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Graduate Student Seminar - 11/14/2014

"High Spatial Resolution Characterization of Al Anodes in All-Solid-State Batteries"

by Chen Gong

Friday, November 14, 2014 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Advisor:  Assistant Professor Jeremy Munday

All-solid-state Li-ion batteries are a promising and rapidly growing power source for mobile devices. However, the mechanisms of lithiation/delithiation for many anode materials are still not well understood. We investigate the mechanism of lithiation in all-solid-state thin film batteries with Al anodes for ultra-lightweight and compact device application. We resolve the chemical composition and morphological changes of the anode upon lithiation by confocal Raman microscopy and X-ray photo-electron spectroscopy (XPS). We find that a Li-Al-O thin layer forms at the top surface of the anode, confirmed by the emerging Raman peaks after cycling at 1380, 1585 cm-1. This oxide layer covers stable LiAl alloy mounds by a surface driven reaction (with (Fd3m) phase, Raman shift at 2890 cm-1). The XPS measurements corroborate the Raman results, showing an atomic ratio of Li:Al equal to 4:1. A simple thermodynamic model for the lithiation of Al suggests that LiAlO2 and Li5AlO4 are expected to form at 3.35 V and 0.17 V, respectively, indicating that different Li-Al-O coexist at the surface of the anode. During lithiation, Li is trapped at the intrinsic Al2O3 on the top of the anode, preventing Al diffusion. This unique behavior of Al is caused by Li surface diffusion: Li diffuses ≈ 9 orders of magnitude faster than Si than it does in Al. The addition of a thin and inert metallic cap layer could prevent the surface driven reactions observed, and the design of a new device architecture will be presented.

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