Fictitious phase separation in Li layered oxides driven by electro-autocatalysis

Area of Science:

• Materials Science
• Electrochemistry
• Chemical Engineering

Background:

• Layered oxides are crucial lithium-ion battery electrodes, typically operated within single-phase composition ranges to avoid phase transitions.
• Operando diffraction studies have indicated phase separation during delithiation in porous electrodes, but this observation is inconsistent and not seen during lithiation.

Purpose of the Study:

• To investigate the anomalies observed in layered oxide electrodes during cycling, specifically the apparent phase separation during delithiation.
• To propose and validate a new model explaining these phenomena as a dynamical artifact rather than a true phase transition.

Main Methods:

• Experimental validation using the single-phase material Liₓ(Ni₁/₃Mn₁/₃Co₁/₃)O₂ (0.5 < x < 1).
• Utilizing operando diffraction and nanoscale oxidation-state mapping.
• Employing a population-dynamics model driven by autocatalytic electrochemical reactions.

Main Results:

• Demonstrated that apparent phase separation is a dynamical artifact caused by autocatalytic electrochemical reactions, where interfacial exchange current increases with delithiation.
• Validated the population-dynamics model experimentally, showing consistency across different transition-metal compositions.
• Confirmed that this fictitious phase separation is a repeatable non-equilibrium effect.

Conclusions:

• The study reveals that apparent phase separation in layered oxide electrodes is a dynamical artifact driven by electro-autocatalysis, not a material phase transition.
• Highlights the critical role of population dynamics and non-equilibrium effects in battery electrode behavior.
• Provides a new framework for understanding and potentially controlling ensemble stability in battery materials.