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

Report on high Li+ conduction in cubic phase (space group Ia (3) over bard) 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 Ia (3) over bard) at room temperature was unsuccessful. Systematic investigation on synthesis, phase, microstructure and Li+ conducting properties of Li7La3Sn2O12 (LLS), Li6.28Al0.24La3Sn2O12 (LALS) and Li7-xLa3Sn2-xTaxO12 (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., Li6.28Al0.24La3Sn2O12 (LALS) did not promote the transformation from tetragonal phase to high Li+ conductive cubic phase at the sintering temperatures of 950 degrees C and 1075 degrees C. Nevertheless, the partial substitution of Ta for Sn in LLS transformed it from tetragonal to high Li+ conductive phase. Among the investigated samples, Li7-xLa3Sn2-xTaxO12 (x = 0.50) (LLST) sintered at 1075 degrees C exhibited maximized total (bulk + grain-boundary) Li+ conductivity of 2.41 x 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.