Synthesis, structural, and electrochemical properties of NaCo(PO3)3 cathode for sodium-ion batteries

A new Co-base sodium metaphosphate compound, NaCo(PO 3 ) 3 , has been synthesized here by solid-state method. The crystal structure is refined by the Rietveld method, and the results reveal that NaCo(PO 3 ) 3 has an orthorhombic structure with the space group of P2 1 2 1 2 1 and lattice parameters of a  = 14.2453(2) Å, b  = 14.2306(1) Å, and c  = 14.2603(2) Å. Its typical morphology and chemical composition are confirmed by scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS). The valence states of all elements and the internal/external vibrational modes of NaCoP 3 O 9 compound are measured by X-ray photoelectron and vibrational spectrum, where a typical feature of the (PO 3 ) − polyanion group is observed. Meanwhile, the electrochemical properties of NaCo(PO 3 ) 3 cathode for sodium-ion batteries are also elevated and an initial discharge capacity of 33.8 mAh/g can be obtained at 0.05 C within 1.5–4.2 V. After 20 cycles, a discharge capacity of 26.7 mAh/g can be obtained and a well-kept oxidation–reduction plateau is still observed for NaCo(PO 3 ) 3 cathode, indicating the good reversibility of this metaphosphate electrode.