Theoretical Study on the Dynamics of Lithium-Ion Transport in PPS-Based Polymer Electrolytes
Authors: Chae Somin , Lee Sangheon 
Published: August 2024

The development of solid-state lithium-ion batteries (LIBs) is a key advancement in energy storage technology. Solid electrolytes are important in this development because they are safer and more stable than liquid electrolytes, and they have higher energy density. Among the various types of solid electrolytes, Polyphenylene Sulfide (PPS)-based solid-state polymer electrolytes (SPEs) are notable for their ability to conduct ions as well as liquid electrolytes across a wide range of temperatures. This ability is particularly important because other solid polymer electrolytes, like those based on polyethylene oxide (PEO), struggle with ion conduction at different temperatures. PPS-based SPEs are different because their ion transport does not depend on the movement of the polymer chains, which means their ion conduction remains steady from high to low temperatures.

The previous study conducted a series of first-principle calculations, revealing that the introduction of neutral molecules, referred to as agent molecules, can significantly enhance the movement of lithium ions within a solid matrix when these are mixed with lithium salts. This is because the intermolecular interactions within a binary system comprising an agent molecule and lithium salt are governed by a strong bond formation between lithium and oxygen atoms. When these agent molecules are introduced, they replace the anionic species around lithium in the salts, leading to a weakened Coulomb force between lithium and oxygen. This reduction is essential for rapid lithium-ion movement through the easy separation of lithium salts and the subsequent generation of ion-hopping sites characterized as lithium-free oxygen cages. According to the previous study, the strategic selection of neutral molecules with functional groups that bolster chemical resonance is imperative. Such molecules have been identified as promising candidates for agent molecules.

Our study builds upon previous research by directly incorporating PPS polymers into our computational simulations. We utilized molecular dynamics and quantum mechanics to examine the role of neutral molecules in PPS in facilitating lithium-ion mobility. We parameterized our system with quantum mechanics calculations, which were crucial in informing our molecular dynamics simulations. Using VASP program for interaction assessment and LAMMPS program for energy and structure analysis, we refined our models to understand the interactions between PPS polymers, Li+ ions, TFSI- ions, and Chloranil molecules. Our simulations explored the structural dynamics and the effects of fillers and PPS polymer layers on ion transport.

Our work enhances the basic understanding of SPEs and helps in the creation of new lithium-ion batteries that perform better and are safer. We've pinpointed important factors that control how lithium ions move in PPS-based electrolytes, laying the groundwork for future improvements in solid-state electrolytes and leading to lithium-ion batteries that are more effective and safer.

Reference:

[1] Jiwon Yu, Myungsuk Lee, Yeonseo Kim, Hyung-Kyu Lim, Jonghyun Chae, Gyeong S. Hwang, Sangheon Lee, “Agent molecule modulated low-temperature activation of solid-state lithium-ion transport for polymer electrolytes”, Journal of Power Sources 505, 229917 (2021).