Investigation on step overcharge to self-heating behavior and mechanism analysis of lithium ion batteries

To obtain intrinsic overcharge boundary and investigate overcharge mechanism, here we propose an innovative method, the step overcharge test, to reduce the thermal crossover and distinguish the overcharge thermal behavior, including 5% state of charge (SOC) with small current overcharge and resting until the temperature equilibrium under adiabatic conditions. The intrinsic thermal response and the self-excitation behaviour are analysed through temperature and voltage changes during the step overcharge period. Experimental results show that the deintercalated state of the cathode is highly correlated to self-heating parasitic reactions. Before reaching the upper limit of Negative/Positive (N/P) ratio, the temperature changes little, the heat generation is significantly induced by the reversible heat (endothermic) and ohmic heat, which could balance each other. Following that the lithium metal is gradually deposited on the surface of the anode and reacts with electrolyte upon overcharge, inducing self-heating side reaction. However, this spontaneous thermal reaction could be "self-extinguished". When the lithium in cathode is completely deintercalated, the boundary point of overcharge is about 4.7 V (∼148% SOC, >40 °C), and from this point, the self-heating behaviour could be continuously triggered until thermal runaway (TR) without additional overcharge. The whole static and spontaneous process lasts for 115 h and the side reaction heat is beyond 320,000 J. The continuous self-excitation behavior inside the battery is attributed to the interaction between the highly oxidized cathode and the solvent, which leads to the dissolution of metal ions. The dissolved metal ions destroy the SEI (solid electrolyte interphase) film on the surface of the deposited Li of anode, which induces the thermal reaction between lithium metal and the solvent. The interaction between cathode, the deposited Li of anode, and solvent promotes the temperature of the battery to rise slowly. When the temperature of the battery reaches more than 60 °C, the reaction between lithium metal and solvent is accelerated. After the temperature rises rapidly to the melting point of the separator, it triggers the thermal runaway of the battery due to the short circuit of the battery.