NEW COMPOSITE MATERIAL

Description

[0001] The present invention relates to a novel composite material, as well as the method for its production and its use in lithium-ion batteries.

[0002] Lithium-ion batteries are rechargeable energy storage systems (secondary batteries) that, among chemical and electrochemical energy storage devices, boast the highest energy density, currently reaching, for example, up to 250 Wh / kg. They are primarily used in portable electronic devices, such as laptops, computers, and mobile phones, and in transportation systems, such as bicycles and electric vehicles.

[0003] For electromobility, higher energy densities of lithium-ion batteries are needed to increase the range of vehicles, and for portable electronic devices to extend the usage time with a single battery charge.

[0004] Lithium-ion batteries comprise an anode (negative electrode), a cathode (positive electrode), a separator that separates the anode and cathode, and an electrolyte that permeates the anode, cathode, and separator.

[0005] In the following, the term anode refers to the composition of active material, also called anode material, binder (such as sodium carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), lithium polyacrylic acid or polyacrylic acid (PAA), polyvinylidene fluoride (PVDF) and variants thereof), conductive additives (such as carbon black, carbon black, graphene, single-wall carbon nanotubes (SW-CNTs), multi-wall carbon nanotubes (MW-CNTs), carbon nanohorns, carbon fibers, vapor-grown carbon fibers, porous carbons or mixtures thereof) and current collector (e.g. copper foil).

[0006] The active material is the material that participates in the electrochemical reaction.

[0007] US 2013 / 0004846 A1 describes a negative active material comprising a silicon-based nanocore and a first amorphous carbon-containing coating layer.

[0008] US 2012 / 244428 A1 describes a negative electrode for a rechargeable lithium battery comprising a negative active material layer containing a polymer binder with a repeating unit and a Si-based negative active material.

[0009] US 2015 / 340687 A1 describes a powder for a negative electrode for a rechargeable lithium-ion battery comprising a mixture of carbon and SiOx with 0 < x < 1, wherein the SiOx consists of a nanometric composite of crystalline SiO2 and amorphous Si.

[0010] To meet the demands of higher energy density, lithium alloy anodes with metals or semimetals such as silicon, germanium, tin, or lead would be desirable, as these have a significantly higher theoretical specific capacity (i.e., storage capacity for lithium) than graphite, which is currently commonly used as the anode material in lithium-ion batteries. However, lithium-ion batteries with lithium alloy anodes exhibit poor capacity retention after many charge / discharge cycles, meaning lower cycle stability. This capacity loss is due to the large volume change of up to approximately 300% during alloy formation between lithium and the alloying metal. The constant volume change during each lithiation or delthiation step places considerable stress on the active material.This can lead to a loss of electrical contact between individual particles and / or between the particles and the current collector. As a result of this loss of electrical contact, the particles can no longer participate in the charging and discharging processes within the battery and thus become inactive. This leads to a loss of capacity. Another problem is the surface area change of the particles caused by the volume change and the resulting formation of a passivating (electrically insulating but permeable to lithium ions) surface layer (the so-called solid electrolyte interphase, SEI) on the particles through reaction with the electrolyte. Since the SEI is not sufficiently flexible, it cannot compensate for the surface changes according to current explanatory models.Therefore, with each charge-discharge cycle, a partial regeneration of the SEI occurs, leading to an irreversible loss of electrolyte and lithium. The loss of lithium, whether originating directly or indirectly from the cathode, results in an irreversible loss of cell capacity. The loss of electrolyte, as well as the accumulation of SEI residues around the particles and in the electrode pores, both lead to an increase in cell resistance, for example, through gradual "drying out" of the cell or clogging of the porosity, which further reduces the cell's performance and, indirectly, its cycle stability. Various approaches are being pursued to prevent the significant volume expansion and reaction with the electrolyte: Applying external pressure to the anode (US 20100035128 A, US 20140266066 A1) Use of nano-silicon as anode material (in pure form or as a physical mixture with graphite) (WO 2008139157 A1, WO 2001096847 A1, DE 102013211388 A1, EP 1730800 A2) Si alloys as anode material (US 7871727 B2, US 7906238 B2) Silicon oxide particles (SiOx) as anode material (US 9281129 B2, JP2011142021; for example, WO 2014 / 095823 A1 describes a powder comprising a mixture containing carbon and SiOx, with 0 <x<1, wobei das SiO x aus einem nanometrischen Komposit aus kristallinem SiO 2 und amorphem Si besteht.) C-Si Komposite Verwendung von Elektrolytadditiven (EP 2430686 B1) Poröse Elektroden als Anodenmaterial (US 6143216 A) Kohlenstoffbeschichtetes nano-Silicium als Anodenmaterial von nano-Si (US 20140234721 A1, JP2012084521)

[0011] The use of pure nano-silicon has several disadvantages. The high surface area of ​​nano-silicon leads to increased SEI formation and thus higher irreversible losses. Furthermore, only an unstable SEI forms on silicon, resulting in continuous regeneration and growth of the SEI, which in turn creates electrical resistance. Additionally, nano-silicon powders are difficult to process (e.g., safety, production of homogeneous electrode pastes, higher binder requirements due to the larger surface area). Moreover, there is an increased likelihood of contact losses with the current collector, necessitating a higher requirement for conductive additives, which in turn leads to higher irreversible losses.

[0012] Another possibility is to combine silicon particles with a carbon matrix, ideally embedding the silicon within the carbon. The resulting C-Si composite material, which can be used as an active material, mitigates the volume change of the silicon and largely avoids direct contact between the silicon and the electrolyte. In the following, C-Si composite material and composite material are used synonymously.

[0013] Another important advantage of C-Si composite materials compared to physical mixtures of graphite and nano-silicon is the ability to achieve a medium particle size in the micrometer range, despite using nano-silicon in the composite materials. This makes the composite particles comparable in size to currently used graphite particles, thus facilitating handling and processing of the composite material on production equipment already designed for graphite anodes, compared to a pure nanomaterial. Furthermore, the volume expansion of the nano-silicon is dampened in the C-Si composite material. Another advantage over physical mixtures is the formation of a more stable SEI (solid electrochemical reinforcement).

[0014] However, the partial separation of silicon and the carbon matrix during thermal treatment has proven to be a difficulty. Even if a homogeneous / uniform mixture of silicon, the other additive particles, and the carbon precursor used (so-called green mixture) was produced in the preceding mixing process, the observed separation during the thermal treatment of the green mixture leads to a partial enrichment and agglomeration of silicon, as well as a corresponding depletion of silicon in other areas of the material after thermal treatment. Thus, an inhomogeneity in the distribution of silicon in carbon ultimately occurs; that is, there are local / microscopic areas with less or no silicon, and local / microscopic areas with significantly more silicon than would be expected on average, for example, based on the average macroscopic composition.If this inhomogeneity occurs in order of magnitude greater than or equal to the desired mean particle size of the final product, this inhomogeneity in the distribution of silicon will have adverse effects both in the further processing of the material and in the final application of the product.

[0015] Segregation is observed particularly when using thermoplastic carbon precursors, such as pitches, and especially with those pitches that are advantageous for the production of active materials for batteries and have particularly low impurities (low ash content (< 1%) and low quinoline-insoluble content (< 10%)). When using these pitches, pronounced (meso-)phase growth and thus the formation of anisotropic regions can be observed during the thermal treatment for conversion to carbon (carbonization), leading to segregation during the carbonization process.

[0016] For use as an active material, the C-Si composite material is milled to a desired particle size. During milling, particles containing little or no silicon are also produced from the silicon-free or silicon-depleted regions. Due to the lack of silicon, these particles have a significantly lower specific capacity than targeted on average and thus contribute only a below-average amount to the overall anode capacity. Nevertheless, SEI formation also occurs on these particles, meaning they too contribute to irreversible formation losses. Therefore, such silicon-free or silicon-depleted particles are undesirable in the active material of the anode for lithium-ion batteries.

[0017] To the extent that silicon is locally depleted or completely absent in some areas of the carbon matrix due to inhomogeneities, it is enriched in other areas of the composite material. This, in turn, leads to the formation of particles during milling that have a significantly higher silicon content or a significantly lower carbon content than desired. As a result, the silicon in these particles is less effectively embedded in the carbon matrix. This leads to a very high specific capacitance in the anode, causing a very strong volume change during charging and discharging due to the high silicon content. This can lead to, for example, the fracture of the C-Si particle and / or disruption of the electrode structure. Furthermore, the strong silicon enrichment...The lithiated silicon in some particles of the anode leads to a corresponding locally uneven current density distribution to and from the anode, which can, for example, promote the formation of lithium dendrites, making the battery less safe and, in extreme cases, even damaging and destroying it in the event of an internal short circuit.

[0018] Another problem arising from the accumulation of silicon in certain areas after the thermal treatment of the mixture concerns the milling process itself: During milling, increased amounts of silicon are released from the silicon-rich regions of the composite material. This is because the silicon in these regions is less effectively embedded in the carbon / carbon matrix, and these areas therefore act as potential mechanical weak points or points of failure. Consequently, the silicon can be easily broken out and released from these silicon-rich regions during the mechanical stress of the composite material during the milling process. In mills with a particle retention system (e.g., classifier) ​​that allows even the smallest particles (e.g., < 1 µm) to pass through, this leads to a significant loss of silicon in the final product, as the nano-silicon used, for example, can pass through these retention systems after its release.Due to the high cost of nano-silicon, this loss is disadvantageous. In mills that retain all the material (e.g., closed ball mills or mills with fine particle filters), the release of nano-silicon leads to an enrichment of the product with unbound nano-silicon. Even if no silicon is lost as a result, this is also disadvantageous, since the higher proportion of nanoparticles impairs, for example, the handling of the product powder (e.g., safety) and complicates the processing of the material into an anode (e.g., a higher specific surface area leads to a higher binder requirement, increased effort for particle dispersion, a greater tendency to agglomerate, potentially poorer electrode adhesion, and reduced electrode compactability).Finally, the released nano-silicon also leads to a deterioration of the electrochemical properties of the anode due to the higher exposed surface area of ​​silicon in the electrode, because this leads, for example, to an uneven load during lithium storage, which in turn leads to a loss of capacity or to increased SEI formation and thus reduces the lifespan of the lithium-ion battery.

[0019] The object of the invention is to provide an easily accessible novel composite material for anodes of lithium-ion batteries, which has a significantly higher specific capacity than graphite, can be processed on production facilities already designed for graphite anodes like conventional graphite materials, and enables a high operational reliability of the battery.

[0020] The problem is solved by a composite material comprising silicon and carbon, wherein the proportion of silicon is 1–80 wt.%, preferably 2–60 wt.%, particularly preferably 5–50 wt.%, and at least 90 wt.% of the composite material lies in a density range between the lower density limit ρ* < 1 and the upper density limit ρ* < 2, where the density limits ρ* 1, 2 ρ 1 , 2 ∗ = 1 ± δ ⋅ ρ where p describes the mean density of the composite material and ± δ determines the range of variation between the upper density limit ρ *< 2 and the lower density limit ρ *< 1, where the magnitude of δ < 0.10 holds, solved.

[0021] In a composite material that is particularly preferred according to the invention, the proportion of silicon is 10 - 75 wt. %, in particular 15 - 70 wt. %, most preferably 20 - 65 wt. %, e.g. 20 to 60 wt. %.

[0022] The composite material according to the invention for lithium-ion batteries has a substantially homogeneous distribution of the components, thereby avoiding uneven loads during lithium storage, uneven volume effects and unnecessary material losses during grinding.

[0023] Although a maximum degree of homogeneity of the composite material would be desirable, it has been found that the composite materials obtained according to the inventive method generally only achieve complete homogeneity approximately. Frequently, at least 3 wt.%, 5 wt.%, or even 10 wt.% of the composite material lies in density range(s) outside a range extending from a lower density limit ρ*< 3 to an upper density limit ρ*< 4, where the density limits ρ* 3, 4 ρ 3 , 4 ∗ = 1 ± δ min ⋅ ρ where ρ describes the mean density of the composite material and ± δ min determines the range of variation between the upper density limit ρ *< 4 and the lower density limit ρ *< 3, where the magnitude of δ min ≥ 0.005, e.g. 0.01, applies.

[0024] The average density ρ of the composite material is given by the following equation: ρ = ∏ i = 1 n ρ i ∑ i = 1 n x i ⋅ ∏ j = 1 j ≠ i n ρ j where: 0 ≤ xi ≤ 1 and 1 = ∑ i = 1 n x i and n > 1 and j ≠ i.

[0025] For example, for a composite material with 4 components (n = 4): ρ = ρ 1 ρ 2 ρ 3 ρ 4 x 1 ρ 2 ρ 3 ρ 4 + x 2 ρ 1 ρ 3 ρ 4 + x 3 ρ 1 ρ 2 ρ 4 + x 4 ρ 1 ρ 2 ρ 3

[0026] The four components can be, for example, silicon, graphite, carbon black and amorphous carbon from the carbon precursor, as shown in embodiment 3c.

[0027] For the density limits (ρ *< 1,2 ) with the range of variation ± δ, the following applies: ρ 1 , 2 ∗ = 1 ± δ ⋅ ρ = 1 ± δ ∏ i = 1 n ρ i ∑ i = 1 n x i ⋅ ∏ j = 1 j ≠ i n ρ j where: 0 ≤ xi ≤ 1 and 1 = ∑ i = 1 n x i and n > 1 and j ≠ i.

[0028] For example, for a composite material with 4 components (n = 4 and δ = 0.2): ρ 1 ∗ = 1 − 0 , 2 ⋅ ρ = 0 , 8 ⋅ ρ 1 ρ 2 ρ 3 ρ 4 x 1 ρ 2 ρ 3 ρ 4 + x 2 ρ 1 ρ 3 ρ 4 + x 3 ρ 1 ρ 2 ρ 4 + x 4 ρ 1 ρ 2 ρ 3 ρ 2 ∗ = 1 + 0 , 2 ⋅ ρ = 1 , 2 ⋅ ρ 1 ρ 2 ρ 3 ρ 4 x 1 ρ 2 ρ 3 ρ 4 + x 2 ρ 1 ρ 3 ρ 4 + x 3 ρ 1 ρ 2 ρ 4 + x 4 ρ 1 ρ 2 ρ 3

[0029] xi denotes the mass fractions of the individual components i that make up the composite material after it has undergone all necessary thermal treatments. These mass fractions result from the composite material formulation, taking into account the respective mass yields of the individual components achieved under the thermal treatment conditions (e.g., via the carbon yields of the carbon precursors used, or more generally, the mass yields of the mixtures of all carbon precursors used in the composite material and all miscible additives in the same proportions as in the composite material itself).

[0030] ρi denotes the densities of the individual components i of which the composite material consists, when these components have been thermally treated under the same conditions as the composite material itself. For example, p(silicon) is the density of the silicon used (including all possible impurities), p(additive a, b, c, etc.) is the density of the insoluble additives a, b, c, etc. used in the composite material (e.g., graphite, carbon black, titanium dioxide, etc.), and p(amorphous carbon) is the density of the amorphous carbon obtained after thermal treatment from the mixture of all miscible carbon precursors used in the composite material and all miscible additives, provided the raw materials are used in the same proportions as in the composite material.The densities ρ i are determined pycnometrically using xylene as a liquid (DIN 51901-2006), whereby finely ground powders of the individual components with a mean particle size d50 in the range of the mean particle size d50 of the composite material are used for their determination in order to keep the influence of pores inaccessible to the liquid as small as possible.

[0031] The parameter δ denotes the inventive range of density variation, which is defined by the lower density limit ρ*< 1 when using -δ and the upper density limit ρ*< 2 when using +δ. The magnitude of the parameter δ has a value of < 0.10, in particular < 0.09, preferably < 0.08, particularly preferably < 0.07, most preferably < 0.06, e.g. < 0.05.

[0032] If the value δ ≥ 0.12, then, as in Figure 1As shown, significant areas with an inhomogeneous distribution of components have formed within the material, resulting in particle fractions with varying densities that deviate considerably from the average density. This inhomogeneous distribution of components in the material, in turn, leads to disadvantages in the production and processing of the composite material, for example, due to undesirable material fractions with very high or very low silicon content, as well as silicon-free particles. Consequently, the product has a lower usable capacity than intended. Furthermore, the uneven distribution of silicon, for example, in the electrode, leads to additional disadvantages in the lithium-ion battery, such as locally uneven current densities, which can, for example, lead to accelerated aging.

[0033] Within the scope of the invention, silicon is understood to mean porous, non-porous, amorphous, semi-crystalline, crystalline silicon in any form and shape and in any mixtures thereof, wherein metallic impurities or components may amount to up to 10 wt.% in total and non-metallic impurities or components (e.g. comprising the elements hydrogen, boron, carbon, nitrogen, oxygen, phosphorus, fluorine and / or sulfur) may amount to up to 50% in total.

[0034] Preferably, the silicon is semi-crystalline or crystalline. It is therefore at least partially crystalline. Whether the silicon is at least partially crystalline can be easily determined, for example, by X-ray diffractometric analysis (XRD) of the silicon or the composite material containing the silicon, based on the presence of at least one X-ray diffraction signal typical of any crystalline silicon. A powder diffractometer can be used for the X-ray diffractometric analysis, such as the EMPYREAN instrument from PANalytical with the associated High Score Plus software. A Cu Kα cathode (λ = 1.54056 Å) serves as the X-ray source, and the measurement of the powder sample can be carried out at room temperature in the range of 2θ = 5–90°C.

[0035] Alternatively, the silicon or the silicon-containing composite material can be examined using transmission electron microscopy (TEM) to determine whether crystalline regions are visible in the silicon that constitute at least 1 wt.% of the total silicon content. For this purpose, a JEOL JEM-2100F transmission electron microscope at a voltage of 200 kV can be used, for example. The silicon particles can be analyzed directly if they are sufficiently small; however, the composite material or larger particle samples must be prepared, for example, by microtome sectioning, to allow for TEM examinations.

[0036] With more than 80 wt.% silicon in the composite material, the proportion of carbon is too low to adequately encapsulate or surround the silicon as required by the invention. The invention achieves a material with a more homogeneous distribution of silicon within the carbon material, which is evident from the fact that at least 90 wt.% of the composite material lies within the density range between the lower density limit ρ* < 1 and the upper density limit ρ* < 2.

[0037] Within the scope of the invention, a material is understood to have a homogeneous distribution in which there are no areas with a significantly higher or lower accumulation of silicon particles, but rather the silicon particles are uniformly distributed – ideally statistically distributed – in the carbon material formed from the carbon precursor. Figure 2d For example, it shows a homogeneous distribution.

[0038] The homogeneous distribution of silicon particles in the composite material improves further processing and end applications. For example, less silicon material is lost during milling, and the composite material is subjected to a more uniform load during lithium deposition when used in lithium-ion batteries. This results in a more even distribution of the volumetric effects of silicon within the composite material and thus within the electrode, compared to strong local concentrations of silicon. Furthermore, a more uniform distribution of silicon within the composite material and therefore within the electrode leads to a more even distribution of current density during charging and discharging of the electrode / battery. This reduces the risk of local damage to the electrode material of the anode or cathode, as well as the risk of lithium dendrites forming due to local current density maxima.

[0039] The composite material is in powder form; it is therefore a composite powder material.

[0040] The composite powder material is in the form of particles, in particular in the form of (sub)microparticles. According to the usual usage of the syllables "micro" and "submicro", "in the form of (sub)microparticles" and "(sub)microparticulate" in connection with the present invention denotes a composite material with a mean particle size (d50) in the range of 100 nm to 500 µm.

[0041] Advantageously, the composite material has a particle size distribution with a mean particle size (d50) of 0.5 to 60 µm, preferably 1 to 40 µm, particularly preferably 1 to 25 µm, and most preferably 2 to 15 µm. Within the scope of the invention, the particle size distribution is understood to be the volume-based particle size distribution. The mean particle size d50 is understood to be the median value X50,3, where the median value X50,3 describes the value at which the cumulative distribution curve Q3(X) of the particle size distribution is 50%. The median value X50,3 was determined using the laser granulometric method (ISO 13320-2009), employing a measuring device from Sympatec GmbH with associated evaluation software. Within the scope of the invention, the volume-based particle size distribution is considered to be the same as the mass-based particle size distribution, since the density of the particles is considered to be independent of size.

[0042] According to the invention, the silicon in the composite material is present in the form of particles that are at least partially encased by carbon. Preferably, the ratio of the average particle size of the silicon to the average particle size of the composite material is at most 0.1; generally, it is in the range of 0.0005 to 0.1, and preferably in the range of 0.0008 to 0.05. Adherence to the upper limit of 0.1 facilitates the achievement of a largely homogeneous density distribution of the composite material. Individual silicon particles that are just barely adhering to one or another composite particle do not contribute significantly to the mass and density of the respective composite particle, because the respective composite particle is much larger and heavier than the silicon particle.

[0043] According to the invention, the composite material fulfills the following condition: d 50 Si ⋅ p c p Si > s where d50 Si represents the average size of the silicon particles in the unit "µm", p C represents the proportion of carbon in the composite material, expressed in wt. %, p Si represents the proportion of silicon in the composite material, expressed in wt. %, and its safety parameter is 0.02, in particular 0.03, preferably 0.04, particularly preferably 0.05.

[0044] The proportions of silicon and carbon can be determined by elemental analysis.

[0045] The size of the silicon particles in the composite material will be determined by electron microscopy by creating a large number of composite material particle sections, e.g. by Fast Ion Bombardment (FIB), and measuring and averaging the size of the Si particles in the section surfaces to form a number-average size.

[0046] Therefore, if small silicon nanoparticles with a d50Si of 0.01 µm (= 10 nm) were present, the proportion of carbon would have to be more than twice as high as the proportion of silicon to satisfy the above inequality. With a d50Si of 0.03 µm (= 30 nm), the above inequality is already satisfied if there is slightly more silicon than carbon present.

[0047] If the proportions of carbon and silicon in the composite material are adjusted to the average size of the silicon particles according to the condition above, sufficient silicon coverage with carbon, or embedding in carbon, is also adequately ensured. This is because the inequality then necessitates a corresponding increase in the proportion of carbon, which, even when using smaller silicon nanoparticles with a particularly large surface area, limits the exposed silicon surface of the composite material to a small extent. It is assumed that a minimum of exposed silicon surfaces leads to more uniform and lower SEI formation, as well as a more uniform current density distribution to and from the anode, ultimately making the battery safer. Furthermore, the silicon coverage—or embedding—makes it more difficult for silicon particles to break out of the composite material.The electrical contact between the individual silicon particles and the current collector then remains more reliable. Due to this continued electrical contact, the particles participate more effectively in the charging and discharging processes within the battery and thus remain active. This, along with the reduction of SEI formation, also minimizes capacity losses that occur during repeated charging and discharging cycles.

[0048] Another aspect of the present invention is a method for producing a composite material.

[0049] This process includes the following steps a) providing silicon, which is preferably at least partially crystalline and is e.g. in the form of particles, b) providing at least one carbon precursor, c) producing a mixture comprising the components from steps a) and b), and d) producing the composite material by further processing the mixture from step c) comprising thermal treatment and comminution.

[0050] The terms thermal treatment and carbonization are used synonymously within the scope of the invention.

[0051] Advantageously, the silicon from step a) has a particle size distribution with a mean particle size (d50) of 10 nm to 1 µm, preferably 20 nm to 0.5 µm, particularly preferably 20 nm to 0.3 µm, most preferably 30 nm to 0.2 µm, and most preferably 40 nm to 180 nm. With a mean particle size of less than 10 nm, the silicon is very reactive, so there would be a risk that it could spontaneously ignite in air if it is not already passivated by oxidation, which is why it can only be handled with great difficulty. If, on the other hand, the material is already surface passivated, for example by oxidation, the surface area to volume ratio would be very unfavorable due to the very small mean particle size; that is, the material would then typically consist of too high a proportion of the passivation layer, and too little pure silicon would remain as the active material.With a mean particle size greater than 1 µm, the absolute volume change of the particles, even with only partial charging and discharging, would be so high that within a few cycles (e.g., within the first 50 cycles), the particles would inevitably break down into smaller fragments. This would lead to electrical / mechanical contact loss with the electrode and further SEI formation, thus significantly reducing the lifespan / cycle stability of a cell with such a material. The particle size distribution with a mean particle size (d50) is measured using dynamic light scattering according to ISO 22412-2017.

[0052] Advantageously, the carbon precursor from step b) is selected from the group consisting of pitch, tar, biomaterials, polymers and resin-based raw materials with a carbon yield >5% or mixtures thereof, preferably pitches, carbohydrates, polyacrylonitriles, polyvinyl chlorides, polyimides, phenolic resins or mixtures thereof, particularly preferably pitches with softening temperatures < 400 °C and ash content < 1%.

[0053] A carbon precursor is defined as any material from which a carbon material can be obtained by the thermal treatment in step d), with a carbon yield of at least 5%. The carbon yield of the carbon precursors is determined according to the Alcan method (ISO 6998-97).

[0054] Within the scope of the invention, biomaterials are understood to mean carbohydrates and lignins.

[0055] A preferred carbon precursor is a thermoplastic carbon precursor. This is a mass that is both fusible and carbonizable. Fusible and carbonizable means that the mass can exist as a melt when heated at a rate of at least 0.001 K / min to 1000 K / s before transitioning into a solid carbonization product upon further temperature increases or after prolonged exposure to a constant temperature above 100 °C. Fusibility can be tested, for example, by heating a sample of the mass in a controlled manner and regularly checking for the presence of a melt, which is indicated, for instance, by the sample softening. Examples of thermoplastic carbon precursors include pitches, tars, bitumen, asphalts, and polymers and copolymers such as polyvinyl chloride, thermoplastic polyacrylonitriles, and certain resins and biomaterials.

[0056] Advantageously, the mixture obtained after step c) is homogeneously distributed. It is particularly advantageous if, when using nano-silicon, it is finely dispersed, i.e., not present in agglomerated form in the mixture.

[0057] The homogeneous distribution in step c) is a beneficial prerequisite for obtaining a product that is as homogeneous as possible after the subsequent step d).

[0058] Advantageously, the thermal treatment in step d) is carried out under exclusion of oxygen at a temperature of up to 400 - 1600 °C, preferably 400°C - 1450 °C, particularly preferably 600 °C - 1450 °C, most preferably 800 °C - 1350 °C and is carried out for a duration of 1 s to 240 h, preferably 1 s to 72 h, particularly preferably 1 s to 24 h, most preferably 1 s to 12 h.

[0059] In the context of this invention, exclusion of oxygen means that, through reactions with oxygen, a maximum of 10% of the expected product reacts in side reactions with oxygen.

[0060] In the context of this invention, a thermal treatment is understood to be a process in which the sample is heated to the selected temperature. The duration, in the context of this invention, is understood to be the time required to reach the desired temperature. In addition to the thermal treatment, the production of the composite material may also include other process steps, such as mixing and / or gassing the mixture.

[0061] Through thermal treatment, the carbon precursor is converted into a carbon material via pyrolysis.

[0062] At temperatures below 400°C, the carbon precursor is not yet completely converted to the carbon material, which can lead to various problems depending on the precursor, such as toxic residues, insufficient purity, foreign elements, insufficient electrical conductivity, etc. At temperatures above 1600°C, the reaction of silicon with carbon is so rapid that the formation of silicon carbide can no longer be ruled out.

[0063] For durations of less than 1 second, such high quantities of volatile decomposition products are released from the carbon precursor in a very short time that their removal / handling becomes difficult on an industrial scale. For durations exceeding 240 hours, the separation can no longer be adequately prevented.

[0064] Advantageously, the ground particles from step d) have a particle size distribution with a d50 of 0.5 to 60 µm, preferably 1 - 40 µm, particularly preferably 1 - 30 µm, most preferably 2 - 25 µm, e.g. 4 - 20 µm.

[0065] With a particle size distribution of less than 0.5 µm (d50), the composite particles are only marginally larger than nano-silicon, so that it can no longer be considered a preferred composite material within the meaning of the invention. Furthermore, such a small average particle size would make the material difficult to process into electrodes using conventional methods and equipment, and the material would also exhibit an undesirably high specific surface area, which in turn would lead to unacceptably high irreversible losses during anode formation. With a particle size distribution greater than 60 µm (d50), the particles would be so large that, compared to the usual anode coating thicknesses (e.g., 40–120 µm), they would negatively affect the electrode homogeneity (thickness, surface loading, porosity) and / or the electrode's calenderability, thereby reducing the stability of the electrode and thus the cell.The particle size distribution is measured as described above, using the laser granulometric method (ISO 13320-2009), with a measuring device from Sympatec GmbH and associated evaluation software.

[0066] In an alternative embodiment of the process for producing a composite material, at least one additional additive is provided alongside the components supplied in steps a) and b). The mixture produced in c) then generally also includes the additive. However, part or all of the additive can also be added during the thermal treatment.

[0067] The additive is defined as the component that, according to calculations, contributes a smaller proportion of carbon to the total amount of carbon produced during the thermal treatment compared to the carbon precursor.

[0068] The following describes the different types of additives, namely particulate additives (insoluble additives) and miscible additives.

[0069] The use of at least one additional additive results in better suppression of segregation during carbonization and / or improved carbon yield from the precursor. The reduction in segregation achieved through the additive allows for a simpler carbonization process and / or a reduction in silicon loss during milling of the material in step d) and / or an improvement in material homogeneity, leading to better material properties such as improved processing of the material for anode production or improved cycle stability.

[0070] In one embodiment, the at least one additive is a component insoluble in the at least one carbon precursor, preferably selected from the group of inorganic materials, particularly preferably titanium dioxide, silicon dioxide, aluminum oxide, zirconium dioxide, boron oxide, silicon carbide, natural graphite, synthetic graphite, expanded graphite, carbon black, carbon black, amorphous carbons, graphene, single-wall carbon nanotubes (SW-CNTs), multi-wall carbon nanotubes (MW-CNTs), carbon nanohorns, vapor-grown carbon fibers, or any mixtures thereof, most preferably graphite, natural graphite, synthetic graphite, expanded graphite, carbon black, carbon black, graphene, amorphous carbons, single-wall carbon nanotubes (SW-CNTs), multi-wall carbon nanotubes (MW-CNTs), carbon nanohorns, vapor-grown carbon fibers, or mixtures thereof.

[0071] Within the scope of the invention, inorganic materials are understood to include all metals and metalloids, as well as their non-gaseous compounds under standard conditions. Likewise, all carbon materials and allotropic forms of the element carbon are considered inorganic materials.

[0072] Within the scope of the invention, amorphous carbon is understood to mean all non-graphitic carbon materials, such as hard carbon, soft carbon, activated carbon, porous carbons, and coke. Within the scope of the invention, soft carbon is understood to mean graphitable non-graphitic carbon materials, and hard carbon is understood to mean non-graphitable non-graphitic carbon materials.

[0073] The smaller the particle size of the silicon used for the C-Si composite material, the more the silicon can take over the function of the additive.

[0074] Advantageously, the at least one additive has a mean particle size d50 or a shortest axis of less than 10 µm, preferably less than 8 µm, particularly preferably less than 6 µm, most preferably less than 4 µm, and most preferably less than 2 µm. If the particle size is greater than 10 µm, there are not enough additive particles per product particle. For this reason, the at least one additive has no positive effect. When using several additives, it is sufficient if one has a mean shortest axis of less than 10 µm, preferably less than 8 µm, particularly preferably less than 6 µm, most preferably less than 4 µm, and most preferably less than 2 µm.

[0075] The shortest axis is measured using an electron microscope and the mean particle size d50 is measured using laser granulometric method (ISO 13320-2009) or dynamic light scattering (ISO 22412-2017), depending on the particle size expected for the additive.

[0076] Advantageously, the at least one additive is used in a ratio such that the proportion of the at least one additive is 90 - 1 wt. %, preferably 70 - 1 wt. %, particularly preferably 50 - 1 wt. %, most preferably 40 - 1 wt. %, of the non-silicon-containing part of the composite material.

[0077] In certain embodiments, only a small amount of additive is used, so that the total additive content is less than 1 wt.% of the non-silicon portion of the composite material. Often, very low additive concentrations are sufficient to suppress the mesophase formation that occurs during thermal treatment. This is particularly true when the additive is very finely dispersed in the mixture, for example, when the additive has a mean particle size d50 or a shortest axis of less than 10 µm.

[0078] If the proportion of at least one additive exceeds 90 wt.% of the non-silicon portion of the composite material, the remaining portion of the non-silicon component, i.e., the carbon matrix, is present only in a very small amount in the composite material. Consequently, the silicon-to-carbon matrix ratio is unfavorable, and the composite material is no longer a preferred composition in which the silicon is distributed throughout the carbon matrix.

[0079] Advantageously, the thermal treatment according to step d) is carried out using an additive, under exclusion of oxygen, at a temperature of up to 400 - 1600 °C, preferably 400°C - 1450 °C, particularly preferably 600 °C - 1450 °C, most preferably 800 °C - 1350 °C and for a duration of 1 s to 720 h, preferably 1 s to 360 h, particularly preferably 1 s to 240 h, most preferably 1 s to 72 h, most preferably 1 s to 24 h.

[0080] At temperatures below 400°C, the carbon precursor is not yet completely converted to carbon, which can lead to various problems depending on the precursor, such as toxic residues, insufficient purity, foreign elements, insufficient electrical conductivity, etc. At temperatures above 1600°C, the reaction of silicon with carbon is so rapid that the formation of silicon carbide can no longer be ruled out.

[0081] For durations of less than 1 second, such high quantities of volatile decomposition products are released from the carbon precursor in a very short time that their removal / handling becomes difficult on an industrial scale. For durations exceeding 720 hours, separation can often no longer be adequately prevented.

[0082] Advantageously, the ground particles from step d) have a particle size distribution with a mean particle size d50 of 0.5 to 60 µm, preferably 1 - 40 µm, particularly preferably 1 - 30 µm, most preferably 2 - 25 µm, e.g. 4 - 20 µm.

[0083] The particle size distribution is determined, as described above, using the laser granulometric method (ISO 13320-2009), with a measuring device from Sympatec GmbH and associated evaluation software.

[0084] In an alternative embodiment, the at least one additive is a component miscible with the carbon precursor, selected from the group consisting of resins, polymers, polymerization initiators, polymer crosslinking agents, or mixtures thereof, preferably phenolic resins, resorcinol resins, cresol resins, alkylphenol resins, cyanate ester resins, epoxy resins, furan resins, polyester resins, alkyd resins, unsaturated polyester resins, vinyl ester resins, acrylic resins, bismaleimide resins, silicone resins, silicone rubbers, polyacrylonitriles, polyimides, polyisoprenes, polybutadienes, polychloroprenes, ethylene propylene diene monomer rubbers, polyvinyl alcohols, polyvinyl chlorides, polystyrenes, 2,3-dimethyl-2,3-diphenylbutane, Friedel-Crafts catalysts, urotropin, sulfur, bismaleimides, or mixtures thereof. from it.

[0085] The term "mixtures" encompasses both physical and "chemical" mixtures (e.g., copolymers). Within the scope of the invention, "miscible" is understood to mean both soluble and emulsifiable.

[0086] The carbon precursor materials mentioned above can also be used as additives for this alternative embodiment. The additive, which is miscible with the carbon precursor, can react with itself or the carbon precursor during thermal treatment and be pyrolytically modified. For the additive to be reactive, it must be polymerizable and / or crosslinkable.

[0087] Advantageously, the proportion of the at least one additive is 90-1 wt. %, preferably 70-1 wt. %, particularly preferably 50-1 wt. %, most preferably 40-1 wt. %, of the non-silicon-containing portion of the composite material.

[0088] If the proportion of at least one additive exceeds 90 wt.% of the non-silicon portion of the composite material, the remaining portion of the non-silicon component, i.e., the carbon matrix, is present in the composite material only in a very small proportion. Consequently, the silicon-to-carbon matrix ratio is unfavorable, and the composite material no longer meets the invention's definition of a homogeneously distributed silicon matrix. At less than 1 wt.%, the effect of the at least one additive is insufficient to achieve the desired improvement in material homogeneity as defined by the invention.

[0089] Advantageously, when using an additive, the thermal treatment in step d) is carried out under exclusion of oxygen at a temperature of up to 400 - 1600 °C, preferably 400°C - 1450 °C, particularly preferably 600 °C - 1450 °C, most preferably 800 °C - 1350 °C and for a duration of 1 s to 720 h, preferably 1 s to 360 h, particularly preferably 1 s to 240 h, most preferably 1 s to 72 h, most preferably 1 s to 24 h.

[0090] At temperatures below 400°C, the carbon precursor is not yet completely converted to carbon, which can lead to various problems depending on the precursor, such as toxic residues, insufficient purity, foreign elements, insufficient electrical conductivity, etc. At temperatures above 1600°C, the reaction of silicon with carbon is so rapid that the formation of silicon carbide can no longer be ruled out.

[0091] For durations of less than 1 second, such high quantities of volatile decomposition products are released from the carbon precursor in a very short time that their removal / handling becomes difficult on an industrial scale. For durations exceeding 720 hours, separation can often no longer be adequately prevented.

[0092] Advantageously, the ground particles from step d) have a particle size distribution with a d50 of 0.5 to 60 µm, preferably 1 - 40 µm, particularly preferably 1 - 30 µm, most preferably 2 - 25 µm, e.g. 4 - 20 µm.

[0093] The particle size distribution is determined, as described above, using the laser granulometric method (ISO 13320-2009), with a measuring device from Sympatec GmbH and associated evaluation software.

[0094] The composite material according to the invention can be used as the sole component or as at least one component of the active material for the anode of lithium-ion batteries, lithium-sulfur batteries, and / or sodium-ion batteries. Other possible components of the active material include, for example, natural graphites, synthetic graphites, expanded graphites, amorphous carbons (hard carbon, soft carbon), or any mixtures thereof.

[0095] The present invention is described below by way of example, using advantageous embodiments and with reference to the accompanying drawings. The invention is not limited by the figures.

[0096] The figures show schematic representations that can be obtained when viewed under a polarizing light microscope. Figures 1a - 1f schematically show the different stages of the formation of the C-Si composite material (basic problem). Figures 2a - 2dschematically show the different stages of the formation of the C-Si composite material with thermal treatment according to the invention without additives or with miscible additives with thermal treatment according to the invention. Figures 3a - 3e schematically show the different stages of the formation of the C-Si composite material with insoluble additives using thermal treatment according to the invention.

[0097] Figure 1a - 1f schematically shows the reaction of a homogeneous green mixture of nano-silicon (1) with a mesogenic carbon precursor (2).

[0098] Figure 1a shows a homogeneous green mixture before the start of thermal treatment.

[0099] Figure 1b shows the beginning of the formation of mesophases (3) during thermal treatment.

[0100] Figure 1cshows the silicon-free mesophases (3) growing during thermal treatment, which leads to a concentration of nano-silicon in the still isotropic phase.

[0101] Figure 1d The structure exhibits silicon-free anisotropic regions that arise from the mesophases through partial coalescence, and silicon-enriched isotropic regions. Upon completion of the thermal treatment, the structure solidifies into a fully carbonized C-Si composite material with silicon-free carbon material regions (5) originating from the mesophase and silicon-enriched regions (6) within the C-Si composite material, which arise from the silicon-enriched isotropic regions.

[0102] Figure 1e shows the schematic division of the thermally treated material by drawn fracture lines into a particle size corresponding to the drawn grid spacing of the lines.

[0103] Figure 1f The figure schematically shows the powder obtained after milling, consisting of silicon-free particles made of pure carbon material (5), silicon-enriched particles (6), and particles with varying proportions (4) between the extremes of silicon-free particles made entirely of carbon material and particles made almost exclusively of silicon without any discernible carbon material. The wide range of possible compositions of the individual particles means that their respective densities can differ, depending on their carbon or silicon content.

[0104] Figures 2a - 2d schematically show the reaction of a homogeneous green mixture of nano-silicon (1) with a mesogenic carbon precursor (2).

[0105] Figure 2a shows a homogeneous green mixture before the start of thermal treatment.

[0106] Figure 2bFigure 1 shows the formation of mesophases (3) during the thermal treatment according to the invention. The individual mesophases remain smaller than in the figure 1. Figure 1b-f , the number of which can increase through enhanced nucleation during thermal treatment.

[0107] Figure 2c shows the schematic division of the composite material after thermal treatment by means of drawn fracture lines to a particle size corresponding to the drawn grid spacing of the lines.

[0108] Figure 2d schematically shows the powder obtained after grinding.

[0109] Compared to Figure 1f The narrower range of possible compositions of the individual particles means that the particles differ less in their respective density.

[0110] The Figures 2a-2d also represent the process with a miscible additive, which is homogeneously mixed with the carbon precursor (2).

[0111] Figures 3a to 3e schematically show the reaction of a homogeneous green mixture of nano-silicon (1) with a mesogenic carbon precursor (2) and an insoluble additive (7).

[0112] Figure 3a shows a homogeneous green mixture before the start of thermal treatment.

[0113] Figure 3b shows the beginning of the formation of mesophases (3) during thermal treatment.

[0114] Figure 3c shows the growing mesophases, which are, however, prevented from growing further by the additive, so that the mesophase growth is lower compared to the case in Figure 1 The hindrance can be reduced by an influence of the additive on the viscosity of the mixture during thermal treatment that is beneficial for the purpose of this invention and / or by an increased nucleation of mesophases during thermal treatment, so that the mesophases are smaller and / or more uniformly distributed in the material.

[0115] Figure 3d The schematic diagram shows the division of the composite material into a particle size corresponding to the indicated grid spacing of the lines, represented by drawn fracture lines.

[0116] Figure 3e schematically shows the powder obtained after grinding.

[0117] Compared to Figure 1f or 2d The insoluble additive (7) leads to additional variability in the possible composition of the individual particles. The density of the individual particles results from the proportions of carbon, silicon, and insoluble additive (7) in the particles. Due to the reduced mesophase growth achieved through a suitable choice of insoluble additives (7), a more homogeneous particle composition can be obtained than for the one described in [reference missing]. Figure 1f The outlined powder material will be obtained.

[0118] The present invention is explained below with reference to exemplary embodiments, which do not constitute a limitation of the invention.

[0119] General procedure i) Providing silicon and carbon precursor. ii) Heating the carbon precursor until it is, for example, melted and sufficiently fluid for the mixer used. To reduce the processing temperature or to dissolve the carbon precursor, a suitable solvent can be used in which the carbon precursor is (predominantly) soluble, at least at elevated temperatures. Alternatively, the silicon can be pre-dispersed in a solvent suitable for the carbon precursor, and the carbon precursor can then be added. To support the deagglomeration of the nano-silicon, treatment with ultrasound (e.g., with a sonotrode), the use of a high-shear stirrer at high speed (e.g., dissolver / toothed discs, Ultra-Turrax®), or another special dispersing device (e.g.,Basket mills, colloid mills, stirred ball mills, high-pressure jet and nozzle processes) are used, as are also known, for example, from the paint, coatings, and ceramics industries. iii) Mixing the silicon as a powder or dispersion and the molten, liquefied, softened, and / or dissolved carbon precursor using at least one suitable stirring tool, e.g., a propeller stirrer, a dissolver, a kneader, a planetary mixer, or combinations thereof. The aim of the mixing process is to produce the most homogeneous green mixture possible with the most deagglomerated nano-silicon particles possible, i.e., that the nano-silicon is homogeneously distributed in the carbon precursor or the carbon precursor-solvent mixture. iv) Optional evaporation / distillation of volatile components of the carbon precursor and / or the solvent added to the mixture to reduce the amount of material for the subsequent thermal treatment step.This step may be necessary, for example, to prevent uncontrolled boil-over or foaming of the material during the subsequent thermal treatment, to recover and recycle the solvent, to reduce the load of the thermal afterburner in the following step, or to reduce sedimentation until the completion of the thermal treatment. Evaporation or distillation of the volatile components or the solvent can be achieved by heating the green mixture, optionally assisted by reduced pressure or vacuum and / or by purging the headspace of the vessel with gases or gas mixtures such as air, nitrogen, argon, carbon dioxide, etc. v) Thermal treatment of the green mixture produced in step iii) or iv) to carbonize the carbon precursor. The thermal treatment of the green mixture takes place in a suitable container (e.g., an open crucible made of steel, graphite, or ceramic) under exclusion of oxygen (e.g.,Under a nitrogen or argon atmosphere or in a vacuum) by heating to a final temperature within a certain time in a suitable furnace, preferably equipped with a thermal afterburner for combusting the volatile exhaust gases. After reaching the final temperature, the produced mixture can optionally be kept at that temperature in the furnace for a further period of time to complete the thermal treatment. The furnace can be, for example, a muffle furnace, tube furnace, chamber furnace, bogie hearth furnace, rotary kiln, annular furnace, tunnel furnace, push-through furnace, or any other continuously or batch-operated furnace with any method of heat generation (e.g., electrical, solar thermal, or by combustion of solid, liquid, and / or gaseous fuels).The thermally treated C-Si composite material is then cooled in the oven to < 400 °C by controlled or natural cooling before being removed. Cooling can also take place outside the actual oven, as long as the C-Si composite material is protected from air or oxygen until it reaches temperatures below 400 °C to prevent oxidation. vi) The resulting C-Si composite material is then crushed / ground to the desired particle size by crushing and grinding, e.g., by pre-crushing with a jaw crusher, cone crusher, roller crusher, etc., followed by single- or multi-stage grinding, e.g., with an impact mill, rotor mill, beater mill, hammer mill, jet mill, ball mill, etc., e.g., with an integrated classifier to adjust the particle size distribution.

[0120] Exemplary embodiment 1 (comparative example, prior art): For steps i)-iii) of the general procedure, 0.23 kg of nano-silicon (mean particle size d50 of approx. 100-200 nm, metallic impurities < 3 wt. %, non-metallic impurities 5-15 wt. %, commercially available e.g. from Alfa Aesar or Sigma-Aldrich) was intensively dispersed in a 5 L beaker in a heatable oil bath in 1.0 kg of tetrahydrofuran (for synthesis, stabilized, commercially available e.g. from VWR) using a dissolver stirrer (commercially available e.g. from IKA), the dispersion in the oil bath was heated to approx. 50 °C, and then a total of 1.0 kg of pitch granules, powder, or pellets with a softening temperature of approx. 60-120 °C (commercially available e.g. from Deza, Koppers, etc.) were gradually added. Rütgers, Bilbaina de Alquitranes) added to the heated dispersion. The mixture was stirred with a dissolver stirrer (commercially available, e.g., from IKA) for approximately...The mixture was stirred intensively for one hour to deagglomerate the nano-silicon as much as possible until a homogeneous dispersion of the nano-silicon in the pitch-solvent mixture was achieved. For certain batches, propeller or anchor stirrers were used instead of the dissolver stirrer. If a mixture proved difficult to stir, additional tetrahydrofuran was added to dilute it until the viscosity of the mixture was low enough for the mixing process.

[0121] For step iv) of the general procedure, the tetrahydrofuran was then distilled off from the mixture at ambient pressure while the mixture continued to be stirred. For this purpose, a distillation setup was used, for example, with a 4–5 L three-necked flask with an attached water-cooled Liebig condenser and a stirrer with a stable metal or PTFE impeller. The distillation was carried out until either at least 70% of the initial amount of tetrahydrofuran had been removed from the green mixture or until the viscosity of the mixture had increased to such an extent that complete circulation and mixing of the mass was no longer possible with the stirrer at a temperature of less than 160–180 °C.

[0122] The condenser and stirrer were then removed, and in preparation for step v) of the general procedure, the hot green mixture was either transferred while still hot – provided it was still fluid – into a sufficiently large solvent-, pitch-, and temperature-resistant container, or the glass flask was broken after cooling to ambient temperature, and the solidified green mixture was placed cold in any desired shape into a sufficiently large solvent-, pitch-, and temperature-resistant container. Suitable containers included, for example, a large tin can, a tin bucket, or a graphite crucible with at least twice the volume of the green mixture.

[0123] Subsequently, in step v) of the general procedure, the green mixture in the container was heated in a chamber furnace with a thermal afterburner to a final temperature of 700–1000 °C under a nitrogen atmosphere within approximately 50–200 hours and held at this final temperature for a further 6–12 hours. Afterward, the furnace heating was switched off, allowing it to cool passively. During cooling, the furnace chamber continued to be purged with nitrogen to prevent oxidation. Once the temperature in the furnace had dropped below 200 °C, the nitrogen purging was stopped, the furnace was opened, and the material was removed.

[0124] For step vi) of the general procedure, the thermally treated material was coarsely mechanically crushed and broken up, e.g., with a hammer, and then broken into fragments no larger than 1 cm using a hammer, mortar, or jaw crusher. The crushed material was then pre-ground in an impact mill to a particle size of approximately 200 µm and subsequently ground to the target particle size distribution in a second impact mill with a classifier. Result of example 1:

[0125] 0.51 kg product powder, mean particle size d50: 5 µm, reversible capacity 1st cycle: 910 mAh / g, efficiency 1st cycle: 84%, reversible capacity 40th cycle: 690 mAh / g, density variation range δ: 0.12.

[0126] For electrochemical investigation of the materials, electrodes were produced from the ground, powdered products and subsequently analyzed in laboratory half-cells (button cells 2016) using a Maccor 4000 battery tester. For electrode production, the powders were mixed with CMC binder (approx. 700,000 g / mol, commercially available from Acros), conductive carbon black Super P (commercially available from Imerys), conductive graphite SFG6 (commercially available from Imerys), and deionized water to create an ink with a viscosity suitable for the subsequent coating process. The ink was then applied to a copper foil (rough, 20 µm thick, commercially available from Schlenk) using a table-top film-drawing machine with a defined doctor blade gap height to achieve the desired areal loading. The composition of the electrode coating was: active material:conductive carbon black:conductive graphite:binder = 88:2:3:7 by mass. Water was added as needed to adjust the viscosity of the ink.The coating was dried under controlled conditions, and then round electrode plates (14 mm diameter) suitable for 2016 button cells (tools and housing parts for button cell construction are all commercially available from Hohsen) were punched out. The electrode mass on the copper foil was determined by weighing, subtracting the mass of the copper from the weight of the electrode plate. The electrode plates were dried (vacuum, > 110°C) and then assembled into button cells (half-cells) in an argon-filled glovebox using a round, punched-out lithium foil (16 mm diameter, commercially available from Alfa Aesar) as the counter electrode, a separator (GF / D, commercially available from Whatman), and an electrolyte. The electrolyte (ready-mixed, commercially available from UBE) had the following composition: 1 mol / L lithium hexafluorophosphate dissolved in ethylene carbonate:ethyl methyl carbonate (1:1, vol.) + 2% vinylene carbonate + 10% fluoroethylene carbonate.

[0127] The cells were charged and discharged ("cycled") under controlled conditions using a battery tester (Maccor, Series 4000), with the lithium counter electrode in the half-cell setup also serving as a potential reference. The battery test was performed as follows: Formation (3 cycles): Charging: 0.1 C CC to 20 mV, CV to C / 100; Discharging: 0.1 C CC to 1.5 V. Cycling (after formation): Charging: 0.5 C CC to 20 mV, CV to C / 20; Discharging: 0.5 C CC to 1.5 V (CC = constant current, CV = constant voltage).

[0128] For the use of C-Si composite materials as anode material for lithium-ion batteries, these can be used either alone or, for example, mixed with graphite anode material to form the anode material (active material), whereby the specific capacity can be adjusted via the mixing ratio.

[0129] The electrochemistry of the following embodiments was also determined according to the described method. The determination of the density range of the composite material was carried out by separating the particles in liquids with defined density, based on the suspension method or the gravity separation of minerals according to their density in heavy liquids.

[0130] To determine the density range of the composite material, the density of the heavy liquid was varied around the range of the average density of the composite material. This allowed for the determination of the achievable separation of the composite material into a "lighter" and a "heavy" fraction. Particles with a density greater than that of the liquid settle, while particles with the same density as the liquid remain suspended, and particles with a density lower than that of the liquid float. Since the separation of the particles according to their density in the gravitational field occurs very slowly due to the very small density difference between the particles and the liquid, a thermostatically controlled centrifuge was used to accelerate the process, thus preventing temperature from influencing the density of the heavy liquid.

[0131] Suitable heavy liquids include, for example, defined mixtures of high-density liquids (e.g., 1,2-dibromopropane, 1,3-dibromopropane, 1,2,3-tribromopropane) with lower-density liquids (e.g., toluene, xylene, decane, dodecane). The density of the liquids or their mixtures is measured using the vibration method according to DIN EN ISO 15212-1 (2009) at the temperature at which the actual particle separation experiment is conducted. In all experiments, care was taken to ensure that the composition, and thus the density, of the mixed liquids did not change due to the evaporation of more volatile components.

[0132] To determine the density range of the powder, approximately 1-2 g of powder were weighed into centrifuge tubes with a volume of approximately 15 ml, ensuring that each tube contained exactly the same amount. The tubes were then filled to a total volume of at least 10-12 mL with a heavy liquid of a precisely known density close to the average density of the material and subsequently sealed tightly. The liquid volume was at least four times the bulk volume of the powder to allow for spatial separation of the particles according to their density within the liquid. To prevent agglomeration or air inclusions, the suspension was treated in an ultrasonic bath for 15 minutes.

[0133] A series of samples, each containing slightly different heavy liquids with varying densities in 0.01 g / cm³ increments, were prepared. All samples were then centrifuged at maximum speed for at least 48–72 hours.

[0134] In samples where the powder settled completely at the bottom, the density of the heavy liquid was less than the density of all the powder particles. In samples where the powder floated completely to the top, the density of the heavy liquid was greater than the density of all the powder particles. In both cases, the density of the respective heavy liquids thus indicated a lower or upper limit for the possible density of the powder particles.

[0135] In the density range between the two extremes, the liquid containing the suspended particles was divided into three equal volume fractions (upper third, middle third, and lower third) by sequentially extracting the individual volumes of the suspensions from the centrifuge tube using pipettes. The three individual fractions were then dried to constant weight, and the powder residue in each fraction was weighed (accuracy at least 0.1% of the total mass of the composite material used). This allowed the determination of the proportion of particles that settled (lower third) and those that remained suspended (upper third). The middle fraction, in which the particles have exactly the density of the heavy liquid, serves to check and, if necessary, correct the masses in the lower and upper fractions, as it was assumed that particles of this density were evenly distributed in all three fractions.

[0136] By varying the density of the heavy liquids, successive analogous experiments were conducted to find the densities of the heavy liquids at which only 5 wt.% of the composite material floated or settled. For this purpose, the mass fractions in the individual fractions from the experiments with slightly different densities of the heavy liquids were plotted against the respective densities of the heavy liquids, and the densities of the heavy liquids for which 5 wt.% of the composite material was found in the upper or lower fraction were determined from the data. These densities thus corresponded to the lower density limit ρ* < 1 or the upper density limit ρ* < 2, and 90 wt.% of the composite material particles lay within the density range between ρ* < 1 and ρ* < 2. The magnitude of the difference |ρ - ρ* < 1.2| between the mean density ρ of the composite material and the lower density limit ρ *< 1 orThe upper density limit ρ *< 2 is the magnitude of the range of variation δ of the density. If the magnitudes of the differences |ρ-ρ *< 1,2 | between the mean density ρ of the composite material and the lower density limit ρ *< 1 or the upper density limit ρ *< 2 were not equal, the larger of the two magnitudes was used to determine the magnitude of the range of variation δ.

[0137] The density ranges of the following embodiments were also determined using the described method. Example 2:

[0138] As in embodiment 1, but with faster thermal treatment in step v). For this purpose, steps i) to iv) and vi) in embodiment 2 were carried out as in embodiment 1. However, the thermal treatment of step v) in embodiment 2 takes place within 3–12 hours. For this purpose, the green mixture was heated in the container in a chamber furnace with a thermal afterburner for the combustion of the resulting exhaust gases under a nitrogen atmosphere within approximately 3–12 hours to a final temperature of 800–1000 °C and then optionally held at this final temperature for a further 1–3 hours. Due to the significantly faster heating to the final temperature, large quantities of pyrolysis gases were released in a relatively short time, for which the furnace and its thermal afterburner were designed. Furthermore, the risk of overflow was greater, which is why a sufficiently large container was used.The furnace heating was then switched off, allowing it to cool passively. During the cooling process, the furnace chamber continued to be purged with nitrogen to prevent oxidation. Once the temperature inside the furnace had dropped below 200 °C, the nitrogen purging was stopped, the furnace was opened, and the material was removed. Result of example 2:

[0139] 0.53 kg product powder, mean particle size d50: 5 µm, reversible capacity 1st cycle: 1110 mAh / g, efficiency 1st cycle: 85%, reversible capacity 40th cycle: 780 mAh / g, density variation range δ: 0.06. Examples 3a-c: Example 3a:

[0140] In contrast to embodiment 1, in embodiment 3a, a powdered additive insoluble in the carbon precursor was additionally added in step iii) of the general procedure and mixed into the green mixture, so that the additive was ultimately distributed homogeneously in the green mixture like the nano-silicon.

[0141] For embodiment 3a, in steps i) to iii) of the general procedure, 0.23 kg of nano-silicon (mean particle size d50 of approx. 100-200 nm, metallic impurities < 3 wt. %, non-metallic impurities 5-15 wt. %, commercially available e.g. from Alfa Aesar or Sigma-Aldrich) was intensively dispersed in a 5 L beaker in a heatable oil bath in 1.2 kg of tetrahydrofuran (for synthesis, stabilized, commercially available e.g. from VWR) using a dissolver stirrer (commercially available e.g. from IKA), the dispersion in the oil bath was heated to approx. 50 °C, and then a total of 0.8 kg of pitch granules, powder, or pellets with a softening temperature of approx. 60-120 °C (commercially available e.g. from Deza, Koppers, Rütgers, etc.) were gradually added. Bilbaina de Alquitranes) and 0.1 kg graphite powder with a mean particle size d50 of 3 - 6 µm (commercially available e.g.The nano-silicon and graphite powder (from Imerys, Graphit Kropfmühl, SGL Carbon) were added as an additive to the heated dispersion. The mixture was then stirred intensively for approximately one hour using a dissolver stirrer (commercially available, e.g., from IKA) to deagglomerate the nano-silicon and graphite powder as thoroughly as possible, until a homogeneous dispersion of the nano-silicon and graphite powder was achieved in the pitch-solvent mixture.

[0142] Subsequently, steps iv) - vi) of the general procedure were carried out as in embodiment 1. Result of example 3a:

[0143] 0.58 kg product powder, mean particle size d50: 5 µm, reversible capacity 1st cycle: 1120 mAh / g, efficiency 1st cycle: 86%, reversible capacity 40th cycle: 800 mAh / g, density variation range δ ~ 0.04. Example 3b:

[0144] As in embodiment 3a, but with an nm-scale powdered additive insoluble in the carbon precursor in step iii). Instead of the graphite powder of embodiment 3a, carbon black was used as the additive in embodiment 3b.

[0145] For embodiment 3b, in steps i) to iii) of the general procedure, 0.23 kg of nano-silicon (mean particle size d50 of approx. 100-200 nm, metallic impurities < 3 wt. %, non-metallic impurities 5-15 wt. %, commercially available e.g. from Alfa Aesar or Sigma-Aldrich) was intensively dispersed in a 5 L beaker in a heatable oil bath in 2.0 kg of tetrahydrofuran (for synthesis, stabilized, commercially available e.g. from VWR) using a dissolver stirrer (commercially available e.g. from IKA), the dispersion in the oil bath was heated to approx. 50 °C, and then a total of 0.8 kg of pitch granules, powder, or pellets with a softening temperature of approx. 60-120 °C (commercially available e.g. from Deza, Koppers, Rütgers, etc.) were gradually added. Bilbaina de Alquitranes) and 0.1 kg of Carbon Black with a mean primary particle size d50 of < 1 µm (commercially available e.g. from Imerys, Orion, Cabot) was added as an additive to the heated dispersion.The mixture was stirred intensively for about 1 hour using a dissolver stirrer and / or a propeller or anchor stirrer (commercially available e.g. from IKA) to deagglomerate the nano-silicon and carbon black powder as thoroughly as possible until a homogeneous dispersion of the nano-silicon and carbon black was achieved in the pitch-solvent mixture.

[0146] Subsequently, steps iv) - vi) of the general procedure are carried out as in embodiment 3a. Result of example 3b:

[0147] 0.56 kg product powder, mean particle size d50: 5 µm, reversible capacity 1st cycle 1100 mAh / g, efficiency 1st cycle: 83%, reversible capacity 40th cycle: 780 mAh / g, density variation range δ: 0.03. Example 3c:

[0148] As in embodiment 3a or embodiment 3b, but with two different additives insoluble in the carbon precursor in step iii), so that the two additives were ultimately homogeneously distributed in the green mixture like the nano-silicon.

[0149] In embodiment 3c, a combination of graphite powder as an additive in embodiment 3a and carbon black as an additive in embodiment 3b was used in a mass ratio of 1:1, i.e., 0.05 kg of graphite powder from embodiment 3a and 0.05 kg of carbon black from embodiment 3b. The remaining materials (types, quantities) and steps of embodiment 3c were used and carried out as in embodiment 3b. Result of example 3c:

[0150] 0.57 kg product powder, mean particle size d50: 5 µm, reversible capacity 1st cycle: 1110 mAh / g, efficiency 1st cycle: 85%, reversible capacity 40th cycle: 790 mAh / g. Density variation range δ: 0.04. Examples 4a-c: Example 4a:

[0151] In contrast to embodiment 1, in embodiment 4a a non-mesophase-forming additive was additionally added in step iii) or iv) of the general procedure and mixed into the green mixture, so that the additive was ultimately homogeneously distributed in the green mixture.

[0152] For embodiment 4a, in steps i) to iii) of the general procedure, 0.25 kg of nano-silicon (mean particle size d50 of approx. 100-200 nm, metallic impurities < 3 wt. %, non-metallic impurities 5-15 wt. %, commercially available e.g. from Alfa Aesar or Sigma-Aldrich) was intensively dispersed in a 5 L beaker in a heatable oil bath using a dissolver stirrer (commercially available e.g. from IKA), the dispersion in the oil bath was heated to approx. 50 °C, and then a total of 0.55 kg of pitch granules, powder, or pellets with a softening temperature of approx. 60-120 °C (commercially available e.g. from Deza, Koppers, Rütgers, etc.) were gradually added. Bilbaina de Alquitranes) was added to the heated dispersion. The mixture was then stirred with a dissolver stirrer (commercially available, e.g., from IKA) for approximately...The mixture was stirred intensively for 1 hour to deagglomerate the nano-silicon as thoroughly as possible until a homogeneous dispersion of the nano-silicon in the pitch-solvent mixture was achieved. Then, 0.5 kg of phenolic resin (Novolak powder with a softening temperature of approximately 60–120°C) (commercially available, for example, from Süd-West-Chemie, Allnex, or Hexion) was gradually added as an additive and stirred intensively for another hour. Subsequently, steps iv)–vi) of the general procedure were carried out as in Exemplary 1.

[0153] If an additive was difficult to mix into the pitch-solvent mixture below the melting temperature of the additive, it was helpful to begin with step iv) of the general procedure during the mixing process, i.e., to gradually evaporate the solvent and raise the temperature of the mixture above the melting temperature of the additive, in order to assist the mixing with the additive. Result of example 4a:

[0154] 0.45 kg product powder, mean particle size d50: 5 µm, reversible capacity 1st cycle: 1070 mAh / g, efficiency 1st cycle: 82%, reversible capacity 40th cycle: 770 mAh / g, density variation range δ: 0.04. Example 4b:

[0155] In contrast to embodiment 1, in embodiment 4b a non-mesophase-forming crosslinkable additive was additionally added in step iii) or iv) of the general procedure and mixed into the green mixture, so that the additive was ultimately homogeneously distributed in the green mixture.

[0156] For embodiment 4b, in steps i) to iii) of the general procedure, 0.23 kg of nano-silicon (mean particle size d50 of approx. 100-200 nm, metallic impurities < 3 wt. %, non-metallic impurities 5-15 wt. %, commercially available e.g. from Alfa Aesar or Sigma-Aldrich) was intensively dispersed in a 5 L beaker in a heated oil bath in 1.0 kg of tetrahydrofuran (for synthesis, stabilized, commercially available e.g. from VWR) using a dissolver stirrer (commercially available e.g. from IKA), the dispersion in the oil bath was heated to approx. 50 °C, and then a total of 0.8 kg of pitch granules, powder, or pellets with a softening temperature of approx. 60-120 °C (commercially available e.g. from Deza, Koppers, Rütgers, etc.) were gradually added. Bilbaina de Alquitranes) was added to the heated dispersion. The mixture was then stirred with a dissolver stirrer and / or a propeller or anchor stirrer (commercially available, e.g., from IKA) for approximately...The mixture was stirred intensively for one hour to deagglomerate the nano-silicon as thoroughly as possible until a homogeneous dispersion of the nano-silicon in the pitch-solvent mixture was achieved. Subsequently, 0.45 kg of unsaturated polyester resin (commercially available, for example, from Reichhold, Synthopol, BÜFA) was added as an additive, along with 6 g of 2,3-dimethyl-2,3-diphenylbutane (commercially available, for example, from Acros, AkzoNobel) as a radical initiator. The mixture was then stirred intensively for another hour. Steps iv) to vi) of the general procedure were then carried out as in Exemplary 1. During the thermal treatment in step v), the additive and its pyrolysis caused significant foaming, necessitating a sufficiently large container for the material in this step. Result of example 4b:

[0157] 0.49 kg product powder, mean particle size d50: 5 µm, reversible capacity 1st cycle: 1100 mAh / g, efficiency 1st cycle: 84%, reversible capacity 40th cycle: 780 mAh / g, density variation range δ: 0.04. Example 4c:

[0158] In contrast to embodiment 1, in embodiment 4c a crosslinking agent was additionally added as an additive in step iii) or iv) of the general procedure and mixed into the green mixture, so that the additive was ultimately homogeneously distributed in the green mixture.

[0159] In embodiment 4c, unlike embodiment 1, 0.85 kg of pitch granules, powder, or pellets with a softening temperature of approximately 60–120 °C (commercially available, for example, from Deza, Koppers, Rütgers, and Bilbaina de Alquitranes) were used in steps i)–iii). The remaining materials (types and quantities) and steps of embodiment 4c are used and carried out as in embodiment 1. In step iii) of the general procedure, after the nano-silicon dispersion was complete, 0.09 kg of sulfur (powder, purity min. 99%, commercially available, for example, from VWR, Carl-Roth, and Sigma-Aldrich) was added and mixed into the green mixture for about half an hour so that the additive was homogeneously distributed in the green mixture before step iv) was started. The reaction of the sulfur with the pitch during heating in step iv) released hydrogen sulfide from about 120 - 150 °C.To prevent over-foaming in step iv) or in step v) of the general procedure, a slow temperature increase, a sufficiently large reaction vessel and a powerful stirrer were helpful.

[0160] The reaction gases can be introduced into a washing bottle containing sodium hydroxide solution during step iv) by purging the flask with nitrogen in order to absorb the hydrogen sulfide, or the hydrogen sulfide can be combusted during the thermal treatment in step v) in a suitably designed thermal afterburner with exhaust gas cleaning. Result of example 4c:

[0161] 0.55 kg product powder, mean particle size d50: 5 µm, reversible capacity 1st cycle: 1090 mAh / g, efficiency 1st cycle: 82%, reversible capacity 40th cycle: 750 mAh / g. Density variation range δ: ~ 0.03.

[0162] Based on the examples and figures, composite materials according to the invention with particularly narrow density distributions are obtained, especially when the process for producing the composite material is carried out in a manner that is appropriate to the specific requirements of the inventive method. The temperature of the mixture should only be kept within a temperature window in which part of the mixture forms mesophases or is molten for as short a time as possible, because mixture components present in mesophases or molten promote the formation of anisotropic regions, and / or a particulate additive (insoluble additive) is added, and / or a miscible additive is added.

[0163] If, due to the limitations of the production facilities, the temperature of the mixture can only be increased very slowly to convert the carbon precursor into carbon, the addition of additives to the mixture is recommended to achieve the density distribution according to the invention. This may be necessary, for example, if a large quantity of the mixture with a large volume and comparatively small surface area is heated in an oven where the thermal treatment is to take place. The mixture will then be heated only slowly, for example, due to its limited thermal conductivity and high heat capacity, both of which result in thermally inert behavior. Therefore, there is a risk of pronounced mesophase formation if no additives are added. The additives then lead to a reduction in mesophase formation and thus, ultimately, generally to a more homogeneous density of the resulting composite material.

[0164] If, however, a faster increase in the temperature of the mixture is possible (e.g., if the mixture is distributed in small portions or finely in a heating zone or introduced into a hot oven to increase the possible heating rate of the mixture, for example by increasing the heat-exposed surface area in relation to the mass of the mixture or by heating only a very small volume of the mixture at a time in a rapidly heating oven or in a hot zone), then usually no additive needs to be added to suppress mesophase formation to such an extent that a density distribution according to the invention is achieved.

[0165] Of course, an additive can also be added to the mixture if the temperature is rapidly increased. This generally leads to a further increase in the homogeneity (i.e., an even narrower density distribution) of the resulting composite material. Reference symbol list

[0166] 1 Nano-silicon 2 Carbon precursor 3 Mesophase 4 Particles with carbon and silicon 5 Particles with pure carbon material 6 Silicon-enriched particles 7 Insoluble additive

Claims

1. Pulverulent composite material comprising silicon particles at least partially coated with carbon, wherein the proportion of silicon in the composite material is 1-80% by weight, characterised in that at least 90% by weight of the composite material is in a density range between a lower density limit value ρ*1 and an upper density limit value ρ*2, wherein ρ * 1 , 2 = 1 ± δ ⋅ ρ applies to the density limit values and ρ describes the average density of the composite material and ±δ determines the range of variation between the upper density limit value ρ*2 and the lower density limit value ρ*1, wherein < 0.10 applies to the amount of δ, wherein the density range of the pulverulent composite material is determined by separating its particles into liquids of defined density; and wherein the composite material satisfies the following condition: d 50 Si ⋅ p c p Si > s wherein d50Si is the electron-microscopically determined number-average size of the silicon particles in the unit "µm", pc is the proportion of carbon in the composite material, expressed in % by weight, pSi is the proportion of silicon in the composite material, expressed in % by weight, and s is a safety parameter that amounts to 0.02.

2. Composite material according to claim 1, wherein the composite material has a volume-based particle size distribution with an average particle size (d50) of 100 nm to 500 µm, in particular of 0.5 to 60 µm, wherein the average particle size (d50) is to be understood as a median value X50.3, in which the cumulative distribution curve Q3(X) of the particle size distribution, determined using a laser-granulometric method according to ISO 13320-2009, is 50%.

3. Composite material according to claim 1, wherein at least 3% by weight of the composite material is in density range(s) outside a range extending from a lower density limit value ρ*3 to an upper density limit value ρ*4, wherein ρ * 3 , 4 = 1 ± δ min ⋅ ρ applies to the density limit values ρ*3.4 and ρ describes the average density of the composite material and ± δmin determines the range of variation between the upper density limit value ρ*4 and the lower density limit value ρ*3, wherein ≥0.005 applies to the amount of δmin.

4. Composite material according to claim 1, wherein the silicon is at least partially crystalline.

5. Method for producing a composite material according to any of claims 1 to 4, comprising the following steps: a) providing silicon b) providing at least one carbon precursor c) preparing a mixture comprising the components of steps a) and b) and d) producing the composite material by further processing the mixture from step c) comprising a thermal treatment and comminution.

6. Method according to claim 5, wherein the carbon precursor of step b) is selected from the group consisting of pitch, tar, biomaterials, polymers and resin-based raw materials having a carbon yield of >5%, or mixtures thereof.

7. Method according to claim 5, wherein the mixture obtained in step c) is homogeneously distributed and the temperature profile of the thermal treatment and the comminution in step d) are matched to the mixture obtained in step c) in such a way that a composite material according to claim 1 is obtained.

8. Method according to claim 5, wherein, in addition to the substances provided in step a) and b), at least one additive is provided and the mixture prepared in step c) comprises the additive.

9. Method according to claim 8, wherein the at least one additive is a component insoluble in the at least one carbon precursor.

10. Method according to claim 8, wherein the at least one additive is a component miscible with the carbon precursor selected from the group of resins, polymers, polymerisation initiators, polymer crosslinking agents, or mixtures thereof.

11. Use of the composite material according to any of claims 1 to 4 as the only component or at least one component of the active material for the anode of lithium-ion batteries, lithium-sulphur batteries and / or sodium-ion batteries.