Experimental and computational study of the structure and electrochemical properties of LixM2(PO4)3 compounds with the monoclinic and rhombohedral structure

This paper presents a combined computational and experimental study of the structural and electrochemical properties of monoclinic and rhombohedral LixM2(PO4)(3) (with a focus on M = V). The preferred sites for dilute Li occupation and stable Li ordered phases are identified. Features of the voltage curve are understood as emerging from site energetics, Li ordering, and redox couples. These features are found to be largely independent of alloying and a simple additive model is proposed to analyze the voltage curve for any cation substitution in the monoclinic structure. The model is shown to be very useful for understanding experimental results for a number of substituted compounds. Voltages for most important cations are calculated from first principles and can be combined with the simple model to predict voltage curves for new alloyed monoclinic systems.