Surface Lattice Regulation Drives Energy Coupling to Stabilize High-Energy Mn-based Prussian Blue Analogue Cathode

Abstract

Sodium-ion batteries (SIBs) are considered as competitive candidates for energy storage applications due to their abundant resources and low cost. Na₂Mn[Fe(CN)₆] (NaMnPB) is an ideal cathode material for SIBs because of its high theoretical energy density. However, it usually suffers from sluggish reaction kinetic and rapid capacity fading due to manganese (Mn) Jahn-Teller distortion. To address these issues, a surface lattice contraction strategy induced by surface [Fe(CN)₆] vacancies is proposed. The surface [Fe(CN)₆] vacancies lead to a charge imbalance and facilitate d-electron compensation from Fe to Mn, which drives the energy coupling of low-spin Fe²⁺ and high-spin Mn²⁺ and promotes the redox activity of Mn²⁺/Mn³⁺. Consequently, the π-backdonation in the low-spin Fe−C unit and the π-donation in the high-spin Mn−N unit work synergistically, mitigating the continuous electron delocalization of Mn²⁺ and the associated Jahn-Teller distortion. This approach stabilizes the NaMnPB structure while maintaining its inherent high discharge potential without elements doping. The as-prepared NaMnPB□-3 delivers a high specific capacity of 148.97 mAh/g with a corresponding energy density of 464.40 Wh/kg at a current density of 15 mA/g. Furthermore, the full-cell assembled with NaMnPB□-3 and hard carbon demonstrates high energy density, superior rate capability, and excellent cycling performance, indicating its potential for large-scale energy storage systems. This research provides valuable insights into stabilizing high-energy Mn-based cathodes.