Structure-Stability Correlations in Li-ion Battery Cathode Materials
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Structure-Stability Correlations in Li-ion Battery Cathode Materials Christopher Patridge,1 Corey Love,2 Wojtek Dmowski,3 David Ramaker,2 Michelle Johannes,4 and Karen Swider-Lyons2 1
Chemistry Dept., D’youville College, Buffalo, NY 14201 Chemistry Division, U.S. Naval Research Laboratory, Washington DC 20375 3 Department of Materials Science, University of Tennessee, Knoxville, TN 37996 4 Center for Computational Materials Science, U.S. Naval Research Laboratory, Washington DC 20375 2
ABSTRACT Detailed structural studies of two lithiated metal oxides, Li2CuO2 and nanoscale LiCoO2, have been carried out using ex situ high-energy X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) with the objective of understanding structural changes that might cause capacity loss during cycling. XRD on the cuprate was studied at various states of charge and phase composition, and the bulk state was determined by Rietveld refinement and pair density function (PDF) analysis. Results showed a largely irreversible structural change of the material upon oxidation of Cu2+ as well as CuO formation. The in-situ XAS of the LiCoO2 was analyzed through a difference method to extract the changes in the local structure that occur upon cycling in both the near edge (XANES) and extended region (EXAFS). Results suggest that cycling causes site exchange of the Co and Li ions near the surface of the nanoscale LiCoO2.
INTRODUCTION Understanding the predominant processes responsible for capacity fade and irreversible loss in Li-ion batteries is a critical step toward the development of stable power sources with good longevity. Both the transition-metal-oxide-based cathode and carbon-based anode contribute to capacity fade in a battery. Herein we attempt to develop structure-stability correlations for two different cathode materials: (1) lithiated cuprate, Li2CuO2, and (2) nanoscale LiCoO2. Li2CuO2 possesses a theoretically high energy density, but suffers from a large irreversible loss and continued fade on subsequent cycles. The structural instabilities occurring during electrochemical cycling of Li2CuO2 still remain unclear, and this work probes the local structural changes at various states of charge using high-energy X-ray diffraction (XRD) and 3b density function (PDF) analysis to better discern the dominant phases and mechanisms of the dramatic capacity losses seen in Li2CuO2. The initial orthorhombic Immm structure of Li2CuO2 contains long planar CuO4 chains running along the b-axis forming an open framework for Li+ to occupy[1]. One full Li+ per formula unit can be removed giving a theoretical capacity of nearly 256 mAh/g for the material. This high capacity is observed during the initial charge cycle between ~ 2.5 – 4.2 V with a large plateau near 3.4 V consistent with the Cu2+/3+ redox couple and the formation of LiCuO2, a monoclinic structure with C2/m symmetry[2]. The LiCuO2 structure contains the same CuO4 chains, but the framework collapses due to the vacant Li+ sites.
Upon discharge, Li2CuO2 is difficult to refo
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