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Conformational Dynamics in an Organic Ionic Plastic Crystal Liyu Jin, Kate M. Nairn, Chris D. Ling, Haijin Zhu, Luke A. O'Dell, Jiaye Li, Fangfang Chen, Adriano F. Pavan, Louis A. Madsen, Patrick C. Howlett, Douglas R. MacFarlane, Maria Forsyth, and Jennifer M. Pringle J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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The Journal of Physical Chemistry

Conformational Dynamics in an Organic Ionic Plastic Crystal Liyu Jin,∗,†,‡,△ Kate M. Nairn,∗,¶ Chris D. Ling,§ Haijin Zhu,k,‡ Luke A. O’Dell,k Jiaye Li,⊥,∇ Fangfang Chen,†,‡ Adriano F. Pavan,§ Louis A. Madsen,# Patrick C. Howlett,†,‡ Douglas R. MacFarlane,@,‡ Maria Forsyth,†,‡ and Jennifer M. Pringle∗,†,‡ †Institute for Frontier Materials, Deakin University, Burwood, VIC 3125, Australia ‡ARC Centre of Excellence for Electromaterials Science, Australia ¶Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia §School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia kInstitute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia ⊥School of Chemistry, Monash University, Clayton, VIC 3800, Australia. #Department of Chemistry and Macromolecules Innovation Institute, Virginia Tech, Blacksburg, VA 24061, USA @School of Chemistry, Monash University, Clayton, VIC 3800, Australia △Present address: Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK ∇Present address: School of Chemistry, University of Melbourne, 30 Flemington Road, Parkville, Victoria 3010, Australia. E-mail: [email protected]; [email protected]; [email protected]

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Abstract Understanding the short-range molecular motions of organic ionic plastic crystals is critical for the application of these materials as solid-state electrolytes in electrochemical devices such as lithium batteries. However, the theory of short-range-motions was originally developed for simple molecular plastic crystals and does not take account of strong inter-ionic interactions that are present in organic ionic plastic crystals. Here we report a fundamental investigation of the dynamic behaviour of an archetypal example triethyl(methyl)phosphonium bis(fluorosulfonyl)amide ([P1222][FSI]) through calorimetry, impedance spectroscopy, synchrotron X-ray diffraction, solid-state NMR and Raman spectroscopies. For the first time, we show the presence of conformational dynamics in the solid state for the FSI anion. We relate the dynamics to a unique second-order displacive phase transition of [P1222][FSI]. This detailed analysis suggests a new disorder mechanism involving cooperative motion between the cation and FSI anion in the plastic crystal due to strong inter-ionic interactions.

Introduction Replacing the current flammable liquid electrolytes by solid ion conductors, is believed to be an important aspect for the development of safe lithium batteries, 1–3 which are urgently needed for large-scale applications such as electric vehicles and renewable energy storage. In practice, however, solid electrolytes face at least two major problems: 1) high internal resistance (e.g., as commonly associated with polymeric electrolytes) and 2) poor solid-solid contact between the electrodes and electrolyte during volume changes upon charging and discharging (usually associated with inorganic ion conductors). Organic ionic plastic crystal (OIPC) ion conductors have the potential to address both of these problems simultaneously, thanks to their relatively high ionic conductivity (especially with the addition of a lithium salt) in combination with their mechanical plasticity. Chemically, OIPCs may be considered as solid state analogues of ionic liquids (ILs). 4 Therefore, OIPCs are generally considered 2


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safe in battery application, due to their non-volatility and negligible vapour pressure even at temperatures well above their melting points. OIPCs are composed entirely of cations and anions located on long-range ordered crystalline lattices. However, unlike traditional crystalline materials, either the cations or the anions, or both, exhibit rotational or reorientational short-range motions within their crystalline lattices. It is this short-range disorder that fundamentally leads to enhanced translational motion for individual ions as well as to lattice defects that allow easily activated plastic deformation (hence ‘plastic’ in their description 5 ). Therefore, understanding of the short-range behaviour, particularly short-range rotation/reorientation motions, serves as a fundamental foundation for the development of OIPC ion conductors. In the last decade, research into the development of new OIPCs as practical solid-state ion conductors has been increasing and making significant progress, 6–16 as well as beginning to attract attention from industry. 17 Recent development of new cations and anions has benefited from research into ionic liquids for electrochemical applications. 18 One example is the bis(fluorosulfonyl)imide anion, commonly denoted as FSA or FSI, which is a lighter and cheaper version of the very popular bis(trifluoromethanesulfonyl)imide anion, denoted as NTf2, TFSI or TFSA. FSI-based ILs 4,19–23 have already exhibited advantageously low viscosity, high conductivity and high solubility for lithium and sodium salts: all effects of the lower molecular weight and greater freedom in conformational isomerism of the anion compared with NTf2. 24,25 Some FSI-based ILs have been used in prototype batteries with Li metal anodes (coupled with LiNi1/3Mn1/3Co1/3O2, 26 LiFePO4 27 or LiCoO2 28 cathodes) that demonstrated fast discharge capability because the FSI anion forms excellent interphases between the electrodes and electrolytes. 21,29,30 Following the success of these FSI-based ILs, the applicability of FSI-based OIPCs in solid-state LiFePO4 | Li metal cells was recently demonstrated, 10 showing a capacity at room temperature of 120 mAh g-1 at 1C discharge rate. Nevertheless, the fundamental study of these relatively new materials is still in its in-

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fancy compared with our relatively comprehensive knowledge of polymeric or inorganic ion conductors. Our limited knowledge about the nature of the intrinsic short-range motions of OIPCs is derived from the understanding of molecular plastic crystals (MPCs), which generally possess centrosymmetric molecular shapes (e.g., cyclohexane and adamantane) leading to lowered activation energies for isotropic motions of molecules upon ordered lattices. 5 For example, we know that centrosymmetric anions, such as PF6 and BF4, 9,31,32 are able to isotropically tumble with relatively low energy barriers, fitting well with the conventional mechanism. However, we do not know the nature of molecular motions for the elongated ions such as NTf2 and FSI anions in crystals that have shown plastic behaviour. Studies of the NTf2 anion do not find evidence for conformational dynamics in the solid state (NTf2 has anti and gauche conformers in the liquid state); this is probably because the potential energy landscape of the molecular structure is dictated by the strong intermolecular forces within the ionic lattice. 24,33 The FSI anion also exhibits isomerism in ILs 34 but it is not yet known if the conformational flexibility of the FSI ion is preserved in an OIPC. In addition, to the best of our knowledge, the important effect of strong inter-ionic interactions — absent from MPCs — on short-range motions in OIPCs has not yet been studied. In this context, we have thoroughly characterised triethyl(methyl)-phosphonium bis(fluorosulfonyl)imide or [P1222][FSI] — a promising ion conductor with a large and accessible temperature window of plasticity and conductivity. Here we compile results obtained from differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), synchrotron Xray diffraction (SXRD), solid-state nuclear magnetic resonance (NMR) and Raman spectroscopies. As the first three analyses give an overall picture of the ‘plasticity’, while the latter two techniques contrast the dynamic and conformational behaviour between the cations and the anions, we can gain deep insights for the first-time into the conformational dynamics of the FSI anion in solid state. From this we are able to propose a mechanism of ‘collaborative motion’ between the cations and the anions.

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Methods [P1222][FSI] was synthesised following the previously published procedure. 8 The [P1222] phosphonium tosylate salt was purchased from CYTEC (Canada). KFSI was purchased from Fluolyte (China). All the chemicals were used as received. The purity was checked and confirmed with published DSC trace and solution 1 H NMR spectrum. 8 The DSC experiments were performed with a TA-Q100 instrument. All samples were weighed and sealed in aluminium crucibles under a N2 atmosphere. The samples were first cooled from room temperature to -80 ◦ C, and subsequently heated to 80 ◦ C at a rate of 2 ◦ C min-1 . The first run was performed to remove the effects of thermal history of the samples. A second repeat cooling and heating cycle was carried out and used to obtain the phase transition temperatures. The conductivity measurement was carried out by using platinum (Pt) wire electrodes. The two Pt wires were dipped into the plastic crystal melts and the cell constant was calibrated with 0.01 M KCl solution at 25 ◦ C. All samples were packed and sealed under a N2 atmosphere. Ionic conductivity data were measured by impedance spectroscopy using a Solartron SI1260 impedance/gain phase analyser, which was connected to a Solartron 1296 dielectric interface, driven by Solartron impedance measurement software. Data was collected over a frequency range from 10 MHz to 1 Hz using a signal amplitude of 0.1 V and at 5 ◦ C intervals. The cell temperature was controlled by a Eurotherm 2204, within 0.1 ◦ C tolerance. The ramp rate was 0.33 ◦ C per minute, and the samples were held at each temperature for 5 min to equilibrate prior to measurement. Synchrotron X-ray diffraction (S-XRD) patterns of polycrystalline powder samples were collected on the Powder Diffraction beamline at the Australian Synchrotron at a wavelength of λ = 0.999 522(2) Å (calibrated against a LaB6 standard). Samples were loaded into sealed 0.3 mm diameter glass capillaries for data collection and cooled using a nitrogen cryostream. Unit cell indexing and Le Bail fits to S-XRD data were carried out using GSAS program 35 with the EXPGUI front-end. 36 Unit cell indexing and fitting unit cell parameter β by Landau 5



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theory are included in the Supporting Information. Solid-state NMR measurements were performed using fine powders, which had been packed and sealed under N2 in a 4 mm NMR rotor.

1

H and

31

P NMR line width mea-

surements were performed on a Bruker Avance III NMR spectrometer operating at Larmor frequencies of 500.06 and 202.43 MHz, for 1 H and 31 P respectively. 1 H spectra were acquired using a single pulse excitation sequence. 1 H spectra were acquired using π/2 a pulse length of 3 µs, recycle delays of 10 s, and 8 scans. 1 H decoupled

31

P spectra were obtained using a

single pulse excitation pulse sequence with continuous wave 1 H decoupling. The

31

P pulse

length was 3.2 µs, and 1 H decoupling strength was 78 kHz. The recycle delay was 4 s, and 8 scans were accumulated. The

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F spectra were acquired using a spin-echo pulse sequence

with a π/2 pulse length of 2.5 µs, an interval between π/2 and π pulses of 50 µs, a recycle delay of 10 s, and 16 scans. Details on modelling

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F CSA powder pattern are included in

the Supporting Information. Raman spectra of the powder samples were obtained using a Renishaw Invia Raman microscope system equipped with a Coherent Inc Innova I70c spectrum laser with a wavelength of 514 nm. For each acquisition, a laser power of 5 mW, an exposure time of 10 s, and 5 accumulations were used; the spectral resolution was 1.5 cm-1 . The wavelength was calibrated using Si at a shift of 520.5 cm-1 . The spectra were normalised by the most intense peak at about 600 cm-1 . For variable temperature experiments, a Linkam FTIR-600 cooling stage was employed, to control the temperature to within ±0.5 ◦ C. Before each measurement, the sample was kept at the selected temperature for 5 min to ensure thermal equilibrium. The operating software used was Renishaw WiRE 2.0.

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which the specific heat drops by 0.33(1) J K-1 g-1 . The ionic conductivity of the material as a function of temperature is also shown in Figure1. Phase I exhibits two different Arrhenius activation energies (Ea), for Phase I-a and I-b. In Phase I-a, it is 110(3) kJ mol-1 , which is significantly higher than in Phase I-b (36(1) kJ mol-1 ). Note that if the sample is slowly cooled/solidified from the molten OIPC before the test, the conductivity can show anisotropy (with respect to the direction of the applied field, with conductivity variation within an order of magnitude 37 ). However, conductivity trend with temperature remains the same. Figure 2 shows synchrotron X-ray diffraction (SXRD) patterns collected over a range of temperatures. Figure 2a presents the patterns between 6 to 20 degrees two-theta. Shortrange molecular disorder results in no significant Bragg peaks at higher angles (i.e., the Debye-Waller effect), which is typical for plastic crystals. 38 In Figure 2b, two particular temperature regions are closely examined: -60 to -55 ◦ C (across the transition from Phase II to I) and from -40 to -30 ◦ C (transition from Phase I-a to I-b). An abrupt structural transition can be observed between -60 and -58 ◦ C: one distinct set of peaks disappears and another appears, while the pattern at -59 ◦ C shows both set of peaks simultaneously. This clearly corresponds to the Phase II to I transition revealed by DSC and suggests a major reconstructive change in crystal structure between Phase II and I. On further heating, a continuous evolution in diffraction peaks is observed (accompanied by temperature modulated birefringence as shown in Figure S3 and merging of diffraction spots in single-crystal X-ray diffraction patterns in Figure S4), with peaks merging in the manner typical of a continuous and displacive phase transition. 39 Microscopic evidence of this displacive phase transition is recorded and included as Web Enhanced Object. Displacive phase transitions are also known as diffusionless phase transitions, and do not involve transport of the atomic or molecular centres of mass during nucleation and growth. The martensitic phase transition 40 and the biaxial nematic liquid crystal phase transition 41,42 are examples of this type of phase transition. Further indexing of the diffraction patterns was carried out using the Le Bail method. This reveals that a likely triclinic unit cell transformed into a C-centred monoclinic one at

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rigid phase (Phase II) to a plastic phase (Phase I) featuring a significantly higher level of molecular mobility. The continuous reduction in line width over Phase I-a indicates a gradual evolution of the molecular mobility of the cations. The asymmetric peaks of the

31

P NMR spectra exhibit typical chemical shift anisotropy

(CSA). The size of the CSA reduces as the temperature increases. This suggests that at least one additional degree of freedom in the motion is activated at -55 ◦ C and intensifies through heating into Phase I-a. The shape of the

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P CSA pattern suggests the axial rotation of the

cation in this phase, with the axis presumably parallel to the phosphorus-methyl bond. The width of the CSA pattern also seems to decrease continuously across Phase I-a. This would not be caused merely by faster rotation, but rather indicates a change in the C-P-C bond angles, potentially due to changes in the free volume around the cation. As expected, the FSI anion also displays an abrupt increase in motion upon transition from Phase II to I, as evidenced by the multiple overlapping CSA patterns at -60 and -70 ◦ C suddenly merging into a single clear CSA pattern at -55 ◦ C. Subsequently, this CSA pattern gradually transforms from a shape with a CSA symmetry parameter (η) 43 of about 1 at 55 ◦ C to an almost axially symmetric CSA shape with η close to 0 at -30 ◦ C. However, further temperature increases into Phase I-b cause only negligible changes in the CSA parameters. This 19 F CSA evolution implies that the anion undergoes anisotropic motion about a single axis (see Methods), which is maintained throughout Phase I and exhibits a temperaturedependent rate constant within Phase I-a. One very likely model of anisotropic motion is the rotation of the sulfonyl (FSO− 2 ) about the N-S bond (Figure 4 inset), which has previously been observed in FSI-based ionic liquids. 24,25,34,44 The

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F CSA patterns were computation-

ally modelled with various rates based on a 3-site jump process (i.e., 120 ◦ rotation) of FSO− 2 groups about N-S bonds. The observed and simulated patterns, shown in red, match well, supporting the validity of the dynamic model and enabling the jump rate at each temperature to be determined. An Arrhenius plot of these rates as a function of temperature (Figure S5) allowed the activation energy of this rotation be determined as 80.6(2) kJ mol-1 .

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It is worth noting that we were not able to model the spectra with 2-site jump (i.e., 180 ◦ flip) process. Meanwhile, models of more-than-three-site jumps of FSO− 2 fit equally well to this

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F CSA evolution (the corresponding jump rate varies). These strongly suggest the

presence of multi-fold (or possibly smooth rotational) motion of the FSO− 2 group of the anion. Based on this suggestion we speculate that this rotation may promote the interconversion between the two known conformational configurations (see below) in Phase I-a. To investigate this, we used Raman spectroscopy.

Conformational Transformation Two conformers of FSI, cis (C1 symmetry) and trans (C2 symmetry) (Figure 5a) observed in the liquid state. 24,25,34,44 The difference between the conformers arises from the angles of the S-F bonds with respect to the plane in which the S-N-S ‘backbone’ resides. In the cis conformers, the two angles are both 72 ◦ while in the trans conformer, one is 72 ◦ and the other is -72 ◦ (Figure 5a). They can potentially interconvert from one to the other with the internal rotation of the FSO− 2 groups as suggested by

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F NMR.

Figure 5b shows the Raman spectra from -70 ◦ C to 40 ◦ C within the range of 200 to 400 cm-1 (the full spectra are included in Figure S6). According to Fujii et al., 34 peaks at 297, 318 and 353 cm-1 are ascribed to the cis conformer; corresponding peaks at 290, 325 and 358 cm-1 are ascribed to the trans conformer. Figure 6a shows the integrated areas of the normalised peaks at 318 cm-1 (cis) and 325 cm-1 (trans), plotted as a function of temperature. In general, peaks corresponding to the cis conformer (in red) grow monotonically in intensity with temperature, while those from the trans (in blue) decrease. Therefore, the trends indicate that the population of trans conformer is decreasing while proportion of cis conformer is increasing. The trans conformers are (dynamically) interconverting to cis from -60 ◦ C until melting. From the relative Raman intensities of the conformers, the enthalpy change for the interconversion from trans to cis can be calculated using the Van’t Hoff equation coupled with 13




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the Beer-Lambert law, 45 as shown in Equation 1:

−R ln Ic1 (ν)/Ic2 (ν) = ∆H ◦ /T − ∆S ◦ − R ln εc1 (ν)/εc2 (ν)

(1)

where I(ν) and ε(ν) denote the integrated intensity and Raman scattering coefficient of a single peak at ν cm-1 , respectively, and I(ν) = ε(ν) × m, where m is the molarity of a conformer. R is the universal gas constant. ∆H ◦ is obtained from linear regression of the Van’t Hoff plot (Figure 6b). The ∆H ◦ values between the cis and trans conformers within Phase I-a and Phase I-b were thereby determined to be 10(2) and 6(1) kJ mol-1 , respectively. The ∆H ◦ value of 6 is consistent with those observed in ionic liquids 25,34 and predicted from ab initio calculation. 24 This suggests that the cis and trans conformers within Phase I-b are adopting their most energetically favourable configurations, as they possess energy differences similar to those in liquid and ideal gas states. However, the significantly larger ∆H ◦ value of 10 kJ mol-1 within Phase I-a, which is nonetheless significantly lower than the rotational activation energy determined above from NMR measurements, seems to suggest a deviation from their energetically preferred cis and trans configurations (labeled as cis* and trans* in Figure 6c for a simple illustration of potential energy states). This deviation presumably relates to the increased conformational constraints on the FSI anion due to intermolecular (inter-ionic) interactions in the Phase I-a region. The strong Raman peak at 336 cm-1 in Figure 5 is most likely from a cation conformer (the molecular configuration is not known), as it is absent from the reported Raman spectrum of [C3mpyr][FSI]. 34 The intensity of the peak grows with temperature: from no peak below 60 ◦ C, increasing in intensity up to -30 ◦ C and then levelling off (Figure 6a). This implies that during the continuous transition this cation conformer gains in relative population, consistent with the decreasing CSA observed in the

31

P NMR analysis. Above -30 ◦ C, however, as the

anisotropic motions become very close to isotropic, the conformational interconversion also

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ceases to have net effect on relative populations of the cation conformers.

‘Cooperative Motions’ Compiling the results above highlights three noteworthy features of Phase I-a: 1) the specific heat is significantly higher than that of Phase I-b; 2) in terms of structure, the monoclinic unit cell gradually transforms into orthorhombic, and this transition is displacive and second-order, and 3) on the molecular level, FSI anions are trapped in non-ideal, or strained, cis* and trans* configurations with wider energy gaps than the equilibrium configurations. Meanwhile, the rotation of sulfonyl groups on the FSI anion is activated. Facilitated by this sulfonyl group rotation, the trans* conformer increasingly converts to the cis*. The cations also adopt an anisotropic motion, consisting of axial rotation in combination with a gradual change in conformation as the temperature increases. Thus, during Phase I-a, the cations and the anions appear to both exhibit anisotropic dynamics. We speculate that these anisotropic rotations are the results of significant mutual steric interactions. This strong coupling can also be viewed as cooperative motions between cations and anions. These collective and restricted motions explain the observed greater specific heat, as more intermolecular ‘bonds’ are disrupted and motional degrees of freedom are activated. We thus conclude that the rather large activation energies of ionic transport (110(3) kJ mol-1 ) and FSI rotation (80.6(2) kJ mol-1 ) are signs of limited dynamics in Phase I-a of [P1222][FSI] that can be accounted for by the model of cooperative motions. Indeed, these interactions are so strong that they facilitate the displacive transition, which requires strong coupling forces between local ordering processes. 39 Phase I-b, on the other hand, shows conventional plastic behaviour with relatively free rotation of the ions and much lower energy barrier for translational motions, both of which result from weaker inter-ionic interactions.

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Conclusions In conclusion, this work has elucidated the ion dynamics in the plastic crystal [P1222][FSI]. For the first time, we observed and examined the conformational dynamics in the solid state for FSI anion due to the rotation of its sulfonyl groups. An unusual second-order, displacive structural transition is strongly associated with the escalation in molecular motions and the conformational dynamics of both the cation and the anion. We suggest a physical model of ‘cooperative motions’ between the ions due to the strong inter-ionic interactions.

Associated Content • Supporting Information. Extended characterisation results, including DSC, optical microscopic images, single-crystal diffraction patterns and Raman data (PDF). • Web Enhanced. A video (AVI) file showing evidence of the displacive transition. The video was taken by Nikon D600 via a cross-polarised microscope, the time frame was sped up by about 26 times.

Acknowledgement L. Jin gratefully acknowledges Mr. Finlay Shanks and Dr. Craig Forsyth at the School of Chemistry, Monash University, for instrumental help with Raman spectroscopy and crystallography, respectively. The authors also thank Prof. Wayne Cook, Prof. Michel Armand, Mr. Zaiquan Xu and Dr. Anthony Hollenkamp for constructive discussions. This research was undertaken on the powder diffraction beamline at the Australian Synchrotron with assistance from beamline scientist Dr. Justin Kimpton. The authors also acknowledge funding from the Australian Research Council (ARC) through its Centre of Excellence program and financial support from Monash University and the ARC for L. Jin (McNeil PG Research Scholarship and IPRS) and for M. Forsyth and D. R. MacFarlane (Australian Laureate Fel18


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lows), respectively. This work was supported in part (L. A. Madsen) by the US National Science Foundation under award number DMR 1057764.

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Conformational Dynamics in an Organic Ionic Plastic Crystal - The

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