Study On The Safety Performance Of Lithium Ion Power Batteries In Whole Life Cycle

Since 2014,the amount of electric vehicles using rechargeable lithium-ion batteries in the world has shown an explosive growth,and the energy density of the power batteries has also doubled.Being a chemical power source with high energy density,lithium-ion batteries have certain safety risks,along with the increment of energy density of batteries.Nowdays,the studies on the safety of lithium-ion batteries was mainly focused on fresh-new batteries,while the research concerned on the safety of cycled batteries within the service-life process is quite few.On the other hand,the safety of batteries during the service-life process is extremely important which related to people's lives and property loss.Therefore,it is of great significance to investigate the safety of power batteries in the whole service-life(capacity retention rate after cycles is not less than 80%).In this paper,the safety of vehicle batteries constituted by ternary materials anode and graphite cathode in the whole service-life cycle were studied,and the reasons affecting the safety of batteries were also analyzed.The main results of this word are summarized as follows:(1)Firstly,cycled batteries with capacity retention rate of 90% after 600 cycles was tested by means of the national standard GBT/31485.The test results show that the overall test passing rate of the batteries is higher than that of batteries after capacitance separation.However,cycled batteries with capacity retention rate of 80% shows a larger area of lithium precipitation.Compared with the fresh battery,the test passing rate of the cycled batteries in needle-punching and over-charging tests were increased,while the test passing rate in heating and short circuit tests were decreased.(2)Secondly,the cycled batteries were disassembled and then analyzed in order to explore the mechanism of safety deterioration of batteries after cycling.Cycled batteries with capacity retention rate of 90% passed the short circuit and overcharge tests,due to the increment of the thickness of the solid electrolyte interface(SEI)film during charge-discharge cycles,which resulting in an increase in the resistance of the electrodes.Therefore,the short circuit resistances were increased during needle-punching test and made it easier to polarize to the upper limit voltage when overcharge occurs.For the cycled batteries with capacity retention rate of 80%,the rapid decay of the battery capacity was mainly due to the large area of lithium precipitation occurred during service cycles,which resulted in the loss of lithium ions.Due to these lithium precipitation,the thermal stability of the cycled battery decreased,leading to the batteries not passed the heating and short circuit tests.(3)Thirdly,the influence of lithium precipitation on the safety of cycled battery was further investigated.Results of X-ray photoelectric spectroscopy(XPS)analysis carried out on the lithium precipitation area and the normal area of the cycled batteries with capacity retention rate of 80% show that the lithium atoms precipitated from the anode became lithium compound,which causing less risk of short circuit safety of both electrodes.Differential scanning calorimetry(DSC)thermal analysis results show that the heat release peak in the lithium was ahead of schedule,while the increased area of the heat release peak caused the decrease of the thermal stability of the anode.Charging at low temperature may also lead to lithium precipitation.The risk of short-circuit caused by lithium precipitation at low temperature was higher,accompanied by the reduction of the thermal stability of the battery.Under the same conditions at low temperature,however,lithium was more difficult to precipitate in the cycled batteries than in fresh batteries.