Nitridation effect on lithium iron phosphate cathode for rechargeable batteries

Area of Science:

• Materials Science
• Inorganic Chemistry
• Electrochemistry

Background:

• Lithium iron phosphate (LiFePO4) is a widely studied cathode material for lithium-ion batteries.
• Improving the electrochemical performance of LiFePO4 remains a key research objective.
• Doping with nitrogen to form oxynitrides is an emerging strategy to enhance material properties.

Purpose of the Study:

• To synthesize and characterize a novel lithium iron oxynitride (Li0.94FePO3.84N0.16) with an olivine structure.
• To investigate the structural and electrochemical properties of the synthesized oxynitride.
• To compare the performance of the oxynitride with the parent LiFePO4 material.

Main Methods:

• Synthesis of LiFePO4 precursor via citrate chemistry.
• Nitridation of the precursor in flowing ammonia at 650 °C.
• Characterization using X-ray and neutron powder diffraction (NPD), elemental analysis, thermal analysis, scanning electron microscopy (SEM), and electrochemical measurements.
• Rietveld refinement of NPD data for structural analysis.
• Bond-valence energy landscape study.

Main Results:

• A novel oxynitride Li0.94FePO3.84N0.16 with olivine structure was successfully synthesized.
• Neutron diffraction revealed preferential occupation of O2 sites by nitrogen.
• The refined structure indicated an Fe oxidation state of 2.2+.
• SEM showed carbon embedding the olivine particles.
• Electrochemical measurements demonstrated enhanced capacity and cyclability compared to unnitrided LiFePO4.
• A 6% decrease in percolation activation energy was observed for the oxynitride.

Conclusions:

• Nitridation of LiFePO4 is an effective method to improve its electrochemical properties.
• The synthesized oxynitride exhibits superior capacity and cyclability.
• The improved performance is attributed to reduced percolation activation energy.
• Pure, well-crystallized olivine phases can be obtained at milder temperatures using this nitridation method.