Operando Optical Imaging of Cathode Swelling Process Inside Lithium Primary Batteries: Comparative Studies between Different Structured CFx.

Abstract

As a crucial cathode material with ultrahigh theoretical capacity (865 mAh/g) and energy density (>2100 Wh/kg), fluorinated carbon (CFx) is promising for lithium primary batteries. However, the operating performance of CFx cathode is hindered by nonuniform volume expansion during discharging. To investigate this, we used operando confocal microscopy to visualize the thickness evolution of two different CFx cathodes and quantified their swelling ratios as a function of the discharge depth. Our findings show that CFx synthesized from hard carbon (FHC), featuring a disordered structure and abundant pores, exhibits a swelling ratio lower than that of conventional layered fluorinated graphite (FG). This is due to different electrochemical mechanisms: lithium enters FG interlayers nonuniformly through "edge propagation" pathways while lithiation occurs homogeneously in FHC, unraveled by single-particle Raman and photoluminescence measurement. This work enhances our understanding on CFx volume expansion, offering important opportunities to address the cathode swelling issue and optimize electrochemical performance in Li/CFx batteries.