LiNiO2 is one of the most attractive materials with the lower cost and higher discharge capacity to replace commercial LiCoO2 cathode material for Li-ion batteries. Nevertheless, due to the undesired capacity fading upon cycling and lower thermal and structural stability, transition metals, such as cobalt and manganese, are substituted for nickel to improve the properties LiNi0.75-xCo0.25MnxO2 materials. In this study, the structural changes for LiNi0.75-xCo0.25MnxO2 (x =0.1, 0.15 and 0.25) were investigated by synchrotron based in situ X-ray technique, along with the electrochemical measurements. In situ XRD data of LiNi0.65Co0.25Mn0.1O2 upon charge showed that there were two hexagonal phases, H1 and H2, which could be recognized by tracking the changes of (003) peak. The lattice parameter “c” of new H2 phase was larger than that of original H1 phase, while the parameters “a” and “b” of H2 were smaller than those of H1. In fact, the phase boundary of H1 and H2 was smeared in LiNi0.6Co0.25Mn0.15O2 and LiNi0.5Co0.25Mn0.25O2, and a single-phase reaction was observed. The difference in phase transition was attributed to more degree of defects induced by the increase in Mn content. In addition, the changes of lattice parameters could be explained by the balance between ionic radius and the repulsive force of the layer-structured material. The X-ray absorption near edge structure (XANES) indicated the initial valences in Li1-xNi0.5Co0.25Mn0.25O2 were +2/+3, +3 and +4 for Ni, Co, and Mn, respectively. The main redox reaction during delithiation was achieved by Ni (i.e. Ni2+□Ni3+ followed by Ni3+ □Ni4+). The oxidation states of Co and Mn remained Co3+ and Mn4+, respectively. The bond length of Ni-O decreased drastically, while the Co-O and Mn-O distances exhibited a slight change with the decrease of Li content in the electrode. It was further revealed all the second shell metal-metal (Ni-M, Co-M, and Mn-O) distances decreased due to the oxidation of metal ions. For further improving the electrochemical performance and thermal stability, magnesium was chosen as dopant in Li[Ni0.6-yMgyCo0.25Mn0.15]O2 cathode materials. LiNi0.6-yMgyCo0.25Mn0.15O2 (y =0~0.08) were successfully synthesized via the mixing hydroxide method. These materials exhibited □-NaFeO2 structure as indicated by the XRD patterns. The intensity ratio of (003) to (104) showed that Mg substitution could reduce the cation mixing. The electrochemical performance, such as capacity retention, was also improved in both room temperature and 55 oC. The initial capacity of LiNi0.57Mg0.03Co0.25Mn0.15O2 was 214 mAh/g and had an excellent cyclic performance with only 7% capacity loss after 30 cycles. A combination of in situ synchrotron X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) was used to study the phase transition and local environment change for LiNi0.57Mg0.03Co0.25Mn0.25O2 material charged to 5.2 V. In situ synchrotron X-ray diffraction patterns showed that the boundary of phase transition for H1 to H2 was more distinct in Mg-doped sample, indicating that the LiNi0.57Mg0.03Co0.25Mn0.15O2 material exhibited higher structural integrity. The improvements of both electrochemical retention and thermal stability were possibly attributed to the reduced cation mixing and complete structural changes. The high energy synchrotron XAS results showed that the main redox reaction during delithiation was achieved by Ni (i.e. Ni3+ □Ni4+), while the oxidation states of Co and Mn remained Co3+ and Mn4+ even charged to 5.2 V.