Crystal field–driven local structure engineering enables high-voltage redox and structural durability in polyanion cathode for sodium-ion batteries

Abstract

The (Na Super Ionic Conductor) NASICON-type Na<inf>4</inf>MnCr(PO<inf>4</inf>)<inf>3</inf> (NMCP) cathode, while attractive for its high operating voltage, faces critical challenges including sluggish electron transport, unstable cycling behavior, Cr redox inactivity, and structural deterioration. To address these issues, a Ti-substituted derivative, Na<inf>3.55</inf>Mn<inf>0.85</inf>Cr<inf>0.85</inf>Ti<inf>0.3</inf>(PO<inf>4</inf>)<inf>3</inf> (NMCTP), was developed through strategic cation engineering. The partial replacement of Mn and Cr with Ti optimizes the local transition metal coordination environment, activating Cr redox reactions, reinforcing structural integrity, and enhancing both electronic conductivity and Na+ transport kinetics. As a result, NMCTP delivers a high-rate capacity and long-term stability, achieving 82.2 mAh g−1 at 10 C with 80.5 % capacity retention after 2000 cycles. Even under ultrafast cycling at 50 C, it maintains a capacity of 40.7 mAh g⁻1. In-situ X-ray analysis reveals a hybrid Na+ storage mechanism involving solid-solution and biphasic transitions with only 5.6 % volume change, underscoring the structural robustness of the material. when paired with a hard carbon (HC) anode, the NMCTP//HC full cell delivers a discharge capacity of 131.2 mAh g−1 at 80 mA g−1 and achieves a high energy density of 403.7 Wh kg−1 (based on cathode mass). This study demonstrates the efficacy of targeted cation substitution in optimizing polyanionic frameworks and provides a viable route toward high-energy, long-life sodium-ion batteries.