Crystal Chemistry and Ionic Conductivity of the NASICON-Related Phases in the Li3-XNaXV2(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, P2(1)/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) (R3(-); 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.

Automated summary

In this study, crystal phases in the Li3-xNaxV2(PO4)(3) system were investigated using four synthesis methods. It was found that mechanochemically assisted solid-state and sol-gel synthesis methods failed to produce the single-phase Na3-xLixV2(PO4)(3). In contrast, chemical and electrochemical ion exchange methods achieved deep substitution degrees, especially in the NASICON framework. The Li2NaV2(PO4)(3) phase, formed by replacing 1/3 of Li+ ions with Na+ in the anti-NASICON framework, was shown to be metastable and decomposed into two NASICON-type compounds after high-temperature treatment. The ionic conductivity and contribution of the electronic component to total conductivity were also analyzed.