Thermal runaway (battery thermal runaway)

Thermal runaway is the self-accelerating exothermic chain reaction in a lithium-ion battery cell where internal heat triggers further chemical decomposition, which releases more heat, which accelerates the decomposition further — typically ending in venting, fire or explosion. It is the dominant safety hazard in lithium-ion battery recycling and e-waste handling, and the principal reason that battery recyclers operate under stricter fire codes than other recycling industries.

The mechanism cascades through three temperature thresholds. Above 80°C, the solid electrolyte interphase (SEI) on the anode breaks down, exposing lithiated graphite to the electrolyte. Above 110-130°C, the separator (typically polyethylene-polypropylene at 130-160°C melt) softens or melts, internal short circuit develops between anode and cathode, and the electrolyte (LiPF6 in EC/DMC) begins to vaporise — releasing flammable hydrocarbon vapour and toxic HF gas. Above 180-220°C, the cathode material (NMC, LFP, LCO) decomposes and releases oxygen, sustaining combustion even in the absence of external air. Once at this stage, the cell vents at 8-15 bar internal pressure, ejecting flame and shrapnel; propagation to adjacent cells in a pack is near-certain unless physical barriers stop it.

Triggers in a recycling context are predictable. Mechanical damage from shredding a charged cell creates an instant internal short. Overcharge or deep discharge during pre-discharge testing of incoming packs can drive plating of metallic lithium on the anode, lowering the onset temperature by 30-50°C. Contact with conductive scrap — copper bus bar, aluminium chassis — short-circuits the terminals at the storage bin. Water contact with a damaged cell hydrolyses LiPF6 to HF + POF3.

Mitigation in Indian recycling plants is layered. Pre-discharge — packs and cells discharged to under 2% SOC before mechanical processing, either by salt-water immersion (5-10% NaCl, 24-72 hours) or by controlled resistive discharge in a temperature-monitored cabinet. Inert atmosphere shredding — N2 or CO2 blanket at under 2% O2 in the shredder enclosure prevents flame from forming even on cell breach; capex adder Rs 35-80 lakh per shredder. Wet shredding — a brine spray inside the shredder both cools and dilutes any vapour; produces a salty water cake but eliminates fire risk. Cell-level segregation — every incoming pack is opened, BMS-tested, and individual cells routed by chemistry before bulk processing. Detection — VESDA air-sampling smoke detectors and IR thermal cameras give 30-60 second early warning; fire suppression typically uses water mist (most effective) plus dry powder for backup. The recurring failure pattern is mixing — a damaged cell hidden in a bulk drum quietly heats over hours and ignites mid-shift in storage.