Covalent-Bond Engineering Stabilizes Layered Oxide Cathodes for Sodium-Ion Batteries

Developing practicable layered oxide cathodes remains a critical challenge for sodium-ion batteries. Here, by systematically varying Cu contents to tune TM–O covalency in Nay[CuxFe3/9Mn(6/9–x)]O2, we show that an optimal degree of covalency simultaneously boosts energy density and structural resilience. The optimized Na7/9[Cu2/9Fe3/9Mn4/9]O2 delivers 131.6 mAh g–1 at 0.1 C and retains 80% capacity after 500 cycles at 0.5 C. Mechanistically, the enhanced Cu–O covalency converts trivalent Cu from a conventional d8 to a heavily mixed d9L configuration, delocalizing holes across the extended TM–O network, enlarging polaron size, and “flattening the ripple” of local lattice distortions. Side-by-side comparison with an ionic Ni analogue confirms the suppressed cation migration and polarization. Crucially, the covalency-driven stabilization is not unbounded–excessive covalency rigidifies the lattice, defining a covalency threshold for optimal performance. These findings establish covalency modulation within a defined threshold as a predictive design principle for durable layered oxide cathodes.