8C, retaining 84% of its initial capacity in its micrometer-sized particle morphology. Na 7 V 3 (P 2 O 7 ) 4 was successfully synthesized for the fi rst time using the Na-phosphate fl ux method (see the Experimental Section in Supporting Information for details). Note that excess amount of Na-phosphate were used and then washed with water. [ 27 ] The structural characterization based on Rietveld refi nement of X-ray diffraction (XRD) in Figure 1 a reveals that Na 7 V 3 (P 2 O 7 ) 4 has the C2/ c space group with a = 9.7219(4) Å, b = 8.3185(4) Å, c = 27.6051(11) Å, and β = 86.724(4)°, which is isostructural with the previously known Na 7 Fe 3 (P 2 O 7 ) 4 . [ 27 ] No contamination or second phases are detected in the XRD pattern. Additionally, through the XRD measurement in the air, it is also identifi ed that this material is stable in the air or humid condition without any structural changes or phase separation, unlike most of Na x MO 2 [M = Fe, Mn, Co, Ni, etc.]. [ 28 ] The detailed structure information is summarized in Table T1 (Supporting Information), showing the low structural R -factors ( R p = 7.69%, R I = 7.99%, R F = 7.06%, χ 2 = 8.14%). The morphology of the Na 7 V 3 (P 2 O 7 ) 4 sample was examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), revealing a primary particle size of a few micrometers (inset of Figure 1 and Supporting Information, Figure S1). Figure 2 a shows the schematic crystal structure of Na 7 V 3 (P 2 O 7 ) 4 based on the refi ned information from the XRD pattern. The structure consists of a succession of two layers, where quasi-layer 1 consists of infi nite chains of V(1)[P(1)P(2)O 7 ] 2 , and quasi-layer 2 consists of those of [V(2)P(3)P(4)O 7 ] 2 . Figure 2 b shows the arrangement of VO 6 and P 2 O 7 groups in quasi-layers 1 and 2. This connection of VO 6 and P 2 O 7 creates large tunnels, which serve as 3D Na diffusion paths in the structure. The Na(2), Na(3), and Na(4) sites are interconnected in quasi-layer 1, and each Na (2) and Na(4) ion in quasi-layer 1 is connected with Na(1) ions in quasi-layer 2. A notable feature of the crystal structure is that VO 6 octahedra in Na 7 V 3 (P 2 O 7 ) 4 share every corner with P 2 O 7 , which is rather unique among vanadium-based polyanion compounds. [ 6,8,11,13,19 ] Because the redox potential is signifi cantly affected by the interaction between the transition metal and polyanions and generally increases via the inductive effect, it is expected that Na 7 V 3 (P 2 O 7 ) 4 may exhibit a higher redox potential than other vanadium-based cathode materials.To investigate the de/sodiation mechanism and theoretical redox potential, fi rst-principles calculations of Na 7 V 3 (P 2 O 7 ) 4 were performed considering the extraction of Na ions from symmetrically different Na sites. The Na2, Na3, and Na4 sites were observed to have similar site energies of approximately 3.8 eV; however, that of the Na1 site in quasi-layer 2 was approximately 1 eV higher than those of the other sites (≈4.9 eV) ( Table T2, Suppor...
