Inner-Stress-Optimized High-Density Fe3O4 Dots Embedded in Graphitic Carbon Layers with Enhanced Lithium Storage.

The volume variation of electrode materials will lead to poor cyclability of lithium-ion batteries during the lithiation/delithiation process. Instead, inner-stress fragmentation is creatively used to change carbon-layer-capped Fe3O4 particles ∼30 nm in diameter into high-density Fe3O4 dots ∼4 nm in size embedded in ultrathin carbon layers. The optimized structure shows a remarkable 45.2% enhancement of lithium storage from 804.7 (the 10th cycle) to 1168.7 mA h g-1 (the 250th cycle) at 500 mA g-1, even retaining 1239.5 mA h g-1 after another 550 cycles. The electrochemical measurements reveal the enhanced capacitive behavior of the high-density Fe3O4 dots@C layers, which have more extra active sites for the insertion/extraction of Li+ ions, confirmed by the differential capacity plots, leading to remarkably increased specific capacity during cycling. The restructured electrode also shows a superior rate capacity and excellent cycling stability (938.7 and 815.4 mA h g-1 over 2000 cycles at 1000 and 2000 mA g-1, respectively). X-ray photoelectron spectroscopy and transmission electron microscopy characterizations show that the optimized structure has stable structural and componential stability even at large rates. This work presents an MOF-guided synthesis of high-density Fe3O4-dots' anode material optimized by inner-stress fragmentation, showing a feasible route to design high-efficiency electrode materials.