The
structural asymmetry of two-dimensional (2D) Janus transition-metal
dichalcogenides (TMDs) produces internal dipole moments that result
in interesting electronic properties. These properties differ from
the regular (symmetric) TMD structures that the Janus structures are
derived from. In this study, we, first, examine adsorption and diffusion
of a single Li atom on regular MX2 and Janus MXY (M = Mo,
W; XY = S, Se, Te) TMD structures at various concentrations using
first-principles calculations within density functional theory. Lithium
adsorption energy and mobility differ on the top and bottom sides
of each Janus material. The correlation between Li adsorption energy,
charge transfer, and bond lengths at different coverage densities
is carefully examined. To gain more physical insight and prepare for
future investigations into regular TMD and Janus materials, we applied
a supervised machine learning (ML) model that uses clusterwise linear
regression to predict the adsorption energies of Li on top of 2D TMDs.
We developed a universal representation with a few descriptors that
take into account the intrinsic dipole moment and the electronic structure
of regular and Janus 2D layers, the side where the adsorption takes
place, and the concentration dependence of adatom doping. This representation
can easily be generalized to be used for other impurities and 2D layer
combinations, including alloys as well. At last, we focus on analyzing
these structures as possible anodes in battery applications. We conducted
Li diffusion, open-circuit voltage, and storage capacity simulations.
We report that lithium atoms are found to easily migrate between transition-metal
(Mo, W) top sites for each considered case, and in these respects,
many of the examined Janus materials are comparable or superior to
graphene and regular TMDs. In addition, we report that the side with
higher electronegative chalcogen atoms is suitable for Li adsorption
and only MoSSe and MoSeTe can be suitable for full coverage of Li
atoms on the surface. Bilayer Li adsorption was hindered due to negative
open-circuit voltage. Bilayer Janus structures are better suited for
battery applications due to less volumetric expansion/contraction
during the discharge/charge process and having higher storage capacity.
Janus monolayers undergo a transition from semiconducting to metallic
upon adsorption of a single Li ion, which would improve anode conductivity.
The results imply that the examined Janus structures should perform
well as electrodes in Li-ion batteries.