Dual-configuration photonic distributed sensing for internal temperature monitoring and strain-sensitive analysis in Li-ion batteries

Accurate, spatially resolved measurements of temperature and mechanical deformation inside lithium-ion pouch cells are essential for improving safety, performance, and lifetime. However, optical fiber sensors embedded in electrodes and electrolyte are inherently cross-sensitive to temperature and strain, making quantitative interpretation challenging. Here, we present a dual-configuration optical frequency domain reflectometry (OFDR) approach for in-situ distributed monitoring that enables strain-sensitive differential analysis under defined assumptions. In the first configuration, fused-silica capillaries are hermetically integrated through the cell, housing sensing fibers under loose conditions that are thermally coupled but mechanically isolated, allowing strain-mitigated internal temperature measurement. In the second configuration, sensing fibers are directly embedded in the cell and exposed to electrolyte, providing combined temperature and strain responses. Subtracting the capillary-derived temperature signal from the embedded-fiber response yields a residual strain-sensitive differential signal across the cell. The impact of fiber and capillary integration on electrode integrity and cell capacity is also evaluated. Comparison of the two configurations further isolates thermo-chemical effects of electrolyte exposure, revealing ∼2.3-times increase in temperature sensitivity. Measurements during cycling demonstrated distributed temperature mapping with 3 cm spatial resolution with short-term noise-equivalent temperature sensitivity less than 1 mK/√Hz under stable conditions, enabling detection of internal gradients and localized hot spots.