Ligand-field-mediated d-π* orbital synergy and electron re-localization in a medium-entropy Prussian blue cathode for superior sodium-ion storage

Manganese-based Prussian blue analogues (PBAs) for sodium-ion batteries (SIBs) are plagued by sluggish kinetics and structural instability stemming from Jahn-Teller distortion. Herein, an entropy-mediated Prussian blue (Na_0.98Mn_0.16Fe_0.13Ni_0.71[Fe(CN)₆]_0.90·2.73H₂O, NaMFNHCF) is proposed as cathode material for SIBs. A ‘ligand field-regulated d-π* orbital synergy and electron re-localization’ effect is generated because the unique electronic configuration of Ni2+ enables effective overlap between its e_g orbitals and the empty anti-bonding π orbitals of the cyanide group. This not only forms a highly conjugated and delocalized “Fe(t_2g)–C≡N(π*)–Ni(e_g)” electron network, thus significantly enhancing electron transport dynamics, but also achieves electron re-localization on the Ni–N bonds with stronger rigidity, thereby strengthening structural stability. Thus, the complex phase evolution of “monoclinic + rhombohedral ↔ cubic ↔ tetragonal” can be completely restrained by entropy stabilization and electron re-localization effects. Thus, Na-ion storage in NaMFNHCF is proceeded via a zero-stress solid-solution mechanism using Mn and Fe-ions as redox centers for charge compensation with ultra-fast electron and Na-ion transfer kinetics. Consequently, NaMFNHCF contributes a high initial energy density of 262.2 Wh kg−1, superior rate capability, and ultra-long lifespans of over 6000 and 5000 cycles with ultra-low decay rates of 0.007% and 0.0058% per cycle in half- and full-batteries, respectively.