Coordination Tuning of [VO₆] and [PO₄] Units Induced Solid-Solution Behavior in Na₃V₂(PO₄)₃ towards Aqueous...

The widespread application of Na₃V₂(PO₄)₃ (NVP) in aqueous sodium-ion batteries is severely limited by irreversible capacity loss caused by vanadium dissolution and structural collapse due to the inherent instability of the V³⁺/V⁴⁺ redox couple and sluggish Na⁺ diffusion. Moving beyond conventional morphology and carbon-compositing strategies, we introduce a coordination engineering strategy through the simultaneous substitution of Zr⁴⁺ at the V site ([VO₆] octahedron) and W⁶⁺ at the P site ([PO₄] tetrahedron), aiming to synergistically stabilize the lattice and optimize the charge transfer. Theoretical and experimental results reveal that Zr/W pairs expand the lattice spacing, promoting rapid Na⁺/electron migration and, more importantly, shifting the Na⁺ storage mechanism from a conventional two-phase reaction to a highly reversible solid-solution process. This transition effectively alleviates phase transition-induced strain and suppresses vanadium dissolution. Consequently, the optimized electrode delivers excellent cycling stability in full cells, retaining 62.6 mAh g^-1 (71% retention) with activated carbon and 84.9 mAh g^-1 (93.8% retention) with sodium titanium phosphate after 1000 cycles at 5.0 A g^-1. This work highlights the power of synergistic anion-cation coordination tuning to overcome fundamental material challenges, establishing a general design principle beyond the NASICON family.