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A lithium-ion battery is like a multi-story building where lithium ions live on every floor. When you charge the battery, the ions move from floor to floor, and the whole building warms up a bit. Usually, this heat dissipates evenly, like from underfloor heating. But sometimes one spot gets too hot, and then the whole building can go up in flames.

Scientists built a model that shows how heat spreads inside a battery. They found that during charging, three things change: the material's ability to conduct heat, its heat capacity, and the heat generated by the ion insertion process itself. These changes cause the heat to stop flowing evenly and instead gather into hot spots. These are especially strong at grain boundaries — the microscopic crystals that make up the material.

Previously, scientists thought that heat was carried by lithium ions rattling like maracas. But it turns out the real story is different: when ions insert, the charge distribution and the strength of bonds between atoms change. This alters thermal conductivity. Internal heat sources create thermal waves that collide and amplify each other, like ripples from two stones thrown into water. These collisions cause mechanical stress and destroy the material from within.

Now, knowing exactly how hot spots form, engineers can tweak the material's composition and grain structure to make heat spread evenly. This is especially important during fast charging, when the battery is under severe stress. A safe battery isn't just about good material: it's about smart architecture at the grain level.

Original abstract

as written by the authors arXiv:2604.20424

Thermal runaway in lithium-ion batteries is governed by the poorly-understood initiation phase, where localised heating introduces instability. Here we identify the three key components that trigger thermal runaway, decreases in local conductivity, heat capacity changes, and intercalation heating, which significantly increase temperature gradients that accelerate battery degradation. Using a multiscale framework that links atomistic phonon calculations with grain-resolved thermal modelling, we identify large thermal gradients across grain boundaries arising from external heating events and intercalation-dependent thermal properties of Li$$_x$$ZrS$$_2$$. The observed changes in thermal conductivity are due to charge redistribution and bond-strength modulation of the host, in contrast to the existing theory of lithium rattler mechanics. Internal heating events driven by intercalation gives rise to local thermal gradients, finite-speed thermal wave interference, and internal thermal fluctuations that generate mechanical strain and sub-grain thermal breakdown. These results show that the trigger for thermal runaway is controlled by internal grain architecture and composition, as well as the external environment. Our findings establish materials and electrode design rules for suppressing hotspot formation and improving battery safety during fast charging.