β-NaVP2O7 as a Superior Electrode Material for Na-Ion Batteries
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 reversible capacity of 104 mAh/g at C/10 current density, average operating voltage of 3.9 V vs. Na/Na+ and only 0.5% volume change between the charged and discharged states. Electrode material exhibits excellent C-rate capability and cycling stability, providing the capacity of 90 mAh/g at 20C discharge rate and < 1% capacity loss after 100 charge-discharge cycles. At low voltage region (≈1.5 V vs. Na/Na+), β-NaVP2O7 reversibly intercalates additional sodium cations leading to unprecedented overall Na-ion storage ability exceeding 250 mAh/g within the 1.5 – 4.4 V vs. Na/Na+ voltage region. This material is one of only a few materials that exhibits reversible sodium ion storage capabilities over such a large potential window.