The magnetic properties and structure of Lix Co O2 for 0.5<1.0 are reported. Co4+ is found to be high spin in Lix Co O2 for 0.94≤x≤1.00 and low spin for 0.50≤x≤0.78. Weak antiferromagnetic coupling is observed, and at x 0.65 the temperature-independent contribution to the magnetic susceptibility and the electronic contribution to the specific heat are largest. Neutron diffraction analysis reveals that the lithium oxide layer expands perpendicular to the basal plane and the Li ions displace from their ideal octahedral sites with decreasing x. Comparison of the structures of Nax Co O2 and Lix Co O2 reveals that the Co O2 layer changes substantially with alkali content in the former but is relatively rigid in the latter, and that the Co O6 octahedra in Lix Co O2 are less distorted. © 2008 The American Physical Society.