Pulse Polarization for Li-Ion Battery under Constant State of Charge: Part I. Pulse Discharge Experiments

Given the increased dependence on battery-powered devices, it is necessary to develop new and robust methods to evaluate and predict battery performance and state of charge. Batteries, including Li-Ion batteries, are unsteady state systems and a need exists to evaluate performance under constant state of charge (SOC). The purpose of this work was to develop pulse polarization curves (PPC) for a Li-Ion battery under constant SOC in order to identify individual overvoltages, such as charge transfer kinetics and mass transport, and their SOC dependence. Consequently, we elected to conduct pulse discharges at various duration and discharge current densities, under well maintained SOCs. Based on the results we were able to construct pulse polarization curves for constant SOCs of 10, 40 and 70% at various pulse durations of 2, 10 and 30 seconds. The experimentally obtained pulse polarizations suggest that the kinetic overpotential is captured in the 2 second pulse, while electrolyte Li+ concentration and Li solid state diffusion gradients are established in the 10 and 30 second pulse times, respectively. This allowed us to identify the individual overvoltages experimentally. An important consideration was that as the battery's SOC changes, the open circuit voltage (OCV) also changes. Therefore, the pulse discharge method we used needed to end at the same SOC in all cases to enable the construction of a PPC with the same OCV. To ensure that this method applied to various sizes and chemistries of batteries reliably, we ran pulse discharge experiments on a high Amp hour (15 Ah) large Li-Ion pouch battery (power) that used a NMC LMO cathode, as well as on a low Amp hour (0.04 Ah) small electrode coin battery (energy) with a CoO2 cathode. In a subsequent paper we employed the Newman method to model the pulse discharges, allowing prediction of the performance of Li-Ion batteries and constructing their PPCs under constant SOC.