HK40021276A - Novel composite material - Google Patents

Description

Novel composite material

Technical Field

The invention relates to a novel composite material, to a method for the production thereof and to the use thereof in lithium ion batteries.

Background

Lithium ion batteries are rechargeable energy storage systems (secondary batteries) and among chemical and electrochemical energy storage have the highest energy density, for example, up to 250Wh/kg at present. Lithium ion batteries are used in particular in the field of portable electronic devices, for example in laptops, computers or mobile telephones, and in the field of vehicles, for example in bicycles or cars with an electric drive.

In the electric automobile, the lithium ion battery requires a higher energy density to increase the driving distance of the vehicle. For portable electronic devices, the useful life of a primary battery charge must be extended.

A lithium ion battery includes an anode (negative electrode), a cathode (positive electrode), and a separator separating the anode and cathode from each other, and an electrolyte saturating the anode, cathode, and separator. The combination of active material (also referred to as anode material), binder (e.g. sodium carboxymethylcellulose (CMC), Styrene Butadiene Rubber (SBR), lithium polyacrylate or polyacrylic acid (PAA), polyvinylidene fluoride (PVDF) and variants thereof), conductive additive (e.g. carbon black, graphene, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon nanohorns, carbon fibers, vapor grown carbon fibers, porous carbon or mixtures thereof) and current collector (e.g. copper foil) is referred to hereinafter as anode.

The active material is a material that participates in an electrochemical reaction.

In order to be able to meet the demand for higher energy densities, lithium alloy anodes composed of metals or metalloids, such as silicon, germanium, tin or lead, are desirable, since they have a considerably higher theoretical specific capacity, i.e. the storage capacity of lithium, than graphite, which is currently used as anode material for lithium ion batteries. However, lithium ion batteries with lithium alloy anodes have a lower capacity retention, i.e., lower cycle stability, after many charge/discharge cycles. The capacity loss is a result of a volume change during the formation of an alloy between lithium and the alloying metal of up to about 300%. The constant volume change in each lithiation or delithiation step results in the active material being subjected to large stresses. This can lead to the disappearance of the electrical contact between the individual particles and/or between the particles and the current collector, as a result of which the particles are no longer able to participate in the charging or discharging process in the battery and thus become inactive as a result of the loss of electrical contact. This results in a loss of capacity. Another problem is the change in the surface of the particles associated with the change in volume, and the resulting reformation of the passivating coating (the so-called solid electrolyte interphase, SEI), which is electrically insulating but permeable to lithium ions on the surface of the particles due to reaction with the electrolyte. Surface variations cannot be compensated according to conventional interpretation models due to insufficient SEI flexibility. Thus, each charge/discharge cycle results in partial reformation of SEI, which causes irreversible loss of electrolyte and lithium. The loss of lithium directly or indirectly from the cathode results in an irreversible loss of battery capacity. Both the loss of electrolyte and the accumulation of SEI residues around the particles and in the electrode pores lead to an increase in the resistance of the battery, for example due to gradual "drying out" or pore blocking of the battery, which additionally reduces the performance of the battery and indirectly its cycling stability. In order to avoid large volume expansion and reaction with the electrolyte, the following various methods can be employed:

pressurizing the anode by external pressure (US 20100035128A, US 20140266066A 1)

Use of nanosilicon as anode material (in pure form or as physical mixture with graphite) (WO 2008139157A 1, WO 2001096847A 1, DE 102013211388A 1, EP 1730800A 2)

Si alloy as anode material (US 7871727B 2, US 7906238B 2)

Silicon oxide particles (SiO)_x) As anode material (US 9281129B 2, JP 2011142021; for example, WO 2014/095823A 1 describes a composition comprising a mixture of carbon and SiO_xOf the mixture of (1), wherein 0<x<1, the SiOx is made of crystalline SiO₂And amorphous Si. )

C-Si composite material

Use of electrolyte additives (EP 2430686B 1)

Porous electrode as anode material (US 6143216A)

Carbon-coated nanosilicon as anode material for nanosilicon (US 20140234721A 1, JP2012084521)

The use of pure nanosilica has some disadvantages. The large surface area of nanosilicon results in larger SEI formation and thus increases the amount of irreversible loss. In addition, only unstable SEIs are formed on silicon, which results in the continued reformation and growth of SEIs, with the result that electrical resistance is produced. Furthermore, poor processability of the nano-silicon powder (e.g. reliability, production of a homogeneous electrode paste, higher binder requirements due to larger surface area) should be mentioned. In addition, the likelihood of loss of contact with the current collector increases, with the result that there is a greater demand for conductive additives, which in turn results in a greater number of irreversible losses.

Another possibility is to combine silicon particles with a carbon matrix, ideally embedding the silicon in the carbon. The C-Si composite material obtained in this way, which attenuates the volume change of silicon and largely avoids direct contact of silicon with the electrolyte, can be used as an active material. C-Si composites and composites are used synonymously below.

Another important advantage of C-Si composites compared to physical mixtures of graphite and nanosilica is that despite the use of nanosilica, the average particle size of the composite may still reach the micron scale range. The size of the composite particles is therefore similar to that of graphite particles currently in use, with the result that handling and processing of the composite material may be easier in a production facility already designed for graphite anodes than is possible with pure nanomaterials. In addition, the volume expansion of the nano-silicon in the C-Si composite material is reduced. Another advantage over physical mixtures is that a more stable SEI is formed.

However, partial separation of the silicon and carbon matrix during the heat treatment has proven to be a problem. Even if a homogeneous/homogeneous mixture of silicon, other additive particles and carbon precursor used is produced in the previous mixing process, the so-called raw mix, the segregation observed during the heat treatment of the raw mix to produce the product leads to a partial enrichment of silicon and to the formation of agglomerates and thus to a consumption of silicon in other areas of the material after heat treatment. Thus, a non-uniformity of the distribution of silicon in the carbon occurs, i.e. there are local/micro areas with less or no silicon and local/micro areas with significantly more silicon than the expected average, e.g. according to the average macroscopic composition. Such non-uniformity of the silicon distribution can have a detrimental effect in the further processing of the material and in the final application of the product if it occurs in a range of orders of magnitude greater than or equal to the desired average particle size of the final product.

When using thermoplastic carbon precursors, a separation is particularly observed, for example in the case of pitch, here again particularly in the case of pitch with particularly low amounts of contaminants (low ash content (< 1%) and small amounts of quinoline-insoluble (< 10%)), which is advantageous for the use for producing active materials for batteries. When these pitches are used, significant (intermediate) phase growth and thus formation of anisotropic regions can be observed during the heat treatment (carbonization) for conversion to carbon, which leads to segregation during carbonization.

In order to use the C-Si composite as an active material, the C-Si composite is ground to a desired particle size. When the material is milled, particles are also produced from the silicon-free or silicon-poor regions, which contain little silicon. Due to the absence of silicon, these particles therefore have a specific capacity which is significantly lower than the expected average value and therefore contribute only to the overall capacity of the anode to a degree lower than the average level. Nevertheless, SEI formation also occurs on these particles, with the result that these particles also cause irreversible formation losses. Thus, such silicon-free or silicon-depleted particles are undesirable in active materials for anodes of lithium ion batteries.

When non-uniformity occurs, silicon is enriched in other regions of the composite to the extent that silicon is locally consumed or completely absent in certain regions of the carbon matrix. This in turn leads to the formation of particles from these regions when grinding the material, which particles have a significantly higher silicon content or a significantly lower carbon content than is required. As a result, silicon is more difficult to embed into the carbon matrix of these particles. In the anode, this leads to very high specific capacities of the particles, with the result that, for example, when charging and discharging the particles, very pronounced volume changes occur due to the high silicon content, which may lead, for example, to destruction of the C — Si particles and/or destruction of the electrode structure. In addition, the large enrichment of silicon or lithiated silicon in some particles of the anode leads to a corresponding locally inhomogeneous current density distribution to or from the anode, which may for example promote the formation of lithium dendrites, as a result of which the battery becomes less safe and may even damage and destroy the battery if internal short circuits are formed in extreme cases.

Another problem caused by the silicon enrichment in certain areas after the heat treatment of the mixture is related to the milling process that generates the powder itself: it can be said that, since the silicon in the silicon-rich regions is more difficult to embed into the carbon/carbon matrix and since these regions therefore act as mechanical weak points or predetermined breaking points, more and more silicon is released from the silicon-rich regions of the composite material during the grinding process. Thus, when mechanical stress is applied to the composite during grinding, silicon can be particularly easily broken and released from the silicon-rich regions. In mills with particle retention systems (e.g. sieves) allowing the passage of the smallest particles (e.g. <1 μm), this leads to a large loss of silicon in the final product, since e.g. the nanosilicon used can pass through the retention system. This loss is disadvantageous due to the high cost of nano-silicon. In contrast, in mills that retain all the material (e.g., closed ball mills or mills with fine particle filters), the liberation of nanosilica results in an enrichment of unbound nanosilica in the product. This is disadvantageous, even if no silicon is thus lost, because, for example, a higher nanoparticle content can alter the handling of the product powder (e.g. in terms of safety) poorly and can make the processing of the material into anodes more difficult (e.g. higher specific surface area leads to higher binder requirements, greater effort required to disperse the particles, increased tendency to agglomerate, potentially poorer electrode adhesion and poorer electrode compactability). The released nano-si may eventually lead to deterioration of electrochemical performance of the anode due to an increase in exposed surface area of si in the electrode, since this may lead to, for example, non-uniform loading during the incorporation of lithium, which in turn leads to a loss of capacity or an increase in SEI formation, thereby shortening the service life of the lithium ion battery.

Disclosure of Invention

It is an object of the present invention to provide a new, readily available composite material for anodes of lithium ion batteries, which has a significantly higher specific capacity than graphite, can be handled like conventional graphite materials in production facilities already designed for graphite anodes, and makes it possible to improve the operational safety of the battery.

This object is achieved by a composite material comprising silicon and carbon, the content of silicon being 1 to 80 wt.%, preferably 2 to 60 wt.%, particularly preferably 5to 50 wt.%, and at least 90 wt.% of the composite material being at a lower density threshold ρ ·₁And upper limit ofDensity threshold ρ #₂In the density range between, the density threshold p_1,2Is characterized by the following relationship:

ρ*_1,2＝(1±δ)·ρ

wherein ρ is the average density of the composite material and ± δ is the upper density threshold ρ ·₂With said lower density threshold ρ ×₁The range of variation therebetween, the amount of δ<0.10。

In a particularly preferred composite material according to the invention, the amount of silicon is from 10 to 75% by weight, in particular from 15 to 70% by weight, very particularly preferably from 20 to 65% by weight, for example from 20 to 60% by weight.

The composite material according to the invention for lithium ion batteries has a substantially uniform distribution of the components, as a result of which inhomogeneous loading, inhomogeneous volume effects and unnecessary material losses during lithium incorporation during grinding are avoided.

Although a high degree of homogeneity of the composite material is desired, it has been found that the composite material obtained using the method according to the invention generally achieves only approximately complete homogeneity. Typically, at least 3 wt.%, 5 wt.%, or even 10 wt.% of the composite material is at a threshold value ρ from the lower limit density₃Extending to an upper density threshold value p₄In a density range outside the extended range, the density threshold ρ ×_3,4Is characterized by the following relationship:

ρ*_3,4＝(1±δ_min)·ρ

where ρ is the average density of the composite and ± δ_minIs the upper density threshold ρ ×₄With said lower density threshold ρ ×₃Range of variation therebetween, δ_minThe amount of (B) is ≧ 0.005, e.g., 0.01.

The average density ρ of the composite is given by the following equation:

wherein

0≤x_i1 or less andand n>1 and j ≠ i.

For example, the following relationship applies to a composite material having four components (n ═ 4):

as shown in example 3c, the four components may be, for example, silicon, graphite, carbon black, and amorphous carbon from a carbon precursor.

Therefore, the following relationship applies to density threshold values (ρ ×) having a variation range ± δ_1,2)：

Wherein

0≤x_i1 or less andand n>1 and j ≠ i.

For example, the following relationship applies to a composite material having four components (n ═ 4 and δ ═ 0.2):

x_irefers to the mass fraction of the individual components i that make up the composite after subjecting the composite to all necessary heat treatments. The mass fraction is calculated from the formulation of the composite material taking into account the respective mass yields of the individual components obtained under the conditions of the thermal treatment of the composite material (for example, by the carbon yield of the carbon precursor used or, more generally, of the mixture of all carbon precursors used for the composite material and all additives that may be mixed therewith)Rate, in the same quantitative ratio as in the relevant composite).

ρ_iRefers to the density of each component i of the composite material when the components are heat treated under the same conditions as the relevant composite material. Thus, for example, when the raw materials used for this are used in the same quantitative ratio as the composite material concerned, ρ (silicon) is the density of the silicon used (including all possible contaminations), ρ (additives a, b, c, etc.) is the density of the insoluble additives a, b, c, etc. (for example graphite, carbon black, titanium dioxide, etc.) used for the composite material and ρ (amorphous carbon) is the density of the amorphous carbon obtained from the mixture of all the mixable carbon precursors used for the composite material after the thermal treatment and all the additives that can be mixed therewith.

Density p_iPycnometer measurements (DIN 51901 and 2006) were carried out with xylene as liquid, using powders of the individual components which were as finely ground as possible and whose average particle size d50 was in the range of the composite average particle size d50, in order to keep the influence of pores which are inaccessible to liquids to a minimum.

The parameter δ refers to the variation of the density according to the invention, when- δ is used, this variation is limited by a lower density threshold ρ ×₁Covered, and when + δ is used, its range of variation is covered by an upper density threshold ρ ×₂And (4) covering. Value of parameter delta<0.10, especially<0.09, preferably<0.08, particularly preferably<0.07, very particularly preferably<0.06, e.g.<0.05。

If the value δ ≧ 0.12, significant regions with uneven distribution may occur in the material, which regions thereby result in particle fractions with different densities that deviate significantly from the average density, as shown in FIG. 1. The inhomogeneous distribution of the components in the material in turn leads to disadvantages in the production and processing of the composite material, for example due to the undesirable amount of material with a very high or very low silicon content and the absence of silicon particles. As a result, the product has a lower available capacity than desired. For example, an inhomogeneous distribution of silicon in the electrodes can also lead to other disadvantages of lithium ion batteries, such as locally inhomogeneous current densities, which may for example lead to increased ageing.

Within the scope of the present invention, silicon refers to porous, non-porous, amorphous, partially crystalline, crystalline silicon of any shape or form and any mixture thereof, with metallic contaminants or components possibly adding up to 10% by weight and non-metallic contaminants or components (e.g. comprising the elements hydrogen, boron, carbon, nitrogen, oxygen, phosphorus, fluorine and/or sulphur) possibly adding up to 50% by weight.

The silicon is preferably partially crystalline or crystalline. Thus, it is at least partially crystalline. Whether silicon is at least partially crystalline can be easily determined, for example by X-ray diffraction analysis (XRD) of silicon or a composite material comprising silicon, in particular on the basis that at least one X-ray diffraction signal is typically present for any crystalline silicon. Powder diffractometers, such as the EMPYREAN instrument from PANALYtic and associated High Score Plus software, can be used for X-ray diffraction analysis. Cu Ka cathodeServing as an X-ray source. Powder samples can be measured at room temperature in the range of 5-90 ° 2 θ.

Alternatively, Transmission Electron Microscopy (TEM) can also be used to analyze the silicon or silicon-containing composite to see if crystalline regions of at least 1 wt% of the total silicon in the silicon can be identified. For this purpose, a JEOL JEM-2100F transmission electron microscope may be used, for example, at a voltage of 200 kV. If small enough, the silicon can be analyzed directly. For example, a composite or larger particle sample must be prepared using microtome slicing in order to perform TEM analysis.

If the content of silicon in the composite material is more than 80% by weight, the amount of carbon is too low to surround or encapsulate silicon to a sufficient extent for the purpose of the present invention. The invention achieves a material with a homogeneous distribution of silicon in the carbon material, which is clearly determined by the fact that: at least 90% by weight of the composite material is at the lower density threshold ρ ×₁And an upper density threshold value rho₂In the density range therebetween.

Within the scope of the present invention, a material having a uniform distribution is understood to mean that there are no regions with a significantly higher or lower accumulation of silicon particles, but the silicon particles are uniformly distributed (ideally statistically distributed) in the carbon material formed from the carbon precursor. For example, fig. 2d shows a uniform distribution.

The uniform distribution of the silicon particles in the composite material improves further processing and end-use applications, e.g. less material loss of silicon during milling and the composite material is subjected to a uniform loading when used in a lithium ion battery or during lithium incorporation. Thus, the volume effect of silicon is more uniformly distributed in the composite material and thus in the electrode than a significant local accumulation of silicon. Furthermore, a uniform distribution of silicon in the composite material and hence in the electrode, when charging and discharging the electrode/battery, results in a more uniform distribution of the current density, thereby reducing the risk of local damage to the electrode material of the anode or cathode or the risk of formation of lithium dendrites due to local current density maximization.

Typically, the composite material is present in powder form, which means that it is a composite powder material.

The composite powder material is present in the form of particles, in particular (sub) particles. According to the typical use of the prefixes "micro" and "submicron", the "(sub) particulate form" and "(sub) particulate" in combination with the present invention refer to a composite material having an average particle size (d50) in the range of 100nm to 500 μm.

The composite material advantageously has a particle size distribution with an average particle size (d50) of 0.5to 60 μm, preferably 1 to 40 μm, particularly preferably 1 to 25 μm, very particularly preferably 2 to 15 μm. In the context of the present invention, particle size distribution means a volume-based particle size distribution. The mean particle size d50 is the median X_50,3Median value X_50,3Is the cumulative distribution curve Q of the particle size distribution₃(X) is a value at 50%. Determination of the median value X by means of laser particle size analysis (ISO13320-2009)_50,3The method already uses a measuring device from Sympatec GmbH and corresponding evaluation software. Within the scope of the present invention, the volume-based particle size distribution is considered equal to the mass-based particle size distribution, since the density of the particles is not considered to have a size dependence.

According to the invention, the silicon is present in the composite material in the form of particles at least partially surrounded by carbon. Preferably, the ratio of the average particle size of the silicon to the average particle size of the composite is not greater than 0.1. Generally, it is in the range of 0.0005 to 0.1, preferably in the range of 0.0008 to 0.05. Adherence to the upper limit of 0.1 helps to adjust the substantially uniform density distribution of the composite material. The individual silicon particles, which only marginally adhere to one or the other composite particle, do not contribute significantly to the mass and density of the respective composite particle, since the respective composite particle is much larger and heavier than the silicon particle.

According to the invention, the composite material preferably satisfies the following conditions:

wherein

d50_SiRepresents the average size of the silicon particles, in units of "μm",

p_crepresenting the amount of carbon in the composite material, expressed in weight%,

p_Sirepresents the amount of silicon in the composite material, expressed in weight%, and

s is a safety parameter of 0.02, in particular 0.03, preferably 0.04, particularly preferably 0.05.

The amounts of silicon and carbon can be determined by elemental analysis.

The size of the silicon particles in the composite material can be determined using electron microscopy, for example by forming a large number of particle fractions of the composite material by rapid ion bombardment (FIB), and measuring the size of the Si particles in the cross-section and taking the average.

D50 if small silicon nanoparticles_SiAt 0.01 μm (═ 10nm), the amount of carbon must be more than twice the amount of silicon to satisfy the above inequality. d50_SiAt 0.03 μm (═ 30nm), the inequality above is already satisfied if there is slightly more silicon than carbon.

If the amounts of carbon and silicon in the composite material are adjusted to the average size of the silicon particles according to the above conditions, it is also possible to ensure that sufficient silicon is covered with carbon or embedded in carbon to a sufficient extent. This is because the inequality then results in a corresponding increase in the amount of carbon, which also limits the exposed silicon surface of the composite to a small fraction when using smaller silicon nanoparticles with particularly large surface areas. It is assumed that a minimum amount of exposed silicon surface results in more uniform and reduced SEI formation, and uniform current density distribution into and out of the anode, with the result that the battery eventually becomes safer. Furthermore, silicon capping or silicon intercalation makes it difficult for silicon particles to become detached from the composite. The electrical contact between the respective silicon particles and the current collector is more reliably maintained. Due to the continuous presence of the electrical contact, the particles participate in an improved manner in the charging and discharging process in the battery and thus remain active. As well as reducing SEI formation, the occurrence of capacity loss is also reduced in the case where charge and discharge cycles are repeated.

Another aspect of the invention is a method of producing a composite material.

The method comprises the following steps

a) Silicon is provided which is preferably crystalline and is present for example in the form of particles,

b) providing at least one carbon precursor

c) Generating a mixture comprising the components from steps a) and b), and

d) producing a composite material by further processing the mixture from step c), said processing comprising heat treatment and comminution.

The terms heat treatment and carbonization are used synonymously within the scope of the invention.

The silicon from step a) advantageously has a particle size distribution with an average particle size (d50) of from 10nm to 1 μm, preferably from 20nm to 0.5 μm, particularly preferably from 20nm to 0.3 μm, very particularly preferably from 30nm to 0.2 μm, very particularly preferably from 40nm to 180 nm. In the case of average particle sizes of less than 10nm, silicon is highly reactive and there is therefore a risk that it will spontaneously ignite in air unless it has been passivated by oxidation, which is why silicon handling is very laborious. Conversely, if the surface of the material has been passivated, for example by oxidation, the surface area to volume ratio will be very disadvantageous, since its average particle size is very small, i.e. too high amounts of material usually consist of the passivation layer, while too little pure silicon still acts as active material. In the case of particle sizes larger than 1 μm, the absolute volume change of the particles will be so high, even in the case of partial charging and discharging, that in a small number of cycles (e.g. in the first 50 cycles) the particles will be broken down into smaller fragments, which will lead to electrical/mechanical losses in contact with the electrodes and further to SEI formation, as a result of which the service life/cycle stability of the battery with such material will be greatly reduced. The particle size distribution with the mean particle size distribution (d50) was measured by dynamic light scattering according to ISO 22412 and 2017.

The carbon precursor from step b) is advantageously selected from pitch, tar, biomass, polymer and resin based feedstock or mixtures thereof with a carbon yield > 5%, preferably pitch, carbohydrate, polyacrylonitrile, polyvinyl chloride, polyimide, phenolic resin or mixtures thereof, particularly preferably pitch with a softening temperature <400 ℃ and an ash content < 1%.

Carbon precursor is understood to be all materials from which carbon materials can be obtained by heat treatment in step d), with a carbon yield of at least 5%. The carbon yield of the carbon precursor was determined using the Alcan method (ISO 6998-97).

Within the scope of the present invention, biological material is understood to encompass carbohydrates and lignin.

The preferred carbon precursor is a thermoplastic carbon precursor. The thermoplastic carbon precursor is a meltable and carbonizable substance. Meltable and carbonizable means that the substance may be present in the form of a melt when heated at least one heating rate in the range of 0.001K/min to 1000K/s, before being transformed into a solid carbonized product with further increase in temperature, or after a constant temperature above 100 ℃ for a long period of time. The meltability can be tested, for example, by heating a sample of the substance in a controlled manner and periodically checking for the presence of melt, which can be identified, for example, by the fact that the sample is soft. Some thermoplastic carbon precursors are, for example, pitch, tar, bitumen, petroleum pitch, and polymers and copolymers, such as polyvinyl chloride, thermoplastic polyacrylonitrile, and certain resins and biomaterials.

The mixture obtained according to step c) is advantageously homogeneously distributed. It is particularly advantageous if, when using nanosilica, the nanosilica is finely distributed, i.e. is not present in the mixture in agglomerated form. The uniform distribution in step c) is an advantageous prerequisite for obtaining a product which is as homogeneous as possible according to the subsequent step d).

The heat treatment in step d) is advantageously selected at a temperature of up to 400-1600 ℃, preferably 400-1450 ℃, particularly preferably 600-1450 ℃, very particularly preferably 800-1350 ℃ under exclusion of oxygen and for a duration of from 1 second to 240 hours, preferably from 1 second to 72 hours, particularly preferably from 1 second to 24 hours, very particularly preferably from 1 second to 12 hours.

In the context of the present invention, "excluding oxygen" means that up to 10% of the desired product reacts with oxygen in side reactions caused by the reaction with oxygen.

Within the scope of the present invention, heat treatment is a process in which a sample is heated to a selected temperature. Within the scope of the present invention, "duration" refers to the time until the temperature is reached. In addition to the heat treatment, the production of the composite material may also comprise further process steps, such as intensive mixing and/or introduction of gases into the mixture.

The heat treatment converts the carbon precursor into a carbon material by pyrolysis.

At temperatures less than 400 ℃, the carbon precursor has not been completely converted to a carbon material, which may lead to various problems, such as toxic residues, insufficient purity, foreign elements, insufficient conductivity, and the like, depending on the precursor. 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.

In the case of a duration of less than 1 second, such large amounts of volatile decomposition products are released from the carbon precursor that removal/disposal of the decomposition products poses difficulties on a large industrial scale. If the duration exceeds 240 hours, separation can no longer be sufficiently prevented.

The abrasive particles from step d) advantageously have a particle size distribution with a d50 of 0.5to 60 μm, preferably 1 to 40 μm, particularly preferably 1 to 30 μm, very particularly preferably 2 to 25 μm, for example 4 to 20 μm.

In the case of a particle size distribution with a d50 of less than 0.5 μm, the composite particles are only insufficiently larger than the nanosilica, so that it is no longer possible to refer them to as preferred composites for the purposes of the present invention. In addition, due to such low average particle size, the material will be difficult to process into electrodes using conventional methods and facilities, and the material will have an undesirably high specific surface area, which in turn will result in undesirably high irreversible losses during anode formation. In the case of a particle size distribution with a d50 of more than 60 μm, the particles will be so large that they will be so large that the uniformity of the electrode (thickness, load per unit area, porosity) or the drawability of the electrode is adversely affected compared to the typical coating thickness of the anode (e.g. 40-120 μm), with the result that the stability of the electrode and thus of the battery is reduced. As described above, the particle size distribution is measured by means of laser particle size analysis (ISO13320-2009), a measuring device from Sympatec GmbH and corresponding evaluation software have been used.

In another embodiment of the method of producing a composite material, at least one additive is provided in addition to the components provided in steps a) and b). The mixture produced in c) then generally additionally comprises additives. However, it is also possible to add only a portion of the additives or the total amount of additives during the heat treatment.

An additive is defined as a component that mathematically contributes less carbon to the total amount of carbon produced during heat treatment after heat treatment than does a carbon precursor.

Hereinafter, various types of additives, i.e., particulate additives (insoluble additives) and miscible additives, will be described.

The use of at least one additional additive enables improved separation suppression during carbonization and/or better carbon yield from the precursor. The achievable reduction in separation of additives makes the carbonization production process simpler and/or the silicon loss during grinding of the material in step d) is reduced and/or the homogeneity of the material is improved and associated improved material properties, such as improved anode processes for material manufacture or improved cycle stability.

In one embodiment, the at least one additive is a component that is insoluble in at least one carbon precursor, preferably selected from inorganic materials, particularly preferably titanium oxide, silicon dioxide, aluminum oxide, zirconium dioxide, boron oxide, silicon carbide, natural graphite, carbon black, amorphous carbon, graphene, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon nanohorns, vapor-grown carbon fibers or any mixture thereof, very particularly preferably graphite, natural graphite, synthetic graphite, expanded graphite, carbon black, graphene, amorphous carbon, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon nanohorns, vapor-grown carbon fibers or mixtures thereof.

In the context of the present invention, inorganic materials are to be understood as meaning all metals and metalloids under standard conditions and also their non-gaseous compounds. Likewise, inorganic materials include all carbon materials and allotropic forms of elemental carbon.

In the context of the present invention, amorphous carbon is to be understood as meaning all non-graphitic carbon materials, such as hard carbon, soft carbon, activated carbon, porous carbon and coke. Within the scope of the present invention, soft carbon is understood to be a graphitizable, non-graphitic carbon material, whereas hard carbon is understood to be a non-graphitizable, non-graphitic carbon material.

The smaller the particle size of silicon used for the C — Si composite, the more the silicon can assume the function of the additive.

The at least one additive advantageously has an average particle size d50 or shortest axis of less than 10 μm, preferably less than 8 μm, particularly preferably less than 6 μm, very preferably less than 4 μm, very particularly preferably less than 2 μm. In the case of sizes greater than 10 μm, there are not enough additive particles present in each product particle. Thus, the positive effect of the at least one additive does not occur. If a plurality of additives is used, it is sufficient for their average shortest axis to be less than 10 μm, preferably less than 8 μm, particularly preferably less than 6 μm, very particularly preferably less than 4 μm, very particularly preferably less than 2 μm.

The shortest axis is measured by electron microscopy and, depending on the expected particle size of the additive, the average particle size d50 is measured by laser particle size analysis (ISO 13320-.

The at least one additive is advantageously used in a ratio such that the amount of the at least one additive is 90 to 1 wt.%, preferably 70 to 1 wt.%, particularly preferably 50 to 1 wt.%, very particularly preferably 40 to 1 wt.%, of the composite part which does not contain silicon.

In certain embodiments, less additive is used such that the amount of additive is generally less than 1 weight percent of the portion of the composite material that does not include silicon. Very small amounts of additives can generally be used to substantially inhibit mesophase formation occurring during heat treatment. This applies in particular to the case of very fine distribution of the additives in the mixture, for example, when the average particle size d50 or the shortest axis of the additives is less than 10 μm.

If the amount of the at least one additive is more than 90% by weight of the portion of the composite material not containing silicon, the amount of the remaining residue (i.e., carbon matrix) of the portion of the composite material not containing silicon is contained in the composite material in only a very small amount. As a result, the ratio of silicon to carbon matrix is unfavorable, and there is no preferred composite material in which silicon is distributed in a carbon matrix.

The heat treatment according to step d) with additives is advantageously carried out at a temperature of up to 400-1600 ℃, preferably 400-1450 ℃, particularly preferably 600-1450 ℃, very particularly preferably 800-1350 ℃ with exclusion of oxygen for a duration of from 1 second to 720 hours, preferably from 1 second to 360 hours, particularly preferably from 1 second to 240 hours, very particularly preferably from 1 second to 72 hours, very particularly preferably from 1 second to 24 hours.

At temperatures less than 400 ℃, the carbon precursor is not yet fully converted to carbon, which, depending on the precursor, can lead to various problems such as toxic residues, insufficient purity, foreign elements, insufficient conductivity, and the like. 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.

In the case of a duration of less than 1 second, such large amounts of volatile decomposition products are released from the carbon precursor that removal/disposal of the decomposition products poses difficulties on a large industrial scale. In the case of a duration of more than 720 hours, separation is generally no longer sufficiently prevented.

The abrasive particles from step d) advantageously have a particle size distribution with an average particle size d50 of 0.5to 60 μm, preferably 1 to 40 μm, particularly preferably 1 to 30 μm, very particularly preferably 2 to 25 μm, for example 4 to 20 μm.

As described above, the particle size distribution is determined by means of laser particle size analysis (ISO 13320-.

In another embodiment, the at least one additive is a component miscible with the carbon precursor selected from the group consisting of resins, polymers, polymerization initiators, polymeric crosslinkers or mixtures thereof, preferably phenolic resins, resorcinol resins, cresol resins, alkylphenol resins, cyanate resins, epoxy resins, furan resins, polyester resins, alkyd resins, unsaturated polyester resins, vinyl ester resins, acrylic resins, bismaleimide resins, silicone rubbers, polyacrylonitrile, polyimides, polyisoprene, polybutadiene, polychloroprene, ethylene propylene diene rubbers, polyvinyl alcohol, polyvinyl chloride, polystyrene, 2, 3-dimethyl-2, 3-diphenylbutane, Friedel-Crafts catalysts, urotropine, sulfur, bismaleimide or mixtures thereof.

Mixtures are understood to be physical and "chemical" (e.g. copolymer) mixtures. Miscible means, within the scope of the present invention, soluble and emulsifiable.

The foregoing carbon precursor materials may also be used as additives for this alternative embodiment. During the heat treatment, the additive that is miscible with the carbon precursor may react with itself or with the carbon precursor and change pyrolytically. To ensure that the additive is reactive, it must be polymerizable and/or crosslinkable.

The amount of the at least one additive is advantageously from 90 to 1% by weight, preferably from 70 to 1% by weight, particularly preferably from 50 to 1% by weight, very particularly preferably from 40 to 1% by weight, based on the silicon-free fraction of the composite material.

If the amount of the at least one additive is more than 90% by weight of the portion of the composite material not containing silicon, the amount of the remaining residue of the portion of the composite material not containing silicon (i.e. the carbon matrix) is only contained in very small amounts in the composite material. As a result, the ratio of silicon to carbon matrix is unfavorable and for the purposes of the present invention, there is no longer a composite material in which silicon is uniformly distributed in the carbon matrix. At less than 1% by weight, the effect of the at least one additive is insufficient to achieve the desired improvement in the homogeneity of the material according to the invention.

The heat treatment with the additive in step d) is advantageously carried out at a temperature of up to 400-1600 ℃, preferably 400-1450 ℃, particularly preferably 600-1450 ℃, very particularly preferably 800-1350 ℃ under exclusion of oxygen and for a duration of from 1 second to 720 hours, preferably from 1 second to 360 hours, particularly preferably from 1 second to 240 hours, very particularly preferably from 1 second to 72 hours, very particularly preferably from 1 second to 24 hours.

At temperatures less than 400 ℃, the carbon precursor has not been completely converted to carbon, which can lead to various problems, such as toxic residues, insufficient purity, foreign elements, insufficient conductivity, and the like, depending on the precursor. 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.

In the case of a duration of less than 1 second, such large amounts of volatile decomposition products are released from the carbon precursor that removal/disposal of the decomposition products poses difficulties on a large industrial scale. In the case of a duration of more than 720 hours, separation is generally no longer sufficiently prevented.

The abrasive particles from step d) advantageously have a particle size distribution with a d50 of 0.5to 60 μm, preferably 1 to 40 μm, particularly preferably 1 to 30 μm, very particularly preferably 2 to 25 μm, for example 4 to 20 μm.

As described above, the particle size distribution is determined by means of laser particle size analysis (ISO 13320-.

The composite material according to the invention can be used as a single component or at least one component of an active material for the anode of a lithium ion battery, a lithium sulfur battery and/or a sodium ion battery. Other possible components of the active material are, for example, natural graphite, synthetic graphite, expanded graphite, amorphous carbon (hard carbon, soft carbon) or any mixtures thereof.

In the following, the invention will be described, by way of example only, with reference to advantageous embodiments and with reference to the accompanying drawings.

The invention is not limited by the accompanying drawings.

The drawing is a schematic view that can be obtained by observation using an optical polarization microscope.

Drawings

Fig. 1a-1f schematically show different stages of C-Si composite formation (basic problem).

Fig. 2a-2d schematically show different stages of forming a C-Si composite material using a heat treatment according to the invention without additives or using a heat treatment with miscible additives according to the invention.

Fig. 3a-3e schematically show different stages of forming a C-Si composite using a heat treatment with insoluble additives according to the invention.

Fig. 1a-1f schematically show the reaction of a homogeneous raw mix consisting of nanosilicon (1) and mesogenic carbon precursor (2).

Fig. 1a shows the homogeneous raw mix before the start of the heat treatment.

Fig. 2b shows that mesophase (3) starts to form during the heat treatment.

Fig. 1c shows a silicon-free intermediate phase (3) grown during the heat treatment, as a result of which a certain concentration of nano-silicon is present in the still isotropic phase.

Fig. 1d shows the silicon-free anisotropic region and the silicon-rich isotropic region resulting from the mesophase due to partial coalescence of the mesophase. Until the heat treatment is completed, the structure hardens to form a fully carbonized C-Si composite having regions (5) of Si-free carbon material resulting from mesophases; and a silicon rich region (6) in the C-Si composite, the region resulting from the silicon rich isotropic region.

By means of the depicted broken lines, fig. 1e shows a schematic subdivision of the heat-treated material into grain sizes corresponding to the depicted grid spacings.

Fig. 1f schematically shows the powder obtained after milling with silicon-free particles (5) consisting of pure carbon material, silicon-rich particles (6) and particles (4) with a corresponding amount between the extreme possibilities of silicon-free particles consisting of only carbon material and particles consisting almost entirely of silicon without any detectable carbon material. The possible compositional ranges of the individual particles lead to the possibility that the particles differ in their respective densities depending on the amount of carbon or silicon they contain.

Fig. 2a-2d schematically show the reaction of a homogeneous raw mix consisting of nanosilicon (1) and mesogenic carbon precursor (2).

Fig. 2a shows the homogeneous raw mix before the heat treatment starts.

Fig. 2b shows the formation of the mesophase (3) during the heat treatment according to the invention. The individual mesophases remain smaller than in fig. 1b-f, and the number of mesophases can be increased by increasing nucleation during the heat treatment.

By the depicted broken lines, fig. 2c shows a schematic subdivision of the composite material after heat treatment into grain sizes corresponding to the depicted wire mesh spacing.

Fig. 2d schematically shows the powder obtained after grinding.

The smaller range of possible compositions of the individual compositions results in smaller differences in the respective densities of the particles compared to fig. 1 f.

Fig. 2a-2d also illustrate the process of using a miscible additive that is homogeneously mixed with the carbon precursor (2).

Fig. 3a to 3e schematically show the reaction of a homogeneous raw mix consisting of nanosilicon (1) with mesogenic carbon precursor (2) and insoluble additives (7).

Fig. 3a shows the homogeneous raw mix before the heat treatment starts.

Fig. 3b shows that mesophase (3) formation starts during the heat treatment.

Fig. 3c shows the growing mesophase, however, the additive inhibits further growth of the mesophase, and thus the growth of the mesophase is reduced compared to the case of fig. 1. This can be inhibited by the effect of the additive on the viscosity of the mixture during heat treatment, which is beneficial for the purposes of the present invention, and/or by increasing the nucleation of the mesophase during heat treatment, so that the mesophase is smaller and/or more uniformly distributed in the material.

With the depicted broken lines, fig. 3d shows a schematic subdivision of the composite into a granularity corresponding to the depicted line mesh spacing.

Fig. 3e schematically shows the powder obtained after grinding.

The insoluble additive (7) leads to other changes in the possible composition of the individual particles compared to fig. 1f or fig. 2 d. The density of the individual particles is calculated from the amounts of carbon, silicon and insoluble additive (7) in the particles. The growth of the mesophase is reduced due to the appropriate choice of insoluble additives, which makes it possible to obtain a more homogeneous particle composition overall than with the powder material shown in fig. 1 f.

Detailed Description

Hereinafter, the present invention will be described with reference to embodiments, which do not constitute a limitation of the present invention.

General procedure

i) Silicon and carbon precursors are provided.

ii) heating the carbon precursor until the carbon precursor is sufficiently molten and fluidized, for example, for the mixing equipment used. For lowering the treatment temperature or for dissolving the carbon precursor, a solvent suitable therefor may be used, wherein the carbon precursor is (mainly) soluble in the solvent at least at elevated temperatures. Alternatively, the silicon may also have been pre-dispersed in a solvent suitable for the carbon precursor and subsequently added to the carbon precursor. To support the depolymerization of nanosilicon, it is necessary to carry out an ultrasonic treatment (for example using a sonotrode), a particularly high-shear mixer operating at high speed (for example dissolver/toothed disc, Ultra-) Or some other special dispersing equipment (e.g. basket mills, colloid mills, stirred ball mills, high pressure injection and nozzle processes), as is known, for example, from the paint, paint and ceramic industries.

iii) mixing the silicon in the form of a powder or dispersion and the melted, liquefied, softened and/or dissolved carbon precursor by means of at least one suitable stirring means, such as a propeller stirrer, dissolver, kneader, planetary mixer or combinations thereof. The purpose of the mixing process is to produce a raw mix that is as homogeneous as possible and with the silicon nanoparticles disaggregated as possible, i.e. with the silicon nanoparticles homogeneously distributed in the carbon precursor or carbon precursor solvent mixture.

iv) optionally evaporating/distilling off volatile components of the carbon precursor and/or the solvent added to the mixture to reduce the amount of material used for the subsequent heat treatment step. This step may be necessary, for example, to prevent uncontrolled boiling or foaming of the material during the subsequent heat treatment, to recover and recycle solvent, to reduce the thermal post-combustion load in the subsequent step, or to reduce precipitation until the heat treatment is complete. The evaporation or distillation of volatile constituents or solvents can be effected by heating the raw meal mixture and optionally supported by reduced pressure or vacuum and/or by purging the gas space of the vessel with a gas or gas mixture (e.g. air, nitrogen, argon, carbon dioxide, etc.).

v) heat treating the green stock mixture resulting from step iii) or step iv) to carbonize the carbon precursor. The heat treatment of the raw mix is carried out in a suitable vessel (e.g. an open crucible made of steel, graphite or ceramic) under exclusion of oxygen (e.g. in a nitrogen or argon atmosphere or in vacuum) by heating to the final temperature for a specific time in a kiln suitable for the purpose, which is preferably equipped with a thermal afterburner for burning volatile exhaust gases. After reaching the final temperature, the resulting mixture may optionally be left in the kiln at the final temperature for a specified period of time to complete the heat treatment. The kiln may be, for example, a muffle furnace, a tube furnace, a chamber furnace, a car-type furnace, a rotary kiln, a ring kiln, a tunnel kiln, a pushed felt kiln, a continuous pulling kiln or any other continuously or batchwise operated kiln having any kind of heat generation (e.g. electricity, solar-heat or by burning solid, liquid and/or gaseous fuels). Thereafter, the heat treated C-Si composite is cooled in a kiln to <400 ℃ by controlled or natural cooling and then removed from the kiln. Cooling outside the furnace may be carried out until below 400 ℃ to avoid oxidation of the C-Si composite, as long as the C-Si composite is protected from air or oxygen.

vi) the C-Si composite obtained is comminuted/ground to the desired particle size by crushing and grinding, for example by pre-comminution using jaw crushers, cone crushers, roller crushers, etc., and by subsequent single-stage or multistage grinding, for example using impact mills, rotor mills, beater mills, hammer mills, jet mills, ball mills, etc. (e.g. with integrated sieves for setting the particle size distribution).

Embodiment 1 (comparative example, prior art):

for steps i) -iii) of the general procedure 0.23kg of nanosilica (average particle size d50 of about 100-200nm, metal contaminants <3 wt%, non-metal contaminants 5-15 wt%, available from e.g. Alfa Aesar or Sigma-Aldrich) is thoroughly dispersed in 1.0kg of tetrahydrofuran (for synthesis, stabilization, available from e.g. VWR) using a dissolving stirrer (available from e.g. IKA), the dispersion in the oil bath is heated to about 50 ℃, followed by a total of 1.0kg of bitumen particles, powders or pellets (available from e.g. Deza, Koppers, R ü tgers, bilbai de alvine) having a softening temperature of about 60-120 ℃ being added little by little to the heated dispersion, during the procedure the mixture is vigorously stirred using a dissolving stirrer (available from e.g. a) for about 1 hour, so that the nanosilica is stirred as much as possible until the viscosity in the nanosilica-silicon is reduced in the solvent mixture, which it is difficult to carry out further stirring to dilute the stirred batch if the stirring process is performed homogeneously.

For step iv) of the general procedure, the tetrahydrofuran is subsequently removed from the mixture by distillation at ambient pressure, while the mixture is stirred further. For this purpose, for example, a distillation structure is used with a 4-5L three-necked flask with an attached water-cooled Liebig condenser and attached stirrer with a stable metal or PTFE stirring blade. The distillation is carried out until at least 70% of the amount of tetrahydrofuran used is removed again from the raw meal mixture or until the viscosity of the mixture has increased to such an extent that complete circulation and mixing of the substances using the stirrer is no longer possible at a mixture temperature of less than 160-180 ℃.

Subsequently, the cooler and stirrer are removed and, in order to prepare step v) of the general procedure, the hot raw mix is transferred hot into a sufficiently large solvent-, bitumen-and temperature-resistant vessel or, after cooling to ambient temperature, the glass flask is broken and the hardened raw mix is introduced cold into a sufficiently large solvent-, bitumen-and temperature-resistant vessel in any form. For example, a large metal can or a metal bucket or a graphite crucible having at least twice the volume of the raw mix is used as the container.

Subsequently, in step v) of the general procedure, the raw meal mixture is heated in a vessel in a chamber kiln of a thermal after-combustion system for burning the produced exhaust gas to a final temperature of 700-1000 ℃ in a nitrogen atmosphere for about 50-200 hours and is kept at the final temperature for a further 6-12 hours. Then, the furnace heating is turned off, and the furnace is passively cooled. During cooling, the kiln chamber is further purged with nitrogen to prevent oxidation. Once the temperature in the kiln falls to <200 ℃, the nitrogen purge is stopped immediately, the kiln is opened and the material is removed.

For step vi) of the general procedure, the heat-treated material is finally crushed and comminuted by coarse mechanical means (for example using a hammer) and broken into fragments having a size of up to 1cm, for example using a hammer, mortar or jaw crusher. The crushed material was then pre-ground in an impact mill to coarse grind to a particle size of about 200 μm and ground to the target particle size distribution in a second impact mill with a fine screen.

Results of embodiment 1:

0.51kg of product powder, mean particle size d 50: 5 μm, reversible capacity of first cycle: 910mAh/g, efficiency of first cycle: 84%, reversible capacity of cycle 40: 690mAh/g, density variation range δ: 0.12.

for electrochemical analysis of the material, electrodes were fabricated from the milled powder product and subsequently analyzed in a laboratory half cell (coin cell 2016) using a Maccor 4000 cell testing apparatus. To fabricate the electrodes, the powder was mixed with CMC binder (about 700,000g/mol, available from Acros), Super P conductive carbon black (available from Imerys), SFG6 conductive graphite (available from Imerys), and deionized water to prepare an ink with the appropriate viscosity for the subsequent coating process, and the ink was coated onto copper foil (matte, 20 μm thick, available from Schlenk) using a bench top film stretching apparatus with a defined blade gap height to achieve the desired area mass loading. The composition of the electrode coating was as follows: the active materials comprise conductive carbon black, conductive graphite and a binder, wherein the mass fraction of the conductive carbon black to the binder is 88:2:3: 7. Water may be used to adjust the viscosity of the ink, if necessary. The coating was dried in a controlled manner and a circular electrode disk (14 mm diameter) suitable for 2016 coin cells was subsequently punched out of it (tooling and housing parts for coin cell manufacture, all of which are available from Hohsen). The mass of the electrode on the copper foil was determined by weighing, for which the mass of copper was subtracted from the weight of the electrode disk. The electrode disks were dried (> 110 ℃ C.) and subsequently button cells (half-cells) were made in an argon-filled glove box with round punched lithium foil (16 mm diameter, available from AlfaAesar) as counter electrode and separator (GF/D, available from Whatman) and electrolyte. The electrolyte (commercially premixed by UBE) had the following composition: 1mol/L of lithium hexafluorophosphate dissolved in ethylene carbonate: ethyl methyl carbonate (1:1, vol) + 2% vinylene carbonate + 10% fluoroethylene carbonate.

The cell was charged and discharged ("cycled") in a controlled manner on a cell test apparatus (Maccor, series 4000), the lithium counter electrode also serving as a potential reference in the half cell configuration. The cell test was performed as follows: formation (3 cycles): charging: 0.1C CC to 20mV, CV to C/100; discharging: 0.1 CC to 1.5V. Cycle (after forming): charging: 0.5C CC to 20mV, CV to C/20; discharging: 0.5 CC to 1.5V (CC ═ constant current, CV ═ constant voltage).

In order to use the C — Si composite material as an anode material of a lithium ion battery, the C — Si composite material may be used alone, or, for example, mixed with a graphite anode material in any manner to prepare an anode material (active material), and the specific capacity may be adjusted by a mixing ratio.

The electrochemistry of the following embodiments is also determined according to the method.

The density range of the composite material is determined by separating the particles into a liquid with a defined density, either by a floating method or by gravity separation based on the density of the mineral in the heavy liquid.

In order to determine the density range of the composite material, the density of the heavy liquid is varied within the average density range of the composite material, and thus the achievable separation of the composite material into "lighter" and "heavier" fractions is determined in each case. Particles having a density greater than the density of the liquid may settle while particles having exactly the same density as the liquid continue to be suspended therein and particles having a density less than the density of the liquid float therein. Since the density difference between particles and liquid is very small, the separation of particles according to their density only proceeds very slowly in the gravitational field, and therefore the process is accelerated using a centrifuge which is thermostatted to prevent the effect of temperature on the density of the heavy liquid.

Suitable heavy liquids include, for example, defined mixtures of high density liquids (e.g., 1, 2-dibromopropane, 1, 3-dibromopropane, 1,2, 3-tribromopropane) and low density liquids (e.g., toluene, xylene, decane, dodecane). The density of the liquid or of the mixture thereof is measured using a shaking method according to DIN EN ISO15212-1(2009) at a temperature at which the actual experiment for separating the particles also takes place. In all experiments care was taken to ensure that the composition of the mixed liquid and thus the density of the more volatile components did not change due to evaporation.

To determine the density range of the powder, about 1-2g of the powder is weighed into centrifuge tubes having a volume of about 15mL, exactly the same amount is weighed into each centrifuge tube, and filled with a heavy liquid to a total volume of at least 10-12mL in the average density range of the material at exactly known densities, followed by tight sealing. The volume of the liquid is at least four times the total volume of the powder so that spatial separation of the particles can be achieved depending on their density in the liquid. To prevent agglomerates or air entrainment, the suspension was treated in an ultrasonic bath for 15 minutes.

A series of samples each having a slightly different weight fluid were prepared, the density of the weight fluid being 0.01g/cm³Step change of (2). Subsequently, all samples were centrifuged in a centrifuge at maximum speed for at least 48-72 hours.

For the sample where the powder completely settled to the bottom, the density of the heavy liquid was lower than the density of all the powder particles. For samples where the powder completely floated to the top, the density of the heavy liquid was greater than the density of all the powder particles. Thus, in both cases, the density of the heavy liquid used in each case indicates the lower or upper limit of the possible density of the powder particles.

In the density range in between, the liquid with the particles suspended therein was divided into three equal sized volume fractions (upper, middle and lower thirds) by removing successively each partial volume of the suspension from the centrifuge tube using a pipette. The three separate portions were then dried to constant weight and the powder residue of each portion was weighed (to an accuracy of at least 0.1% of the total mass of the composite material used). The number of particles that settle to the bottom (lower third) or float to the top (upper third) can thus be determined. The particles of the middle section are exactly the same density as the heavy liquid and can therefore be used to control and optionally correct the quality of the lower and upper sections, since it has been assumed that particles of such density are uniformly distributed in all three sections.

The density of the heavy liquor was explored in successive similar experiments by varying the density of the heavy liquor, which floated or settled only 5 wt% of the composite material used. For this purpose, the mass fractions of the experiments (slightly different density of the heavy liquid in each case) are plotted against the corresponding density of the heavy liquid in the individual fractions, and the density of the heavy liquid with 5% by weight of the composite material used in the upper or lower part is determined from the data.These densities therefore correspond to a lower density threshold ρ ×₁Or an upper density threshold ρ₂And 90% by weight of the composite particles are in p₁And ρ₂In the density range therebetween. Average density ρ and lower density threshold ρ ×, respectively, of a composite material₁Or an upper density threshold ρ₂The difference | ρ - ρ | between them_1,2| is the amount of the variation range δ of the density. If the average density p of the composite material is equal to the lower density threshold p₁Or an upper density threshold ρ₂The difference | ρ - ρ | between them_1,2If | is not equal, the greater of the two quantities is used to determine the amount of the variation range δ.

The density ranges for the following embodiments are also determined according to the method.

Embodiment 2:

similar to embodiment 1, but with a more rapid heat treatment in step v). For this reason, steps i) to iv) and vi) are performed in embodiment 2 in the same manner as in embodiment 1. However, in embodiment 2, the heat treatment of step v) is performed within 3 to 12 hours. For this purpose, the raw meal mixture is heated in a vessel in a chamber kiln with a thermal post-combustion system for burning the produced exhaust gases in a nitrogen atmosphere to a final temperature of 800-. Due to the faster heating to the final temperature, a large amount of pyrolysis gases can be released in a shorter time, for which the kiln and its thermal post-combustion system are designed. In addition, the risk of excessive foaming is greater, which is why sufficiently large containers are used. Then, the furnace heating is turned off, and the furnace is passively cooled. During cooling, the kiln chamber is further purged with nitrogen to prevent oxidation. Once the temperature in the kiln falls to <200 ℃, the nitrogen purge is immediately stopped, the kiln is opened and the material is removed.

Results of embodiment 2:

0.53kg of product powder, mean particle size d 50: 5 μm, reversible capacity of first cycle: 1110mAh/g, efficiency of first cycle: 85%, reversible capacity of cycle 40: 780mAh/g, density variation range delta: 0.06.

examples 3 a-c:

embodiment 3 a:

in contrast to embodiment 1, in step iii) of the general procedure in embodiment 3a, a pulverulent additive which is insoluble in the carbon precursor is additionally added and mixed into the raw mix, so that the additive is finally present in the raw mix homogeneously like nanosilica.

For embodiment 3a, in steps i) to iii) of the general procedure, 0.23kg of nanosilica (average particle size d50 of about 100-200nm, metal contaminants < 3% by weight, non-metal contaminants 5-15% by weight, commercially available, for example, from Alfa Aesar or Sigma-Aldrich) are thoroughly dispersed in 1.2kg of tetrahydrofuran (for synthesis, stabilization, commercially available, for example, from Alfa Aesar or Sigma-Aldrich) using a dissolving stirrer (commercially available, for example, from IKA) in a 5L glass beaker in a heatable oil bath, the dispersion in the oil bath is heated to about 50 ℃, followed by a total of 0.8kg of graphite powder (commercially available, for example, from Deza, Koppers, R ü tgers, Bilbaina de altrianes) having a particle size d50 of 3-6 μm and 0.1kg of graphite powder (commercially available, for example, from imo, phtoppm ü) being added to the graphite powder dispersion as a dissolving stirrer mixture at one point during the vigorous stirring procedure, the graphite powder is added to the graphite dispersion, for example, as a graphite-vwa solvent, and the dispersing the graphite dispersion is stirred uniformly.

The steps iv) to vi) of the general procedure were subsequently carried out in the same manner as in embodiment 1.

Results of embodiment 3 a:

0.58kg of product powder, mean particle size d 50: 5 μm, reversible capacity of first cycle: 1120mAh/g, efficiency of first cycle: 86%, reversible capacity of cycle 40: 800mAh/g, density variation range δ: 0.04.

Embodiment 3 b:

similar to embodiment 3a, but in step iii) there is a nanoscale, pulverulent additive which is insoluble in the carbon precursor. In embodiment 3b, carbon black is used as an additive instead of the graphite powder of embodiment 3 a.

For embodiment 3b, in steps i) to iii) of the general procedure, 0.23kg of nanosilica (average particle size d50 of about 100-200nm, metal contaminants < 3% by weight, non-metal contaminants 5-15% by weight, commercially available, for example, from Alfa Aesar or Sigma-Aldrich) are thoroughly dispersed in 2.0kg of tetrahydrofuran (for synthesis, stabilization, commercially available, for example, from Alfa Aesar or Sigma-Aldrich) using a dissolving stirrer (commercially available, for example, from IKA), the dispersion in the oil bath is heated to about 50 ℃, followed by a total of 0.8kg of bitumen particles, powders or pellets (commercially available, for example, from Deza, Koppers, R ü tgers, Bilbaina de altranes) having a softening temperature of about 60-120 ℃ and 0.1kg of an average primary particle size d50 of <1 μm (commercially available, for example, from caboy, orb) are added to the dispersion of the nanosilica dispersion in the procedure as a stirring stirrer and the dispersion is stirred thoroughly, as a mixture of nanosilicon and the nano-silicon additive, the dispersion is stirred in a stirring procedure, as a mixture, or until the dispersion is possible.

The steps iv) to vi) of the general procedure were subsequently carried out in the same manner as in embodiment 3 a).

Results of embodiment 3 b:

0.56kg of product powder, mean particle size d 50: 5 μm, reversible capacity of first cycle: 1100mAh/g, efficiency of first cycle: 83%, reversible capacity of cycle 40: 780mAh/g, density variation range delta: 0.03.

embodiment 3 c:

similar to embodiment 3a or embodiment 3b, but with two different additives insoluble in the carbon precursor in step iii), such that the two additives are eventually present in the raw meal mixture as homogeneously as nanosilica.

In embodiment 3c, a combination of graphite powder as the additive in embodiment 3a and carbon black as the additive in embodiment 3b in a mass ratio of 1:1, i.e. 0.05kg of graphite powder from embodiment 3a and 0.05kg of carbon black from embodiment 3b, is used. The remaining materials (types, amounts) and steps of embodiment 3c were used and were performed as in embodiment 3 b.

Results of embodiment 3 c:

0.57kg of product powder, mean particle size d 50: 5 μm, reversible capacity of first cycle: 1110mAh/g, efficiency of first cycle: 85%, reversible capacity of cycle 40: 790mAh/g, density variation range δ: 0.04.

embodiments 4 a-c:

embodiment 4 a:

in contrast to embodiment 1, in step iii) or step iv) of the general procedure of embodiment 4a, additionally additives which do not form mesophases are added and mixed into the raw mix, so that the additives are finally present homogeneously in the raw mix.

For embodiment 4a, in steps i) -iii) of the general procedure, 0.25kg of nanosilica (average particle size d50 of about 100-200nm, metal contaminants < 3% by weight, non-metal contaminants 5-15% by weight, available, for example, from Alfa Aesar or Sigma-Aldrich) are thoroughly dispersed in 1.0kg of tetrahydrofuran (for synthesis, stabilization, available, for example, from Alfa aesr or Sigma-Aldrich) using a dissolver stirrer (available, for example, from IKA), in a 5L glass beaker in a heatable oil bath, the dispersion in the oil bath is heated to about 50 ℃, and then a total of 0.55kg of bitumen particles, powders or chealvanade alvine (available, for example, from Deza, Koppers, R ü tgers, Bilbaina de alikatranes) are added little by little to the heated dispersion, in a total of 0.55kg of bitumen particles, powders or cheimax (available, for example, from dehazer a), having a softening temperature of about 60-120 ℃ until the mixture is vigorously stirred for about 1 hour, in a little more than possible using a dissolver stirrer (available, for example, in a dissolver), and the subsequent step ii) is stirred again as a novolac additive in the general procedure, which is added, in a similar manner to about 60-2 kg of the novolak dispersion.

If it is difficult to mix the additive in a bitumen-solvent mixture below the melting point of the additive, it is helpful to have started step iv) of the general procedure during the mixing process, i.e. evaporate the solvent little by little and raise the temperature of the mixture above the melting point of the additive to support mixing with the additive.

Results of embodiment 4 a:

0.45kg of product powder, mean particle size d 50: 5 μm, reversible capacity of first cycle: 1070mAh/g, efficiency of first cycle: 82%, reversible capacity of cycle 40: 770mAh/g, density variation range δ: 0.04.

embodiment 4 b:

in contrast to embodiment 1, in step iii) or step iv) of the general procedure of embodiment 4b, a crosslinkable additive which does not form a mesophase is additionally added and mixed into the raw mix, so that the additive is finally present homogeneously in the raw mix.

For embodiment 4b, in steps i) -iii) of the general procedure, 0.23kg of nanosilicon (average particle size d50 of about 100-200nm, metal contaminants were introduced using a dissolution stirrer (commercially available from IKA, for example) in a 5L glass beaker in a heatable oil bath<3% by weight, non-metallic contaminants 5-15% by weight, available for example from Alfa Aesar or Sigma-Aldrich, are well dispersed in 1.0kg of tetrahydrofuran (for synthesis, stabilization, available for example from VWR), the dispersion in the oil bath is heated to about 50 ℃, and then a total of 0.8kg of bitumen particles, powders or aggregates (available for example from Deza, Koppers, R ü tgers, Bilbaina de alloys) with a softening temperature of about 60-120 ℃ are added little by little to the heated dispersion, during this procedure the mixture is stirred vigorously using a dissolver stirrer and/or propeller or anchor stirrer (available for example from IKA) for about 1 hour to depolymerize the nanosilica as far as possible until the nanosilica is uniformly dispersed in the bitumen-solvent mixture, then 0.45kg of unsaturated polyester resin (available for example from reholhd, Synthopol,) As additive and 6g of 2, 3-dimethyl-2, 3-diphenylbutane (commercially available, for example, from Acros, akzo nobel) as free-radical initiator, and again stirred vigorously for about 1 hour.

The steps iv) to vi) of the general procedure were subsequently carried out in the same manner as in embodiment 1. During the heat treatment in step v), the additive or its pyrolysis causes a large amount of foaming, which is why the container for the material in this step needs to be large enough.

Results of embodiment 4 b:

0.49kg of product powder, mean particle size d 50: 5 μm, reversible capacity of first cycle: 1100mAh/g, efficiency of first cycle: 84%, reversible capacity of cycle 40: 780mAh/g, density variation range delta: 0.04.

embodiment 4 c:

in contrast to embodiment 1, a crosslinking agent is additionally added as an additive in step iii) or step iv) of the general procedure of embodiment 4c and mixed into the raw mix so that the additive is finally present homogeneously in the raw mix.

Compared to embodiment 1, 0.85kg of particles, powders or granules of bitumen (available for example from Deza, Koppers, R ü tgers, Bilbaina de Alquiries) with a softening temperature of about 60-120 ℃ are used in steps i) -iii) of embodiment 4c, the remaining materials (types, amounts) and steps of embodiment 4c are used and are carried out as in embodiment 1. in step iii) of the general procedure, 0.09kg of sulphur (powder, at least 99% pure, available for example from VWR, Carl-Roth, Sigma-Aldrich) is added after the nanosilicon has been completely dispersed and mixed into the mixture for about 1/2 hours so that the additive is present homogeneously in the raw mix before the start of step iv). during the heating of step iv), the reaction of sulphur with the bitumen results in the release of hydrogen sulphide from about 120-150 ℃ in order to prevent excessive foaming in step iv) or step v) of the general procedure, a sufficiently large vessel for slow, high-power and high-power stirring and high-power reaction vessels to be helpful.

The reaction gas can be introduced into the wash flask containing the sodium hydroxide solution in step iv) by flushing the flask with nitrogen to absorb hydrogen sulphide, or combusting the hydrogen sulphide in a suitably designed thermal post-combustion system with off-gas purification during the heat treatment in step v).

Results of embodiment 4 c:

0.55kg of product powder, mean particle size d 50: 5 μm, reversible capacity of first cycle: 1090mAh/g, efficiency of first cycle: 82%, reversible capacity of cycle 40: 750mAh/g, density variation range delta: 0.03.

According to the examples and the figures, in particular during the production of composite materials, composite materials having a particularly narrow density distribution are obtained according to the invention,

the temperature of the mixture is only kept in a temperature range in which a portion of the mixture forms an intermediate phase or melts for as short a time as possible, since the components of the mixture in the intermediate phase or molten state promote the formation of anisotropic regions, and/or

Addition of particulate additives (insoluble additives), and/or

-adding a miscible additive.

If the temperature of the mixture can only be increased very slowly due to the conditions of the production facility in order to convert the carbon precursors in the mixture into carbon, it is advisable to add additives to the mixture in order to achieve the density distribution according to the invention. This is necessary, for example, if large amounts of a mixture having a large volume and a relatively small surface area are to be heated in a kiln intended for heat treatment. The mixture is then heated only slowly, for example due to its limited thermal conductivity and high thermal capacity, both of which lead to a thermally inert behavior, so that if no additives are added, there may be a significant risk of mesophase formation. The additives then lead to a reduction in mesophase formation and thus ultimately generally to a uniform density of the composite material obtained.

However, if the temperature of the mixture can be increased more quickly (e.g., if the mixture is placed in a small portion or fine distribution in a hot kiln or heating zone to increase the possible heating rate of the mixture by, for example, increasing the surface area exposed to heat (relative to the mass of the mixture), or to heat only a small volume of the mixture in a rapidly heating kiln or hot zone), then it is generally not necessary to add additives to inhibit mesophase formation to the extent that a density profile according to the present invention is produced.

Of course, additives may be added to the mixture even if the temperature is rapidly increased. This generally leads to a further improvement in the homogeneity of the composite obtained (i.e. an even narrower density distribution).

List of reference numerals

1 nm silicon

2-carbon precursor

3 intermediate phase

4 particles with carbon and silicon

5 particles with pure carbon material

6 silicon-enriched particles

7 insoluble additive

Claims (15)

1. Composite material comprising silicon and carbon, characterized in that the proportion of silicon is between 1 and 80% by weight and at least 90% by weight of the composite material is at a lower density threshold p₁And an upper density threshold value rho₂In the density range between, the density threshold p_1,2Is characterized by the following relationship:

ρ*_1,2＝(1±δ)·ρ

wherein ρ is the average density of the composite material and ± δ is the upper density threshold ρ ·₂With said lower density threshold ρ ×₁The range of variation therebetween, the amount of δ<0.10。

2. Composite material according to claim 1, characterized in that it is present as a (sub) particulate composite powder material.

3. The composite material according to claim 1, characterized in that it has a particle size distribution with an average particle size (d50) of 0.5to 60 μ ι η.

4. Composite material according to claim 1 or 2, characterized in that the silicon is present in the form of particles at least partially enclosed in carbon.

5. The composite powder material according to claim 4, wherein the ratio of the average particle size of the silicon to the average particle size of the composite powder material is from 0.0005 to 0.1.

6. Composite material according to claim 1, characterized in that at least 3% by weight of the composite material lies from a lower density threshold p₃Extending to an upper density threshold value p₄In a density range outside the range of (a), the density threshold value ρ ×, and_3,4is characterized by the following relationship:

ρ*_3,4＝(1±δ_min)·ρ

where ρ is the average density of the composite and ± δ_minIs the upper density threshold ρ ×₄With said lower density threshold ρ ×₃Range of variation therebetween, δ_minThe amount of (B) is more than or equal to 0.005.

7. Composite material according to claim 1, characterized in that it satisfies the following condition:

wherein

d50_SiRepresents the average size of the silicon particles in units of "μm",

p_crepresents the amount of carbon in the composite material, expressed in weight%,

p_Sirepresents the amount of silicon in the composite material, expressed in weight%, and

s is a safety parameter, which is 0.02.

8. Composite material according to claim 1, characterized in that the silicon is at least partially crystalline.

9. A method of manufacturing a composite material according to any one of claims 1 to 8, comprising the steps of:

a) providing silicon

b) Providing at least one carbon precursor

c) Generating a mixture comprising the components from steps a) and b), and

d) manufacturing the composite material by further processing the mixture from step c), the processing comprising heat treatment and comminution.

10. The method according to claim 9, wherein the carbon precursor from step b) is selected from pitch, tar, biomass, polymer and resin based feedstock or mixtures thereof with a carbon yield > 5%.

11. Method according to claim 9, characterized in that the mixture obtained in step c) is uniformly distributed and in that the temperature profile and the comminution of the heat treatment in step d) are adapted to the mixture obtained in step c), so as to obtain a composite material according to claim 1.

12. The method according to claim 9, characterized in that at least one additive is provided in addition to the substances provided in steps a) and b), and in that the mixture produced in step c) contains the additive.

13. The method of claim 12, wherein the at least one additive is a component that is insoluble in the at least one carbon precursor.

14. The method of claim 12, wherein the at least one additive is a component miscible with the carbon precursor, the component selected from a resin, a polymer, a polymerization initiator, a polymeric crosslinker, or mixtures thereof.

15. Use of a composite material according to any one of claims 1 to 8 as the sole component or at least one component of an active material for the anode of a lithium ion battery, a lithium sulfur battery and/or a sodium ion battery.