Ionic Liquid Thermoplastic Electrolytes for Energy Storage

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Polymer/ionic liquid thermoplastic electrolytes for energy storage processed by solvent free procedures Alberto Mejia, Esperanza Benito, Julio Guzman, Leoncio Garrido, Nuria García, Mario Hoyos, and Pilar Tiemblo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01574 • Publication Date (Web): 15 Feb 2016 Downloaded from http://pubs.acs.org on February 23, 2016

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ACS Sustainable Chemistry & Engineering

Polymer/ionic liquid thermoplastic electrolytes for energy storage processed by solvent free procedures A. Mejía, E. Benito, J. Guzmán, L. Garrido, N. García, M. Hoyos, P. Tiemblo. a

Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC, Juan de la Cierva 3, 28006 Madrid, Spain

Abstract A series of poly(ethylene oxide) (PEO)/lithium trifluoromethanesulfonate (LiTf)/room temperature ionic liquids (RTIL) composite electrolytes has been prepared by melt compounding, using sepiolite modified with D-α-tocopherol polyethylene glycol 1000 succinate

(TPGS-Sep)

as

filler.

These

electrolytes

have

been

extensively

characterized, including thermal stability, relaxations and transitions, rheology, conductivity, ion diffusivity and salt dissociation. The work shows how the ability of TPGS-S to act as a physical crosslinking site for PEO allows these electrolytes to behave as solids at T>70ºC, while the abundance of an ionic liquid phase makes the ion diffusion coefficients at 25ºC to be considerably high, closer to those of a viscous liquid than to those of a solid phase. This combination of rheological and electrical properties, together with their simple and scalable preparation by melt-compounding makes them a very appealing new class of sustainable electrolytes. This same concept can be applied to electrolytes with other types of salts and therefore electrolytes incorporating Al3+, Mg2+ or Na+ salts can be similarly prepared.

Keywords,

composite polymer electrolytes, melt

compounding, thermoplastic

electrolytes, ionic liquids, sepiolite fillers

ACS Paragon Plus Environment


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Introduction Solid Polymer electrolytes (SPE) for ionic transport possess many advantages in comparison to liquid ones, and a main disadvantage which is the low ionic conductivity inherent to their solid state. Extensive revisions of the different strategies explored to date for the preparation of solid polymer electrolytes with sufficient ionic conductivity and mechanical stability can be found in the scientific literature [1, 2]. These strategies are mostly based on making the electrolyte behave as a liquid at the microscopic scale, so as to increase ionic mobility. For example, employing working temperatures where the electrolyte becomes a very viscous liquid [3] or the introduction of large ratios of a liquid component [4, 5]. To date, these strategies cannot be considered completely successful, and the search for better approaches to this problem persists. In 2013, a gum like electrolyte with solid-like behavior and liquid-like conductivity was proposed [6]. This electrolyte included a polymeric network, a packed network of wax particles, and a percolation network of a liquid electrolyte comprising LiClO4 in propylene carbonate (PC). Further studies on this electrolyte appeared in 2014 [7]. Together with dimensional stability, high ionic conductivity and thermal and, electrochemical stability, other characteristics are highly desirable in an SPE, for example those making it more sustainable, that is, safer, recyclable and processable either in the absence of solvents or employing non-toxic and safe solvents. . As regards industrial processing, the electrolyte should resemble a thermoplastic commodity polymer as far as possible, easy to process and shape by sustainable procedures such as melt compounding and injection. It should possess the conductivity of a liquid but the safety and handling simplicity of a solid. In 2014 we proposed a set of polymer-based electrolytes with the aforementioned characteristics [8, 9]. These electrolytes were thermoplastic, could be processed by extrusion, molded by compression and behaved mechanically as a soft and tough solid polymer up to 90ºC. They comprise polyethylene oxide (PEO), ethylene and/or propylene carbonate (EC, PC), LiTf as salt and sepiolite fibers modified with TPGS [10] (TPGS-S). Modified


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sepiolite fibers were added to create the sites for a successful physical dynamic crosslinking with PEO. The electrolytes thus prepared displayed liquid like conductivity even at 30ºC (≈0.7 mS/cm), and solid-like performance even up to 90ºC. Besides they show very interesting electrochemical stability and performance, including the absence or reduction of Li dendrite formation [11, 12]. However, in these electrolytes the use of EC as liquid phase is something to reconsider. First, because of safety reasons in the particular case of Li-ion or Li batteries, and second because this approach to solid electrolytes may have a wider scope than only Li-ion batteries, for a priori these physically crosslinked solid electrolytes can be equally useful in other batteries such as Li-S[13], Na+, Mg2+ [14], Al3+ [15]. As a rule, ethylene (EC) or propylene (PC) carbonates pose problems in electrochemical batteries because of their low electrochemical and thermal stability, and other liquids are being studied. RTILs have been carefully considered in the literature for Li-S, Na+, Mg2+ or Al3+ batteries as a good choice to replace other solvents, basically because of their very low vapor pressure and virtual inflammability [1,16]. In the ultrafast rechargeable Al3+ battery, recently proposed, AlCl3 is dissolved in an ionic liquid, 1-ethyl-3-methylimidazolium chloride (EMI Cl). The EMImCl/AlCl3 IL has been used also for Mg batteries, allowing to obtain safer devices and enabling the use of high voltage oxides as cathode materials [17]. This solution consisting of varying ratios of AlCl3 and EMI Cl has been for long known [18] as an ionic liquid potentially useful as electrolyte because of its good physical, chemical and electrochemical properties. Besides their low flammability, some RTILs have proved to have other advantageous features as electrolytes, namely the ability to form a solid electrolyte interphase (SEI) able to suppress the formation of Li dendrites [19-21] in the case of Li batteries. There is at this moment strong background as to the performance of RTILs as electrolytes with and without PEO [1, 22-25], and the incorporation of RTILs in the formulation of electrolytes seems then, for many reasons, a worthy challenge. Thus, we



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have undertaken the preparation and study of a new family of thermoplastic solid polymer electrolytes based on those formerly prepared in our laboratory [8, 9] but where carbonates EC and PC are substituted by a set of RTILs chosen among those being lately used in the formulation of electrolytes for diverse electrochemical batteries.

Experimental Materials PEO Mw = 5 × 106 g mol−1 from Aldrich was used to prepare the composites. LiTf, from Aldrich, and neat sepiolite, kindly supplied by TOLSA S.A., were dried under vacuum for 24 h. D-α-tocopherol polyethylene glycol 1000 succinate (TPGS), used to prepare the modified sepiolite TPGS–S, was purchased from Aldrich and used as received. Details on the preparation of TPGS-S have appeared elsewhere [10]. The RTILs employed to prepare the electrolytes listed in Table 1 were purchased from Solvionic, all of them with 99.5% purity. 1-Butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (BMP FSI) has been used to prepare electrolytes M0 and MBMPFSI; 1-Ethyl-3Methylimidazolium bis(fluorosulfonyl)imide (EMI FSI) to prepare MEMIFSI, N-Propyl-NMethylpyrrolidinium bis(fluorosulfonyl)imide (PMP FSI) to prepare MPMPFSI; N-Propyl-NMethylpyrrolidinium Bis(trifluoromethanesulfonyl)imide (PMP TFSI) to prepare MPMPTFSI, 1-Ethyl-3-Methylimidazolium Bis(trifluoromethanesulfonyl)imide (EMI TFSI) to prepare electrolyte MEMITFSI. Some of their relevant physical properties are gathered in Table 1 and compared with those of EC [26].

Table 1. Viscosity at 25ºC (η25ºC) and melting temperature (Tm) of the liquid phases employed in this work. Data from Solvionic Datasheets, except for BMP FSI [27]. RTIL

η25ºC (cp)

Tm (ºC)

BMP FSI

38

-17.7

EMI FSI

24.5

-13


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EMI TFSI

35.5

-16

PMP FSI

52.7

-9.5

PMP TFSI

71.2

12

EC

2.56a

36

a

supercooled

Preparation of the composite electrolytes The electrolytes’ composition appears in Table 2. The components were physically premixed and then melt-compounded in a Haake MiniLab extruder. Processing was carried out at a shear rate of 80 rpm during 25 min and at 120 °C [8, 9]. These conditions ensure optimum dispersion of TPGS-S and minimum PEO degradation. Characterization Scanning electron microscopy (SEM) was performed in a Hitachi SU-8000. Differential scanning calorimetry (DSC) studies were performed in a Perkin-Elmer DSC7, on films of controlled thickness (~300 µm) processed by compression moulding at 75 °C during 5 min. The heat flow was recorded at 10 °C min−1. The curves shown correspond to samples with an identical thermal history (heated up to 100 ºC, stabilized at this temperature for 5 min and cooled down up to 0 ºC at 10 °C min−1). ATR-FTIR spectra were recorded on the surface of the electrolytes using a FTIR Perkin–Elmer Spectrum-One, with 10 scans and resolution 2 cm −1. Thermogravimetric analysis (TGA) measurements were carried out in a TA Q-500 under air atmosphere and at a heating rate of 10 ºC min−1. The samples for TGA were always circular discs of 5 mm diameter which were cut from the original compression molded films. Thermomechanodynamical Analysis (DMTA) was performed in a DMA/SDT861e, from -100ºC to 50 ºC, at four frequencies (1, 5, 10, and 30 Hz) and at a heating rate of 2 ºC min−1 (only 10 Hz data appear in the graphs). The samples geometries have been 19.5 x 5.0 x 0.8 mm3.



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7

Li and

19

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F PFG NMR measurements. The NMR measurements were performed in a

Bruker AvanceTM 400 spectrometer equipped with a 89 mm wide bore, 9.4 T superconducting magnet (Larmor frequencies of 7Li and respectively). The 7Li and

19

19

F at 155.51 and 376.51MHz,

F diffusion reported data were acquired at 25 ± 0.2 ºC with

a Bruker diffusion probe head, Diff60, using 90º radiofrequency (rf) pulse lengths of 11.0 µs. In the diffusion experiments, a pulsed field gradient stimulated spin echo pulse sequence was used [28]. The time between the first two 90º rf pulses (the echo time), τ1, was 3.12 ms and the self-diffusion coefficients of

7

Li and

19

F, DLi and DTf,

respectively, were measured varying the amplitude of the gradient pulse between 0 and 460 G cm-1. The diffusion time, ∆, and length of the gradient pulses, δ, were 50 and 2 ms, respectively. The repetition rate was always five times the spin-lattice relaxation time, T1, of the nuclei being observed. The total acquisition time for these experiments varied from 10 min to 20 h. The decay of the echo amplitude was monitored typically to, at least 50% of its initial value and the apparent diffusion coefficient was calculated by fitting a mono-exponential function to the decay curve. Previously, the magnetic field gradient was calibrated as described elsewhere [29]. Rheological measurements were performed using an Advance rheometer AR2000 with a 25 mm steel plate under nitrogen flow to avoid oxidative degradation. 1000 µm thick films were prepared by compression molding at 75 °C during 5 min and quickly cooled down to room temperature. Prior to the launching of the experiment, samples were annealed in the rheometer for 10 min at the extrusion temperature (120 °C) and then stabilised at 75 °C for 5 min. Oscillatory frequency sweeps were performed in the frequency range of 500–0.01 rad s−1 using a stress amplitude from 10 to 100 Pa. Electric Measurements. The conductivity of the electrolytes was determined in a NOVOCONTROL GmbH Concept 40 broadband dielectric spectrometer in the temperature range −80 °C to 80 °C and in the frequency range between 0.1 and 107


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Hz. Disk films of dimensions of 1 cm diameter and ~400 µm thickness were inserted between two gold-plated flat electrodes and the samples are heated to 80 °C. Then, a frequency sweep is done every 10 °C cooling from 80 to −80 °C, thereafter the same measurements are done but heating from −80 to 80 °C. Ionic conductivity (σ) of the samples was calculated by using the conventional methods based on the Nyquist diagram and the phase angle as a function of the frequency plot. The values of σ which appear in this work correspond to the second on heating σ measurement. Results The electrolytes proposed in this work, and which appear in Table 2, have been prepared using RTILs containing TFSI and FSI as anions, and pyrrolidinium and imidazolium as cations, for they are known to form compatible blends with PEO [30]. In addition, FSI provides lower viscosity than other anions [31] together with chemical and electrochemical stability, and is able to form a SEI capable of suppressing the growth of Li dendrites viscosity [24]. Electrolytes including PEO and a similar set of RTILs have been studied by Passerini and co-workers [22, 23]. These authors chemically crosslink the material by uv irradiation to make the electrolyte solid at the working temperatures. On the opposite, in this work the electrolytes are physically crosslinked by employing the filler TPGS-S. This dynamic crosslinking allows the material to behave as a thermoplastic solid at typical working temperatures, i.e. from RT to about 70 ºC, while the microstructure of the material remains liquid-like [8, 9]. In what follows comparison between electrolytes with similar formulations, uncrosslinked (Table 2, M0 and S17 [9]), and physically crosslinked (Table 2, M series and S20 [9]) is presented. It will be shown that physical crosslinking has a dramatic effect on the material’s rheology but little effect on properties related to the microstructure of the material, such as thermal stability, ionic diffusivity and conductivity or salt solubility.

Electrolyte preparation and characterization



The electrolytes in Table 2 have been prepared by melt compounding and processed by compression molding into films of different thicknesses. The electrolytes named as MRTIL have been conceived to be exactly as S20 in composition [9] but changing the EC by a given RTIL Note that a blank sample M0 has been prepared. M0 is an electrolyte resembling MBMPFSI in everything except the presence of TPGS-S. Comparison of the properties of M0 and MBMPFSI allows to understand the role of TPGS-S on the properties of these electrolytes. Together with electrolytes M0 and MRTIL, the electrolytes prepared with EC, S17 and S20 [9] are included for comparison.

Table 2: Composition and physicochemical characterization of the electrolytes prepared in this work and some previously reported [9] (S17 and S20). All electrolytes in the Table, except M0 and S17, contain 5 wt% of TPGS-S and 44 wt% of PEO. M0 and S17 contain 46 wt% of PEO and no TPGS-S. Rheology (75ºC)

RTIL

LiTf

TPGSS (wt.%)

TGA T5 (ºC)

12

12

10 DLi 25ºC (m2s−1)

10 DTf 25ºC (m2s−1)

σ (25ºC) mS/cm

σLiTf mS/cm

570

1.40

3.1

0.59

0.20

0.31

550

0.89

1.8

0.37

0.12

0.33

tLi (LiTf)

2

Sample

10 G’0.05 Hz

Composition

G’=G”(Hz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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wt.%

wt.%

mol dm−3

MEMIFSI

38

13

1195

5

-

MEMITFSI

38

13

1213

5

227

MPMPFSI

38

13

1191

5

230

0.011

495

0.98

2.3

0.38

0.14

0.30

MPMPTFSI

38

13

1179

5

233

0.013

486

0.64

1.7

0.25

0.10

0.27

MBMPFSI

38

13

1148

5

234

0.010

680

0.97

2.3

0.33

0.14

0.30

M0

40

14

1176

0

233

0.044

206

1.09

3.1

0.57

0.18

0.26

S20

38

13

1172

5

101

0.014

635

5.2

12.1

0.75

0.75

0.30

S17

40

14

0

-

0.24

42

0.90

0.90

0.015

In the SEM images in Figure 1 a characteristic morphology of polymer/IL blends is shown, together with the different crystalline organizations. In M0, PEO spherulites are


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clearly seen, while in the rest of the electrolytes the crystalline entities are smaller and less defined. In MEMIFSI it is not possible to identify crystalline structures, and the matrix seems uniformly amorphous.

Figure 1. 2000x SEM images of the prepared electrolytes. All images are done at the same magnification.

Figure 2 summarises some physical properties such as thermal stability, crystallinity and mechanodynamical behavior of the M series listed in Table 2. Figure 2a compares the thermal stability of all the electrolytes, crosslinked and uncrosslinked, prepared with EC or with RTILs. Electrolytes containing RTILs are much more stable than those containing EC. Note that in Table 2 the thermal stability is quantified by T5, the temperature at which 5 wt.% is lost. All MRTIL have T5 values over 200 ºC, while T5 of S20 barely surpasses 100º C. The electrolytes with FSI are clearly less stable than those with TFSI. These results are very similar to those reported previously for the chemically crosslinked PEO/RTIL electrolytes [23]. Then, the thermal stability is seen to be unaffected by chemical or physical crosslinking (compare TGA of M0 and MBMPFSI). The elastic modulus M’ and tan δ of PEO, M0 and MRTIL, appear in Figure 2b and 2c. While in S20 two Tg are seen at −30 and −52 ºC, in MRTIL the double Tg is not seen, and a single one appears in the range −45 to −35 ºC, being its width and position



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dependent on the RTIL employed in each electrolyte. Neither the EC nor the RTILs seem to plasticise PEO strongly. The double Tg in S20 has already been thoroughly discussed in previous work [8, 9]. It was interpreted in terms of a microphase separation into EC-rich and EC-poor phases, and an unequal distribution of the lithium salt in both of them. In the case of the M series, the separation of the electrolyte into RTIL-rich and RTIL-poor phases seems less clear. Given the good compatibility with PEO of the RTIL chosen in this work, the absence of gross phase separated domains is reasonable. A double Tg can be suspected in M0, the electrolyte prepared without TPGS-S. M0’s Tg is significantly broader than that of the crosslinked analogue MBMPFSI, suggesting

that

the

presence

of

TPGS-S

contributes

to

improving

the

microhomogeneity of the electrolyte. At T>Tg, all electrolytes show a second modulus decrease taking place at different T depending on the electrolyte. The position of the second moduli occurs at higher temperatures the higher the Tm of the liquid phase (see Table 1), and it is very probably caused by the melting of a solid phase which can include crystallised RTILs or EC and a low Tm PEO phase. This second modulus decrease (which in RTILs is not detected in the tanδ) is much stronger in S20 than in the M series, because of the stronger phase separation in the former electrolytes. In agreement with the elastic modulus decrease over 0 ºC seen in DMTA, small melting endotherms with Tm in the range 20ºC-50ºC are seen in the DSC scans collected in Figure 2d. These endotherms correspond to PEO crystalline phases in an environment rich in LiTf salt and ionic liquid. Crystallinity is always very small or even inexistent in electrolyte MEMIFSI, in agreement with the SEM images in fig1a. No notable differences are detected between M0 and its physically crosslinked analogue MBMPFSI.


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Figure 2. Physical properties of the extruded polymer electrolytes. (a) TGA in air, (b) Elastic modulus M’ (dot line refers to PEO) and (c) tan δ in DMTA experiments (same scale for all curves) and (d) DSC scans (same scale for all curves).

No comparison of the DSC endotherms of MRTILs with the S electrolytes is done in this work because, as discussed in previous works [8, 9], in S electrolytes the crystallization rate was so much reduced that routine DSC experiments could detect no crystallinity on a second scan immediately following the first one This was interpreted as an indirect consequence of the strong phase separation taking place in those electrolytes. The domains of this phase separation are so large that they are visually detectable in the EC containing electrolytes where no TPGS-S was added (S17) [8, 9]. This is not the case in the RTIL containing electrolyte in this work, M0



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In summary, the routine characterization of crosslinked and uncrosslinked M electrolytes described in this work finds no remarkable differences as regards crystallinity, crystallization rate, Tg or thermal stability. Comparison with the literature results on chemically crosslinked similar electrolytes [22, 23] finds likewise little differences in these features. Finally, the characterization of the electrolytes strongly suggest that the RTIL employed in this work produce more compatible blends with PEO than EC, either in uncrosslinked or in physically crosslinked blends. Rheology According to the elastic moduli in Figure 2b, the M electrolytes should be liquids at T>60ºC, i.e. over their Tm. However, a simple creep experiment proves that the crosslinked M electrolytes behave as solids at temperatures as high as 90 ºC. Figure 3a shows the physical state of 800 µm disks films, 1 cm diameter prepared with the electrolytes after being hot pressed for several minutes at T>Tm.


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Figure 3. (a) samples after a creep experiment as described in the text, (b) shear moduli G’ (solid) and G”(dash) at 75 ºC for M0, MBMPFSI, S17 and S20, (c) effect of the cation on G’ and G” at 75 ºC and (d) effect of the anion on G’ and G” at 75 ºC.

The creep experiment was done as follows: the electrolyte films were sandwiched between aluminium foils, placed on a heater at 70 ºC with the 0.5 kg on top. Those electrolytes which showed no signs of flow after this experiment were subsequently kept for another 20 min at 90ºC. After few minutes at 70ºC it was evident that the uncrosslinked M0 was flowing and dripping out of the aluminium foil, while the rest of electrolytes kept their dimensions. A picture was taken of the state of the sandwiched M0 by the end of this 70ºC experiment and it appears in figure 3a. In the electrolytes which remained solid at 70ºC (all the crosslinked electrolytes) the experiment continued by raising the temperature to 90 ºC for another 20 min. After this second part of the experiment pictures of the sandwiched electrolytes were taken and are those appearing in figure 3a. As was reported before [8, 9] the same occurred with the EC electrolytes: the uncrosslinked S17 showed viscous flow over the melting point of the blend while the physically crosslinked S20 behaved as a solid even up to 90 ºC. This solid-like behavior has been quantified by rheological characterization. Table 2 collects the rheological crossover frequency, frequency at which the loss (G”) and storage shear moduli are the same (G’=G”), and the values of the storage modulus (G’) at 75 ºC and 0.05 Hz. Both G’ and G” appear in Figure 3b-d, arranged to illustrate the effect on the rheological behavior of i) the crosslinking (fig 3b), ii) the cation (fig 3c) and iii) the anion (fig 3d). In Figure 3b the effect of TPGS-S on the rheology of electrolytes containing either EC (S17, S20) or BMPFSI (M0, MBMPFSI) is shown. S20 and MBMPFSI, both crosslinked with TPGS-S, display very similar G’ and G” and an almost identical crossover frequency (see Table 2), suggesting that in these electrolytes, rheology at low frequencies is dominated by the presence of TPGS-S and not by the nature of the liquid phase, EC or BMPFSI. The behavior of S20 and MBMPFSI is known as



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pseudosolid, which is a phenomenon well described in the literature [32] for polymer nanocomposites and featured by the absence of viscous flow in observable timescales. Pseudo solid behavior arises from the existence of transient 3D network created by a physical, reversible interaction of the polymer with the filler. In the case of S20, this 3D network has already been described and discussed in former work [8, 9]. Electrolytes S17 and M0, with the same formulation as S20 and MBMPFSI respectively but with no TPGS-S behave as typical viscous liquids, flowing at T>Tm. In Figure 3b S17 has the lower moduli and the highest crossover frequency (0.24 rad s-1, Table 2) among the electrolytes described in this work. M0, also uncrosslinked, proves to be more viscous and its crossover is shifted to higher frequencies (0.04 rad s−1 Table 2). The higher viscosity of M0 as compared to S17 may be caused both by the lesser phase separation taking place in the former electrolyte as compared to the latter, and/or by the higher viscosity of the LiTf solutions in RTILs than in EC, as strong viscosity increases are produced when dissolving lithium salts in ionic liquids [33]. For a better insight into this second possibility, the diffusion coefficients of Li in the different electrolytes are compared in a forthcoming section. As illustrated in Figure 3c and 3d, TPGS-S is successful in imparting a solid like behavior to all the electrolytes containing RTILs. Their crossover frequency (see Table 2) is similarly low, and in simple creep experiments at 90ºC, as that illustrated in Figure 3a, none of them behaved as a liquid. Their rheology is then dominated by the presence of TPGS-S rather than by their chemical composition, being all MRTIL (and S20) rheologically very similar. It is noteworthy that the differences in structure described in the previous section between EC and RTIL electrolytes have no influence on the rheological behavior, what on the other hand is an expectable result if the physical crosslinking takes place between TPGS-S and PEO, being the role of the liquid phase in the formation of the network minor. It is remarkable that neither neat sepiolite (S), nor sepiolite modified with other compatibilizers [8-10, 34] are successful in producing a physical network like that produced with TPGS-S.


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Conductivity and Ion diffusion The ionic conductivity (σ) of the electrolytes at 25 ºC is reported in Table 2 and in Figure 4a as a function of T in the range −40 to 80 ºC. Over 40 ºC, σ of all the electrolytes is above 1 mS cm−1, and at 70 ºC they range from 2.5 mS cm−1 the lowest (MEMITFSI) to 5.5 mS cm−1 the highest (M0). These σ values look more impressive if it is recalled that all except M0 are solid up to 80 ºC (Figure 3). Opposite to rheology, the comparison of the σ values of EC and RTIL electrolytes makes little sense, as in the former σ derives from the contribution of two ionic species, while in the latter the panorama is more complicated, with four ionic species contributing. For the purpose of this work it is more relevant to study, not the overall σ, but the ion diffusivity, which reflects the microstructure of the electrolyte. In Table 2, DLi and DTf at 25 ºC appear for the M series and for S20. DLi in S20 is significantly higher than in the M series, a four-fold higher if compared to the “quickest” RTIL electrolyte MEMIFSI, and an eight-fold higher if compared to the slowest MPMPTFSI. According to the Stokes-Einstein relation, the higher ηRTIL as compared to ηEC (Table 1) make lower D values predictable on electrolytes including the former. Not only is ηRTIL>> ηEC, but the addition of Li salts to ionic liquids is known to produce strong η increases which critically depend on Li concentration [33]. Figure 4b represents the DLi at 25 ºC as a function of η-1RTIL for the electrolytes prepared with RTILs, and as a function of η-1EC for S20. As a general trend DLi is seen to increase with η-1. This indicates that, among these electrolytes, the major microstructural feature affecting the ion mobility is the η of the media, and that the η of the media is in direct relation with the η of the pure liquid phase. Other factors, such as the compatibility of each RTIL with PEO, the morphology of the phase separation in each electrolyte or the LiTf solubility and dissociation, which also affect the D values, must then differ only slightly among them or play a minor role in comparison with the η of the pure liquid phase. Small differences in the factors mentioned very probably account for the scatter in the



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relation between DLi and η-1RTIL in Figure 4b. Interestingly, even S20 roughly fits in the general trend relating D with η-1 depicted in Figure 4b, indicating that the microstructure of this electrolyte resembles somewhat that of the MRTIL and that the final diffusivity is in all of them basically governed by the viscosity of the liquid phase employed in combination with PEO. From a practical viewpoint this is a very interesting conclusion, for as far as we are able to keep the morphological and structural features of these ternary electrolytes within certain limits, it is possible to tune in them the ion diffusivity by simply changing the η of the liquid phase.

Figure 4. (a) σ as a function of T as obtained from the conductivity experiments (the inset presents the high temperature range in linear scale), (b) DLi as a function of the η1

of the liquid phase employed to prepare the electrolyte, RTIL or EC (data from Table

1). S20 is included as a green circle, (c) FTIR in the ʋ(SO3) region of Tf illustrating the solubility of LiTf in the medium, and (d) σ as a function of (DLi+ DTf) at 25 ºC for M series electrolytes.


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The Nerst-Einstein equation applied to the M series electrolytes reads:

σ=

F2 RT

ሾሺDLi +DTf ሻαLiTf cLiTf + ሺDRTLIశ +DRTLIష ሻαRTLI cRTLI ሿ

1

where Di are the diffusion coefficients of the different ions present in the M series electrolytes, c and α are the ionic species concentrations and dissociation coefficients,. In the light of this relationship, in a set of electrolytes as those in this work, σ of the electrolytes will vary linearly with the diffusion coefficients of the ions if α does not differ much among them or α=1, and if the structure/morphology of the different electrolytes is similar. As for the second condition, we have already shown that only minor differences exist among the electrolytes discussed in this work. What happens with α? For the sake of simplicity, we will assume that the RTILs are all similarly “ionic”, and then that their αRTIL are close to one another. In the case of αLiTf some indications can be obtained from the FTIR in figure 4c. This figure presents the ʋ(SO3)Tf. region of the M electrolytes with FSI anion. This region is very useful to identify the aggregation state of the Tf, whether it is a free anion, an ionic pair or an aggregate [35]. The spectra show that in all the electrolytes the dominating species are free Tf ions, which appear at 1033 cm−1, though in some cases the aggregate band at 1060 cm−1 can be seen. In particular, in MBMPFSI this band is stronger than in the other electrolytes, i.e., salt dissolution has been slightly lower in this electrolyte. To a lesser extent, this is also the case in MPMPFSI. Figure 4d shows that σ varies in a roughly linear way with (DLi+DTf) for all the electrolytes tested; the scatter observed is caused by several factors. First, the ionic liquid term (Danion+Dcation) αRTIL cRTIL is not included, and second, as deduced from Fig4c, αLiTf is not exactly the same in all the electrolytes. Besides, the small morphological differences among electrolytes can also contribute to the scatter. However, taking into account all these considerations, the linear relationship between α and (DLi+DTf) is quite good and suggests that the major differences among these electrolytes as regards their ionic conductivity are the η of the liquid phase employed to prepare them, and the



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solubility of the Li salt in the media. In effect, figure 4d suggests that the lower αLiTf in the MBMPFSI electrolytes (as shown in figure 4c) is making the σ of these electrolyte slightly lower as compared to the other members of the M series. It may seem surprising that the dissolution of LiTf is, according to Figure 4c, quite good in these electrolytes, for this hydrophobic RTILs are as a rule poor solvents for Li salts [36]. In effect, solubility tests in our laboratory have shown that LiTf is very poorly dissolved in these ionic liquids, however addition of a fraction of polyethylene glycol to them solubilises LiTf. The same occurs in the solid electrolytes, where the well-known ability of PEO to complex Li [37] drives the salt dissolution. To enable comparison with the electrolyte prepared with EC, S20, the last two columns in Table 2 collect respectively an estimation of σLiTf using the Nerst-Einstein equation assuming α=1, and a transference number tLi related to LiTf calculated as (DLi+DTf)/DTf. The σLiTf of S20 at 25ºC is about a five-fold higher than the σRTILS, while tLi is almost the same for S20 and for the M electrolytes containing FSI (about 0.30). Though the σRTILS are lower than that in S20 (about 0.2 mS/cm at 25ºC), the results collected in Figures 3 and 4 show that, in these extruded thermoplastic electrolytes, conductivity can be largely increased without deterioration of the mechanical and rheological properties by choosing a liquid phase with a lower viscosity, the only limitation being that the liquid phase is compatible with PEO and that physical crosslinking takes place between PEO and TPGS-S. The decoupling of the rheological macroscopic properties and the microscopic ion mobilities is then clearly accomplished in these physically crosslinked electrolytes. Given the strong advantages in favor of RTILs, efforts to improve their σ seem worthwhile. The major issues in RTIL containing electrolytes like those presented in this work are the reduction of the liquid phase viscosity and the increasing of the Li salt solubility, which is very poor and relies on the oxyethylene units of PEO rather than on the RTIL itself. Electrolytes such as those presented in this work, with formulations optimised to take into account these considerations should produce stable and efficient


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electrolytes,

with

the

additional

advantages

of

being

thermoplastic,

melt-

compoundable, mechanically tough and recyclable.

Conclusion The physically crosslinked electrolytes presented in this work are characterized by being thermoplastic and processable by simple melt-compounding. The physical crosslinking between the modified sepiolite TPGS-S and PEO makes them behave as solids even at T>70ºC, while the abundance of a liquid phase makes the ion diffusion coefficients considerably high, closer to those of a viscous liquid than to those of a solid phase. These electrolytes are thermally more stable and safer than EC containing electrolytes because of the presence of RTILs. Additional advantages are introduced by the presence of PEO, which enables the Li salt dissolution. The rheology and diffusivity experiment show that conductivity can be largely increased without deterioration of the mechanical and rheological properties by optimization of the liquid phase characteristics, mainly its viscosity and the salt dissociation. It is very important to stress that this same concept can be applied to electrolytes with other types of salts and therefore electrolytes incorporating Al3+, Mg2+ or Na+ salts can be similarly prepared.

Acknowledgements The authors acknowledge financial support from the Spanish Ministry through Project MAT2011-29174-C02-02 and the FPI Grant of A. Mejía in Project MAT2008-06725C03-01. TOLSA is gratefully acknowledged for providing neat sepiolite. The authors also thank the ICTP-CSIC Characterization Service which performed the SEM images and the DMTA experiments.

References



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[1] K. Xu, Electrolytes and Interphases in Li-Ion Batteries and Beyond, Chemical Reviews, 114 (2014) 11503. [2] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chemical Reviews, 104 (2004) 4303. [3] Bluecar, http://www.bluecar.fr/les-batteries-lmp-lithium-metal-polymere. [4] A. Manuel Stephan, Review on gel polymer electrolytes for lithium batteries, European Polymer Journal, 42 (2006) 21. [5] J.Y. Song, Y.Y. Wang, C.C. Wan, Review of gel-type polymer electrolytes for lithium-ion batteries, Journal of Power Sources, 77 (1999) 183. [6] Y. Wang, B. Li, J. Ji, A. Eyler, W.-H. Zhong, A Gum-Like Electrolyte: Safety of a Solid, Performance of a Liquid, Advanced Energy Materials, 3 (2013) 1557. [7] Y. Wang, W.-H. Zhong, T. Schiff, A. Eyler, B. Li, A Particle-Controlled, High-Performance, Gum-Like Electrolyte for Safe and Flexible Energy Storage Devices, Advanced Energy Materials, 5 (2015) n/a. [8] A. Mejía, N. García, J. Guzmán, P. Tiemblo, Extrusion processed polymer electrolytes based on poly(ethylene oxide) and modified sepiolite nanofibers: Effect of composition and filler nature on rheology and conductivity, Electrochimica Acta, 137 (2014) 526. [9] A. Mejía, N. García, J. Guzmán, P. Tiemblo, Thermoplastic and solid-like electrolytes with liquid-like ionic conductivity based on poly(ethylene oxide) nanocomposites, Solid State Ionics, 261 (2014) 74. [10] A. Mejía, N. García, J. Guzmán, P. Tiemblo, Surface modification of sepiolite nanofibers with PEG based compounds to prepare polymer electrolytes, Applied Clay Science, 95 (2014) 265. [11] A. Mejía, D. Shanmukaraj, J.M. Lopez del Amo, J. Guzmán., N. García, M. Armand, T. Rojo, P. Tiemblo, Scalable plasticized polymer electrolytes reinforced with surface modified sepiolite nano fillers-A feasibility study in lithium batteries, J. Power. Sources, 306 (2015), 772. [12] A. Mejía, P. Tiemblo, J. Guzmán., N. García, T. Rojo, D. Shanmukaraj, Thermoplastic Composite Polymer Electrolytes prepared by Melt Compounding for Lithium Secondary Batteries, ABAA8 8th International Conference on Advanced Lithium Batteries for Automobile Applications Bilbao, Spain, 2015. [13] M. Barghamadi, A.S. Best, A.I. Bhatt, A.F. Hollenkamp, M. Musameh, R.J. Rees, T. Rüther, Lithium-sulfur batteries - The solution is in the electrolyte, but is the electrolyte a solution?, Energy and Environmental Science, 7 (2014) 3902. [14] P. Saha, M.K. Datta, O.I. Velikokhatnyi, A. Manivannan, D. Alman, P.N. Kumta, Rechargeable magnesium battery: Current status and key challenges for the future, Progress in Materials Science, 66 (2014) 1. [15] M.C. Lin, M. Gong, B. Lu, Y. Wu, D.Y. Wang, M. Guan, M. Angell, C. Chen, J. Yang, B.J. Hwang, H. Dai, An ultrafast rechargeable aluminium-ion battery, Nature, 520 (2015) 325. [16] A. Lewandowski, A. Świderska-Mocek, Ionic liquids as electrolytes for Li-ion batteries-An overview of electrochemical studies, Journal of Power Sources, 194 (2009) 601. [17] F. Bertasi, C. Hettige, F. Sepehr, X. Bogle, G. Pagot, K. Vezzù, E. Negro, S.J. Paddison, S.G. Greenbaum, M. Vittadello, V. Di Noto, A Key concept in Magnesium Secondary Battery Electrolytes, ChemSusChem, 8 (2015) 3069. [18] J.S. Wilkes, A short history of ionic liquids - From molten salts to neoteric solvents, Green Chemistry, 4 (2002) 73. [19] N. Schweikert, A. Hofmann, M. Schulz, M. Scheuermann, S.T. Boles, T. Hanemann, H. Hahn, S. Indris, Suppressed lithium dendrite growth in lithium batteries using ionic liquid electrolytes: Investigation by electrochemical impedance spectroscopy, scanning electron microscopy, and in situ 7Li nuclear magnetic resonance spectroscopy, Journal of Power Sources, 228 (2013) 237.


Page 20 of 22

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[20] A.I. Bhatt, P. Kao, A.S. Best, A.F. Hollenkamp, Understanding the Morphological Changes of Lithium Surfaces during Cycling in Electrolyte Solutions of Lithium Salts in an Ionic Liquid, Journal of The Electrochemical Society, 160 (2013) A1171. [21] A. Basile, A.F. Hollenkamp, A.I. Bhatt, A.P. O'Mullane, Extensive charge-discharge cycling of lithium metal electrodes achieved using ionic liquid electrolytes, Electrochemistry Communications, 27 (2013) 69. [22] M. Joost, M. Kunze, S. Jeong, M. Schönhoff, M. Winter, S. Passerini, Ionic mobility in ternary polymer electrolytes for lithium-ion batteries, Electrochimica Acta, 86 (2012) 330. [23] H. De Vries, S. Jeong, S. Passerini, Ternary polymer electrolytes incorporating pyrrolidinium-imide ionic liquids, RSC Advances, 5 (2015) 13598. [24] I.A. Shkrob, T.W. Marin, Y. Zhu, D.P. Abraham, Why Bis(fluorosulfonyl)imide Is a “Magic Anion” for Electrochemistry, The Journal of Physical Chemistry C, 118 (2014) 19661. [25] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, Fast cycling of Li/LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI]−, Journal of Power Sources, 160 (2006) 1308. [26] HUNTSMAN, http://www.huntsman.com/performance_products/Media%20Library/global/files/jeffsol_alkyl ene_carbonates_brochure.pdf. [27] Q. Zhou, W.A. Henderson, G.B. Appetecchi, M. Montanino, S. Passerini, Physical and Electrochemical Properties of N-Alkyl-N-methylpyrrolidinium Bis(fluorosulfonyl)imide Ionic Liquids: PY13FSI and PY14FSI, The Journal of Physical Chemistry B, 112 (2008) 13577. [28] E.O. Stejskal, J.E. Tanner, Spin diffusion measurements: Spin echoes in the presence of a time-dependent field gradient, The Journal of Chemical Physics, 42 (1965) 288. [29] L. Garrido, M. López-González, E. Saiz, E. Riande, Molecular Basis of Carbon Dioxide Transport in Polycarbonate Membranes, The Journal of Physical Chemistry B, 112 (2008) 4253. [30] A. Tsurumaki, J. Kagimoto, H. Ohno, Properties of polymer electrolytes composed of poly(ethylene oxide) and ionic liquids according to hard and soft acids and bases theory, Polymers for Advanced Technologies, 22 (2011) 1223. [31] E. Paillard, Q. Zhou, W.A. Henderson, G.B. Appetecchi, M. Montanino, S. Passerini, Electrochemical and Physicochemical Properties of PY14FSI -Based Electrolytes with LiFSI, Journal of The Electrochemical Society, 156 (2009) A891. [32] Q. Zhang, L.A. Archer, Poly(ethylene oxide)/Silica Nanocomposites: Structure and Rheology, Langmuir, 18 (2002) 10435. [33] B.G. Nicolau, A. Sturlaugson, K. Fruchey, M.C.C. Ribeiro, M.D. Fayer, Room Temperature Ionic Liquid−Lithium Salt Mixtures: OpXcal Kerr Eﬀect Dynamical Measurements, The Journal of Physical Chemistry B, 114 (2010) 8350. [34] A. Mejía, PhD Thesis, Universidad Autónoma de Madrid, 2013. [35] W. Huang, R. Frech, R.A. Wheeler, Molecular structures and normal vibrations of CF3SO3 and its lithium ion pairs and aggregates, Journal of Physical Chemistry, 98 (1994) 100. [36] Z.P. Rosol, N.J. German, S.M. Gross, Solubility, ionic conductivity and viscosity of lithium salts in room temperature ionic liquids, Green Chemistry, 11 (2009) 1453. [37] M.B. Armand, Polymer Electrolytes, Annual Review of Materials Science, 16 (1986) 245.



TOC for Polymer/ionic liquid thermoplastic electrolytes for energy storage processed by solvent free procedures , A. Mejía, E. Benito, J. Guzmán, L. Garrido, N. García, M. Hoyos, P. Tiemblo.

Tough, soft and shapeable solid electrolytes 5 4 −1

σ (mS cm )

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3 2 1 0

30

40

50 T (ºC)

60


70

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Ionic Liquid Thermoplastic Electrolytes for Energy Storage

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