Employing a Thin Artificial CEI Layer on Na3V2 (PO4)2F3-2xO2x (0 < x < 1) by Atomic Layer Deposition; Structure Stabilization and Capacity Enhancement

Abstract

Sodium ion battery (SIB), still facing the limit in functionality due to lower energy density; arises from either low capacity or narrow potential window. The capacity improvement within material sustainability is one major challenge which needs to be addressed to satisfy the present demand for SIB. Of late phosphate-based materials [Na3V2(PO4)2F3-2xO2x; (NVPFO2x;0 < x < 1)] are established as prospective cathodes for Sodium-ion battery (SIB) due to high reversible capacity and stability. However, the reversible extraction and insertion of only 2 Na ion per formula unit in these polyanionic cathodes limit their capacity. Herein we demonstrate a strategic approach towards electrochemical activation of 3rd Na ion which leads to higher capacity and preserves structural stability. We synthesized a series of NVPFO2x (0 < x < 1) with well-controlled surface morphology and vanadium oxidation state and studied the electrochemical behavior on the various composition and morphology. Cycling within the potential window of 1.2V to 4.3V vs Na/Na+ to trigger extra Na ion in the electrochemical activity have shown their explicit behavior depending on morphology. The optimized NVPFO2x shows specific capacity >130 mA h g-1, implies 3rd Na ion activation. However, the material suffers rapid capacity fading within ~ 65 cycles which is tremendously improved through formation of a thin artificial cathode electrolyte interface (CEI) layer of TiO2 on the surface of NVPFO2x by atomic layer deposition (ALD) technique. This TiO2 coating enabled even further extraction of sodium through higher voltage domain, but more importantly, it facilitated excellent long-term stability over 200 cycles. The thin coating explicitly restores the structural and morphological integrity during prolonged electrochemical cycling which established by post cycling XRD, HRSEM and XPS study. The study demonstrated that the thin TiO2 layer on electrode material is effectively diminishes the parasitic reaction, reduces carbonate formation, and increase the stability of the formed CEI and lowers electrolyte decomposition within wide-range potential window.