Optimal Heating And Charging Methods For Lithium-ion Batteries Under The Low-temperature Environment

Lithium-ion batteries(LIBs),thanks to their high energy density,low self-discharge rate,and long cycle life,are widely adopted in electric vehicles.However,under the low-temperature environments,the impedance of LIBs dramatically increases,and LIBs suffer from the significant decrease of the peak power and available energy.Moreover,it is difficult to charge the battery.If charging is forcibly performed at low temperatures,lithium-ion could deposit on the surface of the anode electrode,resulting in accelerated degradation of battery lifetime,or even leading to safety hazards.The severe attenuation of the low-temperature performance of LIBs has become one of the bottleneck factors restricting the popularity of electric vehicles.To overcome the challenge of low-temperature poor-performance of LIBs and charging difficulty,and to avoid lithium plating during charging,the following detailed research works are carried out:To achieve the objective of determining the primary cause of low-temperature performance-attenuation of LIBs,the electrochemical model,which can describe the charge and discharge performance of LIBs at different current rates under different temperatures,is developed by experimentally testing the high sensitivity model parameters and identifying the model parameters that are difficult to measure using the adaptive particle swarm optimization.Based on the model,the influencing factors of the low-temperature performance attenuation of the commercial batteries are quantitatively analyzed.The reduced solid-phase diffusivity of the cathode and anode is determined as the main cause of the low-temperature available capacity attenuation of LIBs.The low-temperature available power attenuation of LIBs is mainly attributed to the increased passivation film resistance of the anode and the reduced exchange current density of the cathode.It is revealed that the increased passivation film resistance of the anode is the primary cause of low-temperature lithium plating of LIBs.It is indicated that the different performance attenuation of LIBs depends on different characteristic parameters,breaking through the traditional thinking that the greatly changed parameters are determined as the main cause of battery low-temperature performance attenuation.To achieve the accurate prediction of the time-domain and frequency-domain characteristics of LIBs and the precise description of the nonlinearity of the low temperature characteristics,by removing the influence of the open circuit voltage from the measurement electrochemical impedance spectroscopy(EIS),the modified EIS is obtained to reflect the real dynamics information of LIBs.Based on the modified EIS,the fractional-order equivalent circuit model is proposed to outline the time-domain and frequency-domain characteristics of LIBs simultaneously.The approximate equivalent relationship between the time-domain impedance and the frequency-domain impedance of LIBs is constructed.Combining the thermal model and the electrical model,as well as the temperature-dependent parameters,an electro-thermal coupled model is developed to describe the low-temperature characteristics of LIBs.For the frequency-domain application,the frequency dependence equation of the model parameters is constructed,and the reduced frequency-domain electro-thermal coupled model for LIBs is established,followed with the experimental verification under different operating conditions.Based on the reduced model,the online estimation method of thermal parameters is proposed using the power-frequency alternating current(AC)excitation.Validation and comparison results indicate that the accurate thermal parameters are obtained with a shorter time.For the time-domain application,the physicochemical processes in the middle-high frequency region are taken equivalently as the ohmic resistance,and the relationship between the equivalent ohmic resistance(polarization resistance)and the current is constructed.A reduced time-domain electro-thermal coupled model for LIBs is established and experimentally verified.To achieve the rapid heating with less lifetime reduction of batteries at low temperatures,the AC frequency for the maximum heat generation rate under the constant polarization voltage is deduced using the presented reduced frequency-domain electro-thermal coupled model,thus proposing an optimal low-temperature AC self-heating method for LIBs.The AC and direct current(DC)boundary condition is developed to avoid lithium plating,and the low-temperature heating method of superimposing AC and DC is proposed.The soft-switching resonant circuit is developed to achieve the goal of increasing the temperature of the battery pack evenly and quickly,with substantially no lifetime reduction of the battery pack.The heat generation rate model and the capacity fade model under DC heating are constructed.The heating time and the capacity loss are considered as two objectives to determine the optimal heating voltage,thus proposing an optimal low-temperature DC heating method for LIBs.To make the most use of the discharge energy,the heating method with the internal and external heat is presented.The short heating time,low capacity loss and low battery temperature gradient are considered as the three optimization targets for internal and external heating.The suitable external heating resistance is detemmined,and an optimal low-temperature internal and external heating method is proposed to rapidly increase battery temperature at extremely low temperatures.The proposed four heating methods are‘compared and discussed from the aspects of temperature-rise rate,state of charge(SOC)change and lifetime degradation.The results indicate that the proposed internal and external heating method can achieve rapid heating with less lifetime reduction and relatively simple implementation.It is demonstrated as a promising low-temperature heating method,which helps solve the problem of extremely poor low-temperature performance of LIBs.To overcome the difficulty of low-temperature charging and the challenge of lithium plating during low-temperature charging,based on the lithium plating boundary conditions,the maximum acceptable charging current during isothennal charging at different temperatures is determined.The charging procedure using the echelon current is proposed and experimentally verified.At extremely low temperature,a low-temperature charging method,where the battery is heated firstly and then charged using the echelon current,is proposed for LIBs.The total heating-charging time,total energy consumption and maximum charging SOC are adopted as the evaluation criterion to compare the low-temperature heating charging methods with different switching temperatures under isothermal and non-isothermal conditions.It is found that the charging under non-isothermal conditions is almost the same as that under isothermal conditions.With the three optimization objectives of short total time,low total energy consumption and high charging SOC during low-temperature heating-charging,the best time switching from heating to charging is determined,thus proposing an optimal low-temperature charging method.In a shorter time,the goal of the high charging SOC with less energy consumption and no lithium plating is achieved to effectively promote the application of electric vehicles in extremely cold environments.The quantitative analysis method of low-temperature performance-attenuation for LIBs,electro-thermal coupled modeling for LIBs and its simplified technology,low-temperature optimal heating method and low-temperature optimal charging method for LIBs are proposed to overcome the problem of low-temperature poor-performance for LIBs and to circumvent the difficulty of low-temperature charging and the challenge of lithium plating during low-temperature charging for LIBs.The objective that the low-temperature performance is effectively and rapidly improved and the battery is rapidly charged at low temperature,is achieved for LIBs under extremely cold conditions.A systemized solution for the low-temperature application of LIBs is formed,which strongly supports the promotion and popularization of electric vehicles.