Revealing the Role of Liquid Electrolytes in Cycling of Garnet-Based Solid-State Lithium-Metal Batteries

Solid-state lithium-metal batteries (SS-LMBs) suffer from the very high resistance at the garnet electrolyte/cathode interface that hampers their commercialization. Herein, a carbonate-based liquid electrolyte (LE) is introduced at the interface of a Li6.5La2.9Ba0.1Zr1.4Ta0.6O12 (LLBZTO) garnet/LiNi0.6Mn0.2Co0.2O2 (NMC 622) cathode to lower the interfacial resistance and improve the battery performance. In this work, we conducted a thorough study on the role of liquid electrolytes at the interface using scanning transmission X-ray microscopy (STXM) associated with X-ray absorption spectroscopy (XAS). As a result, we have shown new data related to the formation and the chemical composition of the two formed interphases: a solid–liquid electrolyte interphase (SLEI) and the cathode–electrolyte interphase (CEI). Furthermore, we have presented evidence that LE decomposes during cycling into fluoride species including LiF and LaF3, oxides like Li2O, and carbonates (i.e., Li2CO3) as the main components of in situ-formed SLEI. Based on the synergy between SLEI and CEI, we demonstrate Li|garnet|LE|NMC 622 cells cycled with an initial discharge capacity of 168 mAh g–1 and a capacity retention of ∼82% after 28 cycles. We expect that our study of SLEI will accelerate the implementation of a new hybrid electrolyte (solid garnet and liquid electrolyte) approach in SS-LMBs.