The Syntheses and Characterization of Layered LiNi 1-y-z Mn y Co z O 2 Compounds
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EE6.4.1
The Syntheses and Characterization of Layered LiNi1-y-zMnyCozO2 Compounds J. Katana Ngala, Natasha A. Chernova, Luis Matienzo, Peter Y. Zavalij, M. Stanley Whittingham* Chemistry Department and the Institute for Materials Research, State University of NewYork at Binghamton, Binghamton, NewYork 13902-6016, U.S.A. ABSTRACT The layered compounds of formula LiNi0.4Mn0.6-yCoyO2, for y=0.2, 0.3, and 0.4 and LiNi0.7-yMn0.3CoyO2 for y=0.3, and 0.1 were synthesized at 800oC. X-ray powder diffraction indicates layered structure of R3m symmetry similar to α-NaFeO2. Rietveld refinement data shows that Mn and Ni increase the tendency of transition metal ions to migrate into the interlayer sites relative to LiCoO2. Both magnetic susceptibility and XPS data support a 2+ oxidation state for Ni and 4+ and 3+ for Mn and Co, respectively. The layered compound LiNi0.4Mn0.4Co0.2O2 shows a high initial capacity of about 200mAh/g when cycled between 2.5V and 4.3 V at 20oC.
INTRODUCTION Suitable cathode materials need to be found to replace LiCoO2 in commercial lithium rechargeable batteries. The compound LiCoO2 is expensive and toxic. The new material needs to be cheaper, less toxic and, preferably, have a higher capacity than LiCoO2. Much interest has been on the layered LiMnO2 compound for its prospect of providing not only a cheap but also an environmentally benign cathode material [1-3]. However, it easily converts to the, thermodynamically stable, spinel structure. The conversion is promoted by JahnTeller distortion, which occurs due to the t2g3eg1 configuration of the Mn3+ ion. Two approaches to stabilizing the layered LiMnO2 have been taken. In the geometric stabilization approach, “pillars” between the layers provide the stabilization. We reported [4, 5] on the compounds, KMnO2 and (VO)yMnO2, which are examples of such structures. The two compounds have reasonable stability towards cycling, particularly at low current densities. In the electronic stabilization approach, the electron configuration is increased by substitution of the Mn with more electronegative elements such as Co [6-9] and Ni [10-14]. The more electronegative element helps by keeping the Mn above 3+ oxidation state. For instance, the compound LiMn0.5Ni0.5O2 has indicated a high capacity without formation of the spinel [6]. The layered compounds, LiNi1-y-zMnyCozO2, have demonstrated remarkable electrochemical behavior [15,16] such that they are suitable candidates as cathodes for lithium rechargeable batteries. They have smooth discharge curves, which are devoid of plateaus, over a wide potential range. Thus, they are able to deliver high capacities without going through irreversible phase changes. In this paper we report on the syntheses and characterization of some LiNi1-y-zMnyCozO2 compounds. We report, for the first time, on the magnetic susceptibility and XPS analyses on LiNi0.4Mn0.4Co0.2 O2. *
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EE6.4.2
EXPERIMENTAL The mixed hydroxide of the transition metals was prepared. Stoichiometric amounts of the sol
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