Spatially confined synthesis of a flexible and hierarchically porous three-dimensional graphene/FeP hollow nanosphere composite anode for highly efficient and ultrastable potassium ion storage

With excellent theoretical capacities, transition-metal phosphides (TMPs) have been recognized as promising anode materials for potassium-ion batteries (KIBs) but there remain considerable challenges in improving the structural stability and electrochemical performance. Moreover, the electrochemical reaction mechanism of TMP-based KIB anodes is still unclear and mostly unexplored. Herein, we elaborately design a flexible and hierarchically porous anode consisting of discrete FeP hollow nanospheres well encapsulated within a three-dimensional graphene skeleton (3DG/FeP) via a novel spatially confined one-step thermal transformation strategy using a 3DG/metal-organic framework (MOF) as the precursor for the first time. The unique architecture provides interconnected porous conducting network and tight contact between graphene and FeP hollow nanospheres as well as sufficient stress buffer nano-hollow spaces to greatly promote the charge transport and maintain the structural integrity. Thus, the 3DG/FeP anode delivers a high reversible capacity of 323 mA h g(-1) at 0.1 A g(-1) and ultrastable cycle performance with a capacity retention of 97.6% at 2 A g(-1) after 2000 cycles, which is almost the best result among all the reported FeP anodes for KIBs. Further detailed characterizations not only elucidate 3DG confinement-promoted microphase separation and nanoscale Kirkendall effects of MOF but also explore the multiple reversible intercalation and conversion processes of K+ insertion/extraction in the FeP component.