Crystal Chemistry of Chemically Delithiated Layered Oxide Cathodes of Lithium Ion Batteries
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Crystal Chemistry of Chemically Delithiated Layered Oxide Cathodes of Lithium Ion Batteries A. Manthiram and S. Venkatraman Materials Science and Engineering Program, ETC 9.104 The University of Texas at Austin Austin, Texas 78712 ABSTRACT The structural and chemical stabilities of layered Li1-xCoO2-δ, Li1-xNi0.85Co0.15O2-δ and Li1-xNi0.5Mn0.5O2-δ (0 ≤ (1-x) ≤ 1) cathodes have been investigated by chemically extracting lithium from the corresponding LiMO2 with the oxidizer NO2BF4 in acetonitrile medium. While Li1-xCoO2-δ and Li1-xNi0.85Co0.15O2-δ begin to form a P3-type and a new O3-type (designated as O3’) phases, respectively, for (1-x) < 0.5 and (1-x) < 0.3, Li1-xNi0.5Mn0.5O2-δ maintains the initial O3-type structure without forming any second phase. Chemical analysis with a redox titration indicates that the Li1-xCoO2-δ, Li1-xNi0.85Co0.15O2-δ, and Li1-xNi0.5Mn0.5O2-δ systems begin to lose oxygen from the lattice, respectively, for (1-x) < 0.5, < 0.3 and < 0.4, which is accompanied by an onset of a decrease in the c parameter. The oxygen loss signals chemical instability and the trend in instability correlates with the charging voltage profiles of the cathodes. INTRODUCTION Commercial lithium-ion cells presently use the layered LiCoO2 as the cathode, but only 50% of its theoretical capacity (140 mAh/g) could be practically utilized. In contrast, the layered lithium nickel oxide with a partial substitution of Co for Ni (LiNi0.85Co0.15O2) shows a much higher reversible capacity of 180 mAh/g, which corresponds to 65% of its theoretical capacity. Also, the layered LiNi0.5Mn0.5O2 has recently been found to exhibit higher capacities of 150 – 200 mAh/g depending on the synthesis conditions [1]. However, the reason for the differences in capacities among the three systems has not been fully understood in the literature although they have the same O3-type structure. Most of the studies in this regard have focused invariably on the structural characterization of the electrochemically charged cathodes. Despite the recognition that the highly oxidized redox couples such as Co3+/4+ and Ni3+/4+ are characterized by a nearequivalence of the metal:3d and O2-:2p energies particularly in the case of perovskite oxides, little attention has been paid in the literature to the possible oxidation of O2- ions during the charge/discharge process and the consequent chemical instability leading to oxygen loss from the lattice. One of the reasons for the lack of such information is the contamination of the electrochemically charged samples by carbon, binder, and electrolyte, and the consequent difficulty in analyzing the oxidation states and oxygen contents by wet-chemical analysis. To overcome this difficulty, our group has focused recently on synthesizing bulk samples of Li1-xMO2 (M = Co, Ni, Ni0.5Mn0.5) free from carbon, binder, and electrolyte by chemically extracting lithium from LiMO2 with an oxidizer in non-aqueous media [2,3]. We present here the structural and chemical characterizations by X-ray diffraction and wet-chemica
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