PROTON CONDUCTING SOLID ELECTROLYTES BASED ON DIPHOSPHATES

Abstract

The highly densified A_0.9In_0.1P₂O₇(A=Sn,Ti) sinters prepared through a solid-state reaction route showed an extremely low ionic conductivity of <10^-7 Scm^-1 at 400°C. This indicates that both are nearly insulating materials. The H₃PO₄-modified A_0.9In_0.1P_2.4O₇(A=Sn,Ti) were synthesized by planetary-ball milling with 85 mass% H₃PO₄. The H₃PO₄-modified Ti_0.9In_0.1P_2.4O₇ sample showed the highest proton conductivity to date of 2.0×10^-1 Scm^-1 (161°C), which was higher than that reported by Nagao et al. The sol-gel derived A_0.9M_0.1P_2.4O₇(A=Sn,Ti; M=In,Al,Fe) showed high proton conductivities of 7×10^-2 Scm^-1 -10^-4 Scm^-1 (100-250°C). The conduction mechanism based on a core-shell structure is presented. The conductivities were significantly influenced by heat-treatments in preparation processes and increased with increasing amounts of the amorphous phase, the M-containing impurity and the phosphorus oxide content, which formed the conductive shells. The conductivity results reported to date on the diphosphate-related electrolytes are well explained by the mechanism based on the shell-core structure.