Combined conductivity and electrochemical impedance spectroscopy study of Na2FeP2O7 cathode material for sodium ion batteries

Na₂FeP₂O₇ is a cathode material for sodium-ion batteries, distinguished from many other types of cathodes by its superb cycling stability. However, its electrical conductivity has not been characterized. Moreover, the electrode kinetics in sodium-ion batteries in general are poorly understood. Both are crucial for further optimization of battery materials and electrodes. In this work, we firstly analyze the electrical conductivity of Na₂FeP₂O₇ along with its activation energy, which is approximately 0.48 eV – comparable to that of Na₂MnP₂O₇ and its lithium analogue Li₂FeP₂O₇. Next, we systematically analyze electrochemical impedance spectra of Na₂FeP₂O₇ electrodes as a function of temperature, electrode thickness, presence of conductive carbon additive and cycling history. We find that a simplified equivalent circuit is sufficient to describe the obtained impedance spectra and analyze the two fitted resistances to attribute the correct interpretation. The results give insight into conductivity of Na₂FeP₂O₇ and help interpret the EIS data obtained for sodium-ion battery electrodes, paving a further way towards electrode optimization.