Engineering of yolk-shelled FeSe2@nitrogen-doped carbon as advanced cathode for potassium-ion batteries

Potassium-ion batteries (KIBs) have become the most promising alternative to lithium-ion batteries for large-scale energy storage system due to their abundance and low cost. However, previous reports focused on the intercalation-type cathode materials usually showed an inferior capacity, together with a poor cyclic life caused by the repetitive intercalation of large-size K-ions, which hinders their practical application. Here, we combine the strategies of carbon coating, template etching and hydrothermal selenization to prepare yolk-shelled FeSe2@N-doped carbon nanoboxes (FeSe2@C NBs), where the inner highly-crystalline FeSe2 clusters are completely surrounded by the self-supported carbon shell. The integrated and highly conductive carbon shell not only provides a fast electron/ion diffusion channel, but also prevents the agglomeration of FeSe2 clusters. When evaluated as a conversion-type cathode material for KIBs, the FeSe2@C NBs electrode delivers a relatively high specific capacity of 257 mAh/g at 100 mA/g and potential platform of about 1.6 V, which endow a high energy density of about 411 Wh/kg. Most importantly, by designing a robust host with large internal void space to accommodate the volumetric variation of the inner FeSe2 clusters, the battery based on FeSe2@C NBs exhibits ultra-long cycle stability. Specifically, even after 700 cycles at 100 mA/g, a capacity of 221 mAh/g along with an average fading rate of only 0.02% can be retained, which achieves the optimal balance of high specific capacity and long-cycle stability.