Method for producing a carbon support with nanoscale silicon particles located on the surface, and a corresponding carbon support, especially for use in accumulators

Abstract

Method for producing a carbon support with nanoscale silicon particles on its surface, wherein a silicon precursor in an inert carrier gas is brought into contact with the carbon support and wherein the carbon support is heated by means of a pulsed inductive energy input and thereby deposition of the silicon particles occurs.

Description

[0001] The invention relates to a method for producing a carbon support with nanoscale silicon particles located on the surface and a carbon support with silicon deposited thereon.

[0002] Lithium iron phosphate (LiFePO4) batteries deliver very high discharge currents, with a continuous load of 35c and a peak load of 100c possible. ["c" here does not stand for the SI unit coulomb "C" = A·s, but is a relative measure of the battery load. The current (in A) is equal to the c-value multiplied by the battery capacity (in A·h). For a battery with a capacity of 5 A·h and a load of 20c, the current is therefore 100 A.] Unlike conventional lithium-ion cells, no metallic lithium (Li) is deposited during overcharging, and no oxygen is released, as is the case with conventional lithium-ion batteries. Lithium deposition and oxygen release in older types of lithium-ion batteries lead to thermal runaway, which under unfavorable conditions can even cause the cell to explode. This is not possible with a lithium iron phosphate battery.

[0003] The lithium is stored as lithium iron phosphate on the cathode side of the battery cell. When the cell discharges, the resulting lithium ions (Li) migrate + ) to the anode side and are deposited there. Depending on the battery's design, this occurs between carbon layers (for example, graphene) and / or in fine silicon powder with a particle size in the nanometer to micrometer range.

[0004] Preferably, ultrafine silicon powder, i.e., with particle sizes in the range of 1 to 100 nm, is used. This pure and ultrafine silicon powder is produced by the thermal decomposition of silanes in a tube furnace under protective gas. The silanes are heated either by heating the outer surfaces of the reactor or by microwave radiation, which heats the silanes (DE 103 53 995 A1 and DE 103 53 996 A1). The heating of the silanes by heating the outer surfaces of the reactor or by microwave radiation is also described in patent DE 10 2005 011 940 A1.

[0005] The intercalation (deposition) of Li + Carbon has a higher electrochemical velocity compared to silicon. Therefore, a composite of carbon or graphite with fine silicon powder is preferred for the anode side. The carbon acts as an electrical conductor and as a fast intermediate storage medium for lithium. +The silicon serves to form the majority of the lithium. + The process of incorporating lithium into graphene layers is slower compared to incorporation into graphene layers. When incorporating lithium... + In silicon, the volume of the silicon particles increases two- to threefold. This constant, significant volume change of the silicon particles leads to major structural changes in the anode, which can damage the battery in the long run.

[0006] In an article by Magasinski et al. (“High-performance lithium-ion anodes using a hierarchical bottom-up approach”, Nature Materials 9, 353-358 (2010), doi: 10.1038 / nmat2725), a chemical vapor deposition (CVD) process of silicon onto a carbon support is described. However, this method yields only relatively large silicon particles and, in particular, only large silicon-coated carbon particles. While these particles are intended to compensate for volume variations, they are poorly suited for use in battery cells. Furthermore, this method requires long reaction times (> 1 h) and low pressures (< 50 mbar), rendering it unsuitable for industrial applications. An example of silicon-coated nano-carbon tubes produced in this way is disclosed in US 2008 / 0280207A1.

[0007] It is therefore an object of the invention to create a simple and economical method for depositing silicon on a carbon support, which is particularly suitable for use in accumulator cells.

[0008] It is also an object of the invention to provide a carbon support with nanoscale silicon deposited on it.

[0009] This problem is solved by a method having the features of claim 1 and by a carbon support with nanoscale silicon deposited thereon having the features of claim 12.

[0010] According to the invention, a method for depositing nanoscale silicon, i.e., silicon with an average particle size of 1 nm to 100 nm, onto a carbon support is provided, in which a silicon precursor in an inert carrier gas is brought into contact with the carbon support, wherein the carbon support is heated by means of pulsed inductive energy input, thereby resulting in the release of highly reactive and finely dispersed silicon. The silicon particles exhibit a distinct adhesion to the carbon support, so that they exist as a layer.

[0011] According to the invention, a carbon support with silicon deposited on it is also provided, which has an average particle size of 1 nm to 20 nm and is produced by the process according to the invention. The following descriptions regarding the process according to the invention also apply accordingly to the carbon support with nanoscale silicon particles.

[0012] In the context of the invention, carbon support refers to carbon that serves as a support for nanoscale silicon.

[0013] The carbon support according to the invention with nanoscale silicon particles has many advantages. Firstly, the carbon acting as the carbon support preferably has a porous, sponge-like carbon layer which, when the carbon support is used on the anode side of a battery cell, at least partially compensates for the significant volume expansion of the silicon particles. Due to the high porosity of the carbon support, during the intercalation of Li + In silicon, the extended metal particles fill the pore spaces, thus preserving the carbon layer on the anode side to a near-complete extent. Furthermore, the carbon support simultaneously serves as an electrical conductor, electrically connecting the silicon particles. The carbon support also functions as a rapid intermediate storage medium for lithium. +. Due to the particle sizes of silicon and carbon mentioned below, the carbon support according to the invention is well suited for use in accumulators.

[0014] Since the inductive heating in the process according to the invention only increases the temperature at the surface of the carbon support itself, the silicon precursor only reacts when the carbon support is contacted. This allows the silicon released during the reaction to be deposited in very high yield as a layer consisting of individual nanoscale particles, and the unreacted silicon precursor can optionally be reused.

[0015] The inductive heating is controlled in pulses, so that the energy supply can be controlled simply and very precisely.

[0016] For the deposition of the nanoscale silicon particles, a pulsed inductive energy supply is used for the induction coil, wherein the pulsed energy supply preferably has a pulse frequency (pulse rate of the amplitude) of 1 millihertz to 10 kilohertz and a pulse width of pulse time ein 0.1 milliseconds to 1000 seconds and pulse time aus 0.1 milliseconds to 1000 seconds. Furthermore, the pulse shape of the amplitude can preferably be sinusoidal, rectangular, or triangular.

[0017] The support material is preferably heated to a temperature of 200 to 950 °C, particularly 250 to 500 °C, according to the process. At these temperatures, only a very small amount of silicon carbide is formed as a side reaction, but this is advantageous because it results in improved fixation of the silicon particles to the carbon support.

[0018] The carbon used as a carbon carrier in the invention can be commercially available types of carbon, carbon black (e.g., Vulkan® XC72 (registered trademark of Cabot Corporation) and Ketjenblack® (registered trademark of Nouryon Functional Chemicals BV), high surface area activated carbon (HSAC), various types of graphite, graphene, or carbon nanotubes (CNTs), with turbostratic carbons and carbons with a particle size of 1 nm to 12 µm being preferred.

[0019] It is particularly preferred as a carbon carrier - Graphite with an average particle size of 2 to 10 µm, in particular of 2.5 to 7 µm and particularly preferably of 3 to 5 µm, and a surface area of ​​5 to 30 m² 2 / g, especially from 10 to 27 m 2 / g and especially preferably from 15 to 25 m 2 / G, - Conductive carbon black with an average particle size of 1 to 5 µm, in particular of 1 to 3 µm and especially preferably of 1 to 2 µm, and a surface area of ​​30 to 80 m² 2 / g, especially from 40 to 70 m 2 / g and especially preferably from 50 to 65 m 2 / g, as well as - High-surface-area carbon with an average particle size of 1 to 500 nm, in particular of 30 to 300 nm and especially preferably of 30 to 150 nm, and a surface area of ​​80 to 1200 m² 2 / g, especially from 200 to 1000 m 2 / g and especially preferably from 300 to 900 m 2 / G.

[0020] The preferred ranges specified here also apply to other carbons with the same particle sizes and / or surface areas.

[0021] Suitable silicon precursors include, in particular, silanes such as monosilane SiH4, disilane Si2H6 or polysilanes with the formula -[SiH2] n- with n > 8, and secondly cyclic silanes such as cyclotrisilane Si3H6, cyclotetrasilane Si4H8, cyclopentasilane Si5H 10 or cyclohexasilane Si6H 12 , and secondly halogen derivatives of silanes such as monochlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3) or tetrachlorosilane (SiCl4), and organic silicon compounds such as silicon alcoholates (tetraethoxysilane or tetramethoxysilane) or silicon esters (silicon formate or silicon acetate).

[0022] The silicon precursor is diluted via the inert carrier gas, for example nitrogen, helium, neon, argon, krypton, xenon and / or mixtures thereof, whereby the size and thus also the surface area of ​​the deposited silicon particles is controlled by the mixing ratio, since the dilution of the silicon precursor prevents agglomeration into larger silicon particles or silicon layers.

[0023] Preferably, the average particle size of the silicon particles is 1 to 10 nm, particularly preferably 2 to 8 nm, and most preferably 3 to 7 nm. The surface area of ​​the silicon particles is particularly in the range of 20 to 1000 m². 2 / g, preferably in the range of 30 to 750 m 2 / g and particularly preferably in the range of 50 to 500 m 2 / G.

[0024] When using silane as a silicon precursor and argon as a carrier gas, the silane content is 0.5 to 60%, preferably 5 to 50% and particularly preferably 10 to 30%.

[0025] The process is preferably carried out at a gas pressure of 0.1 to 5 bar, more preferably from 0.3 to 3 bar, and particularly preferably from 0.5 to 2 bar.

[0026] Conventional reactors or reactor types are used to carry out the process, preferably fixed-bed and fluidized-bed reactors. Inductive heating saves energy because the entire reactor chamber does not need to be heated and maintained at temperature. This also reduces the reactor's operating time and the reaction duration.

[0027] With pulsed heating of the support material and the use of fixed-bed or fluidized-bed reactors, the reaction time is significantly reduced to < 1 h; furthermore, the reaction can be carried out at higher pressures (> 100 mbar to 5 bar).

[0028] Further preferred embodiments of the invention result from the other features mentioned in the dependent claims.

[0029] The invention is explained in more detail below with reference to a figure. It shows Fig. 1 in a schematic representation an exemplary setup on a laboratory scale for carrying out the method according to the invention.

[0030] The structure according to Fig. Apparatus 1 comprises a three-necked flask 10 as a reactor, in which tetraethoxysilane or tetrachlorosilane is placed in ethylene glycol or ethanol. A strong hydrogenating agent such as NaBH4 or LiAlH4 is added via a dropping funnel 11. The process is carried out under a protective gas, which also acts as a carrier gas, and which is introduced via a gas valve 12. The silane released by the hydrogenating agent is transported by the carrier gas to a reverse frit 13, which also serves as a reactor and on which carbon or a carbon support 14 is arranged. The region of the reverse frit 13 containing the carbon 14 is surrounded by an induction coil 15, which inductively heats the surface of the carbon. This causes the silane in contact with the carbon or carbon support to react, and silicon particles are deposited on the surface of the carbon or carbon support.Unused silane can be collected by means of a downstream cold trap, which is not shown in the drawing.

[0031] As an alternative to the described setup, commercially available silane from gas cylinders can also be used, which in this form can be mixed with the protective or carrier gas in defined proportions. Furthermore, the process can also be carried out under vacuum to allow for better control of the silicon deposition.

[0032] The following series of experiments are carried out with commercially available silane, wherein 2 g of a carbon, also referred to as a carbon carrier within the scope of the invention, are placed in a 500 ml reverse frit. The setup adapted to the modified experimental conditions according to Fig.The apparatus is evacuated three times, with a vacuum of approximately 30 mbar being applied for 10 minutes. Each time, the setup is then filled with argon. After the third vacuum, the apparatus is no longer filled with pure argon, but with a mixture of silane and argon. After the deposition process is complete, the glass apparatus is thoroughly rinsed with argon.

[0033] The carbon used is: - Graphite with an average particle size of 3 to 5 µm and a surface area of ​​15 to 25 m² 2 / G, - Conductive carbon black with an average particle size of 1 to 2 µm and a surface area of ​​50 to 65 m² 2 / g, as well as - High-surface-area carbon with an average particle size of 30 to 150 nm and a surface area of ​​300 to 900 m² 2 / G.

[0034] The following gas mixtures of silane and argon are used: - 10% silane and 90% argon, - 20% silane and 80% argon, and - 30% silane and 70% argon.

[0035] The gas mixtures are introduced into the apparatus at a pressure of approximately 1.05 to 1.2 bar. Reference symbol list 10 three-necked pistons 11 drip funnels 12 Gas valve 13 Reverse frit 14 Carbon carriers 15 Induction coil

Claims

[1] Method for producing a carbon support with nanoscale silicon particles on its surface, wherein a silicon precursor in an inert carrier gas is brought into contact with the carbon support and wherein the carbon support is heated by means of a pulsed inductive energy input and thereby deposition of the silicon particles. [2] Method according to claim 1, characterized by , that the carbon carrier is heated to a temperature of 200 to 950 °C. [3] Method according to claim 1 or 2, characterized by , that silanes, polysilanes, cyclic silanes, halogen derivatives of silanes or organic silicon compounds are used as silicon precursors. [4] Method according to any one of claims 1 to 3, characterized bythat the carbon carrier is selected from the following group: carbon black, such as Vulkan® XC72 and Ketjenblack®, large surface area activated carbon, graphite, graphene and carbon nanotubes, or mixtures thereof. [5] Method according to any one of claims 1 to 4, characterized by that the carbon carrier is a turbostratic carbon. [6] Method according to any one of claims 1 to 5, characterized by , that the carbon support has an average particle size of 1 nm to 12 µm. [7] Method according to any one of claims 1 to 6, characterized by that the carbon support has a porous structure, at least in some areas. [8] Method according to any one of claims 1 to 7, characterized by that nitrogen, helium, neon, argon, krypton and / or xenon are used as inert gases. [9] Method according to any one of claims 1 to 8, characterized by that the proportion of silicon precursor is between 0.5 and 60%. [10] Method according to any one of claims 1 to 9, characterized by that the process is carried out at a gas pressure of 0.1 to 5 bar. [11] Method according to any one of claims 1 to 10, characterized by that the process is carried out in a fixed-bed or fluidized-bed reactor. [12] Carbon support with silicon particles on the surface, characterized by that the carbon carrier was produced according to the method according to one of claims 1 to 11.

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