Revealing the Na+ (de)intercalation mechanism of Na-free polyanionic K3V3(PO4)4·H2O material as a novel anode for advanced sodium-ion batteries

•Two hydrated vanadium phosphates, KVP-FD and KVP-HD, are used as novel anodes for SIBs to unravel their Na+ intercalation mechanism.•KVP-HD electrode displays a higher discharge capacity, much superior rate capability and faster electrochemical kinetics than KVP-FD.•Ex-situ XRD reveals that robust KVP-HD host framework provides appropriate crystallographic sites for reversible Na+/K+ ion intercalation/deintercalation.•Electrochemical mechanism shifts from a predominant K+ to a mixed Na+/K+ intercalation/deintercalation process.