WO2016091957A1

WO2016091957A1 - Process for producing an electrode containing silicon particles coated with carbon

Info

Publication number: WO2016091957A1
Application number: PCT/EP2015/079137
Authority: WO
Inventors: Andreas Mueller; Matthias Georg SCHWAB; Mingjie ZHONG; Shyam Sundar Venkataraman; Klaus MÜLLEN; Hermann Sachdev; Axel Binder
Original assignee: Basf Se; MAX-PLANCK-Gesellschaft zur Förderung der Wissenschaften e.V.
Priority date: 2014-12-10
Filing date: 2015-12-09
Publication date: 2016-06-16

Abstract

The present invention relates to a process for producing an electrode containing silicon particles which are coated with carbon (SP2). The respective process is carried out under plasma conditions in combination with a fluidized bed process since silicon particles (SP1) to be coated with carbon are fluidized into the reactive zone of an apparatus (A), employing a gaseous stream (G) containing at least one carbon- containing gas. The coating of the silicon particles (SP1) in the reactive zone (RZ) of apparatus (A) is preferably carried out via a chemical vapor deposition (CVD) process. The silicon particles coated with carbon (SP2) as obtained in process step d) of the present invention are further processed in order to obtain an electrode containing such silicon particles coated with carbon (SP2). The present invention further relates to such an electrode as well as to a battery containing such an electrode. The present invention also relates to the use of such an electrode containing silicon particles coated with carbon (SP2) within such a battery which preferably is a lithium-ion-battery.

Description

Process for producing an electrode containing silicon particles coated with carbon Description

The present invention relates to a process for producing an electrode containing silicon particles which are coated with carbon (SP2). The respective process is carried out under plasma conditions in combination with a fluidized bed process since silicon particles (SP1 ) to be coated with carbon are fluidized into the reactive zone of an apparatus (A), employing a gaseous stream (G) containing at least one carbon- containing gas. The coating of the silicon particles (SP1 ) in the reactive zone (RZ) of apparatus (A) is preferably carried out via a chemical vapor deposition (CVD) process. The silicon particles coated with carbon (SP2) as obtained in step d) of the present invention are further processed in order to obtain an electrode containing such silicon particles coated with carbon (SP2). The present invention further relates to such an electrode as well as to a battery containing such an electrode. The present invention also relates to the use of such an electrode containing silicon particles coated with carbon (SP2) within such a battery which preferably is a lithium-ion-battery.

Li ion batteries (LIB) have been considered as the core e-mobility technology. Graphite with the theoretical specific capacity of 372 mAh/g is the most widely used commercial anode material for LIB because of its high Coulombic efficiency and good cycle performance. Desirable improvement levels of 20-35% of the electrode's total capacity are achieved when the negative electrode capacity reaches 1200mAh/g. To achieve this, the current graphite anode must be replaced with Li accommodation materials which can offer higher capacity.

Silicon with its high theoretical capacity of 4200 mAh/g has been considered as the most promising candidate. The drawbacks of the high Li uptake are volume changes of the silicon of up to 300% during charging and discharging. The resulting fracturing of the silicon in the anode leads to contact and thus conductivity losses. Also, new solid electrolyte interface (SEI) layers may be formed on the exposed fresh silicon surface thereby depleting the cell electrolyte. Thus, the use of pure silicon is hindered by high irreversible capacity and short cycle life.

WO 2013/078645 A1 discloses a silicon/carbon composite which comprises mesoporous silicon particles and carbon coating provided on the silicon particles, wherein the silicon particles have two pore size distribution of 2-4 nm and 20-40 nm. Further, a process of preparing the silicon/carbon composite is disclosed, which comprises the steps of preparing mesoporous silicon particles via a mechanochemical reaction between SiCI₄ and Li₁₃Si₄ under ball milling and subsequent thermal treatment and washing process, and coating the mesoporous silicon particles with carbon. Further, an anode for lithium ion battery and a lithium ion battery is disclosed, which comprises the silicon/carbon composite.

US-A 5,620,743 discloses a process for coating solid particles such as granules or fibers composed of thermoplastic or thermoset polymers, pigments or granules composed of organic dyes or active substances in a fluidized bed by application of a gaseous coating agent from a plasma, wherein the plasma is generated outside the fluidized bed under 0.01 -500 mbar, and the plasma-activated gas is passed into the fluidized bed, which is operated under 0.1 -500 mbar, where a. the plasma is generated from the total amount of gaseous coating agent with or without another gas, or

b. the plasma is generated from a portion of the gaseous coating agent with or without another gas, and the remaining portion is introduced directly into the fluidized bed, or

c. the plasma is generated from another gas, and the total amount of gaseous coating agent is introduced directly into the fluidized bed.

H. S. Shin et al. describe in "Deposition of diamond coatings on particles in a microwave plasma-enhanced fluidized bed reactor" (Material Letters 19 (1994) 1 19- 122) how to arrive to diamond coatings on particles (Si0₂ and Si), but nowhere the coating of silicon particles with carbon for the use in batteries.

M. L. Terranova et al. summarizes in "Si/C hybrid nanostructures for Li-ion anodes: An overview" (Journal of Power Sources 246 (2014) 167-177) recent and increasing efforts in the development of novel Li ion cell anode nanomaterials based on the coupling of C with silicon, but does not describe a fluidized bed process.

C. Vahlas et al. describe in "Principles and applications of CVD powder technology" (Materials Science and Engineering R 53 (2006) 1-72) the principles and different applications like in the nuclear power industry, heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat of CVD powder technology, but nowhere the coating of silicon particles with carbon for the use in batteries is mentioned.

CN-A 102332571 describes a silicon-carbon compound cathode material and a manufacturing method thereof. The manufacturing method of the silicon-carbon compound cathode material comprises an etching step, a carbon-layer covering step, a scattering step, a spraying pelletizing step and a carbonizing treatment step. The silicon-carbon compound cathode material prepared by using the method has a nano/micro structure, a first cycle efficiency over 85%, a first discharging capability over 1000mAh/g and a retention rate of 100 times cycle capacity over 90%. US 2006/051670 A1 discloses a metallic silicon powder which is prepared by effecting chemical reduction on silica stone, metallurgical refinement, and metallurgical and/or chemical purification to reduce the content of impurities. The powder is best suited as a negative electrode material for non-aqueous electrolyte secondary cells, affording better cycle performance.

EP 1 363 341 A2 discloses a conductive silicon composite in which particles having a structure in which crystallites of silicon are dispersed in silicon dioxide are coated on their surfaces with carbon affords satisfactory cycle performance when used as the negative electrode material in a non-aqueous electrolyte secondary cell.

The above described processes for the preparation of either carbon-coated silicon particles or an electrode containing silicon particles coated with carbon are complicated and require multiple process steps especially in connection with the coating of such silicon particles. Some of the processes for production of silicon particles coated with carbon make use of solvents and/or acids, which can introduce impurities. Furthermore the liquid waste has to be disposed of. Some of these processes make use of milling and/or high temperatures. Some of these processes are very tedious and/or time consuming. Often, no appropriate mixing of the silicon particles is achieved during the coating step with carbon. As a result these processes for the production of silicon particles coated with carbon are expensive and produce impure and inhomogeneous materials. By consequence, electrodes made from these materials are expensive and do not reach their full potential.

Thus, there is the need to develop better cleaner, cheaper and/or more efficient ways for the production of electrodes containing silicon particles coated with carbon.

It is an object of the present invention to provide a new process for obtaining an electrode based on silicon particles. The object is achieved by a process for producing an electrode containing silicon particles coated with carbon (SP2), wherein the process comprises the following steps a) to e): a) provision of silicon particles (SP1 ) on a sample holder (SH) of an apparatus (A), which comprises a sample holder (SH) and a reactive zone (RZ) and the sample holder (SH) is located below the reactive zone (RZ), b) generation of plasma in the reactive zone (RZ) of apparatus (A),

c) fluidizing the silicon particles (SP1 ) in apparatus (A) with a gaseous stream (G), containing at least one carbon-containing gas and optionally containing at least one further gas, into the reactive zone (RZ),

d) coating of the silicon particles (SP1 ) in the reactive zone (RZ) of apparatus (A) to obtain silicon particles coated with carbon (SP2), and

e) further processing of the silicon particles coated with carbon (SP2) to obtain the electrode containing silicon particles coated with carbon (SP2). A major advantage is that with this process it is possible to produce electrodes containing silicon particles coated with carbon (SP2), which are preferably homogenously as well as completely coated by carbon in a clean and/or fast way within an anhydrous atmosphere. The process is very clean since no solvent has to be used in steps a) to d). The process is also fast since steps a) to d) can be carried out as one synthesis which can take just 2 minutes.

Another major advantage in this process is the high flexibility according to treatment time, treatment energy, possible gases, mixing of gases or sequential use of gases and the number of possible process steps.

The electrodes obtained by the process of the present invention are advantageous since they combine the excellent mechanical, electrochemical and/or electrical properties of carbon with the superior lithium alloying ability of silicon. By consequence, they are to be suitable to overcome the limitations of pure silicon and offer significant improvements in the specific capacity and cycle stability.

By consequence, the process according to the present invention provides the production of an electrode containing silicon particles coated with carbon (SP2) in an advantageous manner and the electrode in turn provides an improvement of cycle stability and capacity of silicon based materials for Li ion batteries.

Another advantage is that any kind of carbon-containing gas e.g. alkanes or derivatives such as CH₄ or CH₂CI₂, alkenes such as C₂H₄, alkynes such as C₂H₂, aromatic compounds such as C₆H₆ or C₇H₈, alcohols such as CH₃OH or C₂H₅OH or any kind of carbon-containing compound that can be used in a controlled way for coating the particles (SP1 ) with carbon, or any mixture thereof, can be used.

Another advantage is that optionally further gases such as inert gases such as Ar and He, or reactive gases such as C0₂, N₂, H₂, NH₃, BCI₃, BF₃ or any other reactive gas which can be used to incorporate nitrogen or boron heteroatoms into the carbon coating layer, or any mixture thereof, can be used together with the above mentioned carbon-containing gas or gas mixture.

Another advantage is the high flexibility according to the process excitation power and treatment/reaction times at variable pressure.

Another advantage is that stable plasma can be maintained using the mentioned different plasma gas mixtures at variable pressure, i.e. at low or at high pressure. Another advantage is that when a low pressure is used, stable plasma can be maintained using the mentioned different plasma gas mixtures. This is possible even at relatively low excitation energies.

The process according to the invention for producing an electrode containing silicon particles coated with carbon (SP2) is defined in detail hereinafter.

In step a) of the process according to the invention, silicon particles (SP1 ) on a sample holder (SH) of an apparatus (A) are provided. The apparatus (A) according to the present invention comprises a reactive zone (RZ) and a sample holder (SH) located below the reactive zone (RZ).

The reactive zone (RZ) is the area inside apparatus (A) in which reactions of particles (SP1 ) take place. This area can consume a large area inside apparatus (A).

The sample holder (SH) in apparatus (A) is freely adjustable to the conditions needed and can be a glass frit or anything which is suitable by any means which is known to a person skilled in the art. Further, apparatus (A) may comprise at least one inlet for gaseous stream (G), and an outlet (O) located at the top of apparatus (A).

All parts and components of apparatus (A) are known to the person skilled in the art. Apparatus (A) may preferably be designed in such way in order to comply with a fluidized bed process under plasma conditions (as defined later in steps c) and d)).

Silicon particles (SP1 ) according to the present invention are different forms of silicon, like e.g. crystalline silicon, amorphous silicon, porous silicon or black silicon. Preferred forms of silicon according to the invention are selected from crystalline silicon and/or amorphous silicon. Further, the silicon particles (SP1 ) according to the invention may have different morphologies such as spheres, rods, fibers, (nano)wires or may be made of arbitrary geometry or may be a mixture of the aforementioned morphologies.

Further, the silicon particles (SP1 ) according to the invention may be solid or hollow, and/or they may have monomodal, bimodal or higher-order particle size distribution.

The silicon particles (SP1 ) may also be non-agglomerated particles or agglomerated.

The silicon particles (SP1 ) may be synthesized by e.g. laser ablation, laser synthesized from vapor phase, plasma synthesized, obtained by ball milling of silicon wafers or by any suitable method known in the state of the art. Silicon particles (SP1 ) with a particle size below 200 μηη, preferably below 100 μηη, more preferably below 50 μηη, even more preferably below 1 μηη, most preferably below 500 nm can be used in the process according to the invention. The silicon particles (SP1 ) are usually not smaller than 1 nm, preferably the silicon particles (SP1 ) are not smaller than 5 nm. For example, silicon particles with a particle size between 44 μηη and 149 μηη or an average size <= 50 nm (as commercially available by Alfa Aesar) may be used.

It is preferred that these silicon particles (SP1 ) predominantly contain pure silicon. Predominantly means that the silicon particles (SP1 ) comprise at least 95 wt.-%, preferably at least 98 wt.-%, most preferably 99.9 wt.-% of pure silicon. However, due to storage of the pure silicon, on the surface of pure silicon a native Si0₂-layer may develop by oxidation.

Further, the silicon particles (SP1 ) may contain impurities, e. g. oxygen, nitrogen, carbon or other elements due to the synthesis process of the silicon particles (SP1 ). These impurities may also be present in form of e. g. Si0₂ or SiC. These impurities may be present inside or on the surface of particles (SP1 ). Therefore, according to the present invention, the silicon particles (SP1 ) may comprise either pure silicon but also silicon with impurities, wherein pure silicon is preferred. Further, the mass percentage of the impurities is usually not higher than 15 wt.-%, preferably not higher than 10 wt.-%, more preferably not higher than 5 wt.-%.

Prior to step a) of the process according to the present invention, a drying step may be carried out with the silicon particles (SP1 ). This additional drying step may be carried out in order to remove volatiles and humidity from the silicon particles (SP1 ). Drying steps as such are known to persons skilled in the art. Preferably the silicon particles (SP1 ) are dried for a period of usually more than 0.5 hours and/or at elevated temperatures of at least 50°C. More preferably the drying step is carried out for at least 12 hours at 120°C in vacuo at a pressure of below 100 mbar, preferably below 10 mbar.

Prior to step a) of the process according to the invention, the surface of the silicon particles (SP1 ) may also be treated to remove surface contaminations (e.g. carbon based compounds) or surface layers (e.g. Si0₂). The treatment may be carried out, e.g. by an etching step using acids, in a plasma containing appropriate reactive gases, e.g. C_xFy compounds for removal of Si0₂ (see for example "Fluorocarbon-based plasma etching of Si0₂: Comparison of C₄F₆/Ar and C₄F₈/Ar discharges, X. Li, X. Hua, L. Ling, G. S. Oehrlein, M. Barela, H. M. Anderson, J. Vac. Sci. Technol. A 20 (2002) 2052- 2061 ), by high temperature annealing in a suitable reactive atmosphere, e.g. in C0₂ atmosphere for removal of amorphous carbon or by any other method suitable to clean the surface of the silicon particles (SP1 ) which is known for the person skilled in the art.

In step b) of the process according to the invention, the plasma in the reactive zone (RZ) of apparatus (A) is generated. The expression "generation of plasma" in the context of the present invention is understood as the ignition of the plasma in step b) and the maintenance of the plasma in steps b), c) and/or d).

The generator is then switched on, and the plasma is ignited in the reactive zone (RZ) without any gas flow at low pressure (P < 0.1 mbar).

For the ignition of the plasma with electromagnetic excitation a generator may be used. This step may be conducted without any gas flow at low pressure (e.g. < 0.1 mbar). The electromagnetic excitation frequency for the plasma generation in steps b), c) and/or d) is in the range selected from below 100 Hz, a low-frequency range between 100 Hz and 10 kHz, a radiofrequency range between 10 kHz and 300 MHz, a microwave frequency range between 300 MHz and 300 GHz, or above 300 GHz.

Further, the power fed via the plasma into the reactive zone (RZ) in steps b), c) and/or d) may be between 0.05 kW and 50 kW, preferably between 0.1 kW and 5 kW, more preferably between 0.2 kW and 3 kW, and most preferably between 0.7 kW and 1 .6 kW.

In step c) of the process according to the invention, the silicon particles (SP1 ) are fluidized in apparatus (A) with a gaseous stream (G), containing at least one carbon- containing gas and optionally containing at least one further gas, into the reactive zone (RZ).

Any kind of carbon-containing gas e.g. alkanes or derivatives such as CH₄ or CH₂CI₂, alkenes such as C₂H₄, alkynes such as C₂H₂, aromatic compounds such as C₆H₆ or C₇H₈, alcohols such as CH₃OH or C₂H₅OH or any kind of carbon-containing compound that can be used in a controlled way for coating the particles (SP1 ) with carbon, or any mixture thereof, can be used in the process according to the invention. Optionally, further gases such as inert gases such as Ar and He, or reactive gases such as C0₂, N₂, H₂, NH₃, BCI₃, BF₃ or any other reactive gas which can be used to incorporate nitrogen or boron heteroatoms into the carbon coating layer, or any mixture thereof, can be used together with the above mentioned carbon-containing gas or gas mixture.

The at least one carbon-containing gas is preferably selected from CH₂CI₂, C₂H₄, C₆H₆, C₇H₈, CH₄ or C₂H₂, and/or the optional further gas is preferably selected from Ar or H₂. The at least one carbon-containing gas is most preferably selected from CH₄ or C₂H₂, and/or the optional further gas is most preferably Ar.

The gaseous stream (G) is fed into apparatus (A1 ) from below the sample holder (SH). The gaseous stream (G) may be fed into apparatus (A1 ) at several positions below the sample holder (SH) at the same time. The gaseous stream (G) is usually continuous. However, in one embodiment of the invention in steps c) and/or d), gaseous stream (G) may be blocked for a short period of time, wherein the period of time is not longer than 20 seconds, preferably not longer than 10 seconds, more preferably not longer than 5 seconds, even more preferably not longer than 1 second. The blocking of gaseous stream (G) can be advantageous for the fluidization of the particles (SP1 ) in the reactive zone (RZ).

Another option to improve the fluidization of the particles (SP1 ) in the reactive zone (RZ) is to mix the particles (SP1 ) with e.g. micro-particles, which are between 200 and 1000 μηι. These particles can be filtered out after step d) of the process.

Further, the system pressure in apparatus (A) in steps b), c) and/or d) may be kept between 50 mbar and 0.05 mbar, preferably between 25 mbar and 0.05 mbar, preferably between 10 mbar and 0.05 mbar, most preferably between 1 mbar and 0.05 mbar. The gaseous stream (G) in the process according to the invention may have any suitable process flow rate known to a skilled person, for example it may have a process flow rate of about 100 seem. The process flow rate is a measure for the amount of gas(es) used in the process per minute. However, the process flow rate can be higher e.g. up to 5000 seem, preferably up to 2000 seem, more preferably up to 500 seem. In principle, there is no upper limit for the gas flow.

In step d) of the process according to the invention, the silicon particles (SP1 ) are coated in the reactive zone (RZ) of apparatus (A) to obtain silicon particles coated with carbon (SP2).

The coating as such according to step d) means that the silicon particles (SP1 ), which are fluidized according to step c), react with the carbon-containing gas of gaseous stream (G) under plasma conditions in the reactive zone (RZ) of apparatus (A). By consequence, silicon particles coated with carbon (SP2) are obtained.

The coating can be performed both as a full and as a partial coating on the surface of the silicon particles (SP1 ) to obtain silicon particles coated with carbon (SP2). Full coating of the silicon particles (SP1 ) is preferred. "Partial coating" means that at least 30% of the surface of the silicon particles (SP1 ) is coated, preferably at least 50% of the surface of the silicon particles (SP1 ) is coated, more preferably at least 70% of the surface of the silicon particles (SP1 ) is coated, most preferably at least 90% of the surface of the silicon particles (SP1 ) is coated. The thickness of the coating of the particles coated with carbon (SP2) can be adjusted as necessary. The coating usually has a thickness of less than 1 μηη, preferably the coating has a thickness of less than 500 nm, more preferably the coating has a thickness of less than 150 nm, even more preferably the coating has a thickness of less than 50 nm and most preferably the coating has a thickness of less than 20 nm.

The coating of the particles coated with carbon (SP2) may be porous. Further, the coating of the particles coated with carbon may be amorphous, graphitic or a mixture of both. Further, in the coating of the particles coated with carbon (SP2), heteroelements such as nitrogen, boron, sulfur, phosphorus or mixtures of different heteroelements may be included.

The silicon particles (SP1 ) are kept in the reactive zone (RZ) in steps c) and d) according to the invention for suitable times to fluidize and to coat them. The time of the silicon particles (SP1 ) kept in the reactive zone (RZ) in steps c) and d) is usually between 0.1 seconds and 7 days, preferably between 1 second and 2 days, more preferably between 10 seconds and 12 hours, even more preferably between 30 seconds and 6 hours and most preferably between 1 minute and 3 hours. The coating of the silicon particles (SP1 ) with carbon according to step d) is preferably carried out by chemical vapor deposition of at least one carbon-containing gas on the surface of the silicon particles (SP1 ) under plasma conditions.

As can be found in the literature (e.g. "Principles and applications of CVD powder technology", C. Vahlas, B. Caussat, Ph. Serp, G. Angelopoulos, Mat. Sci. Eng. Reports, 2006, 53, 1 -72), chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials including coatings and can be used for microfabrication processes to deposit materials in various forms. In one embodiment such a CVD process may be performed in a fluidized bed under plasma conditions. A plasma can be considered as an electrically neutral medium comprising electrons, ions and electronically excited species. In such a process the vapour reactants are ionised and dissociated by electron impact, and hence generating chemically active ions and radicals that undergo a chemical reaction at or near a substrate surface and deposit the solid material ("Chemical vapour deposition of coatings", K. L. Choy, Progress in Materials Science, 2003, 48, 57-170.).

The apparatus (A) according to the invention may be designed as a fluidized bed process under plasma conditions. A fluidized bed process is known to the person skilled in the art. Here, particles are brought in contact with the plasma in the reactive zone (RZ) by lifting them (fluidizing) by the process gas flow.

The silicon particles coated with carbon (SP2) produced according to the invention have a particle size below 200 μηη, preferably below 100 μηη, more preferably below 50 μηη, even more preferably below 1 .2 μηη, most preferably below 700 nm. The silicon particles (SP2) are usually not smaller than 2 nm, preferably the silicon particles (SP2) are not smaller than 10 nm.

Further, the silicon particles coated with carbon (SP2) are preferably conducting particles.

In an optional step i), which is performed after step d) and prior to step e) of the process according to the invention, the particles coated with carbon (SP2) may be treated to increase the porosity of the coating, e. g. by C0₂ etching, which is known by the person skilled in the art. In a further optional step ii), which is performed after step d) and prior to step e) of the process according to the invention, the particles coated with carbon (SP2) may be treated at high temperature in inert atmosphere to increase the graphitization degree of the coatings of the particles coated with carbon (SP2).

Furthermore, it is possible to perform both optional steps i) and ii) after step d) and prior to step e) of the process of the present invention in any order or even simultaneously or in parallel. In step e) of the process according to the invention the silicon particles coated with carbon (SP2) are further processed to obtain the electrode containing silicon particles coated with carbon (SP2). Step e) comprises the following partial steps e1 ) to e3).

In partial step e1 ) the silicon particles coated with carbon (SP2) are mixed with graphite, a further conductive additive and a binder in a solution.

The solution used in partial step e1 ) of the process may comprise any organic solvent and/or water. Preferably, the solution used in partial step e1 ) is an aqueous solution. If the solution used in partial step e1 ) of the process is an aqueous solution, it may comprise at least partially an organic solvent. Partially means that the aqueous solution comprises a percentage of at least 5 wt.-%, preferably at least 10 wt.-%, more preferably of 20 wt.- % and most preferably of 50 wt.-% of an organic solvent. Organic solvents are known to the person skilled in the art.

The concentration of the binder in the solution in partial step e1 ) is between 0.1 wt.-% and 20 wt.-%, preferably between 5 wt.-% and 15 wt.-%. The binder is preferably polyacryllic acid. The wt.-%-values of the concentration of the binder are related to the full amount of all components of the solution prepared in partial step e1 ).

The graphite content in partial step e1 ) is between 0.1 wt.-% and 80 wt.-%, preferably between 15 wt.-% and 60 wt.-%, and more preferably between 20 wt.-% and 40 wt.-%. Further, as graphite can be used e.g. Timrex SFG6L (as commercially available by Timcal). The wt.-%-values of the graphite content are related to the full amount of all components of the solution prepared in partial step e1 ).

The content of the further conductive additive in partial step e1 ) is between 0.1 wt.-% and 20 wt.-%, preferably between 5 wt.-% and 15 wt.-%. The further conductive additive is preferably carbon black. Further, as carbon black can be used e.g. Super P (C65) (as commercially available by Timcal). The wt.-%-values of the content of the further conductive additive are related to the full amount of all components of the solution prepared in partial step e1 ).

The mixing in partial step e1 ) may be carried out e.g. by adding the above mentioned components to a beaker and mechanically blending them by planetary mixing using a Thinky (Laguna Hills, California) ARE 200 mixer to obtain a homogeneous slurry. Mixing methods like the aforementioned or other suitable methods for mixing are known to the person skilled in the art. In partial step e2), which follows after partial step e1 ), an electrode support is coated with the mixture obtained in partial step e1 ).

Preferably the electrode support in partial step e2) is a metal foil, more preferably the electrode support in partial step e2) is a copper foil.

The coating (or casting) onto an electrode support may be carried out e.g. by the doctor blade method, which is known to the person skilled in the art, or any other suitable method known in the field. In partial step e3), which follows after partial step e2) the coated electrode support obtained in step partial e2) is dried and punched out to obtain an electrode containing silicon particles coated with carbon (SP2).

The drying of the coated electrode support in partial step e3) may be carried out at room temperature first, e.g. for 2h, followed by drying in a vacuum chamber. Drying in the vacuum chamber, which is generally know to the person skilled in the art, may be carried out for several hours e.g. 12 h at higher temperature, e.g. 100 °C and in vacuo, e.g. 1 mbar. The punching out of the electrode containing silicon particles coated with carbon (SP2) is usually carried out with a cutter, e.g. an EL-Cut electrode cutter (as commercially available from EL-CELL (Hamburg, Germany)).

Storage of the electrode containing silicon particles coated with carbon (SP2) may be assured in an argon-filled glove box.

The person skilled in the art knows that an electrode as e.g. produced by the process according to the invention can be an anode or a cathode. However, preferably here the electrode is an anode.

A preferred embodiment of the invention may comprise: i) gaseous stream (G) containing at least one carbon-containing gas, which is preferably selected from CH₂CI₂, C₂H₄, C₆H₆, C₇H₈, CH₄ or C₂H₂, and/or the optional further gas is preferably selected from Ar or H₂, and/or

ii) silicon particles (SP1 ) preferably having particle sizes below 50 μηη, and/or iii) a system pressure in apparatus (A) in steps b), c) and/or d), which is preferably kept between 25 mbar and 0.05 mbar, and/or

iv) blocking of gaseous stream (G) in step c) at least once for preferably not longer than 20 seconds, and/or

v) the time of the silicon particles (SP1 ) kept in the reactive zone (RZ) in steps c) and d) being preferably between 1 second and 2 days.

Preferably this embodiment contains all options i) to v) as defined above. Another preferred embodiment of the invention contains at least the following options: i) gaseous stream (G) containing at least one carbon-containing gas, which is preferably selected from CH₄ or C₂H₂, and/or the optional further gas, which is preferably Ar, and/or

ii) silicon particles (SP1 ) preferably having particle sizes below 1 μηη, and/or iii) a system pressure in apparatus (A) in steps b), c) and/or d), which is preferably kept between 1 mbar and 0.05 mbar, and/or

iv) blocking of gaseous stream (G) in step c) at least once for preferably not longer than 10 seconds, and/or

v) the time of the silicon particles (SP1 ) kept in the reactive zone (RZ) in steps c) and d) being preferably between 1 minute and 3 hours.

Preferably this embodiment contains all options i) to v) as defined above. Another preferred embodiment of the invention contains at least the following options: i) the concentration of the binder in the solution in step e1 ) is preferably between 5 wt.-% and 15 wt.-%, and/or

ii) the graphite content in step e1 ) is preferably between 15 wt.-% and 60 wt.-%, and/or

iii) the content of the further conductive additive in step e1 ) is preferably between 5 wt.-% and 15 wt.-%, and/or

iv) the solution in step e1 ) is preferably an aqueous solution, and/or

v) the binder in step e1 ) is preferably polyacryllic acid, and/or

vi) the further conductive additive in step e1 ) is preferably carbon black, and/or vii) the electrode support in step e2) is preferably a metal foil. Preferably this embodiment contains all options i) to vii) as defined above.

Another preferred embodiment of the invention contains at least the following options: i) the concentration of the binder in the solution in step e1 ) is preferably between 5 wt.-% and 15 wt.-%,

ii) the graphite content in step e1 ) is preferably between 20 wt.-% and 40 wt.-%, iii) the content of the further conductive additive in step e1 ) is preferably between 5 wt.-% and 15 wt.-%,

iv) the solution in step e1 ) is preferably an aqueous solution,

v) the binder in step e1 ) is preferably polyacryllic acid,

vi) the further conductive additive in step e1 ) is preferably carbon black, and/or vii) the electrode support in step e2) is preferably a copper foil.

Preferably this embodiment contains all options i) to vii) as defined above.

Another subject of the present invention is an electrode containing silicon particles coated with carbon (SP2) as such. An electrode can be an anode or a cathode, preferred is an anode.

Another subject of the present invention is a battery containing silicon particles coated with carbon (SP2) as such, preferably the battery is a lithium ion battery. The electrode containing silicon particles coated with carbon (SP2) according to the present invention can be used in a battery, preferably the battery being a lithium ion battery and/or the electrode preferably being an anode.

All definitions made for the process according to the invention apply as well for the above mentioned further subjects of the invention.

The present invention is further illustrated by the examples as follows. It should be noted that the following examples are illustrative only, and are not construed as limiting the invention in any way. Examples:

Starting material (silicon particles (SP1 )):

Alfa Aesar silicon particles; - 100 + 325 mesh size, which is corresponding to particles with a size between 44 μηι and 149 μηι (Alfa-Si-100-325)

and Alfa Aesar silicon particles; crystalline, laser synthesized from vapor phase, Average Particle Size (APS) <= 50 nm (50 nm Alfa Si)

General experimental procedure:

The hereby disclosed process is designed as a fluidized bed plasma process with standard access to various gases to the reactor (apparatus (A)).

A Huettinger PFG 500 RF generator (13.56 MHz, 5 kW) is used as plasma reactor. The RF power is applied through the matching circuit by an outer coil. The reaction chamber consists of an inner quartz tube of 60 mm diameter and a concentrically arranged outer quartz tube. A quartz frit below the induction coil serves as material support and gas distributor. The space between inner and outer tube is used for the water cooling of the reactor. The base pressure of the system is about 2e^"3 mbar.

All silicon particles (SP1 ) are dried at 120°C in vacuo for at least 12 h prior to loading them in the reactor. The reactor loading depends on the particles to be treated and is typically 30 g for Alfa-Si-100-325 and 1 g for 50 nm Alfa Si. After providing the silicon particles (SP1 ) onto the sample holder (SH), the reactor is evacuated. The generator is then switched on, and the plasma is ignited in the reactive zone (RZ) without any gas flow at low pressure (P < 0.1 mbar). Then the gas flow (gaseous stream (G)) is increased to the process flow rate. This also starts the fluidization of the particles. For the performed experiments the operating pressure during plasma modifications is in the range of 0.1 - 5 mbar. Typical process flow rates (combined gas flow of all used gases) are about 100 seem. The process flow rates have to be adjusted to the used particles to provide satisfactorily fluidization. Blocking of gaseous stream (G) can be used to improve fluidization. The experiments are carried out for 2 - 120 minutes under stable plasma conditions. After synthesis, the product (silicon particles coated with carbon (SP2)) is collected. Table 1 provides an overview over the carried out examples.

Example 1 :

Table 1 : Overview of carried out examples with Alfa-Si-100-325.

(comparison example)

Example 1c,

60 1 80 20 0 0 0 -0.2 Ar/CH₄ plasma

Example 1d,

60 0.8 0 100 0 0 0 -0.2 CH₄ plasma

Example 1e,

120 0.4 0 100 0 0 0 -0.2 CH₄ plasma,

Example 1f,

CH₄ plasma 60 0.8 0 100 0 0 0 3 hP (high Pressure)

Example 1g,

CH₄ plasma 60 0.8 0 100 0 0 0 -0.2 ds (downstream)

Example 1h,

60 80 0 20 0 0 -0.2 Ar/C₂H₂ plasma ¹

Example 1i,

30 80 0 20 0 0 -0.2 Ar/C₂H₂ plasma ¹

Example 1j,

15 80 0 20 0 0 -0.2 Ar/C₂H₂ plasma ¹

Example 1k,

2 80 0 20 0 0 -0.2 Ar/C₂H₂ plasma ¹

Example 11,

60 60 0 20 20 0 -0.2 Ar/C₂H₂/C0₂ plasma ¹

Example 1m,

60 60 0 20 0 20 -0.2 Ar/C₂H₂/H₂ plasma ¹

Results: Table 2: Elemental analysis of examples according to table 1 as well as of untreated particles (example 1v).

Ar/C₂H₂ plasma (Example 1 h, 60min) 1 ,2 < 0,5 < 0,5 98 < 0,5

Ar/C₂H₂ plasma (Example 1 i, 30min) 0,7 < 0,5 < 0,5 99 < 0,5

Ar/C₂H₂ plasma (Example 1j, 15min) < 0,5 < 0,5 < 0,5 99 < 0,5

Ar/C₂H₂ plasma (Example 1 k, 2min) < 0,5 < 0,5 < 0,5 >99.5 < 0,5

Ar/C₂H₂/C0₂ plasma (Example 11, 60 min) 0,5 < 0,5 < 0,5 99 < 0,5

Ar/C₂H₂/H₂ plasma (Example 1 m, 60 min) 1 ,5 < 0,5 < 0,5 99 < 0,5

TEM/EDXS data:

Pristine Alfa-Si-100-325 (untreated) (comparison example): No carbon coating is observed.

Example 1 b, H₂ plasma (comparison example): No carbon coating is observed.

Example 1 d, CH₄ plasma: A carbon coating is confirmed by imaging and EDXS (energy-dispersive X-ray spectroscopy). For some particles the amorphous C-Layer is continuous for other particles not. The thickness is up to about 10 nm.

Example 1 e, CH₄ plasma: Carbon is detected by EDXS. The thickness of the amorphous C-Layer is up to about 20 nm.

Example 1f, CH₄ plasma hP: Carbon coating is observed only occasionally, most probably due to the insufficient particle mixing during the synthesis process.

Example 1 h, Ar/C₂H₂ plasma: The thickness of the amorphous C-Layer is up to about 80 nm.

Example 1 i, Ar/C₂H₂ plasma: The thickness of the amorphous C-Layer is up to about 80 nm. Example 1j, Ar/C₂H₂ plasma: The thickness of the amorphous C-Layer is up to about 80 nm.

Example 1 k, Ar/C₂H₂ plasma: The thickness of the amorphous C-Layer is up to about 50 nm.

Example 11, Ar/C₂H₂/C0₂ plasma: The thickness of the amorphous C-Layer is up to about 80 nm.

Example 1 m, Ar/C₂H₂/H₂ plasma: The thickness of the amorphous C-Layer is up to about 120 nm. XPS analysis

XPS is a surface sensitive analysis method. The information depth is approximately 2- 10 nm (~ 1 -40 monolayers). The size of the measurement spot is approximately 0.3 x 0.8 mm. Because of the surface sensitivity and the limited information depth the quantitative and in the extreme case even the qualitative results of the elemental composition as determined by XPS can differ from results obtained by bulk methods such as elemental analysis.

The surface elemental composition of three examples as determined by XPS analysis is shown in Table 3. The relatively high C-content of the untreated material is due to surface contamination by adsorption of atmospheric carbon based impurities. This is a well known phenomenon. The C-content of the examples 1 d and 1 h is significantly higher (more than double) compared to the untreated example 1 v, thus confirming the successful coating of the silicon particles (SP1 ) by the process according to the invention in examples 1 d and 1 h.

Table 3: Surface elemental composition of examples 1v, 1 d and 1 h.

Example 2:

Table 4: Overview of carried out example with 50 nm Alfa Si.

Results:

Table 5: Elemental analysis of the example according to table 4 as well as of untreated particles (example 2v).

Example 3: Electrode preparation

Composite electrodes are made of above described silicon particles coated with carbon (SP2) as prepared in example 2 and graphite (Timrex SFG6L) as an active material, Super P carbon black (CB, Timcal) as conductive additive, polyacryllic acid (PAA, Mw = 450,000 grmol^"1 Sigma-Aldrich) as a binder. First, the as-prepared silicon particles (60 wt.-%), graphite (15 wt.-%), carbon black (15 wt.-%) and PAA binder in aqueous solution (10 wt.-%) are added to a beaker and mechanically blended by planetary mixing using a Thinky (Laguna Hills, California) ARE 200 mixer. The homogeneous slurry is then cast onto a copper foil using the doctor blade method. After coating, the coated foil is dried at room temperature for 2 h and then transferred to a vacuum drying chamber at a temperature of 100 °C. The coated foil is dried overnight at 1 mbar. Thereafter, 14 mm diameter electrodes are punched out using an EL-Cut electrode cutter from EL-CELL (Hamburg, Germany) and introduced into an argon-filled glove box. Example 4:

Electrochemical testing/characterization Prior to assembling a test cell, the electrodes containing silicon particles coated with carbon (SP2), as prepared according to example 3 are evacuated at 70°C overnight. The as-prepared electrode containing silicon particles coated with carbon (SP2) is tested as an anode material to investigate the effect of carbon coating on the electrochemical behavior (example 2a). For comparison, the bare silicon sample without carbon coating is also tested (example 2v). Two-electrode coin cells are assembled in an argon-filled glove box, using metal lithium as counter and reference electrodes. LiPF₆ 1 M in ethylene carbonate (EC): diethyl carbonate (DEC): fluoroethylene carbonate (FEC) = 3:6:1 are used as electrolyte. The cells are cycled at the potential range 0.01 -1 V at various C-rate using galvonostat-potentiostat (Bio-Logic, France). The C-rate is a measure of the rate at which a battery is charged/discharged relative to its maximum capacity. A 1 C rate e.g. means that the charge/discharge current will charge/discharge the entire battery in 1 hour. A C/10 rate e.g. means that the charge/discharge current will charge/discharge the entire battery in 10 hours. The cells are put in a formation process consisting one C/20, 20 cycles of C/10, and 20 cycles of C/5. The accommodation of Li⁺ (lithiation) into the working electrode characterized by the drop of potential is identified as the charge reaction; while the release of Li⁺ (delithiation, increase in potential) is called the discharge reaction. The galvanostatic cycling performance of samples at C/20, C/10 and C/5 rates are presented in Figure 1 . For the reference cell (example 2v) the initial reversible capacity amounts to 2394.9 mAh/g. The discharge capacity continuously decreases and after 35 cycles only about 15.5% of the initial capacity is recovered. It is suggested that the fast capacity fading is caused by structural failure and pulverization due to the huge volume change of Si during Li⁺ insertion and extraction during cycling process. The performance of the cell containing silicon particles coated with carbon (SP2) (example 2a) is superior compared to the reference anode. The first discharge capacity is lower than that of reference cell, which is associated with lower silicon content (45 wt.-%) in the cell containing silicon particles coated with carbon (SP2). The drop of capacity is much slower than that of reference cell and after 35 cycles approximate 66% of the initial capacity is recovered. The noticeable improvement of stability under cycling might be attributed to the fact that the silicon particles (SP2) are protected by carbon, which buffers the large volume changes and enhances the conductivity during cycling.

Claims

1 . A process for producing an electrode containing silicon particles coated with carbon (SP2), wherein the process comprises the following steps a) to e): a) provision of silicon particles (SP1 ) on a sample holder (SH) of an apparatus (A), which comprises a sample holder (SH) and a reactive zone (RZ) and the sample holder (SH) is located below the reactive zone (RZ),

b) generation of plasma in the reactive zone (RZ) of apparatus (A), c) fluidizing the silicon particles (SP1 ) in apparatus (A) with a gaseous stream (G), containing at least one carbon-containing gas and optionally containing at least one further gas, into the reactive zone (RZ),

e) further processing of the silicon particles coated with carbon (SP2) to obtain the electrode containing silicon particles coated with carbon (SP2).

2. A process according to claim 1 , wherein gaseous stream (G) contains at least one carbon-containing gas selected from an alkane, an alkylene, an alkyne, an aromatic compound, an alcohol or a carbon-containing compound that can be used in a controlled way for coating the particles (SP1 ) with carbon, or any mixture thereof, and/or

gaseous stream (G) optionally contains at least one further gas, selected from Ar, He, C0₂, H₂, N₂, NH₃, BCI₃, BF₃, or a gas which can be used to incorporate nitrogen or boron heteroatoms into the carbon coating layer, or any mixture thereof,

the at least one carbon-containing gas is preferably selected from CH₂CI₂, C₂H₄, C₆H₆, C₇H₈, CH₄ or C₂H₂, and/or the optional further gas is preferably selected from Ar or H₂,

the at least one carbon-containing gas is most preferably selected from CH₄ or C₂H₂, and/or the optional further gas is most preferably Ar.

3. A process according to any of claims 1 and 2, wherein

I) the silicon particles (SP1 ) have particle sizes below 200 μηη, preferably below 100 μηη, more preferably below 50 μηη, even more preferably below 1 μηη, most preferably below 500 nm, and/or

II) the silicon particles coated with carbon (SP2) after process step d) have particle sizes below 200 μηι, preferably below 100 μηι, more preferably below 50 μηι, even more preferably below 1 .2 μηη, most preferably below 700 nm.

A process according to any of claims 1 to 3, wherein i) the system pressure in apparatus (A) in steps b), c) and/or d) is kept between 50 mbar and 0.05 mbar, preferably between 25 mbar and 0.05 mbar, preferably between 10 mbar and 0.05 mbar, most preferably between 1 mbar and 0.05 mbar, and/or

ii) wherein in steps c) and/or d) gaseous stream (G) is blocked at least once for a short period of time, wherein the period of time is not longer than 20 seconds, preferably not longer than 10 seconds, more preferably not longer than 5 seconds, even more preferably not longer than 1 second.

A process according to any of claims 1 to 4, wherein the power fed via the plasma into the reactive zone (RZ) in steps b), c) and/or d) is between 0.05 kW and 50 kW, preferably between 0.1 kW and 5 kW, more preferably between 0.2 kW and 3 kW, and most preferably between 0.7 kW and 1 .6 kW, and/or

wherein the electromagnetic excitation frequency for the plasma generation in steps b), c) and/or d) is in the range selected from below 100 Hz, a low- frequency range between 100 Hz and 10 kHz, a radiofrequency range between 10 kHz and 300 MHz, a microwave frequency range between 300 MHz and 300 GHz, or above 300 GHz.

A process according to any of claims 1 to 5, wherein the time of the silicon particles (SP1 ) kept in the reactive zone (RZ) in steps c) and d) is between 0.1 seconds and 7 days, preferably between 1 second and 2 days, more preferably between 10 seconds and 12 hours, even more preferably between 30 seconds and 6 hours and most preferably between 1 minute and 3 hours.

7. A process according to any of claims 1 to 6, wherein the silicon particles coated with carbon (SP2) are fully coated with carbon, and/or

the coating of the silicon particles (SP1 ) with carbon according to step d) is carried out by chemical vapor deposition of at least one carbon-containing gas on the surface of the silicon particles (SP1 ) under plasma conditions, and/or iii) the silicon particles coated with carbon (SP2) are conducting particles.

A process according to any of claims 1 to 7, wherein

i) the coating of the particles coated with carbon (SP2) has a thickness of less than 1 μηη, preferably the coating has a thickness of less than 500 nm, more preferably the coating has a thickness of less than 150 nm, even more preferably the coating has a thickness of less than 50 nm and most preferably the coating has a thickness of less than 20 nm, and/or ii) the coating of the particles coated with carbon (SP2) is porous, and/or iii) the coating of the particles coated with carbon (SP2) is amorphous, graphitic or a mixture of both, and/or

iv) the coating of the particles coated with carbon (SP2) contains heteroelements selected from nitrogen, boron, sulfur or phosphorus or mixtures thereof.

A process according to any of claims 1 to 8, wherein step e) comprises the following partial steps e1 ) to e3): e1 ) the silicon particles coated with carbon (SP2) are mixed with graphite, a further conductive additive and a binder in a solution,

e2) followed by coating an electrode support with the mixture obtained in partial step e1 ), and

e3) followed by drying and punching out the coated electrode support obtained in partial step e2) to obtain an electrode containing silicon particles coated with carbon (SP2). A process according to claim 9, wherein

I) the concentration of the binder in the solution in partial step e1 ) is between 0.1 wt% and 20 wt%, preferably between 5 wt% and 15 wt%, and/or

II) the graphite content in partial step e1 ) is between 0.1 wt% and 80 wt%, preferably between 15 wt% and 60 wt%, and more preferably between 20 wt% and 40 wt%, and/or

III) the content of the further conductive additive in partial step e1 ) is between 0.1 wt% and 20 wt%, preferably between 5 wt% and 15 wt%.

A process according to any of claims 9 and 10, wherein

I) the solution in partial step e1 ) is an aqueous solution, and/or II) the binder in partial step e1 ) is polyacryllic acid, and/or

III) the further conductive additive in partial step e1 ) is carbon black, and/or

IV) the electrode support in step e2) is a metal foil, preferably the electrode support in partial step e2) is a copper foil.

A process according to any of claims 1 to 1 1 , wherein the electrode is an anode.