DOI: 10.1002/marc.((insert number)) ((or ppap.
, mabi., macp., mame., mren.
, mats.))CommunicationElucidating the Relationship between Mechanical Properties and Ionic Conductivity in a Highly Conductive Solid Polymer Electrolyte Shijia Liu1, Yuanhan Zheng1, Xueqi Koh1, Derrick Wen Hui Fam1* –––––––––Shijia Liu, Yuanhuan Zheng, Xueqi Koh, Dr. Derrick Wen Hui Fam 2 Fusionopolis Way, Innovis#08-03, Singapore 138634 E-mail: [email protected]
edu.sg–––––––––The storage modulus and ionic conductivity of prototypical gel electrolytes consisting of different amounts of poly(vinyl alcohol) and the ionic liquid 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI) were synthesised and characterised. Peak ionic conductivity and room temperature storage modulus of 1.53×?10?^(-2) S cm-1 and 114 MPa respectively were achieved. The ionic conductivity and mechanical properties were closely related to the morphology, crystallinity and extent of hydrogen bonding in the poly(vinyl alcohol) matrix.
It was also found that the optimum ionic conductivity and mechanical properties were obtained with 67 wt% EMI TFSI loading, where an ionic conductivity of 2.51×?10?^(-3) S cm-1 and a room temperature storage modulus of 38.8 MPa were obtained. ? Introduction Solid electrolytes constitute an established field of research because of their improved safety1 and multifunctionality2–4. The development of solid electrolytes makes solid state energy storage a distinct possibility.
With the development of all solid state energy storage composites (ASSESC) like structural supercapacitors and batteries, flexible and structural electronics for wearable5 applications and mechanical support2,4 can now be envisioned. ASSESCs greatly expand the flexibility of design in devices and have the potential to improve device efficiency, for example by providing weight savings with structural energy storage devices.6 However, solid state energy storage composites are still limited to composites demonstrated in literature.7 The lack of a mechanically robust solid electrolyte with high ionic conductivity for fast ion transport between electrodes is a hindering factor to the development of solid state energy storage composites. Innovation on solid electrolytes is therefore necessary to make structurally sound solid-state energy storage devices with good electrochemical performances.
Two popular categories of solid electrolytes are ceramic electrolytes and solid polymer electrolytes. Ceramic electrolytes offer good mechanical properties but suffer from poor room temperature ionic conductivity of about 10-4 S cm-1.8 Germanium-sulfide varieties of lithium-garnet electrolytes exhibit a high room-temperature conductivity of 1.2 x 10-3 S cm-1,9 but are hygroscopic and decompose to form toxic H2S when exposed to moisture.
10 Solid polymer electrolytes enjoy enduring popularity due to their potential for high ionic conductivity of up to 10-2 S cm-1 and their ability to incorporate a large range of fluid electrolytes, ranging from aqueous salts11 to ionic liquids.11,12 Solid polymer electrolytes generally exhibit an inverse relationship between mechanical properties and ionic conductivity due to the opposing microstructural and hence compositional requirements.4 An optimal composition has to be derived, empirically, to obtain the best performance out of any solid polymer electrolyte system. In most solid polymer electrolyte systems, the electrolyte remains fluid and ionic transport and conduction occurs via chain motion; it is theoretically possible to achieve similar conductivities compared to liquid electrolytes while circumventing problems such as leakage, flammability and toxicity.
12 Of the liquid electrolytic phase used in solid polymer electrolytes, organic and aqueous electrolytes usually exhibit narrow electrochemical windows due to electrolysis.1 Aqueous electrolytes can also be corrosive and organic electrolytes can be toxic and volatile.1 Due to these limitations, ionic liquids such as 1-butyl-3-methylimidazolium chloride (BMI Cl)13 and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI)14,15 are gaining popularity as the fluid component in solid polymer electrolytes. Using ionic liquids offer many advantages due to their large electrochemical window, chemical stability, low vapour pressure, and non-flammability compared to aqueous and organic electrolytes.16–18 Poly(vinyl alcohol) (PVA), a widely-available polymer, is an attractive choice for a mechanically robust polymer matrix due to its high elastic modulus of 2,000 MPa (from graph) 19 as compared to other polymers such as poly (methyl metacrylate) PMMA (storage modulus of 3.
2 GPa, from graph).20 PVA is well-studied in literature as a polymer host for electrolytes such as lithium salts21,22 and potassium chloride.11 The incorporation of ionic liquids into PVA is much less explored, with one study achieving a peak room-temperature conductivity of 5.74 mS cm-1 by employing a PVA/NH4CO2CH3/BMI Cl system.23 The conductivity of PVA polymer electrolytes can be improved further by exploring different ionic liquids.
An example would be EMI TFSI, a widely available ionic liquid that is not yet explored in a PVA solid polymer electrolyte system. Development in this area is limited as a protocol to form a homogeneous PVA/EMI TFSI solid electrolyte system has yet to be determined, hence, PVA and EMI TFSI are chosen for this study. To optimize both mechanical strength and ionic conductivity of the PVA/EMI TFSI gel electrolyte system, it is then essential to elucidate the correlations between microstructural and electrochemical difference for the different PVA-EMI TFSI compositions. This paper characterises a series of PVA/EMI TFSI solid polymer electrolytes with different amounts of ionic liquid and relates conductivity and mechanical strength to composition and microstructure. An optimal composition, 67 wt%, was found to have the optimum balance between conductivity and mechanical strength. By probing the correlations between ionic conductivity and mechanical strength of the PVA/EMI TFSI system, the mechanisms of large ion mobility through other porous polymer matrix can be better understood. Experimental Section 2.
1 MaterialsPoly(vinyl alcohol) (MW ~50,000, >99.9% hydrolysed) was purchased from Aldrich Chemical Company, EMI TFSI (>99.8%, with residual water and Cl-), from Yulu Chemicals Pte Ltd and DMSO from Tedia Solvents Pte.
Ltd. Silver paint was obtained from Dupont. All reagents were used as received without further purification.2.2 Synthesis of electrolytesPVA was first dissolved in 15 mL of DMSO at 80 ?C, yielding a colourless solution. EMI TFSI was then added, and the colourless solution was stirred at 80 ?C for 270 mins. The viscous solution was degassed in a vacuum oven for 30 mins before casting onto a glass petri dish. The sample was then dried in a vacuum oven at 60 ?C for 48 h.
The samples were obtained as free-standing discs, ranging from colourless to yellow to white with increasing ionic liquid content. The composites are denoted as PVA’X’ where ‘X’ represents the wt% of EMI TFSI present in the composite (i.e. PVA0 represents pure PVA composites and PVA50 represents a PVA composite with 50 wt% EMI TFSI)2.3 Characterisation of electrolytesThe composition and morphology of PVA/EMI TFSI composites were determined using X-ray diffraction (XRD, Bruker D8 Advanced X-Ray Diffractometer), Fourier-Transform Infrared Spectroscopy (FTIR, PerkinElmer Spectrum 2000 machine fitted with an ATR crystal), Differential Scanning Calorimetry (DSC, Mettler Toledo Differential Scanning Calorimeter from -80 to 250 ?C), Scanning Electron Microscopy (SEM, JEOL FESEM JSM6700F at 5 kV) and Thermo-gravimetric Analysis (TGA, TA Instruments TGA Q500, from 25 ?C to 600 ?C with a ramp of 5 ?C min-1).
Specifically, XRD was employed to investigate the effect of EMI TFSI addition on the crystallinity of the polymer. FTIR was used to investigate the molecular interactions in the electrolytes. The glass transition (Tg) and melting temperature (Tm) of the PVA/EMI TFSI samples were determined using DSC.
The morphologies of the samples were determined by SEM. Lastly, the compositions of the electrolytes were confirmed by TGA.All electrochemical measurements were done on an Autolab potentiostat (PGSTAT302N) with an FRA module in ambient conditions. The samples were coated in silver paint, pressed between copper electrodes and dried at 80 ?C for 10 min prior to measurement. Care was taken to ensure that the silver paint remains on the opposing surfaces of the discs so that the cell was not shorted. Electrochemical Impedance Spectroscopy (EIS) was performed between the frequency of 1 Hz to 1 MHz with an root mean squarerms voltage of 10 mV. The ionic conductivity, ?, was obtained according to the following equation:?=L/(R_b×A) (1)Where L is the thickness of the electrolyte disc, Rb is the resistance of the electrolyte, and A is the contact area between the electrolyte and the copper electrodes.
Dynamic mechanical analysis (DMA) was performed on PVA0, PVA30, PVA50 and PVA80 on a dynamic mechanical analyser (TA Instruments DMA Q800) in film tension mode. A constant temperature of 30.0 ?C was maintained while the sample underwent a sinusoidal tensile deformation at a frequency of 1 Hz, amplitude of 10 µm and preload force of 0.01 N.
3. Results and Discussion 3.1. Electrochemical measurements and mechanical properties Figure 1: Graph of ionic conductivity and storage modulus against EMI TFSI loading respectively. Ionic conductivity and storage modulus increases and decreases respectively with increasing EMI TFSI loading due to their opposing microstructure requirements.According to Figure 1, ionic conductivity increases while the storage modulus decreases with increased ionic liquid loading.
A large step in conductivity from PVA50 (5.32 ×?10?^(-5) S cm-1) to PVA60 (3.32×?10?^(-3) S cm-1) was observed. Storage modulus is an indication of the stiffness of the composite. A rigid solid electrolyte is desired in structural supercapacitor and battery applications as it prevents deformation and short circuiting of the cell. Therefore, storage modulus is used in this work to quantify the mechanical properties of the composites. The storage modulus of PVA improves upon addition of 30 wt% of EMI TFSI.
The increment in storage modulus may result from EMI TFSI acting as a nano-filler and restricting the movement of the polymer chains.19 The storage modulus decreases further linearly with increased EMI TFSI content.An optimal balance between ionic conductivity and storage modulus is determined to be 50 wt% (3.18 ×?10?^(-3) S cm-1, 65.3 MPa); at the intersection of the two graphs (Figure 1).
3.2 Composition and morphology Figure 2 A. XRD spectra of selected PVA/EMI TFSI composites. B. EMI TFSI OH Stretch. C. PVA OH stretch signal in FTIR spectra, deconvoluted from the original spectra with a Gaussian curve fit. XRD spectra (Figure 2 A) shows two main peaks at 19.
9? and 22.3?25, corresponding to the composite peak comprising of 101 ?, 101 peaks and the 200 peak, respectively (lit. 19.4? for 101 ? and 20.0? for 101, 22.
4? for 200). 26 The 101 ? and 101 peaks decrease in intensity and broaden with increasing EMI TFSI content, suggesting that the polymer becomes more amorphous when more EMI TFSI is added. FTIR (Figure 2 C) spectra suggests increasing disruption of the hydrogen bonding network in PVA with increasing EMI TFSI content.
The disruption of the hydrogen bonding network is reflected as a blue-shift in the OH stretching peak, as stronger hydrogen bonds induce red-shifts on the original stretching frequency of the bond.27 The disruption of PVA’s hydrogen bonding network likely led to a reduction in crystallinity of the sample. Table 1: DSC data from PVA0, PVA10, PVA30, PVA67, PVA80. Tg shows an increase from PVA0 to PVA30, while decreasing from PVA30 onwards. This is consistent with trends seen in the storage modulus.
Full curves found in Supporting Information.Sample name Tg ?C Tc ?C Tm?CPVA0 66.2 (lit. 90.0)28189 218 (lit. 216)28PVA10 56.8 171 203PVA30 68.
0 180 215PVA67 53.7 171 234PVA80 19.4 173 205Increment of the Tg between PVA10 and PVA30 corresponds with an increase in storage modulus between PVA0 and PVA30. FTIR data also suggests minimal disruption of hydrogen bonding during this regime. This suggests that ionic liquids could act as nano-fillers at low concentration, preventing segmental motion of the polymer chains and therefore resulting in an increment in the Tg and storage modulus.19 As segmental motion of the polymer is prevented, ionic conductivity remains low. Minimal disruption of hydrogen bonding suggests that the ionic liquid is clustered in the amorphous region, as the low ionic liquid content is was not enough to disrupt the crystalline regions of PVA.Beyond PVA30, Tg and storage moduli both decrease.
Drastic disruption of the hydrogen bonding was also observed. This is due to the plasticising effect of the ionic liquid in the PVA matrix, which is well documented in similar systems.13 As hydrogen bonding between PVA networks arewere disrupted, the polymer chains can move more freely. An increase in chain motion would lead to a lower Tg, as well as cause an increase in ionic conductivity.Otherwise, shifts in infra-red signals which do not exceed 5 cm-1 were observed, indicating that no significant hydrogen bonding interaction is present between the ionic liquid and polymer matrix.29 Figure 3: SEM images of PVA/EMITFSI gels. A.
PVA10; B. PVA30; C. PVA50; D. PVA80 showing larger pores and denser pore distribution with increased ionic liquid loading, resulting in a loose microstructure at high ionic liquid loading.SEM images (Figure 3) show small and sparse pores in PVA10.
Pore distribution becomes denser and enlarged in the 30 wt% samples. When EMI TFSI content was increased to 60 wt%, a higher density of larger pores can be observed (Figure 3C). Finally, at an ionic liquid loading of 80 wt%, polymer clumps were observed. The clumps are weakly connected and the overall structure is extremely porous, resulting in a composite with weak mechanical strength as evidenced in Figure 1. Conclusions and Future WorkPVA/EMI TFSI gel electrolytes were synthesized with a peak ionic conductivity of 1.53×?10?^(-2) S cm-1 and characterized by EIS, SEM, XRD, DSC, FTIR, TGA and DMA. The highest room temperature storage modulus (114 MPa) was obtained with 30 wt% EMI TFSI and the highest ionic conductivity was obtained with 80 wt% EMI TFSI.
It was also ascertained that the addition of EMI TFSI disrupts the hydrogen bonding network in PVA by FTIR, resulting in lower Tg, a decrease in crystallinity and a looser microstructure. Of the eight samples, PVA5067 was found to be the optimal composition with an ionic conductivity and storage modulus of 3.182.51×?10?^(-3) S cm-1 and 65.338.
8 MPa, respectively, and can be suitable as a solid state polymer electrolyte for flexible and structural energy storage devices. PVA/EMI TFSI have the potential to be used in structural supercapacitor applications. Addition of Li TFSI in subsequent studies can also be attempted in order to optimize PVA/EMI TFSI for structural lithium batteries. In the long term, all solid state devices made from this electrolyte can be developed and commercialized.
Supporting InformationSupporting Information is available from the Wiley Online Library or from the author. Acknowledgements: Received: Month XX, XXXX; Revised: Month XX, XXXX; Published online: ((For PPP, use “Accepted: Month XX, XXXX” instead of “Published online”)); DOI: 10.1002/marc.((insert number)) ((or ppap., mabi., macp.
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KGaA, 69469 Weinheim, Germany, 2013.Supporting Information for Macromol. Rapid Commun., DOI: 10.1002/marc.2013##### Elucidating the Relationship between Mechanical Properties and Ionic Conductivity in a Highly Conductive Solid Polymer ElectrolyteAuthor(s), Corresponding Author(s)**