Insight into the limited electrochemical activity of NaVP2O7

Recently, LiVP 2 O 7 has been investigated as a possible high-voltage substitute for Li 2 FeP 2 O 7 . However, its Na-equivalent, NaVP 2 O 7 , as an economic replacement for Li 2 FeP 2 O 7 has not yet been well understood. Here, for the first time, we report the feasibility of NaVP 2 O 7 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, NaVP 2 O 7 , suffers from high intrinsic internal resistance, which limits the phase transition kinetics between pristine NaVP 2 O 7 and desodiated Na 1− x VP 2 O 7 . Recently, LiVP 2 O 7 has been investigated as a possible high-voltage substitute for Li 2 FeP 2 O 7 .