A Novel Gel Polymer Electrolyte by Thiol-Ene Click Reaction Derived from CO2-Based Polycarbonate for Lithium-Ion Batteries

The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province/State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China College of Light Industry and Food, Zhongkai University of Agriculture and Engineering, Guangzhou, China School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China Analytical and Testing Center of Guangzhou University, Guangzhou, China


Introduction
Rechargeable lithium-ion batteries manifest enormous daily influence in electrochemical devices for microchip technology, consumer electronics, battery electric vehicles, and industrial energy storage [1,2]. The electrolyte, providing the passage of ions to create the battery current, is a primary component of lithium-ion battery [3,4]. Liquid electrolytes generally consist of lithium salts and organic solvent, including ethylene carbonate, propylene carbonate, dimethyl carbonate, or diethyl carbonate. These most conventional and typical volatile organic solvents sometimes produce volatilization and leakage issues during long-term operation [5][6][7][8].
Polycarbonates synthesized from CO 2 and epoxides have received considerable attention [35][36][37]. Conversion of CO 2 into economic, environment-friendly, and functional polycarbonates has been achieved since the pioneers Inoue and coworkers in 1969 [38]. Poly(propylene carbonate) (PPC), one of the most extensively studied CO 2 -based polycarbonates, has also been studied as GPE due to the similar structure to conventional carbonate-based solvents applied in electrolytes. Yu and coworkers prepared PPCMA gel polymer electrolyte by the terpolymer of carbon dioxide, propylene oxide, and maleic anhydride. The ionic conductivity of the GPE at room temperature reaches up to 8:43 × 10 −3 S cm −1 [39]. Zhao and coworkers explored a rigid-flexible coupling cellulose-supported PPC polymer electrolyte using LiNi 0.5 Mn 1.5 O 4 salt. The polymer electrolyte exhibited wider electrochemical window up to 5.0 V, higher ion transference number of 0.68, and higher ionic conductivity of 1:14 × 10 −3 S cm −1 with commercial separator at room temperature [40]. Zhou and coworkers prepared a series of ionic liquid polymer electrolytes composed of PPC host, LiClO 4 and 1butyl-3-methylimidazolium tetrafluoroborate (BMIM +-BF 4 − ), which exhibits a high ionic conductivity of 1:5 × 10 −3 S cm −1 at room temperature [41]. Huang and coworkers developed poly(propylene carbonate)/poly(methyl methacrylate)-coated polyethylene gel polymer electrolyte, poly(vinylidene fluoride)/poly(propylene carbonate) gel polymer electrolyte, and polybutadiene rubber-interpenetrating cross-linking poly(propylene carbonate) network gel polymer electrolytes by physical blending methods and the corresponding coin cells achieve good charge-discharge capacity, cyclic stability, and rate performance [42][43][44]. In our previous work, CO 2 -based and fluorinated polysulfonamide single-ion conducting CO 2 -based polymer electrolytes for lithium-ion batteries has also been achieved, but the Li-ion conductivity was still low [45,46].
No relevant research has ever reported GPE conductivity with the same order of magnitude compared to their liquid counterparts (σ ≈ 10 −2 S cm −1 ). In this work, we describe a novel GPE derived from functionalized CO 2base polycarbonate to improve the lithium ionic conductivity and battery performance. This GPE was prepared from the terpolymer of propylene oxide (PO), allyl glycidyl ether (AGE), and CO 2 . Functional thiolated polyethylene glycol (mPEG-SH) was grafted to the side chains through the thiol-ene click reaction. Compared to normal linear polycarbonate, the long, flexible branch group of PEG units benefits segmental mobility, which can improve the transport of lithium ions. The application feasibility in high-performance LIBs of this GPE is explored by electrochemical stability and ionic conductivity test. A skeleton of PVDF serves as a supporter and separator to manufacture composite membrane. The cell performance of the composite GPE was also evaluated.

Synthesis of PPCAGE.
A 500 mL high-pressure autoclave equipped with a programmable temperature controller and a stainless-steel mechanical stirrer was used to carry out the terpolymerization of PO, AGE, and CO 2 . Typically, 1.0 g ZnGA catalyst was placed into the autoclave. Then, the autoclave with the catalyst inside was sealed and further dried for 6 h under vacuum at 120°C. After cooling down to room temperature, PO and AGE with 8 : 2 molar ratio were inhaled into the vacuumed autoclave immediately. Then, the autoclave was pressurized to 5.0 MPa with CO 2 and maintained at 60°C. After 40 hours, the autoclave was cooled down and CO 2 pressure release following. The primeval polymer was dissolved in moderate dichloromethane and 0.5 g of BHT was appended into the solution to prevent the selfcrosslinking phenomenon. The viscous solution polymer solution acid was poured into high-speed stirred cold ethanol detergent with 5% hydrochloric until all albus polymer was precipitated. The purification was repeated twice to completely remove catalyst and monomers. The ultimate polymer precipitates (PPCAGE) were dried under vacuum at 50°C for more than 24 h to pull out the solvent.

Thiolation of mPEG Using Mercaptoacetic Acid.
A quantity of 12.0 g (6 mmol) mPEG and 6.36 g (60 mmol) MPA were dissolved in 100 mL toluene. The mixture was stirred and heated to 50°C. Then, 0.109 g (0.6 mmol) of PTSA was introduced into the solution. The thiolation reaction waged after the solution further heated to 130°C, stirred, and refluxed overnight in a dean-stark apparatus. A small quantity was analyzed by 1 H NMR to verify the complete reaction. The crude mixture was refined by precipitating in 2 L of ether. The mPEG-SH intermediate yield was 94% (11.86 g).

2.4.
In-Situ Preparation of GPE@PVDF Membrane. Electrospun PVDF membrane was obtained according to Huang's work [34]. A typical 10% polymer content gel electrolyte precursor was prepared as follow: 0.202 g (0.33 mmol alkene) PPCAGE, 0.693 g (0.33 mmol) mPEG-SH, and 8.45 mg (0.033 mmol) of DMPA were dissolved in 8.05 g of 1 M LiP-F 6 /EC/DMC liquid electrolyte followed by stirring for 4 h till 2 Advances in Polymer Technology all polymer dissolved. 20 μL precursor was added dropwise onto the PVDF membrane. After 5 min soak, the composite membrane was exposed to UV light (365 nm) for 10 min, then the membrane was ready to test and assemble cell.

Materials
Characterization. 1 H NMR spectra of the polymers were recorded by Bruker DRX-500 NMR spectrometer, using chloroform-d (CDCl 3 ) as solvent and tetramethylsilane as internal standard. Polymer molecular weight (Mw and Mn) of resultant product was measured by gel permeation chromatography (GPC) system (Waters 515 HPLC Pump, Waters 2414 detector) with a set of columns (Waters Styragel 500, 10,000, and 100,000 Å) and chloroform (HPLC grade) as eluent. Glass transition temperature (T g ) of the samples was determined from the second heating run by a differential scanning calorimeter (DSC, Netzsch Model 204).
2.6. Ionic Conductivity Measurement. Ionic conductivities of the PPCAGE-g-mPEG GPEs were demonstrated by electrochemical impedance spectroscopy (EIS). The GPEs were insitu prepared in an electrolytic tank in an argon atmosphere inside a glovebox. The equipment was placed into a selfdesigned vessel alongside a temperature controller. Electrochemical impedance spectroscopy in the 1 Hz to 1 MHz frequency range with an AC excitation voltage of 10 mV A was recorded by a Solartron 1255B frequency response analyzer in a temperature range of 25 to 80°C after maintaining the samples at each test temperature for over 1 h till thermal equilibration state. The bulk resistance can be calculated according to the equivalent fit circuit model to the data. The ionic conductivity (σ) was calculated by the equation as follow: where L is the thickness of the sample, R b is the bulk resistance, and A is the effective overlap area of the sample.

Electrochemical Stability Window Measurement.
The electrochemical stability window was determined by linear sweep voltammetry (LSV). GPE@PVDF membrane was stacked up with lithium foil as a reference electrode between stainless steel working electrodes. The tests were performed between 0 and 6.0 V (vs. Li/Li + ) at the scan rate of 1 mV s -1 .

Assemble LFP/GPE@PVDF/Li Coin Cell.
With a weight ratio of 80 : 10 : 10, lithium iron phosphate (LFP), carbon black (CB), and PVDF were mixed up and grinded in NMP. The above mixture was then cast onto aluminum foil and dried at 80°C to prepare the cathode plate. The loading of LFP is around 1.7 mg cm -2 . The battery was assembled by sandwiched LFP plate, GPE@PVDF membranes (PVDF/liquid electrolyte membrane for comparison), and Li wafer seriatim. Then, the battery was sealed into a 2032 type coin cell by a tablet press. Charge and discharge tests for all batteries were performed at room temperature in a calorstat. The upper cut-off voltage was set at 4.0 V and the lower cut-off voltage at 2.5 V.

Results and Discussion
3.1. Terpolymerization of PO, AGE, and CO 2 . Terpolymerization of PO, AGE, and CO 2 (Scheme 1 and Table S1) was successfully realized using ZnGA as an effective catalyst under 5.0 MPa CO 2 atmosphere at 60°C in a high-pressure autoclave. The 1 H NMR spectrum of purified terpolymers (PPCAGE) is shown in Figure 1. The chemical shift at 5.1 −5.3 ppm and 5.8 ppm correspond to allyl groups -CH=CH 2 from allyl glycidyl ether monomer, showing that the AGE units were incorporated into the PPC mainchain successfully. The resonance peaks at 4.2 and 5.0 ppm are assigned to CH 2 and CH in the carbonate unit, respectively. The tiny peaks at 3.4−3.7 ppm corresponding to ether linkages indicate that the terpolymer is an alternating copolymer of epoxides and CO 2 , consisting of PO-CO 2 carbonate units and AGE-CO 2 carbonate units. The mole percentage of the AGE-CO 2 carbonate unit is 12.2% according to the peak area of the peak at 5.8 ppm and 5.0 ppm. The resulting PO versus AGE ratio within the terpolymers is higher than the feed ratio, which illustrates that the polymerization reactivity of PO is higher than that of AGE.
A unimodal GPC trace of polymer product shown in Figure S1 indicates the molecular weight of 68400 and provides the direct evidence that the resulting polymer is a terpolymer rather than a mixture of PPC homopolymer and PAGEC homopolymer. Usually, the GPC trace of the mixture will more likely to be multiplet.
DSC measurement shows the amorphous character of the terpolymers, without any melting point ( Figure S2). The glass transition temperature of the terpolymers is lower than 11.0°C. This is because that AGE units possess a flexible long pendant group, which favors segmental motion. The PPCAGE polymer is further functionalized to prepare new brand gel polymer electrolytes (GPEs). TGA curve of terpolymer PPCAGE also shows a decomposition temperature around 300°C ( Figure S3).

Thiolation of Polyethylene Glycol Momomethyl Ether.
Synthesis of thiol-functionalized mPEG-SH (Scheme 2) were achieved by esterification coupling between the hydroxyl end of mPEG and the carboxylic acid groups of MPA using PTSA as catalyst and toluene as solvent.
After precipitating the products in ether to remove MPA unit and PTSA catalyst, 1 H NMR spectra (Figure 2(a)) was used for product identification. The proton peaks at 1.6-1.7 ppm correspond to thiol proton. Peaks at 2.7-2.8 ppm correspond to the proton covalent in methylene next to thiol group. Peaks at 3.7 ppm correspond to PEG backbone. In particular, the peaks at 3.7 ppm and 4.3 ppm prove a successful couple between mPEG and MPA. FTIR spectroscopy further confirmed that thiol groups are present in the product (S-H stretch at 2550 cm -1 ), as shown in Figure 2(b).

Grafting mPEG on PPCAGE via Thiol−Ene Click
Reaction. Synthesis of PPCAGE-g-mPEG grafted polymer was successfully realized by a rapid thiol-ene click reaction with high reaction selectivity and conversion, quick reaction 3 Advances in Polymer Technology rate, as well as moderate reaction conditions. Thiolated mPEG-SH can be easily grafted onto PPCAGE side chain via thiol-ene click reaction employing DMPA as photoinitiator in common 1 M LiPF 6 /EC/DMC liquid electrolyte (Scheme 3).
The 1 H NMR spectrum of the resulting mPEG-grafted pendant polymer (PPCAGE-g-mPEG) is shown in Figure 3. The methylene proton signals of the pendant mPEG are distinctly visible at 2.6 ppm and 2.8 ppm, which evidence the objective functionalization reaction. Disappearance of peaks at 5.1 to 5.3 ppm and 5.8 ppm corresponding to the proton in allyl groups -CH=CH 2 confirms the complete conversion of the alkene groups. A photo was taken to show the polymer solution transform before and after UV-irradiation (inserted in Figure 3), showing the mixture changed from cloudy solution into uniform and transparent gel. Another proof is given by IR and TGA spectrum in Figures S4 and S5. 3.4. Ionic Conductivity of GPEs. Figure 4 and Table S2 demonstrate the correspondence of ionic conductivity (σ) with temperature and polymer content in GPE. The ion conductivity of LiPF 6 liquid electrolyte is 2:3 × 10 −2 S cm −1 at 25°C. Apparently from the experiment result, the ionic conductivity of GPE decreases with PPCAGE-g-mPEG polymer content increasing. The value decreases to 2:0 × 10 −2 , 1:7 × 10 −2 , 1:5 × 10 −2 , and 6:6 × 10 −3 S cm −1 when polymer content is 5 wt.%, 10 wt.%, 20 wt.% and 30 wt.%, in turns. According to the ion conduction mechanism in a similar system [43,48], lithium-ion can shuttle back and forth through the gel layer via both liquid state route and polymer state route. It is reasonable since the ion conductivity depends on the concentration of Li + and mobility of Li + . The more the PPCAGE-g-mPEG solid content, the less the concentration and mobility of Li + , thus the lower the ionic conductivity is. To better understand the ion conduction behavior in a polymer electrolyte, the activation energy Ea can be estimated by fitting the curves in Figure 4 to Arrhenius equation:

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Where σ is the ionic conductivity, σ 0 is a pre-exponential factor, R is the Boltzmann constant, and T is the temperature. E a could be calculated from the slopes of the linear relationship. The E a value of the GPE with 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.% polymer contents are in turns incremental 1.02, 1.21, 1.27, and 1.90 kJ mol −1 . On the base of free volume model, the activation energy depends on the mobility of ion carriers [49]. This is reasonable since the increase in PPCAGE-g-mPEG polymer matrix content can affect liquid electrolyte ratio, which involuntarily results in mobility descension of the carriers.
3.5. Fabrication of GPE@PVDF Membrane. Because of sticky nature and weak strength of PPCAGE-g-mPEG gel electrolyte, it is hard to scissors into a desire shape. An electrospun PVDF membrane was utilized to be the scaffold.
In the line of the preparation method given in Section 2.4, the grafted polymer gel was incorporated into surface

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and pores of PVDF separator to fabricate GPE@PVDF composite membrane. The morphologies of electrospun PVDF membrane and the composite gel electrolyte membrane are clearly observed from SEM images shown in Figure 5.
The primary PVDF separator has a visibly fibrous and porous structure. While PVDF skeleton is crop-full by PPCAGE-g-mPEG gel, the surface of PVDF separator is also soaked with gel mass. A notable feature is the transformation of PVDF surface morphology. With increasing polymer gel  Advances in Polymer Technology concentration, the surface pore can be fully covered by a PPCAGE-g-mPEG gel layer. The white porous PVDF membrane becomes transparent as shown in Figure 5(d). Sulphur (root in original thiol group) distribution ( Figure 5(c)) indicates a uniform distribution of PPCAGE-g-mPEG in composite membrane.

Electrochemical Stability of the GPE@PVDF Membrane.
For commercial lithium ion battery, the potential can hit as high as 4.0 V vs. Li/Li + . Therefore, the gel polymer electrolyte (GPE) needs be electrochemically stable at lease over 4.0 V. Figure 6 shows the linear sweep voltammetry curves of the cells employing GPE (10% polymer content) @ PVDF membrane as a separator. No rapid rise in current is detected till 4.5 V vs. Li/Li + for the GPE@PVDF membrane, indicating that the gel polymer electrolyte may be compatible with commercial lithium-ion battery. The conductivity of the GPE@PVDF membrane is   Figure S6. Values of initial and steady-state currents (I 0 , I S ), and interfacial resistances (R 0 , R S ) of symmetric Li/Li + cells with the GPE@PVDF membrane were evaluated by polarization curves and impedance spectra as shown in Table S3 and Figure S7. It is believed that GPE@PVDF possesses better T Li+ (lithium transference numbers) compared to the pure PVDF membrane.

Battery
Performances. The applicability of the GPE@PVDF membrane was further estimated by cell assembly with metallic lithium foil anode and LFP cathode.  C, 2 C, and 5 C, in their turns. The capacity at 5 C current rate retains 66% of original capacity at 0.2 C. As the current rate reverses back to 0.2 C, the initial capacity retains at 161.2 mAh g -1 , very close to initial capacity. Figure 7(b) shows the charge-discharge profiles of rate performance GPE@PVDF membrane cell. Flat voltage plateaus at around 3.5 V for battery charging and 3.4 V for battery discharging correspond to Fe 3+ /-Fe 2+ reversible redox couple reaction on the cathode [33]. At the initial charge or discharge stage, the terminal voltage of the battery rises or drops rapidly. The higher charge-discharge rate is, the faster battery voltage rises or drops. When the battery voltage enters a slowly changing platform area, the lower the charge or discharge rate is, the longer the platform area lasts and the slower the voltage rises or drops. These features are quite similar to liquid electrolyte batteries.
Figure 7(c) shows the cycling performances of liquid electrolyte @ PVDF cell and GPE (10% polymer content) @ PVDF membrane cell at 1 C at room temperature. Both of the two cells displayed good cycling performance with trifling capacity loss over 200 cycles at 1 C rate. The specific capacity of LFP cathode in GPE@PVDF membrane cell remained as high as 136.4 mAh g -1 after 200 cycles with a decay rate of only 0.022% per cycle. On the contrast, LFP cathode in liquid electrolyte @ PVDF cells is 130.4 mAh g -1 after 200 cycles with a decay rate of 0.055% per cycle. This result indicates that GPE @ PVDF membrane cell is more stable. One important reason for the decrease of battery stability is the lithium dendrite formed in the cathode. Polymer gel inhibits the generation of lithium dendrite and protects the cathode thus the stability of the battery is improved. On the other hand, a stable and uniform solid electrolyte interface (SEI) layer between polymer gel and the cathode can be formed in the charge and discharge processes. This passivation layer possesses the electronic-insulating characteristic similar to the  Figure 7: (a) Rate performance and (b) typical charge−discharge curves at various rates of GPE (10% polymer content) @ PVDF membrane cells; (c) cycling performance of liquid electrolyte @ PVDF cells and GPE (10% polymer content) @ PVDF membrane cells at 1 C; (d) typical charge−discharge curves of GPE (10% polymer content) @ PVDF membrane cells at 1 C. 9 Advances in Polymer Technology solid electrolyte as well as excellent lithium-ion conductivity. Lithium-ion can embed and strip freely though the passivation layer without making the cathode corrosion, so as to improve the stability of the battery. Figure 7(d) shows the cycling performance of GPE@PVDF membrane cells. According to the low voltage polarization and stable cycle capacity, as both electrolyte and separator, the GPE@PVDF membrane provides good electrochemical performances for the cell operation. The competitive results for the electrolyte systems demonstrate that the PPCAGE-g-mPEG gel electrolyte has potential applications in lithium-ion batteries applying.

Conclusions
Terpolymer (PPCAGE) with alkene pendent can be readily synthesized by terpolymerization of PO, AGE, and CO 2 employing ZnGA as catalyst. This alkene pendent can be subsequently functionalized via rapid and efficient thiol-ene click reaction with mPEG-SH in liquid electrolyte, to afford a new kind of gel polymer electrolyte. The novel gel polymer electrolyte manifests the highest ionic conductivity as high as 2:0 × 10 −2 S cm −1 at 25°C. Meanwhile, LFP batteries with GPE@PVDF membrane as both separator and electrolyte show good rate performances and cycling performances. In conclusion, this work provides an alternative and efficient way to prepare a brand-new gel polymer electrolyte for lithium battery application utilizing environmentally friendly PEG grafted CO 2 -based polycarbonate. Central Universities (171gjc37), the University Research Project of Guangzhou 1201630241, and the Science and Technology Research Project of Guangzhou (Grant No. 201707010289). Figure S1: GPC curve of terpolymer PPCAGE with feed ratio of 80 : 20 (PO : AGE). Figure S2: DSC curve of terpolymer PPCAGE with feed ratio of 80 : 20 (PO : AGE). Figure S3: TGA curve of terpolymer PPCAGE with feed ratio of 80 : 20 (PO : AGE). Figure S4: IR spectrum of PPCAGE-g-mPEG showing a successful couple between PPCAGE and mPEG-SH. Figure S5: TGA curve of grafted polymer PPCAGE-g-mPEG. Figure S6: EIS plots for GPE @ PVDF membrane at room temperatures. The conductivity of the membrane is calculated to 1:4 × 10 −3 S cm −1 . Figure S7: polarization curves and impedance spectra of (a) liquid electrolyte @ PVDF and (b) GPE@PVDF membrane with 10% polymer gel content. Table S1: results of the terpolymerization of CO2, PO, and AGE catalyzed by ZnGAa. Table S2: conductivity of PPCAGE-g-PEG gel electrolyte at different temperature.