Ratiometric Fluorescence Optical Fiber Enabling Operando Temperature Monitoring in Pouch‐Type Battery

Abstract Thermal characteristics are essential for improving the performance and monitoring the status of Li‐ion batteries (LIBs). However, it is a challenge to design efficient and facile sensing materials for the detection of the in situ temperature of a working LIB. Herein, a ratiometric fluorescence optical fiber is developed and real‐time temperature monitoring is performed with a measurement accuracy of 0.12 °C, and the feasibility based on this polymer optical fiber composed of NaLaTi 2 O 6 :Yb/Er phosphors is verified in a pouch‐type battery. During the charging and discharging cycles, the in situ temperature is instantaneously conveyed, revealing the internal situation of LIBs. This article further dwells on the thermal characteristics in constant current (CC)/constant voltage charging and CC discharging processes at different C‐rates and the battery failure when operated at low temperatures (0 °C). This work demonstrates an innovative strategy for operando solitary temperature monitoring conducted by ratiometric fluorescence optical fiber.