Monoclinic lithium vanadium phosphate, alpha-Li3V2(PO4)(3), is a highly promising material proposed as a cathode for lithium-ion batteries. It possesses both good ion mobility and high lithium capacity because of its ability to reversibly extract all three lithium ions from the lattice. Here, using a combination of neutron diffraction and Li-7 MAS NMR studies, we are able to correlate the structural features in the series of single-phase materials Li3-yV2(PO4)(3) with the electrochemical voltage -composition profile. A combination of charge ordering on the vanadium sites and lithium ordering/disordering among lattice sites is responsible for the features in the electrochemical curve, including the observed hysteresis. Importantly, this work highlights the importance of ion-ion interactions in determining phase transitions in these materials.