Template synthesis of polypyrrole-coated spinel LiMn₂O₄ nanotubules and their properties as cathode active materials for lithium batteries

Tubules of 200 nm outer diameter spinel LiMn₂O₄ were prepared by thermal decomposition of an aqueous solution containing lithium nitrate and manganese nitrate at 1:2 molar ratio using a nanoporous alumina membrane as a template. After dissolving the template membrane, the resulting nanotubule array of LiMn₂O₄ was coated with polypyrrole to investigate the galvanostatic charge-discharge characteristics. The polypyrrole-coated LiMn₂O₄ tubule electrodes exhibited higher capacities than the polypyrrole-coated LiMn₂O₄ thin-film electrode prepared under the same conditions, except for the use of the alumina membrane; it was ca. 2.5 times greater at 0.1 mA cm^-2 and became ca. 12 times greater with an increase in current density to 1.0 mA cm^-2. The observed high capacity of the tubule electrode seems to have resulted from two kinds of effects: a decrease in real current density of high specific surface area at the tubule electrode and a decrease in thickness of LiMn₂O₄ solid phase for Li⁺ ions to diffuse through during the charging and discharging reactions.