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

<p>Herein, we present a novel <i>β</i>-polymorph of sodium vanadium pyrophosphate NaVP<sub>2</sub>O<sub>7</sub> with the KAlP<sub>2</sub>O<sub>7</sub>-type structure obtained via hydrothermal synthesis and further thermal dehydration of a hydrophosphate intermediate. <i>β</i>-NaVP<sub>2</sub>O<sub>7</sub> 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<sup>+ </sup>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<sup>+</sup>), <i>β</i>-NaVP<sub>2</sub>O<sub>7</sub> 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<sup>+</sup> voltage region. This material is one of only a few materials that exhibits reversible sodium ion storage capabilities over such a large potential window. </p>