When Short-Circuits Fall Short: The Thermal Consequences of Lithium Metal Internal Short-Circuits

Lithium metal internal short circuits are often implicated in thermal runaway despite a lack of direct, quantitative evidence supporting a causative link between these two events. A key barrier to quantifying heat release from lithium metal shorting is the challenge associated with isolating shorting from other physical and chemical processes that occur in common carbonate electrolytes. Herein, we develop a system that simultaneously measures heat and voltage using operando isothermal microcalorimetry (ITMC) during lithium metal short circuits. Measurements across various states-of-charge, especially at high voltage where oxygen provides fuel, show that lithium shorting produces very little heat (μW/cm 2 ) and minimal temperature rise (<2 °C), consistent with resistive, passivated lithium metal seen in (electro)chemical analyses. These results suggest that thermal runaway is not driven by the short circuit but aging-related phenomena, such as “dead” lithium buildup and gassing. Our findings support protocols to dissolve lithium filaments to improve battery safety.