Open frameworks in the NaMn(P₂O₇)F fluoro-pyrophosphates system.

Area of Science:

• Solid-state chemistry and materials science.
• Inorganic synthesis and crystal structure determination.
• Ion conductivity and energy storage materials.

Background:

• Development of novel materials for sodium-ion batteries and solid-state electrolytes is crucial for energy storage advancements.
• Fluoro-pyrophosphate compounds offer potential as alternative electrolyte materials due to their structural diversity and ionic conductivity properties.
• Understanding structure-property relationships in these materials is key to designing efficient sodium-ion conductors.

Purpose of the Study:

• To synthesize and characterize new sodium manganese fluoro-pyrophosphate compounds.
• To investigate the crystal structures and sodium ion conductivity of the synthesized materials.
• To analyze the sodium ion diffusion pathways and activation energy in the most promising compound.

Main Methods:

• Synthesis of novel compounds via high-temperature flux method using specific precursor mixtures.
• Structural characterization using single-crystal X-ray diffraction (XRD).
• Estimation of Na+ ion conductivity and activation energy, coupled with bond valence energy landscape analysis for diffusion pathway investigation.

Main Results:

• Three new compounds, Na5.5Mn1.5(P2O7)2F0.5 (I), Na7Mn0.75(P2O7)2F2 (II), and Na9Mn3(P2O7)4F (III), were successfully synthesized.
• Crystal structures revealed diverse arrangements, including a shell-like structure for II and 3D frameworks for I and III with Na+ ions in 1D tunnels.
• Compound III showed a Na+ ion conductivity of 10^-5 S cm^-1 at 400 °C with a high activation energy of 0.89 eV, indicating limited conductivity compared to state-of-the-art materials.

Conclusions:

• The synthesized sodium manganese fluoro-pyrophosphates exhibit complex crystal structures with potential for ion transport.
• Compound III demonstrates a low Na+ ion conductivity and high activation energy, suggesting it is not a high-performance solid electrolyte in its current form.
• Further research into structural modifications or alternative compositions may be necessary to enhance ionic conductivity for practical applications.