Why tail states matter? Impact of defect types on the electrochemical kinetics of CFx cathodes

Abstract

Fluorinated carbon (CF_x) stands out as a promising cathode material for lithium primary batteries, owing to its ultra-high specific capacity and energy density. However, the role of defects in CF_x, which is crucial in shaping its electrochemical properties, remains insufficiently explored, especially for different CF_x forms such as layered fluorinated graphite (FG) and newly emerged fluorinated hard carbon (FHC) with a disordered structure. This study systematically compares these two representative CF_x materials in terms of electronic structures and elucidates how distinct defect types regulate their electrochemical kinetics. Through comprehensive optical spectroscopy and structural characterization, we revealed that FHC exhibits more pronounced excitation-energy dependent photoluminescence shifts, indicative of carbon-cluster-like defects. In contrast, FG displays optical signatures dominated by point-defect induced tail states. The fundamental differences in defect properties correlate well with the cathodes' electrochemical behaviour: FHC's carbon-cluster defects lead to higher electrical conductivity, more favorable Li⁺ ion transport kinetics and higher operational voltages, while FG's point defects result in relatively increased electrochemical polarization and kinetic limitations. These findings establish a direct link between defect engineering and macroscopic battery performance, paving the way for rationally designing high performance CF_x cathodes.