Insight into the limited electrochemical activity of NaVP2O7

Abstract

Recently, LiVP₂O₇ has been investigated as a possible high-voltage substitute for Li₂FeP₂O₇. However, its Na-equivalent, NaVP₂O₇, as an economic replacement for Li₂FeP₂O₇ has not yet been well understood. Here, for the first time, we report the feasibility of NaVP₂O₇ as a 3.4 V cathode material for Na-ion batteries. Having a theoretical capacity of 108 mA h g⁻¹, it shows an initial discharge capacity of 38.4 mA h g⁻¹ at 1/20C (1C = 108 mA g⁻¹) in the voltage range of 2.5–4.0 V. Our study suggests that part of the sodium ions in the lattice structure exist as structural stabilizers and bring lattice distortion upon desodiation. This study also shows that the title compound, NaVP₂O₇, suffers from high intrinsic internal resistance, which limits the phase transition kinetics between pristine NaVP₂O₇ and desodiated Na_1−xVP₂O₇.