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

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.