Structural Characterization of Layered Li_xNi_0.5Mn_0.5O₂ (0 < x)

Christopher S. Johnson, Jeom-Soo Kim, A. Jeremy Kropf, Arthur J. Kahaian, John T. Vaughey, Linda M. L. Fransson,^a Kristina Edstrom,^a and Michael M. Thackeray

Electrochemical Technology and Basic Sciences Program, Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439
^aDepartment of Materials Chemistry, Ångstrom Laboratory, Uppsala University, Sweden

X-ray diffraction and X-ray absorption spectroscopy experiments were used to study chemical and electrochemical Li insertion and extraction reactions of LiNi_0.5Mn_0.5O₂. These results, along with galvanostatic cycling data, suggest that LiNi_0.5Mn_0.5O₂ layered electrodes in lithium batteries operate predominantly off two-electron redox couples, Ni⁴⁺/Ni²⁺, between approximately 4.5 and 1.25 V and Mn⁴⁺/Mn²⁺ between 1.25 and 1.0 V versus metallic Li, respectively. The retention of a stable layered framework structure and the apparent absence of Jahn-Teller ions Ni³⁺ and Mn³⁺ in the high- or low-voltage region is believed to be responsible for the excellent structural and electrochemical stability of these electrodes. The LiNi_0.5Mn_0.5O₂ layered oxide reversibly reacts chemically or electrochemically with Li to form an air-sensitive, dilithium compound, Li₂Ni_0.5Mn_0.5O₂, with a hexagonal structure analogous to Li₂MnO₂. The cycling behavior of Li/LiNi_0.5Mn_0.5O₂ cells over a large voltage window (4.6-1.0 V) and with very slow rates shows that rechargeable capacities >500 mA·h/g can be obtained.

Copyright © 2003, American Chemical Society

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