Phase Transitions in the LiNi_0.5Mn_0.5O₂ System with Temperature

We investigate the phase transformations of layered LiNi0.5Mn0.5O2 at finite temperature with a combined computational and experimental approach. The detailed changes in the ionic configurations with temperature are investigated by Monte Carlo simulations on the basis of a coupled cluster expansion that describes the dependence of the energy on the arrangement of Li+, Ni2+, and Mn4+ in the lithium layer and transition metal layer. First-principles energies in the GGA+U approximation were used to fit the Hamiltonian, as we find that GGA+U better represents magnetic interactions than standard GGA. The simulation results suggest two phase-transition temperatures at approximately 550 and 620 degrees C. Below the first phase-transition temperature, a structure with almost no Li/Ni disorder in the Li layer is energetically favorable. Between the two temperatures, a partially disordered flower structure with about 8-11% Li/Ni disorder is found. Above the second phase transition, a structure that is more disordered but still consistent with a root 3 x root 3 honeycomb model with 8-11% Li/Ni disorder is stable. The results from these simulations are corroborated with DSC, TEM, and XRD measurements on a recently synthesized LiNi0.5Mn0.5O2 with negligible Li/Ni disorder.