AVS 50th International Symposium
    Applied Surface Science Wednesday Sessions
       Session AS-WeA

Paper AS-WeA6
The Chemical Nature of LiCoO@sub 2@, LiNiO@sub 2@ and LiCo@sub 0.5@Ni@sub 0.5@O@sub 2@ Surfaces by X-ray Photoelectron Spectroscopy

Wednesday, November 5, 2003, 3:40 pm, Room 324/325

Session: Fuel Cell & Battery Materials/Corrosion
Presenter: M.A. Langell, University of Nebraska
Authors: M.A. Langell, University of Nebraska
A.W. Moses, University of California
J.G. Kim, University of Nebraska
Correspondent: Click to Email
Many lithium metal oxides find use in lithium batteries as a result of their Li@super +@/Li redox properties coupled to their high lithium conductivity and their ability to be multiply cycled between the near-stoichiometric and severely de-lithiated electrode material. In its stoichiometric form, LiCoO@sub 2@ presents a straightforward picture in charge balance. Lithium is formally Li@super +@, cobalt is Co@super 3+@ and, with the exception of the occasional defect, the lattice oxygen is comparable to that found in 3d transition metal monoxides, O@super 2-@. By simple analogy, LiNiO@sub 2@ should contain nickel formally in its 3@super +@ oxidation state. Ni@super 3+@ is not as thermodynamically favored as the low-spin, octahedrally-coordinated Co@super 3+@, however, and this tends to destabilize the LiNiO@sub 2@ surface composition. We present results from XPS and Auger electron spectroscopy that show LiCoO@sub 2@ forms stable, stoichiometric surface compositions once surface hydroxylation and carboxylation are properly taken into account. These latter surface adsorbates are variable, depending upon the history of the material and the surface pretreatment prior to UHV analysis. LiNiO@sub 2@, however, is not as well behaved, even when XRD indicates that the bulk is well-ordered. The surface is severely depleted in lithium and phase separation to NiO and cubic LiNiO@sub 2@ is common. Adding cobalt to the lattice does not completely stabilize the structure and LiCo@sub 0.5@Ni@sub 0.5@O@sub 2@ substrates behave much like LiNiO@sub 2@ from the viewpoint of the nickel with nickel found predominantly in the Ni@super 2+@ state. In contrast to literature reports that nickel is 2@super +@ in LiNiO@sub 2@, with charge compensation resulting from a localized Ni@super 2+@ - O@super -@ adjacent lattice ion pairs, we see no evidence that the oxygen is significantly different in the three materials, at least within the near-surface area.