14956 J. Phys. Chem. B 2004, 108, 14956-14963 Structure of Nanocrystalline Alkali Metal Manganese Oxides by the Atomic Pair Distribution Function Technique Milen Gateshki,† Seong-Ju Hwang,‡ Dae Hoon Park,‡ Yang Ren,§ and Valeri Petkov*,† Department of Physics, Central Michigan UniVersity, 203 Dow Science, Mt. Pleasant, Michigan 48859, Department of Applied Chemistry and Center for Emerging Wireless Transmission Technology, College of Natural Sciences, Konkuk UniVersity, Chungju Campus, Chungbuk 380-701, Korea, and AdVanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: April 15, 2004 The atomic scale structures of two nanocrystalline K-Li-Mn-O-I materials obtained through Chimie Douce route in aqueous and acetone solutions have been determined using X-ray diffraction and atomic Pair Distribution Function technique. Both samples have been found to possess a layered-type structure, where the layers are made of edge-shared MnO6 octahedra. With the sample prepared in aqueous solution, the layers are well separated and the interlayer space is occupied by both Li and K atoms. With the sample prepared in acetone solution the Mn-O layers are not so well separated and encapsulate mostly Li atoms. This material exhibits some Li/Mn substitutional disorder as well. The new structural information has been used to explain the electrochemical behavior of the two nanocrystalline materials. Some methodological aspects of the atomic pair distribution function technique and its applicability to study the structure of crystalline and nanocrystalline materials have been discussed as well. Introduction In this paper, we describe a determination of the atomic scale structure of two nanocrystalline lithium manganese oxides by Recently, lithium manganese oxides have been extensively the nontraditional Atomic Pair Distribution Function (PDF) studied due to their potential use as cathode materials in Li ion technique. It has emerged recently as a powerful tool for batteries.1 The main advantage of the manganese based cathode structural characterization of materials of limited structural materials over the widely used LiCoO is that manganese is 2 coherence including nanocrystals.15,16 The strength of the nontoxic and relatively cheap. The studies have led to the technique comes from the fact that it takes all components of discovery of a number of lithium manganese oxides with the diffraction data (Bragg peaks and diffuse scattering) into different stoichiometries, crystal structures, and electrochemical account and thus reveals both the longer range atomic order properties. Among them, cubic spinel LiMn O , tetragonal spinel 2 4 and the local deviations from it. In contrast, techniques employed Li Mn O , orthorhombic LiMnO , layered monoclinic Li MnO , 2 2 4 2 2 3 to study crystalline materials (e.g., Rietveld method17,18) rely and LiMnO2 have attracted special attention as promising - only on Bragg peaks in the diffraction data and are not so candidates.2 10 Despite the demonstrated structural and chemical sensitive to the local structural imperfections in materials. Here flexibility and many obvious advantages, crystalline lithium we show that both nanocrystalline lithium manganese oxides manganese oxides have not found a widespread application due studied have a layered structure that is very well defined on to their structural instability. It has been reported that most of the nanometer length scale and may well be described with a lithium manganates rapidly transform to a material with spinel- small number of parameters, such as unit cell and symmetry. type structure and lose their useful properties during the Our PDF studies also reveal other important structural details electrochemical charge-discharge cycle. Work toward solving in the nanocrystalline materials such as the interlayer distance the problem has resulted in the development of a new class of and the type of atomic species encapsulated between the layers. nanocrystalline lithium manganese oxides with better structural - The new structural information reported here well explains stability, larger capacity, and smoother discharge curves.11 14 material’s properties facilitating further progress in Li batteries Understanding the improved performance of the nanocrystalline research. materials requires a good knowledge of their atomic ordering and that is why several studies have been carried out by Experimental Section employing structure-sensitive techniques such as Raman and X-ray absorption spectroscopy (XAS).11,14 However, no com- Sample Preparation. Two different nanocrystalline lithium plete 3D structure determination has been done so far. The manganese oxides were studied in the present work: one reason is that nanocrystalline materials show diffraction patterns obtained via aqueous and the other via nonaqueous route. with a very few, if any, Bragg peaks and a pronounced diffuse Respectively, they were prepared by reacting water and acetone component rendering conventional crystallography inapplicable. solutions of KMnO4 with 1.5 equiv of LiI at room temperature, similarly to the previously reported Chimie Douce method.11,19,20 * To whom all correspondence should be addressed. E-mail: petkov@ The mixed solutions were maintained for 1 day under constant phy.cmich.edu. stirring. The resulting precipitates were washed with water for † Central Michigan University. ‡ Konkuk University. the water-based sample, and acetone and methanol for the § Argonne National Laboratory. acetone-based sample. Then the samples were dried at 130 °C 10.1021/jp048340k CCC: $27.50 © 2004 American Chemical Society Published on Web 09/03/2004 Structure of Nanocrystalline Alkali Metal Manganese Oxides J. Phys. Chem. B, Vol. 108, No. 39, 2004 14957 Å-1. These materials are obviously perfectly crystalline solids. The XRD patterns of the two nanocrystalline materials, on the other hand, contain a few Bragg-like features and a pronounced diffuse component. Such diffraction patterns are practically impossible to tackle by ordinary techniques for structure determination. However, once reduced to the corresponding atomic PDFs, they become a structure-sensitive quantity lending itself to structure determination. The frequently used reduced atomic PDF, G(r), is defined as follows: ) F -F G(r) 4πr[ (r) 0] (1) where F(r) and F0 are the local and average atomic number densities, respectively, and r is the radial distance. It peaks at characteristic distances separating pairs of atoms and thus reflects the atomic structure. The PDF G(r) is the Fourier transform of the experimentally observable total structure Figure 1. Experimental powder diffraction spectra for crystalline function, S(Q), i.e., LiMn0.9Cr0.1O2, LiMn2O4, and nanocrystalline lithium manganese oxides obtained through aqueous and nonaqueous routes. Q G(r) ) (2/π)∫ max Q[S(Q) - 1] sin(Qr)dQ (2) Q)0 (80 °C for the water-based sample) in a vacuum. The chemical compositions of the samples were determined by atomic where Q is the magnitude of the wave vector Q ) (4π sin θ)/λ, absorption (AA) spectrometry, inductive coupled plasma (ICP) 2θ is the angle between the incoming and outgoing radiation spectrometry, thermogravimetric analysis (TGA), and electron beams, and λ is the wavelength of the radiation used. The probe microanalysis (EPMA). The chemical compositions of structure function is related to the coherent part of the total the nanocrystals were found to be Li0.54K0.31MnO3.0-δI0.10 for diffracted intensity of the material as follows: the water-based sample and Li1.28K0.36MnO3.0-δI0.03 for the ) + coh - | |2 | |2 acetone-based sample, respectively. Due to its relatively small S(Q) 1 [I (Q) ∑ci fi(Q) ]/ ∑ci fi(Q) (3) concentration, iodine was not explicitly taken into account in the structure determination studies. TGA results showed that where Icoh(Q)is the coherent scattering intensity per atom in the water-based sample contains about 0.2 mol of water per electron units and ci and fi are the atomic concentration and unit formula. Due to their weak scattering power, water species X-ray scattering factor, respectively, for the atomic species of were not taken into account either. However, as the results of type i.21,22 As can be seen from eqs 1-3, G(r) is simply another our studies show, water plays a certain role in determining representation of the powder diffraction data. However, explor- important features in the atomic-scale structure and thus has an ing the diffraction data in real space is advantageous, especially impact on the electrochemical properties of the nanocrystalline in the case of materials with limited structural coherence. First, materials studied. eq 2 implies that the total scattering, including Bragg scattering In addition to the nanocrystalline samples, we prepared and as well as diffuse scattering, contributes to the PDF. In this studied two crystalline lithium manganese oxides: LiMn0.9- way both the longer range atomic structure, manifested in the Cr0.1O2 and LiMn2O4. The former was prepared by heating a sharp Bragg peaks, and the local nonperiodic structural imper- stoichiometric mixture of Li2CO3,Mn2O3, and Cr2O3 at 940 fections, manifested in the diffuse components of the diffraction °C under argon flow to suppress the oxidation of manganese. pattern, are reflected in the PDF. Second, by accessing high The latter was also prepared by a conventional solid-state values of Q, experimental G(r) functions with high real-space reaction involving heating Li2CO3 and Mn2O3 in ambient resolution can be obtained