Negative electrode material and secondary battery

Abstract

A negative electrode material and a secondary battery. The negative electrode material includes a core and a shell. The core includes a silicon matrix and a filler, and at least part of the filler is distributed within the silicon matrix. The filler includes a transition metal carbide and a carbon matrix. The shell is disposed on at least part of the surface of the core. Based on the negative electrode material, the mass ratio of the transition metal element is 0.5% to 5%. The above-mentioned negative electrode material has high particle strength, can reduce the cyclic swelling effect of the obtained negative electrode sheet, and improve the cyclic stability of the obtained secondary battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, and particularly to a negative electrode material and a secondary battery. Background Art

[0002] Silicon negative electrode materials have a high theoretical specific capacity and a low de-lithiation potential, and are regarded as one of the potential candidates for the next-generation high-capacity lithium-ion battery negative electrode materials. However, silicon negative electrode materials have problems such as volume expansion, unstable SEI film, and poor electrical conductivity in practical applications. Especially during the process of lithium de-insertion and insertion during battery charge and discharge, a large volume expansion will occur, resulting in easy fragmentation and pulverization of silicon negative electrode material particles, seriously affecting the cycle performance of the battery. Summary of the Invention

[0003] In view of this, this application provides a negative electrode material to solve at least one of the above technical problems.

[0004] To achieve the above object, this application provides a negative electrode material, which includes a core and a shell. The core includes a silicon matrix and a filler, at least part of the filler is distributed in the silicon matrix, the filler includes a transition metal carbide and a carbon matrix, the shell is provided on at least part of the surface of the core, and based on the negative electrode material, the mass ratio of the transition metal element is 0.5% to 5%.

[0005] In some possible implementation manners, based on the negative electrode material, the mass ratio of the transition metal element is 0.8% to 3.7%.

[0006] In some possible implementation manners, the filler is also distributed on at least part of the surface of the core to form an intermediate shell layer.

[0007] In some possible implementation manners, the thickness of the intermediate shell layer is 50 nm to 100 nm.

[0008] In some possible implementation manners, the particle strength of the negative electrode material is defined as Cs, wherein, F is the pressure received when the particles of the negative electrode material break, D is the particle diameter of the negative electrode material, is the pi; Cs is 170 Mpa to 350 Mpa.

[0009] In some possible implementation manners, the transition metal carbide includes one or more of molybdenum carbide, titanium carbide, tungsten carbide, tantalum carbide, vanadium carbide, zirconium carbide, chromium carbide, niobium carbide, and hafnium carbide.

[0010] In some possible implementation manners, the morphology of the transition metal carbide includes one or more of nanoparticles, nanowires, nanotubes, and nanosheets.

[0011] In some possible implementation manners, the carbon matrix includes one or more of crystalline carbon, amorphous carbon, and transitional carbon.

[0012] In some possible implementation manners, the material of the outer shell includes at least one of carbon materials, titanium nitride, boron nitride, silicon nitride, silicon oxide, magnesium oxide, aluminum oxide, silicon carbide, and polyaniline.

[0013] In some possible implementation manners, the thickness of the outer shell is 50 nm to 500 nm.

[0014] In some possible implementation manners, the average particle size of the negative electrode material is 1 μm to 15 μm.

[0015] In some possible implementation manners, the specific surface area of the negative electrode material is less than or equal to 8 m 2 / g.

[0016] In some possible implementation manners, the porosity of the negative electrode material is less than or equal to 2.5%.

[0017] In some possible implementation manners, based on the negative electrode material, the mass ratio of silicon element is 50% to 80%.

[0018] In some possible implementation manners, based on the negative electrode material, the mass ratio of carbon element is 15% to 35%.

[0019] This application also provides a secondary battery, including the above-mentioned negative electrode material.

[0020] In this application, the transition metal carbide in the inner core itself has relatively high structural strength, and can be doped on the carbon matrix to form a carbon matrix doped with transition metal carbide, which is dispersed or embedded in the silicon matrix, capable of alleviating the stress generated by the expansion of the silicon matrix and improving the structural stability of the inner core; this application also sets an outer shell, which is beneficial to reducing the contact between the electrolyte and the silicon matrix, reducing side reactions, and also has a certain increase in the particle strength of the negative electrode material, thereby further improving the cycle performance of the battery.

[0021] Moreover, the particle strength of the negative electrode material and the mass ratio of the transition metal element in the negative electrode material show a relationship similar to a quadratic function, that is, as the mass ratio of the transition metal element in the negative electrode material increases, the particle strength of the negative electrode material shows a trend of first increasing and then decreasing. Only by controlling the mass ratio of the transition metal element within the above range can the particle strength of the negative electrode material be effectively improved. Description of the Drawings

[0022] Figure 1 It is a schematic structural diagram of the secondary battery provided by an embodiment of this application during charging.

[0023] Figure 2Schematic diagram of the structure of a secondary battery provided by an embodiment of the present application during discharge.

[0024] Figure 3 SEM cross-sectional image of the negative electrode material provided by Example 2 of the present application.

[0025] Figure 4 EDS surface scan image of the negative electrode material provided by Example 2 of the present application.

[0026] Figure 5 XRD image of the negative electrode material provided by Example 2 of the present application.

[0027] Description of the main component reference numerals:

[0028] Electrode assembly, 100; positive electrode sheet, 101; negative electrode sheet, 102; separator, 103. Specific embodiments

[0029] The embodiments of the present application will be described in detail below. The embodiments described by referring to the accompanying drawings are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application; it should be noted that unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present application belongs; without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other; many specific details are set forth in the following description to facilitate a thorough understanding of the present application, and the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments.

[0030] An embodiment of the present application provides a secondary battery, including a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located inside the housing.

[0031] The housing can be a packaging bag obtained by encapsulating with a packaging film (such as an aluminum-plastic film), for example, a soft-pack battery. In other embodiments, it can also be a steel-shell battery, an aluminum-shell battery, etc.

[0032] Please refer to Figure 1 and Figure 2 , the electrode assembly 100 includes a positive electrode sheet 101, a negative electrode sheet 102, and a separator 103, and the separator 103 is disposed between the positive electrode sheet 101 and the negative electrode sheet 102. When there is an electrolyte (not shown in the figure), during charging, please refer to Figure 1 , active ions (such as lithium ions) are deintercalated from the lattice of the positive electrode material (such as a lithiated intercalation compound) of the positive electrode sheet 101, pass through the electrolyte through the separator 103, reach the negative electrode sheet 102, and are inserted into the lattice of the negative electrode material. During discharge, please refer to Figure 2, active ions (such as lithium ions) are deintercalated from the lattice of the negative electrode material of the negative electrode sheet 102, pass through the electrolyte membrane 103, reach the positive electrode sheet 101 and are intercalated into the lattice of the positive electrode material (such as a lithiated intercalation compound), generating electrons that flow from the negative electrode sheet 102 to the positive electrode sheet 101 through the external circuit. The reverse movement of electrons forms an electric current that can be used by electrical appliances.

[0033] In some embodiments, the electrode assembly 100 can be a stacked structure formed by alternately laminating the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102 in sequence. In other embodiments, the electrode assembly 100 can also be a wound structure formed by laminating the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102 in sequence and then winding them.

[0034] Positive electrode sheet

[0035] The positive electrode sheet 101 includes a positive electrode current collector and a positive electrode material active layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of aluminum foil, nickel foil, etc., or can be any composite current collector disclosed in the prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive electrode material active layer includes a positive electrode active material, and the positive electrode active material includes a compound that can reversibly intercalate and deintercalate lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material can include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material can include but not be limited to at least one of lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese ternary material (NCM), lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4), or lithium iron phosphate (LiFePO4).

[0036] The positive electrode material active layer also includes a binder for binding the positive electrode active material particles to facilitate the formation of a film layer and at the same time improve the bonding force between the positive electrode material active layer and the positive electrode current collector. In some embodiments, the binder can include but not be limited to at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0037] The active layer of the positive electrode material may further include a conductive material, which includes but is not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials may include but are not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials may include but are not limited to metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0038] Negative electrode sheet

[0039] The negative electrode sheet 102 includes a negative electrode current collector and a negative electrode material active layer provided on at least one surface of the negative electrode current collector. The negative electrode current collector may be at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector, and may also be a composite current collector disclosed in any prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0040] The negative electrode material active layer includes a negative electrode material, which includes a core and a shell. The core includes a silicon matrix and a filler, at least part of the filler is distributed in the silicon matrix, the filler includes a transition metal carbide and a carbon matrix, the shell is provided on at least part of the surface of the core, and based on the negative electrode material, the mass ratio of the transition metal element is 0.5% to 5%. For example, the mass ratio of the transition metal element may be 0.5%, 0.6%, 0.75%, 0.8%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 3.7%, 4%, 4.2%, 4.3%, 5%, or any value within the range formed by any two of the above values.

[0041] In this application, the transition metal carbide in the core itself has high structural strength and can be doped on the carbon matrix to form a carbon matrix doped with transition metal carbide, which is dispersed or embedded in the silicon matrix, capable of relieving the stress generated by the expansion of the silicon matrix and improving the structural stability of the core; this application also sets a shell, which is beneficial to reducing the contact between the electrolyte and the silicon matrix, reducing the side reaction between the negative electrode material and the electrolyte, and also has a certain gain in the particle strength of the negative electrode material, thereby further improving the cycle performance of the battery including the negative electrode material of this application.

[0042] It is also found in this application that the content of transition metal carbide affects its contribution to the particle strength of the anode material. By regulating the mass ratio of transition metal elements in the anode material, it is helpful to regulate the particle strength of the anode material. And through a large number of studies, the inventor finds that the particle strength of the anode material has a relationship similar to a quadratic function with the mass ratio of transition metal elements in the anode material, that is, as the mass ratio of transition metal elements in the anode material increases, the particle strength of the anode material first increases and then decreases. Only by controlling the mass ratio of transition metal elements within the above range can the particle strength of the anode material be effectively improved.

[0043] It can be understood that when the mass ratio of transition metal elements in the anode material is too low, during the process of lithium insertion and extraction, the volume expansion of the silicon matrix is significant, which easily leads to the fragmentation or pulverization of the particles of the anode material including the silicon matrix, destroys the structure of the anode sheet, and also causes an increase in the internal stress of the battery, which may lead to the fracture of the anode sheet. When the mass ratio of transition metal elements in the anode material is too high, excessive transition metal carbide will destroy the silicon matrix structure during the subsequent heat treatment process, resulting in cracking of the anode material including the silicon matrix, reducing the particle strength of the anode material, and exacerbating the volume expansion effect.

[0044] In some embodiments, based on the anode material, the mass ratio of transition metal elements is 0.8% to 3.7%. For example, the mass ratio of transition metal elements can be 0.8%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 3.7% or any value within the range formed by any two of the above values. When the mass ratio of transition metal elements meets the above range, it can further improve the particle strength of the anode material, more effectively reduce the cyclic expansion effect of the obtained anode sheet, and further improve the cyclic stability of the obtained secondary battery.

[0045] In some embodiments, the filler is also distributed on at least part of the surface of the inner core to form an intermediate shell layer.

[0046] When the transition metal carbide and carbon matrix in the filler are distributed on the surface of the inner core, the transition metal carbide can be doped on the carbon matrix to form a transition metal carbide-doped carbon matrix, which can protect and support the inner core to maintain a more stable structure, is beneficial to further improving the particle strength of the anode material, so that the anode material can better resist the stress caused by volume change during the charge and discharge process, thereby reducing the risk of fragmentation or pulverization of the anode material particles, reducing the volume expansion effect, and improving the cyclic performance of the obtained secondary battery.

[0047] In some embodiments, the thickness of the intermediate shell layer is from 50 nm to 100 nm. For example, the thickness of the intermediate shell layer can be 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm. Controlling the thickness of the intermediate shell layer within the above range is beneficial to further improving the supporting effect of the intermediate shell layer on the negative electrode material.

[0048] In some embodiments, the particle strength of the negative electrode material is defined as Cs, wherein, F is the pressure received when the particles of the negative electrode material break, and D is the particle size of the negative electrode material, is pi; Cs is from 170 Mpa to 350 Mpa. For example, the particle strength Cs can be 170 Mpa, 180 Mpa, 190 Mpa, 200 Mpa, 220 Mpa, 250 Mpa, 300 Mpa, 330 Mpa, 350 Mpa or any value within the range composed of any two of the above values. The negative electrode material with higher particle strength can better resist the stress caused by volume change during charge and discharge, thereby reducing particle breakage and pulverization, and reducing the cycle expansion rate of the battery including the negative electrode material of the present application.

[0049] In some embodiments, the transition metal carbide includes one or more of molybdenum carbide, titanium carbide, tungsten carbide, tantalum carbide, vanadium carbide, zirconium carbide, chromium carbide, niobium carbide and hafnium carbide. When the negative electrode material includes the above transition metal carbides, it is beneficial to improve the particle strength of the negative electrode material, thereby being beneficial to reducing the volume expansion effect of the negative electrode material.

[0050] In some embodiments, the morphology of the transition metal carbide includes one or more of nanoparticles, nanowires, nanotubes and nanosheets. The present application has no special limitation on the morphology of the transition metal carbide. When it is any of the above morphologies or a combination of multiple morphologies, it can play the role of improving the particle strength of the negative electrode material by the transition metal carbide.

[0051] In some embodiments, the carbon matrix includes one or more of crystalline carbon, amorphous carbon and transitional carbon.

[0052] In some embodiments, the material of the outer shell includes at least one of carbon materials, titanium nitride, boron nitride, silicon nitride, silicon oxide, magnesium oxide, aluminum oxide, silicon carbide and polyaniline.

[0053] In some embodiments, the thickness of the outer shell is from 50 nm to 500 nm. For example, the thickness of the outer shell can be 50 nm, 60 nm, 80 nm, 100 nm, 150 nm, 200 nm, 300 nm, 450 nm, 500 nm, or any value within the range formed by any two of the above values. Controlling the thickness of the outer shell within the above range is beneficial to better reducing the contact between the silicon material and the electrolyte and reducing side reactions.

[0054] In some embodiments, the average particle size of the negative electrode material is from 1 μm to 15 μm. For example, the average particle size of the negative electrode material can be 1 μm, 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 14 μm, 15 μm, or any value within the range formed by any two of the above values. Controlling the average particle size of the negative electrode material within the above range is beneficial to reducing the migration distance of active ions (such as lithium ions) in the secondary battery, reducing the risk of deterioration of the capacity or rate performance in the secondary battery, and at the same time, a suitable particle size is beneficial to maintaining good particle strength of the negative electrode material.

[0055] In some embodiments, the specific surface area of the negative electrode material is less than or equal to 8 m 2 / g. For example, the specific surface area of the negative electrode material can be 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, or any value within the range formed by any two of the above values. Controlling the specific surface area of the negative electrode material within the above range can reduce the formation of SEI on the surface of the negative electrode material, thereby improving the initial Coulomb efficiency of the battery prepared from the negative electrode material.

[0056] In some embodiments, the porosity of the negative electrode material is less than or equal to 2.5%. For example, the porosity of the negative electrode material can be 0.5%, 1%, 1.5%, 1.8%, 2.0%, 2.2%, 2.4%, 2.5%, or any value within the range formed by any two of the above values. Porosity refers to the percentage of the pore volume in the material to the total volume of the material in its natural state. Controlling the porosity of the negative electrode material within the above range indicates that the pores of the silicon matrix are well filled with the carbon matrix doped with transition metal carbides, which is beneficial to improving the particle strength of the negative electrode material and thus beneficial to reducing the volume expansion effect.

[0057] In some embodiments, based on the negative electrode material, the mass proportion of silicon element is 50% to 80%. For example, the mass proportion of silicon element can be 50%, 55%, 60%, 65%, 70%, 75%, 80% or any value within the range formed by any two of the above values. When the mass proportion of silicon element is controlled within the above range, the lithium-ion battery composed of the negative electrode material can store a relatively high amount of electricity, which is beneficial to improving the initial discharge specific capacity.

[0058] In some embodiments, based on the negative electrode material, the mass proportion of carbon element is 15% to 35%. For example, the mass proportion of carbon element can be 15%, 20%, 25%, 30%, 35% or any value within the range formed by any two of the above values. When the mass proportion of carbon element is controlled within the above range, a sufficient carbon-based substrate can be established to provide sufficient distribution sites for the active substances, which is beneficial to forming an effective conductive network and improving the conductivity and cycle stability.

[0059] In some embodiments, the swelling rate of the negative electrode sheet based on the above negative electrode material is 34% to 60%. For example, the swelling rate of the negative electrode material can be 34%, 35%, 36%, 37%, 38%, 39%, 40%, 45%, 50%, 55%, 60% or any value within the range formed by any two of the above values. The swelling rate of the negative electrode sheet within the above range indicates that the negative electrode material particles have good anti-crushing or anti-pulverization ability, which is beneficial to maintaining good cycle stability of the secondary battery.

[0060] The negative electrode material active layer further includes a binder, which is used to bond the negative electrode active substance particles to facilitate the formation of a film layer, and at the same time can also improve the bonding force between the negative electrode material active layer and the negative electrode current collector. In some embodiments, the binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0061] The negative electrode material active layer may further include a conductive material, and the conductive material includes but is not limited to carbon-based materials, metal-based materials, conductive polymers or any combination thereof. In some embodiments, the carbon-based materials may include but are not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, the metal-based materials may include but are not limited to metal powder or metal fiber, such as copper, nickel, aluminum or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0062] Separator

[0063] The separator 103 includes a membrane layer with a porous structure, and its material includes but is not limited to at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide or aramid. For example, the separator 103 can be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite membrane, etc.

[0064] Electrolyte

[0065] The electrolyte functions to conduct ions between the positive electrode sheet 101 and the negative electrode sheet 102. The state of the electrolyte can be one or more of gel state, solid state and liquid state. In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution functions to conduct active ions between the positive electrode sheet 101 and the negative electrode sheet 102. In some embodiments, the electrolytic solution includes a lithium salt and an organic solvent. The lithium salt can be selected from but is not limited to one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethylsulfonyl)imide (LiN(SO2CF3)2, lithium tris(trifluoromethylsulfonyl)methide (LiC(SO2CF3)3), lithium bis(oxalato)borate (LiBOB) and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is selected as the lithium salt because it can provide a high ionic conductivity and improve the cycling performance. The organic solvent can be a carbonate compound, a carboxylate compound, an ether compound, a nitrile compound, other organic solvents or a combination thereof. Examples of carbonate compounds include but are not limited to diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethyl ethylene carbonate or a combination thereof.

[0066] Another embodiment of the present application provides a method for preparing a negative electrode material, including:

[0067] The first step: Provide porous silicon.

[0068] In some embodiments, a silicon matrix with a porous structure can be obtained by etching with an acid solution. For example, using silicon oxide as an example, micron-sized silicon oxide is placed in a box furnace and heat-treated at approximately 900°C in an inert gas atmosphere, causing a disproportionation reaction to produce particles composed of silicon and silicon oxide. The resulting particles are then etched with a 2 mol / L hydrofluoric acid solution to remove the silicon oxide component. The particles are then washed and vacuum-dried to obtain a silicon matrix with a porous structure, i.e., porous silicon.

[0069] Step 2: applying pressure to allow the impregnation solution to enter the interior of the porous silicon, the impregnation solution including a transition metal salt solution and a carbon source solution, and then heating and drying the porous silicon treated with the impregnation solution to obtain a composite material.

[0070] In some embodiments, while pressure is applied to allow the impregnation liquid to enter the porous silicon, a small amount of impregnation liquid that has not entered the porous silicon remains on the surface of the porous silicon. The impregnation liquid remaining on the surface of the porous silicon can form an intermediate shell layer during the heat treatment reaction, coating the surface of the inner core. It is understood that when both the intermediate shell layer and the subsequently formed outer shell are present, the intermediate shell layer is closer to the inner core than the outer shell.

[0071] In some embodiments, this step specifically includes placing the porous silicon prepared in step 1 in a pressure impregnation device, evacuating to 50 Pa, then inputting an impregnation liquid, the impregnation liquid including a transition metal salt solution and a carbon source solution, then applying a pressure of 5 MPa, heating to 120°C, and keeping warm for 0.5 h to 1 h, so that the pores of the porous silicon are filled with the impregnation liquid, and obtaining a composite material after drying.

[0072] In some embodiments, the concentration of the transition metal salt solution is 2 mol / L, and the volume fraction of the transition metal salt solution, based on the volume of the impregnation solution, is greater than or equal to 10% and less than or equal to 70%. Controlling the volume fraction of the transition metal salt solution within the above range is beneficial for controlling the mass fraction of the transition metal element in the resulting negative electrode material, thereby facilitating regulation of the particle strength of the resulting negative electrode material.

[0073] In some embodiments, the transition metal element in the transition metal salt solution includes one or more of molybdenum, titanium, tungsten, tantalum, vanadium, zirconium, chromium, niobium, and hafnium. Based on the above transition metal elements, the above transition metal salt solution can illustratively include sodium molybdate solution, ammonium molybdate solution, metatitanic acid solution, titanium tetrachloride solution, tetrabutyl titanate solution, sodium tungstate solution, ammonium metatungstate solution, tungsten chloride solution, tantalum fluoride solution, sodium metavanadate solution, zirconium chloride solution, chromium chloride solution, sodium chromate solution, niobium oxalate solution, hafnium tetrachloride solution, etc.

[0074] In some embodiments, the carbon source in the carbon source solution includes one or more of liquid asphalt, glucose, polyethylene, polyvinyl alcohol, polyethylene glycol, polyaniline, epoxy resin, phenolic resin, and acrylic resin.

[0075] Step 3: Mix the composite material with the carbon source gas and perform heat treatment in a reducing gas atmosphere to obtain the negative electrode material.

[0076] During the heat treatment process, the impregnation liquid inside the porous silicon undergoes an oxidation-reduction reaction in the inner core area to form a transition metal carbide-doped carbon matrix. If there is a small amount of impregnation liquid remaining on the surface of the porous silicon, an intermediate shell layer of the transition metal carbide-doped carbon matrix is formed. The carbon source gas will decompose and deposit on the surface of the material to form an outer shell, which is mainly pure carbon.

[0077] In some embodiments, the carbon source gas includes one or more of propane, propylene, methane, and acetylene.

[0078] In some embodiments, the reducing gas in the reducing gas atmosphere includes one or more of hydrogen, carbon monoxide, and ammonia.

[0079] In some embodiments, the flow rate of the carbon source gas or reducing gas is 0.5 L / min to 2 L / min. Controlling the flow rate of the carbon source gas or reducing gas within the above range facilitates more complete contact and reaction with the composite material.

[0080] In some embodiments, the heat treatment temperature is 600°C to 800°C, the heat treatment time is 1 hour to 4 hours, and the heating rate during the heat treatment is 3°C / min to 5°C / min. Controlling the heat treatment temperature and time within the above ranges is beneficial for promoting the conversion of the impregnation solution inside and on the surface of the porous silicon, and is also beneficial for the sufficient cracking of the carbon source gas, thereby facilitating the formation of the predetermined core and shell.

[0081] The present invention will be explained below in conjunction with the embodiments. It will be understood by those skilled in the art that the following examples are only used to explain the present invention and are not to be construed as limiting the present invention. Unless otherwise indicated, the reagents, software, and instruments not specifically described in the following examples are all conventional commercially available products or open source.

[0082] Embodiment 1:

[0083] A negative electrode material, the preparation method comprising:

[0084] S1. Place 100 g of silicon monoxide with an average particle size of 3 μm in a box furnace, introduce nitrogen, heat it to 900 °C at a heating rate of 3 °C / min, hold for 4 h, cool to room temperature, mix it with 500 mL of hydrofluoric acid solution with a concentration of 2 mol / L, etch for 40 min, then wash and dry it under vacuum to obtain porous silicon.

[0085] S2. Place the porous silicon obtained in S1 in a pressure impregnation device, evacuate to 50 Pa, input 60 mL of impregnating solution. The impregnating solution includes a carbon source and a metal ion salt solution. The carbon source is liquid-phase asphalt, and the metal ion salt solution is sodium molybdate solution. The sodium molybdate solution accounts for 20% of the volume of the impregnating solution. Then apply a pressure of 5 Mpa, heat to 120 °C, and hold for 1 h, and then dry to obtain a composite material.

[0086] S3. Place the obtained composite material in a rotary furnace, introduce a carbon source gas and a reducing gas, and conduct heat treatment. The carbon source gas is propylene, and the flow rate of propylene is 1.0 L / min. The reducing gas is hydrogen, and the flow rate of hydrogen is 0.5 L / min. The heat treatment temperature is 700 °C, the heating rate is 5 °C / min, and the holding time after reaching the heat treatment temperature is 2 h. Then cool to room temperature to obtain the negative electrode material.

[0087] Example 2:

[0088] The difference from Example 1 is that in S2, the metal ion salt solution is metatitanic acid solution.

[0089] Example 3:

[0090] The difference from Example 1 is that in S2, the metal ion salt solution is sodium tungstate solution.

[0091] Example 4:

[0092] The difference from Example 1 is that in S2, the metal ion salt solution is tantalum fluoride solution.

[0093] Example 5:

[0094] The difference from Example 1 is that in S2, the metal ion salt solution is sodium metavanadate solution.

[0095] Example 6:

[0096] The difference from Example 1 is that in S2, the metal ion salt solution is zirconium chloride solution.

[0097] Example 7:

[0098] The difference from Example 1 is that in S2, the metal ion salt solution is chromium chloride solution.

[0099] Example 8:

[0100] The difference from Example 1 is that in S2, the metal ion salt solution is a sodium chromate solution.

[0101] Example 9:

[0102] The difference from Example 1 is that in S2, the metal ion salt solution is a niobium oxalate solution.

[0103] Example 10:

[0104] The difference from Example 1 is that in S2, the metal ion salt solution is a hafnium tetrachloride solution.

[0105] Example 11:

[0106] The difference from Example 1 is that in S3, the heat treatment holding time is 1 h.

[0107] Example 12:

[0108] The difference from Example 1 is that in S3, the heat treatment holding time is 3 h.

[0109] Example 13:

[0110] The difference from Example 1 is that in S3, the heat treatment temperature is 600 °C.

[0111] Example 14:

[0112] The difference from Example 1 is that in S3, the heat treatment temperature is 800 °C.

[0113] Example 15:

[0114] The difference from Example 1 is that in S2, the sodium molybdate solution accounts for 10% of the volume of the impregnation solution.

[0115] Example 16:

[0116] The difference from Example 1 is that in S2, the sodium molybdate solution accounts for 30% of the volume of the impregnation solution.

[0117] Example 17:

[0118] The difference from Example 1 is that in S2, the sodium molybdate solution accounts for 40% of the volume of the impregnation solution.

[0119] Example 18:

[0120] The difference from Example 1 is that in S2, the sodium molybdate solution accounts for 50% of the volume of the impregnation solution.

[0121] Example 19:

[0122] The difference from Example 1 is that in S2, the sodium molybdate solution accounts for 60% of the volume of the impregnating solution.

[0123] Example 20:

[0124] The difference from Example 1 is that in S2, the sodium molybdate solution accounts for 70% of the volume of the impregnating solution.

[0125] Comparative Example 1:

[0126] The difference from Example 1 is that in S2, the impregnating solution does not contain sodium molybdate solution.

[0127] Comparative Example 2:

[0128] The difference from Example 1 is that in S2, the sodium molybdate solution accounts for 5% of the volume of the impregnating solution.

[0129] Comparative Example 3:

[0130] The difference from Example 1 is that in S2, the sodium molybdate solution accounts for 90% of the volume of the impregnating solution.

[0131] For the partial preparation conditions of Examples 1 - 20 and Comparative Examples 1 - 3 of this application, please refer to Table 1.

[0132] Table 1. Partial preparation conditions of Examples 1 - 20 and Comparative Examples 1 - 3 of this application

[0133]

[0134] The physical properties and electrochemical properties of the negative electrode materials obtained from the above Examples 1 - 20 and Comparative Examples 1 - 3 were tested, and the test methods are as follows:

[0135] Test for the particle strength of the negative electrode material: Under the action of electromagnetic force, the force corresponding to the indenter gradually increases at a certain speed. When the breaking point of the particle is reached, the particle breaks; at this time, due to the disappearance of the resistance received by the indenter, its displacement drops rapidly. The instrument judges its breaking point (the displacement increases rapidly) by recording the displacement of the indenter dropping in real time, and records the pressure received by the particle at this time as pressure F. The relationship among pressure F, particle diameter D, and particle strength Cs is:

[0136]

[0137] Test for the mass fraction of transition metal elements in the negative electrode material: Measure the mass fraction of transition metal elements in the negative electrode material by methods such as ICP.

[0138] Testing of transition metal carbides in the anode material: The anode material was analyzed using X-ray diffraction (XRD). In the XRD image, the metal elements in the anode material mainly showed peaks of transition metal carbides, indicating that the main form of existence of the metal elements in the anode material was transition metal carbides.

[0139] Testing of the thickness of the outer shell of the anode material: The cross-section of the anode material was analyzed using a scanning electron microscope (SEM), and the thickness of the corresponding outer shell was measured to obtain it.

[0140] Testing of the average particle size of the anode material: The material particles were observed through a field emission scanning electron microscope or a transmission electron microscope. The particle sizes of 5 to 10 material particles were randomly measured through a scale, and the average value of the particle sizes was taken as the average particle size of the final material particles.

[0141] Testing of the specific surface area of the anode material: The specific surface area of the powder sample was tested by the gas adsorption method (nitrogen adsorption multi-point BET) at a low temperature controlled by liquid nitrogen cooling using the TriStar3000&3020 specific surface area and pore size analyzer of Micromeritics in the United States.

[0142] Testing of the porosity of the anode material: P = Vo / (Vo + V) × 100%, where P is the porosity of the material (%), Vo is the pore volume of the material (cm 3 ), and V is the absolute dense volume (cm 3 ). Vo was obtained through testing with the TriStar3000&3020 specific surface area and pore size analyzer of Micromeritics in the United States, and V was obtained through conversion by testing with a true density tester AccuPyc 1340.

[0143] Testing of the mass percentage of carbon element in the anode material: The carbon element in the anode material was analyzed and determined using an infrared carbon and sulfur analyzer, and the obtained mass percentage of the carbon element was the mass percentage of the carbon element in the anode material.

[0144] Testing of the mass percentage of silicon material in the anode material: Through the SA2-9-17TP box-type atmosphere furnace of Nanyang Xinyu, the anode material was first placed in an oxygen atmosphere and burned to make the silicon in the sample react to form silicon dioxide, and the carbon burned and turned into carbon dioxide and was discharged. Then, the burned residue was placed in a nitric acid solution to react fully to remove the metal. After the residue was washed and dried and weighed, the mass percentage of silicon was calculated.

[0145] Electrochemical performance test: The anode material, conductive agent, and binder were mixed in a solvent at a mass percentage of 90:5:5. The conductive agent was Super P, and the binder was PAA. The obtained slurry was coated on a copper foil current collector, dried, and pressed to obtain a pole piece. Subsequently, a CR2025 button cell was assembled, and the electrochemical performance was tested using a BlueTec test system. The electrolyte used was 1 mol / L LiPF6 / EC / DMC (1:1). The capacity and initial Coulombic efficiency of the anode material were tested at a current density of 0.1 C, the 50-week cycle retention rate was tested at a current density of 1 C, and the thickness of the battery pole piece was measured using a micrometer after repeating 50 cycles to obtain the volume expansion rate.

[0146] Please refer to Table 2 for the test results of the above part.

[0147] Taking Example 2 as an example, the cross-section of the anode material was analyzed using a scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).

[0148] Please refer to Figure 3 , in the SEM image, an intermediate shell layer and an outer shell with obvious interface differentiation were observed on the outer layer of the anode material core.

[0149] Please refer to Figure 4 , a surface scan was performed on the cross-section of the anode material sample to obtain the element distribution information of the entire region. In the EDS scan image, it was observable that the anode material included Si, Ti, and C elements.

[0150] Please refer to Figure 5 , in the XRD image, the metal elements in the anode material mainly showed peaks of transition metal carbides (TiC), indicating that the main form of existence of transition metal elements in the anode material was transition metal carbides.

[0151] Table 2. Performance test results of Examples 1-20 and Comparative Examples 1-3 of this application

[0152]

[0153] In Examples 1-20, different transition metal salt solutions (such as Examples 1-10) or different process conditions (such as Examples 1, 11-20) were used to prepare the anode material, so that the obtained anode material had a transition metal element with a preset range of mass percentage, forming a distribution of a carbon matrix doped with an appropriate amount of transition metal carbides in the anode material, providing a higher particle strength for the anode material, enabling the obtained anode sheet to maintain a lower expansion after charge and discharge cycles, and enabling the obtained secondary battery to have a higher capacity, initial Coulombic efficiency, and cycle stability.

[0154] Compared with Example 1, in the preparation process of Comparative Example 1, no transition metal salt solution was added, and the resulting negative electrode material contained almost no transition metal elements, resulting in significantly lower particle strength of the negative electrode material, obvious swelling effect of the electrode sheet, and affecting the cycle stability of the secondary battery.

[0155] Compared with Example 1, in the preparation process of Comparative Example 2, a smaller volume fraction of a metal ion salt solution was added to form an impregnating solution. The content of transition metal elements in the prepared negative electrode material was relatively low, which might lead to a relatively low content of transition metal carbides in the negative electrode material. The mass ratio of transition metal elements in the negative electrode material was lower than the preset lower limit value, resulting in obvious volume expansion of the silicon matrix during the process of lithium deintercalation and insertion in the negative electrode material. As a result, the particles of the negative electrode material including the silicon matrix were prone to breakage or pulverization, damaging the structure of the negative electrode sheet, and also causing an increase in internal stress of the battery, which was not conducive to the capacity performance. The initial Coulomb efficiency of the obtained secondary battery was reduced and the cycle stability was poor.

[0156] Compared with Example 1, in the preparation process of Comparative Example 3, a larger volume fraction of a metal ion salt solution was added to form an impregnating solution. The mass ratio of transition metal elements in the negative electrode material was higher than the preset upper limit value, which might result in a relatively high content of transition metal carbides in the negative electrode material. Excessive transition metal carbides would damage the structure of the silicon matrix during the subsequent heat treatment process, leading to cracking of the particles of the negative electrode material including the silicon matrix, reducing the particle strength of the negative electrode material, aggravating the volume expansion effect of the obtained secondary battery, being not conducive to the capacity performance, and reducing the initial Coulomb efficiency and cycle stability of the obtained secondary battery.

[0157] In summary, through the improved preparation method of the present application, the obtained negative electrode material has high particle strength, the volume expansion effect is significantly improved, which is conducive to the capacity performance of the obtained secondary battery, and the obtained secondary battery has a high initial Coulomb efficiency and good cycle stability. The present application also controls the preparation process conditions, so that the mass ratio of transition metal elements in the obtained negative electrode material is controllable, thereby further improving the electrochemical performance of the obtained secondary battery.

[0158] The above embodiments are only used to illustrate the technical solutions of the present application and not to limit them. Although the present application has been described in detail with reference to the above preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A negative electrode material, characterized in that, The negative electrode material includes a core and a shell. The core includes a silicon matrix and a filler. The filler is distributed on at least part of the surface of the core to form an intermediate shell layer. At least part of the filler is distributed within the silicon matrix. The filler includes a transition metal carbide and a carbon matrix. The transition metal carbide includes one or more of molybdenum carbide, titanium carbide, tungsten carbide, tantalum carbide, vanadium carbide, zirconium carbide, chromium carbide, niobium carbide, and hafnium carbide; The shell is disposed on at least part of the surface of the core. Based on the negative electrode material, the mass ratio of the transition metal element is 1.12% to 3.1%; Define the particle strength of the negative electrode material as Cs, , where F is the pressure exerted when the particles of the negative electrode material rupture, D is the particle size of the negative electrode material, and π is pi; Cs is 170 Mpa to 350 Mpa.

2. The negative electrode material according to claim 1, wherein The thickness of the intermediate shell layer is 50 nm to 100 nm.

3. The negative electrode material according to claim 1, wherein, The morphology of the transition metal carbide includes one or more of nanoparticles, nanowires, nanotubes, and nanosheets.

4. The negative electrode material according to claim 1, characterized in that, The carbon matrix includes one or more of crystalline carbon, amorphous carbon, and transitional carbon.

5. The negative electrode material according to claim 1, wherein The shell satisfies at least one of the following conditions: (1) The material of the shell includes at least one of carbon material, titanium nitride, boron nitride, silicon nitride, silicon oxide, magnesium oxide, aluminum oxide, silicon carbide, and polyaniline; (2) The thickness of the shell is 50 nm to 500 nm.

6. The negative electrode material according to claim 1, characterized in that, The negative electrode material satisfies at least one of the following conditions: (1) The average particle size of the negative electrode material is 1μm to 15μm; (2) The specific surface area of the negative electrode material is less than or equal to 8 m 2 / g; (3) The porosity of the negative electrode material is less than or equal to 2.5%; (4) Based on the negative electrode material, the mass ratio of silicon element is 50% to 80%; (5) Based on the negative electrode material, the mass ratio of carbon element is 15% to 35%.

7. A secondary battery, characterized in that, Comprising the negative electrode material according to any one of claims 1-6.

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