Achieving High-Voltage Sodium-Ion Batteries with Phase-Transition-Free High-Entropy Oxide Cathodes via Altering Local...

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Sodium-ion batteries (SIBs) are a promising alternative to lithium-ion batteries.
• High-entropy oxides (HEOs) offer potential as cathode materials for SIBs due to the "cocktail effect."
• However, HEOs often suffer from irreversible phase transitions and voltage hysteresis at high voltages, limiting their practical use.

Purpose of the Study:

• To develop a phase-transition-free cathode material for SIBs.
• To suppress voltage hysteresis and improve the structural stability of HEOs at high states of charge.
• To enhance the electrochemical performance of sodium-ion battery cathodes.

Main Methods:

• Synthesis of a P2-type fluorine-substituted Na0.85Mn0.4Li0.1Ni0.2Fe0.1Cu0.1Co0.1O1.9F0.1 (PNHEOF) cathode.
• Experimental characterizations (e.g., XRD, SEM, TEM) to analyze material structure and morphology.
• Electrochemical testing (e.g., galvanostatic cycling, rate capability) to evaluate battery performance.
• Theoretical calculations (e.g., DFT) to understand the underlying mechanisms.

Main Results:

• The PNHEOF cathode exhibited phase-transition-free operation within the voltage range of 2.0-4.5 V.
• Fluorine substitution alleviated the covalency of TM-O bonds, suppressing slab migration and voltage hysteresis.
• The material delivered a high specific capacity of 182 mAh g−1 with negligible volume change (0.43%).
• A commercial pouch cell using PNHEOF achieved an energy density of 154 Wh kg−1 with a stable midpoint voltage of 3.2 V.

Conclusions:

• The fluorine-substituted P2-type HEO (PNHEOF) is a highly stable and high-performance cathode material for sodium-ion batteries.
• The strategy of fluorine substitution effectively suppresses phase transitions and voltage hysteresis.
• PNHEOF demonstrates significant potential for practical energy storage applications due to its excellent electrochemical properties and structural integrity.