The Role of Ion Hydration in the Performance of Li2SO4

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The Role of Ion Hydration in the Performance of LiSO-Polyacrylamide Electrolyte Systems: Material Characterizations Under Real-Time Conditions Alvin Virya, and Keryn Lian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10436 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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The Journal of Physical Chemistry

The Role of Ion Hydration in the Performance of Li2SO4-Polyacrylamide Electrolyte Systems: Material Characterizations Under Real-Time Conditions

Alvin Virya and Keryn Lian*

*Corresponding Author: E-mail: [email protected]

Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3E4

ACS Paragon Plus Environment

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Abstract Li2SO4-polyacryalamide (PAM) neutral pH polymer electrolytes with different salt:polymer molar ratios (5,000:1 and 10,000:1) were characterized for their ionic conductivity and material properties. Their ionic conductivity over time showed different trends: Li2SO4-PAM(5,000:1) increased while Li2SO4-PAM(10,000:1) decreased. Materials characterizations of the freestanding films were conducted to identify the cause of the difference in conductivity trends. X-ray diffraction and IR spectroscopy suggested slight differences in film crystallinity and sulfate ion bonding structure, but they were not conclusive. Raman spectroscopy proved to be a better tool as it revealed distinct characteristic peaks for different sulfate ion and water interactions. To correlate the film properties with electrolyte performance, a real-time tracking technique to correlate ionic conductivity and the vibrational spectroscopic responses was developed. Leveraging this approach, the level of hydration surrounding the salt molecule was identified as the determining factor of ionic conductivity in this polymer system. This approach can be extended to predict the shelf and service life of this Li2SO4-PAM system and to optimize the next generations of electrolytes.


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1

Introduction

Polymer electrolytes are enablers for solid-state, thin, flexible, and wearable energy storage devices. The performance of polymer electrolytes depends strongly on the material properties, including film crystallinity, hydration, and solvent-polymer-ionic conductor interactions.1 Understanding the correlation between material properties and electrochemical performance can provide key insights to improve and optimize polymer electrolyte systems. Amorphous polymers are preferred as polymer matrix since they facilitate ion movement within the polymer matrix more readily than their crystalline counterparts.2-6 Sufficient hydration of the electrolyte is a crucial factor for high ionic conductivity as reported in numerous studies on proton and hydroxide-conducting electrolytes.7-15 Although amorphous structures can be achieved through polymer selection and proper operating conditions (i.e. above its glass transition temperature), maintaining hydration of the polymer electrolyte is more difficult as it depends on water-ion-polymer interactions. While the level of hydration on a freestanding film can be studied with NMR13, 16-17, IR18-21, or Raman20 spectroscopy, these results may not reflect the progressive changes in the electrolyte films under sandwiched cell configurations. A nondestructive technique along with criteria to predict and track material properties in real-time would be highly valuable in developing high performance and long-lasting polymer electrolytes. Neutral pH polymer electrolytes22-24 are particularly promising due to their wide operating voltage window as well as their leakage-free and non-corrosive nature. For example, a Li2SO4polyacrylamide (PAM) system was recently reported with a 2.0 V window in a symmetric carbon nanotube-graphite cell.25 In that study, the ionic conductivity of Li2SO4-PAM with molar ratios of 5,000:1 and 10,000:1 were compared at their pristine condition and after 3 weeks of


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storage. The electrolyte with the higher ionic conductor concentration underwent degradation of ionic conductivity and, as a result, had a shorter shelf life. Although hydration was the prime suspect for such degradation, no evidence supporting this hypothesis was available. It is necessary to understand the origins of the degradation to shed light on the correlation between material properties and electrochemical performance. In this work, the structural and chemical bonding characteristics of Li2SO4-PAM electrolyte films were investigated and tracked in parallel with their ionic conductivity in cells, using a realtime tracking technique developed for this purpose. Our objective was to identify key signatures in the material properties that would allow predicting the performance and failure modes of Li2SO4-PAM electrolytes in their cells. Tracking the progressive change in the materials and matching those changes to the changes in ionic conductivity pinpointed the cause of performance degradation. This technique could be applied to better understand a wide range of electrolyte systems in applications such as sensor, fuel cell, batteries, etc.

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Experimental

2.1. Preparation of Polymer Electrolyte The preparation of the Li2SO4-PAM electrolyte was described in a previous report.25 The precursor solution was prepared by mixing a 3% PAM solution (Scientific Polymer, Mw: 5-6 000 000) with Li2SO4 powder (Aldrich, 99.99% purity). Mixtures with salt-to-polymer molar ratios of 5,000:1 and 10,000:1 were studied and are referred to as Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1), respectively. Ignoring water content, the Li2SO4-PAM(5,000:1) film had a composition of 90.9 wt% PAM + 9.1 wt% Li2SO4, whereas Li2SO4-PAM(10,000:1) had a composition of 83.3 wt% PAM + 16.7 wt% Li2SO4.


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2.2. Cells Construction and Electrochemical Characterization Test cells with an apparent area of 1 cm2 were constructed using titanium foil (McMaster Carr, 127 µm thick) as electrodes and current collectors. The solid cells were assembled by casting the precursor solution onto each electrode followed by sandwiching the electrolyte between the two electrodes and protecting with tape. The thickness of the test cells was measured using a Mitutoyo digital micrometer; the average cell thickness was 0.55 mm with a ca. 0.1 mm thick electrolyte. Electrochemical impedance spectroscopy (EIS) was performed using a CHI760C bipotentiostat. EIS spectra were recorded from 100 kHz to 1 Hz with 5 mV amplitude at 0 V DC bias. The ionic conductivity σ was calculated from the equivalent series resistance (ESR) obtained from EIS measurements using σ = t / (A×ESR), where A is the apparent area and t is the electrolyte thickness. Values were reported based on 8 cells for each electrolyte composition. 2.3. Material Characterizations The Li2SO4-PAM films were studied utilizing thermogravimetric analysis (TGA) for their thermal properties and water contents, x-ray diffraction (XRD) for the film crystallinity, and both Fourier transform infrared (FTIR) and Raman spectroscopy for their chemical bonding. Unless specified otherwise, characterizations were performed on freestanding films (completely exposed to the environment) conditioned for 7 days in 45 %RH desiccator. For real-time tracking, the films were investigated under emulated sandwich cell conditions, with mock cells fabricated similar to the electrochemical test cells (see supplementary Fig. S1). Two transparent micro cover glasses (VWR, 22×22 mm) were used in place of titanium so that the electrolyte films could be investigated using optical and vibrational spectroscopic techniques. 3 mock cells were


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prepared for each polymer composition. All freestanding films and mock cells were stored under the same conditions as the electrochemical test cells in a desiccator with a saturated K2CO3 solution to maintain a 45 %RH environment at ambient temperature. TGA thermograms were obtained from a TA instrument Q50 system. The thermograms were recorded in an nitrogen environment from ambient temperature to 250 °C with a heating rate of 5 °C min-1. XRD patterns were obtained using a Phillips XRD system. The electrolyte film was mounted on a zero diffraction silicon plate coated with a thin layer of vacuum grease. The analysis was carried out with a monochromatized copper K-α anode source operating at 40 kV, 30mA. The XRD pattern was acquired from 5° to 50° 2θ with a step scan of 0.02° and a scan step time of 2 seconds. FTIR spectra were recorded on a Thermo Scientific Nicolet iS5 spectrometer with iD5 attenuated total reflectance (ATR) module. Pressure was applied to ensure good contact between the sample and the ATR crystal. The FTIR spectra were recorded between 400 and 4 000 cm-1 wavenumbers with 2 cm-1 resolution. Raman spectra were recorded on a Horiba XploRA™ PLUS Raman microscope system with a 532 nm laser and diffraction grating of 1,800 gr/mm. The Raman spectra were recorded between 15 and 4 000 cm-1 wavenumbers. Images were captured for each Raman analysis site with the same equipment. Where Raman mapping was necessary, a 10×10 grid was generated by the equipment software within a selected area.


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Results and discussion

The ionic conductivity of the Li2SO4-PAM electrolytes was tracked for a period of 4 weeks. Material characterizations on the freestanding films were performed to identify a technique that can detect the differences between the electrolytes. An optical and vibrational spectroscopic study was performed on the mock cells simultaneously with ionic conductivity tracking. Tracking the material behavior from the mock cells in real-time and correlating the observations with the behavior of the actual cells captures any change in ionic conductivity caused by a change of material properties. 3.1. Ionic Conductivity Tracking of Li2SO4-PAM The ionic conductivity of Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) was tracked for 28 days and showed different trends (Fig. 1). Initially, the conductivity of Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) were 4.1±0.7 and 6.4±2.3 mS cm-1, respectively. After storage in 45 %RH for 21 days, the conductivity of Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) were 10.5±2.1 and 4.5±4.5 mS cm-1, respectively. As Li2SO4-PAM(10,000:1) contains more ionic conductors, it was expected to exhibit higher ionic conductivity, which was observed over the first two weeks of storage. This relationship, however, inverted after 14 days. The ionic conductivity of Li2SO4-PAM(5,000:1) continued to increase while the conductivity of Li2SO4PAM(10,000:1) decreased. Accompanying the decline in conductivity, the standard deviation in ionic conductivity values of Li2SO4-PAM(10,000:1) also became increasingly larger, suggesting an inhomogeneous degradation process throughout the cells.


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Figure 1. Ionic conductivity of Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) tracked over a period of 28 days (average from 8 cells each).

A change in hydration of the film was hypothesized as the cause of the difference in behavior of Li2SO4-PAM(10,000:1) and Li2SO4-PAM(5,000:1). To test this hypothesis, structural, thermal, and chemical characterizations of both Li2SO4-PAM freestanding films were performed (section 3.2). Tracking for real-time hydration was conducted (section 3.3) to correlate the changes in the polymer electrolyte films to the observed variations in ionic conductivity as shown in Fig. 1. 3.2. Material Characterizations of Li2SO4-PAM Films The water content of the Li2SO4-PAM films was estimated using TGA (Fig. 2). The thermograms of both Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) were free of any distinct thermal events up to 200 °C. The degradation temperature of PAM was previously reported at 220 °C with the completion of water evaporation at 200 °C.26 Accordingly, using the weight loss data at 200 °C, the water content of the film can be calculated as weight or atomic composition (Table 1). The water content in Li2SO4-PAM(5,000:1) is higher at 14.1 wt%,


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compared to 10.9 wt% in Li2SO4-PAM(10,000:1). Assuming a complete dissociation of Li2SO4 salt (i.e. 2 Li+ and 1 SO42-), the difference in ion hydration can be associated with the molar ratio between water and Li2SO4, 1.5 H2O per ion in Li2SO4-PAM(10,000:1) compared to 3.7 H2O per ion in Li2SO4-PAM(5,000:1). The thermograms also show higher temperature for water evaporation in Li2SO4-PAM(10,000:1) than in Li2SO4-PAM(5,000:1). This could be the result of fewer initial water that are more tightly bound in Li2SO4-PAM(10,000:1). The variation in waterto-ion ratio is significant, as it may affect film crystallinity or bonding structure of the saltpolymer system. These were subsequently investigated with XRD and vibrational spectroscopy studies, respectively.

Figure 2. TGA thermograms of Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) freestanding films for water content estimation.

Table 1. Film composition including the water content expressed in both weight and atomic percentage Component

Li2SO4-PAM(5,000:1)

Li2SO4-PAM(10,000:1)

Li2SO4

7.8 wt%

8.3 at%

14.8 wt%

18.2 at%

PAM

78.0 wt%

0.0017 at%

74.2 wt%

0.0018 at%

H2O

14.1 wt%

91.7 at%

10.9 wt%

81.8 at%


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The XRD patterns of both Li2SO4-PAM films (Fig. 3) were compared against the pure components (i.e. dry Li2SO4 powder and PAM film) under the same storage conditions. PAM as an amorphous polymer displayed a typical broad pattern whereas pure Li2SO4 exhibited several peaks corresponding to its crystalline structure. The XRD pattern for Li2SO4-PAM(5,000:1) clearly suggested an amorphous structure similar to that of pure PAM. On the other hand, Li2SO4-PAM(10,000:1) showed minor peaks that corresponded to the Li2SO4 crystal structure.

Figure 3. XRD patterns of Li2SO4-PAM (5,000:1), Li2SO4-PAM (10,000:1), PAM freestanding films, and Li2SO4 salt.

The bonding structure of both Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) were compared against the individual components using IR spectroscopy, see Fig. 4(a). The vibrational assignments for both pure PAM and Li2SO4 are summarized in Table S1. In the PAM film spectra, various vibrational modes of PAM functional groups were identified at their


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corresponding wavenumbers.27 The corresponding PAM functional groups in both Li2SO4PAM(5,000:1) and Li2SO4-PAM(10,000:1) peaks in Fig. 4(a) were identical to that of pure PAM, indicating an excellent chemical stability of PAM with Li2SO4. On the other hand, the spectra of the pure Li2SO4 salt showed a single broad peak at 1,010 cm-1 associated to sulfate stretching.28 The (SO4)2- stretching in Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) were slightly different, at 1,080 and at 1,100 cm-1, respectively. The dipole changes measured by IR spectroscopy29 during the S-O bond stretching can be overshadowed by the symmetric geometry of the sulfate ions and then result in the observed broad peak. Due to the small difference and the broad peak, IR spectroscopy could not pinpoint the actual reason of this difference. Raman spectroscopy is better suited for symmetric molecules as it measures the polarizability of the molecule29, and thus both Li2SO4-PAM films were further investigated using Raman spectroscopy.

Figure 4. Vibrational spectra of (a) IR and (b) Raman for Li2SO4-PAM(5,000:1), Li2SO4PAM(10,000:1), PAM freestanding films, and Li2SO4 salt


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The Raman spectra of both Li2SO4-PAM films are presented in Fig. 4(b) and are similarly compared against the individual components. As in the IR spectra, the vibrational peaks of the various PAM functional groups30-31 were unaffected in both Li2SO4-PAM(5,000:1) and Li2SO4PAM(10,000:1). The major peak at 1,100 cm-1 in the Raman spectra of pure Li2SO4 salt can be attributed to the first mode of sulfate stretching, ν1(SO4)2-, whereas the internal stretching modes of the ion were revealed as various minor peaks.32-33 Interestingly, the ν1(SO4)2- peak in Li2SO4PAM(5,000:1) and Li2SO4-PAM(10,000:1) were noticeably different, where the former showed only one peak at 984 cm-1 while the latter exhibited two peaks at 985 and 1,008 cm-1. The Raman peak for ν1(SO4)2- was known to be highly sensitive to the salt hydration of sulfate compounds such as CaSO434-36, FeSO437, and CuSO438-39. The peak shifted to a lower wavenumber with more water due to the higher amount of hydrogen bonding. This shift was also demonstrated in pure Li2SO4 powders that were subjected to different RH conditions (supplementary fig. S2). In short, the peak at ca. 980 cm-1 is related to the solvated ions whereas the peak at >1,000 cm-1 corresponds to dehydrated ions. Therefore, Raman spectra conclusively revealed that Li2SO4PAM(5,000:1) contains only hydrated ions whereas Li2SO4-PAM(10,000:1) contains both hydrated and dehydrated ions. 3.3 Real-time Tracking of (SO4)2- Hydration in the Electrolyte Films We hypothesized that ion dehydration in Li2SO4-PAM(10,000:1) caused the degradation in ionic conductivity. To prove the correlation between ionic conductivity and ion hydration, real-time analysis was performed. While the Raman spectra of the freestanding film can be monitored over time, the detection of the dehydrated sulfate signature will not reflect the progressive changes in the electrolyte within a sandwiched solid cell. To establish the actual timeline of film dehydration in the cell configuration, mock cells that sandwiched the electrolyte film in between


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glass slides for each composition were assembled and their Raman spectra were tracked in parallel with conductivity tracking (Fig. 1). Raman tracking of Li2SO4-PAM(5,000:1) is depicted in Fig. 5 together with the corresponding micrographs of the Raman acquisition site as insets. Throughout the 28 days of tracking, the Raman spectra showed only the hydrated (SO4)2- signatures. This result was consistent with the spectra of a freestanding film. A translucent homogenous electrolyte film free of crystals was also observed in optical micrographs throughout the tracking period.

Figure 5. Raman tracking of a mock cells containing Li2SO4-PAM (5,000:1), insets are the micrographs of the spectra acquisition site.


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Raman tracking of Li2SO4-PAM(10,000:1) along with the micrographs of the corresponding acquisition sites is shown in Fig. 6. In its pristine condition, Li2SO4-PAM(10,000:1) spectra revealed only hydrated (SO4)2- signatures. Dehydrated (SO4)2- signatures appeared in the spectra after 14 days. Simultaneously with the appearance of dehydrated ion peaks, the micrographs also showed dendritic Li2SO4 salt crystals that grew with longer conditioning time. The appearance of dehydration in Li2SO4-PAM(10,000:1) coincided with the start of the conductivity degradation. The small crystals within the electrolytes impeded ion movement and resulted in the decrease in ionic conductivity.

Figure 6. Raman tracking of a mock cells containing Li2SO4-PAM (10,000:1), insets are the micrographs of the spectra acquisition site.


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Raman mapping was performed at the vicinity of the crystals at 28 days (Fig. 7) to observe the homogeneity of the dehydration within the electrolyte films. In this mapping, the Raman spectra were divided into blue for the hydrated and red for the dehydrated region, Fig. 7(a). The mapping site covered a selected region with two observed crystals, see Fig. 7(b); the resulting mapping is illustrated in Fig. 7(c). The dehydrated sulfate ions were detected only in regions where crystals were observed, while the surrounding area remained hydrated. The inhomogeneity induced by the crystallization not only reduced the conductivity but also created highly irregular interfaces which manifested themselves in a larger standard deviation in the ionic conductivity.

Figure 7. Raman mapping where (a) the spectra were marked blue and red for hydrated and dehydrated ion signatures, respectively. The mapping (b) was performed on a spot with 2


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observed crystals from Li2SO4-PAM(10,000:1) and (c) the micrograph were overlaid with the resulting mapping and indicated inhomogeneity of the electrolyte film.

Although hydration tracking showed a difference for electrolytes with different salt content, it offered little explanation of the increase in ionic conductivity of Li2SO4-PAM(5,000:1). A probable cause for the increasing conductivity is the higher humidity during storage than during cell constructions. All cells were assembled under ambient conditions with humidity around 5-10 %RH, while the cells were stored and conditioned at 45 %RH. Due to the dehydration of ions in Li2SO4-PAM(10,000:1), the ionic conductivity degraded over time. Li2SO4-PAM(5,000:1) maintained a high level of hydration and the electrolytes continued to equilibrate with the higher humidity under storage conditions, leading to an increase in ionic conductivity over time. At the end of tracking period, the ionic conductivity of Li2SO4-PAM(5,000:1) reached 12.5±2.5 mS cm1

. This value is similar to that reported for neutral LiCl-PAM23 and various hydroxyl-conducting

polymer electrolytes40-41 while possessing a wider operating voltage window of 2 V 23. This study leveraged the sensitivity of Raman spectroscopy to differentiate the hydration state of (SO4)2- ion and pinpointed the key attribute affecting the performance of Li2SO4-PAM polymer electrolytes. The concurrent timeline between ion dehydration/Li2SO4 crystallizations and conductivity degradation (Fig. 1 vs. Fig. 6) reflect real-time conditions and provides solid evidence of the strong influence of ion hydration on electrolyte performance. This approach can be a powerful tool to investigate and develop high performance Li2SO4-based polymer electrolytes or similar polymer electrolytes which rely on retaining or increasing the hydration level for applications in fuel cells, sensors, and others.

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Conclusion


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The ionic conductivity of Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) was tracked for 28 days and showed different trends. The ionic conductivity of Li2SO4-PAM(5,000:1) increased while the conductivity of Li2SO4-PAM(10,000:1) decreased. XRD patterns revealed Li2SO4PAM(5,000:1) remained amorphous while Li2SO4-PAM(10,000:1) began crystallization of Li2SO4 salt. IR and Raman spectroscopy were utilized to investigate the reason for the different trends. While the PAM bonding structure remained unaltered in both films, sulfate stretching suggested a difference in ion hydration between the two films. Li2SO4-PAM(5,000:1) contained only hydrated ions whereas Li2SO4-PAM(10,000:1) contained both hydrated and dehydrated ions. A real-time tracking approach leveraging the higher sensitivity of Raman spectroscopy to changes in ion hydration was employed to correlate the ionic conductivity trend with the progressive material changes in the electrolyte film. The hydration of the electrolyte in a sandwiched cell configuration was emulated using mock cells and was correlated to ionic conductivity. The dehydration/crystal formation of Li2SO4-PAM(10,000:1) film was observed to exactly coincide with the degradation of ionic conductivity. This study provides strong evidence that ionic dehydration negatively affects the performance of polymer electrolytes.


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ASSOCIATED CONTENT The following files are available free of charge as supplementary information: Table S1: Peak assignments for both IR and Raman spectra of PAM and Li2SO4 as individual components of the electrolyte films Figure S1: Comparison of the electrochemical cell and mock cell constructions for characterization of electrolytes under simulated cell conditions Figure S2: Raman spectra of (a) wetted Li2SO4 powder, (b) dehydrated Li2SO4, (c) Li2SO4 conditioned in a high humidity desiccator, and (d) as-received Li2SO4 powder AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Acknowledgment We appreciate the financial support from NSERC Canada (Discovery RGPIN-2016-06219, CREATE 397899-11, and Discovery Accelerator Supplements 493032). A.V. would also like to thank the University of Toronto, Department of Materials Science and Engineering for a Haultain scholarship.


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20. Kimura, K.; Motomatsu, J.; Tominaga, Y., Correlation between solvation structure and ion-conductive behavior of concentrated poly(ethylene carbonate)-based electrolytes. J. Phys. Chem. C 2016, 120, 12385-12391. 21. Zhang, J.; Wilkinson, D. P.; Wang, H.; Liu, Z.-S., FTIR and electrochemical observation of water content reduction in a thin Nafion® film induced by an impregnation of metal complex cations. Electrochim. Acta 2005, 50, 4082-4088. 22. Du, L.; Yang, P.; Yu, X.; Liu, P.; Song, J.; Mai, W., Flexible supercapacitors based on carbon nanotube/MnO2 nanotube hybrid porous films for wearable electronic devices. J.Mater. Chem. A 2014, 2, 17561-17567. 23. Virya, A.; Lian, K., Polyacrylamide-lithium chloride polymer electrolyte and its applications in electrochemical capacitors. Electrochem. Commun. 2017, 74, 33-37. 24. Wang, G.; Lu, X.; Ling, Y.; Zhai, T.; Wang, H.; Tong, Y.; Li, Y., LiCl/PVA gel electrolyte stabilizes vanadium oxide nanowire electrodes for pseudocapacitors. ACS Nano 2012, 6, 10296-10302. 25. Virya, A.; Lian, K., Li2SO4-polyacrylamide polymer electrolytes for 2.0 V solid symmetric supercapacitors. Electrochem. Commun. 2017, 81, 52-55. 26. Leung, W. M.; Axelson, D. E.; Van Dyke, J. D., Thermal degradation of polyacrylamide and poly(acrylamide-co-acrylate). J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 1825-1846. 27. Jonathan, N., The infrared and Raman spectra and structure of acrylamide. J. Mol. Spectrosc. 1961, 6, 205-214. 28. Durie, R. A.; Milne, J. W., Infrared spectra of anhydrous alkali metal sulphates. Spectrochim. Acta, Part A 1978, 34, 215-220. 29. Colthup, N., Introduction to Infrared and Raman Spectroscopy. Elsevier Science: 2012, 1-75. 30. Gupta, M. K.; Bansil, R., Laser Raman spectroscopy of polyacrylamide. J. Polym. Sci., Part B: Polym. Phys. 1981, 19, 353-360. 31. Baldock, C.; Rintoul, L.; Keevil, S. F.; Pope, J. M.; George, G. A., Fourier transform Raman spectroscopy of polyacrylamide gels (PAGs) for radiation dosimetry. Phys. Med. Biol. 1998, 43, 3617. 32. Frech, R.; Cazzanelli, E., Raman spectroscopic studies of Li2SO4. Solid State Ionics 1983, 9, 95-99. 33. Cazzanelli, E.; Frech, R., Temperature dependent Raman spectra of monoclinic and cubic Li2SO4. J. Chem. Phys. 1984, 81, 4729-4736. 34. Bhagavantam, S., Interpretation of Raman spectra in crystals: Anhydrite and gypsum. Proc. - Indian Acad. Sci., Sect. A 1938, 8, 345-348. 35. Sarma, L. P.; Prasad, P. S. R.; Ravikumar, N., Raman spectroscopic study of phase transitions in natural gypsum. J. Raman Spectrosc. 1998, 29, 851-856. 36. Berenblut, B. J.; Dawson, P.; Wilkinson, G. R., A comparison of the Raman spectra of anhydrite (CaSO4) and gypsum (CaSO4).2H2O). Spectrochim. Acta, Part A 1973, 29, 29-36. 37. Chio, C. H.; Sharma, S. K.; Muenow, D. W., The hydrates and deuterates of ferrous sulfate (FeSO4): a Raman spectroscopic study. J. Raman Spectrosc. 2007, 38, 87-99. 38. Fu, X.; Yang, G.; Sun, J.; Zhou, J., Vibrational spectra of copper sulfate hydrates investigated with low-temperature Raman spectroscopy and terahertz time domain spectroscopy. J. Phys. Chem. A 2012, 116, 7314-7318.


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TOC Graphic


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Figure 1. Ionic conductivity of Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) tracked over a period of 28 days (average from 8 cells each). 82x77mm (300 x 300 DPI)



Figure 2. TGA thermograms of Li2SO4-PAM(5,000:1) and Li2SO4-PAM(10,000:1) freestanding films for water content estimation. 82x66mm (300 x 300 DPI)


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Figure 3. XRD patterns of Li2SO4-PAM (5,000:1), Li2SO4-PAM (10,000:1), PAM freestanding films, and Li2SO4 salt. 177x98mm (300 x 300 DPI)



Figure 4. Vibrational spectra of (a) IR and (b) Raman for Li2SO4-PAM(5,000:1), Li2SO4-PAM(10,000:1), PAM freestanding films, and Li2SO4 salt. 177x89mm (300 x 300 DPI)


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Figure 5. Raman tracking of a mock cells containing Li2SO4-PAM (5,000:1), insets are the micrographs of the spectra acquisition site. 125x88mm (300 x 300 DPI)



Figure 6. Raman tracking of a mock cells containing Li2SO4-PAM (10,000:1), insets are the micrographs of the spectra acquisition site. 177x125mm (300 x 300 DPI)


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Figure 7. Raman mapping where (a) the spectra were marked blue and red for hydrated and dehydrated ion signatures, respectively. The mapping (b) was performed on a spot with 2 observed crystals from Li2SO4PAM(10,000:1) and (c) the micrograph were overlaid with the resulting mapping and indicated inhomogeneity of the electrolyte film. 172x138mm (150 x 150 DPI)


The Role of Ion Hydration in the Performance of Li2SO4

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