Effects of Non-Uniform Temperature on In-Situ Current Distribution and Non-Uniform State of Charge Measurements for LiFePO4 and LiNiMnCoO2 Cells

Abstract

Understanding internal state non-uniformity occurring across the electrodes in large-format Lithium-ion batteries, and among parallel-connected cells, is a critical part of the cell and battery module design process. Two five-cell parallel groups were tested using LiFePO4/C6 (LFP), and LiNiMnCoO2/C6 (NMC) chemistries. Pulse and full-capacity discharges were performed at various States of Charge (SOC), C-rates, average temperatures, and levels of temperature non-uniformity. In pulse testing, the current non-uniformity was lower for the LFP group compared to the NMC group. The hottest cell in the LFP group produced up to 40% more current than average, while this was up to 80% for NMC. Conversely, under charge depleting conditions the NMC group experienced less current non-uniformity, and in certain cases provided a nearly uniform current distribution in the presence of non-uniform temperature. Higher temperature sensitivity in the impedance causes larger current non-uniformity under pulse conditions. However, due to the presence of non-uniform SOC in charge depletion conditions, the Open Circuit Voltage (OCV) versus SOC gradient plays a significant role in controlling the current distribution, where steeper OCV versus SOC minimizes the effect of the non-uniform impedance.