Unveiling the Microscopic\nOrigin of Ultrahigh Initial\nCoulombic Efficiency of High-Entropy Metal–Organic Frameworks\nfor Sodium Storage

The initial irreversible capacity loss during the first\ncharging\nprocess largely reduces the affordable energy and power density of\nsodium storage devices, and developing advanced materials is the efficient\nway to solve this problem, which is fraught with challenges. Herein,\ninspired by theoretical calculations and the high-entropy concept,\na series of fewer layers of high-entropy metal–organic frameworks\n(FLHE-MOFs) are successfully fabricated, delivering an ultrahigh initial\nCoulombic efficiency (ICE) of 86.1% and excellent cycling performance,\nwhich is far more than that of the other electrode materials (generally\n<70%). Greatly, the storage behavior of high-, medium-, and low-entropy\nMOFs is clarified by theoretical calculations and <i>in-/ex-situ</i> characterization, revealing that Co and Fe species can provide substantial\nsodium storage sites and largely enhance the charge transfer rate,\nwhereas high-entropy effect can enable structural reversibility. Sodium\nion capacitors constructed with FLHE-MOFs as the anode can provide\nan ultrahigh energy density of 121.8 W h kg<sup>–1</sup> (200\nW kg<sup>–1</sup>) and an extremely long-term cycle lifespan.\nThis work not only breaks the limitation of MOF materials with poor\nperformance for sodium storage but also provides an effective strategy\nfor the fabrication and application of high-performance MOF-based\nanode materials with high ICE, in which this idea may also be applied\nin other fields.