## 3 Results and Discussion
Upon DCE addition in the concentrated PC-based electrolyte solution (molar ratio of PC/DCE = 3:0.5 or 3:1), the reversible capacity increased to about 80 and 200 mAh g\({}^{-1}\) (Figure 2b,c), indicating a remarkable improvement in the electrochemical performance of the graphite negative electrode. However, a large irreversible capacity was observed during the first cycle in PLD312 (Figure 2d) as well as a lower reversible capacity than those in PLD621 (Figure 2b) and PLD311 (Figure 2c). A new plateau was confirmed at about 1.8 V when DCE was added in the concentrated PC-based electrolyte solution (Figure 2b-d), implying the decomposition of DCE. Furthermore, the reversible capacity gradually decreased with the increasing cycle number in the concentrated PC-based electrolyte solutions containing various amounts of DCE. Although this plateau disappeared after the first cycle, the irreversible capacity increased with the increasing DCE amount during the first cycle. To sum up, the addition of a co-solvent (i.e., DCE) to concentrated PC-based electrolyte solutions helped preserve the solvation structure of the lithium ion, enabling effective SEI formation, and increased the ionic conductivity, but the decomposition of DCE was unavoidable. Raman spectroscopy was performed to investigate the structures of the electrolyte solutions (Figure 3). The peak at 713 cm\({}^{-1}\) could be attributed to the symmetric ring deformation in the pure PC solvent, implying that the PC molecules exist freely without interacting with the cations or anions (Figure 3a) [17; 18; 19; 20]. Figure 2: Charge and discharge curves of graphite negative electrode in (**a**) PLD310, (**b**) PLD621, (**c**) PLD311, and (**d**) PLD312. C-rate: 0.5 C.  
In pure DCE, the peaks observed at 657, 681, and 757 cm\({}^{-1}\) were attributed to the Cl-C stretching modes of the gauche conformer with A and B symmetry, and the stretching mode of the trans conformer, respectively (Figure 3b) [21,22]. Significant changes such as peak shifts were not observed, indicating no interaction between PC and DCE in the mixed solvent, as shown in Figure 3c. The peak at 713 cm\({}^{-1}\) disappeared and shifted to 723 cm\({}^{-1}\), which means that most PC molecules coordinated to the lithium ions, and the new peak at 745 cm\({}^{-1}\) corresponding to the PF\({}_{6}\)\({}^{-}\) anion appeared in the concentrated PC-based electrolyte solution (Figure 3d) [17]. Moreover, the peak at 723 cm\({}^{-1}\) related to the solvated PC was unchanged in the spectrum of the concentrated PC-based electrolyte solutions containing DCE (Figure 3e-g). No peak shift occurred at 657, 681, and 757 cm\({}^{-1}\) for DCE as compared with the case of pure DCE. This indicated that the solvation structure of lithium ions in the concentrated electrolyte solution was maintained despite the DCE addition with decreasing viscosity, as shown in Table 1. The effect of DCE addition on the solvation structure was negligible in PLD621, PLD311, and PLD312 because of its low solvation ability. Consequently, an effective SEI was formed and reversible lithium intercalation/de-intercalation took place in the concentrated electrolyte solutions containing DCE. Although the reason for the lowering of the viscosity was not clearly understood, it was confirmed that the electrochemical performance improved due to the increased ionic conductivity in the concentrated electrolyte solution upon the addition of DCE.