Transport Properties of Lithium Hectorite-Based Composite Electrolytes

Abstract

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...

Nanocomposite Electrolytes with Fumed Silica and Hectorite Clay Networks: Passive versus Active Fillers

“…The ethylene carbonate (EC) was obtained from Aldrich Chemicals and dried in identical fashion as the PEGdm. The procedures for generating Li-exchanged hectorite and preparing nanocomposite electrolytes with exfoliated Li± hectorite initially involved converting native hectorite to Li±hectorite via serial mixing with LiCl in deionized water, followed by centrifugation and drying in a conventional oven [30]. The resultant dry Li±hectorite powder was then dispersed in a high-dielectric solvent by a Silverson high-shear mixer.…”

“…The temperature was controlled to within ± 0.5 C using a Fisher Isotemp 1016S circulating water bath. Conductivity cells with Pt wire electrodes were used [29,30]. Lithium-ion transference numbers were measured by two different methods.…”

“…It is important to recognize that batteries with T Li = 1.0 would be particularly useful for high-discharge applications since they would be less susceptible to deleterious effects arising from concentration polarization. [8,35] Riley et al [30] have measured T Li values as high as 0.98 for Li±hectorite in PC.…”

“…Since the hectorite serves the dual role as anion and network-forming filler, high Li transference numbers and mechanical strength are simultaneously realized. [30] In this case, the mixed EC/PEGdm or EC/PC solvent provides for relatively high ion transport at ambient temperature with r > 10 ±4 S cm ±1 .…”

“…Previous work on these nanocomposites have addressed some aspects of their electrochemistry [29,30] and rheology. [26,28] Missing, however, is a detailed morphological analysis of these systems together with a comprehensive understanding of the relationship between observed morphology, electrochemistry, and rheology.…”