Exploring the Interaction between EC and Ta-Doped LLZO
Authors: Tronstad Zachary , McCloskey Bryan D. 
Published: December 2024

Solid-state electrolytes are a primary strategy for unlocking the use of lithium metal anodes and enabling batteries with higher energy density.1 Garnet-type Li7La3Zr2O12 (LLZO) is a leading candidate among ceramic-type materials due to its high ionic conductivity and kinetic stability against lithium metal.2 However, LLZO is air sensitive, spontaneously reacting with CO2 in ambient air to form lithium carbonate layers.3–6 Additionally, we have recently collected evidence, as described below, that suggests that LLZO may even be reactive towards ethylene carbonate (EC), a key liquid electrolyte solvent typically used in battery systems. This reactivity has broad implications for hybrid liquid-solid electrolytes, which have been suggested to alleviate prevalent contact issues between ceramic electrolytes and active materials in battery cathodes.7–9

Previous work has shown that a solution of 1.2M LiBF4 in EC/DEC (diethyl carbonate) can remove surface Li2CO3 from LLZO pellets through an acid-base reaction involving the displacement of labile F- in the BF4 anion.4 Our initial studies expanded this carbonate removal strategy to ceramic powders primarily used in polymer composites. By monitoring the CO2 evolution from particles immersed in a 1.8M LiBF4 in EC solution, we discovered a unique 2-peak structure (see Figure 1) when washing LLZO powders with the procedure described above. Additionally, we evolved 2x more CO2 than would be expected given the amount of Li2CO3 present in our sample. Intriguingly, this behavior seems unique to solutions with EC as the solvent, suggesting some reaction occurring between EC and LLZO. Given the ubiquity of EC as a liquid electrolyte, and potential use in hybrid solid-state/liquid electrolytes, it is critical to inform the solid-state battery community of this previously unknown reaction.
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Figure 1