Structural and electrochemical properties of Na2VMn2(PO4)3/C as cathode materials for sodium-ion batteries

The structural information (including the lattice parameters, atomic sites and coordinates (x, y, z)) of Na2VMn2(PO4)(3)/C has been presented for the first time. It has a monoclinic lattice with the space group of C2/c and cell parameters of a = 12.0494 angstrom, b = 12.6198 angstrom, c = 6.5170 angstrom, beta = 114.7 degrees. The valence states of elements and the vibrational modes of Na2VMn2(PO4)(3)/C are performed by X-ray photoelectron spectroscopy, Raman and FTIR. Furthermore, at 5 mA g(-1), Na2VMn2(PO4)(3)/C as cathode for 5113s shows a reversible discharge capacity of 80.4 mAh g(-1). At 20 mA g(-1), Na2VMn2(PO4)(3)/C shows an initial charge capacity of 46.1 mAh g(-1) with the capacity retentions of 63.7% after 100 cycles. The poor cycle stability may be attributed to its low intrinsic Na+ diffusion coefficient (DNa+) and Jahn-Teller effect during the sodiation/desodiation process.