It has been previously hypothesized that the enhanced rate capability of Li-Ni-PO 4-treated xLi 2MnO 3(1-x)LiMO 2 positive electrodes (M Mn, Ni, Co) in Li-ion batteries might be associated with a defect Ni-doped Li 3PO 4 surface structure i.e., Li 3-2yNi yPO 4 (0 < y < 1), thereby promoting fast Li-ion conduction at the xLi 2MnO 3(1-x)LiMO 2 particle surface. In this paper, the solubility of divalent metals (Fe, Mn, Ni, Mg) in -Li 3PO 4 is predicted with the first-principles GGAU 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 xLi 2MnO 3(1-x)LiMO 2 electrodes that show that, after annealing at 550C, a Li 3PO 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 2MnO 3 component during the annealing process. However, electrochemical studies of Li 3-2yNi yPO 4-treated xLi 2MnO 3(1-x)LiMO 2 electrodes indicate that their rate capability increases as a function of y over the range y 0 (Li 3PO 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Renewable Energy, Sustainability and the Environment
- Surfaces, Coatings and Films
- Materials Chemistry