Electrochemical impedance spectroscopy as a tool to study the aging of a carbon xerogel-based anode in sodium-ion batteries

The performance as anode in sodium-ion batteries of a carbon xerogel (CX-500) using NaPF 6 salt-based electrolytes with different carbonate solvent mixtures (EC:DEC, EC:DMC, EC:EMC) is investigated by prolonged galvanostatic cycling with the support of the Electrochemical Impedance Spectroscopy (EIS). To this end, the CX-500 semi-cell internal ohmic (flow of ions by the electrolyte, R s ) and faradic (solid-electrolyte interface layer, R SEI ; charge transfer at the electrode/electrolyte interface, R CT ) resistances as well as the parameters associated with the diffusion processes of the Na + ions at the electrode/electrolyte interface and inside the electrode (Warburg coefficient, σ; diffusion coefficient, D) along cycling were calculated by this non-destructive technique. The long-term electrochemical stability of the carbon xerogel anode in sodium-ion batteries was found to be highly dependent on electrolyte composition. By comparing the evolution of the capacity and the EIS parameters of the CX-500 semi-cell during cycling, it can be inferred that the use of EC:EMC carbonate mixture in the electrolyte is the most suitable since the other mixtures, EC:DEC and EC:DMC, induce abrupt increases of the internal resistances, particularly as regards R SEI and R CT , whose increases are related to the loss of active area accessible to the electrolyte as a consequence of the continuous growth of the SEI layer, thus leading to a significant capacity fading as a consequence of electrode degradation. These findings establish quantitative relationships between solvent chemistry and electrochemical degradation mechanisms in hard carbon anodes, advancing mechanistic understanding of electrolyte design for sodium-ion battery optimization. • EIS proved to be a useful tool to diagnose the CX-based anode performance in SIBs • Long-term stability of CX anodes in SIBs highly depends on electrolyte composition • By using NaPF 6 salt, the EC:EMC carbonate mixture result the most suitable solvent • Anode degradation was related to the loss of active area accessible to electrolyte