Na₃V(PO₄)₂: A New Layered-Type Cathode Material with High Water Stability and Power Capability for Na-Ion Batteries

We introduce Na3V(PO4)2 as a new cathode material for Na-ion batteries for the first time. The structure of Na3V(PO4)2 was determined using X-ray diffraction and Rietveld refinement, and its high water stability was clearly demonstrated. The redox potential of Na3V(PO4)2 (∼3.5 V vs Na/Na+) was shown to be sufficiently high to prevent the side reaction with water (Na extraction and water insertion), ensuring its water stability in ambient air. Na3V(PO4)2 also exhibited outstanding power capability, with ∼79% of the theoretical capacity being delivered at 15C. First-principles calculation combined with electrochemical experiments linked this high power capability to the low activation barrier (∼433 meV) for the well-interconnected two-dimensional Na diffusion pathway. Moreover, outstanding cyclability of Na3V(PO4)2 (∼70% retention of the initial capacity after 200 cycles) was achieved at a reasonably fast current rate of 1C.