Development of a new alluaudite-based cathode material with high power and long cyclability for application in Na ion batteries in real-life

We report Na₂Fe_1.96V_0.96(PO₄)₃ (NFVP) as a new cathode material for Na ion batteries with outstanding cycle life and high power density for the first time. Structural characterization of NFVP was performed using Rietveld refinement of X-ray diffraction and neutron diffraction patterns, which revealed large Na diffusion paths in the structure that enabled rapid Na (de)intercalation without a large volume change. Using X-ray diffraction and X-ray photoelectron spectroscopy with charge/discharge measurements, it was confirmed that based on redox reactions of Fe and V ions, ∼2 Na ions are reversibly (de)intercalated from the structure with an average voltage of ∼3.1 V (vs. Na⁺/Na). At 30C, which denotes the rapid charge/discharge, ∼74% of the theoretical capacity was delivered. Furthermore, NFVP exhibited an outstanding cycle life of over 2500 cycles (∼70% retention of the initial capacity), which can be attributed to the low volume change (∼1%) during charge/discharge.