Symmetric Cell Electrochemical Impedance Spectroscopy of Na2FeP2O7 Positive Electrode Material in Ionic Liquid Electrolytes

Symmetric cell electrochemical impedance spectroscopy (SCEIS) is a powerful method to analyze electrode materials for secondary batteries. The EIS results are used to obtain information related to electrochemical processes such as charge-transfer resistance. In this study, SCEIS is employed to investigate the electrochemical performance of the Na2FeP2O7 positive electrode for sodium secondary batteries operating at temperatures ranging from room to intermediate temperatures using the ionic liquid (IL) electrolytes, Na[FSA]-[C(2)C(1)im][FSA] (ILFSA) (C(2)C(1)im = 1-ethyl-3-methylimidazolium, FSA = bis(fluorosulfonyl)amide). The obtained SCEIS result for Na metal, acetylene black, alpha-Al2O3, and V2O5 revealed that the resistance of the high-frequency region in the Nyquist plots is a combination of several factors (the Na[FSA] fraction, ionic conductivity of the electrolyte, and electronic conductivity of the composite electrode). The activation energies obtained by the Arrhenius plots for both the high-frequency and charge-transfer resistances of Na2FeP2O7/ILFSA/Na2FeP2O7 SCEIS showed that a significant decrease in the charge-transfer resistance contributes to the high rate performance in the intermediate temperature range.