Structural Transformation Induced Twinning for Enhanced Conversion Reaction of Vacancy-Ordered Metal Oxides with Li Ions

Metal oxide anode materials based on conversion reaction usually deliver a capacity much higher than that of commercial graphite anodes in lithium-ion batteries (LIBs), and the porous forms of the materials can effectively alleviate volume change associated with (de)lithiation. In this work, tetragonal γ-Fe2O3, which is a vacancy-ordered superstructure containing large vacancy clusters, is studied as the representative of intrinsic nanoporous metal oxide anode materials of LIBs. γ-Fe2O3 exhibits a reversible capacity higher than the theoretical value in initial cycles, but a steady capacity same as that of α-Fe2O3. In lithiation, γ-Fe2O3 first transforms irreversibly to an ordered rock salt Li1-xFe1+xO2 (LiTiO2 type), and the Li1-xFe1+xO2 is converted reversibly into Li2O and Fe0 upon further lithiation: γ-Fe2O3+Li→Li1-xFe1+xO2+Li ↔ Fe0+Li2O. The alternate stacking of dense tetrahedral layers and porous octahedral layers in γ-Fe2O3 enables the simultaneous formation of Li1-xFe1+xO2 phases with different Li contents in a single particle and triggers structure twinning, accounting for fast reaction and likely high capacity in initial cycles. The structural evolution disclosed in γ-Fe2O3 not only updates the understanding of conversion reactions of vacancy-ordered metal oxides, but also offers an innovative approach for the fabrication of twinning structures in metal oxides including cathode materials.