Introduction to Solid Polymer Electrolytes (SPE)

The main theme of Tominaga Group is "to create solid polymers that can express high ionic conductivity comparable to liquid and gel-like substances and apply it to various appliations". Ionically conductive materials seem like very special and difficult materials, but when you look carefully there are lots of things around you. For example, KOH aqueous solution has long been used as an electrolyte for batteries. In recent years, organic electrolytes using organic solvents capable of dissolving many metal salts like water are used in chargeable/dischargeable lithium-ion secondary batteries such as mobile phones, and it is indispensable for recent our society. The electrolyte material that enables such " utilization of ions" can be applied to various functional materials other than batteries because of its rich variety of ions. However, electrolytes that have been studied over the years have drawbacks as well. First of all it is safety. Recently ignition accidents such as laptop computers reported in news etc. are new to memory. By using organic solvent for electrolyte, there is always a high risk of ignition and explosion, and careful attention must be paid to the manufacture of the device. Second is processability. From the viewpoint of reliably preventing leakage, it is necessary to severely seal with metal or the like, resulting in heavy workability deterioration. Such problems are expected to be overcome by the " use of polymer (to solidify)" for the electrolyte.

In lieu of the existing liquid electrolyte, the polymeric electrolyte material as shown in the left figure has been receiving much attention in recent years. This polymeric material is called Solid Polymer Electrolytes, SPE. Polymers used for SPE can dissolve metal salts such as NaCl like water. Dissociated ions can move through thermal motion of polymer chains even in solid polymer. However, like the figure on the left, the appearance is solid like plastic or rubber which is very interesting substance. Unlike organic solvents, SPE has low risk of ignition and no leakage, so it can be a safe and secure electrolyte material. Furthermore, it is also a great advantage that it is flexible, such as thin, flat, and can be processed into a free form. It is possible to realize a thin and foldable battery that has been difficult to date.

The figure on the left shows the most basic materials for the SPE composition. Poly(ethylene oxide) (PEO), which has been studied for more than 40 years, is an example of ion-conductive polymers. PEO is a waxy solid or viscous liquid at room temperature when the molecular weight is 1000 or less, and a powder solid at a molecular weight higher than that. The crystallinity is very high, the melting point is around 65 to 70 degree C. The glass transition temperature (Tg) of PEO in the amorphous state is very low (around -65 degree C), and it shows excellent ion transport properties in the molten state.
On the other hand, alkali metal salt is usually used as an ion source. PEO can dissolve almost all alkali metal salts except for some. Recently, lithium salts excellent in solubility and ionic conductivity, such as LiFSI (lithium bis (fluorosulfonylimide)) and LiTFSI (lithium bis (trifluoromethanesulfonylimide)), are used together with LiPF6 and the like already in practical use. It is widely used as an excellent electrolyte salt.
The cation formed by dissociation of the metal salt forms a stable solvation structure like the solvated structure in water by the interaction with the dipole of the ether chain and exists stably even in PEO. Dissociated ions are considered to be carrier ions and are thought to be carried by cooperative transport through thermal motion of polymer chains as shown in the left figure in PEO. This can be explained also from the fact that the temperature dependence of the ionic conductivity applies well to the VTF (Vogel-Thamman-Fulcher) formula or the WLF (Williams-Landel-Ferry) equation. In order to develop higher ionic conductivity, it is essential to select polymers with lower Tg.

Unfortunately, practical application of SPE to actual batteries is still difficult, and we need to overcome many challenges. In particular, the value of ionic conductivity which determines the performance of SPE is lower than that of liquid electrolyte by one digit or more, and significant improvement is required. Tominaga Group believes that unprecedented research development is necessary for high performance of SPE, and we are working on various original research themes. We are also considering new applications of SPE and development of different fields. For example, permanent antistatic prevention, fine forming technology, artificial ion channel, etc. can be mentioned. Tominaga Group aims at consistent research from the synthesis and application of SPE based on the movement and diffusion of substances (ions) in solid polymer.
In the study of PEO system SPE reported so far, ionic conductivity is improved by various methods such as reduction of crystallinity by branching and networking of PEO, improvement of added metal salt and addition of inorganic fine particles. However, in a completely dry solid polymer, the diffusion of ions is much lower than in liquids, so it is difficult to transport ions faster. This is because ionic transfer is mass transfer and its mobility is dominated by the viscosity of the system. Therefore, application of solid polymer as SPE material is limited to gel impregnated with polar organic solvent and so on. In order to obtain the SPE that takes advantage of the solid polymer material, it seems necessary to propose new novel ionic transport mechanisms and basic research, evaluation and applied technology based on the molecular structure of macromolecules.
Tominaga Group aims at the creation and application of electrolyte materials sticking to "solid polymer". Specifically, we conduct fundamental and applied research as SPE of conventional systems and novel macromolecules from all directions by "molecular design", "physical property evaluation / structural analysis", "high performance and high function". Ultimately, achieving high ionic conduction of SPE which has never been seen, realizing all solid polymer batteries, and developing new applications making use of material properties as solid polymer is one of the goals.

Introduction to CO2 Utilization

Research on carbon dioxide (CO2), one of the greenhouse gases, is an important issue for protecting the global environment in the future. Carbon Capture and Storage (CCS) for large-scale sequestration of CO2 has already been put to practical use, and it is actively studied in Europe, America and Australia. Many of them are targeted for CO2 generated during natural gas refining, and researches targeting CO2, which is relatively low in concentration and concentration, discharged from power plants, steel works, etc. are also expanding in recent years.
On the other hand, research on carbon capture and utilization (CCU) using CO2 as a carbon source has also been reported. CCU is still at the basic research stage, and currently it is limited to practical application of only a small part, such as oil recovery promotion and urea production increase. Research and development of a new CCU that reduces CO2 to alcohol etc. has been drawing attention in recent years and studies using CO2 as a carbon source are considered to be an important issue for future greenhouse gas reduction. Tominaga Group focuses on CO2/epoxide copolymers among them. Studies on this copolymer began with research on the synthesis of alternating copolymers of ethylene oxide and CO2 reported by Tsuruta et al. in 1969. Tominaga Group considers effective use of CO2 as solvent and monomer, and applies it to various functional materials.