Gel-type polymer electrolytes (GPE) have attracted great attention due to their application to solid-state rechargeable lithium ion batteries with high-energy density, geometrical variability, and safety of use. [1][2][3][4][5] Compared to the well-defined electrochemical behavior of liquid electrolytes, it is difficult to describe the properties of GPE, because they are influenced by the polymer as well as by the liquid electrolyte. There are a lot of investigations on GPE, however, most of them focus on the improvement of some practical properties with putting little emphasis on the thermodynamic relationship between the constituents of GPE. In many previous studies, the electrochemical properties of GPE are discussed on the assumption of the single glass transition temperature (T g ), [6][7][8][9][10][11][12][13][14] which might imply that ion charge carriers are fully miscible with polymers and experience a similar thermodynamic environment regardless of the used materials and compositions. However, it has brought about some ambiguity for explaining the properties of GPE, in particular, when ion transport properties are considered. Therefore, a fundamental understanding of the relationship between the components in GPE is needed. The aim of this study is to perform a quantitative analysis on the thermodynamic phase behavior of GPE and, on the basis of this analysis, provide insight into the physical and electrochemical state of ion charge carriers in GPE. In this elaboration, it is expected that the ion transport properties such as the temperature-dependent ion conductivity and the activation volume (DV) for ion transport can be consistently discussed.In this study, two representative GPE employing PVdF-co-HFP [poly(vinylidene fluoride-co-hexafluoropropylene)] and PMMA [poly(methyl methacrylate)] are prepared with propylene carbonate (PC) as plasticizer and with LiClO 4 (lithium perchlorate) as lithium salt. PVdF-co-HFP-based GPE have been known to show high ion conductivity and good mechanical properties but poor plasticizer retention abilities. On the other hand, PMMA-based GPE have been found to exhibit superior plasticizer retention abilities with relatively low ion conductivity and poor mechanical properties.In Figure 1, the differential scanning calorimeter (DSC) thermograms of PMMA-based GPE are compared with those of PVdF-co-HFP-based GPE. PMMA-based GPE show a single T g over all composition ranges, which manifests their superior miscibility (Figure 1 a). For polymer/low-molecular-weight substance systems, it has been reported that a small addition of a low molecular weight substance decreases the T g value of the polymer, indicating a plasticization effect. [15][16][17] In this study, the decrease of T g with increasing PC content presents a clear evidence that PC acts as a plasticizer. The Fox equation [16] describing the composition dependence of T g is considered for comparison. The calculated T g values from the Fox equation are also plotted in Figure 2. In PMMA-based GPE, a relatively good agre...