Fast sodium intercalation in Na3.41£0.59FeV(PO4)3: A novel sodium-deficient NASICON cathode for sodium-ion batteries

Sodium-ion battery technology is one of the best alternative candidates to the lithium analogue due to the low cost and the abundance of sodium. Extensive research effort is dedicated to the development of low-cost and highperformance cathodes. Here, a new sodium-deficient NASICON material Na-3.41 0.59FeV(PO4)(3) pound is synthesized by a simple sol-gel method. This new material delivers high initial discharge capacity of 170 mAh g(-1) in the voltage range of 1.5-4.4 V vs. Na+/Na, originating from the intercalation of about 3 Na+ per formula unit. Furthermore, when cycled in the range of 2.0-3.8 V vs. Na+/Na, excellent rate capability and outstanding cycle life are obtained. The remarkable electrochemical performances are attributed to the small volume change (2.36 %) during the sodium extraction through a single-phase mechanism proved by in situ X-ray diffraction (XRD). Refined XRD and 23Na solid-state Nuclear Magnetic Resonance (NMR) combined with Density functional theory (DFT) calculations reveal that the sodium extraction during charge process occurs preferably from Na2 sites. Moreover, this new cathode exhibits high sodium diffusion kinetics confirmed by Galvanostatic Intermittent Titration Technique (GITT). These findings highlight the beneficial use of non-stoichiometry in electrodes for batteries and provide rational design of high-performance cathode materials for sodium-ion batteries.