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

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.