New Polymer Boosts Safety, Longevity of Lithium Batteries

Abstract

Owing to their flexibility and cost-effectiveness, solid polymer electrolytes (SPEs) offer a promising alternative to inorganic solid electrolytes. Here, we present a highly oriented relaxor ferroelectric SPE based on a polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (PVDF-TrFE-CFE) matrix achieved through external elongation. The incorporation of a large Cl atom into the ferroelectric polymer chain effectively increases the amorphous regions, allowing for up to 300 % stretching and facilitating the alignment of the polymer chains. This enhanced orientation, confirmed by 2D wide-angle X-ray diffraction, reduces tortuosity and improves Li-ion transport compared to the unstretched sample. Moreover, molecular dynamics (MD) simulations and electrochemical evaluations further demonstrate the advantages of this structure. The aligned amorphous regions, as revealed by MD simulations, provide favorable and continuous pathways for Li-ion transport, facilitating the stable electrochemical performance observed in both Li//Li symmetric cells and full cells with Li iron phosphate cathodes. Additionally, the incorporation of tantalum-doped Li lanthanum zirconate as an active filler further enhances the mechanical strength and electrochemical properties of the SPE, achieving a high ionic conductivity of approximately 3.63 × 10−4 S cm−1 and extended cycling stability. These results highlight the potential of highly oriented PVDF-based SPEs for next-generation Li-ion battery applications.

A research team, affiliated with UNIST has demonstrated a simple yet effective method to extend the lifespan of all-solid-state batteries-by simply stretching film-shaped electrolytes to improve safety and performance.

Led by Professor Seok Ju Kang from the School of Energy and Chemical Engineering at UNIST, in collaboration with Professor Se Hun Joo from Sookmyung Women's University, the research team announced the development of a new type of film-forming electrolyte, capable of enabling longer-lasting solid-state batteries.

Electrolytes serve as the medium for lithium ions to move between the cathode and anode within a battery. Currently, commercial electric vehicle batteries and large-scale energy storage systems primarily use flammable liquid electrolytes. While replacing these with solid polymer electrolytes reduces fire and explosion risks, they have historically suffered from lower lithium-ion mobility, resulting in capacity loss over repeated charge and discharge cycles.

The research team developed a fluorinated polymer-based film electrolyte (PVDF-TrFE-CFE) that significantly enhances lithium-ion transport. The key innovation lies in the application of a uniaxial stretching process, which aligns the polymer chains in a single direction. This physical stretching unfolds the convoluted polymer structure, opening up continuous pathways for lithium-ion movement. Additionally, incorporating ceramic powder (LLZTO) into the polymer matrix enhances mechanical flexibility, flame retardancy, and ion conductivity.

Experimental results showed that the lithium-ion diffusion rate in the stretched polymer electrolyte increased by 4.8 times compared to unstretched samples, with ionic conductivity improving by 72%. When integrated into lithium-metal batteries with lithium iron phosphate (LFP) cathodes, the stretched electrolyte contributed to a notable increase in battery lifespan. After 200 charge-discharge cycles, these batteries retained approximately 78% of their initial capacity, compared to only 55% in batteries using unstretched electrolyte.

Flame retardancy tests confirmed the safety benefits of the new electrolyte; flames extinguished within just four seconds of ignition.

"This research demonstrates that the inherent issues of polymer electrolytes-such as hindered lithium-ion transport-can be effectively addressed through a simple physical process like stretching," said Jonggeon Na from the School of Energy and Chemical Engineering at UNIST, the first author of the study.

Professor Kang added, "Polymer electrolytes are more flexible and easier to produce at scale compared to inorganic solid electrolytes. The method developed in this study can be applied to various types of polymer electrolytes, accelerating the commercialization of safer, longer-lasting all-solid-state batteries."

This research was supported by the National Research Foundation of Korea (NRF), UNIST, and the Ministry of Science and ICT (MSIT)'s InnoCORE program. The findings were published online in the Energy Storage Materials, a leading international journal in the field of energy materials (IF 20.2), on October 31, 2025.

Journal Reference

Minju Lee, Jonggeon Na, Seongeun Oh et al., "Uniaxially Aligned Relaxor Ferroelectric polymer electrolyte for high-performance solid-state lithium batteries," Energy Storage Mater., (2025).