Halogen-Free Electrolytes Based on Modified Boranes for Alkali-Ion Batteries

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, YBn-1Hn- (Y = C, Si; n = 5-14) anions, and their respective alkali metal salts, that is, MYBn-1Hn (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), SiB(11)H(12)(- )anion surpasses the properties of CB11H12- anion (VDE = 6.02 eV, and anionic size = 1547.93 Bohr(3)), 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 MYBn-1Hn (M = Li, Na, K) salts. In moving from Li -> Na -> K salts, v(M+-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 (similar to-0.66), while that on Si in MSiB11H12 is positive (similar to+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.