| The lithium-ion battery(LIB)industry is expanding fast in the background of the current energy crisis and environmental pollution.The benefits of lithium-ion batteries with lithium nickel cobalt manganese oxide(NCM)cathode including high energy density,good cycle performance,outstanding cycle performance,and cheap production costs.Their range of applications is growing.However,NCM ternary lithium-ion batteries are unsafe,and their occasional "thermal runaway" failures often lead to serious safety accidents,which pose a great threat to people’s lives and property safety.Furthermore,the majority of real battery accidents occur during usage,such as fire and explosion incidents during high-rate discharging.Therefore,the thermal danger of NCM ternary LIB in discharge mode must be investigated.By using the 18650-type NCM battery,this paper focuses on the heat generation and thermal runaway characteristics of batteries in discharge,and thermal stability change of lithium-ion batteries after over-discharge cycles.Combining experimental observation with theoretical analysis,the thermal danger of NCM ternary LIB in discharge mode are studied,including the following three aspects:First,the electrochemical and thermal behaviors of LIB at various depths of discharge(DOD)were evaluated to compare the heat generation in of two systems,NCM523/Graphite battery and NCM811/Si-C battery.At the initial stages of discharge,the voltage of a lithium-ion battery is relatively steady,and heat production is modest.When the depth of discharge approaches 80%,the battery’s heat generation increases dramatically due to polarization.When the depth of discharge surpasses 100%,the battery is overcharged,and the battery exhibits significant heat generation and is rapidly damaged.Compared to the NCM523 battery,the NCM811 battery has a higher overdischarge tolerance.The overdischarge behavior of the two batteries vary at various depths of discharge,but they show a high similarity in the voltage dimension,suggesting that voltage is more appropriate for assessing the state of LIB in overdischarge.Furthermore,the internal resistance and voltage temperature coefficient of LIB are employed to investigate the heat generation mechanism of LIB,showing the roles of reversible heat,ohmic heat,and side reaction heat in overdischarge.It is demonstrated that side reaction heat generation accounts for a relatively small proportion of lithium-ion battery discharge,and that reversible heat generation and ohmic heat generation are the primary heat generation in lithium-ion battery overdischarge,and an effective prediction method for lithium-ion battery overdischarge heat generation is proposed.Secondly,the influence of discharge behavior on the thermal runaway characteristics of LIB is explored.To mimic the use of the Li-ion battery,different multiplier discharge currents are supplied to it.Meanwhile,a lithium-ion battery thermal runaway test bench is utilized to supply a continuous external heat source to trigger the lithium-ion battery’s thermal runaway.The battery capacity experience inevitable decrease during discharge,therefore,the impact of capacity decline and the discharge current were contrasted.The experimental results show that the capacity has a significant effect on the thermal runaway behavior of LIB,with the decrease of the battery capacity,the trigger time and trigger temperature of safe venting and thermal runaway increases,while the mass loss and the maximum temperature rise rate in thermal runaway decrease.Which the discharge current only accelerated the thermal runaway process,and there is no significant change in the safety venting and thermal runaway trigger temperature and mass loss.In addition,the internal energy of the LIB during thermal runaway in discharge was quantified.The results reveal that when the discharge current reaches 4 A,the energy provided by the current alone is already comparable to the energy generated by the side reactions,leading the battery’s thermal runaway to be significantly earlier.Third,the aging mechanism and thermal safety changes of LIB after overdischarge cycles are studied.It is proved that the aging of LIB will be accelerated under the over-discharge condition,and its capacity decay is directly related to the DOD.There is a certain threshold value of over-discharge voltage between 0.5～0.0 V.When the over-discharge of battery exceeds this threshold value,its capacity decay will increase sharply.The aging mechanism of lithium-ion batteries during over-discharge cycles was analyzed by electrochemical impedance spectroscopy(EIS),and it was clarified that loss of lithium inventory(LLI)due to solid electrolyte interface film(SEI)growth is the main aging mode during over-discharge cycles.Extended Volume accelerating rate calorimeter(EV-ARC)was used to expose the thermal stability change of the batteries after over-discharge cycle and acquire the kinetic parameters of the thermal runaway processes.After the overdischarge cycle,the activation energy of the battery decrease and the heat generation per unit mass of combustion increase,which means the LIB have more safety hazards.In addition,the classic Semeov model is employed to analyze the criticality of battery thermal runaway and the critical ambient tempareture of thermal runaway is calculated,which can provide guidance for the safety use and storage of the overdischarged LIB. |
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