Highly promoted solvent-co-intercalation process in pencil graphite anode and Na₃V₂(PO₄)₃ cathode in full-cell Na-ion...

The electrochemical performance of graphite recovered from 6H-pencil with the highest content of SiO₂ is evaluated in both Na-ion half and full-cell assemblies. The concept of sodium co-intercalation into graphite is exploited by fabricating cells with electrolytes based on tetraethylene glycol dimethyl ether (G4) and diethylene glycol dimethyl ether (G2). The capacity at high current rates is maximum when the G2-based electrolyte is used, both in half and full cells, while the capacity retention after high current rates is better in a G4-based system. Upon calculating the capacity contribution, the G2-based system shows prominent capacitance-based charge storage, whereas the G4-based system has a higher contribution from the Faradaic mechanism. The former also shows a faster diffusion mechanism. While G2 based system has higher capacity retention in half-cell, G4 based system has higher capacity retention in full-cell. When G2 is used as the electrolyte solvent, the irreversibility during cycling is high, affecting cell performance. The full cells with G4 and G2 electrolytes show maximum energy/power densities of 33 Wh kg^-1/2.7 kW kg^-1 and 23 Wh kg^-1/1.4 kW kg^-1, respectively. Our study shows that the charge storage mechanism can be varied by tuning the electrolyte solvent. This study is the first to explore pencil graphite for sodium-ion storage.