Although straightforward, these “external” measurements can only supply inadequate and time-delayed information about the internal source of the onset of thermal runaway processes and of its true time evolution. The commonly used indicators observed during thermal runaway include surface temperature and temperature rise rate, voltage drop, venting, mass loss, gas release and fire, which are mostly detected through external electrical, heat, sound, and gas measurements and analysis 9, 10. There is an urgent need to understand this thermal runaway mechanism in order to reduce its occurrence and develop early warning and prevention measures. Lithium-ion battery safety issues originate from thermal runaway within a cell, which is defined as an uncontrolled temperature rise 7 provoked by exothermic chain reactions that often become catastrophes in the form of fire and explosions 8.

In this regard, it is essential to improve our understanding of the mechanism of lithium-ion battery thermal runaway and its evolution to inform methods to prevent fires and explosions or at the very least to provide means for early warnings before catastrophic events occur 5, 6. This has not been without issues, however, as recurrent fire safety incidents have become a serious source of concern impacting their further development 3, 4. By raising an alert even before safety venting, this new operando measurement tool can provide crucial capabilities in cell safety assessment and warning of thermal runaway.Īs leading electrochemical energy storage and conversion devices in our daily lives 1, 2, lithium-ion batteries have been identified as critical components in the transition from depleted fossil fuels to sustainable worldwide energy production and use. Further analysis reveals that a scalable solution for predicting imminent thermal runaway is the detection of the abrupt turning range of the differential curves of cell temperature and pressure, which corresponds to an internal transformation between the cell reversible and irreversible reactions. The sensor’s signal shows two internal pressure peaks corresponding to safety venting and initiation of thermal runaway. We observe a stable and reproducible correlation between the cell thermal runaway and the optical response. To address this, we develop a compact and multifunctional optical fiber sensor (12 mm in length and 125 µm in diameter) capable of insertion into commercial 18650 cells to continuously monitor internal temperature and pressure effects during cell thermal runaway. However, most existing sensors cannot survive during such extremely hazardous thermal runaway processes (temperature up to 500 ☌ accompanied by fire and explosion).

Operando monitoring of complex physical and chemical activities inside rechargeable lithium-ion batteries during thermal runaway is critical to understanding thermal runaway mechanisms and giving early warning of safety-related failure.

Anode or cathode led