Incorporation of Fe3+ into MnO2 birnessite for enhanced energy storage: Impact on the structure and the charge storage mechanisms

Abstract

Birnessite δ-MnO2, with its low cost, high theoretical capacity, and stable cycling performance in aqueous electrolytes, holds promise as an electrode material for high-power and cost-effective electrochemical energy storage devices. To address its poor electronic conductivity, we incorporated environmentally friendly iron into birnessite and conducted a comprehensive study on its influence on crystal structure, electrochemical reaction mechanisms, and energy storage performance. In this study, a series of birnessite samples with varying iron content (δ-Mn1-xFexO2 with 0 ≤ x ≤ 0.20) were synthesized using solid-state reactions, resulting in well-crystallized particles with micrometric platelet morphology. Through X-ray absorption and Mössbauer spectroscopies, we clearly demonstrated that Fe replaces Mn in the metal oxide layer, while X-ray diffraction revealed that iron content significantly affects interlayer site symmetry and the resulting polytype. The sample with the lowest iron content (δ-Mn0.96Fe0.04O2) exhibits a monoclinic birnessite structure with an O-type interlayer site, while increasing iron content leads to hexagonal symmetry with P-type interlayer sites. Electrochemical investigations indicated that these P-type sites facilitate the diffusion of partially hydrated alkaline ions and exhibit superior rate capabilities compared to the O-type phase. Furthermore, operando XAS revealed that Fe is electrochemically inactive and that the charge storage in birnessite-type phases in a 0.5M K2SO4 electrolyte primarily relies on the redox reaction of Mn. Finally, we determined that P-type δ-Mn0.87Fe0.13O2 achieved the best compromise between enhancing electrical conductivity and maintaining a maximum content of electrochemically active Mn cations.

Manganese–cobalt geomimetic materials for supercapacitor electrode

“…Due to the intercalation mechanisms that can take place in the interlayer space during cycling in neutral electrolyte, 55,56 capacity values of 72 and 96 F g −1 , respectively, are obtained for birnessite in K 2 SO 4 and Na 2 SO 4 . Although larger specific surface area values were measured for asbolane (46 m 2 g −1 ) compared to the initial structure (28 m 2 g −1 ), much lower capacity performance was obtained in K 2 SO 4 (54 F g −1 ) and in Na 2 SO 4 (65 F g −1 ).…”