Na-ions batteries (NIBs) is greatly hindered by the limited capacity delivery and voltage output due to the high redox potential of Na + /Na and large radius of Na + ions. [1][2][3][4][5] Among the previously exploited cathode materials for NIBs, the polyanion-type cathodes with a Na superionic conductor (NASICON) structure proposed by John B. Goodenough since 1976 have received extensive interests, considering their robust 3D framework, unique inductive effect and thereby enhanced comprehensive performance. [2,3,5] The open framework structure not only furnishes the 3D Na + diffusion pathway, but also renders restricted lattice volume variations during Na + extraction/insertion, which is responsible for the decent rate capability and superior cycle stability. [2,5] Besides, the strong inductive effect and covalent bonds from the polyanionic groups (e.g., PO 4 3− ) allows access to a higher working potential and inhibits oxygen evolution, thus contributing to the increased energy density and high safety of NASICON polyanionic cathodes. [4] Na superionic conductor structured Na 3 V 2 (PO 4 ) 3 cathodes have attracted great interest due to their long cycling lifespan and high thermal stability rendered by the robust 3D framework. However, their practical application is still hindered by the high cost of raw materials and limited energy density. Herein, a doping strategy with low-cost Fe 2+ is developed to activate V 4+ /V 5+ redox, in an attempt to increase the energy density of phosphate cathodes. It is also revealed that reversible activation of V 4+ /V 5+ redox is related to the Na positions (Na1, 6b; Na2, 18e). Only the V-based compounds with enough Na2 content can activate the V 4+ /V 5+ reversibly. More importantly, without presodiation treatment and addition of any sodiation agent, Na 3.4 V 1.6 Fe 0.4 (PO 4 ) 3 is delicately designed as both cathode and the Na self-compensation agent in full cells, allowing a promising energy density of ≈260 Wh kg −1 . This work sheds light on enhancing the energy density, and designing Na self-compensation for practical Na-ions batteries.