Integrating N-Doped Porous Carbon-Encapsulated Ultrafine SnO₂ with MXene Nanosheets via Electrostatic Self-Assembly as a Superior Anode Material for Lithium Ion Capacitors

MXene nanosheets (MNSs) with high conductivity, high flexibility, and rich surface functional groups have gained renewed attention due to their potential applications for energy storage devices. However, the low theoretical specific capacity and easy self-aggregation of MNSs severely restrict their practical application. Herein, a superior anode material with a unique hierarchical structure and optimal composition (SnO₂@NDPC/MNSs-2, NDPC = nitrogen-doped porous carbon) for lithium-ion capacitors (LICs) is successfully fabricated by an electrostatic self-assembly strategy. Specifically, the contrary charges of the SnO₂@NDPC and MNSs enable a feasible and convenient route for the preparation of the as-achieved nanocomposite. Moreover, the SnO₂@NDPC with ∼5 nm SnO₂nanoparticles offering a high specific capacity and pores structure can simultaneously prevent the restack of MNSs and withstand the volume expansion during the repeated Li⁺insertion-extraction processes, and the two-dimensional MNSs can improve the overall conductivity of the nanocomposite and facilitate the Li⁺transport kinetics. As a result, the as-prepared anode material displays excellent rate performance and extraordinary cycle stability, with a high specific capacity of 465 mAh g^-1after 500 cycles at 2 A g^-1and 90.2% capacity retention capability. More importantly, an assembled LIC with the biomass-derived nitrogen-rich porous carbon and SnO₂@NDPC/MNSs-2 as respective cathode and anode can deliver a high energy density of 55.9 Wh kg^-1at a high power density of 6097 W kg^-1and excellent cycle performance, making it potential an anode material for practical LICs with high power and energy densities.