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

chemengineering

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

Xinyi Mei 1, Wendy Zhao 1, Qiang Ma 1, Zheng Yue 1, Hamza Dunya 1 , Qianran He 2, Amartya Chakrabarti 3, Christopher McGarry 1 and Braja K. Mandal 1,* 1 Department of Chemistry, Illinois Institute of Technology, Chicago, IL 60616, USA; [email protected] (X.M.); [email protected] (W.Z.); [email protected] (Q.M.); [email protected] (Z.Y.); [email protected] (H.D.); [email protected] (C.M.) 2 Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA; [email protected] 3 Department of Physical Sciences, Dominican University, River Forest, IL 60305, USA; [email protected] * Correspondence: [email protected]

Received: 3 May 2020; Accepted: 6 July 2020; Published: 16 July 2020

Abstract: 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 (Tg) 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. × − − ◦ Keywords: crosslinked polystyrene; polymer-bound lithium salts; lithium-ion batteries; solid polymer electrolytes; low lattice energy lithium salts

1. 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 1 energy density (150–350 Wh kg− ), 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 LiPF6 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

ChemEngineering 2020, 4, 44; doi:10.3390/chemengineering4030044 www.mdpi.com/journal/chemengineering ChemEngineering 2020, 4, 44 2 of 17 ionic conductivities. However, they suffer from several disadvantages such as poor mechanical properties and increased reactivity toward the metal electrode, leading to significant decrease in battery lifetime and safety. In contrast, SPEs have the potential to solve the safety issue in LIBs [3]. SPEs possess many advantageous features, among which are lightweight, shape versatility, ease of processability, superior interfacial stability [4], and mechanical properties. SPEs were first developed by P.V. Wright in the early 1970s by preparing an ionic complex of PEO with alkali metal salts [5,6]. Since then, numerous studies involving PEO-LiTFSI-based SPEs have been conducted to prepare thinner, lighter, and safer LIBs [7]. The most widely studied low lattice energy lithium salt for SPEs is LiTFSI, which is attributed to several intrinsic features of the large anion TFSI− including (1) the high flexibility of –SO2–N–SO2– of TFSI−, which is favorable for reducing the crystallinity of the PEO matrix; (2) the highly delocalized charge distribution of TFSI− is pivotal + for effectively reducing the interactions between Li and TFSI−, thus increasing the dissociation and solubility of LiTFSI in the PEO matrix [8]; and (3) excellent thermal, chemical, and electrochemical stability, which is required for stable electrolytes. The Tg of electrolytes with LiTFSI is ordinarily lower than that of electrolytes with other salts such as LiClO4 and LiCF3SO3. These significant properties of TFSI− are helpful in developing reliable conductive SPEs for LIBs and other electrochemical devices [9]. However, PEO–LiTFSI-based SPEs show acceptable ionic conductivity only at a temperature higher than the melting point (Tm) of PEO. It is well established that the segmental mobility of a polymer matrix is frozen in the crystallinity phase, and the lithium ion transportation occurs only in the amorphous region of the polymer hosts [9,10]. Although the excellent chain flexibility of PEO causes lithium salts to be easily dissolved and promotes the smooth movement of dissociated ions, PEO-based 7 5 1 electrolytes still exhibit relatively low ionic conductivity (10− ~10− S cm− ) due to the high degree of crystallinity present in PEO at room temperature [11]. Consequently, finding an ideal SPE with good ion transport properties is still a challenging task. Several research groups have improved the performance of the polymer electrolytes by reducing the crystallinity of the polymer, increasing the concentration of ions, and increasing the proportion of the amorphous regions [12]. Some of the notable strategies include decreasing glass transition temperature (Tg) of the polymer electrolyte system and improving the magnitude of lithium ion dissociation [13] through the methods of grafting, crosslinking, mixing with ionic liquids, and blending various polymers materials [14–17]. Recent studies involving polymer nanoparticles containing composite polymer electrolytes (CPEs) have received great attention because of their superior electrolytic properties such as high amorphous content, low Tg, and high thermal and mechanical properties [18,19]. In light of this development, in this research study, we designed and synthesized three types of crosslinked polystyrene nanoparticles containing covalently functionalized lithium salt moieties (PSLSs) for the purpose of destroying the crystallinity of PEO-based electrolytes, leading to higher ionic conductivity. In order to achieve superior SPE properties (viz., compatibility, electrolytic and mechanical properties), we blended these polymer-bound lithium salts with a traditional polymer electrolyte system made of high molecular weight PEO and LiTFSI [15]. Polymer composites containing nanoparticles reinforce the polymer matrix, leading to enhancement of the mechanical properties. The high surface area of the nanoparticles is responsible for more physical interaction with the polymer matrix. We also included a low molecular weight polyethylene glycol dimethyl ether (PEGDM) plasticizer in the SPE formulations. The addition of a plasticizer has been extensively used to improve the ionic conductivity of PEO-based SPEs [16].

2. Experimental

2.1. Materials and Characterization Styrene (St), divinyl benzene (DVB), ammonium persulfate (APS), Triton X-100 (emulsifier), sodium dodecyl sulfonate (SDS), chlorosulfonic acid, trifluoromethane sulfonamide, PEGDM 1 1 (Mw = 1000 g mol− ), PEO (Mw = 4,000,000 g mol− ), and lithium hydroxide mono were all purchased ChemEngineering 2020, 4, 44 3 of 17 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).

2.2. Synthesis of PS–SO2Cl 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. ± − − − − − 2.3. Synthesis of PSTFSILi

PS–SO2Cl (2.50 g, 12.2 mmol), triﬂuoromethanesulfonamide (1.97 g, 13.2 mmol), and lithium hydroxide monohydrate (1.11 g, 26.5 mmol) were placed into a 50 mL ﬂask containing 30 mL of anhydrous acetonitrile. The mixture was stirred at room temperature overnight and then heated to 50 ◦C for 2 h and cooled down. The mixture was sonicated in 15 mL methanol and centrifuged. The washing process was repeated with water and acetone. The product was dried in a high vacuum 1 1 oven at 70 ◦C overnight (Yield: 2.55 g). FTIR: 2900–2950 cm− (m), 1350–1300 (s) cm− and 1180–1140 (s), 1 1 1 1 1000–1400 cm− , 1639.23 cm− (sh), 795.44 cm− (m), 677.52 cm− (m).

2.4. Synthesis of PSPhSILi

PS–SO2Cl (2.50 g, 12.3 mmol), benzenesulfonamide (1.94 g, 12.3 mmol), and lithium hydroxide monohydrate (1.04 g, 24.7 mmol) were placed in a 50 mL ﬂask containing 30 mL of anhydrous acetonitrile. The mixture was stirred at room temperature overnight and then heated to 50 ◦C for 2 h and cooled. The mixture was sonicated in 15 mL methanol and centrifuged. The washing process was repeated with water and acetone. The product was dried in a high vacuum oven at 70 ◦C overnight 1 1 1 1 (Yield: 2.55 g). FTIR: 3063.53 cm− (s), 2850–2950 cm− (sh), 1400–15,000cm− (s), 1000–1200 cm− (s), 1 1 1 1 1 1638.34 cm− (sh), 1600.48 cm− (m), 832.79 cm− (m), 678.43 cm− (m), 580.10 cm− (m).

2.5. Synthesis of PSDTTOLi Synthesis of polystyrenesulfonyl-1,3-dithiane. The 1,3-dithiane (0.32 g) was placed in a 5 mL 3-neck flask, then 1.8 mL of 2.5 M n-butyllithium solution was added to the flask under argon. The mixture was stirred at 0 ◦C for 1 hour before PS–SO2Cl (0.50 g) was added, which was soaked with THF for 3 h. The reaction was continued overnight. The mixture was sonicated in 15 mL methanol and centrifuged. The washing process was repeated with water and acetone. The product was dried in a 1 1 1 high vacuum oven at 70 ◦C overnight (Yield: 0.51 g). FTIR: 2900–2950 cm− , 1638.61 cm− , 1595.89 cm− , 1 1 1 1 1 1 1350–1495 (s) cm− , 1000–1180 cm− , 831.89 cm− , 775.60 cm− , 673.52 cm− , 578.62 cm− . ChemEngineering 2020, 4, x FOR PEER REVIEW 4 of 17 highChemEngineering vacuum oven2020, 4 at, 44 70 °C overnight (Yield: 0.51 g). FTIR: 2900–2950 cm−1, 1638.61 cm−1, 1595.89cm4 of−1 17, 1350–1495 (s) cm−1, 1000–1180 cm−1, 831.89 cm−1, 775.60 cm−1, 673.52 cm−1, 578.62 cm−1. Synthesis of polystyrenesulfonyl-1,3-dithiane-1,1,3,3-tetraoxide (PSDTTO). Polystyrenesulfonyl-1, 3- Synthesis of polystyrenesulfonyl-1,3-dithiane-1,1,3,3-tetraoxide (PSDTTO). Polystyrenesulfonyl-1, dithiane (0.50 g) was placed in a 50 mL flask containing 15 mL of acetic acid. Hydrogen peroxide (10 3-dithiane (0.50 g) was placed in a 50 mL flask containing 15 mL of acetic acid. Hydrogen peroxide mL) was added to the flask. The mixture was heated and stirred at 60 °C. The reaction was conducted (10 mL) was added to the flask. The mixture was heated and stirred at 60 C. The reaction was for three days with further additions of 1 mL of hydrogen peroxide/day. The ◦product was filtered conducted for three days with further additions of 1 mL of hydrogen peroxide/day. The product was and washed with 15 mL water. The product was dried in a high vacuum at 70°C overnight (Yield: filtered and washed with 15 mL water. The product was dried in a high vacuum at 70 C overnight 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 (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, cm−1, 831.89 cm−1, 775.61 cm−1, 673.52− cm−1, 578.62 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 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 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 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 g). FTIR: 2900– by acetone.−1 The product−1 was dried−1 in a high vacuum−1 at 70 ◦C overnight−1 (Yield:−1 0.35 g). FTIR:−1 2950 cm , 1705.731 cm , 1637.341 cm , 1600.751 cm , 1410–14951 (s) cm , 1000–11901 cm , 832.79 cm 1, 2900–2950−1 cm− , 1705.73−1 cm− , 1637.34−1 cm− , 1600.75 cm− , 1410–1495 (s) cm− , 1000–1190 cm− , 776.25 cm 1, 678.43 cm , 1582.81 cm . 1 1 832.79 cm− , 776.25 cm− , 678.43 cm− , 582.81 cm− . 2.6. Thin Film Processing and Cell Fabrication 2.6. Thin Film Processing and Cell Fabrication PEGDM, PEO, and PSLS were mixed in a mortar and pestle. The mixture was then placed in 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 between two Teflon coated sheets, then hot pressed in a Carver press at 100 ◦C under 10 psi pressure for for 5 min [21]. The resulting SPE film was then folded, refolded, and subjected to further hot pressing 5 min [21]. The resulting SPE film was then folded, refolded, and subjected to further hot pressing to to achieve a well-dispersed electrolyte film. Two thin stainless-steel plates were used as a spacer to achieve a well-dispersed electrolyte film. Two thin stainless-steel plates were used as a spacer to control control the thickness of the film. The polymer films were cut circularly in a 2.04 cm2 area and the thickness of the film. The polymer films were cut circularly in a 2.04 cm2 area and sandwiched sandwiched between two steel electrodes and subjected to impedance analysis. between two steel electrodes and subjected to impedance analysis.

3. Results Results and and Discussion Discussion

3.1. Synthesis Synthesis of of PS PS-Bound-Bound Lithium Lithium Salts Salts As outlined in Figure 11,, thethe synthesissynthesis beganbegan withwith thethe preparationpreparation ofof crosslinkedcrosslinked polystyrenepolystyrene nanoparticles following the method of Brijmohan et al. [22] [22].. Chlorosulfonation Chlorosulfonation of of the the PS PS nanoparticles nanoparticles was successfully carriedcarried outout with with a a minor minor modiﬁcation modification of of a reporteda reported procedure procedure [22 [22].]. The The new new TFSI-like TFSI- likePS-bound PS-bound lithium lithium salt, PSTFSILi, salt, PSTFSILi, was synthesized was synthesized in a single-step in by a reacting single-step the chlorosulfonated by reacting the PS chlonanoparticlesrosulfonated with PS CF nanoparticles3SO2NH2 in with the presence CF3SO2NH of2 LiOH. in theThe presence incorporation of LiOH. of The a similar incorporation lithium of salt a similarmoiety (–SOlithium2NLiSO salt moiety2CF3) on (–SO a benzene2NLiSO2 ringCF3) hason beena benzene previously ring has reported, been previously but involved reported three steps, but involvedstarting from three the steps benzene starting sulfonyl from the chloride benzene analog sulfonyl [23]. chloride analog [23].

Figure 1. SynthesisSynthesis of PSTFSILi PSTFSILi..

The aforem aforementionedentioned one one-step-step synthesis of the lithium sulfonimide derivative was successfully employed to prepareprepare thethe otherother newnew PS-bound PS-bound lithium lithium salt, salt, PSPhSILi PSPhSILi (Figure (Figure2). 2). The The special special feature feature of ofthis this solid–liquid solid–liquid phase phase synthesis synthesis included include ad very a very simple simple work-up work-up (ﬁltration (filtration and and washing). washing). ChemEngineering 2020, 4, x FOR PEER REVIEW 5 of 17 ChemEngineering 2020, 4, 44 5 of 17 ChemEngineering 2020, 4, x FOR PEER REVIEW 5 of 17

Figure 2. SynthesisSynthesis of PSPhSILi PSPhSILi.. Figure 2. Synthesis of PSPhSILi. The final ﬁnal new PS-boundPS-bound lithium salt, PSDTTOLi PSDTTOLi,, was prepared in three steps: (1) coupling of 1,31,3-dithiane-dithianeThe final with new PS PS–SO PS–SO-bound2ClCl using using lithium the the standardsalt, standard PSDTTOLi BuLi activation , was prepared protocol protocol; in ;three (2) oxidation steps: (1) of coupling thioether of groups1,3-dithiane to sulfones with PS using –SO 2hydrogen Cl using the peroxide;peroxide standard; and BuLi (3) lithiation activation using protocol LiOH; (2) (Figure(Figure oxidation3 3).). of thioether groups to sulfones using hydrogen peroxide; and (3) lithiation using LiOH (Figure 3).

Figure 3. SynthesisSynthesis of of PSDTTOLi PSDTTOLi.. Figure 3. Synthesis of PSDTTOLi. 3.2. Structural Characterization 3.2. Structural Characterization 3.2. StructuralFTIR Spectroscopy: CharacterizationFTIR spectra of the nanoparticles were obtained by dispersing the sample FTIR Spectroscopy: FTIR spectra of the nanoparticles were obtained by dispersing the sample in in anhydrous potassium bromide. For PSTFSILi, the peak at 1000–1400 cm 1 corresponded to the anhydrousFTIR Spectroscopy: potassium bromide.FTIR spectra For of PSTFSILi, the nanoparticles the peak were at 1000 obtained–1400 b cmy dispersing−1− correspond the edsample to the in stretching of the C–F bond. The broad peaks in the range of 1350–1300 cm 1 (s) and 1180–1140 cm 1 stretchinganhydrous of potassium the C–F bond. bromide. The broad For PSTFSILi,peaks in the the range peak of at 1350 1000–1300–1400 cm cm−−1 −1(s) correspond and 1180–1140ed to cm the−−1 1 (s) were due to the O2S–N-bond. For PSPhSILi, strong peaks around 3063.53 cm− corresponded to (s)stretching were due of tothe the C– OF 2bond.S–N-bond. The broad For PSPhSILi, peaks in strongthe range peaks of 1350around–1300 3063.53 cm−1 (s) cm and−1 correspond 1180–1140ed cm to−1 1 aromatic C–H stretching and 1500–1400 cm− related to aromatic C=C stretching. The peak around aromatic(s) were dueC–H to stretching the O2S– Nand-bond. 1500 For–1400 PSPhSILi, cm−1 relate strongd to peaksaromatic around C=C 3063.53stretching. cm−1 The correspond peak arounded to 1630 cm 1 represents the S–N–S combination bond, which indicates that the reaction was complete. 1630aromatic cm−1− Crepresents–H stretching the S and–N– S1500 combination–1400 cm−1 bond, relate whichd to aromatic indicates C=C that stretching. the reaction The was peak complete. around For PSDTTOLi, after lithiation of PSDTTO, the peak at 1717.43 cm 1 shifted to 1705.73 cm 1, and For1630 P cmSDTTOLi,−1 represents after the lithiation S–N–S ofcombination PSDTTO, thebond, peak which at 1717.43 indicates cm −that−1 shifted the reaction to 1705.73 was cmcomplete.−−1, and 1639.34 cm 1 shifted to 1637.34 cm 1, which can be assigned to the stretching vibration band of carbon 1639.34For PSDTTOLi, cm−−1 shifted after to lithiation1637.34 cm of−1− , PSDTTO,which can the be assigned peak at 1717.43to the stretching cm−1 shifted vibration to 1705.73 band of cm carbon−1, and with the existence of three sulfone electron withdrawing groups. These data partially support the with1639.34 the cm existence−1 shifted of to three 1637.34 sulfone cm−1 , electronwhich can withdrawing be assigned groupto the s.stretching These data vibration partially band support of carbon the proposed structures of the lithium bound polymer salts. proposedwith the existencestructures of of three the lithium sulfone bound electron polymer withdrawing salts. groups. These data partially support the Energy Dispersive X-ray Spectroscopy (EDS): This technique for elemental analysis was performed proposedEnergy structures Dispersive of X the-ray lithium Spectroscopy bound (EDS): polymer This salts. technique for elemental analysis was performed with a scanning electron microscope (SEM, Phenom Pro-X, Nanoscience Instruments) by selecting with Energya scanning Dispersive electron X-ray microscope Spectroscopy (SEM, (EDS): Phenom This technique Pro-X, Nanoscience for elemental Instruments) analysis was by performed selecting three different mapping areas in the secondary electron mode with an acceleration voltage of 15 kV. threewith differenta scanning mapping electron areas microscope in the secondary (SEM, Phenom electron Pro mode-X, Nanoscience with an acceleration Instruments) voltage by ofselecting 15 kV. All samples were deposited on a silicon wafer for SEM/EDS measurement after they were dispersed in Allthree samples different were mapping deposited areas on in a siliconthe secondary wafer for electron SEM/EDS mode measurement with an acceleration after they voltage were dispersed of 15 kV. pure ethyl alcohol and sonicated for 1 h. EDS measurement of each mapping area took about seven inAll pure samples ethyl werealcohol deposited and sonicate on a dsilicon for 1 hwafer. EDS formeasurement SEM/EDS measurement of each mapping after area they took were about dispersed seven minutes, so the total collecting time was 21 min for the three different mapping areas to obtain better minin pureutes ethyl, so the alcohol total collecting and sonicate timed forwas 1 21h. EDSmin formeasurement the three different of each mappingmapping areaareas took to obtain about betterseven statistics or consistency. Figures4–6 show the EDS elemental composition of each sample, while the statisticsminutes, orso consistency.the total collecting Figures time 4–6 wasshow 21 the min EDS for elementalthe three different composition mapping of each areas sampl to obtaine, while better the obtained elemental concentrations from the EDS studies are presented in Table1. The analysis of the obtainedstatistics elementalor consistency. concentrations Figures 4– from6 show the the EDS EDS studies elemental are presented composition in Table of each 1. The sampl analysise, while of thethe data indicates that all of these salts had a different ratio of carbon, nitrogen, oxygen, and sulfur, while dataobtained indicates elemental that all concentrations of these salts hadfrom a differentthe EDS studies ratio of arecarbon, presented nitrogen, in Table oxygen 1. The, and analysis sulfur, whileof the only PSTFSILi exhibited 8.2% F. Lithium ions were too small to be detected by the instrument. onlydata PSTFSILiindicates thatexhibit all edof these8.2% F.salts Lithium had a differentions were ratio too smallof carbon, to be nitrogen, detected oxygenby the instrument., and sulfur, while only PSTFSILi exhibited 8.2% F. Lithium ions were too small to be detected by the instrument. ChemEngineeringChemEngineering 20202020,,, 44,,, xxx FORFORFOR PEERPEERPEER REVIEWREVIEWREVIEW 66 ofof 1717

TableTable 1.1. ElementalElemental concentrationsconcentrations determineddetermined usingusing thethe energyenergy dispersivedispersive xx--rayray spectroscopyspectroscopy ((EDSEDS).).

SampleSample At.At. %% CC AtAt %% NN At.At. %% OO At.At. %% SS At.At. %% FF PSTFSILiPSTFSILi 48.948.9 13.513.5 26.426.4 3.13.1 8.28.2 ChemEngineeringPSPhSILiPSPhSILi 2020 , 4, 44 58.458.4 5.75.7 26.226.2 9.79.7 -- 6 of 17 PSDTTOLiPSDTTOLi 40.440.4 -- 26.926.9 26.926.9 --

FigureFigure 4.4. EnergyEnergy dispersivedispersive X-rayxx--rayray spectroscopy spectroscopy ( (EDS)(EDSEDS)) of of the the PSTFSILi PSTFSILi nanoparticles. nanoparticles.

FigureFigure 5.5. EnergyEnergy dispersivedispersive X-rayxx--rayray spectroscopy spectroscopy ( (EDS)(EDSEDS)) of of the the PSPhSILi PSPhSILi nanoparticles. nanoparticles.

FigureFigure 6.6. EnergyEnergy dispersivedispersive X-rayxx--rayray spectroscopy spectroscopy ( (EDS)(EDSEDS)) of of the the PSDTTOLi PSDTTOLi nanoparticles. nanoparticles.

Table 1. Elemental concentrations determined using the energy dispersive X-ray spectroscopy (EDS). ScanningScanning ElectronElectron Microscopy:Microscopy: TheThe morphologymorphology ofof thethe PSPS--boundbound lithiumlithium saltsalt nanoparticlesnanoparticles waswas analyzedanalyzedSample usingusing scanningscanning At. electronelectron % C microscopmicroscop At %yy N (SEM)(SEM) ((Leica, At.Leica, % O CambridgeCambridge At.,, %UKUK S)) withwith an At.an acceleratingaccelerating % F voltagevoltage valuePSTFSILivalue equalequal toto 1515 kV.kV. 48.9 TheThe samplessamples 13.5 werewere coatedcoated withwith 26.4 aa thinthin goldgold layerlayer 3.1 usingusing aa sputtersputter 8.2 coatercoater (Polaron(PolaronPSPhSILi,, modelmodel SC502SC502,, Williston,Williston, 58.4 VT,VT, USAUSA 5.7)) [13][13].. TFSITFSI andand 26.2 PhSIPhSI modifiedmodified 9.7 nanoparticlesnanoparticles - displayeddisplayed PSDTTOLi 40.4 - 26.9 26.9 -

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 ChemEngineering 2020, 4, 44 7 of 17

ChemEngineering 2020, 4, x FOR PEER REVIEW 7 of 17 (Polaron, model SC502, Williston, VT, USA) [13]. TFSI and PhSI modiﬁed nanoparticles displayed similar particle sizesizess at around 500 nm, whereas the DTTO modiﬁedmodified nanoparticles measured at around 350 nm (Figure7 7).). In In either either case, case, the the particle particle size size waswas muchmuch larger larger that that the the starting starting PS PS nanonanoparticlesparticles (50 nm). This This indicates indicates that that PS PS particles agglomerated during during the incorporation of functional groups [[24]24]..

Figure 7. (a) Scanning electron microscopy ( (SEM)SEM) of the PS nanoparticles, where the diameter of the particles waswas around around 50 nm;50 nm; (b) SEM (b) SEM of the of PSTFSILi the PSTFSILi nanoparticles, nanoparticles, where thewhere diameter the diameter of the particles of the wasparticles around was 500 around nm; ( c 500) SEM nm; of ( thec) SEM PSPhSILi of the nanoparticles, PSPhSILi nanoparticles, where the diameter where the of the diameter particles of was the particlesaround 500 was nm; around (d) SEM 500 ofnm; the (d PSDTTOLi) SEM of the nanoparticles, PSDTTOLi nanoparticles, where the diameter where ofthe the diameter particles of wasthe particlesaround 350 was nm around [21,25 350]. nm [21,25]. 3.3. SPE Formulations 3.3. SPE Formulations The various weight ratios of the PEO-based SPE films along with the PSLSs are listed in Tables2–4. The various weight ratios of the PEO-based SPE films along with the PSLSs are listed in Tables A SPE film containing PEO:PEGDM:LiTFSI (PPL) at the weight ratio of 3:4:2 max limit was prepared 2–4. A SPE film containing PEO:PEGDM:LiTFSI (PPL) at the weight ratio of 3:4:2 max limit was and served as the standard SPE sample. The SPE films maintained good ductility and mechanical prepared and served as the standard SPE sample. The SPE films maintained good ductility and strength and did not snap upon manual bending or stretching. The PPL complex at the weight ratio mechanical strength and did not snap upon manual bending or stretching. The PPL complex at the of 3:4:3 presented challenges to making a nice film, but all PSLS-based SPEs displayed excellent weight ratio of 3:4:3 presented challenges to making a nice film, but all PSLS-based SPEs displayed film quality. excellent film quality.

Table 2. Formulations of the SPE films with various weight ratios of PSTFSILi and LiTFSI.

Components Weight [mg] Abbreviation of the PSTFSILi PEO PEGDM LiTFSI Formulation 0 75 100 50 PPL = 3:4:2 25 75 100 25 TFSI–PPL = 1:3:4:1 25 75 100 50 TFSI–PPL = 1:3:4:2 25 75 100 75 TFSI–PPL = 1:3:4:3 ChemEngineering 2020, 4, 44 8 of 17

Table 2. Formulations of the SPE ﬁlms with various weight ratios of PSTFSILi and LiTFSI.

Components Weight [mg] Abbreviation of PSTFSILi PEO PEGDM LiTFSI the Formulation 0 75 100 50 PPL = 3:4:2 25 75 100 25 TFSI–PPL = 1:3:4:1 25 75 100 50 TFSI–PPL = 1:3:4:2 25 75 100 75 TFSI–PPL = 1:3:4:3 25 37.5 50 12.5 TFSI–PPL = 2:3:4:1 25 37.5 50 25 TFSI–PPL = 2:3:4:2 25 37.5 50 37.5 TFSI–PPL = 2:3:4:3 25 25 37.5 8.3 TFSI–PPL = 3:3:4:1 25 25 37.5 16.7 TFSI–PPL = 3:3:4:2 25 25 37.5 25 TFSI–PPL = 3:3:4:3

Table 3. Formulations of the SPE ﬁlms with various weight ratios of PSPhSILi and LiTFSI.

Components Weight [mg] Abbreviation of PSTFSILi PEO PEGDM LiTFSI the Formulation 25 75 100 25 PhSI–PPL = 1:3:4:1 25 75 100 50 PhSI–PPL = 1:3:4:2 25 75 100 75 PhSI–PPL = 1:3:4:3 25 37.5 50 12.5 PhSI–PPL = 2:3:4:1 25 37.5 50 25 PhSI–PPL = 2:3:4:2 25 37.5 50 37.5 PhSI–PPL = 2:3:4:3 25 25 37.5 8.3 PhSI–PPL = 3:3:4:1 25 25 37.5 16.7 PhSI–PPL = 3:3:4:2 25 25 37.5 25 PhSI–PPL = 3:3:4:3

Table 4. Formulations of the SPE ﬁlms with various weight ratios of PSDTTOLi and LiTFSI.

Components Weight [mg] Abbreviation of the PSTFSILi PEO PEGDM LiTFSI Formulation 25 75 100 25 DTTO–PPL = 1:3:4:1 25 75 100 50 DTTO–PPL = 1:3:4:2 25 75 100 75 DTTO–PPL = 1:3:4:3 25 37.5 50 12.5 DTTO–PPL = 2:3:4:1 25 37.5 50 25 DTTO–PPL = 2:3:4:2 25 37.5 50 37.5 DTTO–PPL = 2:3:4:3 25 25 37.5 8.3 DTTO–PPL = 3:3:4:1 25 25 37.5 16.7 DTTO–PPL = 3:3:4:2 25 25 37.5 25 DTTO–PPL = 3:3:4:3

3.4. 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 (Figure8). 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 (Table5). 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 ChemEngineeringthey did not decompose 2020, 4, x FOR prior PEER toREVIEW PEO. 9 of 17

Figure 8. Thermogravimetric analysis (TGA) curves under air flow for PSTFSILi, PSPhSILi, Figureand PSDTTOLi. 8. Thermogravimetric analysis (TGA) curves under air flow for PSTFSILi, PSPhSILi, and PSDTTOLi. Table 5. Thermal degradation of the PSLSs at different temperatures. Table 5. Thermal degradation of the PSLSs at different temperatures. Decomposition Percentage Weight Loss [%] at Various Temperature Sample Temperature,Decomposition Td [ ◦C] Percentage200 C Weight 300 CLoss [%]400 at VariousC Temperature 500 C Sample ◦ ◦ ◦ ◦ PSTFSILiTemperature 376, Td [°C] 200 4.5 °C 300 6.6 °C 11.1400 °C 29.9500 °C PSTFSILiPSPhSILi 376 450 0.74.5 2.16.6 2.711.1 22.5 29.9 PSPhSILiPSDTTOLi 450 449 0.50.7 1.52.1 4.82.7 23.6 22.5 PSDTTOLi 449 0.5 1.5 4.8 23.6 3.5. Differential Scanning Calorimetry 3.5. Differential Scanning Calorimetry Differential scanning calorimetry (DSC) is one of the most convenient approaches to shed light on the degreeDifferential of crystallinity scanning calorimetry along with the(DSC) thermal is one behavior of the most of polymericconvenient samples, approaches and to was shed used light to oncharacterize the degree our of crystallinity electrolyte samples along with [26]. the The thermal Tg of a behavior polymer of indicates polymeric the samples, transition and from was a rubberyused to characterizeto a glassy state our andelectrolyte vice versa. samples Therefore, [26]. The the T polymerg of a polymer is flexible indicates above the the transition Tg, and hardfrom anda rubbery brittle tobelow a glassy the Tstateg [13 and]. This vice is versa. an endothermic Therefore, the transition polymer while is flexible the crystalline above the melting Tg, and pointhard and (Tm )brittle is an belowexothermic the T transition.g [13]. This In is ouran study,endothermic DSC analysis transition was while carried the out crystalline with a Mettler melting Toledo point di (Tffmerential) is an 1 exothermicscanning calorimeter transition. instrumentIn our study, under DSC an analys argonis atmospherewas carried out with with a flow a Mettler rate at Toledo 70.0 mL differential min− [4] sbetweencanning calorimeter100 C and instrument 150 C[27]. under Samples an a ofrgon around atmosphere 5–7 mg werewith sealeda flow inrate a 40 at µ70.0L aluminum mL min−1 pan [4] − ◦ ◦ withbetween a perforated −100 °C and lid to 150 allow °C [27] the. release Samples and of removal around of5– the7 mg decomposition were sealed in products a 40 µL [28aluminum]. Two stages pan withof DSC a perforated scanning were lid to set: allow (1) the100 releaseC isothermal and removal for 10.0 of the min, deco andmposition (2) heating products rate of 10[28]C. Two min stages1 and − ◦ ◦ − oftemperature DSC scanning range were from set: 100(1) − to100 150 °C isothermalC. An empty for aluminum 10.0 min, and pan ( served2) heating as therate reference. of 10 °C min The−1DSC and − ◦ temperatureresults are presented range from in − Figure100 to9 150 and °C. the An essential empty aluminum glass transition pan served and melting as the reference. points for The various DSC resultselectrolyte are samplespresented are in listed Figure in Table9 and6 .the essential glass transition and melting points for various electrolyte samples are listed in Table 6. ChemEngineering 2020, 4, 44 10 of 17 ChemEngineering 2020, 4, x FOR PEER REVIEW 10 of 17

Figure 9. Heat ﬂow versus temperature of the diﬀerential scanning calorimetry DSC curves: (a) weight

ratio of PEO:PEGDM:LiTFSI (PPL) = 3:4:2, (b) weight ratio of PSTFSILi:PEO:PEGDM:LiTFSI (TFSI–PPL) =Figure1:3:4:2, 9. ( cHeat) weight flow ratio versus of TFSI–PPL temperature= 1:3:4:3, of the (d )differential weight ratio scanning of TFSI–PPL calorimetry= 2:3:4:3, DSC (e) weight curves: ratio (a) ofweight TFSI–PPL ratio= of3:3:4:3, PEO:PEGDM:LiTFSI (f) weight ratio of(PPL) PSPhSILi:PEO:PEGDM:LiTFSI = 3:4:2, (b) weight ratio of (PhSI–PPL)PSTFSILi:PEO:PEGDM:LiTFSI= 1:3:4:3, (g) weight ratio(TFSI of–PP PSDTTOLi:PEO:PEGDM:LiTFSIL) = 1:3:4:2, (c) weight ratio of TFSI (DTTO–PPL)–PPL = 1:3:4:3,= 1:3:4:3, (d) weight (h) four ratio best of performing TFSI–PPL membranes,= 2:3:4:3, (e) (weighti) various ratio weight of TFSI ratios–PPL of PSTFSILi.= 3:3:4:3, (f) weight ratio of PSPhSILi:PEO:PEGDM:LiTFSI (PhSI–PPL) = 1:3:4:3, (g) weight ratio of PSDTTOLi:PEO:PEGDM:LiTFSI (DTTO–PPL) = 1:3:4:3, (h) four best Table 6. Thermal properties of the PSLS-based electrolytes membranes. performing membranes, (i) various weight ratios of PSTFSILi. Sample a b c d e f g Table 6. Thermal properties of the PSLS-based electrolytes membranes. Tg [ C] 49.2 48.5 49.6 50.3 48.9 50.6 50.9 ◦ − − − − − − − SampleTm [◦C]a 61.7b 55.2 44.8c 52.2d 43.7e 52.8f 47.7 g Tg [°C] −49.2 −48.5 −49.6 −50.3 −48.9 −50.6 −50.9

TheT mg of[°C] all electrolytes61.7 was55.2 found to be44.8 concentrated 52.2 around43.750 C. In52.8 contrast, 47.7 Tm showed − ◦ some separations. We presume that this was due to the addition of PSLSs, which caused a change in the shapeThe T ofg of the all endothermic electrolytes peak,was found and the to peaksbe concentrated were slightly around shifted −50 toward °C. In contrast, lower temperatures Tm showed comparedsome separations. to the control We presume (Figure9 h,i).that t Thesehis was observations due to the clearlyaddition suggest of PSLSs that, which a signiﬁcant caused contribution a change in tothe conductivity shape of the enhancement endothermic comespeak, and from the structural peaks were modiﬁcations slightly shifted associated toward with lower the polymertemperatures host causedcompared by the to salts. the control The reorganization (Figure 9h,i) of. theThese polymer observations chain may clearly be hindered suggest by that the a crosslinking significant centerscontribution formed to byconductivity the interaction enhancement of the functional comes from groups. structural modifications associated with the polymer host caused by the salts. The reorganization of the polymer chain may be hindered by the 3.6.crosslinking Ionic Conductivity centers formed by the interaction of the functional groups. The ionic conductivities of the formulated electrolytes were measured by the complex impedance 3.6. Ionic Conductivity method using an impedance analyzer (Solartron model SI-1287) coupled to a Solartron model-1260 frequencyThe ionic response conductivities analyzer [29 of]. The the solid formulated electrolyte electrolytes membranes were were measured sandwiched by between the complex two stainlessimpedance steel method electrodes using for an conductivity impedance measurements analyzer (Solartron [10,30 ].model All measurements SI-1287) coupled were to performed a Solartron in anmodel environmental-1260 frequency chamber response in the analyzertemperature [29] range. The of solid 25–70 electrolyte◦C[31]. 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]. ChemEngineering 2020,, 4,, 44x FOR PEER REVIEW 11 of 17

To compare the performance of our electrolytes, we compared the data with that of similar work To compare the performance of our electrolytes, we compared the data with that of similar work published in the literature. For example, Armand et al. reported a single-ion BAB triblock copolymer, published in the literature. For example, Armand et al. reported a single-ion BAB triblock copolymer, −05 −1 P(STFSILi)–PEO–P(STFSILi) that displayed very good ionic conductivity of 1.3 × 10 05S cm [32]1 . In P(STFSILi)–PEO–P(STFSILi) that displayed very good ionic conductivity of 1.3 10− S cm− [32]. another study, Feng et al. developed a similar single lithium-ion conducting polymer× electrolyte In another study, Feng et al. developed a similar single lithium-ion conducting polymer electrolyte based on the poly[(4-styrenesulfonyl)(trifluoromethane sulfonyl)imide] anion. However, the ionic based on the poly[(4-styrenesulfonyl)(trifluoromethane sulfonyl)imide] anion. However, the ionic conductivities could only reach 7.6 × 10−06 S cm−1 at 25 °C and 10−04 S cm−1 at 60 °C [23]. In this study, conductivities could only reach 7.6 10 06 S cm 1 at 25 C and 10 04 S cm 1 at 60 C[23]. In this study, − − ◦ − − ◦ −03 −1 the ionic conductivity of TFSI–PPL× (Figure 9) at the weight ratio of 1:3:4:3 displayed 3.0 × 1003 S cm 1 the ionic conductivity of TFSI–PPL (Figure9) at the weight ratio of 1:3:4:3 displayed 3.0 10− S cm− at 70 °C, which is superior to that of the PhSI-PPL at 1.27 × 10−03 S cm−1. The best ionic conductivity× of at 70 C, which is superior to that of the PhSI-PPL at 1.27 10 03 S cm 1. The best ionic conductivity ◦ −03 −1 − − 3.23 × 10 S 03cm at 701 °C was achieved with DTTO–PPL ×at the weight ratio of 1:3:4:3. of 3.23 10− S cm− at 70 ◦C was achieved with DTTO–PPL at the weight ratio of 1:3:4:3. Among× the three PSLSs, the compounds containing PSTFSI and PSPhSI showed structural 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 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 electrolyte was due to the presence of the CF group, which induced a strong electron-withdrawing effect when compared to an aromatic ring.3 In contrast, the existence of three strong electron effect when compared to an aromatic ring. In contrast, the existence of three strong electron withdrawing withdrawing sulfone (SO2) groups provided the PSDTTO anion with the best dissociating ability to sulfone (SO ) groups provided the PSDTTO anion with the best dissociating ability to enable faster enable faster2 lithium ion transport. As the temperatures were elevated, the movement of the polymer lithium ion transport. As the temperatures were elevated, the movement of the polymer chain segments chain segments rapidly increased, and all of the salts followed the well-established trend of increasing rapidly increased, and all of the salts followed the well-established trend of increasing conductivity conductivity with the increase in temperatures (Figures 10–13). Nevertheless, with the increasing with the increase in temperatures (Figures 10–13). Nevertheless, with the increasing weight ratio of weight ratio of PSLSs, the conductivity decreased. This trend can likely be attributed to the high salt PSLSs, the conductivity decreased. This trend can likely be attributed to the high salt concentration with concentration with the additional influence of the ion pairs, ion triplets, and the higher ion the additional influence of the ion pairs, ion triplets, and the higher ion aggregations, which reduced aggregations, which reduced the overall mobility and the number of effective charge carriers [1]. It is the overall mobility and the number of effective charge carriers [1]. It is important to note that our important to note that our assessment was based on the formulation that provided the highest ionic assessment was based on the formulation that provided the highest ionic conductivity. PPL displayed conductivity. PPL displayed its highest ionic conductivity with the formulation described in Table 2 its highest ionic conductivity with the formulation described in Table2 (i.e., PEO:PEGDM:LiTFSI::3:4:2). (i.e., PEO:PEGDM:LiTFSI::3:4:2). The TFSI–PPL and DTTO–PPL based electrolytes displayed the best The TFSI–PPL and DTTO–PPL based electrolytes displayed the best ionic conductivity with this ionic conductivity with this formulation (1:3:4:3, see Tables 2 and 4). formulation (1:3:4:3, see Tables2 and4).

Figure 10. Ionic conductivity versus temperature for the four best performing electrolytes. Figure 10. Ionic conductivity versus temperature for the four best performing electrolytes. ChemEngineering 2020, 4, x FOR PEER REVIEW 12 of 17 ChemEngineering 2020, 4, 44 12 of 17 ChemEngineering 2020, 4, x FOR PEER REVIEW 12 of 17

Figure 11. Ionic conductivity versus temperature of various weight ratios of PSTFSILi–PEO–PEGDM– Figure 11. Ionic conductivity versus temperature of various weight ratios of PSTFSILi–PEO–PEGDM– LiTFSI (TFSI–PPL) electrolytes. FigureLiTFSI 11. (TFSI–PPL) Ionic conductivity electrolytes. versus temperature of various weight ratios of PSTFSILi–PEO–PEGDM– LiTFSI (TFSI–PPL) electrolytes.

Figure 12. Ionic conductivity versus temperature of various weight ratios of PSPhSILi–PEO–PEGDM– Figure 12. Ionic conductivity versus temperature of various weight ratios of PSPhSILi–PEO–PEGDM– LiTFSI (PhSI–PPL) electrolytes. FigureLiTFSI 12. (PhSI Ionic–PPL) conductivity electrolytes. versus temperature of various weight ratios of PSPhSILi–PEO–PEGDM– LiTFSI (PhSI–PPL) electrolytes. ChemEngineeringChemEngineering2020 2020, 4, ,4 44, x FOR PEER REVIEW 1313 of of 17 17

Figure 13. Ionic conductivity versus temperature of various weight ratios of PSPhSILi–PEO–PEGDM– LiTFSI (DTTO–PPL) electrolytes. Figure 13. Ionic conductivity versus temperature of various weight ratios of PSPhSILi–PEO–PEGDM– 3.7. CyclicLiTFSI Voltammetry (DTTO–PPL) electrolytes.

3.7. ElectrolytesCyclic Voltammetry 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 voltammetryElectrolytes (CV) in using lithium a Solartron-ion batteries potentiostat need /togalvanostat retain sufficiently impedance wide/gain-phase reversible analyzer electrochemical (SI 1260) + coupledstability with against a Solartron Li/Li . The electrochemical electrochemical interface stability (SI 1287).of the ASPE testing membranes cell was was assembled determined to determine by cyclic thevoltammetry oxidation potential (CV) using of stainless a Solartron steel as potentiostat/galvanostat the working electrode and impedance/gain lithium foil serving-phase as analyzer the counter (SI 1260) coupled with a Solartron electrochemical interface (SI 1287). A testing cell 1was assembled to and reference electrode. All scans were conducted at 25 ◦C at a scan rate of 10 mV s− with the voltage rangedetermine of 0 V the to 5.0oxidation V versus potential Li/Li+ [ 33of ].stainless steel as the working electrode and lithium foil serving −1 as theFor counter practical and applications, reference electrode. an electrolyte All scans system were should conducted exhibit at excellent 25 °C at electrochemical a scan rate of 10 stability mV s + inwith excess the voltage of 4.5 V range since of secondary 0 V to 5.0 lithium-ionV versus Li/Li batteries [33]. typically operate in the voltage range of 2.8–4.35For V practical [34]. The applications, characteristic an CV electrolyte properties system for each should of the exhi threebit excellent best electrolyte electrochemical membranes stability are depictedin excess in of Figure 4.5 V 14since. According secondary to lithium the CV- curves,ion batteries both PhSI–PPL typically andoperate DTTO–PPL in the voltage films at range the weight of 2.8– ratio4.35 ofV 1:3:4:3[34]. The displayed character electrochemicalistic CV properties stability for upeach to 4.2of the V. Thethree PhSI–PPL best electrolyte membrane membranes showed anare anodicdepicted peak in Figure at 1.26 14. V, whileAccording the DTTO–PPL to the CV curves, membrane both exhibitedPhSI–PPL another and DTTO anodic–PPL peak films at at 1.31 the Vweight [35]. Theratio TFSI–PPL of 1:3:4:3 electrolyte displayed at electrochemical the weight ratio stability of 1:3:4:3 up displayed to 4.2 V. the The best PhSI electrochemical–PPL membrane stability showed as no an significantanodic peak electrochemical at 1.26 V, while activity the DTTO appeared–PPL belowmembrane 4.8 V exhibited versus Li /anotherLi+. 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, 44x FOR PEER REVIEW 14 of 17 ChemEngineering 2020, 4, x FOR PEER REVIEW 14 of 17

Figure 14. Cyclic voltammetry (CV) curves of the selected electrolyte membranes: (a) TFSI–PPL = FigureFigure 14. 14. CyclicCyclic voltammetry voltammetry (CV (CV)) curves curves of of the the selected selected electrolyte electrolyte membranes: membranes: (a ()a )TFSI TFSI–PPL–PPL = = 1:3:4:3; (b) PhSI–PPLPhSI–PPL = 1:3:4:3;1:3:4:3; ( (cc)) DTTO DTTO–PPL–PPL = 1:3:4:3.1:3:4:3. 1:3:4:3; (b) PhSI–PPL = 1:3:4:3; (c) DTTO–PPL = 1:3:4:3. The rationale for designing PSTFSILi is the striking similarity with LiTFSI (as illustrated in The rationale for designing PSTFSILi is the striking similarity with LiTFSI (as illustrated in Figure 15)15) with regard to the negative charge on the nitrogen atom being delocalized by two two sulfone sulfone Figure 15) with regard to the negative charge on the nitrogen atom being delocalized by two sulfone groups andand oneone triﬂuoromethanetrifluoromethane group. group. This This creates creates less less interionic interionic attractions attractions between between the cationthe cation and groups and one trifluoromethane group. This creates less interionic attractions between the cation andanion, anion, commonly commonly described described as a low as alattice low energylattice energy salt [36 ]. salt Therefore, [36]. Therefore, this TFSI–PPL this TFSI solid–PPL polymer solid and anion, commonly described as a low lattice energy salt [36]. Therefore, this TFSI–PPL solid polymerelectrolyte electrolyte also shows also potential shows usepot withential high-voltage use with high cathode-voltage materials, cathode which materials, cannot which be safely cannot tested be polymer electrolyte also shows potential use with high-voltage cathode materials, which cannot be safelywith liquid tested electrolytes with liquid due electrolytes to their lower due to electrochemical their lower electrochemical stability. stability. safely tested with liquid electrolytes due to their lower electrochemical stability.

Figure 15. StructuralStructural comparisons comparisons of PSTFSILi PSTFSILi with LiTFSI. Figure 15. Structural comparisons of PSTFSILi with LiTFSI.

ChemEngineering 2020, 4, 44 15 of 17

4. 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.

Author Contributions: X.M. (Conceptualization, Methodology, Investigation, Writing original draft); W.Z. (Data curation, Investigation); Q.M. (Writing, Data curation, Visualization); Z.Y. (Writing, review & editing, Software, Methodology, Data curation); H.D. (Software, Methodology, Data curation); Q.H. (Software, Methodology, Data curation); A.C. (Data curation, Visualization, Investigation); C.M. (Data curation, Writing, review & editing); B.K.M. (Conceptualization, Methodology, Supervision, Writing, review & editing). All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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