## 3 Results  
### Electrochemical Properties  
The _close_-borate salt was prepared by a solid state synthesis route previously reported [41]. This solid-state annealing process is known to yield a product that contains a mixture of _close_-borates with the majority of the sample containing Li\({}_{2}\)B\({}_{12}\)H\({}_{12}\) (85 wt. %), Li\({}_{2}\)B\({}_{10}\)H\({}_{10}\) (12 wt. %), and a small fraction of other borates via \({}^{11}\)B NMR [42]. Although, this process produces a mixture of _close_-borates it has been well established in the literature that mixtures of _close_-borates offer higher ionic conductivity in the solid-state than the two components individually [43]. Additionally, our investigation suggests that the need for a high purity _close_-borate salt is not necessary for excellent electrochemical performance when paired with the electrode materials evaluated. Since Li\({}_{2}\)B\({}_{12}\)H\({}_{12}\) is the major component of the _close_-borate salt mixture, it will be referred to as \"Li\({}_{2}\)B\({}_{12}\)H\({}_{12}\)\" herein. The solubility of the Li\({}_{2}\)B\({}_{12}\)H\({}_{12}\) is relatively low and forms an opaque light-yellow suspension which resembles a \"soggy sand\" type of electrolyte for the three electrolyte systems tested. In a soggy sand type electrolyte, a solid-state inorganic filler (i.e., TiO\({}_{2}\), SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\)) is dispersed in a liquid electrolyte containing a soluble lithium salt (i.e., LiPF\({}_{6}\), LiClO\({}_{4}\), LiBF\({}_{4}\)) [44; 45; 46]. In this type of electrolyte, it has been determined that the addition of the insoluble particles results in the formation of a solid-liquid interface that supports fast lithium-ion condition at this two-phase interface. This is attributed to the partial desolvation of the Li\({}^{+}\) leading to higher mobility at these boundary layers. In the electrolytes presented herein, a portion of the Li\({}_{2}\)B\({}_{12}\)H\({}_{12}\) is soluble, while another portion remains solid producing a solid-liquid interface that can facilitate fast ion transport along that boundary. This system is unique because the lithium salt and the solid-state filler (Li\({}_{2}\)B\({}_{12}\)H\({}_{12}\)) are both the same material. The ionic conductivity of the electrolytes as a function of weight % Li\({}_{2}\)B\({}_{12}\)H\({}_{12}\) was first evaluated with electrochemical impedance spectroscopy (EIS) in a modified Swagelok type cell (Figure 1a). A maximum conductivity was obtained with 5 weight % Li\({}_{2}\)B\({}_{12}\)H\({}_{12}\) dispersed in the liquid electrolyte. The EC-DMC electrolyte shows the highest room temperature ionic conductivity (4.3 mS/cm) followed by PC electrolyte (2.4 mS/cm), and the BMIM-TFSI electrolyte (1.1 mS/cm) as shown in Figure 1b. As expected, the ionic conductivity increases as a function of temperature due to the added thermal energy in the system which helps to facilitate the dissociation of the salt, increasing the concentration of free Li\({}^{+}\) in solution. This also helps to reduce ion pairing leading to a higher concentration of charge carriers in solution and the observed higher ionic conductivity. The observed trends for the ionic conductivity are likely attributed to a combination of the dielectric constant (\(\varepsilon\)) of the solvent and its viscosity (\(\eta\)). For PC, EC, DMC, and BMIM-TFSI the dielectric constants are 64.9, 95.3, 3.12, and 14.0, respectively, at 25 \({}^{\circ}\)C (for EC the value is 89.8 in the liquid state at 40 \({}^{\circ}\)C; EC mp = 37 \({}^{\circ}\)C) [47; 48]. The viscosity of PC, EC, DMC, and BMIM-TFSI is 2.51, 1.93, 0.59, and 61.14 cp, respectively, at 25 \({}^{\circ}\)C (the value for EC is at 40 \({}^{\circ}\)C). The EC-DMC likely has the highest overall ionic conductivity because EC has the highest \(\varepsilon\) while the DMC has the lowest \(\eta\) values out of the series.