Next-generation rechargeable batteries are promising candidates for state-of-the-art lithium-ion batteries owing to their high energy density and preferred cost efficiency. For instance, Lithium-sulfur batteries, which are featured by their theoretically 10 times higher capacity and 5 times higher energy density, are reviving in both the academic and the industry. Shu Lei Chou and colleagues from the Institute for Superconducting and electronic materials, University of Wollongong, presented a review article and proposed a new concept of 4S (stable, safe, smart, sustainable) batteries.

They reviewed the latest development of functional membrane separators in liquid-electrolyte next-generation batteries and based on which they reported the four important criteria for guiding the advancement of novel battery systems. This work, entitled "Functional membrane separators for next-generation high-energy rechargeable batteries", was recently published in National Science Review.

Compared to conventional lithium-ion batteries capable of thousands of cycles, next-generation batteries are plagued by the poor cycling behavior, which is normally caused by the active material loss and the electrode degradation. Functional membrane separators provide an effective approach to extend the cycling stability of several important battery systems.

As can be seen from Figure 2, this work breaks the boundaries of five types of next-generation batteries, i.e., Li-S, room-temperature Na-, Li-organic, organic-based redox-flow and Li-air batteries. Ion-selective materials are applied as the separator to retard the unwanted shuttling of some specific species, e.g., polysulfide diffusion in Li-S batteries.

The applied functional membrane materials are Nafion (protonated, lithiated or sodiated), polymer of intrinsic microporosity (PIM), polyurethane (PU), metal organic frameworks (MOF), graphene oxide and lithium superionic conductor (LISICON). All these materials, whether polymers or inorganics, possess characteristic pore structures for the transport of the component ions but reject others, therefore prevent the side reactions and greatly enhance the cycling stability.

The safety performance of batteries closely relates to the life and property security of customers, hence is also a key criterion for battery development. Separators with important properties of high thermal/dimensional stability, good wetting performance and excellent thermal conductivity help improve the battery safety. With regard to the notorious lithium dendrite problem, separator approaches that create homogeneous environment for lithium deposition enhance the battery safety.

Besides, this article reviews the latest works of smart and sustainable separators. For instance, a voltage-responsive smart membrane system was constructed using a doped polypyrrole. When the applied electric field is zero, the membrane allows no ionic current.

Otherwise, when a certain reducing electric field is applied, the transport of positive ions is facilitated because the polymer is negatively charged and provides hopping pathways for cations, the pore size expanded and the polymer turns from hydrophobic to hydrophilic.

In addition, renewable polymers like cellulose are studied as promising candidates for fossil-based polyolefin materials to enable sustainable separators. The paper concludes that functional separators need further investigation and are expected to play a key role in advancing next-generation batteries towards the goal of 4S: stable, safe, smart, and sustainable.

Research paper: Functional membrane separators for next-generation high-energy rechargeable batteries

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