In Situ and in Operando Investigation of Hard Carbon Anodes Using Galvanostatic Fourier Transform Electrochemical Impedance Spectroscopy

Abstract

Electrochemical energy storage systems, such as batteries, play an indispensable role in modern society and our daily lives. Lithium-ion batteries (LIBs) have dominated the energy storage market due to their high capacity, superior energy density, and portability. Nevertheless, LIBs face significant challenges, including resource scarcity and reliance on environmentally detrimental mining practices to secure key metals like lithium, cobalt, and nickel. Sodium-ion batteries (SIBs) are emerging as a promising alternative to LIBs, benefiting from similar operational principles and the natural abundance of sodium. Just as in the development of LIBs, researchers working on SIBs aim to achieve higher capacities, enhanced energy and power densities, and improved electrochemical stability, especially under rapid charging and discharging conditions. The performance and stability of SIBs are significantly influenced by the anode material. Among potential candidates, hard carbon (HC) stands out due to its excellent sodium storage capability and cost-effectiveness. Despite these advantages, key challenges still remain, including understanding the complex structure of HC, clarifying the detailed mechanisms of sodium insertion and extraction, and addressing sodium plating issues under fast charging conditions. These phenomena are closely linked to electrochemical processes occurring within the HC anode during battery cycling. Electrochemical impedance spectroscopy (EIS) is a powerful analytical tool providing detailed information on electrochemical systems through equivalent circuit modeling and frequency-dependent analysis. However, conventional EIS cannot analyze dynamic systems such as battery cells undergoing active charging or discharging, as it requires steady-state conditions during frequency sweeps. Fourier transform electrochemical impedance spectroscopy (FTEIS) circumvents this limitation by applying multisine perturbation signals, enabling instantaneous acquisition of impedance spectra through Fourier transformation. In this study, we employ galvanostatic FTEIS to monitor real-time impedance variations of HC anodes during cycling. By correlating impedance data with physical characterization techniques and systematically altering cell parameters, such as electrolyte composition, salt type, or current density, we provide deeper insights into the electrochemical behavior of HC anodes. This approach could facilitate further optimization and improved performance of sodium-ion full cells.