Study Of Energy Type Lithium-ion Cell Degradation Behavior Under Multiple Stresses

Lithium-ion cell has been extensively used as power source for electric vehicles due to its superior specific energy and specific power over other types of secondary batteries.However,the aging rate of a cell is much faster and its lifespan is much shorter than conventional internal combustion,limiting the widespread diffusion of electric vehicles.In addition,as the performance of a cell changes with time,the control strategy has to be adjusted accordingly so as to optimize the usage of the cell.Therefore,it is necessary to understand the aging behavior of a cell,including the variation of cell aging rate,the according factors,and the aging mechanism.The cell aging behavior is mainly affected by the stresses it is subjected to,such as current,voltage,and temperature,etc.In this study,the effect of stress type,stress level,and stress sequence on the aging behavior of an energy type 18650 cell was investigated.In the study of the stress type,a general constant-current-constant-voltage(CCCV)charge-CCCV discharge protocol accommodating seven stresses was devised.The L18(2×37)orthogonal array was applied to reduce the experiment units,while ensuring that the main effects were unaffected.The capacity fade rate in each group was used as the key aging indicator.The effects and significance of the seven stresses were obtained using statistic methods,including main effect analysis and analysis of variance(ANOVA).This conclusion was used to guide the selection of stresses in further research.In the study of the stress level,normal accelerated stress test(AST)and abuse were distinguished,and two types of aging model were built to quantitatively relate cell aging rate to various stresses.First,based on main aging stresses that had been obtained,the L27(313)orthorgonol array was used for the design of experiments.In addition,three methods were proposed to differentiate lithium plating from normal AST.Then,the operating boundary of lithium plating were deduced based on the results of the previous 45 groups of tests.A second order polynomial model was proposed to describe the relation between cell aging rate and five main stresses.Regression orthogonal design of experiments with 28 groups of tests was applied to fit the parameters in the model.The applicability of the model was validated using four additional experiments.In the study of the effect of stress sequence,the cell aging behaviors under stress-varying conditions were discussed.The effect of both the temperature sequence and the state of charge(SOC)sequence on calendar aging were studied.The ANOVA method was used to compare the difference in capacities and resistances during aging under different paths.In addition,a rate-based aging model was developed under the assumption of no memory effects.The applicability of the model was validated using results in the stress-varying tests.Finally,the model was further proved to be applicable to the prediction of cell rate capacity and module capacity under storage aging.The results in this work could guide the design of AST,the development of aging model,and the optimization for management strategy.