Functional-oriented design of gradient composite fluoride interphase for enhanced silicon anode performance

Silicon (Si) is considered a promising next-generation anode due to its ultrahigh theoretical capacity, yet the severe volume changes during cycling that cause interfacial instability and rapid capacity fade remain a major challenge. Conventional fluoride-based interfacial engineering seeks to enhance performance by inducing a LiF-rich SEI, but often overfocuses on LiF content while neglecting the structural and multifunctional requirements of the interphase. Herein, we propose an in situ fluorination strategy to construct a composite coating layer comprising crystalline Li₂SiF₆ and amorphous Li₃AlF₆ (denoted as LSAF) on porous silicon (p-Si). This design creates a physicochemical barrier that simultaneously offers high ionic conductivity, superior mechanical strength, and effective electrolyte isolation. The LSAF-1 anode exhibits outstanding cycling stability, retaining 1238.0 mAh g^-1 after 400 cycles at 2 A g^-1. Its advantages are more pronounced under high-temperature and high-rate conditions. Furthermore, it shows remarkable performance in full cells paired with LiFePO₄. Mechanistic studies reveal that this coating not only suppresses the accumulation of P/F-containing by-products at the electrode interface but also alleviates volumetric strain by enhancing interfacial mechanical strength. This research provides novel insights for rational interface engineering of Si anodes, advancing the design and development of high-performance anode materials for lithium-ion batteries.