Concerted Ion-Exchange Mechanism for Sodium Diffusion and Its Promotion in Na₃V₂(PO₄)₃ Framework

Design and development of novel cathode materials for room-temperature sodium ion batteries is of great importance to solve the shortage of lithium resources. As a promising candidate, the Na3V2(PO4)3 cathode (NVP) exhibits stable structure and rapid Na diffusion kinetics. A detailed insight to the transportation behavior of Na+ ions in the interstitials of NVP lattice should be of great importance for understanding the ionic conductivity as well as the electrochemical performances. In this paper, we proposed three different sodium diffusion pathways, among which the concerted ion-exchange route is found to be energetically most favorable. During the migration process, Na ions at both Na(1) and Na(2) sites are engaged in the transportation. Several dopants, including Li, K, Ca, Mg, and Al, were introduced at the Na(1) site to promote the electrochemical performance of the NVP cathode. It was found that the K-doped NVP exhibits the highest voltage and lowest lattice variation during the charge/discharge process. Moreover, the Na diffusion kinetics could be intensively promoted upon K doping. Our results provide another perspective on the Na migration mechanism in the NVP lattice and suggest that K doping should be a promising solution to enhance the electrochemical performances of the NVP cathode.