Enhancing batteries and fuel cells with dispersibility estimation

Modern-day electronics rely on lithium-ion batteries, while fuel cells are a promising candidate for sustainable energy devices. An important factor affecting the performance of both lithium-ion batteries and fuel cells is the dispersibility of carbon slurries, suspensions made of conductive carbon particles dispersed in a solvent. They can be coated on a metal collector to mass-produce electrodes. But the carbon particles in the slurry must be homogenously dispersed for reliable battery performance.

However, evaluating the dispersibility of thick slurries with high particle concentrations is difficult, as the large number of particles prevent peering into the internal structure of the slurries using direct spectroscopic techniques. Moreover, there are no methods to evaluate the dispersibility and conductive properties of slurries in response to shear stress applied during the coating process.

Researchers led by Associate Professor Isao Shitanda from Tokyo University of Science (TUS) have developed a technique to estimate the dispersibility of carbon slurries. The study was published in ACS Applied Electronic Materials.

The researchers combined a rheometer — an instrument to measure the flow/deformation behaviour of fluids in response to applied stress — with a spectroscopy set up to measure the electrochemical impedance of acetylene black slurries with methylcellulose (a cellulose-derived compound used as a thickener and emulsifier in food and cosmetic products) as a dispersant. They conducted experiments under the influence of shear stress at various frequencies to obtain the rheo-impedance spectra, which provide information about the internal structure of carbon particles in a slurry. The researchers noted that the impedance spectra did not change under applied shear stress for a carbon slurry with good dispersibility.

The researchers also developed an equivalent circuit model consisting of three types of contact resistances and capacitances: those between acetylene black particles, those of particle bulk and those arising from the design of the measurement set-up. The bulk resistance of acetylene black showed no dependence on shear rate but decreased with increase in the methylcellulose concentration. The resistance measured at each methylcellulose concentration increased with the shear rate, an observation that was attributed to a partial breakdown of the carbon–carbon network and the decreasing conductivity with rising shear rate.

These results show that it is possible to evaluate the dispersibility of electrode slurries based on a combination of viscosity (measured with the rheometer) and electrochemical impedance measurements. Dr Shitanda said the research findings could prove useful for improving the efficiency of large-scale electrode manufacturing processes in which the internal structure of the slurry must be controlled. Preparing slurries with higher dispersibility could also lead to improved Li-ion battery performance and enhanced functional materials.

“The proposed method can be used to evaluate the dispersibility of not just carbon dispersions, but a wide variety of slurries. In future studies, we plan to conduct further measurements and equivalent circuit verifications by changing the particle type and binder combinations,” Shitanda said.

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