Full Length ArticleA facile method to enhance the uniformity and adhesion properties of water-based ceramic coating layers on hydrophobic polyethylene separators
Introduction
Ceramic coating layers on either separators have become highly necessary components to ensure the safety of mid- and large-format lithium-ion batteries (LIBs) with a high energy density [1], [2], [3], [4], [5]. In general, the major components of ceramic coating layers, such as ceramic materials and polymeric binders, are electrochemically inert, resulting in degradation of the electrochemical properties of LIBs by interrupting the porous structures of separators as well as the electrodes [6], [7]. As a result, many efforts have been made to maintaining the original permeability properties in pristine separators by decreasing (1) the coating thickness or (2) minimizing the binder content of coating layers [8], [9], [10], [11]. However, the latter method cause insufficient adhesion properties between the ceramic coating layer and the pristine separators during the cell lifetime, thereby leading to the delamination of the coating layer from the separators. The reduced mechanical integrity of the ceramic coating layer cannot fulfill its role of withstanding thermal and mechanical stresses resulting in safety issue [12], [13], [14]. Besides, from the industrial point of view, the detachment of ceramic materials from ceramic-coated separators (CCSs) during battery production play as defects that cause crucial malfunction and safety issue of LIBs. As can be seen from battery recalls of Sony and Samsung recently, regardless of the types of battery constituents, even the battery design and manufacturing of batteries can cause enormous financial damage to the company [15], [16], [17]. Thus, improving the adhesion properties of ceramic layers on separator surface is very important issue to guarantee not only the battery safety but also the quality of the LIBs.
The commercialized battery separators are mainly based on polyolefin polymers. Owing to the super hydrophobic surface properties of the polyolefin separators, CCSs have been prepared based on non-aqueous slurry coating solutions [18], [19], [20], [21]. However, the use of water as a dispersing solvent is preferred because of the environmental and cost reasons [22], [23], [24].
Although the precise adhesion mechanisms and polymerization mechanisms are not currently understood clearly, mussel-inspired materials such as polydopamine have attracted many attentions of researchers because of their exceptional adhesion properties even on wet surfaces [25], [26], [27], [28]. Based on these properties of polydopamine, we demonstrated a variety of application approaches using polydopamine in lithium battery applications for the first time. We have developed a facile polydopamine surface coating technique for battery separators that changes the hydrophobic surface of polyolefin separators to hydrophilic surface without compromising their morphological structure [10], [25], [29], [30], [31], [32]. In addition, we have shown that the catechol-functionalized polymeric binders showed enhanced adhesion strength of Si anodes resulting in prolonged cycle life [26], [33].
Herein, we investigated the synergistic effect between polydopamine coating layers and ceramic coating layers on the adhesion properties of CCSs. To achieve this goal, we measured the adhesion strength change of CCSs using a surface and interfacial cutting analysis system (SAICAS®) tool as changing the composition aqueous slurry for alumina (Al2O3) ceramic coating layers. Due to improved hydrophilicity of the bare polyethylene (PE) separators after polydopamine surface modification, the aqueous ceramic slurry was well dispersed on the separator resulting in uniformity of ceramic coating layers. The electrochemical properties of the CCSs based on polydopamine treatment were also evaluated by using 2032-type coin full cells (LiMn2O4/artificial graphite).
Section snippets
Materials
Aluminum oxide (Al2O3, AES-11, Sumitomo Chemical Co., ca. 430 nm) and sodium carboxymethyl cellulose (CMC, WS-C, Dai-ichi Kogyo Seiyaku Co., Ltd.) were used without further purification. Deionized (DI) water was obtained from a Milli-Q system (Millipore Co., USA, >18.2 MΩ cm−1). Trizma® base (99.9%), Trizma® hydrochloride (99%), 2-(3,4-dihydroxyphenyl) ethylamine hydrochloride (dopamine hydrochloride, 98%), methanol (CH3OH) and N-methyl-2-pyrrolidone (NMP) were purchased from Sigma-Aldrich and
Results and discussion
The hydrophilicity of PDA-PE separators was evaluated by measuring contact angles with water and aqueous ceramic coating slurry (Al2O3/CMC/DI water = 38.8/1.2/60). As shown in Fig. 1, PDA-PE showed smaller contact angles than bare PE for both liquid medium (120–52° for water droplet, 110–63° for aqueous ceramic coating slurry). This implies that PDA surface modification efficiently converts the surface properties of bare PE to hydrophilic. In addition, PDA surface treatment did not interfere with
Conclusion
Polydopamine layers on PE separators (PDA-PE) resulted in a uniform ceramic coating layers originating from water-based coating slurries to fabricate ceramic-coated separators (PDA-PE/CCS). The interfacial adhesion strength (Fsepa-Al2O3) between the ceramic coating layer and PDA-PE separator was accurately measured with the help of the SAICAS. PDA-PE/CCS showed superior adhesion strength compared to bare PE because of strong interactions between the polydopamine and CMC binder. As a result, the
Acknowledgements
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A3B03933293). This work was also supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-2014H1C1A1066977).
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These authors contributed equally.