Improvement of Li ion conductivity of Li 5 La 3 Ta 2 O 12 solid electrolyte by substitution of Ge for Ta

Abstract

Abstract Li 5 La 3 Ta 2 O 12 (LLTa) is a promising solid electrolyte for all-solid-state batteries due to its high stability in contact with Li metal, however, low Li ion conductivity of LLTa has restricted its application. In this study, improvement of the Li ion conductivity of LLTa solid electrolyte by substitution of Ge 4+ for Ta 5+ is studied because the improvement is thought to be achieved by increase of charge carrier concentration caused by the substitution of low valence Ge 4+ for high valence Ta 5+ . The Ge substitution shrinks a lattice of cubic LLTa due to small ion radius of Ge 4+ (0.530 A) compared with Ta 5+ (0.640 A). The Li ion conductivity of LLTa is improved by the Ge substitution. The highest bulk and total Li ion conductivities are obtained in Li 5.25 La 3 Ta 1.75 Ge 0.25 O 12 prepared by spark plasma sintering at 1100 °C and the values are 1.3 × 10 −4 and 8.4 × 10 −5 S cm −1 at 28 °C, respectively. The lithium transference number of the Ge-substituted LLTa determined by Hebb-Wagner (HW) polarization method is ≈ 1. Also, it is verified that the new solid electrolyte is stable in a potential range of 0–10 V vs. Li/Li + , indicating that the Ge-substituted LLTa is a promising solid electrolyte for all-solid-state battery application.