Understanding the Effect of UV-Induced Cross-Linking on the

Article Cite This: Langmuir 2019, 35, 8210−8219

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Understanding the Effect of UV-Induced Cross-Linking on the Physicochemical Properties of Highly Performing PEO/LiTFSI-Based Polymer Electrolytes Marisa Falco,*,† Cataldo Simari,‡ Chiara Ferrara,§ Jijeesh Ravi Nair,∥ Giuseppina Meligrana,† Federico Bella,† Isabella Nicotera,‡ Piercarlo Mustarelli,⊥ Martin Winter,∥ and Claudio Gerbaldi*,†

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†

GAME Lab, Department of Applied Science and Technology (DISAT), Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy ‡ Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, Via P. Bucci, 87036 Rende (CS), Italy § Department of Chemistry/INSTM, University of Pavia, Via Taramelli 16, 27100 Pavia, Italy ∥ Helmholtz-Institute Münster (HI MS), IEK-12, Forschungszentrum Jülich GmbH, and MEET Battery Research Center, University of Münster, Corrensstraße 46, 48149 Münster, Germany ⊥ Department of Materials Science, University of Milano − Bicocca, Via Cozzi 55, 20125 Milano, Italy S Supporting Information *

ABSTRACT: We report a thorough, multitechnique investigation of the structure and transport properties of a UV-crosslinked polymer electrolyte based on poly(ethylene oxide), tetra(ethylene glycol)dimethyl ether (G4), and lithium bis(trifluoromethane)sulfonimide. The properties of the crosslinked polymer electrolyte are compared to those of a noncross-linked sample of same composition. The effect of UVinduced cross-linking on the physico/chemical characteristics is evaluated by X-ray diffraction, differential scanning calorimetry, shear rheology, 1H and 7Li magic angle spinning nuclear magnetic resonance (NMR) spectroscopy, 19F and 7Li pulsed field gradient stimulated echo NMR analyses, electrochemical impedance spectroscopy, and Fourier transform Raman spectroscopy. Comprehensive analysis confirms that UV-induced cross-linking is an effective technique to suppress the crystallinity of the polymer matrix and reduce ion aggregation, yielding improved Li+ transport number (>0.5) and ionic conductivity (>0.1 mS cm−1) at ambient temperature, by tailoring the structural/morphological characteristics of the polymer matrix. Finally, the polymer electrolyte allows reversible operation with stable profile for hundreds of cycles upon galvanostatic test at ambient temperature of LiFePO4-based lithium-metal cells, which deliver full capacity at 0.05 or 0.1C current rate and keep high rate capabilities up to 1C. This enforces the role of UV-induced cross-linking in achieving excellent electrochemical characteristics, exploiting a practical, easy up-scalable process.

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requirements, while dangerous electrolyte leak, high flammability, and related hazards5 are inherently ruled out or at least significantly reduced, even in the case of short circuit or abuse. These qualities make them an ideal candidate for operation in electric vehicles (EVs).6,7 Among different classes of solid electrolytes, polymer matrixes offer several advantages over glassy and crystalline ceramics, including the ability to buffer the active materials volume variations during operation and high flexibility. The latter facilitates the arrangement in different geometries and the achievement of an optimal interfacial contact at the

INTRODUCTION Li-ion batteries (LiBs) have currently taken over the market of high-tech personal devices and are aggressively advancing toward the evolution of sustainable transport and grid storage.1,2 Li metal batteries are considered as the next technology leap toward safe and high-energy-density batteries.3,4 Different materials and cell technologies are being investigated to meet the manifold requests of energy storage systems in a scenario where energy sources other than fossil/ combustible fuels are becoming relevant. Besides the need of improving batteries in terms of costs, energy density, charge time, and cycle life, a major and challenging target is safety, which is hardly accomplished without undermining the electrochemical performance. All-solid-state batteries are promising systems which are being conceived to fulfill these © 2019 American Chemical Society

Received: January 5, 2019 Revised: May 17, 2019 Published: May 24, 2019 8210

DOI: 10.1021/acs.langmuir.9b00041 Langmuir 2019, 35, 8210−8219

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Langmuir

Table 1. Composition of the Electrolyte Samples in Percentage by Weight (%) and Molality (m), along with Data Extracted from DSC Analyses of the Polymer Membranesa BP

χc

G4

PEO

sample name

%

%

m

%

m

LiTFSI %

% tot. wt.

% PEO wt.

crystallinity reduction %

LG4 LG4_XL LG4Li LG4Li_XL SP SP_XL SPE_NC SPE_XL

83.8 83.8 63.3 63.3 45.6 45.6 38.8 38.8

0.0 0.0 0.0 0.0 45.6 45.6 38.7 38.7

1.06 1.06 1.06 1.06 0.53 0.53 0.53 0.53

16.2 16.2 12.2 12.2 8.8 8.8 7.5 7.5

0.00 0.00 1.35 1.35 0.00 0.00 0.67 0.67

0.0 0.0 24.5 24.5 0.0 0.0 15.0 15.0

34 23 29 15

75 51 75 39

32 48

a LG4: liquid solution of G4 and BP. LG4Li: liquid solution of G4, BP, and LiTFSI. SP: solid polymer film containing G4, BP, and PEO. SPE_NC: non-cross-linked solid polymer electrolyte membrane containing G4, BP, LiTFSI, and PEO. _XL suffix indicates cross-linked samples. Molality (moles of solute over kilograms of solvent, mol kg−1) is given with respect to the sum of PEO and G4 masses when they are both present. χc = content of crystalline PEO with respect to the total mass of the sample (% tot. wt.) and with respect to PEO mass in the sample (% PEO wt.). Details about computation of the crystallinity reduction upon UV curing are given in the Supporting Information.

electrode/electrolyte interface.8 Poly(ethylene oxide) (PEO)based materials have been the most studied polymeric systems as lithium battery electrolytes since the discovery of their ability to complex and transport alkali metal ions.9,10 Roomtemperature ionic conductivity of 2−3 orders of magnitude lower than the threshold (∼1 mS cm−1) represents the major drawback of PEO-based solid polymer electrolytes (SPEs).11 In previous works,12,13 we demonstrated that type II freeradical photoinitiators such as benzophenone (BP) can effectively induce the cross-linking of PEO-based electrolytes under UV-light irradiation. This strategy, together with the plasticizing effect of lithium salt and low-vapor-pressure liquid, allows the easy formation of highly amorphous systems, where PEO crystallization is hindered. The polymer electrolytes yielded by this single-step process have promising lithium-ion transport properties, such as ambient temperature ionic conductivity above 0.1 mS cm−1 and lithium-ion transport number (t+) approaching 0.5. Moreover, they are homogeneous, flexible, and shape-retaining even at temperatures well above the melting point of PEO, which minimizes the risk of short circuits and allows their direct use as safe separating electrolytes without the need of any spacers. From an application point of view, the cross-linked SPEs have already demonstrated a wide electrochemical stability window, improved interfacial stability with lithium-metal electrodes, and subambient temperature cycling in lithium-metal cell configuration with LiFePO4 (LFP)-based cathodes.13 In the present work, we report a thorough and multitechnique investigation of the structure and transport properties of an UV-cross-linked three-dimensional (3D) network based on PEO and tetra(ethylene glycol)dimethyl ether (G4) doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI), compared to a non-cross-linked polymer electrolyte sample with the same composition. The effect of UV curing on crystallinity is investigated by X-ray diffraction (XRD) patterns and differential scanning calorimetry (DSC) traces. BP reactivity and lithium-ion environment are unraveled by 1H and 7Li magic angle spinning MAS-NMR. Further information about the network structure is elucidated by oscillatory rheological tests.14−16 Ion transport properties are investigated by comparing the ionic conductivity values obtained from electrochemical impedance spectroscopy (EIS) and pulsed field gradient stimulated echo (PFGSE)-NMR analysis. Fourier transform FT-Raman spectroscopy analysis is carried out at

different temperatures to investigate the polymer matrix and ion aggregation. Finally, the cross-linked polymer electrolyte membrane is submitted to galvanostatic cycles at ambient temperature in LFP-based lithium-metal cells, which deliver full capacity at 0.05 or 0.1C current rate and keep high rate capabilities up to 1C. Cross-linking homogenizes the polymer matrix and enhances the performance of the polymer electrolyte as it allows the transport properties of a gel-like system while concurrently retaining the robustness of a solid network. Thus, it is fundamental to understand the characteristics of cross-linked membrane and the role and influence of cross-linking in detail, which has been never deeply investigated, so far.

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EXPERIMENTAL SECTION

Sample Preparation. To prepare the SPE, BP (Sigma-Aldrich, vacuum-dried for 24 h) and LiTFSI (battery grade, Solvionic, vacuum-dried at 120 °C for 24 h) were dissolved in G4 (SigmaAldrich, distilled under reduced pressure over NaH) in an Ar-filled glovebox (O2 < 5 ppm, H2O < 1 ppm). Subsequently, PEO (Mn 200 000, Sigma-Aldrich, vacuum-dried at 50 °C for 24 h) was added stepwise into the solution at 70 °C under constant stirring. The mixture was hand-ground in an agate mortar and reheated at 70 °C on a hot plate for 1 h. After several mixing repeated operations, the sample was placed between two poly(ethylene terephthalate) (PET, Mylar) sheets and sealed inside a poly(propylene) (PP) bag. The resulting paste was hot-pressed at 50 °C for 15 min to yield a solid, self-standing, nontacky, elastic, dry membrane (thickness ≈ 150 μm). Eventually, the non-cross-linked sample (SPE NC) was peeled off from the PET foils in a dry room (10 m2, average RH ≈ 2% at 20 °C). For comparison purpose, a lithium-salt-free solid polymer membrane containing PEO, G4, and BP (labeled SP) was prepared following the same procedure. The weight ratio between G4, PEO, and BP in SP is the same as in SPE_NC. Furthermore, the MASNMR technique was used to analyze the liquid samples LG4Li and LG4, which do not contain PEO. LG4Li and LG4 are subsamples collected from the solutions used, respectively, to prepare samples SPE_NC and SP, prior to the addition of PEO. LG4Li consists of G4, BP, and LiTFSI, and LG4 consists of G4 and BP. The weight ratio among the components in LG4Li and LG4 is the same as in SPE_NC and SP, respectively. One subsample from each of the solutions LG4Li and LG4 was transferred into a quartz tube and, subsequently, UV-cured. The resulting samples were labeled LG4Li_XL and LG4_XL. All of the cross-linked samples (labeled with “_XL” extension) were irradiated by UV light (UV curing) for 6 min at 40 mW cm−2 using a medium-pressure Hg lamp (Helios Quartz). UV curing was performed soon after the hot-pressing step. In general, 8211


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Langmuir PET sheets can act as a filter in the UV range (Figure S1 in the Supporting Information); thus, we have carried out some crosslinking experiments using transparent PP sheets as well. Nevertheless, we did not find any notable difference between the samples hotpressed and cross-linked in either PET or PP sheets (Figure S2 in the Supporting Information). Therefore, the data presented here are referring to the UV-cured polymer electrolytes prepared using PET sheets. All of the samples were stored under vacuum or in the glovebox. Their composition is given in Table 1. Characterization. The influence of cross-linking on the structural and physicochemical properties of the samples was assessed by XRD, DSC, FT-Raman spectroscopy, oscillatory rheology, and 1H and 7Li MAS-NMR. Ion transport and association were investigated using EIS PFGSE-NMR analyses. Lab-scale cells with Li metal anodes and LFP cathodes were assembled in standard Li/electrolyte/LFP sandwiched configuration using either SPE_NC or SPE_XL sample as separator. The electrochemical performance was assessed by potential limited galvanostatic (constant current) cycling. The information about the experimental conditions and the methods used for data extraction as well as details about cell assembly and material preparation are given in the Supporting Information.

melting temperature, which is also confirmed by rheological characterization (see below). Figure 1d shows the heating scans of different samples between −60 and 90 °C. The thermograms of the salt-free samples SP_XL and the non-cross-linked SP sample display two endothermic peaks corresponding to the melting of crystalline G4 (peak at subambient temperature in Figure 1d) and PEO (peak at higher temperature in Figure 1d). The temperature corresponding to the melting peak minimum (Tpeak) of PEO in SP is lower (50 °C, Figure 1d) than that of pure PEO (65−70 °C for PEO of Mn = 200 000),17 possibly due to the decrease in size of PEO crystallites.18 Considering the salt-free samples SP and SP_XL, upon UV irradiation, Tpeak is lowered, the degree of crystallinity (χc) of PEO is reduced by 32% (Table 1), and the specific heat of melting of G4 is decreased from 87.8 to 49.3 J g−1 (where only the mass of G4 is considered). In the case of PEO, it is arising from hindered crystallization upon cooling. For G4, this behavior arises from the formation of oligomers13 and/or covalent bond formation between G4 and PEO chains upon cross-linking by UV light, which effectively prevents the crystallization of part of the G4 molecules. Both the heating traces of the polymer electrolytes SPE_NC and SPE_XL containing 15 wt % of LiTFSI do not display any melting peak for G4 (Figure 1d) around −30 °C. This observation is consistent with the literature data.19,20 The thermograms of the polymer electrolytes show endothermic peaks with Tpeak at 42 and 22 °C for SPE_NC and SPE_XL, respectively. The attribution of the melting peaks to G4LiTFSI complexes can be ruled out based on the literature.20,21 Therefore, the crystalline phase must be arising from PEO and/or the presence of mixtures of pure PEO and PEO6LiTFSI phase.22 The XRD patterns (Figure S3 in the Supporting Information) of SPE_NC and SPE_XL are consistent with those of a diluted mixture at EO/Li = 50,23 where the PEO6-LiTFSI phase is absent or negligible. Assuming that the crystal phase in the electrolytes consists of pure PEO (i.e., by neglecting PEO6−LiTFSI, if present), the comparison of the reduction of crystallinity for SPE_XL and SP_XL (see Table 1) shows that the amorphization process upon cross-linking appears to be more efficient in the presence of LiTFSI. It is worth noting here that the initial degree of crystallinity with respect to PEO mass fraction is the same for both SP and SPE_NC (see χc % PEO wt. in Table 1). Comparing samples SP and SPE_NC, it is also noted that although the presence of LiTFSI does not decrease PEO degree of crystallinity, it allows lowering Tpeak from 50 °C (SP) to 42 °C (SPE_NC) in the non-cross-linked samples. A further decrease of Tpeak is observed for SPE_XL (Tpeak = 22 °C), which likely depends on changes in the size of PEO crystallites or their organization.24 In any case, the UV-curing process could not completely hinder crystallization. The DSC profile and χc of SPE_XL after 6 months of storage in an environmentally controlled dry room is similar to that of a fresh SPE_XL sample (Figure 1d). These results confirm that UV-curing process is highly effective for hindering the growth of crystalline phases over time at ambient temperature, as also previously demonstrated.25 It also demonstrates that the G4 is not leaking out from the cross-linked polymer system, pointing at a stable and compact truly quasi-solid polymer electrolyte film. Within the range of −60 to 90 °C, it was not possible to determine the glass-transition temperature (Tg). The DSC

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RESULTS AND DISCUSSION Influence of Cross-Linking on the Structural, Mechanical, and Physicochemical Properties. The macroscopic features of pristine SPE_NC (a) and cross-linked SPE_XL (b, c) polymer electrolyte samples are shown in Figure 1a−c. Sample SPE_NC in Figure 1a has a translucent

Figure 1. Digital photographs of SPE_NC (a) and SPE_XL (b) polymer electrolytes under investigation. The latter displays excellent robustness and elasticity even well above the melting temperature of PEO (c). DSC heating traces (d) of SPE_NC and SPE_XL, both fresh and after 6 months of storage, (the DSC traces of the salt-free samples SP and SP_XL are shown for comparison). Frequency sweep tests for SPE_NC (e) at 20 (circles) and 50 °C (triangles) and for SPE_XL (f) at 20 (circles) and 120 °C (triangles).

appearance at ambient temperature, which arises from the high degree of crystallinity of the sample, as revealed by XRD and DSC analyses. The translucence disappears above the melting temperature of crystalline PEO. In the molten state, SPE_NC flows and completely loses its mechanical integrity (shape, thickness, and size) without memory of the previous shape. In contrast, SPE_XL is transparent even at ambient temperature (Figure 1b) due to the prevalent amorphous characteristics. Moreover, SPE_XL is self-standing, dry, nontacky, and exhibits improved physical properties in terms of elasticity and excellent thickness/shape retention (Figure 1c) above PEO 8212


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Figure 2. 1H MAS-NMR spectra for all of the samples listed in Table 1 (a). Best fits and tentative attributions for LG4 (b). Chemical shift (c.s.) range where BP (c) and G4 (d) signals are observed.

analysis was repeated in the range of −120 to 80 °C for samples SPE_XL and SPE_NC (Figure S4 in the Supporting Information). Tg was found to increase from −89 (SPE_NC) to −81 °C (SPE_XL) upon cross-linking, evidencing a slight inhibition of local segmental motion in SPE_XL. The mechanical properties of the two polymer electrolytes SPE_NC and SPE_XL were quantified by shear-strain rheological tests, as described in the Supporting Information. Figure 1e,f shows the effect of frequency ω on G′ (storage modulus) and G″ (viscous modulus) at two temperatures (20 °C and higher T), for the SPE_NC (e) and SPE_XL (f), respectively. For a “strong gel”, G′ > G″ and both moduli (especially G′) are nearly independent of ω over a large frequency range. Differently, a “weak gel” shows a nonlinear response (Winter’s theory).14−16,26 At 20 °C, for sample SPE_NC (circles in Figure 1e), G′ is close to G″, and the crossover, which identifies the gel−sol transition,15 is at high ω. Upon heating to 50 °C (triangles in Figure 1e), the typical sol behavior (G″ > G′) is evidenced in the entire frequency range explored. This outcome clearly indicates that SPE_NC film is a poorly structured gel, which loses its shape even at moderate temperatures. The gel−sol transition is also clearly seen upon increasing the temperature from 20 to 40 °C in the FT-Raman spectra in the range 900− 760 cm−1 (see Figure S5 in the Supporting Information).27,28 A different behavior is observed in the case of SPE_XL (Figure 1f), where at 20 °C and even at 120 °C, G′ is about 1 order of magnitude higher than G″ in the entire frequency range and both moduli are basically constant vs ω. This is the typical trend of an elastic structure, proving that the UV-induced cross-linking leads to a polymer electrolyte film that is able to maintain the mechanical properties of a strong gel even at high temperature. The temperature dependence of the moduli for both samples is reported in Figure S6 in the Supporting Information. For SPE_XL, G′ is much higher than G″ in the whole investigated temperature range (20−130 °C). We

observe a weak and constant decrease of both modules only after 70 °C, proving a slight loss of elasticity as well as of viscosity. However, no crossover occurs up to 130 °C, meaning that the strong gel character of SPE_XL is fully preserved. This is definitely important under thermal stress. Conversely, in the case of SPE_NC, the crossover between moduli occurs below 40 °C, denoting the loss of the gel structure. To summarize, in this first section, we have evidenced that UV-induced cross-linking hinders PEO crystallization even upon long-term storage so that the amount of crystalline phase is halved and the melting point is decreased (smaller and/or poorly organized crystalline domains) in SPE_XL compared to the non-cross-linked sample SPE_NC. UV-induced crosslinking provides the polymer electrolyte film with excellent flexibility and mechanical robustness up to 130 °C. The stability of the system over time and upon thermal stress are crucial for application in real battery operational conditions to avoid leaks and short circuits, especially in systems containing plasticizers. Thus, we tried to gain more insight into the role of G4 in the cross-linking process by MAS-NMR analysis, and the effect of LiTFSI salt is also discussed. 1 H spectra for all samples and mixtures listed in Table 1 are shown in Figure 2a, and best fits with tentative attributions for LG4 sample (solution of BP in G4) are shown in Figure 2b. The observed chemical shifts (c.s.) for G4 and BP signals in LG4 are in good agreement with those reported for the liquid NMR of the two pure samples.29 The three resonances in the region 7.9−7.3 ppm are attributed to the three magnetically different H species on the aromatic ring of BP. The four distinct resonances observed in the 3.7−3.2 ppm region are due to G4 molecules. Similar spectra were also observed for the nonirradiated samples LG4Li (i.e., the liquid solution of BP and LiTFSI in G4) and SPE_NC. Considering the 3.7−3.2 ppm range, PEO signals overlap those of G4, as the monomeric unit is the same, resulting in the broadening of signals in the 3.7−3.2 ppm region due to the broader 8213


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We have also evidenced the active participation of G4 in the free-radical reaction and the enhanced BP reactivity and PEO amorphization upon UV curing in the presence of LiTFSI. We will now focus on how cross-linking influences the ions, their environment, and ultimately the ion transport properties. The longitudinal (or spin-lattice) relaxation times (T1) from NMR experiment for 7Li and 19F nuclei in the polymer electrolytes are plotted vs temperature in Figure 3a,b. The

distribution of environments in the polymer matrix. In polymer system including salt, the structured peak at 3.4 ppm, which is attributed to the protons of the PEO/G4 chains, undergoes a small (∼40 Hz) shift downfield due to the inductive effect of Li+-ion complexation. Differently, new spectral features appear in the 7.4−7.0 ppm region for all of the samples exposed to UV radiation (Figure 2c). These signals can be attributed to BP derivatives formed after the radical reactions activated by UV light. As previously reported,12,13 under UV excitation, BP can extract a proton from a methylene group of G4 and/or PEO units so that the >CO group is reduced to an alcohol. The free radical on G4 and/or PEO chain leads to the formation of a cross-linked network. BP radical species can undergo different processes, such as pinacol coupling,30 further H abstraction yielding biphenyl methanol, or coupling with a polymeric chain. The new features observed in the 1H spectra of the UV-cured samples are compatible with the signal expected for these new species, as the signals due to aromatic protons in biphenyl methanol derivatives are expected in the 7.49−7.11 ppm range.31 The results of the quantitative analysis of the 1H MAS-NMR spectra are shown in Table 2, together with the estimated Table 2. Relative Intensities Determined by 1H-MAS-NMR Analyses

Figure 3. Arrhenius plot of 7Li (a) and 19F (b) spin-lattice relaxation times between 20 and 80 °C. Behavior of the center of mass of 7Li MAS-NMR spectra with temperature (c) for SPE_NC and SPE_XL. The lines are linear best fits. FT-Raman spectra of SPE_NC and SPE_XL (d) at 20 °C.

1

H-MAS-NMR intensity (%)

sample

BP

G4

LG4 LG4_XL LG4Li LG4Li_XL SPE_NC SPE_XL

9.3 9.3 9.6 9.4 7.2 7.1

90.7 90.7 90.4 90.6

a

G4 + PEO

reacted BP 8.9 14.3

92.8 92.9

spin-lattice relaxation time T1 reflects more localized motion compared to those affecting diffusion from PFGSE-NMR experiment (see below); T1 quantifies the energy-transfer rate from the nuclear spin system to the neighboring molecules (the lattice). The stronger the interaction, the quicker the relaxation (shorter T1). For both 7Li and 19F carriers, the relaxation times in SPE_NC are higher compared to SPE_XL, indicating stronger electrostatic interactions and therefore, higher coordination degree in the cross-linked system, which reflects the higher local stiffness of this sample. Figure S7a,b (Supporting Information) shows the 7Li MASNMR spectra at different temperatures of SPE_NC and SPE_XL. SPE_NC spectrum is more structured and larger than SPE_XL. This calls for more homogeneous Li + environments in SPE_XL with respect to SPE_NC and/or faster Li+ relaxation in agreement with T1 measurements (Figure 3a), leading to better motional averaging in the timescale of the NMR experiment (∼100 ms).33 By increasing the temperature, both the spectra undergo further narrowing, with a downfield shift (less shielded region) of ∼20 Hz from 25 to 55 °C. The center of mass of 7Li for SPE_NC and SPE_XL is plotted vs temperature in Figure 3c. In the case of SPE_NC, there is a clear slope change in correspondence to the melting of crystalline PEO phase(s), whereas with SPE_XL, a clear linear behavior is observed. This confirms that Li+ environment changes as a consequence of the gel−sol transition in SPE_NC, whereas such change is not observed for SPE_XL at the investigated temperature range, in agreement with T1 measurements (Figure 3a). FT-Raman spectra of SPE_NC and SPE_XL at 20 °C in the range 725−755 cm−1 are shown in Figure 3d. As an attempt to

25.6

a

For SPE_NC and SPE_XL, it was not possible to determine the PEO and G4 amount separately as the relative signals are overlapped. Therefore, only the total polymer fraction was evaluated.

fraction of reacted BP in upon UV-curing (see Table 2). Considering all of the liquid solutions of BP 1.06 m in G4 (rows 1−4 in Table 1), only 9% of BP is reduced in the absence of LiTFSI (LG4_XL), whereas the reacted fraction is 14% when LiTFSI is present (LG4Li_XL). The relative amount of reacted BP is even higher for SPE_XL, where EO/ BP ratio is doubled compared to the solutions (see Table 1). Therefore, a more diluted condition seems to be beneficial for H abstraction, possibly because a statistically larger number of EO units surround the BP units. Overall, MAS-NMR results on the liquid solutions demonstrated that G4 does not only act as a plasticizer in the cross-linked samples, but it is actively involved in the radical reaction with BP induced by UV light. This is important to avoid leaks and preserve the solid-like character of the electrolyte membrane SPE_XL. Moreover, the relative amount of reacted BP upon irradiation of the liquid samples with UV light was found to be higher in the presence of LiTFSI than in the salt-free solution. This result suggests that the improved reduction of crystallinity in SPE_XL compared to SP_XL previously evidenced by DSC (see Table 1) is due to more efficient cross-linking in the presence of LiTFSI. Ion Environment, Association, and Transport. So far, we have characterized the effect of cross-linking in terms of reduction of crystallinity and improved mechanical properties. 8214


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polymer phase.19 Differently, in this work, we observed a temperature-induced change in the echo-signal decay, the origin of which is not clear at present. Above 40 °C, singleexponential echo-signal decays were detected in all cases. In the case of SPE_NC, this outcome might be ascribed to the semicrystalline structure of PEO below 40 °C. Indeed, in the proximity of crystalline domains, the ion diffusivity is likely lower than in the amorphous regions. Anyway, this effect was also observed with SPE_XL; therefore, in these ternary systems, other factors such as the confinement of the liquid phase and mixed solvation may affect diffusion mechanisms, as reported for ternary systems including the equimolar solution of LiTFSI and G4 (i.e., a solvate ionic liquid) with PEO.42 Anyway, from the analysis of D(7Li), we can assert that UVinduced cross-linking does not strongly affect the cation mobility, since the diffusion values are only slightly lower than those found in SPE_NC. This phenomenon can be ascribed to the strong coordination of Li+ ions with −EO− units of the polymer matrix, where a slight reduction of short-range mobility is induced by cross-linking, which is also reflected by a change in Tg values. Conversely, for 19F nucleus, a marked reduction of diffusivity is observed in SPE_XL with respect to SPE NC. This result is also reflected by the activation energies (Ea) extracted from the Arrhenius plots in Figure 4a,b, in the temperature range where the echo-signal decays are single exponentials: Ea values for D(7Li) are quite similar in both the electrolytes (30.2 and 30.7 kJ mol−1 for SPE_XL and SPE, respectively). Differently, in the case of 19F, the Ea value is lower for SPE_NC than for SPE_XL (22.9 vs 27.1 kJ mol−1, respectively), meaning that TFSI− mobility is hindered in the cross-linked matrix, resulting in higher Li+-ion transference number (see Table 3). It is widely recognized that weakly coordinated TFSI− ions in PEO-LiTFSI electrolytes do not interact strongly with the ethylene oxide moieties of the polymer matrix. This weak interaction induces faster diffusion characteristics for TFSI− ions compared to Li+ ions facilitated by a hopping mechanism possibly governed by the redistribution of free volume.43,44 We did not characterize the free volume or the possible influence of the mesh size on diffusivity in SPE_XL; nevertheless, we can speculate that UV-induced cross-linking might have prompted a reduction in free volume compared to SPE_NC, resulting in a decreased TFSI− ion self-diffusion coefficient detected by PFGSE-NMR. Nearly equivalent D(7Li) and D(19F) in SPE_XL yield lithium transport number (tLi) ≈ 0.5 in the range 40−80 °C. In the same temperature range, tLi is in the interval 0.3−0.4 for SPE NC. No straightforward trend as a function of temperature could be observed in either of the samples. The tLi estimation was not performed at 20 and 30 °C due to the biexponential decay of the spin-echo signals. The ionic conductivity from EIS measurements (σEIS) and the conductivity values calculated from the self-diffusion coefficients (σNMR) are shown in Figure 4c,d. Prior to both analyses, the samples were heated at 80 °C and subsequently equilibrated at 20 °C overnight. This thermal treatment was adopted to ensure a good interfacial contact between the electrodes and the polymer electrolytes before EIS analysis. The characteristics of the samples are not supposed to differ significantly from those of the fresh electrolytes submitted to DSC analysis. Indeed, the traces of fresh samples were collected after overnight storage at ambient temperature following the preparation, which included hot pressing at 50

gain information on ion pairing near room temperature, the curves were tentatively fitted using three Gaussian functions corresponding to the contributions of weakly coordinated TFSI− anions, contact ion pairs, and aggregates (see Figure S8a,b in the Supporting Information). The relative ratio obtained after the fitting seems to indicate that the crosslinking process increases only marginally the percentage of weakly coordinated TFSI− ions, which give a signal at 740 cm−1 associated with the breathing of the whole anion and related conformation when dissolved in a solvent.34−36 More insight into ion association and transport is given by PFGSE-NMR and EIS experiments, as shown in Figure 4 and

Figure 4. Arrhenius plots of 7Li (a) and 19F (b) self-diffusion coefficients from 20 to 80 °C. Straight lines result from the fitting operation. Comparison of the ionic conductivities extracted from the PFGSE-NMR self-diffusion coefficients and the EIS analyses for SPE_NC (c) and SPE_XL (d).

Table 3. Ionicity Index and tLi for SPE_XL and SPE_NC ionicity index

tLi (PFGSE-NMR)

T (°C)

SPE_XL

SPE_NC

SPE_XL

SPE_NC

40 50 60 70 80

0.89 0.67 0.75 0.72 0.79

0.25 0.60 0.58 0.56 0.55

0.491 0.479 0.484 0.477 0.498

0.383 0.282 0.296 0.310 0.338

Table 3. PFGSE-NMR technique is a powerful method to investigate the molecular dynamics, since it allows direct measurement of self-diffusion coefficients (D) of diffusing species.37−41 In this work, we have investigated the transport properties of both Li+ and TFSI− (detecting the 7Li and 19F spin signals, respectively) in SPE_NC and SPE_XL, as shown in the Arrhenius plots of Figure 4a,b. Near ambient temperature (i.e., 20 and 30 °C), 7Li and 19F echo-signal decays were found to be biexponential (see the echo-decay plots in Figure S9 in the Supporting Information) in both SPE_NC and SPE_XL, suggesting the presence of two distinct molecular diffusion mechanisms for the ions. This phenomenon was observed in similar ternary electrolytes based on PEO and lithium salts having different anions, with biexponential decays over a wide range of temperature, which was ascribed to the distribution of lithium salt among the plasticizer and the 8215


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Langmuir °C. For SPE_NC, the melting process of the crystalline phase results in the increase of σEIS by about 1 order of magnitude between 40 and 50 °C, with a marked change in the slope of data trend. This behavior is typical of PEO-based electrolytes, with a high-temperature regime corresponding to the high mobility of the fully amorphous matrix, and a low-temperature regime where the segmental motion of EO chains is hindered because of the presence of crystalline domains.45 Differently, σEIS values for SPE_XL do not display two clearly distinct temperature regimes. This is consistent with the higher amorphous fraction and the fusion Tpeak near 20 °C from DSC analysis. As a result, the conductivity near ambient temperature is markedly improved for the cross-linked electrolyte (at 20 °C, σEIS values for SPE_XL and SPE NC are 0.2 and 0.03 mS cm−1, respectively). Above 50 °C, the latter displays slightly higher σEIS values compared to SPE_XL, reflecting the slightly reduced short-range mobility induced by cross-linking. The activation energy extracted from the Vogel− Tamman−Fulcher (VTF) plots (Figure S10a,b in the Supporting Information) in the high-temperature regime is found to be 6.9 kJ mol−1 for SPE_NC. In the case of SPE_XL, all of the conductivity data could be fitted by a single line in the whole temperature range investigated, resulting in a slightly higher activation energy of 10.5 kJ mol−1. From the point of view of application in Li-ion and Li-metal cells, this drawback arising from cross-linking is compensated by the increase of the ionic conductivity near room temperature and the ability to retain the structure and shape hindering the risk of leaks and short circuit at high temperatures. The comparison between σEIS and σNMR allows giving further insight into ion association behavior based on the ionicity index, which is computed as the ratio of σEIS/σNMR. For clarity, it should be emphasized that σEIS only refers to the mobility of charged species, whereas σNMR is affected by the movement of all species containing 7Li and 19F, including neutral ion pairs. Therefore, σNMR is usually higher than σEIS. To avoid artifacts, the values of σNMR were not computed at 20 and 30 °C, where 7 Li and 19F echo-signal decays were found to be biexponential. Indeed, the estimation of σNMR from self-diffusion coefficients requires the knowledge of concentration, which may not be homogeneous all over the sample volume below 40 °C due to phase segregation27 and the presence of crystalline PEO, especially in sample SPE_NC. At 40 °C and above, the discrepancy between the measured and calculated conductivity values is much higher for SPE_NC (see the ionicity indexes in Table 3). This result is related to the lower occurrence of neutral ion pairs in SPE_XL with respect to SPE_NC. Overall, the information collected on ionic environment suggests that in spite of cross-linking, the amorphous (i.e., above 40 °C for SPE_NC) PEO/G4 system is highly homogeneous and the techniques used in this work did not provide obviously distinct responses depending on whether Li+ ions are complexed either to EO from the solid PEO phase or liquid G4 phase. In both SPE_NC and SPE_XL, TFSI− ion environment is not affected by the gel−sol transition, which is in agreement with most of the literature reporting poor interactions between EO and the anions.32 Cross-linking results in lower spin-lattice relaxation times for both the ions and the matrix, possibly because of the reduced local segmental mobility of EO chains. Due to the decrease of the anion diffusivity, cross-linking is an effective way to improve Li+-ion transport number.

Electrochemical Behavior in Li/Electrolyte/LFP Polymer Cells. To demonstrate the promising prospects of the UV-cross-linked membrane as advanced polymer electrolyte in real cell configuration conceived for low-temperature application, it was assembled in lab-scale all-solid-state Li-polymer cell and tested at ambient temperature (different current rates upon prolonged cycling) and compared to the corresponding hot-pressed sample. The cells were assembled by combining a Li-metal anode with a LFP-based composite electrode with the solid polymer electrolyte separator in a simple sandwiched configuration (details in Experimental Section). The electrochemical behaviors of laboratory cells upon constant current (galvanostatic) charge/discharge cycling tests with the crosslinked SPE_XL and the hot-pressed SPE_NC electrolytes are shown in Figure 5.

Figure 5. Galvanostatic charge/discharge cycling behavior of [Li metal/polymer electrolyte/LFP] cells working with SPE_XL (a, c) or SPE_NC (b, d) polymer electrolytes: voltage vs specific capacity profiles at different C rates and evolution of the specific capacity and the Coulombic efficiency at 1C rate with the cycle number; the corresponding voltage profiles vs the specific discharge capacity are shown as insets. All of the measurements were performed at ambient laboratory temperature (≈21 °C), setting the same C rate for both the charge and discharge steps.

In the former case (Figure 5a), the very high specific capacity delivered by the cell (full capacity at C/20 and >87% capacity retention up to C/5 rate) and the very flat voltage vs specific capacity profiles point out at excellent performance at ambient laboratory temperature (≈21 °C). At 1C rate, the discharge specific capacity is about 75 mAh g−1 and still 40 mAh g−1 after 200 cycles. This latter value is then well retained over prolonged cycling (400 cycles, Figure 5c). In addition, the SPE_XL-based LFP/Li metal cell demonstrates excellent Coulombic efficiency, exceeding 99.5% during the whole long-cycling test, stability, and reversibility upon very prolonged operation for an all-solid polymer system. The constant current charge/discharge voltage vs specific capacity profiles shown in Figure 5A point at the highly interesting properties of the cross-linked electrolyte, showing flat potential plateaus during charge and discharge cycles up to C/5 rate, which are typical of the biphasic Li+ extraction/ insertion mechanism from/into the crystal LiFePO4/FePO4 structure, with a steep potential rise/drop at its end. It is worth noting that there is no evidence of any steep capacity fading. This is remarkable for an all-ethylene-oxide-based polymer electrolyte working at ambient temperature in a lithium-metal 8216


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hot-pressed system. All of these characteristics contribute to the excellent cell performance with LFP up to C/5 rate and good long-term stability even at high C rate, which is remarkable for a polymer electrolyte at ambient temperature.

cell, particularly considering the simple sandwich assembly with standard cathode formulation (no polymer electrolyte used as the binder for the active material particles). Despite there are several literature reports on stable cell cycling with all-solid polymer electrolytes,46−48 it is worth stressing that the cells built with SPE_XL can reversibly operate at 1C rate and ambient temperature for several hundreds of cycles. The cell overvoltage is highly stable up to C/5 rate; the polarization is rather limited, which accounts for an efficient redox reaction kinetics, due to the limited internal resistance at the electrode/ electrolyte interface as well as the limited cell overpotential contributions. It increases at 1C rate for the obvious diffusion limitations at high current regimes compared to standard cell operation with common liquid electrolytes. A slight excess of capacity with respect to the theoretical one was observed during the first charge. Wetting issues related to the electrolyte/cathode interface along with possible electrolyte degradation reactions may be responsible for this.49 Anyway, in the following cycles at C/20 rate, this phenomenon was not observed; therefore, we assume that the electrode/electrolyte interface was stabilized after the first charge. As already demonstrated in a previous work, the in situ UV-curing process, where the electrolyte is directly deposited over the electrode, can circumvent this problem, as well as the issue related to relatively low capacity output at high C rate.13 However, such an approach is out of the scope of the present work. Wetting and diffusional issues are much more pronounced in SPE NC. As this latter is not able to retain its shape above the melting temperature, the cell was stabilized at ambient temperature for 10 days prior cycling (Figure S11 in the Supporting Information) to avoid short circuits. Therefore, wetting may represent a major issue. During the first cycle, it took a long time before the cell could reach the operational potential to allow LFP delithiation, resulting in unreliable capacity values (data not shown here). During the following cycles, the Coulombic efficiency is poor (Figure 5d), pointing at possible degradation processes at the electrolyte/cathode interface. At 1C rate, the huge capacity fading leads to the failure of the SPE NC-based cell in about 15 cycles. On the contrary, the cell operated with the SPE_XL displays improved discharge capacities as well as excellent Coulombic efficiency at each of the tested C rates, and capacity retention upon longterm cycling at high 1C rate. Notwithstanding, the active material loading is higher in the cell with SPE_XL (1.3 vs 0.8 mg cm−2 for the cells with SPE_XL and SPE NC, respectively), which accounts for a nearly double current density in the former case at any given C rate. Although the specific capacity achieved is lower compared to the same LFP electrode material at the same current density in common liquid electrolyte, the cross-linked polymer cell shows stable operation and excellent capacity retention, which accounts for the good interfacial contact between the electrodes and the electrolyte separator. In this regard, it is worth mentioning that the scope of this work is not the achievement of record performances, but the deep investigation of the physicochemical properties of the electrolytes. Summarizing, together with notably enhanced mechanical robustness at moderately high (up to 130 °C) temperature, thus greater safety, the cross-linked electrolyte exhibits superior transport properties near ambient temperature in terms of ionic conductivity and lithium-ion transport numbers, together with a lower degree of ion pairing, compared to the standard

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CONCLUSIONS We have demonstrated that UV-induced cross-linking is an effective technique to hinder the crystallite formation in PEObased polymer electrolytes, thus allowing improved ionic conductivity/transport at ambient temperature. The comprehensive analyses performed by DSC, NMR, EIS, and rheology confirmed that the cross-linked polymer electrolyte is homogeneous, highly conducting at room temperature, and mechanically much more robust than the corresponding noncross-linked sample. The cross-linked polymer electrolyte demonstrated Li+-ion transport number close to 0.5, and the lowered anion mobility was confirmed by PFGSE-NMR investigations. The Li+-ion conductivity is influenced largely by the physical nature of the polymer matrix and the presence of crystalline moieties. FT-Raman spectroscopy analysis confirmed that the cross-linked polymer membrane with 15 wt % of LiTFSI content was highly amorphous at ambient temperature and induced more noncoordinated TFSI− anions. Galvanostatic cycling of the cross-linked polymer electrolyte in lithium-metal battery configuration at ambient laboratory temperature (≈21 °C) against a LiFePO4-based composite cathode delivered full capacity at low current rates, excellent cycling stability and Coulombic efficiency at 1C current rate for more than 400 cycles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00041. Detailed description of materials and methods; additional results for rheological experiments; UV−vis spectroscopy; DSC scans; XRD spectra; FT-Raman spectra; temperature sweep tests at 1 Hz; 1H-7Li MASNMR spectroscopy and 7Li-19F PFG NMR; VTF plots of the ionic conductivities; and Nyquist plots of Li/ SPE_XL/LFP cell (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.F.). *E-mail: [email protected] (C.G.). https://www. facebook.com/GAMELabPoliTO. ORCID

Federico Bella: 0000-0002-2282-9667 Piercarlo Mustarelli: 0000-0001-9954-5200 Claudio Gerbaldi: 0000-0002-8084-0143 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was carried out through the financial support of Toyota Motor Europe. In this respect, M. Falco and C. Gerbaldi thank Dr. Laurent Castro, Dr. Fanny Bardé, and Dr. Isotta Cerri. Part of this work was carried out within the activities "Ricerca Sistema Elettrico" funded through contribu8217


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tions to research and development by the Italian Ministry of Economic Development. Prof. Claudia Barolo, University of Torino, is gratefully acknowledged for lab availability, time and support in G4 distillation.

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Understanding the Effect of UV-Induced Cross-Linking on the

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