End-of-life for lithium-ion batteries is commonly defined as 80\% state-of-health (SOH). In some batteries and applications, this coincides with a steep increase in degradation rate. This is referred to as nonlinear aging or an aging knee. If the operating conditions are left unchanged the battery continues on the steep aging trajectory until it is unusable. In this work operating conditions are changed shortly after entering the nonlinear aging phase. Samsung SDI INR21700-48X cells with a typical capacity of 4.80 Ah are used. These presumably have an NMC cathode and a silicon-graphite anode.
After an initial checkup, the cells cycle for 1200 cycles at charge and discharge rates of 1C. Cycles are done at 100% depth-of-discharge. A checkup is performed every 50 cycles. The checkup measures CC-CV discharge capacity as well as R_DC, using the average from six 10s current pulses at ±0.25C, ±0.50C, and ±0.75C. All tests are carried out at 25°C. After 1200 cycles the operating conditions are adapted. The discharge cut-off voltage is increased from 2.5 V to 3.35 V, the charge cut-off voltage is decreased from 4.2 V to 4.08 V. The charge C-rate is decreased from 1C to 0.5C. The cells are then cycled for another 1200 cycles, while checkups are performed every 25 cycles.

Adapting the operating conditions leads to the cells transitioning out of their nonlinear aging trajectory. Capacity loss is reduced significantly. The averaged capacity loss over three cells is comparatively smaller during cycles 1200 to 1800 (0.19 Ah) than during cycles 0 to 600 (0.49 Ah). Adapting the operating conditions limits the usable capacity. Due to the smaller voltage window, the averaged discharge capacity of the three cells is reduced by about 31\%. 3.54 Ah was the measured average during the last checkup with original operating conditions. 2.43 Ah was measured for the first checkup with adapted operating conditions. The increase in resistance is slowed as well.
The alteration of operating conditions is a possible way to transition out of nonlinear aging trajectories in Samsung SDI 48X lithium-ion cells. This leads to significantly longer lifetime for older Battery Energy Storage Systems and second-life systems. However, the application range is reduced: e.g. less usable capacity, less dischargeable energy as well as fast charging no longer being available.
This work can be seen as a proof of concept. The detection of knee point onset is relevant. Counting cycles is not a sufficient measurement.
Future work will be done to find easily measurable variables that indicate knee point onset. The influence of reducing charge cut-off voltage, increasing discharge cut-off voltage, and reducing charging rate will be quantified.