Negative electrode material, preparation thereof, negative electrode plate, secondary battery and device

Abstract

The invention provides a negative electrode material, preparation thereof, a negative electrode plate, a secondary battery and a device, the negative electrode material comprises a silicon-based material, a first coating layer located on at least one part of the surface of the silicon-based material, and a second coating layer located on at least one part of the surface of the first coating layer, the first coating layer comprises amorphous carbon; the second coating layer comprises lithium titanate; and the number of particles of the silicon-based material in the first coating layer is 1-15. The negative electrode material disclosed by the invention has the core-shell structure, so that the cyclic expansion of the silicon-based material can be improved, different electrical property requirements can be met by different core-shell gap blending, and meanwhile, the mechanical strength in the needling process can be improved by coating the surface of the silicon-based material with lithium titanate, so that the electrical property of the silicon-based material can be improved. After lithium titanate is rapidly discharged to form Li4Ti5O12 under the condition of short circuit, the internal resistance is increased, and the generation of Joule heat is reduced, so that the mechanical safety performance can be obviously improved.

Description

Technical Field

[0001] The present application relates to a negative electrode material, and in particular to a negative electrode material, its preparation, a negative electrode sheet, a secondary battery and a device, belonging to the field of batteries. Background Art

[0002] Lithium-ion batteries are currently one of the most promising energy storage and conversion media. Among them, silicon is one of the most promising negative electrode material candidates due to its theoretical specific capacity of 4200 mAh / g. It should be noted that the lithium storage mechanism of silicon-based materials is different from that of graphite. Graphite belongs to the interlayer free lithium deintercalation and intercalation mechanism, while silicon-based materials belong to the alloying lithium storage mechanism. Moreover, due to the different lithium storage mechanisms of the two, silicon-based materials exhibit obvious advantages and disadvantages, and the specific advantages are as follows: 1. Silicon is rich in reserves, cheap in price, low in toxicity, and environmentally friendly; 2. The theoretical specific capacity is the highest, reaching 4200 mAh / g; 3. The lithium intercalation potential is slightly higher than that of graphite, which is beneficial to solving the problem of lithium deposition; the specific disadvantages are as follows: 1. Silicon is a semiconductor material, and its conductivity and lithium ion diffusion rate are inferior to those of graphite; 2. The volume effect is obvious. Volume expansion easily leads to problems such as interruption of electrical contact, continuous rupture and recombination of the SEI film, collapse of the electrode structure, and pulverization of particles, ultimately affecting the battery cycle performance; 3. The first film formation consumes more active lithium, resulting in a low first Coulombic efficiency.

[0003] Therefore, there is an urgent need to develop a novel negative electrode material, which has improved mechanical safety performance when applied to secondary batteries. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present application provides a negative electrode material, its preparation, a negative electrode sheet, a secondary battery and a device. The present application solves the problem of simultaneously taking into account the improvement of mechanical safety problems caused by the increase in energy density.

[0005] In the first aspect of the present application, a negative electrode material is provided, which includes a silicon-based material, a first coating layer on the surface of the silicon-based material, and a second coating layer on the surface of the first coating layer, wherein the first coating layer includes amorphous carbon; the second coating layer includes lithium titanate; there is a gap between the first coating layer and the silicon-based material.

[0006] The second aspect of the present application provides a method for preparing a negative electrode material, which includes the following steps: Step S1: Add an initiator and methacrylic acid to a dispersion containing a modified silicon-based material, and after reacting for 2 h to 3 h, obtain a silicon-based material coated with polymethacrylic acid; Step S2: Mix lithium acetate and a chelating agent to obtain a first mixture, and then add the first mixture to tetrabutyl titanate to obtain a second mixture; Step S3: Add the silicon-based material coated with polymethacrylic acid obtained in Step S1 to the second mixture to obtain a negative electrode material precursor; Step S4: Calcinate the negative electrode material precursor obtained in Step S3 at 700 °C to 900 °C for 5 h to 8 h to obtain a negative electrode material.

[0007] The third aspect of the present application provides a negative electrode material prepared by the above method.

[0008] The fourth aspect of the present application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode material layer provided on the surface of the negative electrode current collector, and the negative electrode material layer includes the above negative electrode material.

[0009] The fifth aspect of the present application provides a secondary battery, which includes the above negative electrode sheet, a positive electrode sheet and an electrolyte.

[0010] The sixth aspect of the present application provides a device, which includes the above secondary battery.

[0011] The negative electrode material of the present application has a core-shell structure, which helps to improve the cyclic expansion of the silicon-based material. Different core-shell gaps are adjusted to meet different electrical performance requirements. At the same time, by coating lithium titanate on the surface of the silicon-based material, it helps to improve the mechanical strength during the acupuncture process. In this way, when a short circuit occurs, lithium titanate will quickly discharge to form Li4Ti5O 12 After that, the internal resistance increases, which helps to reduce the generation of Joule heat, thereby significantly improving the mechanical safety performance. Specific Embodiments

[0012] For the sake of brevity, the present application only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly recorded; and any lower limit can be combined with other lower limits to form a range not explicitly recorded. Similarly, any upper limit can be combined with any other upper limit to form a range not explicitly recorded. In addition, each individually disclosed point or single value itself can be used as a lower limit or an upper limit and combined with any other point or single value or combined with other lower limits or upper limits to form a range not explicitly recorded.

[0013] Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in this application can be measured by various commonly used measurement methods in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0014] The list of items connected by the terms "at least one of", "at least one", "at least one kind of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single component or multiple components. Item B may include a single component or multiple components. Item C may include a single component or multiple components.

[0015] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and not to limit the scope of the present application.

[0016] I. Anode material

[0017] The anode material provided by the present application includes a silicon-based material, a first coating layer located on the surface of the silicon-based material, and a second coating layer located on the surface of the first coating layer. Among them, the first coating layer includes amorphous carbon; the second coating layer includes lithium titanate; there is a gap between the first coating layer and the silicon-based material. The anode material of the present application has a core-shell structure, which helps to improve the cyclic expansion of the silicon-based material. Different core-shell gap formulations meet different electrical performance requirements. At the same time, by coating lithium titanate on the surface of the silicon-based material, it helps to improve the mechanical strength during the acupuncture process. In this way, when a short circuit occurs, lithium titanate will quickly discharge to form Li4Ti5O12, and the internal resistance increases, which helps to reduce the generation of joule heat, thereby significantly improving the mechanical safety performance.

[0018] In some embodiments, the number of silicon-based material particles in the first coating layer is 1 to 15; if there are too many silicon-based material particles in the first coating layer, it is easy to cause damage to the core-shell hollow structure due to the expansion of silicon particles during charge and discharge; if there are too few silicon-based material particles in the first coating layer, the utilization rate of the reserved gap space is low. In some embodiments, the number of silicon-based material particles in the first coating layer is 3, 4, 5, 6, 7, 8, 9, 10, 11, or any range between them. In some embodiments, the number of silicon-based material particles in the first coating layer is 3 to 11.

[0019] In some embodiments, the silicon-based material includes at least one of nano-silicon and nano-silicon oxide.

[0020] In some embodiments, the thickness of the first coating layer is 5 nm to 35 nm; if the thickness of the first coating layer is too large, it will affect the ion transport ability of lithium ions; if the thickness of the first coating layer is too small, the core-shell hollow structure cannot withstand the stress and strain of the expansion of silicon particles during charge and discharge. In some embodiments, the thickness of the first coating layer is 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm or any range therebetween.

[0021] In some embodiments, the thickness of the second coating layer is 80 nm to 400 nm; if the thickness of the second coating layer is too large, it will affect the increase of the ion transport impedance of the material body; if the thickness of the second coating layer is too small, in the case of a short circuit, rapid discharge forms Li4Ti5O 12 Insufficient later, and it cannot play a role in reducing the generation of Joule heat due to the increase in internal resistance.

[0022] In some embodiments, the thickness of the second coating layer is 80 nm, 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, 230 nm, 280 nm, 330 nm, 380 nm, 400 nm or any range therebetween.

[0023] In some embodiments, the mass ratio of the first coating layer to the silicon-based material is 1:9 to 1:33; if the mass ratio of the first coating layer to the silicon-based material is too large, it will affect the advantage of the specific capacity of the material under the same mass; if the mass ratio of the first coating layer to the silicon-based material is too small, a core-shell hollow structure cannot be formed. In some embodiments, the mass ratio of the first coating layer to the silicon-based material is 1:9, 1:14, 1:19, 1:24, 1:29, 1:33 or any range therebetween.

[0024] In some embodiments, the mass ratio of the second coating layer to the silicon-based material is 1:4 to 1:19; if the mass ratio of the second coating layer to the silicon-based material is too large, it will affect the advantage of the specific capacity of the material under the same mass; if the mass ratio of the second coating layer to the silicon-based material is too small, in the case of a short circuit, due to the shortage of Li4Ti5O12, the internal resistance cannot be effectively increased, and thus the rapid discharge cannot be carried out to reduce the problems caused by the generation of Joule heat; in some embodiments, the mass ratio of the first coating layer to the silicon-based material is 1:4, 1:7, 1:10, 1:13, 1:16, 1:19 or any range therebetween.

[0025] In this application, polymethyl methacrylate (PMMA) is coated on the surface of the silicon-based material, and PMMA materials with different coupling degrees are dissolved by acetone to form PMMA coating layers with different concentration gradients on the surface of the silicon-based material. At the same time, by adjusting the ratio of benzoyl peroxide (BPO) initiator to methacrylic acid (MAA), the mixing reaction time, the mass of the silicon-based material added (for example, nano-silicon particles), the solvent treatment time, etc., the thickness of the carbon shell coating the negative electrode material, the size of the reserved voids in the core-shell structure, and the number of silicon particles in the carbon shell coating layer can be flexibly controlled, so as to obtain a negative electrode material with a core-shell structure to meet different electrical performance requirements, such as cycle life, storage life, etc. Moreover, in this application, through the method of gradient coating, the silicon-based material is coated with the polymer PMMA and lithium titanate respectively to prepare a core-shell structure of the silicon-based material to reserve enough expansion space for silicon, and at the same time, the surface is coated with lithium titanate to improve the safety performance of the system against pinprick.

[0026] In some embodiments, the method for preparing the negative electrode material of the present application includes the following steps: Step S1: Disperse the silicon-based material in an organic solvent containing a silane coupling agent, ultrasonically disperse for 0.5 h to 2 h, stir for 6 h to 8 h, then wash and dry to obtain an alkylated silicon-based material, wherein the mass ratio of the silicon-based material to the silane coupling agent is 100:2 to 3; Step S2: Place the above-modified silicon-based material in an organic solvent, disperse for 0.5 h to 1 h, introduce nitrogen, stir and heat in a water bath to 65°C to 85°C, add an initiator and a certain amount of methacrylic acid (MAA), and carry out a mixed reaction for 2 h to 3 h. Wash and dry the reaction product to obtain a PMMA-coated silicon-based material. Step S3: Dissolve the obtained PMMA-coated silicon-based material in an organic solvent, soak for 10 min to 40 min, selectively dissolve PMMA with a low degree of polymerization (<6000) to obtain a negative electrode material precursor. Through Step S3, a PMMA coating layer with a gradient change on the surface of the silicon-based material (the degree of polymerization of PMMA inside and on the outer surface of the silicon-based material particles is different) can be obtained. This is because the silicon-based material is dissolved in the organic solvent, and MAA on the outer surface of the silicon-based material is in full contact with the initiator (fully polymerized), resulting in a higher degree of polymerization of the obtained PMMA. And the PMMA inside the silicon-based material particles is limited in contact with the initiator due to the outer layer being preferentially coated with PMMA with a higher degree of polymerization. Therefore, the present application adjusts the reaction time to control the degree of polymerization of the internal PMMA, thereby realizing a coating layer with a gradient change on the surface of the silicon-based material. Step S4: Dissolve lithium acetate and a chelating agent in an organic solvent (mix into a transparent solution), and slowly add it to tetrabutyl titanate under stirring to obtain a yellow sol (lithium titanate gel). Step S5: Add the above-obtained negative electrode material precursor to the above yellow sol, stir well, cure in air for 10 to 12 h, and dry in a vacuum oven at 60°C to 80°C for 24 h to 36 h to obtain a white precursor. The white precursor is calcined at 700°C to 900°C in an N2 atmosphere for 5 h to 8 h to obtain a negative electrode material (grayish black).

[0027] In some embodiments, before Step S1, it further includes: dispersing the silicon-based material in an organic solvent containing a silane coupling agent to obtain a modified silicon-based material;

[0028] In some embodiments, the degree of polymerization of polymethacrylic acid is greater than 6000. In some embodiments, the initiator includes benzoyl peroxide; in some embodiments, the chelating agent includes oxalic acid.

[0029] In some embodiments, the mass ratio of the methacrylic acid to the modified silicon-based material is 1 to 4:1;

[0030] In some embodiments, the mass ratio of the initiator to the methacrylic acid is 1:6 to 8;

[0031] In some embodiments, the molar ratio of the chelating agent to tetrabutyl titanate is 1:1 to 2.

[0032] In some embodiments, the mass ratio of the silicon-based material to the silane coupling agent is 100:2 to 3.

[0033] In some embodiments, the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of nickel-cobalt-based ternary materials and phosphate-based materials.

[0034] In some embodiments, the nickel-cobalt-based ternary material includes at least one of LiNi m Co n A (1-m-n) O2 materials, where A is selected from at least one of manganese, aluminum, magnesium, chromium, calcium, zirconium, molybdenum, silver, or niobium, 0.5 ≤ m ≤ 1, 0 ≤ n ≤ 0.5, and m + n ≤ 1.

[0035] In some embodiments, m is 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range formed by any two of these values. In some embodiments, n is 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or a range formed by any two of these values.

[0036] In some embodiments, the nickel-cobalt-based ternary material includes at least one of NCA, NCM111, NCM523, NCM622, NCM811, Ni90, Ni92, or Ni95.

[0037] In some embodiments, the phosphate-based material includes at least one of LiMn k B (1-k) PO4, where 0 ≤ k ≤ 1, and the B element is selected from at least one of iron, cobalt, magnesium, calcium, zinc, chromium, or lead. In some embodiments, k is 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range formed by any two of these values. In some embodiments, the phosphate-based material includes lithium iron phosphate, LiMn 0.6 Fe 0.4 PO4, or LiMn 0.8 Fe 0.2 PO4, or at least one of them.

[0038] In some embodiments, the positive electrode active material includes at least one of lithium nickel oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese cobalt magnesium oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, and lithium manganese iron phosphate.

[0039] In some embodiments, the positive electrode active material layer further includes a binder, and optionally includes a conductive material. The binder improves the binding between the positive electrode active material particles and also improves the binding between the positive electrode active material and the current collector.

[0040] In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0041] In some embodiments, the conductive material includes, but is not limited to: carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based materials are selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials are selected from metal powders, metal fibers, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0042] In some embodiments, the positive electrode further includes a positive electrode current collector, and the positive electrode current collector can be a metal foil or a composite current collector. For example, aluminum foil can be used. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer substrate.

[0043] In some embodiments, a separator is provided between the positive electrode and the negative electrode to prevent short circuit. There are no particular limitations on the material and shape of the separator that can be used in the embodiments of the present application, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or an inorganic substance formed of a material that is stable to the electrolyte of the present application.

[0044] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film, or a composite film having a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be selected.

[0045] A surface treatment layer is provided on at least one surface of the base material layer. The surface treatment layer can be a polymer layer, an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0046] The inorganic layer includes inorganic particles and a binder. The inorganic particles include at least one of alumina, silica, magnesia, titania, hafnium dioxide, tin dioxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene.

[0047] The polymer layer contains a polymer, and the materials of the polymer include at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene ether, polyvinylidene fluoride, or poly(vinylidene fluoride - hexafluoropropylene).

[0048] In some embodiments, the secondary battery is a lithium secondary battery or a sodium secondary battery. In some examples, the lithium secondary battery includes, but is not limited to: a lithium metal secondary battery, a lithium - ion secondary battery, a lithium polymer secondary battery, or a lithium - ion polymer secondary battery.

[0049] In some embodiments, the secondary battery may include an outer package, and the outer package can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch - type soft package. The material of the soft package can be a plastic, such as one or several of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0050] In some embodiments, the shape of the secondary battery is not particularly limited, and it can be cylindrical, square, or any other arbitrary shape.

[0051] In some embodiments, the present application also provides a battery module. The battery module includes the above - mentioned secondary battery. Since the battery module of the present application adopts the above - mentioned secondary battery, it has at least the same advantages as the secondary battery. The number of secondary batteries included in the battery module of the present application can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0052] In some embodiments, the present application also provides a battery pack, which includes the above - mentioned battery module. The number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0053] II. Device

[0054] The present application also provides a device, which includes at least one of the above secondary battery, battery module or battery pack.

[0055] In some embodiments, the device includes, but is not limited to: electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, energy storage systems, etc. In order to meet the device's requirements for high power and high energy density of the secondary battery, a battery pack or battery module can be used.

[0056] In some other embodiments, the device can be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires being thin and light, and a secondary battery can be used as the power source.

[0057] Examples and Comparative Examples

[0058] Example 1

[0059] Preparation of the positive electrode sheet

[0060] Disperse the positive electrode active material NCM811, the conductive agent Super P / multi-walled carbon nanotubes and the binder PVDF in an appropriate amount of NMP according to a mass ratio of 97.5:0.9 / 0.5:1.1 to form a uniform positive electrode slurry; coat the positive electrode slurry on the aluminum foil, and after processes such as drying and rolling, a positive electrode sheet is prepared.

[0061] Preparation of the negative electrode sheet

[0062] Step S1: Disperse 100 g of nano-silicon particles in a toluene solution containing 2 g of silane coupling agent, ultrasonically disperse for 0.5 - 2 h, stir rapidly for 6 - 8 h, and then through washing and drying, alkylated nano-silicon particles are obtained;

[0063] Step S2: Place the above-obtained alkylated nano-silicon particles (modified nano-silicon particles) in a toluene solvent, ultrasonically disperse for 0.5 - 1 h, introduce nitrogen, stir while heating in a water bath to 85 °C for 3 h, add 50 g of benzoyl peroxide (BPO) initiator and 400 g of methacrylic acid (MAA), and carry out a mixed reaction for 3 h. The reaction product is washed and dried to obtain PMMA-coated nano-silicon particles;

[0064] Step S3: Dissolve the above-obtained PMMA-coated nano-silicon particles in an acetone solution, soak for 10 min, selectively dissolve PMMA with a low degree of polymerization (n < 6000), then wash with isopropyl alcohol and rinse with deionized water to obtain a Si@Void@PMMA precursor;

[0065] Step S4: Dissolve 14.37 g of lithium acetate and 46.33 g of chelating agent (oxalic acid) in an appropriate amount of absolute ethanol (to form a transparent solution), and slowly add it dropwise to 92.65 g of tetrabutyl titanate solvent under magnetic stirring to obtain a yellow sol (lithium titanate gel);

[0066] Step S5: Add the above-obtained Si@Void@PMMA precursor to the above yellow sol, stir well, cure in air for 12 h, and dry in a vacuum oven at 80 °C for 36 h to obtain a white precursor;

[0067] Step S6: Calcinate the above white precursor in a muffle furnace at 900 °C for 8 h under a N2 atmosphere, and cool to room temperature to obtain negative electrode material Si@Void@C / LTO particles (grayish-black).

[0068] Among them, based on the mass of the above negative electrode material, the mass content of lithium titanate is 25 g.

[0069] (3) Preparation of negative electrode sheet

[0070] Homogenize the above Si@Void@C / LTO particles, conductive agent carbon black / carbon nanotubes, thickening agent CMC, binder SBR and PAA according to a mass ratio of 96:0.9 / 0.1:0.6:1.2:1.2 to obtain a uniformly dispersed slurry. Coating the above slurry on a 6 μm or 8 μm copper foil, drying at 100 °C, and then roll-pressing to obtain a negative electrode sheet.

[0071] Preparation of electrolyte

[0072] Dissolve LiPF6 in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate (volume ratio 1:1:1), and at the same time add 10 wt% of fluoroethylene carbonate as a film-forming additive to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0073] Separator: Polyethylene separator.

[0074] Outer packaging: Aluminum-plastic film.

[0075] Preparation of lithium-ion battery

[0076] Stack the prepared positive electrode sheet, separator, and negative electrode sheet in sequence, with the separator in the middle of the positive and negative electrode sheets, and wind to obtain a bare battery cell; Place the bare battery cell in an aluminum-plastic film outer packaging, inject the prepared lithium-ion battery electrolyte after sufficient drying, and after the battery is left standing at 45 °C for 48 h, subjected to high-temperature formation and secondary sealing, perform 0.33C CC (constant current) + CV (constant voltage to 0.05C) conventional grading.

[0077] Examples 2 to 4 and Comparative Examples 1 to 2

[0078] Examples 2 to 3 and Comparative Example 1 were achieved by adjusting the type of silicon-based material, the mass of lithium acetate, the mass of the chelating agent (oxalic acid), the mass of tetrabutyl titanate, etc. on the basis of Example 1. The specific adjustment measures and detailed data are shown in Table 1.

[0079] Measurement method:

[0080] 1. Measurement of the number / thickness of silicon-based material particles in the first coating layer:

[0081] Use a transmission electron microscope (TEM) to microscopically observe the lattice image and distribution of the material.

[0082] 2. Measurement of the penetration depth of acupuncture:

[0083] Fully charge the lithium-ion battery to 100% SOC, use a high-temperature resistant steel needle with a diameter of 1 mm and a tip taper of 28° - 36°, start in the direction perpendicular to the battery at a speed of 1 mm / s, observe for 1 h when the penetration depth reaches 80%, and if there is no out-of-control situation, continue to observe for 1 h when the penetration depth reaches 100%.

[0084] 3. Measurement of storage performance:

[0085] Fully charge the lithium-ion battery to 100% SOC, place it at 60°C for 100 days, measure the thickness of the battery cell before and after storage, and calculate the thickness expansion rate of the battery cell. Among them, the thickness expansion rate of the battery cell = (the thickness of the battery cell after storage - the thickness of the battery cell before storage) / the thickness of the battery cell before storage.

[0086] Table 1

[0087]

[0088]

[0089] As can be seen from Table 1, the negative electrode material of the present application has a core-shell structure, which helps to improve the cyclic expansion of the silicon-based material. Different core-shell gap formulations meet different electrical performance requirements. At the same time, coating lithium titanate on the surface of the silicon-based material helps to improve the mechanical safety performance.

[0090] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those of ordinary skill in the art will recognize that some modifications and changes can be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims

1. A negative electrode material, which comprises a silicon-based material, a first coating layer located on the surface of the silicon-based material, and a second coating layer located on the surface of the first coating layer, wherein, The first coating layer includes amorphous carbon; The second coating layer includes lithium titanate; There is a void between the first coating layer and the silicon-based material.

2. The negative electrode material according to claim 1, wherein, The negative electrode material satisfies at least one of the following conditions: The silicon-based material includes at least one of nano-silicon and nano-silicon oxide; The thickness of the first coating layer is 3 nm to 40 nm; The thickness of the second coating layer is 65 nm to 450 nm; The mass ratio of the first coating layer to the silicon-based material is 1:9 to 1:33; The mass ratio of the second coating layer to the silicon-based material is 1:4 to 1:19 The number of silicon-based material particles in the first coating layer is 1 to 15.

3. The negative electrode material according to claim 1, wherein, The negative electrode material satisfies at least one of the following conditions: The number of silicon-based material particles in the first coating layer is 3 to 11.

4. A method for preparing the negative electrode material according to any one of claims 1-3, which comprises the following steps: Step S1: Adding an initiator and methacrylic acid to a dispersion liquid containing a modified silicon-based material, and reacting for 2 h to 3 h to obtain a silicon-based material coated with polymethacrylic acid; Step S2: Mixing lithium acetate and a chelating agent to obtain a first mixture, and then adding the first mixture to tetrabutyl titanate to obtain a second mixture; Step S3: Adding the silicon-based material coated with polymethacrylic acid obtained in Step S1 to the second mixture to obtain a negative electrode material precursor; Step S4: Calcining the negative electrode material precursor obtained in Step S3 at 700 °C to 900 °C for 5 h to 8 h to obtain a negative electrode material.

5. The method according to claim 4, wherein, The method satisfies at least one of the following conditions: Before step S1, it further includes: dispersing the silicon-based material in an organic solvent containing a silane coupling agent to obtain a modified silicon-based material; In step S1, the degree of polymerization of the polymethacrylic acid is greater than 6000; In step S1, the initiator includes benzoyl peroxide; In step S1, the mass ratio of methacrylic acid to the modified silicon-based material is 1 to 4:1; In step S1, the mass ratio of the initiator to methacrylic acid is 1:6 to 8; In step S2, the chelating agent includes oxalic acid; In step S2, the molar ratio of the chelating agent to tetrabutyl titanate is 1:1 to 2.

6. The method according to claim 5, wherein, The mass ratio of the silicon-based material to the silane coupling agent is 100:2 to 3.

7. A negative electrode material prepared by the method according to any one of claims 4-6.

8. A negative electrode plate comprising a negative electrode current collector and a negative electrode material layer provided on the surface of the negative electrode current collector, wherein the negative electrode material layer comprises the negative electrode material according to any one of claims 1 and 2 or claim 7.

9. A secondary battery comprising the negative electrode plate according to claim 8, a positive electrode plate and an electrolyte.

10. A device comprising the secondary battery according to claim 9.

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