Rational design of ultrafine FeSe_(2) nanocrystals emb e dde d within hollow mesoporous carbon bowls for potassium-ion batteries with long-term cycling stability and high volumetric capacity

Hollow structures are commonly used to alleviate the mechanical stress on electrode materials and to provide more active sites in potassium-ion batteries(KIBs).Nevertheless,the excessive internal voids within these structures significantly reduce the packing density of particles,resulting in a relatively low volumetric energy density of the fabricated electrodes,which is undesirable for practical use.We designed a hollow mesoporous carbon bowl embedded with ultrafine bis(selanylidene)iron(FeSe 2)nanocrystals(FeSe 2@HMCB)via a controllable impregnation method and subsequent selenization pro-cess for high-performance KIBs.The as-obtained FeSe 2@HMCB can inherit the advantages of conventional hollow carbon-based composites,such as alleviation of volume variation in active materials,abundant ion storage sites,and high electrical conductivity.Simultaneously,the bowl structure has a higher pack-ing density than the conventional hollow structure,resulting in a significant increase in the volumet-ric energy density of the fabricated electrodes.Because of these advantages,the FeSe 2@HMCB exhibits a high,stable reversible capacity of 326 mA h g^(-1) even after 1000 cycles at 0.5 A g^(-1),and excellent rate capacities(182 mA h g^(-1) at 3.0 A g^(-1)).Compared with the hollow structured counterpart,the vol-umetric capacity(mA h cm−3)of FeSe 2@HMCB increased by 60%.Furthermore,a full cell consisting of FeSe 2@HMCB//Prussian blue(PB)exhibits excellent electrochemical performance(99 mA h g^(-1) after 100 cycles at 0.1 A g^(-1)).