WO2015071189A1

WO2015071189A1 - Process for producing tin nanowires

Info

Publication number: WO2015071189A1
Application number: PCT/EP2014/074051
Authority: WO
Inventors: Heino Sommer; Rihab AL-SALMAN; Torsten Brezesinski; Jürgen Janek
Original assignee: Basf Se; Karlsruher Institut Für Technologie (Kit)
Priority date: 2013-11-14
Filing date: 2014-11-07
Publication date: 2015-05-21

Abstract

The present invention relates to a process for producing tin nanowires comprising at least the process step of electrochemically depositing tin directly onto at least one surface of an electrode from a solution comprising at least one tin compound (A), at least one silicon compound (B) and at least one ionic liquid (C).

Description

Process for producing tin nanowires Description The present invention relates to a process for producing tin nanowires comprising at least the process step of electrochemically depositing tin directly onto at least one surface of an electrode from a solution comprising at least one tin compound (A), at least one silicon compound (B) and at least one ionic liquid (C). Secondary batteries, accumulators or "rechargeable batteries" are just some embodiments by which electrical energy can be stored after generation and used when required. Owing to the significantly better energy density, there has in recent times been a move away from the water- based secondary batteries toward development of those batteries in which the charge transport in the electrical cell is accomplished by lithium ions.

Tin nanowires/nanostructures possess high surface area and activity and therefore find many applications in e.g. electronics, catalysis and sensors as described in Appl. Phys. Lett., 102 (2013), 093105 and Adv. Mater., 15 (2003) 997-1000. In addition, tin is considered to be an attractive anode material for Li-ion batteries since it delivers a theoretical capacity of 993 mAh/g which is about three times higher than that of the traditionally used graphite anodes as described in Electrochim. Acta, 45 (1999) 31 -50. Tin nanostructures are expected to solve for the structure degradation normally associated with the large volume change during the lithiation and de-lithation of bulk Sn as described in Nano Lett., 13 (2013) 470-474. Moreover, single- crystalline Sn nanowires show superconductivity behaviors as described in J. Phys. Chem. B, 109 (2005) 4398-4403.

High quality Sn nanostructures are normally synthesized by vacuum techniques like PVD as described in J. Phys. Chem. B, 109 (2005) 4398-4403 or by template-assisted methods as de- cribed in Appl. Phys. Lett., 102 (2013), 093105 or C. R. Chimie, 1 1 (2008) 995-1003. However, the former method is considered to be high-cost technique and the latter normally includes extra steps, namely pre-sputtering of the nonconductive membrane with a conductive layer and post- dissolution of it to get the free nanowires. These pre- and post-treatments might affect/harm the nanostructures and would be disadvantageous for a technological process. WO 2012/17031 1 discloses a method of making tin nanoneedles on the surface of a substrate material via electrodeposition. The SEM images of the nanoneedles show needles with an uneven surface and also needles being tapered along the axis.

WO 2013/052456 describes a method for producing nanostructured materials such as silicon nanowires and their application as anode component for lithium ion batteries. The preparation of tin nanowires is experimentally not disclosed. Proceeding from this prior art, the object was to find a flexible and more efficient synthesis route to tin nanowires useful as anode material for lithium ion batteries, especially for lithium ion secondary batteries. This object is achieved by a process for producing tin nanowires comprising at least the process step of

(a) electrochemically depositing tin directly onto at least one surface of an electrode from a solution comprising

(A) at least one tin compound,

(B) at least one silicon compound and (C) at least one ionic liquid, wherein the concentration of the silicon compound in the solution is in the range from 0.01 M to 1 M. The tin nanowires obtainable or obtained by the inventive process are preferably crystalline. The thickness of tin nanowires obtainable or obtained by the inventive process is usually in the range from 1 nm to 100 nm, preferably in the range from 5 nm to 50 nm, in particular in the range from 10 nm to 30 nm. In a preferred embodiment of the present invention the tin nanowires consist essentially of tin, that means that the tin-content of the tin nanowires is preferably at least 90 %, more preferably in the range of from 95 % to 100 %, in particular from 97 % to 100 % by weight based on the total weight of the tin nanowires. The length of the tin nanowires can be varied in a wide range, depending on the time tin is electrochemically deposited onto at least one surface of an electrode.

In a preferred embodiment of the invention the tin nanowires show an aspect ratio of at least 500, more preferably an aspect ratio in the range from 750 to 4000, in particular in the range from 1500 to 3000.

The definition of the aspect ratio as used herein is for example given in WO 2013/052456, page 8, paragraph [0050]. Depending on the concentration of silicon compound (B) the tin nanowires obtainable or obtained by the inventive process can show different morphological arrangements, like a 3- dimensional network of cross-linked nanowires or closely-stacked tin nanowires having a high- aspect-ratio. The density of the obtained tin nanowires on the surface of the electrode is at least 5 x 10⁹ nanowires per cm². The density of the nanowires can be estimated from SEM images.

The length, the thickness, the aspect ratio or the morphological arrangement of the tin nan- owires obtained by the inventive process can be determined from the SEM images of the corresponding samples.

In process step (a) of the inventive process tin nanowires are directly electrochemically deposited onto at least one surface of an electrode from a solution comprising at least one tin com- pound (A), at least one silicon compound (B) and at least one ionic liquid (C).

The method of electrochemical deposition is well known as mentioned in WO 2013/052456, page 17, paragraph [0074] and referring the literature cited therein. The solution from which tin nanowires are electrochemically deposited comprises at least one tin compound (A), also referred to hereinafter as component (A) for short. The tin of component (A) is usually in the oxidation state +2 or +4, preferably in the oxidation state +4. Component (A) is preferably at least partly, preferably completely soluble in the formed solution. Examples of tin compounds (A) in the oxidation state +2 are tin(ll) halides like SnC , tin(ll) tri- flate, tin(ll) oxalate, tin(ll) acetate, tin(ll) acetylacetonate or tin(ll) stearate.

Examples of tin compounds (A) in the oxidation state +4 are tin(IV) halides like SnCU or SnBr₄, tin(IV) acetate, tin(IV) bis(acetylacetonate) dichloride or tin(IV) tetrakis(trifluoromethanesulfon- imide).

Instead of using a single tin compound (A) it is also possible to use two or more different tin compounds (A) in the solution including mixtures of at least two tin compounds (A) in the oxidation state +2, mixtures of at least two tin compounds (A) in the oxidation state +4 or mixtures of at least one tin compounds (A) in the oxidation state +2 and at least one tin compounds (A) in the oxidation state +4.

Preferred tin compounds (A) are tin(ll) halides or tin tetrahalides (tin(IV) halide) like tin dichloride, tin dibromide, tin tetrachloride (SnCI₄) or tin tetrabromide (SnBr₄), in particular tin dichloride or tin tetrachloride.

In one embodiment of the present invention, the inventive process is characterized in that the tin compound (A) is a tin(ll) halide or a tin tetrahalide, in particular tin dichloride or tin tetrachloride. Preferred tin compounds (A) are tin tetrahalides (tin(IV) halide) like tin tetrachloride (SnCI₄) or tin tetrabromide (SnBr₄), in particular tin tetrachloride.

In one embodiment of the present invention, the inventive process is characterized in that the tin compound (A) is a tin tetrahalide, in particular tin tetrachloride. The concentration of component (A) in the solution can be varied in a wide range depending on the solubility of component (A) in the solution. Preferably the concentration of component (A) in the solution is in the range from 0.01 M to 0.5 M, more preferably in the range from 0.01 M to 0.3 M, in particular in the range from 0.05 M to 0.2 M. The concentration of component (A) in the solution, wherein component (A) is a tin tetrahalide, in particular tin tetrachloride, is preferably in the range from 0.01 M to 0.3 M, in particular in the range from 0.05 M to 0.2 M.

The solution from which tin nanowires are electrochemically deposited comprises further at least one silicon compound (B), also referred to hereinafter as component (B) for short. The silicon of component (B) is usually in the oxidation state +4. Component (B) is preferably at least partly, more preferably completely soluble in the formed solution.

Examples of silicon compounds (B) are silicon tetrahalides like SiCU or SiBr₄, organo halo silanes like dimethyldichlorosilane, trichloro(phenyl)silane or trimethylchlorosilane.

Instead of using a single silicon compound (B) it is also possible to use two or more different silicon compounds (B) in the solution. Preferred silicon compounds (B) are silicon tetrahalides like silicon tetrachloride or silicon tetra- bromide, in particular silicon tetrachloride.

In one embodiment of the present invention, the inventive process is characterized in that the silicon compound (B) is a silicon tetrahalide, in particular silicon tetrachloride.

In one embodiment of the present invention, the inventive process is characterized in that either the tin compounds (A) or the silicon compound (B) is a tetrahalide, in particular a tetrachloride.

The electrochemical deposition of tin from a solution comprising only one tin compound (A) and an ionic liquid (C) and no silicon compound (B) results in the formation of thin sheets of metallic tin, while tin nanowires are formed by the electrochemical deposition of tin from a solution comprising component (A), component (B) and at least one ionic liquid (C), wherein the concentration of the silicon compound (B) in the solution is in the range from 0.01 M to 1 M, preferably in the range from 0.05 M to 1 M, more preferably the range from 0.1 M to 0.9 M, in particular in the range from 0.4 M to 0.8 M.

In one embodiment of the present invention, the inventive process is characterized in that the concentration of the silicon compound (B) in the solution is in the range from 0.4 M to 0.8 M. In addition to the at least one component (A) and to the at least one component (B) the solution, from which tin nanowires are electrochemically deposited, comprises further at least one ionic liquid (C), also referred to hereinafter as component (C) for short. Ionic liquids (C) are known to the person skilled in the art. Several ionic liquids, which are liquid salts with a melting point be- low 100 °C, in particular below room temperature, are commercially available or can be prepared according to known protocols. The ionic liquid (C) can be varied in a wide range as long as component (C) is liquid at the temperature of the deposition and dissolves the components (A) and (B) sufficiently and does not chemically react with them. In addition the ions of the ionic liquid (C) preferably do not react under the conditions of the electrochemical deposition.

Examples of suitable ionic liquids (C) are salts comprising a cation selected from the group of cations consisting of substituted imidazolium, substituted pyrrolidinium, substituted piperidinium, substituted pyridinium, substituted phosphonium and substituted ammonium, preferably consist- ing of substituted imidazolium and substituted pyrrolidinium, wherein substituted means the presence of at least on organic radical, and an anion selected from the group of anions consisting of (CF₃S0₂)₂N- (TFSI-), CF3SO3- (TFO-), ROSCV, RSCV (R= e.g. Me or Et), tosylate, acetate, dialkylphospates and hydrogensulfate, preferably consisting of of (CFsSC^N^", CF3SO3^", ROSO3- and RSO3- with R = Me or Et.

Preferred examples of ionic liquids (C) are 1 -butyl-1 -methylpyrrolidinium bis(trifluoromethyl- sulfonyl) imide (BMP-TFSI), 1 -ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EM- Im-TFSI) and 1 -butyl-1 -methylpyrrolidinium tris(pentafluoroethyl)-trifluorophosphate (BMP-FAP).

Instead of using only one ionic liquid (C) it is also possible to use two or more different ionic liquids (C) in the solution.

In one embodiment of the present invention, the inventive process is characterized in that the ionic liquid (C) is selected from the group consisting of 1 -butyl-1 -methylpyrrolidinium

bis(trifluoromethylsulfonyl) imide (BMP-TFSI), 1 -ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl) imide (EMIm-TFSI) and 1 -butyl-1 -methylpyrrolidinium

tris(pentafluoroethyl)-trifluorophosphate (BMP-FAP), preferably BMP-TFSI and EMIm-TFSI.

In addition to the at least one component (A), to the at least one component (B) and to the at least one ionic liquid the solution might comprise further components, which are inert under the conditions of the electrochemical deposition reaction like polar aprotic solvents which are usually used in electrolytes of electrochemical cell. Preferably the solution is essentially free of water, i.e. the water content in the solution is below 0.1 % by weight, preferably below 500 ppm, in particular in the range from 0.1 ppm to 10 ppm. If component C, for example as technical grade, comprises more water than desired, the water can be removed by known methods, like stripping the water from component A by heating it under reduced pressure, or by adding drying reagents like molecular sieves or by adding scavengers like aluminum alkyls, magnesium alkyls or lithium alkyls. It is also possible to remove excess water by adding additional amount of silicon tetrachloride or tin tetrachloride, which form insoluble compounds by reacting with water. Preferably the sum of the weight of all components (A), (B) and (C) is at least 90% by weight, preferably in the range from 95% to 100% by weight, in particular in the range from 98% to 100% by weight based on the total weight of the solution. The electrochemical deposition of tin nanowires from a solution comprising at least one tin compound (A), at least one silicon compound (B), wherein the concentration of the silicon compound (B) in the solution is in the range from 0.01 M to 1 M, preferably in the range from 0.1 M to 1 M, more preferably the range from 0.2 M to 0.9 M, in particular in the range from 0.4 M to 0.8 M and at least one ionic liquid (C) is also possible in the presence of at least one organic solvent (D). Preferably the organic solvent (D) is a polar aprotic solvent, more preferably a polar aprotic solvent selected from the group consisting of cyclic carbonates, in particular propylene carbonate, ethylene carbonate and fluoroethylene carbonate, acetonitrile, dimethylformamide, tetrahydrofurane, acetone and dimethyl sulfoxide. In one embodiment of the present invention, the inventive process is characterized in that the solution comprises at least one organic solvent (D), preferably at least one polar aprotic solvent (D), more preferably a polar aprotic solvent selected from the group consisting of cyclic carbonates, in particular propylene carbonate, ethylene carbonate and fluoroethylene carbonate, acetonitrile, dimethylformamide, tetrahydrofurane, acetone and dimethyl sulfoxide.

The concentration of component (C) in the solution, which comprises beside component (A) and component (B) also component (D), can be varied in a wide range. Preferably the concentration of all ionic liquids (C) in the solution is at least 0.05 M, more preferably at least 0.1 M, in particular at least 0.2 M up to the maximal concentration of the sum of all ionic liquids (C) in a solution comprising no organic solvent (D).

In one embodiment of the present invention, the inventive process is characterized in that the concentration of all ionic liquids (C) in the solution is at least 0.05 M, more preferably at least 0.1 M, in particular at least 0.2 M up to the maximal concentration of the sum of all ionic liquids (C) in a solution comprising no organic solvent (D).

For economic reasons the amount of ionic liquids, which are usually more expensive than suitable organic solvents, is reduced in the solution, which is the electrolyte, as far as possible. In one embodiment of the present invention, the inventive process is characterized in that the solution comprises at least one organic solvent (D), preferably at least one polar aprotic solvent (D), more preferably a polar aprotic solvent selected from the group consisting of cyclic carbonates, in particular propylene carbonate, ethylene carbonate and fluoroethylene carbonate, acetonitrile, dimethylformamide, tetrahydrofurane, acetone and dimethyl sulfoxide, and wherein the concentration of all ionic liquids (C) in the solution is at least 0.05 M, more preferably at least 0.1 M, in particular at least 0.2 M up to the maximal concentration of the sum of all ionic liquids (C) in a solution comprising no organic solvent (D). The solution used in process step a) is usually prepared by simply mixing the components (A), (B) and (C) preferably under inert and dry, i.e. water-free, conditions, using Schlenk technique or working in a glove-box. The electrochemical deposition can be take place in a wide temperature range. Preferably process step (a) takes place at a temperature in the range from 0 °C to 100 °C, more preferably in the range from 15 °C to 50 °C, in particular in the range from 20 °C to 35 °C.

In one embodiment of the present invention, the inventive process is characterized in that pro- cess step (a) takes place at a temperature in the range from 15 °C to 50 °C, in particular in the range from 20 °C to 35 °C.

The time of electrochemically depositing tin can be varied in a broad range and is preferably adjusted to the desired length of the tin nanowires deposited.

The electrochemical deposition can be take place in a wide range of deposit potentials which are given by reference to a Pt quasi-reference electrode. Preferably process step (a) takes place at a deposit potential in the range from -1 .9 V to - 2.5 V vs. Pt quasi-reference electrode. In one embodiment of the present invention, the inventive process is characterized in that process step (a) takes place at a deposit potential in the range from -1 .9 V to - 2.5 V vs. Pt quasi- reference electrode.

During the electrochemical deposition the surface of the electrode and the solution can be static to each other or the solution is in motion relative to the surface of the electrode, e.g. by simply stirring the solution or by continuously supplying the surface of the electrode with new solution using a pump around system.

The surface of the electrode, where the tin nanowires are deposited during the electrochemical deposition, can be selected from a large number of electrically conductive materials like metals and conductive carbons. Preferably the electrochemical deposition of the tin nanowire takes place on the surface of an electrode, wherein the surface is composed of a material selected from the group consisting of copper, tin, aluminum and glassy carbon. In one embodiment of the present invention, the inventive process is characterized in that the surface of the electrode, where the tin nanowires are deposited, is composed of a material selected from the group consisting of copper, tin, aluminum and glassy carbon.

Tin nanowires with a high aspect ratio in the range from 1500 to 3000 and a high number of nanowires per electrode area are preferably obtained in process step a) of the inventive process under conditions wherein the tin compound (A) is tin tetrachloride, wherein the concentration of tin tetrachloride in the solution is in the range from 0.01 M to 0.3 M, in particular in the range from 0.05 to 0.2 M, and the silicon compound (B) is silicon tetrachloride, wherein the concentra- tion of silicon tetrachloride in the solution is in the range from 0.4 M to 0.8 M, and wherein process step (a) takes place at a temperature in the range from 15 °C to 50 °C, preferably 25 °C to 35 °C, and at a deposit potential in the range from -1 .9 V to - 2.5 V vs. Pt quasi-reference electrode.

In one embodiment of the present invention, the inventive process is characterized in that the tin compound (A) is tin tetrachloride, wherein the concentration of tin tetrachloride in the solution is in the range from 0.05 to 0.2 M, and the silicon compound (B) is silicon tetrachloride, wherein the concentration of silicon tetrachloride in the solution is in the range from 0.1 M to 0.8 M, and wherein process step (a) takes place at a temperature in the range from 15 °C to 50 °C and at a deposit potential in the range from -1 .9 V to - 2.5 V vs. Pt quasi-reference electrode.

The tin nanowires obtained in process step a) of the inventive process are usually isolated by separation them mechanically from the surface of the electrode, for example by cutting.

The isolated tin nanowires can be used in different applications e.g. in electronics, catalysis and for the preparation of nanowires comprising tin oxide, which can be used in sensors and solar cells. The present invention further also provides tin nanowires, preferably tin nanowires having an aspect ratio in the range from 750 to 4000, in particular in the range from 1500 to 3000 obtainable by a process for producing tin nanowires as described above. This process comprises the above-described process step (a) especially also with regard to preferred embodiments thereof. The present invention likewise also provides tin nanowires, preferably tin nanowires having an aspect ratio in the range from 750 to 4000, in particular in the range from 1500 to 3000, wherein the tin nanowires are prepared by a process comprising at least the process steps of

(A) at least one tin compound,

(B) at least one silicon compound and

(C) at least one ionic liquid, wherein the concentration of the silicon compound in the solution is in the range from 0.01 M to 1 M.

The process step a) has been described above. In particular, preferred embodiments of the process step have been described above. The tin nanowires, preferably tin nanowires having an aspect ratio in the range from 750 to 4000, in particular in the range from 1500 to 3000, which are obtainable or obtained by the inventive process, are preferably crystalline. The thickness of tin nanowires obtainable or obtained by the inventive process is usually in the range from 1 nm to 100 nm, preferably in the range from 5 nm to 50 nm, in particular in the range from 10 nm to 30 nm.

In a preferred embodiment of the present invention the tin nanowires consist essentially of tin, that means that the tin-content of the tin nanowires is preferably at least 90 %, more preferably in the range of from 95 % to 100 %, in particular from 97 % to 100 % by weight based on the total weight of the tin nanowires.

Due to its properties, the inventive tin nanowires are particularly suitable as a material for anodes in electrochemical cells, preferably in Li ion cells, especially in Li ion secondary cells or batteries. More particularly, in the case of use in anodes of Li ion cells and especially of Li ion secondary cells, the inventive tin nanowires are notable for high capacity and good cycling stability, and ensure low impedances in the cell. In addition anodes comprising the above described tin nanowires show high coulombic efficiency. Moreover, the inventive tin nanowires can be produced in a simple manner and with reproducible quality. The present invention further also provides for the use of the inventive tin nanowires as described above as part of an electrode for an electrochemical cell.

The present invention likewise accordingly also provides an electrode for an electrochemical cell comprising the inventive tin nanowires as described above. This electrode is typically incor- porated and used as the anode in an electrochemical cell. Therefore, the electrode which comprises the inventive tin nanowires is also referred to hereinafter as the anode.

In addition to the inventive tin nanowires, the anode generally comprises at least one suitable binder for consolidation of the inventive tin nanowires, and optionally further electrically conduc- tive or electroactive constituents. In addition, the anode generally has electrical contacts for supply and removal of charges. The amount of inventive tin nanowires, based on the total mass of the anode material, minus any current collectors and electrical contacts, is generally at least 5% by weight, frequently at least 50% by weight and especially at least 60% up to 97.5% by weight.

Useful further electrically conductive or electroactive constituents in the inventive anodes include carbon black (conductive black), graphite, carbon fibers, carbon nanofibers, carbon nano- tubes or electrically conductive polymers. Typically about 2.5 to 40% by weight of the conductive material are used in the anode together with 50 to 97.5% by weight, frequently with 60 to 95% by weight, of the inventive tin nanowires, the figures in percent by weight being based on the total mass of the anode material, minus any current collector and electrical contacts. Useful binders for the production of an anode using the inventive tin nanowires include especially the following polymeric materials: polyethylene oxide, cellulose, carboxymethylcellulose, polyvinyl alcohol, polyvinylidene fluoride, polyethylene, polypropylene, polytetrafluoroethylene, polyacrylonitrile-methyl methacrylate copolymers, styrene-butadiene copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-tetrafluoroethylene copolymers, perfluoroalkyl vinyl ether copolymers, ethylene-tetrafluoroethylene copolymers, vinylidene fluoride-chlorotrifluoroethylene copolymers, ethylene-chlorofluoroethylene copolymers, polyacrylic acid or ethylene-acrylic acid copolymers, optionally at least partly neutralized with alkali metal salt or ammonia, ethylene-methacrylic acid copolymers, optionally at least partially neutralized with alkali metal salt or ammonia, ethylene-(meth)acrylic ester copolymers, polyi- mides and/or polyisobutene, and mixtures thereof. The selection of the binder is often made with consideration of the properties of any solvent used for production. For example, polyvinylidene fluorides are suitable when N-ethyl-2- pyrrolidone is used as the solvent while polyvinyl alcohol can be processed in aqueous solution. The binder is generally used in an amount of 1 to 20% by weight, based on the total mass of the anode material. Preference is given to using 2 to 15% by weight, especially 7 to 10% by weight.

The inventive electrode comprising the inventive tin nanowires, also referred to above as anode, generally comprises electrical contacts for supply and removal of charges, for example an output conductor, which may be configured in the form of a metal wire, metal grid, metal mesh, expanded metal, a metal foil and/or a metal sheet. Suitable metal foils are especially copper foils.

In one embodiment of the present invention, the anode has a thickness in the range from 15 to 200 μηη, preferably from 30 to 100 μηη, based on the thickness excluding output conductor. The anode can be produced in a manner customary per se by standard methods as known from relevant monographs. For example, the anode can be produced by mixing the inventive tin nanowires, optionally using an organic solvent (for example N-methylpyrrolidinone, N-ethyl-2- pyrrolidone or a hydrocarbon solvent), with the optional further constituents of the anode material (electrically conductive constituents and/or organic binder), and optionally subjecting it to a shaping process or applying it to an inert metal foil, for example Cu foil. This is optionally followed by drying. This is done, for example, using a temperature of 80 to 150°C. The drying operation can also take place under reduced pressure and lasts generally for 3 to 48 hours. Optionally, it is also possible to employ a melting or sintering process for the shaping.

The present invention further provides an electrochemical cell, especially a lithium ion secondary cell, comprising at least one electrode which has been produced from or using tin nanowires as described above. Such cells generally have at least one inventive anode, a cathode, especially a cathode suitable for lithium ion cells, an electrolyte and optionally a separator.

With regard to suitable cathode materials, suitable electrolytes, suitable separators and possible arrangements, reference is made to the relevant prior art (see, for example, Wakihara et al.: Lithium Ion Batteries, 1 st edition, Wiley VCH, Weinheim (1998); David Linden: Handbook of Batteries, 3rd edition, McGraw-Hill Professional, New York (2008); J. O. Besenhard: Handbook of Battery Materials, Wiley-VCH (1998)). Useful cathodes include especially those cathodes in which the cathode material comprises lithium transition metal oxide, e.g. lithium cobalt oxide, lithium nickel oxide, lithium cobalt nickel oxide, lithium manganese oxide (spinel), lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide or lithium vanadium oxide, or a lithium transition metal phosphate such as lithium iron phosphate. If the intention, however, is to use those cathode materials which comprise sulfur and polymers comprising polysulfide bridges, it has to be ensured that the anode is charged with Li° before such an electrochemical cell can be discharged and recharged.

The two electrodes, i.e. the anode and the cathode, are connected to one another using a liquid or else solid electrolyte. Useful liquid electrolytes include especially nonaqueous solutions (wa- ter content generally less than 20 ppm) of lithium salts and molten Li salts, for example solutions of lithium hexafluorophosphate, lithium perchlorate, lithium hexafluoroarsenate, lithium trifluoromethylsulfonate, lithium bis(trifluoromethylsulfonyl)imide or lithium tetrafluoroborate, especially lithium hexafluorophosphate or lithium tetrafluoroborate, in suitable aprotic solvents such as ethylene carbonate, propylene carbonate and mixtures thereof with one or more of the following solvents: dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, fluoroethylene carbonate, dimethoxyethane, methyl propionate, ethyl propionate, butyrolactone, acetonitrile, ethyl acetate, methyl acetate, toluene and xylene, especially in a mixture of ethylene carbonate and diethyl carbonate. The solid electrolytes used may, for example, be ionically conductive polymers.

A separator impregnated with the liquid electrolyte may be arranged between the electrodes. Examples of separators are especially glass fiber nonwovens and porous organic polymer films, such as porous films of polyethylene, polypropylene etc. Particularly suitable materials for separators are polyolefins, especially porous polyethylene films and porous polypropylene films.

Polyolefin separators, especially composed of polyethylene or polypropylene, may have a porosity in the range from 35 to 45%. Suitable pore diameters are, for example, in the range from 30 to 500 nm.

In another embodiment of the present invention, separators composed of polyethylene tereph- thalate nonwovens filled with inorganic particles may be present. Such separators may have a porosity in the range from 40 to 55%. Suitable pore diameters are, for example, in the range from 80 to 750 nm.

Inventive electrochemical cells further comprise a housing which may be of any shape, for ex- ample cuboidal, or the shape of a cylinder. In another embodiment, inventive electrochemical cells have the shape of a prism. In one variant, the housing used is a metal-plastic composite film elaborated as a pouch.

The cells may have, for example, a prismatic thin film structure, in which a solid thin film electro- lyte is arranged between a film which constitutes an anode and a film which constitutes a cathode. A central cathode output conductor is arranged between each of the cathode films in order to form a double-faced cell configuration. In another embodiment, a single-faced cell configuration can be used, in which a single cathode output conductor is assigned to a single anode/separator/cathode element combination. In this configuration, an insulation film is typically arranged between individual anode/separator/cathode/output conductor element combinations.

The inventive electrochemical cells have high capacity, cycling stability, efficiency and reliability, and low impedances leading to high possible charge and discharge rates. The inventive electrochemical cells can be combined to form lithium ion batteries.

Accordingly, the present invention further also provides for the use of inventive electrochemical cells as described above in lithium ion batteries. The present invention further provides lithium ion batteries comprising at least one inventive electrochemical cell as described above. Inventive electrochemical cells can be combined with one another in inventive lithium ion batteries, for example in series connection or in parallel connection. Series connection is preferred. Inventive electrochemical cells are notable for particularly high capacities, high power even after repeated charging, and significantly delayed cell death. Inventive electrochemical cells are very suitable for use in devices. The use of inventive electrochemical cells in devices also forms part of the subject matter of the present invention. Devices may be stationary or mobile devices. Mobile devices are, for example, vehicles which are used on land (preferably particularly auto- mobiles and bicycles/tricycles), in the air (preferably particularly aircraft) and in water (preferably particularly ships and boats). In addition, mobile devices are also mobile appliances, for example cellphones, laptops, digital cameras, implanted medical appliances and power tools, especially from the construction sector, for example drills, battery-powered screwdrivers and battery- powered tackers. Stationary devices are, for example, stationary energy stores, for example for wind and solar energy, and stationary electrical devices. Such uses form a further part of the subject matter of the present invention. The present invention further provides for the use of inventive electrochemical cells as described above in motor vehicles, bicycles operated by electric motor, aircraft, ships or stationary energy stores. The use of inventive electrochemical cells in devices gives the advantage of prolonged run time before recharging and a smaller loss of capacity in the course of prolonged run time. If the intention were to achieve an equal run time with electrochemical cells with lower energy density, a higher weight for electrochemical cells would have to be accepted. The present invention therefore also further provides for the use of inventive electrochemical cells in devices, especially in mobile devices. Examples of mobile devices are vehicles, for example automobiles, bicycles, aircraft, or water vehicles such as boats or ships. Other examples of mobile devices are those which are portable, for example computers, especially laptops, telephones or electrical power tools, for example from the construction sector, especially drills, battery-driven screwdrivers or battery-driven tackers.

The present invention further provides a device comprising at least one electrochemical cell as described above. The invention is illustrated by the examples which follow, but these do not restrict the invention. Figures in percent are each based on % by weight, unless explicitly stated otherwise. I. Electrochemical deposition of tin

Used Chemicals:

The ionic liquids (ILs) 1 -butyl-1 -methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMP- TFSI, lo-Li-Tec), 1 -ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMIm-TFSI, lo- Li-Tec), 1 -butyl-1 -methylpyrrolidinium tris(pentafluoroethyl)-trifluorophosphate (BMP-FAP, Merck) 1 -Butyl-1 -methylpyrrolidinium trifluoromethanesulfonate (BMP-TFO) and 1 -ethyl-3- methylimidazolium methylsulfate (EMIm-MeOSOs) were purchased in the highest available quality and used after drying under vacuum at 100 °C for several hours to water content below 3 ppm. SnCI₄ (99.995%, Sigma-Aldrich), Sn(IV) acetate (Sigma-Aldrich), SiCI₄ (99.998%, Alfa Aesar), SiBr₄ (99.995%, Sigma-Aldrich), Si(IV) acetate (98%, Sigma-Aldrich) and GeCI₄ (99.9999%, Alfa Aesar) were used for the electrodeposition experiments. Anhydrous propylene carbonate (PC) and anhydrous acetonitrile (CH3CN) were utilized as the organic solvents.

General Electrochemical Setup:

Cu foil (> 99.9%, GOULD Electronics) was mainly used as a working electrode. Other sub- strates like Al foil, Sn foil (99.95%, Goodfellow) and glassy carbon plate (Alfa Aesar) have also been used for the electrodeposition of Sn nanowires. Pt wires (99.997%, Alfa Aesar) were used as quasi-reference and counter electrodes. The substrates were cleaned ultrasonically in acetone for 5 minutes before use. However, the use of Cu foil as-received seems not to affect the growth of Sn nanowires. The electrochemical cell was made of Teflon and clamped over a Teflon-covered O-ring yielding a geometric surface area of 0.5 cm² of the used substrate. A Pt wire was coiled into three rings with a diameter of ~ 1 .5 cm and was embedded into the Teflon cavity (0.6 cm deep and 0.5 cm thick) which surrounds the reaction area of the working electrode. In other words, the Teflon cell looks like a small cylinder (8 mm in diameter, where the working electrode is underneath) surrounded by a bigger cylinder (18 mm in diameter, where the coiled Pt wire is placed on its Teflon ground). This coiled wire was serving as a counter electrode. A Pt wire was immersed into the reaction solution near from the working electrode (about 2 mm away from it) to serve as a quasi-reference electrode. Larger electrochemical cells have also been utilized. In this case we used a 50 ml glass-container, a 5 cm^* 5 cm Pt gauze (99.9%, Alfa Aesar) as a counter electrode, 2.5 cm ^* 5 cm Cu foil as a working electrode and a Pt wire as a quasi-reference electrode. The distance between the working- and counter electrode in this setup was kept at 0.5 cm. General Performance of Electrochemical Experiments:

Due to the hygroscopic nature of the Sn- and Si-compounds (A) and (B) respectively, all of the used solutions were prepared inside an argon-filled glove-box with oxygen and water content below 1 ppm. The Si-compound was first dissolved in the ionic liquid and then the Sn- compound was added. All of the electrochemical measurements were performed inside the glove-box as well. These measurements were performed by using BioLogic potenti- ostat galvanostat controlled by EC-Lab software.

1.1 Comparative and inventive examples for the electrochemical deposition of tin Example 1 (comparative)

The above described Teflon cell was filled with about 2 ml solution of 0.1 M SnCU in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.1 V vs. Pt quasi-reference electrode for 1.5 hour at 30 °C. A total charge of ~ 1 .44 C/cm² was con- sumed during this period of time. Figure 1 (Fig 1 ) shows SEM images of the obtained deposit. No nanowires were obtained in the absence of a silicon compound.

Example 2 (inventive) The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI ionic liquid and the substrate was a Cu foil with a geometric surface area of 0.5 cm². The deposition was performed at 30 °C by applying a constant potential of - 2.0 V vs. Pt quasi-reference electrode for 2 hours which corresponds to a total charge flow of ~ 3 C (6 C/cm²). The obtained deposit (Sn nanowires) was then directly removed from the reaction solution and was carefully rinsed with dried acetone for several times inside the glove box to remove the traces of the ionic liquid solution which was trapped inside the Sn nanostructure. This must be done directly after the completion of the deposition process as SnCI₄ chemically oxidizes Sn when the cell is switched off. Figure 2 shows typical SEM images of the obtained Sn nanowires. Sn nanowires/nanostructures were also obtained under other different reaction parameters as will be presented in the following examples. For simplicity the parameter(s), which have been varied compared to Example 2, which is considered as the reference experiment, is(are) underlined.

Example 3 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.3 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of ~ 14 C/cm²was consumed during this period of time.

Example 4 (inventive) The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -1 .75 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of ~ 1.3 C/cm² was consumed during this period of time. Example 5 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.7 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 0.5 hour at 30 °C. A total charge of ~ 1.6 C/cm² was consumed during this period of time.

Example 6 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.1 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of ~ 2.2 C/cm² was consumed during this period of time.

Example 7 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.01 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of 2.0 C/cm² was consumed during this period of time. Example 8 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.05 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of ~ 5.7 C/cm² was consumed during this period of time.

Example 9 (inventive) The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.5 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 0.5 hour at 30 °C. A total charge of ~ 1.9 C/cm² was consumed during this period of time. Example 10 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiBr₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 1 hour at 30 °C. A total charge of ~ 5 C/cm² was consumed during this period of time.

Example 1 1 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M Si(IV)acetate + 0.1 M SnCI₄) in BMP- TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of - 2.1 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of ~ 3.2 C/cm² was consumed during this period of time.

Example 12 (inventive)

The Teflon cell was filled with about 2 ml of freshly prepared solution of (0.5 M SiCI₄+ 0.1 M Sn(IV)acetate) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 1 hour at 30 °C. A total charge of - 3.4 C/cm² was consumed during this period of time.

Example 13 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiBr₄+ 0.1 M Sn(IV)acetate) in BMP- TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of - 2.1 V vs. Pt quasi-reference electrode for 1 hour at 30 °C. A total charge of ~ 1.1 C/cm² was consumed during this period of time. Example 14 (comparative)

The Teflon cell was filled with about 2 ml solution of (0.01 M GeCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of ~ 2.3 C/cm²was consumed during this period of time. Figure 3 (Fig 3) shows SEM images of the obtained deposit. No nanowires were obtained in the absence of a silicon compound.

Example 15 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in EMIm-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -1.9 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of ~ 18 C/cm²was consumed during this period of time.

Example 16 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in BMP-FAP IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.5 V vs. Pt quasi-reference electrode for 2 hours at 30 °C. A total charge of ~ 2.8 C/cm²was consumed during this period of time.

Example 17 (inventive) The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Al foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 1 hour at 30 °C. A total charge of ~ 1.4 C/cm² was consumed during this period of time. Example 18 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Sn foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 10 minutes at 30 °C. A total charge of ~ 0.6 C/cm² was con- sumed during this period of time.

Example 19 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on glassy carbon was then performed by applying a constant potential of -2.3 V vs. Pt quasi-reference electrode for 1 hour at 30 °C. A total charge of ~ 0.8 C/cm² was consumed during this period of time. Example 20 (inventive)

The Teflon cell was filled with about 2 ml solution of (0.5 M SiCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.3 V vs. Pt quasi-reference electrode for 1 hours at 50 °C. A total charge of ~ 0.8 C/cm²was consumed during this period of time.

Example 21 (comparative):

To confirm the effect of the ionic liquid on the growth of Sn as nanowires, the Sn deposition was done in the absence of the ionic liquid. The above described Teflon cell was filled with about 2 ml solution of 0.05 M SnCU and 0.2 M SiCI₄ in propylene carbonate (PC) solvent containing 0.1 M tetrabutylammonium chloride (TBAC) as a supporting electrolyte. The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 1 .5 hour at 25 °C. A total charge of 1 1 .36 C/cm² was consumed during this period of time. Figure 4 (Fig 4) shows SEM images of the obtained deposit.

Example 22 (comparative):

To confirm the effect of SiCI₄ on the growth of Sn as nanowires, we deposited Sn in the absence of SiCI₄. The Teflon cell was filled with about 2 ml solution of 0.03 M SnCI₄ in propylene carbonate (PC) solvent containing 0.2 M BMP-TFSI ionic liquid as a supporting electrolyte. The substrate is a Cu foil with a geometric surface area of 0.5 cm². The deposition was performed by applying a constant potential of - 2.0 V vs. Pt quasi-reference electrode for 1.5 hours.

Figure 5 (Fig 5) shows SEM images of the obtained deposit. No nanowires were obtained in the absence of the ionic liquid.

Examples 23 - 33:

The Teflon cell was filled with about 2 ml solution of A + B + C in PC solvent (the concentrations are given in Table 1 ). The Sn deposition on Cu foil was then performed by applying a constant potential of -2.0 V vs. Pt quasi-reference electrode for 1 hour at 25 °C. As an example (Example 25), a total charge of ~ 46 C/cm² was consumed during this period of time for the system containing 0.05 M SnCI₄ + 0.2 M SiCI₄ + 0.2 M BMP-TFSI.

Figure 6 shows SEM images of Sn nanowires obtained from a solution of (0.2 M SiCI₄ + 0.05 M SnCI₄) in PC containing 0.2 M BMP-TFSI. Table 1 concentrations of the reagents (A + B + C) which are used for the deposition of Sn NWs in the PC solvent.

Examples 34 - 40:

The Teflon cell was filled with about 2 ml solution of A, B, and C in CH3CN (the concentrations are given in Table 2). The Sn deposition on Cu foil was then performed by applying a constant potential of -1 .9 V vs. Pt quasi-reference electrode for 15-20 minutes at 25 °C. As an example (Example , a total charge of 17.8 C/cm² was consumed during 15 minutes deposition time for the system containing 1.0 M SiCI₄ + 0.1 M SnCI₄ + 0.2 M BMP-TFSI.

Table 2 concentrations of the reagents (A + B + C) which are used for the deposition of Sn NWs in the CH₃CN solvent.

Example A (SnCI₄ conB (SiCI₄ conC (BMP-TFSI

number centration) centration) concentration)

34 0.05 M 0.2 M 0.2 M

35 0.05 M 0.5 M 0.2 M

36 0.05 M 1 .0 M 0.2 M

37 0.01 M 1 .0 M 0.2 M

38 0.03 M 1 .0 M 0.2 M

39 0.1 M 1 .0 M 0.2 M

40 0.2 M 1 .0 M 0.2 M Example 41 (inventive):

The Teflon cell was filled with about 2 ml solution of 0.05 M SnCI₄ + 1 .0 M SiCI₄ and 0.2 M BMP-TFSI in CH3CN solvent. The Sn deposition on Cu foil was then performed by applying a constant potential of -1 .9 V vs. Pt quasi-reference electrode for 10-20 minutes at 25 °C. A total charge of 5.2 C/cm²was consumed during 7 minutes deposition time.

Example 42 (inventive):

The Teflon cell was filled with about 2 ml solution of 0.05 M SnCI₄ + 1 .0 M SiCI₄ and 0.2 M BMP-TFO in CH3CN solvent. The Sn deposition on Cu foil was then performed by applying a constant potential of -1 .9 V vs. Pt quasi-reference electrode for 10-20 minutes at 25 °C.

Example 43 (inventive):

The Teflon cell was filled with about 2 ml solution of 0.05 M SnCI₄ + 1 .0 M SiCI₄ and 0.2 M BMP-CH3OSO3 in CH3CN solvent. The Sn deposition on Cu foil was then performed by applying a constant potential of -1 .9 V vs. Pt quasi-reference electrode for 10-20 minutes at 25 °C.

Figure 1 .: SEM images of Sn deposit obtained from a solution of 0.1 M SnCI₄ in BMP-TFSI IL.

Deposition potential: -2.1 V, deposition time: 1.5 hour. Temperature: 30 °C. Figure 2.: SEM images of Sn nanowires obtained from a solution of (0.5 M SiCI₄ + 0.1 M

SnCI₄) in BMP-TFSI IL. Deposition potential: -2.0 V, deposition time: 2 hours. Temperature: 30 °C.

Figure 3.: SEM images of Sn deposit obtained from a solution of (0.01 M GeCI₄ + 0.1 M SnCI₄) in BMP-TFSI IL. Deposition potential: -2.0 V, deposition time: 2 hours. Temperature:

30 °C.

Figure 4.: SEM image of Sn deposit obtained from a solution of 0.05 M SnCI4 and 0.2 M SiCI4 in propylene carbonate (PC) solvent containing tetrabutylammonium chloride (TBAC) as a supporting electrolyte. Deposition potential: -2.0 V, deposition time: 1 .5 hour. Temperature: 25 °C.

Figure 5.: SEM images of Sn deposit obtained from a solution of 0.03 M SnCI4 in propylene carbonate (PC) solvent containing 0.2 M BMP-TFSI as a supporting electrolyte. Deposition potential: -2.0 V, deposition time: 1.5 hour. Temperature: 25 °C.

Figure 6. SEM images of Sn nanowires obtained from a solution of (0.2 M SiCI₄ + 0.05 M

SnCI₄) in PC containing 0.2 M BMP-TFSI IL. Deposition potential: -2.0 V, deposition time: 1 hour. Temperature: 25 °C.

Claims

1 . A process for producing tin nanowires comprising at least the process step of

(a) electrochemically depositing tin directly onto at least one surface of an electrode from a solution comprising (A) at least one tin compound,

(B) at least one silicon compound and

2. The process according to claim 1 , wherein the tin compound (A) is a tin tetrahalide.

3. The process according to claim 1 or 2, wherein the silicon compound (B) is a silicon tetrahalide.

4. The process according to any of claims 1 to 3, wherein the concentration of the silicon compound (B) in the solution is in the range from 0.4 M to 0.8 M.

5. The process according to any of claims 1 to 4, wherein the solution comprises at least one organic solvent (D).

6. The process according to any of claims 1 to 5, wherein the concentration of all ionic liquids (C) in the solution is at least 0.05 M.

7. The process according to any of claims 1 to 4, wherein the solution comprises at least one organic solvent (D) and wherein the concentration of all ionic liquids (C) in the solution is at least 0.05 M.

8. The process according to any of claims 1 to 7, wherein process step (a) takes place at a temperature in the range from 15 °C to 50 °C.

9. The process according to any of claims 1 to 8, wherein process step (a) takes place at a deposit potential in the range from -1 .9 V to - 2.5 V vs. Pt quasi-reference electrode.

0. The process according to any of claims 1 to 9, wherein the surface of the electrode, where the tin nanowires are deposited, is composed of a material selected from the group consisting of copper, tin, aluminum and glassy carbon.

1 . The process according to claim 1 , wherein the tin compound (A) is tin tetrachloride,

wherein the concentration of tin tetrachloride in the solution is in the range from 0.05 to 0.2 M, and the silicon compound (B) is silicon tetrachloride, wherein the concentration of silicon tetrachloride in the solution is in the range from 0.4 M to 0.8 M, and wherein process step (a) takes place at a temperature in the range from 15 °C to 50 °C and at a deposit potential in the range from -1 .9 V to - 2.5 V vs. Pt quasi-reference electrode.