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

Sodium-ion batteries (SIBs) are prospective and fascinating for stationary energy storage systems (ESS) because of the abudance of sodium (Na) resource and their excellent cycle stability. Exploring for advanced anode materials with robust framework and appropriate crystallographic sites, is of great significance to effectively boost the practical applications of SIBs and to largely decrease the potential safety issues of alloy-type anodes with large volume changes during cycling. Herein, two hydrated vanadium phosphates [K3V3(PO4)(4)center dot H2O, KVP] are delicately prepared by freeze-drying and sol-gel heating drying, denoted as KVP-FD and KVP-HD, respectively, and used as novel anodes for SIBs to reveal their Na+ intercalation mechanism. KVP-HD electrode possesses a much superior discharge capacity and a higher capacity retention (185.6 mAh g(-1), 72.2 %) at 1C for 300 cycles, and more excellent rate capability at high C-rates (greater than1C) than KVP-FD (127.4 mAh g(-1), 45.5 %). Cyclic voltammetry, galvanostatic intermittent titration and electrochemical impedance spectroscopy, manifest a more alleviated polarization, smaller impedance and faster diffusion kinetics of KVP-HD electrode than KVP-FD. ExsituX-ray diffraction (exsitu-XRD) further reveals that polyanionic KVP-HD material possesses a robust structure and provides fast ion diffusion tunnels to reversibly (de)intercalate the Na+/K+ due to the pillar K+ residing and holding the structure. The electrochemical mechanism switch successively from an initial predominant K+ to a subsequent mixed Na+/K+ (de)intercalation in the polyanionic KVP during cycling. This work innovatively lights up the future to use Na-free KVP polyanion as robust framework for advanced anode in SIBs, which gives insightful reference for using Na-free materials for next-generation SIBs anodes towards a much safer and a more cost-effective battery system.