Unique Chemistry of Atomic Layer Deposition for Protection of Cathodes and Li-Metal Anodes

D. Kang, R.E. Warburton, A.U. Mane, J.P. Greeley, J.W. Elam
Argonne National Laboratory,
United States

Keywords: atomic layer deposition, lithium manganese oxide, LMO, lithium metal, cathode, anode

Summary:

Ultra-thin coatings of protecting layer by atomic layer deposition (ALD) on various cathode and anode materials have been shown to enhance the battery cycling stability of Li-ion and Li-metal batteries. For cathode materials, ultra-thin Al2O3 ALD coating is widely used as a protecting layer, however, not all cathode materials are significantly enhanced by Al2O3 ALD coating. For example, while Al2O3 coating on lithium cobalt oxide (LCO) using 1-4 TMA/H2O ALD cycles dramatically improves cycle performance, less positive effect on lithium manganese oxide (LMO) was observed. We revealed the TMA surface reactions on LMO are unusual in that they do not involve hydroxyls, ethane is released, and the manganese undergoes redox chemistry from our previous study. Furthermore, density functional theory (DFT) calculations reveal that this unique mechanism is driven by the large free energy changes upon methyl loss from TMA. This information leads us to speculate that the surface reactions and subsequent electrochemistry might also depend on different Al precursors. To evaluate our hypothesis, we are exploring a range of aluminum precursors including trimethyl aluminum (TMA), tris(dimethylamido) aluminum (TDMAA), aluminum trichloride (AlCl3), dimethyl aluminum isopropoxide (DMAI), and aluminum triisopropoxide (ATIP) on LMO cathode materials. Our results showed that totally different TMA/H2O ALD chemistries between LCO and LMO cathode materials. It suggests a correlation between cation reduction on the cathode surface and the relative Lewis acidity of the Al precursor ligands. We will elaborate on these findings using results from XPS measurements, DFT calculations, electrochemical characterizations, and battery cycling studies. Concerning Li-metal anode, many Al-based layer coatings by ALD using TMA have been reported to enhance the battery cycling stability in Li-metal batteries and to reduce dendrite formation. For example, 1 nm of Al2O3 coating on Li-metal anode enhanced battery cycling performance. Interestingly, further application of Al-based coating onto Li-metal and in-depth understanding, we recently unveiled there is a unique chemistry between Li-metal and TMA. The high reactivity of TMA with Li-metal, it is revealed that TMA and Li-metal produce Li-Al alloy at the surface. Based on our observation, we investigate detailed surface chemistry between TMA and Li-metal, which is not reported to date. Using several in situ techniques such as Quartz Crystal Microbalance (QCM) and Quadrupole Mass Spectroscopy (QMS), we reveal the reaction mechanism. By the end of the study, we suggest appropriate Al-precursor other than TMA for ALD coating of Al-based materials, which is able to produce a protecting layer in a stable manner.