Detailed Studies of a High-Capacity Electrode Material for Rechargeable Batteries, Li₂MnO₃−LiCo_1/3Ni_1/3Mn_1/3O₂

TLDR

Lithium‑excess manganese layered oxides, represented by the formula zLi₂MnO₃−(1−z)LiMeO₂, are key positive electrode materials for rechargeable lithium batteries and have been shown to undergo structural rearrangements consistent with literature mechanisms. The study aims to investigate the reaction mechanism of LixCo₀.₁₃Ni₀.₁₃Mn₀.₅₄O₂₋δ during charging and discharging across the 4.5 V plateau by preparing samples via electrochemical oxidation/reduction. Samples were prepared by electrochemical oxidation/reduction, and changes in bulk and surface structures were examined using synchrotron X‑ray diffraction, X‑ray absorption spectroscopy, X‑ray photoelectron spectroscopy, and time‑of‑flight secondary ion mass spectrometry. Simultaneous oxygen and lithium removal at the 4.5 V plateau triggers cation migration and generates oxygen molecules that are electrochemically reduced during discharge below 3.0 V, providing extra capacity, while surface‑derived oxygen species such as lithium carbonate form and decompose upon subsequent high‑voltage charging, indicating that reversible capacity partly originates from oxygen redox at the electrode surface.

Abstract

Lithium-excess manganese layered oxides, which are commonly described by the chemical formula zLi2MnO3−(1 − z)LiMeO2 (Me = Co, Ni, Mn, etc. ), are of great importance as positive electrode materials for rechargeable lithium batteries. In this Article, LixCo0.13Ni0.13Mn0.54O2−δ samples are prepared from Li1.2Ni0.13Co0.13Mn0.54O2 (or 0.5Li2MnO3−0.5LiCo1/3Ni1/3Mn1/3O2) by an electrochemical oxidation/reduction process in an electrochemical cell to study a reaction mechanism in detail before and after charging across a voltage plateau at 4.5 V vs Li/Li+. Changes of the bulk and surface structures are examined by synchrotron X-ray diffraction (SXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (SIMS). SXRD data show that simultaneous oxygen and lithium removal at the voltage plateau upon initial charge causes the structural rearrangement, including a cation migration process from metal to lithium layers, which is also supported by XAS. This is consistent with the mechanism proposed in the literature related to the Li-excess manganese layered oxides. Oxygen removal associated with the initial charge on the high voltage plateau causes oxygen molecule generation in the electrochemical cells. The oxygen molecules in the cell are electrochemically reduced in the subsequent discharge below 3.0 V, leading to the extra capacity. Surface analysis confirms the formation of the oxygen containing species, such as lithium carbonate, which accumulates on the electrode surface. The oxygen containing species are electrochemically decomposed upon second charge above 4.0 V. The results suggest that, in addition to the conventional transition metal redox reactions, at least some of the reversible capacity for the Li-excess manganese layered oxides originates from the electrochemical redox reaction of the oxygen molecules at the electrode surface.