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