Reversible Conversion Reactions of Mesoporous Iron Oxide with High Initial Coulombic Efficiency for Lithium-Ion Batteries

Conversion reaction-based transition metal oxides have been considered as advanced anode materials for lithium batteries because of their high storage capacities; however, the initial lithiation/delithiation mechanism remains poorly understood. In this study, we synthesized single-crystalline spindle-type mesoporous Fe2O3 (MS-Fe2O3), which contained a high fraction of textural porosity that appears as a unique tunnel structure. The MS-Fe2O3 electrode exhibited a remarkably high initial Coulombic efficiency of 85.4% and stable cycling performance with a specific capacity of 1250 mA h g(-1) after 100 cycles. During the lithiation process, the initial alpha-Fe2O3 phase was transformed to nanograin-Fe embedded in the (LiO)-O-2 matrix, while subsequent delithiation changed the Fe phase into gamma-Fe2O3. Despite the initial irreversible phase conversion, a reversible electrochemical reaction (Fe3+ -> Fe-0 -> Fe3+) was retained in the first cycle, leading to the high ICE and discharge capacity. This study provides crucial information on the lithiation/delithiation mechanism of transition metal oxides and benefits the design of advanced materials for lithium batteries.