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

Abstract

In the present work, we studied crystal phases in the Li 3−x Na x V 2 (PO 4 ) 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 Li 3 V 2 (PO 4 ) 3 (anti-NASICON, P2 1 /c S.G.), and Na 3 V 2 (PO 4 ) 3 (NASICON, C2/c S.G.). EIE was studied by operando and ex situ XRD in Li 3 V 2 (PO 4 ) 3 vs Na and Na 3 V 2 (PO 4 ) 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 Na 3−x Li x V 2 (PO 4 ) 3 . In contrast, CIE and EIE lead to deep substitution degrees and proceed much easier in the NASICON framework (Na 3 V 2 (PO 4 ) 3 ), where more than 2/3 of Na + ions per f.u. are replaced with Li + resulting in Na 0.6 Li 2.4 V 2 (PO 4 ) 3 (R3̅ S.G.), while in the anti-NASICON framework (Li 3 V 2 (PO 4 ) 3 ), only 1/3 of Li + ions are replaced with Na + resulting in Li 2 NaV 2 (PO 4 ) 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.