Lithium-ion Battery Aging Deep Insight: Analyzing the Effect of the Rest Time and the Current Dynamic

Summary:

The aging behavior of Lithium ion batteries is generated by the convolution of many different aging mechanisms, making it nonlinear and complex to model. Researchers used to make assumptions that simplify this modelling task. This work provides deep insights into two commonly simplified aspects: the effect of rest periods in between charges-discharges (commonly neglected) and the effect of the dynamics of a current profile (commonly simplified to constant current profiles). For this aim, we have developed a special aging test matrix that has been applied to a commercial cylindrical lithium ion battery. The test matrix contains 13 tests to describe the cycling aging behavior with constant current profiles, 4 tests to describe the calendar aging behavior, 5 tests to describe the effect of rest periods with different lengths in between charges-discharges and 4 tests to describe the effect of the dynamics of different current profiles. The proposed tests have given relevant insight into two concerns.
Firstly, the proposed tests have given relevant insight into the minimum resting time that is required to consider this resting time as calendar aging. We have observed that any resting period inferior or equal to 3h in-between charges-discharges does not generate any calendar aging contribution unless it is done at 100% state of charge (SOC). We have also observed that any resting period equal or superior to 8h in-between charges-discharges generate a calendar aging. In addition to this, we have observed that rest periods of 48 hours at 0% SOC does not generate a capacity recovery effect, but rather a transitory chemical modification of the inner battery chemic structure.
Secondly, the proposed tests tried to give relevant insight into the dimensionless number that links the aging contribution of constant current profiles and realistic current profiles. We have observed that the higher the constant-current charge/discharge is, the higher the degradation is. Nonetheless, this state does not apply to the observed tests done at constant current charge and realistic current discharge. The results have shown that realistic tests done with a 100% DOD do have the same degradation even though the mean discharge current, the max discharge current and the mean operating temperature are different. It shows that there are cases where stress factors are predominant in face of others, making insignificant the effect of the current dynamics on the degradation of the cell.