Phase segregation and nanoconfined fluid O₂ in a lithium-rich oxide cathode

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Lithium-rich (Li-rich) oxide cathodes offer high energy density but suffer from capacity fade.
• Atomic disordering and nanoscale structural rearrangements are key challenges in understanding Li-rich cathode degradation.
• Characterizing these dynamic processes in Li-rich materials is experimentally difficult.

Purpose of the Study:

• To elucidate the kinetics and thermodynamics of structural changes in Li-rich oxide cathodes.
• To identify the mechanisms responsible for energy density loss during cycling.
• To provide insights for stabilizing these advanced cathode materials.

Main Methods:

• Utilized a combined approach of ab initio molecular dynamics (AIMD) and cluster expansion-based Monte Carlo (CE-MC) simulations.
• Investigated an exemplar layered Li_1.2-xMn_0.8O₂ cathode material.
• Analyzed atomic rearrangements and phase segregation phenomena.

Main Results:

• Identified a mechanism for bulk oxygen (O₂) molecule formation involving manganese (Mn) migration and interlayer oxygen dimerization.
• Observed local phase segregation into MnO₂-rich regions and Mn-deficient nanovoids at high charge states.
• Found O₂ molecules forming a nanoconfined fluid within interconnected nanovoids, suggesting a pathway for oxygen transport.

Conclusions:

• The study reveals a bulk mechanism for O₂ formation and nanovoid generation in Li-rich cathodes.
• These nanovoids and associated O₂ formation are linked to capacity fade and potential surface O₂ loss.
• Developing strategies to kinetically stabilize the bulk structure is crucial for maintaining the high energy density of Li-rich O-redox cathodes.