XAFS Analysis of Layered Li_xNi_0.5Mn_0.5O₂ (0 < x ≤ 2) Electrodes for Lithium Batteries

A. J. Kropf, C. S. Johnson, J. T. Vaughey, and M. M. Thackeray

X-ray absorption fine-structure spectroscopy was used to ascertain the individual metal oxidation states involved during the electrochemical insertion and extraction of lithium with layered Li_xNi_0.5Mn_0.5O₂ electrodes in lithium cells. These results, along with electrochemical cycling data, suggest that the Li_xNi_0.5Mn_0.5O₂ (0 < x ≤ 2) electrodes operate predominantly via two-electron redox couples, Ni⁴⁺/Ni²⁺ and Mn⁴⁺/Mn²⁺, between 4.5 and 2.0 V (0< x ≤ 1) and below 1.07 V (1 < x ≤ 2) versus metallic Li, respectively. In the upper voltage region, the NiO bond changes by nearly 0.14 Å, while the Mn-O bond remains at nearly a fixed length. In the low-voltage region, the LiNi_0.5Mn_0.5O₂Ni_0.5Mn_0.5O₂; during this reaction, charge transfer occurs at the Mn metal center.

© Physica Scripta 2005