Cathodes with high energy density and safety are sought to improve the performance of Li ion batteries for electric vehicle and consumer electronics applications. In this study, we examine the properties of the potential new cathodes Li9M3(P2O7)(3)(PO4)(2) for M = V, Mo with density functional theory calculations. These compounds emerged as potentially interesting cathodes from a high-throughput computational search. In this work, we investigate computationally the voltage, volume change, stability, safety, and diffusivity of LixV3(P2O7)(3)(PO4)(2) and find that extracting the final Li in this material will be difficult due to voltage and safety concerns. We suggest the yet-unreported class of compounds LixV3-3yMo3y(P2O7)(3)(PO4)(2) as a potential improvement over the pure V compound. In particular, our computations indicate that y = 2/3 deserves further computational and experimental attention. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.080205jes] All rights reserved.