Ordered LiNi0.5Mn1.5O4 was synthesized through a solid-state reaction. Even though the material has a particle size of 3-5 mu m, it shows very high rate capability and excellent capacity retention. The capacity is as high as approximate to 78 mAh/g at a 167C discharge rate. This high discharge rate performance is consistent with first-principles calculations of the activation barrier for lithium motion, which predict the lithium diffusivity in this material to be around 10(-9)-10(-8) cm(2)/s. We also systematically investigated the effect of several cell components and electrode construction on the measured rate performance and conclude that care has to be taken to remove all other rate limitations from the cell to measure the rate performance of an electrode material. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3439678] All rights reserved.