Optimisation of Formation and Conditioning Protocols for Lithium-Ion Electric Vehicle Batteries

The formation process and subsequent conditioning (cell ageing) protocols for a commercial EV lithium-ion cell chemistry have been studied to 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 conditioning temperature was shown to be dependent on the previous formation conditions. After formation at room temperature, a longer cycle life was observed when conditioning was performed at 5 degrees C. After formation at 5 degrees C, conditioning at 45 degrees C gave the best cycle life. These results show that the conditioning process is important for cell longevity, and is dependent upon the initial formation. The formation process creates an initial interface layer from reduction of the electrolyte, which rearranges chemically during conditioning. The rearrangement has been followed though impedance studies and XPS analysis. After conditioning at 45 degrees C, surface analysis of the graphite showed increased quantities of boron and phosphorus in the interface layer, and the fluorine content increased by 20 % during the conditioning process. For low temperature formation, greater levels of lithium and oxygen were observed, which subsequently decreased during conditioning.