Spatially-confined magnetite nanoparticles for superb potassium-ion storage performance

Abstract

In a potassium-ion battery, concentration gradients of K+ and electrons along electrode thickness usually result in uneven potassiation of electrode material. It is highly possible that the electrode material on the collector side will never reach full stage of charge like that on the separator side especially for thick electrodes. Herein, we develop a unique and huge cobblestone-shaped Fe3O4/C heterostructure in hope to address this problem by converting unidirectional concentration gradient into a three-dimensional one. This structure that comprises labyrinthine carbon skeleton and encapsulated Fe3O4 nanoparticles is propitious to reduce tortuosity beyond particles and accelerate K+ transportation to collector on potassiation. In battery use, Fe3O4 nanoparticles boost K+-storage capability through conversion mechanism, while carbon capsules physically isolate Fe3O4 nanoparticles and serve as nano-reactors for their conversion reactions. Moreover, the carbon skeleton also improves electronic conductivity of the structure and enhances K+-storage capability/cyclability through electrical double-layer capacitive mechanism. As a result, this structure delivers high capacity of ∼406 mAh g−1 at 0.1 A g−1 after 200 cycles and exhibits exceptional cyclability over 700 cycles at 2 A g−1 with capacity of 309 mAh g−1. Besides, the full-cell test shows capacity of ∼112 mAh g−1 at 0.2 A g−1 after 500 cycles.