Property Evolution of ALD-Al2O3 Coated and Uncoated Si Electrodes --- A first Principle Investigation

Introduction

Rechargeable energy storage device are evolving as a major technological challenge when people are aiming its applications on a large scale, such as environmentally friendly electric vehicles or renewable energy storages. In spite of great deal of research, at present, reported energy density of lithium-ion batteries are significantly insufficient for the electric vehicle applications. Therefore, to meet the demand for the large scale applications, Silicon (Si) has long been considered a promising high capacity negative electrode in lithium ion batteries. However, the volume change up to ~300% for Si during lithiation and dilithiation leads to fracture of Si and/or loss of electrical contact with the conductive phase or the current collectors.

The native SEI layer formed due to electrolyte decomposition can effectively passivate the graphite electrode (currently used in all Li-ion batteries) surface to prevent further side reactions with electrolyte, providing high coulombic efficiency.^1,2 However, stabilizing SEI is much more difficult on Si than graphite due to the large volume change. A stable SEI should be able to mechanically accommodate the large volume in the underlying Si, while maintaining the passivation of the Si surfaces. Because the naturally formed native SEI cannot meet both criteria above, various artificial SEI layer have been designed to improve the performance and life of Li-ion batteries. Especially, atomic layer deposition (ALD) coatings have shown promising results to slow down or change the reduction process of EC electrolyte solvent and can also prevent mechanical failure of Si electrode.³

In order to develop a validated model to design coated Si electrode, it is critical to understand the mechanical and chemical stability of SEI in electrochemical environment, the correlation between coulombic efficiency and the dynamic process of SEI evolution and the structural optimization of both the SEI and the Si electrode. Therefore, to elucidate the underlying intricate chemistry, rigorous analysis at atomic level becomes necessary. This study will focus on comparing the ALD-Al₂O₃ coated Si surface with non-coated surface, which is typically terminated by native SiO₂layer.  

Calculations and Results

First, the composition evolution of Al₂O₃ and SiO₂ layer within the voltage of 1~0.1V in a Li ion battery half-cell was computed from density functional theory (DFT). As shown in Table I, both SiO₂ and Al₂O₃will be lithiated at the voltage above EC decomposition (~0.8V), contributing to the initial irreversible capacity loss.

Thus, the mechanical property and Li diffusion were investigated for the lithiated aluminum oxide and lithiated Si oxide phases. The DFT predicted Youngs’ Modulus for crystalline Al₂O₃ coating is softened due to lithiation, while the change in SiO₂layer is minimal. Both Li interstitial and Li vacancy can diffuse easily in Lithaited aluminum oxide and Si oxide phases.

These DFT results were then used to guide the development of the reactive force field (ReaxFF) of Li-Si-O and Li-Al-O systems. More specifically, the ReaxFF was developed against to the open circuit voltage curve, vacancy formation energy, Li interstitial formation energy and their diffusion barriers in crystalline SiO₂ and Al₂O₃structures.   

In order to understand the mechanical response and the diffusion properties of the coating layer on Si electrode upon litigation and delithiation, we further performed MD simulations with the ReaxFF.

Table 1: Illustration of Al₂O₃ and SiO₂covered Si electrode surface. (The data was based on crystalline form via DFT calculations).

Observations	Bare Si electrode	ALD coated electrode
Lithiation Reaction	2Li + SiO2 --> 0.5Li4SiO4 + 0.5Si <V> = 1.2V	1.5Li+Al2O3 --> 1.5LiAlO2+0.5Al <V>=0.93V
Young’s Modules (GPa)*	E(SiO2) = 140 E(Li4SiO4 ) = 143	E(Al2O3) =360 E(LiAlO2)=150

Discussion

                The simulations results are compared with experimental modulus and kinetics measurements on ALD-Al₂O₃coated and uncoated Si thin film electrodes.  Further exploration of the failure mechanism of the ALD coating on Si electrode will be discussed.

References

1. Y.S. Jung, A.S.Cavanagh, R.A. Leah, S.H.Kang, A.C.Dillon, M.D. Groner, S.M. George, and Y.H.Lee, Adv. Mater. 22, 2172 (2010)
2. X. Xiao, P. Lu, D. Ahn, Adv. Mater. 23, 3911 (2011).
3. K. Leung, Y. Qi, K. R. Zavadil, Y. S. Jung, A. C. Dillon, A. S. Cavanagh, S. Lee, and S. M. George, J. Am. Chem. Soc., 133, 14741 (2011)