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Sodium-ion batteries (SIBs) are attractive alternatives for electrochemical energy storagedue to sodium’s natural abundance and cost-effectiveness when compared to theirlithium counterparts. Among the various Na-ion cathode classes, sodium superionicconductor (NASICON) structured Na3V2(PO4)3 (NVP) has emerged as a promising candidatefor SIB applications owing to its high structural stability, 3−D Na+ diffusionnetwork, and high operating voltage of 3.4 V vs. Na+/Na0. Although promising, theimpact of temperature dependent Na+/ vacancy ordering transitions, potential cationmigration, and influence of phase separation upon Na (de)intercalation on Na+ diffusionis still poorly understood.Here, we focus on the effects of isovalent (Al3+) and aliovalent (Mg2+) substitutionfor V into the NVP framework and investigate a series of Na3+yV2−yMgy(PO4)3 (Mg-NVP) (y = 0−1.0) and Na3V2−yAly(PO4)3 (Al-NVP) (y = 0, 0.5) cathodes to betterunderstand the impact of V by substitution on the Na (de)intercalation mechanics bycomplementing short and long range characterization techniques such as synchrotronX-ray diffraction, 23Na, 31P, 51V solid-state NMR, and scanning electron microscopy(SEM) with first principles calculations to unravel the complex electrochemical cyclingbehavior of these materials. We demonstrate the vastly different effect of each dopant onelectrochemical performance with an emphasis on (de)intercalation mechanics and causesof structural degradation during high voltage cycling.In the case of Mg-NVP, Na extraction and reinsertion results in a two-phase reactionmechanism when y < 0.5 and transitions to a solid-solution mechanism abovey = 0.5, when cycled over a potential window of 3.8−2.75 V. Conversely, when Al3+is introduced into the NVP framework, it exacerbates the formation of an intermediaryNa2.24V1.5Al0.5(PO4)3 phase leading to successive biphasic reactions during Na(de)intercalation, in stark contrast to the solid-solution mechanism that emerges at higherMg concentrations. It is found that the formation of the intermediary phase in Al-NVPhelps reduce the propensity for particle cracking during long term cycling and lowersoverpotentials associated with Na extraction during electrochemical cycling. Thus, wehave highlighted the significant role of transition metal dopants to modify the reactionmechanisms associated with Na (de)intercalation in NASICON structured Na3V2(PO4)3.