Highly Reversible Anionic Redox without Voltage Decay

Introduction High-capacity positive electrode materials are needed to further increase energy density of rechargeable lithium batteries. Recently, Li-enriched materials, Li 2 M O 3 -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 Li 2 M O 3 -type oxides, Li 2 MnO 3 and its derivatives, e.g ., Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , have been the most widely studied, and Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , 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, Li 2 RuO 3 which possesses the same crystal structure with Li 2 MnO 3 , 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 Li 2 RuO 3 , the design concept of next generation high capacity Li-excess positive electrode materials with anionic redox is discussed. Experimental Li 2 RuO 3 were prepared by conventional calcination method from a mixture of Li 2 CO 3 , and RuO 2 . Acetylene black (HS-100, Denka) was mixed with Li 2 RuO 3 and used as a conductive material. PVdF (#1100, Kureha) was used as a binder. A mixture of Li 2 RuO 3 , 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 Li 2 RuO 3 and Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 are compared in Fig. 1. Both samples were classified as the cation-ordered rocksalt-type structure with a monoclinic symmetry. Electrochemical properties of Li 2 RuO 3 and Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 in Li cells are also compared in Fig. 2. A characteristic feature of Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 is found as a voltage plateau associated with anionic redox during an initial charge. An initial discharge capacity of Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 exceeds >300 mA h g -1 at 50 o C. However