Optimisation of formation and cell ageing protocols for lithium ion EV batteries

The formation process and subsequent cell ageing (conditioning) protocols for a commercial EV lithium ion cell chemistry have been studied understand their effect on the electrochemical performance and chemical interface. The temperature and duration were varied for both the formation and conditioning steps, and the state of charge was investigated for the conditioning step. The optimum cell ageing temperature was shown to be dependent on the formation conditions. After formation at room temperature, a longer cycle life was observed when ageing was performed at 5 o C. After formation at 5 o C, ageing at 45 o C gave the best cycle life. The formation process creates an initial interface layer from reduction of the electrolyte, which rearranges chemically as the cell ages. Surface analysis of the graphite showed increased quantities of boron and phosphorus in the interface layer after ageing at 45 o C, and the fluorine content increased by 20% during ageing. For low temperature formation, greater levels of lithium and oxygen were observed, which subsequently decreased during ageing.