Abstract
Sodium superionic conductor (NASICON) cathodes are attractive for Na-ion battery applications as they exhibit both high structural stability and high sodium ion mobility. Herein, a comprehensive study is presented on the structural and electrochemical properties of the NASICON-Na3+yV2−yMny(PO4)3 (0 ≤ y ≤ 1) series. A phase miscibility gap is observed at y = 0.5, defining two solid solution domains with low and high Mn contents. Although, members of each of these domains Na3.25V1.75Mn0.25(PO4)3 and Na3.75V1.25Mn0.75(PO4)3 reversibly exchange sodium ions with high structural integrity, the activity of the Mn3+/Mn2+ redox couple is found to be absent and present in the former and latter candidate, respectively. Galvanostatic cycling and rate studies reveal higher capacity and rate capability for the Na3.75V1.25Mn0.75(PO4)3 cathode (100 and 89 mA h g−1 at 1C and 5C rate, respectively) in the Na3+yV2−yMny(PO4)3 series. Such a remarkable performance is attributed to optimum bottleneck size (≈5 Å2) and modulated V- and Mn-redox centers as deduced from Rietveld analysis and DFT calculations, respectively. This study shows how important it is to manipulate electronic and crystal structures to achieve high-performance NASICON cathodes.