Sodium-ion batteries are gaining attention as an alternative to lithium-ion batteries, because of the more abundant and less toxic ma-
terials used for their construction. However, this technology is novel and requires extensive experimental research to fully understand its behavior. Electrochemical impedance spectroscopy is commonly used to identify the internal battery state. For this reason, electrochemical impedance spectroscopy is applied to two commercial sodium-ion batteries in the present work. The effects of different values of the state-of-charge and relaxation times are investigated. The results show that the measurements performed on the two cells are almost the same, showing good reproducibility of the EIS values. In addition, the EIS values decrease for state-of-charge levels from 100 % to 75 %, and increase from 75 % to 0 %. Finally, the results show a strong influence of relaxation time on the cell impedance.

Keywords: {EIS, sodium-ion battery, relaxation time, state of charge}