Quaternary phase diagrams of spinel Liy□1-yMnxNi2-xO4 and composite cathode voltages for concentration gradient materials

Core-shell coating structures and concentration gradient materials may enhance Li-ion battery performance by integrating advantages of core and shell components without introducing unfavorable problems associated with general coatings. The fundamental thermodynamic properties of concentration gradient composite materials are complex due to the multicomponent nature of the problem. We systematically study the thermodynamics of ordering and phase separation in the quaternary spinel Li-y square 1-yMnxNi2-xO4 (square means vacancy) system by density functional theory calculations, together with the coupled cluster expansion method with interactions within and between (Li/square) and (Mn/Ni) sublattices. On the basis of coupled cluster expansion interactions and Monte Carlo simulations, we calculate quaternary phase diagrams as a function of temperature as well as voltage profiles of single ordered phases and multiphase composite structures. The phase diagram and voltage results are in good agreement with available experimental observations. We also predict a stable high-voltage ordered compound LiMnNiO4, with a very high delithiation voltage of 4.76 V. For the composite (Mn-rich + Ni-rich) cathode materials, the voltage profiles show combinations of plateaus from each component compound. The computational strategy of combining quaternary phase diagrams with voltage calculations provides a pathway to understand and design concentration gradient materials.