A Bio-Inspired Methylation Approach to Salt-Concentrated Hydrogel Electrolytes for Long-Life Rechargeable Batteries

Hydrogel electrolytes hold great promise in developing flexible and safe batteries, but the presence of free solvent water makes battery chemistries constrained by H2 evolution and electrode dissolution. Although maximizing salt concentration is recognized as an effective strategy to reduce water activity, the protic polymer matrices in classical hydrogels are occupied with hydrogen-bonding and barely involved in the salt dissolution, which sets limitations on realizing stable salt-concentrated environments before polymer-salt phase separation occurs. Inspired by the role of protein methylation in regulating intracellular phase separation, here we transform the inert protic polymer skeletons into aprotic ones through methylation modification to weaken the hydrogen-bonding, which releases free hydrogen bond acceptors as Lewis base sites to participate in cation solvation and thus assist salt dissolution. An unconventionally salt-concentrated hydrogel electrolyte reaching a salt fraction up to 44 mol % while retaining a high Na+/H2O molar ratio of 1.0 is achieved without phase separation. Almost all water molecules are confined in the solvation shell of Na+ with depressed activity and mobility, which addresses water-induced parasitic reactions that limit the practical rechargeability of aqueous sodium-ion batteries. The assembled Na3V2(PO4)3//NaTi2(PO4)3 cell maintains 82.8 % capacity after 580 cycles, which is the longest cycle life reported to date. Methylation modification of polymer skeletons releases hydrogen bond acceptors as Lewis base sites to assist salt dissolution, which enables a salt-concentrated hydrogel electrolyte and addresses water-induced parasitic reactions in aqueous batteries.+image

Automated summary

In this study, hydrogel electrolytes with high salt concentration were achieved by modifying the polymer skeletons through methylation to weaken hydrogen bonding and release hydrogen bond acceptors as Lewis base sites to assist salt dissolution. This addresses water-induced parasitic reactions in aqueous batteries and improves their rechargeability.