Chaney Gracie, Ibrahim Akram, Ersan Fatih, Çakır D, Ataca Can
Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circ., Baltimore, Maryland 21250, United States.
Department of Physics, Aydın Adnan Menderes University, Aydın 09100, Turkey.
ACS Appl Mater Interfaces. 2021 Aug 4;13(30):36388-36406. doi: 10.1021/acsami.1c05508. Epub 2021 Jul 26.
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 MX 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.
二维（2D）Janus过渡金属二硫属化物（TMDs）的结构不对称性产生内部偶极矩，从而导致有趣的电子特性。这些特性不同于Janus结构所源自的常规（对称）TMD结构。在本研究中，我们首先使用密度泛函理论中的第一性原理计算，研究了单个锂原子在不同浓度下在常规MX和Janus MXY（M = Mo、W；XY = S、Se、Te）TMD结构上的吸附和扩散情况。锂在每种Janus材料的顶面和底面的吸附能和迁移率有所不同。我们仔细研究了不同覆盖密度下锂吸附能、电荷转移和键长之间的相关性。为了获得更多物理见解并为未来对常规TMD和Janus材料的研究做准备，我们应用了一种监督机器学习（ML）模型，该模型使用聚类线性回归来预测锂在二维TMDs上的吸附能。我们开发了一种通用表示法，使用了一些描述符，这些描述符考虑了常规和Janus二维层的固有偶极矩和电子结构、吸附发生的面以及吸附原子掺杂的浓度依赖性。这种表示法可以很容易地推广用于其他杂质和二维层组合，包括合金。最后，我们着重分析这些结构作为电池应用中可能的阳极的情况。我们进行了锂扩散、开路电压和存储容量模拟。我们报告称，在所考虑的每种情况下，锂原子都很容易在过渡金属（Mo、W）顶位之间迁移，在这些方面，许多研究的Janus材料与石墨烯和常规TMDs相当或更优。此外，我们报告称，具有较高电负性硫属原子的一侧适合锂吸附，并且只有MoSSe和MoSeTe适合锂原子在表面上的完全覆盖。由于负开路电压，双层锂吸附受到阻碍。双层Janus结构在放电/充电过程中体积膨胀/收缩较小且具有较高的存储容量，因此更适合电池应用。Janus单层在吸附单个锂离子后会从半导体转变为金属，这将提高阳极导电性。结果表明，所研究的Janus结构在锂离子电池中作为电极应表现良好。
