Gel Polymer Electrolytes Composed of Highly Concentrated Li Salt/Sulfolane Electrolytes and Homogeneous Polymer Network Having Weakly Coordinating Ability
Authors: Sakakieda Riko , Miyagawa Kazuki , Kokubo Hisashi , Dokko Kaoru 
Published: December 2024

Lithium-ion batteries (LIBs) use flammable organic electrolytes, and there are issues of ignition and leakage of liquid electrolytes. Sulfolane solvent is a non-flammable solvent. We previously reported that the sulfolane-based highly concentrated electrolytes exhibit unique transport properties and high Li+ ion transference numbers. 1) Gelation of liquid electrolyte is useful in preventing leakage and improves the safety of LIBs. Conventional gel polymer electrolytes (GPEs) have low mechanical reliability due to the structural defects such as loop chains and dangling chains. The mechanical toughness of GPE depends on the homogeneity of the polymer network. To achieve good mechanical toughness of GPE, the use of tetra-arm poly(ethylene glycol) (Tetra-PEG) has been proposed.2) The end-coupling reaction of two symmetrical tetra-PEGs can form homogeneous network structure, and tetra-PEG gels exhibit good mechanical toughness.
In this work, we synthesized tetra-arm poly(2,2,3,3-tetrafluoropropyl acrylate) (Tetra-PTFPA), and GPEs composed of a sulfolane-based highly Li salt-concentrated electrolyte and tetra-PTFPA. Tetra-PTFPA was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization using a tetrafunctional chain transfer agent in THF solvent. This polymerization solution was mixed with a highly concentrated electrolyte, LiN(SO2CF3)2 (LiTFSA) : sulfolane (SL) = 1:3 (molar ratio), then the THF was removed. Then, GPE with a uniform network structure were prepared by adding 1,3-propanediyl diacrylate (PDA) and further RAFT polymerization of both ends of PDA to tetra-PTFPA in the electrolyte to crosslink the different tetra-polymers. A self-standing GPE membrane could be prepared by RAFT polymerization at a polymer concentration of 20 wt%, while self-standing membrane could not be obtained through free radical polymerization at the same crosslinking density. This suggests that the homogeneous network structure in the tetra-PTFPA gel was effective in improving the mechanical reliability of GPE. The Li ion transference number (t
Li+) of the tetra-PTFPA gel was evaluated by DC polarization and determined to be 0.64. This value is higher than t
Li+ of the mother liquid electrolyte [LiTFSA]/[SL] =1/3 (t
Li+ = 0.55). This is probably due to the interaction between TFSA anion and the acidic protons surrounded by fluorine atoms in the polymer side chains of tetra-PTFPA.TFSA anions are assumed to be trapped by polymer side chains in the gel, while Li+ ion less interact with the side chains. This results in a higher transference number of Li+ in the gel electrolytes compared to that in the mother liquid electrolyte. Further results including mechanical and ion transport properties of the tetra-PTFPA gel will be presented.

Acknowledgements

This study was partially supported by Japan Science and Technology Agency (JST) GteX Program (Grant No. JPMJGX23S0) and JSPS KAKENHI (Grant No. 22H00340) from the Japan Society for the Promotion of Science (JSPS).
References
1) Dokko K., et al, J. Phys. Chem. B, 2018, 122, 10736-10745.
2) Sakai T., et al, Macromolecules, 2008, 41, 5379–5384.