Lithium-ion storage devices have been widely studied for practice application in electric devices, especially for mobile or portable electric devices and electric vehicle (EV) or hybrid EV (HEV). For these industrials needs, the development of electrodes with high specific capacities (=high energy density) and high current densities (=high power density) is necessary. Therefore, the fabrication of materials with high specific capacity at high charge/discharge current rate has been expected.

The advantage of the lithium-ion secondary battery as an energy storage device is its high specific capacity. Poizot et al. reported materials such as nanocrystalline transition metal oxides such as NiO, CoO, and FeO for negative electrodes, which indicate the high specific capacity of around 700  mAh/g at the low rate of 0.07  A/g.¹ Many researchers have studied the transition metal oxide cells.^{2,3,4,5,6,7,8,9,10,11,12,13,14} However, the weak points of the lithium-ion battery, namely, the low power density, has not been overcome.

Electrodes using transition metal oxide with high specific capacity at high charge/discharge current rate have not been reported. There are four problems due to the nature of a lithium-ion battery that should be solved in order to apply a high-rate lithium storage device^15,16,17: (i) Increasing the electronic conductivity of the electrode materials, (ii) decreasing the particle size to reduce the required diffusion length in the active materials, (iii) reducing the effective specific current density in the rapid charge-discharge process, and (iv) realizing high cycle performance even at rapid charge-discharge process.

In order to solve these problems, general processes such as pressing of the powder, physical vapor deposition, and chemical vapor deposition are not suitable because nanostructure control is difficult in these processes. Synthetic processes for nanostructured materials have been developed through the bottom-up process by a chemical bath deposition (CBD),¹⁸ which can control the real nanosize in several nanometers.¹⁹ The resultant nanoparticulate porous materials apply for electrochemical devices making good use of improved and accelerated charge transport phenomena due to high surface areas and high porosity.²⁰

The CBD, in which water is typically used as a solvent, has been utilized for preparing various kinds of metal oxides and sulfides because thin-film materials can be fabricated at low temperatures without expensive and special apparatus required for vapor-phase techniques.^18,21 It is suitable as a low-cost and low-energy consumption process for the environmental problem. Moreover, CBD can deposit the materials on the substrate with complicated morphology due to the reaction in solution. However, the fabrication of metal oxide films that are both nanocrystalline and nanoporous by general CBD was difficult because most metal oxides grow as single crystals.

We have reported the self-template method by CBD.^20,22,23 The deposited metal hydroxide nanosheets instead of metal oxide are easily converted into nanocrystalline and nanoporous metal oxides by pyrolysis as a self-template without nanostructural deformation.^20,22,23

Here, we report the nanostructured Fe₂O₃ on nickel mesh, which are low-cost materials, as negative electrodes for Li-ion cells with the high specific capacity at high charge/discharge current rate to solve the above-mentioned problems. Nanocrystalline and mesoporous Fe₂O₃ film with specific nanostructure was formed on the nickel mesh, knitted nickel micrometer wires, via pyrolytic transformation of iron oxyhydroxide film, which was directly deposited on the entire surface of the nickel mesh by the CBD. The frameworks of the nickel mesh and mesopores of Fe₂O₃ film filled with electrolyte provide ideal electrolyte and lithium ion paths. The nickel wires with low resistivity play a role of electronic path. Moreover, the high surface area of the Fe₂O₃ film reduces the required diffusion length in the active materials and the effective specific current density. The resultant Fe₂O₃ negative electrode shows the high specific capacity at high charge/discharge current rate, which indicates the possibility for an energy storage device with high energy density at high power density.