Kanno-Suzuki Laboratory

Lithium Battery

　Lithium ion secondary battery is a new battery system and is expected to be a contribution to solve environmental and energy issues. The discovery of the inorganic solid material by using intercalation reaction was the key to success of this battery system. Since lithium is a very base metal, it may create high electric potential if used as a conductor of a battery. Also, because it is light in volume, lithium has a high energy density per weight. Therefore, lithium is suitable for high electric potential and high energy density batteries. However, it was not developed as a practical battery component until 1991, because of its need of non-aqueous solvent electrolyte and problems with the anode dendrite formation. In this battery system, intercalation reaction (insertion reaction into structures) is operated in both cathode and anode. Only lithium goes in and out in electrochemical reaction by not changing the structure of crystal lattice. This reaction has high reversibility and does not have problems of dendrite formation. Therefore, materials that have this reaction mechanism are used in all of lithium secondary batteries. From a solid chemistry point of view, the way that crystal structures and physical properties of these materials change phantasmagorically along with lithium intercalation is fascinating and interesting. Since this phenomenon affects the battery’s charge-discharge property, the structural change with the battery reaction should be elucidated significantly. Although Cathode LiCoO2 is mainly used today, materials like Fe, that have bigger capacity and are lower cost and more friendly to environment, are investigated as next generation materials.

参考文献：構造からみたリチウム電池電極材料（GS Yuasa Technical Report）

All-Solid-State Battery

It is a dream for all chemists who involved in battery research to create a battery with only solid materials. Solid materials have advantages in many ways. They are high in reliability, stability, safety, and reversibility with no liquid leakage. Also, they operate in a wide temperature range from low to high temperatures. However, the actual research of this area has not been progressed far enough. The reason for this is that solid electrolytes that are practically endurable and have a wide potential window and as high ionic conductivity as liquid electrolytes are not discovered.

We have discovered LGPS(Li₁₂GeP₂S₁₂)-type compounds which have enough features as of solid electrolytes for all solid-state batteries and have been developing all-solid-state batteries using LGPS-type solid electrolytes. We have various tasks that we need to work on, such as connection problems of solid-solid interface and technical problems with making electrolytes and electrode sheets for batteries.

Solid Oxide Fuel Cells (SOFC) and Metal-Air Batteries

Fuel cells are now in the limelight as future energy resources and have been widely studied with the usage of polymer electrolytes. Solid Oxide Fuel Cells (SOFC) that we discuss here are considered as even more futuristic fuel cells. We operate oxygen ion conductor, zirconia as an electrolyte, oxide perovskites as air electrodes, and metals as fuel electrodes at more than 800 degrees Celsius. Therefore, we have been researched fuel cells, which operate at lower temperature.

Our research is mainly focused on electrode materials. The electrode reaction progresses at the interface of solid phase (electrode and electrolyte) and gas phase. To operate at as lower temperature as possible, we need to disclose the details of the electrode reaction of fuel cells. We construct the ideal two-dimensional interface with the epitaxial electrode film and research on the electrode reaction with the electrochemical measurement and the scattering method.

The air electrode reaction proceeds at room temperature at the electrode-liquid electrolyte interface, which is used as a metal-air battery. The model film studies have been applied to the interfacial analysis to improve the reaction rate and reversibility of the metal-air batteries.

Chemistry and Materials that Allow Ions to Move Rapidly Inside Them

Solid state ionics is the study of the phenomena of rapid ion diffusion, the materials, and their uses. The word, “electronics” is widely used in the studies of electrons moving inside solid materials and their related devices. Whereas, the word “ionics” which was named by Professor Takehiko Takahashi, refers to the studies of ions moving inside solid materials. Ever since silver iodide (α-AgI), silver ion that moves rapidly at the temperature of more than 147 degrees Celsius, was known to be a high ion conductor, the mechanisms of ion conductors have been researched and various other ion conductors were created. In the research of ion conductors up to today, Rb₄Cu₁₆I₇Cl₁₃, which was discovered in 1980, is considered to have the best ion conductivity. This material has the extremely high ion conductivity of 0.37 Scm^-1. None of other materials are found to have as high ion conductivity as Rb₄Cu₁₆I₇Cl₁₃. Some materials that use lithium as an ionic conductive agent are also discovered. However, none of them have as high ion conductivity as of silver and copper ion conductors. Their ion conductivities are a bit higher than 10^-3 Scm^-1.

We have been researching on lithium ion conductors, which are expected to serve as the electrolytes for lithium all solid-state batteries. However, they cannot be used unless they have high conductivity to some extent and a broad electrochemical window. Until recently, materials with both features had not been discovered. Sulfide lithium ion conductors have the highest lithium ion conductivity and a broad electrochemical window, while they are crystalline materials. They are expected to serve as the electrolytes for solid batteries.

Inside the solid materials, constituent elements hardly move. Normally, they finally move at near the melting point. If you think carefully, it is amazing that the one ion moves around like a fluid. It is interesting to think why it is able to move around and how to create such materials.

Oxide Electronics Chemistry

Material Development for Frustrated Compounds
　We have been researching mainly on oxides of electronics to discover new electronic physical properties that have high flexibilities in spin, electrical charge, and orbit. A feature of the studies of the physical properties of aggregates from chemistry perspective is to consider materials are to change forms freely. By fully using the synthetic methods, we precisely control the compositions and constructions in order to control the carrier concentrations. For example, when pyrochlore oxides are compositionally controlled by the high-pressure synthesis, the disproportionation of electrical charges causes the metal semiconductor transition. We revealed that a slight amount of oxygen deficiency affects the transition. In the oxides, including Fe4+, Ni3+, and Mn3+, which have the triangular and tetrahedral lattices that cause spin frustration, we have been researching the relations between the electrical charges and orbits ordered arrays and metal semiconductor transition, and the structural changes.