Unexpectedly high electrochemical performances of a monoclinic Na2.4V2(PO4)(3)/conductive polymer composite for Na-ion batteries

We report a monoclinic Na2.4V2(PO4)(3)/poly(3,4-ethylenedioxythiophene) (M-NVP/PEDOT) composite as a novel cathode material for sodium-ion batteries. The active materials were homogenously encapsulated by thin layers of the electro-conductive polymer PEDOT, and X-ray diffraction analysis indicated that the final products were crystallized into a monoclinic structure. The composite exhibited much more outstanding power capability and cyclability than pristine M-NVP. The discharge capacity of the M-NVP/PEDOT electrode was maintained up to similar to 109.8 mA h g(-1) at 10C (1C = similar to 178 mA g(-1)), which was similar to 9 times higher than that of pristine M-NVP under the same conditions. Moreover, whereas severe capacity degradation was observed for pristine M-NVP, the M-NVP/PEDOT composite delivered similar to 81% capacity retention compared with the initial capacity over similar to 500 cycles at 1C. In particular, the available capacity of M-NVP/PEDOT was similar to 143.3 mA h g(-1), corresponding to similar to 2.4 mol Na+ (de)intercalation, which is larger than the reversible capacity of rhombohedral Na3V2(PO4)(3), at which only 2 mol Na+ can be (de)intercalated. This unexpectedly high capacity of the M-NVP/PEDOT composite was clearly confirmed through combined studies using several experiments and first-principles calculation.