Impact of Protonation on the Electrochemical Performance of Li7La3Zr2O12 Garnets.

Li7La3Zr 2O12 (LLZO) garnet ceramics are promising electrolytes for all-solid-state lithium-metal batteries with high energy density. However, these electrolytes are prone to Li+/H+ exchange, that is, protonation, resulting in degradation of their Li-ion conductivity. Here, we identify how common processing steps, such as surface cleaning in alcohol or acetone, trigger LLZO partial protonation. We deconvolute the contributions to the electrochemical impedance spectra of both the protonated LLZO phase (HLLZO) and its decomposition products forming upon annealing. While the mixed conduction of H+/Li+ ions in HLLZO decreases the contribution of the electrolyte to the overall impedance, it deteriorates the transport of Li+ ions across the LLZO/Li interface. This is also the case after thermal decomposition of HLLZO, which occurs at significantly lower temperature than that for pristine LLZO. As a result, symmetric Li/LLZO/Li cells suffer from inhomogeneous lithium electrodeposition within the first three cycles when stripping and plating a capacity of 1 mA·h/cm2 per half-cycle at 0.1 mA/cm2. We demonstrate that LLZO protonation is avoided when applying solvents with very low acidity, such as hexane. Such Li/LLZO/Li cells provide stable cycling over more than 300 h, demonstrating the importance of selecting an appropriate solvent for LLZO processing to prevent dendrites formation.