1Fast-charging a cold cell (−10 °C)
The car has stood overnight at −10 °C and is plugged straight into a fast charger. In the cold the electrolyte is thick and slow, and lithium creeps into graphite far more slowly than at room temperature. The charger’s current arrives faster than the graphite can take it in, so lithium ions pile up at the graphite surface.
2The graphite drops below 0 V against lithium: plating
Normally charged graphite sits about 0.1 volts above lithium metal. Pushed this hard, its surface falls below 0 volts, and the ions find it easier to become lithium metal on top of the particles than to slip inside them. A silvery, mossy layer grows on the graphite facing the separator. The cell’s voltage looks normal; only a lab cell with an extra reference electrode can see the anode potential directly.
3At rest: some lithium returns, some is lost
When the charge stops, part of the plated lithium dissolves again and slips into the graphite. While it does, the voltage sags along a short flat step before settling, a tell-tale that chargers can look for. The rest stays as metal cut off from the electrode (“dead lithium”) wrapped in fresh SEI film: capacity that is gone for good.
4Weeks later: the deposit grows into the separator
Each cold fast charge adds a little more. The deposit is not a flat film but moss and needles (dendrites), because lithium grows fastest at its own sharp tips. A needle starts pushing into the separator’s pores toward the cathode, 14 micrometres away.
5A soft short: the needle touches the cathode
The dendrite bridges the separator and current leaks straight across: a soft internal short. The thin needle carries only a little current and heats a spot of a few millimetres, here to about 70 °C. The cell slowly drains itself even when parked. If the needle thickens, or the separator melts around the hot spot, the short can grow into thermal runaway, the fire shown in the smartphone explorer.
6How it’s prevented and caught
- The battery management system limits charging current by temperature: at around 0 °C only a fraction of the normal rate, and below that often none at all.
- Cars warm the pack before fast charging, often when you navigate to a charger, using heaters or the motor’s own losses.
- Cells are designed so the anode has about 10 per cent spare capacity and overhangs the cathode by about a millimetre on every edge.
- Chargers and labs look for the flat voltage step after charging (the plated lithium dissolving) and for cells whose capacity falls faster than expected.
- A cell that self-discharges faster than its neighbours while parked is flagged: the pack spends more effort keeping it balanced, which gives the short away.
- Ceramic-coated separators resist piercing and keep their shape when a hot spot forms.