Fluorination Engineering of CNT-Integrated SiO_x Anodes for Controlled Swelling and Enhanced Rate Capability

Silicon oxide (SiO_x) anodes exhibit low electronic conductivity, large volume changes, and unstable interphases. Herein, we report fluorination engineering, which induces coupled structural and chemical transformations in commercial SiO_x. The etching-induced fluorination of commercial SiO_x via NF₃ treatment generates porous silicon oxyfluoride (pSiO_xF_y), which is followed by thermal defluorination during carbon nanotube (CNT) in-pore growth to form pSiO_xF_y-CNT. This defluorination lowers the silicon oxidation state, induces recrystallization, and retains fluorine for the formation of a LiF‑rich solid‑electrolyte interphase, thereby enhancing the electrochemical performance. The resulting embedded CNT network provides fast electronic pathways and mechanical buffering, with the porous framework shortening the Li-ion diffusion distance. Consequently, the optimized pSiO_xF_y-CNT delivers an enhanced capacity of 1525 mAh g^-1. It also retains 731 mAh g^-1 after 500 cycles at 1 A g^-1 while limiting the electrode swelling to 38%, compared with 116% for SiO_x. The fluorination duration serves as a practical control parameter, with 20 min balancing porosity generation and active Si retention. Moreover, full-cell tests using LiFePO₄ cathodes confirm that the designed anodes can operate in a coin-cell configuration. Overall, this work suggests a new design for SiO_x anodes that goes beyond conventional strategies relying on carbon coating or fluorine doping.