Fiber Optic Sensors for Monitoring of Lithium- and Sodium-ion Batteries

Abstract: Rechargeable batteries, particularly lithium-ion batteries, have rapidly evolved since their introduction and now dominate the market, owing primarily to their high energy and power densities. With growing demand for high-performance batteries in portable electronics and electric vehicles, the need for safe, efficient, and reliable batteries is crucial. Conventional battery management systems, which generally rely on parameters such as current, voltage, and temperature, provide limited information on the chemical and physical processes taking place in the battery during operation. The understanding of degradation processes and how they evolve with time is also limited due to the complex nature of batteries. In order to enhance the battery lifetime, safety, and reliability of current batteries as well as for emerging battery technologies, more detailed information from the cells is required. Developing sensors that can be used to probe the batteries could allow for optimized performance and a more accurate determination of cell state. In this regard, fiber optic sensors are promising candidates.This work explores the use of fiber optical evanescent wave (FOEW) sensors for monitoring chemical and electrochemical reactions in lithium- and sodium-ion batteries under working conditions. The sensor response and battery performance were compared with the sensor either fully embedded in a lithium iron phosphate cathode or positioned at the electrode surface. The optical response was further linked to the oxidation and reduction of the active material during cycling by means of galvanostatic and voltammetric experiments. The influence of cycling rate, sensor position, and electrolyte salt concentration was also discussed. The work also shows the ability of the FOEW sensors to detect lithium and sodium plating, both as a result of insufficient storage capacity and high cycling rates. This is an important finding as plating poses a serious risk for short circuit in batteries. A correlation with the sensor response and lithium staging in graphite anodes could also be seen.These findings highlight the value of optical sensors for monitoring batteries under working conditions. The concept of fiber optic sensing in batteries is still in its early stages, but the research field is gaining more interest. This work has aimed to advance the understanding of FOEW sensors in particular, and the results could help to provide directions for the research community for the realization of fiber optic sensing in commercial batteries.