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 NaPF₆ electrolyte) and (de)lithiation (using EC/EMC + 1 M LiPF₆ 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.