Hierarchical Porous Block Copolymer Separator Membranes for Ultrahigh Rate Li-Ion Batteries
Authors: Kim Jung-Hwan , Kim Ju-Myung , Kim Se-Hee , Lee Dong-Gue , Lee Sang-Young 
Published: February 2020

High-performance energy storage systems with reliable and sustainable electrochemical properties are urgently needed to satisfy the continuously surging demand in consumer electronics, electric vehicles (EVs), and grid-scale energy storage systems (ESSs). Among the enormours energy storage systems reported to date, lithium-ion rechargeable batteries (LIBs) have still garnered a great deal of attention. The rational design and fabrication of major battery components such as anodes, cathodes, electrolytes, and separator membranes are imperative prerequisites for the development of advanced batteries. Most research activities on battery components have been devoted to the electrochemically active materials, with a especially focus on electrode materials and electrolytes. Representative results include those related to high-voltage spinel nickel manganese oxides, overlithiated layer oxides, silicon- or metal alloys, functional electrolyte additives, and solid-state electrolytes. Taking into consider the fact that electrochemical performance of the batteries is basically governed by  electron/ion transport phenomena, particular attention should be paid to battery separators as well as electrodes/electrolytes, because (i) all ions particiapted in Faradaic reaction of batteries should pass through electrolyte-ﬁlled porous separator membranes and (ii) Internal short-circuit failure occurring between electrodes (which is considered as a primary cause to trigger cell ﬁre or explosion) is basically prevented by separator membranes.

Currently, commercially available separators in LIBs are manufactured using polyoleﬁn materials. These polyoleﬁn separators have some advantageous attributes that render them suitable for practical use in LIBs. However, their intrinsic limitations (speci ﬁcally, sluggish/nonuniform ionic ﬂow and poor thermal stability) often raise concerns regarding ion transport and electrical isolation between the electrodes. A large number of approaches to overcome these drawbacks have been undertaken, which include ceramic-coated separators, nonwoven separators, and electrospun nano ﬁber separators. Unfortunately, little attention has been devoted to controlling the ion transport phenomena in a separator, despite its important role in activating the electrochemical performance of cells.

Here, as a new strategy to address these challenging issues, we demonstrate new surface energy-tailored amphiphilic polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) membranes with hierarchical multiscale hyperporous structures and chemical functionality (Figure 1), as a revolutionary membrane to enable remarkable advances in cell performance far beyond those achievable with conventional separators. Among the various building blocks for use in porous membranes, BCPs have been extensively investigated because of their self-assembly-enabled nanodomains. Through spinodal decomposition, breath figures, solvent swelling, and nonsolvent-induced phase separation (NIPS), BCPs can produce precisely defined porous structures in versatile shapes.

Owing to facile and effective control of membrane morphologies, NIPS process, which is also known as immersion precipitation, has attracted great attention as a promising way to fabricate large-scale membranes. In the NIPS method, final morphologies (e.g., dense or porous, symmetric or asymmetric) of the membranes can be adjusted by combining the involved mass transport phenomena with corresponding phase separation. For membrane-driven purification and separation processes, asymmetric porous structure is preferable because it endows the membranes with both permeability and selectivity. Asymmetric structures with a well-organized skin layer were explored using self-assembled BCP films. Time-controlled evaporation of volatile solvent produces a skin layer with vertically aligned cylindrical structure supported by a graded porous layer, when the BCP solution was immersed into a nonsolvent. Meanwhile, porous membranes with symmetric structures are suitable for allowing two-way ionic transport, which is essentially required for separation membranes of rechargeable power sources. The macropore size of NIPS-based BCP membranes is basically determined by polymer concentrations and solvent-nonsolvent exchange kinetics, while their nanopore size strongly depends on the compatibility between pore-forming block and nonsolvent. In particular, construction of well-defined nanoporous morphology is highly required for advanced BCP membranes. However, even though BCPs, good solvent and nonsolvent are appropriately chosen, conventional NIPS-based BCP membranes normally generate poorly-developed nanopores due to a limited compatibility between pore-forming block and nonsolvent. To date, the most common way to tune nanoporous structure is limited to variation of molecular weight of pore-forming blocks.

Our strategy for tailoring high porous BCP separator membrane is based on the introduction of a surface-energy-modifying agent, which allows fine-tuning of nanoscale phase separation between the surface-modified BCPs and nonsolvent. The surface-energy-modifying agent can be easily introduced into a pore-forming block of BCPs via nucleophilic substitution reaction. Depending on the degree of substitution, the surface-energy-modifying agent contributes to tuning dual (macro-/nano-) phase separation of the BCPs, which eventually enables the fabrication of hierarchical multiscale porous block copolymer membranes. The novel block copolymer separator membrane exhibited superior rate capability and cycle performance, owing to its precisely tuned, highly developed porous structure far beyond those achievable with conventional separator membrane technologies.

Figure 1