K.-F. Chiu

Plasma Induced Low Temperature Crystallization of Lithium Transition-Metal Oxide Thin Films for Lithium Microbatteries

The choices of substrates for lithium microbatteries or thin-film batteries have been limited by the high annealing temperatures, which are generally required in the fabrication of polycrystalline thin-film cathodes. In this paper, a plasma-assisted technique was developed for the low temperature fabrication of polycrystalline lithium transition-metal oxides as thin-film cathodes. The thin films were deposited by magnetron sputtering, followed by thermal annealing, and plasma treatments. By applying the plasma treatments, polycrystalline films of LiCoO 2 , LiMn 2 O 4 , and LiFePO 4 were obtained under relatively lower annealing temperatures (<400°C). The low temperature prepared films with/without plasma treatments were compared in terms of morphologies, crystal structures, and electrochemical performances. The plasma treatments lifted the discharge voltages and increased the capacity of the low temperature prepared films. The results demonstrated the feasibility of fabricating thin-film batteries on polymers or glass substrates, which generally can only endure temperatures lower than 400°C.

Characteristics of a VHF H2 plasma at high pressures for different discharge gaps

A capacitively coupled VHF H2 plasma (60 MHz) was produced at high pressures, and plasma parameters were examined as a function of pressure and power at different discharge gaps (10, 15, and 20 mm). Here, a balanced power feeding method was adopted to avoid anomalous discharges. It was found that the electron density peaked at a certain pressure. As the discharge gap decreased, the peak pressure increased. The electron temperatures at the discharge gaps of 10 and 20 mm were approximately 8 and 3 eV at high pressures, respectively. In addition, we found that the sheath potential corresponding to the ion bombardment energy with the discharge gap of 10 mm was lower than the theoretical value calculated on the basis of the sheath theory, suggesting the existence of negative ions. Estimation of negative ion concentration was attempted on the basis of the sheath theory taking into consideration negative ions.

Mechanism of VHF H2 plasma production at high pressures

A VHF H2 plasma was produced by a narrow-gap discharge at high pressures, and the plasma parameters were examined with the Langmuir probe. A bi-Maxwellian electron distribution was observed near the discharge electrode at a discharge gap of 10 mm, while a Maxwellian distribution was seen near the center. When the discharge gap was 15 mm, electrons had a Maxwellian distribution independent of the position. It was found that there must be a threshold in the discharge gap for stochastic heating to occur. The plasma potential near the discharge electrode was higher than that near the center of the interelectrode gap, suggesting the existence of negative ions. The simulation using the plasma hybrid code was carried out. The spatial profiles of the density and temperature of electrons were similar to the experimental results. The plasma potential had a hill-like profile that was quite different from the measured one. The negative ion density was negligible.