are avoided, thereby causing the fluorine content to be too high. Therefore, through the single or synergistic regulation of the above factors, the fluorine content of the fluorine-containing area on the cross section of the negative electrode can be controlled to be 3% to 12%.

[0099] The application further optimizes the relationship between the fluorine content on the cross section of the silicon-carbon particles and the fluorine content on the cross section of the graphite particles, thereby maintaining a higher Li +The migration number is reduced, and the impedance increase caused by the volume expansion of the silicon-carbon particles is reduced. The composition and ratio of the above-mentioned fluorine-containing lithium salt and fluorine-containing electrolyte additive, the Dv50 particle size of the graphite particles and the silicon-carbon particles, the specific surface area, and the porosity of the negative electrode coating are also factors that affect the different distribution states of fluorine in the cross sections of the silicon-carbon particles and the graphite particles. Through single or synergistic effects, the fluorine content in the cross section of the graphite particles is further greater than the fluorine content in the cross section of the silicon-carbon particles. First, through the type selection and dosage matching of the fluorine-containing lithium salt, when there is a fluorine-containing lithium salt that has better graphite infiltration ability and eliminates the by-products of the silicon-carbon particle side reaction, the fluorine element is more widely distributed in the graphite particles. At the same time, due to the elimination of HF and other by-products produced by the silicon-carbon interface side reaction during the charging and discharging process of the battery by the specific fluorine-containing lithium salt, a dense protective film is formed on the surface of the silicon-carbon particles, which hinders the further penetration of the electrolyte, so that more fluorine is distributed on the surface of the silicon-carbon layer, reducing the fluorine content in the interior of the silicon-carbon particles. In addition, through the selection and dosage matching of the electrolyte additive, when the electrolyte additive contains specific groups and specific electrolyte additives, the capture of HF and the fluorine immobilization effect can make the fluorine element accumulate on the surface of the silicon-carbon, rather than enter the interior of the silicon-carbon particles, thereby reducing the distribution of fluorine in the silicon-carbon particles, alleviating the particle rupture, side reaction increase and impedance increase caused by volume expansion. In addition, the distribution of fluorine in different negative electrode materials can also be affected by optimizing the particle size, specific surface area of the silicon-carbon particles and graphite particles, and the porosity of the negative electrode coating. When the surface topography is the same, the silicon-carbon Dv50 particle size increases, the specific surface area decreases, and the fluorine penetration depth is significantly shallower. At this time, the fluorine infiltration ability in the electrode is reduced, which affects the fluorine content in the cross section of the silicon-carbon particles; when the graphite Dv50 particle size decreases, the specific surface area increases, and the fluorine penetration depth increases. At this time, the fluorine infiltration ability in the graphite is enhanced, which increases the fluorine content in the graphite particles. Therefore, by selecting silicon-carbon particles with higher Dv50 particle size and lower specific surface area, and by selecting graphite particles with lower Dv50 particle size and higher specific surface area, the fluorine content and distribution characteristics in the cross sections of different negative electrode particles can be further optimized. At the same time, the porosity is indirectly related to the particle size and specific surface area of the graphite particles and the silicon-carbon particles, and also reflects the contact degree between different particles. Generally, as the porosity increases, it is beneficial to the deep infiltration of the fluorine-containing electrolyte into the particles, while weakening the contact between the fluorine-containing lithium salt and the particles. The sensitivity of the graphite particles to the fluorine penetration from the electrolyte infiltration source is higher than that of the fluorine-containing lithium salt contact source, so that the regulation of the porosity also becomes one of the main factors affecting the different distribution of fluorine in the cross sections of the graphite particles and the silicon-carbon particles. Therefore, under the single or synergistic regulation of the above factors, the fluorine content in the cross section of the graphite particles is greater than the fluorine content in the cross section of the silicon-carbon particles.

[0100] The phosphorus element in the composite negative electrode coating in the application can be derived from a phosphorus-containing lithium salt and a phosphorus-containing electrolyte additive, the concentration of the phosphorus-containing lithium salt and the amount of the phosphorus-containing electrolyte additive have a fundamental effect on the phosphorus content in the composite negative electrode coating, and the phosphorus-containing electrolyte additive and its decomposition products have a higher contribution to the phosphorus content in the negative electrode than the phosphorus-containing lithium salt due to their stronger electrode infiltration capacity. When the amount of the phosphorus-containing electrolyte additive in the electrolyte additive is high, it can promote a higher phosphorus content in the composite negative electrode coating.

[0101] The oxygen source in the composite electrode coating in the application is relatively wide, and the lithium salt containing oxygen (including the phosphorus-containing lithium salt), the electrolyte solvent, the electrolyte additive and the oxygen-containing bonding material can all be the main source of oxygen. For example, the carbonate solvent is generally used in the electrolyte solvent, which can contribute to the main oxygen element, and the mass proportion of the electrolyte solvent in the electrolyte is one of the important factors affecting the oxygen content in the negative electrode coating of the battery. When the mass proportion of the electrolyte solvent in the electrolyte is high, the oxygen content in the negative electrode coating is high. In addition, different electrolytes have different infiltration and film-forming abilities, and the contribution of oxygen and the distribution characteristics on the particles are different. Through reasonable combination and amount ratio of the electrolyte solvent, a certain content of oxygen element on the negative electrode particles can be achieved and its distribution on the particle surface can be limited. By selecting a specific electrolyte additive, the free and infiltration of oxygen elements in the electrolyte solvent can be limited through the binding and chelation of oxygen elements, the amount of electrolyte infiltration into the electrode coating is controlled, the diffusion of oxygen in the composite negative electrode is limited, and the oxygen element in the composite electrode coating is provided by optimizing the type and amount of the negative electrode bonding material containing oxygen. By using the negative electrode bonding material with limited oxygen entering the negative electrode particles, the penetration of oxygen is further limited, thereby reducing the dispersion degree of oxygen in the composite negative electrode coating.

[0102] The dispersion degree of phosphorus element in the application is greater than that of oxygen element, which can be regulated by various single or synergistic means, including but not limited to: first, selecting different phosphorus-containing electrolyte additives to release active phosphorus radicals, which can be cross-linked with the phosphorus-containing lithium salt in the electrolyte, and the electrolyte additive containing specific groups can convert harmful phosphides into BPO4, eliminating phosphorus segregation and promoting uniform distribution of phosphorus; and oxygen element, as described above, can be uniformly distributed on the surface of graphite by controlling the type and amount of electrolyte solvent, and by matching the negative electrode slurry preparation process in the negative electrode, the adhesive material can reduce disordered oxides to make oxygen uniformly distributed on the surface of graphite, thereby further reducing the penetration of oxygen in the battery; at the same time, the electrolyte additive containing certain specific groups can bind oxygen in the interface layer through bonding, thereby reducing the dispersion degree of oxygen, and the electrolyte additive containing certain specific groups can realize the uniform diffusion of oxygen on the interface through the formation of a nano-thin film, thereby reducing the diffusion of oxygen into the interior of the particles. Thus, the dispersion degree of phosphorus is greater than that of oxygen.

[0103] Due to the deep fluorine enrichment in the graphite region in the present application, the fluorine in the surface and near-surface SEI film improves the interfacial ionic conductivity, accelerates Li + The diffusion between the dense graphite layers significantly reduces the diffusion impedance, limits the penetration of fluorine into the interior of the silicon-carbon particles, and avoids the excessive decomposition of fluorine-containing lithium salts or electrolyte additives, which leads to the excessive rigidity of the SEI film, thereby relieving the SEI rupture caused by the volume expansion of the silicon-carbon particles. The elastic SEI formed by the specific fluorine-containing electrolyte additive can adapt to the volume change and suppress the interfacial side reactions, thereby reducing the impedance and improving the cycle performance. On the one hand, the volume expansion effect of the silicon-carbon particles in the cycle cannot be completely eliminated, and the expansion leads to poor contact between the particles, Li + The solid-phase diffusion path inside the electrode is lengthened, and micro-cracks are generated at the graphite / silicon-carbon interface due to the expansion stress, which increases the Li + transport resistance. The present application maintains efficient diffusion in the graphite region through fluorine distribution, partially offsets the influence of silicon-carbon expansion, and enhances the mechanical integrity of the electrode through a higher content and more uniform distribution of phosphorus, thereby suppressing crack propagation and controlling the Rssweek growth rate to <1.2%. On the other hand, the lower degree of oxygen diffusion distribution reduces the generation and distribution of high-impedance byproducts (such as Li2CO3), and the uniform phosphorus network maintains the balance between the electronic insulation and ionic conductivity in the SEI, the elastic SEI in the silicon-carbon region adapts to the volume change, and the interface exposure is reduced, thereby controlling the direct current impedance to <1.2% and lower than Rss, which reflects the significance of interface protection and improves the interface stability. Under the above factors, the problems of SEI film instability and Li + migration blockage in high-silicon-carbon proportion composite negative electrodes can be solved, thereby obtaining a composite negative electrode with low impedance and long cycle life.

[0104] As a further scheme, the specific surface area of the graphite particles is ≥2 m 2 / g, and the specific surface area of the silicon-carbon particles is ≥2 m 2 / g.

[0105] As a further scheme, the Dv50 particle size of the graphite particles is < the Dv50 particle size of the silicon-carbon particles ≤10 μm.

[0106] By adjusting the specific surface area of the graphite particles or the silicon-carbon particles, and the Dv50 particle size of the graphite particles and the silicon-carbon particles, the fluorine element can be more inclined to enrich and distribute in the graphite particle region and the surface of the appropriate amount of silicon-carbon particles during the charging and discharging process, thereby more effectively playing a key role in the interface stability and protection of the silicon-carbon particles.

[0107] When the graphite particle Dv50 particle size is small, a larger specific surface area is obtained, and fluorine on the surface of the graphite particle is more inclined to be uniformly distributed in the entire thickness direction of the electrode. Therefore, fluorine on the graphite electrode will form a thinner but more uniform cover layer. In the silicon-carbon layer, due to the large Dv50 particle size of the silicon-carbon particle, the specific surface area is relatively small, and the distribution of fluorine may form a relatively dense and tough layer on the surface of the electrode, but the possibility of penetrating into the interior is small. At the same time, the volume expansion of silicon in the silicon-carbon particle will cause the SEI layer to break and regenerate, so that fluorine is redistributed during the cycle process, and therefore the fluorine content in the fluorine-containing region of the silicon-carbon particle cross section is relatively low.

[0108] As a further preferred solution, the specific surface area of the graphite particle is greater than the specific surface area of the silicon-carbon particle.

[0109] As a further preferred solution, the porosity of the composite negative electrode coating is 21-65%.

[0110] As a further preferred solution, the thickness of the composite negative electrode layer is 40-245 μm, and the coating area density is 0.005-0.040 g / cm 2 .

[0111] In the secondary battery containing the composite negative electrode, an electrolyte, a positive electrode, and a separator film are further included.

[0112] As a further solution, the electrolyte includes a fluorine-containing lithium salt, an electrolyte solvent, and an electrolyte additive.

[0113] As a further preferred solution, the fluorine-containing lithium salt includes one or more of a fluorine-containing lithium phosphate salt, a fluorine-containing lithium sulfonyl salt, and a fluorine-containing lithium oxalate phosphate salt.

[0114] As one of the key factors to achieve accurate control of the fluorine content in the fluorine-containing region of the particle cross section in the composite negative electrode (both the graphite region and the silicon-carbon region are stably in the range of 3wt% to 12wt%), the present application optimizes the mass proportion of different fluorine-containing lithium salts by compounding specific fluorine-containing lithium salts, effectively solving the core problem that the fluorine content in the fluorine-containing region of the cross section is unstable and the distribution is out of control due to the reaction of fluorine-containing lithium phosphate with the electrolyte, hydrolysis to produce HF to corrode the silicon-carbon particle. By compounding fluorine-containing lithium sulfonyl salt and fluorine-containing lithium oxalate phosphate salt, stable, uniform, and high-content distribution of fluorine elements on the composite negative electrode is achieved. Due to the higher thermal stability and chemical stability of the lithium sulfonyl salt, the decomposition of the fluorine-containing lithium phosphate salt is significantly inhibited, and the fluorine loss caused by the generation of HF is further inhibited, providing a stable and sufficient fluorine source for the negative electrode interface and participating in the construction of a high-stability fluorine-containing interface layer. The fluorine-containing lithium oxalate phosphate salt on the silicon-carbon particle can significantly inhibit the disorderly expansion of the fluorine-containing region caused by HF corrosion and repeated rupture / growth of the SEI, and the decomposition products (such as Li x PO yF z )Promote the uniform dispersion of fluorine, thereby protecting silicon carbon. After long-term cycling, the range and fluorine content distribution of the fluorine-containing region of the composite negative electrode are relatively more stable and controllable, which is an important basis for achieving 12wt%≥Fluorine content of the fluorine-containing region on the cross-section of the graphite particle or the cross-section of the silicon-carbon particle≥3wt%.

[0115] As a further preferred aspect, the concentration of the fluorine-containing lithium salt in the electrolyte is 3.2-12.5mol / L.

[0116] The fluorine-containing lithium salt is one of the important sources of fluorine elements in the composite negative electrode, and its concentration affects the infiltration speed of fluorine elements in the composite negative electrode, indirectly affecting the overall distribution state of fluorine at the negative electrode interface.

[0117] As a further preferred aspect, the concentration of the fluorine-containing lithium salt in the electrolyte is 3.2-12.5mol / L.

[0118] Further optimizing the concentration range and ratio of different fluorine-containing lithium salts can stabilize the fluorine content in the particles within a certain range and the fluorine content on the cross-section of the graphite particle>the fluorine content on the cross-section of the silicon-carbon particle. When there is a certain amount of fluorine-containing lithium sulfonate with better graphite infiltration ability and eliminating the byproduct of the side reaction of silicon-carbon particles, the fluorine elements are more widely distributed in the graphite particles. Due to the elimination of HF and other byproducts produced by the side reaction of the silicon-carbon interface during the charging and discharging process of the battery by the fluorine-containing lithium oxalate phosphate, a dense protective film is formed on the surface of the silicon-carbon particles, hindering the further penetration of the electrolyte, and more fluorine is distributed on the surface of the silicon-carbon layer, reducing the fluorine content inside the silicon-carbon particles. By balancing the concentration of each fluorine-containing lithium salt, the distribution characteristics on different negative electrode particles can be improved, thereby achieving the purpose of the fluorine content on the cross-section of the graphite particle>the fluorine content on the cross-section of the silicon-carbon particle.

[0119] As a further aspect, the electrolyte additive includes one or more of fluorinated carbonate, sulfonate, borate, tantalate, and silicon-containing phosphate.

[0120] As a further preferred aspect, the fluorinated carbonate includes one or more of fluorinated ethylene carbonate (FEC), difluoroethylene carbonate (DFEC), fluorinated propylene carbonate, fluorinated vinylene carbonate, and trifluoromethyl ethylene carbonate.

[0121] As a fluorine-containing electrolyte additive, fluorinated carbonate has stronger wettability than fluorine-containing lithium salt, and is preferentially reduced and decomposed at the negative electrode interface. The fluorine atoms carried in the molecule (such as FEC provides -F group, DFEC provides -CF2- group) are effectively integrated into the solid electrolyte interface film (SEI) and infiltrated into the interior of the composite negative electrode particles, significantly increasing the proportion of fluorine-containing compounds in the graphite and silicon-carbon particle cross-section, achieving precise control of the fluorine content in the fluorine-containing region, and stabilizing the fluorine content in the fluorine-containing region of the graphite particle cross-section and the silicon-carbon particle cross-section.

[0122] As a further preferred aspect, the sulfonate includes one or more of 1,3-propylene sulfite, methane disulfite, 1,3-propane sulfite, 1,4-butane sulfite.

[0123] As a further preferred aspect, the borate includes one or more of polymeric borate, tris(trimethylsilyl) borate (TMSB), tri(ethylene glycol) diborate (BEG), tris(2,2,2-trifluoroethyl) borate (TFEB).

[0124] Borate electrolyte additives can bind with free F-, and then capture HF to generate tetrafluoroborate, making the distribution of fluorine more uniform at the silicon particle interface, on the one hand, further adjusting the content of fluorine element, reducing the loss of fluorine element in the side reaction process, further reacting with the silicon surface OH- to generate LiBF4 and borosilicate, on the other hand, simultaneously converting harmful phosphides to stable BPO4, eliminating P segregation, and making P distribution more uniform.

[0125] As a further preferred aspect, the tantalate includes one or more of dimethyl tantalate, diethyl tantalate, and pentanormal butyl tantalate.

[0126] Tantalate electrolyte additives, as important electrolyte additives affecting oxygen distribution, hydrolyze-polymerize to form amorphous nano Ta2O5 film in electrolyte containing trace water, which selectively adsorbs free oxygen in the electrolyte through oxygen vacancies (Vo 2+ ), making oxygen uniformly distributed at the interface without entering the interior of the particles.

[0127] The silicon-containing phosphates can be oriented on the silicon-carbon surface to preferentially react, consume free F- to generate heat-stable fluorosilane, which has a certain volatility, and can appropriately reduce the fluorine deposition of the silicon-carbon layer, while the silicon-containing phosphates in the graphite region have low reactivity, and the fluorine source LiF decomposed from the fluorine-containing lithium salt and fluorine-containing carbonate electrolyte additives can be deposited without hindrance. The silicon-containing phosphates release active phosphorus radicals during decomposition, which synergistically crosslink with the lithium salt containing phosphorus elements P=O to achieve uniform dispersion of phosphorus, and the Si-O produced by hydrolysis of the silicon-containing phosphates can capture active oxygen on the negative electrode surface through chemical adsorption, chelate electrolyte dissolved oxygen, and generate a Si-O-Si three-dimensional network, locking oxygen in the interface layer and reducing the degree of oxygen dispersion.

[0128] As a further scheme, the mass fraction of the electrolyte additive in the electrolyte is 1.5-8.5wt%

[0129] As a further scheme, the mass ratio of fluorocarbonates, sulfonates, borates, tantalates, and silicon-containing phosphates in the electrolyte additive is (1.2-4.5):(0.2-2.4):(0.2-1.8):(0.1-0.8):(0.1-0.8).

[0130] As a further preferred scheme, the mass ratio of fluorocarbonates, sulfonates, borates, tantalates, and phosphates is (3.5-4.2):(0.8-1.5):(0.4-1.1):(0.1-0.5):(0.1-0.4).

[0131] The electrolyte solvent is selected from one or more of cyclic carbonate solvents and chain carbonate solvents.

[0132] The carbonate solvents in the electrolyte solvent contribute the main oxygen element, and the mass fraction of the electrolyte solvent in the electrolyte is one of the important factors affecting the oxygen content in the negative electrode coating of the battery.

[0133] As a further scheme, the mass fraction of the electrolyte solvent in the electrolyte is 55-90wt%.

[0134] As a further scheme, the composite negative electrode coating includes graphite particles, silicon-carbon particles, and further includes a negative electrode conductor material and a negative electrode adhesive material.

[0135] As a further scheme, the positive electrode includes a positive electrode coating.

[0136] As a further scheme, the positive electrode coating includes a positive electrode material, a positive electrode conductor material, a positive electrode adhesive material, and an electrolyte.

[0137] As a further provision, the cathode material is selected from one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, lithium-rich manganese-based, lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium cobalt oxide, lithium nickel manganese oxide.

[0138] As a further provision, the cathode conductor material and the anode conductor material are each independently selected from one or more of conductive carbon black, conductive ketjen black, conductive acetylene black, conductive graphene, single-walled carbon nanotubes, multi-walled carbon nanotubes, few-walled carbon nanotubes.

[0139] As a further provision, the electrolyte is selected from one or more of lanthanum lithium zirconate (LLZO), lanthanum lithium tantalate (LLTO), lanthanum lithium zirconium tantalate (LLZTO), lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP), polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polymethyl methacrylate (PMMA), lithium indium chloride (Li3InCl6), lithium indium bromide (Li3InBr6), lithium yttrium chloride (LiYCl), lithium yttrium bromide (LiYBr), lithium phosphorous sulfide chloride (LiPSCl), lithium germanium phosphide sulfide (LiGePS).

[0140] As a further provision, the cathode binder material and the anode binder material are each independently selected from one or more of polyacrylic acid metal salt and polyacrylate derivatives, polystyrene and its derivatives, carboxyalkyl cellulose and its derivatives, polyvinylidene fluoride, polyacrylic block copolymer derivatives.

[0141] As a further provision, the polyacrylic acid metal salt and polyacrylate derivatives are selected from one or more of polyacrylic acid sodium / lithium, polymethyl methacrylate, polyethyl acrylate;

[0142] As a further provision, the polystyrene and its derivatives are selected from one or more of polystyrene-butadiene, polystyrene-butadiene-methyl / ethyl / propyl acrylate, polystyrene-butadiene-acrylic acid-methyl / ethyl / propyl acrylate, polystyrene-butadiene-sodium / lithium-acrylic acid-methyl / ethyl / propyl acrylate-acrylonitrile.

[0143] As a further provision, the carboxyalkyl cellulose salt and its derivatives are selected from one or more of carboxymethyl cellulose, carboxymethyl cellulose lithium, carboxymethyl cellulose sodium.

[0144] As a further provision, the polyacrylic block copolymer derivatives are selected from one or more of polyacrylic acid sodium / lithium-methyl / ethyl / propyl acrylate, polyacrylic acid sodium / lithium-acrylonitrile, polyacrylic acid sodium / lithium-butadiene-acrylonitrile, polyacrylic acid sodium / lithium-methyl / ethyl / propyl acrylate-acrylonitrile-styrene.

[0145] As a further solution, a solvent is also used in the preparation of the composite negative electrode layer or the positive electrode, the solvent being selected from one or more of N-methylpyrrolidone, dimethyl carbonate, diethyl carbonate, ethylene carbonate, water, ethanol.

[0146] The negative electrode adhesive material comprises adhesive material a, adhesive material b and adhesive material c.

[0147] As a further preferred solution, the adhesive material a is a polyacrylic acid metal salt and / or a polyacrylic acid ester derivative, the adhesive material b is a carboxyalkyl cellulose and derivative thereof, and the adhesive material c is a polyacrylic acid block copolymer derivative.

[0148] The adhesive material a can interact with the fluorine-containing components in the electrolyte (such as fluorine-containing lithium salt, fluorine-containing electrolyte additive decomposition product), promoting the preferential deposition and stabilization of fluorine-containing compounds (such as LiF) on the surface and near-surface of the graphite particles, which helps to achieve the goal of "high fluorine content in the cross-section of the graphite particles", and enhances the ion conductivity of the adhesive material. The adhesive material b is a carboxyalkyl cellulose and derivative thereof, which has a binding effect of oxygen. As a key functional adhesive for the composite negative electrode, the carboxyl (-COOH) and hydroxyl (-OH) on the molecular chain of the adhesive material b cooperatively impart excellent oxygen binding capacity, and in the alkaline slurry environment, the -COOH is dissociated into -COO-, which captures the free O 2 -O free radicals in the electrolyte through electrostatic interaction; and forms a strong hydrogen bond (bond energy ≈ 25 kJ / mol) with the carbonyl oxygen (C=O) in the solvent molecule, inhibiting oxygen migration. The linear β-1,4-glucan skeleton stretches in the aqueous slurry and forms a dense three-dimensional network (pore size < 1 nm) after drying, physically blocking the diffusion of oxygen to the interior of the particles. The block structure (such as hard segment-soft segment) of the adhesive material c can form a more uniform and dense coating layer on the surface of the active material, especially on the surface of the silicon-carbon particles. This dense coating layer can physically hinder the deep penetration of the electrolyte into the interior of the silicon-carbon particles, thereby limiting the diffusion of fluorine elements to the bulk of the silicon-carbon and the formation of internal SEI, further supporting the strategy of "reducing the fluorine content in the interior / cross-section of the silicon-carbon particles".

[0149] As a further solution, the composite negative electrode satisfies: 9wt% ≥ fluorine content in the cross-section of the graphite particles > fluorine content in the cross-section of the silicon-carbon particles ≥ 5wt%.

[0150] As a further solution, the phosphorus content in the composite negative electrode coating is 1.5wt% to 3.4wt%, the oxygen content is 8.5wt% to 15wt%, and the degree of dispersion of oxygen is less than the degree of dispersion of phosphorus.

[0151] The optimized content, spatial position distribution and dispersion degree of fluorine (F), phosphorus (P) and oxygen (O) elements in the composite negative electrode coating at this time further hinders the ion migration of the graphite region and stabilizes the SEI film of the silicon-carbon region, thereby further improving the comprehensive performance of the secondary battery.

[0152] As a further preferred aspect, the fluorine-containing lithium salt is lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonylimide (LiFSI) and lithium difluorophosphate dioxalate (LiDFOP).

[0153] As a further preferred aspect, the molar ratio of lithium hexafluorophosphate, lithium bisfluorosulfonylimide and lithium difluorophosphate dioxalate in the lithium salt is (7.2-9.4):(0.8-1.6):(0.3-0.7).

[0154] The electrolyte additive is fluoroethylene carbonate, tris(trimethylsilyl) phosphate, 1,3-propylene sulfite, dimethyl tantalate, polymeric borate, methane disulfonate methylene;

[0155] The molar ratio of fluoroethylene carbonate, 1,3-propylene sulfite, polymeric borate, dimethyl tantalate, tris(trimethylsilyl) phosphate is (3.5-4.2):(0.8-1.5):(0.4-1.1):(0.1-0.5):(0.1-0.4);

[0156] Preferably, the molar ratio of fluoroethylene carbonate, 1,3-propylene sulfite, polymeric borate, dimethyl tantalate, tris(trimethylsilyl) phosphate is 4:1.35:0.6:0.3:0.2.

[0157] As one of the important influencing factors of the fluorine content in the graphite particle cross-section fluorine-containing region and the silicon-carbon particle cross-section fluorine-containing region, and the content size relationship between them, the present application uses a high content of lithium hexafluorophosphate, which guarantees the supply of basic fluorine source, and a relatively small amount of lithium bisfluorosulfonylimide, which can selectively insert into the graphite interlayer through anion (FSI-). -The C-F bond of LiDFOB facilitates the penetration of fluorine into the graphite and between the graphite layers; a smaller amount of LiDFOB inhibits the excessive enrichment of fluorine on the silicon-carbon surface, and its reduction potential is generally higher than that of LiPF6 and LiFSI. During the voltage drop of the first charge, LiDFOB will preferentially reduce and decompose on the surface of the silicon-carbon particles at a relatively high potential (close to or slightly higher than the lithium intercalation potential of silicon), forming a polymer-like or inorganic-organic composite SEI layer rich in phosphorus, oxygen and a small amount of fluorine. This initial layer can partially passivate the silicon surface, reduce the subsequent generation of LiF on the surface of the silicon-carbon particles, and thus control the fluorine content at the silicon-carbon particle interface. When the molar ratio of LiPF6, LiFSI, and LiDFOB is (7.2-9.4):(0.8-1.6):(0.3-0.7), uniform distribution of fluorine elements is achieved. Avoiding excessive LiDFOB, the risk of silicon-carbon fluorine overload, deviation from the target fluorine content range, and increased particle pulverization, as well as reduced electrode oxidation resistance and reduction potential, is avoided. Also avoided is the lack of LiFSI, which leads to insufficient fluorine content and uneven distribution in the silicon-carbon layer due to the corrosion of the byproduct HF, thereby affecting the fluorine content in the fluorine-containing region of the graphite particle cross-section and the fluorine content in the fluorine-containing region of the silicon-carbon particle cross-section. Therefore, optimizing the concentration and molar ratio of LiPF6, LiFSI, and LiDFOB in the electrolyte is of great significance and is an important material basis for achieving a fluorine content in the fluorine-containing region of the graphite particle cross-section greater than that in the fluorine-containing region of the silicon-carbon particle cross-section. It is also an important foundation for improving the oxidation resistance and reduction potential of the composite negative electrode. In addition, fluorine-containing lithium phosphate salts and fluorine-containing lithium oxalate phosphates provide phosphorus elements for the negative electrode, and the lithium phosphate salts (such as Li3PO4) and inorganic phosphorus oxides produced by decomposition are important sources of phosphorus components in the SEI layer. Fluoroethylene carbonate in the electrolyte additive undergoes reduction and decomposition at the interface of the electrode material (especially the negative electrode), and the fluorine atoms carried in its molecular structure are effectively incorporated into the formed solid-state electrolyte interface film (SEI). Fluoroethylene carbonate provides a single fluorine substituent group (-F), directly increasing the proportion of fluorine-containing organic or inorganic compounds in the SEI film, thereby precisely controlling and increasing the fluorine content in the interface region. Fluoroethylene carbonate as a flexible fluorine source preferentially reduces to form elastic organic matter containing C-F bonds on the silicon surface, adapts to volume changes, and alleviates the volume changes caused by expansion.

[0158] At the interface of graphite particles, higher fluorine content promotes the generation of lithium salt decomposition products (such as LiF), and improves the binding state of organic components and LiF. This makes the formed SEI film structure more uniform and dense, with good ionic conductivity and significantly enhanced stability. The fluorine content in the fluorine-containing region of the graphite particle cross-section is higher than that in the fluorine-containing region of the silicon-carbon particle cross-section, and is within a certain range. Ensuring uniform distribution of the fluorine-containing region in the full electrode meets the needs of graphite particles for high fluorine content and low impedance SEI, and adapts to the requirements of silicon-carbon particles for flexible and anti-cracking SEI, providing a key guarantee for the cycle stability of the composite negative electrode.

[0159] As a phosphoric acid ester electrolyte additive containing silicon, tris (trimethylsilyl) phosphate (TMSP) has a silane group that can be oriented on the surface of silicon-carbon to preferentially react and consume free F - A thermally stable fluorosilane is generated, which has a certain volatility and can appropriately reduce the fluorine deposition of the silicon-carbon layer, while the TMSP reaction activity in the graphite region is low, and the fluorine source LiF from the decomposition of LiPF6 / LiFSI can be deposited without hindrance. The active phosphorus free radicals released during the decomposition of tris (trimethylsilyl) phosphate cross-link with LiDFOP to achieve uniform dispersion of phosphorus, and the Si-O generated by the hydrolysis of tris (trimethylsilyl) phosphate (TMSP) can bond to the active oxygen on the surface of the negative electrode, chelate the dissolved oxygen in the electrolyte, and generate a -Si-O-P-O- three-dimensional network, locking the oxygen in the interface layer and reducing the degree of oxygen dispersion.

[0160] As a borate electrolyte additive, polyborate can combine with free F ～ and further capture HF to generate tetrafluoroborate, making the distribution of fluorine more uniform at the interface of silicon particles, on the one hand further adjusting the content of fluorine elements and reducing the loss of fluorine elements in the side reaction process, and on the other hand transforming harmful phosphides into stable BPO4 and eliminating P segregation to make P more uniformly distributed.

[0161] As an important electrolyte additive that affects oxygen distribution, dimethyl tantalate generates a nano Ta2O5 film that can achieve uniform diffusion of oxygen on the interface.

[0162] As a further preferred place, the electrolyte solvent is vinylene carbonate, vinyl carbonate, propylene carbonate, and methyl ethyl carbonate.

[0163] As a further preferred place, the mass ratio of the electrolyte solvent vinylene carbonate, vinyl carbonate, propylene carbonate, and methyl ethyl carbonate is (2-10):(20-30):(3-10):(30-40).

[0164] Vinylene carbonate (VC) as an efficient film-forming additive. It is preferentially reduced and polymerized on the surface of the negative electrode, forming an SEI layer rich in oxygen-containing polymers such as polyalkyl lithium carbonate. The layer has excellent stability and is an important contributor to oxygen elements, and can effectively inhibit the continuous decomposition of solvents; ethylene carbonate (EC) and propylene carbonate (PC) as the main solvent of the electrolyte. Its reduction and decomposition products are the main source of inorganic / organic oxygen-containing components (such as lithium carbonate, alkyl lithium carbonate) in the SEI film, and constitute the basic framework of the coating oxygen elements; methyl ethyl carbonate (EMC) as a co-solvent, its decomposition also contributes to the alkoxy oxygen-containing species, and helps to adjust the solvation structure. By limiting the content of vinylene carbonate, the content of oxygen in the negative electrode particles is controlled, and by adding ethylene carbonate and propylene carbonate, the distribution of oxygen elements on the surface of the particles is improved.

[0165] As a further preferred aspect, the mass ratio of the binder material a in the negative electrode coating is 1.2-4.0%, the mass ratio of the binder material b in the negative electrode coating is 0.2-1.2%, and the mass ratio of the binder material c in the negative electrode coating is 0.4-1.8%.

[0166] As a further aspect, the mass ratio of the positive electrode material, the conductor material, the binder material, and the electrolyte in the positive electrode coating is (90-99.3):(0.05-6):(0.6-10):(0.02-0.8).

[0167] As a further preferred aspect, the mass ratio of the positive electrode material, the conductor material, the binder material, and the electrolyte in the positive electrode coating is 98.2:1.0:0.5:0.2.

[0168] As a further aspect, the mass ratio of the negative electrode material (graphite particles and silicon-carbon particles), the conductor material, and the negative electrode binder material in the composite negative electrode coating is (90-98.6):(0.2-10):(1.2-8).

[0169] As a further preferred aspect, the mass ratio of the negative electrode material (graphite particles and silicon-carbon particles), the conductor material, and the negative electrode binder material in the composite negative electrode coating is 95:1.4:3.6.

[0170] The application also provides a preparation method of a secondary battery, comprising:

[0171] S1: pre-mixing the conductor material, graphite particles, and silicon-carbon particles; adding the binder material, the conductor material, and the solvent, and stirring to obtain a composite negative electrode slurry;

[0172] S2: pre-mixing the conductor material and the positive electrode material, adding the binder material, the conductor material, the solvent, and the electrolyte, and kneading to obtain a positive electrode slurry.

[0173] S3: the composite negative electrode slurry is coated on the foil, and solvent is removed by drying, rolling, drying, and die cutting to obtain a composite negative electrode sheet;

[0174] S4: the positive electrode slurry is coated on the foil, and solvent is removed by drying, rolling, drying, and die cutting to obtain a positive electrode sheet;

[0175] S5: the positive electrode sheet, the separator, and the composite negative electrode sheet are stacked, the tab is welded, the shell is assembled, the liquid is injected, the formation is performed, and the capacity is divided, thereby obtaining a secondary battery.

[0176] As a further preferred scheme, the S1 is to pre-mix the conductor material a, the graphite particles, and the silicon-carbon particles; add 30% to 60% of the bonding material a, 30% to 60% of the bonding material b, and the solvent to mix and knead for 90 min; then add the remaining 70% to 40% of the bonding material a, the remaining 70% to 40% of the conductor material b, and the solvent to mix and stir for 120 min, control the solid content to be 45%, and then add the bonding material c and the solvent to stir for 60 min, control the solid content to be 40%, and control the fineness to be ≤45 μm, thereby obtaining the composite negative electrode slurry.

[0177] As a further preferred scheme, the S1 is to pre-mix the conductor material a, the graphite particles, and the silicon-carbon particles; add 30% to 60% of the bonding material a, 30% to 60% of the bonding material b, and the solvent to mix and knead for 90 min; then add the remaining 70% to 40% of the bonding material a, the remaining 70% to 40% of the conductor material b, and the solvent to mix and stir for 120 min, control the solid content to be 45%, and then add the bonding material c and the solvent to stir for 60 min, control the solid content to be 40%, and control the fineness to be ≤45 μm, thereby obtaining the composite negative electrode slurry.

[0178] A secondary electrical equipment comprising the secondary battery prepared according to the preparation method of claim 9.

[0179] The application has the characteristics and benefits that: under the synergistic effect of multiple factors, by optimizing the electrolyte additives, the type and content of lithium salt, and combining the physical properties of the negative electrode surface, the reasonable concentration distribution and position distribution of fluorine (F), phosphorus (P) and oxygen (O) elements are formed, which overcomes the ion migration obstruction of graphite area and the instability of SEI of silicon-carbon area caused by traditional homogeneous fluorine distribution, and significantly improves the comprehensive performance of the negative electrode. The graphite particles and silicon-carbon particles in the composite negative electrode coating both contain fluorine and phosphorus with strong electron absorption capacity, and the fluorocarbon layer inside and on the surface of the graphite particles has more distribution, followed by the surface SEI film layer of the silicon-carbon particles, and the carbon layer inside is more uniform, the phosphorus is more uniformly dispersed in the composite negative electrode, which improves the oxidation resistance and reduction potential of the composite negative electrode, and the graphite particle carbon layer can maintain a high Li+ migration number; it is helpful to form a good SEI film on the negative electrode; in addition, the SEI film formed by the graphite particles is thin, dense and rich in LiF, thereby reducing the charge transfer impedance, having high ionic conductivity, reducing the polarization of the electrical equipment, making up for the high lithium diffusion impedance of the vapor deposited silicon-carbon, maintaining the high charge and discharge efficiency of the composite negative electrode, and improving the rate and cycle performance of the electrical equipment.

[0180] As a specific example of the application, the detailed case is provided as follows:

[0181] Embodiment 1:

[0182] (1) The conductor material a conductive acetylene black, graphite particles and silicon-carbon particles are pre-mixed; 50% of the bonding material a lithium polyacrylate, 50% of the bonding material b lithium carboxymethyl cellulose, and the solvent N-methyl pyrrolidone are mixed and kneaded for 90 min; then the remaining bonding material a lithium polyacrylate, the remaining bonding material b lithium carboxymethyl cellulose, the conductor material b single-walled carbon nanotube and the solvent N-methyl pyrrolidone are mixed and stirred for 120 min, and the solid content is controlled at 45%; then the bonding material c lithium polyacrylate-butadiene-acrylonitrile and the solvent N-methyl pyrrolidone are stirred for 60 min, the solid content is controlled at 40%, and the fineness is ≤45 μm, to obtain a composite negative electrode slurry; wherein the weight percentage of graphite particles in the composite negative electrode coating / the weight percentage of silicon-carbon particles in the composite negative electrode coating = 5.

[0183] The mass percentage of the bonding material a, the bonding material b and the bonding material c in the negative electrode coating is 2.6%, 0.7% and 1.1%, respectively;

[0184] The specific surface area of the graphite particles is 3.2 m 2 / g, and the specific surface area of the silicon-carbon particles is 2.6 m 2 / g;

[0185] The Dv50 of the graphite particles is 6 μm, and the Dv50 of the silicon-carbon particles is 8 μm.

[0186] (2) The conductor material c conductive acetylene black, the positive electrode material is premixed, 50% of the adhesive material d polyvinylidene fluoride, the solvent is mixed for 90 min for kneading, then add the remaining adhesive material d polyvinylidene fluoride, conductor material d multi-walled carbon nanotube, solvent N-methyl pyrrolidone, electrolyte lithium aluminum titanium phosphate is mixed for 90 min stirring, control the solid content at 82%, then add the solvent N-methyl pyrrolidone for 60 min stirring, control the solid content at 70%, fineness ≤ 12 μm, get the positive electrode slurry;

[0187] (3) The composite negative electrode slurry is coated on the foil, and the surface density is controlled at 0.006 g / cm 2 , drying to remove the solvent, rolling, drying, die cutting to get the composite negative electrode;

[0188] The porosity of the composite negative electrode coating is 43%

[0189] (4) The positive electrode slurry is coated on the foil, and the surface density is controlled at 0.019 g / cm 2 , drying to remove the solvent, rolling, drying, die cutting to get the positive electrode;

[0190] (5) The positive electrode sheet, separator, composite negative electrode sheet are stacked, the tab is welded, the shell is assembled, the liquid is injected, and the formation is formed: 0.1C charging to 3.5V + 0.5C charging to 4.0V; The capacity is divided: 0.5C charging to 4.5V, 0.5C discharging to 3.0V, which is a secondary power equipment.

[0191] The lithium salt in the electrolyte is composed of lithium hexafluorophosphate, lithium bisfluorosulfonylimide and lithium difluorodioxalate phosphate, wherein the concentration of lithium hexafluorophosphate in the electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.5 mol / L, and the concentration of lithium difluorodioxalate phosphate in the electrolyte is 0.5 mol / L.

[0192] The electrolyte additive in the electrolyte is composed of fluoroethylene carbonate, 1,3-propylene sulfone, polymeric borate, dimethyl tantalate and tris (trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.6:0.3:0.2;

[0193] The electrolyte solvent in the electrolyte is composed of vinylene carbonate, ethylene carbonate, propylene carbonate and methyl ethyl carbonate, and the mass ratio is 6:25:7:35;

[0194] From Figures 1-4 It can be seen that in example 1, the mixed negative electrode material formed by the graphite particles and the silicon-carbon particles, the cross section fluorine content of the graphite particles is greater than that of the silicon-carbon particles, and the degree of oxygen dispersion distribution is less than that of phosphorus.

[0195] Example 2: Different from example 1, the lithium salt in electrolyte is composed of lithium tetrafluorophosphate, lithium bisfluorosulfonylimide, lithium difluorodioxalate phosphate, wherein the concentration of lithium tetrafluorophosphate in electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.5 mol / L, the concentration of lithium difluorodioxalate phosphate in electrolyte is 0.5 mol / L.

[0196] Example 3: Different from example 1, the lithium salt in electrolyte is composed of lithium difluorophosphate, lithium bisfluorosulfonylimide, lithium difluorodioxalate phosphate, wherein the concentration of lithium difluorophosphate in electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.5 mol / L, the concentration of lithium difluorodioxalate phosphate in electrolyte is 0.5 mol / L.

[0197] Example 4: Different from example 1, the lithium salt in electrolyte is composed of lithium difluorophosphate, lithium bisfluorosulfonylimide, lithium difluorodioxalate phosphate, wherein the concentration of lithium difluorophosphate in electrode is 7.2 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.5 mol / L, the concentration of lithium difluorodioxalate phosphate in electrolyte is 0.5 mol / L.

[0198] Example 5: Different from example 1, the lithium salt in electrolyte is composed of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, lithium difluorodioxalate phosphate, wherein the concentration of lithium hexafluorophosphate in electrode is 8 mol / L, the concentration of lithium bis(trifluoromethylsulfonyl)imide is 1.5 mol / L, the concentration of lithium difluorodioxalate phosphate in electrolyte is 0.5 mol / L.

[0199] Example 6: Different from example 1, the lithium salt in electrolyte is composed of lithium hexafluorophosphate, lithium bis(pentafluoroethylsulfonyl)imide, lithium difluorodioxalate phosphate, wherein the concentration of lithium hexafluorophosphate in electrode is 8 mol / L, the concentration of lithium bis(pentafluoroethylsulfonyl)imide is 1.5 mol / L, the concentration of lithium difluorodioxalate phosphate in electrolyte is 0.5 mol / L.

[0200] Example 7: Different from example 1, the lithium salt in electrolyte is composed of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium difluorodioxalate phosphate, wherein the concentration of lithium hexafluorophosphate in electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 0.6 mol / L, the concentration of lithium difluorodioxalate phosphate in electrolyte is 0.5 mol / L.

[0201] Example 8: Different from example 1, the lithium salt in electrolyte is composed of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium difluorodioxalate phosphate, wherein the concentration of lithium hexafluorophosphate in electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.8 mol / L, the concentration of lithium difluorodioxalate phosphate in electrolyte is 0.5 mol / L.

[0202] Example 9: Different from example 1, the lithium salt in the electrolyte is composed of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium difluorophosphate, wherein the concentration of lithium hexafluorophosphate in the electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.5 mol / L, and the concentration of lithium difluorophosphate in the electrolyte is 0.5 mol / L.

[0203] Example 10: Different from example 1, the lithium salt in the electrolyte is composed of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium tetrafluorophosphate, wherein the concentration of lithium hexafluorophosphate in the electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.5 mol / L, and the concentration of lithium tetrafluorophosphate in the electrolyte is 0.5 mol / L.

[0204] Example 11: Different from example 1, the lithium salt in the electrolyte is composed of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium difluorophosphate, wherein the concentration of lithium hexafluorophosphate in the electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.5 mol / L, and the concentration of lithium difluorophosphate in the electrolyte is 0.2 mol / L.

[0205] Example 12: Different from example 1, the lithium salt in the electrolyte is composed of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium difluorophosphate, wherein the concentration of lithium hexafluorophosphate in the electrode is 8 mol / L, the concentration of lithium bisfluorosulfonylimide is 1.5 mol / L, and the concentration of lithium difluorophosphate in the electrolyte is 0.8 mol / L.

[0206] Example 13: Different from example 1, the electrolyte additive in the electrolyte is composed of fluoroethylene carbonate, 1,3-propylene sulfone lactone, polymeric borate, dimethyl tantalate, and tris(trimethylsilyl) phosphate, and the mass ratio is 3.5:1.35:0.6:0.3:0.2.

[0207] Example 14: Different from example 1, the electrolyte additive in the electrolyte is composed of fluoroethylene carbonate, 1,3-propylene sulfone lactone, polymeric borate, dimethyl tantalate, and tris(trimethylsilyl) phosphate, and the mass ratio is 4.2:1.35:0.6:0.3:0.2.

[0208] Example 15: Different from example 1, the electrolyte additive in the electrolyte is composed of fluoroethylene carbonate, 1,3-propylene sulfone lactone, polymeric borate, dimethyl tantalate, and tris(trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.6:0.1:0.2.

[0209] Example 16: Different from example 1, the electrolyte additives in the electrolyte are composed of fluoroethylene carbonate, 1,3-propene sultone, polymeric borate, dimethyltantalum oxide, tris(trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.6:0.4:0.2;

[0210] Example 17: Different from example 1, the electrolyte additives in the electrolyte are composed of fluoroethylene carbonate, 1,3-propene sultone, polymeric borate, dimethyltantalum oxide, tris(trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.5:0.3:0.2;

[0211] Example 18: Different from example 1, the electrolyte additives in the electrolyte are composed of fluoroethylene carbonate, 1,3-propene sultone, polymeric borate, dimethyltantalum oxide, tris(trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.8:0.3:0.2;

[0212] Example 19: Different from example 1, the electrolyte additives in the electrolyte are composed of fluoroethylene carbonate, 1,3-propene sultone, polymeric borate, dimethyltantalum oxide, tris(trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.6:0.3:0.1;

[0213] Example 20: Different from example 1, the electrolyte additives in the electrolyte are composed of fluoroethylene carbonate, 1,3-propene sultone, polymeric borate, dimethyltantalum oxide, tris(trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.6:0.3:0.4;

[0214] Example 21: Different from example 1, the specific surface area of the graphite particles is 2.9 m 2 / g, the specific surface area of the silicon-carbon particles is 2.7 m 2 / g; the Dv50 particle size of the graphite negative electrode is 7.1 μm, and the Dv50 particle size of the silicon-carbon negative electrode is 7.5 μm

[0215] Example 22: Different from example 1, the specific surface area of the graphite particles is 4.0 m 2 / g, the specific surface area of the silicon-carbon particles is 2.3 m 2 / g; the Dv50 particle size of the graphite negative electrode is 5 μm, and the Dv50 particle size of the silicon-carbon negative electrode is 8.5 μm

[0216] Example 23: Different from example 1, the mass ratio of the binder materials a, b, and c in the negative electrode coating is 2.6%, 0.4%, and 1.1%, respectively.

[0217] Example 24: Different from example 1, the porosity of the composite negative electrode coating is 60%

[0218] Example 25: Different from Example 1 is that the porosity of the composite negative electrode coating is 25%

[0219] Comparative Example 1: Different from Example 1 is that the lithium bisfluorosulfonylimide is not contained

[0220] Comparative Example 2: Different from Example 1 is that the lithium difluorodioxalate phosphate is not contained

[0221] Comparative Example 3: Different from Example 1 is that the concentration of the lithium bisfluorosulfonylimide is 2.5 mol / L

[0222] Comparative Example 4: Different from Example 1 is that the concentration of the lithium difluorodioxalate phosphate in the electrolyte is 1.2 mol / L

[0223] Comparative Example 5: Different from Example 1 is that the concentration of the lithium bisfluorosulfonylimide is 0.3 mol / L

[0224] Comparative Example 6: Different from Example 1 is that the concentration of the lithium difluorodioxalate phosphate in the electrolyte is 0.05 mol / L.

[0225] Comparative Example 7: Different from Example 1 is that the lithium bisfluorosulfonylimide and the lithium difluorodioxalate phosphate are not contained. The lithium salt further comprises lithium tetrafluoroborate. The concentration is 0.5 mol / L.

[0226] Comparative Example 8: Different from Example 1 is that the electrolyte additive in the electrolyte does not contain dimethyltantalate, but consists of fluoroethylene carbonate, 1,3- propenesultone, polymeric borate, tris(trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.6:0.2;

[0227] Comparative Example 9: Different from Example 1 is that the electrolyte additive in the electrolyte consists of fluoroethylene carbonate, 1,3-propenesultone, polymeric borate, dimethyltantalate, trimethyl phosphate, and the mass ratio is 4:1.35:0.6:0.3:0.2;

[0228] Comparative Example 10: Different from Example 1 is that the electrolyte additive in the electrolyte does not contain borate, but consists of fluoroethylene carbonate, 1,3-propenesultone, dimethyltantalate, tris(trimethylsilyl) phosphate, and the mass ratio is 4:1.35:0.3:0.2.

[0229] Comparative Example 11: Different from Example 1 is that the porosity of the composite negative electrode coating is 70%.

[0230] Comparative Example 12: Different from Example 1 is that the areal density of the composite negative electrode coating is 0.004 g / cm 2Comparative Example 13: Different from Example 1 is that the bonding material b is selected to be styrene butadiene rubber;

[0231] Comparative Example 14: Different from Example 1 is that the specific surface area of the graphite negative electrode = 2.6 m 2 / g, the specific surface area of the silicon-carbon negative electrode = 3.2 m 2 / g; the Dv50 particle size of the graphite negative electrode = 15 μm, and the Dv50 particle size of the silicon-carbon negative electrode = 6 μm

[0232] Comparative Example 15: Different from Example 1 is that the specific surface area of the graphite negative electrode = 5 m 2 / g, the specific surface area of the silicon-carbon negative electrode = 1 m 2 / g, the Dv50 particle size of the graphite negative electrode = 3 μm, and the Dv50 particle size of the silicon-carbon negative electrode = 13 μm.

[0233] Other relevant parameters of Examples 1-25 and Comparative Examples 1-15 are shown in the following table:

[0234] Table 1

[0235]

[0236]

[0237] The following tests were performed on Examples 1-25 and Comparative Examples 1-15:

[0238] 1. Cycle test:

[0239] 1) In a room environment of 25±1℃ and relative humidity less than 65%, the secondary-use electrical equipment of each of the above examples and comparative examples was subjected to the following charge and discharge without fixture clamping: 3.0C CC (constant current) to 4.05V, static 300s; 2.9C CC to 4.15V, CV (constant voltage) to 2.30C, static 300s; 2.50C CC to 4.25V, CV to 1.80C, static 300s; 2.0C CC to 4.40V, CV to 1.55C, static 300s; 1.55C CC to 4.50V, CV to 1.20C, static 300s; 0.5C DC to 3.0V. Continue 3.0C CC to 4.05V. Record the discharge capacity of 0.5C DC to 3.0V in the first week as C0.5. When the discharge capacity of 0.5C DC to 3.0V of each group of secondary-use electrical equipment meets = 80% x C0.5, stop the charge and discharge, and record the number of charge and discharge cycles of the secondary-use electrical equipment of each example and comparative example.

[0240] 2. Diffusion resistance Rss and direct current resistance DCR test:

[0241] 1) The DCR of each group of secondary electrical equipment is calculated as follows: the secondary electrical equipment is charged to 3.95V at 0.5C current and left for 300s; 2) continue discharging at 0.1C current for 10s and record the voltage at the discharging cutoff; 3) discharge at 1C current for 1s and record the voltage at the discharging cutoff; 4) DCR = (voltage at the discharging cutoff in step 3 - voltage at the discharging cutoff in step 2) / 0.9C;

[0242] 2) The Rss of the secondary electrical equipment is calculated as follows: 1) the secondary electrical equipment is charged to 3.95V at 0.5C current and left for 300s; 2) continue discharging at 0.1C current for 1800s; 3) leave for 3600s and record the voltage before leaving and at the cutoff; 4) Rss = (voltage before leaving in step 3 - voltage at the cutoff in step 3) / 0.1C;

[0243] Verification result analysis

[0244] Table 2

[0245]

[0246]

[0247] As can be seen from the comparison of Examples 1-25 and Comparative Examples 1-15, by limiting the concentration of fluorine-containing lithium phosphate salt, fluorine-containing lithium sulfonate salt and fluorine-containing lithium oxalate phosphate salt in the fluorine-containing lithium salt in the electrolyte, and limiting the components of the electrolyte additive, the use amount of each component in the electrolyte is limited, and an adhesive with certain oxygen binding function is selected, at this time, good distribution of fluorine in the cross section of graphite particles and silicon-carbon particles is obtained, and the content of phosphorus and oxygen in the composite negative electrode coating is reasonably distributed, the purpose of realizing the dispersion degree of oxygen in the composite negative electrode coating being less than the dispersion degree of phosphorus is achieved, based on the above technical features, the present application realizes higher charge and discharge efficiency of the composite negative electrode, reduces the diffusion resistance and direct current resistance, improves the cycle performance of the electrical equipment, and the cycle number is greater than 800 cycles.

[0248] As can be seen from the comparison of Examples 1 and Comparative Examples 1, 3 and 5, when the fluorine-containing lithium salt does not contain fluorine-containing lithium sulfonate salt or has a low content, at this time, due to the lack of the function of the anion (FSI-) in the fluorine-containing lithium sulfonate salt selectively intercalating between the graphite layers, and the lack of the function of the C-F bond in FSI- between the graphite layers promoting the penetration of fluorine into the graphite body and between the graphite layers, the fluorine content in the fluorine-containing region of the graphite negative electrode cross section is low, and the cycle number is significantly reduced.

[0249] As can be seen from the comparison of Example 1 and Comparative Examples 2, 4, 6, when the fluorine-containing lithium salt does not contain or contains a low amount of fluorine-containing lithium oxalate phosphate salt, the lack of reduction and decomposition of the fluorine-containing lithium oxalate phosphate salt on the surface of the silicon-carbon particles forms a polymer or inorganic-organic composite SEI layer rich in phosphorus, oxygen and a small amount of fluorine. At this time, the fluorine content of the fluorine-containing region in the cross-section of the silicon-carbon negative electrode is less than that of the fluorine-containing region in the cross-section of the graphite negative electrode, and the content of phosphorus in the coating of the composite negative electrode is low. At this time, the oxidation resistance and reduction potential of the composite negative electrode cannot be improved, and the Li + The migration number, such as the cycle number of Comparative Example 2 and Comparative Example 6, is less than that of Example.

[0250] As can be seen from the comparison of Example 1 and Comparative Example 7, when lithium bisfluorosulfonylimide and lithium difluorodioxalate phosphate are replaced by lithium tetrafluoroborate, the fluorine in the silicon-carbon is more deposited at this time due to the lack of contribution of fluorine-containing lithium sulfonate and fluorine-containing lithium oxalate phosphate to the distribution of fluorine in the graphite negative electrode and the silicon-carbon negative electrode. At this time, the electrochemical performance is poor.

[0251] As can be seen from the comparison of Example 1 and Comparative Examples 8-10, when the electrolyte does not contain a tantalate or a borate, or a phosphonate containing no silicon is used; due to the lack of dimethyl tantalate as an important electrolyte additive affecting oxygen distribution, the generation of a nano Ta2O5 film to reduce the dispersion degree of oxygen is not possible, and due to the lack of a silane group that can be oriented on the surface of silicon-carbon to preferentially react, the effect of the lack of a borate to react with free F ～ In combination, the fluorine is more uniformly distributed at the interface of the silicon particles, and at the same time, the harmful phosphide is converted into stable BPO4, eliminating P segregation and making P more uniformly distributed. At this time, due to the phosphorus content and oxygen content or distribution of the coating of the composite negative electrode not being within the limited range, the battery impedance is high, and the cycle performance is poor.

[0252] As can be seen from the comparison of Example 1 and Comparative Example 13, when carboxyalkyl cellulose and its derivatives are used in Example 1 of the application, they have an oxygen binding effect. As a key functional adhesive for the composite negative electrode, the carboxyl (-COOH) and hydroxyl (-OH) on the molecular chain cooperatively impart excellent oxygen binding ability and inhibit oxygen migration. Due to the lack of oxygen binding ability, the oxygen dispersion ability of Comparative Example 13 is poor.

[0253] It can be seen from the comparison of Examples 1 and Comparative Examples 11-12 and Comparative Examples 14-15 that when the physical characteristics of the graphite particles and the silicon-carbon particles, such as the Dv50 particle size, the specific surface area, and the porosity of the negative electrode coating, are simultaneously controlled, the fluorine-containing lithium salt and the fluorine-containing electrolyte additive can better cover the surface of the particles and infiltrate into the interior of the particles when the particle size and the specific surface area are of moderate size and sufficient porosity is provided, at which time the insufficient infiltration due to the excessively large particle size, the excessively small specific surface area, and the excessively small porosity is avoided, thereby affecting the fluorine content of the cross section of the particles, and the excessively high fluorine content due to the excessively small particle size, the excessively large specific surface area, and the excessively large porosity is also avoided. Therefore, by single or synergistic control of the above factors, the fluorine content of the fluorine-containing region in the cross section of the negative electrode can be controlled to be 3% to 12%.

[0254] In summary, the present application discloses a composite negative electrode and a secondary electrical equipment thereof. In the present application, the graphite particle region and the silicon-carbon particle region of the composite negative electrode coating both contain fluorine and phosphorus with strong electron-withdrawing ability, and the fluorine-carbon layer of the graphite particles has more fluorine distributed in the interior and on the surface, followed by the SEI film layer on the surface of the silicon-carbon particles, and the phosphorus is more uniformly dispersed in the interior of the composite negative electrode. The oxidation resistance and the reduction potential of the composite negative electrode are improved, the Li + The migration number is helpful to form a good SEI film in the negative electrode. In addition, the SEI film formed by the graphite particles is thin, dense, and rich in LiF, thereby reducing the charge transfer impedance, having a high ionic conductivity, reducing the polarization of the electrical equipment, solving the problem of high lithium diffusion impedance of the vapor-deposited silicon-carbon, maintaining a high charge and discharge efficiency of the composite negative electrode, and improving the rate and cycle performance of the electrical equipment.

[0255] It should be noted that the above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A composite negative electrode, characterized in that, Including composite negative electrode coating; The composite negative electrode coating includes graphite particles and silicon carbide particles, and the graphite particles and silicon carbide particles satisfy the following condition: 1.6 ≤ weight percentage of graphite particles in the composite negative electrode coating / weight percentage of silicon carbide particles in the composite negative electrode coating ≤ 8.

5. The composite negative electrode coating contains fluorine and satisfies the following conditions: 12wt% ≥ the mass percentage of fluorine in the cross section of graphite particles relative to the total mass of carbon, oxygen, fluorine, silicon, and phosphorus elements on the cross section > the mass percentage of fluorine in the cross section of silicon-carbon particles relative to the total mass of carbon, oxygen, fluorine, silicon, and phosphorus elements on the cross section ≥ 3wt%. The composite negative electrode coating contains 0.8–3.4 wt% phosphorus and 6.5–18 wt% oxygen, with the oxygen dispersion distribution being less than the phosphorus dispersion distribution. During the 400–1200 charge-discharge cycles, the diffusion impedance Rss growth rate per cycle is <1.2%, the DC impedance DCR growth rate per cycle is <1.2%, and the diffusion impedance Rss growth rate per cycle is > the DC impedance DCR growth rate per cycle.

2. The composite negative electrode according to claim 1, characterized in that, The specific surface area of ​​the graphite particles is ≥2m². 2 / g, silicon-carbon particles with a specific surface area ≥2m² 2 / g; Preferably, the specific surface area of ​​the graphite particles is greater than that of the silicon-carbon particles.

3. The composite negative electrode according to claim 1, characterized in that, The Dv50 particle size of the graphite particles is less than the Dv50 particle size of the silicon carbide particles, which is ≤10μm. Preferably, the porosity of the composite negative electrode coating is 21-65%; Preferably, the thickness of the composite negative electrode coating is 40–245 μm; Preferably, the coating surface density of the composite negative electrode coating is 0.005–0.040 g / cm³. 2 .

4. A secondary battery comprising the composite negative electrode according to any one of claims 1 to 3, characterized in that, Secondary batteries also include electrolyte, positive electrode, and separator. Preferably, the electrolyte comprises a fluorinated lithium salt, an electrolyte solvent, and an electrolyte additive; Preferably, the fluorinated lithium salt includes one or more of fluorinated lithium phosphate, fluorinated sulfonyl lithium salt, and fluorinated oxalate phosphate; Preferably, the concentration of the fluorinated lithium salt in the electrolyte is 3.2–12.5 mol / L; Preferably, the concentration of the fluorinated lithium phosphate in the electrolyte is 2.5–9.7 mol / L, the concentration of the fluorinated sulfonyl lithium salt in the electrolyte is 0.5–1.8 mol / L, and the concentration of the fluorinated oxalate lithium phosphate in the electrolyte is 0.2–1.0 mol / L.

5. The secondary battery according to claim 4, characterized in that, The electrolyte additive includes one or more of fluorocarbonate, sulfonate, borate, tantalate, and silica-containing phosphate; the electrolyte additive accounts for 1.5 to 8.5 wt% of the electrolyte by mass. Preferably, the mass ratio of fluorocarbonate, sulfonate, borate, tantalate, and silica-containing phosphate in the electrolyte additive is (1.2–4.5):(0.2–2.4):(0.2–1.8):(0.1–0.8):(0.1–0.8). Preferably, the mass ratio of the fluorocarbonate, sulfonate, borate, tantalate, and phosphate ester is (3.5–4.2):(0.8–1.5):(0.4–1.1):(0.1–0.5):(0.1–0.4). Preferably, the electrolyte solvent is selected from one or more of cyclic carbonate solvents and chain carbonate solvents; Preferably, the electrolyte solvent accounts for 55-90 wt% of the electrolyte by mass.

6. The secondary battery according to claim 4, characterized in that, The composite negative electrode satisfies the following conditions: 9wt% ≥ fluorine content in the cross section of graphite particles > fluorine content in the cross section of silicon-carbon particles ≥ 5wt%; The composite negative electrode coating contains 1.5wt%-3.4wt% phosphorus and 8.5-15wt% oxygen, with the oxygen dispersion distribution being less than the phosphorus dispersion distribution. Preferably, the fluorinated lithium salt is lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), or lithium difluorodioxanol phosphate (LiDFOP); The molar ratio of lithium hexafluorophosphate, lithium difluorosulfonylimide, and lithium difluorodioxarate phosphate in the fluorinated lithium salt is (7.2–9.4):(0.8–1.6):(0.3–0.7). The electrolyte additives are fluoroethylene carbonate, tris(trimethylsilane) phosphate, 1,3-propenesulfonate lactone, dimethyl tantalate, polyboronic acid ester, and methane disulfonate. The molar ratio of the fluoroethylene carbonate, 1,3-propenesulfonate lactone, polyboronic acid ester, dimethyl tantalate, and tris(trimethylsilane) phosphate is (3.5–4.2):(0.8–1.5):(0.4–1.1):(0.1–0.5):(0.1–0.4). Preferably, the molar ratio of the fluoroethylene carbonate, 1,3-propenesulfonyl lactone, polyboronic acid ester, dimethyl tantalate, and tris(trimethylsilane) phosphate is 4:1.35:0.6:0.3:0.

2.

7. The secondary battery according to claim 4, characterized in that, The composite negative electrode coating further includes a negative electrode conductor material and a negative electrode adhesive material; the positive electrode includes a positive electrode coating. Preferably, the positive electrode coating comprises a positive electrode material, a positive electrode conductor material, a positive electrode adhesive material, and an electrolyte; Preferably, the positive electrode adhesive material and the negative electrode adhesive material are each independently selected from one or more of the following: metal salts of polyacrylate and polyacrylate derivatives, polystyrene and its derivatives, carboxyalkyl cellulose and its derivatives, polyvinylidene fluoride, and block copolymer derivatives of polyacrylate. Preferably, the positive electrode adhesive material is polyvinylidene fluoride; Preferably, the negative electrode adhesive material includes adhesive material a, adhesive material b, and adhesive material c; Preferably, adhesive material a is a metal salt of polyacrylate and a polyacrylate derivative, adhesive material b is carboxyl cellulose and its derivatives, and adhesive material c is a block copolymer derivative of polyacrylate.

8. The secondary battery according to claim 7, characterized in that, The adhesive material a accounts for 1.2% to 4.0% of the mass of the negative electrode coating, the adhesive material b accounts for 0.2% to 1.2% of the mass of the negative electrode coating, and the adhesive material c accounts for 0.4% to 1.8% of the mass of the negative electrode coating. Preferably, the mass ratio of the positive electrode material, conductor material, adhesive material, and electrolyte in the positive electrode coating is (90-99.3):(0.05-6):(0.6-10):(0.02-0.8). Preferably, the mass ratio of graphite particles and silicon carbide particles, conductor material and negative electrode adhesive material in the composite negative electrode coating is (90-98.6):(0.2-10):(1.2-8).

9. A method for preparing a secondary battery according to any one of claims 4 to 8, characterized in that, Includes the following steps: S1: Premix the conductor material, graphite particles, and silicon carbide particles; add the binder, conductor material, and solvent, mix and stir to obtain a composite negative electrode slurry; S2: Premix the conductor material and the positive electrode material, add the binder material, conductor material, solvent and electrolyte, mix and knead to obtain the positive electrode slurry; S3: The composite negative electrode slurry is coated onto the foil, dried to remove solvent, rolled, dried, and die-cut to obtain the composite negative electrode sheet; S4: The positive electrode slurry is coated onto the foil, dried to remove solvent, rolled, dried, and die-cut to obtain the positive electrode sheet; S5: The positive electrode sheet, separator, and composite negative electrode sheet are stacked, and the tabs are welded, the casing is installed, the electrolyte is injected, the formation is completed, and the capacity is tested, which constitutes a secondary battery.

10. A secondary electrical device comprising a secondary battery prepared according to the preparation method of claim 9.

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