B2P6: A Two-Dimensional Anisotropic Janus Material with Potential in Photocatalytic Water Splitting and Metal-Ion Batteries

Both anisotropic and Janus two-dimensional materials are known for extraordinary properties. We predict a two-dimensional material, B2P6, which combines anisotropy with the Janus geometry, based on evolutionary search and first-principles calculations. High stability of the material is demonstrated in terms of the cohesive energy, phonon spectrum, and melting point. B2P6 turns out to be an indirect band gap semiconductor with anisotropic electronic transport and strong absorption of solar radiation. Importantly, its Janus structure results in an intrinsic electric field, which significantly suppresses the recombination of photogenerated carriers. We demonstrate high efficiency of photocatalytic water splitting with a solar-to-hydrogen efficiency of 28.2%, by far in excess of the conventional theoretical limit of 18%. The structural anisotropy is found to be promising for application in metal-ion batteries. We observe directional diffusion with Li, Na, and K diffusion barriers of only 0.07, 0.04, and 0.03 eV, respectively, suggesting ultrafast charge/discharge characteristics.