Abstract
Contrary to a wide-spread belief that alkali metal (AM) atoms intercalated into layered materials form single-layer structures only, recent experiments [Nature 564 (2018) 234] showed that multi-layer configurations of lithium are possible in bi-layer graphene. Using state-of-the-art first-principles calculations, we systematically study the intercalation energetics for various AMs (Li, Na, K, Rb, Cs) in bi-layer graphene and MoS 2 . We demonstrate that for bi-layer graphene as host the formation energy of multi-layer structures is negative for K, Rb and Cs and only slightly positive for both Li and Na. In view of the previous experimental data on lithium, a multi-layer of Na might therefore form, while it is well-known that single-layers of Na in graphitic hosts are energetically very unfavorable. In MoS 2 , multi-layer structures are considerably higher in energy than the single-layer ones, but the formation of the former can still occur, especially for the AMs with the lowest electro-negativity. To rationalize the results, we assess the charge transfer from the intercalants to the host material and analyze the interplay between the ionic and covalent bonding of AM and host atoms. While our theoretical effort primarily focuses on the fundamental aspects of AM intercalation, our findings may stimulate experimental work addressing multi-layer intercalation to maximize the capacity of anode materials in AM ion batteries. • Theoretically shown that multi-layer configurations of alkali metals in bi-layer graphene and MoS 2 are possible. • Charge transfer from the intercalants to the host material is assessed. • Intercalation of Na as multilayers should be energetically favorable, contrary to single-layer configuration.
Highlights
A fossil fuel-free society is hardly possible without the development of light-weight but high-capacity rechargeable electrical power sources, such as alkali metal (AM)-ion batteries [1,2], which had extensively been studied since 1970s and entered the market in early 1990s
While our theoretical effort primarily focuses on the fundamental aspects of AM intercalation, our findings may stimulate experimental work addressing multi-layer intercalation to maximize the capacity of anode materials in AM ion batteries
We did not consider the AMC8 phase, as it has a lower concentration of AM atoms, and overall our goal was to compare the energetics of multi-layer and single-layer AM structures
Summary
A fossil fuel-free society is hardly possible without the development of light-weight but high-capacity rechargeable electrical power sources, such as alkali metal (AM)-ion batteries [1,2], which had extensively been studied since 1970s and entered the market in early 1990s. The significance of this scientific and technological breakthrough was re flected by the 2019 Nobel Prize in Chemistry awarded for the develop ment of Li-ion batteries (LIBs). Understanding TMD behavior upon AM intercalation is important for controlling phase transitions from the semiconducting H to metallic
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