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Jul 2, 2015 - LiCl·N,N‑Dimethylformamide: A New Paradigm for Solid Electrolytes. Based upon the Pearson Hard−Soft Acid−Base Concept. Parameswar...
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Bulk-phase, low-barrier ion conduction in cocrystalline LiCl•N,Ndimethylformamide: A new paradigm for solid electrolytes based upon the Pearson Hard-Soft Acid-Base concept. Parameswara R. Chinnam, Rebecca N. Clymer, Abdel Aziz Jalil, Stephanie L. Wunder, and Michael J. Zdilla Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00940 • Publication Date (Web): 02 Jul 2015 Downloaded from http://pubs.acs.org on July 5, 2015

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Chemistry of Materials

Bulk-phase, ion conduction in cocrystalline LiCl·N,Ndimethylformamide: A new paradigm for solid electrolytes based upon the Pearson Hard-Soft Acid-Base concept. Parameswara R. Chinnam, Rebecca N. Clymer, Abdel Aziz Jalil, Stephanie L. Wunder,* and Michael J. Zdilla* th

Department of Chemistry, Temple University, 1901 N. 13 St., Philadelphia, PA 19122

Supporting Information Placeholder ABSTRACT:

Cocrystallization of LiCl with N,Ndimethylformamide (DMF) results in crystals with the composition LiCl·DMF. These crystals form linear, parallel channels of ions in the crystallographic a direction. Conductivity measurements by electrochemical impedance spectroscopy -4 give a value of 1.6 x 10 S/cm, the highest room temperature conductivity for an organic solid electrolyte. The material also exhibits an activation barrier of 86 kJ/mol in the temperature region from -60 to 25 °C, and a lithium ion transference number of 0.25. Conductivity measurements on pellets prepared under different pressures and on samples containing the binders dimethylsiloxane-(80% ethylene oxide) block copolymer (PDMS-PEG) or pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane-polyethyleneglycol (POSS-PEG), indicate that conduction occurs predominantly in the bulk crystalline phase, with a percolating liquid layer likely at the grain boundaries that cannot be detected by impedance spectroscopy, but which results in abrupt decreases in conductivity that correlate with their freezing or glass transition temperatures.

Design of new materials with architectures that foster enhanced ion migration are motivated by the need for high ionic conductivity solid state electrolytes over a wide temperature range to replace flammable liquid electrolytes in electrochemical devices such as lithium/lithium ion batteries, hydrogen ion fuel cells and solar cells, due to the in1 creased safety associated with all solid state devices. The highest ionic conductivities for solid electrolytes are found in ceramic/glass conductors, with conductivities approaching -3 -2 2-5 10 -10 S/cm, but these are brittle, have poor adhesion to electrodes, and are difficult to process. Soft-solid electrolytes, 6 including polyethylene oxide (PEO) , PEO/composite 7-13 14-19 blends and PEO copolymers/blends , molecular or ionic 20-25 26-33 plastic crystals, and low molecular weight glymes exhibit desirable flexibility and processibility, but have lower -7 -5 ambient temperature conductivities (10 -10 S/cm). NafiTM on has a hydrophobic perfluorinated matrix that contains pendant anion (often -SO3 ) percolating clusters that form 34 channels through which oppositely charged ions migrate.

The low conductivity of aliphatic oxide solid electrolytes is + in part due to significant affinity of Li ions for ether oxygen atoms. Based upon the Pearson Hard-Soft Acid-Base (HSAB) 35 concept, hard (charge dense, non-polarizable) lithium cations are expected to have a high affinity for hard ether oxygen + atoms, resulting in low Li mobility, particularly as the num+ ber of O-Li contacts increases. For PEO systems, conductivity has been shown to occur primarily through the amorphous phase, where ion migration is coupled to slow back36 6, bone segmental motions , so that decreases in crystallinity 9, 10, 12 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 be37 37-39 tween organized ; aligned polymer or liquid crystalline 40-47 chains; along polymeric/inorganic nanoparticle polymer 48, 49 + interfaces , possibly due to weakening of the ether O -Li 50 bond, and along ion channels in low molecular weight 51 52-55 33 56 57 glymes and trilithium compounds . Decreased + interactions between mobile cations such as Li and their associated anions and/or solvating matrix, such as in mi15 crophase separated solid polymer electrolytes (SPEs) have also been shown to increase cation mobility and conductivity. A viable approach to increase the conductivity of soft solid electrolytes is the design of crystals that have channel walls with low affinity for the enclosed ions. We report here the preparation of crystalline, solid electrolytes with organic matrices containing soft (charge diffuse, polarizable) N,Ndimethylformamide (DMF) donors. The soft C=O functional+ ity is expected to interact poorly with hard Li ions, based on HSAB. Cocrystallization of these molecules with LiCl results in low-affinity ion channels and superior conductivity properties for an organic solid-state electrolyte. +

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The preparation of solid solutions of Li and Cl in crystalline organic matrices is achieved by precipitation of the cocrystals from a LiCl solution. LiCl(s) is dissolved in a minimum of DMF. Pure, white co-crystals of LiCl·DMF are + formed upon addition of diethyl ether (Et2O) precipitant. Li

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and Cl ion-solvent adducts presumably aggregate in the presence of Et2O to precipitate LiCl not as a pure salt, but as a solid LiCl solution in a crystalline, solid solvent matrix. The resulting crystals are highly pure based on powder diffraction (Fig. S1), but very hygroscopic due to the strongly + Lewis acidic Li ions, and absorb water to form a biphasic liquid over the course of minutes when exposed to air. When stored in an airtight vessel at -30°C in a drybox, the crystals are stable indefinitely. Single-crystal X-ray diffraction data identifies the solid as a 1:1 adduct of LiCl with a molecule of DMF (Figure 1, Fig. S2, Tables S1-S5). The structure shows the formation of 1-D ionically bonded Li2Cl2 rhombs interacting with the DMF matrix through weak Lewis acid/base adducts between the lithium atoms and soft Lewis donors of the DMF-oxygen atom. The -1 red-shifted C=O stretch (1640 cm ) in the solid state AT-IR spectrum of DMF:LiCl confirms the presence of C=O--Li contacts between lithium ions and soft carbonyl oxygen donors (Fig. S3). The DMF•LiCl crystal exhibits a linear arrangement of alternating Li-Cl and Li-O(DMF) rhombs arranged end to end through lithium atoms, such that a chain of closely spaced (2.9 Å) lithium ions exists parallel to the a axis of the unit cell. The chloride ion channels are therefore oriented parallel to a as well, but with twice the spacing between these ions. When viewed along the a axis of the unit cell, the linear channels of ions are apparent (Figure 2). As in the case of inorganic lithium superionic conductors such as Li10GeP2S12 and Li10SnP2S12 crystals, where fast ion 4 transport occurs in the more disordered 1D sublattice , but + where Li migration is also predicted to occur perpendicular 58, 59 to these channels, forming 3D pathways , here also, the possibility of ion migration across channels (for example a 6.3 Å migration in the direction of the crystallographic b axis) cannot be ruled out (Fig. S2).

Figure 1. Thermal ellipsoid plot of DMF:LiCl illustrating interactions of the Li2Cl2 rhomb with symmetrically equivalent neighboring rhombs and matrix molecules. Ellipsoids are set at 50% probability level and hydrogen atoms are shown as + open circles. Cl (● green), Li (● magenta), O(● red), N(● blue). Remarkably, a pressed pellet of microcrystalline LiCl·DMF exhibits excellent conductivity compared with other soft solid electrolytes reported to date, with a room temperature -4 conductivity of 1.6 x 10 S/cm at 25 °C (Figure 3). A linear fit of the data gives an Arrhenius activation energy of 85 kJ/mol

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(Figure 4), similar to that observed for typical polymer electrolytes (50- 100kJ/mol) and higher than values measured for inorganic lithium conductors (25-60 kJ/mol). The compound identity and crystallinity are well maintained over the course of the measurement based upon powder diffraction studies before and after the conductivity measurements (Fig. S1). The compounds decompose at elevated temperatures (> 40 °C). Thermal data, i.e. thermogravimetric analysis (TGA) (Fig. S4), on DMF•LiCl indicates that the decomposition temperature is slightly less than Tb of the pure liquid (Tvap of DMF = 153 ˚C, Tmax of LiCl·DMF ~ 140 °C, but starts to come off at lower temperatures). This can be attributed to the exotherm resulting from the formation of pure LiCl(s) from + solid-solution phase Li and Cl ions upon sublimation of the matrix molecule from the solid lattice (Ulatt(LiCl) = -829 kJ/mol), which lowers the decomposition temperature relative to the Tb of the pure liquid.

Figure 2. Crystal packing diagram of DMF:LiCl viewed along + a axis. Channels of Li (●magenta) and Cl (●green) ions are apparent.

Figure 3. Conductivity of LiCl·DMF compared with 2, 60 PEO/LiX ; Li(BETI)= LiN(SO2CF2CF3)2; Li(TFSI)= LiN(SO2CF3)2. Nyquist plots of impedance data for LiCl•DMF are composed of a single semicircle and a spike (Fig S5), which we attribute to combined bulk and Warburg resistance and electrode polarization due to ion blocking electrodes, respective+ ly. In the case of inorganic Li conductors, two semicircles 4 are sometimes (but not always) observed , and are attributed to bulk and grain boundary resistances. Unlike the case for inorganic crystals, where high temperature annealing (at ~

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500 C) is used to improve contact between the grains, such approaches are not possible with the cocrystalline samples under study here. To assess the contribution of the grain boundary resistivity, sample pellets were prepared under different pressures and with different drying protocols. Invariably, pellets prepared under higher pressure (1500 psi) resulted in data with improved signal to noise ratios. Nyquist plots were less noisy in the entire frequency region as the pressure used to make the pellets increased, consistent with significant resistivity resulting from poor grain contact. To understand more about the contribution of bulk and grain boundary resistances to the overall resistance of DMF•LiCl pellets, Nyquist plot data for the samples pressed at 1500 psi have been processed and 61 represented in complex electric modulus formalism (Fig. S6). Surprisingly it was found that there was only a single contribution, which from the temperature independent capacitance of 40 ± 6 pF, was consistent with bulk, not grain 61 boundary resistance . Since the grain boundaries are not observable using impedance spectroscopy, this suggests that there is only a thinboundary layer of liquid-like LiCl/DMF that provides contact between the grains when they are pressed. A similar mechanism of percolating liquid mixed salt phase domains at grain boundaries, rather than solid state diffusion, has been proposed for conductivity increases in plastic crystalline saltlithium salt mixtures, and also for plastic crystalline solvent62 lithium salt mixtures and crystalline polymer electrolytes .

o

dropped below the pure DMF•LiCl at about -60 C. The glass transition temperature (Tg) of POSS-PEG increases from -81 o o 17 + C to -40 C with added Li salt . Although the Li concentration is unknown in the boundary region here, the drop in o conductivity at -60 C suggests that glassy material in the grain boundaries limits the conductivity. In contrast, for o PDMS-PEG, the lower Tg of -100 C (which will also increase + with Li ), increases the conductivity compared with DMF•LiCl (Figure 4), since it is still in the liquid phase. These results suggest that conduction of ions is through LiCl·DMF in the bulk crystalline phase, where crystallite sizes are on the order of microns, and that the the grain boundary thickness is very small, on the nanometer length scale (See SI). +

The lithium ion transference number (tLi ) is 0.25 based o upon impedance measurements using Li (s) electrodes (Fig. o S8). Interfacial resistance (Fig. S9) with Li is high due to the 63 known reactivity of lithium metal with DMF . Although 64 65 DMF has been investigated by itself , in mixed solvents 66 and in polymer gel electrolytes for conductivity studies of lithium ion batteries, it is not used not in practical applications due to problems in electrode surface reactivity and 67 passivation . Nevertheless, DMF•LiCl is a paradigm for the formation of soft-solid crystals with low-affinity ion channels. In summary, we have presented a new type of solid electrolyte based upon “soft” Lewis base donors with the intention of generating channels of decreased affinity for “hard” Lewis acid lithium ions. The result is a solid electrolyte with record conductivity for an organic solid electrolyte. Variablepressure and binder-included pellet preparation suggests bulk conduction through the crystalline phase with grain boundaries becoming an influence below the melt or glass transition of this phase. Future work will explore single crystal anisotropic conductivity, a more detailed analysis of grain boundaries, the synthesis of materials with improved thermal and electrochemical stability, and crystals with lithium ion channels in more than one dimension since point defects in 68 1D crystals limit macroscopic ionic diffusion of lithium .

ASSOCIATED CONTENT Supporting Information

Figure 4. Conductivity of LiCl•DMF compared with 1M LiCl•DMF solution electrolytes, and with added 10% POSS0 o PEG (Tg ~ -40 C) or PDMS-PEG (Tg ~ -100 C) Comparison of conductivity plots of 1 M LiCl/DMF solutions (Fig. 4) with the LiCl•DMF crystals indicates that the o drop in conductivity at ~-70 C is due to solidification of the liquid phase at the grain boundaries (Fig. S4); it was not possible to collect conductivity at temperatures below the rightmost data points. Another test of the role of grain boundaries was the preparation of pellets with 10% of the binders dimethylsiloxane-(80% ethylene oxide) block copolymer (PDMS-PEG) or pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane-polyethyleneglycol (POSS-PEG) (Fig. S7). Conductivity plots (Fig. 4) of these materials exhibited essentially identical conductivity values as the binder-free samples at higher temperatures. However, the conductivity of the samples with POSS-PEG binder

Experimental details, Single Crystal X-ray structural data, FTIR, thermal, calorimetric and electrochemical characterization. “This material is available free of charge via the Internet at http://pubs.acs.org.”

ACKNOWLEDGMENT Support of this research by the National Science Foundation through grants CBET 1437814 and DMR 1207221 is gratefully acknowledged.

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