Correlations between Transition-Metal Chemistry, Local Structure, and Global Structure in Li₂Ru_0.5Mn_0.5O₃ Investigated in a Wide Voltage Window

Li₂Ru_0.5Mn_0.5O₃, a high capacity lithium-rich layered cathode material for lithium-ion batteries, was subject to comprehensive diagnostic studies, including in situ/ex situ X-ray diffraction, X-ray absorption spectroscopy (XAS), pair distribution function, and high resolution scanning transmission electron microscopy analysis, to understand the correlations between transition-metal chemistry, structure, and lithium storage electrochemical behavior. Ru–Ru dimers were identified in the as-prepared sample and found to be preserved upon prolonged cycling. Presence of these dimers, which are likely caused by the delocalized nature of 4d electrons, is found to favor the stabilization of the structure in a layered phase. The in situ XAS results confirm the participation of oxygen redox into the charge compensation at high charge voltage, and the great flexibility of the covalent bond between Ru and O may provide great reversibility of the global structure despite the significant local distortion around Ru. In contrast, the local distortion around Mn occurs at low discharge voltage and is accompanied by a layered to 1T phase transformation, which is found to be detrimental to the cycle performances. It is clear that the changes of local structure around individual transition-metal cations respond separately and differently to lithium intercalation/deintercalation. Cations with the capability to tolerate the lattice distortion will be beneficial for maintaining the integrality of the crystal structure and therefore is able to enhance the long-term cycling performance of the electrode materials.