An all-solid-state battery is a battery in which the cathode, electrolyte, and anode are all composed of solid materials. Currently, the all-solid-state lithium battery is considered the closest to practical use. In the system, lithium-conductive solid materials replace the organic liquid electrolyte of a lithium-ion battery. At the Research Center for All-Solid-State Battery, we are conducting various studies to developing not only all-solid-state lithium batteries but also new energy devices that have yet to be named. Here we introduce a few specific details of our research.
・Exploration of sulfide-based lithium conductors
We have been developing sulfide-based materials for the fast diffusion of lithium ions. Superionic conductor Li10GeP2S12 (LGPS), which we reported in 2011, is one of the representative materials. We investigated the crystal structure of LGPS to clarify the details of ion diffusion.
We developed the LGPS material group by elemental substitution using various synthesis routes and methods based on this information.
The solid-state batteries using the developed LGPS materials exhibited significantly higher power characteristics than the conventional lithium-ion batteries at 100 °C. The advantages of solid-state batteries have gradually become clear. On the other hand, these sulfide materials are unstable in the ambient air, so we are also studying how the materials decompose and how to suppress their decomposition.
“A lithium superionic conductor“, Nature Materials, 10, 682-686 (2011)
“High power all-solid-state batteries using sulfide superionic conductors“, Nature Energy, 1, Article number: 16030 (2016)
・Exploration of oxide-based lithium conductors
Oxide materials are also attractive because of their high stability in the air. However, their ionic conductivities are still lower than those of sulfide materials. On the other hand, because of their stability, wide varieties of compositions have already been examined, and finding new materials is not easy. Therefore, we are collaborating with theoretical and computational chemistry researchers to develop new material search methods. The material recommender system learns chemical compositions registered in databases as stable compositions. It predicts the probability of the existence of unreported chemical compositions. Combining this guidance with synthetic chemistry allows us to find new materials efficiently. We are also building machine learning models to predict chemical compositions with high ionic conductivity and are using these cutting-edge methods to accelerate the discovery of new materials.
“Fast material search of lithium ion conducting oxides using a recommender system“, J. Mater. Chem. A, 8(23), 11582-11588 (2020)
“Search for Lithium Ion Conducting Oxides Using the Predicted Ionic Conductivity by Machine Learning“, J. Jpn. Soc. Powder Powder Metallurgy, 69(3), 108-116 (2022)
“Extending the Frontiers of Lithium-Ion Conducting Oxides: Development of Multicomponent Materials with γ-Li3PO4-Type Structures“, Chem. Mater., 34(9), 3948–3959 (2022)
・Reaction analysis in the solid-state battery
In a solid-state battery, charge and discharge proceed by diffusion of ions and electrons among the solid materials. Observations of the diffusion phenomena from various viewpoints using various analytical methods are essential because we can identify factors that interfere with battery reactions and use the obtained information to improve battery characteristics.
To this end, it is crucial to construct as simple a model battery as possible and design experiments suitable for the purpose. Therefore, we utilize surface and interface crystal structure analysis, electronic structure analysis, X-ray/neutron beam reflectivity analysis, ion beam analysis, and in situ microscopy. We have clarified the improvement of lithium diffusivity due to changes in the crystal structure of electrodes, changes in the potential at the electrode-electrolyte interface, and changes in lithium distribution at the interface during charging and discharging. Therefore, we utilize methods such as surface and interface crystal structure analysis, electronic structure analysis, X-ray/neutron beam reflectivity analysis, ion beam analysis, and in situ microscopy. We have clarified the improvement of lithium diffusivity due to changes in the crystal structure of electrodes, changes in the potential at the electrode-electrolyte interface, and changes in lithium distribution at the interface during charging and discharging.
Our plan is not limited to upgrading our analytical methods. As the next step, we will also change the combination of materials, conductive ion species, and other analytical targets to gain a bird’s eye view of the phenomena inside solid-state batteries from a unified perspective. Finally, we will propose future visions for solid-state batteries and energy devices.
“Reaction Mechanism of Li2MnO3 Electrodes in an All-Solid-State Thin-Film Battery Analyzed by Operando Hard X-ray Photoelectron Spectroscopy“, J. Am. Chem. Soc., 144(1), 236–247 (2022)
“Operando analysis of electronic band structure in an all-solid-state thin-film battery“, Communications Chemistry, 5, Article number: 52 (2022)
・Search for anionic conductors
The basic properties of energy storage devices are determined by the type of ions involved in battery reactions. We are developing solid electrolytes based on new ion species different from Li+, H+, O2−, etc., to realize new energy storage devices. In particular, hydride ion (H−), an anion of hydrogen, is a new mobile ion. It has high polarizability (soft), moderate size, and valence; therefore, it is expected to diffuse at high speed. We have reported a hydride ion conductor in which hydride ions diffuse rapidly in a metal oxide framework. Recently, we have been exploring new materials to improve the ion conductivity of hydride ionic conductors.
We aim to establish design guidelines for hydride ionic conductors.
For this purpose, we are synthesizing new materials, visualizing the distribution of hydride in the structure by neutron diffraction measurements, and then determining the diffusion barrier of hydride using first principles calculations. Finally, we clarify the correlation between crystal structure and hydride ionic conductivity.
In addition, we are working on the development of fluoride ion (F−) conductors. The development of fluoride ion conductors has a long history, with many fluoride ion conductors reported in the 80s and 90s. In recent years, under the exit strategy of all-solid-state fluoride batteries, the importance of fluoride solid electrolytes has increased again. Therefore, new electrochemically stable and highly ion-conductive materials are desired. We have been working on efficiently finding promising solid electrolyte materials from the vast space of material search using first-principles calculations and informatics methods.
“Pure H– conduction in oxyhydrides“, Science, 351(6279), 1314-1317 (2016)
“Hydride-ion-conducting K2NiF4-type Ba–Li oxyhydride solid electrolyte“, Nature Materials, 21, 325–330 (2022)
“Reversible Charge/Discharge Reaction of a Ternary Metal Fluoride, Pb2CuF6: A Highly Conductive Cathode Material for Fluoride-Ion Batteries“, ACS Appl. Energy Mater. 5, 1, 1002–1009 (2022)