Phase transition, lithium ion conductivity and structural stability of tin substituted lithium garnets

Abstract

Report on high Li⁺ conduction in cubic phase (space group Iad) Li₇La₃Zr₂O₁₂ (LLZ) instigated significant research in related systems like Li₇La₃Sn₂O₁₂ (LLS) and Li₇La₃Hf₂O₁₂ (LLH). However, effort to stabilize Li₇La₃Sn₂O₁₂ (LLS) in high Li⁺ conductive cubic phase (space group Iad) at room temperature was unsuccessful. Systematic investigation on synthesis, phase, microstructure and Li⁺ conducting properties of Li₇La₃Sn₂O₁₂ (LLS), Li_6.28Al_0.24La₃Sn₂O₁₂ (LALS) and Li_7−xLa₃Sn_2−xTa_xO₁₂ (x = 0.25 and 0.50) (LLST) have been carried out in this work for further understanding of stabilization of high Li⁺ conductive cubic phase, correlation between structure and microstructure on bulk and total (bulk + grain-boundary) Li⁺ conductivity. It was suggested that at least 0.24 mole of Al is required for the stabilization of high Li⁺ conductive cubic phase LLZ at room temperature. However, an optimal substitution of Al for Li in LLS i.e., Li_6.28Al_0.24La₃Sn₂O₁₂ (LALS) did not promote the transformation from tetragonal phase to high Li⁺ conductive cubic phase at the sintering temperatures of 950 °C and 1075 °C. Nevertheless, the partial substitution of Ta for Sn in LLS transformed it from tetragonal to high Li⁺ conductive phase. Among the investigated samples, Li_7−xLa₃Sn_2−xTa_xO₁₂ (x = 0.50) (LLST) sintered at 1075 °C exhibited maximized total (bulk + grain-boundary) Li⁺ conductivity of 2.41 × 10⁻⁴ S cm⁻¹. Also investigations have been carried out to understand the structural stability of tetragonal phase LLS and high Li⁺ conductive cubic phase LLST (x = 0.50) with proton-exchange under acidic medium, distilled water and humid condition.