The structural changes and voltages of LiMn2O4 spinel as a function of lithium content were investigated with density functional theory (DFT) in the generalized gradient approximation (GGA) and in the GGA with Hubbard U correction (GGA+U). The GGA+U approximation can distinguish the charge separation between Mn3+ and Mn4+, which GGA fails to capture. Therefore with this method the effects of local environments on Li diffusion activation energy barriers in the Li-rich phase could be systematically investigated. Our results showed that the different valences states of Mn ions and their arrangement surrounding the lithium ions have a profound effect on the activation barrier of lithium diffusion in the spinel structure.