Operando\nOptical Imaging of Cathode Swelling Process\nInside Lithium Primary Batteries: Comparative Studies between Different\nStructured CF<sub><i>x</i></sub>

As\na crucial cathode material with ultrahigh theoretical capacity\n(865 mAh/g) and energy density (>2100 Wh/kg), fluorinated carbon\n(CF<sub><i>x</i></sub>) is promising for lithium primary\nbatteries.\nHowever, the operating performance of CF<sub><i>x</i></sub> cathode is hindered by nonuniform volume expansion during discharging.\nTo investigate this, we used operando confocal microscopy to visualize\nthe thickness evolution of two different CF<sub><i>x</i></sub> cathodes and quantified their swelling ratios as a function\nof the discharge depth. Our findings show that CF<sub><i>x</i></sub> synthesized from hard carbon (FHC), featuring a disordered\nstructure and abundant pores, exhibits a swelling ratio lower than\nthat of conventional layered fluorinated graphite (FG). This is due\nto different electrochemical mechanisms: lithium enters FG interlayers\nnonuniformly through “edge propagation” pathways while\nlithiation occurs homogeneously in FHC, unraveled by single-particle\nRaman and photoluminescence measurement. This work enhances our understanding\non CF<sub><i>x</i></sub> volume expansion, offering important\nopportunities to address the cathode swelling issue and optimize electrochemical\nperformance in Li/CF<sub><i>x</i></sub> batteries.