Material comprising hard carbon

Abstract

The invention relates to a process of preparing a material comprising hard carbon, in which the process comprises the utilisation of a composition including hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in an electrochemical cell.

Description

[0001] MATERIAL COMPRISING HARD CARBON

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a process of preparing a material comprising hard carbon. The present invention also relates to a material comprising hard carbon which is obtainable by the process of the present invention. The present invention also relates to uses which include hard carbon.

[0004] BACKGROUND OF THE INVENTION

[0005] Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion) battery is charging, Na+(or Li+) ions deintercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.

[0006] A sodium-ion cell comprises an active material layer coated on a current collector foil to form a cathode, and a similar arrangement exists for a corresponding anode. The cathode and anode are physically separated by a separator which allows for a flow of ions within a liquid electrolyte medium, which is present uniformly within the cell and wets the entire cathode, anode and separator. During charging, Na+ions shuttle from the cathode active material and are inserted in the anode active material (electrons flow through the external circuit) and the reverse process occurs during discharging (sodium ions are extracted from the anode active material and are inserted into the cathode active material with the electrons flowing through the external circuit, doing the useful work).

[0007] Carbon, in the form of graphite, has been favoured for some time as an anode material in lithium-ion batteries due to its high gravimetric and volumetric capacity; graphite electrodes deliver reversible capacity of more than 360 mAh / g, comparable to the theoretical capacity of 372 mAh / g. The electrochemical reduction process involves Li+ ions being inserted in between the graphene layers, to yield LiC6. Unfortunately, however, graphite is much less electrochemically active towards sodium and this, coupled with the fact that sodium has a significantly larger atomic radius compared with lithium, results in the intercalation between graphene layers in graphite anodes being severely restricted in sodium-ion cells.

[0008] Anodes made using hard carbon materials, on the other hand, (such as described in PCT / GB2020 / 050872, US2002 / 0192553A1 , US9,899,665B2, US2018 / 0287153A1) are found to fare much more favourably in sodium-ion cells.

[0009] Hard carbons have disordered structures which overcome many of the insertion issues for sodium ions. The exact structure of hard carbon materials has still to be resolved, but in general terms hard carbon is described as a non-graphitisable carbon material lacking long range crystalline order. Hard carbon has layers, but these are not neatly stacked in long range, and it is a microporous material. Although lacking a definable crystallographic structure, hard carbon is isotropic at the macroscopic level. One of the reasons why it is difficult to construct a universal structural model of hard carbon is that short-range order, domain size, fraction of carbon layers and micropores depend on the synthesis conditions, such as carbon sources, carbonisation and pyrolysis temperatures.

[0010] Further still, unlike graphite, which has a graphite crystal structure in which carbon layer planes are stacked in layers, hard carbon has a turbostratic structure in which carbon layer planes are stacked in a state of being three dimensionally displaced. Therefore, the heat treatment of hard carbon, even at high temperature (e.g. 3000 °C) does not result in a transformation from the turbostratic structure to the graphitic structure or the development of graphite crystallites. Thus, hard carbon is structurally quite distinct from graphite and can be said to comprise one or more non-graphitised domains as well as one or more non-graphitisable domains.

[0011] Sodium-ion cells have generated considerable interest and are considered as promising candidates for next-generation battery technology. Therefore, with the expected growth of ‘spent’ sodium-ion cells, the effective recycling of components from such cells is environmentally attractive. Thus, one area that needs more attention is the development of novel processes to enable effective recycling of key components such as negative electrode active materials from ‘spent’ sodium-ion cells.

[0012] Prior art techniques for recycling graphite from ‘spent’ lithium-ion cells are summarised in a review titled ‘The Review of Existing Strategies of End-of-Life Graphite Anode Processing Using 3Rs Approach: Recovery, Recycle, Reuse’’, Batteries 2023, 9(12), 579. Notably, in ‘Mechanistic insights into the lattice reconfiguration of the anode graphite recycled from spent high-power lithium-ion batteries’-, Journal of Power Sources 2021 ; 481 ; 229159, Yu et al. highlight the crystal lattice defects that present in reclaimed graphite anodes of spent lithium-ion cells. By comparing the physical characteristics, i.e., graphite’s crystallite size, degree of graphitisation, d-spacing and defect density, as well as the electrochemical performance, the authors concluded that annealing at high temperatures (optimally 3000 °C) under certain inert atmospheres (optimally N2) for relatively long periods (optimally 6 hr) are required to restore the characteristics of reclaimed graphite back to ‘battery-grade’. A high degree of graphitisation is deemed desirable and is often achieved by annealing at high temperatures as set out in a brochure titled “The Capacity Boosters: ourgraphite based battery solutions" by SGL Carbon which is available at https: / / www.sglcarbon.com / pdf / SGL-Brochure- SIGRACELL-The-Capacity-Boosters-EN.pdf on the priority date of the present application.

[0013] Furthermore, CN114583315A discloses a method of recycling graphite anodes which have been cycled in a lithium-ion battery. In particular, Examples 1 and 2 disclose recycling methods for natural and artificial graphite anodes respectively. These processes include a coating step wherein the used graphite material is heated with a carbon source to repair the structural damage that arises during cycling of a graphite anode. CN114583315A further applies this teaching to alternative carbon-based anode materials such as hard carbon and mesoporous carbon. However, it is known that coating processes affect the microporosity of hard carbon which must be carefully controlled in a hard carbon anode to minimise first cycle loss and maximise second cycle capacity. CN114583315A does not provide any first cycle loss or capacity data for its recycled materials. As such, it is not clear whether the method of CN114583315A can be used to recycle hard carbon which has been cycled in an electrochemical cell, in particular a sodium-based electrochemical cell, to provide a negative electrode active material which exhibits low first cycle loss and high second cycle capacity.

[0014] Additionally, it is not known whether hard carbon anode materials which have been prepared from ‘recycled’ or ‘recovered’ materials would be expected to show acceptable electrochemical performance when used in an electrochemical cell such as a sodium-ion cell. Furthermore, it is not known whether ‘recycled’ or ‘recovered’ hard carbon anode materials would need to be ‘repaired’ in the same manner as graphite as taught by CN 114583315A.

[0015] The present invention therefore aims to overcome the aforementioned issues.

[0016] In particular, the aim of the present invention is to provide a material comprising hard carbon which can be prepared using materials from ‘spent’ sodium-based electrochemical cells, particularly sodium-ion cells. Importantly, such a material will enable a preferable sodium-ion cell to cycle as efficiently as it would if it were to use an existing prior art material such as an unspent ‘virgin’ hard carbon. Ideally, the material will enable a preferable sodium-ion cell to cycle more efficiently when compared to an existing prior art material such as an unspent ‘virgin’ hard carbon.

[0017] The process of preparing said material will also contrast prior art processes because it will enable effective recycling of hard carbon from ‘spent’ sodium-based electrochemical cells, preferably sodium-ion cells. Further still, the processes of the present invention will also enable efficient and cost-effective production of a material comprising hard carbon.

[0018] The present invention achieves these aims by providing a process of preparing a material comprising hard carbon, in which the process comprises the utilisation of a composition including hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in an electrochemical cell.

[0019] The phase ‘subject to one or more charge and / or discharge operations’ as used herein is intended to encompass one or more ‘partial’ or ‘incremental’ charge and / or discharge operations in an electrochemical cell, as well as one or more ‘full’ or ‘complete’ charge and / or discharge operations in an electrochemical cell.

[0020] Preferably, the hard carbon included in the composition including hard carbon has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell.

[0021] As used herein, the term “sodium-based electrochemical cell” refers to any electrochemical cell in which the charging and / or discharging processes involve the transfer of sodium and / or sodium ions between the positive electrode and the negative electrode. Preferable sodium- based electrochemical cells include sodium-ion cells, sodium metal cells, and anode-free sodium cells.

[0022] Ideally, the sodium-based electrochemical cell is a sodium-ion cell, most preferably a sodium- ion full cell.

[0023] It will be appreciated that a hard carbon composition which has been subject to one or more charge and / or discharge operations in a sodium-based cell, in particular a sodium-ion cell, can be identified by one or more of a variety of techniques known to the skilled person, such as inductively coupled plasma atomic emission spectroscopy (ICP), energy-dispersive x-ray spectroscopy (EDX), or x-ray photoelectron spectroscopy (XPS). Preferably, a hard carbon composition which has been subject to one or more charge and / or discharge operations in a sodium-based cell can be identified by inductively coupled plasma atomic emission spectroscopy (ICP) and / or energy-dispersive x-ray spectroscopy (EDX), ideally by inductively coupled plasma atomic emission spectroscopy (ICP).

[0024] In one embodiment, a hard carbon sample which has not been cycled in a sodium-based electrochemical cell will contain between 0 ppm and 0.1wt% of sodium as measured by any suitable technique known to the skilled person, optionally one of those set out above. Therefore, in this embodiment, a hard carbon sample which has been cycled in a sodium- based electrochemical cell will preferably contain more than 0.1 wt% of sodium as measured by any suitable technique known to the skilled person, optionally one of those set out above.

[0025] In an alternative embodiment, a hard carbon sample which has not been cycled in a sodium- based electrochemical cell will contain between 0 ppm and 0.2wt% of sodium as measured by any suitable technique known to the skilled person, optionally one of those set out above. Therefore, in this embodiment, a hard carbon sample which has been cycled in a sodium- based electrochemical cell will preferably contain more than 0.2wt% of sodium as measured by any suitable technique known to the skilled person, optionally one of those set out above.

[0026] In a further alternative embodiment, a hard carbon sample which has not been cycled in a sodium-based electrochemical cell will contain between 0 ppm and 0.3wt% of sodium as measured by any suitable technique known to the skilled person, optionally one of those set out above. Therefore, in this embodiment, a hard carbon sample which has been cycled in a sodium-based electrochemical cell will preferably contain more than 0.3wt% of sodium as measured by any suitable technique known to the skilled person, optionally one of those set out above.

[0027] Preferably, the charging and / or discharging processes of a sodium-based electrochemical cell do not involve the transfer of lithium and / or lithium ions between the positive electrode and the negative electrode.

[0028] Preferably a hard carbon sample which has been cycled in a sodium-based electrochemical cell will contain between Oppm and 0.2wt% of lithium as measured by any suitable technique known to the skilled person, optionally one of those set out above. The composition including hard carbon is a ‘feedstock’ in the process of the present invention. In particular, it is a feedstock which comprises hard carbon that has been subject to one or more charge and / or discharge operations in an electrochemical cell.

[0029] The process is ideally for preparing a material comprising hard carbon for use in a metal-ion cell, particularly a sodium-based electrochemical cell, very particularly a sodium-ion cell, and most particularly a sodium-ion full cell.

[0030] As used herein the term “hard carbon” means a non-graphitisable carbon material lacking long-range crystalline order. Hard carbon has layers, but these are not neatly stacked in long range, and it is a microporous material. Although lacking a definable crystallographic structure, hard carbon is isotropic at the macroscopic level. One of the reasons why it is difficult to construct a universal structural model of hard carbon is that short-range order, domain size, fraction of carbon layers and micropore structure depend on the synthesis conditions, such as carbon sources, carbonisation and pyrolysis temperatures.

[0031] Further still, unlike graphite, which has a layered crystal structure in which carbon planes (graphene) are stacked, hard carbon has a turbostratic structure in which carbon layer planes are stacked in a state of being three dimensionally displaced. Therefore, the heat treatment of hard carbon, even at high temperature (e.g. 3000 °C) does not result in a complete transformation from the turbostratic structure to the graphitic structure or the development of graphite crystallites. Thus, hard carbon is structurally quite distinct from graphite and can be said to comprise one or more non-graphitised domains as well as one or more non- graphitisable domains.

[0032] Unlike the present invention, usual methods for producing hard carbon materials involve heating carbon-rich starting materials such as minerals, for example petroleum coke and pitch coke; secondary plant-based materials such as sucrose and glucose; man-made organic materials such as polymeric hydrocarbons and other organic compounds such as resorcinol formaldehyde; animal-derived materials such as manure; and primary plant-derived materials such as coconut shells, coffee beans, straw, bamboo, rice husks, banana skins, etc., to temperatures greater than 500 °C in an oxygen-free atmosphere.

[0033] In one embodiment, the hard carbon included in said composition has been subject to four or more charge and / or discharge operations in an electrochemical cell. In another embodiment, the hard carbon included in said composition has been subject to twenty five or more charge and / or discharge operations in an electrochemical cell. In some embodiments, the hard carbon included in said composition has been subject to one hundred or more charge and / or discharge operations in an electrochemical cell. In another embodiment, the hard carbon has been subject to five hundred or more charge and / or discharge operations in an electrochemical cell.

[0034] Preferably, the electrochemical cell is a metal-ion cell such as a sodium-ion cell, which is ideally a sodium-ion full cell. Preferably, the resulting material comprising hard carbon is a negative electrode active material, preferably for use in an electrochemical cell such as a sodium-based electrochemical cell. Ideally, the resulting material comprising hard carbon is a negative electrode active material for use in a sodium-ion cell.

[0035] The composition including hard carbon in which the hard carbon in said composition has been subject to one or more charge and / or discharge operations as defined above provides a ‘recyclable’ source of hard carbon. Ideally, this ‘recyclable’ or ‘recoverable’ source of hard carbon is purified using the process of the present invention to form a material (preferably, a negative electrode active material), which can readily be used in an electrochemical cell such as a sodium-ion cell.

[0036] Preferably, the process of the present invention further includes: a) providing a composition including hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in an electrochemical cell, preferably a sodium-based electrochemical cell; b) treating the composition in step a) to remove any unwanted inorganic impurities; and c) pyrolysing the treated composition, at a temperature from about 600 °C to 3000 °C, to form the material comprising hard carbon.

[0037] The composition provided in step a) is highly likely to contain unwanted inorganic impurities. Such impurities may be due the hard carbon included in said composition previously been subject to one or more charge and / or discharge operations in an electrochemical cell. Additionally, or alternatively, it may be due to the way in which the composition in step a) has been provided. Indeed, as explained in greater detail below, the composition in step a) is ideally provided from the disassembly of one or more electrochemical cells meaning the composition could be contaminated with impurities that have arisen from further components in one or more electrochemical cells during the disassembly process. For instance, the composition in step a) may contain impurities which have been derived from one or more positive electrode active materials; and / or one or more positive electrode active materials themselves; and / or one or more binding components; and / or one or more electrolytes in such one or more electrochemical cells.

[0038] In one embodiment, the composition provided in step a) may contain 0.2% or more by weight of inorganic impurities, optionally 1 % or more by weight of inorganic impurities. In another embodiment, the composition provided in step a) may contain 10% or more by weight of inorganic impurities. In another embodiment, the composition provided in step a) may contain from about 1 to 50% by weight of inorganic impurities.

[0039] Inorganic unwanted impurities means, for example metal-containing and / or non-metal containing impurities which may cause a detrimental effect to electrochemical performance when the resulting hard carbon is used as a negative electrode active material, for instance. Examples of these may include metal or non-metal elements, metal-ions (e.g., transition metal ions, alkali metal ions, or alkaline earth metal ions), metals in elemental form, metals in compound form (e.g., oxides), non-metal ions, compositions, salts, and ligands.

[0040] In a preferred embodiment, step a) comprises providing a composition (i.e., an initial composition) including hard carbon contaminated with one or more elements selected from the group consisting of Na, Cu, Ni, Mn, Mg, Ca, B, K, F, Ti, Al, Fe, P, Si, Zr, V, N and O. Such elements may exist in ion- form, or in non-ion form. Therefore, in this embodiment, step b) may comprise treating the composition to remove one or more elements selected from the group consisting of Na, Cu, Ni, Mn, Mg, Ca, B, K, F, Ti, Al, Fe, P, Si, Zr, V, N and O.

[0041] In one embodiment, elements selected from Ni, Mn, Mg, Ti, Al, Cu, and Fe may be impurities which have been derived from one or more positive electrode active materials such as sodium transition metal oxides. Similarly, elements selected from Na, F and P, particularly F and P, may be impurities which have been derived from one or more electrolytes such as NaPFe. More particularly, F may also be an impurity from other electrolytes such as NaBF4, NaFSI, and NaTFSI.

[0042] Ideally, step b) may comprise treating the composition to remove one or more elements selected from the group consisting of Na, Ni, Mn, Mg, Fe, Ti, Al, and Cu. Indeed, such elements (which may include salts of Ni, Mn, Mg, Fe, Ti, Al, and Cu for example), can be isolated and then converted to more useful forms (e.g., metallic Ni, via electroplating or reduction using NaBF , or NiO / NiCO3, via ion exchange reaction with Na2COs or NaOH i.e., by standard routes). Thus, in preferred embodiment, the process of present invention advantageously not only recycles hard carbon that has been subject to one or more charge and / or discharge operations in an electrochemical cell, but it also it recycles elements such as Ni, Mn, and Mg, for example.

[0043] In a further embodiment, the process comprises providing a composition (i.e., an initial composition) comprising hard carbon contaminated with two or more elements selected from the group consisting of Na, Cu, Ni, Mn, Mg, Ca, B, K, F, Ti, Al, Fe, P, Si, Zr, V, N and O. Therefore, in this embodiment, step b) may comprise treating the composition to remove two or more elements selected from the group consisting of Na, Cu, Ni, Mn, Mg, Ca, B, K, F, Ti, Al, Fe, P, Si, Zr, V, N and O.

[0044] In a further embodiment, step b) may comprise treating the composition to remove two or more elements selected from the group consisting of Ni, Mn, Mg, Fe, Ti, Al, and Cu. In another embodiment, step b) may comprise treating the composition to removal all of Ni, Mn, Mg, Fe, Ti, and Al.

[0045] However, for the avoidance of any doubt, whilst the composition (i.e., the initial composition) including hard carbon may be contaminated with the impurities as described above, the composition may also contain further unwanted impurities in addition to impurities as described above. Further impurities could include carbon-containing impurities (for example carbonate- and / or organic acid salts).

[0046] Thus, a key aim of the present invention is to provide a final material comprising hard carbon, which contains no, or a very low amount of unwanted inorganic impurities. Therefore, preferably, in step b) of the present invention, the composition including ‘recycled’ or ‘recovered’ hard carbon in step a) is treated to substantially remove any unwanted inorganic impurities.

[0047] Ideally, the resulting treated composition from the end of step b) will preferably be at least 90% pure, i.e., it will contain no (0%) or low (< 10wt%) levels of inorganic impurities. In one embodiment, the resulting treated composition from the end of step b) will contain less than 10% by weight of inorganic impurities. Highly preferably, the amount of inorganic impurities in the material produced at the end of step b) will be <5wt%, further preferably <2wt% and highly preferably from about 0 to about 0.6wt%.

[0048] Treatment step b) preferably involves any suitable treatment process or the use of any suitable apparatus known to the skilled person, to separate and remove any unwanted organic and / or inorganic impurities from the composition provided in step a). Preferably, step b) of the present invention comprises the use of chemical digestion. This ideally results in elements such as Ni, Mn, Mg, Ti and Al being present in the treated composition at the end of step b) in an amount from about 0 to about 0.6wt%.

[0049] Ideally, step b) comprises contacting the composition in step a) with acid and / or alkaline conditions. Highly preferably, step b) comprises contacting the composition in step a) with a washing solution, to release at least a portion of the unwanted inorganic impurities to the washing solution to form the treated composition.

[0050] Thus, step b) of the present invention ideally involves chemical digestion in which the composition in step a) is contacted with a washing solution to dissolve the unwanted inorganic impurities. Step b) of the present invention may therefore use a washing solution which is an aqueous and / or a non-aqueous solvent. Ideally, the washing solution is an aqueous solution and / or solvent which may be selected from an acid solution, an alkali solution, a salt solution, and water. The preferred step b) of the present invention uses an acid solution. Suitable acids may include one or more acids selected from the group consisting of hydrochloric acid, hydrofluoric acid, nitric acid and sulfuric acid.

[0051] Ideally, the acid solution is preferably dilute. Preferably, the concentration of the preferred acid solution is from about 1.0M to about 3.0M, and most preferably the acid solution has a concentration of about 2.0M. A 2.0M hydrochloric acid solution is especially preferred.

[0052] Preferably, the washing solution (ideally, acid solution) is hot, further preferably boiling. A suitable temperature may be up to a maximum of 150 °C, further preferably 50 °C to 120 °C.

[0053] Ideally, the treatment in step b) is performed in air and under atmospheric pressure conditions. Such a treatment is preferably performed over a period of time of 30 minutes or more, optionally 1 hour or more, and preferably 2 hours or more. In one embodiment, step b) comprises contacting the composition in step a) with a washing solution for a contact time of 30 minutes or more, ideally 1 hour or more, and most ideally 2 hours or more, to release at least a portion of the unwanted inorganic impurities to the washing solution.

[0054] A contact time from about 1 hour to about 2 hours is especially preferred particularly when using an acid solution, such as a hydrochloric acid solution. The resulting treated composition from the end of step b) is optionally separated (e.g., by filtration, preferably vacuum filtration) and preferably washed with one or more solvents until the pH of the filtrate reached a certain range. Ideally, the one or more solvents include water (preferably deionised water), and the pH of the filtrate is preferably from about 5 to 6, more preferably 5.5 to 6. The resulting acid treated material from step b) is also optionally dried, optionally using heating (preferably up to a maximum of 150 °C, further preferably 50 °C to 120 °C) and further optionally, ground or milled prior to it being pyrolysed in step c).

[0055] For the avoidance of doubt, step b) may comprise one or more, optionally two or more treatment processes as disclosed above. These may be conducted concurrently or sequentially.

[0056] Pyrolysis step c) is ideally required to remove oxygen-containing compounds from the surface of the hard carbon at the end of step b) which has been treated as described above. Indeed, such oxygen groups are liable to act as permanent anchor points for incoming charge carriers and contribute towards first cycle loss. Furthermore, it has been found that the specific surface area of the final negative electrode active material comprising hard carbon determines its level of irreversible capacity, and that this disadvantage is reduced in materials that have a low specific surface area (100 m2 / g or less); pyrolysis has the effect to reduce the specific surface area of the final material comprising hard carbon to less than 50 m2 / g, preferably less than 10 m2 / g.

[0057] Thus, preferably, the final material comprising hard carbon which results from step c) has a specific surface area of less than 50 m2 / g, preferably less than 10 m2 / g, more preferably from 2 to 5 m2 / g. All specific surface area values given in the present application have been determined by BET N2 analysis.

[0058] Pyrolysis involves heating to a temperature of from about 600 °C to about 3000 °C, preferably from 750 °C to 3000°C, further preferably from 800 °C to 2000 °C, further preferably from 1000 °C to 1800 °C, further preferably from greater than 1000 °C to 1750 °C, further preferably from 1100 °C to 1700°C and ideally at a temperature of around 1200 °C, over a period of 30 minutes to 8 hours. Oxygen is advantageously absent from the pyrolysis process and is preferably replaced with one or a mixture of gases selected from nitrogen, carbon dioxide, and an inert gas such as argon. Advantageously, a higher pyrolysis temperature of from 750 °C to 3000 °C, preferably greater than 1000 °C, may enable the process of the present invention to remove a greater amount of impurities.

[0059] Following pyrolysis, the conditions are cooled / allowed to cool to enable handling of the resulting pyrolysed material. Then, the resulting pyrolysed material is optionally milled to provide the pyrolysed material with a d50 particle size of about 1 pm to about 25 pm, and more preferably about 8 pm to about 25 pm. An ideal d50 particle size is from about 8 pm to about 12 pm.

[0060] Optionally between steps (b) and (c) of the process of the present invention, a further step of carbonising and / or drying the treated composition from the end of step b) may occur. This may occur at a temperature from about 60 °C to about 950 °C.

[0061] Indeed, it may be beneficial to perform a drying step on the treated composition from the end of step b), prior to pyrolysis, to drive any volatile compounds away (e.g., by evaporation) from the mixture. More particularly, the drying step may be performed at a temperature of from about 60 to 200 °C, or it may be performed a temperature of from about 80 to 250 °C. It is preferred that any drying is performed under a non-oxidating atmosphere which may comprise one or more selected from nitrogen, carbon dioxide, and argon.

[0062] Alternatively, or additionally, it may be beneficial to perform a carbonisation step prior to pyrolysis, to oxidise and / or carbonise (i.e., carbonisation) the treated composition from the end of step b). More particularly, the carbonisation step in these instances may be performed at a temperature of from about 250 to about 950 °C, preferably from about 400 °C to about 850 °C, further preferably from about 450 °C to about 800 °C, ideally at about 600 °C. Oxygen is advantageously absent from the carbonisation process and is preferably replaced with one or a mixture of gases selected from nitrogen, carbon dioxide, and an inert gas such as argon.

[0063] Preferably, the carbonisation process does not comprise contacting the composition including hard carbon with one or more further carbon-based materials. In particular, it is preferred that the composition comprising hard carbon is not contacted with any one of graphite, graphene, or carbon nanotubes during the carbonisation process.

[0064] Very preferably, the carbonisation process does not comprise contacting the composition including hard carbon with carbon nanotubes at a temperature of about 400 °C. Following pyrolysis, the process of the present invention may also include step d) which comprises contacting the material comprising hard carbon with one or more carbon-containing materials at a temperature of up to 950 °C. Ideally, this will yield a material comprising hard carbon that has an outer surface comprising one or more carbonised materials chemically bonded on the hard carbon material. Preferably, the material comprising hard carbon has an open micropore specific surface area of 0 m2 / g to 5 m2 / g, as determined using nitrogen gas BET analysis.

[0065] Preferably, step d) is conducted after step c). Step d) may be conducted immediately following step c), or after any number of intermediate steps after step c).

[0066] Indeed, the net result from step d) will be that a chemical bond such as a covalent bond is formed between the hard carbon material and the one or more carbonised materials. Therefore, the one or more carbonised materials will be chemically bonded, preferably chemically deposited, on the surface of the hard carbon material. In a highly preferred embodiment, chemical vapour deposition is used in the contacting step d) of the present invention. Alternatively, the contacting step d) of the present invention include plasma- enhanced deposition, atomic-layer deposition and physical vapour deposition.

[0067] The carbon-containing materials from which the carbonised material is preferably derived comprise a vapour and / or a liquid and / or gaseous phase at, at least one temperature from about 950 °C or less. Preferably, a vapour and / or a liquid and / or a gaseous phase at, at least one temperature between about 200 °C or more to about 950 °C or less.

[0068] Step d) involves a temperature of up to 950 °C. A maximum temperature of 930 °C is preferred, a maximum temperature of 900 °C is highly preferred and a maximum temperature of 880 °C is particularly preferred. An ideal temperature is 800 °C. The process is performed over a period of 10 minutes to 2 hours. Oxygen is advantageously absent from the process of step d) and is preferably replaced with one or a mixture of gases selected from nitrogen, carbon dioxide, and an inert gas such as argon.

[0069] Suitable carbon-containing materials from which the carbonised material is derived, may be selected from one or more organic and / or hydrocarbon materials, for example alkanes, alkenes, alkynes or arenes, which may be straight chained, branched, cyclic, aliphatic or aromatic. Secondary carbon-containing materials which comprise one or more gaseous hydrocarbons with the general formula: CnH2n+2 where 1 < n < 10, are particularly preferred. Preferred total pressure, total flow rate and individual partial pressure and individual flow rate of the carbon-containing material are in the range of 10'6to 3x107Pa, 0.001 to 1000 L / min, 10'6to 3X107Pa and 0.001 to 1000 L / min respectively and further preferably in the range of 104to 106Pa, 0.01 to 100 L / min, 104to 106Pa and 0.01 to 100 L / min and highly preferably in the range 5X104to 5X105Pa, 0.1 to 10 L / min, 5X104to 5X105Pa and 0.1 to 10 L / min respectively.

[0070] The resulting formed material comprising hard carbon from the end of step d) is optionally dried, optionally using heating (preferably up to a maximum of 150 °C, further preferably 50 °C to 120 °C) and further optionally, using vacuum, prior to it being used as a negative electrode active material in an electrochemical cell.

[0071] Preferably, the resulting hard carbon material has a d50 particle size of about 1 pm to about 25 pm, and more preferably about 8 pm to about 25 pm. Preferably, the resulting hard carbon material has a d99 particle size of about 10 pm to about 50 pm, preferably about 20 pm to about 40 pm, and more preferably about 20 pm to about 30 pm.

[0072] In one embodiment, the resulting hard carbon material may have a specific surface area of 100 m2 / g or less. Preferably, the resulting hard carbon material has a specific surface area of less than 50 m2 / g, preferably less than 10 m2 / g, more preferably from 2 to 5 m2 / g, as determined using nitrogen gas BET analysis. Preferably, the resulting hard carbon material has an open micropore specific surface area of 0 m2 / g to 5 m2 / g, as determined using nitrogen gas BET analysis.

[0073] Preferably, the resulting hard carbon material contains less than 5%, or less than 2% by weight of inorganic impurities. Ideally, the resulting hard carbon material has from about 0 to about 0.6% by weight of inorganic impurities. Preferably, the resulting hard carbon material has less than 0.1 % by weight of inorganic impurities.

[0074] Preferably, the resulting hard carbon has a carbon content of 95% or greater, ideally 99% or greater.

[0075] As set out above, the composition including hard carbon in step a) is ideally provided from the disassembly of one or more electrochemical cells, and preferably from the disassembly of one or more sodium-ion cells and / or anode-free sodium cells. In one embodiment, the composition including hard carbon in step a) is provided from the disassembly of multiple electrochemical cells, e.g., ten or more electrochemical cells. In a preferred embodiment, the composition in step a) of the present invention is provided by: i) providing one or more electrochemical cells comprising one or more current collectors which include hard carbon, in which the hard carbon has been subject to one or more charge and / or discharge operations; and ii) separating the hard carbon from at least a portion of the one or more current collectors to form the composition including hard carbon.

[0076] The one or more electrochemical cells provided in step i) are optionally one or more metal-ion cells and / or one or more sodium-based electrochemical cells, preferably one or more sodium- ion cells, such as sodium-ion full cells. Alternatively, the one or more electrochemical cells are one or more anode-free sodium cells and / or one or more sodium metal cells.

[0077] Preferably, the one or more current collectors of step i) include hard carbon, in which the hard carbon has been subject to one or more charge and / or discharge operations in a sodium- based electrochemical cell. Ideally, the hard carbon has been subject to one or more charge and / or discharge operations in a sodium-ion cell. Alternatively or additionally, the hard carbon has been subjected to one or more charge and / or discharge operations in a sodium metal cell and / or an anode-free sodium cell.

[0078] Preferably, the one or more electrochemical cells may include one or more negative (anode) electrodes that includes a negative electrode current collector. Ideally, the negative electrode current collector includes hard carbon as a negative electrode active material formed on one or more surfaces of the negative electrode current collector. Such hard carbon has been subject to one or more charge and / or discharge operations because one or more electrochemical cell have been charged and / or discharged.

[0079] Preferably, the one or more electrochemical cells may also include one or more positive (cathode) electrodes that includes a positive electrode current collector. The positive electrode current collector may include one or more positive active materials formed on one or more surfaces of the negative electrode current collector. Such a positive electrode active material may have been subject to one or more charge and / or discharge operations because one or more electrochemical cell have been charged and / or discharged.

[0080] The negative electrode current collector, the positive electrode current collector, or both the positive electrode current collector and the negative electrode current collector can be made from aluminium or aluminium alloy (e.g., an alloy of aluminium and one or more of Mg, Mn, Cr, Zn, Si, Fe, and Ni).

[0081] Preferably, the one or more current collectors comprise aluminium. Preferably, the negative electrode current collector comprises an aluminium current collector. Highly preferably, both the negative electrode current collector and the positive electrode current collector comprise aluminium. Alternatively, copper, magnesium, carbon paper / foil / substrate and tin might also be used as current collector materials.

[0082] Preferably, the one or more electrochemical cells may also include one or more binding components, ideally one or more polymeric binding components. Such compounds are used to enable the negative electrode active material and / or positive electrode active material to bind to one or more surfaces of the negative and / or positive electrode current collector.

[0083] Typically, the polymeric binding component is selected from polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and carboxymethylcellulose (CMC).

[0084] Preferably, the one or more electrochemical cells may also include one or more separators (typically polymeric separators) and / or electrolyte compositions (typically non-aqueous electrolyte compositions).

[0085] The unwanted inorganic impurities (e.g. compounds of Ni, Fe, Mn, Mg, Ti, Al, Na and Cu) which are ideally removed in step b) of the present invention are typically derived from the one or more positive electrode active materials which may be included in the one or more electrochemical cells provided in step i).

[0086] Preferably, the one or more positive electrode active materials are sodium-containing positive electrode active materials which are ideally adapted to allow the insertion / removal of sodium ions during charge / discharge. Examples of these may include sodium transition metal oxides, polyanionic compounds (including fluorinated polyanionic compounds), Prussian blue analogue (PBA) compounds (such as Prussian White or Berlin Green), materials storing sodium via a conversion reaction, sodium transition metal fluorides, oxyfluorides, phosphates, sulfates, and silicates (and their fluorinated versions).

[0087] Preferred one or more positive electrode active materials include sodium transition metal oxides. Preferred sodium transition metal oxides are of the general formula: AI±6 M1VM2WM3X M4YM5ZO2-C wherein

[0088] A is one or more alkali metals selected from sodium, potassium and lithium;

[0089] M1comprises one or more redox active metals in oxidation state +2, preferably selected from the group consisting of nickel, copper, cobalt and manganese;

[0090] M2comprises a metal in oxidation state greater than 0 to less than or equal to +4;

[0091] M3comprises a metal in oxidation state +2;

[0092] M4comprises a metal in oxidation state greater than 0 to less than or equal to +4;

[0093] M5comprises a metal in oxidation state +3; wherein

[0094] 0 < b < 1 ;

[0095] V is > 0;

[0096] W is > 0;

[0097] X is > 0;

[0098] Y is > 0; at least one of W and Y is > 0

[0099] Z is > 0;

[0100] C is in the range 0 < c < 2 wherein V, W, X, Y, Z and C are chosen to maintain electrochemical neutrality.

[0101] Ideally, metal M2comprises one or more transition metals, and is preferably selected from manganese, titanium and zirconium; M3is preferably one or more selected from magnesium, calcium, copper, tin, zinc and cobalt; M4comprises one or more transition metals, preferably selected from manganese, titanium and zirconium; and M5is preferably one or more selected from aluminium, iron, cobalt, tin, molybdenum, chromium, vanadium, scandium and yttrium.

[0102] An especially preferred sodium transition metal oxide will have an 03 and / or a P2 structure and ideally, the especially preferred sodium transition metal oxide will comprise a compound with the general formula detailed above comprising an 03 phase. Preferably, the sodium transition metal oxide will comprise a compound with the general formula detailed above in a mixture of 03 and P2 phases. The ratio of O3:P2 phases may be from 1 to 99: 99 to 1.

[0103] As the composition in step i) comprises one or more current collectors it is necessary to separate the hard carbon from the one or more current collectors prior to step b) as required by step ii). In particular, in step ii), the hard carbon is separated from at least a portion (ideally substantially all) of the one or more current collectors to form the composition including hard carbon as used in step a) of the present invention. For the avoidance of any doubt, the hard carbon that is separated in step ii) is not solely hard carbon but is typically a mixture of components that includes hard carbon. This is because the hard carbon may include one or more inorganic impurities which have been derived from one or more positive electrode active materials; and / or one or more positive active materials themselves; and / or one or more binding components; and / or one or more electrolytes, in the one or more electrochemical cells provided in step i).

[0104] Ideally, the separation in step ii) comprises the use of one or more separation techniques selected from the group consisting of filtration, centrifugation, handpicking, magnetic deflection, flotation, fluidisation, crumbling, heat treatment, drying, chemical extraction, chemical treatment (including washing with an aqueous or non-aqueous solvent), flocculation, and combinations thereof.

[0105] A preferred heat treatment technique may include a carbonisation step which may be performed at a temperature of from about 250 to about 950 °C, preferably from about 400 °C to about 850 °C, ideally at about 600 °C. The purpose of such carbonisation is to induce the hard carbon to crumble away from the one or more current collectors due the decomposition of one or more binding components.

[0106] A preferred separation technique includes a heat treatment in combination with one or more of handpicking, crumbling, or filtration. Alternatively, a preferred separation technique may include filtration without any heat treatment, for instance using a coarse filter, to ideally remove substantially all of the portions of the one or more current collectors.

[0107] In the case where the one or more electrochemical cells additionally contain one or more negative electrode current collectors and / or one or more positive electrode current collectors and / or one or more separators, the hard carbon is also separated, ideally using the techniques discussed above, from at least a portion (ideally substantially all) of these components to form the composition including hard carbon as used in step a) of the present invention.

[0108] The resulting composition including hard carbon formed from step ii) as used in step a) is as set out above is typically impure and may contain 1 % or more by weight of inorganic impurities. For instance, the resulting composition including hard carbon formed from step ii) may contain inorganic impurities which have been derived from one or more positive electrode active materials; and / or one or more positive active materials themselves; and / or one or more binding components; and / or one or more electrolytes, in the one or more electrochemical cells provided in step i). These inorganic impurities maybe mixed in with the hard carbon provided in step i) and / or these inorganic impurities may be mixed in with the hard carbon as it is separated from at least a portion (ideally substantially all) of the one or more current collectors, at step ii).

[0109] In a preferred embodiment, step ii) further includes: iii) disassembling the one or more electrochemical cells provided in step i) to provide at least a portion of one or more current collectors which include hard carbon, in which the hard carbon has been subject to one or more charge and / or discharge operations in an electrochemical cell, preferably a sodium-based electrochemical cell; and iv) separating the hard carbon from the at least a portion of the one or more current collectors by using a heat treatment and / or a chemical treatment, to form the composition including hard carbon.

[0110] It will be appreciated that step iii) and iv) may take place concurrently. A preferred heat treatment technique in step iv) may include a carbonisation step which may be performed at a temperature of from about 250 to about 950 °C, preferably from about 400 °C to about 850 °C, ideally at about 600 °C. Preferably, step iv) therefore comprises contacting at least a portion of the one or more current collectors which include hard carbon with a temperature of from about 250 to about 950 °C, preferably from about 400 °C to about 850 °C, ideally at about 600 °C.

[0111] The purpose of such a heat treatment is to induce the hard carbon to crumble away from the one or more current collectors due the decomposition of one or more polymeric binding components.

[0112] A preferred chemical treatment in step iv) may include the use of one or more solvents to dissolve one or more polymeric binding components and thus induce the hard carbon to crumble away from the one or more current collectors. Step iv) therefore comprises contacting at least a portion of the one or more current collectors which include hard carbon with one or more solvents. Ideally, the one or more solvents dissolve the one or more polymeric binding compounds which enable the hard carbon to bind to one or more surfaces of the one or more current collectors.

[0113] Preferably, the one or more solvents used in step iv) include water (preferably deionised water). Preferably, the one or more solvents (ideally, water) are hot. A suitable temperature may be up to a maximum of 150 °C, further preferably 50 °C to 100 °C, most preferably 70 °C to 80 °C. Step iv) is preferably performed over a period of time of 30 minutes or more, optionally 1 hour or more, and preferably for around 1 hour.

[0114] A preferred chemical treatment in step iv) may also the use of agitation (for example by stirring) to induce the hard carbon to break away from the one or more surfaces of the one or more current collectors. Depending upon the composition of the binding component, in order for the hard carbon to successfully break away from the one or more surfaces of the one or more current collectors, step iv) may also require the use of pulverisation to enable an aqueous dispersion to form. In particular, when the binding component is water soluble, it is preferable that step iv) comprises a pulverisation treatment.

[0115] Preferably, the composition including hard carbon is recovered at the end of a heat treatment and / or a chemical treatment in step iv) by using a technique selected from one or more of handpicking, crumbling, or filtration. Ideally, filtration is used.

[0116] For the avoidance of any doubt, the hard carbon that is separated and eventually recovered in step iv) is not solely hard carbon but is typically a mixture of components that includes hard carbon. This is because the hard carbon may include one or more impurities which have been derived from one or more positive electrode active materials; and / or the one or more positive electrode active materials themselves; and / or one or more binding components; and / or one or more electrolytes, in the one or more electrochemical cells provided earlier in the process.

[0117] In a more preferred embodiment, because step iv) comprises a chemical treatment and / or a heat treatment it is helpful to induce the hard carbon away from the one or more current collectors by increasing the contact area for chemical treatment and / or heat treatment.

[0118] Thus, in a more preferred embodiment, step ii) further includes: iii) disassembling the one or more electrochemical cells provided in step i) to provide one or more current collectors which include hard carbon, in which the hard carbon has been subject to one or more charge and / or discharge operations in an electrochemical cell, preferably a sodium-based electrochemical cell; iv) reducing, into one or more portions, the one or more current collectors which include hard carbon; v) contacting the product of step iv) with a heat treatment and / or a chemical treatment; and vi) separating the hard carbon from the product of step v) to form the composition including hard carbon.

[0119] It will be appreciated that step iii) and iv) may take place concurrently. The preferred features of the heat treatment and / or a chemical treatment in step v) may be further defined in the same way as in step iv) with respect to the embodiment above.

[0120] Preferably, step iii) and iv) take place concurrently and step v) comprises a chemical treatment.

[0121] It will be appreciated that the word “contacting” in step v) has the same meaning as “subjecting”. Therefore, it would be equally appropriate to define step v) as “subjecting the product of step iv) to a heat treatment and / or a chemical treatment”.

[0122] Step iv) in this embodiment preferably involves any suitable process or the use of any suitable apparatus known to the skilled person, to reduce, into one or more portions, the one or more current collectors. Preferably, such processes include may include shredding, cutting, chopping, grinding, grating, pulverising, chewing, macerating etc. Such processes may be performed manually or with a suitable machine known to the skilled person to perform such processes (e.g., a shredder).

[0123] The term ‘portion’ as used herein is intended to mean a part of a whole. Therefore, the term as used herein is not intended to apply any specific size to the ‘portion’ because the one or more current collectors could be ground into a powder or pulverised into fine particles. Alternatively, the one or more current collectors may be simply cut or chopped in half. Similarly, the one or more portions may be of different sizes, or they may each be of the same size.

[0124] Ideally, however, the one or more portions are each approximately from about 0.5 to about 20 cm2in size, preferably from about 1 to about 15 cm2in size.

[0125] Step vi) in this embodiment is required to separate the hard carbon from the product of step v) to form the composition including hard carbon. However, for the avoidance of any doubt, the hard carbon that is separated in step vi) is not solely hard carbon but is typically a mixture of components that includes hard carbon. This is because the hard carbon may include one or more impurities which have been derived from one or more positive electrode active materials; and / or the one or more positive electrode active materials themselves; and / or one or more binding components; and / or one or more electrolytes, in the one or more electrochemical cells provided earlier in the process.

[0126] Step vi) may use a separation technique selected from one or more of handpicking, crumbling, classification (e.g. automated), flotation, magnetic deflection or filtration. Ideally, when using a chemical treatment in step v), the separation in step vi) is performed by filtration, for instance using a coarse filter and / or vacuum filtration.

[0127] Ideally, when using a chemical treatment in step v), the separation in step vi) is performed in two steps, in which case optionally the first step comprises separating the hard carbon from substantially all of the portions of the one or more current collectors, and in which case optionally the second step comprises separating the hard carbon from the one or more solvents used in step v). In both the first and the second step, the separation may be performed by filtration. However, in the first step a coarse filter may be used, whereas in the second step a finer filter maybe used, and ideally the second step may comprise the use of vacuum filtration.

[0128] Of course, in the case where the one or more electrochemical cells additionally contain one or more negative electrode current collectors and / or one or more positive electrode current collectors and / or one or more separators, the hard carbon may also be separated in step vi) from at least a portion (ideally substantially all) of these components to form the composition including hard carbon as used in step a) of the present invention.

[0129] The resulting composition from the end of step vi) is optionally washed with one or more solvents such as water (preferably deionised water), and then optionally dried, optionally using heating ideally for 6 hours or more, preferably 12 hours or more.

[0130] In some cases, it may also be necessary to treat the resulting composition from the end of step vi) with one or more further solvents either prior to, or after such optional washing and / or drying steps. This may be useful to remove any remaining polymeric binding compounds that are present in the resulting composition from the end of step vi). This is particularly the case when step v) comprises a chemical treatment. A remaining polymeric binding compound may include polyvinylidene fluoride (PVDF).

[0131] In this case, this optional step may include contacting the product of step vi) with one or more solvents to ideally enable the removal of PVDF. Preferably, the one or more solvents includes dimethyl sulfoxide (DMSO), which is ideally heated to a temperature of between 30 °C and 120 °C, ideally between 60 °C and 70 °C. Such a step is preferably performed over a period of 1 hour or more. Then, the resulting solvent treated material is optionally filtered and dried, optionally using heating (preferably up to a maximum of 150 °C, further preferably 50 °C to 120 °C) and further optionally under reduced pressure (preferably a dynamic vacuum), prior to it being used in step a) of the present invention.

[0132] In some cases, following step vi), the resulting composition including hard carbon is optionally milled to provide a d50 particle size of about 1 pm to about 25 pm, and more preferably about 8 pm to about 25 pm. An ideal d50 particle size is from about 8 pm to about 12 pm. The milled composition including hard carbon is then ideally used in step (a) of the present invention.

[0133] The present invention also provides, in another aspect, a material comprising hard carbon obtainable by the process as disclosed herein.

[0134] The present invention also provides, in another aspect, the use of a material comprising hard carbon as disclosed herein, in an electrochemical cell. Preferably, this aspect provides the use of a material comprising hard carbon as disclosed herein in a sodium-based electrochemical cell. Ideally, this aspect provides the use of a material comprising hard carbon as disclosed herein in a sodium-ion cell. Alternatively, this aspect provides the use of a material comprising hard carbon as disclosed herein in an anode-free sodium cell and / or a sodium metal cell.

[0135] The present invention also provides, in another aspect, an electrode comprising a material comprising hard carbon as disclosed herein.

[0136] The present invention also provides, in another aspect, an electrochemical cell comprising a material comprising hard carbon as disclosed herein. Preferably, the electrochemical cell is sodium-based electrochemical cell comprising a material comprising hard carbon as disclosed herein.

[0137] Preferably, the electrochemical cell is selected from the group consisting of a sodium metal cell, a sodium-ion cell, and an anode-free sodium cell. Ideally, the electrochemical cell is a sodium-ion cell.

[0138] The present invention also provides, in another aspect, the use of a composition including hard carbon to prepare a material comprising hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in an electrochemical cell. Preferably, the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell.

[0139] Preferably, the material comprising hard carbon has a specific surface area of 100 m2 / g or less. Preferably, the material comprising hard carbon has a specific surface area of less than 50 m2 / g, preferably around 10 m2 / g, more preferably from 2 to 5 m2 / g. All specific surface area values given in the present application have been determined by BET N2 analysis.

[0140] Preferably, the material comprising hard carbon has an open micropore specific surface area of 0 m2 / g to 5 m2 / g, as determined using nitrogen gas BET analysis.

[0141] Preferably, the material comprising hard carbon has less than 2% by weight of inorganic impurities.

[0142] The present invention also provides, in another aspect, the use, in an electrochemical cell, of a material comprising hard carbon, in which the material comprising hard carbon is made using a composition including hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in an electrochemical cell. Preferably, the material comprising hard carbon is used in a sodium- based electrochemical cell such as a sodium-ion cell. Preferably, the hard carbon in the composition including hard carbon has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell.

[0143] Preferably, the material comprising hard carbon is used as a negative electrode active material in an electrochemical cell, ideally in a sodium-based electrochemical cell such as a sodium- ion cell.

[0144] Preferably, the material comprising hard carbon has a specific surface area of 100 m2 / g or less. Preferably, the material comprising hard carbon has a specific surface area of less than 50 m2 / g, preferably around 10 m2 / g, more preferably from 2 to 5 m2 / g. All specific surface area values given in the present application have been determined by BET N2 analysis.

[0145] Preferably, the material comprising hard carbon has an open micropore specific surface area of 0 m2 / g to 5 m2 / g, as determined using nitrogen gas BET analysis. Preferably, the material comprising hard carbon has less than 2% by weight of inorganic impurities.

[0146] In one embodiment, the hard carbon included in said composition has been subject to four or more charge and / or discharge operations in an electrochemical cell. In another embodiment, the hard carbon included in said composition has been subject to twenty-five or more charge and / or discharge operations in an electrochemical cell.

[0147] In some embodiments, the hard carbon included in said composition has been subject to one hundred or more charge and / or discharge operations in an electrochemical cell. In another embodiment, the hard carbon has been subject to five hundred or more charge and / or discharge operations in an electrochemical cell.

[0148] Preferably, the electrochemical cell is a metal-ion cell such as a sodium-ion cell, ideally a sodium-ion full cell. Preferably, the resulting material comprising hard carbon is a negative electrode active material, ideally for use in an electrochemical cell such as a sodium-ion cell.

[0149] The present invention also, provides, in another aspect, the use, as a negative electrode active material of a material comprising hard carbon, in which the material comprising hard carbon is made using a composition including hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in an electrochemical cell. Preferably, the hard carbon in the composition including hard carbon has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell.

[0150] Preferably, the electrochemical cell is a metal-ion cell such as a sodium-ion cell, ideally a sodium-ion full cell.

[0151] Preferably, the material comprising hard carbon is used as a negative electrode active material in an electrochemical cell, ideally in a sodium-based electrochemical cell such as a sodium- ion cell.

[0152] Preferably, the material comprising hard carbon has a specific surface area of 100 m2 / g or less. Preferably, the material comprising hard carbon has a specific surface area of less than 50 m2 / g, preferably around 10 m2 / g, more preferably from 2 to 5 m2 / g. All specific surface area values given in the present application have been determined by BET N2 analysis. Preferably, the material comprising hard carbon has an open micropore specific surface area of 0 m2 / g to 5 m2 / g, as determined using nitrogen gas BET analysis.

[0153] Preferably, the material comprising hard carbon has less than 2% by weight of inorganic impurities.

[0154] The present invention also provides, in another aspect, a metal-ion cell comprising one or more negative electrodes that comprises at least one material comprising hard carbon, in which the material comprising hard carbon is made using a composition including hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in an electrochemical cell.

[0155] The metal-ion cell will also ideally comprise one or more positive electrodes which preferably comprise one or more positive electrode active materials which preferably include sodium- containing positive electrode active materials. Ideally, such materials are adapted to allow the insertion / removal of sodium ions during charge / discharge. Examples of these may include sodium transition metal oxides, polyanionic compounds (including fluorinated polyanionic compounds), Prussian blue analogue (PBA) compounds (such as Prussian White or Berlin Green), materials storing sodium via a conversion reaction, sodium transition metal fluorides, oxyfluorides, phosphates, sulfates, and silicates (and their fluorinated versions).

[0156] Preferred one or more positive electrode active materials include sodium transition metal oxides. Preferred sodium transition metal oxides are of the general formula as set out above.

[0157] BRIEF DESCRIPTION OF THE DRAWINGS

[0158] The present invention will now be described with reference to the following figures in which:

[0159] Figure 1A: shows a scanning electron micrograph of the black mass recovered from a shredded 30 Ah sodium-ion cell after 633 cycles according to Example 4.

[0160] Figure 1 B: shows a scaled up scanning electron micrograph of the black mass recovered from a shredded 30 Ah sodium-ion cell after 633 cycles according to Example 4. Figure 1C: shows a scanning electron micrograph of the post-demineralisation hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles.

[0161] Figure 1 D: shows a scaled up scanning electron micrograph of the post-demineralisation hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles.

[0162] Figure 1 E: shows a scanning electron micrograph of the post-carbonisation hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles.

[0163] Figure 1 F: shows a scaled up scanning electron micrograph of the post-carbonisation hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles.

[0164] Figure 1G: shows a scanning electron micrograph of the post-pyrolysis hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles.

[0165] Figure 1 H: shows a scaled up scanning electron micrograph of the post-pyrolysis hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles.

[0166] Figure 11: shows a scanning electron micrograph of the post-CVD (chemical vapour deposition) hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles.

[0167] Figure 1J: shows a scaled up scanning electron micrograph of the post-CVD (chemical vapour deposition) hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles.

[0168] Figure 2A: shows a scanning electron micrograph of the post-demineralisation hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles, after boiling in 2.0M HCI acid solution for 1 hour. Figure 2B: shows a scaled up scanning electron micrograph of the post-demineralisation hard carbon product of Example 4, recycled from the black mass of a shredded 30 Ah sodium-ion cell after 633 cycles, after boiling in 2.0M HCI acid solution for 1 hour.

[0169] Figure 3: shows an X-Ray Diffraction (XRD) pattern of hard carbon samples from various stages of the process of Example 4, recycled from a shredded 30 Ah sodium- ion cell after 633 cycles.

[0170] Figure 4: shows the weight loss upon oxidation under synthetic air in a thermogravimetric analyser (TGA) of hard carbon samples from various stages of the process of Example 4, recycled from a shredded 30 Ah sodium-ion cell after 633 cycles.

[0171] Figure 5A: shows the full-cell electrochemical performance on the 1stand 4thcycles of a control sodium-ion cell having a negative electrode active material wherein the negative electrode active material comprises Kuranode Type 1 hard carbon.

[0172] Figure 5B: shows the full-cell electrochemical performance on the 1stand 4thcycles of a sodium-ion cell having a negative active material according to the present invention wherein the negative electrode active material comprises hard carbon recycled according to Example 2, from a cell which has been subject to 4 formation cycles only.

[0173] Figure 5C: shows the full-cell electrochemical performance on the 1stand 4thcycles of a sodium-ion cell having a negative active material according to the present invention wherein the negative electrode active material comprises hard carbon recycled according to Example 3, removed manually from a 30 Ah sodium-ion cell after 594 cycles.

[0174] Figure 5D: shows the full-cell electrochemical performance on the 1stand 4thcycles of a sodium-ion cell having a negative active material according to the present invention wherein the negative electrode active material comprises hard carbon recycled according to Example 4, from the shredded black mass of a 30 Ah sodium-ion cell after 633 cycles.

[0175] Figure 6A: shows the cycling performance of a sodium-ion cell having a negative active material according to the present invention which has been subject to 550 cycles in said sodium-ion cell wherein the negative electrode active material comprises hard carbon recycled according to Example 4, from the shredded black mass of a 30 Ah sodium-ion cell after 633 cycles.

[0176] Figure 6B: shows the cycling performance of a sister cell to that of Figure 6A having a negative active material according to the present invention which has been subject to 487 cycles, wherein the negative electrode active material comprises hard carbon recycled according to Example 4, from the shredded black mass of a 30 Ah sodium-ion cell after 633 cycles.

[0177] Figure 7: shows in the top row scanning electron micrographs of hard carbon prepared according to Example 2 at various stages of processing, and below the top row shows corresponding energy dispersive X-ray micrographs, showing the presence or absence of certain elements after each processing step.

[0178] Figure 8: shows in the top row scanning electron micrographs of hard carbon prepared according to Example 3 at various stages of processing, and below the top row shows corresponding energy dispersive X-ray micrographs, showing the presence or absence of certain elements after each processing step.

[0179] Figure 9: shows in the top row scanning electron micrographs of hard carbon prepared according to Example 4 at various stages of processing, and below the top row shows corresponding energy dispersive X-ray micrographs, showing the presence or absence of certain elements after each processing step.

[0180] EXAMPLES

[0181] Hard carbon materials were made according to the process of the present invention as detailed below in Examples 2 to 4. Comparative Example 1 discloses the use of an anode from “virgin” hard carbon (i.e., hard carbon that has not previously been subject to one or more charge and / or discharge operations cycled in an electrochemical cell such as a sodium-ion cell).

[0182] Abbreviations used:

[0183] EC = Ethylene carbonate, DEC = Diethyl carbonate, PC = Propylene carbonate, PCS = 1 ,3- propanediolcyclic sulfate, P123 = Poloxamer (Pluronic) P123, T SB = Tris(trimethylsilyl) borate. General Procedure to Make a Hard Carbon Na-ion Cell

[0184] Sodium-ion cells were fabricated using a mixed phase O3 / P2 oxide cathode, a hard carbon anode, and an electrolyte composition. Aluminium tabs were connected to each of the electrodes and the cell was encased in a polymer-coated aluminium pouch.

[0185] General procedure for cathode preparation

[0186] The cathode electrode was prepared by solvent-casting a slurry of an active material, conductive carbon, binder and solvent. In all examples, the cathode comprised O3 / P2 Nickelate oxide material as cathode active material, polyvinylidene fluoride (PVDF) as binder and C65 as a conductive additive, in 92:03:05 wt. ratio, coated on carbon-coated aluminium foil current collectors.

[0187] Anode Preparation

[0188] The hard carbon negative (anode) electrode was prepared by solvent-casting a slurry of a hard carbon active material, conductive carbon, binder and solvent, by the doctor blade method. The conductive carbon used was C65 (Imerys). A mixture of sodium carboxymethylcellulose (CMC) (Aquaion™ AQU D-5283 from Ashland) and styrene-butadiene rubber (SBR) (BM-451 B from Zeon Europe Gmbh), or polyvinylidene fluoride (PVDF) was used as the binder, and water was employed as the solvent. The slurry was then cast onto carbon-coated aluminium foil and dried at about 120°C under vacuum.

[0189] General procedure for electrolyte preparation

[0190] The electrolyte was prepared as follows. Appropriate amounts of solvents for a desired solvent system were mixed together in a glass vial in the desired weight ratio. Where appropriate, one or more performance additives (e.g., one or more surfactants, one or more boron-containing compounds, one or more sulfur-containing compounds, and combinations thereof) were also added to prepare the solvent system. To dry the solvent-mix, activated 4 A molecular sieves were added and the solvent mix was allowed to dry at least for 18 h.

[0191] In a separate glass bottle with a magnetic pellet, the required weight of NaPFe salt was added, followed by the required quantity of the required solvent mix. The salts and solvent mix were magnetically stirred at least for 18 h, whereupon the resultant electrolyte was ready for use. Please note that the wt.% of the various solvents and / or performance additives are mentioned with respect to the total solvent system weight (they do not account for the weight(s) of the one or more sodium containing salts).

[0192] Cell Construction

[0193] After coating the cathode and anode, they were stamped at the desired dimensions. The coating weights of the cathode and anode have been mentioned as a GSM value (grams per square metres) and refer to the areal density of the layer of electrode active material.

[0194] The separator used was a typical polyolefinic separator, such as Celgard 2500. The cathode / separator / anode assembly was placed within a pouch with two Al-based connecting tabs serving as the terminals, inside the glove box, and filled with the appropriate amount of liquid electrolyte. These pouch cells were then sealed in the glove box and brought out for cell testing.

[0195] Cell testing

[0196] The cells were tested using Constant Current (Galvanostatic) Cycling techniques. Generally speaking, the cells were first charged via the constant current (CC) mode to a pre-defined maximum voltage limit. Afterwards, the cell was made to undergo a constant voltage (CV) step at that maximum voltage limit, to either a pre-defined time or this CV step was made to last until the current dropped to a pre-defined value, as indicated in the examples. In some examples, the cells were made to undergo a series of different CC-modes to different voltage values, before undergoing the CV step only at the maximum voltage value. The discharge process was either conducted at CC-mode to the lower cut-off voltage, or in constant power (CP) mode, as indicated.

[0197] A commercial battery cycler from MTI Inc. (Richmond, CA, USA) or Maccor (Tulsa, OK, USA) was used. On charge, alkali ions are inserted into the carbon-containing anode material. During discharge, alkali ions are extracted from the and re-inserted into the cathode active material.

[0198] All cells were subject to cycling experiments at charge rates such as ±1C (this can also be stated as ±C / 1), ±C / 5 or ±C / 10 (or other rates, such as ±C / 50, as mentioned for each experiment). All cells were rested for at least 4 - 24h prior to formation cycles and postformation cycles.

[0199] Comparative Example 1 : ‘Virgin’ commercial hard carbon (Kuraray’s Kuranode Type 1)

[0200] Negative electrodes were constructed according to general procedure above, with the slurry comprising 95 wt.% hard carbon active material (Kuranode Type 1 , supplied by Kureha), 3.5 wt.% binder, and 1.5 wt.% conductive additive material. The electrode coatweight was 50-60 g / m2. Indeed, the hard carbon that is used as negative electrode active material in this example is ‘virgin’ because it has not been previously subject to one or more charge and / or discharge operations in an electrochemical cell.

[0201] The electrodes using the ‘virgin’ commercial hard carbon were then tested in half-cells and full-cells, using cathodes according to the general procedure above, according to the following procedure:

[0202] Half-cells were tested using Swagelok cells in the voltage window of 0.001-2.0 V at ±C / 20, followed by one cycle of forced sodium plating / stripping to establish the true specific capacity of the hard carbon. The electrolyte in half-cells was 1 M NaPF6 in 1 :2:1 PC: DEC: EC with 1 wt.% PCS and 1wt.% P123.

[0203] Full-cells were tested in A7-size double-layer pouch cells and the electrolyte used was 1M NaPF6 in PC with 20 wt.% DEC, 2 wt.% PCS, 1 wt.% P123 and 1 wt.% TMSB.

[0204] The full-cells were formed in the voltage window of 1 .0-4.1 V at ±C / 10 for 4 cycle and the postformation cycling was in the voltage window of 1.8-4.1 V at ±C / 5, ±C / 3,+C / 3 & -1C each for 5 cycles and ±1C for 1000 cycles with check points at ±C / 5 for 5 cycles after every 100 cycles.

[0205] Example 2: Hard carbon recycled from anode electrode after 4x formation cycles

[0206] A 30 Ah (nominal) pouch cell was constructed from cells made according to the general procedure above, with anode electrodes comprising 95 wt.% hard carbon active material (Kuranode Type 1 , supplied by Kureha), 3.5 wt.% binder, and 1.5 wt.% conductive additive material. The electrolyte used was 1M NaPF6 in PC with 20 wt.% DEC, 2 wt.% PCS, 1 wt.% P123 and 1 wt.% TMSB. The 30 Ah pouch cell was then formed between 1.0-4.0 V for 4 cycles at ±C / 10 before being shorted and dismantled. The separated anodes were then dried in a fume cabinet for at least 12 hours. The dried anode electrodes comprising 95% hard carbon were cut into small pieces of 10-12 cm2and carbonised at 600°C for 2 hours under argon.

[0207] After carbonisation, the coating comprising the negative electrode active material crumbled off the aluminium current collector due to the decomposition of the binder. The current collector was then separated manually, and the recovered anode coating powder was ground using a mortar and pestle.

[0208] The powder was then characterised using an X-ray diffractometer and presence of crystalline inorganic impurities was confirmed. To exclude any unwanted crystalline inorganic impurities (such as elements Na, P, F which have originated from an electrolyte) from the hard carbon, it was demineralised according to the following procedure. 30g of the recovered hard carbon was digested in 350 mL of boiling 2.0 M hydrochloric acid solution (HCI) for 1 h. The demineralised hard carbon was filtered and rinsed using 5L of boiling deionised water until the pH of the filtrate reached 5.5-6. The washed powder was then dried for at least 12 hours.

[0209] The demineralised powder was pyrolysed at 1200 °C for 1 hour under argon. The pyrolysed powder was then analysed using a Malvern Mastersizer 3000 and the particle size distribution was obtained. The powder was then optionally milled using a Retsch PM 100 planetary ball mill using 80 mL Agate jar and ten 10 mm agate balls at 300 rpm for 30 minutes until the median particle size of 8-12 microns was achieved.

[0210] The powder was then surface treated in a chemical vapour deposition reactor at 800-950 °C under argon with an alkane of the formula CnH2n+2 where 1 < n <10). The powder was then vacuum-dried at 120 °C for least for 12 hours and was then coated on an aluminium current collector using the with anode electrode formulation: 95 wt.% active material; 3.5 wt.% binder; and 1.5 wt.% conductive additive. The electrode coat weight was 50-60 g / m2.

[0211] The electrodes were then tested in half-cells and full-cells according to the following procedure:

[0212] Half-cells were tested using Swagelok cells in the voltage window of 0.001-2.0 V at ±C / 20, followed by one cycle of forced sodium plating / stripping to establish the true specific capacity of the hard carbon. The electrolyte in half-cells was 1 M NaPF6 in 1 :2:1 PC:DEC:EC with 1 wt.% PCS and 1wt.% P123. Full-cells were tested in A7-size double-layer pouch cells and the electrolyte used was 1M NaPF6 in PC with 20 wt.% DEC, 2 wt.% PCS, 1 wt.% P123 and 1 wt.% TMSB. The full-cells were formed in the voltage window of 1.0-4.1 V at ±C / 10 for 4 cycle and the post-formation cycling was in the voltage window of 1.8-4.1 V at ±C / 5, ±C / 3,+C / 3 & -1C each for 5 cycles and ±1C for 1000 cycles with check points at ±C / 5 for 5 cycles after every 100 cycles.

[0213] Example 3: Hard carbon recycled from manually delaminated anode electrodes from a 30 Ah pouch cell after 594 cycles

[0214] A 30 Ah pouch (nominal) cell was constructed from cells made according to the general procedure above, with anode electrodes comprising 95 wt.% hard carbon active material (Kuranode Type 1 , supplied by Kureha), 3.5 wt.% binder, and 1.5 wt.% conductive additive material. The electrolyte used was 1M NaPF6 in PC with 20 wt.% DEC, 2 wt.% PCS, 1 wt.% P123 and 1 wt.% TMSB. Anode utilisation was 180-190 mAh / g. The 30 Ah pouch cell was then formed between 1.0-4.0 V for 4 cycles at ±C / 10, with a first-cycle Coulombic efficiency of 87.5% and cycled between 2.0-4.2 V at C / 2 constant current (CC) charge with a constant voltage (CV) to the tenth of the current, followed by 35 W constant powder (CP) discharge for 594 cycles at ±1C before being shorted and dismantled.

[0215] Anode electrodes were then removed. After separation from the cell, the anode electrodes were submerged in deionised water to remove any residual sodium that might have been left in the anode electrodes. The electrodes were then dried at 80°C. The dried anode electrodes were cut into small pieces of 10-12 cm2and carbonised at 600°C for 2 hours under argon.

[0216] After carbonisation, the coating comprising the negative electrode active material crumbled off the aluminium current collector due to the decomposition of the binder. The current collector was then separated manually, and the recovered anode coating powder was ground using a mortar and pestle.

[0217] The powder was then characterised using an X-ray diffractometer and presence of crystalline inorganic impurities was confirmed. To exclude the unwanted crystalline inorganic impurities from the hard carbon, it was demineralised according to the following procedure. The recovered hard carbon was digested in boiling 2.0 M hydrochloric acid solution (HCI) for 1 h. The demineralised hard carbon was filtered and rinsed using boiling deionised water until the pH of the filtrate reaches 5.5-6. The washed powder was then dried for at least 12 hours. The demineralised powder was pyrolysed at 1200 °C for 1 hour under argon. The powder was then analysed using a Malvern Mastersizer 3000 and the particle size distribution was obtained. The powder was then optionally milled using a Retsch PM 100 planetary ball mill using 80 mL Agate jar and ten 10 mm agate balls at 300 rpm for 30 minutes until the median particle size of 8-12 microns was achieved.

[0218] The powder was then surface treated in a chemical vapour deposition reactor at 800-950 °C under argon with an alkane of the formula CnH2n+2 where 1 < n <10). The powder was then vacuum-dried at 120 °C for least for 12 hours and was then coated on an aluminium current collector using the with anode electrode formulation: 95 wt.% active material; 3.5 wt.% binder; and 1.5 wt.% conductive additive. The electrode coatweight was 50-60 g / m2.

[0219] The electrodes were then tested in half-cells and full-cells according to the following procedure:

[0220] Half-cells were tested using Swagelok cells in the voltage window of 0.001-2.0 V at ±C / 20, followed by one cycle of forced sodium plating / stripping to establish the true specific capacity of the hard carbon. The electrolyte in half-cells was 1 M NaPF6 in 1 :2:1 PC:DEC:EC with 1 wt.% PCS and 1wt.% P123.

[0221] Full-cells were tested in A7-size double-layer pouch cells and the electrolyte used was 1M NaPF6 in PC with 20 wt.% DEC, 2 wt.% PCS, 1 wt.% P123 and 1 wt.% TMSB. The full-cells were formed in the voltage window of 1.0-4.1 V at ±C / 10 for 4 cycle and the post-formation cycling was in the voltage window of 1.8-4.1 V at ±C / 5, ±C / 3,+C / 3 & -1C each for 5 cycles and ±1C for 1000 cycles with check points at ±C / 5 for 5 cycles after every 100 cycles.

[0222] Example 4: Hard carbon recycled from black mass of a shredded 30Ah pouch cell after 633 cycles.

[0223] A 30 Ah pouch (nominal) cell was constructed from cells made according to the general procedure above, with anode electrodes comprising 95 wt.% hard carbon active material (Kuranode Type 1 , supplied by Kureha), 3.5 wt.% binder, and 1.5 wt.% conductive additive material. The electrolyte used was 1M NaPF6 in PC with 20 wt.% DEC, 2 wt.% PCS, 1 wt.% P123 and 1 wt.% TMSB. Anode utilisation was 180-190 mAh / g. The 30 Ah pouch cell was then formed between 1.0-4.0 V for 4 cycles at ±C / 10, with a first-cycle Coulombic efficiency of 87.4% and cycled between 2.0-4.2 V at C / 2 constant current (CC) charge with a constant voltage (CV) to the tenth of the current, followed by 35 W constant powder (CP) discharge for 587 cycles before being shorted and dismantled. In this example, the positive electrode (e.g. the cathode electrode) contained at least the following elements present in the positive electrode active material: Ni, Mn, Mg, Ti and Al. Alternative positive electrode active materials incorporating Cu and Fe have also been studied.

[0224] A stack of 10 x anode electrodes, 10 x cathode electrodes, separators and pouch material was randomly extracted from the dismantled cell and was shredded into pieces, approximately 1-3 cm2in size, and was decanted into a beaker, placed on hotplate containing deionised water with a temperature of 70-80°C.

[0225] The mixture was then agitated vigorously for 1 hour resulting in delamination of the coating from both anode and cathode electrodes from the aluminum anode and cathode current collectors. The cathode coating was fully pulverized and dispersed in the deionised water while the cathode coating only delaminated from the current collector without pulverising and dispersing due to the PVDF binder of the cathode electrodes not being water-soluble.

[0226] Insoluble and non-dispersible components such as the separator and stripped aluminum current collector were filtered from mixture using a coarse filter, leaving a black mass. Then the black mass was vacuum filtered, rinsed in hot deionised water trice and dried for at least 12 hours. To remove the residual PVDF, 40 g of the black mass was then dispersed in 150 ml of Dimethylsulfoxide (DMSO) and was stirred for 1 hour at 60-70 °C. It was then vacuum filtered to remove the solvent and was dried and ground using pestle and mortar.

[0227] The powder was then analysed using a Malvern Mastersizer 3000 and the particle size distribution was obtained. The powder was then optionally milled using a Retsch PM 100 planetary ball mill using 80 mL Agate jar and ten 10 mm agate balls at 300 rpm for 30 minutes until the median particle size of 8-12 microns was achieved.

[0228] To demineralise and isolate the hard carbon, 30 g of black mass was dispersed in 350 ml of boiling 2.0 M HCI for 2 hours. The resulting demineralised hard carbon was then vacuum filtered and rinsed with 5 L of boiling deionised water until the pH of the filtrate reached 5.5-6. The obtained demineralised hard carbon was then dried and ground into a fine powder.

[0229] The demineralised hard carbon was then carbonised at 600°C for 2 hours under argon. The resulting carbonised hard carbon powder was then pyrolysed at 1200 °C for 1 hour under argon. The pyrolysed powder was then analysed using a Malvern Mastersizer 3000 and the particle size distribution was obtained.

[0230] The powder was then surface treated in a chemical vapour deposition (CVD) reactor at 800- 950 °C under a flow of argon with an alkane of the formula CnH2n+2 where 1 < n < 10) for 30 minutes. The treated powder was then vacuum-dried at 120 °C for least for 12 hours and was then coated on an aluminium current collector according to the general procedure above with the anode electrode formulation: 95 wt.% active material; 3.5 wt.% binder; and 1.5 wt.% conductive additive. The electrode coatweight was 50-60 g / m2.

[0231] The electrodes were then tested in half-cells and full-cells according to the following procedure:

[0232] Half-cells were tested using Swagelok cells in the voltage window of 0.001-2.0 V at ±C / 20, followed by one cycle of forced sodium plating / stripping to establish the true specific capacity of the hard carbon. The electrolyte in half-cells was 1 M NaPF6 in 1 :2:1 PC:DEC:EC with 1 wt.% PCS and 1wt.% P123.

[0233] Full-cells were tested in A7-size double-layer pouch cells and the electrolyte used was 1M NaPF6 in PC with 20 wt.% DEC, 2 wt.% PCS, 1 wt.% P123 and 1 wt.% TMSB. The full-cells were formed in the voltage window of 1.0-4.1 V at ±C / 10 for 4 cycle and the post-formation cycling was in the voltage window of 1.8-4.1 V at ±C / 5, ±C / 3,+C / 3 & -1C each for 5 cycles and ±1C for 1000 cycles with check points at ±C / 5 for 5 cycles after every 100 cycles.

[0234] Results

[0235] The inorganic impurity content of the materials of Example 4 was estimated at regular intervals throughout the processing steps by calcining portions of (ca. 100 mg) dried powder at 1000°C under atmospheric air and weighing the residual ash. The results of these measurements are set out in Table 1 below and are compared to the ash content of commercially available hard carbon. Table 1

[0236] These data show that treatment of the black mass according to Example 4 yields a hard carbon material after chemical vapour deposition with a lower ash content (and thus a lower inorganic impurity content) than commercially available hard carbon.

[0237] Figures 1A-J show the reduction in impurities as the black mass of Example 4 is subjected to further processing steps. That is, beginning with the black mass recovered from a shredded cell (Figures 1A and 1 B); post-demineralisation (Figures 1C and 1 D); post-carbonisation (Figures 1 E and 1 F); post-pyrolysis (Figures 1G and 1 H); and post-CVD (Figures 11 and 1J).

[0238] Figures 2A and 2B also show that cathode impurities are still present in the black mass of Example 4 after 1 hour in boiling 2.0M HCI acid solution, but that these impurities are removed after 2 hours in boiling 2.0M HCI acid solution (Figures 1C and 1 D).

[0239] Further still, it can be seen from the scanning electron micrographs and energy dispersive X- ray micrographs of Figures 7-9 that various elements, which are initially present in the recycled hard carbon as inorganic impurities, are removed after boiling in 2.0M HCI acid solution (“post demineralisation”).

[0240] Indeed, energy-dispersive x-ray spectroscopy in Figure 7 shows that Na, Al, F, P, and Si (which were present in the recycled hard carbon after the carbonisation step of Example 2) are no longer present after 1 hour in boiling 2.0M HCI acid solution (“post demineralisation”). Energy-dispersive x-ray spectroscopy in Figure 8 shows that P, Na, Al, and F (which were present in the recycled hard carbon after the carbonisation step of Example 3) are no longer present after 1 hour in boiling 2.0M HCI acid solution (“post demineralisation”).

[0241] Energy-dispersive x-ray spectroscopy in Figure 9 shows that Ni, Mn, Mg, Ti, and Al (which were present in the initial black mass of Example 4) are no longer present after 2 hours in boiling 2.0M HCI acid solution (“post demineralisation”).

[0242] More particularly, it is quite surprising that the process of the present invention is able to reduce the inorganic impurity content (e.g.., the ash content as indicated in Table 1 for Example 4) in the initial black mass ‘feedstock’ material so effectively, particularly when such feedstock may contain impurities such as one or more positive active materials, or impurities that are derived from such more positive active materials. Indeed, the final hard carbon material after chemical vapour deposition has a lower level of impurity content than commercially available hard carbon. Having a low level of inorganic impurity content in the resulting final hard carbon material is important because inorganic impurities reduce its ability to be used as a negative active material in a sodium-ion cell.

[0243] Further analysis on the products of each step of Example 4 was conducted by X-ray diffraction (XRD). Analysis by X-ray diffraction techniques was conducted using a Siemens (RTM) D5000 powder diffractometer to confirm that the desired target materials had been prepared, to establish the phase purity of the product material and to determine the types of impurities present. From this information it is possible to determine the lattice parameters of the unit cells.

[0244] The general XRD operating conditions used to analyse the materials are as follows:

[0245] Slits sizes: 2 mm, 2 mm, 0.2 mm

[0246] Range: 20 = 10 ° - 60 °

[0247] X-ray Wavelength = 1 .5418 A (5 Angstroms) (Cu Ka)

[0248] Speed: 1.0 seconds / step Increment: 0.025 °

[0249] The XRD data for the product of each step of Example 4 is shown in Figure 3.

[0250] The TGA plot of Figure 4 shows that the hard carbon fully processed according to the process of the present invention has a purity of >99.97 wt.%. It also shows that the initial black mass contains approximately 53 wt.% used hard carbon and approximately 47 wt.% inorganic impurities such as used cathode active material. It is shown that after two hours of demineralisation, the material is >99 wt.% carbon.

[0251] Table 2 below displays the physical properties of hard carbon prepared according to each of Examples 1-4 above.

[0252] Table 2 These data show that the process of the present invention does not significantly alter the median particle size from that exhibited by Comparative Example 1. Furthermore, although the BET surface area remains largely unchanged, the micropore surface area is reduced by as much as one order of magnitude. Finally, the recycled hard carbon has purity comparable to or slightly better than the ‘virgin’ hard carbon, as proven by the reduction in inorganic / ash content for the materials of Examples 2-4 when compared to that of Comparative Example 1. It is surprising that such purity levels could be achieved as similarly discussed above with respect to Table 1 Electrochemical data for half cells prepared according to each of Examples 1-4 above are set out in Table 3 below.

[0253] Table 3

[0254] These data show that the half-cell reversible and irreversible specific capacities, and first-cycle Coulombic efficiencies for anodes according to Examples 2-4 are comparable to the values obtained for the control anode Example 1 .

[0255] Electrochemical data for full cells prepared according to each of Examples 1-4 above are set out in Figures 5A-5D, Figures 6A-6B, and in Table 4 below.

[0256] Table 4

[0257] These data show that full-cell electrochemical data for cells containing anodes according to Examples 2-4 are comparable to or improved over the values for anodes according to Example 1. Consequently, hard carbon material recycled according to Examples 2-4 is viable for use as a negative electrode active material.

[0258] In particular, a sodium-ion cell containing such a negative electrode active material exhibits electrochemical performance that is equivalent to that of a sodium-ion cell containing “virgin” hard carbon. Thus, the effects of the present invention are particularly surprising because it was previously unknown at the priority date of the present application whether it was possible to ‘recycle’ a negative electrode active material has been subject to one or more charge and / or discharge operations in an electrochemical cell.

Claims

CLAIMS1 . A process of preparing a material comprising hard carbon comprising the steps: a) providing a composition including hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell; b) treating the composition in step a) to remove any unwanted inorganic impurities; and c) pyrolysing the treated composition, at a temperature from about 600 °C to about 3000 °C, to form the material comprising hard carbon.

2. The process according to claim 1 , in which step b) comprises contacting the composition in step a) with acid and / or alkaline conditions.

3. The process according to any one of claim 1 or 2, further including, between steps b) and c), the step of carbonising and / or drying the treated composition from the end of step b) at temperature from about 60 °C to about 950 °C.

4. The process according to any one of claims 1 to 3, further including, after step c), step d) which comprises contacting the material comprising hard carbon with one or more carbon-containing materials at a temperature of up to 950 °C.

5. The process according to any one of claims 1 to 4, in which the composition in step a) is provided by: i) providing one or more electrochemical cells comprising one or more current collectors which include hard carbon, in which the hard carbon has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell; and ii) separating the hard carbon from at least a portion of the one or more current collectors to form the composition including hard carbon.

6. The process according to claim 5, in which step ii) includes: iii) disassembling the one or more electrochemical cells provided in step i) to provide at least a portion of one or more current collectors which include hard carbon, in which the hard carbon has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell; andiv) separating the hard carbon from the at least a portion of the one or more current collectors by using a heat treatment and / or a chemical treatment, to form the composition including hard carbon.

7. The process according to claim 5, in which step ii) includes: iii) disassembling the one or more electrochemical cells provided in step i) to provide one or more current collectors which include hard carbon, in which the hard carbon has been subject to one or more charge and / or discharge operations in a sodium- based electrochemical cell; iv) reducing, into one or more portions, the one or more current collectors which include hard carbon; v) contacting the product of step iv) with a heat treatment and / or a chemical treatment; and vi) separating the hard carbon from the product of step v) to form the composition including hard carbon.

8. The process according to claim 7 in which step iii) and step iv) take place concurrently and step v) comprises a chemical treatment.

9. The process according to any of claims 1 to 8, in which the hard carbon included in the composition of step a) has been subject to one or more charge and / or discharge operations in a sodium-ion cell.

10. The process according to any one of claims 5 to 8, in which the one or more electrochemical cells comprise one or more sodium-based electrochemical cells.11 . The process according to any of claims 1 to 10, in which the pyrolysis of step c) is from a temperature of about 750°C to about 3000°C.

12. A material comprising hard carbon obtainable by the process according to any one of claims 1 to 11.

13. Use of a material comprising hard carbon according to claim 12, in an electrochemical cell.

14. An electrode comprising a material comprising hard carbon according to claim 12.

15. An electrochemical cell comprising a material comprising hard carbon according to claim 12.

16. The electrochemical cell according to claim 15, which is selected from one or more of the group consisting of a sodium metal cell, a sodium-ion cell, and an anode-free sodium cell.

17. Use of a composition including hard carbon to prepare a material comprising hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell.

18. Use, in an electrochemical cell, of a material comprising hard carbon in which the material comprising hard carbon is made using a composition including hard carbon, in which the hard carbon included in said composition has been subject to one or more charge and / or discharge operations in a sodium-based electrochemical cell.

19. The use according to claim 17 or 18, in which the material comprising hard carbon has a BET (N2) specific surface area of 10 m2 / g or less as determined using nitrogen gas BET analysis.

20. The use according to any one of claims 17 to 19, in which the material comprising hard carbon has an open micropore specific surface area of 0 m2 / g to 5 m2 / g, as determined using nitrogen gas BET analysis.21 . The use according to any one of claims 17 to 20, in which the material comprising hard carbon has less than 2% by weight of inorganic impurities.

22. The use according to any one of claims 18 to 21 , in a sodium-based electrochemical cell.

23. The use according to claim 22, in a sodium-ion cell.

24. The use according to any one of claims 18 to 23, in which the material comprising hard carbon is used as a negative electrode active material.

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