Reversible Activation of V⁴⁺/V⁵⁺ Redox Couples in NASICON Phosphate Cathodes

Abstract

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.

Study on sodium storage properties of manganese‐doped sodium vanadium phosphate cathode materials

“…When charged to 3.7 V, two peaks of V 3+ shift to 524.7 and 517.3 eV, matching well with the oxidation of V 3+ to V 4+ . 43 When further charged to 4.2 V, the above peaks further shift to 525.0 and 517.7 eV, matching well with the further oxidation of V 4+ to V 5+ .…”

“…During the charging/discharging process, only Na2 can be reversibly extracted/intercalated in the Na-O polyhedron with zigzag diffusion routes Na2 → Na1 → Na2. 43 The area of bottleneck gradually shrinks with the release of Na + from 3.25 to 1 (Supporting Information: Table S7), indicating a severer volume shrinkage when V 4+ is further oxidized to V 5+ , making it more difficult for Na + to diffuse. When the Na content is 1.25, the bottleneck area is minimized due to the Jahn-Teller effect leading to abnormal lattice distortion.…”

Partial Modification Strategies of NASICON-Type Na3V2(PO4)3 Materials for Cathodes of Sodium-Ion Batteries: Progress and Perspectives

“…Furthermore, part of them has good reversible V 4+ /V 5+ redox couples. Xu et al 105 proposed a low-cost strategy of Fe 2+ doping into Na 3+x V 2−x Fe x (PO 4 ) 3 (NVFP-x, x = 0, 0.2, 0.4 0.6, 0.8, and 1.0) to activate V 4+ /V 5+ redox to achieve promising capacity (e.g., the best capacity of NVFP-0.4: ∼113 mAh g −1 at 20C) (Figure 7a) and further revealed that V 4+ /V 5+ can be reversibly activated only when V-based compounds have sufficient relative content of Na (2). In all of the samples shown in Figure 7b, low-content Fe-doped NVFP-x (0 < x ≤ 0.4) can better reversibly activate V 4+ /V 5+ redox, because its extra x mol Na + prefers to occupy the Na(2) site, resulting in higher Na(2) uptake.…”

“…(c) The operando XRD contour map of NVFP-0.4 electrode collected at a selected angle range. Reproduced with permission from ref . Copyright 2022 John Wiley and Sons.…”

Substituting inert phosphate with redox-active silicate towards advanced polyanion-type cathode materials for sodium-ion batteries

“…In terms of the chemical and crystal structure of NVP, the building blocks include tetrahedral [PO 4 ], octahedral [VO 6 ] and inserted Na + -ions. 33–36 While Na + and V 3+ components are the sites responsible for Na + -ion storage capability, anionic phosphate (PO 4 3− ) is electrochemically inactive during battery cycling and inert for providing capacity. 37–39 Substituting inert PO 4 3− with tetrahedral redox-active anions within the NVP matrix could improve the material capacity and maintain the crystal structure at the same time.…”