Non-invasive application of polymer optical fiber sensors for high fidelity operando monitoring of strain and temperature in Li-ion batteries

Successful assessment of the battery degradation mechanism and safety conditions may be enabled through monitoring dynamic thermal, chemical, and mechanical status of a cell during cycling. In this regard, the convergence of fiber optic sensing with lithium-ion batteries holds great promise for observing key cell parameters in real time, allowing for identifying crucial factors involved in the degradation process. Recently, fiber Bragg grating (FBG) optical fiber sensors have been demonstrated as an ideal tool for measuring these metrics with sufficient temporal and spatial resolution. In this work, we extend the application of fiber Bragg gratings to polymeric optical fibers with markedly greater thermal and strain coefficients than their common silica counterparts. In a non-invasive approach, we demonstrate that pairing polymer optical fiber sensor with conventional silica-based sensor, both externally affixed to the package of a lithium battery, can simultaneously output high fidelity temperature and volumetric expansion data. The quality of this data allows for further assessments as mechanical characteristics associated with dimensional changes of cells exhibit intimate correlations with the electrochemical reactions inside the cell. As an indication of sensor reliability, we obtain demonstrate that the optical sensors can track cell parameters over several hundred cycles with no signs of fidelity loss. While internal monitoring remains essential for future diagnostics, external monitoring using polymer fiber sensors offers a cost-effective and superficial sensing solution that opens a new avenue for real-time state of health (SoH) assessment, prognostics, and condition monitoring.