A Molecular-Sieving Interphase Towards Low-Concentrated Aqueous Sodium-Ion Batteries.

Aqueous sodium-ion batteries are known for poor rechargeability because of the competitive water decomposition reactions and the high electrode solubility. Improvements have been reported by salt-concentrated and organic-hybridized electrolyte designs, however, at the expense of cost and safety. Here, we report the prolonged cycling of ASIBs in routine dilute electrolytes by employing artificial electrode coatings consisting of NaX zeolite and NaOH-neutralized perfluorinated sulfonic polymer. The as-formed composite interphase exhibits a molecular-sieving effect jointly played by zeolite channels and size-shrunken ionic domains in the polymer matrix, which enables high rejection of hydrated Na ions while allowing fast dehydrated Na permeance. Applying this coating to electrode surfaces expands the electrochemical window of a practically feasible 2 mol kg sodium trifluoromethanesulfonate aqueous electrolyte to 2.70 V and affords NaMnFe(CN)//NaTi(PO) full cells with an unprecedented cycling stability of 94.9% capacity retention after 200 cycles at 1 C. Combined with emerging electrolyte modifications, this molecular-sieving interphase brings amplified benefits in long-term operation of ASIBs.