Insights into Reversible Sodium Intercalation in a Novel Sodium-Deficient NASICON-Type Structure:Na 3.40 □ 0.60 Co 0.5 Fe 0.5 V(PO 4 ) 3 .

The rational design of novel high-performance cathode materials for sodium-ion batteries is a challenge for the development of the renewable energy sector. Here, a new sodium-deficient NASICON phosphate, namely Na 3.40 □ 0.60 Co 0.5 Fe 0.5 V(PO 4 ) 3 , demonstrating the excellent electrochemical performance is reported. The presence of Co allows a third Na + to participate in the reaction thus exhibiting a high reversible capacity of ≈155 mAh g -1 in the voltage range of 2.0-4.0 V versus Na + /Na with a reversible single-phase mechanism and a small volume shrinkage of ≈5.97% at 4.0 V. 23 Na solid-state nuclear magnetic resonance (NMR) combined with ex situ X-ray diffraction (XRD) refinements provide evidence for a preferential Na + insertion within the Na2 site. Furthermore, the enhanced sodium kinetics ascribed to Co-substitution is also confirmed in combination with electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT), and theoretical calculation.