WO2015001314A1

WO2015001314A1 - Electrode for use in a lithium-ion electrochemical cell

Info

Publication number: WO2015001314A1
Application number: PCT/GB2014/051942
Authority: WO
Inventors: Ana Belen JORGE; Paul F MCMILLAN; Furio CORÁ; Dan BRETT
Original assignee: Ucl Business Plc
Priority date: 2013-07-01
Filing date: 2014-06-25
Publication date: 2015-01-08
Also published as: GB201311742D0

Abstract

The present disclosure relates to a negative electrode for use in a lithium-ion electrochemical cell comprising a composition comprising graphitic carbon nitride and an optionally modified allotrope of carbon. It also relates to compositions for use in the electrode, methods of making the compositions, and a lithium-ion electrochemical cell comprising the electrode.

Description

Electrode for use in a lithium-ion electrochemical cell

The present invention relates to electrodes for use in a lithium-ion electrochemical cell, such as a rechargeable battery. It also relates to compositions for use in the electrode, methods of making the compositions, and a lithium-ion electrochemical cell comprising the electrode.

Background Energy storage and power density demands of future technologies require further development of lithium-ion battery (LIB) performance. The choice of the electrode materials is of paramount importance for key characteristics including charge storage density, kinetics, safety and cost concerns. Commercially produced LIB anodes are typically graphitic carbon-based systems. Despite a theoretical capacity of 372 mAh g^"1 for fully crystalline material,¹ pure C-graphites are limited in active sites and slow intercalation kinetics. While the effective voltage of graphite, near to that of Li / Li⁺, is attractive to maximise the use of the available cell energy; the fact that its electrochemical potential lies above the lowest unoccupied molecular orbital (LUMO) of the electrolyte (for example, LiPF₆ in 1 :1 ethylene carbonate (EC) / diethyl carbonate (DEC) but also extending to other Li-conducting polymers) results in irreversible electrochemical decomposition due to the transfer of electrons from graphite to the LUMO of the electrolyte.² This reaction leads to the formation of a solid-electrolyte interface (SEI) layer on the graphite surface composed of different Li-containing compounds and irreversible capacity fading is observed. Once the initial SEI layer is formed, it produces an electrically insulating barrier preventing new electrolyte molecules from coming into contact with the anode surface and impeding electron transfers into the electrolyte. Dendrite formation accompanies the reduced Li⁺ diffusion. Alternative LIB anode materials, with various properties that might counter these difficulties, have been proposed. Graphene holds great promise as an electrode in LIBs. With a theoretical surface area of 2630 m² g^"1 , graphene based electrodes have higher specific capacities than many other electrode materials (including graphite).^3,4 Moreover, graphene/Sn0₂ composite electrodes have been shown to exhibit a higher reversible capacity when compared to bare Sn0₂, bare graphene and bare graphite being the optimum graphene: Sn0₂ molar ratio 3.2: 1.⁵ However, recent studies⁶ suggest that graphene is less beneficial as electroactive material than expected due to a low number of electron transfer sites, resulting in slow electrode kinetics. Silicon has also attracted increasing attention as a potential high-capacity anode material because of numerous appealing features such as high theoretical specific capacity of 4212 mA h/g.^7,8 However, Si anode materials suffer from some drawbacks involving the drastic volume change (up to 300%) during the alloying-dealloying reactions with Li and the intrinsic low conductivity. These limitations still challenge the investigation and development of the next generation of LIBs. Some of the approaches to alleviate the volume change during cycling are to utilise Si nanostructures to decrease the absolute volume variation of Si-based materials.^9"12 Some other alternative anode materials include Cu₆Sn₅ (NiAs-type structure with 0.8 interstitial holes/formula unit and specific capacity -400 mAh/g.), Li₄Ti₅0i₂, transition metal phosphides,¹³ graphene-Ti0₂/TiN composites,¹⁴ and lithium iron nitride¹⁵ among others.

Graphitic carbon nitride materials (gCNM) present layered structures related to C- graphite and graphene, but built on heteronuclear C-N bonds containing a high N:C ratio (>1) and a variable quantity of terminal N-H groups. There is growing interest in the catalytic and photocatalytic properties of these new functional materials that combine important properties of polymers and ceramics.^16"23 The layered structures are formed by s-triazine or heptazine (tri-s-triazine) rings linked by -NH- units as the building blocks.²⁴ Voids appear in the graphitic layers providing a third dimension for Li⁺ intercalation and diffusion pathways that can improve the storage capacity and electrode kinetics (Fig.1). The present inventors determined the electrochemical performance of gCNMs synthesized by reacting highly nitrogenated organic precursors (including, but not limited to, cyanamide (CN₂H₂) or dicyandiamide (DCDA, C₂N₄H₄), melamine (C₃N₆H₉), urea (CO(NH₂)₂, cyanuric chloride (C₃N₃CI₃)) at temperature usually in a range 550-650 °C leading to materials with C: N: H ratios and degree of layer condensation controlled by release of NH₃ or HCI. Material prepared at 550 °C had composition C3.0N5.2H1.6 and a microporous texture with interlocking planar microstructures and aggregates with pore sizes of several nm (3-7 nm) that fuse together forming 1-2 μηι pores in the resulting solid.^22,25 The particle sizes were determined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The carbon nitrides exhibit a yellow colour that becomes darker leading to orange and brown as the preparation temperature is increased. The graphitic carbon nitrides were found to have a BET (Brunauer-Emmett-Teller) surface area between 11-28 m²/g with the highest surface area achieved to material prepared at the lowest T (550 °C). Samples prepared at higher temperature had improved crystallinity but lowered porosity, giving rise to blocky structures at a micrometre scale (Fig. 1 and Fig. 2). It would be desirable to provide an alternative to, and ideally an improvement upon, at least some lithium-ion batteries and the materials described above.

Summary of the Invention

In a first aspect, the present invention provides a negative electrode for use in a lithium-ion electrochemical cell comprising a composition comprising graphitic carbon nitride and an optionally modified allotrope of carbon. The optionally modified allotrope of carbon may comprise or be a pure carbon allotrope or an allotrope of carbon that has been modified chemically, e.g. doped with a substance or hydrogenated, as will be described in more detail below.

In a second aspect, the present invention provides a lithium-ion electrochemical cell comprising a negative electrode according to the first aspect, a positive electrode and a lithium-ion transport medium disposed between the negative electrode and the positive electrode.

In a third aspect, the present invention provides a composition comprising graphitic carbon nitride and graphite.

In a fourth aspect, the present invention provides a method for making the composition according to the third aspect, wherein the method comprises mixing graphitic carbon nitride and graphite. The present inventors have found that there are advantages of using graphitic carbon nitride materials (gCNMs) in lithium-ion cell electrodes compared to other technologies. However, the capacity of pure gCNMs has been found to be relatively low. By combining gCNMs with an allotrope of carbon such as graphite, higher capacities (e.g 400 mAh/g) can be achieved compared to gCNM or graphite alone (about 12 mAh/g and 372 mAh/g, respectively).

The advantages of using gCNMs include:

-Graphitic carbon nitride materials are layered structures that can be obtained from reaction and/or condensation of molecular precursors such as melamine, dicyandiamide, cyanamide, urea, cyanuric chloride and other nitrogen-rich compounds, which are very cheap starting materials that are readily available. This contrasts with other alternative technologies such as alloys (Cu₆Sn₅), mixed metal oxides Li₄Ti₅0i₂, transition metal phosphides, graphene-Ti0₂/TiN composites, lithium iron nitride among others. -Graphitic carbon nitride materials are typically metal-free compounds, hence environmental friendly, economically sustainable and suitable for mass scale production.

-Graphitic carbon nitrides can be prepared at relatively moderate temperatures (e.g. 550 °C or below) compared to graphite which usually needs temperatures above 900- 1000 °C. This means that graphitic carbon nitrides require a lower energy preparation process associated with minimal C0₂ production. Due to their compositional flexibility, these materials can be easily tuned, modifying their properties. -The preparative process for the graphitic carbon nitride materials is also easy-to-scale- up.

-gCNM structure is related to graphite or graphene, but built on heteronuclear C-N bonds and with a variable amount of terminal N-H groups. The different valence of C and N causes holes to appear in the graphitic layers, whose texture and composition can be controlled to some extent in the synthesis. The intra-layer holes enable faster and 3D intercalation, not observed in commonly-used graphite.

-Graphitic carbon nitride materials are highly stable showing thermal and chemical inertness. Silicon has attracted increasing attention as a potential high-capacity anode material because of numerous appealing features such as high theoretical specific capacity of 4212 mAh/g (graphite is 372 mAh/g) and higher safety and higher stability than graphite. Si anode materials, however, suffer from some drawbacks involving the drastic volume change (larger than 300%) during the alloying-dealloying reactions with Li, the intrinsic low conductivity, and the unstable solid electrolyte interface (SEI).

-Although pure graphitic carbon nitride materials do not provide much capacity (only up to ~12mAh/g in the present preparation), the gCNM-graphite composites described herein can be easily prepared by mechanical mixing, improving significantly their effective capacity up to 400 mAh/g (Fig.5), which is in the range of state-of-the-art graphite anodes. -Graphitic carbon nitrides exhibit higher operating voltage, improving safety issues observed in current graphite-based lithium-ion batteries where graphite presents a working potential to close to that of Li/Li⁺. - Solid electrolyte interface (SEI) growth on the graphite anode results from irreversible electrochemical decomposition (reduction) of the electrolyte, specifically the reductive decomposition of the organic component of the electrolyte that forms a solid layer on the surface of the active material. This solid layer is made of different Li-containing compounds, including Li salts of the anions formed by reduction of the electrolyte, leading to an irreversible capacity fading due to the Li⁺ that remains in the SEI layer. The SEI is formed at the negative electrode because the electrolyte is not stable at the working potential of the negative electrode during fast charging (the LUMO of the electrolyte is too close to the Fermi energy of the anode). Once an initial SEI layer has formed, it provides a barrier that 1) prevents new electrolyte molecules from coming into contact with the anode surface and 2) is electronically insulating and thus kinetically (not thermodynamically) protects further electron transfers from the anode to the electrolyte, hence prevents further reduction of the electrolyte. The initial SEI formation thus suppresses further SEI growth, although the SEI must be permeable to Li⁺. In the case of gCNMs, the Fermi level is lower than in graphite, so the present invention avoids the need of the SEI layer and therefore, the initial capacity fade.

Brief Description of the Figures

Figure 1 illustrates schematically the porous microstructure of a graphitic carbon nitride material that may be used in the present invention; in this figure are shown two layers 102 of carbon nitride and a lithium ion 101 intercalating into the carbon nitride structure. This diagram is only a schematic model and it represents only one arrangement of the graphitic layers enabling lithium diffusion and intercalation. Other pathways and channels may exist among graphitic carbon nitride materials with less than complete polymerisation within the layers.

Figure 2 shows: (a) X-ray diffraction patterns of graphitic carbon nitride materials prepared at temperatures between 550 and 650 °C, by mixing dicyandiamide (DCDA) and melamine (1 :1 molar ratio) under N₂ (g). (b) SEM images of graphitic carbon nitride materials prepared at temperatures between 550 and 650 °C, by mixing DCDA and melamine (1 : 1 molar ratio) under N₂ (g). (c) UV-vis absorption spectra of graphitic carbon nitride materials prepared at temperatures between 550 and 650 °C, by mixing DCDA and melamine (1 : 1 molar ratio) under N₂ (g). (d) FTIR measurements of graphitic carbon nitride materials prepared at temperatures between 550 and 650 °C, by mixing DCDA and melamine (1 :1 molar ratio) under N₂ (g). Figure 3 shows (a) SEM image of gCNM-550/graphite (40:50) composite before cycling in a lithium cell, (b) SEM image of gCNM-550/graphite (40:50) composite after cycling in a lithium cell using Li metal as reference electrode, (c) TEM image of gCNM- 550/graphite (40:50) composite before cycling in a lithium cell, (d) Electron diffraction pattern of gCNM-550/graphite (40:50) composite before cycling in a lithium cell.

Figure 4 shows the X-ray diffraction pattern of the conductive carbon material (commercially obtained carbon black) used for preparing the composites with gCNM for testing as electrodes in a lithium-ion battery arrangement. Figures 5A and 5B, respectively, show cyclic voltammograms of gCNM-550/graphite (85:5) composite gCNM-550/graphite (40:50) composite cells using Li metal as reference electrode at a scan rate of 10^"3 Vs^"1 in the range 0.01-3.0 V. The materials and the test methods are described in detail in the Examples below. Figure 6 shows charge/discharge curves for gCNM/graphite (40:50)-Li metal cells obtained at a constant current of 0.02 A g^"1.

Detailed Description The present invention provides the first to the fourth aspects described herein. Optional and preferred features will now be described. Unless otherwise stated, any optional or preferred feature can be combined with any aspect of the invention and any other optional or preferred feature. The negative electrode comprises a composition comprising graphitic carbon nitride and an optionally modified allotrope of carbon. In an embodiment, the optionally modified allotrope of carbon may comprise or be a material containing carbon in an elemental form. The optionally modified allotrope of carbon may be selected from an electrically conducting material, and may be selected from graphite, activated carbon, glassy carbon, boron doped diamond, carbon powder, fullerenes, graphene and carbon nanotubes, and combinations thereof. In an embodiment, the optionally modified allotrope of carbon is graphite, which may be selected from edge plane pyrolytic graphite, basal plane pyrolytic graphite and highly ordered pyrolytic graphite, and the graphite may optionally be modified, as described herein.

The allotrope of carbon may optionally have been modified, e.g. doped or substituted with one or more substances, which may increase its electrical conducting properties (compared to the undoped or unsubstituted allotrope of carbon). In an embodiment, the modified allotrope of carbon is or comprises an allotrope of carbon that has been doped with one or more elements, other than carbon, which may be selected from Groups 1 to 17 of the periodic table. In an embodiment, the modified allotrope of carbon is or comprises an allotrope of carbon that has been doped with one or more metals, which may be selected from Groups 1 to 14 of the periodic table, optionally from the alkali and alkali earth metals (groups 1 and 2 of the periodic table) and the transition metals (groups 3 to 12 of the periodic table). In an embodiment, in a modified allotrope of carbon, some of the carbon atoms are bonded solely to other carbon atoms, although species other than carbon may be present in or on the modified allotrope of carbon.

In an embodiment, the optionally modified allotrope of carbon has been at least partially hydrogenated, such that at least some of the carbon atoms are each covalently bonded to one or more hydrogen atoms. In an embodiment, the modified allotrope of carbon comprises a material selected from hydrogenated graphite, hydrogenated graphene, hydrogenated fullerenes and hydrogenated carbon nanotubes.

Optionally, the composition comprises a mixture of particles comprising the graphitic carbon nitride and particles comprising the allotrope of carbon, which may be graphite.

The optionally modified allotrope of carbon is a material containing carbon in an elemental form. The allotrope of carbon may optionally have been modified, e.g. doped or substituted with one or more substances, which may be to increase its electrical conducting properties. The optionally modified allotrope of carbon may be selected from an electrically conducting material, and may be selected from graphite, activated carbon, glassy carbon, boron doped diamond, carbon powder, fullerenes, graphene and carbon nanotubes, and combinations thereof. In an embodiment, the optionally modified allotrope of carbon is or comprises graphite, which may be selected from edge plane pyrolytic graphite, basal plane pyrolytic graphite and highly ordered pyrolytic graphite.

Graphitic carbon nitride is typically a material comprising carbon nitride that contains triazine units that together form a structure similar to graphite. "Graphitic" therefore refers to the structure of this carbon nitride, and does not indicate that it contains graphite. Graphitic carbon nitride may be formed from one or more carbon- and nitrogen-containing precursors compounds. Graphitic carbon nitride may be formed from the polymerization of organic species, which are typically highly nitrogenated, that may be selected from cyanamide, dicyandiamide, melamine, urea, and cyanuric chloride, among other nitrogen-rich precursor compounds. In an embodiment, the graphitic carbon nitride may be formed by mixing dicyandiamide and melamine, and allowing them to react at a temperature T of from 500 °C to 700 °C, optionally a temperature T of from 550 °C to 650 °C, optionally under a gas, which may comprises or be an inert gas such as nitrogen (N₂(g)), until the graphitic carbon nitride is formed. The temperature T may be from 500 °C to 600 °C, optionally 550 °C to 600 °C, or in other embodiments 600 °C to 650 °C. Optionally the gas may comprise or be air or ammonia (NH₃). The dicyandiamide and melamine may be reacted together in a molar ratio of from 1 :0.8 to 1 : 1.2, optionally of from 1 :0.9 to 1 : 1 , optionally in a molar ratio of about 1 :1.

The graphitic carbon nitride may be formed from the reaction of dicyandiamide at a temperature of from 550 to 650 °C, optionally under N₂ (g), NH₃ (g), or in air. The graphitic carbon nitride may be formed from the reaction of melamine at a temperature of from 550 to 650 °C, optionally under N₂ (g), NH₃ (g), or in air.

The graphitic carbon nitride may be formed from the reaction of mixtures of dicyandiamide and melamine at a temperature of from 550 to 650 °C, optionally under N₂ (g), NH₃ (g), or in air.

The graphitic carbon nitride may be formed from the reaction of mixtures of cyanuric chloride and melamine at a temperature of from 550 to 650 °C, optionally under N₂ (g), NH₃ (g), or in air. The graphitic carbon nitride may be formed from the reaction of mixtures of cyanuric chloride and dicyandiamide at a temperature of from 550 to 650 °C, optionally under N₂ (g), N Hs (g), or in air. Synthesis of well crystallised graphitic carbon nitride materials can be achieved by ionothermal routes (use of molten salts acting as solvent medium) under vacuum conditions. These procedures are described in prior art²⁶.

The graphitic carbon nitride may be prepared by numerous routes. These routes can involve the use of, i.e. synthesizing the graphitic carbon nitride under an atmosphere of, inert gas (e.g. N₂ and/or Ar), NH₃ or air. The graphitic carbon nitride can also be prepared under vacuum conditions. The graphitic carbon nitride can be prepared in molten salts, or any other medium, such as ionic liquids to increase their crystallinity. The porosity of the graphitic nitride can be improved by including soft or hard templates in their synthesis. Starting C and N-containing materials include, but are not limited to, cyanamide, urea, melamine, cyanuric chloride and dicyanamide.

The graphitic carbon nitride material may comprise hydrogen within its structure, as well as carbon and nitrogen. In an embodiment, the graphitic carbon nitride material has a stoichiometry of C₃N_xH_y, wherein x is 4.4 to 5.2 and H is 1.3 to 1.6. In an embodiment, the graphitic carbon nitride material has a stoichiometry of C₃N_xH_y, wherein x is 4.8 to 5.2 and H is 1.45 to 1.6. In an embodiment, the graphitic carbon nitride material has a stoichiometry of C₃N_xH_y, wherein x is 4.4 to 4.8 and H is 1.3 to 1.45. In an embodiment, x is 5.2 or less and y is 1.6 or less. In an embodiment, x is 4.4 or more and y is 1.3 or more. In an embodiment, the graphitic carbon nitride material has a stoichiometry of C₃N5.2H1.6. In an embodiment, the graphitic carbon nitride material has a stoichiometry of C₃N_4.4H_{1 3.}

The graphitic carbon nitride typically exhibits a yellow-orange colour depending on the degree of condensation. As the degree of condensation increases, the colour of the material becomes darker (e.g. see Fig. 2c).

The graphitic carbon nitride may be in the form of particles, and optionally at least some of which have a particle size of from 1 nm to 10 nm, optionally from 2 nm to 8 nm, optionally from 3 nm to 7 nm. Particle size, as mentioned herein, may refer to the diameter of the particle, which may be the largest dimension measured across a particle, and the particle size may be determined by a technique selected from X-ray diffraction (Debye-Scherrer formula) and scanning electron microscopy (e.g. see Fig 2a).

The optionally modified allotrope of carbon may be in the form of particles, and optionally at least some of which have a particle size of from 0.01 μηι to 100 μηι, optionally from 0.05 μηι to 10 μηι, optionally from 0.1 μηι to 5 μηι, optionally from 0.1 to 1 μηι, optionally from 0.3 to 0.8 μηι, optionally about 0.5 μηι.

The graphitic carbon nitride may have a BET surface area of from 1 m²/g to 50 m²/g, optionally 5 m²/g to 40 m²/g, optionally 1 1 m²/g to 28 m²/g. The highest surface area material has been found to be prepared the lower the temperature at which the graphitic carbon nitride has been formed. For example graphitic carbon nitride prepared at a temperature of 550 °C has been found to have a higher surface area than graphitic carbon nitride formed at 650 °C. In an embodiment, the graphitic carbon nitride has been formed at a temperature of from 550 °C to 600 °C, optionally of from 550 °C to 580 °C, optionally of from 550 °C to 570 °C, optionally 550 °C to 560 °C In an embodiment, the graphitic carbon nitride has been formed at a temperature of from 600 °C to 650 °C, optionally of from 620 °C to 650 °C, optionally of from 630 °C to 650 °C, optionally 640 °C to 650 °C.

The composition may comprise the graphitic carbon nitride and the optionally modified allotrope of carbon in the relative w/w ratio of 20: 1 to 1 :20, optionally 10: 1 to 1 : 10, optionally 5:1 to 1 :5, optionally 2:1 to 1 :2. Preferably, the composition comprises the graphitic carbon nitride and the optionally modified allotrope in the relative w/w ratio of 1 : 1 to 1 :1.5, more preferably 1 :1.2 to 1 :1.3. In an embodiment the composition comprises the graphitic carbon nitride and the optionally modified allotrope of carbon in the relative w/w ratio of about 1 : 1.25.

The composition may comprise the graphitic carbon nitride and the graphite in the relative w/w ratio of 20: 1 to 1 :20, optionally 10: 1 to 1 : 10, optionally 5: 1 to 1 :5, optionally 2:1 to 1 :2. Preferably, the composition comprises the graphitic carbon nitride and the graphite in the relative w/w ratio of 1 :1 to 1 :1.5, more preferably 1 :1.2 to 1 : 1.3. In an embodiment the composition comprises the graphitic carbon nitride and the graphite in the relative w/w ratio of about 1 : 1.25.

The composition may further comprise a binder. The binder may be present in the composition in an amount of from 1 wt % to 50 wt %, optionally 5 wt %to 30 wt %, optionally 5 wt %to 20 wt%, optionally 5 wt %to 15 wt %, optionally 8 wt %to 12 wt %, optionally about 10 wt %.

The binder comprises may be selected from ethylene propylene diene monomer (EPDM), styrene butadiene rubber (SBR), carboxy methyl cellulose (CMC), a polyvinylidine fluoride (PVDF; sometimes termed polyvinyl diene fluoride), polytetrafluoroethylene (PTFE), polyvinyl acetate or a combination thereof.

The electrode may further comprise a substrate comprising an electrically conducting material, disposed on which is the composition comprising graphitic carbon nitride and an optionally modified allotrope of carbon. The electrically conducting material may be or comprise a material other than graphitic carbon nitride and/or optionally modified allotrope of carbon. The electrically conducting material may be or comprise a metal in elemental form. The electrically conducting material may comprise a metal selected from groups 3 to 12 of the period table. The electrically conducting material copper, nickel, aluminium and cobalt, which may be in elemental form or in the form of an alloy with one or more other metals. The substrate may act as a current collector.

In an embodiment, the composition comprises graphitic carbon nitride and an optionally modified allotrope of carbon, which may be or comprise graphite, in the relative w/w ratio of 1 : 1 to 1 :1.5 and a binder, and the composition is disposed on a metal in elemental or alloy form.

As indicated, the present invention further provides a lithium-ion electrochemical cell comprising the negative electrode described herein. The electrochemical cell may be a capacitor, supercapacitor or battery. Where the electrochemical cell is a battery, this may a secondary, i.e. rechargeable, battery.

The positive electrode may comprise any material suitable for use as a positive electrode in a lithium-ion cell. The positive electrode may comprise a material selected from a layered oxide, such as a lithium cobalt oxide, a polyanion, such as lithium iron phosphate, and a spinel, such as lithium manganese oxide. The positive electrode may comprise a material selected from LiCo0₂, LiMn0₂, LiNiCo0₂, or LiNiAICo0₂. The material of the positive electrode may be disposed on a support, which may be a metal in elemental form, and may be selected from aluminium, copper, tin and gold. The lithium-ion electrochemical cell comprises a lithium ion transport medium disposed between the negative electrode and the positive electrode.

In an embodiment, the lithium-ion transport medium comprises an electrolyte comprising a lithium salt and a material comprising a solvent and/or a lithium-ion conducting polymer. The electrolyte is disposed between the negative electrode and the positive electrode. The lithium salt may be selected from lithium hexafluorophosphate, lithium tetraborate, lithium perchlorate, lithium hexafluoroarsenate, lithium triflate and a combination thereof. The solvent may be selected from ethylene carbonate, diethylene carbonate, dimethyl carbonate, propylene carbonate, mixtures thereof.

As mentioned, in an embodiment, the lithium-ion transport medium comprises a lithium- ion conducting polymer. The lithium-ion conducting polymer may be selected from, but is not limited to, polyethyleneoxide and polyacrylonitrile. The lithium-ion conducting polymer may be a solid at standard temperature (e.g. 25 °) and pressure (e.g. 100 kPa).

The lithium-ion electrochemical cell may further comprise a separator. The separator may be disposed between the negative electrode and the positive electrode, and may act to prevent the electrodes physically contacting one another, while allowing ion transport within the cell. The separator may comprise a material selected from a polypropylene, polyethylene, polyethyleneterephthalate and polyvinylidene chloride, and a combination thereof and the material may be in the form of a porous membrane. Alternatively, the separator may comprise porous glass fibre tissue.

The lithium-ion electrochemical cell may further comprise an appropriate housing to contain its components, e.g. the negative and positive electrode cases, spring and spacer that provide robustness to the cell, and the electrolyte.

As indicated, the present invention further provides a composition comprising graphitic carbon nitride and graphite. The composition, graphitic carbon nitride and graphite may be as described above. The present invention further provides a method for making the composition according to the third aspect, wherein the method comprises mixing graphitic carbon nitride and graphite. The method may comprise mixing together particles comprising graphitic carbon nitride and particles comprising graphite. The particles comprising graphitic carbon nitride and particles comprising graphite may be mixed together with a liquid carrier, and optionally a binder, to form a slurry. The binder may be as described herein. The liquid carrier may be a non-protic solvent, optionally a polar non-protic solvent. The polar non-protic solvent may, for example, be selected from 1-methyl-2-pyrrolidone (NMP, sometimes termed N-Methyl-2-pyrrolidone), dimethylformamide, dimethylacetamide and dimethyl sulfoxide.

The slurry may be deposited on a substrate, and the solvent removed, e.g. by evaporation, which may be effected by heating the substrate for an appropriate amount of time at an appropriate temperature. In an embodiment, the substrate is an electrically conducting substrate, and after the slurry is deposited on the substrate, the liquid carrier is removed, to form a solid composition comprising the particles comprising graphitic carbon nitride, the particles comprising graphite and the binder on the electrically conducting substrate. The substrate formed in the method may be suitable for use as the negative electrode described herein.

Examples

1. Synthesis of the graphitic carbon nitride materials.

In a first Example, polymeric/graphitic carbon nitride materials (gCNMs) were prepared by thermolysis and condensation reactions of a 1 : 1 molar ratio mixture of dicyandiamide (C2N4H4) and melamine (0₃Ν₆Η₉) at 550 °C at an initial heating rate of 5 °C/min and held at temperature for 15 h. Precursor materials were ground together to achieve a particle size of 3-7 nm (determined by X-ray diffraction technique using Debye-Scherrer formula, Fig 2a) and the mixture was loaded in an alumina boat into a quartz tube and placed in a tube furnace under flowing nitrogen. The furnace is allowed to cool to room temperature before samples are removed. The material resulting from this preparation process was light yellow in colour and had a composition close to C3.0N5.2H L6 determined by elemental analysis. The colour of the material becomes darker as T is increased (Fig.2c) The graphitic carbon nitride have a BET surface area between 11-28 m²/g. The graphitic carbon nitride prepared at a temperature of 550 °C has been found to have a higher surface area than graphitic carbon nitride formed at 650 °C.

The graphitic carbon nitride may be formed from the reaction of dicyandiamide at temperatures between 550 °C and 650 °C, under N₂ (g), NH₃ (g), or in air.

The graphitic carbon nitride may be formed from the reaction of melamine at temperatures between 550 °C and 650 °C, under N₂ (g), NH₃ (g), or in air.

The graphitic carbon nitride may be formed from the reaction of mixtures of cyanuric chloride and melamine at temperatures between 550 °C and 650 °C, under N₂ (g), NH₃ (g), or in air.

The graphitic carbon nitride may be formed from the reaction of mixtures of cyanuric chloride and dicyandiamide at temperatures between 550 °C and 650 °C, under N₂ (g), NH₃ (g), or in air. Synthesis of well crystallised graphitic carbon nitride involves the use of ionothermal routes (use of molten salts acting as solvent medium) under high vacuum conditions. Graphitic carbon nitrides may be prepared by numerous routes. These can involve the use of inert gas (N₂, Ar), NH₃ or air. They can also be prepared under vacuum conditions. They can be prepared by using molten salts, or any other medium, such as ionic liquids to increase their crystallinity. The porosity can be improved by including soft or hard templates in their synthesis. Starting C, N-containing materials include cyanamide, urea, melamine, cyanuric chloride and dicyanamide, but other precursors are also possible.

2. Preparation of the pCNM/praphite composites.

The gCNM/graphite composites consisted of a mixture of gCNM prepared as described above, graphite additive and polyvinylidenefluoride (PVdF) binder paired with in 1- methyl-2-pyrrolidinone (C₅H₉NO, NMP) solvent. The graphite used was a conductive crystalline graphite with a particle size of -0.5 μηι (MTI Corporation). In order to prepare the electrode, the slurry mixture is poured onto a Cu foil current collector and dried at 120 °C.

3. Electrochemical performance ofpCNM/praphite composite electrodes in lithium- ion cells.

In order to evaluate the electrochemical performance and determine the working potential range of gCNM/graphite electrodes, cyclic voltammetry (CV) measurements using Li metal as reference electrode were conducted at a scan rate of 10^"3 Vs^"1 where the potential was ramped between 0.01 and 3.0 V and then reversed for several cycles, showing good cyclability (Figs.5A and 5B). Fig. 5A shows the results when the w/w ratio of gCNM/graphite was 85/5, with the remaining weight percentage being binder. Fig. 5B shows the results when the w/w ratio of gCNM/graphite was 40/50, with the remaining weight percentage being binder. Charge-discharge cycles were carried out to investigate the lithium storage capability of the gCNM/graphite electrodes (Fig.6). The best performance obtained so far is for the gCNM/graphite 40:50, exhibiting a capacity of -400 mAh/g vs 372 mAh/g theoretical capacity for graphite, stable over 10 cycles. References mentioned herein or otherwise useful as background: Paek, S.-M.; Yoo, E; Honma, I.; Nano Lett. 2009, 9, 72.

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All references mentioned herein are incorporated herein by reference in their entirety.

Claims

1. A negative electrode for use in a lithium-ion electrochemical cell comprising a composition comprising graphitic carbon nitride and an optionally modified allotrope of carbon.

2. A negative electrode according to claim 1 , wherein the composition comprises a mixture of particles comprising the graphitic carbon nitride and particles comprising the optionally modified allotrope of carbon, which is or comprises graphite.

3. A negative electrode according to claim 2, wherein the composition comprises the graphitic carbon nitride and the graphite in the relative w/w ratio of 20: 1 to 1 :20.

4. A negative electrode according to claim 2, wherein the composition comprises the graphitic carbon nitride and the graphite in the relative w/w ratio of 1 : 1 to 1 : 1.5.

5. A negative electrode according to claim 2, wherein the composition comprises the graphitic carbon nitride and the graphite in the relative w/w ratio of 1 : 1.2 to 1 : 1.3.

6. A negative electrode according to any one of the preceding claims, wherein the composition further comprises a binder.

7. A negative electrode according to claim 6, wherein the binder is selected from ethylene propylene diene monomer, styrene butadiene rubber, carboxy methyl cellulose , a polyvinylidine fluoride, polytetrafiuoroethylene and polyvinyl acetate.

8. A negative electrode according to any one of the preceding claims, further comprising a substrate comprising an electrically conducting material, disposed on which is the composition comprising graphitic carbon nitride and an optionally modified allotrope of carbon.

9. A negative electrode according to claim 8, wherein the electrically conducting material is a metal in elemental form or an alloy of two or more metals.

10. A negative electrode according to claim 9, wherein the metal is selected from from copper, nickel, aluminium and cobalt.

1 1. A negative electrode according to any one of the preceding claims, wherein the composition comprises graphitic carbon nitride and graphite in the relative w/w ratio of 1 :1 to 1 : 1.5 and a binder, and the composition is disposed on a metal in elemental form or an alloy of two or more metals.

12. A lithium-ion electrochemical cell comprising a negative electrode according to any one of the preceding claims, a positive electrode and a lithium-ion transport medium disposed between the negative electrode and the positive electrode.

13. A lithium-ion electrochemical cell according to claim 12, wherein the lithium-ion transport medium comprises a lithium-ion conducting polymer.

14. A lithium-ion electrochemical cell according to claim 12, wherein the lithium-ion transport medium comprises an electrolyte comprising a solvent and a lithium salt.

15. A lithium-ion electrochemical cell according to any one of claims 12 to 14, wherein the electrochemical cell is a rechargeable battery.

16. A composition comprising graphitic carbon nitride and graphite.

17. A composition according to claim 16, wherein the composition comprises a mixture of particles of the graphitic carbon nitride and particles of the graphite.

18. A composition according to claim 16 or claim 17 wherein the composition comprises the graphitic carbon nitride and the graphite in the relative w/w ratio of 20:1 to 1 :20.

19. A composition according to any one of claims 16 to 18, wherein the composition comprises the graphitic carbon nitride and the graphite in the relative w/w ratio of 1 : 1 to 1 : 1.5.

20. A composition according to any one of claims 16 to 19, wherein the composition comprises the graphitic carbon nitride and the graphite in the relative w/w ratio of 1 : 1.2 to 1 : 1.3.

21. A composition according to any one of claims 16 to 20, further comprising a binder.

22. A composition according to claim 21 , wherein the binder is selected from ethylene propylene diene monomer, styrene butadiene rubber, carboxy methyl cellulose , a polyvinylidine fluoride, polytetrafiuoroethylene and polyvinyl acetate.

23. A method for making the composition according to any one of claims 16 to 22, wherein the method comprises mixing graphitic carbon nitride and graphite.

24. A method according to claim 23, wherein the method comprises mixing together particles comprising graphitic carbon nitride and particles comprising graphite.

25. A method according to claim 24, wherein the particles comprising graphitic carbon nitride and particles comprising graphite are mixed together with a binder and a liquid carrier to form a slurry.

26. A method according to claim 25, wherein the slurry is deposited on an electrically conducting substrate, and the liquid carrier removed, to form a solid composition comprising the particles comprising graphitic carbon nitride, the particles comprising graphite and the binder on the electrically conducting substrate.