YIG‐Inspired Fe 3 <i>d</i> Spin Rearrangement to Construct Built‐In Electric Field Achieving Fast‐Charging Layered Cathode for Wide‐Temperature Sodium‐Ion Battery

Abstract O3‐NaNi 1/3 Fe 1/3 Mn 1/3 O 2 cathodes are promising candidates for sodium‐ion batteries benefited of the high theoretical capacity. However, the inherently poor electronic conductivity limits rate performance and aggravates O3‐P3 phase transition, further weakens phase transition reversibility and results in structural degradation. Herein, a localized electronic regulation strategy inspired by yttrium iron garnet (YIG) is employed to address the issue of sluggish electron transport. Specifically, Y 3+ ions are utilized to partially substitute Ni 2+ to rearrange the electronic configuration of Fe 3 d orbitals, which could trigger a transition from high‐spin state ( t 2 g 3 e g 2 ) to low‐spin state ( t 2 g 5 e g 0 ) as well as narrow the band gap due to the asymmetric splitting of the t 2 g ∗ band near the Fermi level. Moreover, the large ionic radius tends to construct a concentration gradient of Y 3+ , thereby generating a long‐range built‐in electric field. Benefiting from the improved electrical conductivity, Y0.25‐NFM performs 64 mAh g −1 reversible capacity at 20 C, also the enhanced reversible phase transitions assist Y0.25‐NFM maintains 80.9% of initial capacity at 1 C for 500 cycles (44.7% for baseline NFM). The thorough understanding of the dual regulatory effects for Y 3+ doping on short &amp; long‐range electronic interactions provides a novel strategy to construct outstanding layered oxide cathodes for advanced SIBs.