Silicon-based composite material, preparation method thereof, negative electrode sheet and battery

Abstract

The application relates to the technical field of batteries, in particular to a silicon-based composite material, a preparation method thereof, a negative electrode sheet and a battery. The silicon-based composite material comprises a core and a coating layer, the coating layer is formed on at least part of the surface of the core; the core comprises graphene oxide-silicon composite particles; the coating layer contains a polymer, wherein the polymer contains a catechol structure; and the thickness of the coating layer is 5 nm-50 nm. The silicon-based composite material can reduce the expansion rate of the negative electrode and realize high cycle stability of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a silicon-based composite material and its preparation method, a negative electrode sheet, and a battery. Background Technology

[0002] As a core component of new energy storage systems, the improvement of lithium-ion batteries' energy density highly depends on the innovation of anode materials. Silicon-based anodes, with a theoretical capacity (4200 mAh / g) ten times that of traditional graphite anodes, are considered a key path to overcome energy bottlenecks. However, silicon materials experience volume expansion of up to 300% during lithium insertion / extraction, leading to two major technological constraints: firstly, repeated volume changes cause active particles to pulverize and detach from the conductive network, resulting in electrode structure collapse; secondly, the nascent silicon surface continuously reacts with the electrolyte to form an unstable solid electrolyte interphase (SEI) film, accelerating active lithium loss and increasing interfacial impedance.

[0003] Therefore, there is an urgent need to develop a silicon-based composite anode that combines structural stability with optimized interface dynamics. Summary of the Invention

[0004] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a silicon-based composite material, its preparation method, a negative electrode, and a battery. When used as a negative electrode in a battery, this silicon-based composite material exhibits a low volume expansion rate and achieves high cycle stability.

[0005] This invention, through the construction of a synergistic structure of a silicon / graphene oxide (GO) composite substrate framework and a polymer flexible coating layer, helps to solve the technical bottleneck of drastic volume change and interface instability in silicon-based anodes. It can achieve dynamic dissipation of expansion stress and synergistic conduction of ions / electrons at the molecular scale, thereby achieving high cycle stability of the battery and ultimately obtaining a lithium battery with high energy density and long cycle life.

[0006] To achieve the objectives of this invention, a first aspect of this invention provides a silicon-based composite material, comprising a core and a coating layer, wherein the coating layer is formed on at least a portion of the surface of the core; the core comprises graphene oxide-silicon composite particles; the coating layer contains a polymer, wherein the polymer contains a catechol structure; and the thickness of the coating layer is 5 nm to 50 nm.

[0007] A second aspect of this invention provides a method for preparing a silicon-based composite material, comprising the following steps:

[0008] (1) Silicon particles were mixed with graphene oxide dispersion and then freeze-dried to obtain graphene oxide-silicon composite particles.

[0009] (2) A monomer containing a catechol structure is subjected to in-situ oxidative polymerization on at least a portion of the surface of the graphene oxide-silicon composite particles to form a polymer-containing coating layer, thereby obtaining a silicon-based composite material.

[0010] The polymer contains a catechol structure.

[0011] A third aspect of the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector; the negative electrode film layer comprises a negative electrode active material; wherein the negative electrode active material comprises the silicon-based composite material described in the first aspect of the present invention, or the silicon-based composite material prepared by the method for preparing the silicon-based composite material described in the second aspect of the present invention.

[0012] A fourth aspect of the present invention provides a battery comprising the negative electrode sheet described in the third aspect of the present invention.

[0013] Through the above technical solution, the core of the present invention, which includes graphene oxide-silicon composite particles, can better construct a continuous conductive network, which is conducive to ion intercalation. At the same time, with the coating layer containing polymer, a more suitable flexible buffer layer can be formed, which can suppress electrolyte side reactions and dissipate stress through the reversible quinoneization of the catechol structure in the polymer. Thus, at the molecular scale, dynamic dissipation of expansion stress and synergistic conduction of ions / electrons through dual channels are achieved, ultimately reducing the expansion rate of the negative electrode and achieving high cycle stability of the battery. Detailed Implementation

[0014] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0015] Silicon-based anode materials are expected to significantly improve the energy density of lithium-ion batteries due to their ultra-high theoretical capacity. However, silicon still has the following defects in actual use: (1) It will expand by about 300% during charging and discharging, resulting in the breakage of the electrode structure and the shedding of active materials; (2) The volume change of silicon will repeatedly damage the solid electrolyte interface (SEI film), resulting in continuous consumption of electrolyte and lithium source.

[0016] In the prior art, modified silicon-based anode materials are prepared by coating the surface of silicon particles with a flexible polymer, such as polydopamine, then combining it with graphene oxide, and finally carbonizing it. The present invention has found that this method has the following defects: (1) Graphene oxide can only be attached to the outer layer of the polymer through van der Waals forces and cannot form a three-dimensional continuous conductive network; (2) During the carbonization process, the pyrolysis and shrinkage of polydopamine produces interfacial pores, which will lead to the interruption of the electron transport path.

[0017] In this invention, by constructing a synergistic structure of graphene oxide-silicon composite particles and a flexible coating layer of polymer containing catechol, graphene oxide and silicon particles are directly bonded to form a rigid conductive framework. The polymer acts as an outer flexible buffer film to achieve stress dissipation in stages. At the same time, functional groups such as catechol can anchor metal ions through strong coordination, effectively inhibiting their dissolution into the electrolyte. Ultimately, this helps to solve the technical bottleneck of drastic volume change and interface instability of silicon-based anodes.

[0018] Silicon-based composite materials

[0019] A first aspect of the present invention provides a silicon-based composite material, comprising a core and a coating layer, the coating layer being formed on at least a portion of the surface of the core; the core comprising graphene oxide-silicon composite particles; the coating layer containing a polymer, wherein the polymer contains a catechol structure; the thickness of the coating layer is 5 nm-50 nm.

[0020] In this invention, a core comprising graphene oxide-silicon composite particles is first constructed, which can build a continuous rigid conductive network and maintain good interlayer spacing, thereby reducing the volume expansion rate of the silicon-based composite material. At the same time, the coating layer contains a polymer containing catechol structure, which gives the coating layer flexible buffer properties, which can suppress electrolyte side reactions and dissipate stress through reversible quinoneization of phenolic hydroxyl groups. Ultimately, this further reduces the volume expansion rate of the silicon-based composite material and increases the capacity cycle performance of the battery containing the silicon-based composite material of this invention.

[0021] According to the present invention, in some embodiments, the polymer is one or more of polydopamine and its derivatives.

[0022] In this invention, when the polymer is polydopamine and its derivatives, the volume expansion rate of the silicon-based composite material can be further reduced.

[0023] In this invention, it is understood that the graphene oxide-silicon composite particles are composite particles formed by graphene oxide and silicon particles. In some embodiments, the mass content of graphene oxide in the graphene oxide-silicon composite particles is 15%-90%, for example, 15%, 30%, 40%, 45%, 50%, 60%, 75%, 85%, 90%, or any two of the above values.

[0024] In this invention, controlling the content of graphene oxide within the above-mentioned range can better reduce the volume expansion rate of silicon-based composite materials during use.

[0025] According to some embodiments of the present invention, the graphene oxide content in the graphene oxide-silicon composite particles is preferably 45%-75% by mass.

[0026] Furthermore, this invention has found that when the content of graphene oxide is too low or too high, the volume expansion rate of the silicon-based composite material during use will increase to varying degrees, and the capacity retention rate of the battery containing the silicon-based composite material will decrease to varying degrees. Controlling the content of graphene oxide within the above-mentioned preferred range can better reduce the volume expansion rate of the silicon-based composite material during use. The reason for this is that when the graphene oxide content is too low, the graphene oxide sheets cannot effectively bridge the gaps between silicon particles, causing stress islands to form in local areas of the graphene oxide-silicon composite particles. When the graphene oxide-silicon composite particles are used, for example during lithiation, the silicon particles not constrained by graphene oxide undergo greater volume expansion, leading to mutual compression and breakage between particles. At the same time, the broken silicon surface continuously regenerates the SEI film, consuming active lithium in the electrolyte and blocking ion channels, thereby reducing the capacity retention rate of the battery containing this silicon-based composite material. Furthermore, when the graphene oxide content is within the above range, the silicon-based composite material has both a continuous conductive network and can maintain a good interlayer spacing, which is more conducive to ion intercalation, thereby better reducing the volume expansion rate of the silicon-based composite material during use and increasing the capacity retention rate of the battery containing this silicon-based composite material.

[0027] According to some embodiments of the present invention, the silicon content in the graphene oxide-silicon composite particles is 10%-85% by mass, for example, 10%, 25%, 35%, 40%, 50%, 55%, 65%, 75%, 85%, or any combination of two of the above values.

[0028] In this invention, controlling the silicon content within a suitable range can better reduce the volume expansion rate of silicon-based composite materials during use and increase the capacity retention rate of batteries containing the silicon-based composite materials.

[0029] According to the present invention, in some embodiments, the silicon content in the graphene oxide-silicon composite particles is preferably 25%-55% by mass.

[0030] In this invention, controlling the silicon content within a suitable range can better reduce the volume expansion rate of silicon-based composite materials during use and increase the capacity retention rate of batteries containing the silicon-based composite materials.

[0031] According to the present invention, in some embodiments, the mass of the polymer is 3%-7% of the core mass, for example, 3%, 4%, 5%, 6%, 7%, or a range of any two of the above values.

[0032] In this invention, when the polymer content is within the aforementioned range, the volume expansion rate of the silicon-based composite material during use can be better reduced, and the capacity retention rate of the battery containing the silicon-based composite material can be increased. Further research has revealed that when the polymer content is too low or too high, the volume expansion rate of the silicon-based composite material during use increases to varying degrees, and the capacity retention rate of the battery containing the silicon-based composite material decreases to varying degrees. Controlling the polymer content within the aforementioned preferred range can better reduce the volume expansion rate of the silicon-based composite material during use. The reason for this is that when the polymer content is too low, a coating layer cannot be formed effectively, while when the content is too high, the catechol structure in the polymer constitutes a high-barrier electron trap, which significantly slows down the charge transfer rate. During charging, lithium ions accumulate outside the coating layer, forming a concentration gradient, inducing shear stress inside the silicon particles, ultimately leading to cracking of the coating layer, thereby reducing the overall performance of the silicon-based composite material.

[0033] According to the present invention, in some embodiments, the average molecular weight of the polymer is 2KDa-50KDa, for example 2KDa, 4KDa, 8KDa, 10KDa, 15KDa, 20KDa, 23KDa, 30KDa, 35KDa, 40KDa, 45KDa, 50KDa, or any range consisting of any two of the above ratios.

[0034] In this invention, the average molecular weight of the polymer can be detected using methods and instruments known in the art. For example, the polymer can be obtained by disassembling and separating the core and coating layers of the silicon-based composite material, and the average molecular weight of the polymer can be determined using gel permeation chromatography (GPC).

[0035] According to the present invention, in some embodiments, the thickness of the coating layer ranges from 5nm to 50nm, for example, 5nm, 8nm, 10nm, 15nm, 20nm, 25nm, 30nm, 33nm, 40nm, 45nm, 50nm, or any combination of two of the above values.

[0036] According to the present invention, in some embodiments, the average particle size of the kernel is 5μm-15μm, for example, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or a range of any two of the above values.

[0037] In this invention, the thickness of the coating layer and the average particle size of the graphene oxide-silicon composite particles can be obtained by any method in the art, such as by using a scanning electron microscope. Specifically, the position of the coating layer is first observed using a scanning electron microscope, and then the particle size of the core and the thickness of the coating layer are determined by energy dispersive spectroscopy.

[0038] According to the present invention, graphene oxide has oxygen-containing groups such as hydroxyl, epoxy, carboxyl and carbonyl groups. In some embodiments, the oxygen content of graphene oxide is 30wt%-45wt%, for example 30wt%, 35wt%, 40wt%, 45wt%, or any combination of two of the above values.

[0039] In this invention, by controlling the oxygen content of graphene oxide within a suitable range, the core can better form a stable core-shell structure with the coating layer, thereby enabling the silicon-based composite material to have excellent stability.

[0040] In this invention, "the coating layer is formed on at least a portion of the surface of the core" means that the coating layer completely covers the surface of the core or that the coating layer does not completely cover the surface of the core. Preferably, the coating layer completely covers the surface of the core, that is, the coating layer is formed on all surfaces of the core.

[0041] The silicon-based composite material of the present invention can be prepared according to the structure of the silicon-based composite material. For example, firstly, graphene oxide-silicon composite particles are constructed by combining silicon particles with graphene oxide, and then a coating layer is constructed by in-situ oxidative polymerization of a monomer containing catechol on the surface of the graphene oxide-silicon composite particles to obtain the silicon-based composite material. Alternatively, it can be prepared according to the preparation method of the silicon-based composite material provided in the second aspect of the present invention below.

[0042] [Preparation methods for silicon-based composite materials]

[0043] According to some embodiments of the present invention, a second aspect of the present invention provides a method for preparing a silicon-based composite material, comprising the following steps:

[0044] (1) Silicon particles were mixed with graphene oxide dispersion and then freeze-dried to obtain graphene oxide-silicon composite particles.

[0045] (2) A monomer containing a catechol structure is subjected to in-situ oxidative polymerization on at least a portion of the surface of the graphene oxide-silicon composite particles to form a polymer-containing coating layer, thereby obtaining a silicon-based composite material.

[0046] The polymer contains a catechol structure.

[0047] In this invention, graphene oxide-silicon composite particles are first constructed, and then a polymer containing a catechol structure is constructed in situ on at least a portion of the surface of the graphene oxide-silicon composite particles. This enables the prepared silicon-based composite material to achieve dynamic dissipation of expansion stress and synergistic conduction of ions / electrons at the molecular scale, thereby enabling the battery containing the silicon-based composite material to have high cycle stability.

[0048] It should be understood that all the features and advantages described above regarding "silicon-based composite materials" also apply to this "preparation method of silicon-based composite materials", and will not be repeated here.

[0049] Preparation of graphene oxide-silicon composite particles

[0050] Silicon particles refer to solid silicon materials at the micron or nanometer scale. The silicon particles in this invention are silicon particles known in the art, preferably nanometer-sized silicon particles. As long as the purpose of this invention can be achieved, the particle size of the silicon particles can be selected within a wide range. In some embodiments, the average particle size of the silicon particles is 10nm-100nm, for example, 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm or 100nm. In this invention, silicon particles with an average particle size of 80nm are used as an example to illustrate the advantages of this invention, but this does not represent a limitation of this invention.

[0051] A graphene oxide dispersion refers to a dispersion of graphene oxide in a solvent (e.g., water). In some embodiments, the solid content of the graphene oxide dispersion is 30wt%-60wt%, for example, 30wt%, 40wt%, 45wt%, 50wt%, or 60wt%.

[0052] In this invention, by controlling the solid content of the graphene oxide dispersion within the above-mentioned range, it is possible to better achieve the composite of silicon particles and graphene oxide to construct a continuous conductive network, and to better maintain the interlayer spacing of graphene oxide, thereby reducing the volume expansion rate of the prepared silicon-based composite material during use and increasing the capacity retention rate of the battery containing the silicon-based composite material.

[0053] According to the present invention, in some embodiments, the average particle size of graphene oxide in the graphene oxide dispersion is 1 μm-14.5 μm, for example, 1 μm, 3 μm, 5 μm, 7 μm, 9 μm, 12 μm or 14.5 μm.

[0054] In this invention, the particle size of graphene oxide is controlled within the above-mentioned range, thereby reducing the volume expansion rate of the prepared silicon-based composite material during use and increasing the capacity retention rate of the battery containing the silicon-based composite material.

[0055] According to the present invention, in some embodiments, the oxygen content of graphene oxide is 30wt%-45wt%.

[0056] According to the present invention, in some embodiments, the mass percentage of graphene oxide in the graphene oxide dispersion is 15%-90%, preferably 45%-75%, based on the total mass of silicon particles and graphene oxide in the dispersion.

[0057] According to the present invention, in some embodiments, the mass percentage of silicon particles is 10%-85%, preferably 25%-55%, based on the total mass of silicon particles and graphene oxide in the graphene oxide dispersion.

[0058] In this invention, by controlling the proportion of graphene oxide or the mass proportion of silicon particles within the above-mentioned range, graphene oxide can better constrain silicon particles, construct a better continuous conductive network, and at the same time avoid the compression of interlayer spacing. The resulting silicon-based composite material is more conducive to ion intercalation when used in batteries, and can better improve the battery capacity retention rate.

[0059] According to the present invention, as long as the purpose of the present invention can be achieved, the mixing in step (1) is not particularly limited. In some embodiments, the mixing method in step (1) includes: mixing silicon particles with graphene oxide dispersion and ultrasonic treatment for 1-3 hours.

[0060] According to the present invention, as long as the purpose of the present invention can be achieved, the conditions for freeze drying are not particularly limited. In some embodiments, the freeze drying conditions in step (1) include: a temperature of -60℃ to -40℃, a pressure of 0.01mbar to 0.5mbar, and a time of 24h to 30h.

[0061] Formation of the coating layer

[0062] According to the present invention, in some embodiments, the mass of the monomer containing the catechol structure is 3%-10% of the mass of the graphene oxide-silicon composite particles, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0063] In this invention, the amount of monomer containing the catechol structure is controlled within the above-mentioned range. This allows for a better reduction in the volume expansion rate of the prepared silicon-based composite material during use and an increase in the capacity retention rate of the battery containing the silicon-based composite material.

[0064] According to the present invention, in some embodiments, the conditions for in-situ oxidative polymerization in step (2) include: reacting at room temperature for 16-30 hours (e.g., 16 hours, 18 hours, 22 hours, 24 hours, 26 hours, 28 hours or 30 hours) in the presence of dissolved oxygen.

[0065] In this invention, "dissolved oxygen" refers to oxygen in the air that dissolves in the reaction system during the reaction.

[0066] According to the present invention, in some embodiments, the method of forming the polymer-containing coating layer in step (2) includes: immersing graphene oxide-silicon composite particles in a mixture of a monomer containing a catechol structure and a buffer solution for in-situ oxidative polymerization, followed by solid-liquid separation, washing, and drying to obtain a silicon-based composite material; in some embodiments, the concentration of the catechol-containing monomer in the mixture is 1 mg / mL-3 mg / mL; in some embodiments, the pH value of the buffer solution is 8-9; there is no particular limitation on the specific type of the aforementioned buffer solution, and in some embodiments, the buffer solution includes Tris-HCl buffer solution; in some embodiments, the aforementioned solid-liquid separation, washing, and drying methods can be conventional methods in the art, such as using centrifugation for solid-liquid separation and vacuum drying for drying.

[0067] According to the present invention, in some embodiments, the monomers providing the catechol structure include dopamine hydrochloride and its derivatives; optionally, the monomers providing the catechol structure may be dopamine hydrochloride.

[0068] As an example, in step (2), the method for forming the polymer-containing coating layer includes: immersing graphene oxide-silicon composite particles in a mixed liquid of dopamine hydrochloride and Tris-HCl buffer (pH=8-9) (the concentration of dopamine hydrochloride in the mixed liquid is 1mg / mL-3mg / mL), the amount of dopamine hydrochloride added is 3wt%-10wt% of the dry weight of the graphene oxide-silicon composite particles, reacting at room temperature for 16-30 hours in the presence of dissolved oxygen, so that dopamine hydrochloride is oxidized and polymerized on the surface of the graphene oxide-silicon composite particles to form a polydopamine (PDA) coating layer, and then performing solid-liquid separation washing and vacuum drying to obtain the silicon-based composite material.

[0069] [Negative electrode plate]

[0070] A third aspect of the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector; the negative electrode film layer comprises a negative electrode active material, the negative electrode active material comprising a silicon-based composite material provided in the first aspect of the present invention, or a silicon-based composite material prepared by the method for preparing a silicon-based composite material provided in the second aspect of the present invention.

[0071] The negative electrode in this invention contains the silicon-based composite material provided above, which enables the negative electrode to simultaneously achieve good electronic conductivity, high energy density and small volume expansion.

[0072] According to the present invention, in some embodiments, the mass content of the silicon-based composite material is 5%-35%, preferably 5%-15%, based on the total mass of the negative electrode film.

[0073] According to the present invention, in addition to the silicon-based composite material described above, the negative electrode active material may also include other negative electrode active materials known in the art for use in batteries, such as at least one of graphite (natural graphite, artificial graphite, or a mixture thereof), soft carbon, and hard carbon. Optionally, other negative electrode active materials include artificial graphite.

[0074] According to the present invention, in some embodiments, the mass content of artificial graphite is 60%-90%, preferably 70%-85%, based on the total mass of the negative electrode film.

[0075] According to the present invention, in some embodiments, the negative electrode film layer further includes a conductive agent, a binder, and a thickener;

[0076] The present invention does not specifically limit the type of the aforementioned conductive agent, including but not limited to at least one of conductive carbon black, conductive graphite, acetylene black, carbon nanotubes, graphene and carbon nanofibers; in some embodiments, the mass content of the conductive agent is 0.5%-5% based on the total mass of the negative electrode film.

[0077] This invention does not impose any particular limitation on the type of binder mentioned above, and any binder used for negative electrode sheets in the art can be used, including but not limited to at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyacrylamide (PAM), and polyvinyl alcohol (PVA). In some embodiments, the mass content of the binder is 0.1%-5% based on the total mass of the negative electrode film layer.

[0078] This invention does not specifically limit the type of thickener mentioned above, and it can be any thickener used for negative electrode sheets in the art, including but not limited to at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, and sodium carboxymethyl cellulose. In some embodiments, the mass content of the thickener is 0.05%-3% based on the total mass of the negative electrode film.

[0079] According to the present invention, "a negative electrode film layer located on at least one surface of the negative electrode current collector" means that the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector, preferably disposed on both surfaces of the negative electrode current collector.

[0080] The negative electrode current collector in this invention can be a metal foil or a composite current collector. As an example of a metal foil, copper foil can be used.

[0081] The negative electrode film layer in this invention is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is usually formed by dispersing negative electrode active materials, conductive agents, binders, and thickeners in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to it; a typical compaction density is 1.45 g / cm³.3 -1.6g / cm 3 .

[0082] [Battery]

[0083] A fourth aspect of the present invention provides a battery comprising the negative electrode sheet provided in the third aspect of the present invention.

[0084] In this invention, the battery comprising the negative electrode sheet described above exhibits excellent high cycle stability.

[0085] According to the present invention, the battery typically includes a positive electrode, a separator, and an electrolyte. The positive electrode, negative electrode, and separator constitute the battery cell. The separator is disposed between the positive and negative electrodes, mainly to prevent short circuits between the positive and negative electrodes, while allowing active ions to pass through. The electrolyte plays a role in conducting active ions between the positive and negative electrodes.

[0086] According to the present invention, the positive electrode sheet can be a conventional positive electrode sheet in the art. For example, the positive electrode sheet includes a positive current collector and a positive active material layer coated on one or both surfaces of the positive current collector; wherein, the positive active material layer may include a positive active material, a conductive agent, and a binder. According to the present invention, in some embodiments, the mass ratio of the positive active material, the conductive agent, and the binder in the positive active material layer is (94-96):(1-3):(1-3).

[0087] According to the present invention, the positive electrode active material can be any positive electrode active material in the art, and in some embodiments, it includes, but is not limited to, lithium nickel cobalt manganese oxide (LiNi). x Co y Mn z M b O2), wherein 0.70≤x≤0.95, 0.15≤y<0.45, 0.05≤z<0.45, 0.0≤b≤0.25, x+y+z+b=1, and element M includes one or more of zirconium (Zr), tungsten (W), titanium (Ti), aluminum (Al), strontium (Sr), boron (B) and neodymium (Nd).

[0088] According to the present invention, in some embodiments, the conductive agent in the positive electrode active material layer includes, but is not limited to, a combination of carbon nanotubes (CNTs) and Ketjen Black (KB).

[0089] According to the present invention, in some embodiments, the binder in the positive electrode active material layer includes, but is not limited to, polyvinylidene fluoride (PVDF).

[0090] According to the present invention, the positive electrode active material is typically formed by coating a positive electrode slurry onto one or both surfaces of a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode active material, a conductive agent, a binder, and any other components in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP), but is not limited thereto. The positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used.

[0091] According to the present invention, as long as the purpose of the present invention can be achieved, the present invention does not have a special limitation on the specific type of electrolyte. In some embodiments, the electrolyte is a lithium-ion electrolyte.

[0092] As an example, the electrolyte can be lithium hexafluorophosphate (LiPF6), an organic solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), and vinylene carbonate (VC) in a mass ratio of 10.0:22.0:53.0:3.0:7.0:5.0.

[0093] The separator is positioned between the positive and negative electrodes, primarily serving to prevent short circuits between them while allowing active ions to pass through. This invention has no particular limitations on the separator's structure, as long as it achieves the intended purpose; any known porous membrane with good chemical and mechanical stability can be used. As an example, a PE base membrane is selected, with a ceramic coating and a PVDF coating sequentially applied to both sides of the base membrane.

[0094] The battery of this invention can be manufactured into a cell by a winding or stacking process using a positive electrode, a separator, and a negative electrode. The cell is then packaged, injected with electrolyte, sealed, and formed to obtain the corresponding battery. Specifically, the N / P ratio of the positive and negative electrodes can be controlled to be 1.02-1.10 during the process.

[0095] The present invention does not impose any special restrictions on the shape of the battery, which can be cylindrical, square or other arbitrary shapes. As an example, the external dimensions of the battery are a diameter of 21.0 mm and a length of 70.0 mm, which conforms to the shape of the 21700 standard specification.

[0096] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0097] Example 1

[0098] 1. Preparation of the positive electrode:

[0099] Take positive electrode active material (LiNi) with a mass ratio of 96:1:1:2 0.8 Co 0.1 Mn 0.1 O2), conductive carbon black, carbon nanotubes and polyvinylidene fluoride (PVDF) are thoroughly mixed in N-methylpyrrolidone to obtain a positive electrode coating material. The positive electrode coating material is then coated onto a 12μm thick aluminum foil, and after drying and cold pressing, a positive electrode sheet is obtained.

[0100] 2. Preparation of the negative electrode:

[0101] Artificial graphite, silicon-based composite material, conductive carbon nanotubes, conductive carbon black, thickener sodium carboxymethyl cellulose (CMC), and binder polyacrylic acid (PAA) were mixed in a mass ratio of 85:10:1.5:1:1:1.5. Deionized water was then added and the mixture was stirred to form a uniform and stable negative electrode slurry with a solid content of 40 wt%. The negative electrode slurry was uniformly coated onto an 8 μm thick copper foil as the negative electrode current collector. After drying and cold pressing, a negative electrode sheet was obtained with a compaction density of 1.5 g / cm³. 3 .

[0102] Preparation of the above silicon-based composite material:

[0103] (1) Take 1 kg of silicon nanoparticles (average particle size 80 nm) and mix with 1.5 kg of graphene oxide (GO) dispersion (solid content 45 wt%, average particle size of graphene oxide 10 μm, oxygen content 40 wt%). The dry weight of GO accounts for 60% of the total mass of silicon nanoparticles and GO. After ultrasonic treatment for 2 h, freeze dry (freeze drying conditions: temperature -50℃, pressure 0.4 mbar, time 24 h) to obtain graphene oxide-silicon composite particles with an average particle size of 11 μm.

[0104] (2) The graphene oxide-silicon composite particles were immersed in a mixture of dopamine hydrochloride and Tris-HCl buffer (pH=8.5) (the concentration of dopamine hydrochloride in the mixture was 2 mg / mL), and the amount of dopamine hydrochloride added was 8 wt% of the dry weight of the graphene oxide-silicon composite particles. The mixture was stirred at room temperature for 24 hours in the presence of dissolved oxygen to allow dopamine hydrochloride to oxidize and polymerize on the surface of the graphene oxide-silicon composite particles to form a polydopamine (PDA) coating layer. After centrifugation and washing, the particles were vacuum dried to obtain the silicon-based composite material.

[0105] 3. Preparation of electrolyte:

[0106] An electrolyte was prepared by mixing lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), and vinylene carbonate (VC) in a mass ratio of 10:22:53:3:7:5.

[0107] 4. Diaphragm:

[0108] A high-porosity membrane is selected, in which the thickness of the PE base membrane is 9μm, the thickness of the ceramic coating on both sides of the base membrane is 1μm, and the thickness of the PVDF coating is 1μm.

[0109] 5. Assembly of lithium-ion batteries:

[0110] After the positive and negative electrode sheets are rolled and slit, they are wound together with the separator according to a set process to form a 21700 cylindrical battery core. The battery core is then fixed to pre-made connecting pieces by welding and installed into a metal battery casing. After completing key processes such as electrolyte injection, sealing, and formation, a lithium-ion battery is obtained.

[0111] The lithium-ion battery uses a cylindrical casing with an external dimension of 21.0 mm in diameter and 70.0 mm in length, conforming to the 21700 standard specification.

[0112] Example 2

[0113] The method of Example 1 differs in that:

[0114] The dry weight of GO accounts for 45% of the total mass of silicon nanoparticles and GO;

[0115] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0116] Example 3

[0117] The method of Example 1 differs in that:

[0118] GO dry weight accounts for 75% of the total mass of silicon nanoparticles and GO;

[0119] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0120] Example 4

[0121] The method of Example 1 differs in that:

[0122] The amount of dopamine hydrochloride added is 5 wt% of the dry weight of the graphene oxide-silicon composite particles;

[0123] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0124] Example 5

[0125] The method of Example 1 differs in that:

[0126] The amount of dopamine hydrochloride added is 10 wt% of the dry weight of the graphene oxide-silicon composite particles;

[0127] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0128] Example 6

[0129] The method of Example 1 differs in that:

[0130] The dry weight of GO accounts for 15% of the total mass of silicon nanoparticles and GO;

[0131] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0132] Example 7

[0133] The method of Example 1 differs in that:

[0134] GO dry weight accounts for 90% of the total mass of silicon nanoparticles and GO;

[0135] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0136] Example 8

[0137] The method of Example 1 differs in that:

[0138] The amount of dopamine hydrochloride added is 12 wt% of the dry weight of the graphene oxide-silicon composite particles;

[0139] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0140] Example 9

[0141] The method of Example 1 differs in that:

[0142] The oxygen content of graphene oxide is 30 wt%.

[0143] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0144] Comparative Example 1

[0145] The method of Example 1 differs in that:

[0146] In the preparation of the negative electrode, the silicon-based composite material was replaced with the graphene oxide-silicon composite particles in Example 1;

[0147] The rest is the same as in Example 1, and the negative electrode and the corresponding lithium-ion battery are finally prepared.

[0148] Comparative Example 2

[0149] The method of Example 1 differs in that:

[0150] The oxygen content of graphene oxide is 25 wt%.

[0151] The rest is the same as in Example 1, and a silicon-based composite material, as well as the corresponding negative electrode and lithium-ion battery, are finally prepared.

[0152] Performance testing

[0153] The lithium-ion batteries in the examples and comparative examples were tested as follows:

[0154] 1. Cyclic performance testing method:

[0155] The lithium-ion battery was placed in a 25°C constant temperature chamber for 6 hours and tested according to the following steps:

[0156] (1) First round of constant current and constant voltage charging: Charge at a constant current of 0.1C to 4.2V, then switch to constant voltage charging until the current drops to 0.01C.

[0157] (2) Let it stand for 30 minutes after charging is complete.

[0158] (3) Perform constant current discharge, and discharge to 2.5V at a rate of 0.1C.

[0159] (4) Cyclic charging and discharging process: Charge at a constant current rate of 1C to 4.2V, let stand for 30 minutes, and discharge at a constant current rate of 1C to 2.5V.

[0160] (5) Repeat the above charging and discharging process for a total of 500 cycles.

[0161] The discharge capacity Q1 and Q500 of the battery after 1 cycle and 1000 cycles were calculated, and the capacity retention rate of the battery was calculated as: Q500 / Q1×100%.

[0162] 2. Electrode Expansion Rate Test Method:

[0163] First, discharge the lithium-ion battery to a constant current of 2.5V to ensure it is in a safe state, reducing the risk of short circuits or thermal runaway during disassembly. Carefully disassemble the lithium-ion battery within a glove box (protected by argon or other inert atmosphere) and remove the negative electrode from the cylindrical cell. Use plastic tweezers to peel off the negative electrode, avoiding damage to the active material layer. Next, cut the removed negative electrode to an appropriate size and soak it in anhydrous dimethyl carbonate (DMC) solution for 30 minutes to dissolve and remove residual electrolyte and possible byproducts. After removing the negative electrode, gently wipe the surface with lint-free paper, then replace with fresh DMC solution, repeating the soaking-wiping process three times to ensure no residual contaminants remain on the negative electrode surface. Subsequently, rinse the electrode with anhydrous ethanol and wipe again to further remove solvent and impurities. After cleaning, place the electrode in a glove box and let it stand for 48 hours to ensure the negative electrode is completely dry, preventing interference from residual solvent in subsequent tests.

[0164] Electrode expansion rate = Electrode thickness after cycling / Initial electrode thickness × 100%.

[0165] 3. Test method for mass energy density:

[0166] Weigh the lithium-ion battery and record the mass as M (unit: kg). Then, place the lithium-ion battery in a 25°C constant temperature chamber for 6 hours and test it according to the following steps:

[0167] (1) Constant current and constant voltage charging: Charge at a constant current of 0.1C to 4.2V, then switch to constant voltage charging until the current drops to 0.01C.

[0168] (2) Let it stand for 30 minutes after charging is complete.

[0169] (3) Perform constant current discharge, and discharge to 2.5V at a rate of 0.1C.

[0170] (4) Repeat the above steps 3 times and record the discharge capacity (denoted as C, unit: Ah) and average working voltage (denoted as V, unit: V) of the 3rd discharge process.

[0171] Mass energy density = C × V / M.

[0172] The test results are shown in Table 1.

[0173] Table 1

[0174]

[0175] Compared with the examples and comparative examples, the present invention can better reduce the expansion rate of the negative electrode and increase the capacity retention rate of the battery by constructing a core including graphene oxide-silicon composite particles and a coating layer containing polydopamine.

[0176] Examples 1-3 and 6-7 demonstrate that controlling the graphene oxide (GO) content within a more favorable range can better reduce the expansion rate of the negative electrode and increase the battery capacity retention rate. This is because when the GO content is too low, the sparse graphene sheets cannot effectively bridge the gaps between silicon particles, leading to the formation of stress islands in localized areas. During lithiation, silicon particles not constrained by GO undergo greater volume expansion, causing them to crush and break apart. Simultaneously, the broken silicon surface continuously regenerates the SEI film, consuming active lithium in the electrolyte and blocking ion channels, ultimately... This resulted in an abnormal expansion of 28.3% and a capacity retention rate of only 84.7%. Conversely, excessive graphene oxide caused the composite framework to form a dense stacked structure, compressing the interlayer spacing and severely hindering the lateral diffusion of lithium ions, leading to intensified electrode polarization. At the end of charging, lithium plating occurred in some areas due to the lag in lithium ion transport, causing irreversible capacity loss and interlayer peeling expansion. Therefore, further control of the graphene oxide content range not only constructed a continuous conductive network but also maintained a good interlayer spacing (facilitating ion intercalation), better reducing the expansion rate of the negative electrode and increasing the battery capacity retention rate.

[0177] Examples 1, 4, 5, 8 and Comparative Example 1 show that the expansion rate of the negative electrode lacking a coating layer increases sharply and the battery capacity retention rate decreases sharply. However, when the coating layer is too thick or too thin, the expansion rate increases to some extent and the battery capacity retention rate decreases to some extent. The reason for this is that the lack of a coating layer allows silicon particles to directly contact the electrolyte, resulting in localized electrochemical corrosion during the first charge and discharge. The uneven lithium intercalation on the silicon surface creates a micro-battery effect, and the anode region dissolves to form SiO2. x Lithium metal dendrites are deposited in the cathode region. This corrosion-deposition cycle leads to rapid pulverization of the active material. At the same time, the dendrites pierce the SEI film, triggering continuous side reactions. The excessive coating layer forms a dense coating layer with excessive thickness. Its benzene ring conjugated structure constitutes a high-energy-barrier electron trap, which significantly slows down the charge transfer rate. During charging, lithium ions accumulate outside the PDA layer, forming a concentration gradient, which induces shear stress inside the silicon particles, eventually leading to cracking of the coating layer. By selecting an appropriate amount of polydopamine to form a more suitable flexible buffer layer, both the side reactions of the electrolyte are suppressed and the stress is dissipated through the reversible quinoneization of phenolic hydroxyl groups are eliminated.

[0178] Examples 1 and 9 and Comparative Example 2 show that controlling the oxygen content of graphene oxide within a suitable range can better increase and reduce the expansion rate of the negative electrode and increase the capacity retention rate of the battery. When the oxygen content of graphene oxide is too low, it cannot form a stable and uniform coating layer with the core anchor in situ polymerization of dopamine hydrochloride to form the PDA outer layer. In long-term operation, this will lead to cracking of the coating layer, resulting in a sharp increase in the expansion rate of the negative electrode and a sharp decrease in the capacity retention rate of the battery.

[0179] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A silicon-based composite material, characterized in that, The device includes a core and a coating layer, the coating layer being formed on at least a portion of the surface of the core; the core comprises graphene oxide-silicon composite particles; the coating layer contains a polymer, wherein the polymer contains a catechol structure, and the polymer is one or more of polydopamine and its derivatives; the thickness of the coating layer is 5 nm-50 nm. In the graphene oxide-silicon composite particles, the mass content of graphene oxide is 30%-90%; the mass of the polymer is 3%-7% of the mass of the core; and the oxygen content of the graphene oxide is 30wt%-45wt%.

2. The silicon-based composite material according to claim 1, wherein, The silicon content in the graphene oxide-silicon composite particles is 10%-85% by mass. And / or, the average molecular weight of the polymer is 2 kDa-50 kDa; And / or, the average particle size of the kernel is 5μm-15μm.

3. The silicon-based composite material according to claim 2, wherein, The graphene oxide-silicon composite particles contain 45%-75% graphene oxide by mass. And / or, the silicon content in the graphene oxide-silicon composite particles is 25%-55% by mass.

4. A method for preparing the silicon-based composite material according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Silicon particles were mixed with graphene oxide dispersion and then freeze-dried to obtain graphene oxide-silicon composite particles; (2) A monomer containing a catechol structure is subjected to in-situ oxidative polymerization on at least a portion of the surface of the graphene oxide-silicon composite particles to form a polymer-containing coating layer, thereby obtaining a silicon-based composite material; The polymer contains a catechol structure, and the monomers providing the catechol structure include dopamine hydrochloride and its derivatives.

5. The method for preparing the silicon-based composite material according to claim 4, wherein, The average particle size of the silicon particles is 10nm-100nm; And / or, in the graphene oxide dispersion, the average particle size of the graphene oxide is 1 μm-14.5 μm; And / or, the solid content of the graphene oxide dispersion is 30wt%-60wt%; And / or, based on the total mass of the silicon particles and the graphene oxide in the graphene oxide dispersion, the mass percentage of graphene oxide in the graphene oxide dispersion is 45%-90%; And / or, the mass of the monomer providing the catechol structure is 3%-10% of the mass of the graphene oxide-silicon composite particles.

6. The method for preparing the silicon-based composite material according to claim 4, wherein, In step (1), the mixing method includes: mixing silicon particles with graphene oxide dispersion and ultrasonic treatment for 1-3 hours; And / or, in step (1), the freeze-drying conditions include: a temperature of -60 to -40°C, a pressure of 0.01 mbar to 0.5 mbar, and a time of 24 h to 30 h.

7. The method for preparing the silicon-based composite material according to claim 4, wherein, In step (2), the conditions for the in-situ oxidative polymerization include: reacting at room temperature for 16-30 hours in the presence of dissolved oxygen; And / or, in step (2), the method of forming a polymer-containing coating layer includes: immersing graphene oxide-silicon composite particles in a mixture of a monomer containing a catechol structure and a buffer solution for in-situ oxidative polymerization, followed by solid-liquid separation, washing, and drying to obtain a silicon-based composite material.

8. The method for preparing the silicon-based composite material according to claim 7, wherein, In the mixture, the concentration of the monomer containing the catechol structure is 1 mg / mL-3 mg / mL; And / or, the pH of the buffer solution is 8-9; And / or, the buffer solution includes Tris-HCl buffer.

9. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector; the negative electrode film layer includes a negative electrode active material; The negative electrode active material includes the silicon-based composite material according to any one of claims 1-3, or the silicon-based composite material prepared by the preparation method of the silicon-based composite material according to any one of claims 4-8.

10. A battery, characterized in that, The negative electrode sheet according to claim 9.

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