Effect of Resting Periods over Cycling Performance of a Li-Ion Battery

Vehicles powered by Li-ion batteries (hybrid- and electric-vehicles) offer an attractive alternative to the typical gasoline fuelled vehicles due to their potential in reducing dependence on fossil fuels, reduction in air pollution and noise-free operation. However, the life of these batteries is limited due to the irreversible or degradation reactions that occur in these energy storage systems. It is widely believed that operational parameters such as charge/discharge rates, resting duration, depth of charge and discharge play an important role in enhancing the cycling life of these battery systems. Providing batteries with a rest period after discharging and charging might be essential for relaxation of gradients generated due to the passage of current. In the present work, the effect of open-circuit time durations, after the completion of each charge and discharge segment, over the performance of Li-ion cells will be analyzed and quantified. The applied current profile used to conduct numerical simulations is shown in Fig. 1. In this figure, t_dch, t_rest,1, t_ch, and t_rest,2 are the discharge, rest after discharge, charge, and rest after charge durations respectively. The impact of different rest durations on the discharge capacity over 1000 cycles is shown in Fig.2. It was observed that resting the cell after discharge has significant influence over cell performance, whereas resting after charge has a marginal effect. In the former case, a relatively thicker film forms at the solid-electrolyte interface in the negative electrode. Moreover, a longer resting period at the end of discharge results in a higher concentration of lithium in the solid matrix of the negative electrode, leading to a higher cell potential during the discharge phase of the subsequent cycle. Similarly, charging the cell following a relatively longer resting period at the end of discharge results in a lower concentration of lithium in the positive electrode indicating a better utilization of cyclable lithium and hence a higher battery capacity. In the future, the effect of resting duration on the solid and electrolyte concentration, and local electrolyte potential will be studied to identify reasons which lead to an improvement in achievable cell capacity and potential which can be foreseen to be helpful in achieving improved cell life and rate capability in these energy storage systems.