Solid polymer electrolytes: ion-conduction enhancement and comprehensive frontiers

Research teams from Lanzhou University, Shandong University of Technology, University of California, Irvine, and Hanyang University have published a comprehensive review on solid polymer electrolytes (SPEs) for next-generation solid-state batteries. Their study, recently accepted in Materials Futures, explores the safety, flexibility, and scalable processability of SPEs, and illustrates how molecular design enables tunable ion-conduction pathways, stable electrode contact, and large-scale manufacturability. Key topics covered include ion-transport mechanisms, polymer chemistry strategies, inorganic filler engineering, and future research directions.

As the global demand for safer, high-energy-density batteries continues increasing, traditional liquid electrolytes face growing scrutiny for their flammability, volatility, and poor compatibility with lithium metal. Solid-state batteries, and particularly solid polymer electrolytes (SPEs), offer a safer and more flexible alternative. Moreover, with the shift toward high-capacity alkali metal anodes, issues like dendrite formation become more critical, highlighting the need for safer and more stable electrolytes like SPEs.

SPEs offer intrinsic adhesion to electrodes, low interfacial resistance, and flexible processing advantages over brittle inorganic solid electrolytes. They eliminate fire risks, enable adaptable film formation, and help suppress dendrite growth while accommodating volume changes during cycling. Their compatibility with scalable production methods makes them promising for high-performance solid-state batteries.

However, current SPEs have still challenges to solve including limited room-temperature conductivity, narrow electrochemical stability windows, and high interfacial resistance, which can cause uneven lithium deposition. The authors explore structural design and in-situ solidification techniques to improve contact, reduce polarization, and prevent dendrites. Scalable fabrication methods such as spray coating are also explored.

Central to SPE performance are Li⁺ transport mechanisms, which rely on polymer segmental motion (enabled by amorphous regions), ion-pair dissociation (influenced by dielectric constant), and percolation effects at polymer-filler interfaces. Even crystalline regions can support fast ion conduction in ordered structures, such as helical PEO channels, underscoring the importance of molecular-level structural design.

This work provides a comprehensive roadmap for tackling the persistent limitations of SPEs. The authors integrate insights from polymer chemistry, material physics, and electrochemistry to offer multi-pronged innovations like: (i) organic-framework engineering (crosslinking, copolymerization, blending, semi-IPNs) to reduce crystallinity and enhance ion mobility; (ii) geometric optimization (ultrathin films, electrospun fiber scaffolds, and engineered solvation cages) to shorten pathways and stabilize interfaces; (iii) single-ion conductors and supramolecular networks to increase the Li⁺ transference numbers and heal mechanical damage; (iv) polymer fluorination to extend oxidation stability and build robust LiF-rich interphases that suppress dendrite growth; and (v) polymer-in-salt electrolytes that decouple ion transport from chain motion. Representative designs include electrospun PAN-reinforced electrolytes (high-temperature structural stability, long cycling) and ultrathin films (enhanced critical current density, better interfacial stability). 

On the filler side, the review contrasts inert fillers (e.g., silica aerogels, TiO_2-x) that expand amorphous regions and strengthen mechanics; active fast-ion conductors (e.g., LLZTO, LZP) that form dual networks for rapid Li⁺ transport; and functional fillers (e.g., defect-engineered chalcogenides or hybrid g-C₃N₄/LLZTO systems) that tune solvation, anchor anions, and build robust SEI chemistry together reducing interfacial resistance, homogenizing ion flux, and suppressing dendrites.

The authors highlight key research priorities: in-situ polymerization within cells, layered/composite structures that simultaneously optimize conductivity and mechanical strength, continuous ion-conduction pathways at interfaces (especially in ultrathin films), and real-time in-situ characterization to monitor polymerization, filler distribution, and interphase formation. Future efforts should also focus on flame-retardant designs and high Young’s modulus systems to balance safety, mechanical integrity, and long-term cycling performance.

In summary, the review notes that SPE development is evolving from a narrow focus on conductivity toward a multi-functional approach, integrating advanced materials design with scalable processing to meet the demands of future solid-state batteries in terms of safety, energy density, and cycle life.

Reference: Yaohui Wang, Peng Li, Bing Liu, Xinyue Wei, Wangyu Fu, Hui Li, Yuxin Cheng, Qinggang Zhang, Changjing Li, Wengao Zhao. Solid polymer electrolytes: ion conduction enhancement and comprehensive frontiers [J]. Materials Futures. DOI:10.1088/2752-5724/ae0ce3