Cationic polymer-in-salt electrolytes for fast metal ion conduction and solid-state battery applications

Polymer electrolytes provide a safe solution for future solid-state high-energy-density batteries. Materials that meet the simultaneous requirement of high ionic conductivity and high transference number remain a challenge, in particular for new battery chemistries beyond lithium such as Na, K and Mg. Herein, we demonstrate the versatility of a polymeric ionic liquid (PolyIL) as a polymer solvent to achieve this goal for both Na and K. Using molecular simulations, we predict and elucidate fast alkali metal ion transport in PolyILs through a structural diffusion mechanism in a polymer-in-salt environment, facilitating a high metal ion transference number simultaneously. Experimental validation of these computationally designed Na and K polymer electrolytes shows good ionic conductivities up to 1.0 × 10−3 S cm−1 at 80 °C and a Na+ transference number of ~0.57. An electrochemical cycling test on a Na∣2:1 NaFSI/PolyIL∣Na symmetric cell also demonstrates an overpotential of 100 mV at a current density of 0.5 mA cm−2 and stable long-term Na plating/stripping performance of more than 100 hours. PolyIL-based polymer-in-salt strategies for new solid-state electrolytes thus offer an alternative route to design high-performance next-generation sustainable battery chemistries. Polymer electrolytes provide a safe solution for future solid-state high-energy-density batteries, but combining high ionic conductivity and a high transference number is a challenge. A polymeric ionic liquid used as a polymer solvent is now shown to be promising for both sodium and potassium batteries.