Transport Properties of Hectorite Based Nanocomposite Single-Ion Conductors.

Abstract

Lithium-ion batteries are an important power source for small electronic technologies because of desirable characteristics including high-energy density, low weight, and excellent cycle performance. We investigated the electrochemical and rheological effects of clay nanocomposite fillers in lithium-ion battery electrolytes. Nanocomposite hectorite, a non-reactive smectite clay filler, was used in this study. Hectorite and other 2:1 layered clays (smectites) are unique in that they are characterized by a large negatively charged plate-like structure (˜250-nm diameter) with exchangeable counter cations sandwiched between thin platelets (˜1 nm). For lithium battery application, the native sodium cations on hectorite are exchanged for lithium ions and the plate-like particles are dispersed in high-dielectric solvents (e.g., ethylene carbonate (EC) and propylene carbonate (PC)) to create a physically gelled structure. The cation mobility is considerable relative to the mobility of the large anion clay platelets. Lithium-ion transference numbers of Li-hectorite in carbonate solvent have shown near unity values indicating efficient Li+ movement in a cell.

Conductivity in our electrolytes is not as high as LiPF6 liquid electrolytes used in today's market, however, we hypothesized that the addition of low-molecular weight polymer compounds would improve conductivity. Our objective was to show improved conductivity in Li-hectorite/ethylene carbonate electrolytes with the addition of polyethylene glycol di-methyl ether (PEG-dm, 250 MW). Several combinations of clay and polymer loading are studied in an attempt to find an electrolyte with the highest conductivity. Finally, a preliminary comparison between hydroxyl terminated PEO (PEG) and PEG-dm as a polymer co-solvent with EC, is made with regards to rheological properties.

We find all samples to exhibit gel-like behavior with room temperature conductivities of order 0.1 mS/cm. A maximum in conductivity is observed with increasing clay concentration. A maximum in clay basal spacing is also observed in the same concentration range, suggesting a direct correlation between conductivity and basal spacing. G' and yield stress increased by two orders of magnitude with increasing clay concentration and conductivity increased by one order of magnitude (from 5 to 25% clay), indicating clearly clay concentration to be the primary factor in determining the characteristics of these single ion conductors. Addition of PEG-dm to the base EC electrolyte shows moderate improvement in conductivity; the elastic modulus and yield stress also increase by a factor of three. Clay concentration had a dominating effect on all results including rheology results, when compared to solvent composition. PEG-dm electrolytes yielded a stronger gel sample when compared to PEG electrolytes. In addition, we found an interesting correlation between clay basal spacing and conductivity as a function of clay concentration.