Synthesis, Characterization and Electrochemistry of Lithium Battery Electrodes: xLi2MnO3·(1 − x)LiMn0.333Ni0.333Co0.333O2 (0 ≤ x ≤ 0.7)
Lithium- and manganese-rich layered electrode materials, represented by the general formula x Li 2 MnO 3 ·(1 − x )LiMO 2 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 x Li 2 MnO 3 ·(1 − x )LiMn 0.333 Ni 0.333 Co 0.333 O 2 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 0.3Li 2 MnO 3 ·0.7LiMn 0.333 Ni 0.333 Co 0.333 O 2 ( x = 0.3) which, if completely delithiated during charge, yields Mn 0.533 Ni 0.233 Co 0.233 O 2, in which the manganese ions are tetravalent and, when fully discharged, LiMn 0.533 Ni 0.233 Co 0.233 O 2, 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 0.3Li 2 MnO 3 ·0.7LiMn 0.333 Ni 0.333 Co 0.333 O 2 composite electrode structures with 0.1 M HNO 3 chemically activates the Li 2 MnO 3 component and essentially eliminates the first cycle capacity loss but damages electrochemical behavior, consistent with earlier reports for Li 2 MnO 3 -stabilized electrodes. Differences between electrochemical and chemical activation of the Li 2 MnO 3 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 LiMO 2 concentration stabilizes a layered Li 2 MnO 3 electrode to reversible lithium insertion and extraction when charged to a high potential.