Embedded 3D Li+ channels in a water-in-salt electrolyte to develop flexible supercapacitors and lithium-ion batteries

Abstract

Electrolytes with high ionic conductivity and intrinsic safety are key to achieving practical high-performance batteries and supercapacitors. Water-in-salt electrolytes (WiSEs) make the performance of an aqueous lithium-ion battery well comparable to that of non-aqueous systems owing to their wide electrochemical windows and non-flammability. Critically, a WiSE is practically limited by the issues of high viscosity and inevitable leakage, restricting its rate performance and safety. Here, we report an approach to achieve ultra-high salt loading of hydrogels by implanting hydrogel 3D ion channels in a WiSE, endowing the WiSE-based electrolyte with versatility and high ionic conductivity. Our polyacrylamide–chitosan-based WiSE (HiSE) affords high ionic conductivity (51.3 mS cm−1) and operating voltage (2.6 V) and excellent flexibility and self-healing ability. These properties are benefitted by polar corona, which is formed by immobilizing water molecules on abundant hydroxyl and amino groups inside the HiSE. The core–corona configuration offers a unique 3D channel to allow Li+ to fast transport at an unsaturated coordination state, as evidenced though differential scanning calorimetry, Raman spectroscopy and first-principles calculations. As expected, these merits impart unprecedented flexibility, reversibility and stability to HiSE-based supercapacitors and lithium-ion batteries. The former delivers a specific energy of 23.54 W h kg−1, while 110.7 W h kg−1 can be reached for the latter, both of which maintain high rate performance and long-term stability.