Influence of the LiFSI and LiPF6 concentrations on the electrochemical behavior of LiFePO4 electrodes at potentials up to 5 V vs. Li+/Li

Herein, the electrochemical performance of LiFePO 4 (LFP) electrodes is studied in electrolytes containing different LiFSI concentrations at room temperature and 40 °C, with upper cut-off potentials up to 5.0 V vs. Li + /Li. LFP-Li half-cells containing 1.0 M LiFSI exhibited stable cycling for more than 250 cycles at 4.0 V but failed rapidly when cycled to 5.0 V vs . Li + /Li. This effect is attributed to the anodic dissolution (corrosion) of the aluminum current collector due to solvent oxidation yielding protons. In contrast, LFP-graphite full-cells enabled reversible cycling up to about 5.0 V (i.e., about 4.9 V vs. Li + /Li) when the LiFSI concentration was increased to about 2.0 M. One possible reason for this improvement is the formation of fluoride-based passive surface layer(s) preventing corrosion upon increasing the LiFSI concentration, as previously seen in 1.0 M LiPF 6 . This hypothesis is further supported by results obtained with electrolytes containing both LiFSI and LiPF 6 . At 40 °C and a 1C-rate, LFP-graphite full-cells containing 2.0 M LiFSI showed higher capacity retention than LiPF 6 -based systems, possibly due to the increased LiPF 6 decomposition at 40 °C. These results demonstrate that electrolytes containing 2.0 M LiFSI can be used in LFP-graphite cells cycled up to 5.0 V.