Decoding Polymer Architecture Effect on Ion Clustering, Chain Dynamics, and Ionic Conductivity in Polymer Electrolytes

Poly(ethylene oxide) (PEO)-based polymer electrolytes are a promising class of materials for use in lithium-ion batteries due to their high ionic conductivity and flexibility. In this study, the effects of polymer architecture including linear, star, and hyperbranched and salt (lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI)) concentration on the glass transition (T _g), microstructure, phase diagram, free volume, and bulk viscosity, all of which play a significant role in determining the ionic conductivity of the electrolyte, have been systematically studied for PEO-based polymer electrolytes. The branching of PEO widens the liquid phase toward lower salt concentrations, suggesting decreased crystallization and improved ion coordination. At high salt loadings, ion clustering is common for all electrolytes, yet the cluster size and distribution appear to be strongly architecture-dependent. Also, the ionic conductivity is maximized at a salt concentration of [Li/EO ≈ 0.085] for all architectures, and the highly branched polymers displayed as much as three times higher ionic conductivity (with respect to the linear analogue) for the same total molar mass. The architecture-dependent ionic conductivity is attributed to the enhanced free volume measured by positron annihilation lifetime spectroscopy. Interestingly, despite the strong architecture dependence of ionic conductivity, the salt addition in the highly branched architectures results in accelerated yet similar monomeric friction coefficients for these polymers, offering significant potential toward decoupling of conductivity from segmental dynamics of polymer electrolytes, leading to outstanding battery performance.

Ultrathin Solid Polymer Electrolyte Design for High-Performance Li Metal Batteries: A Perspective of Synthetic Chemistry

Li metal batteries (LMBs) have attracted widespread attention in recent years because of their high energy densities. But traditional LMBs using liquid electrolyte have potential safety hazards, such as: leakage and flammability. Replacing liquid electrolyte with solid polymer electrolyte (SPE) can not only significantly improve the safety, but also improve the energy density of LMBs. However, till now, there is only limited success in improving the various physical and chemical properties of SPE, especially in thickness, posing great obstacles to further promoting its fundamental and applied studies. In this review, the authors mainly focus on evaluating the merits of ultrathin SPE and summarizing its existing challenges as well as fundamental requirements for designing and manufacturing advanced ultrathin SPE in the future. Meanwhile, the authors outline existing cases related to this field as much as possible and summarize them from the perspective of synthetic chemistry, hoping to provide a comprehensive understanding and serve as a strategic guidance for designing and fabricating high-performance ultrathin SPE. Challenges and opportunities regarding this burgeoning field are also critically evaluated at the end of this review.

Cage-Shaped Polymers Synthesis: A Comprehensive State-of-the-Art

Researchers have dedicated their efforts for the creation of a wide choice of complex and precise macromolecular architectures over the past 100 years. Among them, cyclic polymers benefit from their absence of terminal chains and from their singular topology to minimize their hydrodynamic volume in solution, increase their chemical stability, limit their number of possible conformations as well as a reduce their propensity to crystallize or to form entanglements in comparison to their acyclic counterparts. While monocyclic structures have already been widely investigated and reviewed, reports on more complex polycyclic structures are rare. In this regard, cage-shaped polymers-consisting of at least three polymer chains covalently interconnected through strictly two junction points-have received little attention over the past two decades. Although their synthesis is a worthy challenge, only a few synthetic methodologies of polymer cages were successfully developed so far. Thus, this review intends to highlight the key concepts of the conception of cage-shaped polymers in addition to propose an actual and exhaustive state-of-art concept of their synthesis to rationally promote the next-generation synthesis strategies.

Styrene-Based Poly(ethylene oxide) Side-Chain Block Copolymers as Solid Polymer Electrolytes for High-Voltage Lithium-Metal Batteries

Herein, we report the design of styrene-based poly(ethylene oxide) (PEO) side-chain block copolymers featuring a microphase separation and their application as solid polymer electrolytes in high-voltage lithium-metal batteries. A straightforward synthesis was established, overcoming typical drawbacks of PEO block copolymers prepared by anionic polymerization or ester-based PEO side-chain copolymers. Both the PEO side-chain length and the LiTFSI content were varied, and the underlying relationships were elucidated in view of polymer compositions with high ionic conductivity. Subsequently, a selected composition was subjected to further analyses, including phase-separated morphology, providing not only excellent self-standing films with intrinsic mechanical stability but also the ability to suppress lithium dendrite growth as well as good flexibility, wettability, and good contacts with the electrodes. Furthermore, good thermal and electrochemical stability was demonstrated. To do so, linear sweep and cyclic voltammetry, lithium plating/stripping tests, and galvanostatic overcharging using high-voltage cathodes were conducted, demonstrating stable lithium-metal interfaces and a high oxidative stability of around 4.75 V. Consequently, cycling of Li||NMC622 cells did not exhibit commonly observed rapid cell failure or voltage noise associated with PEO-based electrolytes in Li||NMC622 cells, attributed to the high mechanical stability. A comprehensive view is provided, highlighting that the combination of PEO and high-voltage cathodes is not impossible per se.