Abnormal Growth Behavior of Atomic-layer-deposited Lithium Metal Oxide(LMO)(M=Al, Nb, Ti)

Abstract

Due to their great qualities, including high cell voltage, low self-discharge rate, and extended life, lithium-ion batteries have received more attention from researchers and have been commercialized more successfully than other secondary batteries. The increased energy density of lithium ion batteries and a high degree of stability are required along with the explosive rise in demand. The most preferred method for increasing the energy density of the lithium ion battery is to use lithium metal or silicon instead of graphite as the anode and the cathode material conversion to NCM. However, these anodes have a problem in that their performance is not satisfied due to very large volume expansion contraction during charging and discharging and is broken down. NCM also has the advantage that as the content of Ni increases, the energy density increases and the price becomes cheaper, but has the disadvantage of very poor structural stability. These properties are not easy to control because they are properties of the material itself. As an alternative to that, interest in the protective film is growing. Among them, ALD may overcome the disadvantage of wet deposition due to the reactivity between a lithium active material, which has high reactivity, and a solvent through dry deposition. In addition, since it is easy to control the composition by adjusting the cycle ratio and monolayer deposition proceeding to angstrom scale, a very thin protective film required by the active material can be implemented. For this reason, research on the ALD protective film has been conducted, and metal oxide was initially used, but recently, it is a trend to include lithium even in an as-deposited state without a separate lithiation process through deposition of film including lithium. However, in the case of ALD, some precursors may exhibit abnormal behavior. In particular, it is reported that abnormal behavior occurs when forming an oxide using water in lithium tert-butoxide (LiOtBu), a core precursor, in the deposition of lithium metal oxide. Behavior analysis is essential because these strange behaviors can act as a major obstacle in applying ALD. In this study, an analysis was conducted on the non-ideal behavior of lithium metal oxide using LiOtBu as a lithium precursor. Niobium, titanium, and aluminum were selected as metals to form compounds with lithium. Prior to the deposition of lithium metal oxide, Li, Nb, Ti, and Al were optimized, respectively. Unlike previously reported, a sufficiently long time was set in purge, one of the ALD cycles. Ozone was used as another oxidizing agent to analyze the difference from the water-based process and to determine whether it was abnormal behavior. As a result, it was confirmed that the behavior changes due to water, which was reported only in the existing Al, also appeared in all Nb and Ti. However, there was a difference in behavior change for each metal. Both the cycle ratio and behavior of abnormal behavior were different. There will be a limit to applying the lithium metal oxide protective film without understanding this abnormal behavior. Therefore, it will be important to grasp the behavior in order to successfully apply them, and together with this, it can contribute to improving the battery performance through successful application of the protective film.