Heterolayered Li +-MnO 2-[Mn 1/3Co 1/3Ni 1/3]O 2 nanocomposites with improved electrode functionality: Effects of heat treatment and layer doping on the electrode performance of reassembled lithium manganate

Abstract

Novel heterolayered nanocomposites consisting of interstratified MnO 2 and [Mn 1/3Co 1/3Ni 1/3]O 2 nanosheets are synthesized by a layer-by-layer self-assembly between negatively charged metal oxide nanosheets and lithium cations. According to powder X-ray diffraction and micro-Raman analysis, all of the as-prepared Li +-xMnO 2-(1-x)[Mn 1/3Co 1/3Ni 1/3]O 2 nanocomposites with x = 1, 0.7, and 0.4 have a lamella structure with similar basal spacing of ∼ 7.1 Å, indicating the formation of lithium intercalation structure with cointercalated water bilayers. The nanoscale mixing of MnO 2 and [Mn 1/3Co 1/3Ni 1/3]O 2 nanosheets is confirmed by energy-dispersive spectrometry-elemental mapping analysis. Upon a self-assembly with Li + ions, there occur no marked changes in the octahedral symmetry and mixed oxidation state of M 3+/M 4+ ions (M = Mn, Co, and Ni) in the precursor metal oxide nanosheets. All of the as-prepared nanocomposites commonly experience a structural transformation from hydrated layered structure to dehydrated layered structure at 200 °C, which is followed by the second-phase transition to cubic spinel structure at 600 °C. Despite distinct structural changes of the nanocomposites at elevated temperatures, their porous stacking structure is well-maintained up to 400 °C. The heat-treatment at 400 °C leads to a significant improvement of the discharge capacity of the present nanocomposites because of the dehydration of as-prepared materials and the enhancement of crystallinity. The doping of [Mn 1/3Co 1/3Ni 1/3]O 2 layers enables us not only to increase the discharge capacity of the Li-MnO 2 nanocomposite but also to prevent the phase transition of layered manganese oxide to spinel structure during electrochemical cycling. The present study clearly demonstrates that a postcalcination process as well as a partial doping of [Mn 1/3Co 1/3Ni 1/3]O 2 layer is effective in improving the electrode performance of reassembled Li-MnO 2 nanocomposites. © 2012 American Chemical Society.