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In this work, the structures, phase sequence, and metallic properties of K2S have been systematically explored. We confirm that the P63/mmc phase is the best possible candidate for the stable structure of K2S at low pressure range. Although the phases of P63/mmc and Cmcm K2S are semiconductors, two new structures of P6/mmm and Pm1 emerge with metallic characters at high pressures. The analyses of electronic localization functions reveal that the conductivity mainly comes from the electrons surrounding S atom chains, which supplies a potential way to improve the conductivity of sulfur to enhance the electrode recharge ability and rate capability in alkali sulfide battery under pressure.

Alkali metal compounds exhibit novel characteristics under pressure, such as antimetallization in CLi4, high superconductivity at 80 K in highly compressed Li3S, and the existence of unexpected stable stoichiometries of sodium chlorides, etc., which have greatly prompted us to explore KxS compounds at pressure. We found several stable structures with a variety of stoichiometries and proposed a phase diagram on the K-rich side first. Chemical rules established at ambient pressure are frequently violated when high pressure is applied, Na3Cl and NaCl3 as unusual stoichiometries of sodium chloride have been reported in high-pressure conditions. However, KS, with its counterintuitive chemical formula, has been discovered theoretically even at ambient pressure, and possesses the same stability as K2S. The mechanism of superconductivity in Pmm K3S is deeply investigated, comparing with the reported Pmm Li3S. The weak electron–phonon coupling mainly contributes to the weak superconductivity in K3S, which is fully in contrast to the mechanism of interstitial charge localization which dominates the low Tc in the reported Li3S.

MnO2 nanosheets were successfully grown in situ on the surface of activated carbon fibers (ACFs) via a facile microwave-assisted hydrothermal method. This environmentally-friendly approach displays the advantages of low temperature, short reaction time and low cost. A series of MnO2/ACFs composites with different MnO2 percentages were prepared and their electrochemical performance as an electrode for supercapacitors was investigated. The 63% MnO2 composite showed the optimal charge storage performance, remarkable rate ability, and excellent cycling capability. The enhanced electrochemical performance is attributed to the combination of good electrical conductivity of ACFs and high capacity of MnO2. This work provides a useful insight into the design and fabrication of hierarchical transition metal oxide and carbon material composite electrodes for potential applications in next generation energy storage systems.

Silver (Ag) nanoparticles with an average diameter of 5–10 nm were dotted on the surface of barium titanate (BaTiO3, BT) by electroless plating. The poly(vinylidene fluoride) (PVDF) nanocomposites filled with BT@Ag hybrid particles were prepared by a solution method. Transmission electron microscopy and X-ray diffraction observations revealed that the Ag nanoparticles were well dotted on the surface of BT nanoparticles, and no phase transformation of BT could be observed. Differential scanning calorimetry results showed that the degree of crystallinity and melting temperature decrease with the increasing content of filler. Breakdown strength of the BT@Ag/PVDF composites as well as BT/PVDF decreased with the increasing volume fraction (ffiller) of fillers. The dielectric constant of BT@Ag/PVDF composites was gradually increased with the increasing ffiller of BT@Ag from 0 to 55 vol%, while a little decrease when ffiller was increased to 60 vol%. It was worth noting that the dielectric constant of BT@Ag/PVDF composites was higher than that of BT/PVDF composites with the same ffiller, and the dielectric loss of BT@Ag/PVDF composites remained low. The direct current conductivity of the BT@Ag/PVDF composites had a significant dependence on the concentration of BT@Ag and stayed in the order of 10−12 S cm−1 when ffiller was more than 35 vol%. These results may be ascribed to the small size of Ag nanoparticles and the Coulomb block effect.

Three isostructural 3D metal–organic frameworks [Ln(L)1.5(DEF)]n (Ln = Gd (1), Eu (2), Tb (3), H2L = 9,9-diethylfluorene-2,7-dicarboxylic acid) have been solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction. The main framework of compounds 1–3 show high thermal stability to 400 °C but with a distortion or shrinking of the crystal lattice between 200 °C and 250 °C. The high red emission intensity and the microporous instinct of the solvent-free [Eu(L)1.5]n (2a) indicate that it can be potentially used as luminescent sensor. It was then applied in the detection of organic solvent molecules. Notably, it exhibits high sensitivity for 2,4,6-trinitrophenol (TNP) with Ksv constant 6.24 × 104 M−1 through luminescence quenching experiments.