Equal resistance single and bilayer films decouple role of solid electrolyte interphase from lithium morphology in batteries

Abstract

The solid electrolyte interphase (SEI) is one of the most crucial but least understood performance modulators in lithium metal batteries (LMBs). However, decoupling the effect of interfacial chemistries on the formation of the SEI from the lithium (Li) metal morphology remains a challenge. Here, we develop a platform to control Li morphology independent of the interfacial properties by depositing different metal oxide films of fixed resistance on Cu substrates. While the fixed resistance of the films ensures an analogous, resistance-controlled Li morphology, the different film chemistries result in distinctive chemical compositions of the SEI. Our results show that for a fixed morphology of Li, SEI becomes the key performance determinant, wherein a more stable SEI results in an increased battery cycle life. Moreover, we decouple the importance of the two relevant interfaces—that between the Cu/thin film and between the thin film/electrolyte—by using binary stacks of metal oxide thin films. Our stacked film design establishes the dominance of the thin film/electrolyte interface in controlling behavior for cells with fixed Li morphology. This thin film/electrolyte interface controls both the SEI composition and battery performance, i.e., stack designs containing the same top film result in similar SEI compositions and cycling performance trends in LMBs where Li morphology is fixed. Specifically, by switching the thin film/electrolyte interface to Al₂O₃, significant improvements in cycling stability were observed with coulombic efficiencies above 80% up to ∼130 cycles in carbonate electrolytes.