Rechargeable Li-ion batteries (LIBs) are extensively utilized for sustainable and renewable energy storage,¹ but the high cost and low abundance of lithium have prompted the quest for alternative energy storage systems such as Na and K ion batteries.² An enhanced gravimetric energy density and a high diffusion rate of carrier ions are important aspects to consider in development of alkali metal ion batteries. Recently, 2D layered materials have emerged as anode materials for batteries.¹ Among them, the Group IV monochalcogenides are proposed as anode materials in the forms of nano-crystals³ and sheets.⁴ Herein, by using the density functional theory calculation, we show that the GeSe monolayer can be an anode material for Na and K ion batteries. The calculations of the phonon dispersion and formation energy prove that the GeSe monolayer is a structurally and thermodynamically stable material which can be experimentally exfoliated from its bulk counterpart.⁵ The GeSe monolayer shows adsorption energies of 1.92, 1.22 and 1.32 eV, respectively, for Li, Na and K ions. Upon electrodeposition of an alkali metal ion, substantial charge transfers from the metal atom to the GeSe monolayer and the semiconducting GeSe monolayer turns into a metallic state. The GeSe monolayer gives an anisotropic diffusion of the metal ions with moderate/low energy barriers on GeSe (0.29, 0.11 and 0.10 eV along zigzag, respectively, for Li, Na and K) which ensures fast charge and discharge rates. The low average voltages suggest that the GeSe monolayer is a suitable anode for alkali metal ion batteries. The GeSe monolayer yields a capacity of nearly half (177 mAh g^-1) of the commercial graphite anode. On the other hand, the theoretical capacities of Na and K ions are calculated to be 707 and 353 mAh g^-1, respectively, which are larger than that of typical 2D anode materials.

Reference:

1. H. R. Jiang, W. S., M. Liu, L. Wei, M. C. Wu and T. S. Zhao, Boron phosphide monolayer as a potential anode material for alkali metal-based batteries. J. Mater. Chem. A 2017, 5 (2), 672-679.
2. Joshi, R. P. O., B.; Barone, V.; Peralta, J. E., Hexagonal BC₃ : Robust Electrode Material for Li, Na, and K Ion Batteries. J. Phys. Chem. Lett. 2015, 6 (14), 2728-2732.
3. Im, H. S. L., Y. R.; Cho, Y. J.; Park, J.; Cha, E. H.; Kang, H. S., Germanium and Tin Selenide Nanocrystals for High-Capacity Lithium Ion Batteries: Comparative Phase Conversion of Germanium and Tin. J. Phys. Chem. C 2014, 118 (38), 21884-21888.
4. Ramasamy, P. K., D.; Lim, D.-H.; Ra, H.-S.; Lee, J.-S., Solution synthesis of GeS and GeSe nanosheets for high-sensitivity photodetectors. J. Mater. Chem. C 2016, 4 (3), 479-485.
5. Karmakar, S. C., C.; Datta, A., Two-Dimensional Group IV Monochalcogenides: Anode Materials for Li-Ion Batteries. J. Phys. Chem. C 2016, 120 (27), 14522-14530.