Implantation of Fe7S8 nanocrystals into hollow carbon nanospheres for efficient potassium storage.

As a desirable candidate for lithium-ion batteries, potassium-ion batteries (PIBs) have aroused great interest because of their low cost and high power and energy densities. However, the insertion/extraction of K+ with a large radius (1.38 Å) usually bring about the destruction of the electrode materials. Here, ultrafine Fe7S8 nanocrystals are successfully implanted into hollow carbon nanospheres (Fe7S8@HCSs) via a facile solvothermal method and subsequent novel low-temperature sulfurization, which avoid the aggregation of Fe7S8 nanoparticles produced during high-temperature sulfidation. The ultrafine Fe7S8 nanoparticles and hollow carbon spheres can not only buffer the severe expansion/shrinkage of electrode materials caused by the repeated insertion/extraction of K+, but also provide additional accessible pathways for the high-rate K+ transmission. When tested as an anode material for PIBs, Fe7S8@HCSs achieve impressive K+ storage capacity of 523.2 mAh g-1 at 0.1 A g-1 after 100 cycles and remarkable rate capacity of 176.3 mAh g-1 at 5 A g-1. Further, assembling this anode with a K2NiFe(CN)6 cathode yields stable cycling performance, revealing its great potential for large-scale energy storage applications.