The Aging Mechanism Of Lithium-ion Batteries During Low Temperature Cycling And High Temperature Float Charge

Nowadays,with the widely use of lithium-ion batteries,more and more security problems appear such as burn even explosion of batteries.In order to design a new generation of lithium-ion batteries with more security and longer lifetime,it’s necessary to investigate the aging mechanism of lithium-ion batteries.It is generally assumed that the external factors like temperature,voltage and current cause the degradation of lithium-ion batteries,while only disassemble the battery and find the inner factors of aging with material and electrochemistry methods can solve the problem from the root.In this paper,we chose two working condition the low-temperature cycling and high-temperature float-charge to investigate the aging mechanism of lithium-ion batteries.For low-temperature cycling condition,lithium plating is normally considered to be the reason,we want to find if there exist other reasons during low-temperature cycling and do quantitative analysis.For high-temperature float-charge condition,considering the rate of decay during high SOC float-charge is faster than cycling,it can be an accelerated aging method.So we not only investigate the high-temperature float-charge aging mechanism but also establish an accelerated aging model.In the low-temperature cycling experiment,we investigated the aging performance of Li(Ni_1/3Co_1/3Mn_1/3)O₂/graphite lithium-ion batteries under-10oC and found that except the lithium plating,there exist the loss of NCM active material.Then we did quantitative analysis for the LLI and LAM,and we found that LLI is still the primary cause of the degradation of the batteries,but we can’t ignore the LAM.We also found that both of two mechanism have a behavior of threshold for the current during cycling.At last,we investigated the cause of loss of NCM through the material characterization and found that the reasons are layered structure is destroyed,the increasing of interplanar spacing and appear of spinel phase in some regions.In the high-temperature float-charge experiment,we chose a series of temperature and voltage as the external factors to investigate the degradation mechanism of LiCoO₂/graphite batteries.38 batteries were test and we found that the aging process can be divided into two stages,the first are steady stage（linear decay）,the second are unsteady stage（nonlinear decay）,and the inflection point of steady/unsteady process is consistent with the time of battery beginning to gassing.We compared the cycling and the float-charging condition and found that the CV stage during cycling are the main cause of aging,that is,the aging mechanism of cycling and float-charging under steady stage are the same.We established the float-charge accelerated aging model of both temperature and voltage factors.After the dismantle of the battery,we found that gassing and the side reaction of the electrolyte/negative interface are the main causes of degradation of batteries.The side-reaction products covered on the anode surface and cause the increasing of impedance,which accelerated the decay of batteries.Meanwhile,we found that there exist a few loss of positive electrode.At last,we investigated the cause of gassing and found it is because of the explode of electrolyte.The gas composition are mainly CO₂ and other alkanes gases.After lay up for 6 month,the volume of batteries was reduced obviously and we found the reason is a part of CO₂was absorbed to the anode surface to be lithium carbonate.