WO2023277815A2 - A method of producing metal nitride particles - Google Patents

Abstract

There is provided a method of producing metal nitride particles. There are also provided core-shell particles having a sulfur-containing core and a metal nitride outer shell. There is further provided a positive electrode material comprising the core-shell particles. There are further provided a positive electrode comprising the positive electrode material and a method of preparing the same. There are further provided an electrochemical cell comprising the positive electrode as described herein and a battery comprising at least one electrochemical cell as described herein.

Description

A Method Of Producing Metal Nitride

Particles

References to Related Application

This application claims priority to Singapore application number 10202107235W filed with the Intellectual Property Office of Singapore on 30 June 2021, the contents of which is hereby incorporated by reference.

Technical Field

The present invention generally relates to a method of producing metal nitride particles. The present invention also relates to core-shell particles having a sulfur- containing core and a metal nitride outer shell. The present invention further relates to a positive electrode material comprising the core-shell particles. The present invention further relates to a positive electrode comprising the positive electrode material and a method of preparing the same. The present invention further relates to an electrochemical cell comprising the positive electrode as described herein and a battery comprising at least one electrochemical cell as described herein.

Background Art

Hollow nanoparticles have unique properties that make them good candidates for various energy applications, such as lithium batteries, supercapacitors, solar cells and electrocatalysts. Their structural features include abundant inner void space, high surface-to-volume ratio, short mass and charge transport lengths, and high volumetric loading capacity. Various types of hollow structures, such as single shell and multi shell structures, single-holed and mesoporous structures, and yolk-shell structure have been developed to-date. Conventional hollow nanostructure synthesis approaches are based on the nature of the templates, such as hard templates (e.g. S1O2, or polystyrene sphere), soft templates (e.g. gas bubbles and micelles) and self templates. Other conventional template-free synthesis approaches using Kirkendal effect and Ostwald ripening are also known. The conventional approaches involve complex operational difficulties in the approaches per se and/or in the process of preparing templates.

Li-S batteries, which have high theoretical capacity (1675 mAh/g) and are low in cost, show great potential as a next- generation energy storage device. Using a hollow structure to host S nanocrystals offers an inner void space to buffer large volume changes during the conversion between S and Li2S, and confine the lithium polysulfide to avoid dissolution into the electrolyte, causing a shuttling effect. High volumetric loading capacity would increase practical energy density, which is vital for the commercialization of Li-S batteries. Hollow metal oxides, such as manganese oxide and titanium oxide, have been used conventionally as hosts of S nanocrystals in Li-S battery to prevent the loss of active sulfur and for strong binding to polysulfide through polar-polar interaction to minimize the shuttling effect. However, the low electrical conductivity of these materials results in low sulfur utilization. Transition metal nitrides with high ionic and electrical conductivity have attracted much interest in recent years for Li-S battery application. Different morphologies of TiN nanocomposites, such as mesoporous TiN, pellet TiN and hollow carbon supported TiN, have been synthesized. DFT calculation and experimental data have verified that metal nitride materials with stable structures have better tolerance to volume expansion during the lithiation and delithiation processes, as well as high ionic and electrical conductivity, resulting in fast diffusion of Li ions and electrons in the electrode materials.

Accordingly, there is a need for a method of preparing metal nitride particles that ameliorates one or more disadvantages mentioned above.

Summary

In one aspect, there is provided a method of producing metal nitride particles, comprising the steps of:

(a) reacting a metal oxide precursor with carbon particles in the presence of a solvent to form core-shell particles having a carbon core and a metal oxide shell disposed thereon;

(b) treating the core-shell particles of step (a) thermally to remove at least a portion of the carbon core thereby forming metal oxide particles; and

(c) reacting the hollow metal oxide particles of step (b) with a nitrogen source to form the metal nitride particles.

Advantageously, the method uses carbon particles as the carbon core, which are easily synthesized in large quantities.

Further advantageously, the carbon core may act as a sacrificial core (or template), which may be readily removed by a single thermal treatment step. The carbon core does not require complex physical or chemical treatments to be removed. The carbon core may be completely removed during the thermal treatment step.

In another aspect, there are provided core- shell particles having a sulfur-containing core and a metal nitride outer shell, produced by the method as described herein.

In another aspect, there is provided a positive electrode material comprising the core shell particles having a sulfur-containing core and a metal nitride outer shell as described herein, wherein the sulfur-containing core acts as an electrochemically active material. In another aspect, there is provided a positive electrode comprising the positive electrode material as described herein.

In another aspect, there is provided a method of preparing a positive electrode, comprising the steps of:

(a) mixing the core-shell particles having a sulfur-containing core and a metal nitride outer shell as described herein, a solvent and optionally at least one of an electronically conductive material, a binder and an additive to form a mixture; and

(b) coating the mixture of step (a) onto a substrate to form the positive electrode.

In another aspect, there is provided an electrochemical cell comprising the positive electrode as described herein, a negative electrode and an electrolyte in fluid communication with both the positive electrode and the negative electrode.

In another aspect, there is provided a battery comprising at least one electrochemical cell as described herein.

Definitions

The following words and terms used herein shall have the meaning indicated:

The term “nanoparticles” as used herein refers to particles having an average diameter in the range of about 1 nm to about 500 nm.

The term “nanocomposites” as used herein refers to nanoparticles that comprise more than one distinct compound in them, forming distinct regions in each nanoparticle with different properties. Therefore, the nanocomposites as described herein may also be regarded as multiphase nanoparticles.

The term “average” as used herein when referring to a dimension (such as diameter or thickness) describes an average number of the dimension as measured via transmission electron microscopy.

The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the invention.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

The term "about" as used herein typically means +/- 5 % of the stated value, more typically +/- 4 % of the stated value, more typically +/- 3 % of the stated value, more typically, +/- 2 % of the stated value, even more typically +/- 1 % of the stated value, and even more typically +/- 0.5 % of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Certain embodiments may also be described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

Detailed Disclosure of Embodiments

Exemplary, non-limiting embodiments of a method of producing metal nitride particles will now be disclosed.

The method may comprise the steps of: a) reacting a metal oxide precursor with carbon particles in the presence of a solvent to form core-shell particles having a carbon core and a metal oxide shell disposed thereon; b) treating the core-shell particles of step (a) thermally to remove at least a portion of the carbon core thereby forming metal oxide particles; and c) reacting the metal oxide particles of step (b) with a nitrogen source to form the metal nitride particles.

The metal nitride particles may be hollow metal nitride particles. The hollow metal nitride particles may comprise a shell of the metal nitride that defines a hollow space or a void space within the shell. Where the hollow metal nitride particles are further processed, the hollow space may house other materials such as sulfur, which may be regarded as a core but which does not form a tight fit with the metal nitride shell, that is, there may still be void spaces in between the sulfur core and the metal nitride shell. Where the hollow space is occupied by other materials, the combination of the hollow metal nitride particles and the other materials may be regarded as core-shell particles.

The metal oxide precursor may be a metal salt, a metal oxide, a metal alkoxide, a metal carboxylate, a metal halide or combinations thereof when compatible. The metal of the metal oxide precursor may be a transition or a post-transition metal salt. Non- limiting examples of the metal include titanium, vanadium, aluminium, zirconium, tantalum, hafnium and combinations thereof when compatible. The metal may be selected from the group consisting of titanium, vanadium, zirconium and hafnium. As an example, the metal may be titanium. As an example, the metal oxide precursor may be titanium (IV) butoxide.

The metal oxide precursor may be provided at a concentration in the range of about 2 mM to about 12 mM, about 4 mM to about 12 mM, about 6 mM to about 12 mM, about 8 mM to about 12 mM, about 10 mM to about 12 mM, about 2 mM to about 10 mM, about 2 mM to about 8 mM, about 2 mM to about 6 mM, about 2 mM to about 4 mM or about 4 mM to about 6 mM. The concentration of the metal oxide precursor may be suitably selected, which determines a thickness of the metal oxide shell formed in the reacting step (a). The thickness may be increased with higher concentrations of the metal oxide precursor.

In the reacting step (a), the metal oxide precursor may react with the solvent to form the metal oxide shell. The metal oxide shell may at least partially enclose or encapsulate the carbon particles, which serve as the carbon core. As an example, the carbon core may be partially covered by the metal oxide shell during step (a) to form the core-shell particles. As another example, the carbon core may be completely covered by the metal oxide shell. The carbon core may be made up of a single carbon particle.

The carbon particles may be nanoparticles or microparticles, which may be freshly formed or obtained from commercial sources. The carbon particles may have a morphology that is not particularly limited. As an example, the carbon particles may be substantially spherical. The carbon particles may be spherical.

The carbon particles may have an average diameter in the range of about 100 nm to about 500 nm, about 200 nm to about 500 nm, about 300 nm to about 500 nm, about 400 nm to about 500 nm, about 100 nm to about 400 nm, about 100 nm to about 300 nm, about 100 nm to about 200 nm or about 200 nm to about 400 nm.

The carbon particles may be provided at a concentration in the range of about 0.1 g/L to about 0.3 g/L, about 0.15 g/L to about 0.3 g/L, about 0.2 g/L to about 0.3 g/L, about 0.25 g/L to about 0.3 g/L, about 0.1 g/L to about 0.25 g/L, about 0.1 g/L to about 0.2 g/L, about 0.1 g/L to about 0.15 g/L or about 0.15 g/L to about 0.25 g/L.

The thickness of the metal oxide shell may alternatively or additionally be determined by a ratio between a total volume of the metal oxide precursor and a total weight of the carbon particles. The thickness may be increased by increasing this ratio.

The core-shell particles formed in the reacting step (a) may be dispersed in the solvent. The solvent may determine a thickness of the metal oxide shell. The solvent may be a polar solvent. Non-limiting examples of solvent include methanol, ethanol, propanol, acetonitrile, propionitrile, water, and miscible combinations thereof. As an example, the solvent may be a mixture of ethanol, acetonitrile, and water having a volume ratio of about 187.5:62.5:2.7.

The reacting step (a) may further comprise the steps of (i) dispersing the carbon particles in the solvent; and (ii) adding the metal oxide precursor. The dispersing step may be undertaken by stirring the carbon particles and the solvent at a stirring rate in the range of about 200 rpm to about 400 rpm, about 200 rpm to about 300 rpm or about 300 rpm to about 400 rpm.

The mixture may be stirred for a duration in the range of about 20 minutes to about 40 minutes, about 25 minutes to about 40 minutes, about 30 minutes to about 40 minutes, about 35 minutes to about 40 minutes, about 20 minutes to about 35 minutes, about 20 minutes to about 30 minutes, about 20 minutes to about 25 minutes or about 25 minutes to about 35 minutes.

The method may further comprise a step of pre-treating the core-shell particles formed in step (a) after step (a) but before step (b).

The pre-treating step may comprise stirring, centrifuging, washing and/or drying of the core-shell particles formed in step (a).

The stirring of the core- shell particles may be undertaken for a duration in the range of about 10 hours to 30 hours, about 15 hours to about 30 hours, about 20 hours to about 30 hours, about 25 hours to about 30 hours, about 10 hours to about 25 hours, about 10 hours to about 20 hours, about 10 hours to about 15 hours or about 15 hours to about 25 hours.

The centrifuging of the core- shell particles may be undertaken at a rate in the range of about 7000 rpm to about 9000 rpm, about 7000 rpm to about 8000 rpm or about 8000 rpm to about 9000 rpm.

The centrifuging of the core-shell particles may be undertaken for a duration in the range of about 10 minutes to about 20 minutes, about 10 minutes to about 15 minutes or about 15 minutes to about 20 minutes.

As an example, the drying of the core-shell particles may be undertaken at room temperature under vacuum.

The washing of the core- shell particles may be undertaken using a washing solvent. As an example, the washing solvent may be ethanol.

The method may not comprise a step of treating or pre-treating the core-shell particles with an acid and/or a base. Advantageously, the absence of acid and/or base treatments may result in the core-shell particles having a substantially smoother surface compared to core-shell particles obtained using a method comprising an acid and/or a base treatment.

In the treating step (b), the core-shell particles formed in step (a) may be heated at a temperature and for a duration sufficient to remove at least a portion of the carbon core thereby forming metal oxide particles.

The core-shell particles formed in step (a) may be heated at a temperature in the range of about 300 °C to about 500 °C, about 350 °C to about 500 °C, about 400 °C to about 500 °C, about 450 °C to about 500 °C, about 300 °C to about 450 °C, about 300 °C to about 400 °C, about 300 °C to about 350 °C or about 350 °C to about 450 °C. As an example, the core-shell particles formed in step (a) may be heated at a temperature in the range of about 300 °C to about 450 °C. Where the temperature is higher than about 450 °C, the metal oxide shell may be significantly damaged (such as by shrinking or collapsing). Advantageously, where the temperature is lower than or equal to about 450 °C, the metal oxide shell would not be significantly damaged or would take less damage from heating. Therefore, the metal oxide particles formed by the method as described herein may have better quality as compared to metal oxide particles formed by heating at a temperature that is higher than about 450 °C.

The temperature may be reached at a ramping rate in the range of about 0.5 °C/minute to about 1.5 °C/minute, about 1 °C/minute to about 1.5 °C/minute, or about 0.5 °C/minute to about 1 °C/minute.

The treating step (b) may be undertaken for a duration in the range of about 1 hour to about 3 hours, about 2 hours to about 3 hours or about 1 hour to about 2 hours.

As an example, the treating step (b) may be undertaken in an air atmosphere.

In the reacting step (c), the metal oxide particles formed in step (b) may react with the nitrogen source to form the metal nitride particles while retaining the shape of the metal oxide particles.

Non- limiting examples of the nitrogen source include ammonia, urea, and combinations thereof.

As an example, the nitrogen source may be ammonia. The ammonia may react with the metal oxide particles by flowing through the metal oxide particles at a rate in the range of about 400 cm³/minute to about 600 cm³/minute, about 500 cm³/minute to about 600 cm³/minute or about 400 cm³/minute to about 500 cm³/minute.

The reacting step (c) may be undertaken for a duration to allow for sufficient nitridation of the metal oxide particles formed in step (b). The reacting step (c) may be undertaken for a duration in the range of about 1 hour to about 5 hours, about 2 hours to about 5 hours, about 3 hours to about 5 hours, about 4 hours to about 5 hours, about 1 hour to about 4 hours, about 1 hour to about 3 hours, about 1 hour to about 2 hours or about 2 hours to about 4 hours.

The reacting step (c) may further comprise a step of heating the metal oxide particles and the nitrogen source.

The heating step may be undertaken at a temperature and for a duration to allow for sufficient nitridation of the metal oxide particles. The heating step may be undertaken at a temperature in the range of about 700 °C to about 900 °C, about 750 °C to about 900 °C, about 800 °C to about 900 °C, about 850 °C to about 900 °C, about 700 °C to about 850 °C, about 700 °C to about 800 °C, about 700 °C to about 750 °C or about 750 °C to about 850 °C.

The heating step may be undertaken for a duration in the range of about 1 hour to about 5 hours, about 2 hours to about 5 hours, about 3 hours to about 5 hours, about 4 hours to about 5 hours, about 1 hour to about 4 hours, about 1 hour to about 3 hours, about 1 hour to about 2 hours or about 2 hours to about 4 hours.

The method may further comprise a step of (d) mixing the metal nitride particles formed in the reacting step (c) with a sulfur-containing compound to produce a mixture. As an example, the sulfur-containing compound may be sulfur. In the mixing step, the sulfur-containing compound and the metal nitride particles may be mixed by milling, grounding or combinations thereof.

The sulfur-containing compound and the metal nitride particles may be mixed at a weight ratio in the range of about 1:2 to about 1:4, about 1:2.5 to about 1:4, about 1:3 to about 1:4, about 1:3.5 to about 1:4, about 1:2 to about 1:3.5, about 1:2 to about 1:3, about 1:2 to about 1:2.5 or about 1:2.5 to about 1:3.5.

The method may further comprise a step of (e) heating the mixture produced in the mixing step (d) at an elevated temperature under an inert gas atmosphere for a duration sufficient to cause the sulfur-containing compound to melt diffuse into the metal nitride particles, thereby forming core-shell particles having a sulfur- containing core and a metal nitride outer shell. Therefore, the metal nitride particles produced by the method may be the core-shell particles having the sulfur-containing core and the metal nitride outer shell.

The heating step (e) may be undertaken at a temperature and for a duration sufficient to allow at least a part of the sulfur-containing compound to melt diffuse into the metal nitride particles. The heating step (e) may be undertaken at a temperature and for a duration sufficient to allow a substantially complete melt diffusion of the sulfur- containing compound into the metal nitride particles.

In the heating step (e), the temperature may be an elevated temperature. The elevated temperature may be in the range of about 115 °C to about 165 °C, about 125 °C to about 165 °C, about 135 °C to about 165 °C, about 145 °C to about 165 °C, about 155 °C to about 165 °C, about 160 °C to about 165 °C or about 155 °C to about 160 °C.

The duration in the heating step (e) may be about 15 hours to about 25 hours, about 20 hours to about 25 hours or about 15 hours to about 20 hours.

The mixing step (d) (where present) and the heating step (e) (where present) may be performed sequentially, simultaneously, or partially overlapping in time with each other. As an example, the mixing step (d) (where present) and the heating step (e) (where present) maybe performed sequentially, and the mixing step (d) may be performed before the heating step (e).

Exemplary, non-limiting embodiments of core-shell particles having a sulfur- containing core and a metal nitride outer shell will now be disclosed.

The core-shell particles having the sulfur-containing core and the metal nitride outer shell may be produced by the method as described herein.

The metal nitride outer shell may be substantially spherical with an internal void space. The metal nitride outer shell may be porous. The metal nitride outer shell may allow molecules having a size of less than about 50 nm to diffuse through. The internal void space may host other nanoparticles, such as sulfur nanocrystals. The core- shell particles may be regarded as being hollow due to the presence of the internal void space.

Advantageously, the internal void space of the core-shell particles may buffer large volume variations caused by the sulfur-containing core or the other nanoparticles hosted inside. Further advantageously, the core-shell particles may have a high affinity for polysulfides, thus allowing them to be used in a positive electrode to reduce polysulfides dissolution and shuttling.

Non-limiting examples of the metal nitride include titanium nitride, aluminium nitride, manganese nitride, zirconium nitride, tantalum nitride, hafnium nitride and combinations thereof. As an example, the metal nitride may be titanium nitride.

The core-shell particles may have an average diameter in the range of about 200 nm to about 400 nm, about 250 nm to about 400 nm, about 300 nm to about 400 nm, about 350 nm to about 400 nm, about 200 nm to about 350 nm, about 200 nm to about 300 nm, about 200 nm to about 250 nm or about 250 nm to about 350 nm.

The metal nitride outer shell may have an average thickness in the range of about 22 nm to about 120 nm, about 30 nm to about 120 nm, about 60 nm to about 120 nm, about 22 nm to about 60 nm or about 22 nm to about 30 nm. Advantageously, core shell particles having a substantially lower metal nitride outer shell thickness may result in substantially improved performances when used in a battery.

The metal nitride outer shell may not have an average thickness lower than about 22 nm. Advantageously, the core-shell particles having an average metal nitride outer shell thickness of at least about 22 nm may be substantially stable during charging and discharging processes when used in a battery.

The metal nitride outer shell may have an average thickness of about 23 nm.

The core-shell particles may have an average Brunauer-Emmett-Teller (B.E.T.) surface area, calculated according to their nitrogen sorption at a temperature of 77 K in the range of about 20 m²/g to about 50 m²/g, about 30 m²/g to about 50 m²/g, about 40 m²/g to about 50 m²/g, about 20 m²/g to about 40 m²/g, about 20 m²/g to about 30 m²/g or about 30 m²/g to about 40 m²/g.

The sulfur-containing core may comprise elemental sulfur (Ss), polysulfides, sulfur nanocrystals or combinations thereof.

The sulfur-containing core may be present in the core-shell particles at a concentration in the range of about 75 weight% to about 85 weight%, about 77 weight% to about 85 weight%, about 79 weight% to about 85 weight%, about 81 weight% to about 85 weight%, about 83 weight% to about 85 weight%, about 75 weight% to about 83 weight%, about 75 weight% to about 81 weight%, about 75 weight% to about 79 weight% or about 75 weight% to about 77 weight%, based on the total weight of the metal nitride outer shell.

Exemplary, non-limiting embodiments of a positive electrode material will now be disclosed.

The positive electrode material may comprise the core-shell particles having the sulfur-containing core and the metal nitride outer shell as described herein, wherein the sulfur-containing core acts as an electrochemically active material.

The positive electrode material may further comprise at least one of an electronically conductive material, a binder and an additive. Non-limiting examples of the electronically conductive material include carbon black ( e.g . Ketjen™ black and Super P™), acetylene black (e.g. Shawinigan black and Denka™ black), graphite, graphene, graphene oxide, reduced graphene oxide (RGO), carbon fibers (e.g. vapor grown carbon fibers (VGCFs) and carbon nanofibers), carbon nanotubes (CNTs), carbon nanowires and combinations thereof.

Non- limiting examples of the binder include fluorine-containing polymers (e.g. polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)), synthetic or natural rubber (e.g. ethylene propylene diene monomer rubber (EPDM)), and ion- conductive polymer binders such as a copolymer composed of at least one lithium - ion solvating segment, such as a polyether, and at least one cross-linkable segment (e.g. PEO-based polymers comprising methyl methacrylate units). The binder may be a fluorine-containing polymer binder. As an example, the fluorine-containing polymer binder may be PVDF.

Non-limiting examples of the additive include ionic conductors, inorganic particles, glass or ceramic particles, nanoceramics (for example, AI₂O₃, T1O₂, S1O₂ and other similar compounds), salts (for example, lithium salts) and other similar components. As an example, the similar component may be an ionic conductor selected from the group consisting of sodium superionic conductor (NASICON), lithium superionic conductor (LISICON), thio-LiSICON, garnets, sulfides, sulfur halides, phosphates and thio-phosphates, of crystalline and/or amorphous form, and combinations thereof.

Exemplary, non-limiting embodiments of a positive electrode will now be disclosed.

The positive electrode may comprise the positive electrode material as described herein on a current collector. Alternatively, the positive electrode may comprise the positive electrode material as a self-standing positive electrode.

Non-limiting examples of the current collector include an aluminum foil, a copper foil, an aluminium mesh, a carbon-coated aluminium mesh, a nickel foil or combinations thereof.

Exemplary, non-limiting embodiments of a method of preparing a positive electrode will now be disclosed.

The method may comprise the steps of: a) mixing the core- shell particles having a sulfur-containing core and a metal nitride outer shell as described herein, a solvent and optionally at least one of an electronically conductive material, a binder and an additive to form a mixture; and b) coating the mixture of step (a) onto a substrate to form the positive electrode.

In the mixing step (a), non-limiting examples of the electronically conductive material include carbon black (e.g. Ketjen™ black and Super P™), acetylene black (e.g. Shawinigan black and Denka™ black), graphite, graphene, graphene oxide, reduced graphene oxide (RGO), carbon fibers (e.g. vapor grown carbon fibers (VGCFs) and carbon nanofibers), carbon nanotubes (CNTs), carbon nanowires and combinations thereof. The electronically conductive material may be a combination of RGO and VGCF at a weight ratio of about 2:1, about 1:2 or about 1:1.

In the mixing step (a), the core-shell particles and the electronically conductive material (where present) may be provided at a weight ratio in the range of about 2: 1 to about 5:1, about 3:1 to about 5:1, about 4:1 to about 5:1, about 2:1 to about 4:1, about 2: 1 to about 3: 1 or about 3: 1 to about 4:1.

Non- limiting examples of the binder include fluorine-containing polymers ( e.g . polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)), synthetic or natural rubber (e.g. ethylene propylene diene monomer rubber (EPDM)), and ion- conductive polymer binders such as a copolymer composed of at least one lithium - ion solvating segment, such as a polyether, and at least one cross-linkable segment (e.g. PEO-based polymers comprising methyl methacrylate units). The binder may be a fluorine-containing polymer binder. As an example, the fluorine-containing polymer binder may be PVDF.

The electronically conductive material (where present) and the binder (where present) may be provided at a weight ratio in the range of about 3:1 to about 1:1, about 2:1 to about 1:1 or about 3:1 to about 2:1.

Non-limiting examples of the additive include ionic conductors, inorganic particles, glass or ceramic particles, nanoceramics (for example, AI2O3, T1O2, S1O2 and other similar compounds), salts (for example, lithium salts) and other similar components. As an example, the similar component may be an ionic conductor selected from the group consisting of NASICON, LISICON, thio-LiSICON, garnets, sulfides, sulfur halides, phosphates and thio-phosphates, of crystalline and/or amorphous form, and combinations thereof.

The solvent may be N-Methylpyrrolidone (NMP), N,N-Dimethylformamide (DMF) or a combination thereof.

In the coating step (b), the substate may be a current collector. Non-limiting examples of the current collector include an aluminum foil, a copper foil, an aluminium mesh, a carbon-coated aluminium mesh, a nickel foil and combinations thereof. As an example, the mixture of step (a) may be coated directly onto the current collector. As another example, the mixture of step (a) may be coated as a thin sheet and then punched and pressed onto the current collector. As another example, the positive electrode is a self-standing positive electrode.

The coating step (b) may be undertaken by doctor blade coating, comma coating, reverse-comma coating, gravure coating, slot-die coating or combinations thereof.

Where the current collector is an aluminium mesh, the mixture of step (a) may be coated at an amount sufficient to obtain a sulfur loading in the range of about 0.5 mg/cm² to about 1.5 mg/cm², about 1 mg/cm² to about 1.5 mg/cm² or about 0.5 mg/cm² to about 1 mg/cm².

Where the current collector is a carbon-coated aluminium mesh, the mixture of step (a) may be coated at an amount sufficient to obtain a sulfur loading in the range of about 3 mg/cm² to about 4.5 mg/cm², about 3.5 mg/cm² to about 4.5 mg/cm², about 4 mg/cm² to about 4.5 mg/cm², about 3 mg/cm² to about 4 mg/cm², about 3 mg/cm² to about 3.5 mg/cm² or about 3.5 mg/cm² to about 4 mg/cm². Exemplary, non-limiting embodiments of an electrochemical cell will now be disclosed.

The electrochemical cell may comprise the positive electrode as described herein, a negative electrode and an electrolyte in fluid communication with both the positive electrode and the negative electrode.

The positive electrode may have a sulfur loading in the range of about 0.5 mg/cm² to about 4.5 mg/cm², about 1.5 mg/cm² to about 4.5 mg/cm², about 2.5 mg/cm² to about 4.5 mg/cm², about 3.5 mg/cm² to about 4.5 mg/cm², about 0.5 mg/cm² to about 3.5 mg/cm², about 0.5 mg/cm² to about 2.5 mg/cm² or about 0.5 mg/cm² to about 1.5 mg/cm².

In the electrochemical cell, the negative electrode may comprise an electrochemically active material which may be any known material and is selected for its electrochemical compatibility with the electrochemical cell. The electrochemically active material of the negative electrode may be selected for its electrochemical compatibility with the positive electrode material as described herein. Non-limiting examples of the electrochemically active material of the negative electrode include alkali metals, alkali metal alloys and pre-lithiated or pre-sodiated electrochemically active materials. As an example, the electrochemically active material of the negative electrode may comprise an alkali metal, or an alloy thereof. As an example, the electrochemically active material of the negative electrode may be lithium metal or sodium metal.

In the electrochemical cell, any electrolyte may be considered so long as it is compatible with the electrochemical cell. The electrolyte may be a liquid electrolyte comprising a salt in a solvent. Alternatively, the electrolyte may be a gel electrolyte comprising a salt in a solvent, which may further comprise a solvating polymer. Alternatively, the electrolyte may be a solid polymer electrolyte (SPE) comprising a salt in a solvating polymer. Alternatively, the electrolyte may be a glass or ceramic electrolyte.

Where the electrolyte comprises a salt, said salt may be an ionic salt, such as a lithium salt, a sodium salt, a potassium salt or a magnesium salt. Non-limiting examples of the lithium salt include lithium hexafluorophosphate (LiPF_f,), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium 2-trifluoromethyl-4,5-dicyanoimidazolate (LiTDI), lithium 4,5- dicyano-l,2,3-triazolate (LiDCTA), lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium nitrate (LiNCE), lithium chloride (LiCl), lithium bromide (LiBr), lithium fluoride (LiF), lithium perchlorate (FiC10₄), lithium hexafluoroarsenate (LiAsF_f,), lithium trifluoromethanesulfonate (LiSO¾CF¾) (FiTf), lithium fluoroalkylphosphate Fi[PF3(CF2CF3)3] (FiFAP), lithium tetrakis(trifluoroacetoxy)borate Fi[B(OCOCF3)4] (FiTFAB), lithium bis(l,2-benzenediolato(2-)-0,0')borate Fi[B(C6C>2)2] (FBBB) or combinations thereof.

As an example, the lithium salt may be a combination of FiTFSI and F1NO3 at a molar ratio in the range of about 1:0.2 to about 1:0.4, about 1:0.3 to about 1:0.4 or about 1:0.2 to about 1:0.3. Where the electrolyte comprises a solvent, the solvent may be a non-aqueous solvent. Non-limiting examples of the non-aqueous solvent include cyclic carbonates, such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC); acyclic carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), and dipropyl carbonate (DPC); lactones such as g-butyrolactone (g-BL) and g-valerolactone (g-VL); chain ethers, such as 1,2-dimethoxy ethane (DME), 1,2-diethoxyethane (DEE), ethoxymethoxyethane (EME), trimethoxymethane, and ethylmonoglyme; cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane (DOL) and dioxolane derivatives; and other solvents such as dimethylsulfoxide, formamide, acetamide, dimethylformamide, acetonitrile, propylnitrile, nitromethane, phosphoric acid triester, sulfolane, methylsulfolane, propylene carbonate derivatives and combinations thereof.

As an example, the solvent may be a solvent mixture of DOL and DME at a volume ratio in the range of about 0.5: 1 to about 2: 1, about 1: 1 to about 2: 1 or about 0.5:1 to about 1:1.

Where the electrolyte the salt and the solvent, the salt may be present at a molar concentration in the range of about 1 M to about 2 M, about 1.5 M to about 2 M or about 1 M to about 1.5 M.

Where the electrochemical cell comprises a gel electrolyte or a liquid electrolyte, the electrochemical cell may further comprise a separator. The gel electrolyte or the liquid electrolyte may impregnate the separator. The separator may be a porous membrane of polymers. Non-limiting examples of the polymers include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), and copolymers thereof.

As an example, the separator may be a polypropylene separator.

The electrolyte may be a gel polymer electrolyte. The gel polymer electrolyte may comprise, for example, a polymer precursor, a salt ( e.g . as described above), a solvent ( e.g . as defined above), and a polymerization and/or cross-linking initiator when required.

The electrolyte may be a solid polymer electrolyte comprising a salt in a solvating polymer. The solid polymer electrolyte may be selected from any known solid polymer electrolytes compatible with the electrochemical cell. As an example, the solid polymer electrolyte may be selected for its compatibility with lithium. Solid polymer electrolytes may generally comprise one or more solid polar polymers, optionally crosslinked, and a salt (e.g. as described above). Polyether-type polymers may be used, such as those based on PEO, but several other compatible polymers are also known for the preparation of solid polymer electrolytes. The polymer may be further crosslinked.

As an example, the electrolyte may be a liquid electrolyte comprising a salt as described above in a solvent as described above and impregnate a separator as described above.

The electrolyte may optionally comprise at least one additional component, such as ionically conductive materials, inorganic particles, glass or ceramic particles; for instance, nano-ceramics ( e.g . AI2O3, T1O2, S1O2, and other similar compounds), and the like. The additional component may be selected from NASICON, LISICON, thio- LISICON, garnet, sulfide, sulfide-halide, phosphate, thio-phosphate, and their combinations, in crystalline and/or amorphous form. As an example, the additional component may be substantially dispersed within the electrolyte. Alternatively, the additional component may be in a separate layer.

The electrochemical cell comprising the positive electrode as described herein may have substantially increased electrochemical performances (for example, increased capacity and cyclability) compared to electrochemical cells comprising having a core-shell C@TiN-S nanocomposite or S nanocrystals as a positive electrochemically active material.

As an example, the electrochemical cell may maintain a specific capacity of at least about 750 mAh/g after 500 charge and discharge cycles recorded at a C-rate of 0.5 C and at room temperature.

The electrochemical cell may maintain a specific capacity in the range of about 1000 mAh/g to about 1050 mAh/g, about 1010 mAh/g to about 1050 mAh/g, about 1020 mAh/g to about 1050 mAh/g, about 1030 mAh/g to about 1050 mAh/g, about 1040 mAh/g to about 1050 mAh/g, about 1000 mAh/g to about 1040 mAh/g, about 1000 mAh/g to about 1030 mAh/g, about 1000 mAh/g to about 1020 mAh/g or about 1000 mAh/g to about 1010 mAh/g, after 100 charge and discharge cycles recorded at a C- rate of 0.2 C and at room temperature.

Where the positive electrode material has a sulfur loading of about 3.75 mg/cm², the electrochemical cell may maintain a specific capacity in the range of about 830 mAh/g to about 850 mAh/g, about 840 mAh/g to about 850 mAh/g or about 830 mAh/g to about 840 mAh/g, after 200 charge and discharge cycles recorded at a C- rate of 0.2 C and at room temperature. As an example, the positive electrode material may have a specific capacity of about 1050 mAh/g after 100 charge and discharge cycles recorded at a C-rate of 0.1 C and at room temperature.

Exemplary, non-limiting embodiments of a battery will now be disclosed.

The battery may comprise at least one electrochemical cell as described herein. As an example, the battery may be a lithium sulfur battery.

Brief Description of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

FIG. 1

[Fig. 1] is a schematic diagram of a general procedure for preparing hollow metal nitride particles and core-shell metal nitride particles. FIG. 2A to FIG. 2G

[FIG. 2A] is a low-magnification transmission electron microscopy (TEM) image (at 86000x magnification) of hollow T1O2 nanoparticles prepared according to the method as described herein. [FIG. 2B] and [FIG. 2C] are high-magnification TEM images (at 400000x magnification) of hollow T1O2 nanoparticles prepared according to the method as described herein. [FIG. 2D] is a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (at 80000x magnification) of hollow T1O2 nanoparticles prepared according to the method as described herein. [FIG. 2E] and [FIG. 2F] are Ti and O energy-dispersive X-ray spectroscopy (EDX) maps, respectively, of the hollow T1O2 nanoparticles as shown in the box in FIG. 2D. [FIG. 2G] shows an EDX profile hollow T1O2 nanoparticles prepared according to the method as described herein.

FIG. 3A to FIG. 3E

[FIG. 3 A] shows powder XRD patterns of hollow T1O2, hollow TiN, C@TiN, hollow T1O2-S, hollow TiN-S and C@TiN-S particles prepared by the method as described herein.

[FIG. 3B] shows N2 adsorption-desorption isotherms of hollow T1O2, C@TiN and hollow TiN particles prepared by the method as described herein.

[FIG. 3C] shows TGA curves of hollow T1O2-S, C@TiN-S and hollow TiN-S particles prepared by the method as described herein.

[FIG. 3D] shows X-ray photoelectron spectroscopy (XPS) spectra of hollow T1O2, C@TiN and hollow TiN particles prepared by the method as described herein.

[FIG. 3E] shows Ti spectra of hollow T1O2, C@TiN and hollow TiN particles prepared by the method as described herein.

FIG. 4A to FIG. 4G

[FIG. 4A] and [FIG. 4B] are low-magnification TEM images (at 86000x magnification) of hollow TiN nanoparticles prepared according to the method as described herein. [FIG. 4C] is a high-magnification TEM image (at όOOOOOc magnification) of hollow TiN nanoparticles prepared according to the method as described herein. [FIG. 4D] and [FIG. 4E] are SEM (at 20000x magnification) and HAADF-STEM images (at 80000x magnification), respectively, of the hollow TiN nanoparticles. [FIG. 4F] and [FIG. 4G] are Ti and N EDX maps, respectively, of the hollow TiN nanoparticles as shown in the box in FIG. 4E.

FIG. 5A to FIG. 5G

[FIG. 5A] to [FIG. 5C] are TEM (at 86000x magnification), high-resolution TEM (HRTEM, at 400000x magnification) and HAADF-STEM images (at 80000x magnification), respectively, of C@TiN nanocomposites prepared by the method as described herein. [FIG. 5D] is a HAADF-STEM image (at 80000x magnification) of the C@TiN nanocomposites as shown in the box in FIG. 5C. [FIG. 5E] to [FIG. 5G] are Ti, N and C EDX maps of the C@TiN nanocomposites as shown in the box in FIG. 5C.

FIG. 6A to FIG. 6F

[FIG. 6A] to [FIG. 6C] are TEM (at 860000x magnification), HRTEM (at 600000 x magnification) and HAADF-STEM images (at 80000x magnification), respectively, of hollow TiN-S nanocomposites prepared by the method as described herein. [FIG. 6D] to [FIG. 6F] are Ti, N and S EDX maps of the hollow TiN-S nanocomposites as shown in the box in FIG. 6C.

FIG. 7 A to FIG. 7F

[FIG. 7A] to [FIG. 7F] show electrochemical properties of the Li-S battery as described herein. [FIG. 7A] to [FIG. 7C] show initial charging and discharging curves at 0.1 C, cycling performance at 0.2 C at room temperature, and performance at different current densities, respectively, for hollow TiN-S nanocomposites and core shell C@TiN-S nanocomposites prepared by the method as described herein as well as S nanocrystals. [FIG. 7D] shows cycling performance of hollow TiN-S nanocomposites and core-shell C@ TiN-S nanocomposites with a higher S loading (3.75 mg/cm2) at 0.1 C at room temperature.

FIG. 8

[FIG. 8] is a TEM image of spherical carbon nanoparticles (at _ 40000_x magnification) as described herein.

FIG. 9A to FIG. 9D

[FIG. 9A] to [FIG. 9D] show TEM (at 40000x magnification) and HAADF-STEM images (inset, at 40000x magnification) of C@Ti0₂ core-shell nanocomposites prepared by the method as described herein. The core-shell nanocomposites were prepared with 0.25 ml ([FIG. 9A]), 0.5 ml ([FIG. 9B]), 0.75 ml ([FIG. 9C]) or 1 ml ([FIG. 9D]) of titanium(IV) butoxide (TNB).

FIG. 10A to FIG. 10H

[FIG. 10A] to [FIG. 10H] are characterization results of hollow T1O2 nanoparticles of different shell thicknesses prepared by the method as described herein. The hollow T1O2 nanoparticles were prepared with 0.75 ml ([FIG. 10A] to [FIG. 10D]) or 1 ml ([FIG. 10E] to [FIG. 10H]) of TNB. [FIG. 10A] and [FIG. 10E] are TEM images (at 86000x magnification) of the hollow T1O2 nanoparticles. [FIG. 10B] and [FIG. 10F] are HAADF-STEM images (at 40000x magnification) of the hollow T1O2 nanoparticles. [FIG. IOC] and [FIG. 10D] are Ti EDX map and O EDX map, respectively, of the hollow T1O2 nanoparticles as shown in the box in [FIG. 10B]. [FIG. 10G] and [FIG. 10H] are Ti EDX map and O EDX map, respectively, of the hollow T1O2 nanoparticles as shown in the box in [FIG. 10F].

FIG. 11A to FIG. 11J

[FIG. 11 A] to [FIG. 11 J] are characterization results of hollow TiN nanoparticles of different shell thicknesses prepared by the method as described herein. The hollow TiN nanoparticles were prepared with 0.75 ml ([FIG. 11 A] to [FIG. 11E]) or 1 ml ([FIG. 1 IF] to [FIG. 11J]) of TNB. [FIG. 11 A] and [FIG. 11F] are low-magnification TEM images (at 86000x magnification) of the hollow TiN nanoparticles. [FIG. 1 IB] and [FIG. 11G] are high-magnification TEM images (at 400000x magnification) of the hollow TiN nanoparticles. [FIG. 11C] and [FIG. 11H] are HAADF-STEM images (at 80000x magnification) of the hollow TiN nanoparticles. [FIG. 11D] and [FIG. 1 IE] are Ti EDX map and N EDX map, respectively, of the hollow TiN nanoparticles as shown in the box in [FIG. 11C]. [FIG. 1 II] and [FIG. 11 J] are Ti EDX map and N EDX map, respectively, of the hollow TiN nanoparticles as shown in the box in [FIG. llH]

FIG. 12A to FIG. 12F

[FIG. 12 A] to [FIG. 12F] are TEM (at 86000x magnification), HRTEM (at 400000x magnification) and HAADF-STEM (at 80000x magnification) image, respectively, of C@TiN core-shell nanocomposites prepared by the method as described herein with 0.75 ml of TNB. [FIG. 12D] to [FIG. 12F] are Ti EDX map, N EDX map and C EDX map, respectively, of the C@TiN nanocomposites as shown in the box in [FIG. 12C]

FIG. 13A to FIG. 13L

[FIG. 13 A] to [FIG. 13L] are characterization results of hollow T1O2-S nanocomposites of different shell thicknesses prepared by the method as described herein. The hollow T1O2-S nanocomposites were prepared with 0.5 ml ([FIG. 13A] to [FIG. 13F]) or 0.75 ml ([FIG. 13G] to [FIG. 13L]) of TNB. [FIG. 13A] and [FIG. 13G] are low-magnification TEM images (at 86000x magnification) of the hollow T1O2-S nanocomposites. [FIG. 13B] and [FIG. 13H] are high-magnification TEM images (at 400000x magnification) of the hollow T1O2-S nanocomposites. [FIG. 13C] and [FIG. 131] are HAADF-STEM images (at 80000x magnification) of the hollow T1O2-S nanocomposites. [FIG. 13D] to [FIG. 13F] are Ti EDX map, O EDX map and S EDX map, respectively, of the hollow T1O2-S nanocomposites as shown in the box in [FIG. 13C]. [FIG. 13J] to [FIG. 13L] are Ti EDX map, O EDX map and S EDX map, respectively, of the hollow T1O2-S nanocomposites as shown in the box in [FIG. 131].

FIG. 14A to FIG. 14N

[FIG. 14A] to [FIG. 14N] are characterization results of C@TiN-S nanocomposites of different shell thicknesses prepared by the method as described herein. The C@TiN-S nanocomposites were prepared with 0.5 ml ([FIG. 14A] to [FIG. 14G]) or 0.75 ml ([FIG. 14H] to [FIG. 14N]) of TNB. [FIG. 14A] and [FIG. 14H] are low- magnification TEM images (at 86000x magnification) of the C@TiN-S nanocomposites. [FIG. 14B] and [FIG. 141] are high-magnification TEM images (at 400000x magnification) of the C@TiN-S nanocomposites. [FIG. 14C] and [FIG. 14J] are HAADF-STEM images (at 80000x magnification) of the C@TiN-S nanocomposites. [FIG. 14D] to [FIG. 14G] are Ti EDX map, N EDX map, C EDX map and S EDX map, respectively, of the C@TiN-S nanocomposites as shown in the box in [FIG. 14C]. [FIG. 14K] to [FIG. 14N] are Ti EDX map, N EDX map, C EDX map and S EDX map, respectively, of the C@TiN-S nanocomposites as shown in the box in [FIG. 14J]

FIG. 15A to FIG. 15L

[FIG. 15A] to [FIG. 15L] are characterization results of hollow TiN-S nanocomposites of different shell thicknesses prepared by the method as described herein. The hollow TiN-S nanocomposites were prepared with 0.75 ml ([FIG. 15A] to [FIG. 15F]) or 0.1 ml ([FIG. 15G] to [FIG. 15L]) of TNB. [FIG. 15A] and [FIG. 15G] are TEM images (at 86000x magnification) of the hollow TiN-S nanocomposites. [FIG. 15B] and [FIG. 15H] are HRTEM images (at 400000x magnification) of the hollow TiN-S nanocomposites. [FIG. 15C] and [FIG. 151] are HAADF-STEM images (at 80000x magnification) of the hollow TiN-S nanocomposites. [FIG. 15D] to [FIG. 15F] are Ti EDX map, N EDX map and S EDX map, respectively, of the hollow TiN-S nanocomposites as shown in the box in [FIG. 15C]. [FIG. 15J] to [FIG. 15L] are Ti EDX map, N EDX map and S EDX map, respectively, of the hollow TiN-S nanocomposites as shown in the box in [FIG. 151].

FIG. 16A and FIG. 16B

[FIG. 16A] shows cycling performance of hollow TiN-S nanocomposites of different shell thicknesses prepared by the method as described herein and S nanocrystals.

[FIG. 16B] shows cycling performance of core-shell C@TiN-S nanocomposites and hollow T1O2-S nanocomposites prepared by the method as described herein as well as S nanocrystals.

FIG. 17A and FIG. 17B

[FIG. 17 A] shows photographs of L12S6 solution before and after 12 hours of adsorption of (1) no materials, (2) hollow T1O2 nanoparticles prepared by the method as described herein, (3) core-shell C@TiN nanoparticles prepared by the method as described herein, and (4) hollow TiN nanoparticles prepared by the method as described herein.

[FIG. 17B] shows electrochemical impedance spectra of hollow T1O2-S, core-shell C@ TiN-S and hollow TiN-S nanocomposites prepared by the method as described herein. Detailed Description of Drawings

FIG. 1

[Fig. 1] is a schematic diagram of a general procedure for preparing hollow metal nitride particles and core- shell metal nitride particles. As an example, spherical carbon particles (102) are converted into C@Ti0₂ nanoparticles (104) via reaction with titanium(IV) butoxide. Here, the C@Ti0₂ nanoparticles (104) are heated in air at 400 °C to remove the carbon particles and obtain hollow T1O2 nanoparticles (106). The hollow T1O2 nanoparticles are then reacted with ammonia at 800 °C for 3 hours to obtain hollow TiN nanoparticles (108). In another example, the C@Ti0₂ nanoparticles (104) can directly be reacted with ammonia at 800 °C for 3 hours to obtain C@TiN nanoparticles (110).

Examples

Non- limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1 - Preparation of Hollow T₁O₂ Nanoparticles

Wet chemistry was employed to form the C@Ti0₂ core-shell nanocomposites (104) as shown in FIG. 1. In the first step, carbon nanomaterials were selected as the initial templates in a hydrothermal method.

Generally, all glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia (prepared by mixing hydrochloric acid and nitric acid at a molar ratio of 3:1, both acids were purchased from Sigma-Aldrich, Singapore), followed by copious rinsing with de-ionized water before drying in an oven.

The synthesis of the spherical carbon nanoparticles was as follows. Briefly, 3.6 g of D-glucose (purchased from Sigma-Aldrich, Singapore) was dissolved into 20 ml water, which was then transferred into Teflon autoclave for hydrothermal treatment at 180 °C for 4 hours to form the spherical carbon nanoparticles.

50 mg of spherical carbon nanoparticles was added to a mixture of 187.5 ml of ethanol (purchased from Sigma-Aldrich, Singapore), 62.5 ml of acetonitrile (purchased from Sigma-Aldrich, Singapore) and 2.7 ml of water with vigorous stirring for 30 minutes. Next, 0.5 ml of titanium(IV) butoxide (purchased from Sigma-Aldrich, Singapore) was added and reacted for 20 hours. The resulting C@Ti0₂ core-shell nanoparticles were collected by centrifugation, washed with ethanol and dried in vacuum oven overnight. They were heated to 400°C in air for 2 hours at a ramping rate of l°C/minute to yield hollow T1O2 nanoparticles.

The nanoparticles were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (FEI Tecnai G² F20 electron microscope). Samples for TEM studies were prepared by placing a droplet of the nanoparticle solution on a copper grid coated with a thin carbon film, followed by evaporation in air at room temperature. The material’s composition was determined in situ by energy-dispersive X-ray spectroscopy (EDX attachment) (Oxford Instruments X-Max 80TLE) to the microscope.

X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max-3B diffractometer using Cu K_a radiation (l = 1.54056 A) (Bruker D8 Advance). Brunauer, Emmett and Teller (B.E.T.) surface areas of the samples were calculated from nitrogen sorption at 77 K on a Micrometries ASAP 2020 instrument.

TEM images showed that the carbon sample had a monodispersed spherical morphology with a particle size of about 300 nm (see FIG. 8).

Hollow T1O2 was achieved by calcination of core-shell C@Ti0_2. The hollow structure was confirmed by low- and high-magnification TEM and HAADF-STEM images (see FIG. 2A to FIG. 2D), which clearly indicated the T1O2 shell thickness to be about 23 nm. The HRTEM image in FIG. 2C showed a lattice parameter of about 0.352 nm, which corresponded to T1O2 (101). XRD patterns illustrated only crystalline peaks for antatase T1O2 (JCPDS card No. 00-001-0562) (FIG. 3A). The nitrogen adsorption-desorption isotherms of the as-prepared hollow T1O2 nanoparticle (FIG. 3B) showed a Type IV hysteresis loop, which is characteristic of a mesoporous material. The B.E.T. surface area was 60.3 m²/g.

Example 2 - Preparation of Hollow TiN Nanoparticles and Hollow TiN-S Nanoparticles

In order to convert T1O2 to TiN, the hollow T1O2 nanoparticles as described in Example 1 were annealed at 800 °C in ammonia. Generally, the hollow T1O2 nanoparticles as described in Example 1 were heated to 800 °C in ammonia (purchased from Sigma- Aldrich, Singapore) and kept for 3 hours. For the synthesis of hollow TiN-S nanocomposites, 1 g of hollow TiN nanocomposites was milled with 3 g of S (purchased from Sigma-Aldrich, Singapore), and the resulting mixture was sealed in a Teflon container in an argon-filled glove box and heated at 160°C for 20 hours.

The detailed microstructure and crystalline structure were examined with STEM and TEM. As shown in FIG. 4A to FIG. 4G, all particles had a hollow spherical morphology with a similar particle size as the hollow T1O2 template. The HRTEM image showed a lattice parameter of 0.211 nm (see FIG. 4C), which corresponded to that of TiN (200) facets. Phase purity was confirmed by XRD pattern of the final product (see FIG. 3A); only broad peaks associated with TiN nanocrystals were observed (JCPDS card No. 01-087-063). X-ray photoelectron spectroscopy (XPS) was conducted on the as-prepared hollow TiN nanoparticles. XPS spectra were obtained with an ESCALAB MKII spectrometer (VG Scientific) using Al-K_a radiation (1486.71 eV). FIG. 3E showed that the XPS Ti spectrum could be deconvoluted into two pairs of doublets. The doublet at 455.8 eV was characteristic of TiN 2p3 peak. The second doublet at 461.9 eV could be assigned to TiN 2pl peak. The XPS N spectrum of hollow TiN showed a peak at 396.7 eV, corresponding to Nls in TiN. The porous nature of the hollow TiN nanoparticles was illustrated by the N2 adsorption-desorption isotherms (see FIG. 3B). The as-prepared hollow TiN nanoparticles had a B.E.T. surface area of 33.2 m²/g.

Example 3 - Preparation of Core-shell C@TiN Nanoparticles and C@TiN-S Nanoparticles

Core-shell C@TiN nanocomposites were synthesized by calcination of the core-shell C@Ti0₂ template in ammonia. Generally, the C@Ti0₂ core-shell nanoparticles as described in Example 1 was further heated to 800 °C in ammonia and kept for 3 hours to obtain C@TiN core-shell nanoparticles. For the synthesis of C@TiN-S nanocomposites, 1 g of hollow C@TiN nanocomposites was milled with 3 g of S, and the resulting mixture was sealed in a Teflon container in an argon-filled glove box, and heated at 160 °C for 20 hours.

The core- shell structure was confirmed by TEM and HRTEM and elemental EDX maps of HAADF-STEM images (see FIG. 5A to FIG. 5G). XRD pattern of C@TiN indicated only crystalline peaks for TiN (JCPDS card No. 01-087-063) (Figure 2A). FIG. 3E showed that the XPS Ti spectrum of C@TiN could be deconvoluted into two pairs of TiN doublets. The XPS N spectrum of C@TiN showed a peak at 396.7 eV, corresponding to Nls in the TiN. The mesoporous nature of C@TiN nanocomposites was illustrated by the N2 adsorption-desorption isotherms (see FIG. 3B). C@TiN nanocomposites had a B.E.T. surface area of 169.5 m²/g.

Nanocomposites with a 1:3 weight ratio of hollow TiN and S were achieved by melt diffusion. TEM images and elemental EDX maps of HAADF-STEM image showed that the hollow TiN-S nanocomposites preserved their hollow structure (see FIG. 6A to FIG. 6F). The elemental EDX maps of these nanoparticles in HAADF-STEM image also confirmed that the S nanocrystals were trapped in the hollow TiN nanoparticles. XRD pattern (see FIG. 3A) of the TiN-S nanocomposites showed peaks associated with TiN (JCPDS card No. 01-087-063) and S (JCPDS card No. 01- 073-5056. Further, thermal gravimetric analysis (TGA) was performed in flowing nitrogen on a TA Instruments Discovery TGA 55 (heating rate = 5°C/min). TGA indicated that the loading of S nanocrystals in hollow TiN was ~ 76.8 wt% (see FIG. 3C).

Example 4 - Preparation of Nanoparticles of Different Shell Thicknesses

For comparison, hollow TiN, C@TiN, hollow T1O2-S, C@TiN-S and hollow TiN-S with different shell thicknesses were also synthesized (see FIG. 11A to FIG. 15L). Different amounts of Ti precursor were added to a fixed amount of spherical carbon nanoparticles to achieve different shell thicknesses for the C@Ti0₂ nanocomposites. Generally, 50 mg of the spherical carbon nanoparticles described in Example 1 was added to a mixture of 187.5 ml of ethanol, 62.5 ml of acetonitrile and 2.7 ml of water with vigorous stirring for 30 minutes. Next, 0.25 ml, 0.75 ml or 1 ml of titanium(IV) butoxide (TNB) was added, and reacted for 20 hours. The resulting C@Ti0₂ nanocomposites with different shell thicknesses were collected by centrifugation, washed with ethanol and dried in vacuum oven overnight. They were heated to 400°C in air for 2 hours at 1 °C/minute to yield hollow T1O2 nanoparticles with different shell thickness.

TEM and HAADF-STEM images (FIG. 9A to FIG. 9D) confirmed the core-shell structure of these nanocomposites. The T1O2 shell thickness surrounding the carbon core ranged from ~ 11 nm to 120 nm. Hollow T1O2 nanoparticles of 50 nm and 120 nm shell thicknesses were also obtained from core-shell C@Ti0₂ nanocomposites of different shell thicknesses (see FIG. 10A to FIG. 10H).

FIG. 13 A to FIG. 13L showed that hollow T1O2-S nanocomposites preserved the spherical hollow morphology. The elemental EDX maps in the HAADF-STEM image also confirmed that the S nanocrystals were trapped in the T1O2-S nanocomposites. Hollow T1O2-S showed a similar XRD pattern as S nanocrystals (see FIG. 3A). TGA indicated that the loading of S nanocrystals was ~ 76.9% (see FIG. 3C).

50 mg of the spherical carbon nanoparticles was added to a mixture of 187.5 ml of ethanol, 62.5 ml of acetonitrile and 2.7 ml of water with vigorous stirring for 30 min. Next, 0.25 ml, 0.5 ml, 0.75 ml or 1 ml of titanium(IV) butoxide (TNB) was added, and reacted for 20 hours. The resulting C@Ti0₂ nanocomposites with different shell thickness were collected by centrifugation, washed with ethanol and dried in vacuum oven overnight. They were heated to 400 °C in air for 2 hours at l°C/minute to yield hollow T1O2 nanoparticles with different shell thickness.

Example 5 - Preparation of Hollow TiN Nanoparticles and Hollow TiN-S Nanoparticles of Different Shell Thicknesses

The hollow T1O2 nanoparticles derived with 0.75 ml or 1 ml of TNB (hollow T1O2- 0.75 ml and hollow T1O2-I ml) as described in Example 4 were further heated to 800 °C in ammonia and kept for 3 hours to convert to hollow TiN-0.75 ml and hollow TiN-1 ml, respectively.

To form TiN-S nanocomposites of different shell thicknesses, 1 g of the hollow TiN nanoparticles of the specified shell thickness was milled with 3 g of S, and the resulting mixture was sealed in a Teflon container in an argon-filled glove box and heated at 160 °C for 20 hours.

Example 6 - Preparation of Core-shell C@TiN and C@TiN-S N anocomposites

C@Ti0₂ nanocomposites derived with 0.75 ml or 1 ml of TNB (C@TiO₂-0.75 ml and C@Ti0₂-l ml) as described in Example 4 were further heated to 800 °C in ammonia and kept for 3 hours to convert to C@TiN-0.75 ml and C@TiN-l ml, respectively. 1 g of hollow C@TiN nanocomposites was milled with 3 g of S to form C@TiN-S nanocomposites. The C@TiN-S nanocomposites were sealed in a Teflon container in an argon-filled glove box, and heated at 160 °C for 20 hours to incorporate S into the pores of the nanocomposites.

As shown by TEM and HAADF-STEM images (see FIG. 14A to 14N), the C@TiN- S nanocomposites preserved the spherical core-shell morphology. The elemental EDX maps in the HAADF-STEM image also confirmed that the S nanocrystals were trapped in the C@TiN-S nanocomposites. XRD pattern of C@TiN-S showed crystalline peaks associated with S and TiN (see FIG. 3A). TGA indicated that the loading of S nanocrystals in C@TiN-S was ~ 74.5% (see FIG. 3C).

Example 7 - Adsorption of Lithium Polysulfides

To verify the strong surface affinity in trapping polysulfide, the adsorption of F12S6 on hollow TiN, core-shell C@TiN and hollow T1O2 nanoparticles was examined. Generally, all samples were dried in vacuum oven overnight. Samples of 20 mg each were added to vials containing 2 ml of F12S6 (3 mM, purchased from Sigma- Aldrich) in a solvent mixture of 1,3-dioxolane (DOF, purchased from Sigma- Aldrich) and 1,2- dimethoxy ethane (DME, purchased from Sigma- Aldrich) (1:1 v/v)). A blank glass vial (without sample) was also filled with the same concentration of F12S6 for comparison. All procedures were completed in an argon-filled glovebox. After stirring for 5 minutes, all samples were allowed to stand for 12 hour to obtain full adsorption.

FIG. 17A showed a photograph of the F12S6 solution after adding the same amount of the different nanoparticles. The F12S6 solution containing hollow TiN and core shell C@TiN became almost colorless, whereas that containing T1O2 nanoparticle remained light yellow. This indicated the strong chemical interaction between TiN and polysulfide, as compared to that between T1O2 and polysulfide. Electrochemical impedance spectroscopy (EIS) was performed on the nanocomposites (see FIG. 17B). The depressed semicircle in the high-to-medium frequency region of the Nyquist profiles corresponded to the charge-transfer resistance at the electrode/electrolyte interface: ~ 7 W for hollow TiN-S, ~ 47 W for core-shell C@TiN, and ~ 132 W for hollow T1O2-S. The low transfer resistance of hollow TiN-S nanocomposites indicated high charge transfer, which could be attributed to the high rate capability and stability of this material.

Example 8 - Battery Performances

The battery performance of the nanoparticles as described above was studied using the coil cell configuration. The galvanostatic charge and discharge profiles were shown in FIG. 7 A. A high initial charge capacity of 1,250 mAh/g and a discharge capacity of 1,215 mAh/g were obtained with hollow TiN-S nanoparticles, which were higher than those of core-shell C@TiN-S nanoparticles (1,218 mAh/g and 1,151 mAh/g, respectively) and S nanocrystals (977 mAh/g and 950 mAh/g, respectively), indicating more active S utilization. After 100 cycles at 0.2 C, a reversible discharge capacity of ~ 1,011 mAh/g was retained for hollow TiN-S nanoparticles with good Coulombic efficiency (> 98%) (FIG. 7B), which was superior to that for core-shell C@ TiN-S (851 mAh/g) and S nanocrystals (~ 534 mAh/g). After 500 cycles, hollow TiN-S nanoparticles still retained 754 mAh/g for discharge capacity, which was much higher than that of core-shell C@TiN (493 mAh/g).

FIG. 16A showed that hollow TiN-S with thicker shells had lower capacities after 100 cycles. The thicker shell would increase the resistance of mass transfer and charge transfer during charging and discharging, and lower sulfur utilization. Compared to hollow TiN-S and core-shell C@TiN-S, hollow T1O2-S had a lower capacity (see FIG. 16A and FIG. 16B). The lower capacity of T1O2-S nanoparticles could be attributed to its non-conductivity.

The effectiveness of the nanoparticles in confining S was determined by rate capability studies (see FIG. 7C). The cycling performance of hollow TiN-S nanocomposites, core-shell C@TiN-S nanocomposites and S nanocrystals was investigated by gradually increasing the discharge/charge rate from 0.1 C to 1 C. The hollow TiN-S nanocomposites exhibited good specific capacity (1,170 mAh/g at 0.2 C and 787 mAh/g at 0.5 C), as compared with C@TiN-S nanocomposites (976 mAh/g at 0.2 C and 717 mAh/g at 0.5 C) and S nanocrystals (687 mAh/g at 0.2 C and 443 mAh/g at 0.5 C). Moreover, after cycling at different current densities, the specific capacity of hollow TiN-S nanocomposites was 1,188 mAh/g at 0.1 C, which corresponded to 94% capacity retention, suggesting that the high rates did not substantially alter the structure of the hollow nanostructure. The core-shell C@TiN- S nanocomposites retained 99% of its initial capacity. In contrast, the S nanocrystals recovered only ~ 61% of their capacity after cycling at different current densities. For commercialization of the nanoparticles for Li-S application, battery performance of the high-loading sulfur electrode (3.75 mg of S/cm²) was investigated (see FIG. 7D). After 100 cycles at 0.1 C, a discharge capacity of 890 mAh/g was achieved for hollow TiN-S nanocomposites, which was higher than that of core-shell C@TiN-S nanocomposites (754 mAh/g). After 200 cycles, hollow TiN-S nanocomposites still retained a discharge capacity of 832 mAh/g, which was superior to that of core-shell C@TiN-S nanocomposites (652 mAh/g). This indicated that hollow TiN-S nanocomposites have potential for high-performance Li-S applications.

The superior electrochemical performance of the hollow TiN nanoparticles could be attributed to the high conductivity of nitride materials and the hollow architecture of TiN. The high conductivity of nitride materials would result in fast diffusion of Li ions and electrons in the electrode materials. Its hollow structure could prevent the swelling problem associated with S and polysulfide during charging and recharging. Furthermore, strong chemical adsorption of poly sulfide on TiN could further enhance the S stability in Li-S battery.

Summary of Examples

In conclusion, conductive hollow TiN nanoparticles have been successfully synthesized and compared with core-shell C@TiN nanocomposites and hollow T1O2 nanoparticles. The high conductivity of hollow TiN was achieved by converting non- conductive T1O2 to metallic transition metal nitrides, TiN. Hollow TiN nanocomposites delivered good capacity, rate performance and high capacity at high S loading as cathode material for Li-S batteries. This illustrated that a hollow structure with trapped S would enhance the tolerance for volume expansion during the charging and discharging processes. In addition, the TiN shell of the nanoparticles could enhance conductivity and alleviate the dissolution of polysulfide due to strong interactions.

The high capacity and cycling stability of hollow TiN could be attributed to the hollow microstructure, the high conductivity of nitrides, and the strong chemical interaction between the hollow nitride nanostructure and polysulfide. The novel strategy presented herein could be useful for preparing other metallic transition metal nitrides as hollow nanostructured electrode materials with better capacity and stability for battery applications.

Industrial Applicability

The method of the disclosure may be used to prepare metallic transition metal nitrides as hollow nanostructured electrode materials with better capacity and stability for battery applications.

The batteries of the disclosure may be used in in electrochromic devices, in sensors for organic and bio-organic materials, in field effect transistors, printing plates, portable electronics, drones or electric vehicles.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

Claims

1. A method of producing metal nitride particles, comprising the steps of:

(a) reacting a metal oxide precursor with carbon particles in the presence of a solvent to form core-shell particles having a carbon core and a metal oxide shell disposed thereon;

(b) treating the core-shell particles of step (a) thermally to remove at least a portion of the carbon core thereby forming metal oxide particles; and

(c) reacting the hollow metal oxide particles of step (b) with a nitrogen source to form the metal nitride particles.

2. The method of claim 1, wherein the reacting step (a) further comprises the steps of (i) dispersing the carbon particles in the solvent; and (ii) adding the metal oxide precursor thereto.

3. The method of claim 1 or 2, wherein the metal of the metal oxide precursor is titanium.

4. The method of any one of claims 1 to 3, further comprising a step of pre-treating the core-shell particles formed in step (a) after step (a) but before step (b).

5. The method of claim 4, wherein the pre-treating step comprises stirring, centrifuging, washing and/or drying of the core-shell particles formed in step (a).

6. The method of any one of claims 1 to 5, further comprising a step of (d) mixing the metal nitride particles formed in the reacting step (c) with a sulfur-containing compound to produce a mixture.

7. The method of claim 6, further comprising a step of (e) heating the mixture produced in the mixing step (d) to form core- shell particles having a sulfur-containing core and a metal nitride outer shell.

8. The method of any one of claims 1 to 7, wherein the method does not comprise a step of treating or pre-treating the core-shell particles with an acid and/or a base.

9. Core-shell particles having a sulfur-containing core and a metal nitride outer shell, produced by the method of claim 8.

10. The core-shell particles of claim 9, wherein the metal nitride outer shell has an average thickness in the range of 22 nm to 120 nm.

11. The core shell particles of claim 9 or 10, wherein the metal nitride outer shell has an average thickness of 23 nm.

12. A positive electrode material comprising the core-shell particles having a sulfur- containing core and a metal nitride outer shell of any one of claims 9 to 11, wherein the sulfur-containing core acts as an electrochemically active material.

13. The positive electrode material of claim 12, further comprising at least one of an electronically conductive material, a binder and an additive.

14. A positive electrode comprising the positive electrode material of claim 12 or 13.

15. The positive electrode material of claim 14, further comprising a current collector.

16. A method of preparing a positive electrode, comprising the steps of:

(a) mixing the core-shell particles having a sulfur-containing core and a metal nitride outer shell of any one of claims 9 to 11 , a solvent and optionally at least one of an electronically conductive material, a binder and an additive to form a mixture; and

(b) coating the mixture of step (a) onto a substrate to form the positive electrode.

17. The method of claim 16, wherein the electronically conductive material is a combination of reduced graphene oxide and vapor grown carbon fibers at a weight ratio of 1:1.

18. The method of claim 16 or 17, wherein the binder is polyvinylidene fluoride.

19. An electrochemical cell comprising the positive electrode of claim 14 or 15, a negative electrode and an electrolyte in fluid communication with both the positive electrode and the negative electrode.

20. A battery comprising at least one electrochemical cell of claim 19.