AUTHOR=Inada Ryoji , Yasuda Satoshi , Tojo Masaru , Tsuritani Keiji , Tojo Tomohiro , Sakurai Yoji TITLE=Development of Lithium-Stuffed Garnet-Type Oxide Solid Electrolytes with High Ionic Conductivity for Application to All-Solid-State Batteries JOURNAL=Frontiers in Energy Research VOLUME=4 YEAR=2016 URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2016.00028 DOI=10.3389/fenrg.2016.00028 ISSN=2296-598X ABSTRACT=

All-solid-state lithium-ion batteries are expected to be one of the next generations of energy storage devices because of their high energy density, high safety, and excellent cycle stability. Although oxide-based solid electrolyte (SE) materials have rather lower conductivity and poor deformability than sulfide-based ones, they have other advantages, such as their chemical stability and ease of handling. Among the various oxide-based SEs, lithium-stuffed garnet-type oxide, with the formula of Li₇La₃Zr₂O₁₂ (LLZ), has been widely studied because of its high conductivity above 10⁻⁴ S cm⁻¹ at room temperature, excellent thermal performance, and stability against Li metal anode. Here, we present our recent progress for the development of garnet-type SEs with high conductivity by simultaneous substitution of Ta⁵⁺ into the Zr⁴⁺ site and Ba²⁺ into the La³⁺ site in LLZ. Li⁺ concentration was fixed to 6.5 per chemical formulae, so that the formula 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. As a result, all LLBZT samples have a cubic garnet structure without containing any secondary phases. The lattice parameters of LLBZT decrease with increasing Ba²⁺ contents x ≤ 0.10 while increase with x from 0.10 to 0.30, possibly due to the simultaneous change of Ba²⁺ and Ta⁵⁺ substitution levels. The relative densities of LLBZT are in a range between 89 and 93% and are not influenced in any significant way by the compositions. From the AC impedance spectroscopy measurements, the total (bulk + grain) conductivity at 27°C of LLBZT shows its maximum value of 8.34 × 10⁻⁴ S cm⁻¹ at x = 0.10, which is slightly higher than the conductivity (= 7.94 × 10⁻⁴ S cm⁻¹) of LLZT without substituting Ba (x = 0). The activation energy of the conductivity tends to become lower by Ba substation, while excess Ba substitution degrades the conductivity in LLBZT. LLBZT has a wide electrochemical potential window of 0–6 V vs. Li⁺/Li, and Li⁺ insertion and extraction reactions of TiNb₂O₇ film electrode formed on LLBZT by aerosol deposition are demonstrated at 60°C. The results indicate that LLBZT can potentially be used as a SE in all-solid-state batteries.