Chemically delithiated Li 0.13 Mn 0.54 Ni 0.13 Co 0.13 O 2¹¤ was synthesized by the reaction of Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 with NO 2 BF 4 . Charge and discharge tests for the Li 0.13 Mn 0.54 Ni 0.13 -Co 0.13 O 2¹¤ /Mg alloy cell showed high discharge capacity of 273 mA h g ¹1 . Since the XANES spectra showed a shift toward lower energy in the transition metals after discharge, the reduction of the main phase was confirmed accompanied with Mg intercalation. Rietveld analysis revealed that the mixed phases of layered rock-salt and Li 2 MnO 3 types were generated by Mg insertion during the discharge process. Lithium ion batteries, LIBs, have been widely used in cellular phones, electronic equipment, and electric cars. However, LIBs suffer from limited energy density and safety concern stemming from overheating. Therefore, rechargeable Mg ion batteries (MIB) using Mg metal anodes attract attention as alternative next-generation batteries. The Mg metal as anode delivers approximately 5 times more volumetric capacity (ca. 3837 mA h cm ¹3) in comparison to the graphite anode of conventional LIB involving two electrons per atom.1 In addition, Mg is the fifth most abundant element in the earth crust and is non-toxic. Since the melting point of Mg metal is significantly higher than that of Li metal, the use of Mg anodes is expected to provide higher operational safety.A 9 Although little is known about the insertion and desorption of Mg in the delithiated material, high capacity such as shown in the LIB is expected for rechargeable Mg batteries as well. The objectives of the present study were to evaluate the electrode properties of delithiated Li 1.2¹x Mn 0.54 Ni 0.13 Co 0.13 O 2¹¤ in a Mg battery configuration, to analyze the crystal and electronic structure changes before and after Mg insertion and to reveal the charge/discharge mechanism from the view point of the valence change of transition metals.We synthesized Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 by using a solidstate reaction via coprecipitated precursors. Stoichiometric amounts of Co(NO 3 ) 2 ¢6H 2 O (Wako Pure Chemical Industries, Ltd., 98%), Mn(NO 3 ) 2 ¢6H 2 O (Wako Pure Chemical Industries, Ltd., 98%) and Ni(NO 3 ) 2 ¢6H 2 O (Wako Pure Chemical Industries, Ltd., 98%) were dissolved in double-distilled water and added dropwise to a 0.5 mol L ¹1 solution of LiOH¢H 2 O. The precipitated powders were carefully filtered and washed, then dried in air at 100°C overnight. The dried transition precursors were mixed with a stoichiometric amount of LiOH¢H 2 O. The obtained precursors were annealed at 600°C in air for 15 h, then calcined at 950°C in air for 15 h and subsequently ground.Chemical oxidation was performed to obtain the Li 1.2¹x -Mn 0.54 Ni 0.13 Co 0.13 O 2¹¤ in non-aqueous environment. The NO 2 BF 4 was added as oxidizing agent so that the concentration was 0.2 M in acetonitrile and the reaction time was 24 h. After the NO 2 BF 4 was dissolved, 0.5 g of Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 was added and the mixture was stirred for 24 h at room temperatu...

Chemically-delithiated Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 is regarded as a potential candidate of cathode active materials for magnesium rechargeable batteries owing to its large deliverable capacity and high operation voltage compared to conventional layered transition metal oxides. Our previous study suggested its chemical composition as Li 0.13 Mn 0.54 Ni 0.13 Co 0.13 O 2−δ by X-ray diffraction combined with XAFS analysis. We herein re-analyzed the substantial composition and crystal structure by employing titration technique and combination of neutron and synchrotron X-ray diffractions. Two topotactic phases both belonging to the space group of R 3m were strongly suggested by Rietveld analysis, and the chemical formula was subsequently re-defined as Li 0.17 Mn 0.72 Ni 0.18 Co 0.18 O 2 where oxygen defects were filled by a rearrangement from C2/m structure. Although the battery performance of that active material was poor in the previous study, the discharge capacity greater than 400 mAh g −1 , ca. 95 % of the theoretical capacity, was achieved by using certain anodically stable electrolytes and specific cell configuration. This result strongly implies that the R 3m structure is particularly suitable as a host material for Mg 2+ intercalation.