Negative electrode material, negative electrode sheet, and secondary battery

Abstract

A negative electrode material, a negative electrode sheet, and a secondary battery, the negative electrode material including a core and a coating layer provided on at least part of a surface of the core, the coating layer including a polymer, an infrared spectrum of the negative electrode material including a first characteristic peak, a second characteristic peak, and a third characteristic peak in a wave number range of 1320±10 cm ‑1 to 1880±10 cm ‑1 , the first characteristic peak being a bending vibration peak of a -CH2 bond, the second characteristic peak being a stretching vibration peak of a C-N bond, and the third characteristic peak being a stretching vibration peak of a C=C bond, a ratio of peak areas of the first characteristic peak and the third characteristic peak being Z, Z being 0.35 to 0.8. The negative electrode sheet and the secondary battery include the negative electrode material, and the secondary battery has a high initial coulomb efficiency.

Description

Technical Field

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

[0002] Secondary batteries (such as lithium-ion batteries) are widely used in various electronic devices and electric vehicles. The anode material has always been a crucial factor limiting the electrochemical performance of secondary batteries. Graphite-based materials, with their excellent conductivity, low lithium intercalation potential, layered structure suitable for lithium-ion insertion and extraction, small volume expansion, and low cost, have become the mainstream anode material for commercial lithium-ion batteries. However, existing graphite-based anode materials have poor compatibility with electrolytes, easily leading to the co-intercalation of electrolyte solvents and lithium ions between graphite layers. This results in irreversible lithium-ion consumption and graphite layer peeling, thus affecting the battery's initial coulombic efficiency and cycle performance. Summary of the Invention

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

[0004] To achieve the above objectives, this application provides a negative electrode material, comprising a core and a coating layer disposed on at least a portion of the surface of the core, the coating layer comprising a polymer, and the infrared spectrum of the negative electrode material at 1320 cm⁻¹. -1 Up to 1880cm -1 Within the wavenumber range, there are a first characteristic peak, a second characteristic peak, and a third characteristic peak. The first characteristic peak is the bending vibration peak of the -CH2 bond, the second characteristic peak is the stretching vibration peak of the CN bond, and the third characteristic peak is the stretching vibration peak of the C=C bond. The ratio of the peak area of ​​the first characteristic peak to the third characteristic peak is Z, which is 0.35 to 0.8.

[0005] In some possible implementations, the peak area of ​​the second characteristic peak is between 1100 and 1300.

[0006] In some possible implementations, the Raman scattering spectrum of the negative electrode material is at a wavenumber of 1350 ± 10 cm⁻¹. -1 and 1580±10cm -1 Characteristic peaks D and G exist respectively, and the ratio of the peak intensities of characteristic peaks D and G is I. D / I G I D / I G The value ranges from 0.02 to 0.50.

[0007] In some possible implementations, the specific surface area of ​​the negative electrode material is less than or equal to 8 m². 2 / g.

[0008] In some possible implementations, the tap density of the negative electrode material is greater than or equal to 0.75 g / cc.

[0009] In some possible implementations, the median particle size D of the anode material v 50 ranges from 1μm to 35μm.

[0010] In some possible implementations, the thickness of the coating layer is between 1 nm and 100 nm.

[0011] In some possible implementations, the core comprises graphite, which includes at least one of natural graphite and synthetic graphite.

[0012] In some possible implementations, the median particle size D of the graphite particles v 50 ranges from 1μm to 30μm.

[0013] This application also provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, wherein the negative electrode active material layer includes the aforementioned negative electrode material.

[0014] This application also provides a secondary battery, including the aforementioned negative electrode.

[0015] In this application, the Z-value of the negative electrode material meets a preset range and exhibits a stretching vibration peak of the CN bond, which is beneficial for the polymer monomer coating material to form a polymer with a moderate degree of polymerization and fully coat the surface of the core. During lithium intercalation, the polymer coating layer preferentially undergoes a reduction reaction with the electrolyte components, the CN bond breaks, and further forms the compound Li3N. That is, during lithium intercalation, the coating layer can be transformed into a protective layer containing both polymer and inorganic Li3N. The aforementioned protective layer allows ions to pass through while hindering electrons during lithium intercalation and deintercalation, thereby reducing the occurrence of interfacial side reactions, reducing lithium ion consumption, and improving the initial coulombic efficiency of the negative electrode material and the secondary battery using it. At the same time, the presence of Li3N can also increase the ionic conductivity of the polymer coating layer, thereby improving the kinetic performance of the negative electrode material and further improving the capacity utilization and initial coulombic efficiency of the secondary battery using this negative electrode material.

[0016] When the Z value of the negative electrode material meets the preset range and the stretching vibration peak of the CN bond exists, the polymer monomer coating material provides unsaturated bonds and functional groups such as carboxyl groups. The unsaturated bonds undergo self-polymerization to form a polymer network, and the functional groups such as carboxyl groups can undergo dehydration reactions with the hydroxyl groups on the graphite surface to form chemical bonds, thus building a chemical bridge between the coating layer and the graphite. This is beneficial to improving the bonding force between the polymer network and the graphite, effectively avoiding peeling during charging and discharging, thereby improving the initial coulombic efficiency and cycle performance. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of a secondary battery during charging, according to one embodiment of this application.

[0018] Figure 2 This is a schematic diagram of the structure of a secondary battery during discharge, provided in one embodiment of this application.

[0019] Figure 3 The infrared spectral test results are for the negative electrode material provided in Example 1 of this application.

[0020] Figure 4 The Raman spectroscopy test results are for the negative electrode material provided in Example 1 of this application.

[0021] Figure 5 This is a TEM image of the negative electrode material provided in Embodiment 1 of this application.

[0022] Figure 6 This is a TEM image of the negative electrode material provided in Embodiment 1 of this application at a magnification.

[0023] Explanation of main component symbols

[0024] Electrode assembly 100

[0025] Positive electrode 101

[0026] Negative electrode 102

[0027] Separator 103 Detailed Implementation

[0028] The embodiments of this application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; where there is no conflict, the implementation methods and features of the implementation methods of this application can be combined with each other; many specific details are set forth in the following description to provide a full understanding of this application, and the described implementation methods are only a part of the implementation methods of this application, and not all of the implementation methods.

[0029] Coating the graphite surface can typically reduce the specific surface area and surface defects of graphite materials, improve interfacial stability, and reduce the occurrence of surface side reactions, thereby improving the electrochemical performance of graphite materials. However, existing processes are complex, costly, and the coating layer is not firmly bonded to the graphite surface, failing to effectively suppress interfacial side reactions. Therefore, this patent provides an improved anode material preparation process that helps achieve a firmly bonded polymer coating layer on the core surface of the anode material, thereby better improving the electrochemical performance of graphite materials.

[0030] Based on this, one embodiment of this application provides a secondary battery, including a casing, an electrode assembly, and an electrolyte. Both the electrode assembly and the electrolyte are located within the casing.

[0031] The outer casing can be a packaging bag encapsulated with a sealing film (such as aluminum-plastic film), for example, a pouch battery. In other embodiments, it can also be a steel-cased battery, an aluminum-cased battery, etc.

[0032] Please see Figure 1 and Figure 2 The electrode assembly 100 includes a positive electrode 101, a negative electrode 102, and a separator 103, with the separator 103 disposed between the positive electrode 101 and the negative electrode 102. When an electrolyte (not shown) is present, please refer to [reference needed] for charging. Figure 1 Active ions (such as lithium ions) are extracted from the lattice of the positive electrode material (such as a lithium-ion intercalation compound) in the positive electrode 101, pass through the electrolyte and the separator 103, reach the negative electrode 102, and intercalate into the lattice of the negative electrode material. For discharge procedures, please refer to [link to relevant documentation]. Figure 2 Active ions (such as lithium ions) are extracted from the lattice of the negative electrode material of the negative electrode 102, pass through the electrolyte through the separator 103, reach the positive electrode 101 and are embedded in the lattice of the positive electrode material (such as lithium intercalation compound), generating electrons that travel from the negative electrode 102 to the positive electrode 101 through the external circuit. The reverse movement of electrons forms an electric current, which can be used by electrical appliances.

[0033] In some embodiments, the electrode assembly 100 may be a stacked structure, which is formed by alternatingly stacking a positive electrode 101, a separator 103, and a negative electrode 102. In other embodiments, the electrode assembly 100 may also be a wound structure, which is formed by sequentially stacking and then winding the positive electrode 101, the separator 103, and the negative electrode 102.

[0034] Positive electrode film

[0035] The positive electrode 101 includes a positive current collector and a positive electrode material active layer disposed on at least one surface of the positive current collector. The positive current collector can be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, current collectors formed by combining the aforementioned conductive foil and polymer substrate. The positive electrode material active layer includes a positive electrode active material, which includes a compound that reversibly inserts and extracts lithium ions (i.e., a lithiation intercalation compound). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. This 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 may include, but is not limited to, lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary materials (NCM), lithium manganese oxide (LiMn2O4), and lithium nickel manganese oxide (LiNi). 0.5Mn 1.5 At least one of lithium iron phosphate (LiFePO4) or lithium iron phosphate (LiFePO4).

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

[0037] The positive electrode material active layer 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, 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, 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 102 includes a negative electrode current collector and an active layer of negative electrode material disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be at least one of copper foil, nickel foil, stainless steel foil, titanium foil or carbon-based current collector, or any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and polymer substrate.

[0040] The active layer of the negative electrode material includes a negative electrode material, which includes a core and a coating layer disposed on at least a portion of the surface of the core. The coating layer includes a polymer. The infrared spectrum of the negative electrode material is at 1320 cm⁻¹. -1 Up to 1880cm -1 Within the wavenumber range, there are a first characteristic peak, a second characteristic peak, and a third characteristic peak. The first characteristic peak is the bending vibration peak of the -CH2 bond, the second characteristic peak is the stretching vibration peak of the CN bond, and the third characteristic peak is the stretching vibration peak of the C=C bond. The ratio of the peak area of ​​the first characteristic peak to the third characteristic peak is Z, which is 0.35 to 0.8.

[0041] The Z-value of the anode material meets the preset range and exhibits the stretching vibration peak of the CN bond, which is beneficial for the polymer monomer coating material to form a polymer with a moderate degree of polymerization and fully coat the surface of the core. During lithium intercalation, the polymer coating layer preferentially undergoes a reduction reaction with the electrolyte components, the CN bond breaks, and further forms the compound Li3N. That is, during lithium intercalation, the coating layer can be transformed into a protective layer containing both polymer and inorganic Li3N. This protective layer allows ions to pass through while hindering electrons during lithium intercalation and deintercalation, thereby reducing the occurrence of interfacial side reactions, reducing lithium ion consumption, and improving the initial coulombic efficiency of the anode material and the secondary battery using it. At the same time, the presence of Li3N can also increase the ionic conductivity of the polymer coating layer, which is beneficial to improving the kinetic performance of the anode material, that is, reducing the impedance of lithium ions during the intercalation and deintercalation process, facilitating the full insertion and extraction of lithium ions, and thus improving the capacity utilization and initial coulombic efficiency of the secondary battery using this anode material.

[0042] When the Z value of the negative electrode material meets the preset range and the stretching vibration peak of the CN bond exists, the polymer monomer coating material provides unsaturated bonds and functional groups such as carboxyl groups. The unsaturated bonds undergo self-polymerization to form a polymer network, and the functional groups such as carboxyl groups can undergo dehydration reactions with the hydroxyl groups on the graphite surface to form chemical bonds, thus building a chemical bridge between the coating layer and the graphite. This is beneficial to improving the bonding force between the polymer network and the graphite, effectively avoiding peeling during charging and discharging, thereby improving the initial coulombic efficiency and cycle performance.

[0043] Z can be 0.35, 0.4, 0.45, 0.46, 0.5, 0.6, 0.63, 0.65, 0.67, 0.70, 0.78, 0.8, or any value within the range of any two of the above values. Understandably, when the Z value is too small, it indicates a high content of C=C bonds in the negative electrode material, meaning a low polymer content or degree of polymerization in the coating material. In this case, the coating layer's effect on the core is poor, increasing the risk of side reactions between the core and the electrolyte, which is detrimental to the performance of the negative electrode material. Conversely, when the Z value is too large, it indicates a high content of -CH2 bonds in the negative electrode material, meaning an excessive amount of polymer is formed in the coating material. This excessive coating material tends to accumulate, hindering its uniform distribution on the core surface and potentially leading to an excessively thick coating layer, increasing the interfacial resistance of the negative electrode material and negatively impacting capacity maintenance.

[0044] In one specific embodiment of this application, a NICOLET iS50 Fourier transform infrared spectrometer was used to test the negative electrode material, and a 500 cm⁻¹ spectroscopy was selected. -1 Up to 4000cm -1The wavenumber range was scanned, the infrared spectrum was exported, and peak position analysis was performed, with peaks at 1440 cm⁻¹. -1 A bending vibration peak of the -CH2 bond is present nearby, at 1635 cm⁻¹. -1 There are stretching vibration peaks of C=C bonds nearby. The ratio of peak areas Z is obtained by calculating the peak areas.

[0045] In some embodiments, the peak area of ​​the CN bond in the infrared spectrum of the negative electrode material is between 1100 and 1300. For example, the B peak area can be 1100, 1130, 1134, 1159, 1173, 1175, 1178, 1190, 1225, 1240, 1247, 1300, or any value within the range of any two of the above values. Understandably, a CN bond peak area within the above range indicates that the coating layer has an appropriate amount of CN bonds. During lithium intercalation, the CN bonds break and further form an appropriate amount of N and Li-containing compound Li3N, increasing the ionic conductivity of the polymer coating layer. This is beneficial for improving the kinetic performance of the coating layer, further improving the capacity utilization and initial coulombic efficiency of the negative electrode material and its secondary battery. In a specific embodiment of this application, the negative electrode material is tested using a NICOLET iS50 Fourier transform infrared spectrometer, with a 500 cm⁻¹ range selected. -1 Up to 4000cm -1 The wavenumber range was scanned, the infrared spectrum was exported, and peak position analysis was performed, with peaks at 1560 cm⁻¹. -1 A stretching vibration peak of CN bond is present nearby, and the peak area is calculated.

[0046] In some embodiments, the Raman scattering spectrum of the negative electrode material is at a wavenumber of 1350 ± 10 cm⁻¹. -1 and 1580±10cm -1 Characteristic peaks D and G exist respectively, and the ratio of the peak intensities of characteristic peaks D and G is I. D / I G I D / I G It ranges from 0.02 to 0.50. For example, I D / I G It can be 0.02, 0.09, 0.12, 0.17, 0.23, 0.24, 0.25, 0.32, 0.39, 0.45, 0.48, 0.50, or any value within the range of any two of the above values. Taking graphite core material as an example, graphite product I... D / I G Typically greater than 1, the I of the negative electrode material D / I GWithin the aforementioned range, it is evident that a relatively uniform coating layer is formed on the surface of the anode material core, thereby significantly reducing surface defects and minimizing side reactions between the core material surface and the electrolyte, thus improving the initial coulombic efficiency.

[0047] In some embodiments, the specific surface area of ​​the negative electrode material is less than or equal to 8 m². 2 / g. For example, the specific surface area of ​​the negative electrode material can be 0.5m². 2 / g、1m 2 / g、2.1m 2 / g, 2.7m 2 / g、3m 2 / g, 3.1m 2 / g, 3.2m 2 / g、3.3m 2 / g, 3.5m 2 / g, 4.7m 2 / g、6.2m 2 / g、8m 2 / g or any value within the range of any two of the above values. It is understandable that the specific surface area of ​​the negative electrode material affects the contact area between the negative electrode material and the electrolyte. When the specific surface area of ​​the negative electrode material is within the above range, the amount of lithium ions consumed by the SEI film formed during the first charge-discharge process of the battery prepared with the negative electrode material can be reduced, thus reducing irreversible capacity loss of the battery.

[0048] In some embodiments, the tap density of the negative electrode material is greater than or equal to 0.75 g / cc. For example, the tap density of the negative electrode material can be 0.75 g / cc, 0.95 g / cc, 1.00 g / cc, 1.03 g / cc, 1.05 g / cc, 1.06 g / cc, 1.07 g / cc, 1.09 g / cc, 1.10 g / cc, 1.12 g / cc, 1.15 g / cc, or any value within the range of any two of the above values. Controlling the tap density within the above range is beneficial for the formation of a suitable degree of compactness in the internal structure of the negative electrode material, thereby improving lithium-ion transport and electron conduction, increasing battery energy density, extending cycle life, and improving safety performance.

[0049] In some embodiments, the median particle size D of the negative electrode material v 50 ranges from 1 μm to 35 μm. For example, the median particle size D of the anode material. v 50 can be 1μm, 5μm, 10μm, 15μm, 18μm, 18.2μm, 18.3μm, 18.4μm, 18.5μm, 18.6μm, 19μm, 25μm, 30μm, 35μm, or any value within the range of any two of the above values. v50 refers to the particle size value corresponding to a cumulative distribution percentage of 50% by volume. Controlling the median particle size of the anode material within the above range is beneficial to improving the energy density, rate performance, and cycle performance of the anode material.

[0050] In some embodiments, the thickness of the coating layer is from 1 nm to 100 nm. For example, the thickness of the coating layer can be 1 nm, 3 nm, 6 nm, 10 nm, 15 nm, 20 nm, 40 nm, 70 nm, 100 nm, or any value within the range of any two of the above values. Controlling the coating layer thickness of the negative electrode material within the above range is beneficial for forming sufficient coverage on the surface of the negative electrode material core, while maintaining good interfacial resistance and capacity. In some embodiments, the coating layer can be a single-layer coating layer formed of a single material, a coating layer formed of a combination of multiple materials, a multi-layer coating layer formed of a single material, or a multi-layer coating layer formed of multiple materials, etc., and the layer structure of the coating layer can be selected according to actual needs.

[0051] In some embodiments, the core comprises graphite, which includes at least one of natural graphite and synthetic graphite. In some embodiments, the median particle size D of the graphite particles is... v 50 represents the range from 1 μm to 30 μm. For example, the median particle size D of graphite particles. v 50 can be 1μm, 3μm, 8μm, 10μm, 15μm, 20μm, 25μm, 30μm, or any value within the range of any two of the above values.

[0052] The active layer of the negative electrode material also includes a binder to bond the negative electrode active material particles, thereby facilitating the formation of the film layer and improving the bonding force between the active layer of the negative electrode material 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, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0053] The active layer of the negative 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, 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, 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.

[0054] Separating membrane

[0055] 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 may be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane, etc.

[0056] electrolytes

[0057] The electrolyte serves to conduct ions between the positive electrode 101 and the negative electrode 102. The electrolyte can be in one or more states, including gel, solid, and liquid. In some embodiments, the electrolyte is a liquid electrolyte solution. The liquid electrolyte solution serves to conduct active ions between the positive electrode 101 and the negative electrode 102. In some embodiments, the liquid electrolyte solution includes a lithium salt and an organic solvent. The lithium salt may 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(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium tris(trifluoromethanesulfonyl)methyllithium (LiC(SO2CF3)3), lithium dioxolaneborate (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, LiPF6 is selected as the lithium salt because it provides high ionic conductivity and improves cycling characteristics. The organic solvent may be a carbonate compound, a carboxylic acid ester compound, or an ether. Compounds, nitrile compounds, other organic solvents, or combinations 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), butyl carbonate (BC), vinyl 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, trifluoromethylethylene carbonate, or combinations thereof.

[0058] Another embodiment of this application also provides a method for preparing a negative electrode material, including:

[0059] Step 1: Mix polymer monomers, graphite, oxidation environment conditioner and water. The polymer monomers have at least one of unsaturated double bonds, carboxyl groups and hydroxyl groups. The mass ratio of polymer monomers to graphite is (1% to 20%):1. The mass ratio of oxidation environment conditioner to graphite is (0.1% to 10%):1. Adjust the pH to 2 to 12 to obtain a mixture. The solid-liquid ratio of the mixture is 1:(1.5 to 5).

[0060] Polymer monomers have highly reactive sites. When used as coating materials, they are converted into cationic free radicals in a specific solution environment. Similar cationic free radicals form dimers in a head-to-tail connection manner. The dimers continue to grow to form polymers with higher degrees of polymerization. The polymer preferentially undergoes dehydration condensation and other reactions with the active sites such as the end faces and hydroxyl groups of graphite (core material) and is distributed on the graphite particles to form a surface coating layer.

[0061] The mass ratio of polymer monomer to graphite can be 1%:1, 2%:1, 3%:1, 5%:1, 8%:1, 10%:1, 12%:1, 15%:1, 17%:1, 20%:1, or any value within any two of the above ranges. Maintaining the mass ratio of polymer monomer to graphite within these ranges is beneficial for forming a uniform coating layer on the surface of the negative electrode material core, while maintaining good interfacial resistance and capacity. The pH can be adjusted to 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or any value within any two of the above ranges. The solid-liquid ratio of the mixture can be 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, or any value within any two of the above ranges. Maintaining the pH within the above-mentioned range is beneficial for promoting the polymerization reaction of polymer monomers. The pH is further preferably between 3 and 12, which helps control the content of CN bonds and further improves the performance of the anode material. Maintaining the solid-liquid ratio of the mixture within the above-mentioned range allows for regulation of the concentration of polymer monomers in the reaction system, thereby controlling the reaction rate of the polymer monomers and preventing them from undergoing self-polymerization rather than direct deposition on the surface of the core material. This reduces the risk of increased specific surface area and decreased initial coulombic efficiency of the anode material due to direct deposition of polymer monomers. The ratio of the oxidation environment conditioner to graphite can be 0.1%:1, 0.5%:1, 1%:1, 2%:1, 4%:1, 6%:1, 8%:1, 10%:1, or any value within the range of any two of the above values. Maintaining the oxidation environment conditioner within the above-mentioned range is beneficial for promoting the polymerization reaction of polymer monomers.

[0062] In some embodiments, the polymer monomer includes one or more of acrylamide, methacrylamide, ethylacrylamide, phenylacrylamide, p-toluamide, diamine maleate, aminosulfonic acid, and ammonium benzoate.

[0063] In some embodiments, pH can be adjusted using acidic or alkaline chemicals. For example, the acidic chemicals used can be one or more of hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, and acetic acid, while the alkaline chemicals can be one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, and ammonia.

[0064] In some embodiments, the oxidizing environment conditioner includes one or more of azobisisobutyronitrile, azobisisoheptanenitrile, hydrogen peroxide, ammonium persulfate, sodium persulfate, potassium persulfate, aluminum chloride, and ferric chloride.

[0065] Step 2: Heat and react the obtained mixture, dry and evaporate the solvent to obtain the intermediate product.

[0066] In some embodiments, the reaction temperature is from 1°C to 90°C. For example, the reaction temperature can be 1°C, 5°C, 10°C, 20°C, 30°C, 40°C, 60°C, 80°C, 90°C, or any value within the range of any two of the above values. In some embodiments, the reaction time is from 1 hour to 10 hours. For example, the reaction time can be 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, 8 hours, 10 hours, or any value within the range of any two of the above values. It is understood that controlling the reaction temperature and time within the above ranges is beneficial for the polymer monomers to undergo self-polymerization to form a polymer coating on the surface of the core material.

[0067] Step 3: Dry the obtained intermediate product at a temperature of 50°C to 300°C to obtain the negative electrode material.

[0068] The drying temperature can be 50℃, 100℃, 150℃, 200℃, 250℃, 300℃, or any value within the range of any two of the above values. Controlling the drying temperature within the above range is beneficial for maintaining the polymer structure and preventing the polymer from decomposing and agglomerating into spherical shapes, which would lead to a poorer surface coating effect.

[0069] In some embodiments, the drying process includes one or more of the following: forced-air drying, vacuum drying, freeze drying, and spray drying.

[0070] The preparation method of this application uses polymer monomers as coating materials. Through liquid phase coating, the polymer monomers are polymerized in situ at a certain temperature to form a dense polymer coating on the surface of the core material. This method has the advantages of simple synthesis process, high practicality and applicability to raw materials with different particle sizes.

[0071] The present application's solution will be explained below with reference to embodiments. Those skilled in the art will understand that the following examples are for illustrative purposes only and should not be construed as limiting the present application. Unless otherwise stated, reagents, software, and instruments involved in the following embodiments that are not specifically mentioned are all conventional commercially available products or open-source materials.

[0072] Example 1:

[0073] A negative electrode material, the preparation method of which includes:

[0074] S1. Mix 25g of methacrylamide, 500g of graphite and 1000mL of deionized water. After stirring thoroughly, add 4wt% sodium hydroxide solution dropwise to adjust the pH to 7, forming a mixture.

[0075] S2, the resulting mixture was heated and stirred in a 60°C water bath for 2 hours, and then stirred in a 90°C water bath until the liquid was completely evaporated to obtain the intermediate product.

[0076] S3. The obtained intermediate product was placed in a forced-air drying oven and treated at 130°C for 24 hours, and then placed in a vacuum drying oven and treated at 130°C for 12 hours to obtain the negative electrode material.

[0077] Example 2:

[0078] The difference from Example 1 is that in S1, the amount of methacrylamide is adjusted to 5g.

[0079] Example 3:

[0080] The difference from Example 1 is that in S1, the amount of methacrylamide is adjusted to 100g.

[0081] Example 4:

[0082] The difference from Example 1 is that in S1, the amount of deionized water is adjusted to 1500 mL.

[0083] Example 5:

[0084] The difference from Example 1 is that in S1, the amount of deionized water is adjusted to 2500 mL.

[0085] Example 6:

[0086] The difference from Example 1 is that in S2, the heating and stirring time under a 60°C water bath is adjusted to 4 hours.

[0087] Example 7:

[0088] The difference from Example 1 is that in S2, the water bath temperature condition of heating and stirring for 2 hours is adjusted to 80°C.

[0089] Example 8:

[0090] The difference from Example 1 is that in S2, the water bath temperature condition of heating and stirring for 2 hours is adjusted to 40°C.

[0091] Example 9:

[0092] The difference from Example 1 is that in S1, 4 wt% HCl solution is added dropwise to adjust the pH value to 3.

[0093] Example 10:

[0094] The difference from Example 1 is that in S1, the pH value is adjusted to 12.

[0095] Example 11:

[0096] The difference from Example 1 is that in S3, the processing temperature of the blower drying oven is adjusted to 110°C, and the processing temperature of the vacuum drying oven is adjusted to 110°C.

[0097] Example 12:

[0098] The difference from Example 1 is that in S3, the processing temperature of the blower drying oven is adjusted to 200°C, and the processing temperature of the vacuum drying oven is adjusted to 200°C.

[0099] Example 13:

[0100] The difference from Example 1 is that in S1, 4 wt% HCl solution is added dropwise to adjust the pH value to 2.

[0101] Comparative Example 1:

[0102] The difference from Example 1 is that in S1, methacrylamide is removed.

[0103] Comparative Example 2:

[0104] The difference from Example 1 is that in S1, the amount of methacrylamide is adjusted to 2.5g.

[0105] Comparative Example 3:

[0106] The difference from Example 1 is that in S1, the amount of deionized water is adjusted to 500 mL.

[0107] Comparative Example 4:

[0108] The difference from Example 1 is that the heating and stirring time under a 60°C water bath condition was adjusted to 0.5h.

[0109] Comparative Example 5:

[0110] The difference from Example 1 is that S3 is adjusted to place the obtained intermediate product in a drum furnace and dry it at 350°C for 5 hours under a nitrogen atmosphere.

[0111] The physical and electrochemical properties of the negative electrode materials obtained in Examples 1-13 and Comparative Examples 1-5 were tested using the following methods:

[0112] Morphology testing of negative electrode materials: The morphology of negative electrode materials was tested using transmission electron microscopy (TEM). The sample was loaded onto the TEM stage, and after selecting suitable particles, clear images at different scales were obtained by adjusting the magnification and focal length of the microscope.

[0113] Particle size testing of the negative electrode material: The particle size distribution range of the negative electrode material was measured using a Malvern 3000 laser particle size analyzer. The dispersant (ethanol, pure water, and a low-foaming surfactant) and the test sample were placed in a 50 mL beaker, followed by the addition of a certain amount of pure water. The mixture was stirred thoroughly with a glass rod to ensure uniform dispersion. The pump speed was set to 2400 r / min–2500 r / min, and the frequency was 19.5 Hz for particle size analysis. v 50 refers to the particle size value corresponding to a cumulative distribution percentage of 50% in the volume fraction. That is, the particle size is greater than D. v 50% of the particles are of 50% size, and the particle size is smaller than D. v 50% of the particles are 50%.

[0114] Specific surface area testing of negative electrode materials: The test was conducted using a high-precision micrometer, a DX400 micrometer. The sample was loaded into a sample tube, and an isothermal jacket was placed on the sample tube. A filler rod was placed inside the bubble tube, and retaining rings and O-rings were attached to the bubble tube. The assembled sample bubble tube was then placed in the corresponding analytical station for testing. At a constant low temperature, the amount of gas adsorbed on the solid surface at different relative pressures was measured. Based on the Brownnor-Etter-Taylor adsorption theory and its formula (BET formula), the monolayer adsorption amount of the sample was calculated, thereby determining the specific surface area of ​​the sample.

[0115] Infrared spectroscopy testing of the anode material: A NICOLET iS50 Fourier transform infrared spectrometer was used to test the surface functional groups of the material. The sample was prepared as a thin film and placed in the sample chamber, ensuring close contact between the sample and the sample container. A 500cm² depth was selected. -1 Up to 4000cm -1 The wavenumber range was scanned, the infrared spectrum was exported, and peak position analysis was performed to calculate the peak area.

[0116] Raman spectroscopy of negative electrode materials D / I GTesting: Raman scattering spectra were measured using a HORIBA-XPLORA laser confocal Raman spectrometer with a laser wavelength of 532 nm. Data was collected from 30 points on each sample, and peak fitting was performed on the scattering spectra obtained at each point. The D peak position of the sample was at 1350 cm⁻¹. -1 Nearby, peak G is located at 1580cm. -1 Nearby, after labeling peaks D and G, the area ratio of peak D to peak G is calculated as I. D / I G value.

[0117] Tap density test of negative electrode material: A certain mass of powder sample is placed in a graduated cylinder, and the container is vibrated manually or mechanically for 1000 times. The volume of the sample after vibration is read. The ratio of the mass of the sample to the volume after vibration is the tap density.

[0118] Electrochemical performance testing: Based on the negative electrode materials prepared in the examples and comparative examples of this application, negative electrode materials, carboxymethyl cellulose, and styrene-butadiene rubber were dissolved in deionized water at a mass ratio of 96.5:1.5:2, with the solid content controlled at 50%. The solutions were coated onto copper foil current collectors, vacuum dried, rolled, and stamped to obtain negative electrode sheets. Lithium metal sheets were used as counter electrodes, and the cells were assembled into coin cells in an argon-filled glove box. Charge-discharge tests were conducted at a current density of 0.1C, covering a charge-discharge range of 0.01V to 1.5V, to obtain the initial reversible specific capacity, first-cycle charge capacity, and first-cycle discharge capacity. The initial coulombic efficiency was calculated as: first-cycle discharge capacity / first-cycle charge capacity.

[0119] Please refer to Table 1 for some of the preparation conditions of Examples 1-13 and Comparative Examples 1-5 above. Please refer to the corresponding figures and Table 2 for the above test results.

[0120] Taking Example 1 as an example, infrared spectroscopy testing was performed on the negative electrode material. Please refer to [link to relevant documentation]. Figure 3 Located at 1320cm -1 Up to 1880cm -1 The bending vibration peak of the -CH2 bond is present sequentially (1440 cm⁻¹). -1 Nearby, CN bond stretching vibration peak (1560 cm⁻¹) -1 The stretching vibration peak of the C=C bond (near) and the peak of the C=C bond (1635 cm⁻¹) -1 (Nearby) three peaks.

[0121] Taking Example 1 as an example, Raman spectroscopy was performed on the negative electrode material. Please refer to [link to relevant documentation]. Figure 4 At 1349.9cm -1 A D peak exists at 1579.9 cm⁻¹. -1 A peak G exists at a certain location, and the average peak intensity ratio of peak D to peak G, obtained statistically, is 0.25.

[0122] Taking Example 1 as an example again, the morphology of the negative electrode material was tested. Please refer to [link / reference needed]. Figure 5 The negative electrode material prepared in this embodiment exhibits a near-spherical morphology and shows no obvious agglomerates. Please refer to [link / reference]. Figure 6 At magnification, it can be seen that there is a layered structure on the edge of the negative electrode material surface that is different from the graphite layered structure, indicating that the preparation method of this application can achieve uniform distribution of the coating on the graphite material.

[0123] Table 1. Partial preparation conditions of Examples 1-13 and Comparative Examples 1-5 of this application

[0124]

[0125] Table 2. Performance test results of the negative electrode materials of Examples 1-13 and Comparative Examples 1-5 of this application

[0126]

[0127] Under the preparation process conditions of this application, the negative electrode materials of Examples 1-13 form a polymer coating layer on their core surface. This coating layer contains -CH2 bonds, CN bonds, and C=C bonds, and the Z value falls within a preset range. Thus, during the lithium insertion process, the polymer coating layer preferentially undergoes a reduction reaction with the electrolyte components, the CN bonds break, and further form a compound containing N and Li, Li3N. Finally, a protective layer containing both polymer and inorganic Li3N is formed on the coating layer, similar to an SEI film. During the lithium insertion / extraction process, it allows ions to pass through while hindering electrons, thereby reducing the occurrence of interfacial side reactions and reducing lithium ion consumption. As a result, the obtained secondary battery has a significantly higher initial coulombic efficiency. At the same time, the presence of Li3N can also improve the ionic conductivity of the polymer coating layer, thereby improving the kinetic performance of the coating layer and further improving the capacity utilization and initial coulombic efficiency of the obtained secondary battery.

[0128] In the preparation processes of Examples 2, 1, and 3, the mass ratio of polymer monomers (coating material) to graphite (core material) gradually increases within a suitable range, resulting in a gradual increase in the CN stretching vibration peak area and Z value of the obtained negative electrode material. In the preparation processes of Examples 1, 4, and 5, the solid-liquid ratio gradually decreases within a suitable range, causing the polymer monomers to tend to self-polymerize in the reaction system to form polymer coatings, thus gradually increasing the CN stretching vibration peak area and Z value of the obtained negative electrode material. In the preparation processes of Examples 1 and 6, the heating reaction time is extended within a suitable range, increasing the degree of polymerization of the polymer, resulting in a gradual increase in the CN stretching vibration peak area and Z value of the obtained negative electrode material. In the preparation processes of Examples 13, 9, 1, and 10, the pH of the mixed solution ranges from acidic to neutral to alkaline within a suitable range. The Z value of the obtained negative electrode material varies within a preset range with pH conditions, and the CN stretching vibration peak area also gradually increases. Comparing Example 13 with Examples 9, 1, and 10, when the CN stretching vibration peak area further meets the preset range, the initial coulombic efficiency of the negative electrode material can be further improved. In the preparation processes of Examples 11, 1, and 12, the drying temperature gradually increases within a suitable range. At relatively high drying temperatures, the degree of polymerization of the polymer decreases, thus the CN stretching vibration peak area and Z value of the obtained negative electrode material gradually decrease within a preset range.

[0129] Compared to Example 1, no polymer monomers were added in the preparation process of Comparative Example 1, resulting in a negative electrode material mainly composed of graphite. No CN bonds were detected on its surface, and the Z value was too small. The surface of this negative electrode material is prone to side reactions with the electrolyte, which exacerbates lithium ion consumption and results in a significantly lower initial coulombic efficiency.

[0130] Compared to Example 1, the amount of polymer monomer used in the preparation process of Comparative Example 2 was too low, resulting in a low ratio of polymer monomer to graphite. This led to insufficient coating layer, no CN bonds were detected on its surface, the Z value was too small, the coating effect was poor, and the first coulombic efficiency was significantly lower.

[0131] It should be noted that the capacity values ​​of Comparative Example 1 and Comparative Example 2 are higher because the polymer coating itself has no lithium storage activity, and the mass of the coating needs to be taken into account when calculating the specific capacity. Therefore, the calculated capacity of Comparative Example 1 (almost no coating is formed) and Comparative Example 2 (a small amount of coating is formed) is slightly higher, but this does not mean that the performance of Comparative Example 1 and Comparative Example 2 is better than that of the Example.

[0132] Compared to Example 1, the preparation process of Comparative Example 3 used too little water, resulting in a high solid-liquid ratio and an excessively high concentration of polymer monomers in the system. This intensified the self-polymerization reaction of the polymer monomers, reduced the number of C=C bonds, and consequently decreased the area of ​​the C=C peak in the infrared spectrum, leading to an excessively large Z value. Furthermore, no CN bonds were detected on its surface. The intensified self-polymerization reaction easily leads to the accumulation of excessive polymer coatings, which is not conducive to the formation of a uniform coating layer or the maintenance of good interfacial resistance. Therefore, the initial coulombic efficiency was significantly lower.

[0133] Compared to Example 1, the heating reaction time in the preparation process of Comparative Example 4 was insufficient, resulting in insufficient polymerization of polymer monomers or failure to form a stable bond with the graphite surface. The Z value was too small, and the coating effect was poor. Even though the area of ​​the CN stretching vibration peak was not very low, the initial coulombic efficiency was still significantly low.

[0134] Compared to Example 1, the drying temperature in the preparation process of Comparative Example 5 was too high, which caused the polymer coating to not be uniformly coated on the graphite surface. Instead, it underwent a decomposition reaction, and no CN bonds were detected on its surface. The Z value was too small, the degree of polymerization of the polymer decreased, and it agglomerated into small spheres, resulting in a significant increase in the specific surface area of ​​the negative electrode material and a significantly lower initial coulombic efficiency.

[0135] In summary, the preparation method of this application enables the polymer monomers to self-polymerize and form a uniform polymer coating layer on the core surface of the negative electrode material, resulting in a negative electrode material with good electrochemical performance.

[0136] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A negative electrode material, comprising a core and a coating layer disposed on at least a portion of the surface of the core, characterized in that, The coating layer comprises a polymer, and the infrared spectrum of the negative electrode material is at 1320 cm⁻¹. -1 Up to 1880cm -1 Within the wavenumber range, there are a first characteristic peak, a second characteristic peak, and a third characteristic peak. The first characteristic peak is the bending vibration peak of the -CH2 bond, the second characteristic peak is the stretching vibration peak of the CN bond, and the third characteristic peak is the stretching vibration peak of the C=C bond. The ratio of the peak area of ​​the first characteristic peak to the peak area of ​​the third characteristic peak is Z, where Z is 0.35 to 0.

8.

2. The negative electrode material as described in claim 1, characterized in that, The peak area of ​​the second characteristic peak is 1100 to 1300.

3. The negative electrode material as described in claim 1, characterized in that, The Raman scattering spectrum of the negative electrode material is at a wavenumber of 1350±10 cm⁻¹. -1 and 1580±10cm -1 Characteristic peaks D and G exist respectively, and the ratio of the peak intensities of characteristic peak D to characteristic peak G is I. D / I G I D / I G The value ranges from 0.02 to 0.

50.

4. The negative electrode material as described in claim 1, characterized in that, The specific surface area of ​​the negative electrode material is less than or equal to 8m². 2 / g.

5. The negative electrode material as described in claim 1, characterized in that, The tap density of the negative electrode material is greater than or equal to 0.75 g / cc.

6. The negative electrode material as described in claim 1, characterized in that, The median particle size D of the negative electrode material v 50 ranges from 1μm to 35μm.

7. The negative electrode material as described in claim 1, characterized in that, The thickness of the coating layer is 1 nm to 100 nm.

8. The negative electrode material as described in claim 1, characterized in that, The core comprises graphite, and the negative electrode material further satisfies at least one of the following conditions: (1) The graphite includes at least one of natural graphite and artificial graphite; (2) The median particle size D of the graphite particles v 50 ranges from 1μm to 30μm.

9. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, characterized in that, The negative electrode active material layer includes the negative electrode material as described in any one of claims 1-8.

10. A secondary battery, characterized in that, Includes the negative electrode as described in claim 9.

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