Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes

Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes

DOI: 10.1021/acs.chemmater.3c01353

Vitrimers are an important class of materials offering advantages over conventional thermosets due to their self-healing properties and reprocessability. Vitrimers are ideal candidate materials for solid polymer electrolytes because their viscoelasticity and conductivity can be independently tuned by salt addition in distinct ways from linear polymer electrolytes while further providing resistance to lithium dendrite propagation. In this work, the chemical and physical properties of vinylogous urethane (VU) vitrimers were characterized by using a combination of experiments and simulations to develop molecular design rules for controlling material properties. A series of VU vitrimers containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt were synthesized by precisely controlling the VU cross-linking density using defined linker lengths of ethylene glycol (xEG, x = 2, 3, 4, 6, or 12), thereby enabling control over the dynamic bond-to-EG ratio. Viscoelastic measurements show that the characteristic relaxation time τ* of VU vitrimers containing salt decreased by a factor of ∼70 relative to neutral vitrimers due to Li-ion coordination and catalysis of VU bond exchange. Stress relaxation times and shear moduli decrease with lower cross-linking densities in VU vitrimers. Solid-state ⁷Li NMR further reveals that VU vitrimers with longer linker lengths prefer lithium-ethylene oxide (Li-EO) solvation, whereas shorter linkers cannot sufficiently solvate the cation, and Li-VU coordination is preferred. Density functional theory (DFT) simulations were used to elucidate the dominant binding mode of Li-ion interaction as a function of linker length. The preferential partitioning of Li at the VU site leads to an order of magnitude decrease in stress relaxation times with a negligible impact on the conductivity after normalizing to the glass transition temperature T_g. Interestingly, our results show universal behavior for T_g-normalized ionic conductivity data regardless of linker length. Overall, this work provides new avenues for orthogonal tuning of bulk dynamics, recyclability, and conductivity in vitrimer electrolytes.

https://pubs.acs.org/doi/10.1021/acs.chemmater.3c01353.

Solution/solid-state ¹H/¹³C NMR; FT-IR spectra; TGA; DSC; normalized/non-normalized stress relaxation curves; rheological characterization of 2/3/4/6/12EO-LiTFSI as a function of temperature (stress relaxation, temperature sweep); ionic conductivity measured by EIS; DFT structures and delta binding energy as a function of sites; and Li⁺ transference number measurements