A Universal Voltage-Enhancement Strategy Based on Multifunctional Ligand-Mediated Modulation for Na-Layered Oxide Cathode.

Sodium-ion batteries (SIBs) have emerged as promising energy storage solutions due to resource abundance and low cost, yet their practical deployment is hindered by limited operating voltages of cathode materials. Herein, we propose a universal ligand-field engineering strategy to enhance voltage through electronic structure modulation in transition metal oxides. Using the Na0.67Mg0.2Mn0.8O2 as a model system, weak-field ligands are introduced to alter the coordination environment of MnO6 octahedra. Experimental and theoretical results demonstrate the weak-field ligands reduce Mn octahedral splitting energy, inducing a downshift of the e g ∗ $e_g^*$ orbital. This shifts the valence band maximum away from the vacuum level, thereby elevating the Mn3+/Mn4+ redox potential with an average voltage increase of 0.24 V. Concurrently, the lowered initial Mn oxidation state induces an increased discharge capacity by 35.7% (153.2 mA h g-1), while suppressing Jahn-Teller distortions and Mn3+ disproportionation yielding 94.4% capacity retention after 200 cycles (versus 69.9% in pristine sample). This ligand-engineering strategy in regulating redox voltage can be extended to other systems with e g ∗ $e_g^*$ -involved redox systems (such as Ni-, Fe-, and Mn-based cathodes). Our work establishes a direct correlation between e g ∗ $e_g^*$ orbital energy modulation and operating voltage, providing foundational insights for designing Na-layered oxide cathodes with high operating voltage.