Ni-Regulated Energy-Level Matching Enables Enhanced Electronic Delocalization and Fast Na⁺ Kinetics in High-Entropy...

Sodium-ion batteries (SIBs) are promising candidates for large-scale energy storage, yet their development is hindered by sluggish kinetics and severe polarization of cathode materials. Prussian blue analogues (PBAs), despite their open frameworks, suffer from weak -C≡N- covalency and highly localized electronic states, leading to structural instability during Na⁺ insertion/extraction. Herein, entropy engineering is employed to regulate the electronic structure of PBAs by constructing a series of high-entropy Prussian blue analogues (HEPBAs) (HE-FeMnCoMCu, M = Ni, V, Ti, Zn) with a fixed Fe-Mn-Co-Cu backbone and systematically varied fifth metal. We demonstrate that the N-coordination energy levels critically determine the degree of electronic delocalization across the -C≡N- bridges. Among the investigated systems, Ni²⁺ exhibits the optimal energy-level alignment with the C≡N π* states, significantly strengthening metal-ligand-metal electronic coupling and promoting ligand-to-metal and metal-metal charge transfer (LMCT/MMCT). Consequently, the Ni-containing sample shows the strongest and broadest LMCT/MMCT absorption in ultraviolet-visible-near-infrared (UV-vis-NIR) spectra and a more complex local electronic environment in X-ray photoelectron spectroscopy (XPS) analysis. Benefiting from enhanced electronic delocalization, HE-FeMnCoNiCu exhibits reduced polarization, the lowest charge-transfer resistance, and accelerated Na⁺ diffusion kinetics. As a result, it delivers a high reversible capacity of 87.21 mAh g^-1 at 1 A g^-1 and retains 85.45% of its capacity after 1000 cycles at 500 mA g^-1. This work reveals that entropy-driven modulation of N-coordination energy levels fundamentally governs the electronic delocalization and electrochemical kinetics of PBAs, offering a rational design strategy for high-performance SIBs.