Electrolyte-Salts Regulated Hydrogen-Bonding Configuration and Interphase Formation Achieving Highly Stable Anode for Rechargeable Aqueous Sodium-Ion Batteries

Hydrogen evolution reactions that cause the alkalization of aqueous electrolytes generally frustrate the structural stability and cycling performance of NaTi 2 (PO 4 ) 3 /C anode material for rechargeable aqueous sodium-ion batteries (ASIBs). Herein, a novel highly concentrated electrolyte with a large hydrogen-evolution overpotential and hydroxide-capture ability is rationally established by incorporating a bifunctional Mg(Ac) 2 additive into a concentrated NaAc aqueous solution. The highly concentrated electrolyte salts (4 m NaAc+3 m Mg(Ac) 2 ) favor regulation on hydrogen-bonding configurations and kinetically shift the hydrogen evolution potential to a lower value of −1.37 V (vs Ag/AgCl). The Mg(Ac) 2 additive plays particular roles in spontaneously capturing hydroxide ions generated during hydrogen evolution reactions on anode surfaces and simultaneously forming a protective Mg(OH) 2 -like interphase. As a result, the unique electrolyte significantly improves the structural stability and cycling performance of NaTi 2 (PO 4 ) 3 /C anode (94.8% capacity retention after 100 cycles at 100 mA·g −1 ). The effect of salt concentration on hydrogen bonding configurations of aqueous electrolytes is investigated with Raman spectroscopy and FTIR spectroscopy. The interphase is identified by coupling EDS mapping, X-ray photoelectron spectroscopy, and X-ray diffraction. This work provides a new strategy for improving the cycling stability of aqueous sodium-ion batteries.