Covalency modulation in Co-free high entropy cathodes for enhanced stability and performance in sodium-ion batteries

Sodium-ion batteries are progressively scrutinized for their economic viability and natural abundancy of resources. However, their practical implications are hampered by their limited energy density, primarily stemming from cationic redox reactions in transition-metal based cathodes. Achieving higher energy density via anionic redox activation is one of the promising approach but often compromises structural integrity due to lattice oxygen loss and transition metal migration. In this work, we present a strategy of covalency modulation through low-level Ru⁴⁺ doping in a Na-deficient, Co-free high-entropy (HE) layered cathode. By completely substituting Mn⁴⁺ with Ru⁴⁺ in HE cathode model, we enhance TM–O bond covalency and stabilize the oxygen framework. This effectively balances the trade-off between high capacity and structural stability, enabling reversible anionic redox activity while suppressing irreversible spinel formation and lattice strain. The Ru⁴⁺/Ru⁵⁺ couple improves voltage stability and delivers a high capacity of 146 mAh g⁻¹ (2–4.3 V, C/15), while Raman and OEMS studies confirm minimized surface degradation and oxygen release. Our findings demonstrate that the approach of entropy stabilization combined with targeted covalency tuning can supplemented the cathodes with both enhanced performance and longevity, offering a promising design pathway for next-generation sodium-ion batteries.