Thermal Runaway And Failure Mechanism Of Lithium Ion Batteries During Overcharge And Over-discharge

Overcharge and over-discharge,resulting from large current,battery management system failure and inconsistency of internal resistance,can easily cause battery failure or thermal runaway.Meanwhile,in real application,a single cell is often in a poorly ventilated environment.Therefore,study on overcharge and over-discharge of lithium ion cells in extreme ventilation can not only improve our understanding on the origins of heat,clarify the main cause of failure or thermal runaway,but also can quantitatively determine the critical conditions of overcharge induced thermal runaway and establish a semi-failed state prediction method to provide theoretical basis and technical support for the industrial application of lithium ion battery.The research on overcharge was conducted from three aspects.Firstly,commercial LiCoO2 + Li(Ni0.5Co0.2Mn0.3)O2/graphite+SiOx cells were employed to investigate the effects of ventilation condition,overcharging regime and current rate on the thermal behavior by combining a multi-channel battery cycler with an accelerating rate calorimeter(ARC).Results indicate that overcharging with galvanostatic-potentiostatic-galvanostatic regime is more dangerous than that with galvanostatic way.Side reactions contribute 82%,84%,80%,60%and 40%of heat to thermal runaway in 0.1 C,0.2 C,0.5 C,1 C and 2 C tests,respectively.To prevent the thermal runaway,the effective methods should be taken within 2 minutes to cool down the batteries as soon as the cells pass inflection point voltage.Hereinto,the inflection voltage increases linearly with the increasing current rates.Secondly,failure mechanism of overcharge was explored from the chemical reaction products of the battery.By scanning electron microscope and energy dispersive spectrometer,the decomposed products of the mixed cathode materials are found to be soluble with SiOx.The residual lamellar graphite structure on the anode demonstrates that the lithium deposition on the anode is due to the change of the distance between the electrodes,instead of the saturation of the anode capacity.X-ray Diffraction(XRD)results of cathode materials from self-made Li(Ni0.5Co0.2Mn0.3)O2/graphite coin cells before and after overcharge prove the decreased lattice parameter and the production of NiO2.Then,the relationship between the internal resistance and the thermal runaway in the process of overcharge under adiabatic condition was discussed.And the semi-failure state prediction method for any small battery was obtained.The overcharge resistances of commercial pouched lithium ion batteries obtained from voltage-current characteristics and intermittently charging consistently drop with the increasing state of charge(SOC)and amplify from 150%SOC,with corresponding voltage of 4.8 V and open circuit voltage(OCV)of 4.5 V where the high-frequency and the medium-to low-frequency semicircles start to separate due to abnormal thickening of electrode surface films according to electrochemical impedance spectroscopy(EIS).Thus,V-I method and an intermittently overcharging experiment(<1 C)under adiabatic condition are effective to obtain the semi failure state.This method was verified by LiCoO2+Li(Ni0.5Co0.2Mn0.3)O2/graphite+SiOx cells.The growth rate of total resistance was determined as 0.4C ? h-1 to stop thermal runaway.The over-discharge research was focused on thermal behavior and failure mechanism under adiabatic conditions.Results indicate that the temperature of anode lug is always larger than that of cathode lug during over-discharge.Internal short circuit leads to sharp rise of surface temperature.And the larger the current,the higher the short circuit heat releases.ARC result and the incremental capacity analysis show that the exothermic peak observed at 0.5 V is due to the dissolution of the copper current collector.The recoverable capacity was also measured and was proposed to have relation with the state of solid electrolyte interface(SEI)film and separator.C80 tests demonstrate that the SEI film on the anode has decomposed when the cell over-discharged to 0 V.After recharge,new SEI film generates but turns unstable and the decomposition temperature drops to 55.1?.XRD results indicate that the crystal structure of cathode active materials does not change during over-discharge.But,the crystal structure of the anode is damaged to some extent.