Abstract: 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 (FeSe₂) nanocrystals (FeSe₂@HMCB) via a controllable impregnation method and subsequent selenization process for high-performance KIBs. The as-obtained FeSe₂@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 packing density than the conventional hollow structure, resulting in a significant increase in the volumetric energy density of the fabricated electrodes. Because of these advantages, the FeSe₂@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 volumetric capacity (mA h cm^-3) of FeSe₂@HMCB increased by 60%. Furthermore, a full cell consisting of FeSe₂@HMCB//Prussian blue (PB) exhibits excellent electrochemical performance (99 mA h g^-1 after 100 cycles at 0.1 A g^-1).

Key words: Potassium-ion batteries, Iron selenide, Bowl-like structure, Mesoporous carbon, Anode material