Effect of Solid Electrolyte Interphase on Sodium-Ion Insertion and Deinsertion in Non-Graphitizable Carbon

Non-graphitizable carbon allows reversible sodium-ion intercalation and hence enables stable and high-capacity sodium storage, making it a promising material for achieving long-term cycling stability in sodium-ion batteries (SIBs). This study investigated the interfacial reactions between various electrolytes and a non-graphitizable carbon electrode for their use in SIBs. The morphology and particle diameter of the non-graphitizable carbon, HC-2000, remained unchanged after heat treatment, indicating its stability. The X-ray diffraction pattern and Raman spectrum suggested a disordered structure of HC-2000 carbon. The interlayer spacing, Brunauer-Emmett-Teller specific surface area, and density were determined to be 0.37 nm, 5.8 m2 g-1, and 1.36 g cm-3, respectively. Electrochemical impedance spectroscopy analysis showed that the charge transfer resistances differed between the Na salts and other electrolytes. Therefore, the use of a large amount of NaF in the solid electrolyte interphase (SEI) resulted in high charge transfer resistances at the non-graphitizable electrodes. However, there were no apparent differences in the activation energy or reversible capacity. In summary, NaF obstructs the penetration pathway of sodium ions into non-graphitizable carbon, impacting the charge transfer resistance and rate stability of SIBs. Charge-discharge measurements revealed reversible capacities of 260-290 mAh g-1, and the rate performance varied depending on the electrolyte. Therefore, an SEI containing minimal inorganic species, such as NaF, is desirable for efficient sodium-ion insertion into non-graphitizable carbon.