First-principles methods have been used to calculate the phase diagram and volume expansion of spinel LixMn2O4 as a function of lithium content. The calculations confirm the experimentally observed two phase region between x = 1 and 2 and the ordered phase at x = 1/2. In addition, the expected large step in voltage at x = 1 is obtained. It is shown that these phenomena are qualitatively determined by the interactions of Li with each other and the Mn2O4 host and only quantitatively influenced by the more subtle electronic effects such as Jahn-Teller distortions, charge ordering and magnetic excitations. The two-phase region between x = 1 and 2 is found to be driven by strong repulsive interactions between lithium ions occupying adjacent tetrahedral 8a and octahedral 16c sites. The origin of the large volume change upon transforming from LiMn2O4 to Li2Mn2O4 is also investigated from first principles. The possible sources of the volume change are identified to be the intercalated lithium, the Jahn-Teller distortion, and the introduction of anti-bonding e(g) electrons into the Mn d-orbitals. The latter effect is found to be dominant. Some speculation is offered on how the large volume change upon lithiation of manganese dioxide can be prevented. (C) 2000 Elsevier Science B.V, All rights reserved.