Synthesis, electrochemical characterization, and kinetics study of sodium ion insertion/de-insertion at Na3CoCO3PO4 in aqueous Na2SO4 using electrochemical impedance spectroscopy

Abstract

In this work, Na₃CoCO₃PO₄ (NCCP), a cathode material for aqueous sodium ion batteries is synthesized by low temperature ionothermal method using Deep Eutectic Solvent (DES) as reaction medium. A basic electrochemical study on the mechanism of sodium ion de-insertion/insertion process from/into the NCCP in aqueous Na₂SO₄ solution is done using electrochemical impedance spectroscopy (EIS). X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy-dispersive X-ray analysis (EDX) and thermogravimetric analysis (TGA) are used for physical characterizations of the material. The physical characterization revealed that the synthesized material is a nanomaterial with a particle size of 15–30 nm. Cyclic voltammetry (CV) Galvanostatic charge–discharge techniques (GCPL) are used for electrochemical characterization of the material. The impedance data obtained from the kinetic study of the material for Na⁺ insertion and de-insertion process are subjected to simulation with an equivalent circuit. The charge transfer resistance (R_ct), Warburg resistance, double layer capacitance and chemical diffusion coefficient (D_Na+) vary with potentials during de-insertion/insertion processes. R_ct is lowest at the CV peak potentials and the important kinetic parameter, D_Na+ exhibits two distinct minima at potentials corresponding to CV peaks during de-insertion/insertion and it is found to be varying between 10⁻¹¹ and 10⁻¹³ cm² s⁻¹ during sodium de-insertion/insertion processes.