Charge storage behavior and reaction mechanism of α-Fe2O3 as anodes for aqueous batteries

Highlights: • Formation of resoluble Fe²⁺ is proposed for Fe₂O₃ anode in netural solution. • In alkaline solution of LiOH, Fe₂O₃ is irreversibly reduced to be Fe₃O₄. • High reversible capacity of ∼160 mAh g⁻¹ is reached in LiOH. -- Abstract: α-Fe₂O₃ has been widely reported as negative electrodes in aqueous based energy storage systems. Yet, investigation on its electrochemical behavior and energy storage mechanism is still a desolate field, especially as anodes for batteries. Herein, ring-like α-Fe₂O₃ is synthesized and its charge storage behavior is explored in both neutral and alkaline electrolytes. The as-obtained samples exhibit high capacities with low negative charging/discharging plateaus but poor cycling performance. In Li₂SO₄, a reversible capacity of 90 mAh g⁻¹ at 1 A g⁻¹ is achieved, while it declines quickly to about zero at the first 20 cycles. In Na₂SO₄ and K₂SO₄ with larger cations, the initial capacities are lower and the decay speeds are faster, indicating a cation involved charge storage process. With careful analysis, it is proposed that part of Fe₂O₃ is reduced to be resoluble Fe²⁺ followed by the formation of Fe(OH)₂ around the electrode in low potential range owning to the insertion of (H₃O)⁺, and subsequently converted into FeOOH as precipitation, which would cause irreversible mass loss and hence fast capacity decay. While in alkaline solution of LiOH, a higher reversible capacity of ∼160 mAh g⁻¹ at 1 A g⁻¹ is reached accompanied by better cycling performance and much higher Coulombic efficiency of ∼100%. Yet, the durability is still unsatisfied. With operando Raman, it is found that Fe₂O₃ is irreversibly reduced to be Fe₃O₄ during the discharging process. Structural collapse of Fe₃O₄ during the following cycles should take main responsibility for the poor durability. Hence, how to inhibit mass loss of Fe₂O₃ in neutral solution and structure collapse of Fe₃O₄ in alkaline solution is supposed to be effective strategy to enhance the stability of Fe₂O₃ based electrodes.