Carbon-coated ultrathin metallic V5Se8 nanosheet for high-energy-density and robust potassium storage

Abstract Earth-abundant potassium is a promising alternative to lithium in energy-storage systems, but a pivotal limitation of potassium-ion batteries (KIBs) is their relatively low capacity and inferior cycle stability. Here we report the first synthesis of ultrathin metallic V5Se8 nanosheets embedded in porous carbon (graphene-like V5Se8@C) as a superior anode for KIBs, which achieves a high reversible depotassiation capacity along with unprecedented rate performance and outstanding cycling stability (a reversible depotassiation capacity of 145 mAh g–1 after 800 cycles at 4 A g–1 with 82.9% capacity retention). The impressive performances achieved are attributed to the synergistic contributions of the NiAs-type superstructure, ultrathin nanosheet architecture, sufficient accessible active sites, multi-dimensional electronic/ionic transport pathways and significant pseudocapacitive behaviors. Combined experimental analysis and first-principles calculations reveal fast reaction kinetics, high ionic/electronic conductivity and low diffusion barriers of K-ion in graphene-like V5Se8@C hybrid. Ex-situ characterizations confirm that V5Se8@C electrode undergo a reversible phase-evolution by the sequential intercalation and conversion reactions with synergistic K+-storage mechanisms. Furthermore, by coupling with pre-treated K0.5MnO2 cathode, the full-cell is demonstrated to exhibit large energy density of 160.2 Wh kg–1 with average discharge voltage of 2.2 V and capacity retention of 86% over 200 cycles. These desirable findings demonstrate graphene-like V5Se8@C nanosheets hold great practical application in future grid-scale energy storage.