Structure and Transport Properties of a Plastic Crystal Ion Conductor: Diethyl(methyl)(isobutyl)phosphonium Hexafluorophosphate Liyu Jin, †,§ Kate M. Nairn, †,§ Craig M. Forsyth, ‡ Aaron J. Seeber, ∥ Douglas R. MacFarlane, ‡,§ Patrick C. Howlett, §,⊥ Maria Forsyth, §,⊥ and Jennifer M. Pringle* ,†,‡,§ † Department of Materials Engineering, and ‡ School of Chemistry, Monash University, Clayton, Victoria 3800, Australia § ARC Centre of Excellence for Electromaterials Science ∥ CSIRO Process Science & Engineering, Private Bag 33, Clayton South, Victoria 3169, Australia ⊥ Institute for Frontier Materials, Deakin University, Burwood Campus, Burwood, Victoria 3125, Australia * S Supporting Information ABSTRACT: Understanding the ion transport behavior of organic ionic plastic crystals (OIPCs) is crucial for their potential application as solid electrolytes in various electrochemical devices such as lithium batteries. In the present work, the ion transport mechanism is elucidated by analyzing experimental data (single- crystal XRD, multinuclear solid-state NMR, DSC, ionic conductivity, and SEM) as well as the theoretical simulations (second moment-based solid static NMR line width simulations) for the OIPC diethyl(methyl)(isobutyl)phosphonium hexa- fluorophosphate ([P 1,2,2,4 ][PF 6 ]). This material displays rich phase behavior and advantageous ionic conductivities, with three solid−solid phase transitions and a highly “plastic” and conductive final solid phase in which the conductivity reaches 10 −3 S cm −1 . The crystal structure shows unique channel-like packing of the cations, which may allow the anions to diffuse more easily than the cations at lower temperatures. The strongly phase-dependent static NMR line widths of the 1 H, 19 F, and 31 P nuclei in this material have been well simulated by different levels of molecular motions in different phases. Thus, drawing together of the analytical and computational techniques has allowed the construction of a transport mechanism for [P 1,2,2,4 ][PF 6 ]. It is also anticipated that utilization of these techniques will allow a more detailed understanding of the transport mechanisms of other plastic crystal electrolyte materials. 1. INTRODUCTION Plastic crystalline fast-ion conductors represent a unique family of materials with demonstrated merits as solid-state electrolytes for lithium rechargeable batteries, 1−11 fuel cells, 12−16 and dye- sensitized solar cells (DSSC). 17−19 Ionic compounds in which plastic phases are observed, termed “organic ionic plastic crystals” (OIPCs), are a recently developed subset of this family that offer important properties as electrolytes as a result of their nonflammability and negligible vapor pressure, unlike molecular plastic crystals. 20 The macroscopic transport properties of these materials (such as conductivity) have their origins in the molecular motions that occur as a result of the solid−solid phase changes. Understanding these motions is crucial to the development of OIPCs as electrolyte materials, but is also very challenging. Plastic crystals are generally characterized by a low entropy of fusion (<20 J K −1 mol −1 ) 21 and have the unique property of maintaining their long-range ordered crystalline lattice through one or more solid−solid phase transitions that introduce only short-range disorder. The disorder involved is typically associated with rotational and/or orientational changes in the molecules or ions. 20,22 As a consequence of this disorder, not only is fast-ion transport induced for either the matrix ions or doped targeted ions such as Li + , 4 H + , 12 or I − /I 3 − , 23 but also plastic mechanical properties are conferred; the materials are often soft and waxy at ambient temperature. 24 Both of these properties are highly favorable for solid electrolytes, by (i) enabling good ionic conductivity, (ii) allowing the contact between the electrolyte and the electrodes to be maintained over a range of operating temperatures, and (iii) potentially allowing the fabrication of flexible devices such as DSSCs on plastic substrates. Although molecular plastic crystals have been known since the 1960s, plasticity in organic ionic materials is a more recent discovery, and understanding of the transport and phase behavior of these materials is still in its infancy. Ikeda et al. 25−27 reported OIPCs based on quaternary alkylammonium cations with highly symmetric anions in the late 1980s, and in 1999 we reported a new family of OIPCs utilizing the pyrrolidinium cation. 28 In the latter family, the pyrrolidinium ring rotates easily, although the interionic interactions are sufficiently strong to maintain a crystalline structure. Since their original discovery, a large number of pyrrolidinium cation based OIPCs have been described. Alarco et al. 29,30 have studied pyrazolium-based OIPCs, and, through conductivity measurements, suggest that pipe diffusion is the dominant transport mechanism in these systems. 30 Received: February 13, 2012 Published: May 29, 2012 Article pubs.acs.org/JACS © 2012 American Chemical Society 9688 dx.doi.org/10.1021/ja301175v | J. Am. Chem. Soc. 2012, 134, 9688−9697