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 SnO2 nanoparticles 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–1 after 500 cycles at 2 A g–1 and 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–1 at a high power density of 6097 W kg–1 and excellent cycle performance, making it potential an anode material for practical LICs with high power and energy densities.

Lithium-ion capacitors (LICs), by integrating the merits of batteries and supercapacitors, can improve energy/power density compatibly. However, the current understanding of the structure–property–performance relationship has limited the further development of carbon-based electrodes. Here we discuss how the architecture of interlayer channels influences the ion accessibility and selectivity in N-doped porous carbon (NDPC) cathodes. Electrochemical and spectroscopic measurements reveal that large interlayer channels provide high permeability for different anions and rearrange to buffer the volume variation. Conversely, narrow slits show high ion selectivity and suffer from distortion interacting with large anions. The turning point is the tuned channel size after the carbon layer rearrangement in the soaking process. Contrary to the popular opinion that pore size dominates capacitive behavior while d-spacing determines the intercalative process, we demonstrate the importance of interlayer spacing in electrical double-layer mechanism for NDPCs. This size effect is also successfully put into practice for developing advanced LICs.

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