First-Principles Calculations, Electrochemical and X-ray Absorption Studies of Li-Ni-PO₄Surface-TreatedxLi₂MnO₃·(1−x)LiMO₂(M = Mn, Ni, Co) Electrodes for Li-Ion Batteries

It has been previously hypothesized that the enhanced rate capability of Li-Ni-PO 4 -treated x Li 2 MnO 3 ·(1−x)LiMO 2 positive electrodes (M = Mn, Ni, Co) in Li-ion batteries might be associated with a defect Ni-doped Li 3 PO 4 surface structure [i.e., Li 3−2 y Ni y PO 4 (0 < y < 1)], thereby promoting fast Li + -ion conduction at the x Li 2 MnO 3 ·(1−x)LiMO 2 particle surface. In this paper, the solubility of divalent metals (Fe, Mn, Ni, Mg) in γ-Li 3 PO 4 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-PO 4 -treated x Li 2 MnO 3 ·(1−x)LiMO 2 electrodes that show that, after annealing at 550°C, a Li 3 PO 4 -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 Li 2 MnO 3 component during the annealing process. However, electrochemical studies of Li 3-2 y Ni y PO 4 -treated x Li 2 MnO 3 ·(1−x)LiMO 2 electrodes indicate that their rate capability increases as a function of y over the range y = 0 (Li 3 PO 4 ) to y = 1 (LiNiPO 4 ), 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.