Reaction mechanisms of NASICON-type Na4MnV(PO4)3/C as a cathode for sodium-ion batteries

NASCION-type Na4MnV(PO4)3/C was synthesized through a sol-gel method. Two Na+ ions can reversibly (de)intercalation from/into the unit structure, with a reversible capacity of 106.7 mAh g-1. The charge-discharge curves show a voltage slope at 3.4 V, and a plateau at 3.6 V. To elucidate the sodium storage mechanisms, the structure evolution and electron transfer are demonstrated using in-situ X-ray diffraction and ex-situ X-ray absorption spectroscopy. It is found that at different stage of the electrochemical process, it undergoes different phase reaction process with different redox couples. A single-phase reaction occurs when the first sodium-ion extracted from Na4MnV(PO4)3 with a V3+/V4+ redox, while a two-phase reaction takes place when the second sodium-ion extracted with a Mn2+/Mn3+ redox. Galvanostatic intermittent titration technique, GITT, indicates the single-phase reaction process shows a faster kinetic compared to the two-phase reaction process. These findings between the kinetics, chemical and structural evolution provide new insight into the sodium storage mechanisms of NASICON-type cathode, and further the understanding of other materials for sodium-ion batteries.