A Low-Cost and Environmentally Friendly Mixed Polyanionic Cathode for Sodium-Ion Storage.

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Sodium-ion batteries (NIBs) are crucial for renewable energy, but require novel electrode materials.
• Understanding NIB working mechanisms is key to improving battery performance.

Purpose of the Study:

• To develop and characterize a new mixed polyanionic compound, Na₂Fe(C₂O₄)SO₄·H₂O, as a cathode for NIBs.
• To investigate the electrochemical properties and working mechanisms of the new cathode material.

Main Methods:

• Experimental synthesis and characterization of Na₂Fe(C₂O₄)SO₄·H₂O.
• In situ synchrotron X-ray diffraction and density functional theory (DFT) calculations.
• Electrochemical testing including cycling stability and Coulombic efficiency measurements.

Main Results:

• Na₂Fe(C₂O₄)SO₄·H₂O demonstrated reversible sodium ion insertion/desertion at 3.5 V and 3.1 V.
• The material maintained high performance over 500 cycles with ~99% Coulombic efficiency.
• Fe²⁺/Fe³⁺ redox reactions were identified as the mechanism for electron compensation.

Conclusions:

• Mixed polyanionic frameworks offer promising, low-cost, and environmentally friendly materials for sodium-ion storage.
• The developed Na₂Fe(C₂O₄)SO₄·H₂O cathode exhibits excellent stability and electrochemical performance for NIBs.