Sodium-ion battery using a NASICON-type Na3V2(PO4)3 cathode: quantification of diffusive and capacitive Na+ charge storage

Abstract

NASICON-type sodium vanadium phosphate (Na₃V₂(PO₄)₃) as a cathode for sodium-ion batteries has attracted widespread research interest due to its high operating voltage (∼3.3 V) and stable three-dimensional structural framework. However, it suffers from low specific capacity due to its poor electronic conductivity and limited redox features. To increase the specific discharge capacity of Na₃V₂(PO₄)₃, structural modifications are necessary. Thus, it is important to probe the influence of synthetic routes on the electrochemical performance of NASICON-type Na₃V₂(PO₄)₃ (NVP). Herein, sodium vanadium phosphate was synthesized using a sol–gel method (NVP-SG) and a solid-state route (NVP-SS). NVP cathodes were tested and examined for laboratory prototype CR-2032 coin-type sodium-ion batteries. The NVP-SG cathode exhibited a passable discharge capacity of 130 mA h g⁻¹ at a 0.1C rate, whereas the NVP-SS cathode delivered a high discharge capacity of 160 mA h g⁻¹ at a 0.1C rate. The detailed charge storage modes of NVP synthesized through solid-state (NVP-SS) and sol–gel (NVP-SG) synthesis were examined by means of Dunn's analysis. Dunn's analysis confirmed that the charge storage is dominated by the diffusive mode at the peak potential region and the capacitive mode at the non-peak potential regions.