A Single Li-Ion Conductor Based on Cellulose

Polymeric single-ion conductors, also known as ionomers, are are touted as an enabling solution for stable alkali metal deposition, necessary for the realization of alkali metal batteries. Such ionomer electrolytes can potentially prevent ion concentration gradient build-up in the electrolyte during battery charge/discharge, which may otherwise lead to polarization losses and ramified metal deposition. Although typically Li+ ionomers are designed from complex synthetic polymers, here we introduce one based on the most abundant natural polymer–cellulose. Evaporation-induced self-assembly of aqueous cellulose nanocrystal (CNC) suspension results in free-standing porous CNC membranes, which transform into Li+ conductor upon lithiation of the -OSO3H groups on the surface of the CNCs. Infused with organic solvent, the CNC membrane shows a good Li-ion conductivity of 0.2 × 10–4 S cm–1 at room temperature, a near unity Li+ transference number (δLi+ = 0.93), and a wide operational window (≥4.5 V) against Li. These properties of the CNC ionomer electrolyte enable smooth Li metal deposition and stable cycling of Li-LiFePO4 cells.