Fiber Optic Monitoring of Composite Lithium Iron Phosphate Cathodes in Pouch Cell Batteries

Developing techniques for real-time monitoring of the complex and dynamic environment in lithium-ion batteries is crucial for optimal use of the cells and to develop the next generation of batteries. In this work, we demonstrate the use of fiber optic evanescent wave (FOEW) sensors for monitoring lithium iron phosphate (LFP) composite cathodes in pouch cells. The fiber optic sensors were placed on top of the LFP electrodes, and the pouch cells were found to cycle well with significantly improved electrochemical performance compared to fully embedded fibers in Swagelok cells. Galvanostatic, voltammetric, and pulsed current techniques demonstrated that the optical response correlated well with the capacity, and a clear difference in sensor response was seen when the sensors were placed at the surface of composite electrodes compared to fibers embedded in the cathode. The optical response from LFP at different rates was also investigated, but no apparent influence on intensity output was found even though polarization was observed in the voltage profiles at higher currents. It was also demonstrated that the electrolyte itself functioned as a fiber cladding and that the salt concentration in the electrolyte did not influence the optical signal. In addition, given the short penetration depth of the evanescent waves, the sensor response is most likely dominated by the surface conditions of electrode particles near the sensing region. These findings provide further insight into the application and performance of FOEW sensors integrated into batteries, as well as the possibility of developing low-cost fiber optic sensors for battery monitoring under working conditions.