Insights into Reversible Sodium Intercalation in a Novel Sodium-Deficient NASICON-Type Structure:Na3.400.60Co0.5Fe0.5V(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 Na3.400.60Co0.5Fe0.5V(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 & AP;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 & AP;5.97% at 4.0 V. Na-23 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.

Automated summary

A novel sodium-deficient NASICON phosphate with the presence of Co demonstrates excellent electrochemical performance in sodium-ion batteries. It exhibits a high reversible capacity of 155 mAh g(-1), with a preferential Na+ insertion within the Na2 site. Enhanced sodium kinetics due to Co-substitution is confirmed by electrochemical impedance spectroscopy, galvanostatic intermittent titration technique, and theoretical calculation.