GB2516318A

GB2516318A - Etched silicon structures, method of forming etched silicon structures and uses thereof

Info

Publication number: GB2516318A
Application number: GB1312984.6A
Authority: GB
Inventors: Jonathon Speed
Original assignee: Nexeon Ltd
Current assignee: Nexeon Ltd
Priority date: 2013-07-19
Filing date: 2013-07-19
Publication date: 2015-01-21
Anticipated expiration: 2033-07-19
Also published as: GB201312984D0; GB2516318B; WO2015008093A1

Abstract

A method of etching silicon of a material 101 comprising silicon at a surface 103 thereof comprises forming a functionalized surface comprising a functional group 105 comprising a metal-binding group 107 on the silicon surface, binding a metal 109 or metal ion to the metal-binding group, if necessary reducing the metal ion to elemental metal and etching the silicon by metal-assisted chemical etching. The metal binding group preferably comprises a thiol or amine. The metal ion may be bound to the functional group by bringing the functionalized surface into contact with metal nanoparticles or with a solution of metal ions. The method may further comprise the step of growing a metal particle on the reduced metal ion. The reduced metal ions are preferably exposed to a capping agent (115, figure 3) before or during contact with a solution of metal ions. The material to be etched may be in the form of bulk silicon, such as a silicon wafer, or may be a powder having silicon at a surface thereof.

Description

Etched Silicon Structures. Method of Forming Etched Silicon Structures and Uses Thereof

Field of the Invention

The present invention relates to methods of etching silicon, etched silicon structures, elecirodes conlaining etched silicon structures and devices including elched silicon structures.

Background of the Invention

Etched silicon structures comprising pores or clongated pillar-like structures may be used in a wide range of applications including clectrochemical cells, metal ion batteries such as lithium-ion battcries, lithium air batteries, flow cell battcries, other encrgy storage deviccs such as fuel cells, thermal batterics, photovoltaic devices such as solar cclls, filters, sensors, clcctrical and thermal capacitors, microfluidic devices, gas/vapour sensors, thcrmal or diclectric insulating deviecs, devices for controlling or modifying the transmission, absorption or reflectance of light or other forms of electromagnetic radiation, chromatography or wound dressings.

Porous silicon particles may also he used for the storage, controlled delivery or timed release of ingredients or aclive agents in consumer care producis including oral hygiene and cosmetic products,food or other nutritional products, or medical products including pharmaceutical products that deliver drugs internally or externally to humans or animals.

Etched silicon may also form architeetured conducting or semiconducting components of electronic circuitry.

The use of silicon as the anode for a metal ion battery, for example a lithium ion battery, is known. Silicon has a substantially higher maximum capacity than graphite. however, unlike aclive graphile which remains suhstaniiafly unchanged during insertion and release of metal ions, the process of insertion of metal ions into silicon results in substantial structural changes, accompanied by substantial expansion. For example, inserlion of lithium ions into silicon resulls in formation of a Si-li alloy. The died of Ii ion insertion on the anode material is described in, br example, "Inserlion Electrode Materials for Rechargeahk lithium Baileries", Winier et al, Adv. Mater. 1988, ID, No. 1 0, pages 725-763.

W02009/01 0758 discloses the etching of silicon powder in order to make silicon material for use in lithium ion hatteries. The resulting etched particles contain pillars on their surface. These siructured silicon elecirodes show-good capacily relention when subjected to repealed charge/discharge cycles and this good capacity retenlion is believed Lo be due to the ahilily of the silicon pillars lo absorb Ihe volumetric expansion/coniraclion associated with lithium inserlion/exiraclion from the hosi silicon without Ihe pillars being broken up or destroyed.

Huang et a!, "Melal-Assisled Chemical Elching of Silicon: A Review-", Advanced Materials 2010, 1-24, discloses elching of silicon wherein elemenlal metal is deposiled on the silicon surface by a process of electroless deposilion followed by eiching of silicon undcriying the deposited silver.

In electroless deposition, the silicon is exposed to a solution of a metal salt, for example silver nitrate, and a source of fluoride ions, for example 1W to reduce the metal ions and form elemental metal on the silicon surface.

The size, shape and distribution of metal formed on a silicon surface by electroless deposition, and as a result the size and shape of etched structures formed by etching following electroless deposition, may be difficult to control.

It is an object of the invention to provide an improved process for etching silicon, including hulk silicon and silicon powder, and in particular a process for providing improved control over etching of silicon.

Summary of the Invention

In a first aspect the invention provides a method of etching silicon of a material comprising silicon at a surface thereof, ihe method comprising the sleps of: lorming a Iunctionaliíed surface comprising a lunctional group on the silicon surface wherein the lunctional group comprises a metal-binding group; binding a metal or metal ion to the metal-binding group; if a metal ion is bound to the functional group, reducing the metal ion to elemental metal; and etching the silicon by metal-assisted chemical elching.

in a sccond aspect the invention provides etched silicon obtainable by a mcthod according to the first aspcct.

in a third aspect thc invention provides an clcctrode comprising an active material of etched silicon according to the sccond aspect. Optionally, thc electrode further comprises a conductivc current collector in electrical contact with the active material.

In a fourth aspect the invention provides a method of forming an electrode according to thc third aspect, thc method comprising the step of depositing onto thc conductive current collector a slurry comprising an etched silicon powder according to thc second aspect and at least one solvcnt, and evaporating thc at least one soNent.

In a fifth aspcct the invention provides a rechargeable metal ion hattery comprising an anode, the anode comprising an electrode according to the second aspect capable of inserting and releasing metal ions; a cathode formed from a metal-containing compound capable of releasing and reabsorbing the metal ions; and an electrolyte hetween the anode and the cathode.

Description of the Drawings

The invention will now he described in more detail with reference to the Figures, in which: Figure 1 is a schematic illustration of a method according to an embodiment of the invention wherein elemental metal is bound to a functionafized silicon surface: Figure 2 is a schematic illustralion of a method according to an embodiment of Ihe invenlion wherein melal ions are hound Lo a lunctionalt'ed silicon surface; Figure 3 is a schematic illustration of a functionalized silicon surface of a material used in combination with a capping layer in a method according to an embodiment of the invention; Figure 4 is a schematic illustraLion of a silicon surface having a functional group derived from APTS hound thereto; Figure 5 is a schematic illustration of a metal ion battery according to an embodiment of the invention; Figure 6A is a SEM image of a silicon particle after electroless metal deposition and etching; and Figure ÔB is a SEM imagc of a silicon particle after etching by a method according to an cmbodiment of the invention.

Detailed Description of the Invention

Figure 1 illustrates an etching process according to an embodiment of the invention.

In a first slep, a silicon surface 103 of a material 101, which in this embodiment is a particulate material, is modified to form frmnctional groups 105 bound to the silicon surface.

The funcLional groups 105 each have a melal-hinding group 107 thaI is capable of binding to a melal. The bond may he a dalive bond.

Following formation of the lunclional groups 105 on the silicon surface, parLicles 109 of elemental metal M are bound to the functional groups. the source of the elemental metal may he metal particles,for example metal nanoparticles that are brought into contact with the functionalized silicon surface.

The lunclional group maybe selecled such Ihat ii hinds strongly to the melal. For example, the lunctional group may have an amino group br binding to silver and may he a ihiol group br gold.

Tic functionalized silicon surfacc carrying mctal is then ctched by metal-assistcd chemical etching, which is an dectroless etching process without the application of an exiernal bias, Lo produce silicon pillars 111 exiending from an etched surface 113.

Allernaiively, the silicon may be etched lo produce a porous surface. In the process of melal-assisled chemical etching, the metal forms an electrode of a local galvanic cell and the silicon surface is etched in Ihe presence of a fluoride, for example HF, and an oxidani.

It is believed thai the density and form of melal particles on the surface influences ihe type of etched strucLures ihai will he formed.

The elehing process may be as described in Huang et al, "Metal-Assisted Chemical Elehing of Silicon: A Review", Advanced Malerials 2010, 1-24, the conlenis of which are incorporated herein by reference. Exemplary oxidants indude 02; 03: hydrogen peroxide; the acid or saiL of NO, S20t, NOI, B40t or CO4 and mixiures thereoL Preferred oxidants include hydrogen peroxide and nitrates, for example alkali metal nitrates and ammonium nitrate.

The silicon-containing material may be irradiated during etching.

The etching process may be carried out in any suitable reaction vessel, for example a vessel formed from a TIP-resistant material, such as polyethylene or polypropylene or a reaction vessel lined with a hF resistant material such as a TIP resistant rubber. If the silicon is irradiated Ihen (be vessd may he Hght-iransmissive.

Figure 2 illustrates another embodiment of the invention. A silicon surface is thnctionalized as described with reference to Figurc I and is thcn brought into contact with a solution containing metal ions M'1, whcrcin n is 1, 2 or 3.

The metal ion is reduced to form elemental metal that provides a nucleation point for the metal ions in the solution, enabling formation of a metal particle. Etching may then be performed as described wilh reference to Figure 1.

The silicon surface carrying melal ions maybe hroughi into contaci with a Iluoride, for example HF, lo release elecirons (Equalion I) lo reduce ihe melal ions Lo elemental melal (Equation 2): Si° + GE SiIY + 4c (Equation 1) Ag (aq) + & Ag (s) (Equation 2) Fluoride ions both generate elecirons for reduction of the melal ions and, upon addition of an oxidani, etch the silicon surface. Accordingly, reduction and etching steps may lake place without separation of the silicon from the reaction mixture, and may take place in a single reaclion vessel.

in other embodiments the metal ions may first be reduced before the silicon surface is brought into contact with a fluoride for etching of the silicon. the metal may be reduced by heat treatment and / or by a reducing agent, for example sodium borohydride, trisodium citrate, citric acid, alkyl amines, ascorbic acid or ethylene glycol.

in the embodiment illustrated in Figure 2, the functional group remains hound to the silicon surface during reduclion of the melal ions, however some or all funclional groups may detach &om the silicon surface during reduction and / or during etching. fluoride ions may cause delachmenl of funclional groups from the silicon surface.

in the embodiment illustrated in Figure 2, metal ions bound to the metal binding group are reduced, and remain hound to the metal binding group following reduction. in other embodiments, metal may detach from the metal binding group and deposit on the silicon surface during the reduction process.

it will he appreciated ihal the functional group of the embodimeni of Figure 2 is capable olbinding lo the metal in hoLh ionic and demenial form.

Figure 3 illustrates one stage of a further embodiment of the invention, which follows the same process as described with reference to Figure 2, except that a capping agent is added before or during nucleation. The capping agent forms a capping layer I 15 on the surFace of the meial atom and nucleaied meiäl ions, and may hall Further nucleation or may constrain Further nuclealion Lo uncapped surFace areas of ihe nucleaied metal.

Figure 3 illusiraies capping ihai prevents ouiward growih of the nucleale hut allows lateral access lo ihe nucleate br fttrther lateral nucleaiion.

The use of a capping agent may reduce the amouni of melal needed in melahassisied chemical eiching, and may he used lo conirol ihe shape of meith nucleales.

Exemplary capping agenis include melal citrales; polymers, for example polyvinylpyridine: and ascorbic acid. Conirolled growlh of capped nanoparlicles may be as described in Basius ci a, Langmuir 2011, 27, 11098-11105, ihe conienis of which are incorporated herein by reference.

Figures I, 2 and 3 illustrate etching of particles having silicon at a surface thereof, for example a silicon powder. It will he appreciated that other forms of material having a silicon surface maybe used, such as bulk silicon, for example a silicon wafer or a polycrystalline or amorphous ribbon or sheet of silicon.

Figures 1, 2 and 3 illustrate anisotropic etching of silicon to produce silicon pillars extending from a core. In other embodiments, anisotropic etching may produce porous silicon.

The etched material formed following etching maybe used without further modification, or may he modified prior to use. In the case of an etched material having pillars extending from a core, the pillars may be detached from the etched surface to form silicon fibres.

Silicon surface functionalisation The lunclional group may he formed from a Iunctionaliting material ihai is itacied wiih the silicon surface to form the functional groupS. The functionalizing material may have a silicon-binding group capable of hinding io ihe silicon surface and a metal-binding group capable of Forming Ihe bond wiih ihe metal or melal ion.

The lunclionali ng material used to lorm the lunclional group may have lormula (U: (M13G)-(Sp) -(SBG), (1) wherein MGB is a metal-binding group; SBG is a silicon-binding group; 5p is a spacer group; x and z are each a least 1: and y is 0 or 1 Exemplary groups MBG include amino groups Optionally, xis 1.

Optionally, z is 1, 2 or 3.

Exemplary groups SBU include groups capable 0! lorming a Si-O bond or a Si-C bond al Ihe silicon surface. Exemplary groups capable 0! !orming a Si-O bond include siloxy groups and siloxyl esters. Exemplary groups capable o! !orming a Si-C bond include alkynes and alkenes. The surface of the silicon may be activaled prior to reaction with the functionalizing material to impmve binding of the SBG. For examp'e, the surface of the silicon maybe activated by HF treatment before reaction with a SBG for formation of a Si-C bond.

The funclionalizing material may he formed from a precursor malerial. For example, in compounds of formula (1) the or each MBG and / or SBG may be provided with prolecling groups lo preveni reaclion of MBG and / or SBG prior lo conlaci with the melal and I or silicon respectively. For example, a siloxy SBC may he formed by reading a silanol activaled silicon surface with a Iri-oxy silane.

The funclionalizing material that is conlacled wilh the silicon surface may be dissolved in one or more solvents,for exampk water, alcohols and mixtures thereof.

The precursor material may he converted into the functionalizing material before being brought into contact with the silicon surface, or may be converted in situ.

S

An exemplary precursor ol a lunctionali ng material is (3-aminopropyl)Lriethoxysilane (APTS). Figure 4 iillusiraies a silicon surface 103 having a lunctional group lormed using APTS, and with silver hound to the amino melal-hinding group of Ihe funelional group.

Thc functionalizing matcrial, or a prccursor thereof, may bc dissolvcd in any suitable solvent including organic solvents, water, and mixtures thereof The exieni io which the silicon surface is covered by the meia particles may be controlled by faciors including, without limilalion, -the concentration of functional groups on the silicon surface, which may he controlled by one or more of coneenLraiion of the funetionaliLing material and duration of contact between the functionalizing material and the silicon surface; and -the perccntagc of functional groups on the surfacc that bind to a mctal or metal ion, which may be controlled by one or more of: concentration of mctal or metal ions brought into contact with the functionalized surface: duration of contact between the metal or meta' ions and the functionalized surface; strength of the bond formed between the meta' or metal ion and the metahbinding group: and temperature during ch&ation.

Surface functionalisation maybe as described in Langmuir, 2004, 20, 4720 -4272: Wesicoli et al, Langmuir 1998, 14, 5396-5401 and Buriak, Chem. Rev. 102(5), 2002, 1272-1308 Meulls and meia ions Exemplary melals thai may be hound io ihe funciiona group and used in melal-assisted chemical elching, and metal ions thereof, include silver, gold, platinum and copper ions.

If a metal ion is hound to the functional group then exemplary metal compounds containing these metth ions are AgNO3, AuCI4, silver acetate, copper sulphate pentahydrate, silver oxide, silver fluoride, silver tetrafluorohorate, silver trifluoroacetate, platinum chlorate and copper oxide. The meta' ions may he metal complex ions, for example [Ag(NH3)2 ions, copper (IT) tarlrate ions and copper (TI) ciLrale ions. The metal compounds are preFerably waler soluble, and ihe solutions oF metal ions are oplionally aqueous solulions or a mixlure of water and one or more waler-miscihle organic sol venis.

Metal nanopartieles, either as added directly for binding to the metal binding group or as formed following reduclion of melal ions, may have a widlh of less than 1 micron on average, oplionafly less than 500 nm, less than 250 nm, or less than 100 nm. Metal nanoparlicles preferably have a widlh of al leasl 5 nm on average.

Silicon slartinz malerial The silicon surface to he etched maybe undoped, n-doped, p-doped or a mixture thereof.

Preferably, the silicon is n-or p-doped. Examples of p-lype dopants for silicon include H, A, In, Mg, Zn, Cd and Hg. Examples of n-type dopants for silicon include P, As, Sb and C. l)opants such as germanium and silver can also he used.

The silicon to he etched may be supported on a surface of another material.

The silicon may be pure silicon or may he an alloy or other mixlure of silicon and one or more other materials. The silicon may have a purity of at least 90.00 wt%, optionally at least 99 wl%, oplionally at leasl 99.8 weighl %. Optionally the silicon purity may he less than 99.99 wt%. The silicon maybe metallurgical grade silicon.

The silicon may have a resislivily of heiween 0.000 1 -100 11cm, preferably less than 1 11cm, preferably less than 0.1 11cm.

The starling silicon material may he cryslalline or amorphous. Etching may be carried out on, for example, bulk silicon or on a particulate material, for example a silicon powder. Exemplary hu& silicon structures include silicon sheets such as silicon wafers or of melallurgical grade silicon, and silicon sheds or chips formed by breaking a silicon wafer into smaller pieces, or by breaking other firms of bulk silicon into sheets or flakes.

Powder particles of silicon may he formed from a silicon source such as metallurgical grade silicon by any process known to the skilled person, for example hy grinding or jetmilfing hulk silicon Lo a desired size. Suitable example silicon powders are availahle as "Silgrain'TM" 1mm Elkem of Norway.

Where used, hulk silicon such as a silicon wafer may have first and second opposing faces, the surface of cach face having an area of at least 0.25 cm2, optionally at lcast 0.5 cm2, optionally at least I cm2. Each face maybe substantially pbnar. Bulk silicon may have a thickness of more than 0.5 micron, optionally more than 1 micron, oplionally more than 10 rmcrons, optionally more than 100 microns, oplionally in Ihe range of aboul 100 -1000 microns.

Where used, particles may he in the form of flakes, wires, ribbons, cuboid, subsianlially spherical or spheroid particles. They may he multifaceled or may have substantially continuous curved surfaces. Non-spherical core parlicles may have an aspect ralio of al least 1.5 1, oplionally al leasl 2 1.

Ilic particles may have a size with a largest dimension up to ahout 100pm, preferably kss than 5Opm, more preferably less than 3Opm.

The particles may have at least one smallest dimension less than one micron. Preferably the smallest dimension is at least 0.5 microns.

Particle sites may be measured using oplical methods, for example scanning election microscopy.

In a composition containing a p'urality of paiticles, for example a powder, preferably at east 20%, more preferably at east 50% of the particles have smallest dimensions in the ranges descrihed above. Particle size distribution may he measured using laser diffraction methods or optical digital imaging methods.

A distribution of the particle sizes of a powder of starting particles used to form etched parlides maybe measured by laser diFFraction, in which ihe particles being measured are typically assumed to he spherical, and in which particle size is expressed as a spherical equivalent volume diameler, For example using the MaslersizerTM partide size analyter available from Malvern Insirumenis Lid. A spherical equivaleni volume diameter is Ihe diameter of a sphere with the same volume as Ihat of Ihe partide being measured. TI-all parlicks in the powder being measured have ihe same density ihen the spherical equivalent volume diameler is equal Lo Lhe spherical equivalenl mass diameter which is the diameter of a sphere thai has the same mass as the mass of Ihe particle being measured. For measurement the powder is typically dispersed in a medium with a refractive index that is different to the refractive mdcx of the powder material. A suitable dispersant for powders of the prescnt invention is water. For a powder with different sizc dimensions such a particle size analyser provides a spherical equivalent volume diameter distribution curve.

Size distrihution of particles in a powder measured in this way maybe expressed as a diameter value Dn in which at least n ft of the volume of the powder is termed from partides have a measured spherical equivalent volume diameter equal to or less than D. Preferred size disirihulions for a powder of starling silicon particles include D50 25pm, optionally lSpm, optionally 10pm.

BET (Brunauer, Emmett and Feller) surface area per unit mass of a starting material may he at least 0.5 m2/g, preferably aL least 1, 2 or 3 m2/g.

It will he appreciated that etching a starting material particle to produce a pillared particle, for example as described with reference to Figures 1-3, then the resultant pillared particle will have a pillared particle core that is smaller than the starting material particle.

A porous particle produced by etching a starting material may be substantially the same size as, or smaller than, the starting material.

The material to be etched may consist essentially of silicon as described above, for example silicon having a purity of at least 90%, such as metallurgical grade silicon as described above, or it may contain one or more further materials. The material to be etched may have a non-silicon core, for example a core of graphite, and a silicon shell wherein the shell is etched.

Whcrc thc starting material has a silicon shell, thc shell thickness may be grcatcr than 0.5 microns, optionally in thc range of 1-10 microns or 1-5 microns. The material having a non-silicon core may he a powder, and Ihe non-silicon core of this material may have a diameter grealer Ihan 5 microns.

The starting silicon th he etched may have a surface layer of a silicon compound, for example a silicon oxide layer. Silicon may havc a native silicon oxide surface laycr which may have a thickness of about 1-2 nm This may he increased by heating to a thickness of no more than 20 nm.

The surface of the silicon-containing materia' may include non-silicon materials.

Preferably, at least 5 weighl % of the slarling material is silicon.

Etched silicon structures Pillars formed by etching of the silicon surface may have any shape. For example, pillars may he branched or unbranched; substantiafly straight or bent; and of a substantially constant thickness or tapering. Pillars may contain steps.

Pillars extend outwardly from, and may he spaced apart on, an etched sificon surface. In one arrangemcnt, substantially all pillars may bc spaced apart. in another arrangemcnt, some or substantially all of thc pillars may be clustered togcther.

The cross-sections of the pillars may form regular shapes (e.g. circular, square or triangular) or be irregular in shape (e.g. may contain one or more concave or convex curved sides or branches or spurs extending outwards or combinations thereof. it will be appreciated that the shape of the pillars is at least partly determined by the shape of the exposed surface areas of silicon after metal deposition.

In the case where particles are elehed to form pillars extending from a parlicle core, the parlicles may have a pillar mass fraction (PMF) of al leasi 5 %, optionally al least 15 % or aL eas1 20 % wherein PMF given by the loflowing equalion: PMF = [(Total mass oF pillars exiending Irom the particle core) I (Total mass 0! pillared particle)1 x 100.

Tie PMF may be dctermincd by measuring mass of the particles beforc and after separation of pillars from the core. Pillars may bc separated from thc core by sonication.

Pillar volume Iraclion (volume ol pillars / volume ol pillared parlicles) is Ihe same as PMF ii the densilies of Ihe core and the pillars are Ihe same.

If the particle core of a pillared particle is not silicon then the PYF and PMF values may he differenl. In this case, a PMF value may he converted Lo a PVF value using the densities of the pillars and the core.

The pillars may have a diameter or thickness in the range of about 0.02 to 0.70 tm, e.g. 0.1 to 0.5pm, for example 0.1 to 0.25pm, preferably in the range 0.04 10 0.50pm. The pillars may have an aspect ratio (defined as the height of the pillar divided by the average thickness or diameter of the pillar at us base) in the range 5:1 to 100:1, preferably in the range 10:1 to 100:1. The pillars maybe subsiantially circular in cross-section hui they need not be. Where the pillars have irregular cross-seclions comprising a pluralily of extended sections with changing direclion and/or with branches or spurs then the average thickness of the pluralily of such section is used in the calculation of the aspect ratio. The pillars may extend outwards from the silicon in any direction and may comprise kinks or changes in direclion along their length.

Pillars may be formed by etching the silicon surface to a depth of more than 0.25 microns, more than 0.5 microns, optionally at least I micron, optionally at least 2 microns, optionally more than 10 microns. Optionally, the pillars are formed by etching the silicon surface to a depth in the range of 1-10 microns.

Silicon may be etched silicon to produce porous, eg mesoporous silicon (pores SOnm) or macroporous silicon (i.e. silicon with pores of diameter> 5Onm). The process of etching silicon to form porous silicon may be substantially the same as described with reference to Figures 1-3, except that etching results in formation of pores on the surface of the silicon to be etched and extending downwards into the silicon material, rather than pillars extending from an etched surface of the etched silicon. Porous silicon may have a substantially continuous connected network of silicon walls at the outer surface of the silicon thai has been etched.

The surface of the etched silicon may comprise both regions of porous silicon and regions with pillars. The etched silicon may also combine regions of porous and pillared silicon in an inward extending direction. Thai is, an outer shell region of ihe eLched silicon may comprise pillared silicon whflsi the inner region comprises porous silicon and vice versa.

Pores may extend at least 100 rim, optionally at least 0.5 microns into the silicon from silicon surface, optionally at lcast 1 micron, optionally at least 2 microns. The pores may have a diameter of at least 10 nm, 20 nm, or 100 rim, optionally at least 300nm, oplionally al leasl 0.5 microns. The pores may extend inwards perpendicular lo the silicon surface or may exiend inwards al any intermediate angle. Nol all pores may exiend in the same direclion, instead ihe plurality of pores may extend in a plurality of direclions. The direciion in which the pores exiend inwards may chmge pariway down. Two or more pores may join 10 form an irregular network of pores below-the surface of the silicon.

The surfaces of pores or pillars may he relatively smooth or they may he rough. The surfaces may be pitied or comprise pores or voids with diamelers less than 5Onm. The pillar siruciures may he solid; mesoporous: mieroporous or a comhinalion thereof. The pillar structures may have a solid core with a mesoporous outer shdl.

The porosity of the etched silicon may he defined as the percentage ratio of the total volume of the void space or pores introduced into the etched silicon to the volume of the silicon before etching. A higher porosity may provide a higher surface area which may increase the reactivity of the silicon in a device, for example in electrochemical cells, sensors, detectors, filters etc. or it may provide a larger volume for containing ingredients or active agents in medical or consumer product compositions. however, if the porosity is too large the structural integrity (or mechanical strength) of the silicon may be reduced and for example, in devices such as a lithium ion battery, the volume of electrochemically active silicon material is reduced. The porosity of the etched silicon may be at least 5%, optionally at least 10%. Preferably it is at least 2Ot/e, at least 40 %, at least 50 % or at least 50 9k The porosity maybe less than 95%, less than 90%, optionally less than 80%.

Dimensions of pores and pillars may be measured using optical methods, for cxample scanning electron microscopy. Porosity may be measured using known gas or mercury porosimetry techniques or by measuring thc mass of the silicon material before and after ctching.

Batiery lormation Etched silicon formed as described herein may be used as an active material in an anode of a reehargeaHe metal ion hattery ("active material" or "eleetroactive material" as used herein means a material which is able to insert into its structure, and release therefrom, metal ions such as lithium, sodium, potassium, calcium or magnesium during the respeclive charging phase and discharging phase of a hatlery. Preferably the malerial is able lo inserl and release lithium.) The structure of an exemplary rechargeable metal-ion ballery is shown in Fig. 5. The haltery cell includes a single cell hul may also include more than one cell. Batleries of other melal ions are also known, for example sodium ion and magnesium ion batleries, and have essentially the same cell structure.

The battery cell comprises a current collector for the anode 10, for example copper, and a current collector for the cathode 12, for example aluminium, which are both externally connectable to a load or to a recharging source as appropriate. A composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12 (for the avoidance of any doubt, the terms "anode" and "cathode" as used herein are used in the sense that the battery is placed across a load -in this sense the negative electrode is referred to as the anode and the positive electrode is referred to as the cathode).

The cathode comprises a material capable of releasing and reahsorbing lithium ions for example a lithium-based metal oxide or phosphate, LiCoO2, LiNi08Co015A1005O2, liMnNiCoi,O2 or JAFePO4.

A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and the lithium containing metal oxide-based composite cathode layer 16.

An electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some eases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composile cathode layer 16. The polymer electrolyte material can he a solid polymer electroyLe or a gel-type polymer electro'yte and can incorporate a separalor.

Whcn the battery ccli is fully chargcd, lithium has bcen transported from the lithium containing metal oxide cathode layer 16 via the electrolyte into the anode layer 14.

In the case where hulk silicon is etched, an anode current colleclor may he formed on one side of the hulk silicon and another side of the bulk silicon having an etched surface may come mb conbaci with the electrolyte of Ihe battery. The curreni collector may he a melal foil, for example copper, nickel or aluminium, or a non-melallic curreni collector such as carbon paper In the case where the silicon is in the form of an etched powder, a slurry comprising the etched powder and one or more solvents may he deposited over an anode cunent collector to form an anode ayer. Ihe slurry may further comprise a hinder material, for example polyimide, polyacrylic acid (PAA) and alkali metal salts thereof, polyvinylalchol (PVA) and polyvinylidene fluoride (PVDF), sodium carhoxymethylcellulose (Na-CMC) and optionally, non-active conductive additives, for example carbon black, carbon fibres, ketjen black or carbon nanotuhes. In addition to providing the silicon powder to act as an active material in the battery, one or more further active materials may also be provided in the slurry. Exemplary further active materials include active forms of carbon such as graphite or graphene. Active graphite may provide for a larger number of charge I discharge cycles without significant loss of capacity than active silicon, whereas silicon may provide for a higher capacity than graphite. Accordingly, an electrode composition comprising a silicon-containing active material and a graphite active material may provide a lithium ion battery with the advantages of both high capacity and a large number of charge I discharge cycles. The slurry may be deposiled on a currenb collector, which may he as described above. Further treatments may be done as required, for example to directly bond the silicon particles Lo each other and/or to the current collector. Binder material or other coatings may also he applied bo the surface of the composibe elecirode layer after inibial formation.

Examples 0! sui able caihode malerials include LiCoO2, TACo0 99A10 0102, LiNiO2, LiMnO2, LiCo0sNi05O2, TiCo0 7Ni0 302, TACo0 8Ni0 202, JACogs2Niojs02, LiCo08Ni0i5AIgo502, JANi04Co03Mn03O2 and TANi033Co033Mn03402. The caihode curreni colleclor is generally of a thickness of heiween 3 In 500iim. Examples ol materials ihal can he used as Ihe cathode curreni collecior include aiuminium, slainless steel, nickel, titanium and sintercd carbon.

The elecirolyle is suilably a non-aqueous elecirolyle conlaining a lilhium salt and may include, wilhout limilalion, non-aqueous electrolytic solulions, solid elecirolyles and inorganic solid electrolyles. Examples of non-aqueous elecirolyte solutions that can he used include non-probe organic solvenis such as propylene carbonale, ethylene carbonate, hutylenes carbonate, dimethyl carbonale, diethyl carhonale, gamma butyro lactone, 1,2-dimeihoxy elhane, 2-meihyl letrahydrofuran, dimelhylsulphoxide, 1,3-dioxolane, formamide, dimethylformamide, acelcnilrile, nilromethane, methylformale, methyl acelale, phosphoric acid trimesler, Irimethoxy methane, sulpholane, methyl sulpholane and 1,3-dimethyl-2-imidazolidione.

Examples of organic solid electrolytes include polyethylene derivatives polyethyleneoxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, polyester sulphide, polyvinyl alcohols, polyvinylidine fluoride and polymers containing ionic dissociation groups.

Examples of inorganic solid electrolytes include nitrides, halides and sulphides of lithium salts such as Li5NI2, Li3N, Lii, LiSiO4, Li2SiS3, Li4SiO4, LiOTI and Li3PO4.

The lilhium saIl is suitably soluble in Ihe chosen solveni or mixiure of solvenis. Examp'es of suitable lithium salts include LiCI, LiBr, LiT, LiCIO4, LiBF4, LiBC4O8, LiPF6, LiCF3SO3, TAAsF6, LiShF6, TAAICI4, CH3SO3II and CF3SO3Li.

Where the electrolyte is a non-aqueous organic solution, the battery is provided with a separator interposed between the anode and the cathode. the separator is typically formed of an insulating material having high ion permeability and high mechanical strength. the separator typically has a pore diameter of between 0.01 and 100pm and a thickness ol heiween 5 and 300pm. Exampks of suitable electrode separators include a micro-porous p0] yeLhy]ene him.

Examples

Example 1

APTS (2 g) was added lo water (5 rnL) and elhanol (95 mL). 50 mL of 10% v/v acetic acid was added lo give a pH in the range of 4.5 -5.5. The solution was stirred and set aside for al leasl 5 mm to form the silanol.

Silgrain (R) HQ metallurgical grade powder available from Elkern (10 g, D50 = 12 urn, BET = 0.7 m2 g') was suspended in 20% v/v HNO3 and sonicaled for 30 s and then sd aside for al leasl 10 mm, before the solid was collected by filiration, and washed with water. Ic silicon powdcr was thcn resuspended in 100 mL ethanol and sonicated for 1 mm.

The two solutions were combined and rehluxed overnight with stirring.

g of the rnodified Si was suspendcd in 100 mL water and 5 g AgNO3, and stirred for 30 mm. This slurry is poured ink a solution containing 600 mL water and 300 mL hF (49%), and is stirred for 1 rnin. The solution was rested for 15 mm, before the addilion of 4 aliquois of 50% w/w N114N03 solution, of 2.5 g, one every 15 mm. After the last addilion Ihe solution was rested for 1 hr, the TIP poured away, and the Ag recovered using 30% v/v hINO3, before filiration to coiled the Si.

Comparative Example I Silver was deposiled onlo the surface of the silicon starting parlicles described in Example 1 by electroless deposition as described in W02009/010758 followed by rnelal-assisted chemical etching of the silicon to form pillared silicon particles.

Figure 6A is a SEM image of etched silicon formed by the process of Comparative Example 1, and Figure 6B is a SEM image of etched silicon formed by the process of

Example I.

Properlies oF the etched silicon are set oul in laNe 1, wherein the yield is Ihe remaining mass 0! silicon slarling maleriad lollowing lormalion of the pillared pariicles

Table 1

Example Yield PMF Pillar lenglh BET Comparative 28% 15.79% 1.6 ± 0.39 6.71

Example 1

Example I 10% 27A7% L63 ± 0.29 12.23 pm As apparent from the values shown in Table I, the PMF of Example 1 is significantly higher than that of Comparative Example 1. This may he partly explained by a denser coverage of pillars on some particle surfaces. The pillars may also he Ihicker, making them morc robust but it is also bclicved that a morc uniform ctching of the particles surFaces has been achieved, reducing the number oF unetched or poor'y etched surfaces in the powdcr on Comparative Example 1.As the piHars are intcndcd to provide the majorily oF the active material in the electrode Ihen the higher PMF provides a higher capacity clectrode material.

Tic invention has bccn dcscribcd with reference to anodes of rcchargeable batteries that operate by absorption and desorption of lithium ions, however it will be appreciated that etched silicon structures as described hercin may be applicable to other metal ion batteries, for example sodium or magnesium ion batteries. Moreover, it will he apprecialed that etched silicon as described herein may he used in devices other than melal ion balleries, for example filters, other energy sicrage devices such as fuel cells, photovollaic devices such as solar cells, sensors, and capacilcrs. Elched silicon as described herein may also form conducting or semiconducting componenis of electronic circuilry.

Alihough the present invenlion has been described in terms of specific exemphtry embodiments, ii will he appreciated that various modificalions, alleralions and/or comhinations of Features disclosed herein will he apparent to Ihose skilled in Ihe art wilhouL deparling from Ihe scope 0! the invenlion as sd!orth in the lo]]owmg claims.

Claims

Claims 1. A method of etching silicon of a matcrial comprising silicon at a surface thereoL the method comprising the steps of: forming a functionalized surface comprising a functional group on the silicon surface wherein the functional group comprises a metal-binding group; binding a metal or metal ion to the melal-binding group; if a metal ion is bound to the functional group, reducing the metal ion to elcmcntal metal; and etching the silicon by mctal-assisted chemical etching.
2. A method according to claim 1 wherein the functional group comprises a group bound to the silicon surface and the metal binding group.
3. A method according to claim 1 or 2 wherein the metal binding group comprises a thio or amine.
4. A method according to any preceding claim wherein a metal is bound to the metal binding group by bringing the functionalized surface into contact with metal nanoparticles.
5. A method according to any of claims 1-3 wherein a metal ion is bound to the functional group by bringing the functionalized surface into contact with a solution of melal ions.
6. A method according to claim 5, the method comprising the further step of growing a meta' partidc on the reduced metal ion.
7. A method according to claim 6 wherein the reduced metal ions are exposed to a capping agent before or during contact with the solution of metal ions.
8. A method according to any preceding claim wherein the metal or metal ions are selected From silver, copper, phulinum, palladium, gold and ions thereoli 9. A method according to any preceding claim wherein the metal-assisted chemical etching comprises contacting thc silicon with an etching composition comprising a fluoride and an oxidant.10. A method according to claim 9 wherein the fluoride and the oxidani are provided in an aqueous elching composition.11. A method according to claim 9 or 10 wherein the fluoride is hydrogen fluoride.12. A method according to any of claims 9-11 wherein the oxidant is selected from the group consisting of 02; th; hydrogen peroxide; and the acid or salt of N0, S2052, N02, B4072 or Cl04, and mixtures thereof 13. A method according to claim 12 wherein the oxidant is selected from the group consisting of hydrogen peroxide, alkali metal nitrates, ammonium nitrate and mixtures thereof 14. A method according to any of claims 5-13 wherein the metal ion is reduced by exposing the silicon surface to a solution of fluoride ions.15. A method according to claim 14 wherein an oxidant is added to the solution of fluoride ions to form the etching composition according to any of claims 9-13.16. A method according to any preceding claim wherein the functional group is formed by reacting a funclionalizing material with the silicon surface.17. A method according to claim 16 wherein the funclionalizing material comprises the metal-binding group and a silicon-binding group.18. A method according to claim 17 wherein the silicon-binding group comprises one or more siloxy groups.19. A meihod according to any preceding claim wherein the etched silicon comprises piflars extending oul Irom an etched surface lormed by etching Ihe silicon surface.20. A method according to any preceding claim wherein the material to he etched is in the form of bulk silicon, optionally a silicon wafer.21. A method according to any of claims 1-20 wherein the material lobe etched is a powder having silicon at a surface thereof.22. Etched silicon oblainahie by a method according to any preceding claim.23. An electrodc comprising an active material of ctched silicon according to claim 22.24. An electrodc according to claim 23, whcrein thc electrode further compriscs a conductive currcnt collcctor in clectrical contact with the active material.25. A method of forming an electrode according to claim 24, the method comprising the step of deposiling onto the conduelive current collector a slurry comprising an etched silicon powder according to claim 22 and at least one solvent, and evaporating the at least one solvent.26. A rechargeable metal ion battery comprising an anode, the anode comprising an elcctmde according to claim 23 or 24 capable of inserting and releasing metal ions: a cathode formed from a metal-containing compound capable of releasing and reabsorhing the metal ions: and an ekctrolytc hetween the anode and the cathode.27. A rechargeable melal ion batlery according to claim 26 wherein the metal ion haltery is a lithium ion ballery.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS: Claims I A method of etching silicon of a material comprising silicon at a surface thereof, the method comprising the steps of: forming a functionalized surface comprising a functional group on the silicon surface wherein the functional group comprises a metal-binding group; binding a metal or metal ion to the metal-binding group; if a metal ion is bound to the functional group, reducing the metal ion to elemental metal; and etching the silicon by metal-assisted chemical etching.2. A method according to claim I wherein the functional group comprises a group bound to the silicon surface and the metal binding group.3. A method according to claim 1 or 2 wherein the metal binding group comprises a thiol or amine.0') 4. A method according to any preceding claim wherein a metal is bound to the metal binding group by bringing the functionalized surface into contact with metal nanoparticles. aD5, A method according to any of claims 1-3 wherein a metal ion is botmd to the functional group by bringing the functionalized surface into contact th a solution of metal ions.6. A method according to claim 5, the method comprising the further step of nucleating a metal particle on the reduced metal ion.7, A method according to claim 6 serein the reduced metal ions are exposed to a capping agent before or during nucleation, 8. A method according to any preceding claim erein the metal or metal ions are selected from silver, copper, platinum, palladium, gold and ions thereof.
9, A method according to any preceding claim wherein the metal-assisted chemical etching comprises contacting the silicon with an etching composition comprising a fluoride and an oxidant.
10. A method according to claim 9 wherein the fluoride and the oxidant are provided in an aqueous etching composition.
11. A method according to claim 9 or 10 wherein the fluoride is hydrogen fluoride.
12. A method according to any of claims 9-it wherein the oxidant is selected from the group consisting of 02; 03; hydrogen peroxide; and the acid or salt of NO3-, S202, N02, B40Y or ClO4, and mixtures thereof.
13. A method according to claim 12 wherein the oxidant is selected from the group consisting of hydrogen peroxide, alkali metal nitrates, ammonium nitrate and mixtures thereof
14. A method according to any of claims 5-13 wherein the metal ion is reduced by 0') exposing the silicon surface to a solution of fluoride ions.00
15. A method according to claim 14 wherein an oxidant is added to the solution of fluoride ions to form the etching composition according to any of claims 9-13.
16. A method according to any preceding claim serein the functional group is formed by reacting a functionalizing material th the silicon surface.
17. A method according to claim 16 wherein the functionalizing material comprises the metal-binding group and a silicon-binding group.
18. A method according to claim 17 wherein the silicon-binding group comprises one or more siloxy groups.
19. A method according to any preceding claim wherein the etched silicon comprises pillars extending out from an etched surface formed by etching the silicon surface.
20. A method according to any preceding claim serein the material to be etched is in the form of bulk silicon, optionally a silicon wafer,
21. A method according to any of claims 1-19 wherein the material to be etched is a powder having silicon at a surface thereof a) aD