Method for manufacturing silicon-based electrode material

Abstract

The present invention relates to a method for producing a silicon-carbon composite material. The composite material can be used as an anode active material for silicon-based lithium-ion batteries, or can be further processed into such an active material. When used for lithium storage, the composite material is characterized by a particularly high specific capacity and a particularly long life span related to charge and discharge cycles.

Description

Technical Field

[0001] The present invention relates to silicon-carbon composite materials (Si / C composites) and methods for producing them. The composites can be used as active materials for negative electrodes in silicon-based lithium-ion batteries, or can be further processed into such active materials. In the case of lithium storage, the composites are characterized by a particularly high specific capacity and a long life span associated with charge-discharge cycles.

[0002] The provided method enables the production of cost-effective active materials for storing lithium ions in industrial-scale lithium-ion batteries. In existing lithium-ion battery production plants, this material can serve as a "drop-in replacement" for prior art materials, such as graphite. Because the material can reduce the production cost of lithium-ion batteries while increasing the battery's volumetric and gravimetric energy density, it can be used to store electrical energy in all known applications, with particular benefit in mobile applications such as electric vehicles or any type of portable electronic device.

[0003] The present invention aims to provide novel materials and production methods for lithium-ion battery anodes, significantly reducing the cost of lithium-ion batteries while simultaneously increasing the energy stored per unit weight or volume. In theory, silicon is well-suited as anode material. However, silicon undergoes chemical and mechanical changes during battery cell operation. Consequently, after repeated charging and discharging of the battery cell, the silicon's ability to absorb lithium decreases.

[0004] Traditionally, lithium ions in lithium-ion batteries bind to graphite during charging. In this manner, each gram of graphite in the battery can store a maximum of 372 mAh of charge. In recent years, battery manufacturers, in their search for new materials to increase battery energy density, have focused on replacing graphite with silicon. Silicon opens the possibility of a lithium ion-to-silicon mass ratio exceeding tenfold. In this case, the theoretical limit for the specific gravimetric capacity of the active material is approximately 4200 mAh / 1 gram of silicon. While this value can be roughly achieved in practice, the usable capacity decreases significantly after just a few cycles due to the significant volume expansion of the silicon-lithium alloy as lithium ions incorporate into the silicon structure (Zhang L et al.: Silicon-Containing Precursors for Silicon-Based Anode Materials for Lithium-Ion Batteries: A Review, Energy Storage Materials 4 (2016), pp. 92-102). This process further mechanically fragments the silicon particles. Developing silicon alloys with aluminum-based metallurgy has indeed significantly improved these material properties, but material degradation remains relatively significant. Background Art

[0005] In the method disclosed in US 2015 / 0295233 A1, sucrose is thermally decomposed to coat silicon particles with carbon. The material produced in this way is suitable for lithium-ion batteries. In this case, carbon particles are mixed with the starting mixture. In addition, carboxylic acid can be added. A very high proportion of graphite particles are used in this method, and the coating method is a single step. The discharge capacity that can be achieved by the composite material is less than 500 mAh / g. Li Y et al.: Growing compliant graphene cages on micron-sized silicon particles as stable battery anodes (Nature Energy 1, 15029 (2016)) addresses the problem of collapse of silicon microparticles caused by lithium adsorption. To solve this problem, the silicon microparticles are surrounded by graphene cages, and the graphene cages have pores to accommodate the expansion of the microparticles. In this case, the collapse of the microparticles cannot be avoided, but the fragments can be retained in the cage.

[0006] One of the key criteria for the function of lithium-ion batteries is the formation of a suitable passivation layer on the surface of the negative electrode active material. If silicon is used as a host to store lithium ions without further processing, an unsuitable layer will form on the silicon surface, which is mainly composed of lithium silicate. Due to the volume expansion when lithium is incorporated, the silicon partially breaks down, and new silicon surfaces will continue to appear, allowing new unsuitable layers to be generated on the new silicon surfaces during subsequent charging cycles. The unsuitable layer that continues to grow during each charging process consumes lithium, and the consumed lithium is no longer active for charge carrier transport and consumes energy. The number of charge carriers available in the battery decreases, and the transport of lithium ions into the silicon particles is hindered. Therefore, the storage capacity of the battery decreases with the number of cycles until it becomes unusable. This situation must be avoided during the expected life of the battery (the expected number of cycles), so that the battery always has a minimum charge capacity. Summary of the Invention

[0007] The method provided by the present invention covers the silicon particles with a suitable carbon coating to protect the silicon particles before they are used in batteries, thereby minimizing the gradual formation of an unsuitable passivation layer that gradually removes the lithium required for charge carrier transport.

[0008] If a suitable carbon coating is chosen, direct chemical reactions of the electrolyte with the silicon can be avoided. Instead, the so-called SEI (solid electrolyte interface) layer is formed only on the surface of the carbon coating that is in direct contact with the electrolyte. Therefore, the amount of boundary layer (initial growth) is very limited, similar to the prior art of graphite-based anode materials, so as to implement stable conditions for charge carrier transport through these layers without further increasing the internal resistance of the battery as the number of cycles increases. After only a few cycles, relatively stable conditions appear, in which the electrolyte does not decompose further and does not continue to consume large amounts of lithium to form a continuously growing passivation layer (the passivation layer can no longer be used for the storage capacity of the battery). Compared with the SEI layer of silicon particles not coated with carbon, favorable conditions for battery cycle stability can be achieved through a suitable coating. In this way, an ideal passivation layer can be generated once on the surface of the carbon coating, which is conducive to increasing the battery life. This stable SEI layer is known in the prior art of graphite anodes and is mainly composed of lithium carbonate, lithium methyl carbonate, and lithium ethylene dicarbonate (Decomposition reaction of anode solid electrolyte interface (SEI) components with LiPF6, J. Phys Chem. C 2017, pp 22733-22738).

[0009] Suitable electrolyte additives known in the art can be selected to stabilize the SEI boundary layer between the carbon and the electrolyte. Simultaneously, the carbon coating of the silicon particles maintains the electrical conductivity between the individual composite particles of the material over time, preventing it from being continuously degraded by continued SEI growth, thereby improving the energy efficiency of the resulting battery. It is even possible to omit conductive additives from the battery electrodes, further increasing the battery's overall energy density. Lithium ions can penetrate a carbon layer composed at least partially of structured carbon, such as graphene or graphene-like compounds, thus enabling the operation of the battery cell while protecting the silicon from chemical attack.

[0010] Therefore, the present invention aims to provide improved materials for lithium-ion battery anodes, thereby achieving longer-lasting and more efficient batteries. A further object is to provide a simple and particularly low-cost method for producing silicon-carbon composite materials for batteries. These objects are achieved by the subject matter of the present invention.

[0011] One of the important criteria for the function of lithium-ion battery cells is the formation of a suitable passivation layer (such as a solid electrolyte interface, SEI) on the surface of the negative electrode active material. If silicon is used as the main body for storing lithium ions without further measures, layers (including lithium silicate and other reaction products) will form on it that are detrimental to the battery function and interact with the electrolyte. Due to the significant volume expansion when lithium combines with silicon, the silicon particles will break or form fragments in them. The formation of these unfavorable layers will further lead to the continuous consumption of silicon and lithium during each charging process to grow these unwanted layers. The ratio of the number of available carriers to the active material in the battery cell is therefore reduced. The growth of unfavorable layers will further inhibit the transport of lithium ions into the silicon particles and back to the cathode side, and significantly reduce the electronic conduction between the particles. As a result, the internal resistance of the battery may become increasingly higher.

[0012] Because the silicon particles are coated and protected with a suitable carbon layer before use in battery electrodes, the method provided by the present invention can particularly avoid the extensive formation of undesirable passivation layers on the silicon surface. Therefore, ideally, the passivation layer can be formed on the carbon surface all at once when the battery is first charged. The passivation layer improves cycling durability or maintains battery capacity over a large number of cycles, similar to the effect of using graphite instead of silicon. The carbon coating maintains electrical conductivity between the individual particles of the material, extending battery life and ensuring charge carrier transport between the battery electrodes over a significantly greater number of charge and discharge cycles. As a result, batteries produced from composite materials of carbon-coated silicon particles have significantly improved energy efficiency compared to batteries produced from silicon particles alone. Consequently, battery cells can be charged significantly faster. The carbon layer (coating) can particularly consist, at least in part, of structured carbon, such as graphene or graphene-like structures, which may have a scale-like arrangement on the silicon surface. The carbon layer is permeable to lithium ions, thereby enabling battery cell operation and protecting the silicon from chemical attack. In this way, a composite material having one or more silicon particles embedded in the matrix of the carbon material can be produced.

[0013] The specific size of the active surface further impacts the usability of the composite material in a battery, as it also determines the amount of passivation layer formed, a key factor in the battery's Coulombic efficiency. The term "active surface" as used herein refers to the surface of the composite particle that interacts with the battery cell's electrolyte. This size can be determined by BET measurements (e.g., adsorption / desorption characteristics). In the present method, process parameters can be controlled to influence the specific size of the active surface. Those skilled in the art will appreciate that the specific surface area of a body is the surface area of the body divided by its mass. Thus, the specific active surface area of the resulting composite particle can be lower than the specific surface area of the silicon particles contained within the composite particle in its initial state. This allows for the use of silicon particles that are small enough to avoid rupture during lithium absorption, while the larger specific surface area (as measured by BET) of the small particles within the composite material does not negatively impact the battery cell's Coulombic efficiency. In one embodiment, the specific surface area of the composite material is no greater than twice the specific surface area of the silicon particles within the composite material, particularly less than 50% greater than the specific surface area of the silicon particles, and even more particularly less than the specific surface area of the silicon particles.

[0014] One of the preferred features of the present invention is that the active silicon surface of the composite particles in the battery cell that is exchanged with the electrolyte is reduced by at least a factor of 10 compared to the case where no carbon coating is applied around the silicon.

[0015] The silicon particles used are preferably approximately spherical. Specifically, the ratio of the largest diameter to the smallest diameter of the particles is at most 1.5:1, preferably at most 1.3:1, and particularly preferably at most 1.2:1 or at most 1.1:1. This applies in particular to the majority of the particles, i.e., more than half of the particles, or even more than two-thirds or more than 90% of the particles.

[0016] Silicon particles are mixed with a carbon compound (preferably a carbohydrate or, in another preferred embodiment, a liquid or solid hydrocarbon), followed by controlled thermal conversion or carbonization of the carbon compound to produce a composite material. The term "thermal conversion" refers to thermally treating the carbon compound (particularly in step A) to cause one or more of the following changes: polymerization, mutarotation change, inversion, caramelization, oxidation, H2O splitting, OH group splitting, condensation reaction, intramolecular covalent bond formation, redistribution, isomerization, partial pyrolysis, and decomposition. The terms "thermal treatment" and "temperature treatment" are synonymous. The term "conversion temperature" is the lowest temperature at which the compound is converted under the conditions of the present method. Depending on the initial composition of the composite material, the temperature of the temperature treatment step A can be selected over a wide range of temperatures. After the conversion of the carbon compound in the heat treatment step A is completed, there is typically a corresponding loss of mass associated with the carbon compound. The intermediate products of the heat treatment are in different chemical and / or mechanical states for use in the second heat treatment step B. These other states also affect the initial composition after the heat treatment step A and its reactivity with (other) components within the system and tool used for the temperature conversion. The term "carbonization" refers to the production of a carbon-containing intermediate product by heat treatment, particularly the thermal conversion of the carbon compound in step B, which undergoes one or more of the following: pyrolysis, decomposition of water vapor, decomposition of OH groups, decomposition of CO, decomposition of CO2, decomposition of H2, or decomposition of hydrocarbons. Venting and / or actively removing the reaction gases generated or escaping from heat treatment step A may be beneficial during heat treatment step B. It is further advantageous if the converted components (i.e., silicon particles and at least one carbon compound) do not interact with the container or conveying means from heat treatment step A during a second heat treatment step B following heat treatment step A. It may be advantageous to transport the heat-treated intermediate product (after heat treatment step A) to another container or conveying means having different reaction characteristics than those used in heat treatment step B. Specifically, it is desirable and advantageous for the silicon-carbon composite material obtained from heat treatment step B to undergo minimal or no material reaction with objects or solids with which the silicon-carbon composite material came into contact during heat treatment step B. It is thus further avoided that reaction gases which are harmful or detrimental to these materials are generated or escape and deposited on the walls of the heating space enclosing or separating the heating space used for the thermal conversion or transport of the materials.

[0017] Thermogravimetric measurement of the generated or evolved reaction gases with downstream mass spectrometry analysis can be used to adjust the appropriate temperature range for thermal treatment steps A and B. Furthermore, temperature and thermal treatment can be studied during the method to allow for targeted adjustments to the gas atmosphere.

[0018] The process temperature in the second heat treatment step of the thermal synthesis process, as well as the process (such as grinding, deagglomeration, rolling, crushing, fragmentation, mixing) optionally performed to produce the desired particle size distribution, influence the size of the specific active surface.

[0019] In addition, the synthesis temperature can be further selected to reduce any oxides such as silicon oxide on the surface of the silicon particles in a carbothermal manner (due to the production of carbon monoxide), or another reducing atmosphere can be selected to reduce any oxides on the surface of the silicon particles. Whether necessary or not, carbides can be produced on the surface of the silicon particles. At the high synthesis temperature of 1300°C, evidence of the formation of carbides can be provided by XRD. The carbon can also take on the structure of synthetic graphite as appropriate. Another optional measure of the method of the present invention is to crush the intermediate product of the first heat treatment step into a defined particle size (or a suitable intermediate size) of the final product before the second heat treatment step (such as a high temperature process step). This has advantages when performing a high temperature process step:

[0020] - Before the high-temperature process step, the material is less hard and easier to grind; this is particularly true when carbohydrates are used as carbon sources, which often form very hard composite particle aggregates after both thermal treatments;

[0021] - Due to the predefined particle size distribution in the milling process, the subsequent high-temperature process is more reproducible and a wider selection of suitable production systems for mass production methods is available; in particular, subsequent printing or slot nozzle coating methods require a suitable initial particle size distribution in the printed paste, slurry, hot melt compound, or ink;

[0022] - In particular, when using a rotary furnace, the temperature-time curve can be better and more reproducibly controlled during the straight-through feeding process. Furthermore, by means of the first thermal treatment step A (i.e., conversion), it is possible to prevent undesirable concentrations of cracking products in the gas atmosphere, which would negatively affect the results of the high-temperature treatment. Consequently, it is also possible to prevent an increasing accumulation of residues on the furnace tube walls.

[0023] - The flushing / process gas in the high-temperature process steps can more evenly and effectively flush the surface of the pulverized particles. The fragmentation products of the thermal conversion can be better and more reproducibly extracted and transported away, and unwanted side reactions of these fragmentation products can be avoided or minimized.

[0024] The grinding process after the second heat treatment step of the method may undesirably form new open silicon surfaces and negatively affect the configuration or structure of the silicon / carbon composite particles, thereby impairing battery function. Properly pulverizing the silicon / carbon composite after step A can suppress or minimize this problem.

[0025] On the other hand, when hydrocarbons are used as carbon sources, oxidation of silicon during the heat treatment step can be minimized or suppressed, even on surface oxides that are stored therein. In addition, with the appropriate choice of hydrocarbons (e.g. paraffin wax), the formation of very hard, large, and tightly adherent particle aggregates can be avoided. Therefore, a grinding step between the first and second temperature treatments is not required. However, it may be necessary to feed the intermediate product into another container or to feed the intermediate product into a rotary kiln (e.g. as a matrix material) via a conveying process for the second temperature treatment step. In this case, when hydrocarbons are used as carbon source and dispersant, the composite matrix material is preferably and automatically deagglomerated or pulverized.

[0026] The method has the following steps:

[0027] mixing silicon particles with at least one carbon compound,

[0028] The mixture is heat treated by performing at least two steps in the following order:

[0029] A. heat-treating the mixture at a temperature corresponding to at least the conversion temperature of the carbon compound, particularly 120° C. to 700° C., preferably 120° C. to 500° C., more particularly 120° C. to 350° C., to obtain a heat-treated intermediate product;

[0030] B. Heat-treating the intermediate product at a temperature above 750°C to obtain a silicon-carbon composite material. In this case, carbonization and / or dissociation of compounds or elements from the intermediate product preferably occur, which typically escape in gaseous form and are removed by evacuation. As the temperature increases, an increasingly ordered structure is produced.

[0031] The method of the present invention is essential for performing at least two heat treatment stages. This means that the treatment is performed at at least two different temperatures, without cooling between stages. Conversely, additional heating can be performed after the first heat treatment stage A without significant cooling to allow for the second heat treatment stage B. The terms "heat treatment stage" and "heat treatment step" are used synonymously herein. It has been found that in the case of gradual heat treatment, an initial temperature above the transition temperature followed by a temperature exceeding that of the first stage can yield particularly favorable product properties in the second heat treatment stage. Furthermore, the present invention preferably performs the two heat treatment stages in separate apparatuses, systems, and / or vessels (e.g., furnaces). This allows for a continuous production process. Depending on the material composition of the starting materials, particularly the choice of carbon compound or dispersant, spatially separating the two heat treatment steps may facilitate different process atmospheres, different process pressures, and different means for extracting reaction gases generated or released during the two heat treatment steps. The steps of combining or transferring the starting materials can be further separated within the two heat treatment steps.

[0032] Specifically, when using hydrocarbons such as paraffin, they can serve as both a carbon source and a dispersant. Pre-treatment of the dispersed silicon particles is advantageous, as the interface between the dispersed silicon particles and the gaseous atmosphere is as large as possible, to facilitate the escape of generated or escaping reaction gases. This avoids significant gradients in the interaction between the synthesis product and the gaseous atmosphere, ensuring that the material's properties are substantially uniform across the height of the container, or along any gradients that occur.

[0033] It is advantageous to continuously convey the dispersed silicon particles along a temperature gradient. In this case, the dispersed silicon particles are initially applied as a thin layer to a conveying medium (e.g., a continuous conveyor belt) so that the reactant gases can be uniformly exhausted, extracted, or conveyed away from the heating system that generates the reactant gases whenever they are generated or escape.

[0034] At the end of temperature treatment step A, the intermediate product formed in this manner can be collected again, for example as a powder with a roughly suitable particle size distribution. For example, the powder can be collected in a container that can then be easily introduced into a second process chamber, which can, on the one hand, be subjected to a higher temperature treatment and, on the other hand, have a completely different process atmosphere composition, process atmosphere pressure, and alternative transport or handling concepts for the intermediate product to be further processed in temperature treatment step B.

[0035] In a preferred embodiment, a powdered intermediate product with a particle size distribution of 10 microns or less, preferably 3 microns or less, is initially collected in a container. In a second temperature step (B), the powdered intermediate product is continuously conveyed from the container into a high-temperature furnace, such as a rotary kiln, in a process atmosphere that varies in pressure, either negative or positive, relative to the ambient atmosphere. In this embodiment, the powdered intermediate product can be continuously conveyed along a temperature gradient, preferably heated to a higher process temperature or cooled. Another preferred embodiment utilizes a rotary kiln, whose rotation substantially continuously mixes the powdered intermediate product and propels the powdered intermediate product forward via the kiln's adjustable inclination. In this embodiment, the kiln is preferably only partially filled, preferably less than 50%, and even more preferably less than 30%, relative to the individual tube diameters along the entire axis of the rotary kiln. This allows for rapid escape of generated or escaping reaction gases. Furthermore, the lances in the rotary kiln can be positioned above the product to allow for different gas feed and extraction points to be located at different points along the propulsion motion. This allows for extraction of reaction gases at the point where they are generated or escaping, rather than only at higher temperatures. The material of the rotary kiln may be chosen such that the intermediate product does not come into contact with the rotary kiln during the second temperature treatment, which would adversely affect or even destroy the rotary kiln.

[0036] Furthermore, the two separate heat treatment stages A and B allow for possible intermediate processing of the heat-treated intermediate product after the first heat treatment stage, such as grinding the heat-treated intermediate product. It is advantageous if the grinding step is carried out in a separate system or apparatus, preferably after cooling.

[0037] In a preferred embodiment, the grinding step is carried out under controlled atmosphere, temperature, and extraction conditions, depending on the starting composition of the synthesis product and / or the proportion of the carbon compound and / or any other materials in the synthesis, such as lithium or lithium-containing compounds, so that the intermediate products between temperature treatment A (conversion) and temperature treatment B (high-temperature step) are always under these well-controlled conditions, and controlled or integrated transportation is performed between temperature treatment A and temperature treatment B, that is, the comminution step (such as grinding) is well controlled. However, in particular, it is possible to adopt a relatively low-cost production method in terms of equipment because there are no pressure conditions that significantly deviate from atmospheric pressure and no preferred process steps such as spray drying that require no equipment are necessary.

[0038] In one embodiment, at least step B (and optionally step A) is performed in a substantially oxygen-free atmosphere, in particular in a process gas atmosphere containing less than 100 ppmv, less than 10 ppmv, less than 1 ppmv, or less than 0.1 ppmv of oxygen. The atmosphere may be an inert gas atmosphere, in particular a nitrogen or inert gas atmosphere. However, other atmospheres are also possible, such as a reducing atmosphere, which may contain hydrogen and / or carbon monoxide. A reducing atmosphere is advantageous for reducing silicon oxide or reducing oxidation. Once silicon comes into contact with air, a silicon oxide layer forms very quickly on the silicon surface, in particular at higher process temperatures. In one embodiment, the size of the silicon oxide layer is reduced or very thin, in particular, it is substantially absent. A low-oxygen atmosphere may replace a substantially oxygen-free atmosphere, in particular, an oxygen content of less than 5 vol% or less than 1 vol%. As an alternative to or in addition to a low-oxygen or substantially oxygen-free atmosphere, a process liquid may be used that prevents silicon from coming into contact with air. In a preferred embodiment, a hydrocarbon-based process liquid, such as paraffin or paraffin oil, is added to the mixture of silicon particles and at least one carbon compound to minimize or completely prevent the dispersed solid components from contacting air and / or oxygen and / or nitrogen and / or moisture and / or other undesirable gases (such as generated or escaping reaction gases). In this embodiment, the process liquid wets the solid components of the dispersion and escapes only when the process atmosphere is changed or the temperature is increased during temperature treatment step A (conversion process), but preferably escapes completely only during temperature treatment step B. More preferably, the silicon particles and the carbon compound (and any other possible synthesis starting materials) are mixed in a low-oxygen or substantially oxygen-free atmosphere and / or in the process liquid itself.

[0039] The preferred atmosphere can be or contain nitrogen, carbon dioxide, carbon monoxide, hydrogen, an inert gas such as argon or helium, or a mixture thereof. Preferred process liquids are liquids suitable for keeping atmospheric oxygen away from the silicon surface. Particularly suitable substances are liquid at room temperature (20°C) and / or have a solubility for carbon compounds at 20°C of at least 1 g / L, in particular at least 10 g / L or at least 50 g / L. Suitable liquids are liquid at room temperature and atmospheric pressure and wet the surface of silicon and / or silicon oxide. In one embodiment, the liquid is miscible with water, i.e., forms a single liquid phase with water at room temperature. Preferred liquids dissolve the carbon compounds in the mixture, in particular completely. Preferred process liquids are water, monohydric or polyhydric alcohols such as isopropanol, ethanol, or in particular dihydric alcohols such as ethylene glycol, or mixtures thereof. Particularly preferred process liquids are based on paraffin wax. The process liquid used has an evaporation temperature above the transition temperature of the carbon compound. For example, if carbohydrates such as sugars form the main carbon source, a corresponding selected liquid hydrocarbon can be used. The process liquid is preferably one that allows good wetting of the silicon particles with carbon compounds or converted carbon compounds. Since water favors the oxidation of silicon, a preferred embodiment does not use water. A person skilled in the art can select a suitable process liquid. In one embodiment, no liquid is added to produce a mixture. The mixture thus contains silicon and at least one carbon compound, such as a hydrocarbon such as paraffin, toluene, or the like. In one embodiment, a dispersant can be used, which does not evaporate or does not evaporate completely at the end of the first treatment step (step A). If the dispersant does not evaporate completely, the intermediate product of the heat treatment can be comminuted at low cost, and the paraffin or other suitable hydrocarbon can further exclude atmospheric influences.

[0040] In another preferred embodiment, paraffin wax is used as the process liquid, which is solid at room temperature and becomes liquid at moderate temperatures (preferably 30°C to 90°C), and the components are precisely dispersed at these temperatures so as to solidify again after dispersion (preferably in the absence of oxygen). In this way, the dispersed object can contain lithium or lithium-containing starting materials without reacting with oxygen, nitrogen, and / or water vapor or moisture. After the dispersion cools and solidifies, it can be transported in air and the risk of atmospheric reaction with lithium-containing compounds is effectively avoided. In particular, highly exothermic reactions of lithium or lithium-containing starting materials with their potential fire consequences can be suppressed and avoided.

[0041] In a preferred embodiment, at least 10 wt % (particularly at least 20 wt %) of the dispersant is still present after the first heat treatment step. The dispersant preferably has a boiling point at atmospheric pressure greater than 120° C., particularly greater than 150° C., greater than 160° C., or greater than 180° C. The process liquid and the dispersant may be the same or different. In one embodiment, the intermediate product produced after step A no longer contains at least 90 wt %, particularly at least 95 wt %, or at least 99 wt % of the dispersant and / or process liquid, particularly if the dispersant and / or process liquid has evaporated or reacted.

[0042] Preferably, cooling is performed in a low-oxygen or substantially oxygen-free atmosphere (such as an inert gas atmosphere, particularly a nitrogen or inert gas atmosphere or a reducing atmosphere) after step B and / or after step A. Other preferred embodiments may use a reducing atmosphere (such as a hydrogen or carbon monoxide atmosphere).

[0043] In one embodiment, at least one additive is added to the mixture. Suitable additives include structural and / or catalytic additives, in particular selected from graphene, graphene oxide, graphite, fullerenes, nanotubes, and combinations thereof. Suitable catalytic additives may be used instead or in addition, such as various iron compounds or other catalytic additives known to those skilled in the art. The proportion of the additive in the mixture may be 0.01 to 10.0 wt %, in particular 0.05 to 5.0 wt % or 0.1 to 2.5 wt %. In the case of adding such additives, the time of the first and / or second heat treatment step and / or the temperature of the first and / or second heat treatment step may be reduced. The method may also be carried out without the use of additives.

[0044] In a preferred embodiment, the mixture of starting materials may contain the following components:

[0045] silicon 1.5 to 99.0 wt% Carbon compounds 1.0 to 50.0 wt% dispersants 0.0 to 90.0 wt% additive 0.0 to 10.0 wt%

[0046] In one embodiment, the dispersant is used in a mixture, and the mixture contains the following ingredients:

[0047]

[0048]

[0049] In other embodiments, the mixture contains a dispersant, and the mixture contains the following ingredients:

[0050] silicon 5.0 to 35.0 wt% Carbon compounds 15.0 to 40.0 wt% dispersants 40.0 to 70.0 wt% additive 0.0 to 5.0 wt%

[0051] In another embodiment, no dispersant is used at all or only a very small amount is used in the mixture, and the mixture contains the following ingredients:

[0052] silicon 50.0 to 90.0 wt% Carbon compounds 10.0 to 50.0 wt% dispersants 0.0 to 5.0 wt% additive 0.0 to 10.0 wt%

[0053] In another embodiment, the mixture has a small amount of dispersant or no dispersant and contains the following ingredients:

[0054] silicon 60.0 to 90.0 wt.-% Carbon compounds 10.0 to 40.0 wt% dispersants 0.0 to 2.0 wt% additive 0.0 to 5.0 wt%

[0055] In another embodiment, no dispersant or only a very small amount of dispersant is used in the mixture, and the mixture contains the following ingredients:

[0056] silicon 50.0 to 70.0 wt% Carbon compounds 30.0 to 50.0 wt% dispersants 0.0 to 5.0 wt% additive 0.0 to 10.0 wt%

[0057] In another embodiment, a small amount of dispersant or no dispersant is used in the mixture, and the mixture contains the following ingredients:

[0058]

[0059]

[0060] In another embodiment, a small amount of dispersant or no dispersant is used in the mixture, and the mixture contains the following ingredients:

[0061] silicon 70.0 to 90.0 wt% Carbon compounds 10.0 to 30.0 wt% dispersants 0.0 to 2.0 wt% additive 0.0 to 5.0 wt%

[0062] In another embodiment, other liquid or solid carbon compounds from the group of hydrocarbons, in particular paraffin waxes, are used in the mixture, which can simultaneously act as dispersants at room temperature or at least slightly elevated temperature.

[0063] silicon 50.0 to 99.0 wt.-% Carbon compounds = dispersants 1.0 to 50.0 wt.-%

[0064] In a preferred embodiment of the other embodiment, the mixture of starting materials may contain the following components, wherein paraffin wax is used as the carbon compound and the dispersant.

[0065] silicon 9.0 to 33.0 wt% Carbon compounds = paraffin 67.0 to 91.0 wt%

[0066] In a preferred embodiment of the other embodiment, the mixture of starting materials may also include sucrose in addition to silicon and paraffin.

[0067] silicon 9.0 to 33.0 wt% paraffin 67.0 to 91.0 wt% sucrose 0.9 to 33.0 wt%

[0068] In a preferred embodiment of the other embodiment, the mixture of starting materials may include a suitable lithium compound in addition to silicon and paraffin, wherein the material ratio of silicon atoms to lithium atoms is between 1:0.5 and 1:5.0.

[0069] silicon 9.0 to 33.0 wt% paraffin 67.0 to 91.0 wt%

[0070] Compared to the prior art, the mixture employed has a relatively high solids content. This refers to the proportion of solids that remains after evaporation of the dispersant and / or process liquid. Specifically, this refers to the combined proportions of silicon, carbon compound, and any additives used. The solids content, relative to the mass of the mixture, can be at least 9.0 wt%, at least 16.5 wt%, or at least 20.0 wt%. In variations with low levels of dispersant or no dispersant, the solids content can be significantly higher. In mixtures containing dispersant, the solids content is preferably at most 70.0 wt% or at most 60.0 wt%. In a preferred embodiment, the solids content is as high as 90 wt%. In cases where the solids content is too high, achieving a uniform distribution of the carbon compound on the silicon becomes more difficult. If the dispersant content is too high, excessive time and energy are required to remove the dispersant. Furthermore, to minimize costs and potentially harmful emissions, high dispersant content should be avoided. In this case, there is a risk of further oxidation on the surface of the silicon particles. Because this method does not rely on spray drying, a higher solids content can be achieved, reducing energy requirements, equipment costs, and solvent usage. It is therefore preferred not to spray dry the mixture, and in particular to use a production method that does not require spray drying and uses highly viscous dispersions that are completely unsuitable for spray drying. In this case, the proportion of dispersant is reduced to such an extent that the starting materials can still be easily dispersed, but the generated or escaping reaction gases can escape, while at the same time minimizing the costs of the dispersant and any post-processing.

[0071] Since the dispersant also ensures that the carbon compound is distributed as evenly as possible on the silicon, the ratio of these two components is important. The mass ratio of carbon compound to dispersant can be from 0.1 to 0.7, in particular from 0.1 to 0.4, or from 0.3 to 0.7. These ratios have proven to be advantageous. The goal of one embodiment is to keep the proportion of dispersant as low as possible while still achieving a sufficiently uniform dispersion. In a preferred embodiment, a high viscosity of the mixture of this example, such as greater than 5000 mPa·s, in particular greater than 15000 mPa·s or greater than 25000 mPa·s, meets the requirements for dispersion purposes. In one embodiment, the viscosity is not higher than 50000 mPa·s. In particular, the viscosity decreases with increasing shear rate (shear thinning properties). A small amount of dispersant can positively affect the cost and environmental friendliness of the method and avoid unwanted parasitic oxidation of particles during the oxygen elimination of the dispersant or the degassing of the degassing products during the heat treatment stage. The viscosity can be determined at 21.5° C. using a rotational viscometer (plate / plate with a gap width of 0.3 mm and counter-rotating motion, shear rate of 100 / s).

[0072] It has been discovered that silicon-carbon composite materials, when used as or in lithium-ion battery anode materials, can exhibit outstanding properties, particularly in terms of efficiency during the first operating cycle (first cycle efficiency), and can utilize non-toxic and environmentally friendly binders and solvents. In one embodiment, this advantage may be related to the fact that the interface between silicon and carbon is substantially free of silicon dioxide, or contains only a thin layer of silicon dioxide.

[0073] In XPS measurements (X-ray Photoelectron Spectroscopy), it was shown that silicon carbide is not located on the surface ( Figure 10 XPS results further revealed that the SiO2 particles on the surface were functionalized with graphite. However, the surface SiO2 particles were not completely functionalized with graphite. This finding is related to the fact that the thickness of the graphite layer is likely less than 3 nm, thus allowing the detection of specific areas of the Si surface that were initially oxidized but still had a thin oxide layer.

[0074] When using materials in battery cells, the formation of lithium silicate is not conducive to Li + The diffusion characteristics of ions are improved, and reducing the SiO2 content in this area can inhibit the formation of lithium silicate. In addition, the present invention particularly provides a simple method for preparing the silicon-carbon composite material.

[0075] The silicon used as the starting material is related to silicon particles. Porous or porous silicon particles known to those skilled in the art can also be used as silicon particles. The silicon can be amorphous silicon or crystalline silicon, particularly polycrystalline silicon. Silicon with a particle size D90 of less than 300 nm or less than 200 nm can be used.

[0076] Silicon having at least a portion of its surface composed of silicon dioxide can serve as the starting material for the method. Silicon particles, which have formed an oxide layer on their surface due to contact with an oxidizing environment, are particularly contemplated. The method may optionally include a step of removing silicon dioxide from the silicon surface. This step can be performed, for example, by grinding, plasma treatment, and / or etching. A reducing atmosphere may alternatively or additionally be used for this purpose.

[0077] Acid or base etching is preferably used to remove the silicon dioxide. Preferred materials include HF, KOH, NH₄F, NH₄HF₂, LiPF₆, H₃PO₄, XeF₂, SF₆, and mixtures thereof. HF is particularly preferred. The acid or base may be mixed with additives that impart structural or catalytic properties (e.g., metal-assisted etching). In one embodiment, a plasma treatment is used to remove the oxide layer. In one embodiment, etching is used during the thermal treatment, particularly during step A.

[0078] The silicon employed herein is preferably elemental silicon, particularly in the form of silicon particles. The silicon particles may optionally further have other substances, particularly other metals, oxides, carbides, or dopants (particularly phosphorus, boron, gallium, or aluminum which increase the conductivity of silicon), preferably in small amounts such as <10 wt%, and particularly preferably <1.0 wt%. In one embodiment, the silicon particles are composed of elemental silicon, silicon oxide, or binary, ternary or multi-component silicon / metal alloys (such as Li, Na, K, Sn, Ca, Co, Ni, Cu, Cr, Ti, Al, Fe). In a preferred embodiment, Si alloyed with lithium x Li y particles can be produced from Si particles at a lower temperature, and the formation step preferably excludes oxygen, nitrogen, and water vapor, such as being produced in a paraffin dispersion. Si x Li y particles are preferably produced during temperature treatment step A. In this example, the proportion of Li relative to Si x Li y in the alloy is preferably up to 35 wt%, and even more preferably between 10 wt% and 30 wt%.

[0079] The silicon preferably has at most a small proportion of contaminants, particularly less than 10 wt% and particularly advantageously less than 1.0 wt% of contaminants (such as B, P, As, Ga, Fe, Al, Ca, Cu, Zr, C). Phosphorus, boron, aluminum, tin, antimony, and / or gallium can be added for the purpose of increasing the conductivity of the Si particles. In a preferred embodiment, the silicon has a typical dopant concentration of 10 15 to 10 21 dopant atoms per cubic centimeter. Doped silicon is advantageous for specific applications. In a method of a preferred embodiment, the corresponding dopant portion has been added to the mixture of silicon particles and at least one carbon compound during temperature treatment A or alternatively during temperature treatment B. In terms of an economical method, adding aluminum is particularly advantageous because doped Al-Si particles can be produced at a temperature above 577 °C (i.e., the eutectic temperature) or above 660 °C (the melting point of Al).

[0080] If the silicon particles contain silicon oxide, the stoichiometry of the oxide SiO x is preferably 0 < x < 1.3. If the silicon particles contain silicon oxide with a higher oxygen stoichiometry (such as x = 2), the layer thickness on its surface is preferably less than 10 nm.

[0081] In the example of an alloy of silicon particles and a metal M (such as an alkali metal), the stoichiometry of the alloy M y Si can be 0 < y < 5. The silicon particles can be alloyed with lithium. In this example, the alloy Li zThe stoichiometry of Si is preferably 0 < z < 2.2. However, in another preferred embodiment, a Li z z Si alloy with 2 ≤ z ≤ 4.3 is used.

[0082] In a preferred embodiment, alloying of silicon and lithium can also be carried out during temperature treatment steps A and B. One of the key points to note is the addition of a lithium source before the individual temperature treatment steps of the synthesis, and how the lithium source interacts with the individual process atmosphere, dispersant, or other synthesis components. For example, if pure lithium is added to the synthesis or dispersion, it should be strictly ensured that lithium does not react with oxygen, nitrogen, or even water vapor. For example, this can be avoided by using paraffin oil as a dispersant to wet and treat lithium. Alternatively or additionally, at least step B (and possibly step A) can be carried out in an atmosphere that is substantially free of oxygen, nitrogen, and water vapor, or in an atmosphere where the content of O2, N2, or H2O is less than 100 ppmv, preferably less than 10 ppmv, more preferably less than 1 ppmv, or less than 0.1 ppmv, and...

[0083] In order to obtain alloy particle sizes in the micron or sub - micron range after the temperature treatment in steps A and B, at least one alloy component should be present in a finely dispersed manner, which has a small particle size in the starting state (i.e., before the temperature treatment), ideally two alloy starting materials. Depending on the melting point of the lithium source and the dispersant used, it may be advantageous to first carry out carbon coating of the silicon particles in temperature treatment step A and only add the lithium source in temperature treatment step B. A very large number of lithium starting materials can be used as the lithium source for this alloy synthesis. In addition to lithium itself, lithium salts (such as lithium halides, especially lithium bromide), lithium hydride, lithium hexafluorophosphate, lithium stearate, lithium nitride, lithium amide, lithium carbide, and lithium soaps are particularly suitable.

[0084] In a preferred embodiment, the silicon particles consist of at least 90 wt% (especially ferrosilicon), preferably at least 95 wt%, more preferably 98 wt% of silicon (especially metallurgical silicon) relative to the total weight of the silicon particles. The silicon particles are preferably substantially carbon - free.

[0085] In one embodiment, the surface of the silicon particles can have Si - OH - or Si - H - groups, or covalently bonded organic groups such as alcohols or alkenes. Thus, the dispersant or liquid carbon compound can be specifically influenced during the synthesis. <s

[0086] In addition, in certain cases, ALD (atomic layer deposition), PVD (sputtering, evaporation deposition), CVD (chemical vapor deposition), or PECVD (plasma assisted CVD) methods are used to form a coating, which may and is beneficial for suppressing the formation of a solid electrolyte boundary layer and / or improving lithium transport properties. The coating may include aluminum oxide, titanium oxide, zirconium oxide, silicon carbide, and / or other carbon-containing (also organic) coatings or lithium-containing coatings. These coatings can be formed before and after the heat treatment step, or in the last step of adjusting the particle size distribution, or even on the anode surface after the coating is completed. In the case of powder-type particles, a fluidized bed ALD method is preferably used. In this case, a suitable fluidized bed reactor can be constructed to minimize the phenomenon of particle escape.

[0087] Furthermore, particles or liquids of a lithium-containing component or lithium itself may be added to the mixture or intermediate product prior to step A and / or step B in order to produce a lithium-silicon alloy during the temperature treatment. Suitable components may be selected from lithium-containing salts (e.g., LiF, LiCl, LiBr, LiI, Li3N, LiNH2, LiPF6, or Li2CO3), lithium hydrides (e.g., LiH, LiBH4, LiAlH4), organolithium compounds (e.g., n-butyllithium, tert-butyllithium, methyllithium, phenyllithium, lithium diisopropylamide, lithium bis(trimethylsilyl)amide); lithium soaps, and combinations thereof. Specifically, in one embodiment, the lithium-containing component may be added to the composite material produced prior to the third temperature treatment only after the second temperature treatment step B has concluded or has concluded. This requires a well-controlled gas atmosphere, particularly one that is substantially oxygen-free, nitrogen-free, and water-free or water vapor-free, during and after the first two temperature treatment steps A and B and any other temperature treatment steps that may be performed.

[0088] After the temperature treatment, the carbon-coated lithium-containing synthetic particles may be protected from unwanted reactions with the atmosphere or binders. Such protection may be achieved by further processing and / or storage in a protective atmosphere or inert gas atmosphere, in a vacuum, or in a dispersion with a suitable liquid or binder to inhibit unwanted reactions.

[0089] In one embodiment, the method of the present invention provides the possibility of producing a silicon-carbon composite material having a silicon dioxide (SiO2) layer of reduced thickness between the silicon and the carbon. Before the mixture with the carbon compound is produced, the silicon particles can be pretreated in HF or another fluorine compound to remove silicon oxide. The silicon particles can then be directly mixed with a liquid carbon compound (such as paraffin) or a dispersant for heat treatment. This method can be carried out in an atmosphere that excludes air, is low in oxygen, or is essentially oxygen-free. In a preferred embodiment, before heat treating the mixture or mixing the silicon particles with at least one carbide, the silicon particles are moved to a dispersion of hydrofluoric acid to remove oxides on their surface, after which the silicon particles can be transferred to a dispersion containing liquid paraffin in a suitable container. As expected, complete phase separation occurs between the HF and the silicon particles dispersed in the paraffin. The HF can then be separated, for example, by a filter.

[0090] The carbon compound is suitable for forming a carbon-containing coating on a silicon surface, in particular a coating containing structured carbon or even consisting of structured carbon. The compound is characterized in that it forms carbon, in particular at least partially structured carbon, when subjected to the heat treatment described herein. Preferred carbon compounds according to the present invention are carbohydrates, in particular sugars, and mixtures of different carbohydrates or hydrocarbons that are solid or liquid at room temperature. In a preferred embodiment, the carbon compound is selected from monosaccharides, disaccharides, polysaccharides, and mixtures thereof. Preferred sugars used as carbon compounds according to the present invention are glucose, fructose, galactose, sucrose, maltose, lactose, starch, cellulose, glycogen, or mixtures or polymers thereof.

[0091] Other biopolymers, such as lignin, can be used as alternative or additional carbon compounds, thereby significantly avoiding crude oil-based products. The carbon compound is preferably a non-polymer plastic. Renewable raw materials with low material costs and no environmental impact are preferably used as the carbon compound in the required / necessary amounts. For example, the carbon compound is preferably selected from waxes, vegetable oils, fats, oils, fatty acids, rubbers, and resins.

[0092] In a preferred embodiment of the method, the carbon compound may alternatively or additionally comprise at least one carbon compound selected from lignin, wax, vegetable oil, fat, oil, fatty acid, rubber, and resin. This facilitates biocompatibility, avoids environmental damage, and minimizes environmental impact.

[0093] In another preferred embodiment, the carbon compound is paraffin or a related hydrocarbon. The combination of silicon particles and paraffin as the carbon compound is advantageous because it can significantly shorten and simplify the process, i.e., a grinding step as an intermediate step is no longer required.

[0094] The term "structured carbon" is known to those skilled in the art and specifically includes graphene, graphene oxide, carbon nanotubes, fullerenes, fumed graphite, "hard carbon," and graphite.

[0095] The amount of carbon compound is preferably selected so that the mass ratio of carbon to silicon in the composite material is from 3:1 to 1:90, particularly from 3:1 to 1:20, more preferably from 1.2:1 to 1:10, and particularly preferably from approximately 1:5 to approximately 1:9. The coating on the silicon preferably comprises more than one carbon layer, particularly at least two carbon layers, at least three carbon layers, or at least five carbon layers. The present invention preferably provides that the mass ratio of the carbon compound in the mixture is from 5% to 110% by weight of the silicon, or from 1000% to 200% by weight of the silicon. In a preferred embodiment, the mass ratio in the mixture is from 25% to 80% by weight of the silicon, particularly from 35% to 70%, and particularly preferably from 40% to 60%. Selecting the appropriate amount of carbon compound helps to achieve the desired configuration of the silicon-carbon composite material.

[0096] In a preferred embodiment, a liquid hydrocarbon such as paraffin is used as the sole carbon source and at the same time as a dispersant. The mass ratio of the carbon compound in the mixture is between 1000% and 100% of the mass of the silicon. For example, 15 mL of paraffin is mixed with 3 g of Si. The specified range of liquid hydrocarbon is necessary to ensure good dispersion of the silicon particles, and a high proportion of liquid hydrocarbon will escape during heat treatment. It is preferred to choose a carbon source ratio that is sufficient to easily disperse the silicon particles therein. Other materials can also be added to the mixture here. When a hydrocarbon such as paraffin is used, it can be a compound containing lithium or elemental lithium. Due to the use of paraffin, air can be excluded during the mixing and dispersion process, and unwanted reactions of lithium compounds with nitrogen, oxygen, and / or moisture, or water vapor can be avoided.

[0097] In a preferred embodiment, paraffin wax serves as both the sole carbon source and the dispersant, with the paraffin wax to silicon ratio ranging from 1:1 to 10:1, preferably from 3:1 to 6:1. The silicon-carbon composite material produced after heat treatment steps A and B typically has a silicon mass ratio greater than 80%, preferably greater than 90%, more preferably greater than 95%, and most preferably greater than 99%. In other preferred embodiments, a lithium compound or lithium may be additionally used in the initial synthesis, with the ratio typically selected such that the resulting composite material has a lithium to silicon atomic ratio of, for example, 0.5:1 to 4:1, preferably 1:1 to 3:1. Using a hydrocarbon such as paraffin wax as both the carbon source and the dispersant is advantageous because the heat treatment is separated into two separate temperature treatment steps, A and B, and the intermediate cooling and grinding steps after the first temperature treatment can be omitted. The two temperature treatment steps, A and B, need only be separated spatially and / or temporally to remove substances generated and released during heat treatment step A, rather than adversely affecting the atmosphere of the second heat treatment step, B. A further advantage is that the treatment times of the separate heat treatment steps can be significantly shortened.

[0098] In another preferred embodiment, a carbohydrate (such as sucrose) and paraffin wax are used simultaneously, with the latter serving as a dispersant and carbon source, to produce a silicon-carbon composite material while excluding air. The purpose of adding a suitable small amount of paraffin wax is to disperse the viscous dispersion, which is not suitable for spray coating.

[0099] In a preferred embodiment, the mixing step comprises contacting the silicon surface or silicon oxide surface with the carbon compound. The mixing may comprise dispersing the silicon particles in a dispersant / process liquid. Specifically, the steps comprise preparing a dispersion, a carbon compound, silicon, and a dispersant. In this case, the silicon contacts the dispersion, in particular applying the dispersion to the silicon to integrate the silicon into the dispersion. The step of integrating the silicon into the dispersant may be performed together with, before, or after the carbon compound. In a preferred configuration of the method of the present invention, the dispersion comprises a carbon compound and silicon in addition to the dispersant. The silicon may in particular be a plurality of silicon particles. This makes the method particularly simple and cost-effective. In a preferred embodiment, the carbon compound is dissolved in the dispersant and additives such as structured carbon and silicon particles are dispersed therein. In one configuration of the method, the contacting occurs before the step of optionally grinding the mixture, and the dispersant preferably serves as a grinding medium and protective liquid during grinding.

[0100] An advantage of using the above-described dispersion is that the dispersant protects the silicon surface from atmospheric oxygen and other oxidizing environments. The dispersant in this context preferably serves as a protective liquid. The dispersant further ensures that the carbon compound is evenly distributed on the silicon. In a preferred embodiment, the dispersant can be partially or completely removed during heat treatment step A, and the carbon compound precipitates on the silicon surface and is at least partially converted. Advantageously, the above-described heat treatment step B can then be performed to convert the carbon compound precipitated on the silicon surface into a carbonaceous coating. Specifically, this converts the essential components of the carbon compound other than carbon and silicon. If a lithium source is used in the starting compound, components other than lithium are substantially removed from the resulting composite, potentially forming a structured carbon compound that is in close contact with the silicon particle surface, or a similar structured Si-Li-C alloy. The term "substantially removed" may be understood to mean that the proportion of components in the composite material that are not carbon or silicon, and may be lithium, is at most 15 wt%, at most 10.0 wt%, at most 5.0 wt%, at most 3.0 wt%, or at most 1.0 wt%.

[0101] Silicon may advantageously be the body, the particles, or a majority of the particles, the surface of which at least partially (especially at least 90% or at least 95%, especially substantially completely) consists of silicon and / or silicon oxide. The body or particles preferably consist of silicon.

[0102] In particular, the silicon may have a particle size D90 of less than 500 nm or less than 300 nm. In one embodiment, the particle size D90 is at least 50 nm. The particle size can be measured by dynamic light diffusion or REM. In the case of spherical particles, the particle size corresponds to the diameter of the particle. In this case, D90 refers to the particle size distribution in which 90% of the particles have a particle size smaller than or equal to D90. Other D values should be understood in a similar way. If the D value relates to a mass distribution, D90 means that 90% of the mass of all particles consists of particles smaller than or equal to D90. Other D values should be understood by analogy. Unless otherwise stated, the D value relates to the distribution of the number of particles.

[0103] Heat treatment

[0104] The temperature in step A is higher than the transformation temperature of the carbide, in particular, at least 5°C, at least 10°C, or at least 20°C higher than the transformation temperature of the carbide. If a mixture of different compounds is used as the carbon compound, the temperature is particularly higher than the transformation temperature of the compound with the highest transformation temperature. Specifically, the temperature in step A is higher than the temperature in step B. The temperature in step A can be between 120°C and 700°C, preferably between 120°C and 500°C, even more preferably between 120°C and 350°C, and can be between 150°C and 250°C. In one embodiment, the temperature in step A is between 175°C and 200°C and / or higher than 180°C. The temperature during the heat treatment need not be constant at a specific temperature and can also vary temporarily around the set value (based on planning or technical deviations). However, in the context of the present invention, the heat treatment at least provides for a certain time (in particular, the time specified herein) during which the mixture in step A is exposed to the temperature within the specified limits. For example, this step can be carried out in a furnace. It is not excluded that the heat treatment according to step A is initially carried out under the specified temperature conditions for a first period and then for a second period, as long as the temperatures and times described herein are fully complied with. Preferably, the heat treatment of step A is carried out in one step, i.e., the temperature does not fall below the minimum temperature during step A.

[0105] The heat treatment pressure of step A is preferably 95 kPa to 110 kPa, in particular atmospheric pressure. In another embodiment, it can be carried out at an elevated pressure, in particular an overpressure compared to atmospheric pressure, such as exceeding 5 kPa or exceeding 15 kPa. If the operation is carried out in a low-oxygen or substantially oxygen-free atmosphere, the overpressure helps to maintain the ambient atmosphere outside the furnace used. The elevated pressure also affects the enthalpy, so elevated pressure can save energy. In this embodiment, the overpressure compared to the ambient pressure needs to be 10 kPa to 1000 kPa. In other embodiments, step A is carried out in the form of hydrothermal carbonization. This example particularly uses a high process pressure, in particular above 0.5 MPa. With proper process management, the energy required for process step A can be reduced.

[0106] In another preferred embodiment, a negative pressure or even a vacuum relative to the ambient atmosphere is desired. This requires hermetically sealing the furnace interior from the ambient atmosphere. This has the advantage that, even if degassing occurs during the conversion, decomposition, or carbonization process, the process atmosphere within the furnace can be largely oxygen-free or low in oxygen. Degassing products resulting from the breakdown of the initial carbon compounds can thus be immediately extracted and discharged into the atmosphere surrounding the process material. The pressure generally preferably ranges from 0.01 kPa to 95 kPa relative to ambient pressure. In one embodiment, the negative pressure relative to ambient pressure is at least -5 kPa or at least -15 kPa. Negative pressure can also be used to replace the protective atmosphere, depending on the situation.

[0107] In one embodiment, the specified temperature in step A is maintained for at least 1 minute or at least 5 minutes, in particular for 5 to 1000 minutes. Heat treatment step A preferably lasts for at least 15 minutes, in particular at least 25 minutes, or at least 1 hour, or at least 2 hours, and particularly preferably at least 5 hours or at least 12 hours. A minimum time is recommended to ensure partial or complete removal of the liquid or substantial conversion of the carbon compounds. Heat treatment step A can be terminated after these processes are completed. According to the present invention, it is preferably terminated at the latest after 20 hours, in particular at the latest after 10 hours, and preferably at the latest after 6 hours or at the latest after 2 hours.

[0108] The heat treatment according to step A can be used to prepare for the thermal decomposition of the carbon compound. Specifically, during the corresponding heat treatment, any solvents present, such as dispersants, grinding media, and / or process liquids, will partially or completely evaporate, and the carbohydrate or alternative or additional carbon compound used as the carbon compound will be at least partially converted. Since the loading of the process atmosphere with escaping substances can vary significantly locally during carbon source conversion and / or solvent or dispersant escape, proper drainage of these substances from the furnace interior or temperature treatment system should be ensured, and condensation of the exhaust reaction gases in the outflowing gas stream due to cooler surfaces and their subsequent descent or refluxing into the interior of the thermal system should be avoided. This further prevents the formation of condensate from blocking or damaging the gas outflow channels.

[0109] In a preferred embodiment of the method, a dispersed (starting) mixture of silicon particles and at least one carbon compound is prepared. The method includes a subsequent step of applying the mixture over its entire surface and / or in a thin layer to a conveyor belt or other suitable transport medium. This facilitates the rapid entry of escaping reaction gases into the process atmosphere during the subsequent heat treatment of the mixture, without a large amount of the (starting) mixture having previously flowed through. In another preferred embodiment of the method, the method includes an additional step of conveying the mixture during the heat treatment, in particular through one or more heat treatment systems. This allows the reaction gases escaping along the temperature-time profile of the heat treatment A to be extracted locally in a spatially separated manner and independently of the temperature increase, resulting in a different atmosphere composition at the subsequent higher temperature than during the previous, lower temperature passage through the space. This is particularly advantageous if water vapor or oxygen-containing compounds are generated, as they can be extracted at relatively low temperatures and no longer oxidize silicon or lithium at high temperatures.

[0110] The temperature in step B can be from >750°C to 2600°C. Specifically, the temperature in step B is higher than the temperature in step A. In one embodiment, the temperature in step B is limited to at most 2000°C, or at most 1800°C, or at most 1400°C. The temperature in step B can be at least 800°C, at least 1000°C, greater than 1000°C, or at least 1050°C. In one embodiment, the temperature is from 1000°C to 1600°C, and can be from 1050°C to 1500°C. The temperature can be below the melting point of the silicon particles, in particular below the melting point of pure silicon. The temperature during the heat treatment process does not have to be constant at a specific temperature, but can also assume other values or temporarily vary around the set value (based on planning or technical deviations). In the context of the present invention, the heat treatment provides at least a certain time (in particular, the time described herein) for exposing the intermediate product of the heat treatment in step B to an ambient temperature within the stated limits. In a preferred embodiment, the temperature of step B is 800° C. to 1200° C., more preferably 800° C. to 1100° C. In a particularly preferred embodiment, the temperature of step B can be adjusted so that substantially no silicon carbide is formed. This is particularly advantageous if paraffin wax or paraffin oil is used as the carbon source.

[0111] The heat treatment can be carried out as appropriate, with the heating rate selected in a targeted manner to the target temperature required for the individual steps, so that the volatile components are released before the target temperature is reached. A preferred average heating ramp rate is between 1 K / min and 100 K / min, more preferably between 2 K / min and 20 K / min, and even more preferably between 3 K / min and 15 K / min. For example, the heat treatment can be carried out in a furnace. The maximum temperature of step B is preferably greater than the maximum temperature of step A. It is not excluded that the heat treatment according to step B is initially carried out under the specified temperature conditions for the first period and then for the second period, as long as the temperatures and times specified herein are fully complied with. However, it is preferred that the heat treatment according to step B be carried out in one step, i.e., the minimum temperature is not dropped during step B.

[0112] The heat treatment pressure in step B is 95 kPa to 110 kPa, particularly atmospheric pressure.

[0113] In another embodiment, the steps may be performed at elevated pressure, particularly at an overpressure exceeding 5 Pa relative to ambient pressure. If operating in a low-oxygen or substantially oxygen-free atmosphere, the overpressure helps maintain the ambient atmosphere outside the furnace being used. Elevated pressure also affects enthalpy, thus saving energy. In this embodiment, an overpressure of 10 kPa to 1000 kPa relative to the atmosphere surrounding the thermal processing apparatus is desired.

[0114] In another preferred embodiment, a negative pressure or even a vacuum is required relative to the ambient atmosphere. This has the advantage of maintaining a substantially oxygen-free or low-oxygen atmosphere within the furnace during the conversion, decomposition, or carbonization process, as opposed to requiring a hermetically sealed furnace interior. Consequently, outgassing products resulting from the decomposition of the original carbon compounds can be extracted and removed directly from the atmosphere surrounding the process material. Typical pressures preferably range from 0.01 kPa to 95 kPa absolute. In one embodiment, the vacuum is at least -5 kPa or at least -15 kPa relative to ambient pressure.

[0115] In a preferred embodiment, the temperature in step B can be maintained for a period of time, such as at least 1 minute or at least 5 minutes, particularly from 5 minutes to 600 minutes. In other embodiments, step B lasts for at least 15 minutes or at least 25 minutes. Step B can be limited to a duration of at most 500 minutes or at most 400 minutes. In one embodiment, it lasts up to 150 minutes or up to 90 minutes. In a preferred embodiment, heat treatment step B is performed after heat treatment step A, particularly after the first heat treatment step, directly heating to a second heat treatment step at a higher temperature without an intermediate cooling step.

[0116] In another preferred embodiment, the first and second heat treatment steps may be performed in separate furnaces. In one embodiment, the first heat-treated intermediate product is pulverized after step A and before step B. This facilitates or renders unnecessary the optional further pulverization step after step B. Omitting the pulverization after step B is particularly advantageous because the composite material after step B is significantly harder and pulverization is very expensive. In one embodiment, the heat-treated intermediate product may be partially or completely cooled (i.e., to room temperature, 20°C) after step A and before step B. Specifically, maintaining the intermediate product in a controlled process atmosphere (e.g., an atmosphere excluding oxygen, even a pure inert gas atmosphere such as argon, or under negative pressure or vacuum) is advantageous for transporting the intermediate product after step A. In one embodiment, before step B, the method may include steps such as transporting the heat-treated intermediate product in an atmosphere excluding water vapor and / or oxygen, transporting the heat-treated intermediate product in a pure inert gas atmosphere such as argon, or transporting the heat-treated intermediate product under negative pressure or vacuum.

[0117] The specified minimum temperature for heat treatment step B should not be lowered to ensure complete conversion of the carbon compounds into a carbon-containing coating. However, the specified maximum temperature should not be exceeded to prevent the formation of carbides. Heat treatment step B is preferably carried out for at least 30 minutes, in particular for at least 90 minutes, and preferably for at least 180 minutes or at least 300 minutes. Heat treatment step B should be carried out for no more than 15 hours, in particular for no more than 10 hours, and particularly preferably for no more than 8 hours. Choosing the correct duration preferably results in a multilayer composed of structured carbon and preferably separates substantially all OH groups.

[0118] Preferably, heat treatment step B is performed after the aforementioned heat treatment step at a lower temperature. The purpose of heat treatment step A is, in particular, to at least partially remove any liquid and at least partially converted carbon compounds. Heat treatment step B preferably aims to at least partially convert the carbon compounds into structured carbon and to pyrolyze or carbonize the carbon compounds remaining after step A. The temperature range indicated has proven advantageous, as it achieves substantial conversion and reduces or avoids the formation of silicon carbide (SiC). When heat treating or converting the carbon compounds, a carbonaceous coating containing or consisting of structured carbon is preferably produced. Decomposition is preferably performed in an atmosphere excluding oxygen and, preferably, other oxidizing gases or liquids.

[0119] The term "coating" or "carbon-containing coating" refers to a carbon-containing product of heat-treating a carbon compound that at least partially surrounds or covers silicon, and particularly substantially completely covers silicon. This includes thin coatings as well as carbon matrices to embed silicon.

[0120] The specific surface area of the composite material can be adjusted in a predetermined manner by selecting an appropriate temperature in step B. Lower temperatures result in a larger specific surface area, while higher temperatures result in a smaller specific surface area.

[0121] crush

[0122] In one embodiment, the silicon is pulverized prior to the thermal process, particularly prior to step A. The pulverization may include crushing, decomposing, deagglomerating, rolling, shredding, fragmenting, and / or grinding. The pulverization may be performed in a low-oxygen environment as described above and / or in a suitable process liquid (e.g., a dispersant). In a preferred embodiment, paraffin wax serves as a dispersant. During the grinding of the silicon, the paraffin wax ensures that gases are removed from the newly generated silicon surfaces during grinding. In another preferred embodiment, the paraffin wax serves as both a dispersant and a carbon source. The silicon may be ground to a particle size D90 of less than 500 nm or less than 300 nm. In one embodiment, the particle size D90 is at least 50 nm. Particle size may be measured by dynamic light diffusion or REM. Pulverizing the silicon prior to thermal treatment is advantageous, particularly immediately prior to thermal treatment, because pulverization creates new silicon surfaces. These surfaces are not initially covered by an oxide layer, and the native oxide layer that is immediately formed can be expected to be thinner or substantially absent due to the pulverization, which can be advantageous. The silicates formed by silicon oxide increase the internal resistance of the battery and lithium during battery charging. Silicon oxide reacts with lithium to convert the active materials (silicon and lithium) until the silicon oxide layer is completely converted. In this case, the battery loses lithium during carrier transport, causing the battery's internal resistance to increase. However, this is an undesirable phenomenon and is best minimized.

[0123] The step of comminuting the silicon before step A can optionally be replaced or additionally performed by comminuting the first heat-treated intermediate product, in particular by grinding it. Specifically, a free-flowing intermediate product can be obtained, and further processing of the intermediate product can be more efficient. Another advantage of comminuting the heat-treated intermediate product is that it makes it easier to further comminute the silicon-carbon composite material after step B. The heat-treated intermediate product is advantageously comminuted to a particle size D90 of less than 50 microns or less than 35 microns. The particle size D10 relative to the particle mass distribution can be greater than 500 nm.

[0124] In one embodiment, graphite can be added to the heat-treated intermediate product before or after the comminution step to obtain a blend. The blend can then be heat-treated according to step B. The preferred ratio of graphite is selected to maintain the desired carbon ratio in the composite material.

[0125] In one embodiment, the silicon-carbon composite is pulverized after step B, particularly to a particle size D90 greater than 1 micron (or greater than 1 micron to 35 microns). However, the target particle size distribution may be set to a D50 value greater than the D50 value of the silicon particles prior to heat-treating the carbon compound. Pulverization advantageously yields a composite material in particulate form that can be efficiently further processed into a paste or slurry. This slurry or paste can be combined with suitable binders and additives to apply the silicon / carbon composite to a metal foil (preferably copper foil) to produce the anode surface. The pulverization step is preferably limited to breaking up smaller silicon / carbon composite aggregates, particularly pulverizing intermediate products of the heat treatment, and particularly also pulverizing hydrocarbons such as hydrocarbons (e.g., paraffin wax) as a carbon source. It has been shown that pulverizing the intermediate products, particularly when paraffin wax is used as a carbon source, results in less solid sintered mass formed in step B. This significantly reduces the overall energy requirements of the process and further minimizes or avoids the generation of new, uncoated silicon surfaces during the pulverization process after the heat treatment.

[0126] In one embodiment, the composite material is sieved, in particular to largely or completely exclude particles with a particle size of less than 500 nm and greater than 35 μm. The composite material preferably has a mass-related particle distribution with a D10 value greater than 500 nm and / or a D90 value less than 35 μm.

[0127] Composite materials

[0128] Silicon-carbon composite materials are also according to the present invention, particularly those obtained according to the methods described herein. The composite materials are characterized by particularly low coulombic losses when used in batteries. Specifically, in embodiments where the charge capacity is at least 1000 mAh / 1 g silicon, and particularly 1200 mAh / 1 g silicon or more, the average coulombic efficiency of the materials in half-cell testing after more than 1000 charge / discharge cycles is at least 99.5%. In half-cell testing, the composite materials can have a specific discharge capacity of at least 1000 mAh / 1 g silicon, or at least 1200 mAh / 1 g silicon, after more than 1000 charge / discharge cycles.

[0129] The proportion of silicon in the composite material relative to the total weight of the material may be at least 20 wt%, at least greater than 40 wt%, at least 51 wt%, particularly at least 60 wt%, at least 70 wt%, or at least 80 wt%, preferably at least 90 wt%, and most preferably at least 95 wt%. In one embodiment, the proportion is at most 99 wt% or at most 95 wt%.

[0130] The carbon ratio of the composite material relative to the total mass of the material may be 1 wt% to 60 wt%, in particular at least 5 wt% or at least 9 wt%. In a specific embodiment, the carbon ratio of the composite material relative to the total mass of the material may be less than 1 wt%. A sufficiently high carbon ratio can reduce the coulombic loss of a battery cell having a composite material. Compared with a lower carbon ratio with a higher silicon ratio, the initial capacity of a higher carbon ratio is indeed lower. However, the capacitance can stabilize quickly after the initial drop and is at a surprisingly high level. Nevertheless, the upper limit of the carbon ratio should still be limited, specifically to a maximum of 55 wt%, a maximum of 50 wt%, or a maximum of 25 wt%. In addition to carbon derived from carbon compounds, the carbon ratio may include a certain proportion of graphite.

[0131] In a preferred embodiment, the silicon and carbon content of the composite material used on the anode side can be adjusted to balance its charge capacity and cycling stability with the charge capacity and cycling stability of the battery's cathode side. In this case, it is conceivable that the composite material produced by the heat treatment can be mixed in this manner with conventional graphite materials as a "drop-in replacement" and used as a blend to produce the desired balance (anode and cathode side) of battery capacity.

[0132] The material is preferably in the form of a composite material, particularly comprising particles having a D90 size of less than 50 microns, less than 20 microns, or less than 10 microns. In one embodiment, the particle size D90 is greater than 1 micron. In one embodiment, the composite material is substantially free of particles smaller than 500 nm. Specifically, the D10 value relative to the mass distribution of the particles is greater than 500 nm.

[0133] The composite material may optionally be substantially free of particles larger than 35 microns, or larger than 25 microns, or larger than 20 microns, or larger than 10 microns. The composite material may optionally be filtered to remove a majority of particles smaller than 500 nm and / or larger than 35 microns.

[0134] In one embodiment, at least a plurality (particularly a majority or all) of the composite particles each comprise at least two silicon particles. The silicon particles have a specific particle size D90 of less than 300 nm or less than 200 nm. Particle size in the composite material can be measured using REM. In case of doubt, this refers to the Martin diameter.

[0135] The specific surface area of the composite particles can be up to 300m 2 / g, especially 40m 2 / g to 300m 2 / g. Specifically, the specific surface area can be 10m 2 / g to 100m 2 / g. Specific surface area can be measured by the BET method (e.g., according to DIN ISO 9277:2014). A smaller specific surface area has been shown to be more advantageous, as it can reduce parasitic reactions.

[0136] Applications and battery cells

[0137] The present invention employs the composite material described herein as the anode material in a battery cell, optionally with the addition of other additives such as graphite. The mass ratio of carbon to silicon in the anode material can be at most 1:1, preferably at most 4:10, and particularly at most 2:10 or at most 1.5:10. The anode material comprises the composite material of the present invention. A high proportion of silicon significantly increases the maximum charge capacity of a correspondingly equipped battery. Preferably, the amounts of silicon and carbide are selected to adjust the mass ratio according to the method of the present invention. Lithium-ion battery cells comprising the anode material also encompass the present invention.

[0138] According to the present invention, a battery cell includes an anode composed at least partially of the composite material. The anode may optionally consist of at least 10 wt%, particularly at least 20 wt%, or at least 60 wt%, of the composite material. In addition to the composite material, the anode may contain other carbon, such as graphite and / or carbon black, and / or in the form of a binder.

[0139] The battery preferably has a battery housing, a cathode, a separator, and an electrolyte. Consider a standard electrolyte such as LP30, in which the conductive salt LiPF6 is dissolved in a 1M solution of ethylene carbonate and dimethyl carbonate (EC:DMC=1:1). The electrolyte added to the battery cell / half-cell may include additives, in particular, the total proportion relative to the weight of the electrolyte may be up to 15 wt% or up to 12 wt%. Specifically, the electrolyte (such as LP30) may contain up to 10 wt% FEC (fluoroethylene carbonate) and / or up to 2 wt% VC (vinyl carbonate) relative to the weight of the electrolyte. The additives may be selected from FEC (fluoroethylene carbonate), VC (vinyl carbonate), LiBOB (lithium-bis(oxalato)borate), or a combination thereof. These additives can improve the conductivity of the intermediate phase (SEI) between the active material and the electrolyte. However, these additives may form gases at higher temperatures (e.g., 50°C to 60°C). One advantage of the present invention is that the amount of additives in battery cells containing composite materials can be reduced, particularly to less than 10 wt%, less than 3.0 wt%, or less than 0.5 wt% relative to the mass of the electrolyte. The proportion of additives can optionally be at least 0.1 wt% or at least 1 wt%. In preferred embodiments, battery cells containing the present invention are substantially free of such additives.

[0140] The procedure for producing the carbon-coated silicon anode is as follows:

[0141] -M1: A solution of silicon particles mixed with ethylene glycol and sucrose (as a carbon compound). In other embodiments, ethylene glycol (as a dispersant) can be replaced with a carbohydrate such as paraffin. In this example, paraffin can also be used as a carbon source, and sucrose or other carbohydrates can be largely or completely omitted.

[0142] Process T1 (Step A): Heat the dispersion to approximately 180°C and maintain the temperature until the solvent evaporates, causing the sugars to caramelize and structure. This process can be performed under a nitrogen and / or protective gas atmosphere (the protective gas can be omitted if necessary). When paraffin wax is used as the carbon source and / or dispersant, the dispersant can be heated to 120°C to 700°C, preferably 150°C to 600°C.

[0143] - Z1: After process T1, the intermediate product can be optionally comminuted. In this case, a defined particle size distribution can be targeted. The specific goal is to approximate the desired particle size of the final product. In theory, no further comminution steps are necessary after T2 (at most, loose aggregates can be comminuted). When paraffin wax is used as a carbon source and / or dispersant, the final particle size can be achieved even without an intermediate comminution step, as any large aggregates that may have formed can be easily broken down again in the subsequent steps of the method.

[0144] -M2: At this point, the product can be mixed with graphite as appropriate to complete the subsequent thermal process used to form the mixed material.

[0145] Process T2 (Step B): For higher temperature heat treatment of the material, various process equipment is used. Under nitrogen and / or argon and / or protective gas atmospheres and / or reducing atmospheres, the temperature is raised to 750°C to 2600°C (preferably 750°C to 1100°C) and maintained there until the desired amount of carbon compound converted to structured carbon is achieved. This produces a composite material of silicon particles embedded in a carbon matrix.

[0146] - Cooling the material to room temperature in a nitrogen environment, or alternatively and preferably an argon environment, or in vacuum.

[0147] -Z3: Material may be crushed and / or sieved as appropriate. BRIEF DESCRIPTION OF THE DRAWINGS

[0148] Figure 1 Comparison of Raman spectra of the composite material of Example V of the present invention and silicon and graphene nanosheets;

[0149] Figure 2 Shows the XRD spectrum of graphene nanosheets;

[0150] Figure 3A and 3B Performance test results showing two varying degrees of side reactions of the composite material in a battery test cell;

[0151] Figure 4 Showing degradation characteristics of unprotected versus protected silicon in a battery test cell;

[0152] Figure 5 shows the specific discharge capacity of a battery cell according to the present invention in a half-cell test after multiple charge cycles;

[0153] Figure 6A Shows the REM image of the silicon-carbon composite material obtained by the present invention;

[0154] Figure 6B The REM image of the silicon-carbon composite material obtained in the present invention is shown, which is used for EDX measurement (further described below).

[0155] Figure 7 The specific charge capacity of the battery cell according to the invention is shown in a half-cell test with multiple charge cycles based on paraffin as the carbon compound.

[0156] Figure 8A Displays the temperature-time curve of the TGA measurement process.

[0157] Figure 8B Shown are TGA profiles as mass changes from 100% of the starting synthesis mass as a function of the temperature reached for the synthesis of silicon nanopowder, sucrose, and a simple diol as dispersant (mass ratio of Si:sucrose:dispersant 1:0.556:1.666).

[0158] Figure 8C Display and Figure 8B Mass spectrometric analysis of the exhaust gas escaping from the measuring chamber and of the nitrogen throughflow fed into the measuring chamber.

[0159] Figure 8D Shown are the TGA profiles as mass changes from 100% of the starting synthesis mass as a function of the temperature reached for the synthesis of silicon nanopowder with white oil (paraffin) (mass ratio Si:paraffin 1:5).

[0160] Figure 8E Display and Figure 8D Mass spectrometric analysis of the exhaust gas escaping from the measuring chamber and of the nitrogen throughflow fed into the measuring chamber.

[0161] Figure 9 Shows the viscosity of a dispersion of silicon nanoparticles and white oil.

[0162] Figure 10Shown are carbon compounds found on the surface of a synthesis product obtained from silicon nanoparticles and paraffin oil in a mass ratio of 1:4.3, as measured by XPS, using typical temperature treatment steps A and B and ensuring a nitrogen atmosphere for the starting materials. DETAILED DESCRIPTION

[0163] Manufacturing silicon-carbon composites

[0164] The method of the present invention can be implemented in a variety of settings. Specifically, it can be performed with or without a dispersant. The mass ratio of silicon to carbon can be varied. The present invention is not limited to the following examples.

[0165] Example 1

[0166] Silicon and sucrose are mixed in a dispersant. Ethylene glycol serves as the dispersant. Sucrose serves as the carbon compound. The mass ratio of silicon to sucrose to dispersant is approximately 2:10:15.

[0167] The mixture is transferred to a crucible for heat treatment. The crucible's capacity exceeds the volume of the mixture to prevent the mixture from overflowing due to bubbling.

[0168] The crucible was transferred to a pusher furnace for the first heat treatment (step A) and treated at 180° C. for 15 hours. The transition temperature of sucrose is 160° C. The heat treatment was carried out in a nitrogen atmosphere. During the heat treatment, the sucrose lost approximately 15% of its mass and caramelized.

[0169] The product of the first heat treatment (intermediate product of the heat treatment) was then pulverized, and the average matrix material density obtained in this way was 1.09 g / cm 3 .

[0170] The pulverized intermediate product was then transferred to a rotary kiln and subjected to a second heat treatment at 1600° C. for 6 hours (step B). The heat treatment was carried out in a nitrogen atmosphere. The mass loss was approximately 60%. The resulting silicon-carbon composite was then ground in a multi-stage roller mill to a particle size D90 of 10 to 20 microns. The composite had a silicon content of 50% by weight and a carbon content of 50% by weight.

[0171] Example II

[0172] Silicon and sucrose were mixed without a dispersant. Sucrose served as the carbon compound. The mass ratio (silicon:sucrose) in the mixture was approximately 9:5.

[0173] The mixture is transferred to a crucible for heat treatment. The capacity of the crucible corresponds to the volume of the mixture. Since there is no dispersant, there is no need to worry about the mixture overflowing.

[0174] The crucible was transferred to a pusher furnace for a first heat treatment (step A) and treated at 180° C. for 1 hour. The heat treatment was carried out in air. During the heat treatment, the sucrose lost approximately 15% of its mass and caramelized.

[0175] The product of the first heat treatment (intermediate product of the heat treatment) was then pulverized, and the average matrix material density obtained in this way was 1.05 g / cm 3 .

[0176] The pulverized intermediate product was then transferred to a rotary kiln and subjected to a second heat treatment at 1400° C. for 1 hour (step B). The heat treatment was carried out in a nitrogen atmosphere. The mass loss was approximately 25%. The resulting silicon-carbon composite was then ground in a multi-stage roller mill to a particle size D90 of 10 to 35 microns. The composite had a silicon content of 90% by weight and a carbon content of 10% by weight.

[0177] Example III

[0178] Silicon (average particle size of 100 nm), graphene oxide, and sucrose are mixed in a dispersant such as isopropyl alcohol. Sucrose serves as the carbon compound. The mass ratio of silicon:sucrose:graphene oxide:isopropyl alcohol in the mixture is approximately 20:100:1:850.

[0179] The mixture was first ground together in a ball mill. The dispersant was then evaporated substantially completely at 80° C. in air, and the remaining mixture was transferred to a crucible.

[0180] The crucible was transferred into a synthesis furnace for a first heat treatment (step A) and treated at 180° C. for 15 hours. The heat treatment was performed in a nitrogen atmosphere.

[0181] The intermediate product was then subjected to a second heat treatment at 1280° C. for 6 hours (step B). The heat treatment was performed in a nitrogen atmosphere. The resulting silicon-carbon composite material was then ground in a mortar. The composite material had a silicon ratio of 50 wt % and a carbon ratio of 50 wt %.

[0182] Example IV

[0183] Silicon (31.01 wt%), graphene oxide (0.09 wt%), and sucrose (17.23 wt%) were mixed with each other in a dispersant (51.67 wt%). Ethylene glycol was used as the dispersant. Sucrose was used as the carbon compound.

[0184] The mixture was transferred to a crucible for a first heat treatment (step A) and treated at 180° C. for 15 hours. The heat treatment was performed in a nitrogen atmosphere.

[0185] Then, a second heat treatment is performed at 1100° C. for 6 hours (step B). The heat treatment is performed in a nitrogen atmosphere.

[0186] Example V

[0187] An arbitrary amount of silicon powder with a particle size D90 of 150 microns (having a native oxide layer on its surface) is mixed with enough ethylene glycol to completely cover the powder. The ethylene glycol protects the silicon from oxygen during subsequent grinding.

[0188] The corresponding amount of sucrose was added to the above mixture and stirred until the sugar dissolved in the ethylene glycol. The calculated or experimental yield of thermal conversion of sucrose to structured carbon at 850°C was approximately 20% relative to the mass of sucrose used. The mixture was selected to achieve a silicon to structured carbon ratio of 9:1.

[0189] The mixture was transferred to a zirconia grinding cup with zirconia grinding balls with a diameter of 3 microns until the mixture just covered the grinding balls. The grinding cup was then closed and placed in a planetary ball mill and ground at a speed of 500 rpm.

[0190] The final dispersion was filled into a ceramic crucible and placed in a synthesis furnace. The thermal synthesis process was then carried out in a protective nitrogen atmosphere using the following sub-steps:

[0191] a. Raise the temperature to 180°C and maintain the temperature for 15 hours to evaporate the ethylene glycol and caramelize the sucrose.

[0192] b. Raise the temperature to 850°C and maintain the temperature for 6 hours in order to completely decompose the carbon source and form a multilayer structure composed of structured carbon around the silicon particles.

[0193] c. Cool to room temperature in a protective atmosphere.

[0194] The product was pulverized in a mortar to break up aggregates, and carbon black, a binder, and deionized water were added to a three-roll mill to form a compressible paste.

[0195] The paste is applied with a doctor blade to form a layer on the copper foil and then dried.

[0196] Components are punched or cut from the coated copper foil and further processed to form battery cells (see half-cell testing below for details).

[0197] The Raman spectrum of the product of this method is as follows Figure 1 shown. Figure 1The resulting material is shown with its superimposed Raman spectra, along with the spectra of pure silicon and pure graphene nanosheets (GNS) used for comparison. Clearly, the material synthesized here possesses all the characteristics of both silicon and GNS (see S. Stankovich et al., Carbon 45 (2007) 1558–1565). Clearly, the resulting material consists of silicon coated with multiple, discontinuous graphene layers. Figure 2 Shown are the XRD spectra of GNS.

[0198] Outcome evaluation

[0199] Test cells were produced using the composite material according to the invention. The battery test cells were repeatedly charged and discharged, and the amount of charge flowing during charging and discharging was measured. Of particular interest here is the extent to which the specific charge level that can be removed at most during discharge has decreased (battery degradation), as well as the ratio between the charge supplied during charging and the charge removed during subsequent discharge. This allows the extent to which unwanted side reactions have occurred to be determined. This value is particularly important in the case of the first cycle, as unavoidable side reactions (formation of a passivation layer on the negative electrode) occur at this point. The method according to the invention significantly reduces the extent of these side reactions. Figure 3A and Figure 3B Based on two different variations of a test cell with a Si / C composite anode, it is shown how the method of the invention can be used to significantly limit the extent of side reactions. Example with variation 1 ( Figure 3A ) compared to the change 2( Figure 3B ) significantly reduces charge loss, thereby lowering the extent of side reactions. This effect is achieved through improved process management. This approach also significantly reduces material degradation related to the available specific storage capacity. Figure 4 The difference in degradation characteristics of cells with unprotected silicon (one with electrolyte additives and one without) compared to carbon-coated silicon is shown. Clearly, the cell with unprotected silicon shows a significant decrease in usable capacity over charge / discharge cycles, while the cell with the carbon protective layer shows only a slight decrease in usable capacity over cycling.

[0200] Half-cell test

[0201] To test cycling stability, a coin cell half-cell was produced using the silicon-carbon composite material in a Si / C-based electrode. In this case, the half-cell served as a test cell, while the Si / C-based electrode was tested with lithium as the counter electrode. This specifically tested the functionality of the Si / C composite material as an electrode material.

[0202] To prepare a Si / C electrode from the composite material, crushed Si / C composite (e.g., D90 <35 μm) and carbon black were added to a water-soluble sodium alginate binder and mixed uniformly in a high-speed mixer at speeds up to 3000 rpm. The alginate binder was prepared according to the method described by Liu et al. (Liu, Jingquan et al., High-performance alginate hydrogel binder for Si / C anodes in lithium-ion batteries, Chemical Communications 50 48 (2014): 6386-9). Deionized water: sodium alginate: CaCl2 (mass ratio of 100:3:0.03) was added and the mixture was evaporated at approximately 80°C with continuous stirring to a residual solid (containing 10 wt% water).

[0203] Relative to the pure solids content of the alginate binder, 65% of the Si / C composite, 25% of the alginate binder solids, and 10% of the carbon black were mixed in a high-speed mixer. If necessary, the water content of the binder can be adjusted to influence the rheological properties of the resulting paste / slurry.

[0204] After mixing the ingredients, the mixture is homogenized in a three-roll mill and particle agglomerates are broken up. The initial gap of the three-roll mill is preferably set to 20 microns, which then ensures that the applied layer (wet applied thickness) when printed is approximately 30 microns.

[0205] The resulting paste from the roller grinding process is printed onto thin copper foil and then dried. A large amount of the environmentally friendly solvent (water) is removed from the printed layer, allowing the adhesive to effectively bond and crosslink with the copper foil surface. The printed material is dried to virtually completely remove moisture and air from the printed material.

[0206] Coins of defined diameter (e.g., 14 mm, 16 mm, or 18 mm) were punched out from the copper foil printed with the Si / C composite material. The Si / C active material ratio in the coins was measured, and the silicon to carbon ratio in the active material was calculated based on the synthesis conditions.

[0207] The theoretical maximum specific capacitance per gram of active material (Si / C composite material) can be calculated from this.

[0208] The air is then removed in an inert atmosphere (e.g., argon) and the button cell (half-cell) is assembled. The composite coin is placed in the center of the copper foil, and the copper side is placed into the first half of the button cell casing. In this example, the casing diameter is larger than the punched coin diameter.

[0209] A separator (e.g., a 1 mm thick fiberglass separator from Whatman) is inserted concentrically onto the composite material and also into the first housing half. The separator's diameter is typically greater than or equal to the diameter of a coin containing the Si / C material, but also smaller than the inner diameter of the button cell's battery housing. The separator is then showered / immersed in the electrolyte mixture (see below). An amount of electrolyte, such as 100-200 μl, is generally sufficient.

[0210] In a half-cell setup, a sufficiently thick Li counterelectrode is concentrically inserted onto the dripping electrolyte. The counterelectrode thickness is chosen so that the availability of Li does not limit the half-cell performance. The diameter of the Li coin also tends to be smaller than, or at most the same as, the separator diameter.

[0211] A spacer of appropriate thickness and a spring are placed on the lithium counter electrode. The second half of the housing is then placed concentrically and pressed onto the housing cover with a pressure of 6 tons to seal the housing tightly.

[0212] A standard electrolyte (trade name LP30) is used, which, in addition to the conductive salt (LiPF6), also contains ethylene carbonate (EC) and dimethyl carbonate DMC (ratio 1:1) and two additives such as fluoroethylene carbonate FEC (10wt%) and vinylene carbonate VC (2wt%).

[0213] The half-cells were then subjected to the so-called forming process. In this case, target charge / discharge conditions were first implemented at a low charge rate before cycling tests were performed. In each case, a CC method of approximately 1 / 30C (twice) was used with a voltage limit of 100mV.

[0214] For testing purposes, 1,300 1C cycles can then be performed using the CC / CV method at room temperature (20°C) with a voltage limit of 100 mV / 1.5 V. CC stands for charging with "constant current" (fixed current value) to a defined final voltage. CV stands for "constant voltage" (fixed charging voltage). The C rate (C / 30 or 1C) indicates the period over which the battery capacity is charged. 1C corresponds to a full charge in one hour. C / 30 means a charging process lasting 30 hours. In the case of a defined voltage limit of 1.5 V, only a portion of the maximum available capacity of the Si / C-based electrode can be recharged and / or discharged.

[0215] For half-cell testing, the specific charge capacity is limited to 1200 mAh per gram of silicon.

[0216] A battery cell tester from the manufacturer Neware was used for the half-cell tests.

[0217] discharge capacity

[0218] Figure 5The specific charge capacity was limited to 1200 mAh / g and the discharge capacity in the half-cell test was plotted as a function of cycle number to show the performance of the composite material of the present invention. Obviously, the initial value of the specific discharge capacity can be maintained for more than 1000 cycles.

[0219] EDX-REM analysis

[0220] Figure 6A Shown is a REM image of the silicon-carbon composite material obtained in the present invention. The composite material was synthesized using a mass ratio of silicon to paraffin wax of 1:4.3. The REM image was recorded at 5.0 kV, a magnification of 15050, and a working distance of 4.8 mm.

[0221] Figure 6B EDX images. The composition of the silicon-carbon composite was confirmed at four measurement points near the surface using energy-dispersive X-ray spectroscopy (EDX; Quantax, Bruker Nano GmbH) at 7.0 kV, 10,000 magnification, and a working distance of 4.5 mm. The wide variation in the carbon ratio indicates that it depends on the choice of measurement point and, therefore, on the particle surface orientation. The detected oxygen is assumed to originate from the initial oxide layer on the silicon particles.

[0222]

[0223] Thermogravimetric analysis (TGA)

[0224] Thermogravimetric measurement of the generated or escaping reaction gases for downstream mass spectrometry analysis. By way of example, two measurement results according to the invention are reproduced here. In both cases, a nitrogen atmosphere with a low volume flow was used for the thermogravimetric analysis (TGA). Additional tests on pure silicon wafers in the same atmosphere (without other additives) demonstrated the presence of small amounts of residual oxygen in the measuring device, which came from very small leak paths in the seals of the high-temperature measuring chamber or were introduced by the nitrogen flushing gas itself. According to the same temperature-time profile used for the TGA, an increase in the silicon oxide layer was shown on the surface of the silicon wafer (in a nitrogen atmosphere). However, this test was advantageously carried out in an almost pure nitrogen atmosphere.

[0225] The temperature-time profile of TGA is consistent with the measurement process and its limitations, such as Figure 8A shown.

[0226] In the first example, a synthesis was performed using silicon nanopowder, sucrose, and a simple diol as a dispersant. The mass ratio of the starting materials was silicon:sucrose:dispersant = 1:0.556:1.666. This total mass was chosen to be relatively low to facilitate rapid removal of evolved gases in industrial implementations of the method.

[0227] The associated TGA profile shows the percent change in mass from 100% of the starting synthesis mass as a function of the temperature reached by the temperature-time profile used ( Figure 8B ). The temperature carried out in the first temperature treatment step A according to the invention is carried out to a temperature slightly above 300° C. It can be seen from this that the various conversion processes, which are usually carried out in the first temperature treatment step A according to the invention, are carried out to a temperature slightly above 300° C. The further conversion in the second temperature conversion step B, which is usually separate, no longer leads to a sudden change in mass. Nevertheless, a further conversion of the synthesis product is still carried out here. In this case, the process gas separates or escapes from the synthesis volume and starts to cause a continuous further reduction in the mass of the synthesis volume. The fact that the residual oxygen atmosphere reacts with the Si particles or the carbon source may lead to a slight increase in the synthesis mass above approximately 900° C. As can be seen from the separate evidence of the TGA using pure silicon wafers in a nitrogen atmosphere, a thin silicon oxide layer may form on the silicon surface as long as residual oxygen cannot be completely excluded from the system.

[0228] In addition to TGA, mass spectrometry analysis was performed on the exhaust gas escaping from the measurement chamber and the input nitrogen gas ( Figure 8C ). It should be noted that a relatively low nitrogen flushing gas flow rate was selected. Qualitative mass spectrometry analysis was performed and the absolute proportions of the corresponding escaping substances were omitted. In addition, the analysis was limited to a few key compounds or escaping gases that were analyzed as important and related to the mass spectrometer. Regarding the temperature-time profile of TGA ( Figure 8A ) and the mass spectrometry analysis of the outflowing gas stream, water vapor, methane, hydrogen, and CO2, as well as OH groups and methyl groups, are still largely separated or split from the synthesis volume at temperatures significantly above 300°C (time curve>>200 minutes) ( Figure 8C ).

[0229] In the second example, two starting materials (silicon nanoparticles (Si) and white oil (paraffin)) were used for synthesis, wherein the white oil served as both a dispersant and a carbon source. The mass ratio of silicon to paraffin in the synthetic composition was 1:5 ( Figure 8D ). Compared to the first example, the first conversion of the synthetic composition is only carried out at temperatures above 200°C, and then the temperature is continuously increased to approximately 350°C. This is the reason why the maximum temperature of the first temperature treatment step A is selected to be slightly higher than this temperature. However, those skilled in the art will appreciate the possibility of appropriately selecting another hydrocarbon as a dispersant and carbon source for the synthesis. In this example, the first conversion can be shifted to higher (up to 700°C) or lower temperatures. When selecting another hydrocarbon, the maximum temperature of the first temperature treatment step A is generally selected accordingly so that the maximum mass reduction is approximately complete before the start of the temperature treatment step B.

[0230] Compared with the synthesis of silicon, sucrose, and diols as dispersants, the water vapor, methane, methyl, and OH groups in the synthesis of silicon using white oil have no or only a small degree of splitting ( Figure 8E ). Even hydrogen escapes into the atmosphere only at a slightly increased rate within both temperature ranges. At temperatures significantly above 400°C, only CO2 escapes into the process atmosphere again to a significant extent. It is assumed that the process atmosphere favors reducing rather than oxidizing effects during temperature treatment step B. However, it was also observed in the case of TGA that low residual oxygen concentrations above 900°C can lead to oxidation of the still exposed Si surface. In contrast to TGA measurement structures, those skilled in the art know how to suppress residual oxygen concentrations in suitable synthetic production processes.

[0231] Rheology

[0232] Viscosity was determined using an Anton Parr MCR 702 MultiDrive rotational rheometer. Measurements were performed in dual-drive mode, with the upper and lower plates rotating at the same speed (50% / 50%) in opposite directions to investigate the high shear rate range. A plate-on-plate measurement geometry with a contoured surface prevents or minimizes sliding effects during measurement. The plate profile has a pyramidal structure (0.2 mm x 0.1 mm). The measurement parameters were as follows: a measuring gap of 0.3 mm in the plate-on-plate geometry, room temperature of 21.5°C, logarithmic shear rates of 0.05–100,000 (in dual-drive mode), and logarithmic measuring point durations of 60 seconds to 1 second.

[0233] During the measurement, the air supply to the chamber may be reduced at the discretion of those skilled in the art to avoid rapid drying of the dispersed powder mixture. The volumetric air flow rate during the measurement was 0.35 m 3 h -1 In addition, a temperature chamber is used to protect the measurement from external influences.

[0234] Before each measurement, a waiting time is observed (optionally between 1 and 10 minutes) at the discretion of a person skilled in the art, since the application step slightly shears the paste to its actual state.

Claims

1. A method for producing a silicon-carbon composite material, comprising the steps of - mixing a plurality of silicon particles and at least one carbon compound; - thermally treating a mixture by performing at least two steps in the following order: A. heat treating the mixture at a temperature at least corresponding to the transition temperature of the carbon compound to obtain a heat-treated intermediate product; B. heat treating the heat-treated intermediate product at a temperature higher than 750° C. to obtain the silicon-carbon composite material; The silicon-carbon composite material has a silicon mass percentage greater than 80%, The particle size D90 of the silicon particles is less than 500 nm and at least 50 nm.

2. The method of claim 1, wherein at least step B is performed in a substantially oxygen-free atmosphere.

3. The method of claim 1 , wherein at least step B is performed in an atmosphere having less than 100 ppmv of oxygen. The method of claim 2 , wherein the oxygen-free atmosphere is a nitrogen or inert gas atmosphere.

5. The process according to claim 1 , wherein the temperature in step A is from 150° C. to 700° C.

6. The process according to any one of claims 1 to 3, wherein the temperature in step B is >750°C to 2600°C.

7. The method of claim 6, wherein the temperature of step B is adjusted so that substantially no silicon carbide is formed.

8. The method according to any one of claims 1 to 3, wherein the heat-treated intermediate product is comminuted to a particle size D90 of less than 50 microns.

9. The method of any one of claims 1 to 3, wherein the mixture of silicon and carbon compounds additionally comprises an additive imparting a structural and / or catalytic effect, and the additive is selected from graphene, graphene oxide, graphite, fullerenes, nanotubes, and combinations thereof.

10. The process of any one of claims 1 to 3, wherein the solids content of the mixture is at least 9 wt%.

11. The method according to any one of claims 1 to 3, wherein - Maintaining the specified temperature in step A for at least 1 minute; and / or - Maintain the specified temperature in step B for at least 1 minute.

12. The method of any one of claims 1 to 3, wherein the carbon compound is a carbohydrate, and the carbohydrate comprises sugars.

13. The process of any one of claims 1 to 3, wherein the product after step A or the intermediate product between steps A and B is filtered to remove most of the particles smaller than 500 nm and / or larger than 35 microns.

14. The method according to any one of claims 1 to 3, wherein the mixture of the silicon particles and the at least one carbon compound has a viscosity greater than 5000 mPa·s when measured using a rotational viscometer with counter-rotation, a shear rate of 100 / s, and a temperature of 21.5°C.

15. The method of any one of claims 1 to 3, wherein the carbon compound alternatively or additionally comprises at least one carbon compound selected from the group consisting of lignin, wax, fat, oil, fatty acid, rubber, and resin.

16. The method according to any one of claims 1 to 3, wherein the mixture further comprises a dispersant, and the carbon compound and / or the dispersant is paraffin or paraffin oil, wherein paraffin or paraffin oil is a single carbon compound. 17 . The method of claim 1 , wherein the mixture of the silicon particles and the at least one carbon compound further comprises lithium or a lithium-containing starting material.

18. The method of any one of claims 1 to 3, wherein silicon is pulverized prior to the heat treatment, wherein the mixture further comprises paraffin wax as a dispersant.

19. The method of any one of claims 1 to 3, wherein the method comprises a step of removing silicon dioxide from the silicon surface by etching, wherein the etching material used to remove the silicon dioxide is HF, KOH, NH4F, NH4HF2, LiPF6, H3PO4, XeF2 or SF6.

20. A silicon-carbon composite material obtainable by the method of any one of claims 1 to 3, having an average coulombic efficiency of at least 99.5% over 1000 charge / discharge cycles in a half-cell test having a specific charge capacity of at least 1000 mAh / g relative to the mass of silicon in the composite, The composite material exists in the form of composite particles with a particle size D90 of less than 50 microns. 21 . The composite material of claim 20 , wherein relative to the mass distribution of the particles of the composite material, a particle size D10 of the composite material is greater than 500 nm.

22. The composite material of claim 20 or 21, wherein most or all of the composite particles have at least two silicon particles per composite particle.

23. The composite material according to claim 20 or 21, wherein the specific surface area is at most 300 m 2 / g.

24. The composite material of claim 20 or 21, wherein the specific surface area thereof is not greater than twice the specific surface area of the silicon particles in the composite material.

25. The composite material of claim 20 or 21, having a specific discharge capacity of at least 1000 mAh / g relative to the mass ratio of silicon in the composite material after more than 1000 charge / discharge cycles in a half-cell test.

26. The composite material of claim 20 or 21 as an anode material in a battery cell.

27. A battery cell comprising an anode, wherein the anode is composed at least in part of the composite material of claim 20 or 21.

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