Influence of Electrode Thickness and Electrolyte Composition on SEI Formation and Cycle Stability in Binder-Free Hard Carbon Anodes for Na-Ion Batteries

Hard carbon self-supported electrodes (HC SSEs) have gained significant interest as an alternative to classical binder-based anode materials for sodium-ion batteries (SIBs), eliminating the need for binders, solvents, conductive carbon, and current collectors. In this work, we investigate the impact of HC thickness and macroporosity, as well as the presence/absence of an electrolyte additive, on the formation of the solid electrolyte interphase (SEI) and electrochemical performance. Three cellulose-based filter paper (FP) precursors with different thicknesses were used to prepare HC SSEs through a pyrolysis process at 1500 °C, and then these were compared with an HC SSE obtained from commercial carbon fabric. The results highlight the importance of appropriate structural, morphological, porosity, and surface chemical properties of the materials to achieve high initial Coulombic efficiency (up to 93%) and high reversible capacity (up to 328 mAh g^-1 at 37.2 mA g^-1). However, the cycle stability was greatly impacted by the electrode thickness/macroporosity and the electrolyte additive. In NaPF₆ without fluoroethylene carbonate (FEC) additive, the maximum number of cycles (50) was obtained for the thinnest electrode (79 μm), while rapid fading (after ≈1-5 cycles) was observed for electrode thicknesses >185 μm. By using the FEC additive, the fading is mitigated and cycling stability (100 cycles) is reached for all the electrodes, except the commercial one (5 cycles), due to its highest thickness (1042 μm). The improvement in stability could be explained by the buildup of a thin homogeneous inorganic-rich SEI containing more NaF in the presence of FEC, which is more stable, protective, and electronically conductive.