Article pubs.acs.org/JPCC
Atomic Layer Deposition and in Situ Characterization of Ultraclean Lithium Oxide and Lithium Hydroxide Alexander C. Kozen,*,†,‡ Alexander J. Pearse,†,‡ Chuan-Fu Lin,†,‡ Marshall A. Schroeder,†,‡ Malachi Noked,‡,§ Sang Bok Lee,§ and Gary W. Rubloff†,‡ †
Department of Materials Science & Engineering, ‡Institute for Systems Research, and §Department of Chemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *
ABSTRACT: We demonstrate the ultraclean atomic layer deposition (ALD) of Li2O and LiOH using lithium tert-butoxide (LiOtBu) precursor with H2O and plasma O2 as oxidants, along with conversion of Li2O and LiOH products to Li2CO3 upon CO2 dosing. Using LiOtBu and H2O results in LiOH below 240 °C and Li2O above 240 °C for otherwise identical process parameters. Substituting plasma O2 as the oxidation precursor results in a combination of Li2CO3 and Li2O products, indicating modification of the ALD reaction preventing volatilization of the C from the Li precursor. The chemistry of the films is definitively characterized for the first time with XPS utilizing an all-UHV transfer procedure from the ALD reactor. We use in situ UHV gas dosing to investigate the reaction mechanisms of ALD Li2O and LiOH with H2O and CO2 to simulate reactions upon air exposure. Lastly, we employ in situ spectroscopic ellipsometry to determine the reaction kinetics of thermal LiOH decomposition, and we report an activation energy of 112.7 ± 0.6 kJ/mol.
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fundamental understanding of the film growth processes and surface chemistry during deposition is incomplete.13,14 Utilizing a unique ultrahigh-vacuum deposition and characterization apparatus,15 we deposit and characterize the first carbon-free ALD films of LiOH and Li2O and discuss the implications of the deposition parameters and gas exposure on resulting film chemistry and morphology.
INTRODUCTION With the push for ever higher energy density and power density in Li-ion batteries, innovative fabrication procedures are necessary to fully realize advances in the power and energy density of nanostructured devices. Indeed, recent nanostructured energy storage systems have demonstrated substantial increase in both power density and available gravimetric energy density of these heterostructured systems.1−3 As these devices approach the size limits of scalable architectures, they indicate a need for new alignment, deposition, and patterning techniques to realize device performance.4 Atomic layer deposition (ALD) is ideally suited to fabricate these nextgeneration nanostructures due to its low-temperature deposition, highly conformal and tunable nature, and self-alignment to a nanopatterned scaffold.5−7 Lithium compounds play multiple critical roles in electrochemical energy storage. Typically Li is incorporated into either anode or cathode structures in a battery, so the development of ALD processes for Li-containing electrode compounds is highly desirable. Li-based solid electrolytes are of growing importance as well, both for solid state batteries and as protection layers for metal anodes. Li2O represents an attractive starting point for ALD of Li compoundsa superionic conductor in its own right8 and a model process for incorporating Li into multicomponent ALD materials for electrodes and electrolytes. Previously, ALD films deposited using lithium tert-butoxide (LiOtBu) and H2O have been reported,9 but the chemistry of the deposited films has not been conclusively identified due to the extreme air sensitivity of lithium oxides and hydroxides and the lack of in situ characterization. Others have combined the LiOtBu and H2O chemistry in ternary ALD processes for metal oxide cathodes10 and solid electrolytes,11−13 but here again a © 2014 American Chemical Society
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EXPERIMENTAL PROCEDURES All films were deposited and measured in a unique experimental configuration that links ultraclean (i.e., high vacuum) ALD process capability with in situ X-ray photoelectron spectroscopy (XPS) and a variety of other surface analysis techniques. The entire deposition, transfer, and characterization sequence takes place under high vacuum conditions (
