Electrode-Electrolyte Interfacial Engineering and Failure-mode Analysis of Cellulose Nanocrystals-Montmorillonite Composite for Solid-State Sodium Batteries

Electrolytes and the associated interphases are critical components supporting emerging battery chemistries that promise tantalizing energy storage options but involve drastic phase transitions and structural complications. Consequently, designing better electrolytes and interphases is key to the success of these batteries with sustainable performance parameters. In most advanced batteries, the two electrodes operate at potentials well beyond the thermodynamic stability limits of electrolytes through ingenious stoichiometry tuning. Therefore, stability must be achieved kinetically through an interphase formed from sacrificial reactions between the electrolyte and electrodes. Here, we report a unique polymer composite electrolyte using abundant Montmorillonite and cellulose nanocrystals (CNC), which creates a stable electrolyte interphase with Na metal, alleviating common degradation issues in standard liquid electrolytes. This electrolyte reveals a stability window of 2.3–5.3 V with a transference number of ~ 0.87. FT-IR and Raman spectroscopy provide valuable insights into interfacial chemistry, as evidenced by a prominent hydroxyl stretching band associated with CNC. While the hydroxyl groups may disrupt interfacial stability at the cathode, potentially leading to cell degradation, they simultaneously enhance sodium-ion mobility at the anode by enabling favourable coordination with sodium metal. This dual role underscores the critical need for functional group tuning in electrolyte design.