Synthesis, electrochemistry, and structural studies of lithium intercalation of a nanocrystalline Li ₂MnO ₃-like compound

A nanocrystalline lithium-rich manganese (IV) oxide, synthesized by a low-temperature sol-gel route, is reported as a surprising lithium intercalation host. The composition of the material is very close to Li ₂MnO ₃. X-ray diffraction (XRD), electron diffraction, and high-resolution transmission electron microscopy analyses establish that the material possesses a nanocrystalline structure, similar to that of the rock salt monoclinic Li ₂MnO ₃, with a crystallite size of about 5 nm. X-ray absorption spectroscopy (XAS) analysis at the Mn K edge indicates that the Mn is in the 4+ oxidation state and in a local atomic/ electronic environment similar to that in the rock salt monoclinic Li ₂MnO ₃. Unlike the microcrystalline Li ₂MnO ₃, which is known to be electrochemically inactive for lithium intercalation or deintercalation, this nanocrystalline counterpart surprisingly yields a reversible intercalation capacity of 0.71-0.87 Li per formula or 163-200 (mA h)/g and a specific energy density of 400-450 (mW h)/g, at different current rates, with excellent capacity retention over repeated cycling. In situ XAS analysis, conducted during discharge and charge, shows the occurrence of highly reversible Faradaic processes, with reduction and oxidation of Mn. Electrochemical data, namely, low hysteresis in the charge/discharge voltage profiles, excellent cycling performance, and the charge coefficient being constantly at unity, in conjunction with structural data, namely, XAS and XRD collected during/after electrochemical tests showing that the material retains its original structure, establish the excellent electrochemical and structural stability and reversibility of the compound. The role of nanocrystallinity, lack of long-range order, defects, disorder, and nanostructured morphology are discussed to relate to the surprising intercalation properties of this nanocrystalline compound.