Prof. Masahiro Tatsumisago has focused on the superiority of glassy materials in the solid state ionics field and has created numerous glass-based inorganic solid electrolyte materials having high ionic conductivity comparable to that of liquid electrolytes and ease in construction of solid-solid interface. In addition, he has created prototype all-solid-state secondary batteries using the solid electrolyte materials and pioneered the way of total solidification in electricity storage devices.
All- solid-state lithium secondary batteries have reached the eve of commercialization. The outline of his main research achievements is as follows.
1. Fabrication of highly ionic conductive glass-based electrolyte materials
Since inorganic solid electrolyte materials are nonflammable and single ion conduction has been realized, all-solid-state batteries using inorganic solid electrolytes are thought to be one of the ultimate goals of rechargeable energy sources. Prof. Tatsumisago focused on the open structure of disordered glass in an atomic arrangement, presented guidelines for searching the glass composition favorable to ionic conduction, and demonstrated numerous examples in experiments. In particular, he discovered that oxysulfide glasses in the systems Li2S-SiS2-LixMOy (M=Si,P,Ge,S) have high room temperature conductivity exceeding 10-3 S cm-1, and a guideline for enhancing ionic conductivity called the "mixed anion effect."
Based on the concept that direct formation of electrolyte particles should be advantageous, he succeeded in mechanochemical synthesis using a planetary ball mill of various glassy sulfide electrolyte materials. He also discovered that highly conductive glass-ceramics could be obtained by subsequent heat treatment. For example, new superionic conducting Li7P3S11 crystals were precipitated from 70Li2S·30P2S5 (mol%) glass, and due to optimization of heat treatment conditions for crystallization and improvement of sinterability, it reached 1.7×10-2 S cm-1 at room temperature, comparable to the conductivity of organic liquid electrolytes.
Prof. Tatsumisago constructed bulk type all-solid-state batteries composed of a composite electrode that combines a sulfide glass-based electrolyte and various electrode active materials. Charge and discharge tests of all-solid-state batteries using various positive and negative electrode active materials showed that sulfide-type batteries have excellent cycle performance and are extremely reliable.
The sulfide-based solid electrolytes have a serious problem of poor stability in air atmosphere. Prof. Tatsumisago found an extremely stable composition in the Li2S-P2S5 system and identified ion species with low reactivity with water molecules using various spectroscopic techniques such as Raman scattering. In addition, he succeeded in obtaining a solid electrolyte with high atmospheric stability while maintaining high conductivity through small amounts of oxygen and iodine substitution.
2. Formation and evaluation of solid-solid interface for construction of all-solid-state batteries
Unlike conventional batteries, in all-solid-state batteries, electrochemical reactions should occur at the solid-solid interface between the electrode active material and the solid electrolyte. How well a solid-solid interface is formed is the key to the success of all-solid-state batteries, and reduction of the interface resistance is the biggest problem in all-solid-state batteries. Prof. Tatsumisago succeeded in forming a good interface and greatly reducing the interface resistance by various creative methods. "Interface formation by gas phase and/or liquid phase methods" is a good example, and he succeeded in obtaining an excellent interface by coating of Li2S-P2S5 based sulfide electrolyte on the surface of electrode active material particles including LiCoO2 by gas phase or liquid phase method.
Microscopic observation of various active material-electrolyte interfaces using electron microscope before and after electrode reaction was conducted. The positive electrode composites using LiCoO2 particles coated with the Li2S-P2S5 film were sintered by pressing at room temperature, and an effective ion pathway among the active material particles was formed on their surface. This peculiar phenomenon was named "cold pressure sintering." Since the active material repeatedly changes in volume during charging and discharging, the mechanical properties within the elastic limit of the solid electrolyte material are also important. Prof. Tatsumisago succeeded in measuring the Young's modulus of various sulfide-based solid electrolyte materials, and revealed that the values at around 20 GPa were intermediate between oxides and organic polymers.
3. Fabrication of high energy density all-solid-state lithium secondary batteries
The most promising active material for all solid-state batteries is elemental sulfur for the positive electrode and metallic lithium for the negative electrode. Elemental sulfur is inexpensive, non-toxic and has extremely large theoretical capacity, but it is difficult to use with conventional liquid electrolyte batteries. Prof. Tatsumisago succeeded in fabricating a new high-capacity electrode composite by milling sulfur, copper or nano-carbon as a conductive additive, and also a sulfide solid electrolyte. Lithium sulfide, which is a discharge product of sulfur, also succeeded as a composite with copper or nano-carbon as well as sulfur. It was revealed that these lithium-sulfur based all-solid-state batteries exhibited excellent charge/discharge cycle performance even at a relatively high current density. With respect to the active material-electrolyte solid interface in the obtained electrode composite, structural analysis during charge and discharge was performed by use of microscopy, diffraction, and spectroscopy to clarify the reversible change of the sulfur-based active materials between microcrystalline and amorphous phases. He succeeded in remarkably improving charge/discharge performance of all-solid-state batteries using metallic lithium as the negative electrode by conducting interface analysis and generating In and/or Au thin films as buffer layers at the lithium-electrolyte interface.
As mentioned above, Prof. Tatsumisago clearly demonstrated guidelines for designing glass-based solid electrolyte materials and developed innovative inorganic solid electrolyte materials one after another. Also, while fabricating all-solid-state batteries and analyzing and evaluating the structure of the interface, he has increased his field's science and technology to the level on the eve of practical use of all-solid-state batteries. These achievements are highly appreciated both at home and abroad. With all-solid-state lithium batteries coming close to practical use, it is greatly expected that other new innovations will be brought about by Prof. Tatsumisago's research achievement. Therefore, his achievement was recognized as worthy of the Chemical Society of Japan Award.