When recharging, lithium ions (red particles) move through electrolyte (grey) from the cathode to the anode but stop when they touch a stationary ion (green particles). These "dendrites" are roughly fractal trees, resulting from diffusion limited aggregation. These branching protrusions form at the negative electrode when battery is charging. When they reach the counter-electrode, they can interact with flammable electrolytes, causing a short circuit, and may make the battery catch fire.
Lithium-based batteries are powerful and lightweight, but potentially explosive. Soon after development of lithium batteries, scientists discovered the lithium ions form dendrites when recharged repeatedly. The dendrites form from the anode (green), short circuiting (shown by flashing), often catching fire. In 2019, a team led by C. Wang discovered an electrolyte solvent prompts the dendrite growth. A similar dendrite pattern is seen in crystal formation, coral, fungi growth, liquid seeping through soil, lightning, or what you see when you drop dye into glue, which is called viscous fingering. A process called diffusion-limited aggregation happens to create the same patterns as seen in viscous fingering, in which a less viscous liquid presses through a more viscous liquid. The finger-like projections result from pressure being somewhat greater at the tips.
