Combined 6,7 Li NMR and Molecular Dynamics Study of Li Diffusion in Li 2 TiO 3 M. Vijayakumar, Sebastien Kerisit, Zhenguo Yang, Gordon L. Graff, Jun Liu, Jesse A. Sears, Sarah D. Burton, Kevin M. Rosso,* and Jianzhi Hu* Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: July 28, 2009; ReVised Manuscript ReceiVed: September 15, 2009 Understanding lithium diffusion properties in electrode materials is important for designing rechargeable lithium- ion batteries with improved performance. In this work, the lithium dynamics in layered Li 2 TiO 3 were characterized using a combination of 6,7 Li nuclear magnetic resonance (NMR) over a wide temperature range (150-500 K) and molecular dynamics (MD) simulations. The 7 Li static NMR and stimulated echo experiments show slow and partial lithium diffusion in Li 2 TiO 3 . The high-field (21.1 T) 6 Li magic-angle spinning NMR shows a new tetrahedral lithium site along with the three crystallographic octahedral sites in Li 2 TiO 3 sample. MD simulations predict that lithium can occupy a tetrahedral site if two or more vacancies exist in the vicinity, which may result, for example, from the presence of a Ti defect in the LiTi 2 layer. 6 Li two-dimensional (2D) exchange NMR experiments show evidence of lithium diffusion between the pure Li and LiTi 2 layers along the c axis. Although the 2D exchange NMR data are not sensitive to lithium diffusion in the ab plane, MD simulations show that lithium diffusion in the pure Li layer is equally probable. Combining these results, a detailed picture of the lithium diffusion pathways in Li 2 TiO 3 is presented. Introduction Lithium metatitanate (Li 2 TiO 3 ) is a technologically important material with many applications. For example, it has been used as an electrode material in lithium-ion batteries, 1,2 as a double layer cathode material in molten carbonate fuel cells, 3 and as a solid breeder material in the blanket of fusion reactors. 4 In the field of lithium-ion batteries, Li 2 TiO 3 has been reported to be capable of stabilizing the structure of high-capacity cathode materials such as LiFeO 2 , LiMnO 2 , LiCrO 2 , and LiNiO 2 . 5-8 The layered solid solutions, with formulas xLiMO 2 - (1 - x)Li 2 TiO 3 (M ) Fe, Mn, Cr, Ni), lead to promising cathode materials, which provide higher Coulombic efficiencies on extended cycling between 4.6 and 2.3 V. 1,6 When used as a breeder material, Li 2 TiO 3 produces tritium atoms by lithium transmutation and transports the heat generated by the nuclear reaction to the coolant. 9 Because of its many important applications, the mechanical and thermal properties of Li 2 TiO 3 have been extensively investigated. 10-12 The electrical and thermal con- ductivity of Li 2 TiO 3 has also been probed using traditional impedance measurements. 13,14 It has been reported that Li 2 TiO 3 shows poor electrical conductivity, that is, on the order of 10 -7 S cm -1 at 400 K. 13 Despite all of the prior investigations, the detailed diffusion pathways of lithium in the bulk and the oc- cupancy of lithium in the lattice as well as the effects of the defects that are critically important for understanding the poor electrical conductivity of Li 2 TiO 3 materials are not known. This is largely because the impedance measurements only probe the overall conductivity, which is a combination of lithium diffusion and electronic contributions. In particular, in conductivity measurements, the intrinsic properties of the material of interest are sometimes masked by those of grain boundaries or impuri- ties. Therefore, a better analytical tool is needed for identifying the detailed lithium diffusion properties of Li 2 TiO 3 . Nuclear magnetic resonance (NMR) is nucleus specific (i.e., only the local environment of the particular nucleus under study is probed by NMR), nondestructive, and stringently quantitative, and therefore can offer valuable information about diffusion processes in ion conducting materials. Indeed, details in local structure and dynamics of lithium-ion conducting materials have been unraveled with solid-state NMR. 15 Although NMR can offer a conspicuous view of the diffusion properties of complex materials, interpretation of NMR data is theoretically complex and thus often relies on complementary information from computational models to elucidate the ion dynamics and diffusion pathways. One example of choice is the use of potential-based molecular dynamics (MD) simulations, which offer several advantages. First, it allows one to perform dynamical simulations of lithium diffusion for long periods of time. This makes the direct calculation of lithium diffusion coefficients and diffusion pathways possible and allows for directly identifying any mixing between different lithium sites. Second, it allows one to treat much larger lattices than used with first-principles techniques, which, in turn, allows one to consider a wide range of conditions from the infinite dilution limit to high lithium contents with small concentration intervals. Therefore, by combining information obtained from NMR and MD simulations, we can draw a prominent picture of the diffusion processes in ion conducting materials. In this Article, we present results from solid-state 6,7 Li NMR and computational studies of the lithium local structure and diffusion processes in Li 2 TiO 3 . The lithium dynamics are analyzed with powerful NMR methods such as traditional relaxation NMR, 16 two-dimensional (2D) exchange NMR, 17 and newly established quadrupolar spin-alignment echo (SAE) technique. 18 The lithium local structure and probable diffusion pathways are computed using potential-based molecular dynam- ics simulations. These results constitute a first step toward understanding the lithium local diffusion mechanisms in lithium metatitanate and will provide a basis for further investigations of lithium diffusion in layered cathode materials. * Corresponding authors. J.H.: phone, (509) 371-6544; fax, (509) 371- 6546; e-mail, jianzhi.hu@pnl.gov. K.M.R.: phone, (509) 371-6357; e-mail, kevin.rosso@pnl.gov. J. Phys. Chem. C 2009, 113, 20108–20116 20108 10.1021/jp9072125 CCC: $40.75  2009 American Chemical Society Published on Web 10/27/2009