Excellent Electrolyte Wettability and High Energy Density of B₂S as a Two-Dimensional Dirac Anode for Non-Lithium-Ion Batteries

Two-dimensional (2D) Dirac materials with ultrahigh electronic conductivity exhibit great promise for application as an anode material in non-lithium-ion batteries (NLIBs) with a reduced conductive additive and a binder as a nonactive material additive. Graphene is one of the most prominent 2D Dirac materials with high electrolyte wettability; however, it cannot be used as an anode material in NLIBs owing to its poor affinity toward metal ions such as Na, K, Ca, Mg, and Al. In this work, we investigated the use of recently developed boron sulfide (B2S) as a new lightweight 2D Dirac anode for NLIBs on the basis of first-principles calculations. We demonstrate that B2S delivers excellent electronic conductivity and has a unique “self-doping” effect by forming S or B vacancies. The ultrahigh energy densities of 2245 and 1167 mWh/g, a product of capacity and open-circuit voltage referenced by standard hydrogen electrode potential (Cao Nat. Nanotechnology. 2019, 14, 200−207), could be achieved for the B2S anode in Na- and K-ion batteries, respectively, significantly larger than those of graphene. More importantly, the B2S presents graphene-like wettability toward commonly used electrolytes in Na- and K-ion batteries, i.e., the solvent molecules and metal salt, indicating excellent compatibility. Moreover, the minimum energy path for Na- and K-ion diffusion on the B2S surface shows energy barriers of 0.19 and 0.04 eV, which indicates high ionic conductivity. Furthermore, a small contraction of the B2S lattice upon ion intercalation has been observed due to the adsorption-induced corrugation of the electrode, which offsets the lattice expansion. The results suggest that the B2S electrode can be used as a lightweight 2D Dirac anode material with excellent energy density, desirable rate performance, and robust wettability toward the electrolytes.