Solid Polymer Electrolytes Derived from Crosslinked Polystyrene Nanoparticles Covalently Functionalized with a Low Lattice Energy Lithium Salt Moiety

: Three new crosslinked polystyrene nanoparticles covalently attached with low lattice energy lithium salt moieties were synthesized: poly(styrene lithium triﬂuoromethane sulphonyl imide) (PSTFSILi), poly(styrene lithium benzene sulphonyl imide) (PSPhSILi), and poly(styrene lithium sulfonyl-1,3-dithiane-1,1,3,3-tetraoxide) (PSDTTOLi). A series of solid polymer electrolytes (SPEs) were formulated by mixing these lithium salts with high molecular weight poly(ethylene oxide), poly(ethylene glycol dimethyl ether), and lithium bis(ﬂuorosulfonyl)imide. The crosslinked nano-sized polymer salts improved ﬁlm strength and decreased the glass transition temperature (T g ) of the polymer electrolyte membranes. An enhancement in both ionic conductivity and thermal stability was observed. For example, the SPE ﬁlm containing PSTFSILi displayed ionic conductivity of 7.52 × 10 − 5 S cm − 1 at room temperature and 3.0 × 10 − 3 S cm − 1 at 70 ◦ C, while the SPE ﬁlm containing PSDTTOLi showed an even better performance of 1.54 × 10 − 4 S cm − 1 at room temperature and 3.23 × 10 − 3 S cm − 1 at 70 ◦ C. The the salt


Introduction
Over the past two decades, rechargeable lithium-ion batteries (LIBs) have been strongly considered worldwide as the most reliable sustainable energy storage systems [1,2]. LIBs display good specific energy density (150-350 Wh kg −1 ), long cycle life, high open-circuit voltage, low self-discharge rate, and high efficiency [3]. These properties make them attractive for portable electronics, electric vehicles, and renewable energy storage systems. Currently, most commercial LIBs use organic liquid electrolytes composed of~1 M LiPF 6 in a mixture of organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) because liquid electrolytes provide very high ionic conductivity. However, the liquid electrolytes pose a serious safety issue due to their high flammability and potential for leakage. This makes solid polymer electrolytes (SPEs) a promising alternative to enhance the safety performance of LIBs.
Gel polymer electrolytes (GPEs) with characteristics of both solid and liquid electrolytes have also been investigated to address the safety issue of LIBs. GPEs formed by incorporating a significant amount of organic liquid electrolytes into a polymer framework display very high ambient temperature from Sigma-Aldrich Chemical Co. Ltd (St. Louis, MO, USA). Acetone, acetonitrile, dichloromethane, and methanol were supplied by Fisher Scientific Co. Ltd., (Waltham, MA, USA).
Fourier transform infrared (FTIR) of all SPEs were recorded in a high resolution Perkin-Elmer (Frontier Optica) instrument. The morphology of the PS-bound lithium salt nanoparticles was analyzed using scanning electron microscopy (SEM) (Cambridge, Leica) with an accelerating voltage value equal to 15 kV. Thermogravimetric measurements were carried out with a TGA/SDTA851e thermal analyzer (Mettler Toledo, Columbus, OH, USA). Differential scanning calorimetry (DSC) analysis was carried out with a Mettler Toledo Differential Scanning Calorimeter instrument under an argon atmosphere with a flow rate at 70.0 mL min −1 between −100 • C and 150 • C. The ionic conductivities of the formulated electrolytes were measured by the complex impedance method using an impedance analyzer (Solartron model SI-1287, Ametek, Inc., Berwyn, PA, USA) coupled to a Solartron model-1260 (Ametek, Inc., Berwyn, PA, USA) frequency response analyzer. The electrochemical stability of the SPE membranes was determined by cyclic voltammetry (CV) using a potentiostat/galvanostat (Solartron impedance analyzer).

Synthesis of PS-SO 2 Cl
A total of 4.30 g of PS and 40 mL of dichloromethane was placed in a flask and stirred overnight for solvent absorption. A mixture of 12.5 mL of nitromethane and 13 mL of chlorosulfonic acid was added drop by drop. The reaction mixture was heated for 7 h at 40 • C. After isolating the crude product, 10 mL of dichloromethane was added. The solution was filtered on a sintered funnel, washed twice with 10 mL of acetonitrile, and then washed twice with 10 mL of acetone. The entrapped solvent was removed under vacuum at 70 • C overnight (Yield: 7.99 g) [20]. FTIR: 2928.05 cm −1 , 1365 ± 5 (as) and 1180 ± 10 (s) cm −1 , 1160-1140 (s) and 1350-1300 (s) cm −1 , 1450-1500 cm −1 , 772.50 cm −1 , 672.09 cm −1 .
Synthesis of PSDTTOLi. Polystyrenesulfonyl-1,3-dithiane-1,1,3,3-tetraoxide (0.34 g) and lithium methoxide (0.08 g) were placed into a 50 mL flask containing 20 mL of methanol. The mixture was stirred at room temperature for two days. The product was washed with methanol twice, followed by acetone. The product was dried in a high vacuum at 70 • C overnight (Yield: 0. 35

Thin Film Processing and Cell Fabrication
PEGDM, PEO, and PSLS were mixed in a mortar and pestle. The mixture was then placed in between two Teflon coated sheets, then hot pressed in a Carver press at 100 • C under 10 psi pressure for 5 min [21]. The resulting SPE film was then folded, refolded, and subjected to further hot pressing to achieve a well-dispersed electrolyte film. Two thin stainless-steel plates were used as a spacer to control the thickness of the film. The polymer films were cut circularly in a 2.04 cm 2 area and sandwiched between two steel electrodes and subjected to impedance analysis.

Synthesis of PS-Bound Lithium Salts
As outlined in Figure 1, the synthesis began with the preparation of crosslinked polystyrene nanoparticles following the method of Brijmohan et al. [22]. Chlorosulfonation of the PS nanoparticles was successfully carried out with a minor modification of a reported procedure [22]. The new TFSI-like PS-bound lithium salt, PSTFSILi, was synthesized in a single-step by reacting the chlorosulfonated PS nanoparticles with CF 3 SO 2 NH 2 in the presence of LiOH. The incorporation of a similar lithium salt moiety (-SO 2 NLiSO 2 CF 3 ) on a benzene ring has been previously reported, but involved three steps starting from the benzene sulfonyl chloride analog [23]. Synthesis of polystyrenesulfonyl-1,3-dithiane-1,1,3,3-tetraoxide (PSDTTO). Polystyrenesulfonyl-1, 3dithiane (0.50 g) was placed in a 50 mL flask containing 15 mL of acetic acid. Hydrogen peroxide (10 mL) was added to the flask. The mixture was heated and stirred at 60 °C. The reaction was conducted for three days with further additions of 1 mL of hydrogen peroxide/day. The product was filtered and washed with 15 mL water. The product was dried in a high vacuum at 70°C overnight (Yield: 0.34 g). FT-IR: 2900-2950 cm −1 , 1717.43 cm −1 , 1639.34 cm −1 , 1600.46 cm −1 , 1350-1495(s) cm −1 , 1000-1225 cm −1 , 831.89 cm −1 , 775.61 cm −1 , 673.52 cm −1 , 578.62 cm −1 .
Synthesis of PSDTTOLi. Polystyrenesulfonyl-1,3-dithiane-1,1,3,3-tetraoxide (0.34 g) and lithium methoxide (0.08 g) were placed into a 50 mL flask containing 20 mL of methanol. The mixture was stirred at room temperature for two days. The product was washed with methanol twice, followed by acetone. The product was dried in a high vacuum at 70 °C overnight (Yield: 0.

Thin Film Processing and Cell Fabrication
PEGDM, PEO, and PSLS were mixed in a mortar and pestle. The mixture was then placed in between two Teflon coated sheets, then hot pressed in a Carver press at 100 °C under 10 psi pressure for 5 min [21]. The resulting SPE film was then folded, refolded, and subjected to further hot pressing to achieve a well-dispersed electrolyte film. Two thin stainless-steel plates were used as a spacer to control the thickness of the film. The polymer films were cut circularly in a 2.04 cm 2 area and sandwiched between two steel electrodes and subjected to impedance analysis.

Synthesis of PS-Bound Lithium Salts
As outlined in Figure 1, the synthesis began with the preparation of crosslinked polystyrene nanoparticles following the method of Brijmohan et al. [22]. Chlorosulfonation of the PS nanoparticles was successfully carried out with a minor modification of a reported procedure [22]. The new TFSIlike PS-bound lithium salt, PSTFSILi, was synthesized in a single-step by reacting the chlorosulfonated PS nanoparticles with CF3SO2NH2 in the presence of LiOH. The incorporation of a similar lithium salt moiety (-SO2NLiSO2CF3) on a benzene ring has been previously reported, but involved three steps starting from the benzene sulfonyl chloride analog [23].  The final new PS-bound lithium salt, PSDTTOLi, was prepared in three steps: (1) coupling of 1,3-dithiane with PS-SO2Cl using the standard BuLi activation protocol; (2) oxidation of thioether groups to sulfones using hydrogen peroxide; and (3) lithiation using LiOH ( Figure 3).

Structural Characterization
FTIR Spectroscopy: FTIR spectra of the nanoparticles were obtained by dispersing the sample in anhydrous potassium bromide. For PSTFSILi, the peak at 1000-1400 cm −1 corresponded to the stretching of the C-F bond. The broad peaks in the range of 1350-1300 cm −1 (s) and 1180-1140 cm −1 (s) were due to the O2S-N-bond. For PSPhSILi, strong peaks around 3063.53 cm −1 corresponded to aromatic C-H stretching and 1500-1400 cm −1 related to aromatic C=C stretching. The peak around 1630 cm −1 represents the S-N-S combination bond, which indicates that the reaction was complete. For PSDTTOLi, after lithiation of PSDTTO, the peak at 1717.43 cm −1 shifted to 1705.73 cm −1 , and 1639.34 cm −1 shifted to 1637.34 cm −1 , which can be assigned to the stretching vibration band of carbon with the existence of three sulfone electron withdrawing groups. These data partially support the proposed structures of the lithium bound polymer salts.
Energy Dispersive X-ray Spectroscopy (EDS): This technique for elemental analysis was performed with a scanning electron microscope (SEM, Phenom Pro-X, Nanoscience Instruments) by selecting three different mapping areas in the secondary electron mode with an acceleration voltage of 15 kV. All samples were deposited on a silicon wafer for SEM/EDS measurement after they were dispersed in pure ethyl alcohol and sonicated for 1 h. EDS measurement of each mapping area took about seven minutes, so the total collecting time was 21 min for the three different mapping areas to obtain better statistics or consistency. Figures 4-6 show the EDS elemental composition of each sample, while the obtained elemental concentrations from the EDS studies are presented in Table 1. The analysis of the data indicates that all of these salts had a different ratio of carbon, nitrogen, oxygen, and sulfur, while only PSTFSILi exhibited 8.2% F. Lithium ions were too small to be detected by the instrument.

Structural Characterization
FTIR Spectroscopy: FTIR spectra of the nanoparticles were obtained by dispersing the sample in anhydrous potassium bromide. For PSTFSILi, the peak at 1000-1400 cm −1 corresponded to the stretching of the C-F bond. The broad peaks in the range of 1350-1300 cm −1 (s) and 1180-1140 cm −1 (s) were due to the O2S-N-bond. For PSPhSILi, strong peaks around 3063.53 cm −1 corresponded to aromatic C-H stretching and 1500-1400 cm −1 related to aromatic C=C stretching. The peak around 1630 cm −1 represents the S-N-S combination bond, which indicates that the reaction was complete. For PSDTTOLi, after lithiation of PSDTTO, the peak at 1717.43 cm −1 shifted to 1705.73 cm −1 , and 1639.34 cm −1 shifted to 1637.34 cm −1 , which can be assigned to the stretching vibration band of carbon with the existence of three sulfone electron withdrawing groups. These data partially support the proposed structures of the lithium bound polymer salts.
Energy Dispersive X-ray Spectroscopy (EDS): This technique for elemental analysis was performed with a scanning electron microscope (SEM, Phenom Pro-X, Nanoscience Instruments) by selecting three different mapping areas in the secondary electron mode with an acceleration voltage of 15 kV. All samples were deposited on a silicon wafer for SEM/EDS measurement after they were dispersed in pure ethyl alcohol and sonicated for 1 h. EDS measurement of each mapping area took about seven minutes, so the total collecting time was 21 min for the three different mapping areas to obtain better statistics or consistency. Figures 4-6 show the EDS elemental composition of each sample, while the obtained elemental concentrations from the EDS studies are presented in Table 1. The analysis of the data indicates that all of these salts had a different ratio of carbon, nitrogen, oxygen, and sulfur, while only PSTFSILi exhibited 8.2% F. Lithium ions were too small to be detected by the instrument.

Structural Characterization
FTIR Spectroscopy: FTIR spectra of the nanoparticles were obtained by dispersing the sample in anhydrous potassium bromide. For PSTFSILi, the peak at 1000-1400 cm −1 corresponded to the stretching of the C-F bond. The broad peaks in the range of 1350-1300 cm −1 (s) and 1180-1140 cm −1 (s) were due to the O 2 S-N-bond. For PSPhSILi, strong peaks around 3063.53 cm −1 corresponded to aromatic C-H stretching and 1500-1400 cm −1 related to aromatic C=C stretching. The peak around 1630 cm −1 represents the S-N-S combination bond, which indicates that the reaction was complete. For PSDTTOLi, after lithiation of PSDTTO, the peak at 1717.43 cm −1 shifted to 1705.73 cm −1 , and 1639.34 cm −1 shifted to 1637.34 cm −1 , which can be assigned to the stretching vibration band of carbon with the existence of three sulfone electron withdrawing groups. These data partially support the proposed structures of the lithium bound polymer salts.
Energy Dispersive X-ray Spectroscopy (EDS): This technique for elemental analysis was performed with a scanning electron microscope (SEM, Phenom Pro-X, Nanoscience Instruments) by selecting three different mapping areas in the secondary electron mode with an acceleration voltage of 15 kV. All samples were deposited on a silicon wafer for SEM/EDS measurement after they were dispersed in pure ethyl alcohol and sonicated for 1 h. EDS measurement of each mapping area took about seven minutes, so the total collecting time was 21 min for the three different mapping areas to obtain better statistics or consistency. Figures 4-6 show the EDS elemental composition of each sample, while the obtained elemental concentrations from the EDS studies are presented in Table 1. The analysis of the data indicates that all of these salts had a different ratio of carbon, nitrogen, oxygen, and sulfur, while only PSTFSILi exhibited 8.2% F. Lithium ions were too small to be detected by the instrument.    Scanning Electron Microscopy: The morphology of the PS-bound lithium salt nanoparticles was analyzed using scanning electron microscopy (SEM) (Leica, Cambridge, UK) with an accelerating voltage value equal to 15 kV. The samples were coated with a thin gold layer using a sputter coater (Polaron, model SC502, Williston, VT, USA) [13]. TFSI and PhSI modified nanoparticles displayed (EDS).

Sample
At.    Scanning Electron Microscopy: The morphology of the PS-bound lithium salt nanoparticles was analyzed using scanning electron microscopy (SEM) (Leica, Cambridge, UK) with an accelerating voltage value equal to 15 kV. The samples were coated with a thin gold layer using a sputter coater (Polaron, model SC502, Williston, VT, USA) [13]. TFSI and PhSI modified nanoparticles displayed (EDS).

Sample
At.    Scanning Electron Microscopy: The morphology of the PS-bound lithium salt nanoparticles was analyzed using scanning electron microscopy (SEM) (Leica, Cambridge, UK) with an accelerating voltage value equal to 15 kV. The samples were coated with a thin gold layer using a sputter coater (Polaron, model SC502, Williston, VT, USA) [13]. TFSI and PhSI modified nanoparticles displayed   [13]. TFSI and PhSI modified nanoparticles displayed similar particle sizes at around 500 nm, whereas the DTTO modified nanoparticles measured at around 350 nm (Figure 7). In either case, the particle size was much larger that the starting PS nanoparticles (50 nm). This indicates that PS particles agglomerated during the incorporation of functional groups [24].
ChemEngineering 2020, 4, x FOR PEER REVIEW 7 of 17 similar particle sizes at around 500 nm, whereas the DTTO modified nanoparticles measured at around 350 nm (Figure 7). In either case, the particle size was much larger that the starting PS nanoparticles (50 nm). This indicates that PS particles agglomerated during the incorporation of functional groups [24].

SPE Formulations
The various weight ratios of the PEO-based SPE films along with the PSLSs are listed in Tables  2-4. A SPE film containing PEO:PEGDM:LiTFSI (PPL) at the weight ratio of 3:4:2 max limit was prepared and served as the standard SPE sample. The SPE films maintained good ductility and mechanical strength and did not snap upon manual bending or stretching. The PPL complex at the weight ratio of 3:4:3 presented challenges to making a nice film, but all PSLS-based SPEs displayed excellent film quality.

SPE Formulations
The various weight ratios of the PEO-based SPE films along with the PSLSs are listed in Tables 2-4. A SPE film containing PEO:PEGDM:LiTFSI (PPL) at the weight ratio of 3:4:2 max limit was prepared and served as the standard SPE sample. The SPE films maintained good ductility and mechanical strength and did not snap upon manual bending or stretching. The PPL complex at the weight ratio of 3:4:3 presented challenges to making a nice film, but all PSLS-based SPEs displayed excellent film quality.

Thermogravimetric Analysis (TGA)
The thermal stability of the PSLS nanoparticles was determined by thermogravimetric analysis (TGA). This analysis was carried out with a TGA/SDTA851e thermal analyzer (Mettler Toledo). A sample (5-7 mg) was placed in an aluminum pan, and then heated from 25 • C to 100 • C for 10 min and cooled rapidly to 25 • C for another 10 min. The samples were then heated from 25 • C to 500 • C at the rate of 10 • C/min [12]. All measurements were done under air flow through the system.
The TGA curves of the PSLS nanoparticles displayed two mass loss steps (Figure 8). The initial mass loss below 400 • C was due to the gradual evaporation of absorbed moisture. The second mass loss in the range of 450 to 480 • C was a result of the decomposition of the polystyrene matrix. Our results show that the thermal stability of in the order PSTFPhLi > PSDTTOLi > PSTFSILi. PSTFSILi displayed more moisture sensitivity (11% weight loss before 400 • C) due to its structural similarity ChemEngineering 2020, 4, 44 9 of 17 with LiTFSI, which was found to be hygroscopic (Table 5). In contrast, PSTFPhLi and PSDTTOLi were less hygroscopic and did not exhibit significant mass loss below 400 • C. Polystyrene backbone degradation occurred very rapidly in the temperature range of 450-500 • C. The TGA of PEO-based electrolytes is well documented in the literature and they begin to decompose around 250 • C. TGA is not recommended for PEGDM plasticized SPEs as PEGDM begins to escape (evaporate) from the sample at around 150 • C. This is why we conducted a TGA of the salt nanoparticles to make sure that they did not decompose prior to PEO.

Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) is one of the most convenient approaches to shed light on the degree of crystallinity along with the thermal behavior of polymeric samples, and was used to characterize our electrolyte samples [26]. The Tg of a polymer indicates the transition from a rubbery to a glassy state and vice versa. Therefore, the polymer is flexible above the Tg, and hard and brittle below the Tg [13]. This is an endothermic transition while the crystalline melting point (Tm) is an exothermic transition. In our study, DSC analysis was carried out with a Mettler Toledo differential scanning calorimeter instrument under an argon atmosphere with a flow rate at 70.0 mL min −1 [4] between −100 °C and 150 °C [27]. Samples of around 5-7 mg were sealed in a 40 µ L aluminum pan with a perforated lid to allow the release and removal of the decomposition products [28]. Two stages of DSC scanning were set: (1) −100 °C isothermal for 10.0 min, and (2) heating rate of 10 °C min −1 and temperature range from −100 to 150 °C. An empty aluminum pan served as the reference. The DSC results are presented in Figure 9 and the essential glass transition and melting points for various electrolyte samples are listed in Table 6.

Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) is one of the most convenient approaches to shed light on the degree of crystallinity along with the thermal behavior of polymeric samples, and was used to characterize our electrolyte samples [26]. The T g of a polymer indicates the transition from a rubbery to a glassy state and vice versa. Therefore, the polymer is flexible above the T g , and hard and brittle below the T g [13]. This is an endothermic transition while the crystalline melting point (T m ) is an exothermic transition. In our study, DSC analysis was carried out with a Mettler Toledo differential scanning calorimeter instrument under an argon atmosphere with a flow rate at 70.0 mL min −1 [4] between −100 • C and 150 • C [27]. Samples of around 5-7 mg were sealed in a 40 µL aluminum pan with a perforated lid to allow the release and removal of the decomposition products [28]. Two stages of DSC scanning were set: (1) −100 • C isothermal for 10.0 min, and (2) heating rate of 10 • C min −1 and temperature range from −100 to 150 • C. An empty aluminum pan served as the reference. The DSC results are presented in Figure 9 and the essential glass transition and melting points for various electrolyte samples are listed in Table 6.  The Tg of all electrolytes was found to be concentrated around −50 °C. In contrast, Tm showed some separations. We presume that this was due to the addition of PSLSs, which caused a change in the shape of the endothermic peak, and the peaks were slightly shifted toward lower temperatures compared to the control (Figure 9h,i). These observations clearly suggest that a significant contribution to conductivity enhancement comes from structural modifications associated with the polymer host caused by the salts. The reorganization of the polymer chain may be hindered by the crosslinking centers formed by the interaction of the functional groups.

Ionic Conductivity
The ionic conductivities of the formulated electrolytes were measured by the complex impedance method using an impedance analyzer (Solartron model SI-1287) coupled to a Solartron model-1260 frequency response analyzer [29]. The solid electrolyte membranes were sandwiched between two stainless steel electrodes for conductivity measurements [10,30]. All measurements were performed in an environmental chamber in the temperature range of 25-70 °C [31].  The T g of all electrolytes was found to be concentrated around −50 • C. In contrast, T m showed some separations. We presume that this was due to the addition of PSLSs, which caused a change in the shape of the endothermic peak, and the peaks were slightly shifted toward lower temperatures compared to the control (Figure 9h,i). These observations clearly suggest that a significant contribution to conductivity enhancement comes from structural modifications associated with the polymer host caused by the salts. The reorganization of the polymer chain may be hindered by the crosslinking centers formed by the interaction of the functional groups.

Ionic Conductivity
The ionic conductivities of the formulated electrolytes were measured by the complex impedance method using an impedance analyzer (Solartron model SI-1287) coupled to a Solartron model-1260 frequency response analyzer [29]. The solid electrolyte membranes were sandwiched between two stainless steel electrodes for conductivity measurements [10,30]. All measurements were performed in an environmental chamber in the temperature range of 25-70 • C [31].
Among the three PSLSs, the compounds containing PSTFSI and PSPhSI showed structural similarity as both contained a nitrogen anionic site. The higher ionic conductivity of the TFSI-PPL electrolyte was due to the presence of the CF 3 group, which induced a strong electron-withdrawing effect when compared to an aromatic ring. In contrast, the existence of three strong electron withdrawing sulfone (SO 2 ) groups provided the PSDTTO anion with the best dissociating ability to enable faster lithium ion transport. As the temperatures were elevated, the movement of the polymer chain segments rapidly increased, and all of the salts followed the well-established trend of increasing conductivity with the increase in temperatures (Figures 10-13). Nevertheless, with the increasing weight ratio of PSLSs, the conductivity decreased. This trend can likely be attributed to the high salt concentration with the additional influence of the ion pairs, ion triplets, and the higher ion aggregations, which reduced the overall mobility and the number of effective charge carriers [1]. It is important to note that our assessment was based on the formulation that provided the highest ionic conductivity. PPL displayed its highest ionic conductivity with the formulation described in Table 2 (i.e., PEO:PEGDM:LiTFSI::3:4:2). The TFSI-PPL and DTTO-PPL based electrolytes displayed the best ionic conductivity with this formulation (1:3:4:3, see Tables 2 and 4).
Among the three PSLSs, the compounds containing PSTFSI and PSPhSI showed structural similarity as both contained a nitrogen anionic site. The higher ionic conductivity of the TFSI-PPL electrolyte was due to the presence of the CF3 group, which induced a strong electron-withdrawing effect when compared to an aromatic ring. In contrast, the existence of three strong electron withdrawing sulfone (SO2) groups provided the PSDTTO anion with the best dissociating ability to enable faster lithium ion transport. As the temperatures were elevated, the movement of the polymer chain segments rapidly increased, and all of the salts followed the well-established trend of increasing conductivity with the increase in temperatures (Figures 10-13). Nevertheless, with the increasing weight ratio of PSLSs, the conductivity decreased. This trend can likely be attributed to the high salt concentration with the additional influence of the ion pairs, ion triplets, and the higher ion aggregations, which reduced the overall mobility and the number of effective charge carriers [1]. It is important to note that our assessment was based on the formulation that provided the highest ionic conductivity. PPL displayed its highest ionic conductivity with the formulation described in Table 2 (i.e., PEO:PEGDM:LiTFSI::3:4:2). The TFSI-PPL and DTTO-PPL based electrolytes displayed the best ionic conductivity with this formulation (1:3:4:3, see Tables 2 and 4).

Cyclic Voltammetry
Electrolytes in lithium-ion batteries need to retain sufficiently wide reversible electrochemical stability against Li/Li + . The electrochemical stability of the SPE membranes was determined by cyclic voltammetry (CV) using a Solartron potentiostat/galvanostat impedance/gain-phase analyzer (SI 1260) coupled with a Solartron electrochemical interface (SI 1287). A testing cell was assembled to determine the oxidation potential of stainless steel as the working electrode and lithium foil serving as the counter and reference electrode. All scans were conducted at 25 °C at a scan rate of 10 mV s −1 with the voltage range of 0 V to 5.0 V versus Li/Li + [33].
For practical applications, an electrolyte system should exhibit excellent electrochemical stability in excess of 4.5 V since secondary lithium-ion batteries typically operate in the voltage range of 2.8-4.35 V [34]. The characteristic CV properties for each of the three best electrolyte membranes are depicted in Figure 14. According to the CV curves, both PhSI-PPL and DTTO-PPL films at the weight ratio of 1:3:4:3 displayed electrochemical stability up to 4.2 V. The PhSI-PPL membrane showed an anodic peak at 1.26 V, while the DTTO-PPL membrane exhibited another anodic peak at 1.31 V [35]. The TFSI-PPL electrolyte at the weight ratio of 1:3:4:3 displayed the best electrochemical stability as no significant electrochemical activity appeared below 4.8 V versus Li/Li + .

Cyclic Voltammetry
Electrolytes in lithium-ion batteries need to retain sufficiently wide reversible electrochemical stability against Li/Li + . The electrochemical stability of the SPE membranes was determined by cyclic voltammetry (CV) using a Solartron potentiostat/galvanostat impedance/gain-phase analyzer (SI 1260) coupled with a Solartron electrochemical interface (SI 1287). A testing cell was assembled to determine the oxidation potential of stainless steel as the working electrode and lithium foil serving as the counter and reference electrode. All scans were conducted at 25 • C at a scan rate of 10 mV s −1 with the voltage range of 0 V to 5.0 V versus Li/Li + [33].
For practical applications, an electrolyte system should exhibit excellent electrochemical stability in excess of 4.5 V since secondary lithium-ion batteries typically operate in the voltage range of 2.8-4.35 V [34]. The characteristic CV properties for each of the three best electrolyte membranes are depicted in Figure 14. According to the CV curves, both PhSI-PPL and DTTO-PPL films at the weight ratio of 1:3:4:3 displayed electrochemical stability up to 4.2 V. The PhSI-PPL membrane showed an anodic peak at 1.26 V, while the DTTO-PPL membrane exhibited another anodic peak at 1.31 V [35]. The TFSI-PPL electrolyte at the weight ratio of 1:3:4:3 displayed the best electrochemical stability as no significant electrochemical activity appeared below 4.8 V versus Li/Li + . ChemEngineering 2020, 4, x FOR PEER REVIEW 14 of 17 The rationale for designing PSTFSILi is the striking similarity with LiTFSI (as illustrated in Figure 15) with regard to the negative charge on the nitrogen atom being delocalized by two sulfone groups and one trifluoromethane group. This creates less interionic attractions between the cation and anion, commonly described as a low lattice energy salt [36]. Therefore, this TFSI-PPL solid polymer electrolyte also shows potential use with high-voltage cathode materials, which cannot be safely tested with liquid electrolytes due to their lower electrochemical stability.  The rationale for designing PSTFSILi is the striking similarity with LiTFSI (as illustrated in Figure 15) with regard to the negative charge on the nitrogen atom being delocalized by two sulfone groups and one trifluoromethane group. This creates less interionic attractions between the cation and anion, commonly described as a low lattice energy salt [36]. Therefore, this TFSI-PPL solid polymer electrolyte also shows potential use with high-voltage cathode materials, which cannot be safely tested with liquid electrolytes due to their lower electrochemical stability. The rationale for designing PSTFSILi is the striking similarity with LiTFSI (as illustrated in Figure 15) with regard to the negative charge on the nitrogen atom being delocalized by two sulfone groups and one trifluoromethane group. This creates less interionic attractions between the cation and anion, commonly described as a low lattice energy salt [36]. Therefore, this TFSI-PPL solid polymer electrolyte also shows potential use with high-voltage cathode materials, which cannot be safely tested with liquid electrolytes due to their lower electrochemical stability.

Conclusions
In this paper, the synthesis of three new PS-bound lithium salt-based SPEs was discussed for applications in lithium-ion batteries. The polymer salts were made from relatively cheap starting materials. The resulting PSLSs-based SPEs showed very good ionic conductivity at ambient temperature with excellent thermal stability. The highest ionic conductivity was able to reach 1.54 × 10 −04 S cm −1 at room temperature and 3.23 × 10 −03 S cm −1 at 70 • C for the DTTO-PPL membrane at the weight ratio of 1:3:4:3. These PSLS-based electrolytes also displayed very good electrochemical stability, which ensures their potential for further use in high-voltage lithium-ion cells.