Na₆Li₄MO₄(CO₃)₄ (M = W and Mo): An Alternative Electrolyte for High-Temperature Electrochemical Cells

Two new acentric oxycarbonates Na₆Li₄MO₄(CO₃)₄ (M = W and Mo) were synthesized via a conventional solid-state route. Their structure was determined from X-ray diffraction data on single crystals. Na₆Li₄MO₄(CO₃)₄ (M = W and Mo) crystallizes in the acentric cubic P-43m space group (a ≈ 7.15 Å). It is composed of MLi₄O₁₆ units built from MO₄ and LiO₄ tetrahedra and linked by CO₃^2- groups to form a three-dimensional framework in which Na⁺ ions are inserted. We showed from differential scanning calorimetry and powder X-ray diffraction experiments that the melting is congruent (T ∼525 °C). In the solid and molten forms, conductivity was measured for both oxycarbonates by electrochemical impedance spectroscopy with three various gas compositions (CO₂ 100 vol %, CO₂-air 70-30 vol %, and CO₂-air 20-80 vol %). Each time, the stability of the electrical behavior was checked via heating and cooling cycles. The conductivity of both solid and molten phases is purely ionic and in the same order of magnitude as for the classical molten alkali electrolyte made of Li-Na or Li-K carbonates. As activation energies are also comparable, those new oxycarbonates appear to be promising electrolytes for electrochemical devices.