Understanding the superior sodium-ion storage in a novel Na3.5Mn0.5V1.5(PO4)3 cathode

Na3V2(PO4)3 is one of the most promising cathodes for sodium-ion batteries (SIBs) due to its three-dimensional framework. To meet practical feasibility, the substitution of vanadium with low-cost active elements is urgent. Here the Na+ superionic conductor (NASICON)-type Na3.5Mn0.5V1.5(PO4)3 nanoparticles homogeneously embedded in porous carbon matrix are prepared and investigated as a novel cathode for SIBs. The as-prepared Na3.5Mn0.5V1.5(PO4)3/C could deliver a desirable average discharge potential of 3.43 V vs. Na+/Na with fascinating rate capability of 92.7 mA h g−1 at 60C and impressive capacity retention of 87.2% after 4000 cycles at 20C. In situ X-ray diffraction reveals that the electrochemical process undergoes highly reversible biphasic transition and single phase change with a relatively small volume change (8.21%), ensuring the high structural stability and excellent cyclic capability. The reversible evolution of Mn2+/3+ and V3+/4+ redox couples upon Na+ extraction/insertion has been revealed by the confirmation of valence state via both ex situ X-ray absorption near-edge structure spectra and X-ray photoelectron spectroscopy. The superior performance of Na3.5Mn0.5V1.5(PO4)3 could be attributed to the high electronic/ionic conductivity and low sodium-ion diffusion energy barrier, which is supported by the electrochemical impedance spectroscopy and cyclic voltammetry measurements, as well as density functional theory computations.