Solid Polymer Electrolytes for Lithium Metal Battery via Thermally

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Solid polymer electrolytes for lithium metal battery via thermally induced cationic ring-opening polymerization (CROP) with an insight into the reaction mechanism Jijeesh Ravi Nair, Ishamol Shaji, Niloofar Ehteshami, Andreas Thum, Diddo Diddens, Andreas Heuer, and Martin Winter Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04172 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Chemistry of Materials

Solid polymer electrolytes for lithium metal battery via thermally induced cationic ring-opening polymerization (CROP) with an insight into the reaction mechanism Jijeesh Ravi Nair*†, Ishamol Shaji†, Niloofar Ehteshami†, Andreas Thum‡, Diddo Diddens†, Andreas Heuer†,‡, Martin Winter*†,‡,§ † Helmholtz

Germany. ‡Institute

Germany.

Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster,

of Physical Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstraße 28/30, 48149 Münster,

§ University

of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstraße 46, 48149 Münster, Germany. ABSTRACT: We report the synthesis of solid polymer electrolytes (SPEs) using a thermally induced and a lithium salt catalyzed cationic ring-opening polymerization (CROP) technique. A synergistic approach using two salts such as lithium tetrafluoroborate-LiBF4, and lithium bis(trifluoromethane sulfonyl)imide -LiTFSI has assured a complete monomer to polymer conversion and fast reaction kinetics during the CROP process. The initiation mechanism of lithium salt-induced CROP is elucidated using Molecular Dynamic simulation, Quantum Chemical calculation, real-time FT-Raman spectroscopy, Nuclear Magnetic Resonance spectroscopy, X-ray Photoelectron Spectroscopy, and Thermogravimetry–Mass spectrometry analysis techniques. The crosslinked 3D network of ethylene oxide (EO) based SPE is prepared without the use of any solvents or external catalysts. In particular, a mixture of poly(ethylene glycol) diglycidyl ether (PEGDGE), LiBF4 and LiTFSI in appropriate proportions after a baking process produced a freestanding, flexible and non-tacky film. The synthesized SPEs exhibit low glass transition temperature (0.1 mScm-1), and excellent oxidation stability (>5.5 V vs. Li/Li+). The SPE is polymerized directly onto a carbon-coated LiFePO4 cathode film and successfully cycled in lithium metal battery configuration at 40 and 60°C. As evidence, the SPE is galvanostatically cycled against high-voltage LiNi1/3Mn1/3Co1/3O2 cathode, and the preliminary results indicated exciting characteristics in terms of specific capacity and Coulombic efficiency.

1. INTRODUCTION Polymer electrolytes emerge as a safe alternative to existing and widely used organic carbonate-based liquid electrolytes1 due to the possibility of enabling an all solidstate energy storage device. For lithium-metal batteries, the transformation from a liquid state to a solid-state construction is expected to advance safety, architectural ease, high-temperature stability, and economic fabrication2,3. However, several constraints such as poor ionic conductivity, low cation transport and stringent processing condition (use of costly or toxic organic processing solvents) have impeded their intrusion into the mainstream4. Thus, researchers have proposed several approaches5 including ultraviolet (UV) or thermally induced in situ preparation6,7 of polymer electrolytes. These reactions are generally carried out using processing techniques such as free radical and ionic polymerizations. Even though several research works have been carried out via UV and thermally induced free radical polymerization techniques, transforming the laboratory-scale approach into an industrially viable process is still a challenge8. The

free radical polymerization reaction requires an initiator that generates free radicals9, and the common issue is that these initiating molecules generate side products, i.e., either O2 or N2, during the initiation stages10. Other classes of initiators require high temperature for effective polymerization to take place. UV induced free radical polymerization is well-established, economic, and fast, however when the precursor is deposited over the electrode a complete conversion from monomer to polymer cannot be achieved due to the presence of unexposed (dark) areas within the active materials. Thus, it is essential also to look beyond such classic techniques and related monomers so that the polymer electrolyte can be industrially produced and employed in lithium polymer batteries. Cationic ring-opening polymerization (CROP) is an alternate technique, which can produce target-specific polymers for the development of solid polymer electrolytes (SPEs). Indeed, ring-opening polymerization reactions, where the initiating and propagating species are a positively charged intermediate is called cationic ring-

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opening polymerization. CROP technique is used to produce several industrial polymers such as polyacetals, polyoxymethylene, polytetrahydrofurans, polysiloxanes, polyethyleneimines, polylactides, and polyphosphazenes11. It is industrially well-established, and the reaction can be carried out using heat or UV irradiation,14,15 indeed, this technique has not been explored to full potential in the field of lithium polymer batteries. In general, Brønsted acids (e.g., HCl, H2SO4, HClO4, and HOSO2CF3), Lewis acids (BF3) with a co-initiator, onium ions12, photoinitiators, carbenium ions, and covalent initiators are employed as initiating species13 for a CROP11 reaction. BF3 with its precursor BF3OEt213 is one of the widely utilized Lewis acids for CROP. Commonly used lithium salts such as lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) are based on the anion of superacids14,16,17 and such molecules could initiate the cationic polymerization reaction. Dreyfuss et al.18 demonstrated in 1976 that alkali metal salts are effective as an initiator for the cationic ring-opening polymerization of heterocyclic molecules. Miwa et al.19,20 reported that the ring-opening polymerization of oxetanes could be carried out in the presence of lithium salts such as LiPF6, LiBF4, and LiN(CF3SO2)2 when a suitable solvent is employed. Later, they also demonstrated that the solvent plays a critical role in polymerization temperature, reaction kinetics, and yield. These initial works laid the foundation for alkali salt-induced ring-opening polymerization of cyclic monomers. Recently, Cui et al. explored the idea of polymerization of epoxide oligomers using lithium salt,21,22,23 and demonstrated the performance of a solid polymer electrolyte (SPE) in LiFePO4 based lithium metal cells. They reported the preparation of SPEs using catalytic amounts of LiDFOB salt to initiate the ringopening polymerization reaction, thus producing at 80°C within 4 hours a polymer electrolyte membrane, which exhibits21 an ionic conductivity of 0.089 mS cm-1 at ambient conditions. They also demonstrated that these membranes have an electrochemical stability window of 4.5 V vs. Li/Li+21. In this respect, we are proposing a facile and rapid synthesis of solid polymer electrolytes from epoxide-based oligomers using a thermally induced CROP. A fast and a reliable process has been developed, which works in tandem under the synergy of a mix of lithium salts that produce mechanically stable, self-standing and ion conducting membrane. An attempt is made to elucidate the polymerization mechanism using Molecular Dynamic (MD) simulation, Quantum Chemical (QC) calculation, real-time FT-Raman spectroscopy, Nuclear Magnetic Resonance (NMR) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and Thermogravimetry–Mass Spectrometry (TGA-MS) analysis techniques. Complementary to the experimental techniques, the MD simulations provide valuable insights into the microscopic ion coordination within the monomers from which

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polymerization is initiated. In this way, we identify likely scenarios for the underlying reaction mechanism. The complete ring-opening reaction of the cyclic epoxide monomers is carried out within 1 hour at 80 °C under nitrogen atmosphere. No solvents or external catalysts are used at any stages of polymer electrolyte preparation. The optimized SPE membrane exhibits ionic conductivity >0.11 mS cm-1 at ambient conditions. The SPE demonstrated oxidation stability >5.5 V vs. Li/Li+ and excellent cycling stability at an elevated temperature for hundreds of cycles against C-LiFePO4 based cathode. The results achieved in our labs indicating that CROP is a viable and industrially upscalable technique with enormous potential as a primary tool for SPEs production. 2. EXPERIMENTAL SECTION 2.1 Materials Unless otherwise stated separately, the materials used in the present work were purchased from Sigma Aldrich. A bifunctional oligomer poly(ethyleneglycol) diglycidyl ether (PEGDGE, Mn 500), and lithium salts such as lithium bis(trifluoromethane)sulfonimide (LiTFSI), and lithium tetrafluoroborate (LiBF4) were used during various stages of polymerization. The oligomer was vacuum dried under molecular sieves (3Å), and lithium salts were also vacuum dried at 120°C for 48 hours before using in any formulations. Carbon-coated lithium iron phosphate (CLiFePO4 LifePower®) was purchased from Phostech Lithium Inc., and the binder polyvinylidene difluoride (PVdF, Solef® 5130) was purchased from Solvay. LiNi1/3Mn1/3Co1/3O2 was purchased from Hunan Shanshan Energy Technology Co. Ltd. The carbon black, Super P® (TIMCAL) was used as a conductive additive. 2.2 Preparation of polymer electrolyte The sample preparation was carried out in an environmentally controlled dry-room (100 m2, R.H. < 0.02% at 20 °C). The general procedure involved the mixing of an oligomer, PEGDGE, with various amounts of lithium salts. The mixture was stirred using a disperser (IKA T25 digital ULTRA-TURRAX®) until a clear solution (precursor) was obtained. The electrolyte precursor solution was cast between two Mylar foils and polymerized at 80°C for 1 hour under nitrogen flux. The resulted membranes were vacuum dried to remove any impurities that were trapped during the mixing and/or casting. The formulation is given in Table 1 with the name of the components used for the membrane preparation and their weight percentage, sample name, and EO:Li ratio. Once the polymerization was complete, the membranes were directly cut into a disk and used for further characterization. The membrane thickness was tunable between 50 and 300 m, however, the membranes used in this work had a thickness of 200±10 µm. 2.3 Characterization methods and techniques The gel content (insoluble fraction) of thermally crosslinked polymer electrolytes was calculated as follows: a known weight of polymer membrane was enclosed in a thin stainless steel mesh. The polymer sample with the mesh was soaked in acetonitrile for 18 hours to extract the


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unreacted or loosely bound oligomer/polymer chains. Then the metal mesh with the insoluble fraction was vacuum dried at 60°C for 3 hours to get the residual polymer weight after the extraction process. The difference in weight was determined and the gel content was calculated using the equation (1), where Wo and We were the initial and after extraction (residue) weight of the polymer matrix. W0 was calculated with respect to the weight of PEGDGE as the lithium salt was soluble in the extracting solvent. Gel content (%) = 𝑊𝑒 𝑊0―1 ∗ 100

(1)

The crosslinking (curing) temperature of the precursor and the glass transition temperature (Tg) of the resulting polymer electrolyte were determined using differential scanning calorimetry (DSC) analysis using a DSC 2500 (TA Instruments, Inc) instrument. To minimize moisture uptake and contamination, the sample preparation was carried out in a dry room. A known weight of polymer sample (10 mg) was hermetically sealed in an aluminum pan (Tzero), and during the measurement these sample pans were heated (from 20 °C to 120 °C), cooled (120°C to 150°C), and then heated again (from 150°C to 120 °C). The heating and cooling steps were carried out at a scan rate of 10 °C min-1 under helium flux. Thermogravimetric analysis (TGA) was carried out using a Discovery TGA 5500 instrument (TA Instruments, Inc), which was additionally equipped with a Discovery Mass Spectrometer (MS). The test was carried out in the presence of helium carrier gas at a flow rate of 25 ml.min-1. The MS has the sensitivity to detect compounds and gases in parts per billion (ppb), which was ensured with a quadrupole detection system, including a closed ion source, a triple mass filter, and a dual (Faraday and Secondary Electron Multiplier) detector system. The weight of the sample used for the analysis was less than 2 mg. The conversion of a liquid precursor to a solid crosslinked polymer electrolyte was investigated using real-time FTRaman spectroscopy (Bruker, RAM II FT-Raman module Vertex 70 FT-IR spectrometer) technique by following the area of the peaks centered at 912cm-1 and 1257cm-1 (corresponding to the cyclic epoxide vibrations). The Raman instrument was equipped with an Nd:YAG laser source (λ=1064 nm). The real-time FT-Raman analysis was performed on various samples at 80 °C using a LinkamFTIR600 (Linkam Scientific Instruments) temperature control stage with a quartz window. The precursor solution that contains PEGDGE and lithium salt were sealed in an NMR tube and exposed to heating until the required temperature was reached. The heating rate was 30 °C min1 so that the required temperature of 80 °C could be achieved in 2 minutes from 20 °C. The samples were scanned between 0 cm-1 and 4000 cm-1 with 200 scans at a resolution of 12 cm-1, thus a spectrum was taken in every 30 seconds, and the polymerization reaction was followed for 4000 seconds. The intensity of the laser was kept stable to 500 mW, and whenever necessary, separate scans were made at a high resolution of 2 cm-1 between 0 cm-1 and 4000 cm-1, especially on fully polymerized samples and precursor solutions. The oligomer to polymer conversion was

calculated using the equation (2), where the 𝐴01257 is the normalized area of the peak at a time ‘t=0’, and 𝐴𝑡1257 is the normalised area of the peak at a chosen time ‘t’. Reference peak centered at 1450 cm-1 corresponding to -CH2 deformation was used to normalize area of the peak at 1257cm-1 before and during the polymerization at a time ‘t’. Percentage of conversion (%) =

(𝐴01257

― 𝐴𝑡1257)

𝐴01257

∗ 100

(2)

NMR measurements were performed on LiBF4, pure PEGDGE, PEGDGE+LiBF4 (P95 after polymerization) and the extract of P95 in CD3CN to investigate the interaction of LiBF4 and PEGDGE. The measurements were performed by employing an Avance III HD spectrometer (Bruker, USA) at 400 MHz (1H) and a broadband probe (PA BBO 400 MHz, Bruker). The 1H and 13C NMR signals were referenced to the signals of EC at 4.63 ppm (1H) and 67.1 ppm (13C), while the 19F signal was referenced concerning the signals of PF6 at 72.7 ppm (19F). 11B NMR measurements were referenced to boron trifluoride etherate (46.5 % BF3 basis, Aldrich) in CDCl3 (99.99 %, 99.96 atom % D, Aldrich) at 0 ppm. Note that SiMe4 (1H and 13C), and CCl3F (19F) were used as primary standards24. XPS measurements were conducted using an Axis UltraDLD (Kratos, U.K.). The analysis conditions were as follows: (i) X-ray source: a monochromatic Al Ka source with a 15 mA filament current, 15 kV filament voltage source energies with charge neutralizer (to compensate for the charging of samples); (ii) pass energy: 20 eV for core spectra; (iii) analysis area: approx. 700 µm x 300 µm; (iv) Fitting: as reference C-C was set to 284.5 eV; (v) the used XPS equipment was equipped with a small antechamber, allowing limiting atmospheric exposure when loading samples; (vi) for each XPS sample, at least three measurement spots per sample were selected to test the reproducibility; (vii) sputter depth profiling was carried out using an argon ion gun, where coronene was used as the ion source. Operating conditions: filament voltage of 0.5 kV and an emission current of 8 mA. The sputter crater diameter set to 1.1 mm. The angle between the surface normal and the ion gun beam was 45°. Measurement field of view 2 with a 110 μm aperture and pass energy of 40 eV. The sputter times: 1 x 60 s, 1 x 120 s, and 1 x 600 s. The XPS fitting was accomplished with the CasaXPS software (Version 2.3.16 PR 1.6, Casa Software Ltd., U.K.) and core peaks were analyzed using a nonlinear Shirley-type background. The peak positions and areas were optimized by a weighted least-square fitting method using 70 % Gaussian, 30 % Lorentzian line shapes.25 The ionic conductivity of crosslinked polymer electrolytes was calculated from electrochemical impedance spectroscopy (EIS) analysis data using an Autolab (PGSTAT204-FRA32M, Metrohm) potentiostat. Two electrodes coin cell (2032) was assembled by sandwiching an SPE membrane between two stainless steel blocking electrodes (area 1.54 cm2). EIS measurements were carried out between 0 °C and 70 °C at open circuit voltage (OCV) in the 500 kHz to 1 Hz frequency range. The coin cells were assembled in a dry room and conditioned in the climatic



chamber (BINDER MK-53) before the analysis. The impedance response was taken at every 10 °C and the thermal equilibrium was maintained at every temperature for 2 hours. The ionic conductivity (, S cm-1) was calculated using the equation (3) where ‘l’ is the thickness, ‘Rb’ is the bulk resistance, and ‘A’ is the area of the test sample. 𝜎 = 𝑙 𝐴 ―1𝑅𝑏―1

(3)

The Li-ion transference number (TLi+) was measured at 60 °C by using combined AC impedance spectroscopy and DC polarization measurements for a Li/polymer electrolyte/Li cell. The experimental details of the measurements are reported elsewhere26. The oxidation (anodic) stability of the SPE was evaluated by linear sweep voltammetry (LSV) of three electrodes test cells (ECC-Ref, EL-Cell, Germany) using a VMP3 (Bio-logic, Switzerland) potentiostat. Aluminum or Stainless Steel (SS) was used as a working electrode, and lithium metal was used as a counter and a reference electrode. The reduction (cathodic) stability of the polymer electrolyte membrane was determined using cyclic voltammetry (CV) of a 3-electrode ECC-Ref cell where Cu was used as a working electrode, and lithium metal was used as a counter and a reference electrode. In both measurements, a scan rate of 0.1 mV s-1 was employed. The LSV tests were run between OCV and 7 V vs. Li/Li+ while the CV tests were carried out between OCV and -0.5 V vs. Li/Li+. 2.4. Molecular Dynamics Simulations Molecular Dynamics (MD) simulations were performed with GROMACS-2018.1.27 The molecular interactions for the ether moieties of PEGDGE were described via the OPLS-AA force field28, while the parameters for LiBF4 and LiTFSI were taken from the CL&P force field.29,30 The partial charges of the epoxy group of PEGDGE were determined by the CHelpG method31 using Gaussian 0932, by following a previously established procedure30,29. In particular, we find values of -0.250 e for the oxygen atom of the epoxy group (with ‘e’ being the elementary charge), while the terminal carbon atom and the carbon atoms connected to the remainder of the PEGDGE molecules had charges of -0.095 e and 0.015 e, respectively. For all hydrogen atoms of the epoxy group, we used 0.110 e. The force field parameters for bonds, angles, and dihedral interactions were taken from the original OPLS-AA parameters29 or determined by adiabatic scans of the potential energy landscape. Initially, systems consisting of 500 PEGDGE molecules and 300, 150, and 60 LiBF4 or LiTFSI ion pairs were created via Packmol33, corresponding to EO:Li ratios of 20:1, 40:1, and 100:1. Subsequently, these systems were first equilibrated for 1 ns at constant volume, and temperatures of 500 and 373 K using the Nosé-Hoover thermostat34. Another equilibration step of 5 ns at a pressure of 1 bar followed using the Berendsen thermostat and barostat35. After equilibration, production runs of 45 ns were performed, in which temperature and pressure were maintained by the Nosé-Hoover thermostat34 and the Parrinello-Rahman barostat36,37, respectively. Only the last 30 ns were analyzed

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and periodic boundary conditions were applied in all three dimensions. The electrostatic interactions were computed by the Particle-Mesh Ewald method38,39 using a cut-off radius of 1.6 nm, and a grid spacing of 0.08 nm with sixthorder interpolation. All partial charges of the ions were scaled by a factor of 0.8, as reported and validated previousely40. Lennard-Jones interactions were truncated at 1.6 nm. All bonds involving hydrogen atoms were constrained by the LINCS algorithm41, and a time step of 1 fs was used in all simulations. 2.5. Quantum Chemical Calculations QC calculations have been performed with Gaussian 09 32. Deprotonation free energies have been calculated for various proton positions of a PEGDGE model compound, i.e., methoxyethyl glycidyl ether, with the highly accurate G4MP2 method42. Additional calculations for epoxide ringopening were performed at the PBE/6-31+G(d,p) level. All compounds were immersed in the SMD solvation model43 using the solvent parameters for ‘ether’ as implemented in Gaussian 09. 2.6. Electrodes characterization

fabrication

and

electrochemical

Laboratory-scale lithium metal cells were assembled using C-LiFePO4 or LiNi1/3Mn1/3Co1/3O2 as a cathode, lithium metal as an anode and the SPE as a separator. A typical cathode was composed of 90 wt.% of active material, 5 wt.% of PVdF as binder and 5 wt.% of Super P as conductive carbon additive. An NMP based slurry of active material, PVdF, and Super P in 90:5:5 ratio was prepared and then cast on to an aluminum foil, which was then dried at 60 °C overnight. Later, the electrodes were cut into a disk of 14 mm diameter and dried at 120°C under high vacuum for 12 hours. The mass loading of the C-LiFePO4 electrode was 3.2 mg cm-2 whereas the LiNi1/3Mn1/3Co1/3O2 electrode had a mass loading of 2.4 mg cm-2. Coin cells, 2032 type were prepared in the dry room using the cell components, and their electrochemical characteristics were investigated at 40 and 60°C in terms of constant current charge/discharge cycling at different current intensities using a MACCOR cycler (Series 4000). The galvanostatic charge-discharge studies were carried out in the range of 2.5 - 4 V vs. Li/Li+ for C-LiFePO4 and 3 - 4.4 V vs. Li/Li+ for LiNi1/3Mn1/3Co1/3O2. Before cycling, all cells were equilibrated to 60°C. 3. RESULTS AND DISCUSSIONS A possible mechanism of formation of an initiating complex in a lithium salt-induced CROP process leading to a crosslinked network is proposed in Scheme 1. In the present system, we have employed LiBF4 as an initiating complex generator in the presence of moisture. According to the scheme, H2O, which is present in the precursor as an impurity or the oligomer itself that bears acidic protons44 in every EO repeating units can give away a proton to generate the initiating species similar to H+BF4. This initiating species can attack the cyclic epoxide ring, and then continue the propagation and polymerization reactions as shown in Scheme 1. The optimum polymerization temperature and the salt concentration required for an effective epoxide ring-


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opening reaction were determined from DSC analysis profiles of precursor solution between 20 °C and 150 °C. To prepare a precursor solution for DSC analysis, LiBF4 salt (2, 5 or 10 wt.% with respect to total weight) was added to PEGDGE and stirred under high shear rate until a clear solution was obtained.

increase in salt concentration, which directly indicates that the total polymerization is declining with an increasing amount of LiBF4 salt. Such behavior is not common in classic polymerization systems, indeed, the rate of initiation, propagation and eventual total polymerization are proportional to initiator concentration45,46,47. This phenomenon indicates that the amount of salt plays a crucial role in determining the conditions for CROP. The SPE compositions that contain LiBF4 and LiTFSI as lithium salts and PEGDGE as oligomer are reported in Table 1 along with their Tg values and gel content data. Table 1. Summarizes the composition of SPEs in weight percentage (wt.%), corresponding EO:Li ratio, gel content, and glass transition temperature values. Sample PEGDGE LiBF4 LiTFSI EO:Li Gel/% Tg/°C

Scheme 1. Illustrates the CROP initiation mechanisms induced by LiBF4 salt complex and eventual formation of a crosslinked solid polymer electrolyte based on PEGDGE oligomer. The polymerization initiation temperature (PiT), and the peak polymerization temperature (PpT) were calculated from the reaction exotherm observed in DSC profiles (see Figure S1A). In particular, the PiT was considered as the sudden deflection point of the heat flow whereas the PpT was calculated from the exothermic peak maxima during the heating scan. In Figure S1B, PEGDGE to LiBF4 combination is reported as EO to Li ratio. The results reported in Figure S1B demonstrates that an increase in LiBF4 salt concentration decreases the PiT from 90.8 °C to 44.8 °C. Interestingly, similar behavior is observed in the case of PpT, where the temperature at which maximum reaction occurs is decreasing from 101.6 °C to 69.4 °C. However, the area under the exotherm decreases with an

P98

98

02

-

104

-

-

P95

95

05

-

40

78.7

-55.8

P90A

90

05

05

29

74.4

-57.2

P85A

85

05

10

22

66.5

-54.8

P80A

80

05

15

17

55.1

-52.0

NMR measurements were carried out on pure LiBF4 salt, PEGDGE oligomer, P95 precursor and CD3-CN (deuterated acetonitrile) extract of polymerized P95 and P80A samples. The resulting spectra are reported in Figure S2A-E. LiBF4 salt was dissolved in DMSO-d6 (deuterated dimethyl sulfoxide), and 1H, 7Li, 11B, and 19F NMR spectra were acquired. A singlet at 1.05 ppm and 1.31 ppm was observed in 7Li and 11B NMR spectrum, respectively, which corresponds to the characteristic peak of LiBF448. Besides, 19F NMR spectra exhibited two singlets separated by 0.05 ppm at -148 ppm with an intensity ratio of 1:4 (boron isotopes: 10B and 11B).48 In general, 11B NMR peaks (B-O bond) corresponding to a boron atom attached to one or more oxygen atoms are observed above 0 ppm, and no such peaks were observed in the sample49. In addition, when 19F spectra were evaluated, no extra split peaks were observed in the complete range. Indeed, BF3 and BF3OH molecules in 19F spectra exhibit multiplets below and above the doublet peaks ofBF4 anion. LiBF4 salt was also heated at 80°C for 1 hour in DMSO-d6 solvent and NMR peaks were recorded. Even in that case, no peaks corresponding to a BO bond was observed. The absence of clear peaks corresponding to a B-O bond in the 11B and 19F NMR spectra at ambient conditions, and also at polymerization conditions indicated that LiBF4 salt used in the polymer electrolyte hasBF4 ions as the only anionic component along with trace amounts of H2O. The NMR peaks were in agreement with the literature data50. Pure PEGDGE corresponding to its structure exhibited neat 13C, and 1H NMR peaks (see Figure S2B), and no extra peaks or impurities were detected. Similarly, the NMR spectra of the P95 precursor was analyzed, and the resulting peaks are reported in Figure S2C. The broadening of the peaks at 70.5 ppm, corresponding to the ethylene oxide repeating units, confirmed the occurrence of partial polymerization



during the precursor sample preparation step or during the long wait before the NMR spectra analysis. The new peak at 67.20 ppm was assigned to the methylene group carbon formed due to the ring-opening of the PEGDGE molecule. The P95 membrane sample was soaked in CD3-CN solvent and the extract soluble in CD3-CN was analyzed by 11B, 13C and 19F NMR spectroscopy (Figure S2D). The resulting 13C spectrum did not exhibit any peaks corresponding to the strained epoxide ring at 52.5 ppm, which confirmed the complete ring-opening of epoxide moieties. Similar results were also observed in the NMR spectra of P80A membrane extract when measured in DMSO-d6 (Figure S2E) solvent. These results further confirmed the complete conversion of epoxide monomer to a crosslinked polymer electrolyte. Nevertheless, for P95 and P80A membrane sample extracts, the 13C spectra showed new peaks at 202.2ppm, which was corresponding to a –CHO group. In general, during CROP reaction several side reactions occur, which could terminate the growing chain. For example, the elimination of CH3+ molecule from an initiating/propagating carbocation chain leads to the formation of aldehydes, and such reactions affect the overall crosslinking density. The formation of >C=O group is further confirmed by FT-Raman spectroscopy (see Figure S3) of polymerized samples P95 and P90A, where the polymerization reaction induced the formation peaks between 1680 cm1 and 1740 cm1. The NMR spectra corresponding to 11B did not change; indeed it was similar to the pure LiBF4 salt spectra with 19F attached to 10B and 11B in 1:4 ratio and the peaks were at the same position, which confirmed thatBF4 ions as the only boron moieties in the solid polymer electrolyte. In addition, the boron peak corresponding to a B-O bond was not detected and this result is in agreement with the pure LiBF4 salt spectra discussed above. H2O molecules present in the precursor system as an impurity or chemisorbed to LiBF4 salt may induce the generation of the initiating complex H+BF4 or H+ BF4(H2O). In these samples, no singlet peak at −123.0 ppm was observed in the 19F NMR spectrum, which was corresponding to the presence of LiF51 molecule. Thus, NMR spectroscopy analysis results indicate that the polymerization process occurs in the presence ofBF4 ions, and not byBF3OH ions. In addition, the results also indicated that the major initiating species was H+BF4 acid species and not the adductBF3OH ion formed by the interaction between BF3 and H2O. An attempt was made to elucidate the mechanism of polymerization by MD simulation and QC calculations. When employing lithium salts such as LiBF4, in which the anion is based on a Lewis acid (BF3), both the Li+ and potential degradation products ofBF4 (mainly BF3) could in principle initiate the polymerization. Alternatively,BF4 might deprotonate trace water or PEGDGE to form HBF4, which may trigger a cationic polymerization in a subsequent step via protonation of the epoxide group (see Scheme 1). In a first step, we performed classical MD simulations of the initial PEGDGE-salt solution before polymerization to unravel the coordination of Li+ ions, anions and, epoxy and ether oxygen atoms for LiBF4 and

Page 6 of 20

LiTFSI at various concentrations. The temperature was maintained at 373 K, which falls into the experimentally determined range of polymerization temperature (Figure S1A). Additionally, we compute the tendency of the epoxy hydrogen atoms to coordinate to the fluorine atoms of BF4 anion, which may eventually lead to deprotonation and HBF4 formation, resulting in the mechanism shown in Scheme 1.

Figure 1. The fraction of Li+-ions coordinated to at least one of the respective anions for different concentrations of (A) LiBF4 and (B) LiTFSI at 373 K. (C) Shows the radial distribution functions between the fluorine atom ofBF4 ion or the oxygen atom of TFSI ion and the hydrogen atoms of PEGDGE backbone. (D) A simulation snapshot (created with VMD52) of typical Li+-ion coordination. (E) & (F) Shows the deprotonation free energies (in kcal mol1) for specific proton positions of a PEGDGE model compound with and without Li+-ion coordination. Figure 1A and 1B show the fraction of Li+-ions that coordinated to at least one oxygen atom of a specific type of the PEGDGE chain as well as the fraction of lithium-ions coordinated to at least one of the respective anions for different concentrations of LiBF4 and LiTFSI (EO:Li = 20:1, 40:1 and 100:1). Here, OEP1 and OEP2 denote the terminal epoxy oxygen atoms, while OG1 to OG10 denote the consecutively numbered ether (or glyme) oxygen atoms (with OG1 and OG10 being the outmost and OG5 and OG6 the innermost oxygen atoms). We observe that in the PEGDGE monomer-lithium salt solutions, Li+-ion is mainly coordinated to the central ether oxygen atoms of PEGDGE, while the coordination to ether oxygen at the chain ends are significantly less frequent. This is in line with the wellknown coordination of lithium-ions by oligo- or polyether chains, in which the backbone wraps around the ion as


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shown in Figure 1D). Moreover, we observe from Figure 1A that LiBF4 forms minor ion clusters (especially at high concentrations), while this tendency is even less pronounced for LiTFSI. Remarkably, the epoxy oxygen atoms are only marginally coordinated by Li+-ions for both salts at all concentrations. Also, the coordination of the central ether oxygen atoms only shows a weak dependence on the salt concentration, which can mainly be attributed to the effect of the anion. These findings indicate that the choice of the anion alone cannot account for the experimentally observed differences between LiBF4 and LiTFSI, as it would be the case if the anions would compete with the epoxy oxygen for interactions with Li+-ion. Rather, the reaction ofBF4 ion with water or R-H (proton bearing organic molecule such as PEGDGE) and the resulting formation of HBF4 seems to play a key role in the polymerization process, as suggested in Scheme 1. Since the lithium ions are well solvated by PEGDGE for either salt (Figure 1A and 1B), we also expect this scenario to hold in the experimentally studied mixed-salt systems containing both LiBF4 and LiTFSI, that is P90A, P85A, and P80A (see Table 1). As mentioned above,BF4 may deprotonate trace water or other compounds with acidic protons at elevated temperatures. To quantify the latter effect (deprotonation of R-H), we computed the radial distribution functions (RDF) between the hydrogen atoms of the epoxy moiety and the fluorine atoms ofBF4 ion (Figure 1C). For comparison, the same quantity has been calculated for the hydrogen atoms of the ether monomers, as well as the interaction of these two hydrogen types with the oxygen atoms of TFSI ion. Indeed, we observe a clear coordination peak at distances shorter than 0.36 nm for the coordination of the fluorine atoms ofBF4 ion with the hydrogen atoms at both the epoxy (red curve) and the ether groups (black curve). However, for the epoxy moiety, this peak is significantly larger than the central monomers. Similar observations can be made for the coordination of the TFSI ion oxygen to two types of hydrogen atoms, although in this case, the difference between epoxy (blue curve) and ether hydrogen atoms (green curve) is less pronounced. Thus, at least from a kinetic point of view, this coordination suggests that deprotonation might in principle occur under experimental conditions. The increased overall lithium ion concentration in the experimentally studied mixed-salt systems P90A, P85A and P80A (Table 1) may additionally polarize the PEGDGE molecule due to progressive Li+-ion coordination, and thus shift the tendency towards deprotonation, resulting in an increased polymerization rate (see below). To probe for the possibility of such a deprotonation mechanism triggered byBF4 ions, we performed additional QC calculations at the G4MP2 level. We find that HBF4 was neither stable in a vacuum nor within the implicit solvation model (SMD) but rather dissociated into HF and BF3. However, from the NMR measurements, no indication for this dissociation was found (Figure S2C and S2D). Therefore, exclusively describing the solvation environment by an implicit solvent is too simplistic,

especially when protons are involved53. To improve the description of theBF4 ion solvation we included one or two water molecules (representing trace water) in the coordination shell and focused on the deprotonation reactions (4). R-H + [BF4(H2O)n] → R + [BF4(H2O)n-1(H3O)]

(4)

with n=1 or 2. For these clusters, we observed no dissociation into HF and BF3, compatible with the experimental findings. For R-H, we chose methoxyethyl glycidyl ether (MGE, see Figure 1E) as a small and computationally efficient but representative model compound for PEGDGE. Besides, we also performed analogous calculations for MGE coordinated to Li+-ion. The deprotonation free energies for various proton positions of MGE and MGE-Li+ are summarized in Figure 1E (MGE) and Figure 1F (MGE-Li+). In Figure 1(E and F), the first numbers correspond to equation (4) with n=1 and the second values to n=2. We note that for MGE, all deprotonation free energies were in the range of 220-270 kcal mol-1, rendering these reactions very unlikely. Coordination of Li+ ions substantially lowers these energies to 10-70 kcal mol-1, depending on the proton position and if the [BF4(H2O)] or the [BF4(H2O)2] cluster acts as a base. The lowest deprotonation free energy of 10 kcal mol-1 was found for protons connected to the third carbon atom of the PEGDGE backbone with [BF4(H2O)2]. This specific position was particularly beneficial for deprotonation, as the proton abstraction was stabilized by subsequent ringopening and double bond formation compatible with an aldehyde, ketone and enol formation observed experimentally (see below). Moreover, a comparison between [BF4(H2O)] and [BF4(H2O)2] suggests that larger clusters composed ofBF4 ion and trace water were more basic. As opposed to the deprotonation of MGE-Li+, the reaction showed in equation (5) displays a reaction free energy of 81 kcal/mol, thus rendering an anionic polymerization via OH ion is unfavorable. [BF4(H2O)] + H2O → [BF4(H3O)] + OH

(5)

Most importantly, the impact of Li+ on the deprotonation free energies (Figure 1F) indicates that both Li+ andBF4 ions are essential for catalysis. Analogous calculations with [TFSI(H2O)2] yielded deprotonation free energies that are even lower by about 2 kcal mol-1 as compared to [BF4(H2O)2]. Thus, the deprotonation step is not limited to the presence ofBF4 ions. Therefore, we hypothesize that the bulky TFSI ion is stabilizing the intermediate carbocation and sterically hinders the polymerization. The RDFs between epoxide oxygen and both TFSI andBF4 ions show significant peaks in the MD simulations (not shown), which likely become even more pronounced after epoxide protonation. Moreover, we observed ketone, aldehyde or enol formation in QC calculations of the epoxide ringopening for protonated MGE at the PBE/6-31+G(d,p) level, in agreement with the experimental data (see below). Since this side reaction predominantly occurs in the presence of



Page 8 of 20

TFSI ions in the experiments, this also hints towards the specific interaction of the carbocation with the anion. In addition to this scenario, a bimolecular reaction mechanism between MGE and protonated MGE also seems viable, as reflected by a negative reaction free energy of -17 kcal mol-1 and an energetic barrier of 16 kcal mol-1 at the PBE/6-31+G(d,p) level. This is also supported by the MD simulations, which show marked coordination between distinct epoxide monomers (RDF peak height ~1.6, not shown), however, no difference between TFSI andBF4 ions was found. Nonetheless, an anion-stabilized transition state could still rationalize the experimentally observed differences between TFSI andBF4 ions. This is further corroborated by QC calculations. In particular, the barrier mentioned above decreases to 5 kcal mol-1 whenBF4 ion is located near the protonated epoxide group, while for TFSI ion no such stabilized cluster could be identified. In contrast, the barrier for a nucleophilic attack ofOH ion at an epoxide (deprotonated) group is significantly larger (7 kcal mol-1), again illustrating that an anionic polymerization mechanism is unlikely. However, we expect thatOH – even though rarely formed due to a high reaction free energy of 81 kcal mol-1 (see above) – could in principle react with the carbocation and terminate the polymerization. This termination reaction byOH ion will have an impact on the crosslinking density and the resulting transport properties. Based on these insights, we propose the mechanism in Scheme 1. The reduced reactivity for electrolytes containing bothBF4 and larger amounts of TFSI ions could be related to the stronger interaction of water with TFSI ions as compared toBF4 ions (about 1 kcal mol-1 at the G4MP2 level). Figure 2. (A) Raman spectra of P95 and P90A membranes in comparison with pure PEGDGE oligomer showing the disappearance of cyclic epoxide peaks at wave number 912 cm-1 (*) and 1257 cm-1 (#). (B) Percentage of monomer to polymer conversion vs. time plot for the series of polymer electrolyte membranes (Table 1), data extracted from realtime FT-Raman spectroscopy analysis. Inset of Figure 2A: showing the non-tacky and transparent nature of a P90A membrane after the polymerization process. The polymerization reaction produced a freestanding, transparent and non-tacky membrane as shown in the inset of Figure 2A. The polymerization reaction was evaluated by FT-Raman spectroscopy analysis, which ensured that the chosen polymerization temperature and time were sufficient for a complete ring-opening of the cyclic epoxide moieties as shown in Figure 2A. Figure 2A shows the FT-Raman peaks corresponding to pure PEGDGE oligomer, P95, and P90A polymer membranes. The peaks centered at 912 cm-1 (*, epoxide ring deformation) and 1257 cm-1 (#, epoxide ring breathing) were completely disappeared for P95 and P90A polymer samples after the baking process at 80°C for 1 hour. If HF was formed during the polymerization then the unstableF ion would terminate the carbocation initiating species to form a –CH2–F bond. However, –C–F bonds were not detected in FT-Raman and/or 19F NMR spectra (see Figure


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S2C). The influence of LiBF4 and LiTFSI contents on the monomer to polymer conversion, and polymerization rate behavior was analyzed by an in situ and real-time FTRaman spectroscopy technique, which is coupled with a heating unit. The area of the peak centered at 1257 cm-1 was followed at 80°C for 4000 seconds. The results for the polymer electrolyte samples P98, P95, P90A, P85A, and P80A are reported in Figure 2B. The percentage of conversion of monomer to polymer was calculated using the equation (2).54,55 For a precursor solution in which only 2 wt.% of LiBF4 was added, the reaction started slowly with an induction period close to 700 seconds. This delay may arise from a low concentration of the initiating species, increased solubility (dissociation) of LiBF4 salt at different temperatures, the coordination between the EO chains and the Li+ cation, the cluster shape, coordination number, etc., which may differ during the baking process. Such rearrangements and the time required for the system to absorb heat uniformly may also influence the polymerization reaction. In the case of P98, the rate of polymerization was slow; the polymerization reaction took place slowly at a rate of 0.07 % s-1 (slope of the curve corresponds to respective polymerization reactions shown in Figure 2B) between 800 sec and 2000 sec. However, the polymerization was complete within 1 hour and monomer to polymer conversion of 99 % was achieved. When the quantity of LiBF4 was increased from 2 wt.% to 5 wt.%, the reaction rate was increased to a great extent along with a decrease in induction time. In the case of P95, the rate of polymerization was 0.25 % s-1, which was at least 3.5 times faster than the P98 polymerization. In the case of P95, the polymerization reaction started when the heating was started, and the process was extremely fast in the first 400 seconds. In this case, the monomer to polymer conversion (ring-opening of cyclic epoxide group) reached almost 100 % after 3600 seconds. When an extra 5 wt.% of LiTFSI salt was added to the P95 system to form P90A precursor, the rate of reaction was increased further and reached a maximum of 0.49 % s-1. This means that the reaction occurred rapidly, at least twice that of P95. Indeed, this result confirmed that the LiTFSI salt also influences the polymerization reaction rate. However, a further increase in LiTFSI amount (for P85A) witnessed a marginally reduced polymerization rate than the P90A (0.46 % s-1). Nevertheless, in all three cases of P95, P90A and P85A a conversion of above 90 % was achieved within 1000 seconds (including the resting and heat ramp periods). In the case of P80A, a drastic change in the polymerization behavior was observed and the polymerization conversion rate was decreased by 10 fold as compared to P90A or P85A. The slow polymerization rate may arise from the increased amount of LiTFSI salt, which increases the viscosity of the precursor solution. The TFSI ion, which may also compete with theBF4 ions, and hinder the polymerization rate. Interestingly for P80A, the concentration of TFSI ion was close to the concentration ofBF4 anions, which reduced the required amount of free initiator species near to the propagating cationic chain or epoxide moiety. This

reduced amount of initiating complex slow down the polymerization reaction. In addition, TFSI ion is a bulky ion, and it may hinder the accessibility of the cyclic epoxide moieties of the unreacted PEGDGE molecules to the propagating chains (or vice versa) due to the interaction between the carbocation and the oxygen atoms of the TFSI anions (refer to the RDF calculation). The conversion of the bi-functional oligomer, PEGDGE, to a crosslinked three-dimensional network was further evaluated by measuring the gel content using acetonitrile (ACN). ACN can effectively extract LiBF4 and LiTFSI salts used in the formulation, which were not part of the main polymer chain or crosslinked network. Thus, the gel content or the insoluble fraction values reported here correspond to the effective mass of only the PEGDGE oligomer content in the precursor solution. This means W0 was corrected to PEGDGE content in equation 1. The results are listed in Table 1. The gel content study revealed that the P95 polymer has an insoluble content of 78.7 %, which was gradually decreased for other samples by an increase in LiTFSI content. However, for the series of membranes investigated here, the insoluble content was always above 55 %. When the salt concentration was increased, the viscosity was also increased, which in turn reduced the mobility of the entire oligomer chain, thus the propagation reactions may be slower than the P95 precursor. It is interesting to note that even though in all SPEs, the FT-Raman spectra indicated a full ring-opening of the epoxide moieties, conversion into a crosslinked network was not 100%. This may arise from parasitic reactions such as the termination of the initiating oligomer-initiator complex by backbiting, chain transfer reactions or even elimination of the cyclic epoxide moieties11. Such side reactions will surely induce ringopening, however, may not reflect on building an effective crosslinked network. Polymerization reactions were also carried out at 100 °C by varying the reaction time between 15 and 30 minutes. In particular, the fast reaction rate was achieved at 100 °C for a polymerization time of 15 minutes and 30 minutes. P80A, when baked at 100 °C for 15 minutes, showed a gel content of 61.8 %, whereas a baking time of 30 minutes increased the gel content to 64 % for the same precursor composition. Even though, a higher temperature produced a better conversion of the oligomer to a crosslinked network, 80°C was chosen as the optimum temperature in the present work. Indeed, higher polymerization temperature and longer baking time may induce parasitic reactions such as chain transfer11 and termination of the growing chain, which leads to lower insoluble content and yellowing. Besides, it may also limit the use of monomers with a low boiling point when a copolymerization process was considered for practical applications. Thus, in the present work, the polymerization temperature was selected as 80°C while the reaction time was fixed to 1 hour. NMR spectroscopy, real-time and ex-situ FT-Raman spectroscopy, and gel content estimation results, have confirmed that the selected reaction conditions were



Page 10 of 20

sufficient for the production of solid polymer electrolytes. Scheme 2 illustrates the processes adopted for the production of SPE membranes where no solvents were used during the polymerization process.

Scheme 2. Illustrates the SPE sample preparation steps involved in a solvent-free thermally induced CROP. Glass transition temperature (Tg) was evaluated by DSC analysis between -150°C and 120°C for a series of SPEs P95 to P80A. The DSC profiles are reported in Figure S4A and the Tg values are tabulated in Table 1. All polymers showed Tg values well below -50°C, which indicates that at room temperature these SPEs are at least 70 °C above their respective Tg, assuring enough segmental motion for the polymer chains. However, the evaluation evidenced an increase in Tg values by increasing the LiTFSI salt content. This effect may result from the coordination of lithium salt with EO moieties, which might increase the viscosity or stiffness of the polymer chains, thus reduces the segmental mobility. DSC measurements are an effective tool to calculate the percentage of crystallinity of a polymer. In the present case, no melting peaks were observed during the heating cycle between -150 and 120 °C, indicating the formation of a highly crosslinked and fully amorphous SPE.

Figure 3. Shows the AMU of the products released during the TGA-MS analysis. The graph is reported as the ion current vs. test time for different molecules expelled during the TGA analysis of (A) LiBF4 salt and (B) P95 polymer membrane. Thermogravimetric analysis (TGA) was carried out to elucidate the thermal stability of PEGDGE, LiBF4 salt, P95 and P80A samples (see Figure S4B). LiBF4 was thermally stable up to 170 °C and decomposition started soon after that. The decomposition of LiBF4 was a single step process where the major weight loss happened around 200 °C, and 27 % of residue remained at 600 °C, which was corresponding to residual LiF56. However, 1.7 % of weight loss was observed around 100 °C, which refers to moisture that was absorbed during the sample preparation. The thermogram corresponding to PEGDGE oligomer indicated that the major weight loss happened only around 200 °C, and the weight loss was more gradual than the sudden drop that was observed for LiBF4 salt. However, in the case of polymer membranes P95 and P80A, the weight loss began around 135 °C, which limited the application of the polymer electrolyte membrane up to 100 °C. In particular, the major weight loss for P95 and P80A was noticed only above 250 °C, whereas, in the last stages of decomposition, LiTFSI salt decomposes above 350 °C in P80A sample. In general, LiTFSI salt was highly thermally stable and decomposed only above 350 °C. The decomposition mechanism of the polymer electrolyte


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membrane, in particular, P95 was followed by the MS analysis, which was coupled with the TGA instrument. The results concerning the TGA-MS analysis are tabulated in Table S1. The data obtained after the TGA-MS analysis was matched with the NIST Mass Spec Data center database57. LiBF4 salt during the TGA heating step between 30 °C and 600 °C released water above 70 °C, corresponding to the AMU (atomic mass unit, m/z) of 17 (OH+) and 18 (H2O+). Also, a small amount of water was detected above 170 °C, which was corresponding to the crystalline water (crystal water as a hydrate) present in LiBF4 salt (see Figure S4B and Figure 3A). The peaks observed above 70 °C may relate to moisture that was absorbed during the sample preparation as physisorbed water. At 170 °C another molecule with AMU value of 20 (HF+) was observed, which is corresponding to HF molecule58. Indeed, the second stage of water loss was coupled with HF release, which leads to the conclusion that water, that was part of crystalline structure lost in the form of HF by the reaction between H2O and BF459. No peaks were observed corresponding to the molar mass ofBF4 (AMU 87), which indicated that the major decomposition product of LiBF4 was LiF and BF3, however, the presence of physisorbed and/or chemisorbed water influenced the decomposition process by the release of a small amount of HF. BF3 (AMU 68) further decomposed to BF2 (AMU 49) and fluorine with AMU of 19 (F+). A clear distinction was observed for HF (AMU 20) and F (AMU 19, F+), where the peaks were observed at two different temperatures. Indeed, a small peak was observed at AMU 89, which may correspond to the HBF4 molecule. The residual weight of 27.58 % was in agreement with LiBF4 decomposition products (LiF and BF3), and the temperature at which the significant decomposition occurs was also in accordance with literature data56. The TGA-MS analysis was further extended to pure PEGDGE oligomer, and the corresponding MS spectra of the decomposition is reported in Figure S5. The peaks that were related to the PEGDGE were for methyl and ethyl cations at AMU 15 and 29, respectively. The AMU of 26 and 27 correspond to ethylene ion, and these peaks were observed only above 150 °C. The release of water observed at a high temperature starting from 160 °C, which was released as a decomposition product of the oligomer. Other species such as [CH3O]+ and [HO-CH2-O-CH2-CH2]+ were observed at AMUs 31 and 75. The peaks corresponding to ethylene oxide moieties were observed around AMUs 45 [HO-CH2-CH2]+ and 89 [HO-CH2-CH2-OCH2-CH2]+. In conjunction with AMU 31, a peak at AMU 43 was also observed, which was corresponding to [O=CHCH2]+ molecular ion. In addition, peaks corresponding to C3H5O (AMU 57) and C3H5O2 (AMU 73) were also observed. These peaks were observed only above 150 °C, indicating that these are the decomposition products rather than the impurity. In addition, we have also observed the peaks corresponding to CO2 (AMU 16, 28, and 44), which appeared only at a temperature above 160 °C. The obtained results (Table S1) were in agreement with the reported literature60,61.

Table 2. Summary of the ionic conductivity and activation energy of polymer samples P95 to P80A. The ionic conductivity () and activation energy (Ea) values are reported in mS cm-1 and kJ mol-1, respectively. Sample

 / 30°C

 / 60°C

EaVTF

P95

0.035

0.157

9.97

P90A

0.055

0.264

10.05

P85A

0.113

0.633

11.41

P80A

0.013

0.014

14.87

The TGA-MS analysis was further carried out on the crosslinked P95 polymer electrolyte sample, and the result is reported in Figure 3B and Table S1, which exhibited more or less similar peaks corresponding LiBF4 and PEGDGE. The major change for P95 from the oligomer and the salt was that BF3 peak was absent in the polymer sample. Indeed, for pure LiBF4 solid powder, the decomposition products were LiF and BF3, whereas the LiBF4 salt dissolved in PEGDGE or in a polymer matrix such as that of P95, the major decomposition component wasBF4. The peaks correspond to methyl (AMU 15) for PEGDGE were observed above 150 °C. On the contrary, a small amount of ethylene peak of AMU 27 appeared around 110 °C for P95 polymer, which was also the major decomposition product at elevated temperature. This decomposition at 110 °C restricted the application of P95 to 110 °C, and it was formed from the ether bond break. A small amount of ethyl ion (AMU 29) was also observed. Except for H2O, all other major components were released as decomposition products only above 150 °C. The overall TGA-MS results, corresponding decomposition products, and their assigned AMU values are reported in Table S1, Figure 3A, Figure 3B and Figure S5. The ionic conductivity of a series of crosslinked SPEs, namely, P95, P90A, P85A, and P80A was calculated from the electrochemical impedance spectroscopy (EIS) analysis results. Figure 4A shows the ionic conductivity of SPEs for a wide range of temperature ranging between 0 °C and 70 °C. Initially, it was observed that an increase in lithium salt concentration enhanced ionic conductivity. When the EO:Li ratio was changed from 40:1 to 22:1, the corresponding ionic conductivity was further improved. A further increase to 17:1 from 22:1 reduced the ionic conductivity to lower values. The highest ionic conductivity was observed for P85A polymer membrane, which exhibited an ionic conductivity exceeding 0.1 mS cm1 at 30 °C. The same SPE, P85A, also demonstrated an ionic conductivity of 1 mS cm-1 at 70 °C. This particular composition was comprised of 10 wt.% of LiTFSI, 5 wt.% of LiBF4 and 85 wt.% of PEGDGE. A further increase in LiTFSI to obtain the SPE P80A decreased the ionic conductivity values, which may be influenced by the higher viscosity of the local polymer matrix environment as compared to P85A. Every Li+-ion added in the electrolyte slows down the polymer segments and increases the viscosity. However, the more Li+-ions in the system, the more charge carriers are present, which results in a larger ionic



conductivity. Indeed, there is a competition between these two opposing trends: increase in the number of charge carriers enhances the sigma (), and at the same time reduces the segmental mobility leading to a decreased ionic conductivity. The influence of increased LiTFSI content and its effect on EO to Li+-ion complexation and related reduced chain mobility was also confirmed by an increased Tg in the DSC analysis. Thus, an optimum ionic conductivity was achieved for P85A with an EO:Li ratio of 22:1.

Page 12 of 20 𝐸a

 = 𝐴exp ( ― 𝑅(𝑇 ― 𝑇0))

(6)

The VTF fitting curves corresponding to each SPE are reported in Figure 4B. The attempt to fit the ionic conductivity curves with the Arrhenius equation was not fruitful as the R2 values were around 94 and energy values above 53 kJ mol-1 were obtained. However, the curves fit very well to VTF equation with R2 value around 99.9. The resulted activation energy after the fit with the VTF equation is reported in Table 2 along with their ionic conductivity at 30 °C and 60 °C. The activation energy was improved with an increase in LiTFSI addition, where P95 polymer exhibited activation energy of 9.97 kJ mol-1, and for P80A, it was 14.87 kJ mol-1. Such a high Ea may arise from the restricted mobility of EO chains in a highly crosslinked polymer matrix where the long-range segmental mobility was limited, and the conduction was favored through short-range local segmental mobility. Such restricted ion mobility may require higher activation energy; nevertheless, it was still lower than the typical PEO+LiX salt system above the melting point (generally > 60°C)64. It is worth noting that for PEO based gel polymer electrolytes26, the observed Ea was below 10 kJ mol-1. Moreover, a higher salt concentration may induce the formation of ion pairs and aggregates, which require higher activation energy than the free ions due to their larger size. The lithium-ion transport number evaluation was carried out by employing the A.C. impedance spectroscopy and chronoamperometry, which demonstrated that the P80A sample has a TLi+ value of 0.23. The corresponding Nyquist and chronoamperometry graphs are reported in Figure S6A and S6B.

Figure 4. (A) Ionic conductivity as a function of inverse temperature plot for a series of polymer electrolytes prepared with various amounts of PEGDGE, LiBF4, and LiTFSI. (B) VTF fitting of the polymer electrolyte samples P95 to P80A in natural logarithmic scale. The ionic conductivity behavior was fit with both Arrhenius and Vogel–Tammann–Fulcher (VTF) equation (6)62. In equation (6), ‘A’ is a prefactor corresponding to charge carrier concentration, ‘Ea’ is the activation energy corresponding to segmental motion, ‘σ’ is the overall conductivity, and ‘T’ is the given temperature. The value ‘T0’ is generally 50 °C below the Tg, where the configurational entropy of the polymer system disappears and a threshold of infinitely long relaxation time is reached63.

The oxidation stability of the SPEs was analyzed by LSV tests against aluminum working electrodes at 60 °C. The results are shown in Figure 5A. The anodic sweep was performed towards higher potential values, and the onset of current peak is generally related to the decomposition (oxidation) of the electrolyte. In this case, the limiting oxidative current was set as 1 µA.cm-2, and the highest electrochemical stability was achieved for P95 membrane, up to 7 V vs. Li/Li+. An increase in LiTFSI salt concentration decreased the oxidation stability, and among the tested samples, P80A showed the lowest oxidative stability value. However, P80A sample showed an oxidative stability value above 5.5 V vs. Li/Li+. If one considers electrochemical oxidative stability as one of the critical parameters for evaluating the polymer electrolyte membrane performance then 5.5 V is a significant value. The inset of Figure 5A shows the magnified scale of the same plot between 2.5 and 5.0 V, which exhibited the surface passivation of the Al current collector (in this case working electrode) by theBF4 ion65. Indeed, the addition of LiTFSI salt into the P95 polymer shifted this behavior towards higher potential, nevertheless, the passivation occurred in all the polymer electrolytes. Interestingly for P85A and P80A, there was an extra peak that started to rise above 4.0 V, which might be induced by the LiTFSI salt. However, in all


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cases, the passivation current observed above 3.5 V was small, and the peak current was less than 0.2 A cm-2.

during the XPS analysis.67,68 Nevertheless, the oxidation stability of 5.5 V vs. Li/Li+ was still a significant value, andBF4 ion as compared to the TFSI ion at the surface would surely induce the surface passivation69 during the oxidation process, thus hinder further degradation of the polymer electrolyte. Moreover, such passivation layer can also suppress the dissolution of the aluminum current collector70. Thus, it is worth mentioning that the type of salt used as a Li+-ion source influences the oxidation stability of the polymer electrolyte. XPS analysis of LiBF4 salt powder was performed, and the results are reported in Figure S8A and S8B. The results confirmed that there is onlyBF4 species in pure LiBF4 salt powder, where the B1s peak is centered at 195.8 eV and F1s at 687 eV. No other peaks were observed for B1s (around 190-193 eV), or F1s (around 682-685 eV), which confirmed the absence of B-O bonds and Li-F bonds, respectively.

Figure 5. Linear sweep voltammetry and XPS investigation of selected polymer electrolytes. (A) The oxidative stability analysis (anodic scan) of P95 to P80A in Al/SPE/Li cell configuration; inset Figure 7A: LSV curves magnified between 2.5 V and 5.0 V. (B) Al2p XPS spectra of an Alelectrode after the LSV test between 3.0 V and 6.0 V in Al/P95/Li cell configuration. XPS analysis was performed on the polymer samples P95 (see Figure S7 A-D) and P80A (see Figure S7 E-G) to understand surface characteristics. The results indicated that the surface of polymer membrane P95 was enriched withBF4 ions, whereas the P80A polymer sample surface, was populated with TFSI ions with less amount ofBF4 ions. These results confirmed that the presence ofBF4 ion is one of the reasons for the increased oxidation stability for P95 whereas a higher TFSI ion concentration at the surface of P80A decreases the oxidation stability. In summary, the LiTFSI salt has low oxidation stability than the LiBF4 salt based polymer electrolyte66 and the XPS analysis results confirmed such characteristics. The presence of LiF in the polymer electrolyte surface (Figure S7D-G) may arise from the decomposition ofBF4 ions

Figure 6. Shows the cyclic voltammetry and XPS investigation of selected solid polymer electrolytes, (A) the reduction stability (cathodic scan) of polymer electrolytes P95, P90A and P80A in Cu/SPE/Li cell configuration. B) XPS spectra of Cu2p peak of a Cu-electrode after cyclic voltammetry tests between voltage ranges of 1.0 V - 3.0 V, and -0.5 V – 3.0 V in Cu/P95/Li cell configurations.



To understand the role of LiBF4 salt on the oxidation stability of polymer electrolyte, P95 polymer was tested for oxidation stability using LSV up to 6.0 V vs. Li/Li+ and kept at a constant potential of 6.0 V for 3 hours. The Al2p spectra of the Al-electrode after the LSV analysis is reported in Figure 5B. The working electrode aluminum was removed after the test, and the Al surface that was in contact with the polymer electrolyte was analyzed by XPS technique. The results lead to the conclusion that the highest oxidation stability values were achieved due to the presence of LiBF4 salt, which passivates the Al (see Figure 5B) by forming AlF371 and LiF (Figure S9A). The formation of LiF and AlF3 increased the decomposition resistance of the polymer electrolyte membranes to 7.0 V, especially for the P95 polymer. In general, Al surface is covered with a thin layer of Al2O3. The Al2O3 can react gradually with theBF4 ions to form an AlF3 layer via AlOxFy intermediate. Similar observations were reported for boron fluoridebased polymer electrolytes and LiPF6 salt-based liquid electrolytes.48,72,73 The oxidative stability was also measured against stainless steel as a working electrode, and similar oxidation stability values were observed (not reported here). However, it was important to note that an increased amount of LiTFSI reduces the oxidative stability of the membrane. This result indicated that the synthesized SPEs are stable towards higher potential scans, thus these systems have the potential to be employed for high-voltage applications if appropriately fine-tuned using suitable salts.

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and XPS analyses results lead to the conclusion that the solid polymer electrolytes prepared using the thermally induced CROP are ideal to use as a separator, which can facilitate the mobility of Li+-ions between the electrodes. In summary, a series of solid polymer electrolytes were prepared, which exhibited a low glass transition temperature, an acceptable ionic conductivity, and a wide electrochemical stability window. These are encouraging characteristics for a highly crosslinked polymer electrolyte, however, validation is necessary using galvanostatic cycling in lithium metal cell configuration against various cathode active materials. Thus, the membranes were tested on C-LiFePO4 cathodes in lithium metal cells.

Cyclic voltammetry analysis was employed for the evaluation of the reduction stability of the SPEs at 60 °C. In all cases, a clear and a well-defined lithium plating/stripping reaction was observed on a Cu working electrode, as reported in Figure 6A. Only one irreversible peak was observed at 2.2 V vs. Li/Li+, which indicated the reduction of CuO (present at the Cu foil surface) to Cu2O74,75,76. Nevertheless, the peak at 2.2 V was disappeared in the second cycle. An increase in LiTFSI content improved the total charge that was deposited during the plating process, and a corresponding charge for stripping was retrieved. This increased current during the plating/stripping process was an indication of an improved ionic conductivity that was exhibited by SPEs upon addition of LiTFSI into the polymer matrix. XPS analysis (Figure 6B) was carried out on a pristine Cu-foil, a Cuelectrode cycled between 1.0 V and 3.0 V, and another Cuelectrode cycled between -0.5 and 3.0 V. In all cases, the peaks corresponding to the reduction of Cu2+ observed for pristine Cu-foil were disappeared,77,78,79 and only a small shoulder peak corresponding to Cu+ was detected around 932 eV. The peak intensity was marginally decreased for cycled Cu-foils that was in contact with the SPEs during the CV analysis. However, in all cases, the Cu(OH)2 was disappeared confirming that the peak at 2.2 V is corresponding to the reduction of Cu2+ moieties present in the form of CuO or Cu(OH)2. In all cases, LiF was found to be one of the major decomposition product, which was formed from LiBF4 salt decomposition during the cyclic voltammetry steps (see Figure S9B). The electrochemical


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Chemistry of Materials P90A and P85A, showed comparatively increased overpotential in the first cycles when galvanostatically cycled in C-LiFePO4 based lithium metal cells. This increased overpotential indicated the presence of a resistive interface against lithium metal (see Figure S10 AD). Figure 7A shows the specific capacity vs. cycle number plot at different current rates for a C-LiFePO4 based lithium metal cell. The constant current charge-discharge cycling was carried out between 2.5 V and 4.0 V vs. Li/Li+, and excellent Coulombic efficiency and specific capacity were achieved. The lithium metal cell at the first cycle delivered a discharge capacity of 160.6 mAh g-1 at the 0.05C current rate with a Coulombic efficiency of 98.3 %. In the following cycles, the Coulombic efficiency was increased and reached >99.9 %. The cell delivered acceptable high current rate performance at elevated temperatures. An increased current rate reduced the specific capacity, and the cell delivered a specific capacity of 144.6 mAh g-1 at 0.5C and 73.8 mAh g-1 at 1C. The charge-discharge process at 0.1C current rate was continued and after the 200th cycle, the cell exhibited the capacity of 135.5 mAh g-1, which corresponds to more than 86 % of capacity retention as compared to the 5th cycle where the cell delivered the capacity of 156.8 mAh g-1. This result confirmed that the synthesized electrolyte is stable, and inherits excellent characteristics for a solid-state lithium metal battery construction. Indeed, the lithium metal cell showed reproducible voltage profiles at various current rates as shown in Figure 7B. The specific capacity and the cycling profile showed in Figure 7A and 7B confirmed that the LFP-based cell was considered stable for long-term cycling. Also, the C-LiFePO4-based lithium polymer cell was also tested at 40 °C where the cell delivered a specific capacity of 151.4 mAh g-1 at a 0.05C current rate (Figure 7C). The preliminary results at comparatively low temperatures are encouraging, which confirmed that the P80A sample is stable towards lithium metal, and the limitation due to kinetics is minimal.

Figure 7. Shows the constant current charge-discharge cycling data of solid-state C-LiFePO4/P80A/Li cells, at 60 °C and 40 °C. (A) The specific capacity at various current rates (0.05C - 1C, see also the inset for a magnified view) against a number of cycles, (B) Voltage vs. specific capacity profiles between 0.05C and 1C at 60 °C. (C) Voltage vs. specific capacity profiles of a cell at a current rate of 0.05C for the first 3 cycles at 40 °C between 2.5 V and 4.2 V vs. Li/Li+. For the galvanostatic measurements, P80A sample was chosen as the optimum sample irrespective of the good ionic conductivity and wide electrochemical stability window of P85A. The polymer electrolyte membranes P95,

To understand the utility of high voltage stability of SPEs, lithium metal cells were assembled against LiNi1/3Mn1/3Co1/3O2 cathodes. The obtained results are reported in Figure 8A and 8B. The galvanostatic cycling results, especially at low current rates indicated that the cell delivered a capacity of 186.6 mAh g-1 during the charge and 169.4 mAh g-1 at the first discharge. This value account for a Coulombic efficiency of 90.8 %. In the second cycle itself, the Coulombic efficiency was increased to 96.4 %. The cell at 0.2C current rate delivered a specific capacity of 111.3 mAh g-1 with a Coulombic efficiency of 99.4 %. Indeed, it is worth mentioning that these results were achieved for solid polymer electrolytes with ethylene oxide as the primary polymer matrix. The cells were assembled in a sandwich configuration where the SPE membrane was kept in contact between the electrodes. The results were encouraging, and required further optimization in terms of electrode porosity, improved contact with the active materials and further stabilization towards lithium metal interface. Also, future works are in progress to optimize the cycling behavior of these SPEs at room temperature. Thus,



the overall results achieved for the thermally cured CROP of SPE systems are encouraging and lay the foundation towards realizing the cycling of solid polymer electrolytes at ambient temperature.

Page 16 of 20

ambient conditions and wide electrochemical stability window (>5.5 V vs. Li/Li+). The electrochemical characteristics achieved on the SPEs are encouraging, in particular, the cycling performance results against CLiFePO4 electrodes in lithium metal configuration. The CLiFePO4 based polymer cell delivered a specific capacity of 160.2 mAh g-1 at 0.05C and 102.3 mAh g-1 at 1C where the Coulombic efficiency reached values exceeding 99.8 % after the first cycle. Interestingly, a C-LiFePO4 based lithium metal cell delivered a specific capacity of 151.4 mAh g-1 when cycled at 40 °C at a current rate of 0.05C. The highvoltage stability of SPEs is confirmed by cycling the polymer electrolyte membranes against LiNi1/3Mn1/3Co1/3O2 cathodes and lithium metal anodes. The results demonstrated that the polymer cell is able to deliver a specific capacity of 186.6 mAh g-1 during the first charge and 169.4 mAh g-1 at the first discharge with a Coulombic efficiency exceeding 90 %. Thus, the SPEs produced using a thermally induced CROP process are prominent candidates for the development of solid-state lithium metal polymer batteries.

ASSOCIATED CONTENT Supporting Information. Supplementary data on DSC analysis, TGA analysis, NMR spectroscopy, XPS analysis, TGA GC-MS analysis, impedance analysis, chronoamperometry and galvanostatic cycling of Li/SPE/C-LiFePO4 cells are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Jijeesh Ravi Nair, [email protected]; * Martin Winter, [email protected]; [email protected].

Figure 8. Shows the constant current charge-discharge cycling of a solid-state LiNi1/3Mn1/3Co1/3O2/P80A/Li cell at 60 °C. (A) The specific capacity at various current rates (0.05C - 1C) against number of cycles, and (B) voltage vs. specific capacity profile between 0.05C and 1C. 4. CONCLUSIONS Thermally induced cationic ring-opening polymerization (CROP) has been effectively used for the synthesis of ethylene oxide based solid polymer electrolytes. The polymerization reaction is fast and facile, and no solvents or additives are necessary at any stages of the polymerization process. Using Molecular Dynamics simulation, Quantum Chemical calculation, real-time FTRaman spectroscopy, NMR spectroscopy, and XPS analysis, the polymerization initiation mechanism is elucidated and demonstrated. The results evidenced that either the moisture or an acidic proton bearing molecule can initiate the ring-opening polymerization in the presence of an effective anion such asBF4 and BF3 molecules are not involved in any polymerization steps. The synthesized SPEs exhibited low glass transition temperature, good ionic conductivity (0.11 mScm-1) at

Author Contributions The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

Notes The authors declare the following competing financial interest(s): Patent application at German Patent and Trade Mark Office, reference 10 2018 006 11.7, www.dpma.de. A patent application including contents of this manuscript has been submitted to the German Patent and Trade Mark Office (www.dpma.de) and is in progress.

ACKNOWLEDGMENT J.R.N would like to thank Ms. Debbie Berghus for the DSC measurements.

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Solid Polymer Electrolytes for Lithium Metal Battery via Thermally

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