Deciphering Solid Electrolyte Interface in Cellulose-Montmorillonite Nano-Composites for Sodium Batteries

Abstract

Electrolytes and their interphases are critical for emerging battery chemistries like metal- sulphur, metal-oxygen especially using solid electrolytes, which offer tantalizing energy storage possibilities but involve drastic phase transitions and structural challenges. Therefore, developing improved electrolytes and interphases is a key to achieving sustainable battery performance. In most advanced batteries, the electrodes operate at potentials beyond the thermodynamic stability of electrolytes, achieved through ingenious stoichiometry tuning. Stability is thus maintained kinetically via an interphase formed through sacrificial reactions between the electrolyte and electrodes. Here, we introduce a novel polymer composite electrolyte utilising abundant Montmorillonite and cellulose nanocrystals (CNC), creating a stable interphase with Na metal and alleviating common degradation issues seen with standard liquid electrolytes. For example, this electrolyte exhibits a stability window of 2.3-5.3 V and a transference number of ~0.87 although durability and storage properties need further improvement. FT-IR, XPS, and Raman spectroscopy provide valuable insights into interfacial chemistry, as evidenced by a prominent hydroxyl stretching band associated with CNC. While hydroxyl groups may compromise interfacial stability at the cathode, possibly causing cell degradation, they simultaneously enhance sodium-ion mobility at the anode by facilitating favourable coordination with sodium metal. This dual function underscores the need for tuning functional groups in electrolyte design.