GB2498802A - Anode Composition - Google Patents

Abstract

An anode composition for batteries and fuel cells comprising a first particulate electroactive material, a particulate graphite material and a binder, wherein at least 50% of the total volume of each said particulate materials is made up of particles having a particle size D50 and wherein a ratio of electroactive material D50 particle size : graphite D50 particle size is up to 4.5 : 1.

Description

Comijosition

Field of the Invention

The present invention relates to compositions comprising particles of an electroactive material and additives, and use of said compositions in devices including fuel cells and rechargeable metal ion batteries.

Background of the Invention

Rechargeable metal-ion batteries, for example lithium ion battcrics, arc extensively used in portable electronic devices such as mobile telephones and laptops, and are finding increasing application in electric or hybrid electric vehicles.

Rechargeable metal ion batteries have an anode layer; a cathode layer capable of releasing and re-inserting metal ions, and an electrolyte between the anode and cathode layers. When the battery cell is frilly charged, metal ions have been transported from the metal-ion-containing cathode layer via the electrolyte into the anode layer. In the case of a graphite-based anode layer of a lithium ion battery, the lithium reacts with the graphite to create the compound LiC6 (0 < x <= 1). The graphite, being the electrochemically active material in the composite anode layer, has a maximum capacity of372 niAh/g.

The use of a silicon-based active anode material, which may have a higher capacity than graphite, is also known.

W02009/010758 discloses the etching of silicon powder in order to make silicon material for use in lithium ion batteries.

Xiao et al, S. Electrochem. Soc., Volume 157, Issue 10, pp. A 1047-Al 051(2010), "Stabilization of Silicon Anode for Li-ion Batteries" discloses an anode comprising silicon particles and Ketjenblack carbon.

Lestriez et al, Electrochemical and Solid-State Lefters, Vol.12, Issue 4, pp. A76-A80 (2009) "Hierarchical and Resilient Conductive Network of Bridged Carbon Nanotubes and Nanofibers for High-Energy Si Negative Electrodes" discloses a composite electrode containing multiwall carbon nanotubes and vapour-grown nanofibres.

US 2011/163274 discloses an electrode composite of silicon, a carbon nanotube and a carbon nanofibre.

It is an object of thc invention to provide an anode composition for a metal ion battery that is capable of maintaining a high capacity.

It is a thrther objection of the invention to provide a composition for forming an anode of a metal ion battery from a slurry.

Summary of the Invention

In a first aspect, the invention provides a composition comprising a particulate electroactive material, a particulate graphite material and a binder, wherein at least 50% of the total volume of each said particulate materials is made up of particles having a particle size D50 and wherein a ratio of electroactive material D50 particle size graphite D50 particle size is up to 4.5: Optionally, the ratio is at least 2: Optionally, the ratio is in the range of 2:1 -4:1, optionally 3:1 -4:1.

Optionally, the particulate electroactive material is a silicon-comprising material.

Optionally, the particulate electroactive material comprises particles having a particle core and electroactive pillars extending from the particle core.

Optionally, the pillars of the silicon-comprising particles are silicon pillars.

Optionally, the core of the silicon-comprising particles comprises silicon.

Optionally, the silicon-comprising particles consist essentially of n-or p-doped silicon and wherein the pillars are integral with the core.

Optionally, the particulate elecfroactive material is provided in an amount of at least 50 vt % of the composition.

Optionally, the composition comprises at least one elongate nanostructure material.

Optionally, the first elongate nanostructure has a mean average diameter of at least 100 nfl.

Optionally, the composition comprises at least two elongate nanostructure materials.

Optionally, a second elongate carbon nanostructure material has a mean average diameter of no more than 90 nm, optionally a mean average diameter in the range of 40-90 nm.

Optionally, the first elongate nanostructure second elongate nanostructure weight ratio isintherange2.5: lto2O: 1.

Optionally, each of the at least one elongate nanostructure materials has an aspect ratio of at least 50.

Optionally, the first and second carbon elongatc nanostructurc matcrials arc each independently selected from carbon nanotubes and carbon nanofibres.

Optionally, the first carbon clongatc nanostructure material is a nanofibre and the second elongate carbon nanostructure material is a nanotube.

Optionally, the at least one elongate carbon nanostructure materials are provided in a total amount in thc rangc of 0.1-15 wcight % of the composition.

Optionally, one or more of the elongate carbon nanostructure materials has a fijnctionalised surface, optionally a surface flinctionalised with a nitrogen-containing group or an oxygen containing group.

Optionally, the graphite is provide in the composition in an amount of 1-30 wt %, optionally 1-20 \vt %.

Optionally, the crystallite length Lc of the graphite is optionally at least 50 nm, optionally at least 100 nm.

Optionally, the composition further comprises carbon black.

Optionally, the carbon black is provided in an amount of at least 0.5 weight % of the composition, and optionally less than 10 wt % of the composition, optionally less than 4 wt % of thc composition.

In a second aspect, the invention provides a metal-ion battery comprising an anode, a cathodc and an electrolyte between thc anode and the cathode whercin thc anode comprises a composition according to thc first aspect.

In a third aspect the invention provides a slurry comprising a composition according to the first aspcct and at least one solvent.

In a fourth aspect the invention provides a method of forming a metal-ion battery according to the second aspect, the method comprising the step of forming an anode by depositing a slurry according to the third aspect onto a conductive material and evaporating the at least one solvent.

Weight percentages of components of a composition described herein are the weight percentages of those components in a porous or non-porous solid composition containing all components of the composition. In the case of a slurry containing a composition, it will be understood that the weight of the one or more solvents of the slurry does not form part of the composition weight as described herein.

Description of the Drawings

The invention will now be described in more detail with reference to the drawings, in which: Figure I illustrates schematically a metal ion battery according to an embodiment of the invention; Figure 2 illustrates schematically a composite electrode according to an embodiment of the invention; Figure 3A illustrates schematically a process of forming a pillared particle by an etching process; Figure 3B illustrates schematically a process of forming a pillared particle by growing pillars on a core; Figure 4A is a scanning electron microscope image of a composition according to an embodiment of the invention; Figure4B is a magnification of a region of the image of Figure 4A; Figure 4C is a magnification of a region of the image of Figure 4B; Figure 5A illustrates variation of electrode capacity density with cycle numbcr for cells according to embodiments of the invention; Figure 5B illustrates variation of end charge voltage with cycle number for the cells of Figure 5A; Figure 6A illustrates variation of electrode capacity density with cycle number for cells according to embodiments of the invention; and Figure 6B illustrates variation of end charge voltage with cycle number for the cells of Figure 6A.

Figure 7 illustrates variation of specific discharge capacity as a function of the product of the cycle numbcr and electrode capacity density in mAh/cm2 for cells according to embodiments ofthe invention and comparative devices;

Detailed Description of the Invention

The structure of a rechargeable metal ion battery cell is shown in Fig. I, which is not dra\vn to any scale. The battery cell includes a single cell but may also include more than one cell. The battery is preferably a lithium ion battery, but may be a battery of another metal ion, for example sodium ion and magnesium ion.

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 containing active silicon particlesl4 overlays the current collector 10 and a lithium

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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. "Active material" or "electroactive 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 respective charging phase and discharging phase of a battery. Preferably the material is able to insert and release lithium. Preferred active materials include materials having silicon surface at a surface thereof, for example silicon particles or a composite of a material having a non-silicon core and a surface that is partly or wholly a silicon surface.) The cathode 12 comprises a material capable of releasing and reabsorbing lithium ions ibr example a lithium-based metal oxide or phosphate, LiCoO2, LiNio2Coo*15A10*0502, LiMnNiCo1..2O2 or LiFePO4.

A liquid electrolyte may be provided between the anode and the cathode. In the example of Figure I, a porous plastic spacer or separator 20 is provided between the anode layer 14 and the lithium containing cathode layer 16, and a liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. 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 composite cathode layer 16. The polymer electrolyte material can bc a solid polymer electrolyte or a gel-type polymer electrolyte.

When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide cathode layer 16 via the electrolyte into the anode layer 14.

A composition according to an embodiment of the invention comprises silicon-comprising particles, a binder and one or more additives. Each additive is preferably a conductive material. Each additive may or may not be an active material.

The silicon-comprising particles maybe structured particles. One form of structured particles are particles having a core, which may or may not comprise silicon, with silicon-comprising pillars extending from the core. Another form of structured particles is porous silicon, in particular macroporous silicon, as described in more detail below.

Additives may be selected from: a first elongate carbon nanostructure; one or more further elongate carbon nanostructures; carbon black particles including acetylene black and ketjen black particles; and a material containing graphite or graphene particles. Each elongate carbon nanostructure is preferably selected from a nanotube and a nanofibre. A "nanostructure" material as used herein may mean a material comprising particles having at least one dimension less than 1 micron, preferably less than SOOnm, more preferably less than 200nm.

With reference to Figure 2, which is not drawn to any scale, a composition according to an embodiment of the invention comprises silicon-comprising particles 201, a first elongate nanostructure 203, a second elongate nanostructure 205, carbon black particles 207, graphite particles 209 and binder 211. The silicon-comprising particles 201 illustrated in Figure 2 are pillared particles having a core with pillars extending from the core, however the silicon-comprising particles may or may not carry pillars.

The second elongate nanostructure material may become entangled with the pillars of the pillared silicon particles, and each nanostructure may wrap around some or all of the perimeter of one or more of the pillared silicon particle cores, and so may extend electronic conductivity beyond the pillared particle surface and / or lower barrier to conduction between the pillared particle surface and other conductive species, including the binder and other additives of the anode. The second elongate nanostructure may also be entangled with other components of the composition, for example graphite (if present).

[he pillars, or other structural elements,of the silicon-comprising particles 201 may provide anchors for the nanofibres or nanotubes of the second elongate nanostructurc material 205.

The larger diameter of the first elongate nanostructure material 203 may make it more rigid than the second elongate nanostructure material 205. The first elongate nanostructure material 203 may provide conduction paths within the composition that extend along the length of each nanostructure. These conduction paths may form the framework or support for conductive bridges between silicon-comprising particles 201 and between the silicon-comprising particles 201 and other components in the composite such as graphite particles 209.

Compositions of the invention may include only two different elongate nanostructure materials, for example as illustrated in Figure 2, or may include three or more different elongate nanostructure materials.

Silicon-comprising particles The silicon-comprising particles may be structured particles. Structured particles include particles having a core and pillars extending from the core, and particles having pores on the particle surface or pores throughout the particle volume. A surface of a macroporous particle may have a substantially continuous network of the particle material at a surface of the particle with spaces, voids or channels within the material that may have dimensions of at least 5Onm. Such voids may be present throughout the particle volume or may be restricted to regions of the particle. A particle may have regions of pillars and regions ofpores. The pillars themselves may be microporous or mesoporous.

The silicon-comprising particles in compositions of the invention may consist essentially of n-or p-doped silicon or may contain one or more further materials. For example, in the case of pillared particles the particle maybe selected from one of the following: -a particle having a silicon core with pillars extending from and integral with the silicon core -a particle having a non-silicon core of a conductive material, for example a graphite core, with pillars extending from the core; and -a particle having a non-silicon core of a conductive material, for example a graphite core, coated with a silicon shell and having silicon pillars extending from and integral with the silicon shell.

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The pillars may be core-shell structures, the inner core being of a different material to the outer shell material and where the core and/or shell contains silicon. In the case where thc core and pillars arc of diffcrcnt materials, the corc may or may not bc an clcctroactivc material.

Figure 3A illustrates a first method of forming pillared particles wherein a starting material is etched to form a pillared particle wherein a starting material 301 is exposed to an ctching formulation for sclcctivc ctching at thc surfacc of thc starting matcrial to produce a pillared particle 303 having a core 305 and pillars 307.

It will be appreciated that the volume of the particle core ofthe pillared particle formed by this method is smaller than the volume of the starting material, and the surface of the core is integral with the pillars. The size of the pillared particle may be the same as or less than the size of the starting material.

A suitable process for etching a material having silicon at its surface is metal-assisted chemical etching (alternatively called galvanic exchange etching or galvanic etching) which comprises treatment of the stalling material with hydrogen fluoride, a source of metal ions, for example silver or copper, which electrolessly deposit onto the surface of the silicon and an oxidant, for example a source of nifrate ions. More detail on suitable etching processes can be found in, for example, Huang et al., Adv. Mater. 23, pp 285-308 (2011).

The etching process may comprise two steps, including a step in which metal is formed on the silicon surface of the starting material and an etching step. The presence of an ion that may be reduced is required for the etching step. Exemplary cations suitable for this purpose include nitrates of silver, iron (111), alkali metals and ammonium. The formation of pillars is thought to be as a result of etching selectively taking place in the areas underlying the electrolessly deposited metal.

The metal deposition and etching steps may take place in a single solution or may take place in two separate solutions.

Metal used in the etching process may be recovered from the reaction mixture for re-use, particularly if it is an cxpcnsivc metal such as silvcr.

Exemplary etching processes suitable for forming pillared particles are disclosed in WO 2009/010758 and in WO 20101040985.

Other etching processes that may be employed include reactive ion etching, and other chemical or electrochemical etching techniques, optionally using lithography to define the pillar array.

If the pillared particle comprises a first material at its core cenfre with a shell formed from a second material, for example carbon coated with silicon, then this particle may be formed by etching of silicon-coated carbon to a depth of less than the thickness of the silicon shell in order to form a pillared particle with a composite carbon / silicon core.

Etching may be to a depth of less than 2-10 microns, optionally at least 0.5 microns, to form pillars having a height of up to 10 microns. The pillars may have any shape. For example, the pillars may be branched orunbranchcd; substantially straight or bent; and of a substantially constant thickness or tapering.

The pillars may be formed on or attached to a particle core using methods such as gro\ving, adhering or ifising pillars onto a core or growing pillars out of a core. Figure 3B illustrates a second method of forming pillared particles \vherein pillars 307, preferably silicon pillars, for example silicon nanowires, are grown on or attached to a stalling material 301 such as a silicon or carbon (e.g. graphite or graphene) starting material. The volume of the particle core 305 of the resultant pillared particle 303 may be substantially the same as the volume of the starting material 301. In other words, the surface of the stalling material may provide the surface of the particle core 305 from which the pillars 307 extend.

Exemplary methods for gro\ving pillars include chemical vapour deposition (CVD) and fluidised bed reactors utilising the vapour-liquid-solid (VLS) method. The VLS method comprises the steps of forming a liquid alloy droplet on the starting material surface where a wire is to be grown followed by introduction in vapour form of the substance to form a pillar, which diffuses into the liquid. Supersaturation and nucleation at the liquid/solid interface leads to axial crystal growth. The catalyst material used to form the liquid alloy droplet may for example include Au, Ni or Sn.

Nanowires may be grown on one or more surfaces of a starting material.

Pillars may also be produced on the surface of the starting material using thermal plasma or laser ablation techniques.

The pillars may also be formed by nanowire growth out of the starting material using mcthods such as a solid-liquid-solid growth technique. In onc example silicon or silicon-based starting material granules are coated with catalyst particles (e.g. Ni) and heated so that a liquid alloy droplet forms on the surface whilst a vapour is introduced containing another element. The vapour induces condensation of a product containing the starting material and the other element from the vapour, producing growth of a nanowire out of the starting material. The process is stopped before all of the starting material is subsumed into nanowires to produce a pillared particle. In this method the core of the pillared particle will be smaller than the starting material.

Silicon pillars grown on or out of starting materials may be grown as undoped silicon or they may be doped by introducing a dopant during the nanowire growth or during a post-growth processing step.

The pillars are spaced apart on the surface of the core. Tn one arrangement, substantially all pillars may be spaced apart. In another arrangement, some of the pillars may be clustered together.

The starting material for the particle core is preferably in particulate form, for example a powder, and the particles of the starting material may have any shape. For example, the starting material particles may be cuboid, cuboidal, substantially spherical or spheroid or flake-like in shape. The particle surfaces may be smooth, rough or angular and the particles may be multi-faceted or have a single continuously curved surface. The particles may be porous or non-porous.

A cuboid, multifaceted, flake -like, substantially spherical or spheroid starting material may be obtained by grinding a precursor material, for example doped or undoped silicon as described below, and then sieving or elassiing the ground precursor material.

Exemplary grinding methods include power grinding, jet milling or ball milling.

Depending on the size, shape and form of the precursor material, different milling processes can produce particles of different size, shape and surface smoothness. Flake-like particles may also be made by breaking up / grinding flat sheets of the precursor material. The starting materials may alternatively be made by various deposition, thermal plasma or laser ablation techniques by depositing a film or particulate layer onto a substrate and by removing the film or particulate layer from the substrate and grinding it into smaller particles as necessary.

The starting material may comprise particles of substantially the same size.

Alternatively, the starting material may have a distribution of particle sizes. In either case, sieves and/or classifiers may be used to remove some or all starting materials having maximum or minimum sizes outside desired size limits.

In the case where pillared particles are formed by etching a material comprising silicon, the starting material may be undoped silicon or doped silicon of either the p-or n-type or a mixture, such as silicon doped with germanium, phosphorous, aluminium, silver, boron and/or zinc. It is preferred that the silicon has some doping since it improves the conductivity of the silicon during the etching process as compared to undoped silicon.

The starting material is optionally p-doped silicon having l0' to 1020 carriers/cc.

Silicon gnmules used to form the pillared particles may have a silicon-purity of 90.00% or over by mass, for example 95.0% to 99.99%, optionally 98% to 99.98%.

The starting material may be relatively high purity silicon wafers used in the semiconductor industry foniicd into granules. Alternatively, the granules may be relatively low purity metallurgical grade silicon, which is available commercially and which may have a silicon purity of at least 98%; metallurgical grade silicon is particularly suitable because of the relatively low cost and the relatively high density of defects (compared to silicon wafers used in the semiconductor industry). This leads to a low resistance and hence high conductivity, which is advantageous when the pillar particles or fibres are used as anode material in rechargeable cells. Impurities present in metallurgical grade silicon may include Iron, Aluminium, Nickel, Boron, Calcium, Copper, Titanium, and Vanadium, oxygen, carbon, manganese and phosphorus. Certain impurities such as Al, C, Cu, P and B can further improve the conductivity of the starting material by providing doping elements. Such silicon may be ground and graded as discussed above. An example of such silicon is "SilgrainM" from Elkem of Norway, which can be ground and sieved (if necessary) to produce silicon granules, that may be cuboidal and / or spheroidaL The granules wed for etching may be crystalline, for cxainple mono-or poly-crystalline with a crystallite size equal to or greater than the required pillar height. The polycrystalline granules may comprise any number of crystals, for example two or more.

Where the pillared particles are made by a growth of silicon pillars as described above, the starting material may comprise an eleetroactive material, and may comprise metal or carbon based particles. Carbon based starting materials may comprise soft carbon, hard carbon, natural and synthetic graphite, graphite oxide, fluorinated graphite, fluorine-intercalated graphite, graphene.

Graphene based starting materials may comprise particles comprising a plurality of graphene nanosheets (UNS) and/or oxidised graphene nanosheets (ox-ON 5) or nano Graphene Platelets (NOP). Methods of making graphene particles include exfoliation techniques (physical, chemical or mechanical), unzipping of MWCNT or CNT, epitaxial growth by CVD and the reduction of sugars.

The core of the silicon-comprising particle illustrated in Figure 3 is substantially spherical, however the particle core may have any shape, including substantially spherical, spheroidal (oblate and prolate), and irregular or regular multifaceted shapes (including substantially cube and cuboidal shapes). The particle core surfaces from which the pillars extend may be smooth, rough or angular and may be multi-faceted or have a single continuously curved surface. The particle core may be porous or non-porous. A cuboidal core may be in the fonti of a flake, having a thickness that is substantially smaller than its length or width such that the core has only two major surfaces.

The aspect ratio of a pillared particle core having dimensions of length L, width Wand thickness lisa ratio of the length L to thickness I (L: 1) or width W to thickness I (W F) of the core, wherein the thickness T is taken to be the smallest of the 3 dimensions of the particle core. The aspect ratio is 1:1 in the case of a perfectly spherical core. Prolate or oblate spheroid, cuboidal or irregular shaped cores preferably have an aspect ratio of at least 1.2:1, more preferably at least 1.5:1 and most preferably at least 2: I. Flake like cores may have an aspect ratio of at least 3:1.

In the case of a substantially spherical core, pillars may be provided on one or both hemispheres of the core. In the case of a multifaceted core, pillars may be provided on one or more (including all) surfaces of the core. For example, in the case of a flake core thc pillars may be provided on only one of thc inor surfaces of thc flakc or on both major surfaces.

The core material may be selected to be a relatively high conductivity material, for example a material with higher conductivity than the pillars, and at least one surface of the core material may remain uncovered with pillars. The at least one exposed surface of the conductive core material may provide higher conductivity of a composite anode layer comprising the pillared particles as compared to a particle in which all surfaces are covered with pillars.

The silicon particles may have at least one smallest dimension less than one micron.

Preferably the smallest dimension is less than 500nm, more preferably less than 300nm.

The smallest dimension may be more than 0.5 nm. The smallest dimension of a particle is defined as the size of the smallest dimension of an element of the particle such as the diameter for a rod, fibre or wire, the smallest diameter of a euboid or spheroid or the smallest average thickness ibr a ribbon, flake or sheet where the particle may consist of the rod, fibre, wire, cuboid, spheroid, ribbon, flake or sheet itself or may comprise the rod, fibre, wire, cuboid, spheroid, ribbon, flake or sheet as a structural element of the particle.

Preferably the particles have a largest dimension that is no more than 100gm, more preferably, no more than 50Mm and especially no more than 30gm.

Particle sizes may be measured using optical methods, for example scanning electron microscopy.

Preferably at least 20%, more preferably at least 50% of the silicon particles have smallest dimensions in the ranges defined herein. Particle size distribution may be measured using laser diffraction methods, for example using a MasterSizerM described in more detail below, or optical digital imaging methods.

ElonQate carbon nanostructure materials A composition of the invention includes at least two elongate carbon nanostructure materials. A first elongate carbon nanostructure material may have a diameter (or smallest dimension) that is larger than that of the second elongate carbon nanostructure.

The second nanostructure material may have a higher surface area per unit mass than the first nanostructure material. The first elongate nanostructure material may have a large enough diameter so that the nanostructure is relatively straight and rigid whereas the second elongate nanostructure may have a small enough diameter such that it can be flexible and curved or bent within the composite. Preferably the diameter (or smallest dimension) of the first elongate carbon nanostmcture is at least lOOnm. Preferably the diameter (or smallest dimension) of the second elongate carbon nanostructure is less than 100nm, more preferably less than 90 nm, more preferably less than 8Onm. Preferably, both the average thickness and average width of each ofthe first and second elongate carbon nanostructures is less than 500 nm.

Each of the elongate carbon nanostructure materials may have a large aspect ratio, the aspect ratio being the ratio of the largest and smallest dimensions of the material.

Preferably, the aspect ratio of the first elongate carbon nanostructure is in the range of about 40 to 180. Preferably the aspect ratio of the second carbon nanostructure is in the range of 200 to 500.

Elongate nanostructures may be selected from nanoflbres and / or nanotubes and thin ribbons.

Nanotubes may be single-walled or multi-walled. Preferably, carbon nanotubes used in compositions of the invention are multi-walled. Walls of the nanotubes may be of graphene sheets.

Nanofibres may be solid carbon fibres or may have a narrow hollow core, and may be formed from stacked graphene sheets. An example of a suitable nanofibre material is VGCFRTh1 supplied by Showa Denko KK.

Optionally, the elongate nanostructures have a mean average length in the range of 3- 50jm. Preferably the length of the first elongate nanostructure material is in the range 5-3Oum.

Preferably the surface area of each elongate nanosThucture material is no more than 100m2/g and at least 1m2/g.

The first elongate nanostructure may be a nanofibre having a surface area in the range of 10-20 m2/g The second elongate nanostructure may be a nanotube have a surface area in thc range of 40-80 m2/g.

The carbon nanostructures may be functionalised to improve adhesion or connection to other components in the composition, especially the silicon-comprising particles. For example carbon nanotubes can be ftinctionalised with oxygen-containing groups, for example COOF, OH, CO and nitrogen containing groups, for example NH2. The second elongate nanostructure may be a carbon nanotube functionalised with COOH groups which may promote connectivity to the surface of silicon-comprising particles or other electroactive particles.

A composition including a binder, silicon-comprising particles, two or more different elongate carbon nanostructurc materials and any frirther additives may include each of the elongate nanostructure materials in an amount in the range of 0.25-20 weight %, optionally 0.25-10 wt % of the composition. The total amount of the two or more different elongate nanostructure materials in the composition may be in the range of 2-25 weight percent, optionally 3 -13 weight percent.Carbon black The composition may comprise carbon black, which may be characterised as a highly conducting particulate carbon, quasigraphitic in nature, composed of aggregates having a complex configuration (including but not limited to chain-like agglomerates) and of colloidal dimensions. Carbon black is typically made via the thermal decomposition and partial combustion ofhydrocarbons. Various types of carbon black are available, including acetylene blacks. Examples of commercial products include Ketjen BlaekRTht EC600JD or EC300J supplied by AkzoNobel, VulcanRTM XC72R manufactured by Cabot Corp, TokaBlackTM 5500, 4500, 4400 or 4300 manufactured by Tokal Carbon Co., LTD. and DenkaBlackRW FX-35 or HS-1 00 manufactured by Denki Kagaku Kogyo Kabushiki Kaisha. The composition may comprise a single type of carbon black or a blend of one or more types of carbon black. The carbon black particles may have dimensions in the range of 10-lOOnm and a surface area in excess of 50m2/g.

A composition including a binder, silicon-comprising particles, a first elongate carbon nanostructure and a second elongate carbon nanostructure, carbon black additive(s) and any further additives may include carbon black (of a single type or a blend of a plurality of types) in an amount of at least 0.25 weight % of the composition, and optionally less than 10 wt % of the composition. Preferably, the carbon black is present in an amount in the range 0.5 wt% to 4 wt% of the total composition. Ketjen Black EC600JD with an average particle size of 20-4Onm and a surface area of>1000 m2/g is particularly preferred as an additive.

Graphite or Graphene The composition may contain graphite particles or flakes. Optionally the graphite is synthetic graphite.

The crystallite length Lc of the graphite particles or flakes is optionally at least 50 nm, optionally at least 100 nm. Graphite with a higher crystallite length Lc may be preferable as this may provide higher conductivity, and higher overall conductivity of the composite. Suitable commercial products of graphite particles may include TimrexRrM SFG6, SFG1O, SFG1 5, KS4 or KS6 manufactured by Timcal Ltd. Graphite present in an anode of a metal ion battery may function as an active material.

Active graphite may provide for a larger number of charge / discharge cycles without significant loss of capacity than active silicon, whereas silicon may provide for a higher capacity than graphite. Accordingly, an electrode composition having both silicon-comprising active particles and a graphite active material may provide a metal ion battery, for example lithium ion baftery, with the advantages of both high capacity and a large number of charge / discharge cycles. Depending on the type of graphite material and the charge/discharge conditions, the graphite additive in a silicon based composition may not be fully lithiated during charging and may have a negligible or zero contribution to the electrode capacity above that of the silicon based material. It may be used primarily to improve the overall conductivity of the composition.

Graphite present in the composition may also improve coating properties of a slurry of the composition as compared to a composition in which graphite is absent.

Graphite particles or flakes may be provided as a po\vder having a D50 size as measured using laser diffractometry of less than 50 microns, optionally less than 25 microns.

Dn as used herein (for example, D50 or D90) means that at least n % of the volume of the material is formed from particles have a measured spherical equivalent volume diameter equal to or less than the identified diameter.

A composition including a binder, silicon-comprising particles, graphite and any further additives may include graphite in an amount in the range of 2-30 wt %, optionally 2-15 wt %.

The present inventors have surprisingly found that the performance of a metal-ion battery having a composite anode containing both silicon-comprising particles and graphite particles may be affected by the size ratio of the silicon-comprising particles to the graphite particles.A graphite additive as described herein may be replaced in whole or in part with a graphene additive. A graphene additive may have dimensions as described above for a graphite additive.

Binder The binder may be provided to provide cohesion of the particles and, in the case of use in a metal ion battery, for adhesion of the composition to an anode current collector.

The binder material may be a polymeric material, for example polyimide, polyacrylic acid (FAA) and alkali metal salts thereof, polyvinylalchol (PVA), polyvinylidene fluoride (PVDF) and sodium carboxyrncthylccllulosc (Na-CMC) or rubbcr based binders such as SBR. Mixturcs of different binder matcrials may bc used.

The binder may be provided in an amount in the range of5-30 wt % of the composition.

Composition The silicon particles and the carbon additives and any other additives may each be provided in the form of a powder or slurry for ease of mixing and blending. For example a slurry can be made by mixing the silicon particles or carbon additives with an appropriatc amount of aqueous (c.g. water) and / or non-aqucous (c.g. NMP) solvcnt. A slurry of a composition comprising the silicon particles, carbon additives and any other additives may be made by mixing all elements together with a solvent or alternatively may be made by first making more than one slurry, each slurry comprising one or more the individual elements of the composition in a solvent and then combining the separate slurries together to create a slurry containing all elements of the composition. The solvents of the separate starting slurries may be the same or may be different, as long as they are miscible when combincd. A binder material with or without a solvent may also be added and blended to the composition or sluriy. The resulting slurry may be deposited onto a substrate and dried to remove the solvent to form a composition for the electrode of a metal-ion battery.

The inventors have recognised that if a metal ion battery comprising a negative electrode comprising a silicon containing electroactive material is to cycle with a high capacity (for example, in excess of 500mAh per gram of active material) for in excess of 100-300 charge/discharge cycles, then the electrode composite structure should be uniformly porous and electronically well connected and designed to accommodate the volume changes of the clectroactive material during cycling without mechanical or electronic disconnection of the active material from the composite structure.

Tn order to achieve this, the components within the composite may have moderate values of surthce area per unit mass. A high surface area may provide higher reactivity of the active material or improved conductivity from thc additives, however if the surface area of the components is too high, excessive formation of a solid-electrolyte interphase (SET) layer may increase metal ion loss, cause reduction cycle life and cause reduction in porosity. In addition, an excessive surface area of the additives will require a higher content of binder in the composition to effectively bind the components ofthe composite together and to adhere it to the current collector -which may reduce the overall volumetric capacity and make it difficult to provide an appropriate level of porosity in the composition.

When the composition is mixed with a solvent to form a slurry for depositing the composition onto a current collector, the mix of components with different shapes and varying volumes is preferably such that slurry comprise a uniform mixture with all components equally dispersed and of sufficiently low viscosity to enable thin, uniform coatings to be prepared.

The inventors have discovered that a negative electrode with a composition having the following properties may provide improved cycling performance as described above: (a) At least 50 wt% active material and no more than 80 wt%, the active material preferably comprising structured silicon particles (b) Binder in the range of 5-3Owt%, preferably 10-2Owt%.

(c) First elongate carbon nanostructure material comprising nanostructures with a smallest dimension of more than I OOnm in the amount of 0.25 to 2Owt%, preferably 3-7wt% (d) Second elongate carbon nanostructure material comprising nanostructures with a smallest dimension of less than l0Onm, preferably in the range 30-8Onm, in the amount of 0.25 to 2Owt%, more preferably 2-8wt%.

(e) Carbon black in the range 0.25 to I Owt%, preferably 0.5 to 4 wt%.

(f Graphite particles and/or other additives, fillers and spacers in the range 2-3Owt% (g) A porosity of at least 10-80%, preferably 20-60%.

wherein the total percentage of the above components adds up to 100 wt%. Preferably the total amount of the first and second elongate carbon nanostructures (c and d) in the composition is in the range 2-25 wt%, especially 3-l3wt%. Preferably the ratio of the mass of the first elongate carbon nanostructure material to the mass of the second elongate carbon nanostructure material is no more than 5:1, most preferably the ratio is in the range 0.1:1 to 5:1 and especially 0.5:1 to 2:1.

Preferably the composition comprises structured silicon particles as described above. The inventors have discovered that all three carbon components c, d and e, within the weight amounts described above may produce a negative electrode with excellent cyclability.

Without wishing to be bound by theory, it is believed that by using elongate carbon nanostructures such as MWCNT with diameters in the range 30-8Onm and in the amounts described above, the MWCNT can become entangled with the structural features of the silicon structured particles to form short range conductive networks without excessive filling of the voids or spaces between the said structural features that are necessary to provide space for silicon expansion and access of electrolyte. The larger diameter, rigid first elongate carbon nanostructiires, such as VGCF, provide conductive bridges for longer range electronic connections and help to provide a strong mechanical framework within the composition to withstand the volume expansion and contraction of the active material during cycling. It is believed that the highly dispersed carbon black may provide sufficient conductivity in the remaining locations within the composition. However if an excessive amount of any of the carbon additives is used then the effectiveness of the binder may be reduced and the uniformity of the composition may be reduced.

The composition may be formed by mixing the components of the composition, a solvent and optionally one or more of a surfactant, dispersant or porogen, and stirring the mixture. Two or more of the components may first be mixed together in a solvent before being added to the other components for a final mixing stage. The composition may then be deposited on a substrate and dried so that the solvent is evaporated to form a porous composite film.

Examples

Materials Compositions were prepared with components selected from the following materials: Pillared silicon particles formed by etching starting silicon particles available as "Silgrain'M" from Elkem ofNorway, wherein the starting silicon particles have a D50 particle size of 11.5-12.5 microns, or 24.5 -25.5 microns as measured using a MastersizerTM particle size analyzer available from Malvern Instruments Ltd. It will be understood that the resultant pillared particle may have a D50 that is smaller than that of the starting material, for example up to 2 or 4 microns smaller respectively.

VGCF carbon nanofibres available from Showa Denko, having an average diameter of nfl, an average length of 10-20 microns and a surface area of 13 m2/g.

Multiwalled carbon nanotubes from CheapTubes Inc having an average diameter of 50-nm, an average length of 15-20 microns and a surface area of 55-75 m2/g (hereinafter "MW CN T").

Carbon black material available from AzkoNobel as Ketjenblack® ECbOO-JD having a surface area of 1400 m2/g and an average particle size of 20-4Onm.

Carbon black material available from Denka as Denka black having a surface area of 69 m2/g and an average particle size of 35 nm.

Graphite available as TIMCAL TIMREX ® 1(54, 1(56, SF06 and SF010 having D10, D50 and D90 values (measured using aMasterSizerparticle size analyser) and BET values as given in Table 2..

A sodium polyacrylate binder, hereinafter referred to as "NaPAA" was formed by partially neutralising commercially available polyacrylic PAA45OKusing sodium carbonate or sodium hydroxide to a 70% degree of neutralisation.A distribution of the particle sizes ofapowder of starting material particles used to form pillared particles may be measured by laser diffraction, in which the particles being measured are typically assumed to be spherical, and in which particle size is expressed as a spherical equivalent volume diameter, for example using the MastersizerTM particle size analyzer available from Malvern Instruments Ltd. A spherical equivalent volume diameter is the diameter of a sphere with the same volume as that of the particle being measured. If all particles in the powder being measured have the same density then the spherical equivalent volume diameter is equal to the spherical equivalent mass diameter which is the diameter of a sphere that has the same mass as the mass of the particle being measured. For measurement the powder is typically dispersed in a medium with a refractive index that is different to the refractive index of the powder material. A suitable dispersant for powders of the present invention is water. For a powder with different size dimensions such a particle size analyser provides a spherical equivalent volume diameter distribution curve.

Figure4A is a SEM imagc of a composition containing each of the aforementioned components following formation of a sluny of the composition and deposition of the composition onto a copper current collector and evaporation of the sluny solvent to form an anode layer.

The second elongate nanostructures 205, which in this case are multiwalled carbon nanotubes, are entangled with the silicon-comprising particles 201, which in this case are pillared silicon particles. The first elongate nanostructures 203, in this case a nanofibre, provides conductivity over a relatively long range, as shown for the annotated nanofibre 203 bridging two silicon particles.

The nanotubes provide medium range conductivity. Referring to Figures 4B and 4C, it can be seen that nanotubes 205 form a bridge extending across two silicon particles 201.

The nanotubes and nanopartieles also provide for improved conductivity between the silicon particles and graphite flakes 209 of the composition.

General Device Process

TM

Swagelok -style test cells were constructed using an anode comprising a composition comprising silicon pillared particles as the active material deposited with a coat weight of 13.5-15.5 grams of silicon per m2 onto a lOjim thick copper foil, an NCA cathode (Li1 NiogCooisAloosO,) on an aluminium foil and a Tonen separator between the two electrodes. The electrodes and separator were wetted with an electrolyte solution of IM LiPF6 in EC/EMC containing VC (vinylene carbonate, 3wt%, FEC (fluoroethylene carbonate, lOwt%) and CO2 (0.2wt%) as additives. The capacity of the NCA cathode was 3 times higher than the capacity of ppSi in the composite electrode that was designed to operate at 1200 mAh/g. The silicon pillared particles were prepared using metal-assisted etching of metallurgical grade silicon particles (with a silicon purity of 99.7-99.95wt%), to form irregular shaped pillars of lengths 1.5-2.5tm and thicknesses of 40-lSOnm such that the average mass of the pillars was 20-40% of the total silicon mass. The cells were cycled in such a way that the ppSi was charged to l200mAh/g and discharged to a cut-off voltage of 2.5V. The cycling rate was C/2 for both charge and discharge. The electrode area was 1.13cm2.Device ExamI2le 1 Compositions of the following materials in the following weight ratios were prepared: wt % pillared silicon particles (pillared particle Dio= 1 ijim, D50 = 21 microns, D903 9um) 12 wt % binder NaPAA 6 wt % graphite 12 wt % made up of VGCF: multi-wall carbon nanotubes: EC600: Ketjenblack® EC600-SD: Dcnka black in the ratio of 4:1:1:2. Graphite was varied as shown in Table I. cz i 001 98 I9 czi 81 8ZT 99 8t 01D48 I I""'J 001 L8 9 LI 9 c C I 9048 ________________ __________________ 009 ___________ ___________ ___________ _________ OAUUJRdWOJ 001 L8 9 0Z c9 91 98)1 ________________ __________________ 19 ___________ ___________ ___________ _________ OAflRWdUIO3 j jduimq 001 cx c 9Z LI? ________________ __________________ cL ___________ ___________ ___________ _________ MTvJrduTo3 (yr,) siouiogp ( at) U (suodonu) (suoTonu) (suololul) OflVJ adAj 011(0 PJIIU Ph1 von oovpns ocq oTiqdrJ 0c0 o'ci ojduinxj 3T!qdVJD puooos iS.Ti4 _______________ j opqdiij: uooijis °!4-D ouqdai oiiqdno Thc similarities in cfficicncics for diffcrcnt sizcs of graphitc indicatc that graphitc size has littic or no cffcct on first and subscqucnt cycle cfficicncics.

These compositions were used to prepare lithium-ion cells according to the General Device Process. The devices had first, second and third cycle efficiencies as shown in

Table 1.

Figure 5A depicts the evolution of the capacity density of the cells ofComparativc Examples 1-3 and Example 1, and Figure SB shows the evolution of the end of charge voltage for these cells with cycle number. The end of charge voltage was limited to 4.3V.

Figure 5B shows that cdl resistance increases fastest for the anode of Example Comparative Example 3, containing SFG6. In particular, the cell resistance of Comparative Example 3 increases faster than for Comparative Example 1, containing KS4.

Example 1 delivers the highest capacity density over 350 cycles.

Device Example 2

Dcviccs wcrc prcparcd as dcscribcd with rcfcrcncc to Examplc 1 cxccpt that thc pillared silicon particlcs had a D50 size of 11 jim, a D90 size of 2Opm and a D1j size of 6m and thc graphitc was varicd as shown in Tablc 2.

U

z jqj.

OOT TS LL LI PT i'll __________ ci 994S Z OjdUTEXJ ç jdmmcq OO IX U _______ ___________ ct Ct' _________ t'S)l OAPRWdUT0O t' L''d OOT 08 69 9t SiT gct' Ci' __________ l'SN A!TtJPdm0D (iii m) (%) oioAo aDl3JJns (m/is-) oipu (suonnu) (suodDim) piqi put rin iijtha' uoo °rj oiqdw OO Dcg puooas sJ!d opqdt uoiTisodwo3: uooijig OT!4dtJD oi4dRJ9 oiiqdaio ____________ The measured cycling efficiencies in Table 2 indicate that the performance improsement in Example 2 is not simply down to the lower surface area of SFG6 leading to less SET layer being formed in the first few cycles.

In contrast to the relative performance described above with reference to Comparative Examples 1 and 3, Figure 6A shows that Example 2 containing SFG6 with pillared silicon particles having a D50 of 11 microns maintains its capacity for a larger number of cycles than Comparative Examples 4 and 5 containing KS4, and Figure 6B shows that cell resistance increases fastest for the anode of Comparative Examples 4 and 5, indicating a relationship between silicon particle size and graphite size for optimum performance. Preferably, the silicon: graphite D50 ratio is at least 2:1, optionally it is no more than 4.5:1, optionally no more than 4: 1. Most preferably it is in the range 2:1 to 4:l,optionally3:l to 4:1.

Device Examples 3-7

Compositions of the following materials in the following weight ratios were prepared: wt % pillared silicon particles (pillared particle D50 = 11.1 microns) 14 wt % binder NaPAA 4 wt % graphite SFG6 12 wt % made up of elongate nanostructures, VGCF and EC600, as shown in Table 3.

Example Nanotube VGCF (wt %) Carbon black First, second and third (cell number) MWCNT EC600 (wt %) cycle efficiencies (wt %) (%) 3(NG415) 5 5 2 36,70,100 4 (NG598) 8 1 3 73, 100, 79 (N0469) 0 II I 79,76, 100 6 (NG629) 11 0 1 73,99,80 7(NG418) 7 I 4 72,72,100

Table 3

These compositions were used to prepare lithium-ion cells according to the General Device Process. The devices had first, second and third cycle efficiencies as shown in Table 3. The nth cycle efficiency is the ratio of the discharge capacity to the preceding charge capacity and provides an indication of the amount of lithium lost or retained within the anode or other cell components during the nth charge-discharge cycle, for example due to formation of the SET layer.

With reference to Figure 7, normalised capacity starts to decrease at a lower cycle number for devices having compositions in which one of VGCF and MWCNT is not present than for Example 8, in which both VGCF and MWCNT are present.

With reference to Figure 7, decrease in capacity is fastest for Example 7. Without wishing to be bound by any theory, it is believed that the high level of carbon black in Example 7 may result in a high level of absorption of the binder due to the high surface area per unit mass of the carbon black. Preferably, the weight ratio given by the combined mass of the elongate carbon nanostructures to the mass of the carbon black particles is in the range 3:1 to 20:1.

The invention has been described with reference to electroactive silicon, however it will be understood that the invention may be applied to other electroactive materials that have a bulk volume expansion of more than 10% when fully lithiated or is capable of having a specific capacity of greater than 300niAh/g. Other exemplary electroactive materials are tin; aluminium; eleetroaetive compounds including oxides, nitrides, fluorides, carbides and hydrides, for example compounds of tin, aluminium and silicon; and alloys thereof.

The invcntion has been described with reference to rechargeable lithium ion batteries, however it will be understood that the invention may be applied to metal ion batteries generally, and moreover that the invention maybe applied to other energy storage devices, for example fuel cells.

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the scope of the invention as set forth in the following claims. at

Claims (2)

1. <claim-text>1. A composition comprising a particulate electroactive material, a particulate graphite material and a binder, wherein at least 50% of the total volume of each said particulate materials is made up of particles having a particle size I)so and wherein a ratio of electzuactive material D50 particle size graphite I)so particle sizcisupto4.5:1.</claim-text> <claim-text>2. A composition according to claim 1 wherein the ratio is at least 2: 1.</claim-text> <claim-text>3. A composition according to claim 2 wherein the ratio is in the range of 2:1 -4:1, optionally3:l-4:1.</claim-text> <claim-text>4. A composition according to any preceding claim wherein thc particulate electroactive material is a silicon-comprising material.</claim-text> <claim-text>5. A composition according to any preceding claim wherein the particulate clectroaetivc matcrial compriscs particles having a particle core and elcctroactive pillars extending from the particle core.</claim-text> <claim-text>6. A composition according to claim 4 or 5 wherein the pillars ofthe silicon-comprising particles are silicon pillars.</claim-text> <claim-text>7. A composition according to claim 6 wherein the core ofthe silicon-comprising particles comprises silicon.</claim-text> <claim-text>8. A composition according to any of claims 4-7 wherein the silicon-comprising particles consist essentially of n-or p-doped silicon and wherein the pillars are integral with the core.</claim-text> <claim-text>9. A composition according to any preceding claim wherein the particulate electroactive material is provided in an amount of at least 50 wt % of the composition.</claim-text> <claim-text>10. A composition according to any preceding claim wherein the composition comprises at least one elongate nanostructuic materiaL I I. A composition according to claim 10 wherein the first elongate nanostructure has a mean average diameter of at least 100 nm.12. A composition according to claim 10 or I I wherein the composition comprises at least two elongate nanostmcture materials.13. A composition according to claim 12 wherein a second elongate carbon nanostnieture material has a mean average diameter of no more than 90 nm, optionally a mean average diameter in the range of 40-90 nm.14. A composition according to claim 13 wherein the first elongate nanostructure: second elongate nanostructure weight ratio is in the range
2. 2.5: 1 to 20: 1.15. A composition according to any of claims 10-14 wherein each of the at least one elongate nanostnzcture materials has an aspect ratio of at least 50.16. A composition according to any of claims 12-15 wherein the first and second carbon elongate nanostnicture materials are each independently selected from carbon nanotubes and carbon nano fibres.17. A composition according to claim 16 wherein the first carbon elongate nano structure material is a nano fibre and the second elongate carbon nanostnieture material is a nanotube.18. A composition according to any of claims 10-17 wherein the at least one elongate carbon nanostructure materials are provided in a total amount in the range of 0.1-weight % of the composition.19. A composition according to any of claims 10-18 wherein one or more of the elongate carbon nanostructure materials has a flinctionalised surfice, optionally a surface flinctionalised with a nitrogen-containing group or an oxygen containing group.20. A composition according to any preceding claim wherein the graphite is provide in the composition in an amount of 1-30 wt %, optionally 1-20 wt %.2 I. A composition according to any preceding claim wherein the crystallite length Lc of the graphitc is optionally at least 50 nm, optionally at least 100 nm.22. A composition according to any preceding claim wherein the composition ffirthcr comprises carbon black.23. A composition according to claim 22 wherein the carbon black is provided in an amount of at least 0.5 weight % of the composition, and optionally less than 10 wt % of thc composition, optionally less than 4 wt % of the composition.24. A metal-ion battery comprising an anode, a cathode and an electrolyte between the anode and the cathode wherein the anode comprises a composition according to any preceding claim.25. A slurry comprising a composition according to any of claims 1-23 and at least one solvent.26. A method of forming a metal-ion battery according to claim 24, the method comprising the step of fbrming an anode by depositing a shirry according to claim onto a conductive material and evaporating the at least one solvent.</claim-text>