Long-Term Study on the Impact of Depth of Discharge, C-Rate, Voltage, and Temperature on the Lifetime of Single-Crystal NMC811/Artificial Graphite Pouch Cells

Abstract

This work examined the impact of depth of discharge (DOD), C-rate, upper cut-off voltage (UCV), and temperature on the lifetime of single-crystal NMC811/Artificial Graphite (AG) cells. Cells were cycled at C/50, C/10, C/5, or C/3, and 25, 50, 75, or 100% DOD at room temperature (RT, 20 ± 2 °C) or 40.0 ± 0.1 °C. The UCVs were 4.06 or 4.20 V. After 12000 hr of cycling, experiments such as electrochemical impedance spectroscopy (EIS), Li-ion differential thermal analysis (DTA), ultrasonic mapping, X-ray fluorescence (XRF), differential capacity analysis, synchrotron computed tomography (CT) scans, and cross-section scanning electron microscopy (SEM) were carried out. We showed that capacity loss increased slightly with DOD and C-rate, and that cells with 4.06 V UCV have superior capacity retention and impedance control compared to 4.20 V. SEM, CT scans, and differential capacity analysis show that microcracking and positive electrode mass loss did not occur regardless of DOD, C-rate, or UCV. DTA and ultrasonic mapping showed no C-rate or DOD dependency for electrolyte changes or “unwetting.” Finally, a simple square-root time model was used to model SEI growth in 4.06 V UCV cells.