Metrics for evaluating safe electrolytes in energy-dense lithium batteries

Lithium-ion batteries can fail catastrophically through thermal runaway, but the key trigger has remained unclear. Here we show that the most harmful cause is lithium oxidation reaction (LOR). This makes two types of high-energy-density battery surprisingly most dangerous: all-solid-state batteries with cracked solid separators, whether from manufacturing defects, high-pressure assembly or electrochemical cycling, and batteries with non-flammable liquid electrolytes. In both batteries, oxygen evolved from an oxide cathode passes directly to an anode, triggering highly energetic LOR. In contrast, traditional carbonate- and ether-based electrolytes are safer because they can consume O₂ in transit, alleviating or avoiding LOR. These findings apply to both lithium metal and lithiated anodes such as graphite. Safe electrolytes are thus either solid ion conductors that stop O₂ crossover under all conditions or materials that scavenge O₂ through low-exothermic reactions.