High ionic conductivity NASICON based materials for Na-ion batteries: a density functional approach

Sodium ion batteries are now believed to~be the best candidate for large-scale~applications. Simultaneously, it is required to develop solid-state~batteries using solid electrolytes for advancing the safety and reliability~of batteries. The most promising solid-state battery is composed of the~3-D NASICON electrode~Na$_{\mathrm{3}}$V$_{\mathrm{2}}$(PO$_{\mathrm{4}})_{\mathrm{3}}$~(NVP) and electrolyte~Na$_{\mathrm{3}}$Zr$_{\mathrm{2}}$Si$_{\mathrm{2}}$PO$_{\mathrm{12\thinspace }}$(NZSP). In this~work, we aim to theoretically investigate the structures and the diffusion pathway of Na in these~materials. Using density functional theory (DFT) method, we investigated the structures and diffusion mechanism of Na ions in the materials. They are~insulators with large band. The polaron formation is found to occur only in NVP. Na ions can diffuse along three preferable~diffusion pathways; those are, two intra-layer and one~inter-layer pathway that takes place between Na layers via the empty Na~site. In accordance with experiments reported before, the materials~have high ionic conductivity with the activation barrier of about 760meV and~370meV for NVP and NZSP, respectively.~ \newline