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 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 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 –SO
2–N–SO
2– 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 T
g of electrolytes with LiTFSI is ordinarily lower than that of electrolytes with other salts such as LiClO
4 and LiCF
3SO
3. 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 (T
m) 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 electrolytes still exhibit relatively low ionic conductivity (10
−7~10
−5 S cm
−1) 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 (T
g) 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,
15,
16,
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 T
g, 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 (Mw = 1000 g mol−1), PEO (Mw = 4,000,000 g mol−1), and lithium hydroxide mono were all purchased 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 oven at 70 °C overnight (Yield: 2.55 g). FTIR: 2900–2950 cm−1 (m), 1350–1300 (s) cm−1 and 1180–1140 (s), 1000–1400 cm−1, 1639.23 cm−1 (sh), 795.44 cm−1 (m), 677.52 cm−1 (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 (Yield: 2.55 g). FTIR: 3063.53 cm−1 (s), 2850–2950 cm−1 (sh), 1400–15,000cm−1 (s), 1000–1200 cm−1 (s), 1638.34 cm−1 (sh), 1600.48 cm−1 (m), 832.79 cm−1 (m), 678.43 cm−1 (m), 580.10 cm−1 (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 high vacuum oven at 70 °C overnight (Yield: 0.51 g). FTIR: 2900–2950 cm−1, 1638.61 cm−1, 1595.89 cm−1, 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-dithiane (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: 0.35 g). FTIR: 2900–2950 cm−1, 1705.73 cm−1, 1637.34 cm−1, 1600.75 cm−1, 1410–1495 (s) cm−1, 1000–1190 cm−1, 832.79 cm−1, 776.25 cm−1, 678.43 cm−1, 582.81 cm−1.
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 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.