Embedded Fiber Optic Chemical Sensing for Internal Cell Side-Reaction Monitoring in Advanced Battery Management Systems

Abstract

Advanced Lithium (Li)-ion batteries today present attractive options for high-performance energy storage in a number of applications ranging from portable electronics to aerospace vehicles. Battery management systems (BMS) play a crucial role in the safe, efficient utilization of capacity and power from these high-density energy storage devices and ensuring that battery state of health (SOH) is not compromised. In this respect, BMS SOH estimation functions are significantly limited by the use of externally monitored parameters such as voltage, current, and temperature. Consequently, battery packs are often designed and used very conservatively by the BMS to ensure safety and reasonable lifecycles. Present-day commercial Li-ion battery electrolytes, typically consisting of cyclic alkyl carbonate and chain alkyl carbonate solutions with Li-hexafluorophosphate as salt, are known to generate gases and other adverse chemical species when subjected to aggressive cycling or with aging. However, the conditions that cause such adverse side-reactions are not fully understood and can change with different use scenarios. These can accelerate cell aging and in the worst case, lead to safety hazards. Internal sensing within the cell to monitor chemical side reactions to detect such adverse side-reactions at an early stage would be highly desirable to support BMS functions. However, the harsh chemical environment within the cells and tight constraints on acceptable cost and size overheads for BMS sensors have historically challenged viable embedded sensor solutions for cell monitoring. In this respect, fiber optic (FO) sensors present an appealing potential solution. They are lightweight and thin, immune to electrostatic discharge, EMI, and can measure multiple parameters with high sensitivity, such as strain, temperature, pressure, and chemical composition in multiplexed configurations. FO sensors have been demonstrated to accurately and specifically resolve a number of chemical and gas species in various applications. This paper focuses on in situ FO chemical sensing methods tailored for accurate cell side-reaction monitoring. Studies on sensitivity, response time and other characteristics of in situ chemical measurements during different charge cycles in Li-ion cells will be summarized. Some experimentally tested chemical sensing schemes are presented and compared, both during normal cycling conditions and under simulated abuse cycles/aged conditions. These results will be benchmarked with other published data in the literature from controlled lab studies on chemical side reactions in Li-ion cells. The potential of such chemical side-reaction monitoring capabilities for significantly improved SOH estimation algorithms will be highlighted. Finally, the practical use of such embedded side-reaction FO chemical sensors in commercial battery packs to improve safe and optimal utilization of true battery capacity by the BMS will be discussed.