STUDIES OF Ni-RICH POSITIVE ELECTRODE MATERIALS FOR LITHIUM ION BATTERIES
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Ni-rich layered Li transition metal oxides are some of the most promising positive electrode materials for Li-ion batteries due to low cost and high energy density. Increasing the Ni content is one important approach to further increase the energy density and lower the cost. However, it is conventionally believed that high Ni content brings about challenges like poor cycling stability and thermal stability. This thesis focuses on fundamental studies of Ni-rich positive electrode materials, development of novel materials with enhanced properties, and investigations of failure mechanisms. The thesis begins with a study of LiNiO2, the “grandfather” of Ni-rich positive electrode materials. The multiple phase transitions which occur as x varies in LixNiO2 (0 x 1) were thoroughly investigated by X-ray diffraction, neutron diffraction, and electrochemical measurements. Based on this work, a study on how dopants, M, affect LiNi1-xMxO2 was performed. The effects of dopants on structural, electrochemical, and thermal properties were comprehensively studied. It was concluded that Co, commonly believed to be an essential dopant in Ni-rich materials, is actually not required. The development of single crystal LiNi1–x–yMnxCoyO2 (NMC) and LiNi1–x–yCoxAlyO2 (NCA) is another focus of this thesis. Optimal synthesis conditions were developed for single crystal NMC622, and a two-step synthesis method was invented for impurity-free single crystal NCA preparation. Preliminary electrochemical studies of materials made at Dalhousie are included. The last part of this thesis presents an unavoidable challenge for Ni-rich positive electrode materials. On the basis of a large volume of data collected from Ni-rich positive electrode materials having various compositions, a failure mechanism, which relates the cycling stability to the universal structural changes of Ni-rich materials, was proposed. It is hoped that this work can effectively guide further research to overcome this unavoidable challenge.