BACKGROUND TO THE INVENTION Field of the invention

The present invention relates to methods for the manufacture of composite

nanostructured materials. Such materials are of particular (but not necessarily exclusive) interest as anode materials for lithium ion batteries.

Related art

As background to this technical field, we refer in particular to the review provided in Glaize & Genies (2013). Some parts of that disclosure are set out below, to provide the context for the disclosure of the present invention.

Lithium ion (Li-ion) batteries are a commonly used type of rechargeable battery with a global market estimated at $1 1 bn in 2010 and predicted to grow to $50bn by 2020. This large market is divided between various applications, ranging from transport and utility- scale energy storage to consumer electronics.

The main outlet for Li-ion batteries at the time of writing is consumer electronics, both in terms of number of units sold and turnover. Electric vehicles, if they develop as they are predicted to, will ultimately represent the dominant market.

Each of these applications have very different requirements in terms of battery performances. For instance electrical vehicle batteries need to be able to provide a large electric current without degrading to sustain vehicle acceleration phases, whereas consumer electronics batteries would rather benefit from the capability to be flexed, folded, or stretched. Ultimately these specific requirements lead to different technological choices in terms of battery design, especially with regards to the choice of the

electrochemically active materials that store the lithium ions during charge and discharge. For this reason, one technology - e.g. silicon anodes - is unlikely to dominate the entire market. Battery technologies rather need to be examined in the light of how well they perform on a number of metrics, the combination of which will ultimately give one technology a competitive advantage for one specific application. These metrics are described below.

A typical lithium-ion battery is composed of multiple cells connected in series or in parallel. Each individual cell is usually composed of an anode (negative polarity electrode) and a cathode (positive polarity electrode), separated by a porous, electrically insulating membrane (called a separator), immersed into a liquid (called an electrolyte) enabling lithium ions transport. In most systems, the electrodes are composed of an electrochemically active material - meaning that it is able to chemically react with lithium ions to store and release them reversibly in a controlled manner - mixed with an electrically conductive additive (such as graphitic carbon) and a polymeric binder. This slurry is coated as a thin film on a current collector (typically a thin foil of copper or aluminium, or a carbon nanotube mat in emerging applications), thus forming the electrode.

In the known Li ion battery technology, the low theoretical capacity (about 370 mA g ^"1 ) of graphite anodes is a serious impediment to its application in high-power electronics, automotive and industry. Among a wide range of potential alternatives proposed recently, Si, MxSy and Fe _x O _y are the main contenders to replace graphite as the active material of choice. Si has about 10 times more theoretical capacity than graphite but its dramatic volume expansion (up to about 400%) severely limits high-power applications. Although this problem can be partially tackled by carbon coating [Liu et al (2014)], implementation of these in large scale is still problematic. Similarly, metal sulphide (M _x S _y ) electrodes, despite their high theoretical capacity not only suffer from volume expansion but dissolution of polysulfides that form during charge/discharge [Liang et al (2015)] in battery electrolytes. On the other hand, Fe _x Oy-nanocarbon [Tuek et al (2014)] has now emerged as a promising anode material platform because of its higher (600-1000 mAh/g, or 600-800 mAh/g sustained) capacity than graphite, good capacity retention at high rates, environmental-benignity, high corrosion resistance, low-cost, non-flammability and high-safety. However, Fe _x O _y based anodes have some drawbacks, operating via conversion or conversion alloying, as explained in Loeffler et al (2015).

Ren et al (2015) reported the formation of a composite material of carbon fibre with CoFe20 ₄ binary metal oxide particles. The performance of this material as an anode material for a lithium ion battery was investigated. After 20 cycles the capacity reported in Ren et al (2015) is 400 mAh/g. Further improvements in the performance of candidate anode materials would be desirable. Tuek et al (2014) and Ren et al (2015) are two examples of conversion batteries, meaning that the chemical mechanisms leading to lithium ions storage and release is a conversion reaction. The conversion mechanism can be generally described as follows: TMxOy + z e- + z Li ⁺ -> x TM<°> + Li _z O _y

where TM is a transition metal and TM ⁽⁰⁾ refers to is elemental form. Upon battery charging, lithium ions diffuse and react into these materials, and nanoscale metallic domains of TM ⁽⁰⁾ are formed, embedded in an amorphous matrix of Li _z O _y . The reaction is reversed during battery discharge.

Conversion anodes have recently been referred to as the next generation anodes

[Loeffler et al (2015)]. As explained in Loeffler et al (2015), an appealing feature of conversion materials is their ability to store more equivalents of lithium (two to eight per unit formula of the starting material) than any insertion compound (up to two), resulting in substantially higher specific capacities. However, conversion materials exhibit a series of severe drawbacks which necessarily need to be overcome before they can be seriously considered for commercial applications [Cabana et al (2010)]. These drawbacks are also explained in Loeffler et al (2015). The conversion reaction inherently causes a massive structural reorganisation, which potentially leads to a loss of electrical contact and electrode pulverisation. Moreover, conversion materials suffer from a very high reactivity towards commonly used electrolytes and a marked (dis-)charge voltage hysteresis, considerably affecting the energy storage efficiency of such electrodes. The elevated operational potentials of many conversion materials also limit the achievable energy density and the large first-cycle irreversible capacity is considered to be unacceptable for practical applications. Electrode pulverisation refers to the loss of electrode mechanical integrity after charge and discharge cycling. Upon active material lithiation and delithiation, the active material swells and contracts, creating internal stresses that can ultimately lead to structural damage. SUMMARY OF THE INVENTION

Previous work by the inventors' research group has identified Fe _x Cvnanocarbon structures as providing a particularly advantageous basis for the development of new anode materials for lithium ion batteries. This work is unpublished at the time of writing the present disclosure. That previous work was based on a realisation that a known nanostructured material can be used as an anode material for lithium ion batteries.

The known nanostructured material is disclosed in US 8,628,747, which discloses a CVD process for the bulk production of carbon nanotubes (CNTs). First, metal composite Fe- Al particles are generated by spray pyrolysis. The aerosol metal composite particles are then reacted with a suitable hydrocarbon compound (e.g. acetylene) in a suitable thermal reactor, with an inert carrier gas and hydrogen, to facilitate growth of CNTs on the surface of the metal composite particles. The resultant nanostructures have a core particle of Fe-AI-0 with an array of CNTs anchored at the surface of the core particle. Due to their particular morphology, these nanostructures are referred to in the academic literature and in US 8,628,747 as having a "sea urchin" structure.

The present inventors have realised that the process described above for the

manufacture of nanostructured particles, while suitable for laboratory scale investigations, has some drawbacks when considering possible industrial scale implementation. The spray pyrolysis process requires that the precursor materials are available as soluble salts. Therefore the manufacture of suitable precursor materials adds to the cost and complexity of the overall manufacturing process. Furthermore, the use of nitrate salts (which is considered convenient for spray pyrolysis) means that NOx gases are produced during the decomposition of the nitrates into metal oxide particles. NOx gases are harmful and, in particular on an industrial scale, require special measures to remove them from exhaust gases before emission to the atmosphere. Still further, the

requirement in spray pyrolysis for soluble salts limits the compositional range of precursor materials that can be deployed in the manufacture of the nanostructured particles.

The present invention has been devised in order to address at least one of the problems identified above. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems.

In a general aspect, the present invention provides plasma vaporisation of a

core particle precursor material and then nucleation and growth of core particles from the vaporised core particle precursor material before growing carbon nanomaterials from the core particles.

In a first preferred aspect, the present invention provides a method for the manufacture of composite nanostructured particles, the composite nanostructured particles each comprising at least one core particle and carbon nanotubes, wherein the carbon nanotubes are anchored at one end on said core particle, the method including the steps:

generating a plasma in a plasma chamber;

adding a core particle precursor material into the plasma to vaporise the core particle precursor material using the plasma;

flowing the vaporised core particle precursor material away from the plasma to allow the nucleation and growth of core particles from the vaporised core particle precursor material; and

adding a carbon nanotube precursor material to the core particles in conditions to cause the growth of carbon nanomaterials from the core particles.

Compared with the process described above, in which the core particles are formed by spray pyrolysis, the first preferred aspect of the invention provides various advantages in terms of energy efficiency, versatility and scalability. These advantages, and others, are described below in more detail.

In a second aspect, the present invention provides a material comprising composite nanostructured particles obtained by or obtainable by a method according to the first aspect. In a third preferred aspect, the present invention provides an electrochemical device, preferably a lithium ion battery, comprising an anode, cathode and electrolyte, wherein the anode comprises an anode active material comprising a material according to the second aspect. In a fourth preferred aspect, the present invention provides a use of a material according to the second aspect as an anode active material in an anode in conjunction with a cathode and an electrolyte in a lithium ion battery for charging and discharging of the lithium ion battery. In a fifth preferred aspect, the present invention provides a method for processing a material according to the second aspect as an anode active material for a lithium ion battery, the method including diffusing lithium ions into the core particles. The first, second, third, fourth and/or fifth aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.

The plasma may be selected from the group consisting of: microwave plasma; RF plasma; DC plasma.

The plasma may provide a plasma temperature of at least 4000K. More preferably, the plasma may provide a plasma temperature of at least 5000K.

The plasma may be surrounded by a flowing gas sheath. Typically, the plasma is formed in a first region, with the flowing gas sheath defining a second region around the first region and in which the speed of gas flow is higher than in the first region. The differential gas flow speeds may be provided by the provision of a wall member, oriented parallel to the direction of gas flow, with different gas speeds provided on opposing side of the wall member. In a particularly preferred embodiment, the second region may be disposed annularly around the first region.

The core particle precursor material may be in the form of a powder. The core particle precursor material may be one or more elemental powders. Additionally or alternatively the core particle precursor material may be one or more compound powders. Suitable compound powders include oxides. The advantage here is that the plasma can effectively vaporise most materials. Therefore, for a desired core particle composition, the most cost-effective precursor materials may be used, due to the flexibility of the use of plasma to form the core particles. The core particles may comprise Si. Si is an attractive material for anode active materials for Li ion batteries in view of its high energy density.

The core particles may comprise a transition metal. Preferably the transition metal is suitable for catalysing the growth of CNTs.

The core particles preferably comprise O. In some embodiments, this is achieved through the use of one or more oxides in the core precursor material. Additionally or alternatively, this may be achieved through the addition of O to the method. For example, O may be added into the process downstream of the plasma, in order to promote at least partial oxidation of the core particles. Additionally or alternatively O may be added upstream of the plasma. In either case, it is considered that oxidation of the material is likely to take place downstream of the plasma, where the temperature is suitably high. The core particles may comprise Fe-AI-Li-O, wherein Fe is present in an amount of at least 10 wt% to at most 90 wt%, Al is present in an amount of at least 0.1 wt% to at most 90 wt%, and Li is optionally present, in an amount of 0 wt% or higher, wherein wt% is expressed in terms of the total mass of the particles of Fe Al Li O. Preferably, Al is present in an amount of at least 5 wt%. Furthermore, preferably, Al is present in an amount of at most 70 wt%. The amount of Al is chosen in order to promote the formation of a suitable array of CNTs at the core particle surface, as explained in more detail below. Preferably, Li is present in the particles of Fe-AI-Li-0 in an amount of at least 0.1 wt%. In the context of Li ion batteries, such a state would exist, for example, where the anode active material has been used for charging and/or discharging a Li ion battery.

Furthermore, in view of the conversion reaction and/or conversion alloying that takes place during charging, the core particles preferably contain lithium oxide and metallic iron. The carbon nanotubes are anchored at one end on the core particles. As can be noted on careful inspection of the nanostructures, the carbon nanotubes are grown from the core. The core particles may have, on average, at least 10 ¹¹ carbon nanotubes per m ² anchored on the core particles. The core particles may have, on average, at most 10 ¹⁷ carbon nanotubes per m ² anchored on the core particles. These values may be determined by SEM inspection of the nanostructures, by assessing the diameter of the core particle to determine the surface area of the core particle (not including porosity, where present) and counting the anchored carbon nanotubes.

The material may comprise at least 0.1 wt % by weight of carbon nanotubes, expressed in terms of the total weight of the core particles and the carbon nanotubes. The material may comprise not more than 99% by weight of carbon nanotubes, expressed in terms of the total weight of the core particles and the carbon nanotubes. More preferably, the material comprises not more than 50% by weight of carbon nanotubes, still more preferably not more than 40% by weight of carbon nanotubes, more preferably not more than 30% by weight of carbon nanotubes, still more preferably not more than 20% by weight of carbon nanotubes.

The material preferably comprises at least 50% by weight of core particles . More preferably, the material comprises at least 60% by weight of core particles . Still more preferably, the material comprises at least 70% by weight of core particles . Still more preferably, the material comprises at least 80% by weight of core particles

The material may consist of the core particles and carbon nanotubes, and incidental impurities, with any of the compositional ranges set out above and/or below.

In some embodiments, for example, the material comprises about 80% by weight particles and about 20% by weight carbon nanotubes. The benefits of the carbon nanotube content and of the core particle content for the material are explained in more detail below. However, briefly, it is possible to see that where the material has utility as an anode active material, the electrochemical capacity of the material comes from the capacity of the core particles to store and release lithium ions. The carbon nanotubes do not have a direct role in promoting this activity, but they provide advantages in terms of electrical conductivity, cyclability and mechanical integrity for the material. The particles may have a diameter in the range 10 nm to 50 μηη. Within this range, it is found that the material provides suitable performance in particular as an active material for Li ion batteries. It is further preferable that the particles have a diameter of at least 30 nm. The particles may have a diameter of not more than 10 μηη. More specifically, early in the nucleation and growth from the vaporised precursor material, there may be formed particles of relatively small size, such as in the range 10- 50 nm diameter. It is preferred that these particles agglomerate and at least partially sinter in order to form core particles on which CNT growth is carried out. These core particles may have a diameter in the range 30 nm to 50 μηη, or 30 nm to 10 μηη, as set out above. The core particles formed in this method may have a non-uniform shape (in contrast to typical core particles formed by spray pyrolysis). Therefore reference here to core particle diameter is intended to refer to the diameter of a notional spherical particle of equivalent volume to the core particle under consideration. The particles may include a matrix of amorphous Al-Fe-O. Furthermore, AI-Fe-0 crystallites may be embedded in the matrix of amorphous Al-Fe-O. The AI-Fe-0 crystallites may comprise a solid solution of hercynite into magnetite. Where carbon nanotubes are anchored to the core particles, it is preferred that the carbon nanotubes are attached to the core particles at AI-Fe-0 crystallites. It is considered by the inventors, without wishing to be bound by theory, that the AI-Fe-0 crystallites provide nucleation and growth sites for the carbon nanotubes.

Preferably, during the use of the material in the fourth aspect of the invention, during charging, lithium ions diffuse into the core particles, and a conversion reaction and/or a conversion alloying reaction takes place in which lithium oxide and metallic iron are formed.

Capacity, rate capability, cyclability (i.e. durability), safety, cost can potentially be improved by the use of conversion anodes. However, fundamental problems have previously impeded the use of conversion anodes in commercial systems. The present inventors consider that the preferred embodiments of the present invention address these problems, as now explained. One problem which has been considered to be present for conversion anodes is loss of electrical contacts and electrode pulverisation. The present inventors, without wishing to be bound by theory, consider that the sea-urchin structure promotes the so-called buffer effect of carbon nanotubes. This means that the stiff carbon nanotubes act as a structural reinforcement helping to preserve the integrity of active material particles. Also the nature of the carbon nanotube network resulting from the sea-urchin structures, each active core being essentially a node of the network, limits the proportion of active material that may become inaccessible to electrons or lithium-ions during battery operation. Furthermore, the volume expansion/structural changes occurring to the active material seem to be fundamentally mitigated by the use of the AI-Fe-0 alloy, limiting electrode pulverisation for both the AI-Fe-0 particles and the full AI-Fe-0 sea-urchins.

Another problem which has been considered to be present for conversion anodes is a perceived high reactivity towards commonly used electrolytes. However, in the preferred embodiments of the present invention it is considered that the AI-Fe-0 alloy forms a stable solid electrolyte interphase (SEI) with the electrolyte, ensuring stable battery operation.

A further problem which has been considered to be present for conversion anodes is a marked (dis-)charge voltage hysteresis. This is addressed by the sea urchin structures. The nature of the carbon nanotube network resulting from the sea-urchin structures is that each active core is essentially a node of the network. This limits the proportion of active material that may become inaccessible to electrons or lithium-ions during battery operation.

With respect to the elevated operational potentials of many conversion materials being considered also to limit the achievable energy density, this problem is considered to be addressed by the composition of the core particles. Furthermore, high voltage cathodes are now available.

It is considered that the AI-Fe-0 alloy enables higher rates (high current density charge/discharge) and life (cycle number) than pure iron oxide based electrodes. This is at present thought to be due to a stabilising effect of Al. The nature of the carbon nanotube network resulting from the sea-urchin structures, each active core being essentially a node of the network, ensures good electrode electrical conductivity, which contributes to good battery performances at high current rates.

Similarly, the nature of the carbon nanotube network resulting from the sea-urchin structures, each active core being essentially a node of the network, ensures good electrode thermal conductivity, which contributes to enhanced battery thermal management and ultimately battery safety. Further optional features of the invention are set out below.

The indication "Fe-AI-Li-O" is intended to indicate a composition which includes Fe, Al, O and optionally Li. The presence of other elements is not necessarily excluded. However, in some preferred embodiments, the indication "Fe-AI-Li-O" may designate a composition which consists of Fe, Al, O and optionally Li, and optionally up to 10wt% of further components, including incidental impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 shows a schematic illustration of an experimental setup for CNTSU synthesis and characterization according to a reference arrangement, not within the scope of the invention.

Fig. 2 shows an SEM image of salt nanoparticles collected downstream of the silica gel drier in Fig. 1

Fig. 3 shows an SEM image of bimetallic nanoparticles collected downstream of furnace 1 in Fig. 1.

Fig. 4 shows an SEM image of CNTSUs collected downstream of furnace 2 in Fig. 1 . Fig. 5 shows a schematic illustration of an experimental setup for CNTSU synthesis according to an embodiment of the invention.

Fig. 6 shows a schematic illustration of the generation of plasma in a basic configuration. Fig. 7 shows a view from one end of the plasma chamber illustrating the generation of the plasma against the wall of the plasma chamber in the basic configuration of Fig. 6. Fig. 8 shows a schematic illustration of the generation of plasma in a modified

configuration.

Fig. 9 shows a view from one end of the plasma chamber illustrating the generation of the plasma in the modified configuration of Fig. 8. Fig. 10 shows a schematic illustration of an experimental setup for CNTSU synthesis according to another embodiment of the invention.

Fig. 1 1 shows an SEM image of typical material produced following an embodiment of the invention.

Fig. 12 shows an SEM image of further typical material produced following an

embodiment of the invention, with several larger primary particles visible.

Fig. 13 shows the results of TGA analysis carried out on material produced according to an embodiment of the invention.

Fig. 14 shows the results of TGA analysis carried out on various materials produced according to embodiments of the invention.

Fig. 15 shows a schematic cross sectional view of a lithium ion battery according to an embodiment of the invention.

Fig. 16 shows a schematic enlarged view of the transport of lithium ions and electrons relative to the core particles and CNTs in a battery.

Fig. 17 shows a schematic view of an individual nanostructure.

Fig. 18 shows a schematic view of a nanostructured film formed from an assembly of nanostructures.

Fig. 19 illustrates a suitable lab-scale process for manufacturing an anode according to a preferred embodiment of the invention.

Fig. 20 shows the charge/discharge current signature of an embodiment of the invention. Fig. 21 shows the cell capacity over first and second cycles for an embodiment of the invention. The first and second discharge cycle lines are indicated.

Fig. 22 shows the cell capacity over third and fourth cycles for an embodiment of the invention. The third and fourth discharge cycle lines are indicated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER OPTIONAL FEATURES OF THE INVENTION

In the present disclosure, we focus on a continuous gas-phase carbon nanotube (CNT) production process that relies on a three-dimensional organisation of CNTs around a central particle in a structure named Carbon Nanotube Sea Urchin (CNTSU).

Here CNTSUs are broadly defined as microscale structures whereby CNTs are grown radially from a central nano or micro particle, which is usually referred to as the core of the sea urchin. Early accounts of such architecture include magnetic hollow nickel microspheres covered with oriented CNTs [Han et al (2006)], boron nitride/CNT composite particles synthesised using a spray-pyrolysis route [Nandiyanto et al (2009)], CNT forests grown on spherical alumina microparticles via a CVD method [He et al (2010 and 201 1 )], or boundary layer CVD synthesis of radial filled CNT structures [Boi et al (2013)].

Previous work by the inventors' research group has focused on the most studied process to date, which was first reported in Kim et al (201 1 ) whereby an aerosol of CNTSUs with bimetallic nanoscale cores is continuously synthesised in the gas-phase. As shown in Fig. 1 , an aqueous solution of aluminium nitrate AI(NOs)3 and iron nitrate Fe(NOs)3 is atomised in a stream of nitrogen carrier gas, creating a polydisperse microdroplets aerosol. As water evaporates from the droplets, solute concentration increases, eventually leading to precipitation of the metal nitrate salts, thus forming bimetallic salt nanoparticles (Fig. 2) suspended in the gas-phase. These nanoparticles then undergo calcination in a high temperature reducing environment, typically a tube furnace with a small addition of hydrogen to the carrier gas, enabling the formation of bimetallic nanoparticles that will act as CNTSU cores (Fig. 3). Upon addition of acetylene (C2H2) and a new addition of hydrogen, CNTs grow radially from the surface of the cores in a second CVD tube furnace, leading to CNTSUs as shown in Fig. 4. As with most aerosol-based nanomaterial processes, this synthesis route illustrated in Fig. 1 is continuous and solvent-free. It combines two widely used methods - spray pyrolysis and CVD - that are used to manufacture most of the world's industrial production of nanomaterials such as carbon black, ΤΊΟ2 and other metal oxide nanopowders, or bulk CNT powder.

Regarding the fundamental phenomena at play during synthesis, the mechanism leading to catalytic site formation at the surface of the cores can be considered. Kim, Wang et al (201 1 ) briefly investigates the composition of cores collected downstream of the calcination furnace via X-ray diffraction (XRD), leaving an open question as to whether the surface of the cores is composed of nanometer-size catalytically-active crystallites of iron oxide embedded in an inert matrix of aluminium oxide, or whether iron and aluminium oxides form a single amorphous phase of Al _x Fe _y O _z . However the former hypothesis seems to be favoured in the rest of that paper, where it is postulated that this hypothetic segregated structure could originate from the difference in solubilities of aluminium nitrate and iron nitrate. As aluminium nitrate solubility in water is lower than that of iron nitrate, it may be that upon droplet drying, aluminium nitrate preferentially precipitates at the droplet solidification front, so that the surface of cores is aluminium- enriched relative to iron, with small patches of iron nitrate isolated by a matrix of aluminium nitrate. Following that reasoning, this structure would be retained during calcination and CNT growth to form pure iron metal catalytic sites in a matrix of aluminium metal or aluminium oxide. Additionally, Kim, Wang et al (201 1 ) shows that the coverage density of CNTs can be tailored by tuning the AI:Fe ratio in the precursor solution. In Kim, Ahn et al (201 1 ), where iron nitrate is replaced with nickel nitrate, the same hypothesis is accepted on the mechanism of catalytic site formation and shows that variation of core size and process temperatures enable tuning of the resulting particle morphology, from straight and coiled individual CNTs to CNTSUs. Both of these papers base the explanation of their findings on the assumption that iron and aluminium segregate in two separate phases within the cores, starting from the precipitation stage to the CNT growth stage. Owing to their enhanced heat transfer properties, with all CNTs being interlinked via a central thermally conductive core, thus enabling control of CNT-CNT junctions and facilitating dispersion in other media, these CNTSUs have found applications as diverse as nanofluid coolant additives [Han et al (2007), additive to bulk heterojunction polymer-fullerene solar cells as an exciton dissociation medium, or as an optical igniter in the explosion of nanoenergetic thermite materials [Kim et al (2014)]. However, none of these applications take advantage of the fact that CNTSUs are true hybrid materials, where the core could add functionality to the material independently of its role as an inert thermally conductive architecture element. This may be attributed to the lack of material characterisation of the core, including its exact chemical composition and crystallographic structure.

The present inventors' research group has previously found that, contrary to what was assumed so far in the literature, cores are composed of an amorphous Al _x Fe _y O _z alloy which composition is quantified. This has led to the proposition of a new mechanism for catalytic site formation, whereby catalytic sites nucleate at the surface of the cores upon cooling in the downstream temperature gradient of the calcination furnace, effectively decoupling catalytic site formation and CNT growth from the initial metal nitrate precipitation stage. Building on that finding, which indicates that the process is more versatile than what was originally thought as it does not rely on the initial structure created upon droplet precipitation, the present inventors' research group has shown that CNTSU morphology can be tuned independently of core size and composition by changing the operating parameters of the CVD growth furnace, and that CNT length, density, and quality can be increased to improved levels. For reference, the approach taken in Fig. 1 is first explained. A one-jet collision nebulizer is used to atomize an aqueous solution of 2 w.% aluminium nitrate and 2 w.% iron nitrate (Sigma-Aldrich ACS reagent grade and type II deionized water) in a 1500 seem flow of nitrogen carrier gas. The resulting aerosol (geometric mean diameter of about 1 μηη as measured with a PALAS Welas digital 1000H optical aerosol spectrometer) is then passed through a custom-made silica gel drier where close to 100% of the stream moisture content is removed. The resulting dry metal nitrate salt nanoparticles then enter a calcination furnace, furnace 1 , operated at r ₁ =900 C together with an additional flow of hydrogen at a flowrate of 130 seem. Downstream of the first furnace, the bimetallic nanoparticles aerosol is further mixed with a 150 seem flow of hydrogen and a 30 seem flow of acetylene before entering the CVD growth furnace, furnace 2, operated at a temperature i ^* ₂ =800°C. Total process time is about 10 s. These operating conditions (flowrates and temperatures) are referred to as the nominal conditions in what follows. Both furnaces are electrical tube furnaces with about 50 cm heated lengths, equipped with 1 m long, 19 mm inner diameter alumina worktubes. All gases are BOC 99.998% purity compressed gases, HEPA filtered and controlled using Alicat MC series mass flow controllers.

As discussed above, the CNTSU material is of interest for energy storage applications, particularly for the anode of lithium ion batteries. The material consists of metal oxide particles (typically comprising iron and aluminium) stabilized with carbon nanotubes. The metal oxides are the active material, offering sites for the lithium ions to bond, and the carbon nanotubes provide electrically (and thermally) conductive pathways within the material. This material has several advantages over the conventional graphite anodes used in batteries today. First, it is possible to obtain higher storage capacity per unit mass with this material than with graphite (about a 30% increase). Secondly, this material offers a significantly increased service life over the current state of the art. With each charge/discharge cycle, the anode swells and contracts as lithium is stored and released. The repeated changes in physical dimensions cause fatigue failures and cracks, and can often be the mechanism of failure for the battery. The carbon nanotubes included in this novel material serve as a stabilizer and can help damp the expansion and contraction experienced during repeated use.

In the spray drying / calcination process described with respect to Figs. 1 -4, the carbon nanotube "sea urchins" produced comprise a spherical iron and aluminium oxide core particle with many carbon nanotubes grown radially from the surface. The spray drying process inherently requires a feedstock that is soluble so that it can be precipitated into particles during drying. This necessitates particular feedstock materials, such as iron and aluminum nitrates. The present inventors have realised that it is possible to produce a similar material, compositionally and morphologically, using plasma vaporization instead of spray drying / calcination. The plasma-based process has several advantages over the spray drying method. First, the plasma itself is able to maintain an extremely high temperature (about 5000 K) which is sufficient to vaporize nearly any known precursor. This allows the liberty of using a wide range of feedstocks, in particular those which are cost effective and readily available. As is explained below, metal or metal oxide powders are preferably used since they are inexpensive and good candidates for an industrially-sized process. Moreover, the plasma system has a higher energy density than a tube furnace used in the spray drying / calcination process, meaning that more product can be produced in a given volume. This is advantageous when scaling the process because a smaller system (and perhaps building) is then required for a given mass throughput. The plasma system is also more effective at delivering the input energy to the precursors because all the energy is transferred to the gas in the reactor tube directly, rather than having to conduct through the reactor tube first. As a result, a larger fraction of energy is attributed to the gas and reagents and less energy is wasted on heating the tube itself (despite the plasma temperature of around 5000 K and downstream gas temperatures in excess of 1300 K, the reactor tube itself can easily be maintained at less than 1000 K). Finally, since nitrates are not required for this process, gases such as NO _x which are harmful to people and the environment are not generated.

For the above reasons, it is considered that the process disclosed herein is able to produce a competitive and valuable material for the energy storage industry in a cost effective and scalable manner. The experimental setup for this process is illustrated in Fig. 5, which shows one embodiment of a production apparatus 10 for use in implementing an embodiment of the invention. The energy source for the process is a microwave (MW) plasma system (Sairem GMP 60K) identified by reference number 12 in Fig. 5 controlled via power supply 13. The microwave plasma system sustains a surface wave plasma from microwaves at the frequency of 2.45 GHz. The system can supply power in the range of 600 watts to 6 kilowatts. Microwaves are emitted from a magnetron which pass through a port 14 connected to a damping element known as a 3-stub tuner 16. 3-stub tuner 16 contains three copper rods that protrude into the channel housing the microwaves and the depth of the rods can be modulated to regulate the amount of microwave (MW) energy reflected back into the generator. The MW energy then passes into the waveguide 18, which holds the reactor tube 20. MWs penetrate the tube 20 and pass into a sliding short circuit 22 at the end of the MW channel. The short circuit 22 contains a plate which can be moved inwards and outwards to change the length of the MW chamber. This is useful in ensuring there is a strong maximum of MW power at the reactor tube 20.

Downstream of the plasma system, additional gases such as h and C2H2 can be injected at port 24, and the mixture passes to a tube furnace 26. Material is collected after the furnace 26 either with a filter 28 or with other precipitation methods such as thermophoresis or magnetic precipitation.

Powder feeder 21 is controlled to feed Fe and Al powder (in one embodiment) into the plasma chamber 20. The plasma chamber may comprise a quartz tube (30 mm outer diameter, downward flow in this case, although the system can operate in any suitable configuration). When free charges are created (i.e. a spark) from an external source, the oscillating electromagnetic field forces free electrons to collide with neutral atoms, causing them to be ionized which in turn liberates more electrons and a cascade begins. At some point the ionized gas volume reaches an equilibrium of ionization and recombination events and this results in a stable plasma, provided the MWs continue to energize the reaction volume. Various gases and gas mixtures have been tested with this plasma system, including nitrogen, helium, and argon, as well mixtures of the aforementioned gases with hydrogen or oxygen. The plasma region has a temperature of at least 5000 K which is sufficient to vaporize a wide range of precursor materials.

Modifications have also been made to the plasma system geometry to reduce

degradation of components and increase the vaporization efficiency of precursors. In particular, it was discovered that in its default configuration, atmospheric pressure plasma would preferentially adhere to the wall of the quartz tube. Its close proximity caused the quartz to melt or crack, and much of the powder fed into the system would not pass directly through the plasma resulting in little vaporization. This arrangement is illustrated in Fig. 6, in which the quartz tube is indicated at 50 and the plasma 52 is created close to the wall of the quartz tube 50. Fig. 7 shows a view from one end of the plasma chamber illustrating the generation of the plasma against the wall of the plasma chamber (quartz tube 50) in the basic configuration of Fig. 6, with the flow rate of N2 being about 1 -20 slpm substantially uniformly along the quartz tube.

A solution was developed which consists of a ceramic insert or "torch" that is inserted into the quartz tube. This modified configuration is illustrated in Fig. 8, in which plasma torch 54 is inserted coaxially along quartz tube 50. Fig. 9 shows a view from one end of the plasma chamber illustrating the generation of the plasma 52 in the modified configuration of Fig. 8. Separate gas streams can flow through along the torch and through the small gap (about 1 .5 mm) between the torch 54 and the quartz tube 50. A large gas flow rate is passed in the annular sheath region between the quartz tube 50 and the torch 54 (about 6-20 slpm) and a smaller gas flow is passed through the centre of the torch (about 1 -2 slpm). The sheath gas is forced to accelerate as it passes through the narrow sheath region, and it is the relatively high velocity of this gas that discourages plasma formation here. Instead, plasma preferentially forms from the slow-moving gas of the centre flow. This results in a plasma that is stable in the centre of the quartz tube, and ensures that several millimetres of separation is maintained on all sides between the plasma and the quartz tube wall. The second benefit of this configuration is that when introduced through the centre of the torch, vaporization of the precursors is improved since the material is not able to exit the torch without passing directly through the plasma. The difference between this configuration and the basic configuration can be seen by comparing and contrasting Figs. 7 and 9. Powdered precursors are introduced into the plasma. A commercial powder feeder system 21 allows delivery of the powder at a feed rate which is appropriate for this lab- scale system (about 10-100 milligrams of powder per minute) in a steady, repeatable manner. The powder falls into a nitrogen plasma (8 slpm sheath flow, 1 slpm centre flow, 1 .5 kW MW power) where it is vaporized. As the metal vapour exits the plasma region its partial pressure reaches supersaturation and nucleation occurs. Liquid metal droplets are formed which quickly solidify as their temperature decreases below their melting temperature.

Due to Brownian motion (diffusion) these nearly-spherical primary particles collide with one another (a ubiquitous phenomenon in aerosol processes) which almost always results in the particles sticking together via the Van der Waals force. This effect ultimately produces aggregate particles with a fractal-like shape, comprising numerous primary particles. A process known as sintering can be used to convert the fractal aggregate particles into spheres. In other words, the primary particles will merge to form a single large spherical particle, driven by a potential to reduce surface area. The sintering time of a particle shows a strong positive correlation with temperature, so simply maintaining the aerosol temperature as high as possible can be enough to sinter the particles at least partially. This can either be done by insulating the tube downstream of the plasma to reduce heat loss, or by passing the particles into an intermediate furnace to actively maintain a high temperature. At the time of writing, there is no particular preference between fractal aggregate core particles and spherical core particles with regard to performance in energy storage applications. From one possible perspective, spherical particles could be more effective at resisting the degradation from swelling and contracting during charge/discharge cycles. Alternatively, aggregate particles typically have a much higher specific surface area and could be more effective at storing lithium ions: increasing the cell's capacity.

In the results reported here, steps have not been taken to cause substantial sintering of the particles, so they remain aggregates of primary particles. This is in contrast to carbon nanotube sea urchin cores which have previously been formed from a spray dry / calcination process which are highly spherical.

Upon exiting the quartz reactor tube containing the plasma, some of the gas and core particles are vented at vent port 25 into the exhaust. The remaining sample is mixed with hydrogen (200 seem) and acetylene (about 15 seem) before passing into the tube furnace 26. The acetylene is the carbon source and the hydrogen is included as a reducing agent which will react with any oxidizing species that may inhibit carbon nanotube growth. Note that the step to vent a portion of the plasma gas and core material is simply a limitation of the lab-scale setup: the tube furnace 26 used in this work was not able to accommodate the high gas flow rates used by the plasma system. The tube furnace 26 is maintained at 830°C and houses a 30 mm diameter alumina tube. The gas/core particles are passed through the furnace at 1.7 slpm which results in a residence time (nanotube growth time) of approximately 4 seconds.

A modification of the production apparatus of Fig. 5 is illustrated in Fig. 10. Similar features are not described again, for conciseness. In Fig. 10, the production apparatus 10a is modified and simplified by eliminating the growth furnace 26 of Fig. 5 and replacing it with an insulated tube 30, provided with insulating jacket 32. Vent 25a is provided between the plasma chamber and the insulated tube 30. This approach therefore seeks to provide temperature control via insulation, rather than re-heating using a tube furnace. In doing so, the cost of this process both in terms of equipment and energy can be substantially reduced. The location of the insulation on the tube should be adjusted so it maintains the gas temperature within the tube as near as possible to the ideal temperature nanotube growth temperature - at the time of writing it is considered that about 830°C is the preferred nanotube growth temperature.

The material produced in the process and apparatus described above comprises aggregated metal oxide core particles with carbon nanotubes grown radially from the surface. Because the cores are aggregates of smaller primary particles, the final material has a different appearance than nanotube sea urchins produced from a spray drying process. Fig. 4 shows a sea urchin from spray drying: hundreds of nanotubes are seen growing from a spherical core particle of diameter about 400 nm. The sea urchin is a distinct unit, in contrast to the material produced from this plasma process, shown in Figs. 1 1 and 12. This material is more of a mixture of core particles with fewer (but still multiple) nanotubes grown from each particle. The aggregate core particles and their respective nanotubes have further aggregated while being collected on a filter to form a continuous bulk material where it is no longer possible to distinguish individual aggregate particles.

In a lithium ion battery, the anode (this material's intended application) stores energy by bonding the lithium ions to oxygen atoms present in the anode material. This is therefore the incentive for oxidizing the core particles in this process: more oxygen present in the anode allows more lithium to be stored, resulting in a higher energy storage capacity per unit mass of the cell. Oxidizing the material has been accomplished with two methods: with the introduction of oxygen gas into the plasma (about 20 seem) while feeding an elemental metal powder mixture, and by feeding a mixture of powdered metal oxides (also possible is a mixture of elemental and oxide powder). Both of these methods have been shown to produce metal oxide particles with carbon nanotubes grown radially from their surfaces. The metals used in the process are aluminium and iron although it is understood that other combinations of metals are also suitable. The transition metal, in this case iron, is used as a catalyst to grow the carbon nanotubes in addition to storing energy in its oxide form. Other transition metals which can be used are nickel, cobalt, yttrium, and cerium. Iron and aluminium have been chosen as precursors here because they are inexpensive and non-toxic.

In the oxidation methods described above, it is important to oxidize as much of the metal as possible; however a large excess of oxygen must not be present because carbon nanotube growth will not be favourable. It is for this reason that hydrogen is added as nanotube growth is to occur, so it can react with any remaining oxygen. If more oxidation was desired, this could be done either by increasing the time before entering the growth furnace, or by adding another region after the growth furnace in which excess oxygen is added (likely in the form of oxygen gas) but the temperature is kept below approximately 350°C. This encourages oxidation of the metals but prevents the nanotubes from burning.

Even without the use of a dedicated oxidation step, the metals are able to form oxides during the period between formation in the plasma and upon entering the growth furnace. It has been determined that the majority of the core material is amorphous (a

disorganized, randomly oriented solid) as opposed to a regular, crystalline phase. This is not surprising however, since the rapid cooling from the extremely high temperature of the plasma does not allow much time for the metal and oxygen atoms to arrange in uniform patterns. The amorphous structure has not been shown to exhibit any negative effects on energy storage performance. Phases of aluminium oxide, iron oxide, combined oxide phases such as hercynite, as well as elemental iron and aluminium are present in the core particles. Phase identification is difficult for amorphous materials however, so it is not yet known what mass fraction each of these chemical species represents. A technique known as thermogravimetric analysis (TGA) can be used to determine the overall oxygen content of the material, as well as the proportion of metal oxide cores to nanotubes. An example plot is shown in Fig. 13 for a product of an embodiment of the invention. Several milligrams of material are placed in a crucible on a microbalance inside a small furnace. With air as the atmosphere, the temperature of the furnace is slowly increased, and the mass of the sample is recorded. In the example plot of Fig. 13, the increase in mass up to 400°C is due to the metal cores reacting with (and trapping) oxygen from the air in the furnace. It is therefore ideal to see only a small increase in mass, or none, as this would indicate that the sample already contains as much oxygen as possible. The subsequent decrease in mass is attributed to the nanotubes burning (converting to carbon dioxide and leaving the furnace). The amount of mass lost from nanotube burning indicates the mass fraction of nanotubes compared to the residual metal oxide material. The example plot in Fig. 13 was carried out on a material containing approximately 50% nanotubes by mass. It is preferred that the nanotube mass fraction should be about 20% or less. The reason for this is that the function of the nanotubes is to provide electrically conductive pathways in the material but they do not contribute to the energy storage itself. This effect can be accomplished with less than 20% nanotubes (e.g. at least 5 mass% or at least 10 mass% or at least 15 mass%) but any fraction higher than this does not offer substantial benefit. In summary, the TGA plot of the ideal material would be one that does not gain any mass from metal oxidation, and experiences a relatively small mass loss from nanotube burning (e.g. less than 20%).

TGA runs from various feedstocks and process configurations are shown in Fig. 14 with results summarized in Table 1 :. The example run plotted in Fig. 13 is shown again (material produced from elemental powder, without any oxygen added). Other runs include material produced from elemental powder which was then placed in an oxidation furnace for several hours, a run with elemental powder and added oxygen gas, and a run using oxide powder as the feedstock. All subsequent runs showed a smaller oxygen gain than the initial elemental powder run, and with oxygen gas added, the nanotube fraction was reduced below 20% as well. Using oxide powder, there was particularly little oxygen uptake during TGA, and although the nanotube fraction was large in this sample, this property has yet to be optimized and can be lowered below 20% with little difficulty.

Table 1 : Quantitative TGA results

The performance of the material produced according to an embodiment of the invention in a half-cell has been characterized for elemental powder without the addition of oxygen. The profile of the cell current as a function of voltage for several charging and

discharging cycles is shown in Fig. 20. The profile is repeatable, which is a desirable trait. Fig. 21 shows the capacity performance of the cell for the first and second cycles and Fig. 22 shows the capacity performance of the cell for the third and fourth cycles.

As shown in Fig. 21 , the first cycle exhibits a very high capacity although subsequent cycles have lower capacity. This is a common trait of chemical cells, but ideally the decrease is as small as possible. Also important is that the capacity does not decrease noticeably after the first cycle. In this case, the capacity does decrease but not severely. This particular sample was produced with elemental powder and no additional oxygen. Its capacity is lower than that of a typical graphite anode but this is largely due to the high nanotube mass fraction (about 50%) which does not contribute to energy storage. It was also suspected (and confirmed by TGA) that the oxygen content in this sample was relatively low (i.e. a lot of oxygen gained during the TGA run), and this was the motivation for adding oxygen by another means (e.g. oxygen gas or oxide powder). With added oxidation and a reduction in nanotube content, the capacity of the material increases significantly. As explained above, a particular preferred application for the CNTSUs is in the

fabrication of lithium-ion battery electrodes. In such an application, the CNTSUs are arranged as an assembly, either as a film or layer deposited on a substrate. The material reported here outperforms existing lithium-ion battery anode materials for a number of reasons:

- Its capacity (on a mass basis of core particles) at low current is at least 1 10% that of graphite, the current standard for commercial Li-ion battery anodes.

- It maintains a large capacity at high discharge rates, which is a requirement for the next generation of electrical vehicles.

- The performances of the material do not degrade significantly over charge and discharge cycles.

- The material facilitates battery thermal management, which is key to answering the safety issues that currently limit the development of the lithium-ion battery technology. -The material can be synthesised from inexpensive, widely available chemical precursors, using a continuous gasphase process that can be scaled-up industrially. Once

synthesised it can easily be integrated to the existing assembly chains in standard lithium-ion battery factories. Referring now to Figs. 15-18, the principle of operation is that lithium ions are stored in the metal oxide cores, while electrons and heat are conducted out via the carbon nanotubes. The hierarchical nature of the material over many length scales, coupled with the inherent chemical, mechanical, electrical, and thermal properties of the nanostructures enable high performances. In practice, the commercial product can be a powder to be coated on standard battery current collectors during battery assembly in factories.

In a lithium ion battery, the anode is the electrode that stores lithium ions during charge and expels them during discharge. This flow of positive charges between the anode and the cathode has to be matched by a flow of electrons, which produces useful work through an external circuit. In standard commercially available Li-ion batteries, the anode is composed of a graphite powder that acts as a host material for lithium ions, coated on a current collector (metal foil) with a polymeric binder that maintains the mechanical integrity of the electrode. In a preferred embodiment of the present invention, graphite is replaced by a film of metal oxide-carbon nanotube nanostructures as shown in Fig. 18. The film can be coated on a current collector.

Within the realm of lithium-ion batteries, the current technology that is set to dominate the market for another ten years at least, these performances are primarily determined by the choice of the active material for the cathode and anode. The current challenge is to find materials that largely exceed the energy density and rate performances of

commercially available batteries, without sacrificing the other performance parameters.

As explained above, the nanostructure is composed of a metal oxide core with a diameter between 30 nm and 10 μηη, onto which a large number of single-wall and/or multiwall carbon nanotubes are anchored. The metal oxide core consists of an amorphous alloy of aluminium, iron, and oxygen matrix, into which small crystallites of the same composition are embedded. The alloy composition is not necessarily homogeneous. The overall molar proportion of AI:Fe:0 can be 1 .5/1/4. Different proportions can be used for Li-ion battery applications with ease, and give similar or better electrochemical results. Carbon nanotubes are covalently anchored to the core and extend radially, creating a porous, electrically and thermally conducting shell of carbon nanotubes around it, whose thickness is between about 10 nm and 10 μηη on average.

The nanostructured film thickness can range from less than one monolayer to several hundreds of micrometres. The packing density can be varied according to the deposition process, from a very porous film, to a highly compact one. Fig. 19 illustrates a suitable lab-scale process for manufacturing an anode according to a preferred embodiment of the invention. CNTSU powder is manufactured as explained above. The CNTSU powder (10 mg) is mixed with PVDF binder (2 mg) and NMP solvent (5 mL) using a mortar until a uniform slurry is produced. The slurry is dropcast onto an Al foil, which is then heated at 70°C overnight on a glass slide to provide the cured electrode.

In addition to the use as an anode material for lithium ion batteries, there are other uses for the material: active material for other electrochemical devices (alternative battery types, supercapacitors, fuel cells, electrochemical water filters, electrocatalysis, photocatalytic water splitting, etc.), filler additive for enhanced mechanical, thermal, and electrical properties of composites, or production of large area carbon nanotube mat or fibres for a number of applications (composites, actuators, heat sinks, electric cables, electromagnetic shielding, etc.).

In the context of lithium ion batteries, the present invention combines both material advantage and nanostructure engineering to design an anode structure that optimises ion diffusion, electron transport, mechanical stability and Li ion kinetics, as these are the most stringent conditions needing a balance in high energy/power batteries. One main problem impeding the commercialisation of metal oxide electrodes [Cabana et al (2010)] is voltage hysteresis associated with their charge/discharge cycles. This originates from multiple pathways of the conversion reactions between metal oxide and Li ions and complexities in phase transformation of active particles, resulting in unregulated reaction pathways during battery charge-discharge cycles. Thus, maintaining an uninterrupted ion/electron transport across electrodes together with a high mechanical resilience as battery is charged and discharged, remains a challenging task till to date.

One way to mitigate poor ion/electron transport and electrode stability that causes capacity decay is to design electrodes that can couple the active particles (iron oxide) and conductive additive (carbon) at a single particle level with adequate control over particle morphology and carbon-metal oxide interface. In our urchin like structures (FexAh-xOy-MWCNT), the core is made of an alloy of AI-Fe-0 that acts as active particles while CNTs play a dual role by acting as in situ conductive additive and mechanical spacer for active particles that typically tend to swell and disintegrate during charge- discharge cycles. The major advantages of our design are that (i) it in principle can offer high electrical accessibility of active particles for effective conversion reactions, during which the volume expansion of particles is efficiently accommodated by void space and the mechanical buffer effect of CNTs, (ii) the compaction of urchins in electrodes results in a porous network conducive to better electrolyte immersion, and (iii) CNTs are efficient heat dissipaters that can potentially avoid thermal runway of batteries.

Electrical characterization

Method:

Swagelok-type coin cells were fabricated using standard procedures to test the performances of the invented material versus lithium metal (half cell configuration) and versus a common commercially available cathode material (full cell configuration). In both cases the electrode was fabricated according to the steps shown in Fig. 19. These cells were then subjected to thorough electrochemical testing. In preferred embodiments, the full cell has the following construction:

Anode: as described above.

Cathode: LNCO (lithium nickel cobalt oxide) and LiFePC (commercial)

Electrolyte: LiFP6 in polycarbonates and LiTFSI in polycarbonates (commercial)

Binder: PvDF, carboxymethyl cellulose , binder-free

Separator: Poly propylene (Cell guard), Whatman glass microfiber (commercial), paper, ceramic

_{***********************} While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

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