Measurement of the internal temperature distribution of lithium-ion pouch cells with fiber Bragg grating sensors at realistic external compressive loads

Lithium-ion pouch cells perform best under constrained conditions. The pressure applied increases the contact between layers and reduces internal resistance. This research focuses on the effect of different compressive loads on the embedded optical fiber sensors and the influence of different compressive loads on internal temperature. A self-made lithium-ion pouch cell is equipped with two optical fibers with three fiber Bragg gratings encapsulated in a fused silica capillary tube with polyimide coating. Four compressive loads were tested, 40 kPa, 80 kPa and 120 kPa, as well as the unconstrained pouch cell at atmospheric pressure. We can show that the embedded fibers work under compressive loads. Internal temperature has a peak at the beginning of the discharge phase and external temperature peaks at the end of the discharge phase. The highest temperature is observed in the center and on the side of the positive tab of the pouch cell. We can show that internal temperature measurement is not significantly affected by the increased pressure. The discharge capacity increases by 3.5% . 5% from atmospheric pressure to 120 kPa. These findings are crucial for the usage of fibers inside lithium-ion pouch cells. Furthermore, they can be considered in the development of battery packs and thermal management systems.