Developing safe, efficient, low-cost, and high-voltage rechargeable alkali metal (Li, Na, and K) ion batteries (AMIBs) is a research topic of great interest. One of the major challenges in this regard is the design of halogen-free solid-state electrolytes with fast ion conductivity and large electrochemical stability window. Using density functional theory, we have systematically studied the structure and properties of electrolytes based on modified boranes, YB n–1H n – (Y = C, Si; n = 5–14) anions, and their respective alkali metal salts, that is, MYB n–1H n (M = Li, Na, K) as potential candidates. Among all the materials studied, the alkali metal salt composed of SiB11H12 – anion is found to be the best candidate. With the vertical detachment energy (VDE = 6.34 eV) and anionic size (1623.62 Bohr3), SiB11H12 – anion surpasses the properties of CB11H12 – anion (VDE = 6.02 eV, and anionic size = 1547.93 Bohr3), which was earlier found to be a superior electrolyte for Li-, Na-, and Mg-ion batteries. Deeper insights into the stability, bonding, and electronic structure of these systems are obtained by analyzing the natural bond charge, anion volume, and binding energies of the MYB n–1H n (M = Li, Na, K) salts. In moving from Li → Na → K salts, νM+–B frequency mode shows a red shift for the corresponding M+–B bond and an increase in the average d M+–B bond length, resulting a reduction in the binding energy. Furthermore, charge analysis shows that the charge on C in MCB11H12 is negative (∼−0.66), while that on Si in MSiB11H12 is positive (∼+1.05), altering the charge distribution on B atoms. In short, this study suggests that MYB11H12-type compounds have the potential for being safe, nontoxic, and halogen-free high-voltage electrolytes for AMIBs.

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High‐voltage alkali metal‐ion batteries (AMIBs) require a non‐hazardous, low‐cost, and highly stable electrolyte with a large operating potential and rapid ion conductivity. Here, we have reported a halogen‐free high‐voltage electrolyte based on SiB11(BO)12−. Because of the weak π‐orbital interaction of −BO as well as the mixed covalent and ionic interaction between SiB11‐cage and −BO ligand, SiB11(BO)12− has colossal stability. SiB11(BO)12− possesses extremely high vertical detachment energy (9.95 eV), anodic voltage limit (∼10.05 V), and electrochemical stability window (∼9.95 V). Furthermore, SiB11(BO)12− is thermodynamically stable at high temperatures, and its large size allows for faster cation movement. The alkali salts MSiB11(BO)12 (M=Li, Na, and K) are easily dissociated into ionic components. Electrolytes based on SiB11(BO)12− greatly outperform commercial electrolytes. In short, SiB11(BO)12−‐based compound is demonstrated to be a high‐voltage electrolyte for AMIBs.