Mitigating Li-Loss during Processing of Thin-Film Li6.22Al0.26La3Zr2O12 Solid Electrolytes

Abstract

LLZO (Li7La3Zr2O12) garnets are one of the most promising solid electrolytes for All Solid-State Batteries (ASSBs) owing to their high Li-ion conductivity and electrochemical stability against Lithium metal anode[1]. Processability is the greatest challenge with these electrolytes since it requires very thin and highly dense solid electrolytes for better performance of Li-ion batteries. ASSBs will require less than a 20-μm electrolyte to match the cycle rate performance of conventional Li-ion batteries [2]. Currently, most of the research is being carried out on compressed LLZO pellets, which are sintered in dry air at elevated temperatures to achieve higher bulk ionic conductivity. Since lithium is volatile at such high temperatures, Li-loss adversely affects performance of LLZO. Addition of 10 wt% excess lithium while synthesis of LLZO powders and covering the compressed pellets with mother-powder is often adopted to compensate Li-loss [3]. This work has resulted in the development of strategies for processing very thin film electrolytes via slurry processing. Green films as thin as 20 μm were prepared with optimized composition of binder-solvent-plasticizer system to achieve homogeneous coating. It was seen that the Li-loss is significant in thin-green-film sintering compared to compressed pellets. This phenomenon was attributed to higher surface to volume ration compared to compressed pellets, which are generally 1 mm thick. We have taken an approach of non-mother-powder based sintering for ease of post processing, which has never been reported for thin film electrolyte processing. As shown in the left figure, 75 wt% excess Li was required to compensate for Li-loss while sintering 50 μm green film. Excess Li percentages were optimized to get a pure cubic phase of LALZO after sintering green film thicknesses ranging from 20 to 100 μm. Atomic ratios of all the elements were analyzed with inductively coupled plasma - optical emission spectrometry (ICP-OES), and a compound stoichiometry of Li6.22Al0.26La3Zr2O12 was measured after sintering. The results from this work will provide a wealth of information for the first step in development of high performance inorganic solid-state electrolytes.