Fiber Optic Boltzmann Thermometry in a Doped Halide Double Perovskite for Dynamic Temperature Monitoring in Pouch Cell

Abstract Temperature evolution is critical in monitoring the status of Li‐ion batteries (LIBs), however, it is a challenge to develop precise thermometry down to the nanoscale regime and instantly detect the internal temperature of pouch‐type LIBs. Herein, a Boltzmann type luminescence thermometry is designed and prepared in halide double perovskite Cs 2 NaLuCl 6 :Yb/Er upconversion nanocrystals and further fabricate the flexible fluorescence polymer optical fiber (POF) sensor for their in situ and real‐time temperature monitoring. The thermally enhanced upconversion luminescence of the nanocrystals thermometry ensures sensitive temperature sensing in a wide temperature range, and the POF sensor exhibits stable and repeatable responses to temperature with a deviation of ±0.13 at 30 °C. Through the implementation of fluorescence POF sensors into pouch cell, the dynamic thermal state inside the LIBs is instantaneously captured without affecting the normal operations during battery cycling. This work paves the way for fluorescence POF sensors assisting in battery thermal management and evaluating the performance of battery materials for further developing LIBs.