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

Various operational parameters such as charge/discharge rates, relaxation periods, and depth of charge and discharge play an important role in enhancing the cycling life of Li-ion batteries. Providing batteries with a relaxation period after discharging and charging might be essential for removing concentration gradients generated due to passage of current. In the present work, the effect of providing open-circuit time durations after completion of each charge and discharge over the performance of Li-ion cells has been analyzed and quantified. It is shown that relaxing the cell after discharge has significant influence over cell performance, whereas relaxation 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, providing a sufficiently long relaxation to the cell at the end of discharge results in (a) a higher concentration of lithium in the solid matrix of the negative electrode and (b) a lower concentration of lithium in the positive electrode, both leading to a higher cell potential during the discharge phase of the subsequent cycle. Charging the cell following a relaxation period of more than one hour at the end of discharge results in a better utilization of cyclable lithium.