applied sciences  
Article Influence of Solvent System on the Electrochemical Properties of a _closo_-Borate Electrolyte Salt  
[PERSON], [PERSON], [PERSON] and [PERSON], [PERSON]. 1  
international attention through incidents that occurred with the Boeing 787 Dreamliner, Tesla Model S, and Samsung Note 7. The high chemical reactivity and low thermal stability of halogenated electrolytes has led to the search for halogen-free alternatives that offer similar or enhanced electrochemical performance, have low chemical reactivity, and are thermally stable. This has led to an interest in the lithium salts of anionic polyhydroborate clusters as possible candidates. Many of these large boron rich anion (LBRA) clusters are considered superhalogens because they have vertical displacement energies (VDE), the energy needed to remove an electron from the cluster, greater than that of the halogen group [3, 4]. The metal salts containing LBRA have been extensively studied because they are often problematic intermediates or unwanted byproducts resulting from the dehydrogenation of metal borohydrides being investigated for solid-state hydrogen storage [5, 6]. This has led to the discovery that highly mobile cationic and anion species are present in these materials via various methodologies. It has been shown that a number of LBRA containing salts (M\({}_{2}\)B\({}_{12}\)H\({}_{12}\), M\({}_{2}\)B\({}_{11}\)H\({}_{11}\), M\({}_{2}\)B\({}_{10}\)H\({}_{10}\), MCB\({}_{11}\)H\({}_{12}\), MCB\({}_{9}\)H\({}_{10}\), MB\({}_{11}\)H\({}_{14}\), etc.) have cationic conductivities \(>\)10\({}^{-1}\) S/cm in the solid state [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. This behavior is attributed to the unique vibrational and rotational dynamics of the LBRA that can to occur on time scales faster than cation diffusion in the solid state [24, 25, 26]. A series of NMR, QENS, and MD calculations have demonstrated that the cation translational mobility is correlated with the reorientation rate of the LBRA consistent with a \"paddle-wheel\" mechanism [10, 25, 27, 28]. This cation translational motion and anion reorientation rate can be tuned via temperature (phase transitions), cation vacancies, the type of atom attached to the boron cage (i.e., H, Cl, F, Br), and LBRA size [7, 12, 13, 29, 30, 31, 32, 33]. While there has been an extensive experimental and theoretical analysis of LBRA salts in the solid-state, there is limited understanding of how these materials behave and perform when used in a liquid electrolyte system. Some of the initial work investigating the use of lithium LBRA salts as non-aqueous electrolytes were initiated in the early 80s by recent Nobel Laureate [PERSON] and colleagues at Exxon [34, 35, 36]. They utilized the lithium salt of a chlorinated _closo_-borate (Li\({}_{2}\)B\({}_{10}\)Cl\({}_{10}\)). Since this initial work, there are only a few other reports using lithium _closo_-borate salts as either electrolytes or redox shuttles for overcharge protection in LIB [37, 38, 39]. In all of these prior studies, the lithium _closo_-borate was halogenated (typically fluorinated) to improve solubility. Additional MO calculations found that the dissociation energy of the first Li\({}^{+}\) from Li\({}_{2}\)B\({}_{12}\)F\({}_{12}\) (126 kcal/mol) is actually lower than the dissociation energy of LiPF\({}_{6}\) (132 kcal/mol) [40]. However, the dissociation of the second Li\({}^{+}\) from Li\({}_{2}\)B\({}_{12}\)F\({}_{12}\) is significantly higher (192 kcal/mol). This is consistent with a dynamic equilibrium existing in solution with Li\({}^{+}\), LiBi\({}_{2}\)F\({}_{12}\)\({}^{-}\), and B\({}_{12}\)F\({}_{12}\)\({}^{-2}\) present in these electrolyte solutions. LBRA, such as the _closo_-borates, do not readily undergo hydrolysis to produce acidic byproducts even when halogenated unlike the common anion (PF\({}_{6}\)\({}^{-}\)) utilized in current liquid electrolytes for LIB.