## 3 Results and Discussion  
Table 1 shows the physical properties of the PC-based electrolyte solutions. The viscosity of concentrated electrolyte solutions is much higher than that of normal electrolyte solutions because of the stronger interactions among the cations, anions, and solvent molecules, leading to the formation of ion pairs and aggregates [17]. It follows that the aforementioned interactions should be rendered weaker for decreasing the viscosity; this in turn increases the ionic conductivity in concentrated electrolyte solutions. PC-based electrolyte solutions containing various amounts of DCE showed higher ionic conductivity than did the concentrated PC-based electrolyte solution without DCE because of the low viscosity. Moreover, the ionic conductivity increased with increasing amounts of DCE. These results indicate that the addition of DCE effectively decreased the viscosity due to the weak interaction among the ions and solvent molecules in the concentrated PC-based electrolyte solution. In order to evaluate the electrochemical properties, CV was carried out at the graphite negative electrode in the concentrated PC-based electrolyte solutions containing various amounts of DCE (Figure 1). In all cases, reversible reduction and oxidation peaks were confirmed under 0.5 V, which could be attributed to lithium intercalation and de-intercalation into/from graphite. Moreover, a cathodic current was observed above 0.5 V during the first cycle, which decreased after the first cycle. This result revealed that above 0.5 V, the electrolyte solution decomposed to form an SEI. It can also be considered that an effective SEI was formed in the PC and DCE mixed electrolyte because of the reversible lithium intercalation and de-intercalation reactions. In other words, the solvation structure of the lithium ions, which enables the formation of an effective SEI, was maintained even though the total molality of LiPF\({}_{6}\) was decreased with the addition of DCE in the concentrated PC-based electrolyte solutions. The cathodic and anodic currents corresponding to lithium intercalation and de-intercalation also increased with increasing amounts of DCE; that is, the electrochemical performance improved when the ionic conductivity of the electrolyte solutions, which was instrumental in SEI formation, was increased. Figure 2 shows the charge/discharge properties of graphite in the concentrated PC-based electrolyte solutions with and without DCE. The potential plateau at about 0.5 V attributed to electrolyte decomposition was confirmed during the first cycle in the concentrated PC-based electrolyte solution (Figure 2a). In the subsequent cycles, this plateau disappeared, and reversible intercalation and \begin{table}
\begin{tabular}{c c c c c} \hline
**Electrolyte** & **Concentration** & **Molar Ratio** & **Viscosity (Pa s)** & **Ionic Conductivity** \\
**Solution** & **(mol kg\({}^{-1}\))** & **(PC/LiPF\({}_{6}\)/DCE)** & **Viscosity (Pa s)** & **(mS cm\({}^{-1}\))** \\ \hline PLD310 & 3.27 & 3:1:0 & 0.580 & 0.41 \\ PLD621 & 2.81 & 6:2:1 & 0.117 & 0.85 \\ PLD311 & 2.47 & 3:1:1 & 0.059 & 1.31 \\ PLD312 & 1.98 & 3:1:2 & 0.030 & 2.84 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical properties of electrolyte solutions. DCE: 1,2-dichloroethane; PC: propylene carbonate. Figure 1: Cyclic voltammograms of graphite negative electrode in (**a**) PLD310, (**b**) PLD621, (**c**) PLD311, and (**d**) PLD312. Scan rate: 0.5 mV s\({}^{-1}\). de-intercalation occurred at the graphite negative electrode. The effective SEI formation suppressed further decomposition in the concentrated PC-based electrolyte solution. The reversible capacity was estimated to be 75 mAh g\({}^{-1}\), which is lower than the theoretical specific capacity of graphite (372 mAh g\({}^{-1}\)) because of the low ionic conductivity.