Sodium intercalation/de-intercalation mechanism in Na₄MnV(PO₄)₃ cathode materials

Na₄MnV(PO₄)₃ is a sodium ion conducting material with a NASICON type crystal structure. This phase is not much known as an electrode material. The present work focuses on the sodium ion intercalation/de-intercalation mechanism and charge/discharge behavior of the material. The Na₄MnV(PO₄)₃ is synthesized through a sol-gel process and characterized by XRD, SEM, and XPS. The structural analysis confirms the formation of a phase pure crystalline material with nanometric particle size which adopts a trigonal crystal structure. Galvanostatic intermittent titration technique (GITT) measurements indicate that Na₄MnV(PO₄)₃ is electrochemically active having slanting voltage plateaus. Ex-situ and In-situ XRD analysis, as a function of sodium concentration, indicate that the intercalation/de-intercalation of sodium is associated with a single-phase reaction rather than a biphasic reaction when cycled between 1.5 and 4.5 V. The electrochemical measurements on composite electrodes, Na₄MnV(PO₄)₃/CNTS (1 & 3 wt.%), show promising charge/discharge capacity (∼140 mAh/g), good cyclability (100% capacity retention after 40 cycles) and reasonable rate capability. The cyclic voltammetry (CV) and X-ray Photoelectron Spectroscopy (XPS) analyses indicate that the main contributions towards the activity of Na₄MnV(PO₄)₃ can be attributed to the active of Mn²⁺/Mn³⁺ and V³⁺/V⁴⁺ redox couple with partial activity of V⁴⁺/V⁵⁺. The obtained results suggest that Na₄MnV(PO₄)₃ is a promising electrode material which can be achieved better rate performance with long cycling stability and battery performance through engineering of the particle morphology and microstructure.