Experimental research on GEIS of C/LiFePO<sub>4</sub> battery

Lithium-ion batteries are widely considered as the energy storage systems appropriate for use in the battery electric vehicles (BEV) and hybrid electric vehicles (HEV). In order to fulfill the operating time required for such applications, many efforts have been paid to understand the electrochemical behavior and aging mechanisms of these batteries. Electrochemical impedance spectroscopy (EIS) is commonly applied, since it brings substantial information regarding the electrochemical processes that take place inside the battery at different time scales. The nondestructive character of EIS renders this technique promising for in-situ diagnosis of state of charge (SOC). EIS includes Galvano electrochemical impedance spectroscopy (GEIS) and potentio electrochemical impedance spectroscopy (PEIS). In most cases, potentio and Galvano modes are equivalent and result in the same impedance diagrams provided that a sine current amplitude is “equivalent” to the voltage sine amplitude. However, in certain conditions, typically when the system evolves during the measurement, results from the two techniques may be different. At present, PEIS is more often performed than GEIS. However, GEIS with the sine current disturbing is closer to the real working state of the battery, and is more accurate to respond to the internal change of the battery from the impedance spectroscopy. That is because when PEIS is used, a minor voltage disturbing may lead to a big current that is inappropriate for nondestructive diagnosis. Besides, GEIS is more suitable for investigations on batteries with lower 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 (Bio-logic, France). Firstly, the current disturbing value of 1700 mA h C/LiFePO₄ battery was optimized at different temperatures. Then, under the optimized current disturbing value, the GEIS of 1700 mA h C/LiFePO₄ battery at different temperatures and different SOC was investigated, and the variation tendency of ohmic resistance (R_s), charge transfer impedance (R_ct) and diffusion impedance (Z_w) with the change of temperature and SOC was analyzed. The results show that the current disturbing value increases with the increase of temperature. SOC and temperature have an important effect on the state of the battery. With the increase of SOC, R_s slightly increases and the increase becomes slower with the rise of temperature, while Z_w decreases. However, R_ct apparently declines and gets steady at both ends of SOC (0–40% and 60%–100%), and arrives at higher values at SOC of 40%–60%. For different temperatures, the values of R_s, R_ct and Z_w all decrease with the increase of temperature. As a result, GEIS can effectively reveal the polarization state inside the battery, so that it provides an efficient way to predict the degradation state of energy storage lithium-ion battery. In addition, the information about lithium intercalation-deintercalation process and charge transfer process is of significance in developing relevant fundamental theory. The study also provides insights on better development of green rechargeable batteries with high-specific energy density.