Enhanced electrochemical characteristics of lithium manganese oxide thin film cathodes for lithium-ion rechargeable microbatteries.

摘要

Spinel LiMn2O4 (average oxidation state of Mn = 3.5) thin film has been introduced for a promising cathode of thin film lithium-ion microbatteries because of its advantages over other cathodes. Thus, many research groups have been investigating the thin film lithium manganese oxide cathode using several different techniques but only a few of them achieved acceptable electrochemical properties required for a thin film cathode for 4 V region. However, for 3 V application the wide applications of LiMn2O 4 film as a cathode in rechargeable microbatteries have been restricted by electrochemically unfavorable facts such as capacity fade on cycling and poor rate capability at high rates. In this study, we examined the mechanisms responsible for capacity fade on cycling and rate capability of LiMn 2O4 thin film cathodes with the help of pulsed laser deposition (PLD) technique.; In an attempt to address these issues, a three-part experimental procedure has been designed to look at the effect of structure and compositions of the thin film cathodes on their electrochemical characteristics. First, the effect of growth temperature of the thin film cathodes has been investigated. Next, LiMn2O4 thin film cathodes doped with aluminum, which replaces Mn3+ in the spinet structure, have been synthesized and characterized as a function of the amount of aluminum substituted. Finally, ultraviolet radiation was added to a PLD system for in situ ultraviolet-assisted PLD (UVPLD) growth of cathode films. Through the addition of ultraviolet radiation, highly reactive oxygen species are generated which alter the oxygenation conditions and dynamically alter the films properties such as crystallinity and composition.; A variety of characterization techniques indicate that LiMn2O 4 thin film cathodes grown at 400∼500°C exhibit the optimized electrochemical characteristics in terms of capacity, capacity retention, and rate capability. Al-doped LiMn2O4 thin films show a more stable structure, leading to higher capacity retention with some sacrifice of discharge capacity and rate capability. Oxygen-rich spinel (LiMn2O4+delta, delta > 0) thin film cathodes produced by UVPLD result in very high capacity and better rechargeability but lower rate capability.