Novel sodium superionic conductor of the Na1+yTi2SiyP3-yO12 series for application as solid electrolyte

In the search for new materials to be used as solid electrolytes, this paper discusses the substitution of phosphorus with silicon in the NaTi2(PO4)3 NASICON (Na-Super Ionic Conductor) compound, giving rise to the Na1+yTi2SiyP3-yO12 series. In fact, the substitution of P+5 for Si+4 enables the increase in the Na+ charger carrier concentration. The solid electrolytes are synthetized by the glass-ceramic route, which consists in the controlled crystallization of a precursor glass subjected to specific heat treatment. Experimental results indicate that precursor glasses are successfully crystallized in compositions containing y ≤ 1.2. X-ray diffraction patterns show the formation of NASICON phase in the Na1+yTi2SiyP3-yO12 glass-ceramics for y ≤ 0.8. However, with further addition of silicon, the major crystalline phase obtained is the Na(TiO)(PO4) phase. Surprisingly, the electrical characterization reveals that the y = 1.0 sample, whose main phase is the non-NASICON Na(TiO)(PO4), exhibits the lowest activation energy (0.31 eV) and the highest ionic conductivity of 1.0 × 10−4 S cm−1 at room temperature and 1.7 × 10−2 S cm−1 at 300 °C. Rietveld refinement and electrical conductivity results suggest that the increased ionic conductivity in the Na(TiO)(PO4) phase is due to the inclusion of some Si+4 ions in its structure, thus forming a new and highly Na(TiO)((Si)PO4) conductive phase.