Fig. 2

Electrochemical characterization of NVPF samples in NVPF/Na half cells cycled between 4.4 and 1.0 V. a First cycle activation in which the charging potential is used up to 4.8 V and charge process controlled by limiting the Δx (Na) = 2.0, 2.25, 2.50, 2.75, and 3.0 extracted, followed by discharge to 1 V at C/10. The inset shows the amount of Na⁺ extracted at ~4.75 V and reinserted at ~1.6 V. b The charge-discharge profile of the subsequent second cycle in the voltage window of 4.4 and 1.0 V. c The corresponding capacity retention plots