Polymer electrolytes are considered as solid-state ionic conductors for the next generation of lithium metal batteries. Particularly, poly(ethylene oxide)-based electrolytes have been extensively studied due to their excellent ion conductivity and interfacial compatibility. Since the lithium-ion transport is strongly coupled with segmental motion of polymer chains, there usually exists a dilemma between the ion conductivity and mechanical strength enhancement. Other problems of low ion conductivity at low temperature and low cation transference number further impede their practical applications in batteries. Here, we report a type of polythiourea solid-state electrolyte, which exhibits a superionic ion conducting behavior. The strong supramolecular interaction between the thiourea group and perchlorate anions promotes the dissolution of high concentration lithium salts in polythiourea. The conducting channel composed of high-density thiourea and percolating polar groups enables rapid cation transport along the polymer chain and even working at the glass state. The combined merits of high transference number, good ion conductivity, and excellent mechanical strength are achieved in this class of superionic ionic conductors, shedding light on alternative solutions for solid-state electrolytes in batteries.

Solid polymer electrolytes (SPEs) are attractive for next-generation lithium metal batteries but still suffer from low ionic conductivity. Nanostructured materials offer design concepts for SPEs with better performance. Using molecular dynamics simulation, we examine SPEs under nanoscale confinement, which has been demonstrated to accelerate the transport of neutral molecules such as water. Our results show that while ion diffusion indeed accelerates by more than 2 orders of magnitude as the channel diameter decreases from 15 to 2 nm, the ionic conductivity does not increase significantly in parallel. Instead, the ionic conductivity shows a nonmonotonic variation, with an optimal value above, but on the same order as, its bulk counterparts. This trend is due to enhanced ion association with decreasing channel size, which reduces the number of effective charge carriers. This effect competes with accelerated ion diffusion, leading to the nonmonotonicity in ion conductivity.

Graft polymers have been widely investigated as solid polymer electrolytes (SPEs) in the past decades. However, the presence of insulating backbones in the conventional graft polymers damps the overall Li+ conductivity and transport number (tLi+). Herein, a series of polycarbonates (PCs) possessing ethylene oxide (EO) side-chains were designed and synthesized through ring-opening polymerization (ROP), and their ionic conductivities were evaluated as SPEs with LiTFSI. The synergy of conductive backbones and side-chains gives a high tLi+ value of 0.67, while having an ionic conductivity of 210-5 S cm-1 at 30 C. This work provides new insights into the development of high-performance SPEs by combining different conductive polymers.

Electrolyte engineering with fluoroether as solvents offers promising potential for high-performance lithium metal batteries. Despite recent progresses achieved in designing and synthesizing novel fluoroether solvents, a systematic understanding of how fluorination patterns impact electrolyte performance is still lacking. We investigate the effects of fluorination patterns on fluorinated 1,2-diethoxyethane (FDEE) as electrolyte solvents. By employing quantum calculations, molecular dynamics simulations, and interpretable machine learning, we establish significant correlations between fluorination patterns and electrolyte properties. Higher fluorination levels enhance FDEE stability but decrease conductivity. The symmetry of fluorination sites is critical for stability and viscosity, while exerting minimal influence on ionic conductivity. FDEEs with highly symmetric fluorination sites exhibit favorable viscosity, stability, and overall electrolyte performance. Conductivity primarily depends on lithium-anion dissociation or association. These findings provide design principles for rational fluoroether electrolyte design, emphasizing the trade-offs between stability, viscosity, and conductivity. Our work underscores the significance of considering fluorination patterns and molecular symmetry in the development of electrolytes for advanced lithium metal batteries.

Solid polymer electrolytes (SPEs), though widely regarded as materials that can enable next-generation lithium metal battery with improved safety, extended stability, and high capacity, suffer from problems of low ionic conductivity. Myriads of strategies have been proposed to improve the performance of polymer electrolytes, but with little success, and the state-of-the-art polymer electrolytes are still based on LiTFSI dissolved in PEO, which have been proposed for more than 40 years. New design concepts are indispensable to improving the performance of SPEs. Using molecular dynamics simulation, we examine SPEs under nanoscale confinement, which has been demonstrated to accelerate the diffusion of neutral molecules such as water. While ion diffusion indeed shows an acceleration by more than two orders when the channel diameter decreases from 15~nm to 2~nm, the ionic conductivity does not show a paralleling increase. Instead, as the nanochannel diameter decreases, ionic conductivity shows a non-monotonic variation, with an optimal value above yet on the same order as its bulk counterparts. The reason for this trend is due to enhanced ion association with decreasing channel size, which leaves a smaller amount of effective charge carriers. This effect competes with accelerated ion diffusion, leading to the non-monotonicity in ion conductivity. We further show that this trend also appears in two-dimensional nanoslit pores. These findings not only manifest the intricacies of ionic transport behavior but also provide new concepts and necessary implications for designing composite SPEs.