ORIGINAL PAPER Formation of stable phases of the Li–Mn–Co oxide system at 800 °C under ambient oxygen pressure A. Ye. Shpak 1 & S. K. Kumara Swamy 2 & J. Dittmer 2 & N. Ye. Vlasenko 1 & N. I. Globa 3 & A. A. Andriiko 1 Received: 6 June 2015 /Revised: 28 July 2015 /Accepted: 30 July 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract The synthesis method of lithiated d-metal oxides using molten formate mixtures as precursors has been devel- oped and the isothermal (800 °C) cross section of pseudo ternary Li–Mn–Co oxide system under ambient oxygen pres- sure has been investigated by XRD, 7 Li NMR, and galvano- static electrochemical methods. Special attention has been paid to the compositions inside the quadrangle restricted by solid solutions LiCoO 2 –LiCo 0.85 Mn 0.15 O 2 with the layered structure of α-NaFeO 2 and solid solutions LiMn 2 O 4 – LiMnCoO 4 with the structure of spinel. It was found that, depending on the composition, three types of equilibrium phases could be formed: spinels Li[Li,Mn,Co] 2 O 4 with a part of Li atoms in octahedral sites, cation-deficit layered com- pounds Li 1 - δ [Co,Mn]O 2 , and Li 2 MnO 3 . Areas of (co)existence of these phases were plotted on the composition plane of the pseudo-ternary Li–Mn–Co system. Electrochem- ical properties of the compositions inside the quadrangle LiCoO 2 –LiCo 0.85 Mn 0.15 O 2 –LiMn 2 O 4 –LiMnCoO 4 are deter- mined by the content and average oxidation number of Mn atoms, which is higher than in the normal spinels Li[Mn, Co] 2 O 4 . Thus, the specific capacities of the polyphase com- positions are lower in comparison with the binary solid solu- tions Li[Mn,Co] 2 O 4 or pure LiCoO 2 . Keywords Ternary Li–Mn–Co oxides . Phase composition . Structure . Electrochemical properties Introduction The lithiated complex oxides of transition metals have been subject of extensive studies along the last two decades mainly because of their possibilities to be used as active materials for a positive electrode of Li-ion cells. The most common mate- rial for commercial Li-ion batteries is LiCoO 2 with layered structure of α-NaFeO 2 , though high cost and toxicity of Co compounds are the disadvantages of this material [1, 2]. An- other commercially available material, spinel-structured LiMn 2 O 4 , is much cheaper and less toxic. However, it is less stable during cycling [3, 4], and thus, its usage is restricted. Many attempts have been made in order to overcome the drawbacks of the both materials. The most promising way was considered to be the replacement of a part of Co atoms in the layered structure or a part of Mn atoms in the LiMn 2 O 4 spinel with atoms of other transition metals [5–8]. However, most of these works were directed onto the preparation of single-phase homogeneous substances. Only a few researches were found where solid solution series and their structure were investigated [9, 10]. Concerning the oxide Li–Mn–Co system, several mixed spinel oxides were synthesized with a part of the Mn atoms being replaced by Co atoms [6, 11–14]. Some of such com- pounds were found to be electrochemically active in the po- tential range up to 5 V versus Li. Since reliable electrolytes for such high cathodic voltages are not yet available, practical application of such materials is still questionable. We chose the Li–Mn–Co oxide system as a model for the experimental investigation of phase formation in the whole possible range of compositions. The purpose was to establish * A. A. Andriiko Andriiko_aa@ukr.net 1 National Technical University of Ukraine BKyiv Polytechnic Institute^, Kyiv, Ukraine 2 Institut des Molécules et Matériaux du Mans (IMMM), Université du Maine, Le Mans, France 3 Joint Electrochemical Energy Department, National Academy of Science, Kyiv, Ukraine J Solid State Electrochem DOI 10.1007/s10008-015-3001-x