Research

Publication List

Prof. Kaoru Dokko
Prof. Kazuhide Ueno
Dr. Hisashi Kokubo

Materials Science Related to Electrochemical Devices

In our research group, we are investigating the structures, physicochemical properties, and electrochemical reactions of materials for electrochemical devices. We are involved in the research and development of liquid electrolytes, polymer-gel electrolytes, solid-state electrolytes, electrode materials, and other functional materials for Li-ion batteries (LIBs) as well as next-generation batteries, such as Li-S and Na-ion batteries.

Schematic illustration of a Li-ion battery.

Novel Electrolytes

We aim to develop novel electrolytes for next-generation batteries. For instance, we are developing ionic liquids and molten solvates as electrolytes for LIBs and Li-S batteries. Ionic liquids are non-flammable and non-volatile, and the utilization of an ionic liquid electrolyte may be beneficial in achieving high thermal stability in batteries. To understand the physicochemical properties of liquid electrolytes, we are investigating the liquid structures and interactions between cations and anions using various experimental techniques. In liquid electrolytes, cations and anions form ion-pairs and ionic aggregates, and the liquid structure significantly affects the ion conduction, thermal properties, and electrochemical properties of the electrolytes. Therefore, fundamental studies are crucial for the development of new electrolytes for next-generation batteries. Along with liquid electrolytes, other electrolytes, such as polymer gel and composite (composed of inorganic solid electrolytes and polymer electrolytes) electrolytes have also been studied.

Schematic illustration of Li ion hopping conduction in a highly concentrated electrolyte
(K. Dokko et al., J. Phys. Chem. B, 2018, 122, 10736-10745)

Schematic illustration of a gel electrolyte composed of a solvate ionic liquid and a triblock copolymer
(Y. Kitazawa et al., Chem. Mater. 2018, 30, 252–261.)

Electrochemical Reaction Mechanisms in Batteries

During the charging and discharging processes of a battery, electrochemical reactions occur simultaneously with the charge transfer process at the electrode/electrolyte interface and the mass transport process in the electrolyte. For example, the Li-ion transfer process at the electrode/electrolyte interface involves the desolvation of Li ions; moreover, the interaction between the Li ion and solvent significantly affects the electrochemical reaction mechanism. In addition to the interfacial process, the mass transport process in the electrolyte also influences the electrochemical reaction rate. The Li-ion diffusion in the vicinity of the electrode and the Li-ion migration in the bulk of the electrolyte become rate-determining steps at high current densities. Understanding the thermodynamic and kinetic factors and the mass transport processes, and interpreting their effects on the electrochemical reactions is crucial for designing electrochemical devices. We aim to elucidate the mechanisms of the interfacial charge transfer reactions and ion transport in batteries using electrochemical methods.

Schematic illustration of the Li-ion intercalation reaction at the interface between the graphite electrode and solvate ionic liquid.

Schematic illustration of Li-ion transport in a Li-S battery
(T. Seita et al., ACS Energy Lett. 2020, 5, 1-7.)

Ionic-liquid-based soft materials

Ionic Liquids

Ionic liquids (ILs) are solely composed of ions (cations and anions) and are liquid at ambient temperature. Unlike conventional organic solvents, ionic liquids exhibit attractive properties, including low flammability, negligible volatility, and high ionic conductivity. Owing to these unique properties, our group has focused on ionic liquids as new electrolyte materials for electrochemical devices.

~Composite of Ionic Liquids and Polymers/Nanoparticles~

Although ILs exhibit attractive properties as mentioned above, their fluidity sometimes causes a handling problem, which may hinder their use in various applications.
We have developed soft solid-like materials by combining ILs with polymers or nanoparticles. For instance, polymer gels swollen with ILs (i.e., composites of ILs and polymers—ion gels) can be treated as a self-standing and thin membranes (shown in the left figure). Moreover, other types of ion gels based on composites of ILs and nanoparticles were found to exhibit unique rheological properties, including shear thinning and shear thickening. Because these soft solid-like materials exhibit self-supportability and flexibility, in addition to the intrinsic properties of ILs, they are expected to be used in various applications such as quasi-solid electrolytes for electrochemical devices. Our group has also developed novel ‘functional’ ion gels by introducing, for instance, stimuli-sensitivity into ILs and/or polymers.