Crystal Chemistry and Ionic Conductivity of the NASICON-Related Phases in the Li_3–xNa_xV₂(PO₄)₃ System

In the present work, we studied crystal phases in the Li3–xNaxV2(PO4)3 system over a wide range of x prepared by four synthesis methods: mechanochemically assisted solid-state synthesis, ‘soft chemistry’ sol–gel approach, chemical (CIE) and electrochemical (EIE) ion exchange starting from Li3V2(PO4)3 (anti-NASICON, P21/c S.G.), and Na3V2(PO4)3 (NASICON, C2/c S.G.). EIE was studied by operando and ex situ XRD in Li3V2(PO4)3 vs Na and Na3V2(PO4)3 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 Na3–xLixV2(PO4)3. In contrast, CIE and EIE lead to deep substitution degrees and proceed much easier in the NASICON framework (Na3V2(PO4)3), where more than 2/3 of Na+ ions per f.u. are replaced with Li+ resulting in Na0.6Li2.4V2(PO4)3 (R3̅ S.G.), while in the anti-NASICON framework (Li3V2(PO4)3), only 1/3 of Li+ ions are replaced with Na+ resulting in Li2NaV2(PO4)3 (Pbcn 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.