Kinetics of Na|CFx and Li|CFx systems

Abstract

Properties of CFx/Li and CFx/Na cells were examined while using galvanostatic charging/discharging, electrochemical impedance spectroscopy and scanning electron microscopy (SEM). The capacity during the first cycle was as high as ca. 1000 mAh g−1. Such an electrode is suitable for primary CFx/Li and CFx/Na batteries. SEM images of CFx cathode showed that during discharging it was transformed into amorphous carbon and LiF or NaF crystals (of diameter of ca. 5–20 μm). These systems (C + LiF or C + NaF) cannot be reversibly converted back into CFx/Li or CFx/Na, respectively. Exchange current densities are between 10−7 Acm−2 and 10−9 Acm−2 when working with LiPF6 and NaPF6 electrolytes (1.12 × 10−7 Acm−2 and 6.82 × 10−9 Acm−2, respectively). Those values are low and indicate that the charge transfer process may be the rate-determining step. Activation energies for the charge transfer process were 57 and 72 kJ mol−1 for CFx/LiPF6 and CFx/NaPF6 systems, respectively. Higher activation energy barrier for the CF/Na+ + e− → C + NaF reaction results in lower observed exchange current density in comparison to the system with lithium ions.