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.
The conductivity of solution electrolytes containing lithium salts (imide and triflate anions), poly(ethylene glycol), and mono‐ and dimethyl‐terminated poly(ethylene glycol) (Mw 200 to 400), and their corresponding composite electrolytes containing fumed‐silica particulates (0 to 20 weight percent) are reported. At room temperature the ionic conductivity is as high as
1.5×10−3Scm−1
for the composites studied, and they exhibit a gel‐like consistency but flow under shear. The electrochemical stability of the composites and compatibility with lithium metal were also examined. A large potential window (∼5.5 V) was found for Li imide‐based electrolytes. The passive film formed on lithium in contact with the composite electrolyte is relatively more stable and less resistive than that formed in contact with the parent solution electrolyte. Considering the additional advantages of processability and low volatility, these composites should be good candidate electrolytes for lithium and lithium ion batteries.
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...
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