All-solid-state lithium-ion battery (LiB) is expected as one of the next generation energy storage devices because of their high energy density, high safety and excellent cycle stability. Although oxide-based solid electrolyte materials have rather lower conductivity and poor deformability than sulfide-based one, they have other advantages such as their chemical stability and easiness for handling. Among the various oxide-based solid electrolytes, cubic garnet-type oxide with the formula of Li₇La₃Zr₂O₁₂ (LLZ) have been widely studied because of their high conductivity above 10^-4 Scm^-1 at room temperature, excellent thermal performance and stability against Li metal anode. Li₇La₃Zr₂O₁₂ (LLZ) have been widely studied because of their high conductivity above 10^-4 Scm^-1 at room temperature, excellent thermal performance and stability against Li metal anode. LLZ has two different crystal phases, one is cubic phase and another is tetragonal one, but high conductivity above 10^-4 Scm^-1 at room temperature is mostly confirmed in cubic phase sintered at high temperature. The concentration of Li⁺ in a garnet framework has crucial importance to stabilize cubic phase

In this study, we present our recent progress for the development of garnet-type solid electrolytes with high conductivity by simultaneous substitution of Ta⁵⁺ into Zr⁴⁺ site and Ba²⁺ into La³⁺ site in LLZ. Li⁺ concentration was fixed to 6.5 per chemical formulae, so that the formulae of our Li garnet-type oxide is expressed as Li_6.5La_3-xBa_xZr_1.5-xTa_0.5+xO₁₂ (LLBZT) and Ba contents x are changed from 0 to 0.3. We don’t use Al₂O₃ crucible for sample synthesis because Al³⁺ contamination from the crucible into the LLBZT lattice during high temperature sintering may influence on Li⁺ concentration and the contamination level is very difficult to control.

As results, all LLBZT samples have cubic garnet structure without containing any secondary phases. The lattice constants of LLBZT decrease with increasing Ba²⁺ contents x from 0 to 0.1 while increase with x from 0.1 to 0.3, possibly due to the simultaneous change of Ba²⁺ and Ta⁵⁺substitution levels. Relative densities of LLBZT are in the range between 89% and 93% and not influenced so much by the compositions. From AC impedance spectroscopy measurements, the total (bulk + grain) conductivity at 27ºC of LLBZT has its maximum value of 0.83 mS/cm at x = 0.10, which is slightly higher than the conductivity (= 0.80 mS/cm) of the sample without substituting Ba (x = 0). Activation energy for the conductivity is lowered with increasing Ba substitution levels but excess Ba substitution degrades the conductivity in LLBZT.

LLBZT is electrochemically stable at the potential range from 0 to 6.0 V vs. Li⁺/Li. Moreover, Li⁺ insertion and extraction reactions of LiMn₂O₄ (LMO) or TiNb₂O₇(TNO) film electrode formed on LLBZT by aerosol deposition were confirmed successfully. Particularly, TNO film electrode in all solid state cell with LLBZT showed initial capacities of 220 mAh/g for Li⁺ insertion and 170 mAh/g for Li⁺ extraction at 60ºC and 5 mA/g-TNO. These results indicate that cubic garnet-type LLBZT can potentially be used as a solid electrolyte in all-solid-state batteries.