Ostwald Ripening Tailoring Hierarchically Porous Na3 V2 (PO4 )2 O2 F Hollow Nanospheres for Superior High-Rate and Ultrastable Sodium Ion Storage.

Sodium-ion batteries (SIBs) are receiving considerable attention as economic candidates for large-scale energy storage applications. Na3 V2 (PO4 )2 O2 F (NVPF) is intensively regarded as one of the most promising cathode materials for SIBs, due to its high energy density, fast ionic conduction, and robust Na+ -super-ionic conductor (NASICON) framework. However, poor rate capability ascribed to the intrinsically low electronic conductivity severely hinders their practical applications. Here, high-rate and highly reversible Na+ storage in NVPF is realized by optimizing nanostructure and rational porosity construction. Hierarchical porous NVPF hollow nanospheres are designed to modify the issues of inconvenient electrolyte transportation and unfavorable charge transfer behavior faced by solid-structured electrode materials. The individual unique nanosphere is assembled from numerous nanoparticles, which shortens the length of Na+ transport in solid state and thus facilites the Na+ migration. Hollow nanostructure hierarchically porous configuration enables adequate electrolyte penetration, continuous electrolyte supplementation, and facile electrolyte transportation, leading to barrier-free Na+ /e- diffusion and high-rate cycling. In addition, the large electrolyte accessible surface area boosts the charge transfer in the whole electrode. Therefore, the present NVPF demonstrates unprecedented rate capability (85.4 mAh g-1 at 50 C) and long-term cyclability (62.2% capacity retention after 2000 cycles at 20 C).