We report a simple and effective carbon-free nanocoating strategy for large-scale synthesis of Mo₂N nanolayer coated MoO₂ hollow nanostructures. This strategy only involves commercial MoO₃ powders reacted with reduced gas. The carbon-free nanocoating of Mo₂N is highly effective in improving the electrochemical properties of MoO₂, promising advanced batteries with high specific capacity up to 815 mA h g⁻¹, long cycle-life (e.g., >100 cycles) and high rate capability compared with the carbon nanocoating commonly used in electrode materials. The present nitride-nanocoating strategy is facile but effective, and therefore it is very promising for large-scale industrial production. It may be extended to prepare other metal oxides with nitride coating nanolayers to enhance their performances as electrode materials.

Silicon batteries have attracted much attention in recent years due to their high theoretical capacity, although a rapid capacity fade is normally observed, attributed mainly to volume expansion during lithiation. Here, we report for the first time successful synthesis of Si/void/SiO₂/void/C nanostructures. The synthesis strategy only involves selective etching of SiO₂in Si/SiO₂/C structures with hydrofluoric acid solution. Compared with reported results, such novel structures include a hard SiO₂-coated layer, a conductive carbon-coated layer and two internal void spaces. In the structures, the carbon can enhance conductivity, the SiO₂layer has mechanically strong qualities and the two internal void spaces can confine and accommodate volume expansion of silicon during lithiation. Therefore, these specially designed dual yolk-shell structures exhibit a stable and high capacity of 956 mA h g⁻¹after 430 cycles with capacity retention of 83%, while the capacity of Si/C core-shell structures rapidly decreases in the first ten cycles under the same experimental conditions. The novel dual yolk-shell structures developed for Si can also be extended to other battery materials that undergo large volume changes.

There is an urgent need for low-cost, high-energy-density, environmentally friendly energy storage devices to fulfill the rapidly increasing need for electrical energy storage. Multi-electron redox is considerably crucial for the development of high-energy-density cathodes. Here we present high-performance aqueous zinc–manganese batteries with reversible Mn²⁺/Mn⁴⁺ double redox. The active Mn⁴⁺ is generated in situ from the Mn²⁺-containing MnO_x nanoparticles and electrolyte. Benefitting from the low crystallinity of the birnessite-type MnO₂ as well as the electrolyte with Mn²⁺ additive, the MnO_x cathode achieves an ultrahigh energy density with a peak of 845.1 Wh kg⁻¹ and an ultralong lifespan of 1500 cycles. The combination of electrochemical measurements and material characterization reveals the reversible Mn²⁺/Mn⁴⁺ double redox (birnessite-type MnO₂↔ monoclinic MnOOH and spinel ZnMn₂O₄ ↔ Mn²⁺ ions). The reversible Mn²⁺/Mn⁴⁺ double redox electrode reaction mechanism offers new opportunities for the design of low-cost, high-energy-density cathodes for advanced rechargeable aqueous batteries.