The effect of fluorinated ethylene carbonate (FEC) as a cosolvent in alkyl carbonates/LiPF6 on the cycling performance of high-voltage (5 V) cathodes for Li-ion batteries was investigated using electrochemical tools, X-ray photoelectron spectroscopy (XPS), and high-resolution scanning electron microscopy (HRSEM). An excellent cycling stability of LiCoPO4/Li, LiNi0.5Mn1.5O4/Si, and LiCoPO4/Si cells and a reasonable cycling of LiCoPO4/Si cells was achieved by replacing the commonly used cosolvent ethylene carbonate (EC) by FEC in electrolyte solutions for high-voltage Li-ion batteries. The roles of FEC in the improvement of the cycling performance of high-voltage Li-ion cells and of surface chemistry on the cathode are discussed.

The effect of the electrolyte solution composition and the cycling protocol on the long-term cycling performance and surface chemistry of monolithic amorphous columnar silicon film electrodes was investigated using electrochemical tools, XPS, SEM and EDS. An excellent cycling stability of Si electrodes in fluoroethylene carbonate (FEC)-based electrolyte solutions was demonstrated. It relates to the ability of FEC to form polyenes, as well as to a high rate of HF formation in its water contaminated LiPF6 solutions, measured by 19F NMR spectroscopy. We found that excellent passivation in FEC solutions related to a low content of oxygen containing moieties in surface films. Galvanostatic tests and spectroscopic analyzes revealed also a strong dependence of the composition and properties of the surface films on amorphous Si electrodes on the cycling procedure. Repeated deep discharge of Si electrodes down to 10 mV vs. Li/Li+ from the very beginning of the electrodes life ensures the formation of thin effective surface films on their surface and good cycling performance. Cycling stability of Si electrodes in LiPF6 solutions decreases in the following order FEC/DMC > 3,4-trans-difluoroethylene carbonate (DFEC)/DMC > ethylene carbonate (EC)/DMC > propylene carbonate (PC).

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