Multielectron reaction of AlCl_n in borophene for rechargeable aluminum batteries

As a new 2-dimensional material, borophene is expected to be used in energy storage devices because of its unique electronic properties. However, its utilization in rechargeable aluminum batteries (RABs) is limited by high valence of Al (3s²3p¹). Namely, the Al adsorption borophene is too weak to carry on a multielectron reaction. Here, we investigate the origin of unfavorable Al adsorption on borophene that lies at the repulsion of lone pair electrons of 3s orbital with electron-gaining borophene. Because of the existing AlCl_n compounds during the charging and discharging, we introduce AlCl₄⁻, AlCl₂⁺, and AlCl²⁺ ions as possible redox carrier in RABs for the first time. In sharp contrast, multielectron (over 2 electrons per Al) reaction and high capacity (841 mAh/g for [AlCl]_0.33B) can be achieved as a result of Cl coordination. In addition, the diffusion barrier of AlCl²⁺ compound is only 0.08 eV. Notably, the binding of Al and Cl in the form of coordinations liberates the electrons in stable states of 3s orbital, breaking the limit of electron transfer on borophene as cathode materials, thus promoting the adsorption of AlCl_n compounds. In addition, the energy barrier of the reaction between Al 3s electrons and borophene is decreased in the degenerated orbital. Because of the effect of Cl coordination, borophene is suitable for AlCl_n storage, not Al³⁺. These results offer a new insight of the interplay of AlCl_n carrier and multielectron reactions in RABs.