Crystal Structure and Ionic Conductivity of Soft Solid Crystals Composed of NaClO4. N,N-Dimethylformamide
Authors: Chinnam Parameswara Rao , Fall Birane , Jalil Abdel Aziz , Zdilla Michael J , Wunder Stephanie 
Published: February 2020

There have been safety issues with liquid electrolytes used in commercial lithium/sodium batteries. This has motivated the development of solid electrolytes with desired properties to replace liquid electrolytes. Solid electrolytes made with ceramic/glass have ionic conductivities in the range of 10-3 to 10-2 S/cm [1], but are brittle, with poor adhesion to the electrodes and are difficult to process.  Organic solid electrolytes, such as polyethylene oxide (PEO) [2], PEO/composite blends, PEO copolymers/blends [3-4], molecular or ionic plastic crystals, and low molecular weight glymes have lower ambient temperature conductivities (10-7-10-5 S/cm), but are flexible, with better adhesion to electrodes, and are easier to process.

For PEO systems, conductivity has been shown to occur primarily through the amorphous phase, where ion migration is coupled to slow backbone segmental motions, so that decreases in crystallinity and alignment of polymer chains increase conductivity. Other approaches to improve ionic conductivities in soft-solid electrolytes are based on the observation that molecular organization rather than disordered structures foster ion mobility. In particular, this is true for materials in which there are alternative, low activation energy pathways for ion migration, such as along and between organized, aligned polymer or liquid crystalline polymer chains; along polymeric/inorganic nanoparticle interfaces possibly due to weakening of the ether O-Li+ bond and along ion channels in low molecular weight glymes and trilithium compounds.

We have been investigating soft solid electrolytes that are crystalline solids in which the channel walls have low affinity for the enclosed ions. The first such material, DMF:LiCl has the highest conductivity reported to date for a soft solid organic electrolyte [7,8]. The high conductivity can be explained based on the hard and soft acids and bases (HSAB) theory [5,6].  In DMF:LiCl, the soft (charge diffuse, polarizable) C=O functionality in N,N-dimethylformamide (DMF) donors interacts poorly with the hard Li+ ions in LiCl, permitting fast ion migration.

Here we report another soft solid crystal, DMF:NaClO4, which is a sodium ion conductor. Single crystal X-ray data identifies the solid as 2:1 adduct of DMF:NaClO4  (Figure 1). The structure shows the formation of 1-D covalently bonded Nan •DMFnchains in which Na ions interacting with the soft DMF matrix through weak Lewis acid/base interactions of sodium and oxygen ions. The

crystal structure exhibits a linear arrangement of Na-O(DMF) rhombs arranged end to end through sodium atoms, such that a chain of closely spaced (3.232 Å) sodium ions exists parallel to the a axis of the unit cell. The melt temperature of DMF.NaClO4 is 55oC. Unlike DMF:LiCl crystals that degrade before melting, DMF:NaClO4 reversibly melts and recrystallizes. The decomposition of DMF:NaClO4 starts at 70oC. The temperature dependent conductivity of the pressed pellets (1200 psi) of air dried DMF:NaClO4 exhibits excellent room temperature conductivity 8.0x10-6 S/cm (Figure 2). This may be due to an interfacial layer of liquid-like DMF:NaClO4 between the grains, since extra drying decreases the conductivity. The liquid DMF:NaClO4 has a conductivity approximately 1 order of magnitude greater than that of the crystal.

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Figure 1