Unique walnut-shaped porous MnO₂/C nanospheres with enhanced reaction kinetics for lithium storage with high capacity and superior rate capability

Unique walnut-shaped porous MnO₂/carbon nanospheres (P-MO/C-NSs) with high monodispersity have been designed and prepared for lithium storage via in situ carbonization of amorphous MnO₂ nanospheres. Polyvinylpyrrolidone (PVP) is utilized as both the surfactant for morphology control and carbon source for carbon scaffold formation accompanied with MnO₂ crystallization. Such a unique walnut-shaped porous nanostructure with an intimate carbon layer provides a large contact area with the electrolyte, short transport path length for Li⁺, low resistance for charge transfer and superior structural stability. The P-MO/C-NS electrode demonstrates high lithium storage capacity (1176 mA h g^-1 at 100 mA g^-1), very good cycling stability (100% capacity retention versus the second cycle) and excellent rate capability (540 mA h g^-1 at 1000 mA g^-1). We propose that it is the deep oxidation of Mn²⁺ to Mn³⁺ in P-MO/C-NSs, which results in an extraordinarily high capacity of 1192 mA h g^-1 at a current density of 1000 mA g^-1 after a long period of cycling, very close to the maximum theoretical reversible capacity of MnO₂ (1230 mA h g^-1). This is the highest value ever observed for MnO₂-based electrodes at such a rate. The high lithium storage capacity and rate capability can be attributed to the enhanced reaction kinetics owing to the walnut-shaped porous nanostructure with an intimate carbon layer. This work provides a meaningful demonstration of designing porous nanostructures of carbon-coated metal oxides undergoing deep conversion reactions for enhanced electrochemical performances.