Impact of Li⁺/Na⁺ Substitution in the Local Ion Dynamics and Electrochemical Performance of A₃V(PO₃)₃N (A = Li, Na) Nitridophosphates

Li+ and Na+ local ion dynamics in nitridophosphates Na3V(PO3)3N and Li3V(PO3)3N are evaluated here by means of solid-state nuclear magnetic resonance (NMR) and density functional theory (DFT) methods. In particular, the presence of diffusion pathways for the alkali metals within the crystal structures is evaluated experimentally by solid-state NMR and compared with bond valence energy landscape (BVEL) calculations. While no exchange between Na+ ions in Na3V(PO3)3N was detected in the millisecond time scale, Li3V(PO3)3N shows fast exchange between all Li+ positions in the structure. 2D 7Li-7Li EXSY solid-state NMR experiments performed at different mixing times were implemented to determine the Li-exchange rates. The observed diffusion mechanism involves the interconnection of all three alkali positions in the structure in a Li1 ↔ Li3 ↔ Li2 pattern. Nevertheless, the spontaneous Li exchange at room temperature between positions Li1 ↔ Li3 (k13 = 826 Hz) is considerably faster than that between positions Li3 ↔ Li2 (k23 = 207 Hz). These results are in agreement with the activation energies obtained for Li3V(PO3)3N by the BVEL calculations performed in this work. These findings correlate well with the observed differences in the electrochemical performance of nitridophosphates A3V(PO3)3N (A = Li, Na) and might be used to design future strategies for next-generation cathode materials.