Dynamic rheological techniques are used to probe the microstructures present in fumed silica-based composite polymer electrolytes. These electrolytes are obtained by dispersing hydrophobic fumed silica in a poly(ethylene glycol)−lithium salt solution and display high conductivities (σ298K > 10-3 S/cm), mechanical stability, and easy processability. The materials behave as soft solids (gels) under ambient conditions due to the presence of a three-dimensional network of silica entities. Network formation occurs as a result of van der Waals (dispersion) forces between the nonpolar surface layers on silica units. Factors which affect the van der Waals interaction, and hence the gel network density, include the nature of the PEG end group, the presence of ionic species, and the size of the hydrophobic surface group on the silica. The composites also exhibit shear-thinning behavior due to the shear-induced disruption of network bonds, and this behavior can be advantageously utilized in electrolyte processing.

Conductivity and lithium-ion transference numbers are reported for physically gelled composite electrolytes using lithium hectorite clay as the charge carrier and carbonate solvents ͑ethylene carbonate, propylene carbonate, and dimethyl carbonate͒. Results are compared with those of typical lithium-ion battery electrolytes based on lithium hexafluorophosphate ͑LiPF 6 ͒ and carbonate solvents. Room-temperature conductivities of the composite electrolytes as high as 2 ϫ 10 Ϫ4 S/cm were measured. Because of the nature of the anionic clay particulates creating the gel structure, near-unity lithium-ion transference numbers are expected and were observed as high as 0.98, as measured by the dc polarization method using lithium-metal electrodes. Since the carbonates react with lithium and create mobile ionic species that significantly reduce the observed lithium-ion transference number, care must be taken to minimize or eliminate the presence of the reaction-formed ionic species. These hectorite-based composite systems are possible electrolytes for rechargeable lithium-ion batteries requiring high discharge rates.A viable electrolyte for lithium-ion batteries must meet a number of requirements, for example, high conductivity ͑Ͼ10 Ϫ3 S/cm at 25°C͒, low electrode-electrolyte interfacial impedance, and highpotential stability window ͑Ͼ4.5 V͒. It is most often the electrolyte conductivity that receives the majority of attention in electrolyte characterization and design. While the conductivity is certainly an important property in determining the success of a particular electrolyte in a lithium-ion cell, the lithium-ion transference number is also an important property and in recent years has started to receive increased attention. 1-5 A high lithium-ion transference number can significantly reduce the effects of concentration polarization, thus decreasing this potential loss in a cell. Theoretical work has shown that a transference number of unity can offset a decrease in conductivity by up to an order of magnitude, particularly under high discharge rates. 6 Thus, a single-ion conducting electrolyte is particularly attractive for applications requiring high power such as electric vehicles.Single-ion conducting polymer electrolytes intended for lithium batteries have been reported. 7-9 For polyelectrolytes gelled with propylene carbonate, 7 ethylene carbonate, 8 and dimethyl sulfoxide, 9 room-temperature conductivities have been reported in the 10 Ϫ4 S/cm range. However, lithium-ion transference numbers were either not reported 7,8 or were low ͑Ͻ0.3͒ and explained by reaction of the solvent with the lithium metal electrodes which created other mobile ionic species. 9 We have developed single-ion conducting, physically gelled electrolytes based on lithium hectorite clay nanoparticulates dispersed in carbonate solvents suitable for lithium-ion cells. As we report in this communication, room-temperature conductivities of these electrolytes have been measured as high as 2 ϫ 10 Ϫ4 S/cm and lithium-ion transference numbers have be...

The use of nanocomposites constitutes a versatile and robust approach in the development of novel electrolytes with tailored electrochemical and mechanical characteristics. In this study, we examine the morphology, rheology, and ion‐transport properties of two types of nanocomposite electrolyte gels, one consisting of branched silica nanoparticles and the other composed of hectorite clay. In the first system with hydrophobic (fumed) silica, oligomers of poly(ethylene oxide) (PEO), and lithium salt, the silica acts as a passive filler and does not participate in ion transport. The electrochemical properties are controlled by the salt–PEO electrolyte, allowing for ionic conductivities greater than 10–3 S cm–1 at ambient temperature. At sufficiently high concentrations, the silica forms an elastic gel possessing a large open network structure that provides for unimpeded ion mobility. In the second system composed of lithium‐exchanged hectorite filler, the nanoscale platelets serve as the anion. This active filler yields ionic conductivities in excess of 10–4 S cm–1 and lithium transference numbers approaching unity. Similar to fumed silica, the hectorite clay also forms an elastic gel network. However, the morphologies of the two systems are distinctively different both in terms of network structure and characteristic length scale. These morphological differences manifest themselves in different rheological responses with regard to gel modulus and yield stress.

Cathodes based on LiCoO 2 that contain various lithium-conducting species ͑lithium hectorite, lithium Laponite, and lithiumexchanged Nafion͒ are studied in conjunction with lithium metal anodes and composite electrolytes based upon lithium hectorite clays as the charge carrier. Performance is compared to that of cells with a standard liquid electrolyte ͑i.e., LiPF 6 ϩ1:1 w/w ethylene carbonate:ethyl methyl carbonate͒. Effects on cathode capacity are examined for these variables: hot-press force used in construction of the porous cathode, carbon type ͑graphite vs. carbon black͒, and clay particle size. AC impedance spectroscopy is used to probe the cells and equivalent circuits are used to model the physical processes that occur. Cathodes containing 4 wt % lithium hectorite ϩ3 wt % lithium-exchanged Nafion ϩ3 wt % carbon black exhibit discharge capacities approximately 90 mAh/g LiCoO 2 compared to that observed in a standard cell of 110 mAh/g LiCoO 2. These clay-containing cathodes are potentially attractive for use in single-ion conducting lithium-ion batteries designed for high discharge applications.

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