On diffusion-controlled Li-trapping in high energy Li-ion cells under fast discharge and freezing conditions

Li-ion batteries are promising energy storage devices owing to their high energy density which enables their use in different applications. Herein, we report on the electrochemical behavior of commercial high capacity 30 Ah pouch cells with energy density 275 Wh kg⁻¹ over a wide temperature range (60 °C to −20 °C) using different discharging rates. A testing protocol was designed to understand the losses seen in discharge capacities (reduction charges) when the cells were discharged at fast rates (~16 % and 20 % capacity losses at discharge rates of 2C and 3C, respectively) or operated under freezing conditions (~17 % capacity loss at ~ −20 °C and C/5 discharge rate). The protocol involved monitoring the charge capacity (oxidation charge) before and after discharging as well as tracking the changes in open circuit voltage (OCV) for about 30 min after discharging. The accessible capacity was concluded to be limited not only by the developed iR-drop, but also by the diffusion-controlled lithium trapping as a result of the formation of concentration gradient of Li-ions. Notably, the surface temperatures of the cells were raised from 25 °C (environmental chamber temperature) to ~40 °C and 52 °C upon fast discharge at rates of 2C and 3C, respectively. Resting the cells for about 30 min after fast discharge was sufficient to drop the surface temperature back to 25 °C. This work provides insights for understanding the limitations of fast discharge and operating temperatures on industry relevant high energy Li-ion battery cells.