Hysteresis and current dependence of the thickness change of lithium-ion cells with graphite anode

The charging process of a lithium-ion cell causes the graphite anode to expand due to the intercalation of lithium ions in the graphite layers. This leads to an increased cell thickness, which is entirely reversible when moderate charging currents are applied. This paper measures the cell thickness using a dial indicator on the surface of a commercial nickel-cobalt-manganese/graphite pouch cell during the charging and discharging process. By comparing the measurements to the theoretical thickness calculated from published layer parameters, information can be acquired about the insertion and extraction processes in the graphite anode for the charging and discharging directions. One of the peculiarities of the measured thickness during discharging is that the theoretical maximum is exceeded in a specific SOC range. A current dependence of the graphite thickness can also be observed for insertion and extraction. The thickness hysteresis between charging and discharging is analyzed considering different intercalation variants for the same SOC. In addition, it is shown that moving between the discharging and charging curve is possible without a complete charge or discharge of the cell. (C) 2017 Elsevier Ltd. All rights reserved.