(Digital Presentation) Electrochemical Properties of Ta-Doped Li7La3Zr2O12 Ceramic Electrolyte Sintered with Ga2O3 Additive

Abstract

A garnet-type oxide with the formula of Li7La3Zr2O12 (LLZO) has attracted much attention because of its high Li ion conductivity at room temperature, excellent thermal performance, and high stability against Li metal.1,2) However, poor interfacial connection causes non-uniform Li plating and intergranular penetration of Li dendrite in polycrystalline LLZO when the cell is cycled particularly at high current densities, resulting in internal short-circuit failure.3,4) It has been pointed out that the tolerance for Li dendrite growth into LLZO is influenced by many factors such as the interfacial resistance between LLZO and Li anode, cell stacking pressure and microstructure (density and grain size) of LLZO. In this study, we fabricated Ta-doped LLZO (Li6.55La3Zr1.55Ta0.45O12) solid electrolytes with Ga2O3 additive and investigated the effects on the microstructure and electrochemical properties.Ta-doped LLZO with Ga2O3 additive were synthesized via a conventional solid-state reaction process. 1-7 mol % Ga2O3 powder was mixed with calcined Ta-doped LLZO powder. The mixture was pelletized and then sintered at different temperatures with an Al-free crucible. In XRD analysis, each sintered sample has a cubic garnet phase and diffraction peaks from other phases were not detected. In SEM observation, microstructure of sintered sample was strongly influenced by sintering temperature and Ga2O3 addition, due to the sintering with liquid Li-Ga-O phase during high temperature sintering.5, 6) The high total (bulk + grain-boundary) ionic conductivity above 1 mS cm-1 at room temperature was obtained in samples composed of large grains with the size of several 10 to 100 µm. However, the tolerance for Li dendrite growth characterized in a symmetric cell is improved remarkably in the samples with uniform microstructure composed of fine grains with the size of below 5 µm. Since the interfacial resistance between Li and solid electrolyte was adjusted to the same level, the difference in tolerance for Li dendrite growth is mainly attributed to the difference in microstructure depending on both the sintering condition and the amounts of Ga2O3 addition.