Polyanionic materials (phosphates in particular) are of special interest as positive electrodes for Li-in or Na-ion batteries since they offer competitive electro-chemical performances compared to sodiated or lithiated transition metal oxides. Polyanionic crystal structures of Na- or Li-containing materials are based upon stable 3D frameworks, which provide long-term structural stability and demonstrate a unique variety of atomic arrangements in their crystal structures. Recent electrochemical and structural investigations of vanadium-based phosphate compounds (LiVPO4O-LiVPO4F, Na3V2(PO4)2F3, Na3V2(PO4)3 …..), revealed promising perspectives. The NASICON structural family, NaxMM’(XO4)3 (0&lt;x&lt;4 ; M,M’ = Ti, Fe, V, Cr, M,; Nb, ; X = P, S, Si, W, Mo, …) is among the most widely investigated due to its specific three-dimensional framework structure, stable long-term cycling ability and high Na+ mobility. The vanadium phosphate Na3V2(PO4)3 is of particular interest because of its high rate capacity and thermal stability (with a theoretical capacity of 117.6 mAh/g and an energy density of 396 Wh/kg at 3.4 V vs. Na/Na+ by 2 Na+ extraction per formula unit). By substitution of vanadium with Aluminum, towards the new composition Na3AlV(PO4)3, a higher operating voltage is reached at 3.9 V vs. Na/Na+ for the V4+/5+ redox couple. Similar phenomena were recently spotted for Na3FeV(PO4)3 and Na2TiV(PO4)3. Recently, Mn2+ was used as a substituting ion to enhance the capacity of the Na3V2(PO4)3 cathode material. Single phase Na4MnV(PO4)3 powders were synthesized and studied structurally and electrochemically in details. In-operando X-ray diffraction studies during electrochemical operation show that Na4MnV(PO4)3 can deliver 156 mAh/g towards the new composition NaMnV(PO4)3. Tricky redox phenomena were spotted thanks to operando XAS spectroscopy and apparent structural irreversibility occurs when vanadium is oxidized from V4+ to V5+.

Crystal chemistry of NASICON-type positive electrodes for Na-ion batteries

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.