The Principle of Introducing Halogen Ions Into β-FeOOH: Controlling Electronic Structure and Electrochemical Performance.

Coordination tuning electronic structure of host materials is a quite effective strategy for activating and improving the intrinsic properties. Herein, halogen anion (X-)-incorporated β-FeOOH (β-FeOOH(X), X = F-, Cl-, and Br-) was investigated with a spontaneous adsorption process, which realized a great improvement of supercapacitor performances by adjusting the coordination geometry. Experiments coupled with theoretical calculations demonstrated that the change of Fe-O bond length and structural distortion of β-FeOOH, which is rooted in halogen ions embedment, led to the relatively narrow band gap. Because of the strong electronegativity of X-, the Fe element in β-FeOOH(X)s presented the unexpected high valence state (3 + δ), which is facilitating to adsorb SO32- species. Consequently, the designed β-FeOOH(X)s exhibited the good electric conductivity and enhanced the contact between electrode and electrolyte. When used as a negative electrode, the β-FeOOH(F) showed the excellent specific capacity of 391.9 F g-1 at 1 A g-1 current density, almost tenfold improvement compared with initial β-FeOOH, with the superior rate capacity and cyclic stability. This combinational design principle of electronic structure and electrochemical performances provides a promising way to develop advanced electrode materials for supercapacitor.