β‑NaVP₂O₇ as a Superior Electrode Material for Na-Ion Batteries

The feasibility of sodium-ion batteries as an alternative to lithium-ion batteries in large-scale storage systems largely depends on the availability of advanced electrode materials leading to enhanced energy density and operational sustainability. Herein, we present a novel β-polymorph of sodium vanadium pyrophosphate NaVP₂O₇ with the KAlP₂O₇-type structure obtained via hydrothermal synthesis and further thermal dehydration of a hydrophosphate intermediate. β-NaVP₂O₇ demonstrates attractive electrochemical behavior as a Na-ion positive electrode (cathode) material with practically achieved a reversible capacity of 104 mAh/g at C/10 current density, an average operating voltage of 3.9 V vs Na/Na⁺, and only 0.5% volume change between the charged and the discharged states. Electrode material exhibits excellent C-rate capability and cycling stability, providing a capacity of 90 mAh/g at 20C discharge rate and <1% capacity loss after 100 charge–discharge cycles. In the low-voltage region (≈1.5 V vs Na/Na⁺), β-NaVP₂O₇ reversibly intercalates additional sodium cations, leading to extraordinary overall Na-ion storage ability exceeding 200 mAh/g within the 1.5–4.4 V vs Na/Na⁺ voltage region. This material is one of only a few materials that exhibit reversible sodium-ion storage over such a large potential window.