Elucidating the Role of Defects for Electrochemical Intercalation in Sodium Vanadium Oxide

Na1.25+xV3O8 (with x < 0, = 0, and > 0) was synthesized via a wet chemical route involving the reduction of V2O5 in oxalic acid and NaNO3 followed by calcination. It was possible to control the sodium composition in the final product by adjusting the amount of sodium precursor added during synthesis. It was revealed that deficient and excessive sodium contents, with respect to the ideal stoichiometry, are accommodated or compensated by the respective generation of oxygen vacancies and partial transition metal reduction, or cation disordering. When examined as NIB electrode material, the superior performance of the cation disordered material with excessive sodium was clearly demonstrated, with more than 50% higher storage capacity and superior rate capacity and cyclic stability. The formation of oxygen vacancies initially seemed promising but was coupled with stability issues and capacity fading upon further cycling. The disparity in electrochemical performance was attributed to variations in the electronic distribution as promoted through Na-ion interactions and the direct influence of such on the oxygen framework (sublattice); these factors were determined to have significant impact on the migration energy and diffusion barriers.