MnO 2 hollow spheres and hollow urchins are synthesized via a simple hydrothermal process without using any template or organic surfactant. The effect of the reaction time on the microstructure and morphology of samples is observed systemically. Meanwhile, the forming mechanism of hollow-structured R-MnO 2 is carefully investigated by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results of nitrogen adsorption-desorption experiments and electrochemical measurements show that the product obtained by hydrothermal reaction for 6 h has large specific surface area, uniform pore-size distribution, and excellent capacitance performance, which make it have a potential application as a supercapacitor electrode material.

Positive electrodes for the oxygen reduction reaction (ORR) and the oxygen-evolution reaction (OER) play a critical role in fuel cells and metal-air batteries. Tetragonal CoMn2O4 spinel nanoparticles have been grown on the surface of graphene sheets (CMOG) via a two-step synthesis. The ORR/OER catalytic characteristics of CMOG were studied with a rotating-disk electrode. Also a lithium-air primary cell having a non-aqueous electrolyte and a rechargeable lithium-air cell with a Li-ion solid electrolyte separating a non-aqueous anode electrolyte from an alkaline aqueous cathode electrolyte were assembled with a CMOG cathode and tested. The results indicate that a CMOG cathode can provide a catalytic platform of considerable activity for the ORR in both electrolytes and also for the OER in the aqueous electrolyte.

A Na3V2O2(PO4)2F/reduced-graphene-oxide (RGO) sandwich structure has been synthesized by a facile one-step solvothermal method. Cubic Na3V2O2(PO4)2F nanoparticles are homogeneously trapped between conductive RGO sheets during its growth and assembled into a compact sandwich structure, which allows the electrically insulating Na3V2O2(PO4)2F nanoparticles to be wired up to a current collector through the underlying graphene conducting layers. As a sodium-insertion cathode material, the structure exhibits a high reversible capacity of 120 mA h g(-1) at a discharge rate of C/20 with a capacity retention of 100.4 mA h g(-1) at 1 C and an excellent cyclic retention of 91.4% after the 200th cycle at C/10. These results highlight the importance of anchoring Na3V2O2(PO4)2F on a conducting scaffold for maximum utilization of the electrochemically active Na3V2O2(PO4)2F particles in a high-performance sodium-ion battery.

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