Internationale Ausgabe: DOI: 10.1002/anie.201606363 Solid Electrolytes Hot Paper Deutsche Ausgabe: DOI: 10.1002/ange.201606363 A Self-Binding, Melt-Castable, Crystalline Organic Electrolyte for Sodium Ion Conduction Parameswara Rao Chinnam, Birane Fall, Dmitriy A. Dikin, AbdelAziz Jalil, Clifton R. Hamilton, Stephanie L. Wunder,* and Michael J. Zdilla* Abstract: The preparation and characterization of the cocrys- talline solid–organic sodium ion electrolyte NaClO 4 (DMF) 3 (DMF = dimethylformamide) is described. The crystal struc- ture of NaClO 4 (DMF) 3 reveals parallel channels of Na + and ClO 4 À ions. Pressed pellets of microcrystalline NaClO 4 (DMF) 3 exhibit a conductivity of 3  10 À4 Scm À1 at room temperature with a low activation barrier to conduction of 25 kJ mol À1 . SEM revealed thin liquid interfacial contacts between crystal- line grains, which promote conductivity. The material melts gradually between 55–65 8C, but does not decompose, and upon cooling, it resolidifies as solid NaClO 4 (DMF) 3 , permit- ting melt casting of the electrolyte into thin films and the fabrication of cells in the liquid state and ensuring penetration of the electrolyte between the electrode active particles. An increased demand for compact, clean, and efficient energy storage has sparked an increase in demand for high- power-density batteries. [1] Currently, lithium ion batteries (LIBs) dominate the market, but owing to the relative scarcity of lithium, batteries made with more abundant sodium are being explored as an alternative. [2] Ultimately, construction of sodium ion batteries (NIBs) will require the development of improved sodium ion electrolytes. Most recent research in this field has focused on analogues of the salts with the same anions (I À , ClO 4 À , BF 4 À , PF 6 À , CF 3 SO 3 À (Tf), and [N(SO 2 CF 3 ) 2 ] À (TFSI)) and solvents (carbonates and ILs) used for LIBs, [3] but these suffer from the same safety problem, namely flammability. [4] This, as well as the pyro- phoric nature of the alkali metals, makes solid electrolytes desirable for both Li and Na metal or ion batteries. For this reason, solid electrolytes made from inorganic/organic poly- mer materials [5] and organic crystals, [6] which are of interest for LIBs, are also under investigation for NIBs. As in the case of LIBs, Na [7] and Li [8] ceramic electrolytes can be employed but are not flexible. Solid polymer electrolytes (SPEs), which consist of a Na salt in typically a polyethylene oxide matrix, [9] single ion conductors (SICs), [10] gel polymer electrolytes, [11] in which a polymer entraps/encapsulates a solvent/NaX solu- tion, and eutectic salt mixtures (NaX/MX, where X = anion, M = alkali cation) have recently been investigated for NIBs; while they are flexible, they have lower ionic conductivities than the inorganic electrolytes. One of the grand challenges of solid electrolytes is to develop materials with excellent conductivity and favorable mechanophysical properties for device fabrication. We herein report the simple preparation and characterization of a crystalline organic sodium ion conductor that can be pressed and melt-cast into thin films. The material was prepared by slow vapor diffusion of diethyl ether into a solution of NaClO 4 in anhydrous DMF. The insolubility of the salt mixture in ether resulted in the appearance of a white crystalline precipitate, which was identified by X-ray diffraction as NaClO 4 ·(DMF) 3 in the hexagonal space group P6 ¯ 2c (Figure 1). The structure con- tains a linear channel of sodium ions at the intersection of the Figure 1. X-ray crystal structure of NaClO 4 ·(DMF) 3 . Hydrogen atoms omitted for clarity. Top: Thermal-ellipsoid plot of five consecutive asymmetric units. Ellipsoids set at 50 % probability. Bottom: Crystal packing diagram illustrating the ion channels in the crystallographic c direction. [*] Dr. P. R. Chinnam, B. Fall, A. Jalil, C. R. Hamilton, S. L. Wunder, M. J. Zdilla Department Of Chemistry, Temple University 1901 N. 13th Street, Philadelphia, PA 19122 (USA) E-mail: slwunder@temple.edu mzdilla@temple.edu Dr. D. A. Dikin Department of Mechanical Engineering Temple University 1947 N. 12th Street, Philadelphia, PA 19122 (USA) Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.doi.org/10.1002/anie.201606363. A ngewandte Chemie Zuschriften 15480  2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2016, 128, 15480 –15483