Structural and Chemical Evolution of the Layered Li-Excess LixMnO3 as a Function of Li Content from First-Principles Calculations

• Published in: Advanced energy materials Vol. 4; no. 15
• Main Authors: Lee, Eunseok, Persson, Kristin A.
• Format: Journal Article
• Language: English
• Published: Weinheim Blackwell Publishing Ltd 01-10-2014; Wiley Subscription Services, Inc; Wiley
• Subjects: cluster expansion method; ENERGY STORAGE; first-principles calculations; Li-excess materials; Li2MnO3; phase transformations; Principles
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201400498

Li2MnO3 is a critical component in the family of “Li‐excess” materials, which are attracting attention as advanced cathode materials for Li‐ion batteries. Here, first‐principle calculations are presented to investigate the electrochemical activity and structural stability of stoichiometric LixMnO3 (0 ≤ x ≤ 2) as a function of Li content. The Li2MnO3 structure is electrochemically activated above 4.5 V on delithiation and charge neutrality in the bulk of the material is mainly maintained by the oxidization of a portion of the oxygen ions from O2− to O1−. While oxygen vacancy formation is found to be thermodynamically favorable for x < 1, the activation barriers for O2− and O1− migration remain high throughout the Li com­position range, impeding oxygen release from the bulk of the compound. Defect layered structures become thermodynamically favorable at lower Li content (x < 1), indicating a tendency towards the spinel‐like structure transformation. A critical phase transformation path for forming nuclei of spinel‐like domains within the matrix of the original layered structure is proposed. Formation of defect layered structures during the first charge is shown to manifest in a depression of the voltage profile on the first discharge, providing one possible explanation for the observed voltage fade of the Li‐excess materials.
The electrochemical activity, phase transformation, and oxygen stability of LixMnO3 (0 ≤ x ≤ 2) as a function of the Li content are investigated using ab initio calculations and statistical mechanics approaches. In particular, the mechanism of Mn migration from the Mn‐layer to the Li‐layer and the follow‐up process for the formation of spinel‐nucleus is intensively studied.