Hollow Porous\nCo<sub>0.85</sub>Se/ZnSe@MXene Anode\nwith Multilevel Built-in Electric Fields for High-Performance Sodium\nIon Capacitors

Sodium ion capacitors (SICs) are promising candidates\nin energy\nstorage for their remarkable power and energy density. However, the\ninherent disparity in dynamic behavior between the sluggish battery-type\nanodes and the rapid capacitor-type cathodes constrained their performance.\nTo address this, we fabricated a hollow porous Co<sub>0.85</sub>Se/ZnSe@MXene\nanode featuring multiheterostructure, utilizing facile etching and\nelectrostatic self-assembly strategies. The hollow porous structure\nand multiple heterointerfaces stabilize the anode by mitigating the\nvolume changes. Density functional theory (DFT) calculations further\nrevealed that induced multilevel built-in electric fields facilitate\nthe formation of rapid ion diffusion pathways and reduce the Na<sup>+</sup> adsorption energy, thereby boosting Na<sup>+</sup>/electron\ntransport kinetics. The fabricated TA-Co<sub>0.85</sub>Se/ZnSe@MXene\nanode demonstrates outstanding long-term cycling stability of 406\nmA h g<sup>–1</sup> after 1000 cycles at 1 A g<sup>–1</sup>, with an ultrahigh rate performance of 288 mA h g<sup>–1</sup> at 10 A g<sup>–1</sup>. When paired with the active carbon\n(AC) cathode, the SICs deliver extraordinary energy/power densities\nof 144 W h kg<sup>–1</sup> and 12000 W kg<sup>–1</sup>, maintaining over 80% capacity retention at 1 A g<sup>–1</sup> after 10000 cycles. This innovative strategy of engineering multiheterostructured\nanode with the induced multilevel built-in electric fields holds significant\npromise for advancing high-energy and high-power energy storage systems.