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Understanding of Lithium 4,5-Dicyanoimidazolate-PEO System. Influence of the Architecture of the Solid Phase on the Conductivity Piotr Jankowski, Grazyna Zofia Zukowska, Maciej Dranka, Maciej Jozef Marczewski, Andrzej Ostrowski, J#drzej Korczak, Leszek Niedzicki, Aldona Zalewska, and Wladyslaw Grzegorz Wieczorek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07058 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Understanding of Lithium 4,5Dicyanoimidazolate-PEO System. Influence of the Architecture of the Solid Phase on the Conductivity. Piotr Jankowski,a,b Grażyna Zofia Żukowska*,a Maciej Dranka,a Maciej Józef Marczewski,a Andrzej Ostrowski,a Jędrzej Korczak,a Leszek Niedzicki,a Aldona Zalewskaa and Władysław Wieczoreka a

Faculty of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664

Warsaw, Poland b

Department of Physics, Chalmers University of Technology, SE-412 96, Gothenburg,

Sweden *e-mail: [email protected], tel. +48 22 234 5739

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ABSTRACT

Solid polymer electrolytes (SPEs) with high lithium conductivity are very beneficial as a safe material for lithium battery applications. Herein we present new set of SPE based on lithium 2-trifluoromethyl-4,5-dicyanoimidazolate (LiTDI) with wide range of ether oxygen to lithium molar ratios. The phase composition was characterized in details with thermal, diffraction and spectroscopic techniques and its influence on conductivity behavior was examined. Detected two crystalline phases of LiTDI-PEO were simulated with computational methods. The obtained results allowed to give insight into the structure of these electrolytes and help to understand on the molecular level factors influencing electrochemical properties and phase behavior. It was shown that ability to form low-melting phase can be used to lower the temperature window of operation. That made possible to keep such SPEs amorphous at 30°C during 80 days. The thermal stability of the samples was checked to prove safety of electrolytes. Introduction The market of portable devices, power tools and electric vehicles expansion, involves the need to design more advanced energy storage systems.1 In particular, regarding the development of electronic market toward flexible and even wearable devices, it is necessary to develop technology of flexible, ultra-thin, light and pre-eminently safe batteries.2 Replacement of liquid electrolyte by solid one eliminates risk of leaking and the requirement of separator usage. Furthermore, higher energy densities can be achieved by application of a lithium metal as a negative electrode, which is not possible in case of liquid electrolytes. Although SPEs based on simple poly(ethylene oxide) usually show low ionic conductivity at room temperature, PEO has been widely used as an electrolyte matrix for LIBs.3 In a typical polymer electrolyte, the amorphous phase is responsible for conductivity, 2 ACS Paragon Plus Environment

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where ion transport is coupled to segmental motions of polymer backbone. Except some special cases, where the crystalline structure forms tunnels for fast lithium migration4, the conductivity of the crystalline phase of the complex is several orders of magnitude lower than in the case of an amorphous phase, and its contribution is negligible. Obviously, the structure of crystalline phase significantly affects the observed ionic conductivity dependences. Crystallization of the PEO phase leads to the increase of the salt concentration in the amorphous phase, while crystallization of the PEO-salt complexes may result both in formation of salt- enriched or poor in salt amorphous phase, depending on the stoichiometry of the crystalline complex. Melting leads to the formation of totally amorphous system, but above the glass transition temperature some other effects come into consideration, which modify system without transition to the amorphous phase. Design of new SPEs is generally motivated by need for enhancement of ionic conductivity.5 The conductivity value is determined by two key parameters: the ion mobility and the ion concentration.6 The first one is primarily dependent on the movement of the polymer chains and on the interactions of the salt with the polymer matrix, and thus on the glass transition temperature and plasticizing/crosslinking properties of applied compound (salt). Therefore, the ability of the salt to form complex phases with melting temperatures lower than that of pristine PEO is desired. The latter results from the applied concentration of salt, or more precisely, on the effective ion concentration, and the association effects. It is known that above certain level increase in the salt concentration results in ion recombination i.e. ionic association, leading to a decrease of the mobile charge carriers and hence decrease in the conductivity. In the recent years, two families of the salts with weakly associating anions gained in popularity: the imide anions7 – e.g. LiTFSI, LiFSI – and the Hückel anion8 – e.g. LiDCTA, LiTDI. The anions of the salts of the latter are based on heteroaromatic ring. That provides a better delocalization of charge and lower interaction energy between cation and anion.9 Usage 3 ACS Paragon Plus Environment


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of the salt based on cyano-substituted imidazoles or triazoles was first proposed by Armand in early 2000’s. 8 One of these salts, LiTDI, developed by group of Wieczorek, gained particular attention thanks to the electrochemical stability, excellent solubility in organic solvents and weak tendency to aggregate. Moreover, the relatively simple synthesis and low price makes it attractive for commercial application in the solid electrolytes. Crystallographic studies of LiTDI solvates helped us to gain an insight into electrochemical properties of TDI doped electrolytes. 10 The information about the coordination modes existing in glyme based systems was particularly useful for understanding of the association process in electrolytes based on high molecular weight PEO. Our results point also on the possibility to achieve high cation transference numbers at very high salt doping level. Another salt with Hückel anion, LiDCTA, was already reported to „plasticize” crystalline phase and lower the crystalline-toamorphous phase transition from 60 to 40 °C.8 This effect significantly broadens the working temperature window, which is limited by the formation of the crystalline phase. Melting of the PEO-LiDCTA membrane (O/Li = 20) results in a significant increase in the conductivity level, above 10-4 S/cm at 50ºC. Analysis shows that, despite structural similarities of these anions, replacement of a ring nitrogen atom (DCTA) with a C−CF3 group (TDI) significantly reduces the lithium cation ionic association tendency of the anion.

10

These results could

explain why LiTDI electrolytes have clearly higher conductivity of liquid electrolytes than comparable LiDCTA ones – 1 M LiTDI in EC:DMC indicates value of 6.7 mS·cm−1 at 20 °C,11 which is quite high regarding to others modern, safe lithium salts, stable in presence of water and at high temperature (>250°C). Properties of the liquid electrolytes based on LiTDI were extensively studied by Niedzicki,12,13, but little data is available for LiTDI- containing SPEs.14 The understanding of polymer systems requires the knowlege about their molecular structure. The main difficulty is that these systems are complex and comprise both crystalline and amorphous phases. Therefore it was necessary to study properties of these electrolytes in


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an expanded composition range, from very diluted to concentrated. In order to check the behavior of LiTDI salt in polymer matrix, we conducted broad analysis of LiTDI-PEO system. There are available detailed computational15 and structural16,17 analyses based on complexes with short PEO-analogues, which facilitate the understanding of the association process in electrolytes based on LiTDI. Such electrolyte interactions largely govern lithium battery performance. The presence of many different coordination centers of lithium cation, gives TDI anion unique nature. It appeared possible to obtain free cations at very high salt concentration, by appropriate adjusting electrolyte composition.16 In our former work concerning PEO-LiTDI systems we proposed the structure of the found two different crystalline complex phases. Thanks to our previous results, it was possible to simulate intramolecular interactions and explain influence of formed crystalline phases on observed conductivity relationship. Herein we present comprehensive analysis of new Solid Polymer Electrolytes with LiTDI as safe electrolytes of new generation.

Experimental Sample preparation Poly(ethylene oxide) PEO (Mw =5×106 g/mol, Aldrich) was dried under vacuum. LiTDI-PEO membranes were obtained as follows. LiTDI was dried under vacuum and added to poly(ethylene oxide) in acetonitrile solution. The solution obtained was poured onto a teflon dish and a thin foil was formed after vacuum drying. All operations were carried out inside an argon-filled glovebox. Electrochemical measurements The electrochemical measurements were performed using a computer-interfaced multichannel potentiostat with frequency response analyzer option (Bio-Logic Science Instruments VMP3). Electrochemical impedance spectroscopy was carried out within the 500–0.5 kHz frequency



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range with 10 points per decade, 5 measurements per frequency, and 5 mV AC amplitude. Lithium transference number was calculated using polarization method introduced by Bruce and Vincent (29) with ∆V = 15 mV. The presented value of transference number is an average of six measurements. Samples for all electrochemical measurements were prepared inside an argon-filled glovebox. For the relaxation measurements, samples with O/Li ratio equal 4 and 20 were heated in Büchi oven to 100ºC for 3 hours. After subsequent cooling samples were stored at 30oC for 80 days. The conductivity was measured firstly every 2 hours, and then the time interval was gradually increased to 7 days. Next, samples were cooled down to 20ºC and the conductivity measurements were repeated every 15 minutes during 10 days, in order to study the crystallization progress. Powder X-ray Diffraction (PXRD). Laboratory powder X-ray diffraction patterns were recorded at room temperature on a Bruker D8 Advance diffractometer equipped with a LYNXEYE position sensitive detector, using CuKα radiation (λ = 0.15418 nm). The data were collected in the Bragg-Brentano (θ/θ) horizontal geometry (flat reflection mode) between 4° and 60° (2θ) in a continuous scan, using 0.03° steps 384 s/step. The diffractometer incident beam path was equipped with a 2.5° Soller slit, and a 1.14° fixed divergence slit, while the diffracted beam path was equipped with a programmable antiscatter slit (fixed at 2.20°), a Ni β-filter and a 2.5° Soller slit. Data was collected under standard laboratory conditions (temperature and relative humidity).

Theoretical calculations The calculations, due to the large size of the systems, have been carried out with increasing of the theory level. Starting geometries were obtained from semi-empirical method (PM3)18, and then optimized with Hartree-Fock method.19 Final geometries were obtained with M06-2X 6 ACS Paragon Plus Environment

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density functional and 6-31G(d) basis set with application of Periodic Boundary Conditions (PBC)20, as implemented in Gaussian09.21 The frequency calculations for single mers, allowed to consider these geometries as ground states – all of the found imaginary frequencies were negligible and can be attributed to the absence of boundary conditions. To obtain more accurate energies, optimized geometries were subject to single-point calculations, using 631+G(d) basis set.

The energies of creation were calculated as a difference between

optimized complex and relaxed separated polymer chains and ion-pairs. For interaction calculations, due to electroneutrality required by PBC, the boundary conditions were simulated by the use of three structural units in each dimension. The interaction energies, both of cation (ELi-int) and anion (Eanion-int) inside the structure with the rest of the system were calculated as a single ion removal from the middle structural unit, without structure relaxation. For every calculation, the BSSE was considered via counterpoise calculations.

Spectroscopic studies The Raman spectra were collected on a Nicolet Almega Raman dispersive spectrometer. Diode laser with excitation line 532 nm was used. The spectral resolution for all experiments was about 2 cm–1. Temperature-dependent spectra were obtained with the use of a Peltier cooled Linkam stage. The infrared spectra were recorded on a PerkinElmer 2000 FT-IR system with a wavenumber resolution of 2 cm–1. Room temperature measurements were performed with use of the ATR Gladiator accessory with diamond crystal, the temperature dependent experiments were done with HATR accessory equipped with heated ZnSe crystal; the accuracy of the temperature was estimated to be 1 °C. The analysis of Raman and IR spectra was performed with the Omnic software.



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Thermal properties The DSC studies were performed using a TA Instruments Q200DSC apparatus in nitrogen flow. The heating rate was equal to 5 °C/min. The degradation products formed during the heating of the electrolytes were evaluated by thermogravimetry combined with an infrared spectroscopy (TG–IR) measurements. The sample mass was about 6- 8 mg and the heating rate was 10ºC per minute. During the TG/FTIR experiments, spectra were repeatedly collected as interferograms and then processed to build up a Gram–Schmidt reconstruction, each point of which corresponded to the total IR absorbance of the evolved components in the spectral range 4000- 750 cm-1. Consequently, the Gram–Schmidt plot was formed by averaging the intensities of all FTIR peaks over the entire spectral range. Thus, the total absorbance intensity of each mass loss is a function of the concentration of the evolved gases and their corresponding infrared extinction coefficients. Results and Discussion Phase composition analysis The conductivity behavior can be explained on the basis of the complex phase composition of the LiTDI-PEO electrolytes. We have previously reported, that in the studied salt concentration range LiTDI-PEO system is able to form several crystalline phases: two comprising LiTDI (complexes α and β) and one without salt, i.e. pristine PEO.16,17 Figure 1 presents phase diagram constructed on the basis of DSC data.


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Figure 1 Phase diagram of the PEO-LiTDI system. -Tt PEO, -Tt α-LiTDI-PEO, - Tt β-LiTDI-PEO, -Tg

The most diluted sample with O/Li ratio equal 160, exhibits only one melting effect, at 65ºC, corresponding to the melting temperature of the crystalline phase of pure PEO. Increase in the salt content results in the formation of crystalline phases of two PEO-LiTDI complexes with different stoichiometry. On the basis of our former structural studies on LiTDI-glyme solvates we ascribed these complexes structural motifs shown in Figure 2, representing contact ionic pair and aggregate (infinite chain), respectively.



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Figure 2 Coordination modes of the TDI anion found in PEO-LiTDI electrolytes17 : I - "free" anions; II - contact pairs; III - chain aggregates; IV – dimers

Melting temperature of the complex α (II) varies from 39 to 44ºC. The more aggregated complex β (III) forms at higher O/Li ratio and melts at considerably higher temperature 80100ºC. Presence of the PEO-LiTDI phase α was confirmed for O/Li ranging from 4 to 32, while the β phase appeared at O/Li equal 8 and is the only crystalline phase present in the most concentrated system PEO-LiTDI 3. The phase composition significantly affects properties of the amorphous phase: Tg values of the pristine samples are 5-10ºC lower than of the previously molten samples. It is interesting to note that Tg does not change significantly for the samples with O/Li range spanning from 160 to 20. Sample with O/Li ratio equal 15 exhibit significant decrease in Tg which is probably correlated with free ions distributed in a polymer matrix. Higher Tg’s for samples with the lowest salt content can be explained as resulting from their higher crystallinity in comparison to samples with ratio greater than 20. The crystalline phase consists almost exclusively from pure PEO, therefore the crystallization leads to the increase in the salt content in the amorphous phase and to the increase in Tg values. Further increasing in LiTDI content up to O/Li equal 6 leads to more organized 10 ACS Paragon Plus Environment

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amorphous phase with higher Tg. Simultaneously, the crystalline phase α appears at O/Li equal 20 and can be detected at O/Li as high as 4. The kinetics of the crystallization of both complex phases is slower than of PEO. The temperature- dependent Raman experiment reveal that peaks characteristic to crystalline phases α appear after at least one hour storage at room temperature, while phase β requires several days for recrystallization. Determination of the structure of these phases is virtually impossible, due to the difficulty with obtaining such a single crystal. However, the approximate structures were simulated using computational methods and knowledge from our previous studies, that correlates structural and spectroscopic data.16,17 Based on that, we concluded that lithium cation surrounded by TDI anions, is trying to keep coordination number equal 6. Analysis of interactions between cation and anion allowed to mark appearing coordination motives. In α (II), TDI anion interacts with Li+ by ring nitrogen and fluorine atom; remaining 4 coordination sites of lithium cation are occupied by oxygen atoms of PEO (ratio O/Li = 4). Inside β (III) each lithium cation is linked to ring nitrogen, fluorine and also nitrile nitrogen of an another TDI anion. That makes only 3 coordination sides available for polymer oxygens; ratio O/Li equal 3 is in agreement with experimental data suggesting a higher salt concentration for β phase. Optimized structures are shown in Figure 3.



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Figure 3 Optimized structures of α-PEO-LiTDI (3a) and β-PEO-LiTDI (3b); M06-2X/6-31G(d).

The properties of both phases were confronted with supposed by Bruce double helical structure of PEO-LiPF622, and presented in Table 1. Table 1. Calculated energies of creation and ion interactions for proposed LiTDI-PEO phases and LiPF6-PEO as a reference; M06-2X/6-31+G(d)//M06-2X/6-31G(d). Phase

α-LiTDI-

Molecular

Lithium

Molecular

Ecreation

ELi-int

Eanion-int

formula

coordination sphere

assembly

(kJ·mol-1)

(kJ·mol-1)

(kJ·mol-1)

[Li(EO)4TDI]n

4O + NIm+ F

1D

-645

764

417

[Li(EO)3TDI]n

3O + NCN + NIm + F

2D – layer

-648

761

439

[Li(EO)6PF6]n

6O

1D

-411

799

260

–

helix

PEO β-LiTDIPEO LiPF6-PEO

–

double

helix

Formation of crystalline phase, where lithium ion is linked to the TDI anion is more favorable (Ecreation = -645 and -648 kJ·mol-1 for α and β, respectively) than complete coordination of the polymer, as in case of LiPF6 (-411 kJ·mol-1). The lower (more negative) energies of phase creation for LiTDI-PEO phases explain, why typical, double-helical structure is not observed here. Although these phases are more stable, the lithium cations are less bound therein. Interaction energy between lithium cation and crystalline lattice (ELi-int) decreases when oxygen atoms of PEO are replaced by nitrogen atoms of TDI - from 799 kJ·mol-1 for LiPF6PEO, via 764 kJ·mol-1 for α-LiTDI-PEO to 761 kJ·mol-1 for β-LiTDI-PEO. This is consistent with our previous data suggesting that the increase in the system aggregation allows for weaker bonding of part of cations and can lead to disproportionation of the cationic centers.


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Stronger involvement of the anions in the crystalline phase construction, causes their higher interaction energy with crystalline lattice of the phase structure (Eanion-int) – 260 kJ·mol-1 for LiPF6-PEO and 417 kJ·mol-1 for α-LiTDI-PEO. Creation of the additional chain by salt molecules in β-phase, causes further increase of Eanion-int (439 kJ·mol-1). This relationship has a significant influence on the observed melting point of both phases: one dimensional structure of α (low-melting phase) is easier to destroy than two dimensional β (high-melting phase).

Ionic Conductivity

Figure 4 Conductivity as a function of temperature for LiTDI-PEO with Li/O ratio varying from 4 to 20

The phase composition of the studied membranes is well reflected in the conductivity data. Figure 4 presents conductivity of the LiTDI-PEO electrolytes as a function of the 13 ACS Paragon Plus Environment


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reciprocal temperature. It is interesting to note that samples with O/Li spanning from 8 to 15 exhibit almost the same temperature conductivity dependence, with significant increase of the conductivity above 40ºC, corresponding to the melting of the crystalline phase. Also the conductivity values remain close in the entire temperature range. The more concentrated samples, with O/Li ranging from 4 to 6, have comparable ambient temperature conductivities, as the more diluted ones, but considerably lower in a molten state. This effect can be explained as resulting from both an increase of the ionic association, leading to the decrease of the number of charge carriers and a decrease of the mobility of the charge carriers due to the stiffening of the polymer matrix. The difference between the conductivity behavior of the samples with high (4-6) and moderate to low (8-15) O/Li ratio is a consequence of the multiphase character of the samples: the latter ones consist from crystalline phases with the melting temperatures at ~40 and 60ºC, while the more concentrated ones have also two melting temperatures, at ~40 and 90ºC. In our previous paper we postulated, that the temperatures at 40 and 90ºC correspond to the melting of the crystalline polymer complexes α and β.17 Therefore at 70ºC the melting process is completed for the more diluted samples, which at this temperature can be considered fully amorphous. The conductivity behavior of the samples 4:1 and 5:1 can be described by VTF equation in the whole studied temperature range, which can be explained as a result of lower crystallinity in the highly concentrated systems, confirmed by the DSC experiments. Conductivity of the more diluted samples 6:1 and 8:1 is slightly higher below 40ºC; melting of the crystalline phase α results in a significant increase of the conductivity and Arrhenius-like conductivity dependence at elevated temperatures. Two distinct jumps of the conductivity at ~40 and 80ºC should be assigned to the subsequent melting of the crystalline phases α and β. Similarly, two phase transitions, at ~40 and 60ºC, observed in DSC


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traces of the diluted samples and corresponding to melting of the complex phase α and of pure PEO, are reflected as jumps in conductivity data. In the more concentrated samples with 4-8 O/Li ratio the β phase partially dissolves during heating, above the melting point of the phase α, but the samples remain partially crystalline. On the other hand, melting of the crystalline phase of PEO in the diluted samples leads to the increase of the salt dissociation in the amorphous phase. In the concentrated samples, melting of α-PEO-LiTDI phase results in a release of ionic pairs and a decrease of the percentage of mobile charge carriers in the amorphous phase. Particularly interesting effect was found for sample with O/Li equal 6, where three jumps in conductivity were observed. The distinct increase in conductivity around 60ºC corresponds to the melting of PEO phase, which was not present in the membrane at room temperature. It is likely, that presence of PEO is a consequence of disproportionation which takes place above 40ºC, i.e. after melting of α-PEO-LiTDI and leads to formation of both crystalline PEO and β-PEOLiTDI phases. Disproportionation of LiTDI in glyme system was already described in our previous paper and is responsible for high conductivity of crystalline LiTDI-tetraglyme complex.16 Table 2 summarizes lithium transference numbers and lithium cation conductivities for selected samples.

Table 2 Lithium transference numbers (tLi+), ionic conductivities and lithium cation conductivities at 40ºC. O/Li tLi+ σ/S cm-1 σ Li+/S cm-1

4

5

6

8

12

15

20

0.207 6.31x10-7 1.31 x10-7

0.276 1.58 x10-6 4.37 x10-7

0.274 2.00 x10-6 5.47 x10-7

0.270 3.16 x10-6 8.54 x10-7

0.216 2.51 x10-6 5.43 x10-7

0.225 3.16 x10-6 7.12 x10-7

0.148 1.58 x10-5 2.35 x10-6

Measurements were performed at 40ºC, i.e. temperature corresponding to the melting temperature of the α-LiTDI-PEO. The observed trend in the lithium transference numbers 15 ACS Paragon Plus Environment


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with maximum at O/Li ratio equal 8 corresponds to the phase composition, at which β-phase disappears and is replaced by crystalline PEO. Slight decrease in the lithium transference number found for O/Li equal 20 is in agreement with an increase in the Tg for the most diluted samples.

Figure 5 Conductivity of the O/Li = 4 (grey) and 20 (red) samples during relaxation at 30 and 20ºC; insert shows magnification of conductivity curve after decrease of temperature to 20 ºC

Conductivity relaxation measurements for 4:1 and 20:1 samples show that at 30ºC, both of them remain amorphous (Figure 5); the initial increase in the conductivity can be ascribed to slow organization of the system. Depending on the salt content, the resulting crystalline phase can be either PEO (O/Li equal 20), or β-LiTDI-PEO (O/Li equal 4), which


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leads to slight increase in the salt concentration in the amorphous phase in the first case and slight decrease in the second. After decrease in the temperature to 20ºC the fast recrystallization occurs, resulting in the stiffening of the system and rapid decrease in the conductivity. On the basis of the Raman and XRD experiments it was found, that as the first the crystalline phase with lower salt content appears, i.e. pure PEO in the case of the 20:1 sample, and α-PEO-LiTDI for the concentrated system 4:1. XRD

Figure 6 XRD pattern of for LiTDI-PEO with Li/O ratio varying from 4 to 20. Complex nature of the studied electrolytes was confirmed by XRD measurements. Figure 6 shows XRD patterns of the selected LiTDI-PEO membranes with various LiTDI doping level. Signals attributed to LiTDI were not detected in any of the studied membranes, while signals ascribed to the PEO crystalline phase can be distinguished in samples with 20 up to as 5 O/Li ratio. New signals at 9.4, 11.2, 15.5, 17.4, 25.7 and 28.3 indicate the formation of an another phase identified with the α-PEO-LiTDI phase. Signals due to the PEO gradually 17 ACS Paragon Plus Environment


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decline with rise in the salt content and new peaks arise at 10.2 and 22.5 for 8:1 sample. In the diffraction pattern of the 4:1 sample signals of the PEO are no longer present. Hence, it can be concluded that the 4:1 sample consists from at least two crystalline phases, both different than PEO and corresponding to the previously described two types of LiTDI-PEO complexes.17 DSC data indicate that PEO-LiTDI phase β, is the only crystalline phase observed in the most concentrated sample (3:1) after 3 months of storage. Samples with 20 and 4 O/Li ratio were molten at 120ºC and slowly cooled down to 20ºC. After 2 days of storage the only crystalline phase found for 20:1 was PEO, while 4:1 sample exhibited signals characteristic for α and β phases. Further annealing at 20ºC lead to the formation of α phase and further increase of the content of PEO crystalline phase in the 20:1 membrane. The highly hygroscopic β phase decomposed under the experiment conditionfurther annealing of the 4:1 sample at room temperature results in the increase of PEO-LiTDI α phase (see Supporting Figure S1).

Spectroscopy studies Phase assignment Polymer electrolytes based on the high molecular weight PEO usually consist from several crystalline phases and may contain some amounts of the amorphous phase(s). It can be concluded on the basis of the DSC and XRD results, that most of the obtained membranes consist from two crystalline phases. Independent Raman mapping experiments confirmed the multiphase character of the electrolytes obtained and the presence of domains with different structures. Temperature dependent spectroscopy experiments allowed to identify these phases and ascribe to them the previously described coordination modes.16,

17

Tables 3 and 4

summarize the positions of the characteristic bands in Raman and FTir spectra of the studied membranes at room temperature. 18 ACS Paragon Plus Environment

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Table 3. Band assignments in Raman spectra (at 25ºC) of LiTDI-PEO membranes. O/Li

−1

−1

−1

−1

3 4 5

νCN/ cm 2259, 2250, 2232 2231 2255, 2251, 2233

νCN Im/ cm 1316, 1306 (sh) 1318 1315, 1305

δNCN / cm 993 996 992

δCF3 / cm 684 675 683, 675

6

2252, 2232

1319, 1308

996, 979

680 (sh), 674

8 16 32 160

2231 2231 2228 2226

1318 1318, 1306 1308 1304

995 995 979 978

674 674 674 674

−1

ρCH2 / cm 875, 849 878, 839 878, 849 879, 869, 848, 840 878, 840 875, 860, 838 859, 844 859, 844

Table 4. Band assignments in FTir spectra (at 25ºC) of LiTDI-PEO mebranes O:Li

−1

νCN/ cm

ωCH2 / cm

−1

3

2260, 2229

1362, 1353, 1344

4

2255, 2250, 2229

1362, 1353, 1344

5

2250, 2229

1362, 1353, 1344

6

2229

1362, 1353, 1344

8

2229, 2223 (sh)

1360, 1353, 1342

16

2229, 2224 (sh)

1360, 1352, 1341

32 160

2229 (sh), 2224 2224

1360, 1341 1360, 1341

−1

τCH2 / cm 1282, 1278, 1251, 1238 1282, 1277, 1251, 1237 1282, 1277, 1251, 1237 1282, 1277, 1251, 1237 1282, 1277, 1251, 1237 1280, 1251(sh), 1242, 1235 1280, 1242, 1235 1280, 1242, 1235

δNCN / cm

−1

ρCH2 / cm

−1

999, 994

869, 848, 837

993

869, 848, 837

993

869, 848, 837

994

848, 837

994, 985, 978 993 (sh), 985, 978, 985, 978 978

841, 837 841, 837 (sh) 859, 844 859, 844

Certain discrepancies between infrared and Raman spectra should be explained as a result of differences in the experimental techniques. Raman spectra were collected from spots with diameter of several microns, while infrared were recorded from much larger areas. Therefore, in the infrared spectra we observed bands representing both crystalline and amorphous phases, while use of the Raman microscope allowed to distinguish the domains consisting almost exclusively from one of the crystalline phases in multi-component samples. The most diluted sample, 160:1, exhibits only one melting temperature, 66ºC and the spectral characteristic of the polymer is typical for crystalline PEO. However, positions of the peaks originating from the salt indicate on the coordination type I, i.e. free anion. It can be concluded that at this doping level the whole amount of salt is dissolved in the amorphous phase and exist solely in the form of solvent separated ionic pairs. Samples with O/Li ratio spanning from 32 to 8 exhibit two melting temperatures, the first of them attributed to crystalline PEO and the 19 ACS Paragon Plus Environment


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second one to the crystalline phase of LiTDI-PEO complex. Increase in the salt content results in the increasing intensities of the peaks typical for coordination type II (2231, 1318 and 995 cm-1), i.e. crystalline complex α, although bands characteristic for I can still be distinguished in the infrared spectra (see Supporting Figure S3). Samples with O/Li ratios ranging from 3 to 16 are also characterized by two melting temperatures, ascribed to the melting of the two different PEO-LiTDI crystalline phases, at 39°C (phase α) and 80-89°C (phase β). Attribution of the phase β is confirmed by the presence of bands at 2259, 2231, 1316, 999 and 993 cm-1 16,17

in spectra of the most concentrated samples, as shown in Tables 3 and 4 and Supporting

Figures 2 and 3. Split of the νCN and δNCN bands for the crystalline phase β indicates the presence of both free and coordinated nitrile groups and the exchange of the coordination type II by the coordination type III. Further rearrangement of the structure occurs with rise in temperature. It was shown previously, that at moderate (O/Li equal 8 – 16) doping level melting of the α-LiTDI-PEO leads to partial re-dissociation of the ionic pairs. Figures 7 and 8 present FTir and Raman spectra of samples with O/Li equal 6, recorded in the 20-120ºC temperature range. With rise in temperature in Raman spectrum of the more diluted sample bands of the complex at 2231 and 995 cm−1 are replaced by peaks at 2226 and 978 cm−1, typical for free anions. Shoulders at 2245 cm−1 and 985 cm−1 visible at elevated temperatures, indicate the presence of more aggregated species in the amorphous phase. In more concentrated samples with O/Li lower than 6 slow heating up to 50ºC results in a decrease of the intensity of the peaks attributed to both crystalline phases. Band attributed to phase α disappear above 50ºC due to the phase melting. Phase β is then partially dissolved in an amorphous phase which results in the decrease of its content. Between 70 and 90ºC the spectral characteristic is changed again: increase of the intensities of the shoulders at 2245 and 988 cm-1 correspond to the formation of aggregate species in a melt. The spectral pattern closely resembles that of 20 ACS Paragon Plus Environment

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(Li(diglyme)TDI)2,16 representing structural type IV, i.e. dimer. Formation of the dimers in a molten system is also supported by the increase of the intensity of the band in the δCF3 range, with maximum at ~680 cm−1, related to coordination types IV. Further increase of the temperature to 120ºC leads to formation of additional shoulders with maxima attributed to free ions, due to the partial re-dissociation of aggregates at elevated temperatures.

Figure 7 FTir spectra of the LITDI-PEO 6:1 in a 40- 90ºC temperature range.

Figure 8 Raman spectra of the LITDI-PEO 6:1 in a 20- 120ºC temperature range.

Chain conformations



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The room temperature infrared spectral characteristics of the polymer matrix corresponds either to the crystalline phase of the PEO (samples with O/Li equal or above 8) or to the crystalline complex (O/Li equal or below 6). In both cases the basic pattern is modified by presence of the bands attributed to the amorphous phase. The number and position of the bands in the 1300- 1200 cm-1 and 1000- 800 cm-1 spectral ranges is related to the population of the torsional sequences adopted by the polymer chain.23 Studies on various LiX-PEO complexes by Frech 24, 25 Staunton4 and Ducasse26 delivered valuable information concerning the influence of the polymer chain conformation on the vibrational spectra. Figure 7 and 8 present FTir and Raman spectra of the 6:1 LiTDI-PEO samples in various temperatures. According to the DSC results, at this salt concentration two crystalline complex phases α and β co-exist with the amorphous phase. Raman spectra were recorded in spots rich in the crystalline phase α, enabling observation of the local changes in the ionic association during melting. It is interesting to note, that although position and number of the peaks in the spectral region of the twisting vibrations both in Raman and in FTir spectra is similar to that of the PEO6LiX complexes, the spectral region of the rocking vibrations is different and possess some spectral features common with PEO3LiX. The spectral pattern of the α complex bears certain resemblance to the PEO6-LiX complexes, due to the presence of the bands at 1304, 1251 and 848 cm-1(see Tables 3 and 4 and Supporting Figure S3 and S4). On the other hand, position of the peaks at 960, 938 and 837 cm-1, attributed to the coupled CO stretching and CH2 rocking vibrations is similar to that found in PEO3LiX. In contrast to the PEO6LiX, in the α-PEO-LiTDI the salt is present in a form of contact ionic not isolated ions, which excludes the possibility of cylinder-like structures formation described for the former. The cation is most likely coordinated by 4 oxygens from a single polymer chain and by an imidazole nitrogen and fluorine of TDI anion. The configuration of the chain may be similar to that described for β-PEO6:LiAsF626 with anions similarly located along the chain, but coupled


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with cations. As the temperature increases the intensity of the bands gradually decrease and above 60ºC arise at ~1280, 1246, 1233, 960, 939, 928, 848 and 797 cm-1, originating from the β-PEO-LiTDI (see Tables 2 and 3 and Supporting Figure 4). Thermal stability

Figure 9 Thermal stability behavior of PEO-LiTDI membranes. 9a- TG traces, 9b- 1st derivative, 9cGram-Schmidt profile, 9d- exemplary FTir spectra of the degradation products

Thermal stability of the electrolytes as well as knowledge about the volatile products of the degradation is of great importance from the point of view of the application in the batteries. The information about the products arising during the pyrolysis achieved by TG–IR technique allows proposing the steps of thermal degradation. Figures 9a-c show the TG and DTG curves and Gram–Schmidt plots. Both samples exhibit two stages of the decomposition. The initial stage starting at 220ºC and completed at approx. 280ºC corresponds to the evaporation of the 23 ACS Paragon Plus Environment


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water absorbed during placing of the sample in the TGA apparatus and decomposition of the polymer matrix alone. The products formed in the second stage, reaching its maximum at 350ºC, depend on the content of the salt in the sample. The FTIR spectra collected for maximum peaks from Gram–Schmidt graph for both samples are shown in Figure 9d. Spectra were recorded at the time of about 95 and 101 min (which corresponds to temperatures of 360 and 400ºC), for samples 3:1 and 8:1, respectively. The more diluted sample, 8:1, decomposes with formation of oligomers of ethylene oxide and emission of CO2 and HF. The pyrolysis of the highly concentrated sample, 3:1, leads to the formation of 1,4-dioxane instead of the oligoethers and also CO2 and HF. The emission of the HCN was not detected under the experiment condition. The total weight loss at 430ºC, which can be considered as the end temperature of the second degradation step, is 56 and 79 wt % for 3:1 and 8:1 samples. At this stage the decomposition of polymer matrix in both samples is completed: presence of the HF in the gas product is an evidence of a partial degradation of the salt. Further heating leads to much slower process of the remaining salt degradation. It has to be stressed out that the reported thermal stability is much higher, than for LiPF6 based electrolytes which are stable up to 110ºC. 27


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Conclusions

Figure 10 The influence of the phase composition on the conductivity of the samples with O/Li equal 20 (red) and 6 (purple). Insert shows DSC traces of the 6:1 sample.

Detailed analysis reveal an interesting correlation between conductivity behavior and phase composition obtained from DSC, XRD and Raman data. As shown in Figure 10, melting behavior of the diluted samples (e.g. Li/O = 20) is reflected in conductivity plot as inflection points, which can be ascribed to melting temperatures of α-LiTDI-PEO and PEO. The more concentrated sample with O/Li equal 6 exhibits an additional inflection point, although the starting composition of the membrane consisted from α-LiTDI-PEO and β-LiTDI-PEO but no PEO phases. α-LiTDI-PEO was not detected in the second heating run in DSC experiment. It can be concluded, that the PEO phase is formed in situ, as a result of the disproportionation 25 ACS Paragon Plus Environment


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following melting of α-LiTDI-PEO. During this process, PEO and more concentrated βLiTDI-PEO appears, which is confirmed by FTir experiments. Moreover, the content of ionic pairs decreases in favor of free ions dissolved in PEO phase and formation of aggregates in βLiTDI-PEO. The mechanism of the salt disproportionation is an analogue to those observed in LiTDI-glyme systems.16, 17 The conductivity behavior of the studied electrolytes observed during storage, i.e. the initial increase in the conductivity is a clear evidence that there must be some other mechanism that increases the conductivity. Increase in the degree of crystallinity should generally reduce conductivity. However, we may assume that slow crystallization process is related to α phase disproportionation and subsequent formation of more concentrated β and PEO phases. This effect is reflected in the conductivity, as show in Figure 10. The formation of the crystalline PEO at elevated temperatures (40-60oC) in samples with high salt concentration is not obvious. The explanation of such phase behavior was possible thanks to the thorough structural studies supported by theoretical calculations. In this paper, we present application of new generation of lithium salt - LiTDI in polymer electrolytes. It shows that the specific nature of the TDI anion results in the possibility of the creation of two salt-rich crystalline phases – with high and low melting points. Electrolytes combining high conductivity with mechanical stability (up to 60ºC) can be obtained in a wide (5-15) O/Li ratio. Plasticizing effect of the salt, indicated by the formation of low-melting phase, leads to a significant lowering of the operating electrolyte windows from 60ºC for typical polymer electrolytes to 30ºC for SPE containing LiTDI. Theoretical analysis, supported by spectroscopic data allowed to propose their structure. Moreover, they are free of toxic, moisture sensitive salt and liquid solvents. They have also high thermal stability, up to 200ºC. All of this makes them excellent for applications where safety is of central importance. 26 ACS Paragon Plus Environment

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Supporting Information XRD data for samples PEO-LiTDI samples. XRD patterns of the PEO-LiTDI before and after thermal treatment. Raman spectra of PEO-LiTDI with 3 to 160 O/Li ratio. FTir spectra of PEO-LiTDI with 3 to 32 O/Li ratio. FTir spectra of PEO-LiTDI membranes with 4 and 8 O/Li ratios recorded at various temperature (melting regime). Optimized geometries of αPEO-LiTDI and β-PEO-LiTDI in xyz format.

Acknowledgements This work is part of the Warsaw University of Technology and EuroLiion Project (“High energy density Li-ion cells for traction”) supported by European Union, grant agreement no 265368. All calculations have been carried out at the Wroclaw Centre for Networking and Supercomputing (http://www.wcss.pl), grant No. 346. The authors would like to thank Dr Johan Scheers for FT Raman measurements.



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Sources, 2011, 196, 1386-1391 (13) Niedzicki, L., Kasprzyk, M., Kuziak, K., Żukowska, G. Z., Armand, M., Bukowska, M., Marcinek, M., Szczeciński, P., Wieczorek, W., Modern Generation of Polymer Electrolytes Based on Lithium Conductive Imidazole Salts, J. Power Sources, 2009, 192, 612-617 (14) Polu, A. R., Rhee, H-W., The effects of LiTDI salt and POSS-PEG (n = 4) hybrid nanoparticles on crystallinity and ionic conductivity of PEO based solid polymer electrolytes,

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Figure 1 Phase diagram of the PEO-LiTDI system. ■-Tt PEO, ▽-Tt α-LiTDI-PEO, □- Tt β-LiTDI-PEO, ●-Tg Phase diagram of the PEO-LiTDI 41x43mm (600 x 600 DPI)

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Coordination modes of the TDI anion found in PEO-LiTDI electrolytes17 : I - "free" anions; II - contact pairs; III - chain aggregates; IV – dimers Figure 2 Coordination modes of 66x65mm (300 x 300 DPI)



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Figure 3 Optimized structures of α-PEO-LiTDI (3a) and β-PEO-LiTDI (3b); M06-2X/6-31G(d). Figure 3 Optimized structures 112x74mm (300 x 300 DPI)


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Figure 4 Conductivity as a function of temperature for LiTDI-PEO with Li/O ratio varying from 4 to 20 Figure 4 Conductivity as a fun 119x92mm (300 x 300 DPI)



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Figure 5 Conductivity of the O/Li = 4 (grey) and 20 (red) samples during relaxation at 30 and 20ºC; insert shows magnification of conductivity curve after decrease of temperature to 20 ºC Figure 5 Conductivity of the O 99x77mm (300 x 300 DPI)


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Figure 6 XRD pattern of for LiTDI-PEO with Li/O ratio varying from 4 to 20.

119x78mm (300 x 300 DPI)



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Figure 7 FTir spectra of the LITDI-PEO 6:1 in a 40- 90ºC temperature range. 119x76mm (300 x 300 DPI)


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Figure 8 Raman spectra of the LITDI-PEO 6:1 in a 20- 120ºC temperature range. 199x67mm (300 x 300 DPI)



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Figure 9 Thermal stability behavior of PEO-LiTDI membranes. 9a- TG traces, 9b- 1st derivative, 9c- GramSchmidt profile, 9d- exemplary FTir spectra of the degradation products 199x132mm (300 x 300 DPI)


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Figure 10 The influence of the phase composition on the conductivity of the samples with O/Li equal 20 (red) and 6 (purple). Insert shows DSC traces of the 6:1 sample. 80x58mm (300 x 300 DPI)



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Graphical TOC For Table of Contents Only 39x41mm (300 x 300 DPI)


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