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ADVANCED MATERIALS ENABLED BY ATOMIC LAYER DEPOSITION FOR HIGH ENERGY DENSITY RECHARGEABLE BATTERIES
Title
ADVANCED MATERIALS ENABLED BY ATOMIC LAYER DEPOSITION FOR HIGH ENERGY DENSITY RECHARGEABLE BATTERIES
Creator
Chen, Lin
Date
2017, 2017-05
Description
In order to meet the ever increasing energy needs of society and realize the US Department of Energy (DOE)’s target for energy storage,...
Show moreIn order to meet the ever increasing energy needs of society and realize the US Department of Energy (DOE)’s target for energy storage, acquiring a fundamental understanding of the chemical mechanisms in batteries for direct guidance and searching novel advanced materials with high energy density are critical. To realize rechargeable batteries with superior energy density, great cathodes and excellent anodes are required. LiMn2O4 (LMO) has been considered as a simpler surrogate for high energy cathode materials like NMC. Previous studies demonstrated that Al2O3 coatings prepared by atomic layer deposition (ALD) improved the capacity of LMO cathodes. This improvement was attributed to a reduction in surface area and diminished Mn dissolution. However, here we propose a different mechanism for ALD Al2O3 on LMO based on in-situ and ex-situ investigations coupled with density functional theory calculations. We discovered that Al2O3 not only coats the LMO, but also dopes the LMO surface with Al leading to changes in the Mn oxidation state. Different thicknesses of Al2O3 were deposited on nonstoichiometric LiMn2O4 for electrochemical measurements. The LMO treated with one cycle of ALD Al2O3 (1×Al2O3 LMO) to produce a sub-monolayer coating yielded a remarkable initial capacity, 16.4%higher than its uncoated LMO counterpart in full cells. The stability of 1×Al2O3 LMO is also much better as a result of stabilized defects with Al species. Furthermore, 4×Al2O3 LMO demonstrates remarkable capacity retention. Stoichiometric LiMn2O4 was also evaluated with similar improved performance achieved. All superior results, accomplished by great stability and reduced Mn dissolution, is thanks to the synergetic effects of Al-doping and ALD Al2O3 coating.Turning our attention to the anode, we again utilized aluminum oxide ALD to form conformal films on lithium. We elaborately designed and studied, for the first time, the growth mechanism during Al2O3 ALD on lithium metal in-situ quart crystal microbalance (QCM) measurements and found larger growth than expected during the initial cycles. Besides, we discovered that electrolytes show much enhanced wettability on Li with Al2O3 coating, leading to uniform and dense solid electrolyte interphase formation as well as less electrolyte required for battery operations. Also, we achieved more than 2 times longer cycling life with protected Li and obtained Coulombic efficiencies as high as ~98% at a practical current rate of 1 mA/cm2, compared to bare Li. More significantly, when the electrolyte volume is limited (10 μL and 5 μL), the cycling life is about 4 times longer. X-ray photoelectron spectroscopy (XPS) for electrodes after cycles and in-situ transmission electron microscopy (TEM) demonstrate that most of lithium is deposited beneath the film. The more uniform Al2O3 coated lithium after cycling observed by scanning electron microscopy (SEM) verifies that ALD Al2O3 isexceptionally effective to prevent lithium dendrite formation. These results demonstrate that ALD Al2O3 coatings offer a promising route towards energy storage devices that utilize lithium metal anodes, such as Li-S batteries.
Ph.D. in Materials Science and Engineering, May 2017
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