Research On The Aging Mechanism And Model Of Nickel-Cobalt-Aluminum Lithium-Ion Batteries In The Whole Life Cycle

With the increase in the number of internal combustion engine vehicles,the oil shortage and environmental pollution in China are becoming more and more serious.Pure electric vehicles have developed rapidly thanks to their advantages of not consuming oil and being environmentally friendly.However,due to the problems of reduced cruising range and the high cost of pure electric vehicles,their promotion has been limited.Lithium-ion batteries are a key factor affecting the cruising range and cost of pure electric vehicles.How to improve the cycle life of lithium-ion batteries and reduce the cost of lithium-ion batteries has become an urgent problem to be solved.The aging mechanism analysis of lithium-ion batteries can provide a basis for the design and use of lithium-ion batteries,so that the cycle life of lithium-ion batteries can be longer.Meanwhile,it can guide the construction of the lithium-ion battery mechanism aging model,and predict the cycle life under different working conditions through the mechanism aging model,which reduces the cost compared with the experimental method.Therefore,this paper studies the aging mechanism of NickelCobalt-Aluminum lithium-ion batteries at different temperatures,and proposes a corresponding mechanism aging model based the aging mechanism.The specific works carried out in this paper are as follows:(1)Design of cyclic aging experiments and identification of cyclic aging mechanisms for lithium-ion batteries: A fixed-node cyclic aging experiment was designed in this paper to analyze the aging mode evolution of lithium-ion batteries at different temperatures(25°C and 45°C)through non-destructive aging characterization methods,including differential voltage method and electrochemical impedance spectroscopy method.While the characterization of the aging mechanism is further aided by the topography observations of the electrode.(2)Construction of the electrochemical-thermal-side reaction coupled aging model for lithium-ion batteries: Based on the electrochemical-thermal coupled model,a side reaction model for electrolyte decomposition that can be applied to different temperatures is proposed.It can be described that the decomposition products of the electrolyte in the lithium-ion battery are mainly solid components at room temperature,and the decomposition products of the electrolyte are mainly gas components at high temperatures.The lithium plating side reaction model is also coupled,which can describe the capacity plunge phenomenon in the later stage of aging of lithium-ion batteries.Meanwhile,the loss model of electrode active material considering the internal mechanical stress of the lithium-ion battery is coupled,which can describe the fracture phenomenon of electrode active material particles.The proposed electrochemical-thermal-side reaction coupled aging model can accurately reflect not only the relative capacity fading during cycling at 25°C and 45°C,but also the aging mode evolution during cycling at 25°C and 45°C.(3)Life prediction and mechanism analysis based on the coupled aging model:Based on the electrochemical-thermal-side reaction coupled aging model,the effects of discharge rate,charge rate,and ambient temperature on battery aging were studied.The increase of discharge rate and charge rate will not change the proportion of each aging mode of the lithium-ion battery,but will aggravate each aging mode,resulting in faster capacity decay.The increase in temperature will make the main product of electrolyte decomposition change from solid to gas.On the one hand,the loss of lithium ions will be reduced,and on the other hand,the reduction of the internal mechanical stress of the negative electrode will reduce the loss of the negative electrode active material.The loss of positive active material hardly changes with temperature.Therefore,there is a suitable temperature to maximize the cycle life.