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

Although overcharge is in extensive investigation for lithium ion batteries, the underneath mechanism 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 3/7 in 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 chromatograph-mass spectrometer, X-ray diffraction and scanning electron microscope, the fundamental chemistry and underneath degradation mechanism are explored thoroughly. It is found that as the cutoff voltage increasing, the reactions between electrolyte and electrode become more complex. Furthermore, the EC/DEC electrolyte system is more advantageous against overcharge. The EC/DMC system goes through more severe side reactions generating m...