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

Abstract

O3-NaNi_1/3Fe_1/3Mn_1/3O₂ 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, Y³⁺ ions are utilized to partially substitute Ni²⁺ to rearrange the electronic configuration of Fe 3d orbitals, which could trigger a transition from high-spin state (t_2g³e_g²) to low-spin state (t_2g⁵e_g⁰) as well as narrow the band gap due to the asymmetric splitting of the t_2g^∗ band near the Fermi level. Moreover, the large ionic radius tends to construct a concentration gradient of Y³⁺, 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 Y³⁺ doping on short & long-range electronic interactions provides a novel strategy to construct outstanding layered oxide cathodes for advanced SIBs.