Decoding commercial lithium-ion battery ageing via operando optical fiber multi-parameter sensing

Operando multi-parameter monitoring in lithium-ion batteries is essential for accurate state of health estimation, performance enhancement, and sustainable reuse. However, current sensing techniques struggle to simultaneously track key electrochemical parameters during battery operation. To address this limitation, this study introduces an operando diagnostic approach based on a microstructured optical fiber sensing system, designed to enable multimodal monitoring within commercial 18650 lithium iron phosphate (LiFePO 4 , LFP) cells. By incorporating a hybrid sensor combining a tilted fiber Bragg grating and a Fabry-Pérot interferometer, simultaneous monitoring of local electrolyte wetting conditions near the implanted fiber, internal pressure, and temperature is achieved. Experiments conducted on commercial 18650 LFP cells successfully captured dynamic variations in internal air pressure, electrolyte state, and temperature during operation. The results indicate that the battery's internal air pressure and electrolyte level during cycling are significantly correlated with the progression of capacity decay.