γ‑Fe₂O₃ Nanocrystalline Microspheres with Hybrid Behavior of Battery-Supercapacitor for Superior Lithium Storage

Maghemite (γ-Fe₂O₃) nanocrystalline microspheres (MNMs) self-assembled with 52 nm nanocrystals bridged with FeOOH around grain boundaries were formed by solvothermal reaction and thermal oxidation. The unique architecture endows the MNMs with the lithium storage behavior of a hybrid battery-supercapacitor electrode: initial charge capacity of 1060 mAh g^–1 at the 100 mA g^–1 rate, stable cyclic capacity of 1077.9 mAh g^–1 at the same rate after 140 cycles, and rate capability of 538.8 mAh g^–1 at 2400 mA g^–1. This outstanding performance was attributed to the nanocrystal superiority, which shortens the Li⁺ diffusion paths. The mechanism of this hybrid anode material was investigated with experimental measurements and structural analysis. The results indicate that at the first discharge, the MNM nanocrystal microsphere, whose structure can buffer the volume change that occurs during lithiation/delithiation, goes through four stages: Li⁺ insertion in cation vacancies, spinel-to-rocksalt transformation, Li⁺ intercalation of Li_{1.75+<i>x</i>}Fe₂O₃ nanocrystals, and interfacial Li storage around nanocrystal boundaries. Only the latter two stages were reversible at and after the second charging/discharging cycle, exhibiting the hybrid behavior of a battery-supercapacitor with superior lithium storage