Unveiling Hidden Chemistry between an Aluminum ALD Precursor and Li Metal Anodes

Trimethylaluminum (TMA) is a widely utilized aluminum precursor for atomic layer deposition (ALD) in various aluminum-based coatings. For example, TMA produces Al2O3 with H2O,[1] AlF3 with HF-pyridine,[2] and Al2S3 with H2S [3] with well-defined ALD mechanisms. These ALD processes have been widely applied into various materials to introduce ultra-thin Al-based coating layers, including energy storage materials such as battery cathodes, anodes, and solid electrolytes. Our group demonstrated that the Al2O3 ALD process using TMA can be used for Li-metal anode coating to improve electrochemical stability and reduce Li-dendrite formation during battery cycling.[4] Although many studies about ALD on Li metal anodes have been reported using TMA, most of them focus on Li-metal battery performance, instead of ALD chemistry. During deep investigation on ALD chemistry of TMA, we recently discovered an interesting chemistry between TMA and reactive metal substrate (i.e. Li metal). Based on our observation, TMA reacts with Li-metal during ALD cycles then producing a carbon composite layer on the top of Li metal, not following conventional ALD mechanisms. It may come from highly reactive nature both of TMA and Li metal. Despite of wide use of TMA, this phenomenon has not been reported to date because Li metal is passivated by several layers such as Li2CO3, LiOH and Li2O even stored in glovebox environment. In this presentation, we will discuss the chemistry between TMA and Li metal found by in-situ QCM, high-resolution XPS, SEM, and Raman studies. Then we will also demonstrate how this unexpected ‘side reaction’ can be utilized in Li-metal battery applications using liquid and solid electrolytes. [1] 1. A. C. Kozen et al., ACS Nano, 9, 5884–5892 (2015). [2] Y. Lee, H. Sun, M. J. Young, and S. M. George, Chemistry of Materials, 28, 2022–2032 (2016). [3] X. Meng, Y. Cao, J. A. Libera, and J. W. Elam, Chemistry of Materials, 29, 9043–9052 (2017). [4] L. Chen et al., Journal of Materials Chemistry A, 5, 12297–12309 (2017). Figure 1