displays the XRD patterns of the as-prepared product before and after annealing at 350 °C for 3 h. The diffraction peaks of the product obtained after the hydrothermal treatment can be readily indexed to monoclinic crystalline Na2V6O16·3H2O (space group P21/m) with lattice parameters a = 1.2170 nm, b = 0.3602 nm, c = 0.7780 nm and β = 95.03° (JCPDS No. 16-0601). The product obtained after annealing at 350 °C for 3 h can be indexed to Na1.1V3O7.9 (JCPDS No. 045-0498) [22]. Since oxygen was present in sufficient amounts in air during the calcination process, Na1.1V3O7.9 should not be present. Wang et al. [12] investigated the actual formula of this product and this could be written as Na1.08V3O8 based on their atomic absorption spectrophotometer measurements. In comparison, the XRD patterns for Na2V6O16·3H2O and Na1.08V3O8 are analogical, but the intense diffraction peak is (0 0 1) at 2θ = 11° for the former but rather (0 0 2) at 2θ = 12.5° for the latter.