Quantifying Interfacial Reactions in Lithium Metal Batteries for a New Paradigm of Long-Cycling Electrolyte Designs

Abstract Side reactions at electrolyte-electrode interfaces are the key to failures of lithium metal batteries (LMBs). Recent works fell short of rationalizing the whole picture of interfacial reactions quantitatively. Here we develop and combine multiple techniques to systematically study the interfacial reactions in LMBs with an ether-based electrolyte. Reduction, oxidation pathways of lithium salt and organic solvents, formation of key solid-electrolyte interphase (SEI) species such as LiH and Li₂CO₃, gas generation and its cross-talk, are quantitatively mapped out. We find that the reduction of lithium bis(fluorosulfonyl)imide (LiFSI) salt is the dominant cause of both electrolyte “dry-out” and lithium inventory loss. FSI anion-derived SEI has significantly promoted lithium metal cycleablity, but it now becomes the limiting factor for the long cycling of LMBs. Based on these findings, we propose a new paradigm for long-cycling electrolyte design, where LiFSI content is maximized without compromising viscosity and ionic conductivity. Following this strategy, we prolong the life of a LMB to nearly 500 cycles under 20 μm Li and 2.1 g/Ah lean electrolyte conditions. Our work demonstrates a bottom-up design methodology where electrolyte formulations are iterated based on a clear understanding of cell failure mechanisms.