Superstructure Engineering Enables NASICON-Type Phosphate Cathodes with Increased Working Voltage and Energy Density.

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• NASICON-type iron-based phosphates are promising for sodium-ion batteries (SIBs) due to cost and rate capability.
• Strong Fe─O bonds limit their energy density by lowering the Fe2+/Fe3+ redox potential below 2.5 V.
• Achieving higher energy density requires strategies to increase the working voltage of these phosphate cathodes.

Purpose of the Study:

• To enhance the ionic characteristics of Fe─O bonds and increase the working voltage of Fe2+/Fe3+ redox couples in NASICON-type iron-based phosphates.
• To investigate the effect of superstructure engineering on the electrochemical performance of these cathode materials.
• To demonstrate a method for improving the energy density of phosphate cathodes for SIBs.

Main Methods:

• Superstructure engineering was employed by manipulating calcination temperature during synthesis.
• Advanced structural characterization techniques were used to analyze the material structure.
• Theoretical calculations were performed to understand the electronic structure and ion migration.

Main Results:

• Fe3+ ion migration to Na+ vacancies created Fe/Na_v superstructure ordering.
• Fe delocalization and electronic structure rearrangement led to an enlarged energy gap.
• The Na3VFe(PO4)3 cathode with Fe/Na_v superlattice structure showed increased Fe2+/Fe3+ redox potential (2.82 V) and energy density (350 W h kg-1).

Conclusions:

• Superstructure engineering is an effective strategy to increase the working voltage and energy density of NASICON-type iron-based phosphate cathodes.
• The Fe/Na_v superlattice structure improves the electrochemical performance of these materials for SIB applications.
• This research provides a pathway for developing high-performance phosphate cathodes for next-generation sodium-ion batteries.