Aging And Safety Behaviors Of 18650 Ternary Lithium-Ion Battery Caused By Slight Overcharging

In recent years,the electric vehicles and electrochemical energy storage industries have developed rapidly.Lithium-ion batteries are used in the electric vehicles and electrochemical energy storage system due to their superior performance such as high energy density?long life,no memory effect and low pollution.However,the number of fires of lithium-ion batteries has been increasing year by year.Overcharging is a common abuse.When overcharged to a certain voltage,lithium-ion battery will fail,or even thermal runaway,leading to serious fire accidents,which is a serious overcharge.Serious overcharging can be warned by voltage,temperature,impedance and other warning parameters.However,slight overcharging is relatively mild,which will only lead to battery aging and is not easy to be found,which may cause potential safety hazards.Therefore,this paper studies the influence of slight overcharge on the aging and safety of lithium-ion batteries.The specific researches are as follows:The influence of slight overcharge on lithium-ion battery aging and thermal stability was studied.Electrochemical impedance spectroscopy and capacity incremental analysis were used to study the battery aging mechanism,and EV-ARC was used to study the thermal stability of lithium-ion batteries.The results of experiments showed that the aging of the lithium-ion battery was accelerated under 4.5 V slight overcharging,while the one-time slight overcharging under 4.6 V,4.7 V,4.8 V and 4.9 V had little effect on the aging of the lithium-ion battery.The main aging mechanism was the loss of lithium ions during one-time slight overcharge.The main aging mechanism of battery suffering the 4.5 V slight overcharging cycling started with the loss of lithium ions and then become the loss of active materials.The thermal stability of lithium-ion batteries after one-time slight overcharging and slight overcharging cycling was reduced,and exothermic reactions were more likely to occur at high temperatures.The reason for the decrease in thermal stability of lithium-ion batteries was the formation of lithium plating under the condition of slight overcharging,the increase of solid electrolyte interface film thickness and the destruction of the structure of the cathode material.The failure behavior and mechanism of lithium-ion battery suffering slight overcharging cycleing were studied.The battery was cycled at 4.5 V until the battery failure occurs.Electrochemical impedance spectroscopy was used to measure the impedance of the battery,and capacity incremental analysis was used to track the failure behavior of the battery.Industrial computed tomography was used to detect structural changes of battery after battery failure.scanning electron microscope and energy dispersive spectrometer were used to reflect the surface morphology change and elements chagne of battery materials respectively.The results showed that the battery not go into thermal runaway under the 4.5 V slight overcharging cycling,but failure occured.This was due to the formation of lithium dendrites because of lithiu plating caused by the slight overcharging cycling,which lead to an internal micro-short circuit.And the internal micro-short circuit caused the "jelly roll" to break,the battery impedance increases.The results of industrial computed tomography indicated the fracture of the "jelly roll" of lithium-ion batteries.The positive value of the incremental capacity during discharge can be used as a characteristic parameter of the internal micro short circuit of the lithium ion battery.After the battery failure occured,the impedance of the battery increases tenfold,so the impedance can be used as a method to detect the failed battery.The heat generation and thermal runaway behavior and mechanism of lithium-ion batteries under adiabatic conditions were studied.The current interruption test was used to measure the impedance of the battery,and the temperature coefficient of the battery voltage was measured using the stepped heating-standby method to calculate the battery heat generation.The extended volume heating calorimeter(EV-ARC)was used to measure the heat generation during the first charge and discharge cycle and the slight-overcharging-discharging cycling heat generation under different charge cut-off voltages(4.2 V,4.4 V,4.6 V and 4.8 V),thermal runaway under overcharging cycling and thermal stability of overcharged battery.The calculation result of lithium-ion battery heat production was not much different from the EV-ARC measurement result,indicating that the battery heat production can be analyzed by theoretical calculation.The thermal stability of lithium-ion batteries under the high SOC(State of Charge)gradually decreased with the increase of charge cut-off voltage.The thermal runaway results of the slight overcharging cycling of lithium-ion batteries indicate that batteries under high SOC and low temperature conditions were more likely to be thermally runaway than batteries under low SOC and high temperature conditions.Therefore,after the battery surffering slight overcharging,the SOC of lithium ion battery should be lowered first.And then the temperature of battery should be also lowered.