Hexagonal boron phosphide as a potential anode nominee for alkali-based batteries: A multi-flavor DFT study

The adsorption of alkali atoms (Li, Na, and K) on graphene and hexagonal boron phosphide (h-BP) is studied by means of various density functionals including semi-local GGA, dispersion corrected DFT-D, meta-GGA, and many of the available non-local vdW-DF functionals. The accurate interaction energies are crucial for energy storage and battery applications as it is directly related to key properties in electrochemistry, such as storage capacities in general, and open circuit voltages (OCVs), in particular. A wider range of adsorption strength is predicted depending on the choice of non-local vdW-DF functionals. Furthermore, the performance of vdW-DF functionals was found to be independent of the choice of pseudopotentials used and especially if the choice is revPBE. Additionally, an excellent agreement is found between the Gaussian, VASP, and QE codes. Moreover, the h-BP is found to be an exceptional anode material for alkali batteries which can compete with any other available anode material. The small change in the adsorption energies as a function of increasing concentration of alkali atoms is a unique characteristic of h-BP. The exceptional 1283 mAh/g storage capacity not only for Li but in contrary with the previous study, also for Na in addition to the 642 mAh/g for K at vdW-DF/DZP level makes it a prominent candidate to be used as anode material. The average open circuit voltage for Li, Na, and K was also found to be in superb range. However, the values are found to be sensitive to the choice of functional, and in some cases, the storage capacity can be predicted as high as twice of the actual values. Therefore, the accurate description of the interaction is crucial and this study can be used to further refine the non-local DFT functionals. Moreover, by virtue of these properties, h-BP can be the best choice to be used as an anode material not only in LIBS but also in SIBs and KIBs.