Elucidating the origins of rapid capacity fade in hybrid garnet-based solid-state lithium metal batteries

Hybrid solid-state lithium metal batteries (SS-LMBs) incorporate small amounts of liquid electrolyte (LE) to overcome interfacial instabilities between cathodes and solid-state electrolytes. However, hybrid SS-LMBs suffer from a rapid capacity fade after a few cycles that is not yet fully understood. Herein, to shed light on this phenomenon and mechanism, we report the use of ex situ post-mortem analysis of Li|Li_6.5La_2.9Ba_0.1Zr_1.4Ta_0.6O₁₂|LE|LiNi_0.6Mn_0.2Co_0.2O₂ (NMC 622) cells by scanning transmission X-ray microscopy (STXM) combined with ptychography and X-ray absorption spectroscopy (XAS). We find that the mechanism is complex and that microstructural and chemical phase changes, oxygen vacancy formation associated with transition metal dissolution, and contributions from the cathode-electrolyte interphase (CEI) and solid-liquid electrolyte interphase (SLEI) play key roles in the deterioration of battery performance of hybrid SS-LMBs. We expect that our findings will help advance the development of hybrid SS-LMBs by further optimization of the liquid and solid electrolyte formulations and cell structure.