A Mechanically Flexible Necklace‐Like Architecture for Achieving Fast Charging and High Capacity in Advanced Lithium‐Ion Capacitors

Abstract Integration of fast charging, high capacity, and mechanical flexibility into one electrode is highly desired for portable energy‐storage devices. However, a high charging rate is always accompanied by capacity decay and cycling instability. Here, a necklace‐structured composite membrane consisting of micron‐sized FeSe 2 cubes uniformly threaded by carbon nanofibers (CNF) is reported. This unique electrode configuration can not only accommodate the volumetric expansion of FeSe 2 during the lithiation/delithiation processes for structural robustness but also guarantee ultrafast kinetics for Li + entry. At a high mass loading of 6.2 mg cm −2 , the necklace‐like FeSe 2 @CNF electrode exhibits exceptional rate capability (80.7% capacity retention from 0.1 to 10 A g −1 ) and long‐term cycling stability (no capacity decay after 1100 charge–discharge cycles at 2 A g −1 ). The flexible lithium‐ion capacitor (LIC) fabricated by coupling a pre‐lithiated FeSe 2 @CNF anode with a porous carbon cathode delivers impressive volumetric energy//power densities (98.4 Wh L −1 at 157.1 W L −1 , and 58.9 Wh L −1 at 15714.3 W L −1 ). The top performance, long‐term cycling stability, low self‐discharge rate, and high mechanical flexibility make it among the best LICs ever reported.