Reversible Flat to Rippling Phase Transition in Fe Containing Layered Battery Electrode Materials

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Layered sodium transition metal oxides (NaTMO2) are promising cathode materials for sodium-ion batteries.
• The incorporation of iron into the transition metal (TM) layer offers unique structural possibilities not seen in lithium counterparts (LiTMO2).

Purpose of the Study:

• To investigate the spontaneous structural evolution in iron-containing NaTMO2 cathode materials during sodium-ion battery operation.
• To elucidate the mechanism behind the observed TM layer rippling and its impact on electrochemical performance.

Main Methods:

• In situ X-ray diffraction (XRD) to monitor structural changes during cycling.
• Cs-corrected scanning transmission electron microscopy (STEM) for high-resolution imaging of the material's morphology.
• Density functional theory (DFT) simulations to understand the underlying electronic and structural driving forces.

Main Results:

• A novel spontaneous transition metal (TM) layer rippling phenomenon was observed in NaTMO2 cathodes at high voltages.
• The rippling is driven by the softening and distortion of FeO6 octahedra, leading to inhomogeneous interlayer distances.
• This rippling phase results in distinct charge and discharge pathways for sodium ions, creating an unusual hysteresis voltage loop.
• The rippling TM layer demonstrates reversibility, returning to a flat state upon discharge beyond a specific sodium composition.

Conclusions:

• The discovery of the rippling phase provides new insights into the complex structural behavior of layered sodium ion battery cathodes.
• The reversible nature of this structural evolution suggests potential for designing advanced cathode materials with tailored electrochemical properties.
• Understanding this phenomenon is crucial for optimizing the performance and cycle life of sodium-ion batteries.