Lithium electrochemical deintercalation from O2-LiCoO 2 : structural study and first principles calculations
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EE5.9.1
Lithium electrochemical deintercalation from O2-LiCoO2: structural study and first principles calculations D. Carliera,b, A. Van der Vena, G. Cedera, L. Croguennecb, M. Ménétrierb and C. Delmasb a
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave. Cambridge, MA, 02139 (USA)
b
Institut de Chimie de la Matière Condensée de Bordeaux-CNRS and Ecole Nationale Supérieure de Chimie et Physique de Bordeaux 87 av. Dr A. Schweitzer, 33608 Pessac cedex (France)
Abstract We present a detailed study of the O2-LiCoO2 phase used as positive electrode in lithium batteries. This phase is a metastable form of LiCoO2 and is prepared by ionic exchange from P2-Na0.70CoO2. The O2-LiCoO2 system presents interesting fundamental problems as it exhibits several phase transformations upon lithium deintercalation that imply either CoO2 sheet gliding or lithium/vacancy ordering. Two unusual structures are observed: T#2 and O6. The T#2 phase was characterized by X-ray, neutron and electron diffraction, whereas the O6 phase was only characterized by XRD. In order to better understand the structures and the driving forces responsible for the phase transformations involved in lithium deintercalation, we combine our experimental study of this system with a theoretical approach. The voltage-composition curve at room temperature is calculated using Density Functional Theory combined with Monte Carlo simulations, and is qualitatively in good agreement with the experimental voltage curve over the complete lithium composition range. Pseudopotential and thermodynamic calculations both show that two tetrahedral sites have to be considered for Li in the T#2 structure. The calculated voltage curve thus exhibits a two-phase O2/T#2 region that indicates that this phase transformation is driven by the entropy maximization and not by a non-metal to metal transition. We also predict two ordered phases for Li1/4CoO2 (O2) and Li1/3CoO2 (O6) and show that the formation of the O6 phase is not related to Li staging or Co3+/Co4+ charge ordering.
I.
Introduction
LiCoO2 can exhibit two types of layered structures: O3 and O2. The thermodynamically stable structure is prepared by solid-state reaction and exhibits an O3 stacking where the LiO6 and CoO6 octahedra share only edges. The O2-type LiCoO2 is metastable and was prepared for the first time by Delmas et al. by Na+/Li+ exchange from the P2-Na0.70CoO2 phase [1]. In the O2 structure, the LiO6 octahedra share edges but also faces with the CoO6 ones (Fig. 1). The Rietveld refinement of the neutron diffraction pattern of O2-LiCoO2 (S.G. P63mc) indicated that the repulsion between lithium and cobalt ions through the common face of their octahedra is strong enough to displace them from the center of their octahedra [2]. Whereas O3-LiCoO2 has been extensively studied as positive electrode in lithium batteries, to our knowledge, very few papers deal with the O2-LiCoO2 system
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[3-4]. During lithium deintercalation, this system exhibits several
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