A reference electrode for in situ impedance measurements in sodium-ion batteries

In this work, we introduce a newly developed micro-reference electrode (μ-RE) for sodium-ion batteries (SIBs). This μ-RE is based on a 50 μm-sized tin-coated copper wire, manually insulated using polyurethane (PU) spray, such that only the cross-sectional area of the wire tip is in contact with the electrolyte. The tin-coating allows for facile in situ electrochemical sodiation, resulting in a stable potential of the wire that enables in situ Electrochemical Impedance Spectroscopy (EIS). We will show that reliable single-electrode impedance data from SIB cells can be obtained with this new tin wire μ-RE (μ-TWRE) concept, validated by comparing μ-RE-based single-electrode impedance data with those acquired by a symmetrical cell approach. As hard carbons are currently the most promising anode material for SIBs, we evaluate the impedance evolution of a hard carbon anode over extended charge/discharge cycles in a half-cell vs. sodium metal, comparing its impedance measured at the same state-of-charge (SOC) over 50 cycles. Thus, we demonstrate that EIS using a μ-TWRE can be used as a convenient tool to quantify the impedance evolution of SIB anodes and cathodes.