Structural transformation induced twinning for enhanced conversion reaction of vacancy-ordered metal oxides with Li ions

초록

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 gamma-Fe2O3, which is a vacancy -ordered superstructure containing large vacancy clusters, is studied as the representative of intrinsic nano -porous metal oxide anode materials of LIBs. gamma-Fe2O3 exhibits a reversible capacity higher than the theoretical value in initial cycles, but a steady capacity same as that of alpha-Fe2O3. In lithiation, gamma-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: gamma-Fe2O3+Li -> Li1-xFe1+xO2+Li F iota Fe0+Li2O. The alternate stacking of dense tetrahedral layers and porous octahedral layers in gamma-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 gamma-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.

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