Electrostatically Fabricated Three-Dimensional Magnetite and MXene Hierarchical Architecture for Advanced Lithium-Ion Capacitors.

Conversion-type magnetite shrewdly shows abundance, nontoxicity, and high lithium storage capacity. However, either pristine magnetite or nanocomposites with two-dimensional materials cannot prevent restacking, pulverization, and poor structural homogeneity simultaneously because of a lack of universal interfacial interactions. Here, an electrostatic self-assembly strategy is uncovered between hollow Fe3O4/C microspheres (with H+-induced quasi-intrinsic positive charge) and few-layer MXenes (with intrinsic negative charge from terminating functionalities). This strategy realizes the uniform and interconnected architecture of Fe3O4/C@MXene that favors fast Li+ diffusion, easy electron/charge transfer, and suppressed pulverization. Specifically, after the long-term cycling, an undegraded specific capacity of 907 mA h g-1 remains at 0.5 A g-1. Further adoption of such superior anode in 4.0 V lithium-ion capacitors results in a high energy density of 130 W h kg-1, a maximum power density of 25,000 W kg-1, and excellent cycling stability. This work thus sheds light on a generic self-assembly process where intrinsic electrostatic interaction plays an essential role.