Electrochemical (de)lithiation of silver ferrite and composites: mechanistic insights from ex situ, in situ, and operando X-ray techniques

Abstract

The structure of pristine AgFeO₂ and phase makeup of Ag_0.2FeO_1.6 (a one-pot composite comprised of nanocrystalline stoichiometric AgFeO₂ and amorphous γ-Fe₂O₃ phases) was investigated using synchrotron X-ray diffraction. A new stacking-fault model was proposed for AgFeO₂ powder synthesized using the co-precipitation method. The lithiation/de-lithiation mechanisms of silver ferrite, AgFeO₂ and Ag_0.2FeO_1.6 were investigated using ex situ, in situ, and operando characterization techniques. An amorphous γ-Fe₂O₃ component in the Ag_0.2FeO_1.6 sample is quantified. Operando XRD of electrochemically reduced AgFeO₂ and Ag_0.2FeO_1.6 composites demonstrated differences in the structural evolution of the nanocrystalline AgFeO₂ component. As complimentary techniques to XRD, ex situ X-ray Absorption Spectroscopy (XAS) provided insight into the short-range structure of the (de)lithiated nanocrystalline electrodes, and a novel in situ high energy X-ray fluorescence nanoprobe (HXN) mapping measurement was applied to spatially resolve the progression of discharge. Based on the results, a redox mechanism is proposed where the full reduction of Ag⁺ to Ag⁰ and partial reduction of Fe³⁺ to Fe²⁺ occur on reduction to 1.0 V, resulting in a Li_1+yFe^IIIFe^II_yO₂ phase. The Li_1+yFe^IIIFe^II_yO₂ phase can then reversibly cycle between Fe³⁺ and Fe²⁺ oxidation states, permitting good capacity retention over 50 cycles. In the Ag_0.2FeO_1.6 composite, a substantial amorphous γ-Fe₂O₃ component is observed which discharges to rock salt LiFe₂O₃ and Fe⁰ metal phase in the 3.5-1.0 V voltage range (in parallel with the AgFeO₂ mechanism), and reversibly reoxidizes to a nanocrystalline iron oxide phase.