Structural and Electrochemical Properties of LiMn 0.4 Ni 0.4 Co 0.2 O 2
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Structural and Electrochemical Properties of LiMn0.4Ni0.4Co0.2O2 Miaomiao Ma, Natasha A. Chernova, Peter Y. Zavalij, M. Stanley Whittingham Materials Science, State University of New York at Binghamton, Binghamton, NY, 13902, USA ABSTRACT The layered oxide LiMn0.4Ni0.4Co0.2O2 was synthesized by heating the mixed hydroxide precursor. This 442 composition was found to show high capacity. It has the optimum cobalt concentration to both substantially order the lattice, yet leave enough nickel on the lithium sites to minimize conversion to the 1T structure of CoO2 on deep charging. A combined x-ray and neutron diffraction study showed conclusively that only nickel, not manganese or cobalt is found on the lithium sites at room temperature. Magnetic measurements also confirmed the presence of nickel on the lithium sites, and showed the effectiveness of cobalt at minimizing nickel disorder. Heating above 800°C always leads to nickel disorder. The structural and thermal stability of reduced lithium content materials was studied; the structure remains rhombohedral except for x≤0.05, and cobalt substitution improves the thermal stability of the layered compound, but not the chemical stability.
INTRODUCTION Due to the high cost of LiCoO2, cathode materials containing other transition metals have been intensively investigated as an alternative. Among those, LiNiO2 and LiMnO2 are the most promising substitutions. However, pure LiNiO2 without any lithium and nickel cation mixing is hard to synthesis, and LiMnO2 encounters the phase transition, to spinel, and low cyclability problem. Recently, mixed compounds with all three transition metals, cobalt, nickel, and manganese, synthesized by mixed hydroxide method, have been studied [1-4]. The substitution of cobalt by manganese and nickel lowers the cost and improves the structure stability and capacity. Hence, LiMnyNizCo1-y-zO2 has become an attractive cathode material, and many researchers are trying to find the optimum composition with the best performance. Here, we concentrate on the composition LiMn0.4Ni0.4Co0.2O2, determining its structure, electrochemistry and thermal stability, and comparing it to the compositions LiMn0.5Ni0.5O2 and LiMn1/3Ni1/3Co1/3O2.
EXPERIMENTAL DETAILS The LiMnyNiyCo1-2yO2 (y = 0.33, 0.4, 0.5) compounds were synthesized by the mixed hydroxide method, followed by high temperature solid-state heating as described in our previous work [3]. Stoichiometric amount of Mn(OAc)2·4H2O (99+%, Aldrich), Ni(NO3)2·6H2O (98%, Aldrich), and Co(OAc)2·4H2O (Aldrich) were dissolved in distilled water and dripped into LiOH·H2O (98+%, Aldrich) solution with 3 times molar amount of the sum of the metal salts. Precipitates were filtered and washed with distilled water, then heated in an oven at 65oC
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overnight. The powder precipitates were then mixed with LiOH·H2O in molar ratio of Li/M=1.05 and heated at high temperatures, first at 450°C for 12 hours and then at 800°C for 8 hours. The electrochemical properties of LiMn0.4Ni0.4Co0.2O2 as rechargeable lithiu
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