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

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Sodium-ion batteries (SIBs) are crucial for large-scale energy storage but face challenges with cathode material kinetics and polarization.
• Prussian blue analogues (PBAs) offer open frameworks but suffer from weak covalency and localized electronic states, impacting structural stability during sodium ion cycling.

Purpose of the Study:

• To investigate entropy engineering as a method to enhance the electronic structure and electrochemical performance of Prussian blue analogues for SIBs.
• To explore the relationship between N-coordination energy levels, electronic delocalization, and sodium-ion diffusion kinetics in high-entropy Prussian blue analogues (HEPBAs).

Main Methods:

• Synthesis of a series of high-entropy Prussian blue analogues (HEPBAs) with a fixed Fe-Mn-Co-Cu backbone and varied fifth metal (Ni, V, Ti, Zn).
• Characterization using UV-visible-near-infrared (UV-vis-NIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to analyze electronic structure and metal-ligand interactions.
• Electrochemical testing to evaluate sodium-ion insertion/extraction kinetics, polarization, charge-transfer resistance, and cycling stability.

Main Results:

• Entropy engineering successfully modulated N-coordination energy levels, leading to enhanced electronic delocalization across -C≡N- bridges in HEPBAs.
• The Ni-containing HEPBA (HE-FeMnCoNiCu) exhibited optimal energy-level alignment, strengthening metal-ligand-metal coupling and promoting charge transfer.
• HE-FeMnCoNiCu demonstrated reduced polarization, lower charge-transfer resistance, and accelerated Na+ diffusion, delivering a reversible capacity of 87.21 mAh g-1 at 1 A g-1 with 85.45% capacity retention after 1000 cycles.

Conclusions:

• Entropy-driven modulation of N-coordination energy levels is a fundamental mechanism governing electronic delocalization and electrochemical kinetics in PBAs.
• HEPBAs, particularly HE-FeMnCoNiCu, offer a promising strategy for developing high-performance cathode materials for advanced sodium-ion batteries.
• This research provides a rational design approach for optimizing SIB cathode materials through precise control of electronic structure via entropy engineering.