Overcharge Investigations of LiCoO2/Graphite Lithium Ion Batteries with Different Electrolytes

Although overcharging has been under extensive investigation for lithium ion batteries, the mechanism underneath still remains a "dark art" with little rational understanding. In this work, the degradation behaviors of LiCoO2/artificial graphite full cells using 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) electrolyte or 1 M LiPF6 in EC/dimethyl carbonate (DMC) electrolyte (both with a 3/7 weight ratio) after overcharge are investigated in detail for the first time. By the means of combining liquid chromatography–quadruple time-of-flight mass spectrometry, gas chromatography–mass spectrometry, X-ray diffraction, and scanning electron microscopy, the fundamental chemistry and degradation mechanism underneath are explored thoroughly. It is found that as the cutoff voltage increases, the reactions between electrolyte and electrode become more complex. Furthermore, the EC/DEC electrolyte system is more advantageous against overcharging. The EC/DMC system goes through more severe side reactions, generating more gaseous and degradation products. Thus, the use of electrolyte with relatively large steric hindrance can slow down the reaction rate during the overcharge process and should be more advantageous. The overcharge performance and degradation mechanisms revealed in this study should be of serious consideration before implementing the cell chemistry in high-power commercial batteries.