Preparation of graphite-like carbonaceous thin films by plasma CVD

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Lithium ion batteries have been vastly studied for use of portable electric devices. Recently, the batteries are expected to be power sources of hybrid electric vehicles. Carbonaceous materials have been used as negative electrodes in lithium ion batteries, and various carbonaceous materials have been investigated to improve the battery performances of lithium batteries. To elucidate the precise electrochemical properties of carbonaceous materials, thin film electrodes are very ideal, and therefore, we fabricated the very flat and homogeneous carbonaceous thin film electrode by means of plasma assisted CVD. Followings are the results obtained :(1) Carbonaceous thin films with low to high crystallinity have been obtained by plasma CVD at lower temperatures below 1023 K.(2) Diffusion coefficients of resultant carbonaceous thin film electrodes are evaluated, and the values first increased with increasing lithium ion content in carbonaceous and then decreased.(3) Lithium ion transfer at interface between electrode and electrolyte has been studied by AC impedance spectroscopy, and the activation energies for the lithium ion transfer at interface were determined. As a result, the activation energies were dependent on the electron donicity of solvents in electrolytes.(4) Surface modification of carbonaceous thin film electrodes were made in NF_3/Ar plasma, and the electrochemical properties of the surface-treated thin film electrodes was studied. The first irreversible capacities were greatly reduced as compared with those of no surface-treated carbonaceous thin films.