Quantitative Estimation of Turning Point of Ageing Based on a Two-Stage Model for Lithium-Ion Batteries

The capacity fading of lithium-ion batteries (LIBs) is commonly reported by a linear dependency followed by a nonlinear ageing process. The linear ageing process is dominated by solid electrolyte interphase formation and reformation (SEI and SEI-re), whereas the nonlinear ageing process is dominated by lithium plating. The turning point is usually alleged when the remaining capacities of LIBs are 80%. This empirical experience may deviate from the turning points of the LIBs in practice, especially under complicated conditions. In this work, a two-stage model is developed to quantitatively predict the turning point during the capacity fading of LIBs, which features the coupling of electrochemical and thermal models accounting for SEI, SEI-re and lithium plating. On the basis of this model, a quantitative evaluation method of the turning point is proposed by attributing the transition of the capacity fading to the balance of consumption of active lithium for SEI growth and lithium plating per cycle in the two stages. The characteristics of capacity fading of LIBs is quantitatively analyzed and discussed under various operation conditions and design parameters. An NCM111/graphite battery is used to validate the proposed model. The results show that it is effective to quantitatively divide the capacity fading curve into two stages by the proposed model, and the presence of the turning point of the two stages reflects the capacity fading of the LIBs. The turning points of the capacity fading processes are extensively influenced by the operation conditions and design parameters of the LIBs, where the reaction rate of lithium plating and SEI or SEI-re growth reign. According to the effect on the turning point, the order of significance of the factors is charging current, charging cut-off voltage, temperature, and N/P ratio, respectively. Moreover, the SEI and SEI-re growth are influenced by temperature, charging current, charging cut-off voltage and N/P ratio, whereas the lithium plating is influenced by charging current, temperature, charging cut-off voltage, and N/P ratio.