Unveiling degradation mechanisms of anode-free Li-metal batteries

Anode-free Li-metal batteries (AFLMBs), in which Li⁺ ions from the cathode are deposited on a Cu substrate and the deposited Li-metal serves as the anode, exhibit higher energy density compared to Li-metal batteries (LMBs). However, achieving stable cycle performance, even at moderate operating conditions, is difficult and has so far hindered their practical uses. In AFLMBs, the homogeneity of solid electrolyte interphase (SEI), initially created by electrolyte reduction on Cu substrate, is not maintained during Li-metal deposition, leading to uncontrolled electrolyte decomposition. The SEI is therefore not conserved, and uneven Li deposition morphology is induced on the Cu substrate and the eventual instability of SEI leads to the overall degradation of AFLMBs. Here, we report on the failure mechanisms of AFLMBs through a comparative study with LMBs using 3 M lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in N,N-dimethylsulfamoyl fluoride. Our investigation reveals that the SEI inhomogeneity in AFLMBs makes Li⁺ transport through SEI sluggish and non-uniform, triggering local compositional changes of the initially formed SEI on the Cu substrate and unwanted consumption of FSI^– anion from the electrolyte. This work provides clear understanding to the interfacial engineering and important roles of Li-metal on the Cu substrate in AFLMBs, promising the creation of stable SEI, reversible electrochemical reaction of Li-metal, and interfacial stability of the cathode in LMBs.