Important Structural Features to Enhance Na-Ionic Conductivity in Single-Ion-Conducting Polymer Electrolytes
Authors: Kagawa Yuta , Miyagawa Masaya , Takaba Hiromitsu 
Published: December 2024

Polymeric solid electrolytes (SPEs) have excellent properties such as high safety and long life and are expected to be put to practical use as next-generation all-solid-state lithium secondary battery electrolyte. Single ion-conducting polymer electrolytes (SICPEs) are one of SPEs and have a structure in which the anion is covalently bonded to the polymer. They therefore have the advantage of high cation transference number, and a long lifetime because ionic polarization is less likely to occur. However, as the ionic conductivity is comparable to that of conventional SPE, it is necessary to clarify the structure that facilitates ion diffusion to further improve ionic conductivity. In this study, machine-learning combined with first-principles calculations and molecular dynamics (MD) were used to elucidate the important features for high Na-ionic conductivity in SICPEs.

The diffusion of ions within the polymer is related to the desorption rate of ions from the strong interaction site in the polymer. Therefore, the adsorption energies (Eads) for thirteen different SICPEs monomer structures were firstly evaluated by a density functional theory (DFT) calculation. Among of the investigated SICPEs, the monomer structure of poly [lithium malonato oligo (ethylene glycolato) orthoborate] (MEGB) is explained as an example. In this polymer model, cations coordinate near anions with weak interaction to surrounding ether oxygen atoms. To consider multiple possible adsorption structure, DFT calculations were performed for several adsorption configurations and the one with the most stable energy was adopted. In addition, a bulk model of the electrolyte was prepared for SICPEs to perform MD simulations at 298 K to investigated 3D level structural features.

The correlation between calculated Eads and literature values of their ionic conductivity (σ) is summarized and a reasonable correlation between Eads and σ was observed, indicating that the desorption process of ions from strong polymer interaction is dominant for diffusion dynamics. A high interaction energy between ions and polymers would suppress ion diffusion, resulting in a low ionic conductivity. Adsorption energy correlates with the distance between the anion atom and Na ion. The electronic structure of polymers such as HOMO/ LUMO was also investigated, however, there was no significant correlation has been observed. The free volumes of polymers were also estimated using the structure obtained by MD. The ratio of free volume against the total volume of the polymer was between 20 – 30 %. That free volume has no relation with the Na-ion conductivity. These results indicate that the Na-ionic conductivity is mainly governed by the interaction with the polymer chains, and the dynamics of Na-ion diffusion through the polymer chains and adsorption process to the strong interaction site are not significant in determining the Na-ionic conductivity. In the presentation, based on these results, novel polymer structures to enhance the Na-ionic conductivity will be presented with some validation data.