Synthesis and properties of Al-free Li7-xLa3Zr2-xTaxO12 garnet related oxides

We investigated the properties of Al-free garnet related Li7La3Zr2O12 (LLZ) and Li7 - xLa3Zr2 - xTaxO12 (LLZTa) with Ta contents x = 025, 0.5 and 1.0 synthesized by a solid state reaction method. It was confirmed that all Al-free LLZTa have the cubic garnet related structure, while Al-free LLZ without Ta substitution has a tetragonal one. No other secondary phases were observed in all sintered samples. This indicates that partial substitution of Zr4+ by Ta5+ with higher valence increases Li-ion vacancy concentration in a garnet framework and stabilizes cubic garnet structure. The LLZTa grains are in good contact with each other although a certain amount of pores in the boundary area can be observed. Total (bulk + grain boundary) Li-ion conductivity at 27 degrees C for all Al-free LLZTa was well above 10(-4) Scm(-1), while Al-free LLZ showed a much lower room temperature conductivity of 1 x 10(-5) Scm(-1). The highest room temperature conductivity of 6.1 x 10(-4) Scm(-1) was obtained in Al-free LLZTa with x = 0.5, which is approximately 30-40% lower than the value in LLZTa (x = 0.5) with Al inclusion as previously reported. (C) 2013 Elsevier B.V. All rights reserved.