Shape-Conformable, Eco-Friendly Cellulose Aerogels as High-Performance Battery Separators

The ubiquity of portable electronics underlines the importance of high-performance flexible metal-ion batteries and the necessity of their development. Considering their ecological footprint, the application of eco-friendly recyclable battery components has become the greatest challenge and the focal point of research. However, less attention has been devoted to the development of shape-conformable separators with minimal impact on the battery performance and the environment. It is therefore imperative to develop a rational design of next-generation eco-friendly separators with an optimized structure–performance relationship. In this work, a highly flexible and eco-friendly cellulose-nanofiber aerogel (CNF-AG) separator is developed and its dynamic behavior in battery cells is assessed. The tailored channel-like structure with a meso- and macroporosity of 99.5% and good mechanical stability results in superior performance to the commercial glass fiber (GF) membranes and other cellulose-based separators. Its structure with a well-connected pore network and affinity to carbonate-based and ionic liquid electrolytes realize an electrolyte uptake of 12 000%. Furthermore, an effective diffusion coefficient of 1.70 × 10–10 m2 s–1, only 16% lower than that of the bulk electrolyte, yielded an ionic conductivity of 2.64 mS cm–1. Assessing the CNF-AG in lithium-ion batteries (LIBs) revealed a stable interfacial resistance over time, reaching 380 Ω, one-third of that obtained for GF. Accordingly, superior electrochemical performance is observed, achieving good cycling stability up to 200 cycles. Moreover, its applicability in aluminum-ion batteries is demonstrated. The outstanding structure–performance relationships of the developed CNF-AG indicate its superiority as a shape-conformable biodegradable separator suitable for metal-ion batteries.