Research On Cycle Performance Degradation Rule Of LiCoO₂/C Battery And The Accelerated Effect Of Different Conditions

The lithium ion batteries have been widely used in many fields including aerospace,energy storage and electric transportation,which require a long service life up to 10-15 years.However,the lifetime of current lithium ion battery can not meet the demand.Therefore,the evaluation of battery life is quite necessary.The aging tests of lithium batteries under the real operation conditions are both time and cost consuming;and short-term accelerated aging test has been considered as a powerful method.The research on the accelerated stress of performance degradation can not provide only the reference for the rapid evaluation of lithium ion battery life,but also the data and theoretical basis for the life prediction of lithium ion battery.In this work,the effects of charge/discharge rates,depth of discharge?DOD?and ambient temperatures on the accelerated aging of LiCoO₂/MCMB and LiCoO₂/graphite full cells are investigated.Charge/discharge curves,end of discharge voltage and the electrochemical impedance spectroscopy?EIS?are adopted to analyze the capacity degradation mechanism.Half-cell tests are applied to identify the single electrode performance before and after aging as well as to quantify the consumption of active lithium.The degradation mechanism of a single electrode has been investigated by combining the information of surface morphology,composition and structure.The LiCoO₂/MCMB full cells are cycled at different charge/discharge rates?0.6C,1.2C,1.5C,1.8C 2.4C and 3.0C?under 30%DOD,and the accelerated aging effect as well as the degradation mechanism of the full cells under corresponding test conditions are investigated.It shows that the capacity fade is accelerated first with the increasing charge/discharge rate;and the decay rate of capacity slows down when the test rate is higher than 1.8C.The performance degradation of single electrodes and the loss of active lithium are tested quantificationally using half-cells,which are prepared by the harvested aged electrodes.The loss of active lithium in the anode,including the irreversible consumption of active lithium and the lithium content left in anode due to the polarization of full cell,leads to a shift of the available capacity of full cells towards to the direction of high state of charge?SOC?.Meanwhile,the decay in cathode makes the available capacity of full cells shift to low SOC direction.The three aging factors jointly results in the capacity fade of full cells.The extent of the effects of all the three factors has been calculated on the basis of half-cell tests.When the charge/discharge rate is less than or equal to 1.5C,the ratios of capacity fade caused by different aging factors are approximately equal.When the charge/discharge rate is greater than or equal to1.5C,the effect of the polarization in the full cell is increased gradually,and the decay in cathode is reduced,which indicates that the degradatio n mechanism of full cell capacity is changed.According to the disassembly analysis,the cathode performance is mainly affected by the structure reduction.The SEI film on the anode is the primary reason for the anode performance degradation.Based on the above analysis,the charge/discharge rate of 1.5C is the frontier to accelerate the aging of the LiCoO₂/MCMB full cells which are cycled at 0.6C and 25?under30%DOD?100%⁷0%SOC?,and the capacity fade rate is enhanced by nearly 2.6times.The above conclusion is verified by the test results of LiCoO₂/graphite full cells.The LiCoO₂/MCMB full cells are cycled at the temperature of 25?,with different charge/discharge rates?0.6C,1.5C,1.8C and 2.4C?under 30%DOD?from100%SOC to 70%SOC?and 50%DOD?from 100%SOC to 50%SOC?.The accelerated aging effect of the depth of discharge and the degradation mechanisms of full cell capacity under the corresponding test conditions are compared.It shows that the large depth of discharge can accelerate the aging of f ull cells when the charge/discharge rates are higher than or equal to 1.8C,while the effect of DOD can be ignored when the full cells are cycled at low charge/discharge rates.Through the surface morphologies,compositions and structures of single electro des,the large depth of discharge results in the declining cathode capacity due to the decay in cathode structure,which means the degradation mechanism of full cell performance is changed.Therefore,the large depth of discharge is not suitable to accelerate the aging of LiCoO₂/MCMB full cells cycled at 0.6C and 25?under30%DOD?100%⁷0%SOC?.The LiCoO₂/MCMB full cells are cycled at different temperatures?25?,35?and 45??under 30%DOD?from 100%SOC to 70%SOC?with the charge/discharge of 0.6C.The accelerated aging effect of ambient temperatures on the aging of full cells and the capacity degradation mechanisms are researched.The capacity fade is accelerated with the rising ambient temperature,especially for the full cells cycled at 45?.The aging factors affecting the capacity fade of full cells are still the decay in cathode,the irreversible consumption of active lithium as well as the polarization in the full cell.When the full cells are cycled at 45?,the primary reason for the capacity fade is changed to the decay i n cathode instead of the consumption of active lithium.The capacity fade rate of full cells is also determined by the decay rate of cathode capacity.Through the analysis of the surface morphologies,compositions and structures of single electrodes,the e levated ambient temperature accelerates the fracture and repair of the SEI film on cathode,leading to the increased polarization as well as the decreased cathode capacity.The effect of the SEI film and some lithium depositions on the anode performance as well as the effect of material structure are calculated by washing the anodes with distilled water.When the full cells decayed to the end of life under different ambient temperatures,about 75%of the anode capacity loss is attributed to the effect of th e SEI film and some lithium depositions on the anode.Based on the above analysis,the elevated temperature changes the capacity degradation mechanism,and it cannot rationally accelerate the aging of the LiCoO₂/MCMB full cells cycled at 0.6C and25?under 30%DOD?100%⁷0%SOC?.This conclusion is verified by the LiCoO₂/graphite full cells,so the elevated temperature is not suitable to accelerate the aging of the LiCoO₂/C full cells.