Hierarchical Early-Warning Strategy for Overheating-Induced Thermal Runaway in High-Nickel Lithium-Ion Batteries

Lithium-ion batteries (LIBs) can undergo thermal runaway (TR) under thermal abuse, and high-nickel LIBs face more severe safety challenges. Therefore, early warning of TR is crucial for the safe operation of LIBs. Overheating-induced TR experiments were conducted on 15 Ah NCM811 LIBs at different states of charge (SOCs, 10–100%), with synchronous monitoring of expansion force, temperature, voltage, and characteristic gases (CO, CO2, HF, and H2). Based on signals variations, five characteristic moments were defined, dividing TR into five stages: abnormal expansion-force increase (tabn), cell venting (tV), first gas detection (tG), internal short circuit (tISC), and TR (tTR). As SOC increased, deeper cathode delithiation and anode lithiation accelerated internal reactions, shifting all moments earlier. The expansion-force signal enabled the earliest anomaly identification, up to 1275 s before TR onset, while CO detection within 30 s after venting provided chemical confirmation. Voltage signals accurately indicated ISC, and temperature signals directly reflected the essence of TR. Consequently, a three-level early-warning strategy was proposed: Level 1 (dF/dt > 1 N/s), Level 2 (sequential force–gas confirmation), and Level 3 (“voltage + temperature” with duration constraints). The thresholds are SOC-independent, offering a reference for engineering deployment in high-nickel pouch LIBs.