Interfacial Aluminum Corrosion Reactions with Lithium Bis(fluorosulfonyl)imide (LiFSI) in Fluorinated and Non-Fluorinated Ether-Based Solutions for Li-Metal Batteries

Lithium-metal batteries (LMBs) have garnered significant attention as a potential replacement for graphite-based lithium-ion batteries due to their superior energy density. However, a critical challenge in LMBs is stabilizing the metallic lithium (Li) electrodes, which face continuous decomposition of the electrolyte solution and the formation of dendrites. It is crucial to develop a desired solid electrolyte interphase (SEI) that contains lithium fluoride (LiF) to protect the Li surface. For this purpose, the bis(fluorosulfonyl)imide (FSI^-) anion has been widely employed, as it readily forms abundant LiF through defluorination. However, the use of FSI^- is accompanied by the corrosion of aluminum (Al), the current collector in the positive electrode, which often leads to cell failures. Although recent studies reported mitigated Al corrosion through the use of fluorinated ethers and locally high concentrated electrolytes (LHCE), there is a lack of study on the interfacial reactions between these electrolytes and Al, as well as their ability to inhibit corrosion.<br/>Herein we studied the Al corrosion with 1 M LiFSI in 1,2-dimethoxyethane (DME), tetraethylene glycol dimethyl ether (G4), 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB), and LHCE comprising DME and 1,1,2,2-tetrafluorethylene 2,2,3,3-tetrafluoropropyl ether (TTE). The corrosion of Al was evaluated through chronoamperometry tests conducted from room temperature to 60 °C. The applied potential for each test was kept below the oxidation potential of the electrolyte solutions (4~4.3 V for DME and G4, and 4.5 V for FDMB and LHCE). In contrast to a significant increase in current observed with DME, the other electrolyte solutions exhibited low and stable leakage currents at room temperature. X-ray photoelectron spectroscopy revealed the formation of the Al(FSI)₃ complex on Al from G4 and LHCE, indicating a low solubility of Al(FSI)₃. On the other hand, AlF₃ was detected when FDMB was used, which was likely formed through the defluorination of FDMB, leading to the replacement of Li⁺ coordination with Al³⁺ in the presence of LiFSI. At 60 °C, the leakage currents with DME, G4, and LHCE significantly increased, in the order of DME &gt; G4 &gt; LHCE. It is attributed to the increased solubility of Al(FSI)₃ and the exacerbation of Al corrosion. Open circuit potentials (OCPs) measured after tests were 0 to 0.3 V with DME, G4, and LHCE, indicating the significant dissolution of Al³⁺ that was deposited on the Li electrode. In comparison, FDMB with LiFSI provided a low and stable current even at 60 °C and 4.5 V. The OCP also remained above 3 V, suggesting the preservation of the Al₂O₃ surface and the mitigated Al corrosion. I will discuss the details of interfacial reactions between Al and electrolyte solutions in the presentation.