MgCl₂-Derived Li-Mg/LiCl Dual-Phase Interphase for Stable Li Metal Cycling

Li metal offers high capacity and low electrochemical potential; however, its high reactivity leads to unstable solid electrolyte interphase (SEI) formation and dendrite growth. Consequently, Li-halide surface modification has attracted significant attention. Among these approaches, LiCl can suppress electron leakage and stabilize the interface, yet its low Li⁺ conductivity limits Li-ion transport and nucleation control under high current densities. In this study, to address these limitations, an MgCl₂-based thermal conversion reaction was employed to construct a mixed interfacial structure in which a Li-Mg alloy layer and LiCl coexist (Li-Mg/LiCl@Li). This strategy preserves the insulating nature and chemical and electrochemical stability of LiCl, while introducing the high Li-ion diffusivity and lithiophilicity of the Li-Mg alloy, thereby effectively mitigating the unfavorable Li plating behavior observed in conventional LiCl artificial SEI layers that arises from insufficient Li-ion diffusivity under high-current conditions. Furthermore, Li symmetric cells employing Li-Mg/LiCl@Li maintain a low overpotential of ∼28 mV and exhibit stable Li plating and stripping for over 1000 h at 1 mA cm^-2/1 mAh cm^-2. Postcycling X-ray photoelectron spectroscopy further suggests more effective suppression of electrolyte side reactions relative to LiCl-only electrodes. Moreover, full cells employing a LiNi_0.8Co_0.1Mn_0.1O₂ cathode also demonstrate long cycle-life stability and excellent rate performance.