Low Concentration DMF/H2O Hybrid Electrolyte: A New Opportunity for Anode Materials in Aqueous Potassium-Ion Batteries.

Superconcentrated "water-in-salt" electrolytes have greatly widened the electrochemical stable window (ESW) of aqueous electrolytes, but they also generate new problems, including high costs, high viscosity, and low conductivity. Here we report a 2 m low concentration electrolyte using an N,N-dimethylformamide/water (DMF/H2O) hybrid solvent, which provides a wider ESW (2.89 V) than an aqueous electrolyte (2.66 V) and presents nonflammability, high conductivity, and low viscosity characteristics. In 2 m DMF/H2O hybrid electrolyte, the LUMO energy of the DMF solvent (-0.00931 a.u.) is lower than that of H2O (-0.00735 a.u.), which could effectively promote the degradation of FSI- and lead to stable solid electrolyte interphase formation. As a result, the electrochemical reversibility and cyclability of the KTi2(PO4)3@C (KTP@C) anode in the aqueous electrolyte have been significantly enhanced with the help of DMF addition. Moreover, the K2Zn3(Fe(CN)6)2 (KZnHCF)//KTP@C full potassium-ion battery exhibits highly efficient stability and rate capability with a long cycle performance over 10 000 cycles and delivers a specific discharge capacity of 33 mAh g-1 at a high current density of 20 A g-1. Low concentrations of DMF/H2O hybrid electrolytes can inhibit the hydrogen evolution reaction of aqueous electrolytes, providing more opportunities for the practical application of electrode materials. Not limited to DMF solvent, mixing organic and aqueous solvents will provide more available options and perspectives for improving the energy density and long cycle performance of the aqueous metal-ion battery.