WO2018226158A1 - A core-shell composite - Google Patents

Abstract

The present invention relates to a core-shell composite and a method of preparing thereof, wherein said core-shell composite comprising a core of non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide. In a preferred embodiment, the core-shell composite is hollow sulfur particles encapsulated by molybdenum disulfide (MoS2) layers. In another embodiment, the core-shell composite is useful as a cathode material.

Description

A Core- Shell Composite

References To Related Applications

The present application claims the benefit of Singapore Application No. 10201704587U, filed on 5 June 2017, and incorporated herein by reference.

Technical Field

The present invention generally relates to a core- shell composite useful as the cathode material. The present invention also relates to method of preparing the core-shell composite as well as an electrode comprising the core-shell composites.

Background Art

The adoption of intermittent renewable energy sources such as solar power and wind energy, along with the electrification of transportation systems, require the development of rechargeable batteries with higher energy density and lower cost. Lithium- sulfur (Li-S) batteries have been considered as one of the most promising approaches for overcoming the above challenges owing to

(1) the high theoretical capacity of sulfur cathode (1675 mAhg ¹ based on pristine sulfur); and

(2) the high natural abundance of sulfur.

However, the practical application of Li-S batteries faces several obstacles such as the limited cycling life. The problem is predominantly attributed, among others, to the dissolution of intermediate lithium polysulfides (Li₂S_x, x>3) into the electrolytes, large volume change of sulfur particles (-80%) that damage the structural integrity of the electrode, and the insulating nature of sulfur and Li₂S. To date, efforts have been made to address these issues through trapping of dissolvable lithium polysulfides.

Recent work to overcome the above issues involves embedding sulfur into order porous CMK-3 carbon, the most common strategies have been realizing as encapsulation of active sulfur within the conductive matrixes, such as mesoporous/microporous/hollow carbon, carbon nano tubes/fibers, and conducting polymers and so forth. Despite the improved performance, such physical architectures were only partially able to mitigate the leakage of polysulfides due to the imperfect encapsulation of sulfur. Functional groups have also been introduced as the encapsulating building blocks to adsorb lithium polysulfides. In particular, carefully designed polymers, functionalized graphene, and nitrogen-doped carbon matrixes etc. have been used to provide strong interaction between lithium polysulfides and the polar surface. Similar strategy can be realized using hydrophilic metal oxides, sulfides, metal organic frameworks and their derivatives as lithium polysulfide absorbers. However, thus far, the attempts outlined above do not appear to solve the aforementioned issues such as full encapsulation of the sulfur in the cathode.

Therefore, there is a need to provide a suitable cathode material that overcomes, or at least ameliorates, one or more of the disadvantages described above. Summary

In one aspect, there is provided a core-shell composite comprising a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide.

The transition metal chalcogenide may have a layered conformal cage structure. Advantageously, each layer of the conformal cage structure may be bound tightly together by van der Waal's interactions such that the shell is mechanically strong and stable, while retaining the morphology of the composite under extreme conditions (such as when subjected to a vacuum). Further advantageously, the strong van der Waal's interaction between the transition metal chalcogenide layers allows the formation of hermetic nanocages for trapping non-metal particles.

Still advantageously, the hollow structure of the core together with the layered shell (having the aforementioned van der Waal's interactions) may be capable of balancing against each other to accommodate any inward volume change when a stress is applied to the core-shell composite. When the core-shell composite is used as an electrode material, the core-shell composite undergoes lithiation which may account for the stress described above. Hence, the hollow structure of the core may advantageously accommodate the inward volume change during lithiation.

In another aspect, there is provided a method of preparing a core-shell composite comprising a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide, comprising the steps of:

a) mixing a suspension of surfactant-encapsulated non-metal particles with a transition metal chalcogenide to bind the transition metal chalcogenide with the surfactant; and

b) removing the surfactant to obtain said core -shell composite.

In another aspect, there is provided an electrode comprising a plurality of core-shell composites, wherein each composite comprises a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide.

Advantageously, the transitional metal chalcogenide of the shell may be multi-layered and able to conform to the shape of the core, hence, the transition metal chalcogenide may be deemed as having a "layered conformal cage". The transition metal chalcogenide may exhibit good binding with lithium polysulfides in the core thereby preventing the dissolution of intermediate lithium polysulfides (Li₂S_x, x>3) into the electrolytes. Further advantageously, there may not be any obvious volume expansion of the core particles upon lithiation. As mentioned above, the transition metal chalcogenide or transition metal dichalcogenide may comprise multi-layer sheets, wherein the sheets may be of nano size (and therefore termed as nanosheets). The sheets (or nanosheets) may have atomic defects. Advantageously, as the lithium-sulfide batteries go through the charge-discharge cycle, the presence of the above atomic defects may facilitate the interaction between lithium polysulfide and the core of the composite so as to reduce degradation of the non-metal making up the core. Definitions

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

The term "composite" or "composite material" as used herein refers to a material system consisting of a mixture or combination of two or more micro- or macro-constituents that differ in form and chemical composition, and which are essentially insoluble in each other.

The term "encapsulation" as used herein refers to the formation of one or more layers of the shell material on the surface of the core particle. The encapsulation process is used to avoid the dissolution of the core particle in a solvent such as in an electrolyte. Further, such encapsulation is also used to protect the core particle from degradation for example when the core particle is exposed to an extreme condition such as high vacuum.

As used herein, the term "exfoliation" refers to a detachment and shedding of one or more layers of the transition metal chalcogenide from the bulk of the transition metal chalcogenide.

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.

As used herein, the term "about", in the context of concentrations of components of the formulations, 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 core-shell composite comprising a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide, will now be disclosed.

The transition metal chalcogenide as defined herein may have a layered conformal cage structure. Each layer of the conformal cage structure may be bound together by van der Waal's interactions. Due to the van der Waal's interactions between the various layers, the various layers are advantageously bound tightly to each other such that the shell is mechanically strong and stable, while retaining the morphology of the composite under extreme conditions for example when subjected to a vacuum. The strong van der Waals interaction between the transition metal chalcogenide layers may allow the formation of hermetic nanocages for trapping non-metal particles.

As outlined above, the hollow structure of the core together with the layered shell (having the above van der Waal's interactions) can balance against each other to accommodate any inward volume change when a stress is applied to the core-shell composite. When the core-shell composite is used as an electrode material, the core-shell composite undergoes lithiation which may account for the stress described above. Therefore, the hollow structure of the core may advantageously accommodate the inward volume change during lithiation.

The non-metal particle of the core-shell composite as described above may be sulfur particle, silicon particle, phosphorous particle or its oxides thereof.

The transition metal in the transition metal chalcogenide as defined herein may be selected from the element in Group 3 to Group 11 of Periodic Table. Non -limiting examples of such transition metal include zinc (Zn), titanium (Ti), vanadium (V), zirconium (Zr), chromium (Cr), molybdenum (Mo), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and tungsten (W). Other suitable transition metal elements that are not shown here may also be used.

The transition metal chalcogenide may be a transition metal monochalcogenide, transition metal dichalcogenide, or transition metal trichalcogenide. Non-limiting examples of transition metal monochalcogenide include ZnX and CdX, where X is sulfur (S), selenium (Se) or tellurium (Te). Such transition metal monochalcogenides may accordingly be termed as zinc sulfide, zinc selenide, zinc telluride, cadmium sulfide, cadmium selenide and cadmium telluride, respectively.

The transition metal dichalcogenide may have the formula MX₂, where X is similar as defined above. Non-limiting examples of the transition metal dichalcogenide include MoSe₂, WSe₂, NiSe₂, CoSe₂, TiSe₂ MoS₂, WS₂, NiS₂, CoS₂, TiS₂, MoTe₂, WTe₂, NiTe₂, CoTe₂, and TiTe₂. It is to be understood that other suitable transition metal dichalcogenide may also be used as the shell material.

As outlined above, the transition metal trichalcogenide may also be used as the shell material.

It is to be understood that there may be a specific stoichiometry for each transition metal chalcogenide due to the electronic structures in the transition metal chalcogenide. Thus, for a given transition metal chalcogenide, it may not be present in all forms above i.e. as transition metal monochalcogenide, transition metal dichalcogenide and transition metal trichalcogenide. For the sake of clarity, for example zinc sulfide can only be present as ZnS but not as ZnS₂ or ZnS₃. Further, only MoS₂ and MoS₃ forms can be present but not as MoS.

The transition metal chalcogenide, which may be the transition metal monochalcogenide, transition metal dichalcogenide, or transition metal trichalcogenide, may be found in its crystalline form, amorphous form or mixture thereof. In a preferred embodiment, the transition metal dichalcogenide may be in its crystalline form, amorphous form or mixture thereof.

The transition metal chalcogenide may have at least one layer sheet. Where the transition metal chalcogenide has at least two layers sheet, such transition metal chalcogenide may adopt a layered structure and may therefore be termed as a layered transition metal chalcogenide. Accordingly, the transition metal monochalcogenide, transition metal dichalcogenide and transition metal trichalcogenide may be in the form of layered transition metal monochalcogenide, layered transition metal dichalcogenide and layered transition metal trichalcogenide, respectively.

In an embodiment, the transition metal chalcogenide such as the transition metal dichalcogenide may comprise multi-layer sheets, wherein the sheets may be of nano size (and therefore termed as nanosheets). The sheets (or nanosheets) may have atomic defects. The defects may be generated during exfoliation of the sheets or nanosheets. The presence of the atomic defects in the sheets (or nanosheets) may impart the strength and stability to the core-shell composite as defined herein. The exfoliation of the bulk transition metal dichalcogenide may occur due to an increase in the interspacing between the multi-layer sheets. As will be mentioned further below, the increase in the interspacing may be due to the method used to form the core-shell composite.

As aforementioned, the layered transition metal chalcogenide such as the layered transition metal dichalcogenide may be monolayered, bilayered, or multi-layered transition metal dichalcogenide. For avoidance of doubt, the layered transition metal dichalcogenide may have 2, 3, 4, 5, 6, 7, 8, 9, or 10 layers. Preferably, the layered transition metal dichalcogenide has 4 to 10 layers, more preferably 5 to 7 layers.

The layered transition metal chalcogenide such as the layered transition metal dichalcogenide may have an interlayer spacing in the range of about 0.3 nm to about 2 nm, such as about 0.3 nm to about 0.5 nm, about 0.3 nm to about 0.8 nm, about 0.3 nm to about 1 nm, about 0.3 nm to about 1.5 nm, about 0.5 nm to about 0.8 nm, about 0.5 nm to about 1 nm, about 0.5 nm to about 1.5 nm, about 0.5 nm to about 2 nm, about 0.8 nm to about 1 nm, about 0.8 nm to about 1.5 nm, about 0.8 nm to about 2 nm, about 1 nm to about 1.5 nm, about 1 nm to about 2 nm, or about 1.5 nm to about 2 nm. The interlayer spacing between the various layers of the layered transition metal chalcogenide preferably about 0.5 nm to about 0.8 nm, more preferably about 0.6 nm to about 0.7 nm.

The core-shell composite as defined above may be structurally stable when placed under high- vacuum for an extended period of time. The term "high-vacuum" as used herein refers to the vacuum condition having a pressure in the range of between about 10⁴ Pascals (Pa) to about 10 ⁹ Pa such as about 10⁴ Pa, about 10⁵ Pa, about 10⁶ Pa, about 10⁷ Pa, about 10 ⁸ Pa, about 5 x 10⁹ Pa, about 2 x 10⁹ Pa, or about 10⁹ Pa. The "extended period of time" as used above refers to a period of about one minute to about 5 hours such as about one minute, about five minutes, about 10 minutes, about 30 minutes, about one hour, about 2 hours, about 3 hours, about 4 hours or about 5 hours. The hermetic structure of the shell may be capable of protecting the inner core from degradation or phase changes for example sublimation. Hence, advantageously the core-shell composite may increase the sublimation temperature of the core particles.

The mass loading of the non-metal particles in the core-shell composite may be in the range of about 40 wt% to about 90 wt%, such as about 40 wt%, about 45 wt%, about 50 wt%, about 55 wt%, about 60 wt%, about 65 wt%, about 70 wt%, about 75 wt%, about 80 wt%, about 85 wt% or about 90 wt%. The mass loading of the non-metal particles in the core-shell composite is preferably in the range of about 60 wt% to about 90 wt%.

Further, the core-shell composite as defined above may be a regularly shaped particle such as a spherical shaped particle or an irregular shaped particle.

The particle size of the core shell composite as defined herein may be in the range of about 50 nm to about 5,000 nm (or about 5 μπι), such as about 50 nm to about 100 nm, about 50 nm to about 200 nm, about 50 nm to about 500 nm, about 50 nm to about 800 nm, about 50 nm to about 1000 nm, about 50 nm to about 2000 nm, about 50 nm to about 3000 nm, about 50 nm to about 4000 nm, about 100 nm to about 200 nm, about 100 nm to about 500 nm, about 100 nm to about 800 nm, about 100 nm to about 1000 nm, about 100 nm to about 2000 nm, about 100 nm to about 3000 nm, about 100 nm to about 4000 nm, about 100 nm to about 5000 nm, about 500 nm to about 800 nm, about 500 nm to about 1000 nm, about 500 nm to about 2000 nm, about 500 nm to about 3000 nm, about 500 nm to about 4000 nm, about 500 nm to about 5000 nm, about 1000 nm to about 2000 nm, about 1000 nm to about 3000 nm, about 1000 nm to about 4000 nm, about 1000 nm to about 5000 nm, about 2000 nm to about 3000 nm, about 2000 nm to about 4000 nm, about 2000 nm to about 5000 nm, about 3000 nm to about 4000 nm, about 3000 nm to about 5000 nm, or about 4000 nm to about 5000 nm. The particle size may refer to the diameter or equivalent diameter for an irregularly shaped composite.

The particle size of the core shell composite as defined herein is preferably in the range of about 100 nm to about 1000 nm (or about 1 μτη), more preferably is in the range of about 300 nm to about 800 nm.

In the above regard, therefore there is also provided a core-shell composite comprising a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcoge ide, wherein said core is partially or fully (or completely) encapsulated by said shell.

Exemplary, non-limiting embodiments of a method for preparing a core-shell composite comprising a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide, will now be disclosed.

There is provided a method of preparing a core-shell composite comprising a core of a non- metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide, comprising the steps of:

a) mixing a suspension of surfactant-encapsulated non-metal particles with a transition metal chalcogenide to bind the transition metal chalcogenide with the surfactant; and

b) removing the surfactant to obtain said core -shell composite. The above method can be considered as a self-assembly process where no external stimulation or initiator is required for the spontaneous reorganization due to the binding between the surfactant and the transition metal chalcogenide to occur. When the surfactant is removed such as in step (b), the transition metal chalcogenide can be viewed as a shell that at least partially encapsulates the core of the non-metal particle having the hollow inner structure. Accordingly, the shell of the transition metal chalcogenide may substantially encapsulate the core of the non- metal particle having the hollow inner structure fully or completely.

The transition metal chalcogenide used in the above method may be an exfoliated transition metal chalcogenide. Therefore, there is also provided a method of preparing a core-shell composite comprising a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide, comprising the steps of:

a) mixing a suspension of surfactant-encapsulated non-metal particles with an exfoliated transition metal chalcogenide to bind the exfoliated transition metal chalcogenide with the surfactant; and

b) removing the surfactant to obtain said core -shell composite.

Similar as above, such method can be considered as a self-assembly process where no external stimulation or initiator is required for the spontaneous reorganization due to the binding between the surfactant and the exfoliated transition metal chalcogenide to occur. When the surfactant is removed such as in step (b), the exfoliated transition metal chalcogenide can be viewed as a shell that at least partially encapsulates the core of the non-metal particle having the hollow inner structure. Accordingly, the shell of the exfoliated transition metal chalcogenide may substantially encapsulate the core of the non-metal particle having the hollow inner structure fully or completely.

The non-metal suspension may be highly-dispersed such as mono dispersed. The non-metal is as described above, which may be sulfur particle, silicon particle, phosphorous particle or its oxides thereof.

The method described herein may further comprise, before the mixing step (a), the step of a-i) mixing a non-metal precursor with a surfactant under a suitable temperature and a suitable medium.

Specifically, the surfactant-encapsulated non-metal suspension may be prepared by mixing a non-metal precursor with a surfactant at room temperature such as from about 20°C to about 30°C in a suitable medium followed by the addition of an acid to form a mixture. The room temperature may be about 20°C, about 21°C, about 22°C, about 23°C, about 24°C, about 25°C, about 26°C, about 27°C, about 28°C, about 29°C or about 30°C. The mixture may then be stirred for about one hour to 3 hours (such as for about one hour, about 2 hours or about 3 hours) at room temperature to afford a solid product.

Therefore, the step a-i) of the method described herein may further comprise the step of a-ii) adding an acid to said mixture from step a-i).

The step of preparing the surfactant-encapsulated non-metal suspension may be carried out at room temperature with a single step. Therefore, as one may appreciate, this step may be scaled up in industry. The solid product containing surfactant-encapsulated non-metal particles may be separated from the reaction medium using suitable separation technique known in the art. The non-limiting examples of the separation technique used here are filtration and centrifugation.

The surfactant-encapsulated non-metal particles obtained from this step may be re-dispersed in a suitable medium and recollected by an appropriate separation technique. Finally, the surfactant- encapsulated non-metal particles obtained from this step may be re -dispersed in distilled water to afford the surfactant-encapsulated non-metal suspension for further use. The surfactant- encapsulated non-metal suspension may have a concentration from about 1 mg mL^"1 to about 6 mg mL^"1, such as about 1 mg mL^"1, about 2 mg mL^"1, about 3 mg mL^"1, about 4 mg mL^"1, about 5 mg mL ¹ or about 6 mg mL^"1, preferably about 4 mg mL^"1.

The non-metal precursor used for preparing the above surfactant-encapsulated non-metal suspension may be selected from salt, solvate or hydrate of the non-metal precursor. If a surfactant-encapsulated sulphur particle is desired, a sulfur -containing salt may be used such as sodium thiosulfate. It is to be understood that the sulfur-containing salt provided here is not limiting and therefore may extend to other suitable sulfur-containing salt.

In the above method, the acid may be selected from weak acid, strong acid, organic acid, or inorganic acid. Preferred acid is strong acid and more preferably is strong inorganic acid such as hydrochloric acid.

The suitable medium may be selected from an organic solvent, an aqueous solution, or a mixture of organic and aqueous media forming a single phase, preferably an aqueous medium of polyvinylpyrrolidone (PVP) solution. When PVP is used, its concentration thereof in the solution may be in the range of about 0.1 M (1M = 1 mol L^"1) to about 1 M, such as about 0.1 M, about 0.2 M, about 0.3 M, about 0.4 M, about 0.5 M, about 0.6 M, about 0.7 M, about 0.8 M, about 0.9 M, about 1 M, preferably around 0.4 M.

Further, when PVP solution is used, the molecular weight of the PVP may be in the range of about 30,000 Da to about 100,000 Da such as about 30,000 Da, about 40,000 Da, about 50,000 Da, about 60,000 Da, about 70,000 Da, about 80,000 Da, about 90,000 Da, or about 100,000 Da.

The transition metal chalcogenide or the exfoliated transition metal chalcogenide may be prepared by way of contacting a transition metal chalcogenide with a lithium precursor to form the transition metal chalcogenide or the exfoliated transition metal chalcogenide. For avoidance of doubt, before step (a) of the method described herein, the step of a-iii) contacting a transition metal chalcogenide with a lithium precursor to form the exfoliated transition metal chalcogenide may be undertaken. The lithium precursor used may be an organolithium solution as will be defined below.

The transition metal chalcogenide or the exfoliated transition metal chalcogenide may be prepared via lithium intercalation or other suitable process. When lithium intercalation is used, transition metal chalcogenide crystals may be dissolved in the organolithium solution in anhydrous organic solvent under inert atmosphere to produce Li_x-transition metal chalcogenide. This product may be recovered via separation-washing cycle with the anhydrous organic solvent to remove excess lithium and organic residues. The product may be then dried, re-dissolved in an aqueous medium and may be subjected to ultrasonication for about 30 minutes to about 2 hours, for example for about 30 minutes, about 60 minutes, about 90 minutes or about 2 hours, for the exfoliation process to occur. The exfoliation may occur partially or completely. However, it is to be understood that complete exfoliation is desirable.

The exfoliation of Li_x-transition metal chalcogenide may be achieved via the insertion of the solvent molecules such as water molecules into the van der Waal's layers. The insertion of water molecules may refer to the forced hydration of lithium intercalated transition metal chalcogenide. The suspension of the exfoliated transition metal chalcogenide may be collected for further use with a concentration in the range of about 0.5 mg mL ¹ to about 4 mg mL ¹ such as about 0.5 mg mL^"1, about 1 mg mL 1, about 1.5 mg mL^"1, about 2 mg mL^"1, about 2.5 mg mL ^l, about 3 mg mL^"1, about 3.5 mg mL^"1, or about 4 mg mL^"1.

The core-shell composite as defined herein may be produced upon mixing the surfactant- encapsulated non-metal suspension and the exfoliated transition metal chalcogenide suspension under stirring for about 30 minutes to about 2 hours such as about 30 minutes, about 60 minutes, about 90 minutes, or about 2 hours. The core-shell composite may be collected via at least 3 cycles of separation and washing. Thus, the cycle of separation and washing may be repeated at least 3, 4, 5, 6, 7, 8, 9, 10 or more as appropriate. The separation and washing step may remove the surfactant and impurities. Upon removal of the surfactant and impurities, the exfoliated transition metal chalcogenide, which may be in the form of flakes may compress and encapsulate the core. Therefore, when the core-shell composite is formed, the composite may not contain solvent or impurities such as water molecules.

The non-limiting examples of the organolithium solution include butyllithium solution, pentyllithium solution or hexyllithium solution, preferably butyllithium solution. In an embodiment, the organic solvent used may be selected from N-Methyl-2-pyrrolidone, pentane, hexane, heptane or octane. In a further embodiment, the inert atmosphere may refer to the atmosphere that is substantially free of oxygen or oxidant and therefore may refer to the atmosphere under nitrogen or argon. In an embodiment, the drying process may be carried out using a suitable drying technique. The non-limiting examples of drying technique include vacuum and heating.

The exfoliated transition metal chalcogenide may be as described above. The exfoliated transition metal chalcogenide may be in the form of flakes or sheets. The flakes as described above may have lateral dimension in the range of about 200 nm to about 2 um (or 2000 nm), such as about 200 nm to about 400 nm, about 200 nm to about 600 nm, about 200 nm to about 800 nm, about 200 nm to about 1000 nm, about 200 nm to about 1500 nm, about 400 nm to about 600 nm, about 400 nm to about 800 nm, about 400 nm to about 1000 nm, about 400 nm to about 1500 nm, about 400 nm to about 2000 nm, about 600 nm to about 800 nm, about 600 nm to about 1000 nm, about 600 nm to about 1500 nm, about 600 nm to about 2000 nm, about 800 nm to about 1000 nm, about 800 nm to about 1500 nm, about 800 nm to about 2000 nm, about 1000 nm to about 1500 nm, about 1000 nm to about 2000 nm, or about 1500 nm to about 2000 nm. Such lateral dimension refers to the dimension of the side or sides of the flakes.

Upon mixing the non-metal suspension with the exfoliated transition metal chalcogenide as described above followed by separation-washing cycle, the flakes of the exfoliated transition metal chalcogenide may adhere or bind to the surfactant on the surface of hollow structure of non-metal particles and may stack together owing to the van der Waals attractions. The flakes of the exfoliated transition metal chalcogenide may wrap around (or at least partially encapsulate) the hollow structure of non-metal particles upon removal of the surfactant. The flakes may wrap around the hollow structure of non-metal particles due to electrostatic interactions. The surfactant may then be removed during the separation-washing cycle as the surfactant is soluble in the solution.

The suitable surfactant used may be a polymer containing a hydrophilic group that is selected from polyvinylpyrrolidone, polyvinylpyridine, polyvinylimidazole, preferably polyvinylpyrrolidone or non-polymeric surfactants such as sodium stearate, or dodecylbenzenesulfonate.

Exemplary, non-limiting embodiments of an electrode comprising a plurality of core-shell composites, wherein each composite comprises a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide, will now be disclosed.

The transition metal chalcogenide may have a layered conformal cage structure. Each layer of the conformal cage structure may be bound together by van der Waal's interactions as described above.

The electrode comprising the plurality of the core-shell composites as defined above may be used in lithium-sulphide batteries. Advantageously, the layered conformal cage of a transition metal chalcogenide exhibits good binding with lithium polysulfides thereby preventing the dissolution of intermediate lithium polysulfides (Li₂S_x, x>3 such as 4, 5, 6, 7, 8, 9, or 10 into the electrolytes. Further advantageously, there is no obvious volume expansion of the core particles upon lithiation.

As mentioned above, the transition metal chalcogenide or transition metal dichalcogenide may comprise multi-layer sheets, wherein the sheets may be of nano size (and therefore termed as nanosheets). The sheets (or nanosheets) may have atomic defects. Advantageously, as the lithium-sulphide batteries go through the charge-discharge cycle, the presence of the above atomic defects facilitates the interaction between lithium polysulfide and the core of the composite so as to reduce degradation of the non-metal making up the core.

In an embodiment, the core-shell composite may be loaded into the electrode at a loading mass of about 0.5 mg cm ² to about 10 mg cm ² such as about 0.5 mg cm ² to about 1 mg cm ², about

0.5 mg cm 2 to about 2 mg cm 2 , about 0.5 mg cm 2 to about 5 mg cm 2 , about 1 mg cm 2 to about

2 mg cm 2 , about 1 mg cm 2 to about 5 mg cm 2 , about 1 mg cm 2 to about 10 mg cm 2 , about 2 mg cm 2 to about 5 mg cm 2 , about 2 mg cm 2 to about 10 mg cm 2 , or about 5 mg cm 2 to about 10 mg cm ².

The electrode can be made according to known methods in the art except that the material of the electrode is replaced with the presently disclosed core-shell composite. For example, the electrode may be prepared by mixing Vapour Grown Carbon Fiber (VGCF) and polyvinylidene fluoride (PVDF) binder with the core-shell composite in N-methy-2pyrrolidone (NMP) to form a slurry. The slurry may be coated onto copper foil and dried under vacuum overnight.

In an embodiment, the electrode may be used in a battery. The battery may be used in electric vehicles and stationary storage systems. In an embodiment, the electrode may achieve a high specific capacity of up to about 1660 mAhg^"1. The specific capacity may be the discharge capacity. Advantageously, when charging/discharging is conducted at higher rates such as 3C and 5C, the electrode may be able to realize a discharge capacity of about 500 to 600 mAhg ¹ and about 300 to 450 mAhg^"1, respectively. The electrode as defined herein may have a long-term stability with a decay of about 0.049% per cycle over 1000 cycles at 1C. The improved electrochemical performance may be observed using the current to discharge capacity in the range of about 0.1 C to about 5 C.

As a person skilled in the art would understand, the apparent problems associated with the practical application of the Li-S batteries including the dissolution of intermediate lithium polysulfides (Li₂S_x, x>3) into the electrolytes, large volume change of sulfur particles that damage the structural integrity of the electrode and the insulating nature of sulfur and Li₂S are solved by the present disclosure including as shown in the examples provided.

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.l

[Fig. 1] is a number of images and graphs obtained from characterization of MoS₂ flakes described in Example 1. Fig. 1A: scanning transmission electron microscopy (STEM) image of MoS₂ flakes, field of view (FOV) 2048 nm; Fig. IB: High resolution STEM image of MoS₂ flakes, FOV 16 nm; Fig. 1C: Atomic STEM image of MoS₂ flakes, FOV 2.5 nm; Figs. 1D-1F: Images obtained from a simulation to indicate the sulfur vacancy- based defects on MoS₂ flakes. The FOV for fig. ID is 1.5 nm; Fig. 1G: Digital photographs of Vapor Grown Carbon Fiber (VGCF), reduced graphene oxide (rGO) and MoS₂ flakes mixed with 0.005 mol L ¹ Li₂S₉ solution; Fig. 1H Raman spectra of MoS₂ flakes treated with Li₂S₉ solution (marked area is zoomed in as Fig. II).

Fig.2

[Fig. 2] is a Fourier Transform infrared spectroscopy (FTIR) spectra of MoS₂ before and after being treated with 1-undecanethiol as described in Example 1. This figure also depicts the characteristic bands of CH₂ asymmetrical, CH₂ symmetrical stretch, CH₂ scissor and C- S stretch.

Fig.3

[Fig. 3] is a digital photograph of MoS₂ and MoS₂ treated with 1-undecanethiol (blocked MOS₂) mixed with 0.005M Li₂S₉ as described in Example 1.

Fig.4

[Fig. 4] is an image illustrating the schematic of the step (a) synthetic process outlined in Example 2 and step (b) lithiation/de-lithiation process of MoS₂ cages encapsulated hollow sulfur spheres. Fig 4A depicts the overall process (both synthetic process and lithiation de- lithiation process) of MoS₂ cages encapsulated hollow sulfur spheres. The arrows depict the MoS₂ cages that substantially encapsulate the hollow sulfur sphere (or sulfur core) ; Fig. 4B indicates that the soluble lithium polysulfides can be effectively entrapped within the MoS₂ cages. This figure shows the enlarged version of the MoS₂ cages encapsulated hollow sulfur spheres as shown in Fig. 4A (the arrows depict the MoS₂ cages that substantially encapsulate the hollow sulfur sphere (or sulfur core)).

Fig.5

[Fig. 5] is a number of scanning electron microscope (SEM) images obtained from characterization of MoS₂ cages encapsulated hollow sulfur spheres described in Example 2. Figs. 5A-5D: SEM images of MoS₂@S composite at magnifications of ΙΟΟχ, ΙΟ,ΟΟΟχ, 75,000x and 65,000x, respectively; Fig. 5E: SEM image of sulfur spheres, magnification of 20,000x; and Fig. 5F: SEM image of the hollow MoS₂ framework upon removal of sulfur in MoS₂ cages encapsulated hollow sulfur spheres, magnification of 20,000x.

Fig.6

[Fig. 6] is a number of images and a graph obtained from characterization of the MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@S composite) described in Example 2. Fig. 6A: transmission electron microscopy (STEM) image of the MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@S composite), magnification of 87,000x; Fig. 6B: STEM image of the MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@S composite), magnification of 87,000x; Fig. 6C: The energy-dispersive X-ray (EDX) line scan of the MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@ S composite); Figs. 6D-6F: TEM images of the MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@S composite) at the magnifications of 435,000x (D), 435,000x (E) and 620,000x (F), respectively; Fig. 6G-6I: high-resolution TEM (HR-TEM) images of the MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@S composite) at the magnifications of 87,000x (G), 40,000x (H) and 435,000x (F), respectively indicating the layered stacking structures of MoS₂ cages remain continuous throughout the curved regions.

Fig.7

[Fig. 7] is a number of graphs and tables of the EDX analysis of the MoS₂@S composite and MoS₂ empty cages described in Example 2.

Fig.8

[Fig. 8] is a number of X-ray powder diffraction (XRD) patterns of the MoS₂@S composite. Fig 8B is the enlarged XRD pattern of the marked area of Fig. 8 A.

Fig.9

[Fig. 9] is a number of graphs obtained from the thermogravimetric analysis (TGA) of the MoS₂@S composite, pristine sulfur spheres and pristine MoS₂ described in Example 2.

Fig.10

[Fig. 10] is a number of images and a graph obtained from the electrochemical characterization of the MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@S composite) described in Example 3. Fig. 10A: Charge/discharge plots of the MoS₂@S composite at different rates; Fig. 10B: Cycle capability of the MoS₂@S composite at different rates; Fig. IOC: Long-term cycle capability of MoS₂@S composite, rGO mixed with sulfur spheres (rGO@S) and pristine sulfur spheres at 0.5C; Fig. 10D: Long-term cyclability of MoS₂@S composite at 1C; Fig. 10E: Digital photograph of the demo pouch cells based on MoS₂@ S; Fig. 10F: Digital photograph of using demo pouch cells based on MoS₂@ S to power a LED light; Fig. 10G: Digital photograph of using the demo pouch cells based on MoS₂@ S to power a small electrical fan.

Fig.ll

[Fig. 11] is a number of images and a graph obtained from characterization of the MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@ S composite) before and after lithiation as described in Example 4. Fig. 11 A: A digital photograph of a pouch cell with glassy carbon substrate loaded with MoS₂@ S; Figs. 11B and 11C: SEM images of MoS₂@S composite before lithiation at magnifications of 50,000x and ΙΟ,ΟΟΟχ; Figs. 11D and HE: SEM images of MoS₂@ S composite after lithiation at magnifications of 50,000x and ΙΟ,ΟΟΟχ; Fig. 11F: Statistics of the particle diameters of MoS₂@S before and after lithiation.

Examples

Non-limiting examples of the invention and a comparative example 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: Solution-exfoliation and characterization of defects- decorated MoS₂ flakes

To obtain an aqueous suspension of highly dispersed mono-layer/several-layer MoS₂ flakes, commercial MoS₂ powders were firstly intercalated with lithium to form Li_xMoS₂, which was then readily exfoliated through forced hydration with the assistance of sonication. As can be seen in the scanning transmission electron microscopy (STEM) image depicted in Fig. 1A, the typical MoS₂ flakes were from mono to few layers thick with a lateral dimension of up to a few micrometers. Generated during the exfoliation process, typical hole defects could be clearly observed on MoS₂ flakes (refer to Fig. IB). These defects were shown to have large concentration of sulfur vacancies based on the enlarged and simulated STEM images (refer to Figs. 1C and ID).

To examine the chemical adsorption of Li₂S_x to MoS₂ flakes, a 0.005 mol L ¹ Li₂S₉- DOL DME (1 : 1 v/v) solution was prepared and mixed with MoS₂ powder in Argon- filled glove box (DOL: 1 ,3-dioxolane, was purchased from Sigma- Aldrich of Saint Louis, Missouri of the United States of America; DME: dimethyl ether, was purchased from Sigma-Aldrich of Saint Louis, Missouri of the United States of America). As shown in Fig. 1G, the yellow Li₂S₉ solution was completely decolorized upon mixing with a small amount of MoS₂ flakes powder, indicating a strong chemical absorption of Li₂S_x by MoS₂ flakes. After treating with Li₂S solution, Raman spectra of MoS₂ flakes powder reveal new broad peaks centered around 198 cm \ 225cm ¹ and 490 cm , which are very near to the characteristic absorbance of S-S bond in lithium polysulfides (refer to Fig. 1H).

The red-shift of typical MoS₂ absorbance provides evidence of the formation of Li₂S_x-MoS₂ complex (refer to Fig. II). In contrast, no obvious colour change of Li₂Sg solution could be observed after mixing with Vapor Grown Carbon Fiber (VGCF) known as conductive additives. Upon mixing with similar amount of rGO powder, which has been used as lithium polysulfildes blocker, only slight color decay was observed, indicating a more efficient trap of lithium polysulfide to MoS₂ flakes powder than to reduced graphene oxide (rGO) powder. The strong chemical absorption of lithium polysulfide to MoS₂ flakes was attributed to the large concentration of sulfur vacancies on the MoS₂ flakes. To further prove this, 1-undecanethiol, which has been demonstrated to compensate the sulfur vacancies through the sulfydryl group was used to block the sulfur vacancies in MoS₂ flakes (refer to Fig. 2). Further, as shown in Fig. 3, the blocking of sulfur vacancies significantly reduced the binding of lithium polysulfide on the MoS₂ flakes, indicating a strong chemical adsorption of lithium polysulfides to the sulfur vacancies on MoS₂ flakes.

Example 2: Synthesis and characterization of MoS₂ cages encapsulated hollow sulfur spheres

Other than strong chemical adsorption of lithium polysulfides to the sulfur vacancies, efficient physical confinement of sulfur can further provide strong trapping of lithium polysulfides and effective accommodation of outward volume expansion during lithiation. The MoS₂ cage encapsulation must be highly conformal to provide unity protection for each sulfur spheres. Therefore, a unique and facile synthesis approach as outlined in this example was developed using highly dispersed suspensions of MoS₂ flakes and polyvinylpyrrolidone (PVP)- encapsulated hollow sulfur spheres as shown in Fig. 4A.

a) Preparation of the mono-dispersed solution of PVP-sulfur particles

The mono-dispersed solution of PVP-sulfur particles (abbreviated as PVP-S thereafter) was prepared via a solution synthesis method. In a typical synthesis, 50 mL of 80 mM sodium thiosulfate aqueous solution (purchased from Sigma-Aldrich of Saint Louis, Missouri of the United States of America) was mixed with 50 mL of 0.4 M PVP (molecular weight of ~55,000, purchased from Sigma-Aldrich of Saint Louis, Missouri of the United States of America) at room temperature. Following this, 0.4 mL of concentrated hydrochloric acid HC1 was added to the Na₂S₂0₃/PVP solution under magnetic stirring. After stirring for 2 hours at room temperature, the product was collected by centrifugation at 7000 rpm for 10 minutes. The product was then re-dispersed in 0.4 M PVP solution and re-collected by centrifugation at 6000 rpm for 10 minutes. The product was dispersed again in distilled water for use. The resulting solution is referred as Solution A, with a concentration around 4 mg mL^"1.

b) Preparation of the suspension of the exfoliated MoS₂

MoS₂ dispersion was prepared via forced hydration of lithium intercalated MoS₂ under sonication. Lithium intercalation was achieved by stirring 2 grams of natural MoS₂ crystals (purchased from Sigma-Aldrich of Saint Louis, Missouri of the United States of America) in 20 mL of 1.6 M butyl lithium solution (purchased from Sigma-Aldrich of Saint Louis, Missouri of the United States of America) in anhydrous hexane in an argon-filled glove box. The Li_xMoS₂ was then retrieved by filtration and washed with anhydrous hexane to remove the excess lithium and organic residues. After drying in vacuum, Li_xMoS₂ was mixed with water and placed in an ultrasonication bath for one hour. Exfoliation of Li_xMoS₂ was achieved with the insertion of water molecules into the Van der Waal's layers. The suspension of MoS₂ flakes was then collected for use with a concentration around 1 mg mL ¹ (referred as Solution B). c) Preparation of the MoS₂@S composite

In a typical synthesis, 20 mL of Solution A was added into 40 ml Solution B under stirring. After stirring for one hour, the MoS₂ encapsulated sulfur particles product (MoS₂@S composite) was collected through filtration and washed with water/methanol (3 times) to remove polymer and impurity.

Upon mixing with the PVP-S suspension, the MoS₂ flakes adhered to PVP on the surface of hollow sulfur spheres, stacked together owing to the van der Walls attraction and wrapped around the sulfur spheres upon removal of polymer (refer to Figs. 4A and 4B). Figs. 5A-5D illustrate typical scanning electron microscope (SEM) images of MoS₂@S composite which exhibit a similar morphology and spherical diameter with its sulfur sphere precursor shown in Fig. 5E, but with a rough surface having typical wrinkles. The wrinkles are generated from the restacking of 2D flakes on the surface of sulfur spheres. During the characterization of SEM, the sublimation of sulfur in high-vacuum system may generate obvious holes and cracks on bare sulfur spheres as shown in Fig 5E. On the contrary, the conformal MoS₂ coating can protect the inner sulfur from the sublimation in the high vacuum, indicating an effective physical encapsulation of sulfur. The SEM image of the hollow MoS₂ framework upon removal of sulfur in MoS₂ cages encapsulated hollow sulfur spheres is depicted in Fig. 5F.

The transmission electron microscopy (TEM) images shown in Fig. 6A and STEM images of Fig. 6B clearly reveal the spherical morphology of MoS₂@S composite. The energy-dispersive X-ray (EDX) line scan clearly indicated the distribution of sulfur signal along with the high angle annular dark-field (HAADF) intensity depicted in Fig. 6C. It is to be noted that owing to the substantial overlapping of the Mo La and S Ka peaks, the composition of sulfur in MoS₂@S composite can be justified based on additional consideration of the contribution of Mo Ka line in EDX pattern as shown in Fig. 7.

High resolution TEM images indicated that crystalline sulfur spheres are conformably wrapped by few-layer (5 to 7 layers) stacked MoS₂ flakes as can be seen from Fig. 6D and 6E, forming MoS₂ nanocages. Fig. 6F reveals that the van der Waal's gap of the typical MoS₂ nanocage was approximately 0.61 nm; and the crystalline nature of the sulfur sphere was evidenced by the presence of the (026) facet in high resolution TEM (HR-TEM). To date, there is no lattice- resolution characterization of elemental sulfur by using high-vacuum TEM, due to the high vapor pressure of sulfur.

With the hermetic encapsulation of MoS₂ nanocages, the sulfur sphere can survive in high- vacuum TEM for an extended period of time under 200 KV electron irradiation and be characterized for multi-time without structural degradation. Defects and/ or deformation can be observed in the HR-TEM images of MoS₂ nanocages (refer to Figs. 6D and 6F highlighted in blue circle). The multi-layer stacking structure allowed the presence of defects on the MoS₂ nanocages while at the same time preventing the sublimation of sulfur. Other than to act as the efficient chemical absorbers of dissolvable lithium polysulfides, those defects offered unique channels for lithium ion transport during lithiation/delithiation.

X-ray powder diffraction (XRD) patterns as illustrated in Fig. 8 further confirmed the crystallinity of sulfur hollow spheres and homogeneous wrapping of MoS₂ out sulfur particles. Based on the thermogravimetric analysis (TGA) shown in Fig. 9, the sulfur mass loading in the MoS₂@S composite was around 65%, which is a relatively high mass loading. After removal of sulfur spheres by vacuum heating (as proved by the EDX analysis as shown in Fig. 7), the structures of the self-supporting MoS₂ cages remained intact as can be seen in Figs. 6G and 6H. The HR-TEM image in Fig. 61 clearly showed that the layered stacking structures of MoS₂ cages remained continuous throughout the curved regions, which may partially accommodate the outward volume expansion from sulfur spheres during lithiation.

Example 3: Electrochemical properties of MoS₂ cages encapsulated hollow sulfur spheres (MoS₂@S composite)

The electrochemical performance of the MoS₂@S composite was evaluated using Li half -cells (2032-type). Working electrodes with a loading mass of -1.5 mg cm ² were prepared by mixing Vapour Grown Carbon Fiber (VGCF) and polyvinylidene fluoride (PVDF) binder with the MoS₂@S composite in N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was then coated onto copper foil and dried under vacuum overnight. The cells were tested within potential range of 1.6 or 1.8V to 2.6 V at room temperature; and the specific capacity were calculated based on the mass of sulfur only.

MoS2@S exhibited the characteristic Li-S voltage profiles when evaluated at different rate (0.1, 0.2, 0.5, 1 and 2C, wherein 1C=1672 mAhg ') as shown in Fig. 10A. At 0.1C, the MoS₂@S exhibits a discharge capacity of 1660 mAhg ¹ (based on the mass of sulfur), whereas at higher rate of 0.2C, 0.5C, 1C and 2C, a discharge capacity of 1430 mAhg \ 1137mAhg \ 930 mAhg ¹ and 720 mAhg ¹ can be obtained, respectively.

Two-plateau discharge profile of a typical sulfur cathode can be obtained at various charactering rates (Fig. 10B). When charged/discharged at higher rates of 3C and 5C, the MoS₂@S can realize a discharge capacity of 508 mAhg ¹ and 305 mAhg \ respectively as shown in Fig. 10B. Considering the insulating nature of sulfur and its lithiated products, such rate capability of MoS₂@S is considered remarkable.

It is important to note that 2H-MoS₂, a semi-conductor, will be partially transformed to be metallic 1-T phase upon lithiation, which has a high electrical conductivity of 10-100 S cm^"1. Such high electrical conductivity may improve the utilization of insulating sulfur and its lithiated intermediate.

Fig. IOC illustrates the cyclability of MoS₂@S composite and control samples (sulfur spheres, sulfur spheres mixed with rGO, abbreviated as rGO/S) for 300 cycles at 0.5C. The discharge capacity of MoS₂@S exhibited a gradual increase in initial cycles, which may be explained by the increasing of the accessibility of sulfur to lithium ions diffused through the defects/deformation on MoS₂ flakes and the gradual transformation of 2-H MoS₂ into 1-T metallic MoS₂ to increase the utility of sulfur. Following the initial activation process, the cells made from MoS₂@S can still exhibit an impressive discharge capacity up to 956 mAhg ¹ even after 300 cycles, showing a relatively high capacity retention of 83.2% (of the initial discharge capacity) with 0.056% capacity decay per cycle. Without MoS₂ cages, the battery cells made from sulfur spheres show a quick capacity fading at the initial cycles and only 49.1% capacity retention after 300 cycles, demonstrating the essential role of MoS₂ cages in preventing the leakage of active materials during prolonged cycles.

As shown in Fig. IOC, mixture of rGO was able to partially mitigate the serious initial capacity fading of sulfur sphere. However, the capacity retention of the cells made from rGO/S was around 50.2% after 300 cycles with a capacity decay of 0.166% per cycle (much higher compared to that of MoS₂@S), which is similar with sulfur sphere electrodes, indicating that rGO with functional groups on its surface can only partially retained polysulfide in limited cycles. A good cyclability can be reproduced from the cells made from MoS₂@S when cycled at 1C. The cells exhibited a discharge capacity up to 585 mAhg ¹ at 1C, realizing a slight capacity decay of 0.0449% per cycles (refer to Fig. 10D).

Finally, owing to the facile synthesis procedure, the product mass can be easily scaled up. With a loading mass up to 30-40 mg (based on the sulfur mass), a pouch cell can be assembled as illustrated in Fig. 10E, which can light up LED light and even power a small fan as shown in Figs. 10F and 10G.

Example 4: Characterization of MoS₂@S composite after lithiation

To evaluate the volume expansion of MoS₂@S composite after lithiation, the MoS₂@S composite was dispersed and drop-casted on a 20*20*1 mm glassy carbon plate without adding carbon conductive and polymer binder. A pouch cell was assembled in Argon-filled glove box by using the glassy carbon plate loaded with MoS₂@S composite as working electrode and Li foil as counter electrode (Fig. 11 A).

Upon lithiated to 1.5V at 0.5C, the glassy carbon plate was recovered from the pouch cell and rinsed with a mixture solution of 1,3-DOL and DME (1:1 v/v). Typical SEM images of MoS₂@S composite on glassy carbon substrate before and after first lithiation are depicted in Figs 1 lB-1 IE. The size-distribution of MoS₂@S is presented in Figure 1 IF. Based on the above results, it can be concluded that the spherical morphology of the MoS₂@S remained intact after lithiation. Further, no significant size difference can be observed in MoS₂@S before and after lithiation. The observations indicated that the MoS₂ cages can effectively avoid the volumetric expansion of sulfur spheres, while the hollow space in the sulfur spheres can accommodate the inward volume expansion upon lithiation. The well-preserved morphology can aid in maintaining a stable electrode -electrolyte interface, limiting the leakage of lithium polysulfide into the electrolyte.

Industrial Applicability

As outlined above, the Li-S batteries are considered promising owing to the high theoretical capacity of sulfur cathode and the high natural abundance of sulfur. Such Li-S batteries, being rechargeable and having high specific energy, may therefore be used in a wide spectrum of applications such as in electric vehicles (including high altitude long endurance unmanned aerial vehicles or HALE UAVs) and stationary storage systems.

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 core- shell composite comprising a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide.

2. The core-shell composite of claim 1, wherein the non-metal particle is sulfur, silicon, phosphorous particle or its oxides thereof.

3. The core-shell composite of claim 1 or claim 2, wherein the transition metal in the transition metal chalcogenide is selected from the element in Group 3 to Group 11 of Periodic Table.

4. The core-shell composite of any one of the preceding claims, wherein the transition metal chalcogenide is a transition metal dichalcogenide.

5. The core-shell composite of claim 4, wherein transition metal dichalcogenide is selected from the group consisting of MoSe₂, WSe₂, NiSe₂, CoSe₂, TiSe₂ MoS₂, WS₂, NiS₂, CoS₂, TiS₂, MoTe₂, WTe₂, NiTe₂, CoTe₂, and TiTe₂.

6. The core-shell composite of claim 4 or claim 5, wherein the transition metal dichalcogenide is in crystalline form, amorphous form or mixture thereof.

7. The core-shell composite of any one of claims 4 to 6, wherein the transition metal dichalcogenide comprising at least one layer sheet.

8. The core-shell composite of any one of claims 4 to 6, wherein the transition metal dichalcogenide comprising at least two layers sheet.

9. The core-shell composite of claim 7 or claim 8, wherein the at least one layer sheet or the at least two layers sheet of the transition metal dichalcogenide has atomic defects.

10. The core-shell composite of claim 8, wherein the transition metal dichalcogenide comprising at least two layers sheet has an interlayer spacing in the range of 0.3 nm to 2 nm.

11. The core-shell composite of any one of claims 1 to 10, wherein the non-metal particle is present in the range of 40 wt% to 90 wt% based on the total weight of the core-shell composite.

12. The core-shell composite of any one of claims 1 to 11, wherein the particle size is in the range of 50 nm to 5 mm.

13. A method of preparing a core-shell composite comprising a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide, comprising the steps of:

a) mixing a suspension of surfactant-encapsulated non-metal particles with a transition metal chalcogenide to bind the transition metal chalcogenide with the surfactant; and

b) removing the surfactant to obtain said core -shell composite.

14. The method of claim 13, further comprising, before the mixing step (a), the step of a-i) mixing a non-metal precursor with a surfactant under a suitable temperature and a suitable medium.

15. The method of claim 14, further comprising the step of a-ii) adding an acid to said mixture from step a-i).

16. The method of claim 14 or claim 15, wherein the non-metal precursor is in the salt, solvate or hydrate form.

17. The method of claim 15, wherein the acid is a weak acid, strong acid, organic acid or inorganic acid.

18. The method of any one of claims 13 to 17, wherein the transition metal chalcogenide is an exfoliated transition metal chalcogenide.

19. The method of claim 18, further comprising, before step (a), the step of a-iii) contacting a transition metal chalcogenide with a lithium precursor to form the exfoliated transition metal chalcogenide.

20. The method of claim 18 or claim 19, wherein the exfoliated transition metal chalcogenide is in the form of flakes or sheets.

21. The method of claim 20, wherein the flakes form of the exfoliated transition metal chalcogenide have lateral dimension in the range of 200 nm to 2 μπι.

22. An electrode comprising a plurality of core-shell composites, wherein each composite comprises a core of a non-metal particle having a hollow inner structure encapsulated by a shell of a transition metal chalcogenide.