Design and Electrochemical Mechanism of the MgF₂ Coating as a Highly Stable and Conductive Interlayer on the Si Anode for High-Performance Li-Ion Batteries

Silicon (Si) with high specific capacity, abundant reserve, and low cost is a promising replacement for graphite in anodes of next-generation lithium-ion batteries (LIBs). However, practical implementation is still hampered by the poor rate performance and short lifespan due to the unstable electrode/electrolyte interface and low ion/electron conductivity. Therefore, design of a stable and high-conductivity interface for Si anodes is desirable albeit challenging. Herein, a mixed ion/electron conducting interlayer (MIECI) consisting of LiF and Li−Mg alloy is formed in situ from an intermediate MgF₂ layer on the surface of the porous Si electrode in the first lithiation step to produce a robust solid electrolyte interface (SEI). The MIECI formation mechanism is investigated by operando Raman scattering, X-ray diffraction, and Fourier transform infrared spectroscopy. LiF in the MIECI provides high ion conductivity, while the Li−Mg alloy produces fast electron conductivity and high mechanical strength. As a result, the p-Si@MgF₂ anode shows excellent cycling stability with 90% capacity retention after 200 cycles and a superior rate capacity of 70% when the current density is increased from 0.5 to 5.0 A g⁻¹. The novel interfacial modification and engineering strategy has large potential in the design and fabrication of Si anodes for LIBs. © 2023 Wiley-VCH GmbH.