Hectorite-Based Nanocomposite Electrolytes for Lithium-Ion Batteries

Abstract

Rechargeable lithium-ion batteries are becoming an increasingly important technology for energy storage due to their high-energy density and low self-discharge rates compared to batteries. However, issues of reliability, safety, and cycle life among others hamper their acceptance as an energy storage medium for applications beyond portable electronics. Electrolyte concentration polarization becomes a problem at high-discharge rates, making them unsuitable for applications requiring high power such as electric vehicles. Hectorite clay is presented in this work as a promising component for electrolytes for lithium-ion batteries. This negatively-charged, plate-shaped (250 nm diameter by 1 nm thickness) clay has exchangeable cations for which lithium may be substituted. When properly dispersed in high-dielectric solvents such as the carbonates (ethylene carbonate and propylene carbonate) typically used in lithium-ion cells, a shear-thinning physical gel is created possessing a good conductivity (as high as 2×10-4 S⁄cm at room temperature has been measured) with near unity lithium-ion transference numbers. As a result, electrolytes designed around the clay could drastically reduce concentration polarization and possibly present an inherently safer electrolyte as toxic salts such as LiPF6 that are typically used could be eliminated.

Hectorite clay dispersions in aqueous and non-aqueous (1:1 (v:v) ethylene carbonate: poly(ethylene)glycol dimethyl ether 250 MW) solvents have been studied using dynamic and steady rheology, conductivity, and TEM imaging to examine their microstructures and recovery after shear deformation. Two different particle size clays (25 nm and 250 nm) were included in the study. The aqueous dispersions show a highly-exfoliated microstructure (fractal dimension, Dƒ=1.6) created primarily through electrostatic repulsive forces which recovers after shear deformation through reorientation of the clay platelets. The nonaqueous dispersions form gel structures at higher concentrations than the aqueous dispersions with a much higher degree of aggregation (Dƒ= 2.5), and recovery after shear deformation appears to be an aggregation controlled process as well. The use of two different particle size clays (25 and 250 nm diameter) reveals that particle size of the clay platelets does not have a significant impact on the gel modulus, fractal dimension, or recovery after shear deformation, although conductivity measurements indicate a higher degree of aggregation with the smaller clay platelets. TEM imaging of non-aqueous clay dispersions at low magnification shows the clay to be uniformly distributed, while high magnification shows that the platelets exist in aggregates of approximately 5 layers.

Use of the single-ion conducting hectorite-based electrolytes in lithium-ion cells requires an electrode that contains a single-ion conductor in the typically porous structure. Cathodes based on LiCoO2 that contain various lithium-conducting species (lithium hectorite, lithium Laponite®, and lithium-exchanged NAFION®) have been studied in conjunction with lithium metal anodes. Performance was compared to that of cells with a standard liquid electrolyte (i.e., LiPF6 + 1:1 w⁄w ethylene carbonate:ethylmethyl carbonate). Effects on cathode capacity were examined for these variables: hot-press force used in construction of the porous cathode, carbon type (graphite vs. carbon black), and clay particle size. AC impedance spectroscopy was used to probe the cells and equivalent circuits were used to model the physical processes that occur. Cathodes containing 4 wt. % lithium hectorite + 3 wt. % lithium-exchanged NAFION® + 3 wt. % carbon black exhibit capacities approximately 90 mAh⁄g LiCoO2 compared to that observed in a standard cell of 110 mAh⁄g LiCoO2. These hectorite-based electrolytes and clay-containing cathodes are potentially attractive for use in single-ion conducting lithium-ion batteries designed for high-discharge applications.