Gao Fei

Experimental research on GEIS of C/LiFePO 4 battery

Lithium-ion batteries are widely considered as the energy storage nsystems appropriate for use in the battery electric vehicles (BEV) nand hybrid electric vehicles (HEV). In order to fulfill the operating ntime required for such applications, many efforts have been paid to nunderstand the electrochemical behavior and aging mechanisms of these nbatteries. Electrochemical impedance spectroscopy (EIS) is commonly napplied, since it brings substantial information regarding the electrochemical nprocesses that take place inside the battery at different time scales. nThe nondestructive character of EIS renders this technique promising nfor in-situ diagnosis of state of charge (SOC). EIS nincludes Galvano electrochemical impedance spectroscopy (GEIS) and npotentio electrochemical impedance spectroscopy (PEIS). In most cases, npotentio and Galvano modes are equivalent and result in the same impedance ndiagrams provided that a sine current amplitude is “equivalent” nto the voltage sine amplitude. However, in certain conditions, typically nwhen the system evolves during the measurement, results from the two ntechniques may be different. At present, PEIS is more often performed nthan GEIS. However, GEIS with the sine current disturbing is closer nto the real working state of the battery, and is more accurate to nrespond to the internal change of the battery from the impedance spectroscopy. nThat is because when PEIS is used, a minor voltage disturbing may nlead to a big current that is inappropriate for nondestructive diagnosis. nBesides, GEIS is more suitable for investigations on batteries with nlower resistance. Therefore, experimental research on GEIS of C/LiFePO4 battery was carried out in this paper. The EIS experiments were performed using a VMP2 Potentiostat workstation n(Bio-logic, France). Firstly, the current disturbing value of 1700u2005mAu2005h nC/LiFePO4 battery was optimized at different temperatures. nThen, under the optimized current disturbing value, the GEIS of 1700u2005mAu2005h nC/LiFePO4 battery at different temperatures and different nSOC was investigated, and the variation tendency of ohmic resistance n( R s), charge transfer impedance ( R ct) and diffusion impedance ( Z w) with the change of temperature and SOC was analyzed. nThe results show that the current disturbing value increases with nthe increase of temperature. SOC and temperature have an important neffect on the state of the battery. With the increase of SOC, R s slightly increases and the increase becomes nslower with the rise of temperature, while Z w decreases. However, R ct apparently ndeclines and gets steady at both ends of SOC (0–40% and 60%–100%), nand arrives at higher values at SOC of 40%–60%. For different ntemperatures, the values of R s, R ct and Z w all decrease nwith the increase of temperature. As a result, GEIS can effectively nreveal the polarization state inside the battery, so that it provides nan efficient way to predict the degradation state of energy storage nlithium-ion battery. In addition, the information about lithium intercalation-deintercalation nprocess and charge transfer process is of significance in developing nrelevant fundamental theory. The study also provides insights on better ndevelopment of green rechargeable batteries with high-specific energy ndensity.