Used in electrodes for secondary batteries on the commercial market, polymer binders combine cathode materials (that accept and release ions) with a carbon-based electrical conducting material (for improving conductivity) and an aluminum foil current collector. Because of this blending, the voltage distribution and interface state become complex and the mechanisms on which relative capacity and discharge rate depend have not been well understood.

The research group led by Professor Yutaka Moritomo of the Faculty of Pure and Applied Sciences at the University of Tsukuba has been focusing on cathode materials with no binders mixed in, conducting precise experiments using thin-film electrodes with different deposition times. Using these thin-film electrodes, voltage was applied to each size of cathode material grain. What they found is a scaling relation between the renormalized discharge rate and relative capacity. Also, by simulating the 2D diffusion of the ions, they were able to reproduce quantitatively the above-mentioned phenomenon. In other words, Moritomo's team showed that ion diffusion is a predominant factor in battery capacity, based on experimental evidence. In the future, the research group aims to develop battery materials with a high ion diffusion coefficient to produce secondary batteries with high current density.

An example of discharge curves of thin films of P2-Na0.68CoO2 against thickness 80 nm and grain radius 40 nm.

Original Paper

Ayumu Yanagita, Takayuki Shibata, Wataru Kobayashi, Yutaka Moritomo, Scaling relation between renormalized discharge rate and capacity in NaxCoO2 films, APL Materials 3, 106104 (2015)
Doi.org/10.1063/1.4933236