Characterization of Spontaneous Reactions of LiCoO[sub 2] with Electrolyte Solvent for Lithium-Ion Batteries

Positive electrode materials LiCoO 2 , commercial and nanosized, bare and Al 2 O 3 -coated, are soaked in a mixture of battery-grade ethylene carbonate and dimethyl carbonate, with and without LiPF 6 . Strong spontaneous reactions have been observed between LiCoO 2 and the solvent. These reactions become less obvious in practical electrolyte. The composition of the soakage-induced gas, liquid, and solid products are characterized with gas chromatography and mass spectroscopy, Raman and Fourier transformed infrared spectroscopy, respectively. The atomic ratios of Co/Li in the liquid and in the solid are analyzed by inductively coupled plasma technique. The surface morphologies of the solvent- and electrolyte-soaked commercial LiCoO 2 particles are observed with scanning electron microscopy. Significant impacts of the spontaneous reactions on the structure and electrochemical performance of the electrode material are evaluated with X-ray diffraction (XRD) and electrochemical cycling, respectively. Based on these results, other important aspects of the performance improvement mechanism by surface modification to LiCoO 2 and the limitation of surface modification are proposed.