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. However, most existing sensors cannot survive during such extremely hazardous thermal runaway processes (temperature up to 500 °C accompanied by fire and explosion). 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. We observe a stable and reproducible correlation between the cell thermal runaway and the optical response. The sensor’s signal shows two internal pressure peaks corresponding to safety venting and initiation of thermal runaway. 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. 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.

Electrodes are the most important components in the lithium-ion battery, and their design, which ultimately determines the quantity and speed of lithium storage, directly affects the capacity, power density, and energy density of the battery. Herein, an electrochemical–thermal coupling model was established for an 18.5 A h lithium-ion battery, and the model was validated by experiment at four discharge rates. Based on this model, the effects of the electrode design parameters (electrode thickness, volume fraction of active material and particle size) on the battery performance (electrochemical characteristics, thermal behavior, energy density and power density) were initially investigated. It was found that as the electrode thickness and volume fraction of the active material increased, the polarization, heat generation rate and energy density increased, while the power density degraded. In addition, as the particle size decreased, both the energy density and power density improved, which can provide guidance for the design of electrodes. Subsequently, a multi-parameter (thickness of the positive and negative electrodes) and multi-objective (energy density and power density) optimization procedure was performed via two optimization methods, and the positive electrode thickness of 55.335 μm and negative electrode thickness of 63.188 μm were determined to be the optimized parameters.