Experimental Research On The Electrochemical Impedance Spectroscopy Of Lithium Iron Phosphate Cell

Battery technology restricts the large-scale application of electric vehicle. Lithiumiron phosphate （LiFePO₄） was considered to be the most valuable cathode material forits stable structure, non-toxicity and low-cost. However, there were lots of scientificissues which cannot be explained by the traditional theory during the use of LiFePO₄battery, and the SOC of the battery cannot be estimated accurately. Based on the EIS ofLi/LiFePO₄half-cell and C₆/LiFePO₄power cell under different temperatures andSOCs, the relationship between the parameters of EIS with temperature and SOC havebeen discussed and analyzed.The researchers at home and abroad used the EIS to study the battery from twoaspects: to find out electrochemical mechanism and to discuss the impact of externalenvironment change on the EIS. The discovery of electrochemical mechanism wasbased on the EIS of half cell, and the EIS of full cell under different environment wasnecessary for the battery application. So the objects of this research were determined asthe Li/LiFePO₄half-cell and C₆/LiFePO₄10Ah power cell.The parameters which were suitable to such system were obtained according tosome pre-tests.The equivalent circuit model was the best method to analysis the EIS, and differentmodels should be applied to the different systems. Based on the electrochemicalmechanism and the acquired data, the models applied to the half-cell and power cellwere found.Temperature and SOC influenced the process of electrochemical reactions, so theEIS parameters were also influenced by such factors. Then the relationships of ohmresistance, solid electrolyte interphase （SEI） resistance and the charge transfer resistancewith the temperature and SOC were analyzed. The ohm resistance of half-cell andpower cell reduced as the temperature increased. The ohm resistances of half-cell keepconstant during different SOC. The charge transfer resistance of half-cell reduced as thetemperature and SOC increased, and charge transfer resistance of the power cell alsoreduced as the temperature increased. According to the Arrhenius relationship, theactivation energy of charge transfer process and Li-ion diffused the SEI process of half-cell and power cell were calculated. The amplitude of power cell at0.01Hz couldbe used to estimate the SOC of the cell, the function between frequency at thetransitional point from charge transfer to diffusion and temperature could be used toestimate the inner temperature of the cell.