Mussel-Inspired Coating and Adhesion for Rechargeable Batteries: A

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Mussel-Inspired Coating and Adhesion for Rechargeable Batteries: A Review You Kyeong Jeong,† Sung Hyeon Park,†,‡ and Jang Wook Choi*,†,‡ †

Graduate School of Energy, Environment, Water, and Sustainability (EEWS) and KAIST Institute NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, Seoul 08826, Republic of Korea ABSTRACT: A significant effort is currently being invested to improve the electrochemical performance of classical lithiumion batteries (LIBs) or to accelerate the advent of new chemistry-based post-LIBs. Regardless of the governing chemistry associated with charge storage, stable electrode− electrolyte interface and wet-adhesion among the electrode particles are universally desired for rechargeable batteries adopting liquid electrolytes. In this regard, recent studies have witnessed the usefulness of mussel-inspired polydopamine or catechol functional group in modifying the key battery components, such as active material, separator, and binder. In particular, the uniform conformal coating capability of polydopamine protects active materials from unwanted side reactions with electrolytes and increases the wettability of separators with electrolytes, both of which significantly contribute to the improvement of key battery properties. The wet-adhesion originating from catechol functional groups also largely increases the cycle lives of emerging high-capacity electrodes accompanied by huge volume expansion. This review summarizes the representative examples of mussel-inspired approaches in rechargeable batteries and offers central design principles of relevant coating and adhesion processes. KEYWORDS: catechol functional groups, dopamine, lithium-ion batteries, polydopamine, wet-adhesion

1. INTRODUCTION Rechargeable batteries represented by lithium-ion batteries (LIBs) have been successfully developed to power a variety of mobile IT devices and are now expanding their territory to larger scale applications, including electric vehicles (EVs) and grid-scale utility networks.1−4 These emerging applications have promoted research targeting so-called post-LIBs3,5 that adopt electrode materials with higher specific capacities than those of current graphite anodes and lithium metal oxide cathodes. While the incorporation of new electrode materials is expected to considerably increase the energy density, other key electrochemical properties, such as rate capability, cycle life, and safety, are demanded for improvement in parallel. While the selection of each component in batteries is critical for electrochemical performance, the compatibility between cell components, i.e., electrode−electrolyte and electrolyte−separator, is often as critical. The interparticle interactions in the electrode are also important in a similar context. For example, the wetting of separator with electrolyte affects Li-ion transport in the electrolyte, having a considerable effect in rate performance6,7 and cycle life.6,8,9 Likewise, the wetting of electrolyte on the surface of electrode influences the stability of the electrode−electrolyte interface, which is linked to various electrochemical properties.10−12 For these reasons, a vast number of approaches have been introduced to coat separators and active materials to manipulate the interfacial properties © 2017 American Chemical Society

between cell components. In a smaller scale, the interparticle interactions in electrode play a decisive role in maintaining the stability of the solid−electrolyte−interphase (SEI) layer and the mechanical steadiness of the electrode, especially for active materials that undergo large volume change in each charge− discharge cycle. Hence, uniform coating approaches have been engaged for the modification of key battery components to achieve improved surface and interfacial properties. To this end, surface coating with polydopamine (PDA) has received discernible attention13,14 because this method can be versatilely applied to a wide range of surfaces with moderate sensitivity to surface characteristics. Dopamine was identified13−15 from the analysis of mussel’s byssal proteins that exhibit extraordinary wet-adhesion onto coastal rock surfaces (Figure 1a). Those proteins contain a substantial amount of 3,4-dihydroxy-L-phenylalanine (L-DOPA) and lysine amino acids that bear catechol and amine functional groups, respectively. This finding led to an identification of Special Issue: 10 Years of Polydopamine: Current Status and Future Directions Received: June 13, 2017 Accepted: September 22, 2017 Published: September 22, 2017 7562

DOI: 10.1021/acsami.7b08495 ACS Appl. Mater. Interfaces 2018, 10, 7562−7573

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ACS Applied Materials & Interfaces

Figure 1. (a) Dopamine-inspired from mussel’s byssal protein. (b) Single-step polymerization of dopamine to polydopamine for its coating onto various substrates. Reproduced with permission from ref 13. Copyright 2007 The American Association for the Advancement of Science. (c) Application of polydopamine coating or catechol conjugation for various purposes in rechargeable batteries.

Figure 2. PDA coating on separators for improved rate and cycling performance. (a) Effect of PDA coating on the separator in terms of contact angle (left), porosity (middle), and electrolyte wetting (right). (b) Rate performance of LiMn2O4/Li half-cells incorporating the PDA-coated and bare PE separators. Reproduced with permission from ref 7. Copyright 2011 John Wiley & Sons, Inc. (c) Cycle lives of LiCoO2/Li cells incorporating the bare and PDA-coated PE separators as well as gel-polymer electrolyte (GPE, polyvinylidene fluoride-co-hexafluoropropylene); Liionic flux of the cells containing the bare and PDA-coated PE separators (right). (d) Surface morphologies of Li metal electrodes based on the bare and PDA-coated PE separators after 1 and 20 cycles. Reproduced with permission from ref 8. Copyright 2012 John Wiley & Sons, Inc.

organic, ceramic, and metallic (Figure 1b). Due to the wide compatibility with a variety of surfaces and the simplicity of the synthetic route, in fact, PDA coating has been comprehensibly used for battery components. This review covers recent studies that adopt PDA coating onto separators,6−9,22 active materials,23−29 and electrodes25,30 in rechargeable batteries. (Figure 1c) The PDA coating demonstrates its usefulness not only in improving the performance of classical LIBs7,22,25 but also in enhancing the

dopamine as a moiety of versatile coating materials; dopamine is spontaneously polymerized13,16−18 to PDA via an oxidation process under a slightly basic condition in the presence of oxygen. Some studies19−21 revealed the critical role of oxygen in the given polymerization; oxygen catalyzes the sequential formation of dopaminequinone and 5,6-dihydroxyindole (DHI), intermediate compounds toward PDA. The impressive nature of PDA formation is that PDA can be uniformly coated onto the surfaces with different characteristics, including 7563


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Figure 3. PDA coating on separators. (a) SEM images of the bare cellulose and PDA-coated cellulose separators (top); dimensional stability of the bare PP, bare cellulose, and PDA-coated cellulose separators after 30 min at 200 °C (bottom left); and rate performance of LiCoO2/graphite full cells incorporating the same three separators (bottom right). Reproduced with permission from ref 22. Copyright 2014 Royal Society of Chemistry. (b) SEM images of the bare glass-fiber (GF) separator, GF separator covered with PVDF-HFP (poly(vinylidene fluoride)-co-hexafluoropropylene), and GF separator sequentially coated with PVDF-HFP and PDA (top); dimensional stability of the same three separators and bare PP separator after 30 min at 200 °C (bottom left); and rate performance of Na2MnFe(CN)6/Na cells based on the three separator/electrolyte conditions (bottom right). Reproduced with permission from ref 6. Copyright 2015 John Wiley & Sons, Inc. (c) SEM images of the bare PP/PE/PP (polypropylene/ polyethylene/polypropylene) separator in different magnifications (top) and its PDA-coated counterparts (bottom). (d) Polysulfide rejection tests for both separators. The electrolyte is 1,2-dimethoxyethane and 1,3-dioxolane (DME/DOL) in a volumetric ratio of 1:1. Reproduced with permission from ref 9. Copyright 2015 Springer International Publishing AG.

long-term cyclability of emerging post-LIBs6,8,9,23,24,26−28 where novel active materials need distinct surface and interfacial treatments. Owing to a substantial content of nitrogen (N) in PDA, when combined with other active materials, PDA serves as a N-source to provide lone pair electrons for capturing unwanted reaction products,31−33 facilitating efficient storage of carrier ions,33−35 and increasing electronic conductivity.32,34 From a different angle, as early studies found research motivation from mussel’s wet-adhesion, the wet-adhesion based on catechol functional groups was applied to polymeric binder designs.36,37 Since the majority of LIBs are adopting liquid electrolytes, these binders’ wet-adhesion is particularly beneficial in keeping the integrity of the electrode for prolonged cycles when active materials continuously undergo the volume change in contact with liquid electrolyte.

liquid electrolyte and is therefore responsible for ionic conductivity. Besides, the separator should be electrochemically stable without detrimental side reactions with the electrolyte under the application of anodic or cathodic bias. It is wellknown that the key battery performances including energy density, fast charging capability, and cycle life, can be greatly improved by modifying the surface properties of separators.11,38 In particular, the PDA coating of separators was recently found to improve the electrochemical performance of classical LIB electrodes, Li metal anodes, and high-capacity post-LIB electrodes, as will be described in the following paragraphs. Conventional separators consist mainly of porous olefin− polymer films with hydrophobic surfaces. However, simple PDA coating can turn their surfaces lyophilic, increasing the wettability with electrolyte without significantly ruining the original porosity. The distinct wetting of the separator before and after the PDA coating was clearly reflected in different contact angles when tested with distilled water droplet (Figure 2a). As a result of improved wetting, cells integrating the PDAcoated separators exhibit enhanced rate capability (Figure 2b) by taking advantage of more efficient Li-ion transport through the separator.7 This effect is remarkable, as most of the other

2. POLYDOPAMINE COATING ONTO BATTERY SEPARATORS Separators are porous polymer membranes located between the anodes and cathodes in rechargeable batteries. The main role of a separator is to separate the anode and cathode, thus preventing short circuits between them. At the same time, the separator serves as an electrolyte vessel by absorbing the 7564


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Figure 4. PDA coating on battery active materials. PDA coating on (a) few-layered sulfur nanosheets, (b) silicon particles, and (c) interconnected SnO2 particles to protect the active materials and accommodate the volume expansion. Reprinted with permission from refs 23, 26, and 28. Copyright 2013 American Chemical Society, 2016 American Chemical Society, and 2017 John Wiley & Sons, Inc., respectively. (d) PDA coating on graphite anodes to block H+ without sacrificing Li+ transport. Reprinted with permission from ref 25. Copyright 2016 American Chemical Society.

(Figure 2d), in contrast with needle-like morphologies of the bare separator counterparts. The PDA coating was expanded to nonwoven separators. Nonwoven separators have lately emerged, as they can avoid the thermal shrinkage of conventional woven polyolefin separators, which can cause severe fire hazards.38,41 Despite this advantage with regard to the safety, inherently large pore sizes of nonwoven separators result in short circuits between the electrodes on both sides. The PDA coating was adopted for cellulose nonwoven separators22 to decrease the pore size (Figure 3a, top) and thus prevent short circuits. In fulfilling this function, conformal adhesion of PDA onto the cellulose fibers plays a crucial role. Even though nonwoven separators have high thermal stability, the PDA coating further improves the mechanical robustness over thermal treatment at 200 °C for 30 min (Figure 3a, bottom left). Consistent with the study in Figure 2b, the cell with the PDA-coated cellulose separator improves the rate capability (Figure 3a, bottom right) owing to the increased wetting with the electrolyte. In addition, the modified surface properties of the PDA-coated nonwoven separators allow6,42 the wetting with gel-polymer electrolytes such as poly(vinylidene difluoride)-co-hexafluoropropylene

approaches employed so far improved the rate capability at the expense of energy density and other key properties. The enhanced wettability by the PDA coating can also play a critical role in addressing the chronic issues39,40 of Li metal anodes in LIBs, namely, Li dendrite growth and resulting uncontrolled side reactions with electrolyte, which lead to poor Coulomb efficiency (CE) in each charge−discharge cycle. In spite of the high theoretical specific capacity, the sustainable operation of Li metal anodes still remains a formidable challenge because suppressing Li dendrites is technologically nontrivial; Li dendrites can grow from native roughness of Li metal surface due to concentrated electron densities around local microbulges and high surface tension of Li metal. More significantly, the dendrite growth becomes amplified with increasing areal current density on Li deposition, hindering fast charging. The integration of PDA-coated separators can facilitate uniform Li-ion flux over the electrode area as well as the adhesion of the separator onto the Li metal electrode, keeping Li ions from being concentrated toward local spots on the Li metal surface.8 Utilizing these advantages, the PDAcoated cell improves the cyclability remarkably (Figure 2c) by engaging far more rounded morphologies for Li metal surfaces 7565


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Figure 5. PDA coating to produce core−shell structures for battery electrodes. (a) STEM images and elemental mapping of hollow N-doped carbon shells originating from the carbonization of PDA-coated layers (left) and their sulfur-embedded counterparts (middle); cycling performance of the sulfur-embedded N-doped carbon shell electrode in comparison with that of other sulfur electrodes (right). Reprinted with permission from ref 27. Copyright 2014 American Chemical Society. (b) TEM images of PDA-coated SnO2 particles (top left) and their carbonized derivative (bottom left); higher magnification TEM image of the carbonized derivative (middle); and cycling performance of the bare SnO2 and carbonized PDA-coated SnO2 electrodes (right). Reproduced with permission from ref 24. Copyright 2012 Royal Society of Chemistry. (c) SEM and TEM images of a yolk−shell structured silicon anode (left) and its cycling performance (right). Reprinted with permission from ref 29. Copyright 2012 American Chemical Society.

diffuse through a porous separator and destabilize the surface of the Li metal anode. Zhang et al.9 demonstrated that the PDA coating onto a polypropylene/polyethylene/polypropylene (PP/PE/PP) separator can mitigate the migration of polysulfides markedly. This result is attributed to the fact that the decreased pore sizes (Figure 3c) by the PDA coating impose a physical barrier for polysulfide diffusion without hindering Li-ion diffusion severely. Simultaneously, the electrostatic interaction between polysulfides and PDA makes an additional contribution to the rejection of the polysulfides from penetrating through the separator (Figure 3d). In having this electrostatic interaction into effect, the highly electronegative functional groups of PDA such as catechol and amide groups play a key role for the binding with polysulfides.48 The controlled polysulfide dissolution leads to superior cycling performance compared to that of a control cell based on the bare PP/PE/PP separator.

(PVDF-HFP) so that the weak mechanical stability of nonwoven separators can be largely compensated for. In the context of enhancing thermal stability, PDA was also used to immobilize inorganic particles (SiO2 or Al2O3) on the surfaces of polyolefin woven separators.43,44 In a similar context, the PDA coating was applied to glassfiber separators (Figure 3b). Gao et al.6 utilized a glass-fiber network as a nonwoven separator, and a combination of PVDFHFP and liquid electrolyte was integrated with the separator. While the thermal stability of the separator was achieved from a joint effect of the high strength of glass fibers and the nonwoven weaving configuration, the PDA’s excellent wetting with the gel electrolyte leads to an ionic conductivity in the order of 10−3 mS cm−1. The improved wetting was also reflected in high rate and cycling performance when tested for Na2MnFe(CN)6 half-cells in sodium-ion batteries. The PDA coating onto separators was used for a representative post-LIB, namely, lithium−sulfur (Li−S) batteries. Despite the high theoretical gravimetric energy density, which is critical for transportation applications such as allelectric vehicles and drones, the cycle lives of Li−S cells are insufficient due to the dissolution of high-order lithium polysulfides to the electrolyte.45−47 The dissolved polysulfides

3. POLYDOPAMINE COATING ON BATTERY ELECTRODES The facile synthesis and uniform coating of PDA have expanded the scope of the given coating approach to the modifications of various active materials. In particular, the 7566


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moieties, which improves the cycle life of the graphite anode at 60 °C, a critical temperature for electric vehicle applications.

conformal coating of PDA can protect active materials from unwanted side reactions, typically originating from electrolyte decomposition, for emerging high-capacity materials, such as sulfur, silicon, and tin. The surface coating of PDA on the active material can also give rise to the intimate binding of active particles with polymeric binders and conductive materials so that the electrode integrity can be greatly preserved over cycling even with the large volume change of active materials. In bringing these advantages into effect, however, the thickness control of PDA is critical, because an excessively thick PDA layer could increase the electrical resistance, thus impeding the electrochemical performance. Along this direction, Li−S batteries are among those which take substantial benefits from PDA coating. The PDA coating on the sulfur cathodes offers the effect similar to that of its coating on the separator, that is, the mitigation of polysulfides based on the electrostatic interaction between the polar functional groups of PDA and polysulfides. Wang et al.23 prepared PDA-coated few-layered sulfur nanosheets (Figure 4a). This active structure was covalently cross-linked to a poly(acrylic acid) (PAA) binder network and multiwall carbon nanotubes (MWCNTs) via amide bonds (RN−CO), created from the reaction between the amine groups (-NH2) of PDA and the carboxyl groups (-COOH) of PAA or MWCNT. A similar approach based on the covalent cross-linking between PDA and PAA binder network was also adopted for high-capacity Si anodes. Si anodes have been investigated with much attention in the past decade because of their high impact on the energy density of LIBs.5,49,50 However, the short cycle lives of Si anodes originating from the immense volume expansion of Si during lithiation are a well-known shortcoming of the Si anode technology. Bie et al.26 reported that 1−2 nm thick PDA coating onto the native SiO2 surface layers of Si particles can resolve the volume expansion issue to a large extent. Similar to the aforementioned sulfur cathodes, the PDA on the electrode material surfaces can be cross-linked to a PAA binder network via the amide bond formation (Figure 4b, left). This network formation leads to a stable electrode morphology over cycling accompanied by repeated volume change of Si (Figure 4b, right). The enhanced mechanical property of the electrode implies that the cross-linking of PDA to the binder helps the electrode to accommodate the stress generated during the volume change of Si. As a result, the PDA-coated Si electrode retained 1800 mA h g−1 after 100 cycles, whereas the other Si electrode without the PDA coating preserved only 800 mA h g−1 after the same number of cycles. Besides, SnO2 nanoparticle anodes that operate based on the conversion reaction (i.e., SnO2 + 4Li+ + 4e− ↔ Sn + 2Li2O) improve28 the cycle life by the PDA coating onto SnO2 (Figure 4c). Specifically, the PDA coating largely alleviates the agglomeration of SnO2 during cycling (Figure 4c, right), keeping the electric conductivity of the electrode to a great degree. Moreover, the PDA coating improves the electrode− electrolyte interfacial stability by mitigating continuous electrolyte decomposition. The positive effect of PDA coating in the interfacial stability was found for conventional graphite anodes (Figure 4d). Park et al.25 particularly paid attention to the ability of PDA to scavenge hydrofluoride (HF), which is known51 to impair the cycle lives of LIBs by degrading a broad range of active materials and SEI layers. The PDA coating layer on the graphite can capture H+ using the secondary amines of the PDA’s indole

4. PDA FOR NITROGEN-DOPED CARBON COATING OF ACTIVE MATERIALS Taking advantage of the thin and uniform coating capability of PDA, PDA was coated onto a variety of battery active materials to improve the performance of diverse rechargeable batteries. In particular, PDA contains approximately 4.5 wt % nitrogen so that PDA coating provides nitrogen-doping (N-doping), offering a further increase in electric conductivity or binding sites using lone pair electrons at the nitrogen. One representative example is sulfur cathodes in Li−S batteries. Zhou et al.27 demonstrated N-doped carbon hollow spheres (Figure 5a, left) where sulfur occupies the inner space. In developing this pinhole-free structure, the uniform coating of PDA onto SiO2 particle templates plays a crucial role. The PDA coating was conducted once again onto the hollow N-doped carbon spheres to provide continuous conductive pathways over the entire electrode area (Figure 5a, middle). As a result of complete encapsulation of sulfur and well-organized conductive pathways, the given Li−S cell achieved ∼84% improved cycling performance compared to bare sulfur for 150 cycles (Figure 5a, right). Also, the binding capability of PDA with soluble lithium polysulfides based on the electrostatic interaction during the initial discharging period plays an important role for the improved cyclability. Similar effects based on the concurrent use of hollow carbon structures and electrostatic interaction involving the heteroatoms in PDA were also reported in other works52,53 in the same context. The N-doped carbon coating originating from PDA can also be used for metal oxide electrodes that operate based on the conversion process. Kong et al.24 applied PDA coating onto SnO2 particles and subsequently carbonized the PDA to complete N-doped carbon-coated SnO2 active materials (Figure 5b, left). This coating layer provides efficient conductive pathways to enhance the electric conductivity as well as prevents aggregation of the active materials. Similar to the aforementioned sulfur electrode, the uniform coating capability of PDA is critical, as the uniform coating of SnO2 nanoparticles (10−20 nm in diameter) with a limited amount is nontrivial. Utilizing the effect of N-doped coating layer, the carbonized PDA-coated SnO2 electrode retained 700 mA h g−1 after 100 cycles, which corresponds to 99% capacity retention (Figure 5b, right). Distinguished from the direct contact coverage of PDA on active materials, a yolk−shell-like structure with a void between Si and the PDA shell was synthesized29 (Figure 5c, left) in which Si is positioned inside N-doped carbon shells. PDA was first coated onto Si particles (∼100 nm) with the surfaces covered with 40−50 nm of sacrificial SiO2 layers, followed by carbonization of the PDA layers and etching of the sacrificial SiO2 layers. The void space inside the shells can accommodate the volume expansion of Si, retaining the overall electrode structure for prolonged cycles. Taking advantage of this unique structure, the given yolk−shell structure operates for 1000 charge−discharge cycles with preserving 74.0% of the initial capacity (Figure 5c, right). 5. CATECHOL-FUNCTIONALIZED BINDERS In an effort to address the aforementioned volume expansion issue of Si anodes, the critical role of polymeric binders has 7567


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Figure 6. Mussel-inspired battery binders. (a) Mussel-inspired catechol conjugated alginate (Alg-C) and poly(acrylic acid) (PAA-C) binders for Si anodes. (b) Adhesion strength of Si electrodes incorporating Alg-C, Alg, and PVDF binders based on 180° peeling tests. (c) Cycling performances of the same three electrodes as in panel b. Reproduced with permission from ref 37. Copyright 2013 John Wiley & Sons, Inc. (d) Sulfur cathode in which self-assembled carbon black particles are in contact with hollow sulfur spheres utilizing PDA as a “nano binder” intermediate layer. (e) Superb cycling performance of the given hollow sulfur−carbon black core−shell structure. Reproduced with permission from ref 36. Copyright 2017 John Wiley & Sons, Inc.

begun to be recognized.37,54−68 In the classical LIB electrodes, binders occupy only a small content (i.e., below 3 wt %) in the electrodes and play mainly a passive role, focusing on the adhesion of active materials with moderate volume expansion onto a current collector. However, the drastic volume expansion of Si calls for more advanced binders that can keep the integrity of the electrode over cycling. In this direction, strong adhesion of binders with Si particles would be beneficial, and various binder-to-Si binding mechanisms have been introduced, including hydrogen bonding,54,58,60,61,63,65,68 ion-dipole interaction,59,60,62,67 and self-healing.55,66 Musselinspired wet-adhesion of catechol functional groups was utilized to enhance the adhesion of binders with Si particles; Ryou et al.37 conjugated catechol functional groups to alginate and PAA backbones and used them as Si anode binders (Figure 6a).

Utilizing the wet-adhesion of catechol functional groups, those polymeric binders showed enhanced adhesion of the electrode onto a copper current collector (Figure 6b) and longer cycle life compared to those of the bare polymers without catechol conjugation (Figure 6c). The spikes marked with black arrows in Figure 6b indicate the points where the electrode film starts to peel off from the current collector after holding the adhesion by assistance of the wet-adhesive binder-to-Si interactions or binder-to-binder interactions. On the other hand, PDA itself was introduced as a concept called nanobinder36 for sulfur cathodes; carbon black particles and hollow sulfur sphere are linked through a PDA interfacial layer (Figure 6d). With this structure, the carbon black shell layer blocks the dissolution of polysulfides while providing wellorganized conductive pathways. Furthermore, it was claimed 7568


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Figure 7. PDA as a battery active material. (a) PDA as an anode material and (b) PDA/few-walled carbon nanotube composite as a cathode material for lithium-ion batteries. Reproduced with permission from refs 70 and 71. Copyright 2016 John Wiley & Sons, Inc. and 2017 Royal Society of Chemistry, respectively. (c) PDA as an active material for aqueous Mg batteries. Reproduced with permission from ref 72. Copyright 2014 John Wiley & Sons, Inc.

the robust nature of the PDA-originating active material in the long-term cycling period. PDA was also used in a composite with few-walled carbon nanotubes (FWCNTs). Liu et al.,71 incorporated 53 wt % PDA with FWCNTs (Figure 7b, left) and operated the composite in the potential range of 1.5−4.5 V vs Li/Li+. As clear evidence of its robust cyclability, the composite reached 10,000 cycles with a substantial capacity retention (Figure 7b, right). Furthermore, the PDA-derived organic structure was reported72 to store divalent carrier ions such as Mg2+ in an aqueous electrolyte (Figure 7c, left). In the discharging process, the quinone moieties of PDA are first protonated and undergo a cation exchange to bind with a Mg2+ at the fully discharged state. The authors proved that the formation of the quinone moieties is critical for reversible binding with Mg2+ (see CV data in Figure 7c, right). Based on this storage mechanism, the PDA electrode ran for 500 cycles without any capacity loss, and its Coulombic efficiencies for 500 cycles stayed over 99.2%. The

that the elasticity of PDA (estimated elastic constant, 6.748 kN m kg−1) helps accommodate the volume expansion of sulfur during lithiation. As a result, the given Li−S cell exhibited an exceptional capacity retention of 65.4% after 2500 cycles (Figure 6e).

6. PDA AS ACTIVE MATERIALS PDA was used as a precursor of active materials for organic electrodes. Organic electrode materials have been highlighted1,69 as next-generation active materials to ceramic counterparts because of their low cost, ecofriendliness, and structural tunability. In an effort to develop advanced organic electrodes, PDA was reduced to produce quinone functional groups from the catechol moiety (Figure 7a, left). Moreover, the electrode was fabricated in a freestanding form due to the adhesion of the PDA precursor.70 From its structure (Figure 7a, left), in theory, each molecular unit can bind with 11 Li ions. When cycled in the potential range of 0−2.75 vs Li/Li+, the given electrode retained 1414 mA h g−1 for 580 cycles, verifying 7569


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Figure 8. PDA or dopamine as an electrolyte additive. (a) PDA as an electrolyte additive for scavenging superoxide radicals in Li−O2 batteries. Reprinted with permission from ref 79. Copyright 2014 American Chemical Society. (b) Dopamine as an electrolyte additive to electrochemically form passivation layers for high-voltage operation of lithium-ion battery cathodes. Reprinted with permission from ref 30. Copyright 2016 American Chemical Society.

theoretical simulation, dopamine was oxidized to PDA below the operation voltage of LiNi1/3Co1/3Mn1/3O2, thus serving as an electrolyte additive (Figure 8b, left). The main role of the PDA coating was to protect the LiNi1/3Co1/3Mn1/3O2 from detrimental side reactions especially at high voltages. This effect was visualized using SEM (Figure 8b, right); compared to the bare cell without employing dopamine, the dopamine-included cell maintained the original morphology of the active particles after precycling.

excellent cyclability was attributed to the stability of PDA in the electrolytes without raising side reactions.

7. PDA AND DOPAMINE AS ELECTROLYTE ADDITIVES PDA and dopamine were used as battery electrolyte additives for Li−O2 batteries and LIBs. Li−O2 batteries have lately received considerable attention, because the use of oxygen molecule as a cathode active material, instead of solid counterparts, can increase the energy density significantly.73,74 However, the use of oxygen, in turn, generates superoxide radical (O2−) as an intermediate reaction product, which causes unwanted side reactions by decomposing carbon components or electrolytes.75−78 These side reactions impede the round trip efficiency in each cycle, eventually limiting the cycle life. Inspired by melanin in the human eye that scavenges reactive oxygen species (ROS) and protects the eye from its lifetime exposure to sunlight, Kim et al.79 added PDA, an artificial melanin, to the electrolyte (Figure 8a, left). As expected, it was proved from an electron paramagnetic resonance (EPR) experiment that PDA indeed scavenges the excess superoxide radicals. As a result of the inclusion of PDA in the electrolyte, the Li−O2 cell operates beyond 100 cycles, whereas the capacity of a control cell without PDA degrades around the 65th cycle. The morphology of the electrode observed after 10 cycles verifies the effect of PDA (Figure 8a, right); the cell incorporating PDA maintained the original CNT morphology. In contrast, the CNTs in the bare cell were covered by uncontrolled SEI layers. On the other hand, dopamine was added to a carbonatebased electrolyte to generate a PDA passivation layer on the surface of LiNi1/3Co1/3Mn1/3O2 cathode.30 As suggested by a

8. CONCLUSION The key performance of rechargeable batteries is often affected by the stability of electrode−electrolyte interface and particleto-particle interaction in the electrode. In this regard, conformal and universal coating capability of PDA was usefully applied to various battery components, including active materials and separators. The modified separators with PDA coating layers increase the wetting with electrolytes, resulting in the improved rate and cycling performance in classical LIBs and emerging batteries incorporating Li metal anodes. The PDA coating of active materials largely prevents undesirable side reactions or particle aggregation, extending the cycle life significantly. Once again, the uniform coating capability onto diverse surfaces plays a key role in those approaches relating to separators and active materials. Also, the wet-adhesion of PDA largely resolves the issues of upcoming high-capacity active materials, such as silicon and sulfur, as the wet-adhesion can contribute to keeping the integrity of the electrodes even during the repetitive volume expansion of the corresponding electrodes. Since the key properties of PDA, namely, wet-adhesion and uniform coating capability, are commonly desired for 7570


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rechargeable batteries bearing liquid electrolytes, the effect of PDA or catechol functional group is expected to be continuously recognized in various rechargeable batteries, regardless of their detailed chemistries.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jang Wook Choi: 0000-0001-8783-0901 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Research Foundation of Korea (NRF-2014R1A4A1003712 and NRF-2015R1A2A1A05001737), Korea Institute of Science and Technology (KIST) Institutional Program (2E27090), the Climate Change Research Hub Project of the KAIST EEWS Research Center (N11170056), the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which is granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (20152020104870), and the Project of flexible power module and system development for wearable devices funded by the Ministry of Trade, Industry and Energy (10065730).

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