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

We introduce Na 3 V(PO 4 ) 2 as a new cathode material for Na-ion batteries for the first time. The structure of Na 3 V(PO 4 ) 2 was determined using X-ray diffraction and Rietveld refinement, and its high water stability was clearly demonstrated. The redox potential of Na 3 V(PO 4 ) 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. Na 3 V(PO 4 ) 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 Na 3 V(PO 4 ) 2 (∼70% retention of the initial capacity after 200 cycles) was achieved at a reasonably fast current rate of 1C.