Thermo‐Enhanced Ratiometric Optical Fiber Sensor with Superior Anti‐Interference for Operando Temperature Monitoring in Sodium‐Ion Battery

ABSTRACT Ratiometric luminescent‐material‐based optical fiber temperature sensors (OFTSs) are crucial for advancing emerging sensing applications. However, the thermal quenching and laser‐induced thermal effects in sensing materials pose significant challenges to their practical applications. Herein, by utilizing a phonon‐assisted upconversion (UC) mechanism, thermo‐enhanced near‐infrared UC emissions and inhibited thermal effects are presented in Y 2 Mo 4 O 15 :Yb 3+ /Nd 3+ . Based on thermally coupled emissions of 807/880 nm in this material, a thermo‐enhanced ratiometric OFTS with excellent accuracy ( δ max = 0.12°C), high relative sensitivity ( S Rmax = 1.97%°C −1 ) and low detection limit ( δT min = 0.04°C) is demonstrated. The OFTS exhibits exceptional long‐term operational stability and superior anti‐interference performance against mechanical deformations, pump power variations, and ambient light crosstalk. Furthermore, by non‐destructively embedding this OFTS into a sodium‐ion battery (SIB), the internal states of the SIB during operation can be monitored in real time; thus, the battery's failure under a low operating temperature, performance degradation during long‐term cycling, and thermal runaway risk induced by over‐discharge are revealed. This work not only provides new insights into the design of high‐performance OFTS but also presents a thermo‐enhanced OFTS for safety evaluation in commercial batteries.