Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 3904−3909
pubs.acs.org/JPCL
Structure and Ion Dynamics in Imidazolium-Based Protic Organic Ionic Plastic Crystals Haijin Zhu,*,†,‡ Xiaoen Wang,†,‡ R. Vijayaraghava,§ Yundong Zhou,† Douglas R. MacFarlane,§ and Maria Forsyth*,†,‡ †
Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia ARC Centre of Excellence for Electromaterials Science, Deakin University, 221 Burwood Highway, Burwood, Victoria 3125, Australia § School of Chemistry, Monash University, Clayton, Victoria 3169, Australia Downloaded via NEW MEXICO STATE UNIV on July 3, 2018 at 16:54:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: A fundamental understanding of the structure and dynamics of organic ionic plastic crystal (OIPC) materials allows for a more rational design of molecular chemistry toward improved mechanical and electrochemical performances. This Letter investigates the solid-state structure and ion dynamics of two imidazolium-based protic organic ionic plastic crystals as well as the ion-transport properties in both compounds. A combination of DSC, conductivity, NMR, and synchrotron X-ray studies revealed that a subtle change in cation chemistry results in substantial differences in the thermal phase behavior, crystalline structures, as well as the ion conduction mechanisms in the protic plastic crystal compounds. Whereas most of the research nowadays has been focused on the optimization of chemistry of cations and anions, this work highlights the importance of microstructures on the ion transport rate and pathways of the OIPC materials.
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elevated temperatures and anhydrous conditions to avoid the volatility issue associated with water and many other organic solvents and to obtain a good proton conductivity under low or zero humidification.10,18 These works have demonstrated that the POIPCs have great potential as next-generation solidstate electrolytes that may be applied for the medium temperature region of 120−180 °C.10,19,20 Despite significant progress in making new POIPCs and applying them in various devices, the fundamental understanding of these relatively new materials is still in its infancy. For example, from studies of molecular plastic crystals (e.g., cyclohexane and adamantine), we know that the plasticity is originated from the centrosymmetric shape of the molecules, which allows fast isotropic motion of molecules within the ordered lattice.21 Similarly, many of the OIPCs consist of centrosymmetric anions, such as PF6 and BF4, which are able to tumble isotopically at relatively low temperature without breaking the lattice structure.22,23 However, what has not been well understood in OIPCs is the nature of molecular motions of ions with lower molecular symmetry such as FSI and NTf2 that have shown plastic crystal behaviors. In particular, for those that have multiple solid phases, the molecular dynamics and ion-transport properties of each phase are different. It is crucial to understand molecular-level motions and ion transport properties in each solid phase and how these
lastic crystals are a class of crystalline materials that are composed of weakly associated molecules or ions that possess certain orientational or conformational degrees of freedom.1 Similar to liquid crystals, plastic crystal can be considered as an intermediate state between the completely ordered crystalline states and liquids. A unique feature of plastic crystals is that they usually exhibit one or more so-called “rotator phases” where the molecules/ions maintain a longrange ordered crystalline lattice structure and rotational disorder simultaneously.2,3 This rotational disorder is highly desirable for electrolyte applications because (i) it supports the generation of defects within the lattice, which (ii) predisposes these materials to slip-plane deformation under stress and therefore enables a good contact between electrolytes and electrodes, and (iii) leads to enhanced ion transport for not only the matrix ions4 but also the doped target ions such as Li+,5 H+,6,7 and Na+.8 As a result, these materials are rapidly gaining importance and attention in recent years and have found applications in a wide range of electrochemical devices such as lithium-ion batteries, biosensors, fuel cells, and redoxflow batteries, and so on.9−11 Progressing from the conventional molecular plastic crystals such as succinonitrile12 and cyclohexane,13 the last decades have also seen a rapid increase in the number of ionic plastic crystals, particularly organic ionic plastic crystals (OIPCs)1,4 and protic organic ionic plastic crystals (POIPCs).14,15 The POIPCs are a subclass of the plastic crystal family that comprise one or more labile protons in the cation or anion and present proton conductivity.16,17 Several studies have explored the use of POIPCs as proton-conducting electrolytes at © XXXX American Chemical Society
Received: May 12, 2018 Accepted: June 28, 2018 Published: June 28, 2018 3904
DOI: 10.1021/acs.jpclett.8b01500 J. Phys. Chem. Lett. 2018, 9, 3904−3909
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) Molecular structure of the two protic OIPCs: imidazolium triflate (H2IM) and 1-methyl-imidazolium triflate (HMIM). Nuclei sites are labeled numerically in the Figure. (b) DSC heating and cooling thermograms of H2IM and HMIM (second heating scans). The number on the top of each peak indicates the corresponding enthalpy of transition in J/g. The solid phases of each sample have been labeled as I, II, III, and so on. (c) Arrhenius plot of the ionic conductivity of H2IM and HMIM. The dots represent the experimental data and the solid lines represent the best fits by Arrhenius equation.
crystal phases”. Using this empirical criterion, both H2IM and HMIM can be classified as typical organic plastic crystal materials. Comparing the cation structures, one observes that replacing one of the labile proton in H2IM with a covalently bonded methyl group will have two direct consequences to the molecule: (1) the hydrogen bonding density will reduce by half and (2) the axial (mirror) symmetry of the imidazolium cation will be broken. These molecular level modifications then lead to a significantly reduced melting entropy and melting temperature of the material, which is a strong indication of an increase in the degree of rotational and reorientational freedom in the plastic crystal phase due to decreased hydrogen bonding. The solid-state ionic conductivity of POIPC materials is closely related to their molecular-level rotational and translational motions. Figure 1c shows the Arrhenius plots of the ionic conductivity of both samples. It is interesting to note that the H2IM sample shows a curved behavior with an inflection point at ∼40 °C, which corresponds to the major solid−solid transition temperature at ∼34 °C in the thermogram. Various previous studies have shown that the ionic conductivity of organic plastic crystal materials is often coupled to their thermal phase transitions.10,22,24 Each thermal transition may correspond to a (lattice) structure rearrangement, the onset of specific molecular motions, or both. Using this rationale, the curved behavior suggests that the ionic conductivity of H2IM is coupled to the molecular motions in the bulk plastic crystal phase, although we cannot discriminate on the basis of this data between cation or anion mobility. This will be further explored by nuclei-site-specific solid-state NMR techniques in the following discussion. On the contrary, the conductivity of the HMIM sample follows a typical Arrhenius behavior, despite the major solid transition at 86 °C observed in the DSC. This result is consistent with the recent work of Ponomareva and coworkers,25 which shows Arrhenius behavior
properties are related to the molecular chemistry and conformational dynamics of both cation and anion, as this knowledge will allow us to design better POIPC materials with lower enthalpy of fusion, improved ion dynamics, and a superior ion-transport properties. In this context, here we have systematically compared the structure and dynamics of two POIPCs with the same triflate anion and similar cation chemistry, imidazolium triflate, and 1-methyl-imidazolium triflate. In the following discussion, the two samples will be referred to as H2IM and HMIM, respectively. The molecular structures are shown in Figure 1a. Both cations are small and highly symmetrical, with the only difference being the substitution of one labile proton in the imidazolium ring with a methyl group, thereby breaking the axial symmetry of the cation. We found that this subtle change in cation chemistry results in substantial differences in the crystalline structures, proton transport property, cation and anion dynamics, as well as the activation energy for charge transfer. The DSC (second) heating and cooling curves in Figure 1b show completely different thermal phase behaviors for both materials. H2IM exhibits three solid−solid phase transitions, with one strong transition at 34 °C and two weak transitions at 104 and 163 °C, respectively. It is worth mentioning that the purities of both samples have been confirmed by solution 1H NMR (Figure S1); this rules out the possible contributions from impurities. The melting temperature and enthalpy of fusion of H2IM are 193 °C and 39.6 J/g, respectively, corresponding to an entropy of fusion of 18.5 J/(mol·K), whereas the HMIM has a lower melting temperature of 112 °C and a lower enthalpy of 16.2 J/g, corresponding to an entropy of 9.7 J/(mol·K). In the 1960s, Timmermans21 observed that materials that have fusion entropies
