Non-invasive temperature monitoring of operando lithium-ion batteries using polymer capped optical fiber tips

A polymer-capped fiber Fabry–Perot interferometer (PFFPI) is proposed and validated as a compact, and highly sensitive optical sensor for non-invasive temperature monitoring of commercial lithium-ion batteries (LIBs). The interferometer was fabricated by forming a microdome on the cleaved end of a standard single-mode fiber, resulting in a thermally responsive cavity whose interference spectrum was analyzed through phase demodulation. Three sensors were calibrated between 20°C and 60°C, yielding reproducible sensitivities of 0.13 rad/°C. The PFFPIs were externally integrated on a commercial cylindrical INR18650-30Q cell to monitor its temperature changes during galvanostatic charge/discharge at different charge rates (C). Temperature changes below 1.5°C were observed at C/10, increasing to ∼15°C at 1C and ∼30°C at 2C, demonstrating high temporal and spatial resolution of the sensors. During 25 cycles at 2C, the thermal response remained stable without cumulative drift. The relationship of differential analyses revealed a direct correlation between d Δ T / d V with Incremental Capacity Analysis and d Δ T / d Q with Differential Voltage Analysis. Additionally, thermal delays potentially associated with aging were detected. These results demonstrate that PFFPI sensors enable operando thermal-electrochemical coupling analysis, offering a promising approach for real-time diagnostics, safety assurance, and lifetime estimation of LIBs.