In situ real-time monitoring of gas evolution during lithium-ion battery thermal runaway using laser absorption spectroscopy

Timely and accurate early detection of thermal runaway in lithium-ion batteries remains a critical challenge for large-scale energy storage systems. Conventional diagnostic strategies relying on temperature evolution or voltage signatures typically identify hazardous conditions only after irreversible exothermic reactions have already escalated, leaving insufficient time to prevent catastrophic failure. Here, we introduce an in-situ hollow-core waveguide–based CH₄ sensor for tunable diode laser absorption spectroscopy (TDLAS), enabling the first real-time tracking of methane generation kinetics inside a lithium-ion battery. Validated across diverse battery formats, the measured gas-phase signal is systematically compared with conventional temperature and voltage indicators. Experimental results demonstrate that methane release exhibits a pronounced early-warning characteristic: the initial CH₄ peak emerges at 930 s, approximately 679 s earlier than the thermal-runaway temperature onset at 1609 s. Multidimensional analysis reveals the complete evolution pathway from initial chemical destabilization to catastrophic thermal runaway, highlighting the direct correspondence between gas signatures and internal electrochemical decomposition events. This study establishes the unique advantages of hollow-core waveguide TDLAS for proactive battery safety monitoring, providing a transformative diagnostic tool for early-stage hazard detection in high-capacity energy-storage systems. • In-situ CH4 monitoring realized by hollow-core waveguide and TDLAS sensor. • Gas signal warns of thermal runaway 679 s earlier than temperature. • Hollow-core waveguide enables internal sensing in harsh battery environments. • Multi-dimensional analysis reveals the “steady-nucleation-runaway” transition.