Investigation On Cascade Utilization,Capacity Attenuation And Recovery Process Of Retired Lithium Power Batteries

A large number of retired lithium ion(Li-ion)batteries have been produced with the explosion of electric vehicles,it is necessary to investigate the cascade utilization,mechanism of energy decline,scrap and recovery of power batteries,harmless,green and resource utilization maximization of retired power batteries.Considering the discharging rate,battery capacity and resistance,the retired Li-ion power batteries are taken as a research object,and the influence rules of three performance parameters on the capacity of retired batteries are given,and the level of cascade utilization is divided by practical examples.A capacity loss model is proposed as the main degradation mechanism of cycle lifetime of power batteries.Based on the performance test,lifetime test and internal resistance test,the effect of charging cut-off voltage,discharge depth,charge-discharge ratio and ambient temperature on capacity attenuation of cascade power Li-ion batteries are given.Moreover,a lifetime prediction method for cascade utilization Li-ion power batteries is established.A new recovery system is established based on the current retired battery recovery system,which can effectively improve the recovery rate of useful materials for anode and cathode materials.The main content of this dissertation are expressed as follows:(1)Qualified single Li-ion battery is selected by checking the shape and structure of retired Li-ion power batteries and sorting their performance.In order to make the retired batteries with different capacity and internal resistance suitable for cascade utilization,the relationship between discharge rate and capacity and internal resistance of retired Li-ion battery is fitted by binary linear regression model,and the cascade utilization model of retired Li-ion batteries based on capacity and internal resistance is established.The price of Li-ion batteries is analyzed by market conditions and material cost.The effect of cascade utilization on the price of Li-ion batteries is estimated,and the economic feasibility of cascade utilization of Li-ion power batteries is verified.(2)On account of the fracture and repair theory of SEI,a capacity loss model in cyclic process is proposed.The mechanical model of SEI and active materia l particle composition system is deduced,and research results show that material parameters and operating conditions of battery have different effect on discharge and SEI stress.By comparing the calculated results with the experimental data of accelerated aging of Sanyo battery based on graphite/NMC technology,it is shown that the key to the effect of DOD or mean SOC on capacity attenuation is to control the relative change of valence of LixC6 and graphite expansion and the electrode balance.In addition,the minimum degradation of the battery during the cycle corresponds to the minimum stress oscillation during the cycle.The position of the minimum stress is affected by the battery balance,which determines the exact SOC of the graphite parameter expansion range in the whole battery.(3)Based on the performance test,lifetime test and internal resistance test,combined with the capacity attenuation mechanism of Li-ion batteries,the effect of charging cut-off voltage,discharge depth,charge-discharge ratio and ambient temperature on capacity attenuation of cascade power Li-ion batteries are researched.The experimental results show that the discharge capacity of LFP batteries decreases rapidly and the service lifetime decreases obviously at high charge cut-off voltage,the internal resistance of LFP batteries increases faster at high discharge depth;and the performance degradation rate of LFP batteries will accelerate with the increase of charge rate.(4)The second-order Thevenin equivalent circuit model is established by interval pulse test of cascade Li-ion batteries and the parameters of equivalent circuit model are identified by the recursive least square method.According to the research results of the attenuation characteristics of cascade Li-ion batteries,the main factors which affect the lifetime of cascade Li-ion batteries are analyzed and concluded as follows: battery temperature,internal resistance and discharge voltage.The temperature,internal resistance and discharge voltage of cascade Li-ion batteries are taken as BP neural network model parameters,and a BP neural network model is used to predict SOH of cascade Li-ion batteries and its cycle lifetime.(5)Aiming at intelligent screening,intelligent disassembly,cascade utilization and element recovery of retired cascade LFP batteries in the process of reclamation and utilization,the efficient and environmentally friendly reclamation process of retired cascade LFP batteries is discussed.The design of intelligent and efficient battery pack dismantling system and element recovery system for retired cascade LFP batteries is completed,and the recovery efficiency of the system is investigated,and reasonable suggestions for efficient utilization and recovery of Li-ion batteries are put forward.