Measurements of Interdiffusion Coefficients of Transition Metals in Layered Li–Ni–Mn–Co Oxide Core–Shell Materials during Sintering

Core–shell structure positive electrode materials, based on layered Li–Ni–Mn–Co oxides, could be the next generation of positive electrode materials for high energy density lithium-ion batteries. Diffusion of the transition metal cations between the core and shell phases occurs during sintering, which can significantly affect the final core–shell (CS) properties. However, the interdiffusion constants have never been measured. Laminar pellets of the pure core phase and pure shell phase were pressed in contact and then heated to measure the interdiffusivity of the transition metals at various temperatures. The diffusion couples Ni3+/Co3+, Co3+/Mn4+, and Ni3+/Mn4+ were measured by analyzing composition versus position, with respect to the initial interface between the core and shell phase pellets. The transition metal composition profiles were measured with energy dispersive spectroscopy (EDS) line scans. This is the first time interdiffusion constants have been reported in the layered cathode materials to our knowledge. At 900 °C, Ni3+/Co3+ has the highest interdiffusivity, D, of ∼4.7 × 10–12 cm2/s, while Ni3+/Mn4+ has the lowest of ∼0.1 × 10–12 cm2/s. The activation energy barriers for Ni3+/Co3+, Co3+/Mn4+, and Ni3+/Mn4+ interdiffusion were determined from Arrhenius plots of D vs 1/T. Simulations of diffusion in spherical core–shell materials were performed to show how knowledge of the interdiffusion constants can guide rational design of practical core–shell materials.