A new polyanionic cathode with stable structure and superior kinetics for Na-ion batteries

Abstract The discovery of suitable electrode materials is extremely important for the success of Na-ion batteries (NIBs). In this work, a phosphate-typed polyanionic compound Na3(VO)(PO4)(HPO4) is explored as a new cathode material of NIBs for the first time. The as-synthesized Na3(VO)(PO4)(HPO4) sample presents as well-crystallized nanoparticles. Without any carbon coating and high-temperature treatment, it exhibits about one mole Na+ (de)intercalation with an average voltage of ∼ 3.5 V, outstanding cycling stability with 71% capacity retention after 500 cycles at 1C and superior reaction kinetics with a large Na+ diffusion coefficient of around 10-10 cm2 s-1 (comparable with other carbon-coated Na superionic conductors). The excellent Na-storage performance suggests that the Na3(VO)(PO4)(HPO4) compound may be a potential cathode for NIBs. Further analyses demonstrate the as-prepared electrode experiences a series of highly reversible phase-transition and solid-solution reactions during Na+ extraction/insertion with a V4+/V5+ redox couple.