7 Li and 51 V NMR study on Li + ionic diffusion in lithium intercalated Li x V 2 O 5 Koichi Nakamura a, * , Daisuke Nishioka a , Yoshitaka Michihiro a , M. Vijayakumar b,1 , S. Selvasekarapandian b , Tatsuo Kanashiro a a Department of Physics, Faculty of Engineering, The University of Tokushima, 2-1 Minami-Josanjima-Cho, Tokushima 770-8506, Japan b Solid State and Radiation Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore-641 046, Tamilnadu, India Received 19 June 2005; received in revised form 5 September 2005; accepted 9 September 2005 Abstract Li x V 2 O 5 (0.4 < x < 1.4) prepared by solid-state reaction were studied by 7 Li and 51 V NMR spectroscopy. 7 Li NMR spectra showed a narrowing of the line width in relation to Li + ionic diffusion. Analysis of Li x V 2 O 5 using a Debye-type relaxation model showed a low activation energy ¨0.07 eV in the sample of x = 0.4 below room temperature, and revealed a Li + ionic diffusion with larger activation energy ¨0.5 eV above 450 K in lithium-rich samples. The latter is ascribed to the existence of a multi-phase system comprising stable q- and g-phases, resulting from complicated phase transitions at high temperature. These shapes and shifts enable the classification of the h-, q-, y-, and g-phases. The ionic diffusion of Li + ions is discussed in relation to the complicated phase transitions. D 2005 Elsevier B.V. All rights reserved. PACS: 66.30; 76.60 Keywords: NMR; Li + ion diffusion; Lithium vanadium bronze; Phase transition 1. Introduction The development of advanced cathode materials to replace the use of LiCoO 2 , in commercial products, has been particularly intensive over the last decade. Lithium intercalated Li x V 2 O 5 is attractive for the cathode material of 3 V lithium rechargeable batteries. It is known that V 2 O 5 acts as a host, able to intercalate a maximum of 2 Li + ions per V 2 O 5 reversibly. The various valence states of vanadium and the ability of the oxide (V 2 O 5 ) to accept large amounts of Li + ions through intercala- tion, causes complicated phase transitions [1,2]. The host oxide V 2 O 5 has an orthorhombic structure and forms [VO 5 ] V layers parallel to (001), consisting of edge- and corner-sharing VO 5 square pyramids. It is generally considered that in this structure, ravines shaped by the square pyramids along the b-axis, form a one-dimensional thoroughfare for the Li + ions to pass through. However, Li + ionic diffusion is not well known in this system. The bronzes a-A x V 2 O 5 (A = Li, Na, Ag, K, Cu; 0  x  0.10) are isostructural. The chemical lithium insertion in V 2 O 5 gives rise to several structural modifications: a-phase for 0 < x < 0.13, q-phase for 0.32 < x < 0.88, y-phase for 0.88 < x < 1.0 and g-phase for x > 1.0 [3]. In addition, Li x V 2 O 5 prepared at high temperature by solid-state reaction exhibits the h/h V -phase in the range of 0.2 < x < 0.6 [4,5]. The composition limits in the solution are not defined and the neighboring phases are usually coexistent. Li x V 2 O 5 exhibits a very similar framework with only a slight puckering of V 2 O 5 up to x = 1.0, while also showing good reversibility. Beyond x = 1.0 however, this puckered structure is not particularly stable and is typically replaced by the g-phase, which exhibits a remarkably distorted structure. Such a phase transition is closely related to Li + ionic motion. Since a study on the Li + ionic diffusion of the h-phase by Gendell [6–11], intensive NMR studies on lithiated V 2 O 5 have revealed that the crystalline material maintains several structural phases depending on the lithium content as men- tioned above. Although the solid-state reaction method is often used for sample synthesis, a heating/melting process at high tempera- ture can easily cause the loss of oxygen, resulting in non- 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.012 * Corresponding author. E-mail address: koichi@pm.tokushima-u.ac.jp (K. Nakamura). 1 Present address: Department of Physics, College of William and Mary, Williamsburg-23187, Virginia, USA. Solid State Ionics 177 (2006) 129 – 135 www.elsevier.com/locate/ssi