Synthesis, Characterization and Electrochemistry of Lithium Battery Electrodes: <i>x</i>Li₂MnO₃·(1 − <i>x</i>)LiMn_0.333Ni_0.333Co_0.333O₂ (0 ≤ <i>x</i> ≤ 0.7)

Abstract

Lithium- and manganese-rich layered electrode materials, represented by the general formula xLi2MnO3·(1 − x)LiMO2 in which M is Mn, Ni, and Co, are of interest for both high-power and high-capacity lithium ion cells. In this paper, the synthesis, structural and electrochemical characterization of xLi2MnO3·(1 − x)LiMn0.333Ni0.333Co0.333O2 electrodes over a wide compositional range (0 ≤ x ≤ 0.7) is explored. Changes that occur to the compositional, structural, and electrochemical properties of the electrodes as a function of x and the importance of using a relatively high manganese content and a high charging potential (>4.4 V) to generate high capacity (>200 mAh/g) electrodes are highlighted. Particular attention is given to the electrode composition 0.3Li2MnO3·0.7LiMn0.333Ni0.333Co0.333O2 (x = 0.3) which, if completely delithiated during charge, yields Mn0.533Ni0.233Co0.233O2, in which the manganese ions are tetravalent and, when fully discharged, LiMn0.533Ni0.233Co0.233O2, in which the average manganese oxidation state (3.44) is marginally below that expected for a potentially damaging Jahn−Teller distortion (3.5). Acid treatment of 0.3Li2MnO3·0.7LiMn0.333Ni0.333Co0.333O2 composite electrode structures with 0.1 M HNO3 chemically activates the Li2MnO3 component and essentially eliminates the first cycle capacity loss but damages electrochemical behavior, consistent with earlier reports for Li2MnO3-stabilized electrodes. Differences between electrochemical and chemical activation of the Li2MnO3 component are discussed. Electrochemical charge/discharge profiles and cyclic voltammogram data suggest that small spinel-like regions, generated in cycled manganese-rich electrodes, serve to stabilize the electrodes, particularly at low lithium loadings (high potentials). The study emphasizes that, for high values of x, a relatively small LiMO2 concentration stabilizes a layered Li2MnO3 electrode to reversible lithium insertion and extraction when charged to a high potential.