Optimum in ligand density for conductivity in polymer electrolytes

• Published in: Molecular systems design & engineering 2021-11, Vol.6 (12), p.125-138
• Main Authors: Schauser, Nicole S, Richardson, Peter M, Nikolaev, Andrei, Cooke, Piper, Kliegle, Gabrielle A, Susca, Ethan M, Johnson, Keith, Wang, Hengbin, Read de Alaniz, Javier, Clément, Raphaële, Segalman, Rachel A
• Format: Article
• Language: English
• Subjects: Bulk density; Charge transport; Conducting polymers; Design optimization; Electrolytes; Ethane; High temperature; Imidazole; Ion transport; Ligands; Lithium ions; NMR spectroscopy; Normalizing; Percolation; Polymers; Siloxanes; Solubilization; Solvation; X-ray scattering
• ISSN: 2058-9689; 2058-9689
• DOI: 10.1039/d1me00089f

Current design rules for ion conducting polymers suggest that fast segmental dynamics and high solvation site density are important for high performance. In a family of imidazole side chain grafted siloxane polymer electrolytes containing LiTFSI, we conclude that while the presence of imidazole solvation sites promotes solubilization of Li + containing salts, it is not necessary to substitute every monomer in the polymer design. Rather, optimization of Li + conductivity relies on a balance between imidazole presence and the ability of the chains to rearrange locally to facilitate transport. Lowering the imidazole content in the ethane-imidazole series leads to a 10-fold increase in conductivity, while conductivity decreases for the phenyl-imidazole series due to differences in steric bulk. Normalizing conductivity by T g reveals a threshold ligand density above which increased solvation sites do not improve conductivity, but below which the conduction gradually decreases. NMR spectroscopy shows the high temperature Li + transport number increases slightly with increasing grafting density, from around 0.17 to 0.24. NMR T 1 ρ relaxation reveals that the Li + ions are present in two environments with distinct dynamics within the polymer, matching X-ray scattering and PFG results which suggest ion aggregation exists in these polymers. These results emphasize the importance of local re-arrangements in facilitating ion transport at low solvation site density, confirming the role of dynamic percolation, and suggest that an optimum ligand density exists for improved charge transport. Optimization of Li + conductivity relies on a balance between ligand presence and the ability of the chains to rearrange locally to facilitate transport.