Polyether-based solid electrolytes with a homogeneous polymer network: effect of the salt concentration on the Li-ion coordination structure

We report a solid polymer electrolyte with an ideal polyether network that was synthesized by using tetra-functional poly(ethylene glycol) (TetraPEG) and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt. The solid TetraPEG electrolyte had few network defects (<5%) and exhibited high mechanical toughness by enduring approximately 11-fold elongation at a 1 : 10 ratio of Li salt to O atoms of PEG (Li/O_PEG). We found that the mechanical properties strongly depend on the Li/O_PEG ratio, which mainly contributes to the density of crosslinking points in the electrolyte. Raman spectroscopy and high-energy X-ray total scattering were used with all-atom molecular dynamics simulations to visualize the structural effects of Li-ion coordination in the TetraPEG network. At lower salt contents (Li/O_PEG = 1 : 10), Li ions were found to preferentially coordinate with O_PEG atoms rather than the TFSA anions to form crown ether-like Li⁺-PEG complexes as ion pair-free species. With increasing salt content, the TFSA anions partially coordinated with Li ions through O atoms of TFSA (O_TFSA) to afford contact ion pairs surrounded by both O_PEG and O_TFSA atoms. Finally, the ion pairing enhanced mononuclear ion pairs as well as multinuclear ionic aggregates when more Li salt was added. This structural change in the Li-ion complexes was directly reflected by the ion-conducting properties of the electrolyte. The TetraPEG electrolyte composed of the ion pair-free Li⁺ species (Li/O_PEG = 1 : 10) exhibited higher ionic conductivity, and the conductivity gradually decreased with increasing salt content because of extensive ion pairing for both mononuclear contact ion pairs and multinuclear aggregates. Regarding the electrochemical properties, the optimum electrolyte composition to realize a reversible Li deposition/dissolution reaction for a negative electrode was found to be Li/O_PEG = 1 : 4.

Star polymer networks: a toolbox for cross-linked polymers with controlled structure

The synthesis of precisely controlled polymer networks has been a long-cherished dream of polymer scientists. Traditional random cross-linking strategies often lead to uncontrolled networks with various kinds of defects. Recent...

Polymer network formation mechanism of multifunctional poly(ethylene glycol)s in ionic liquid electrolyte with a lithium salt

We report a controlled polymer network gel electrolyte based on a multifunctional poly(ethylene glycol) (PEG) prepolymer (herein, tetrafunctional PEGs (tetra-PEGs) and bisfunctional linear PEGs (linear-PEGs)) and an ionic liquid (IL)-based electrolyte solution containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSA) salt. The gel electrolyte was obtained via a gelation reaction, i.e., the Michael addition reaction between maleimide (MA)-terminated tetra-PEGs and thiol (SH)-terminated tetra- or linear-PEGs (termed tetra/tetra-PEG gel or tetra/linear-PEG gel systems), in a LiTFSA/IL solution under noncatalytic conditions at room temperature. For the tetra/linear-PEG system, the gelation reaction depended on the ratio of tetra-PEG-MA and linear-PEG-SH; an optimum terminal MA/SH ratio of 1 : 1 yielded a reaction efficiency (p) of ∼98% (an ideal polymer network structure). The tetra/tetra-PEG system with an MA/SH ratio of 1 : 1 also achieved a reaction efficiency of ∼98%. Time-resolved rheological measurements revealed that the network formation process can be categorized into three steps: (I) oligomer formation at an early stage of the reaction, (II) formation of a roughly linked polymer network with a large mesh size as the reaction proceeded, and (III) full network formation also at the local scale near the gelation completion time. The resulting tetra/linear-PEG ion gel with an optimum MA/SH ratio of 1 : 1 exhibited high stretchability, enduring approximately 10-fold elongation, and superior ion-conducting properties compared with the corresponding IL-based electrolyte solution.