Abstract
Thermal safety is the main roadblock for applications of high energy density lithium-ion batteries (LIBs) and catastrophic LIB failure has long been a public concern. There have been numerous LIB safety testing standards and many LIB safety management devices, including isolating shutdown separators, protective electrode coatings, safety valve, etc., that are proposed for LIB service safety and reliability improvement. Despite the application of LIB tests and safety devices, thermal runaway is still reported as a major threat to equipment and personal safety. In order to improve the in-service safety, the battery management system (BMS) and safety devices need to examine and analyze the LIB performance in a real-time manner so the rapid thermal runaway progress can be detected and prevented at an early stage. Due to the complex nature and rapid reaction rate of thermal runaway, BMS and safety device with capability of comprehensive in-situ analysis of LIB thermal runaway progress have seldomly been reported. A simple BMS capable of examining the LIB thermal safety and detecting thermal runaway related reactions needs to be developed.
Existing LIB thermal safety management systems typically rely on temperature sensors including thermocouples, thermistors, fiber bragg-grating sensors, etc. Despite their success in LIB surface temperature measurement, they cannot be applied for direct electrode temperature measurement, which is crucial for thermal runaway detection and prevention. Existence of thermal resistance between the battery surface and electrodes also impedes the development of quick responding LIB temperature monitoring systems based on surface temperature measurements. To address this issue, we propose the application of resistance temperature detectors (RTDs) in LIBs with the help of additive manufacturing. By embedding RTDs into operating LIBs, reliable in-situ monitoring of the electrode temperature can be achieved, which is beneficial for early detection and prevention of thermal runaway. In this work, thermal runaway in LIBs was triggered by overcharge and external short circuit (ESC). Battery temperature was recorded by the internal RTD embedded on the electrodes and external RTD mounted on the battery surface. Significant temperature difference of ~9 ºC and ~20 ºC were detected between the electrode and battery surface in ESC and overcharge of small lab-fabricated Li-ion coin cells. The temperature difference can be as high as ~200 ºC for large capacity Li-ion pouch cells. In ESC, the internal RTD detected thermal runaway ~ 10 times faster than the external RTD despite of the battery type. Besides, the internal RTDs can provide accurate electrode temperature contour and comparison of anode and cathode temperatures during regular operation and thermal runaway of LIBs. The solid electrolyte interface (SEI) which is comprised of reduced electrolytic compounds like ROCO2Li, (CH2OCO2Li)2 and ROLi, can contribute to heat generation in LIB thermal runaway. The effect of SEI can be analyzed with in-situ examination of cathode and anode temperatures. Transient temperature difference captured by the internal RTD indicated that anode is the critical electrode and contributes more to the heat generated in thermal runaway. This helps to optimize temperature sensor arrangement for more effective monitoring and prevention of thermal runaway.
The internal RTD also provides opportunity for in-situ analysis of chemical reactions in thermal runaway. Gas generated in thermal runaway has a high concentration of CO, H2, CH4, etc., which is prone to ignition and explosion. The generated gas can affect LIB structure as it causes deformation of electrodes and separators. Although this is hard to be detected with most basic BMS, it can be examined with electrode and battery surface temperature comparison, as the LIB thermal resistance is changed during gas accumulation. Different modes of thermal runaway can be differentiated by analyzing the temperature profiles, which allows for customized thermal runaway prevention and control strategy.