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Na3V2(PO4)3 (NVP) has been considered a promising candidate cathode for sodium - ion battery (SIB) applications, owing to its high structural stability and high Na-ion mobility1. In this study, we investigate the structural effects of Mg doping into the NVP framework and investigate a series of Na3+yV2-yMgy(PO4)3 ( y = 0, 0.25, 0.5, 0.75, 1.0) compounds via solid-state nuclear magnetic resonance (ssNMR) techniques in combination with first principles calculations. Notably, ssNMR spectra collected on battery electrode materials usually consist of broad and overlapping signals, due to strong interactions between the nucleus under study and nearby open-shell transition metal ions (here, vanadium (V) species), and are particularly difficult to interpret. First principles DFT calculations are therefore needed to assist the assignment of individual resonances. This is done here using the all-electron gaussian basis set CRYSTAL17 code, which enables us to compute the unpaired electron spin density at the position of the nucleus of interest (directly proportional to the chemical shift) with high accuracy. In particular, by using the spin-flipping technique (Middlemiss et al2) it is possible to break down the observed chemical shift into individual contributions from nearby V species and obtain further information on the arrangement of V/Mg in the cathode structure, with important consequences on Na+ ion dynamics and electrochemical properties. Using hybrid Hartree-Fock/Density Functional Theory methods, 23Na ssNMR shifts were found within 75-400 ppm (at 300 Mhz and room temperature) corresponding to each crystallographic environment surrounding Na atoms in the alpha and beta polymorphs of Na3V2(PO4)3. Understanding the different mechanisms of Na insertion and extraction is important for improving the efficiency, understanding, and reliability of future SIBs.