Structural Characteristics of Lithium Nickel Phosphate Studied Using Analytical Electron Microscopy and Raman Spectroscopy C. V. Ramana,* ,² A. Ait-Salah, ‡ S. Utsunomiya, ² U. Becker, ² A. Mauger, ‡ F. Gendron, ‡ and C. M. Julien ‡ Nanoscience and Surface Chemistry Laboratory, Department of Geological Sciences, UniVersity of Michigan, Ann Arbor, Michigan 48109, and Institut des Nano-Sciences de Paris (INSP), UniVersite´ Pierre et Marie Curie, CNRS-UMR 7588, campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France ReceiVed May 15, 2006. ReVised Manuscript ReceiVed June 7, 2006 The structural characteristics of lithium nickel phosphate (LiNiPO 4 ) prepared by solid-state chemical reaction have been studied in detail using the analytical electron microscopy and Raman spectroscopy measurements. The high-resolution transmission electron microscopy and selected area electron diffraction measurements indicate that the grown LiNiPO 4 is well-crystallized in olivine structure without any indication of crystallographic defects such as dislocations or misfits. The energy-dispersive X-ray spectrometry coupled with the elemental compositional mapping using high-angle angular dark field scanning electron microscopy confirms the chemical quality of the grown LiNiPO 4 in terms of homogeneity and uniform elemental distribution characteristics. The local structure and chemical bonding between NiO 6 octahedral and (PO 4 ) 3- tetrahedral groups probed by Raman spectroscopy also indicate the high- quality of LiNiPO 4 . Structural analysis of the delithiated Li 0.09 NiPO 4 phase indicates lattice contraction and distortion upon lithium extraction. A detailed analysis and comparison of the pristine and delithiated phases is also reported. I. Introduction The large family of compounds of the ABPO 4 type (with A and B being mono- and divalent cations, respectively) exhibits different frameworks depending on the relative size of the A and B ions, for example, arcanite-, trydimite-, or olivine-type structures. For the A ions of small size, as in the case of Li + , the resulting compounds, lithium transition- metal phosphates (LiMPO 4 with M ) Fe, Ni, Co, Mn) adopt the olivine-like (Mg 2 SiO 4 ) structure containing high-spin M 2+ ions. 1 These phosphates exhibit a variety of structural features, 2-4 the most prominent of which is the existence of tunnels in which small ions can move freely, a property that makes them potential hosts for the insertion and extraction of ions. As a result of their high capacity, good thermal stability, and environmental benignity, these compounds have been proposed as alternative electrode materials for recharge- able lithium-ion batteries. 1,5-10 The crystal structure of LiNiPO 4 is made up of two types of polyhedra, distorted NiO 6 octahedral units that are corner shared and cross-linked with the PO 4 tetrahedral oxo-anions, forming a three-dimensional network with tunnels that are occupied by Li ions along the (010) and (001) directions. In this network, nearly close-packed oxygen atoms in hexagons can be found with Li and Ni ions that are located at the center of octahedral sites. 2 A schematic representation of the olivine structure is shown in Figure 1. As a result of this typical structural configuration, it is believed that the strong covalent PO 4 unit tends to reduce the covalency of the M-O bond, modifying the redox potential for the M 2+/3+ couple and thus producing a useful potential for lithium extraction and reinsertion. 11 Recently, there has been a great deal of interest in lithium nickel phosphates. Different aspects, such as synthesis, structural and vibrational properties, electrochemistry, and magnetism, of these materials have been studied by various research groups using both experimental and theoretical invetigations. 12-35 The advancement in lithium battery tech- nology is envisaged on the basis of lithium metal phosphate * Corresponding author. E-mail: ramanacv@umich.edu. Tel.: 734-763-5344. Fax: 734-763-4690. ² University of Michigan. ‡ Universite´ Pierre et Marie Curie. (1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electro- chem. Soc. 1997, 144, 1188. (2) Se´ller, S.; Duran, J. L. 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