Structural changes and thermal stability: in situ X-ray diffraction studies of a new cathode material LiMg0.125Ti0.125Ni0.75O2

In situ X-ray diffraction studies in using synchrotron radiation were carried out on a new cathode material LiMg0.125Ti0.125Ni0.75O2 in a charging cell. It had been demonstrated [Y. Gao, M.V. Yakovleva, W.B. Ebner, Electrochem. Solid State Lett. 1 (1998) 117] that this new material has thermal stability superior to that of LiNiO2 and LiCo0.2Ni0.8O2 at an over-charged state. In this paper, studies of the relationship between the structural changes during charge and the thermal stability at an overcharged state are presented. For the first time, a hypothesis is proposed to attribute the poor thermal stability of LiNiO2 based materials at overcharged state to the formation of hexagonal phase (H3). An irreversible contraction along crystal `cʹ axis usually occurs during the H3 phase formation. According to this hypothesis, improved thermal stability can be obtained by suppressing the formation of H3 phase. The reversibility of the structural changes during overcharge–discharge cycle for this new material is compared with the irreversible structural changes of the LiNiO2 cathode under the same condition.