Interfacial Engineering of Na V (PO ) F Hollow Spheres through Atomic Layer Deposition of TiO : Boosting Capacity and Mitigating Structural Instability.

To mitigate the associated challenges of instability and capacity improvement in Na V (PO ) F  (NVPF), rationally designed uniformly distributed hollow spherical NVPF and coating the surface of NVPF with ultrathin (≈2 nm) amorphous TiO  by atomic layer deposition is demonstrated. The coating facilitates higher mobility of the ion through the cathode electrolyte interphase (CEI) and enables higher capacity during cycling. The TiO @NVPF exhibit discharge capacity of >120 mAhg , even at 1C rates, and show lower irreversible capacity in the first cycle. Further, nearly 100% capacity retention after rate performance in high current densities and 99.9% coulombic efficiency after prolonged cycling in high current density is reported. The improved performance in TiO @NVPF is ascribed to the passivation behavior of TiO  coating which protects the surface of NVPF from volume expansion, significantly less formation of carbonates, and decomposition of electrolyte, which is also validated through post cycling analysis. The study shows the importance of ultrathin surface protection artificial CEI for advanced sodium-ion battery cathodes. The protection layer is diminishing parasitic reaction, which eventually enhances the Na ion participation in reaction and stabilizes the cathode structure.