Comparison of voltammetric responses over the cathodic region in LiPF ₆ and LiBETI with and without HF

This study reports the effects of HF on the voltammetric responses below 3 V (vs. Li/Li⁺) in LiPF₆ solutions being widely used in rechargeable lithium-ion batteries (LIBs). During the first cathodic scan on a Pt electrode, two reduction peaks are observed between 3.0 and 1.5 V in LiPF₆ solutions irrespective of the solvents used. The reduction reactions are absent in LiBETI solutions but present after the addition of trace amount of HF. Thus, the reduction reactions in LiPF₆ solutions are due to the HF, an inevitable impurity of LiPF₆ solutions. Based on the results of the rotating disk electrode and the rotating ring disk electrode experiments, two reduction peaks are assigned to the hydrogen underpotential deposition and the hydrogen evolution reaction, respectively. HF reduction reactions forms a surface layer of which the main constituent is LiF. The surface layer suppresses the other reduction reactions otherwise occurring below 3.0 V. For example, the electrodeposition of metallic Mn, which takes place below 1.0 V in LiBETI solutions, is severely hampered in LiPF₆ solutions. The passivity by HF reduction is observed not only for a Pt electrode but also for a glassy carbon electrode.