Early-Stage Heat- And Discharge-Induced Transformations in a Lithium-Ion Cell Monitored by an Advanced Battery Management System

Processes at the graphitic anode in lithium-ion (Li-ion) cells affect the solid-electrolyte-interphase (SEI) even when the anode temperature (T a ) is at or below 75 °C. Normal discharge may not raise T a above 50 °C due to endothermic heat absorption by the anode. Under fast (>3C) discharge, exothermic reactions between the anode and the electrolyte increase T a above 75 °C, even though surface-mounted temperature sensors do not measure it. Increase of T a above 75 °C, even for one fast discharge cycle, permanently changes the cell impedance similarly to 15 months of calendar aging. In both cases, impedance measurements suggest loss of lithium ions from the electrolyte during the anode-electrolyte exothermic reaction that contributes to the growth of the SEI layer. Most battery management systems (BMS) monitor only the cell’s surface temperature (T surf ). During fast discharge, most BMS register T surf < 75 °C, providing a false sense of safety, while the anode experiences unimpeded damage from reactions that may affect the SEI. An advanced Battery Internal Temperature Sensor-based—BMS (BITS-BMS) described in this work successfully monitors T a and other intrinsic cell parameters in real time, and records heat- and discharge-induced events affecting the anode and the SEI.