The galvanostatic cycling for Li||Li, Li||LFP/c-LFP, and Li||LMO/c-LMO cells were also carried out. All electrochemical measurements were conducted using CR2032 coin cells (Hohsen Corp.). Cell cycling was carried out at 50 °C with voltage limits of 2.5 to 4 V for Li/c-LFP cells, and 3 to 4.3 V for Li/c-LMO cells. Two formation cycles at C/20 were used before long-term cycling unless mentioned otherwise. The reproducibility of the results has been checked by multiple tests. Figure provides an overview of PiHCIL-SPEs where the relative ratio of PEO and high salt content ionic liquids (HCIL) influences the phase behavior, ionic conductivity, and morphology. It is established that PEO is a semi-crystalline polymer with a melting transition at 65 °C. As shown in Figure , the addition of HCIL to PEO in a 20:1:1 molar ratio reduces the melting transition temperature to 50 °C and introduces a Tg at -55°C, indicating the coexistence of amorphous and crystalline phases in SPE20:1:1. Increasing the HCIL ratio to 10:1:1 further lowers the melting transition in both temperature and enthalpy, accompanied by a significant reduction in Tg (-76 °C). This notable decrease in Tg in SPE10:1:1, compared to SPE20:1:1 or SPE15:1:1, likely results from altered interactions between the polymer chains and the Li + ions in the presence of FSI -anions which will be further discussed. At an even higher HCIL ratio of 8:1:1, the melting peak disappears, and the Tg drops further to -79 °C, suggesting a highly amorphous phase. Notably, in samples with higher HCIL ratios (SPE8:1:1, and SPE10:1:1), a small peak at approximately -20°C appears, attributed to a phase transition in the C3mpyrFSI/LiFSI ionic liquid, which indicates the solubility limit of the IL in PEO and potential phase separation. The impact of HCIL on the ionic conductivity of the prepared SPEs is detailed in Figure and. There is a clear dependency of conductivity on HCIL concentration. The conductivity values for SPE20:1:1 and SPE15:1:1 are remarkably similar, aligning with their comparable melting transitions and Tg values (Fig. ). They exhibit a sharp increase in ionic conductivity from the order of 10 -5 S cm -1 at 30 °C to 10 -3 S cm -1 at 70 °C, consistent with the DSC data showing a melting transition at 35 °C (onset) to 65 °C (end). Furthermore, there is a significant rise in ionic conductivity from SPE15:1:1 (9.1 x10 -5 S cm -1 at 30 °C) to (e) SPE10:1:1 (4 x10 -4 S cm -1 at 30 °C) and SPE8:1:1 (5.56 x10 -4 S cm -1 at 30 °C), suggesting that HCIL enhances overall ionic mobility. This is confirmed by DSC data, where the crystallization peak diminishes for SPE10:1:1 and disappears for SPE8:1:1. No gain in ionic conductivity is observed above 8:1:1 ratio (Fig. and Fig. ) and excess IL starts to exude from the polymer matrix in SPE6:1:1 (Fig. ), indicating a potential HCIL solubility limit in the PEO chain. Therefore, we speculate that SPE8:1:1 represents an optimal EO:Li:IL ratio, favoring high ambient temperature ionic conductivity which is closely approximating that of HCIL liquid electrolytes. The surface morphology of the prepared PiHCIL-SPEs was characterized by SEM and shown in Figure . With high PEO content and low HCIL presence (i.e., SPE20:1:1 and SPE15:1:1), the SPEs show a relatively dense and smooth surface (Fig. and), highlighting the good miscibility of PEO and HCIL. With decreasing PEO content, a heterogeneous microstructure is observed (Fig. ), indicating the formation of separated phases, most likely approaching the miscibility limit of HCIL into the polymer. When the ratio reaches 8:1:1, a porous, interconnected and aligned ion conducting network is formed (Fig. ). It can be proposed that at high polymer content, most LiFSI can be dissociated by the PEO backbone, and most FSI -anions exist as free ions. Li + ions mainly transfer via chain segmental motion. Therefore, ionic conductivity increases slowly with increasing HCIL content due to increased charge carrier amounts; whereas at low polymer content, and high HCIL content, a considerable amount of LiFSI cannot be dissociated, and together with the anions from the IL they can form aggregated ion clusters, resulting in phase separation and pores in the system. The ion conduction behavior in this case can be explained by a "polymer-in-salt" percolation model where high salt content favors the formation of an ion conductive network composed of interconnected ion clusters that provide percolation pathways for fast ion transport. Consistent with the polymer-in-salt system, 36 in SPE8:1:1, the excessive FSI -anions from both LiFSI and C3mpyrFSI form interconnected cluster networks and pores as shown in Fig. . Interestingly, these connected ion clusters and pores also happen to be locally aligned, which can provide direct pathways for Li + transport, reducing tortuosity. This can lead to more efficient ion transport and higher conductivity. The pores may help accommodate volume change during cell cycling, potentially enhancing mechanical stability and longevity of the SPE. Figure shows a proposed morphology model to help understand the structural change and ion dynamics in the prepared PiHCIL-SPEs. FTIR was used to investigate the changes of cation-polymer and cation-anion interactions in the prepared PiHCIL-SPEs as shown in Figure (the full spectrum is given in Fig. , supporting information). In the region between 1000 and 1200 cm -1 , a vibrational peak at 1100 cm -1 from the neat IL (Fig. ) is assigned to the -SNS symmetric stretching from the FSI -anions. Our previous work showed that adding 50 mol% LiFSI salt to the neat IL shifted this peak to 1115 cm -1 which was attributed to the increased coordination between Li + and FSI -anions. After adding PEO (Fig. ), the -SNS stretching peak shifts to lower wavenumbers and splits into free FSI -and coordinated FSI -(blue), indicating a coordination environment change of the FSI -anions that may be due to the introduced Li + -EO coordination in SPE8:1:1. As PEO content increases, the coordinated FSI -peak dramatically decreased leaving mainly free FSI -in the SPE20:1:1 (Fig. ). Additionally, three -CO stretching peaks (red) from PEO shifted to higher wavenumbers, suggesting a coordination environment change of the -CO groups which may be due to increased Li + -EO coordination and this is consistent with PEO-salt systems. Therefore, summarily it can be stated that increasing HCIL concentration in the PEO matrix results in more coordinated FSI -anions, leading to the formation of a connected anion-rich ion conducting phase that can be further supported by the microstructure analysis in Figure . These separated phases might also decrease the crystalline regions of PEO, leading to higher chain mobility, improving the ionic conductivity and thus faster Li + transport.