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

Silicon oxide (SiOx) 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 SiOx. The etching-induced fluorination of commercial SiOx via NF3 treatment generates porous silicon oxyfluoride (pSiOxFy), which is followed by thermal defluorination during carbon nanotube (CNT) in-pore growth to form pSiOxFy-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 pSiOxFy-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 SiOx. 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 SiOx anodes that goes beyond conventional strategies relying on carbon coating or fluorine doping.