The surge in renewable energy adoption and the rapid growth of the electric vehicle market have created a pressing need for high-performance, all-solid-state batteries. These innovative batteries offer higher energy density, improved safety, longer lifespan, and reliable operation over a wide temperature range compared to conventional liquid electrolyte-based batteries.
However, widespread adoption faces challenges such as low ionic conductivity, high interfacial resistance, and particle-particle interfaces in the electrolyte, leading to increased resistance and lower energy density.
High-performance solid electrolyte research has predominantly focused on inorganic and organic solid electrolytes. While inorganic solid electrolytes transport only lithium ions, organic solid electrolytes allow the migration of anions and other species. However, this can lead to side reactions at the electrodes, reducing capacity and adversely affecting battery performance and lifespan.
In contrast, inorganic electrolytes are less prone to side reactions, offering longer battery life and higher performance. However, they come with their own set of challenges. For example, oxide-type inorganic solid electrolytes face reduced stability and require high-temperature sintering, while sulfide-type electrolytes react with atmospheric moisture, producing toxic hydrogen sulfide gas.
To combat these challenges, a team of researchers from Japan embarked on a groundbreaking study, shifting their focus to organic ionic plastic crystals (OIPCs). OIPCs are composed of an organic cation, a suitable inorganic anion, and the lithium salt of the same anion. Made entirely of ions, these materials boast high ionic conductivity, exceptional stability, and minimal flammability, positioning them as ideal solid electrolytes for batteries.
A remarkable characteristic of OIPCs is their ability to transition between the solid crystalline phase and the liquid phase, known as the plastic crystal phase. Despite these advantages, OIPCs still require higher ionic conductivity to be suitable for practical applications.
In their study, Professor Masahiro Yoshizawa-Fujita and his research team from Sophia University, along with collaborators from the Tokyo Institute of Technology, harnessed the power of Material Informatics (MI) to unlock the potential of highly conductive Organic Ionic Plastic Crystals (OIPCs).
“MI leverages informational science, such as statistical science and machine learning, for efficient material development. In this study, we explored OIPCs by combining empirical rules and a machine learning-based MI model,” explains Prof. Yoshizawa-Fujita.
By leveraging a training dataset of chemical structures and conductivity data from OIPC-related literature, the team demonstrated the accuracy of their MI model in predicting the properties of OIPCs. Their findings revealed that the prediction accuracy significantly improves when the training data includes similar chemical structures, leading the researchers to identify promising candidate substances.
Through a combination of cutting-edge MI techniques and empirical rules from previous studies, the team successfully narrowed down the selection of candidate substances, with a particular focus on pyrrolidinium cations. This innovative approach holds immense promise for advancing the development of high-performance OIPCs and opens new doors for future research in the field.
The team has achieved a major breakthrough by successfully synthesizing eight new compounds, including six OIPCs and two ionic liquids. Among these, one compound has demonstrated exceptional ionic conductivity, setting a new benchmark in the field. The results have uncovered groundbreaking insights into the relationship between ionic radius and ionic conductivity of OIPCs, challenging established empirical rules. Additionally, their use of the MI model has revealed the potential for predicting phase transitions in OIPCs, hinting at even greater advancements on the horizon.
“The development of high-performance solid electrolytes will increase the safety of rechargeable batteries, as there will no longer be a concern about liquid leakage,” Prof. Yoshizawa-Fujita says, explaining the potential benefits of the new OIPCs. “Also, it will increase the energy density of these batteries, making devices equipped with batteries lighter and more compact. For example, OIPC-based rechargeable batteries can increase the range of electric vehicles and promote their widespread adoption.”
These findings underscore the tremendous potential of our work to revolutionize the development of safer, high-performance, and next-generation rechargeable batteries.
Journal reference:
- Takuto Ootahara, Kan Hatakeyama-Sato, Morgan L. Thomas, Yuko Takeoka, Masahiro Rikukawa, Masahiro Yoshizawa-Fujita. Efficient Exploration of Highly Conductive Pyrrolidinium-Based Ionic Plastic Crystals Using Materials Informatics. ACS Applied Electronic Materials, 2024; DOI: 10.1021/acsaelm.4c00861
