Article pubs.acs.org/cm
Mussel- and Diatom-Inspired Silica Coating on Separators Yields Improved Power and Safety in Li-Ion Batteries Sung Min Kang,†,‡ Myung-Hyun Ryou,§,‡ Jang Wook Choi,*,§,⊥ and Haeshin Lee*,⊥,#,|| †
Department of Marine Bio-Materials & Aquaculture, Pukyong National University, Busan, 608-737, Korea Graduate School of EEWS (WCU), ⊥KAIST Institute for Nanocentury, #Department of Chemistry, and ||Graduate School of Nanoscience & Technology (WCU), KAIST, Daejeon, 305-701, Korea
§
S Supporting Information *
ABSTRACT: In this study, we developed an integrative bioinspired approach that improves the power and safety of Li-ion batteries (LIBs) by the surface modification of polyethylene (PE) separators. The approach involves the synthesis of a diatom-inspired silica layer on the surface of the PE separator, and the adhesion of the silica layer was inspired by mussels. The mussel- and diatom-inspired silica coating increased the electrolyte wettability of the separator, resulting in enhanced power and improved thermal shrinkage, resulting safer LIBs. Furthermore, the overall processes are environmentally friendly and cost-effective. The process described herein is the first example of the use of diatom-inspired silica in practically important energy storage applications. The improved wetting and thermal properties are critical, particularly for large-scale battery applications. KEYWORDS: biomimetics, silica, surface modification, Li-ion battery
1. INTRODUCTION Bioinspired chemistry and materials have been a source of numerous breakthroughs in many areas. Bioceramic silica components found in diatoms1 and sponges2 are good examples. Kröger et al. discovered that silaffin, a cationic polypeptide and silaffin-mimetic polyamines trigger a biosilicification reaction under mild, aqueous conditions.3,4 Diverse silica structures such as nanoparticles,5−7 thin films,8,9 and composite materials10 have been synthesized by mimicking the biosilicification strategy. Subsequently, a wide variety of silicified materials have been applied to many areas of surface coating,5,9 sensors,11 (bio)catalysis,12,13 nanohybrids,10,14 and protein/enzyme encapsulation.15,16 Among these applications, biotechnology has been a primary application of biosilicification. For example, Stone et al. reported that the activity and thermostability of enzymes can be dramatically enhanced by encapsulation with biomimetic silica.15 Silica/enzyme hybrid materials have been used as sensors with improved shelf life and thermostability.11 Bioapplications of silicification also include cell-surface engineering. Choi et al. reported that yeast cells encapsulated by biosilica showed improved mechanical strength and chemical stability.17 Although its widespread implementation, applications of this process related to energy storage technologies have not been reported. LIBs have been successfully used in various portable electronic devices, and the range of uses of LIBs is rapidly expanding toward large-scale applications such as hybrid electrical vehicles (HEVs) and smart utility grids.18,19 These large-scale applications are expected to play important roles in addressing urgent energy and environmental issues. HEVs and smart utility grids will contribute to the reduction of CO2 emission and the widespread use of renewable energy resources.18,20 Before these future applications are developed, © 2012 American Chemical Society
however, all of the cell components (i.e., electrodes, separators, and electrolytes) need to be further improved. In particular, the role of separators becomes increasingly critical21,22 as separators are directly associated with power performance and cell safety, which are the biggest concerns for the aforementioned largescale applications of LIBs. Given the importance of separators in determining the key properties of batteries, in this investigation, separators were coated with silica through a simple, integrative bioinspired approach. The approach involves on-surface growth of a silica layer inspired by diatoms, and the interfacial adhesion of the layer is inspired by mussel adhesion. The silica coating minimizes thermal shrinkage by the intrinsic nature of ceramics, thus resolving the safety issue of LIBs. Additionally, the coating increases electrolyte wettability because of the hydrophilicity of silica, thus enhancing the power performance of LIBs. A biomimetic silica coating was applied to the most widely used type of separator, the PE separator. The advantages of PE, such as low cost, excellent mechanical strength, and electrochemical stability, have allowed this type of separator to dominate the market.23−25 However, the severe thermal shrinkage of PE separators at high temperatures causes shortcircuits between anodes and cathodes, eventually resulting in cell explosion.25,26 Among the various approaches to overcome the thermal shrinkage,22,25,27−30 the coating of separators with ceramic materials has proven to be effective. These ceramic materials prevent shrinkage by mechanically maintaining the size of the separators.31 However, the robust attachment of ceramic particles onto separator surfaces is challenging due to Received: June 23, 2012 Revised: August 14, 2012 Published: August 14, 2012 3481
dx.doi.org/10.1021/cm301967f | Chem. Mater. 2012, 24, 3481−3485
Chemistry of Materials
Article
separators were preserved after the silica growth (Figure 2a−c). The pD can functionalize the surface of virtually any materials
the low surface energy of the hydrophobic PE membranes. The use of polymer binders has been the only solution developed to date for the attachment of ceramic particles. A major disadvantage of using these binders is the reduction in the ionic conductivity as the result of blocking separator pores due to thick coating of ceramic materials, and this reduced conductivity decreases the power capability of the battery.31 In contrast, the bioinspired silica coating demonstrated in this study is binder-free and is instead based on the direct growth of ceramic layers on the separator surface by taking advantage of recent advances in surface chemistry inspired by mussel adhesion.32 The direct growth of ceramic materials might be suitable because the agglomeration of ceramic particles can be minimized compared with the level of agglomeration that occurs when using the aforementioned binder/ceramic slurry coating processes. In addition, the entire bioinspired ceramic coating process is environmentally friendly. The process is performed in aqueous media, unlike the existing ceramic coating processes that utilize organic solvents in the preparation of binder/ceramic slurries.
Figure 2. SEM images of separator membranes. (a) Unmodified PE, (b) DMAET-PE, and (c) Silica-PE. Static water contact angle images of (d) unmodified PE, (e) DMAET-PE, and (f) Silica-PE.
with good physical and chemical stabilities,33,34 and the surface chemistry was inspired by the amino acid composition of mussel adhesive proteins.19,33,35−39 Recently, the pD coating method was further developed to allow the incorporation of a variety of molecules by a one-step procedure,32 which was used in this study. In principle, the strategy used herein is a general method for the silicification of the surface of any materials. The surface immobilization of DMAET and silicification were analyzed by goniometry. The static water contact angles of the PE separators (119.6 ± 3.5°) decreased after each step of the modification process: 49.8 ± 5.7° for DMAET-PE and
