Crystal Chemistry and Ionic Conductivity of the NASICON-Related Phases in the LiNaV(PO) System.

In the present work, we studied crystal phases in the LiNaV(PO) system over a wide range of prepared by four synthesis methods: mechanochemically assisted solid-state synthesis, 'soft chemistry' sol-gel approach, chemical (CIE) and electrochemical (EIE) ion exchange starting from LiV(PO) (anti-NASICON, 2/ S.G.), and NaV(PO) (NASICON, 2/ S.G.). EIE was studied by operando and ex situ XRD in LiV(PO) vs Na and NaV(PO) vs Li electrochemical cells. It was shown that both mechanochemically assisted solid-state and sol-gel synthesis methods do not result in the single-phase NaLiV(PO). In contrast, CIE and EIE lead to deep substitution degrees and proceed much easier in the NASICON framework (NaV(PO)), where more than 2/3 of Na ions per f.u. are replaced with Li resulting in NaLiV(PO) (3̅ S.G.), while in the anti-NASICON framework (LiV(PO)), only 1/3 of Li ions are replaced with Na resulting in LiNaV(PO) ( S.G.), which was shown to be a metastable phase, and after high-temperature treatment, it decomposes into two NASICON-type compounds. The ionic conductivity was analyzed both theoretically and experimentally, and the results show that in the NASICON framework, the migration of both Na and Li ions is realized, while in the anti-NASICON framework, the Li migration is preferable. The contribution of the electronic component to total conductivity was determined.