Trade-offs between load-bearing capability and ionic conductivity of composite polymer electrolytes

Composite polymer electrolytes show strong potential for load-bearing energy storage systems, such as structural batteries, due to their high mechanical strength, flexibility, interfacial compatibility, low interfacial resistance, ease of manufacturing, and improved safety. However, literature often reports an inverse relationship between mechanical performance and ionic conductivity, requiring careful trade-offs to achieve optimal multifunctionality. This review discusses the thermal (e.g., glass transition temperature, melting point), mechanical (e.g., stiffness, strength), and ionic conductivity properties of commonly used thermoplastic polymers in electrolytes. It examines how viscoelastic behavior, molecular weight, and temperature influence these thermo-electro-mechanical properties and the need for balancing them. The role of additives—including salts, ionic liquids, nano-fillers, and thermoset polymers like epoxy—is evaluated, with a focus on their impact on electrochemical and mechanical performance trade-offs. Additionally, current manufacturing techniques along with their advantages and limitations are discussed. Eventually, the paper highlights strategies to overcome the mechanical–ionic conductivity trade-off, including the use of self-healing networks, functional fillers, and engineered architectures.