Effect of Liquid Electrolyte Soaking on the Interfacial Resistance of Li₇La₃Zr₂O₁₂ for All-Solid-State Lithium Batteries

The impact of liquid electrolyte soaking on the interfacial resistance between the garnet-structured Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte and metallic lithium has been studied. Lithium carbonate (Li₂CO₃) formed by inadvertent exposure of LLZO to ambient conditions is generally known to increase interfacial impedance and decrease lithium wettability. Soaking LLZO powders and pellets in the electrolyte containing lithium tetrafluoroborate (LiBF₄) shows a significantly reduced interfacial resistance and improved contact between lithium and LLZO. Raman spectroscopy, X-ray diffraction, and soft X-ray absorption spectroscopy reveal how Li₂CO₃ is continuously removed with increasing soaking time. On-line mass spectrometry and free energy calculations show how LiBF₄ reacts with surface carbonate to form carbon dioxide. Using a very simple and scalable process that does not involve heat-treatment and expensive coating techniques, we show that the Li–LLZO interfacial resistance can be reduced by an order of magnitude.