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

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.