Highly Reversible Anionic Redox without Voltage Decay
Authors: Matsuzaki Kaito , Yabuuchi Naoaki , Masahiro Yasushi 
Published: December 2020

Introduction
High-capacity positive electrode materials are needed to further increase energy density of rechargeable lithium batteries. Recently, Li-enriched materials, Li2
MO3-type layered materials (M = transition metal ions), classified as a cation-ordered rocksalt-type structure, have been extensively studied as advanced positive electrode materials. Among the series of Li2
MO3-type oxides, Li2MnO3 and its derivatives, e.g., Li1.2Ni0.13Co0.13Mn0.54O2, have been the most widely studied, and Li1.2Ni0.13Co0.13Mn0.54O2, delivers a large initial discharge capacity, over 250 mA h g-1 originating from anionic redox. Nevertheless, oxygen is irreversibly released on charge, leading to voltage decay on continuous electrochemical cycles.
In this study, Li2RuO3 which possesses the same crystal structure with Li2MnO3, is targeted as a model material with highly reversible anionic redox. Ru ions have a much a higher covalent nature with oxide ions, and moreover chemical stability with higher oxidation states is relatively high when compared with 3d-transition metal ions. Therefore, unfavorable charge transfer from oxygen to Ru ions on charge is effectively suppressed, resulting in highly reversible anionic redox.1 Through the detailed study on Li2RuO3, the design concept of next generation high capacity Li-excess positive electrode materials with anionic redox is discussed.
Experimental
Li2RuO3 were prepared by conventional calcination method from a mixture of Li2CO3, and RuO2. Acetylene black (HS-100, Denka) was mixed with Li2RuO3 and used as a conductive material. PVdF (#1100, Kureha) was used as a binder. A mixture of Li2RuO3, acetylene black, and PVdF was casted on aluminum foil. Electrochemical properties of the composite electrodes were evaluated in a two-electrode cell. (Type TJ-AC, Tomcell, Japan).
Results and discussion
X-ray diffraction patterns of Li2RuO3 and Li1.2Ni0.13Co0.13Mn0.54O2 are compared in Fig. 1. Both samples were classified as the cation-ordered rocksalt-type structure with a monoclinic symmetry. Electrochemical properties of Li2RuO3 and Li1.2Ni0.13Co0.13Mn0.54O2 in Li cells are also compared in Fig. 2. A characteristic feature of Li1.2Ni0.13Co0.13Mn0.54O2 is found as a voltage plateau associated with anionic redox during an initial charge. An initial discharge capacity of Li1.2Ni0.13Co0.13Mn0.54O2 exceeds >300 mA h g-1 at 50 oC. However, voltage decay is clearly observed on continuous cycles, which presumably associated with gradual and irreversible oxygen loss.
Similarly, Li2RuO3 with the 4d-transition metal ion exhibits a large reversible capacity of >300 mA h g-1, which corresponds to >90% of the theoretical capacity. Moreover, in contrast to Li1.2Ni0.13Co0.13Mn0.54O2, voltage decay is not observed even at 50 oC. This observation indicates high reversibility of anionic redox even at elevated temperatures. From these experimental results of Li1.2Ni0.13Co0.13Mn0.54O2 and Li2RuO3, factors affecting reversibility of anionic redox are discussed in detail.
Reference

Yabuuchi, Chem. Rec.,
19, 692 (2019).

Figure 1