Advances on Mechanism of Degradation and Thermal Runaway of Lithium-Ion Batteries
Lithium-ion battery(LIB)is widely used in the new energy vehicles since its characteristics of high specific energy,high power density and high working voltage.However,its degradation in energy storage and power capability is inevitable during opera-tion,which highly limits the vehicle driving range and lifespan,and even induces safety risks.With the increasing application of new energy vehicles,fire accidents tend to occur more frequently,most of which can be related to thermal runaway(TR).In addition to the degradation,safety performance of LIB became another major concern for the development of new energy vehicles.Both degradation and safety issues severely restrict the rapid development of new energy vehicles.For better on-board utilization of LIB,it is necessary to understand its internal electrochemical mechanism of aging and TR for the estimation of state of health(SOH)and state of safety(SOS).Therefore,the recent progress on the mechanism of aging was reviewed firstly.During the charge-discharge process of LIB,side reactions were inevitable which lead to capacity fade,power fade,stress-induced material damage and the potential for TR.Those side reactions occurred on different components of the battery,such as anode,cathode,separator,and so on.The existing research re-sults showed that the formation and thickening of solid electrolyte interphase(SEI)film,lithium plating and loss of active materials are the main aging mechanisms of conventional graphite anode.For the cathode,the material structure collapse caused by stress,the dissolution and deposition of transition metal ions,and the phase change of the active material were the main factors affecting its degra-dation.Separator aging,binder and current collector corrosion were also not negligible issues related to aging process.Meanwhile,the degradation mode of LIB was highly affected by different external factors such as ambient temperature,charge/discharge rate(C-rate),state of charge(SOC)and depth of discharge(DOD).At present,great progress had been achieved in the understanding of the aging mechanisms of conventional LIB with lower energy density.But the development of LIB with higher energy density was also pros-perous as it is driven by the market demand of the new energy vehicles.Ni-rich layered oxides cathode and silicon/carbon composite an-ode have been considered as promising materials for LIB due to their higher energy density.The materials of separators and electrolyte were also thoroughly studied in order to improve the battery performance.These improvements also highlighted some more specific ag-ing mechanisms such as structure disorder,particle cracks and dramatic volume expansion.The degradation mechanisms of higher en-ergy density batteries with novel battery chemistries had not yet been deeply understood,and further researches were still urgently re-quired.The main external feature of LIB safety hazarded is thermal runaway.There were two main reasons for the TR of the battery.One called the battery intrinsic safety was closely related to the thermal stability of its material,structure design and production pro-cess.And the other caused by abuse was the safety during on-board utilization.Mechanical abuse,electrical abuse and thermal abuse were the most common abuse conditions of LIB and it was these coupled conditions that may trigger the battery TR.According to the previous researches,it was concluded that the root cause of the thermal runaway could be interpreted by the internal chain reactions,where the most common feature of thermal runaway was internal short circuit(ISC).However,some researchers discovered the phe-nomenon of TR without ISC.So the former study results on mechanism of thermal runaway might need to update due to the utilization of emerging battery chemistries.In addition,the problems and future researches on degradation and safety issues of LIB were discussed.The safety performance and reasonable operating window affected by aging path could also alter during the battery whole lifespan.Therefore,the battery management system(BMS)needed to adapt the strategies of state estimation and safety management of the bat-tery according to these changes to prevent abuse and safety accidents.Currently,fresh LIB could be used properly without abuse under the management of BMS.Nevertheless,the state estimation of the aging battery was unable to meet the needs of the vehicle yet because LIB was a very complicated nonlinear system and the aging mechanism of different batteries could be totally different under various working conditions.The existing research of LIB performance fade was mostly based on accelerated aging tests,and there were few re-searches reporting the aging mechanism under actual complex vehicle conditions.Furthermore,the mechanisms of aging and TR on LIB used emerging material required further investigations to ensure the battery working within safety and high-performance operating window.With the increasing requirements of new energy vehicle,the next-generation BMS based on the cloud computing and digital twin had become a promising topic.Therefore,the researches on the degradation and safety performance of lithium-ion battery under complex on-board working condition were critical to the development of online intelligent BMS.And it was also crucial to conduct the state and safety evaluation of the batteries retired from previous vehicles.
new energy vehiclelithium-ion batteryaging mechanismthermal runawayside reaction
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