Structure and Ionic Conductivity of Li₂S-P₂S₅-P₂O₅ Glasses and Glass-Ceramics Prepared by Mechanical Milling

Abstract

Lithium ion conducting Li₂S-P₂S₅-based solid electrolytes with P₂O₅ and Li₂O as an oxygen source were synthesized by the mechanical milling method. P₂O₅ effectively reacted with Li₂S-P₂S₅ compared to Li₂O. ³¹P MAS-NMR spectroscopy revealed that the 80Li₂S•18P₂S₅•2P₂O₅ (mol%) glass included PO_nS_4-n (n=1, 2, 3) tetrahedral units where phosphorus was coordinated with both sulfurs and oxygens. The 80Li₂S•19P₂S₅•1P₂O₅ (mol%) glass attained a minimum value of the glass transition and crystallization temperatures. In addition, the glass exhibited high ambient temperature conductivity of 1.7×10^-4 S•cm^-1 and low activation energy of 36 kJ•mol^-1 for conduction which were comparable to the glass without P₂O₅. Further increase in P₂O₅ to the Li₂S-P₂S₅-P₂O₅ glasses led to a decrease in conductivities at room temperature and an increase in activation energies. The higher conductivity and lower activation energy for conduction of the 80Li₂S•(20-x)P₂S₅•xP₂O₅ (mol%) glass-ceramics compared to those of the corresponding glasses were caused by the fact that the highly conductive thio-LISICON analog phase was precipitated by heating of the glass over the crystallization temperature.