Capacity Recovery Phenomenon Manifested in Lithium-Ion Batteries Due to over-Discharge
Authors: Ito Shota 
Published: December 2024

【Background】
The need for Lithium-ion batteries (LIBs) with longer life is growing with the popularization of their applications, but LIBs lose capacity with use. These reduction in capacity of LIBs is mainly due to three factors: namely, the reduction of a positive electrode capacity, the reduction of a negative electrode capacity, and the mutual “capacity slippage” between the capacity of a positive and a negative electrodes [1]. The reduction of a positive and a negative electrode capacity is caused by deactivation of active materials due to decrease in the conducting paths of electrons and Li+. The capacity slippage is not only caused by the decreases in the capacity of a positive and a negative electrodes but also by deactivation of Li+. This Li+ deactivation is attributed to the formation of solid electrolyte interfaces and the immobilization of Li+ due to their trapping within a negative electrode [2, 3]. In this study, a method for recovering capacity by repeating short-term over-discharge was investigated, and the effect of this method on a LIB was evaluated.
【Experimental】
Laminated-type cells were fabricated by alternately stacking negative electrodes and positive electrodes. The positive and negative active materials were LiNi1/3Co1/3Mn1/3O2 and graphite, respectively. In the charge/discharge cycle test, the cells were charged and discharged at a constant current of 1 C between 3.0 V to 4.2 V at 50 oC. The rest time was 30 minutes. The capacity recovery treatment consisted of two steps at 25 oC. First, the cells were discharged at a constant current of 1 C until the cell voltage reached 2.5 V or 72 seconds had elapsed, followed by a rest period of 30 minutes, and the cycle was repeated until the behavior of the discharge curve did not change. Second, the cycle of discharging at a constant current of 10 C until the cell voltage reached 0.5 V or 1 second had elapsed and resting for 60/n minutes was repeated n times, where n is an integer from 3 to 6.
【Results & Discussion】
Figure 1a shows discharge capacity retention in the charge/discharge cycle tests with capacity recovery, and Figure 1b shows discharge curves at the specific cycles. Capacity recovery was performed after the 400th and specific cycles. As shown in Fig. 1a, the number of cycles at which the capacity retention fell bellow the value at the 400th cycle was extended by repeating the capacity recovery. As shown in Fig. 1b, because of performing capacity recovery after 400 cycles, the discharge capacity was recovered by 17.0 mAh, and the discharge curve at the 401st cycle is equivalent to that at the 100th cycle. To verify the origin of this capacity recovery, the discharge curve analysis [4] was carried out for these discharge curves.
The results of discharge curve analysis are shown in Figures 2a and 2b. The horizontal axis shows discharge capacity, and the negative region is the surplus charge capacity. The vertical axis is the cell voltage and the potential of the positive and negative electrodes relative to lithium metal. The plots represent actual values during discharge, and the solid lines represent the potential curves of the positive and negative electrodes and the voltage curve of the cell plotted from the difference between them. According to the results of the analysis, the discharge endpoints of the negative electrode curves are located on the lower capacity side than the discharge endpoints of the positive electrodes. This result is due to capacity slippage and means the discharge endpoints of the negative electrode curves determine those of the cell curves. Focusing on the discharge endpoints of the negative electrode curves reveals that the position was shifted toward the higher capacity side by 17.6 mAh (from 201.6 to 219.2 mAh) owing to the capacity recovery. This value agrees well with 17.0 mAh which is the amount of recovery capacity obtained from the charge/discharge cycle tests shown in Fig. 1b. This agreement strongly suggests that the origin of the capacity recovery is the recovery of capacity slippage.
On the day of the presentation, the mechanism by which capacity slippage is recovered will be discussed while the results of the disassembly analysis will be also introduced.
[1] P. Ramadass et al., J. Power Sources, 112, 614 (2002).
[2] H. J. Ploehn et al., J. Electrochem. Soc., 151, A456 (2004).
[3] T. Yoshida et al., J. Electrochem. Soc., 153, A576 (2006).
[4] K. Honkura et al., J. Power Sources, 264, 140 (2014).

Figure 1