It has been previously hypothesized that the enhanced rate capability of Li-Ni-PO4-treated xLi(2)MnO(3) center dot (1-x)LiMO2 positive electrodes (M = Mn, Ni, Co) in Li-ion batteries might be associated with a defect Ni-doped Li3PO4 surface structure [i.e., Li3-2yNiyPO4 (0 < y < 1)], thereby promoting fast Li+-ion conduction at the xLi(2)MnO(3) center dot (1-x)LiMO2 particle surface. In this paper, the solubility of divalent metals (Fe, Mn, Ni, Mg) in gamma-Li3PO4 is predicted with the first-principles GGA+U method in an effort to understand the enhanced rate capability. The predicted solubility (x) is extremely small; this finding is consistent with experimental evidence: 1) X-ray diffraction data obtained from Li-Ni-PO4-treated xLi(2)MnO(3) center dot (1-x)LiMO2 electrodes that show that, after annealing at 550 degrees C, a Li3PO4-like structure forms as a second phase at the electrode particle surface, and 2) X-ray absorption spectroscopy, which indicate that the nickel ions are accommodated in the transition metal layers of the Li2MnO3 component during the annealing process. However, electrochemical studies of Li3-2yNiyPO4-treated xLi(2)MnO(3) center dot (1-x) LiMO2 electrodes indicate that their rate capability increases as a function of y over the range y = 0 (Li3PO4) to y = 1 (LiNiPO4), strongly suggesting that, at some level, the nickel ions play a role in reducing electrochemical impedance and increasing electrode stability at the electrode particle surface. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.098202jes] All rights reserved.