Comparing the Lithiation and Sodiation of a Hard Carbon Anode Using In Situ Impedance Spectroscopy

We present in situ electrochemical impedance spectroscopy data measured during (de)sodiation and (de)lithiation of a commercial hard carbon (HC) anode material. For this purpose, two different systems of micro -reference electrodes ( μ -RE) were used: a gold-wire reference electrode ( μ -GWRE) for Li/HC half-cells and a tin-wire reference electrode ( μ -TWRE) for Na/HC half-cells. We show that for both (de)sodiation (using EC/DMC + 1 M NaPF 6 electrolyte) and (de)lithiation (using EC/EMC + 1 M LiPF 6 electrolyte) the impedance spectra are dominated by a charge transfer resistance (R CT ) which is reversibly decreasing/increasing with increasing/decreasing state-of-charge. The contributions to the HC electrode resistance (R anode ), i.e., charge transfer (R CT ), pore (R pore ), and separator resistance (R HFR ), were obtained by fitting the impedance spectra using a representative equivalent circuit. We conclude that the R CT associated with sodiation of HC is ≈10-fold higher compared to the lithiation of HC at 100% SOC. Furthermore, we compare the evolution of R anode measured in situ over 52 cycles at the same SOC. We find that the higher electrode resistances for sodiated HC result in a considerably reduced rate capability for HC sodiation. For a potential future commercialization of sodium-ion batteries, the fast-charging properties (=HC sodiation) would be a crucial performance indicator.