Crystal and Local Structure Analyses for Layered Rock-Salt Type LixMn0.9Ti0.1O2 Prepared By Soft Chemical Synthesis

Abstract

Introduction There has been considerable interest in the layered rock-salt type oxides for the positive electrode materials in secondary lithium ion batteries (LIBs). The lithium cobalt oxide, LiCoO2, has been attracted much attenstion for many years because of its high electric capacity and excellent cycle life. The high cost and toxicity of cobalt compounds, however, have led to the investigation of alternative materials. So far, the layered rock-salt type LiNiO2, LiNi0.5Mn0.5O2, LiMnO2 and Li2MnO3-LiMO2 compounds (M = Mn, Co, Ni) have been intensively studied. Although Li2MnO3-LiMO2 materials showed high capacity, the the initial charge/discharge process was characterized by low coulombic efficiency. On the other hand, the disadvantage of meta-stable LiMnO2 was to convert to spinel-type structure on charge-discharge cycling. The phase transformation was suppressed by the partial substitution of the titanium ion for manganese ion [1]. Thus, expectation in regard to the modified Mn-rich layered oxide, LiMn0.9Ti0.1O2, is fairly high on the capacity and cycle life. The soft chemical synthesis has opened route for the synthesis of layered rock-salt type LiMn0.9Ti0.1O2 that are difficult or impossible to obtain by high temperature solid-state reactions. The advantage of such a soft chemical method is to induce the preparation of meta-stable phases and to considerably reduce the formation temperature below 500 ºC. In order to alleviate reaction conditions, environmental impact and decrease the economic cost, the synthesis of this material through soft chemical method would be prior if the properties of the material could be retained. In this study, the LixMn0.9Ti0.1O2 was examined about the crystal and local structures. Experimental We have attempted the preparation of LixMn0.9Ti0.1O2 with the O3-type layered structure by Na+/Li+ ion exchange using the corresponding sodium compounds as starting materials. The precursors of Na0.7Mn0.9Ti0.1O2 was prepared by solid-state reaction of stoichiometric mixtures of CH3COONa (purity 98.5%, Wako Pure Chem.) and coprecipitated manganese-titanium hydroxides. The powders were mixed by grinding and heated in a box furnace at 500 °C for approximately 12h in air. The (Li,Na)xMn0.9Ti0.1O2 sample was prepared by Na+/Li+ ion exchange of Na precursor with an eight fold molar excess of LiBr (purity > 99%, Strem Chem.) for 6 h in ethanol at 80 °C. After ion exchange, the samples were washed in ethanol and water. Chemical analyses for Li, Na, Mn, and Ti were carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Powder X-ray diffraction data were collected on a Panalytical X’pert-pro diffractometer. The morphology was observed by scanning electron microscopy (SEM, Hitachi, S-2600N). The crystal structure analysis was performed by using the synchrotron X-ray diffraction at BL02B2 (SPring-8, JAPAN). The local structure was examined by the X-ray total scattering at BL04B2 and the XAFS at BL14B2 (SPring-8, JAPAN). Charge and discharge tests were performed at 25 °C using a positive electrode, which consisted of the sample, conductvie carbon, and poly-tetrafluoroethylene powder between 2.5 and 4.8 V using a Li-metal negative electrode at a fixed current density of 30 mA g-1. A solution of 1 M LiPF6 in a 1:2 mixture of EC/DMC by volume (Kishida Chemical Co., Ltd.) was used as the electrolyte. The cells were constructed in an argon-filled globe box. Results and Discussion The XRD pattern for the precursor, Na0.7Mn0.9Ti0.1O2, was not attributed to any phases as previously shown [1]. The Na+/Li+ ion exchange for Na0.7Mn0.9Ti0.1O2 resulted in the chemical composition of the Li0.6Na0.1Mn0.9Ti0.1O2. After the ion exchange, the XRD pattern mainly showed the O3-type structure with a few unknown weak diffractions. The SEM observation revealed the grain size of about 200 nm. The electrochemical property of ion-exchanged material showed 243 mAh g−1 at first discharge. The synchrotron X-ray diffraction pattern of ion-exchanged material was analysed by Rietveld method and was not fully fitted to layered rock-salt structure with R-3m. Therefore the EXAFS and X-ray total scattering (Fig.) were also examined to analyse the local structure of Li0.6Na0.1Mn0.9Ti0.1O2. The reduced pare distribution function indicated that the structure of Li0.6Na0.1Mn0.9Ti0.1O2 was locally characterized as the spinel type and/or hollandite type structure. Reference [1] N. Ishida, H. Hayakawa, H. Shibuya, J. Imaizumi and J. Akimoto, Chemistry Letters, 10, 1478 (2012) Figure 1