Insights into Reversible Sodium Intercalation in a Novel Sodium-Deficient NASICON-Type Structure:Na_3.40□_0.60Co_0.5Fe_0.5V(PO₄)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□_0.60Co_0.5Fe_0.5V(PO₄)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. ²³Na 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.