The 1T metallic phase of MoS2 shows high volumetric capacitance and electrochemical properties that are attractive for supercapacitor applications. Efficient intercalation of ions in layered materials forms the basis of electrochemical energy storage devices such as batteries and capacitors1,2,3,4,5,6. Recent research has focused on the exfoliation of layered materials and then restacking the two-dimensional exfoliated nanosheets to form electrodes with enhanced electrochemical response7,8,9,10,11. Here, we show that chemically exfoliated nanosheets of MoS2 containing a high concentration of the metallic 1T phase can electrochemically intercalate ions such as H+, Li+, Na+ and K+ with extraordinary efficiency and achieve capacitance values ranging from ∼400 to ∼700 F cm−3 in a variety of aqueous electrolytes. We also demonstrate that this material is suitable for high-voltage (3.5 V) operation in non-aqueous organic electrolytes, showing prime volumetric energy and power density values, coulombic efficiencies in excess of 95%, and stability over 5,000 cycles. As we show by X-ray diffraction analysis, these favourable electrochemical properties of 1T MoS2 layers are mainly a result of their hydrophilicity and high electrical conductivity, as well as the ability of the exfoliated layers to dynamically expand and intercalate the various ions.

Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials

Chemistry of intercalation compounds: Metal guests in chalcogenide hosts

The coordination chemistry of pyrazole-derived ligands

Intercalation of the layer type transition metal dichalcogenides by a variety of organic molecules, alkali metals, or ‘3d’ transition metals, provides a powerful way to finely tune the electron occupation of the relatively narrow ‘d’ bands met in these solids These transition metal dichalcogenides are highly anisotropic solids, sometimes referred to as ‘two-dimensional’ solids, and the intercalant molecules which are electron donors enter between the layers This can result in profound changes in the electronic properties of the host lattice, and these changes can be understood in terms of charge transfer and increased interlayer separation The phenomena discussed include optical properties, transport properties, super-conductivity, order-disorder phenomena and phase changes, staging, magnetic properties, metal-insulator transitions, Anderson localization, and fast-ion conduction Some possible practical applications are also considered

Electronic properties of intercalation complexes of the transition metal dichalcogenides

Organic semiconductors are attracting increasing interest as flexible thermoelectric materials owing to material abundance, easy processing and low thermal conductivity. Although progress in p-type polymers and composites has been reported, their n-type counterpart has fallen behind owing to difficulties in n-type doping of organic semiconductors. Here, we present an approach to synthesize n-type flexible thermoelectric materials through a facile electrochemical intercalation method, fabricating a hybrid superlattice of alternating inorganic TiS2 monolayers and organic cations. Electrons were externally injected into the inorganic layers and then stabilized by organic cations, providing n-type carriers for current and energy transport. An electrical conductivity of 790 S cm(-1) and a power factor of 0.45 mW m(-1) K(-2) were obtained for a hybrid superlattice of TiS2/[(hexylammonium)x(H2O)y(DMSO)z], with an in-plane lattice thermal conductivity of 0.12 ± 0.03 W m(-1) K(-1), which is two orders of magnitude smaller than the thermal conductivities of the single-layer and bulk TiS2. High power factor and low thermal conductivity contributed to a thermoelectric figure of merit, ZT, of 0.28 at 373 K, which might find application in wearable electronics.

Flexible n-type thermoelectric materials by organic intercalation of layered transition metal dichalcogenide TiS2

Layer intercalation compounds of molybdenum disulphide with alkali and alkali-earth metals show cation exchange behaviour after decomposition with water or aqueous ammonia. Mono- and divalent inorganic and organic cations can be taken into exchange positions in the interlayer space of the MoS 2 crystals. The interlayer distance depends on the type of cation exchanged and on cation hydration. Intracrystalline swelling of the compounds is observed with a whole series of polar inorganic and organic molecules. The cation exchange phenomena are explained in terms of the reduction of a part of the Mo(IV) atoms in the sulphide layers by alkali metal. Oxidation and heating restore the original MoS 2 lattice.

Cation exchange reactions and layer solvate complexes of ternary phases MxMoS2

Cathodic reduction of layered transition metal chalcogenides

The topotactic formation and the structure of layered intercalation complexes L y H x MoO 3 obtained by the reaction of hydrogen bronzes H x MoO 3 with organic Lewis base donor molecules L are described. The considerable stability of these complexes is discussed in terms of an ionic bonding model involving proton transfer from the host layers to the guest molecules. It is demonstrated that Lewis base intercalation complexes may also form on ion exchange with hydrated molybdenum bronzes, on cathodic reduction of MoO 3 in aqueous LH + electrolytes and by direct thermal reaction. H x Mo 18 O 52 and Mo 18 O 52 were found to undergo intercalation under conditions similar to those just described.

Layered intercalation complexes of the hydrogen bronze H0.5MoO3 With organic lewis bases

Superconductivity in hydrated ternary tantalum sulfides

The hydrated ternary sulphides K0.5(H2O)yNbS2 and K0.33(H2O)yTaS2 are isotypic and crystallize in hexagonal structures with the parameters a = 3.35 A , c = 17.94 A (Nb) and a = 3.336 A , c = 18.18 A (Ta); the space group is P 6 2c . In the S-M-S sandwich layers the transition metal atoms have trigonal prismatic coordination. With respect to the arrangement of the layer units, the structures of these compounds are similar to those of the unhydrated ternary phases AxMS2 and the pyridine intercalation compound TaS2(py)0.5- Stacking disorder phenomena of the layers and high thermal mobility of the interlayer atoms prevented the location of alkali and oxygen atoms at room temperature.

Robert Schöllhorn

Papers

Cation exchange reactions and layer solvate complexes of ternary phases MxMoS2

Cathodic reduction of layered transition metal chalcogenides

Layered intercalation complexes of the hydrogen bronze H0.5MoO3 With organic lewis bases

Superconductivity in hydrated ternary tantalum sulfides

A crystal structure investigation of the hydrated layered chalcogenides Kx(H2O)yNbS2 and Kx(H2O)yTaS2