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/deintercalation 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 (similar to 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. (C) 2018 Elsevier Ltd. All rights reserved.