Silicon carbon composite and method for manufacturing the same

Abstract

The present invention generally relates to methods for making silicon-carbon composites containing nanoscale silicon and carbon, the methods comprising the steps of preparing a dispersion of silicon nanoparticles and a selected form of carbon; spray-drying the dispersion to form essentially spherical silicon nanoparticles; heat-treating the silicon nanoparticles to pyrolyze and / or burn off the polymer and consolidate the silicon nanoparticles; coating the silicon nanoparticles with carbon to form Si:C composites; and optionally adding additional elements, such as lithium, magnesium, nitrogen, and halogen gases, to the composite during the heating step (c) or the coating step (d) or during a subsequent heat-treatment step. The present invention further relates to composites made by such methods, anodes made from such composites, and batteries containing such anodes.

Description

[Technical Field] 【0001】 Related applications This application claims priority to Australian Provisional Patent Application No. 2020903802, filed on 21 October 2020. The contents of AU'802 are incorporated herein by reference in their entirety. 【0002】 This invention relates to a silicon composite material for use as an anode material in lithium-ion batteries. 【0003】 The present invention is described below with reference to its preferred embodiments, but those skilled in the art will understand that the spirit and scope of the invention can be embodied in many other forms. [Background technology] 【0004】 Nothing discussed in this specification regarding prior art should be considered to acknowledge that such prior art is widely known or forms part of the common general knowledge in the art. 【0005】 As society progresses, the demand for energy in areas such as electronics, renewable energy generation systems, and electric vehicles is increasing more and more significantly. One way to address this ever-growing demand is through improved battery technology. 【0006】 Lithium-ion batteries (LIBs) are considered a promising solution to meet the growing demand for portable electronic devices, electric vehicles, and hybrid vehicles due to their high energy density and stable cycle life. 【0007】 A typical lithium-ion battery (LIB) consists of a lithium metal cathode and anode, separated by a liquid electrolyte that moves lithium between the two electrodes. The battery generates power by discharging lithium from the anode to the cathode through the electrolyte. To date, most lithium-ion batteries use a graphite anode, which is a layer of carbon sheets arranged in a hexagonal pattern. The large spaces between these layers provide an ideal place to store lithium atoms that move in and out of the anode during battery charging and discharging. The maximum amount of lithium that can be stored in the anode determines the battery's capacity and limits the distance a car can travel before needing to be recharged. The capacity of a conventional lithium-ion battery with a graphite anode is approximately 370 mAh / g, which is sufficient to power a laptop but insufficient for long-distance travel. 【0008】 Among various anode materials, silicon has the highest theoretical specific capacity (approximately 4200mAhg), which is 10 times that of conventional carbon anodes. -1 ) and a sufficient potential (Li / Li) for lithium insertion and deinsertion. + It is attracting considerable attention because it is <0.5V. 【0009】 Unfortunately, the practical application of Si anodes is currently hindered by several challenges. The main drawbacks are the large volume change (~300%) during complete lithiation and the resulting expansion / contraction stresses during lithiation / desilitonization, which can cause serious cracking of the Si. This leads to the formation of an unstable solid electrolyte interface (SEI) on the Si surface, trapping lithium in the active Si material, resulting in irreversible rapid capacity loss and low initial Coulomb efficiency (CE). This causes problems with cycle life and also results in electrode swelling, which should be kept below approximately 20% in commercial cells. 【0010】 Furthermore, the slow lithium diffusion rate in Si (10 -14 and 10 -13 cm 2 s -1(diffusion coefficient between) and the low intrinsic electrical conductivity of Si (10 -5 to 10 -3 Scm -1 ) also greatly affect the rate characteristics and the overall capacity utilization of the Si electrode. 【0011】 To improve the cycle life, the use of nano-sized silicon has been shown to result in an acceptable cycle life as the strain during expansion may be absorbed. However, this increases the surface area, leading to a significant reaction with the electrolyte and a decrease in the first-cycle efficiency. Nano-sized silicon may also be somewhat expensive. 【0012】 Silicon nanostructured materials, including nanotubes, nanowires, nanorods, nanosheets, porous and hollow or encapsulated Si particles with protective coatings, have been dedicated to achieving improved structural and electrical performance. 【0013】 On the other hand, the preparation methods of these nanostructures (e.g., vapor-liquid-solid method, magnetron sputtering, and chemical vapor deposition) are generally complex techniques and involve multiple steps. Graphite and porous carbon have relatively small volume changes during the lithiation-delithiation process (e.g., ~10.6% for graphite) and are potential anode materials with excellent cycle stability and electronic conductivity. Carbon materials have properties similar to silicon and can be closely bonded to each other, so they are naturally selected as the substrate material (i.e., dispersion carrier) for dispersing silicon particles. Therefore, silicon-carbon composite anodes have been widely studied due to their higher capacity, better electronic conductivity, and cycle stability. However, it is necessary to overcome the problems of silicon-carbon anode materials such as low initial discharge efficiency, low conductivity, and low cycle performance. 【0014】 Previous research [Li, X., et al. "Mesoporous silicon sponge as a micronization prevention structure for high-performance lithium-ion battery anodes," Nature Communications, 5:4105, 2014] prevented this volume expansion by dividing the silicon anode into many small nanoparticles embedded in another material, giving them space to expand. However, this solution only introduces more problems. The small Si nanoparticles that solve the expansion problem are vulnerable to irreversible reactions with the liquid electrolyte (known as the solid-electrolyte interface) that penetrates the anode. These reactions inhibit silicon's ability to take up lithium ions, reducing the overall battery life. Furthermore, the small particles have low conductivity, reducing the battery's ability to supply enough current to power automobiles and other devices. To date, there is no anode design that can limit volume expansion while preventing undesirable side effects such as electrolyte interactions and low conductivity. 【0015】 Recently, a joint study between the University of Waterloo and General Motors has developed a novel method for protecting tiny silicon particles from electrolytes while maintaining their conductivity. This method creates a structural scaffold around the silicon nanoparticles, allowing for lithium ion intercalation but preventing electrolyte intrusion. The design combines three different materials: Si nanoparticles, graphite sheets in which some carbon atoms are replaced with sulfur (sulfur-doped graphene), and an organic polymer known as polyacrylonitrile (PAN). After all components are mixed together, the silicon nanoparticles tend to covalently bond to the sulfur moieties of the graphite. This strong interaction naturally forms a network of silicon particles bonded to the intermittent sulfur moieties between the graphite layers. 【0016】 When the mixture is slowly heated to approximately 450°C, a structural framework of PAN is formed around and between the graphite layers. The ability of PAN to permeate the entire graphene-Si structure shields the Si nanoparticles from the electrolyte while simultaneously providing a dense network of molecules through which electrons can move. Thus, this anode design solves both the electrolyte and conductivity problems seen in previous anode designs. At the same time, the Si nanoparticles adhere well to the sulfur-doped graphene sheet, providing ample space for expansion between the graphite layers during lithium intercalation. 【0017】 The object of the present invention is to overcome or improve upon at least one of the drawbacks of the prior art, or to provide a useful alternative. 【0018】 The present invention relates to porous carbon / silicon composite particles for addressing one or more of the many problems related to silicon in general. 【0019】 An object of a particularly preferred embodiment of the present invention is to provide composite particles and a method for producing them that improve voids and silicon-to-carbon ratio while being sealable with a coating of appropriate thickness. 【0020】 Although the present invention is described with reference to specific examples, it will be understood by those skilled in the art that the present invention can be embodied in many other forms. 【0021】 definition In describing and defining the present invention, the following terms will be used in accordance with the definitions set forth below. It should also be understood that the terms used herein are solely for the purpose of describing specific embodiments of the present invention and are not intended to limit them. 【0022】 Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art in which this invention pertains. 【0023】 Unless the context clearly requires otherwise, throughout the specification and claims, words such as "comprise", "comprising", etc. should be construed in an inclusive sense as opposed to an exclusive or exhaustive sense. That is, it means "including but not limited to". 【0024】 As used herein, the phrase "consisting of" excludes elements, steps, or components not specified in the claim. When the phrase "consists of" (or its variations) appears in a clause of the body of the claim rather than immediately following the preamble, it limits only the elements specified in that clause, and other elements are not excluded from the claim as a whole. As used in this specification, the expression "consisting essentially of" limits the scope of the claim to those that, in addition to the specified elements or method steps, do not substantially affect the basis and novel features of the claimed subject matter. 【0025】 With respect to the terms "comprising", "consisting of", and "consisting essentially of", when one of these three terms is used in this specification, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of "comprising" may be replaced with "consisting of" or "consisting essentially of". 【0026】 Unless otherwise indicated or except in the case of operating examples, all numerical values representing amounts of ingredients or reaction conditions used in this specification are to be understood as being modified in all instances by the term "about", taking into account the normal tolerances in the art. The operating examples are not intended to limit the scope of the invention. Hereinafter, unless otherwise specified, "%" means "% by weight", "ratio" means "weight ratio", and "parts" means "parts by weight". 【0027】 As used herein, the term "substantially", unless otherwise indicated, shall mean including more than 50% when relevant. 【0028】 The recitation of numerical ranges using endpoints includes all numbers within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). 【0029】 The terms "preferred" and "preferably" refer to embodiments of the invention that can provide certain benefits under certain circumstances. However, in the same or other circumstances, other embodiments may also be preferred. Furthermore, the description of one or more preferred embodiments does not mean that other embodiments are not useful, nor is it intended to exclude other embodiments from the scope of the invention. 【0030】 It should also be noted that, as used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. 【0031】 Those skilled in the art will understand that the embodiments described herein are merely illustrative and that the electrical characteristics of this application can be configured in various alternative arrangements without departing from the spirit or scope of the invention. 【0032】 While exemplary embodiments of the disclosed technology are described in detail herein, it should be understood that other embodiments are contemplated. Therefore, the disclosed technology is not intended to be limited in scope to the details of the arrangement of structures and components shown in the following description or drawings. Other embodiments of the disclosed technology are possible and can be implemented or performed in a variety of ways. [Overview of the Initiative] 【0033】 The applicant has surprisingly discovered that certain composite properties can be achieved using silicon nanoparticles, various forms of carbon, and carbon coatings. Such composite properties make the resulting coated Si:C nanoparticles suitable for use in lithium-ion batteries. 【0034】 Furthermore, the applicant has surprisingly discovered a method for manufacturing the composite, which generally consists of low-cost processes. 【0035】 A preferred embodiment of the present invention comprises low-cost silicon and various amounts of carbon allotropes optimized to achieve a cost- and performance-advantageous combination in the resulting LIB. 【0036】 According to a first aspect of the present invention, a silicon-carbon composite is provided which contains nanoscale silicon and carbon in a weight ratio between about 30:70 and about 70:30, and which has a volume fraction of voids between about 20 and about 70%. 【0037】 In one embodiment, the weight ratio of nanoscale silicon to carbon is approximately 60:40. 【0038】 In one embodiment, the volume fraction of the void is approximately 50%. 【0039】 In one embodiment, the volume fraction of the voids is approximately twice the volume fraction of silicon. 【0040】 In one embodiment, the voids in the composite can accommodate swelling of up to approximately 300% during the lithium-desilitonization process. 【0041】 In one embodiment, the carbon is carbon in the form of fibers such as carbon nanotubes (CNTs), and / or thin nanoplates such as graphene, graphene oxide, or reduced graphene oxide, or a combination thereof. 【0042】 In one embodiment, the complex further comprises carbon produced by the thermal decomposition of polymer precursors such as sugars including glucose, sucrose, and fructose. 【0043】 In one embodiment, the composite is sealed with a carbon coating of appropriate thickness. 【0044】 In one embodiment, the coating reduces the available (effective) surface area of ​​the Si:C particles by between approximately 50% and approximately 80%. 【0045】 In one embodiment, the coating has a thickness of less than approximately 500 nm. 【0046】 In one embodiment, the composite is intended for use as an anode in a lithium-ion battery. 【0047】 According to a second aspect of the present invention, an anode for a lithium-ion battery is provided which includes a silicon-carbon composite according to the first aspect of the present invention. 【0048】 According to a third aspect of the present invention, a half-cell for a lithium-ion battery is provided, comprising the anode, binder, and conductive additive according to the second aspect of the present invention in a composite:binder:conductive additive weight ratio of approximately 8:1:1. 【0049】 In one embodiment, the binder is carboxymethylcellulose (CMC) / styrene-butadiene rubber (SBR), and the conductive additive is Imerys C45 carbon black. 【0050】 In one embodiment, the counter electrode is made of lithium metal. 【0051】 According to a fourth aspect of the present invention, a lithium-ion battery is provided comprising an anode, cathode, electrolyte, and separator according to a second aspect of the present invention. 【0052】 According to a fifth aspect of the present invention, a method for producing a silicon-carbon composite containing nanoscale silicon and carbon is provided, the method comprising the following steps: 【0053】 (a) A step of preparing a dispersion of silicon nanoparticles and a selected form of carbon; 【0054】 (b) A step of spray-drying the dispersion to form essentially spherical micrometer-sized composite particles; 【0055】 (c) A step of heat-treating the composite particles to thermally decompose and / or burn the polymer and strengthen the composite particles; 【0056】 (d) A step of coating the composite particles with carbon to form a Si:C composite; and 【0057】 (e) Optionally, adding additional elements such as lithium, magnesium, nitrogen, and halogen gases to the composite during the heating step (c) or the coating step (d), or during a subsequent heat treatment step. 【0058】 According to a sixth aspect of the present invention, a method for fabricating a silicon-carbon composite comprising nanoscale silicon and carbon is provided, the method comprising the following steps: 【0059】 (a) A step of preparing a dispersion of silicon nanoparticles by grinding in water and holding the mixture of silicon and water; 【0060】 (b) optionally, preparing a separate dispersion of a selected form of carbon in water, which optionally contains one or more surfactants; 【0061】 (c) Adding the carbon dispersion and any surfactant mixture (or carbon in an undispersed form) to the silicon-water dispersion; 【0062】 (d) A step of dispersing the obtained mixture; 【0063】 (e) A step of spray-drying the obtained dispersed Si:C mixture to form essentially spherical particles; 【0064】 (f) A step of heat-treating the essentially spherical particles to thermally decompose and / or burn the polymer and strengthen the spherical Si:C particles; 【0065】 (g) A step of coating the heat-treated spherical Si:C particles with carbon using a chemical vapor deposition method to form a carbon-coated Si:C composite; and 【0066】 (h) Optionally, adding additional elements such as lithium, magnesium, nitrogen, and halogen gases to the carbon-coated Si:C composite during the mixing step (c) or the dispersion step (d), or during the subsequent heat treatment. 【0067】 In embodiments of the fifth or sixth aspect, the selected form of carbon includes carbon nanotubes (CNTs) and / or thin nanoplates, such as graphene or graphene oxide or reduced graphene oxide, and combinations thereof. 【0068】 In embodiments of the fifth or sixth aspect, the weight ratio of nanoscale silicon to carbon is approximately 60:40. 【0069】 In the sixth embodiment, the surfactant is nonionic. 【0070】 In embodiments of the fifth or sixth aspect, the carbon further includes carbon produced by the thermal decomposition of polymer precursors such as sugars including glucose, sucrose, and fructose. 【0071】 In one embodiment of the fifth or sixth aspect, the volume fraction of voids in the particles before coating is approximately 50%. 【0072】 In one embodiment of the fifth or sixth aspect, the volume fraction of the voids is approximately twice the volume fraction of silicon. 【0073】 In embodiments of the fifth or sixth aspect, the composite is sealed with a carbon coating of appropriate thickness. 【0074】 In one embodiment of the fifth or sixth aspect, the coating reduces the available (effective) surface area of ​​the Si:C particles by about 50 to about 80%. 【0075】 In embodiments of the fifth or sixth aspect, the coating thickness is less than approximately 500 nm. 【0076】 According to a seventh aspect of the present invention, a silicon-carbon composite containing nanoscale silicon and carbon is provided when produced by a process according to a fifth aspect of the present invention. 【0077】 According to an eighth aspect of the present invention, a carbon-coated silicon-carbon composite containing nanoscale silicon and carbon is provided when manufactured by a process according to the sixth aspect of the present invention. 【0078】 According to a ninth aspect of the present invention, an anode for a lithium-ion battery is provided, comprising a silicon-carbon composite according to a seventh aspect of the present invention or a carbon-coated silicon-carbon composite according to an eighth aspect of the present invention. 【0079】 According to a tenth aspect of the present invention, a half-cell for a lithium-ion battery is provided, comprising an anode, binder, and conductive additive according to a ninth aspect of the present invention in a composite:binder:conductive additive weight ratio of approximately 8:1:1. 【0080】 According to an eleventh aspect of the present invention, a lithium-ion battery is provided which includes an anode, cathode, electrolyte, and separator according to a ninth aspect of the present invention. 【0081】 According to a twelfth aspect of the present invention, silicon-carbon composite particles are provided which contain at least 40% silicon relative to carbon and have at least 50% pores, wherein the carbon is composed of graphene and carbon nanotubes, and the amount of graphene relative to the total amount of graphene and carbon nanotubes is at least 40%. 【0082】 According to a thirteenth aspect of the present invention, a silicon-carbon composite material is provided which contains at least 50% pores and in which the amount of silicon in the material exceeds 90%. [Brief explanation of the drawing] 【0083】 Herein, preferred embodiments of the present invention will be described merely as examples with reference to the accompanying drawings. 【0084】 [Figure 1] Figure 1 shows scanning electron microscope (SEM) images of uncoated particles of the composite of the present invention. 8.0 kV; scale 1 μm (a) and 100 nm (b). Figure 1 shows scanning electron microscope images of the particles. The porous carbon network (1) contains very well dispersed silicon nanoparticles (2). That is, most of the nanoparticles are not in contact with each other. 【0085】 [Figure 2]Figure 2 shows scanning electron microscope (SEM) images of uncoated and coated particles of the composite material of the present invention. 8.0 kV; scale 1 μm (a) and (b). Figure 2 shows scanning electron microscope images of particles before and after coating. The coating encapsulates at least 90% of the surface. In similar experiments, it was found that the coating reduced the surface area from ~100 m² / g to approximately 5 m² / g, demonstrating that Si nanoparticles are effectively encapsulated by the coating. 【0086】 [Figure 3] Figure 3 shows the pore size distribution of particles from Example 1 before coating. The pore size distribution, measured by mercury porosimetry, was 56%, and the majority of the pores were less than approximately 200 nm in size. 【0087】 Detailed description of preferred embodiments Porous particles containing silicon and carbon are attractive as desirable anode materials because their pores can absorb silicon swelling internally, potentially reducing the swelling of the electrode itself. High levels of voids are desirable because they allow for the incorporation of higher levels of silicon while still accommodating swelling. 【0088】 Carbon can play several roles in the Si:C composite of the present invention. Firstly, it can separate the silicon particles so that they do not collide with each other during swelling. The carbon network can also add strength and elasticity to the composite particles and provide a strong network for the conduction of electrons and lithium ions. However, the weight and volume capacity of carbon are much smaller than that of silicon. Therefore, it is desirable to have a small amount of carbon while allowing the carbon network to perform its various functions. 【0089】 Carbon nanotubes are a good potential source of carbon because their extremely small diameter allows them to provide networks with a very low volume fraction of carbon. Similarly, graphene and / or graphite nanoplatelets are very thin and can also generate networks with low volume fractions. 【0090】 Commercially relevant products require a high level of voids. In some embodiments, the volume fraction of voids (Vf) is approximately 2 relative to Vf silicon, allowing for internal expansion of the silicon. In other embodiments, the ratio of Vf void to Vf silicon is approximately 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, or approximately 4.0. 【0091】 Preferably, the amount of carbon should be minimized while providing a sufficiently strong conductive network. 【0092】 Finally, highly porous structures should be sealable with a sufficiently thin coating so that the coating does not significantly reduce their weight and volume. By sealing the particles, the liquid electrolyte cannot directly access the silicon surface, minimizing the silicon-electrolyte reaction. However, highly porous structures are generally not expected to be suitable for coating. Furthermore, coating relies heavily on the nucleation and growth of the coating, i.e., the surface structure. In nanoscale materials, such structures can be extremely difficult, if not impossible, to predict with respect to the outcome of the coating process. 【0093】 The inventors have discovered that a suitable Si:C composite can be prepared using a method comprising the following steps: 【0094】 (a) A step of preparing a dispersion of silicon nanoparticles and a selected form of carbon; 【0095】 (b) A step of spray-drying the dispersion to form essentially spherical composite particles; 【0096】 (c) A step of heat-treating the silicon nanoparticles to thermally decompose and / or burn the polymer and strengthen the composite particles; 【0097】 (d) A step of coating composite particles with carbon to form a Si:C composite; and 【0098】 (e) Optionally, during the heating step (c) or the coating step (d), or during a subsequent heat treatment step, additional elements that can enhance the first cycle efficiency and / or extend the cycle life. Examples include lithium, magnesium, and nitrogen, and may further include halogen gases. 【0099】 A preferred embodiment of the method of the present invention includes the following steps: 【0100】 (a) A step of preparing a dispersion of silicon nanoparticles by grinding in water and holding the mixture of silicon and water; 【0101】 (b) optionally, preparing a separate dispersion of a selected form of carbon in water, optionally containing one or more surfactants; 【0102】 (c) Adding a carbon dispersion and an arbitrary surfactant mixture (or carbon in an undispersed form) to the silicon-water dispersion; 【0103】 (d) A step of dispersing the obtained mixture; 【0104】 (e) A step of spray-drying the resulting dispersed Si:C mixture to form essentially spherical particles; 【0105】 (f) A step of heat-treating the essentially spherical particles to thermally decompose and / or burn the polymer, thereby strengthening the spherical Si:C particles; 【0106】 (g) A step of coating heat-treated spherical Si:C particles with carbon using a chemical vapor deposition process to form a carbon-coated Si:C composite; and 【0107】 (h) Optionally, during the mixing step (c) or the dispersion step (d), or during the subsequent heat treatment, adding additional elements to the carbon-coated Si:C composite that can increase the first cycle efficiency and / or increase the cycle life. Examples include lithium, magnesium, nitrogen, and halogen gases. 【0108】 In this embodiment, costs are reduced compared to current state-of-the-art techniques by (i) grinding in water instead of organic solvents, and (ii) avoiding drying of silicon nanoparticles. 【0109】 In preferred embodiments, the porous Si:C composite has a high level of voids, which allows for the incorporation of a high level of silicon while still accommodating swelling during complete lithiation and the resulting expansion / contraction stresses during lithiation / desilitonization. 【0110】 Preferably, the volume fraction of voids in the particles is greater than 30%, greater than 40%, greater than 50%, or about 60%. In other embodiments, the volume fraction of voids is greater than approximately 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69 or about 70%. 【0111】 In some embodiments, the volume fraction of voids is about twice the volume fraction of silicon. In other embodiments, the volume fraction of voids is about 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, or about 3.5 times the volume fraction of silicon. 【0112】 The voids in the Si:C composite can withstand swelling up to approximately 300%. In one embodiment, swelling is up to approximately 350, 340, 330, 320, 310, 300, 290, 280, 270, 260, 250, 240, 230, 220, 210, 200, 190, 180, 170, 160, 150, 140, 130, 120, 110, or approximately 100%. 【0113】 In a preferred embodiment, the ratio of silicon to carbon is maximized while achieving the preferred volume fraction of voids described above. Silicon has much higher gravimetric and volumetric capacity than carbon. Therefore, it is desirable to maximize the ratio of silicon to carbon for both gravimetric and volumetric capacity. 【0114】 The ratio of silicon to carbon is a key feature of this invention. In one embodiment, the ratio may be at least 40:60, or at least 50:50, or at least 60:40, or at least 70:30 on a weight basis. Various forms of carbon mixtures can provide the desired performance and cost. In other embodiments, the silicon-to-carbon ratio is approximately 30:70, 31:69, 32:68, 33:67, 34:66, 35:65, 36:64; 37:63, 38:62, 39:61, 40:60, 41:59, 42:58, 43:57, 44:56, 45:55, 46:54; 47:53, 48:52, 49:51, 50:50, 51:49, 52:48, 53:47, 54:46, 55:45, 56:44; 57:43, 58:42, 59:41, 60:40, 61:39, 62:38, The ratios are 63:37, 64:36, 65:35, 66:34; 67:33, 68:32, 69:31, or approximately 70:30 w / w. Most preferably, the silicon-to-carbon ratio is approximately 60:40 w / w. 【0115】 In preferred embodiments, carbon may be provided in the form of fibrous carbon, such as carbon nanotubes (CNTs). Small-diameter CNTs have the advantage of providing a mechanically stable framework with a low volume fraction of carbon. Very thin nanoplates, such as graphene, graphene oxide, or reduced graphene oxide, also help in realizing a framework with a low volume fraction of carbon. In other embodiments, carbon may be a mixture of carbon forms, for example, CNTs interspersed with graphene platelets. 【0116】 In some embodiments, the carbon network can be improved by a small amount of carbon produced by the thermal decomposition of a polymer precursor. Examples of polymer precursors include sugars such as glucose, sucrose, and fructose, and pitch. Such materials can improve the connectivity of the carbon network and provide elastic and / or improved Li-ion conductivity and / or improved electronic conductivity. 【0117】 The amount of carbon thus produced may be less than 20%, less than 10%, or less than 5% of the weight of the uncoated composite. In other embodiments, the amount of carbon thus produced may be less than approximately 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%, 22%, 21%, 20%, 19%, 18%, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or about 1% of the weight of the uncoated composite. 【0118】 In a preferred embodiment, particles having the above attributes of voids, silicon-to-carbon ratio, and carbon type / ratio may be essentially sealed with a coating of appropriate thickness. By essentially sealed, the applicant means that the coating reduces the available (effective) surface area of ​​the Si:C particles by at least 50%, preferably at least 80%. In other embodiments, the coating reduces the available (effective) surface area of ​​the Si:C particles by at least about 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, or at least about 90%. 【0119】 The coating may have a thickness of less than approximately 500 nm, or less than approximately 400 nm, or less than approximately 300 nm, or less than approximately 200 nm. In preferred embodiments, the coating may have a thickness of less than approximately 600, 580, 560, 540, 520, 500, 480, 460, 440, 420, 400, 380, 360, 340, 320, 300, 280, 260, 240, 220, 200, 180, 160, 140, 120, or less than approximately 100 nm. It will be understood that the coatings will vary in thickness, and therefore the cited thicknesses are the average thickness across a selection of coated Si:C nanoparticles. Larger particles may result in thicker coatings because they have a smaller relative volume fraction. However, larger particles may result in reduced rate performance. It can be understood that particle size and coating thickness vary and may be optimized for different applications. In one embodiment, the coating thickness is approximately the same as the spacing between particles in the composite. 【0120】 Lithium ions enter the Si:C composite by solid-state diffusion. In one embodiment, an additive such as glucose and / or sucrose enables solid-state diffusion of lithium ions into the Si:C composite. 【0121】 In a preferred embodiment, the composite utilizes a low-cost form of silicon. In a preferred embodiment, the silicon is in the form of angled nanoparticles manufactured using a grinding process. In another preferred embodiment, the silicon nanoparticles are ground in water, and the silicon nanoparticles have oxides formed on their surface. Current state-of-the-art processes prefer silicon with a minimal oxide layer. However, the applicant has surprisingly found that good performance can also be achieved using oxidized or partially oxidized silicon nanoparticles. 【0122】 In some embodiments, the oxide layer may be modified by introducing elements such as lithium and / or magnesium and / or nitrogen. These layers may improve lithium ion diffusion and may also react with the oxide, thereby reducing reactions with the electrolyte during initial charging and discharging, and thus increasing the first cycle efficiency. 【0123】 In the method of the present invention, the dispersion may be spray-dried to form particles with a diameter of approximately 10 μm. In other embodiments, the dispersion may be spray-dried to form particles with a diameter of approximately 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or approximately 25 μm. This diameter may be modified using known spray drying parameters to achieve the desired particle size. As will be understood by those skilled in the art, the particle diameter can be adjusted to give different performance in terms of energy density and power. 【0124】 The method of the present invention may optionally utilize a step to passivate active sites in the electrolyte within the cell. Such sites may reduce the first cycle efficiency and cycle life. Examples of such steps include high-temperature treatment, introduction of halogen gas during high-temperature treatment, and introduction of lithium via evaporation of lithium metal during a pyrolysis or chemical vapor deposition (CVD) process. 【0125】 Example 1 Silicon nanoparticles were produced by grinding silicon particles in an aqueous medium using a high-speed ball mill. Carbon nanotubes were dispersed in water using a suitable surfactant, such as a nonionic surfactant. Then, the silicon nanoparticle / water mixture, the carbon nanotube / water mixture, and glucose were dispersed in an aqueous solution using a suitable surfactant. The mixture was then spray-dried to obtain particles with an average size of approximately 18 μm in diameter. These particles were then thermally decomposed in a reducing H2 / Ar atmosphere at approximately 850°C. The thermal decomposition of the surfactant and glucose yielded the following properties. 【0126】 The ratio of silicon to carbon nanotubes was approximately 60:40. 【0127】 The voids measured by mercury porosimetry accounted for 56%, and the size of most pores was less than approximately 200 nm (see Figure 3). 【0128】 Figure 1 shows a scanning electron microscope image of the particles. The porous carbon network (1) contains well-distributed silicon nanoparticles (2), i.e., most of the nanoparticles are not in contact with each other. 【0129】 Carbon coatings were applied to particles using fluidized bed chemical vapor deposition (CVD) and propane gas at approximately 1000°C in argon with a 32% propane ratio relative to a 5% H2 carrier gas. Scanning electron microscopy showed that the coating thickness ranged from approximately 200 nm to approximately 300 nm. 【0130】 Figure 2 shows scanning electron microscope images of particles before (a) and after (b) coating. It can be seen that the coating seals at least 90% of the surface. In similar experiments, the coating seals a surface area of ​​~100 m². 2 From / g to approximately 5m 2 It was found that the amount was reduced to / g, indicating that the particles were effectively encapsulated by the coating. 【0131】 Half-cells were fabricated using a composite material and a carboxymethylcellulose (CMC) / styrene-butadiene rubber (SBR) binder, with Imerys C45 carbon black as a conductive additive. The ratio of composite material, binder, and C45 was 8:1:1. Lithium metal was used as the counter electrode. This composite material provided a capacity of ~750 mAh / g and a first-cycle efficiency of approximately 80%. 【0132】 Comparative Example 1 The procedure of Example 1 was used, but glucose was not added. This complex yielded only a capacity of ~240 mAh / g with a first-cycle efficiency of ~70%. The applicant hypothesizes that the capacity was reduced because, in the absence of glucose, lithium ions could not properly diffuse through the carbon solid. 【0133】 Comparative Example 2 The procedure of Example 1 was used, but no coating was applied. The capacity was ~1000mAh / g. However, the first cycle efficiency was only ~60%. This indicates that coating was necessary to give a reasonable first cycle efficiency. 【0134】 Although the present invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that the present invention can be implemented in many other forms.

Claims

[Claim 1] It contains nanoscale silicon and nanoscale carbon in weight ratios between 30:70 and 70:30, and has a volume fraction of voids between 20 and 70%. The volume fraction of the voids is twice that of the silicon volume fraction. It further contains carbon produced by the thermal decomposition of glucose, It is sealed with a carbon coating with a thickness between 100 nm and 500 nm. Silicon-carbon composite. [Claim 2] The composite according to claim 1, wherein the weight ratio of nanoscale silicon to nanoscale carbon is 60:

40. [Claim 3] The composite according to claim 1 or claim 2, wherein the volume fraction of the void is 50%. [Claim 4] The composite according to any one of claims 1 to 3, wherein the voids in the composite are able to accommodate swelling of up to 300% during the lithium-desilitonization process. [Claim 5] The composite according to any one of claims 1 to 4, wherein the nanoscale carbon is in the form of fibers, thin nanoplates, or a combination thereof. [Claim 6] The composite according to claim 1, wherein the coating reduces the available (effective) surface area of ​​the Si:C particles between 50 and 80%. [Claim 7] A composite according to any one of claims 1 to 6 for use as an anode in a lithium-ion battery. [Claim 8] An anode for a lithium-ion battery comprising a silicon-carbon composite according to any one of claims 1 to 6. [Claim 9] A half-cell for a lithium-ion battery comprising the anode, binder, and conductive additive according to claim 8 in a weight ratio of composite:binder:conductive additive of 8:1:

1. [Claim 10] The half-cell according to claim 9, having a counter electrode made of lithium metal. [Claim 11] A lithium-ion battery comprising the anode, cathode, electrolyte, and separator according to claim 8. [Claim 12] A method for producing a silicon-carbon composite according to claim 1, the method comprising the following steps: (a) A step of preparing dispersions of nanoscale silicon and nanoscale carbon; (b) A step of spray-drying the dispersion to form essentially spherical micrometer-sized composite particles; (c) A step of heat-treating the composite particles; and (d) A step of coating the composite particles with carbon to form a Si:C composite. [Claim 13] A method for producing a silicon-carbon composite according to claim 1, the method comprising the following steps: (a) A step of preparing a nanoscale silicon dispersion by grinding in water and holding a mixture of silicon and water; (b) A step of preparing separate dispersions of nanoscale carbon containing one or more surfactants in water; (c) A step of adding the carbon dispersion and surfactant mixture to the silicon-water dispersion to form a mixture; (d) A step of dispersing the mixture formed in step (c); (e) A step of spray-drying the obtained dispersed Si:C mixture to form essentially spherical particles; (f) A step of heat-treating the essentially spherical particles; (g) A step of coating the heat-treated spherical Si:C particles with carbon using a chemical vapor deposition method to form a carbon-coated Si:C composite; and (h) Optionally, adding an element selected from lithium, magnesium, nitrogen, and halogen gases to the carbon-coated Si:C composite during the mixing step (c) or the dispersion step (d), or during the subsequent heat treatment step. [Claim 14] The method according to claim 12 or claim 13, wherein the nanoscale carbon includes carbon nanotubes (CNTs), thin nanoplates, and combinations thereof. [Claim 15] The method according to any one of claims 12 to 14, wherein the weight ratio of nanoscale silicon to nanoscale carbon is 60:

40. [Claim 16] The method according to claim 13, wherein the surfactant is acidic. [Claim 17] The method according to any one of claims 12 to 16, wherein the volume fraction of voids in the silicon-carbon composite is 50%. [Claim 18] The method according to any one of claims 12 to 17, wherein the coating reduces the available (effective) surface area of ​​the Si:C particles between 50 and 80%.

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