Ta-LLZO Garnet Processed By Spark Plasma Sintering for All Solid State Batteries

Abstract

All Solid State Lithium based Batteries (ASSLB) using a stable inorganic solid state electrolyte are envisaged as the most promising answer to solve the safety issues of Li-ion batteries using inflammable organic liquid electrolyte. Garnet Li7La3Zr2O7 showing high ionic conductivity and high stability versus metallic lithium is currently the most appealing solid electrolyte to be used in oxide based ASSLB. However, its high reactivity with moisture and sensitivity to all energetic process lead to strong difficulties in obtaining stable fully dense pellets while mandatory to ensure its role as mechanical barrier to avoid the short-circuit caused by Li-dendrites [1-2]. In this work, Spark Plasma Sintering is used as both a characterization and a fast sintering techniques to determine the influence of side products and efficiency of applied processes to remove them in reaching dense pellets with high ionic conductivity.Commercial Ta-doped LLZO powder is fully characterized using conventional techniques to identify chemical and thermal stability. SPS experiments are conducted to determine the influence of the side products on the sintering. The efficiency of powder pretreatment designed to remove side products is determined based on data collected during SPS experiments. SPS parameters (applied pressure, temperature ramping, densification dwell ...) are then optimized allowing reaching in a reproducible way pellets with high density (>97 %) and high mechanical strength. Analysis using XPS, SEM and XRD of obtained pellets shows that decomposition occurs only at the surface while the core of the pellet is made of pure cubic phase Ta-doped LLZO. Impedance spectroscopy is used to evidence the influence of side products on the electrical properties and confirms high ionic conductivity (0.5 mS/cm) and very low electronic conductivity of optimized electrolyte. The sintering temperature and dwell time are then optimized by advanced sintering techniques (high pressure, Flash SPS) allowing to prevent the chemical instability usually observed between LLZO and active materials (LiCoO2 or Li[Ni1-x-yCoxMny]O2 (NCM)) and to obtain a cathodic half-cell (LLZO/positive electrode) in one step. References : [1] H. Yamada, T. Ito, R. Basappa, Electrochimica Acta 222 (2016) 648–656.[2] M. Kotobuki, M.Koishi, Journal of Alloys and Compounds 826 (2020) 15421.