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

Abstract

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.

Two Layers of Computational Screening on Silaborane‐based Clusters Filter Ca(SiB11H11CH3)2 as the Promising Electrolyte for Calcium‐Ion Batteries

“…Another interesting finding is that the BE values for Ca 2 + /SiB nÀ 1 H À n ionpairs are comparatively higher than the corresponding Ca 2 + / CB nÀ 1 H À n ion-pairs (Figure 9; n = 5-15). Incorporating a larger silicon atom increases the van der Waals (vdW) volume of the silaborane clusters to values higher than the corresponding carboranes, [4] for instance, the vdW volume of the CB 11 H À 12 ion is 1547.93 Bohr 3 , while that of SiB 11 H À 12 is 1623.62 Bohr 3 . This suggests that the Ca 2 + /SiB nÀ 1 H À n ion-pairs should possess less BE than analogous Ca 2 + /CB nÀ 1 H À n ion-pairs, thereby facilitating the easier detachment of Ca 2 + from silaboranes than carboranes.…”

“…The VDE is determined by taking the difference between the energies of the anion and the corresponding neutral species possessing the anion geometry, as shown in equation 1. [4,57,60]…”

“…The calculated VDEs of carboranes and silaboranes are in close agreement with the VDEs reported by Kushwaha et al for a similar set of carborane and silaborane anions. [4] This validates the accuracy of the computational model and methods used in the present study. It can be seen from Table 1 that all the cluster anions apart from CB 4 H À 5 (3.42 eV), SiB 4 H À 5 (3.22 eV) and SiB 7 H À 8 (3.51 eV), are superhalogens since their VDEs exceed the electron affinity of halogens (3.62 eV for chlorine).…”

“…[1][2][3] Nevertheless, the limited sources of raw materials for LIBs, the formation of hazardous dendrites, and the increasing cost of Li are the matters of great concern that call for developing new and better battery technologies. [4] In this direction, the multivalent ion batteries (such as Mg 2 + , Ca 2 + , Zn 2 + , and Al 3 + ) have emerged as strong contenders that can usher in another revolution in energy storage technology. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] This is due to their numerous benefits, such as improved safety features, higher abundance, higher theoretical charge density, and larger volumetric and gravimetric capacities.…”

“…As of now, electrolytes based on carboranes of varying sizes have been investigated for application in alkali-ion batteries (such as LIBs), but they are yet to be explored for Ca-ion batteries. [4,44,47] Besides carboranes ðCB nÀ 1 H À n ), examining the properties of their silicon analogs (SiB nÀ 1 H À n ) also appears to be intriguing because the inclusion of the bigger silicon atom may expand the overall size of the anion. The larger size of silaboranes may lead to easier detachment of the metal ion from the anion, thereby enhancing the ionic conductivity.…”