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.

Automated summary

Silicon (Si) has the potential to replace graphite in next-generation lithium-ion batteries (LIBs) due to its high specific capacity, abundant reserve, and low cost. However, the poor rate performance and short lifespan of Si anodes are still major challenges due to the unstable electrode/electrolyte interface and low ion/electron conductivity. In this study, a mixed ion/electron conducting interlayer (MIECI) consisting of LiF and Li-Mg alloy is formed in situ from an intermediate MgF2 layer to stabilize the electrode/electrolyte interface. The p-Si@MgF2 anode with the MIECI shows excellent cycling stability and superior rate capacity, demonstrating the potential of this interfacial modification and engineering strategy for Si anodes in LIBs.