Mechanistic Insights into Enhanced Capacity and Pure-Phase Formation in Fe-Based Mixed Phosphate Cathodes.

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Sodium-ion batteries (SIBs) are a cost-effective alternative to lithium-ion batteries.
• Fe-based mixed phosphates like Na4Fe3(PO4)2P2O7 show promise as SIB cathodes.
• Limitations include impurity phases and insufficient iron (Fe) redox activity, hindering capacity.

Purpose of the Study:

• To enhance the capacity of Fe-based phosphate cathodes for SIBs.
• To address limitations of impurity phases and insufficient Fe redox activity.
• To establish a design metric for high-capacity cathode materials.

Main Methods:

• Introduced an electroactive coefficient (η = C/I) as a cathode design metric.
• Employed vanadium (V) doping to improve multielectron transfer and suppress impurities.
• Synthesized and characterized optimized Na3.6Fe2.6V0.4(PO4)2P2O7 cathode material.

Main Results:

• The electroactive coefficient (η) was used to guide material design, increasing from 0.72 to 0.85 with V doping.
• V doping induced a high-spin-to-low-spin transition in Fe2+, shortening Fe-O bonds and increasing Fe-vacancy formation energy.
• The optimized cathode achieved a record capacity of 124.6 mAh g-1 at 0.1 C, with suppressed impurity phases.

Conclusions:

• Vanadium doping is an effective strategy to enhance the electrochemical performance of Fe-based phosphate cathodes.
• The electroactive coefficient provides a valuable metric for designing high-capacity electrode materials.
• This work demonstrates a pathway to phase-pure, high-capacity cathode materials for advanced sodium-ion batteries.