Synthesis and Electrochemical Properties of Layered Birnessite MnO2/Activated Carbon Nanocomposite.

Mesoporous MnO₂/activated carbon (AC) nanocomposites are promising materials as anode material in lithium ion batteries which are being considered by researchers due to their low conductivity and considerable irreversible capacity loss. The porous structures of these nanocomposites can facilitate Li ion diffusion into the porous structure. In this paper, MnO₂/AC nanocomposites were synthesized by the coating of a MnO₂ layer on AC using the reduction reaction of KMnO₄ with AC and citric acid. The AC in nanocomposites prepared from walnut shell powder, hydrazine hydrate, and NaCl were used as activating agents. The MnO₂/AC nanocomposites were synthesized with mass ratios of 1:4, 1:1, 4:1, and 1:0 and the effect of the annealing process at the temperature of 300°C was investigated. The x-ray diffraction (XRD) patterns of MnO₂/AC nanocomposites have shown the growth of the layered birnessite-type MnO₂ nanoparticles on the AC. Field emission-scanning electron microscopy (FE-SEM) images of the AC shows cracked surfaces with pieces of sizes from 20 nm to 100 nm and the pore size in the wide range of 20–200 nm. Based on EDS results, decreasing the AC content in MnO₂/AC nanocomposites led to the decrease of the weight ratio of carbon before annealing, but increased the weight ratio of carbon after annealing. Fourier-transform infrared (FTIR) spectroscopy results showed the existence of bands attributed to the lattice vibration of Mn−O and the strengthening of the related carbon bands in composites containing AC. The direct and indirect band gaps of MnO₂/AC nanocomposites were determined by UV-Vis absorption spectroscopy. For the MnO₂/AC nanocomposites with less or equal MnO₂ content, the indirect energy gap of MnO₂ (≈ 2.4 eV) increases with increasing the MnO₂ /AC ratio before annealing, while this gap disappeared after annealing. The direct energy gap of MnO₂ in the nanocomposites was always larger than 3.09 eV, due to the nanoscale size of the MnO₂ nanoparticles. Comparisons of the direct gaps of 1:4, 1:1, and 4:1 composites before and after annealing imply that the direct gap decreases from 5.88 eV, 5.51 eV, and 6.41 eV before annealing to 5.52 eV, 5.44 eV, and 5.68 eV after annealing, respectively. Electrochemical measurements including voltage capacity and dQ/dV indicate that the MnO₂/AC (1:4) nanocomposite anodes demonstrate more than 89% coulombic efficiency and a specific capacity of 1495 mAh/g in 20 mA/g in the first cycle. [ABSTRACT FROM AUTHOR]