Phase Stability and Sodium-Vacancy Orderings in a NaSICON Electrode

TL;DR

This study uses computational methods to map the phase diagram of NaSICON Na$_x$V$_2$(PO$_4$)$_3$, revealing stable phases, structural features, and charge orderings crucial for improving Na-ion battery electrodes.

Contribution

First computational temperature-composition phase diagram of NaSICON Na$_x$V$_2$(PO$_4$)$_3$, identifying stable phases and structural origins of Na/vacancy arrangements.

Findings

Identification of stable phases at Na$_2$V$_2$(PO$_4$)$_3$ and Na$_{3.5}$V$_2$(PO$_4$)$_3$

Structural analysis of Na/vacancy orderings driven by charge orderings

Insights into electronic structure related to ground-state compositions

Abstract

We elucidate the thermodynamics of sodium (Na) intercalation into the sodium super-ionic conductor (NaSICON)-type electrode, Na$_x$V$_2$(PO$_4$)$_3$, for promising Na-ion batteries with high-power density. This is the first report of a computational temperature-composition phase diagram of the NaSICON-type electrode Na$_x$V$_2$(PO$_4$)$_3$. We identify two thermodynamically stable phases at the compositions Na$_2$V$_2$(PO$_4$)$_3$ and Na$_{3.5}$V$_2$(PO$_4$)$_3$, and their structural features are described for the first time based on our computational analysis. We unveil the crystal-structure and the electronic-structure origins of the ground-state compositions associated with specific Na/vacancy arrangements, which are driven by charge orderings on the vanadium sites. These results are significant for the optimization of high-energy and power densities electrodes for sustainable Na-ion batteries