Constructing Cyanogen-Substituted High-Transport NaO₂ Diffusion Layers for High-Capacity Sodium-Ion Battery Cathodes

Sodium-ion batteries are potential alternatives in the lithium-ion battery territory. However, developing practical Na⁺ storage devices that offer high capacity, superior rate performance, and durability remains a significant challenge. Herein, a cyanide-to-oxide substitution strategy was developed via a two-step annealing process, converting Prussian blue analogues to a novel Fe-Mn oxide (HNMFO). The obtained material exhibits a unique phase configuration with an expanded Na⁺ diffusion channel and exceptionally ordered transition metal framework, effectively mitigating the intrinsic limitations of conventional cathodes. The HNMFO cathode delivers a high specific capacity of 171.9 mAh g^-1 at 100 mA g^-1 from 2.0 to 4.2 V, surpassing most previously reported Fe-Mn-based analogues. Crucially, the deep Mn³⁺/Mn⁴⁺ redox activity was successfully activated, synergistically enhancing both the capacity and stability. Pseudocapacitive analysis, density functional theory, and ab initio molecular dynamics simulations corroborate that the NaO₂ layer elongation expands the interlayer spacing, contributes to electron delocalization, and reduces the energy barrier for Na⁺ diffusion, thereby accelerating ion transport and enhancing overall electrochemical performance. These theoretical insights align with experimental observations and validate the critical role of structural engineering in optimizing electrode kinetics.