Al₂O₃ coatings prepared by atomic layer deposition (ALD) using trimethyl aluminum (TMA) and H₂O on various cathode materials have been shown to enhance the battery cycle stability of lithium ion batteries. Interestingly, not all cathode materials are enhanced by Al₂O₃ ALD coating. For example, while Al₂O₃ coating on lithium cobalt oxide (LCO) using 1-2 TMA/H₂O ALD cycles dramatically improves cycle performance [1], much less positive effect on lithium manganese oxide (LMO) was observed [2]. 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 [2]. Furthermore, density functional theory (DFT) calculations reveal that this unique mechanism is driven by the large free energy changes upon methyl loss from TMA [2].

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 (Al-TDMA), aluminum trichloride (AlCl₃), dimethyl aluminum isopropoxide (DMAI), and aluminum triisopropoxide (ATIP) on LCO and LMO cathode materials. Our preliminary results showed that very different ALD chemistries on 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.

[1] Y. S. Jung et. al., J. Electrochem. Soc. 157 (1) A75-A81, (2010).

[2] L. Chen et. al., Chem 4 2418-2435 (2018).