Research On Optimized Charging Strategy Of Lithium-ion Batteries Based On The Electrode Dynamics

Lithium-ion batteries,due to their high energy density,high power density,and long cycle life,are used as the power source of electric vehicles(EVs).They are one of the most important parts in electric vehicles.However,compared with fuel vehicles,it takes a long time to charge for EVs,which cannot meet the requirements of users.Furthermore,unreasonable charging strategies will not only shorten the life of lithium-ion batteries,but even cause safety issues.The optimization of lithium-ion batteries charging has become one of the bottlenecks restricting the large-scale popularity of electric vehicles.Therefore,it is important to explore safe and fast charging strategies for lithium-ion batteries.It is well known that the charging process of lithium-ion batteries corresponds to the charging for positive electrode and discharging for negative electrode,respectively.Thus,it is important to explore the optimization of lithium-ion battery charging strategies based on the intrinsic electrode dynamics,for optimizing battery charging.The main research contents are as follows:(1)The characteristics are tested on positive and negative electrodes through assembling half cells.In order to study the charge and discharge performance of the positive and negative half cells,the low current charge and discharge performance test and the rate performance test are designed,respectively.Based on the second-order equivalent circuit model of lithium-ion batteries,the hybrid pulse power characteristic test under different SOC and rates is designed,and the relevant parameters are identified through Ohm law and nonlinear fitting methods.(2)The electrode dynamics of the positive and negative electrodes are studied based on the charge and discharge performance test.First,according to the charge and discharge curves and incremental capacity curve,the phase change process of the positive and negative electrodes during the lithium-ion batteries charging process is clarified.Then,the charge curve of the positive electrode and the discharge curve of the negative electrode at different rates are analyzed,and the capacity retention rate of the positive and the negative electrode are analyzed.It preliminarily shows that the negative electrode is the limiting electrode.Finally,based on the polarization voltage,the variations of ohmic polarization voltage and electrochemical polarization voltage for the positive and negative electrode under different SOC and rate are analyzed.Moreover,the relationship between polarization voltage and rate are fitted through the linear and the quadratic function.(3)Considering suppressing positive electrode overcharge and negative electrode lithium plating,the maximum allowable charging current is examined.First,based on the relationship between polarization voltage and rate of electrodes at different SOC,and the maximum allowable charging current of positive electrode and the maximum allowable discharging current of negative electrode have been determined with the boundary condition of none positive electrode overcharge and lithium-metal deposition.Then,the relationship of the SOC value between the half cell and full cell is established.By comparing the maximum allowable charge rate of the positive electrode and the maximum allowable discharge rate of the negative electrode under full cell SOC scale,it is proved that the negative electrode is the limiting electrode during the charging process of full cell.Finally,through in-situ detection methods,the maximum acceptable discharge rate of the negative electrode and the maximum acceptable charge rate of the full cell are verified to avoid lithium plating.(4)The research on charging strategies of lithium-ion battery is conducted to optimize charging energy consumption and charging time.Based on the second-order equivalent circuit model,the charging energy consumption and charging time model are established for the lithium-ion battery charging process.With the charging energy consumption and charging time as optimization objectives,the charging strategy is optimized by employing the particle swarm optimization method,where the constraints are SOC and the maximum allowable charging current.By comparing the optimized charging control strategy with the traditional charging strategy,the advantages of the optimized charging strategy have been verified.