Inverse Opal-Inspired, Nanoscaffold Battery Separators: A New

Nanoscaffold Battery Separators: A New Membrane Opportunity for High-Performance Energy Storage Systems ... Publication Date (Web): June 30, 2014...
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Letter pubs.acs.org/NanoLett

Inverse Opal-Inspired, Nanoscaffold Battery Separators: A New Membrane Opportunity for High-Performance Energy Storage Systems Jung-Hwan Kim,† Jeong-Hoon Kim,† Keun-Ho Choi,† Hyung Kyun Yu,‡ Jong Hun Kim,‡ Joo Sung Lee,‡ and Sang-Young Lee*,† †

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, Korea Batteries R&D, LG Chem, Yusong-gu, Daejon, 305-380, Korea

‡

S Supporting Information *

ABSTRACT: The facilitation of ion/electron transport, along with ever-increasing demand for high-energy density, is a key to boosting the development of energy storage systems such as lithium-ion batteries. Among major battery components, separator membranes have not been the center of attention compared to other electrochemically active materials, despite their important roles in allowing ionic flow and preventing electrical contact between electrodes. Here, we present a new class of battery separator based on inverse opal-inspired, seamless nanoscaffold structure (“IO separator”), as an unprecedented membrane opportunity to enable remarkable advances in cell performance far beyond those accessible with conventional battery separators. The IO separator is easily fabricated through one-pot, evaporation-induced self-assembly of colloidal silica nanoparticles in the presence of ultraviolet (UV)-curable triacrylate monomer inside a nonwoven substrate, followed by UV-cross-linking and selective removal of the silica nanoparticle superlattices. The precisely ordered/well-reticulated nanoporous structure of IO separator allows significant improvement in ion transfer toward electrodes. The IO separator-driven facilitation of the ion transport phenomena is expected to play a critical role in the realization of high-performance batteries (in particular, under harsh conditions such as high-mass-loading electrodes, fast charging/discharging, and highly polar liquid electrolyte). Moreover, the IO separator enables the movement of the Ragone plot curves to a more desirable position representing high-energy/high-power density, without tailoring other battery materials and configurations. This study provides a new perspective on battery separators: a paradigm shift from plain porous films to pseudoelectrochemically active nanomembranes that can influence the charge/discharge reaction. KEYWORDS: Separator membranes, lithium-ion batteries, inverse opal, seamless nanoscaffold, one-pot self-assembly, ion transport phenomena

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Given that a key factor governing battery performance is how to facilitate ion and electron transport,1−6 the roles of separator membranes in electrochemical cells should not be underestimated. Fast/uniform ionic flow toward electrodes via electrolyte-filled separators13 needs to be secured to boost redox reaction at the electrodes. In particular, the ion transportinduced cell polarization becomes more critical under harsh conditions, such as fast charging/discharging and high-massloading electrodes, both of which are urgently needed to realize long-range electric vehicle batteries.5,6 Currently, most widely used separators in LIBs are manufactured using polyolefin materials. These polyolefin separators have numerous advantageous attributes that render them suitable for practical use in LIBs. However, their intrinsic limitations (specifically, sluggish/ nonuniform ionic flow and poor thermal stability) often raise

igh-performance energy storage systems with reliable and sustainable electrochemical properties are being pursued as an ultimate power source that will play a central role in the upcoming smart energy era.1−3 Among the diverse energy storage systems reported to date, lithium-ion rechargeable batteries (LIBs) have still garnered a great deal of attention, particularly with the rapid growth of newly emerging applications such as wearable electronics, electric vehicles, and grid-scale energy storage systems.4−6 The rational design and synthesis of major battery components such as anodes, cathodes, electrolytes, and separator membranes are indispensable prerequisites for the development of full-fledged batteries. Most research activities on battery components have been devoted to the electrochemically active materials, with a particular focus on electrode materials and electrolytes. Representative reports include those related to high-voltage spinel nickel manganese oxides,7 overlithiated layer oxides,8 silicon- or tin-based metal alloys,9,10 functional electrolyte additives,11 and solid-state electrolytes.12 © XXXX American Chemical Society

Received: April 16, 2014 Revised: June 21, 2014

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Figure 1. Schematic representation of the overall fabrication procedure for an inverse opal-mimic nanoporous structure: (a) traditional approach requiring a multistep process. (b) IO separator manufactured via the simple one-pot EISA of SiO2 nanoparticles in the presence of a UV-curable ETPTA monomer inside a PET nonwoven substrate, followed by UV-cross-linking and selective removal of the SiO2 nanoparticle superlattices.

proaches24−29 for preparing inverse opal-like porous structures usually require multistep processes (Figure 1a): close-packed assembly of sacrificial colloidal particles on top of a substrate → infiltration or impregnation of matrix-forming materials → selective removal of the colloidal crystal template. Although three-dimensionally (3D) ordered porous structures can be attained using this method, from a macroscopic point of view, the generation of defects or cracks is difficult to avoid because the dimensional/mechanical stability of the colloidal crystal template is weak and easily damaged by capillary force (developed during the drying step after the close packing of colloidal particles) or hydraulic pressure (created during infiltration of matrix-forming materials).26,27 For this reason, the construction of large-area, crack/defect-free 3D porous structures has remained a substantial challenge.28,29 The 3D porous structure-based design concept has been widely exploited in the synthesis of high-surface-area battery electrode materials.30−32 Most of these materials have been used in powder form after being crushed, instead of being retained as large-area sheets. A few attempts to fabricate 3D ordered porous membranes have been reported. However, their poor mechanical/dimensional stability and complicated synthetic procedure have caused critical problems in attaining a uniform porous structure, physical flexibility, and thickness reduction.33−35 To successfully fabricate large-area, thin, and flexible IO separators for potential use in high-performance LIBs, the aforementioned issues need to be fully addressed. In the present study, the first two steps (corresponding to the closepacked assembly and infiltration (or impregnation)) are simply merged into a one-pot self-assembly process (Figure 1b). In a PET nonwoven substrate, vertical deposition-driven, evaporation-induced self-assembly (EISA)22,28,29 of the colloidal SiO2 nanoparticles (average particle size = 100 nm, solvent = methyl ethyl ketone (MEK)) was conducted in the presence of a UV-

significant concerns regarding ion transport (affecting cell performance) and electrical isolation (related to cell safety) between the electrodes. 13−15 A large number of approaches16−19 to overcome these drawbacks (in particular, thermal shrinkage of polyolefin separators) have been undertaken, which include ceramic-coated separators, nonwoven separators, and electrospun nanofiber 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, we demonstrate a new class of battery separator that features an inverse opal-inspired, seamless nanoscaffold structure (“IO separator”), as a revolutionary membrane to enable remarkable advances in cell performance far beyond those achievable with conventional separators. Inverse opals (or inverse colloidal crystals) are known to be a sustainable building block of finely tuned, micro/nanoporous materials applicable to high-surface-area catalysts, electrode materials, optical sensors, and photonic crystals,20−23 owing to their periodically ordered/highly interconnected porous structure. The IO separator presented herein is an inverse replica of hexagonally close-packed SiO2 nanoparticle superlattices embedded in a UV-cross-linked ethoxylated trimethylolpropane triacrylate (ETPTA) polymer matrix, which is integrated with a polyethylene terephthalate (PET) nonwoven substrate serving as a compliant thermomechanical framework. The structural uniqueness (i.e., inverse opal-mimic, seamless nanoscaffolds) of the IO separator, in combination with its good electrolyte wettability, allows substantial improvement in ion transfer between electrodes. We anticipate that this IO separator-driven facilitation of the ion transport phenomena can provide unprecedented membrane opportunity for the development of high-performance batteries. The overall fabrication procedure for the IO separator is schematically illustrated in Figure 1. Traditional apB

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Figure 2. Effect of the matrix-forming materials on the particle size distribution of the SiO2 colloidal solution, packing state of SiO2 nanoparticles, and porous morphology of the resulting membranes for a fixed composition ratio of SiO2/matrix-forming material (= 74:26 (v/v)): (a) UV-curable ETPTA monomer. (b) PVdF-HFP polymer.

PP) separator (Celgard, thickness ≈20 μm) with fewer and randomly distributed slit-like pores (Supporting Information, Figure S2c) underscores the structural superiority of the IO separator in terms of uniformity and number density of pores. Notably, the ETPTA monomer does not impair the welldefined superlattice structure of the colloidal SiO2 nanoparticles during the aforementioned simple one-pot EISA process, which allows the infiltration step for matrix-forming materials to be eliminated. A prerequisite for enabling this one-pot EISA process is the acquisition of a good dispersion state for the SiO2 nanoparticles in the colloidal solution. The mixture solution comprising the SiO2 nanoparticles and ETPTA monomer exhibits a good dispersion state (Figure 2a). A large portion of the dispersed SiO2 nanoparticles were approximately 100 nm in size, indicating that the ETPTA monomer does not interfere with the good dispersion state of the colloidal SiO2 nanoparticles. The well-dispersed SiO2 nanoparticles in the presence of ETPTA monomer enable the formation of hexagonally closepacked superlattice structure in the one-pot EISA process, which serves as a sustainable building template for the construction of the 3D ordered seamless nanoporous structure. To account for the unique role of the ETPTA monomer in the colloidal dispersion, we prepared a control solution incorporating a long-chain linear polymer (polyvinylidene-co-hexafluoropropylene (PVdF-HFP, HFP content = 6 mol %, MW (number-average) = 477 000 g mol−1) was chosen at the same composition ratio (SiO2/PVdF-HFP = 74:26 (v/v)). Figure 2b shows that the control solution exhibits a broad distribution in particle size, with the majority between 1000 and 10 000 nm. This result indicates that the PVdF-HFP chains cause

curable ETPTA monomer. Subsequently, the ETPTA monomer that was preferentially located in interstitial voids of the hexagonally close-packed SiO2 nanoparticle arrays, where the SiO2/ETPTA composition ratio was 74:26 (v/v) and 2hydroxy-2-methyl-1-phenyl-1-propanone (HMPP) was incorporated as a photoinitiator, was exposed to UV irradiation for 15 s, leading to the formation of a UV-cured ETPTA polymer matrix between the close-packed SiO2 nanoparticles. Characteristic peaks assigned to acrylic CC bonds36,37 of the ETPTA monomer disappeared after the UV irradiation process (Supporting Information, Figure S1). In addition, the gel content (i.e., the insoluble polymer fraction after solvent (acetone) extraction) of the UV-cured ETPTA polymer was observed to be greater than 99% by weight. The FT-IR analysis results and gel content verified the successful UV-cross-linking reaction of the ETPTA monomer. Finally, the close-packed SiO2 nanoparticle superlattices embedded in the UV-cured ETPTA polymer matrix were selectively removed using hydrofluoric acid (HF) as an etchant, resulting in a thin and flexible IO separator (separator thickness ≈20 μm) with an inverse opal-mimic, seamless nanoscaffold structure. The average pore size of the nanoscaffold appears to be approximately 100 nm (Supporting Information, Figure S2a), which matches well with the original size of sacrificial SiO2 particles prior to the selective etching process. The crosssectional morphology (Supporting Information, Figure S2b) of the IO separator confirms the successful evolution of 3D reticulated nanoporous channels in the through-thickness direction. A morphological comparison with a commercial trilayer polypropylene/polyethylene/polypropylene (PP/PE/ C

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Figure 3. Effect of the SiO2/ETPTA composition ratio on the EISA-driven structural development of SiO2 nanoparticle arrays and the porous morphology of the resulting membranes: (a) SiO2/ETPTA = 50:50 (v/v); (b) 74:26 (v/v); (c) 90:10 (v/v). (d) Charge/discharge profiles of the cells as a function of the SiO2/ETPTA composition ratio, where the cells (LiCoO2 cathode/graphite anode) were charged and discharged at a constant charge/discharge current density of 0.1 C/0.1 C under a voltage range of 3.0−4.2 V. (e) OCV profiles of the cells, where the cells were charged to 4.2 V at constant current density of 0.2 C and their voltage drop was measured as a function of the elapsed time.

(Supporting Information, Figure S3b). However, the EGDA monomer with two acrylic double bonds is insufficient to allow the fabrication of a physically robust network structure, as compared to the ETPTA monomer with three acrylic double bonds. As a result, the EGDA polymer matrix is too weak to preserve its dimensional integrity during the etching step, leading to the partial collapse of the nanoscaffold structure. In addition to the aforementioned material/processing variables, another important factor is the SiO2/ETPTA composition ratio. Figure 3a shows that, for a ratio of SiO2/ ETPTA = 50:50 (v/v), sacrificial SiO2 nanoparticles are randomly dispersed rather than close-packed in the ETPTA polymer matrix, yielding sparsely scattered and poorly interconnected pores after the removal of SiO2 nanoparticles.

aggregation of the SiO2 nanoparticles in the mixture solution. Moreover, the poorly dispersed SiO2 nanoparticles may be randomly entangled with PVdF-HFP chains during the subsequent solvent evaporation step. Under these conditions, hexagonal close-packing of SiO2 nanoparticles is difficult to achieve, resulting in a poorly developed porous structure after the removal of SiO2 nanoparticles. To further verify the simplicity and excellence of ETPTA monomer-mediated one-pot EISA process, another control sample was prepared (Supporting Information, Figure S3) using UV-curable ethylene glycol diacrylate monomer (EGDA). Similar to the result obtained with the ETPTA monomer (Supporting Information, Figure S3a), the EGDA monomer enables the formation of close-packed SiO2 nanoparticle arrays D

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Other separator properties, including electrolyte wettability, electrochemical stability window, thermal shrinkage, and mechanical properties, were estimated. The IO separator exhibits a higher electrolyte-immersion height than the hydrophobic PP/PE/PP separator (Supporting Information, Figure S5a). This improved electrolyte wettability of the IO separator is attributed to the polar characteristics of its constituents and to its well-developed porous structure, which facilitates capillary intrusion18,19 of the liquid electrolyte into the nanopores. The linear sweep voltammograms (Supporting Information, Figure S5b) indicate that no appreciable decomposition of the IO separator occurred at potentials less than 4.5 V vs Li/Li+. The thermal shrinkage test (Supporting Information, Figure S5c) indicates that the IO separator presents good thermal resistance (thermal shrinkage ≈0%, after exposure to 150 °C/0.5 h) compared to that of the PP/PE/PP separator (thermal shrinkage ≈38%). Such a remarkable improvement in the thermal shrinkage of IO separator is attributed to incorporation of the thermally stable PET nonwoven substrate (melting temperature >250 °C) and the UV-cured ETPTA nanoscaffold, along with the omission of film stretching processes that are commonly used for fabrication of conventional polyolefin separators. Notably, this result stresses the safety-reinforcing effect of the IO separator on preventing separator shrinkage-triggered internal short-circuit failures,17−19 which may result in a fire or the explosion of cells. The mechanical flexibility of the IO separator was investigated to explore its potential applications in cells with various form factors. The IO separator can be wound around a stainless steel rod (diameter = 1.5 mm), and more notably, its seamless nanoscaffold structure is not disrupted after the bending deformation (Supporting Information, Figure S5d). Furthermore, the IO separator is not mechanically broken even after being folded and twisted several times (Supporting Information, Figure S5e). To achieve commercially meaningful separators for use in practical batteries, mechanical properties of separators are high enough to withstand roll-to-roll processes in cell assembly. The tensile properties of the IO separator were examined using a universal tensile tester (at a strain rate of 10 mm min−1) and compared with those of the PP/PE/PP separator (Supporting Information, Figure S5f and Table S2). In overall, the IO separator shows the lower tensile stress and elongation at break than the PP/PE/PP separator. However, when we focused on the tensile modulus, which is another important mechanical property affecting the processability of continuous cell assembly, the higher value was observed at the IO separator. Meanwhile, no significant difference in tensile properties between the IO separator and PET nonwoven substrate was found, indicating that the tensile properties of the IO separator are highly dependent on those of the PET nonwoven substrate. This result demonstrates that, although the tensile stress of the IO separator is lower than that of the PP/PE/PP separator, adoption of an advanced nonwoven substrate with reliable mechanical properties could be a promising way to overcome its weak tensile properties. Another important mechanical requirement of separators is the tolerance against local protrusion stress, which often arises from unwanted solid impurities or uneven-sized/sharp-edged electrode powders that may eventually give rise to internalshort circuit failures in cells. In the battery industry, puncture strength or mix penetration strength is commonly used to evaluate this protrusion resistance.13 Here, the puncture strength of separators was examined using a stainless steel

The theoretical particle volume for constructing the hexagonally close-packed sphere arrays is known to be 74%.38,39 To achieve this ideal arrangement of SiO2 nanoparticles, the SiO2/ ETPTA composition ratio was adjusted to 74:26 (v/v). Figure 3b shows that the SiO2 nanoparticles are densely packed and in close contact with each other as originally designed, thereby enabling the evolution of the 3D interconnected, well-ordered nanopores entrained with the ETPTA polymer. However, a further increase in SiO2 content to 90% disrupts the ordered packing of the SiO2 nanoparticles and also causes particle aggregation, leading to the poorly ordered porous structure with a relatively large pore size and broad pore size distribution (Figure 3c and Supporting Information, Figure S4). This strong dependence of the porous structure on the SiO2/ ETPTA composition ratio is expected to affect cell performance. The control separator (I) (fabricated from SiO2/ETPTA = 50:50) shows a substantial increase in cell polarization during the charge/discharge reactions (Figure 3d) because of the lessporous structure impeding ion transport, where the cell (LiCoO2 cathode/graphite anode) was charged and discharged at a constant charge/discharge current density of 0.1 C/0.1 C under voltage range of 3.0−4.2 V. By comparison, no abnormal charge/discharge behavior was observed at the IO separator (SiO2/ETPTA = 74:26). More details on cell performance for this well-tailored IO separator are discussed in the following section. The control separator (II) (SiO2/ETPTA = 90:10) with a nonuniform porous structure and short-tortuous path may cause unwanted leakage current between the electrodes, resulting in unstable charge behavior in the cell. This poor cell performance was further verified by examining the open-circuit voltage (OCV) drop as a function of elapsed time, which is considered to be an indicator of self-discharge characteristics of cells.17−19 Figure 3e shows that the IO separator (SiO2/ETPTA = 74:26) exhibits a stable OCV profile that is comparable to that of the commercial PP/PE/PP separator. By comparison, the control separator (II) fails to adequately prevent leakage current, giving rise to a sharp decrease in the OCV, which appears to be similar to that of a pristine PET nonwoven with an excessively large pore size and broad pore size distribution (Supporting Information, Figure S4). The relationship between the porous structure (specifically, pore size/pore size distribution) of the separators and the OCV drop behavior has been extensively investigated in previous studies.18,19,40 The major membrane properties of the IO separator were characterized (Supporting Information, Table S1 and Figure S5) for a fixed SiO2/ETPTA composition ratio of 74:26 (v/v). A notable advantage of the IO separator over the PP/PE/PP separator is the higher porosity (IO separator ≈60% vs PP/PE/ PP separator ≈40%) as well as its precisely ordered porous structure, which contributes to an improvement of the ionic conductivity (σ = 1.0 vs 0.6 mS cm−1 for the PP/PE/PP separator) after being filled with the liquid electrolyte (1 M LiPF6 in EC/DEC = 1:1 (v/v)). The MacMullin number (NM = σo/σs, where σo is the ionic conductivity of the liquid electrolyte itself and σs is the ionic conductivity of the liquid electrolyte-filled separator) of a separator is known to represent a loss of ionic conductivity due to the presence of an ionically inert volume in a separator.13,14 The IO separator presents a lower NM (= 7.5) than the PP/PE/PP separator (NM = 12.5), revealing that ion transport in the IO separator is not seriously impaired due to the 3D reticulated, highly porous nanoscaffold structure (Supporting Information, Table S1). E

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Figure 4. Discharge performance of cells, where the cells were assembled with LiCoO2 cathode/graphite anode and IO separator (SiO2/ETPTA = 74:26 (v/v)) or PP/PE/PP separator: (a) Discharge C-rate capability for a low-mass-loading electrode (cathode/anode = 3.5/1.7 mg cm−2). (b) Discharge C-rate capability for a high-mass-loading electrode (cathode/anode = 25.0/11.5 mg cm−2). (c) Nyquist plot used to calculate the ionic conductivity of separators. (d) GITT profiles of the cells assembled with the high-mass-loading electrodes and variation in the internal cell resistance as a function of the state of charge and the depth of discharge. (e) Variation in the peak currents of oxidation/reduction reactions (obtained from the CV profiles) as a function of the square root of scan rate (= [v s−1]1/2). (f) Ragone plot showing the relationship between gravimetric energy and power density of cells, where the cell weight was determined on the basis of the weight of cathode active materials.

because they can simplify the cell configuration and increase the volumetric energy density of a cell by minimizing the volume of electrochemically inert components, such as separators and current collectors.43,44 However, poorly established ion/ electron pathways in high-mass-loading electrodes pose a substantial obstacle in their facile application to electrochemical systems. Here, we scrutinize the effect of ion transport (passing through separators) on cell performance as a function of electrode mass loading. Figure 4 shows the discharge C-rate capabilities of cells assembled with the IO separator (SiO2/ ETPTA = 74:26 (v/v)) at different electrode loadings (LiCoO2 cathode/graphite anode = 3.5/1.7 and 25.0/11.5 mg cm−2). The cells were charged at a constant current density of 0.2 C and were discharged at various current densities under a voltage range of 3.0−4.2 V. At all of the examined electrode mass

rod (diameter = 0.4 mm), where a vertical penetration speed was 10 mm min−1. The IO separator presents the lower puncture resistance than the PP/PE/PP separator (Supporting Information, Figure S5g and Table S2). Similar to the results of tensile properties, the puncture resistance of the IO separator appears to be affected by that of the PET nonwoven substrate. Meanwhile, in the field of fibers and nonwoven fabrics, several promising approaches have been reported to improve mechanical properties of nonwoven substrate.41,42 Our future works will be devoted to further enhancing the mechanical properties of IO separators, with a particular focus on fabrication and incorporation of mechanically robust nonwoven substrates. In energy storage systems including LIBs, a great deal of attention has been devoted to high-mass-loading electrodes F

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Figure 5. Cycling performance of cells, where the cells were assembled with high-mass-loading electrodes (cathode/anode = 25.0/11.5 mg cm−2) and IO separator (SiO2/ETPTA = 74:26 (v/v)) or PP/PE/PP separator. The cells were cycled between 3.0 and 4.2 V at a charge/discharge current density of 1.0 C/1.0 C. (a) Variation in the charge/discharge profiles with cycling. (b) Capacity retention as a function of the cycle number. (c,d) TOF-SIMS images of the cathode surface after the 40th cycle, where the blue color indicates a relatively low concentration of Li2F+ byproducts for the cell incorporating (c) IO separator and (d) PP/PE/PP separator. The influence of the separators on the Faradaic reaction of electrodes is conceptually illustrated in the view of ionic flow toward the electrodes.

The superior discharge C-rate capability of the IO separator can be explained by its higher ion conductivity (Figure 4c and Supporting Information, Table S1), which is attributed to the electrolyte-philic and 3D reticulated nanoscaffold structure. To further verify the advantageous effects of the IO separator, the GITT (galvanostatic intermittent titration technique) measurement45,46 was performed with the high-mass-loading electrodes, where the current was applied for 12 min at a charge (or discharge) current density of 1 C (= 3.54 mA cm−2) and an interruption time of 60 min between each pulse. Figure 4d shows that, in comparison to the PP/PE/PP separator, the IO separator effectively mitigates the build-up in ohmic polarization of a cell from the repeated current stimuli during both the charge and the discharge reactions. The detailed values of internal resistance were also provided as a function of the state of charge and the depth of discharge (the bottom side of Figure 4d). In addition, the CV (cyclic voltammetry)47,48 behavior of

loadings, the IO separator exhibits a better discharge C-rate capability than the PP/PE/PP separator. More details related to the discharge profiles of the cells are depicted in the Supporting Information, Figure S6. The gap in the discharge capacity between the two separators became larger as the discharge current density was increased. Notably, in the case of the lowmass-loading electrode (cathode/anode = 3.5/1.7 mg cm−2, Figure 4a), the IO separator exhibited a satisfactory level of discharge capacity (higher than 60 mAh g−1) even at a fast discharge rate of 20 C (corresponding to a discharge time of 3 min), whereas the PP/PE/PP separator did not exhibit a meaningful discharge capacity. This beneficial effect of the IO separator was also observed for the high-mass-loading electrode (cathode/anode = 25.0/11.5 mg cm−2, Figure 4b), although the improvement in discharge rate capability is not so remarkable compared to that of the low-mass-loading electrode. G

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Figure 6. Fast-charging and electrolyte wettability of cells, where the cells were assembled with high-mass-loading electrodes (cathode/anode = 25.0/11.5 mg cm−2) and IO separator (SiO2/ETPTA = 74:26 (v/v)) or PP/PE/PP separator. The cells were discharged at a constant current density of 0.5 C after being charged at various current densities ranging from 0.2 to 2.0 C (excluding the CV charge mode). (a) Comparison of the charge rate capability between the cells incorporating the IO separator and PP/PE/PP separator. (b) Variation in the overpotential as a function of the charge current density. (c-f) Structural analysis of the anode surface after the fast-charging test. EDS images depicting the distribution and concentration of C and F elements (inset) of (c) PP/PE/PP separator and (d) IO separator. XPS spectra (specifically, F 1s peaks) showing the relative concentration of LiF byproduct: (e) PP/PE/PP separator and (f) IO separator. (g,h) Effect of electrolyte (1 M LiPF6 in EC/PC = 1:1 (v/v)) wettability of the separators on cell performance, where Li4Ti5O12 anode was used and the cells were cycled at a charge/discharge current density of 0.1 C/0.1 C under voltage range of 1.5−2.7 V: (g) PP/PE/PP separator and (h) IO separator.

the cells was analyzed (Supporting Information, Figure S7). Figure 2e depicts the variation in peak currents for the

oxidation/reduction reactions as a function of the square root of scan rate (= [v s−1]1/2). The cell with the IO separator shows H

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Information, Figure S8). The growth of the cell impedance after the 40th cycle was suppressed in the cell with the IO separator (ZRe(40th cycle) − ZRe(1st cycle) = ΔZRe ≈ 20 ohm) compared to that in the cell with the PP/PE/PP separator (ΔZRe ≈ 50 ohm). This result demonstrates that the IO separator effectively prevents the formation of undesirable resistive layers (which cause the increase of cell impedance54−56 on the electrodes. A more detailed analysis of the structural change on electrode surface during charge/discharge reaction and the correlation with the cell polarization will be conducted in future studies. This unusual contribution of the IO separator to the cycling performance, which has never been reported in previous studies, is conceptually illustrated in the right sides of Figure 5c,d. In comparison to the PP/PE/PP separator, which has a low porosity, nonuniform porous structure, and poor electrolyte wettability, the IO separator with the 3D interconnected seamless nanoscaffold structure and good wettability permits fast/uniform ionic flow toward electrodes. As a result, the favorable ionic transport via the IO separator could allow for facile accessibility and uniform concentration distribution of electrolyte on electrodes over a wide area, which thus facilitates the Faradaic reaction of the electrodes and alleviates the buildup of cell polarization in overall during cycling. By comparison, the PP/PE/PP separator showing nonuniform/ sluggish ionic migration could provoke the position-dependent uneven Faradaic reaction of electrodes, which may expedite the local decomposition of electrolyte on electrodes during charge/ discharge reaction. Therefore, it can be speculated that separators themselves are electrochemically inert and do not directly participate in Faradaic reaction of cells, however, their porous structure and membrane properties significantly affect the distribution and accessibility of electrolytes on electrodes, which are believed to play an important role in mitigating the localized, unwanted side reactions between electrolytes and electrodes. In addition, the long-term structural stability of the IO separator after the cycling test was examined. No morphological disruptions or structural defects were observed in the nanoscaffolds on either the cathode or the anode side of the IO separator (Supporting Information, Figure S9). Enormous interest in LIB-powered electric vehicles has continuously spurred researchers to address the issues associated with fast-charging cell.4−8 Figure 6a shows the variation in charge capacities for cells as a function of charge rate, where the cells were assembled with the previously used high-mass-loading electrodes to clearly demonstrate the severity of the fast-charging problem. The cells were discharged at a constant current density of 0.5 C after being charged at various current densities ranging from 0.2 to 2.0 C (excluding the CV (constant voltage) charge mode). At low charge rates, no significant difference in charge capacity was observed between the IO separator and the PP/PE/PP separator. The charge/ discharge profiles of the corresponding cells are shown in the Supporting Information, Figure S10. As the charge current density is increased, the IO separator mitigates the cell polarization and also increases the charge capacity, although substantial increases were not achieved, potentially because of the use of the high-mass-loading electrodes. The advantageous effect of the IO separator was verified by measuring a kind of overpotential (= average cell voltage at a given charge current density − average cell voltage at a fixed charge current density of 0.1 C) generated during the fastcharging reaction. The cell with the IO separator exhibits a

the higher peak currents of oxidation/reduction reactions than that with the PP/PE/PP separator. The difference in the peak currents between the two separators became pronounced at faster scan rates. These GITT and CV results demonstrate that tailoring the structure/properties of the separators, in addition to classical approaches that focus on the electrode active materials and electrolytes, may represent an alternative pathway to boost the electrode charge/discharge reaction kinetics. The pursuit of an ideal energy storage system demands simultaneous improvements in both energy and power density. The influence of the separators on the Ragone plot (which describes the relationship between energy and power density) of the cells is shown in Figure 4f, where the cell weight used to calculate the gravimetric energy and power density of a cell is determined on the basis of the weight of cathode active materials. The previously reported energy/power densities of the cells, which were chosen exclusively from the results with LiCoO2 active materials, were also plotted.49,50 At relatively low power densities, no appreciable difference in the energy density was observed between the IO separator and the PP/PE/PP separator. As the power density was increased, the cells with the IO separator exhibited higher energy densities than those with the PP/PE/PP separator. The Ragone plot is known to be predominantly influenced by electrochemically active materials.4−6 However, we demonstrate that the Ragone plot curves for a cell can be shifted to a more desirable position by controlling the ion transport phenomena in the separators without the need to alter other battery materials and configurations; i.e., the IO separator may represent an additional tool that can be used to change the Ragone plot, which expands the available material selection/design options in the development of high-performance energy storage systems. The effect of the IO separator on the cycling performance of a cell incorporating the high mass loading electrode (LiCoO2 cathode/graphite anode = 25.0/11.5 mg cm−2) was also investigated. The cell was cycled between 3.0 and 4.2 V at a relatively high charge/discharge current density (= 1.0 C/1.0 C). Figure 5a shows that the cell with the IO separator presents stable charge/discharge behavior, even under harsh operating conditions (i.e., the high-mass-loading electrodes and fast charge/discharge rate). By comparison, the cell with the PP/ PE/PP separator exhibits a sharp decrease in capacity and also large cell polarization. The capacity retention after the 40th cycle was approximately 70% for the IO separator and 13% for the PP/PE/PP separator (Figure 5b). The superior capacity retention of the cell with the IO separator was verified by examining the physicochemical change of the cathode surface after the 40th cycle. The time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis revealed that the cell with the IO separator (Figure 5c) produces a smaller amount of Li2F+ (one of the byproducts generated by the electrochemical decomposition of LiPF6 salts in liquid electrolytes51−53) on the cathode surface compared to that in the cell with the PP/PE/ PP separator (Figure 5d). The blue color in the TOF-SIMS images of the IO separator indicates a relatively low concentration of the Li2F+ byproduct. Moreover, in the case of the cell with the IO separator, the Li2F+ byproducts are uniformly dispersed on the cathode surface, whereas in the cell with the PP/PE/PP separator, the byproducts are locally concentrated and unevenly distributed. In addition to this structural elucidation, the variation in the AC impedance spectra of the cells was monitored during cycling (Supporting I

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Nano Letters

Letter

wearable electronics. We envisage that the IO separator, owing to the versatility of its well-defined nanoscaffold structure that is capable of imparting superior ion transport, can be easily applied as a pseudoelectrochemically active nanomembrane to a wide variety of energy storage systems, including supercapacitors, Na-ion batteries, Li-sulfur batteries, and metal-air batteries, in addition to LIBs.

lower overpotential than the cell with the PP/PE/PP separator (Figure 6b). The difference in the overpotential between the two separators becomes more significant at higher charge current densities. In addition, the surface morphology of the graphite anode was analyzed after the fast-charging test. The EDS images show that, in comparison to the cell with the PP/ PE/PP separator (Figure 6c), the cell with the IO separator (Figure 6d) exhibits a higher concentration of carbon (green color, representing graphite anode materials) relative to fluorine (red color, fluorine-containing byproducts) on the anode surface, indirectly demonstrating the suppression of byproduct formation. We further confirmed this beneficial influence of the IO separator on the fast-charging behavior by analyzing the XPS (Figure 6e and f), FE-SEM (Supporting Information, Figure S11a and b), and TOF-SIMS (Supporting Information, Figure S11c and d) results for the anode surface. The IO separator presents the relatively cleaner anode surface and also less LiF byproduct compared to the PP/PE/PP separator. To emphasize the superior wettability of the IO separator, we evaluated the electrochemical performance of a cell with an extremely polar electrolyte (1 M LiPF6 in EC/PC = 1:1 (v/v)) was chosen as a representative electrolyte), where Li4Ti5O12 anode was used instead of a graphite anode which is difficult to wet with the polar electrolyte. The cells were cycled at a charge/discharge current density of 0.1 C/0.1 C under the voltage range of 1.5−2.7 V. This EC/PC-based liquid electrolyte has attracted great attention due to its advantageous characteristics, which include low volatility and good thermal/ electrochemical stability.51,52 However, the excessively high polarity of the EC and PC gives rise to wetting problems in other cell components, especially hydrophobic polyolefin separators.13−15,40 The cell incorporating the PP/PE/PP separator (Figure 6g) was unable to be electrochemically activated during the charging process because of the poor electrolyte wettability. By contrast, both the normal discharge capacity of ≈146 mAh g−1 and the stable cycling behavior were achieved with the cell containing the IO separator (Figure 6h). This result underscores the fact that the IO separator broadens the selection of electrolytes available for cell design. In summary, we have developed the inverse opal-inspired, seamless nanoscaffold battery separator (“IO separator”) as an unprecedented membrane opportunity to improve electrochemical performance of LIBs far beyond those accessible with conventional battery separators. The electrolyte-philic, 3D reticulated nanoporous structure of the IO separator was formed over a wide area via the simple one-pot EISA of colloidal SiO2 nanoparticles in the presence of a UV-curable ETPTA monomer, followed by UV-cross-linking and selective removal of hexagonally close-packed SiO2 nanoparticle superlattices. The IO separator-driven facilitation of the ion transport phenomena was more pronounced under harsh conditions such as the high-mass-loading electrodes, fast charging/discharging, and highly polar liquid electrolyte. Furthermore, the IO separator enabled the movement of the Ragone plot curves to a more desirable position representing high-energy/highpower density, without the need to alter other battery materials and configurations. To the best of our knowledge, this work is the first report to demonstrate the exceptional functionality of a battery separator in controlling the Ragone plot for LIBs. These noteworthy advantages of the IO separator are expected to play a critical role in the development of next-generation highperformance LIBs for use in the rapidly emerging industrial fields of electric vehicles, grid scale energy storage systems, and

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ASSOCIATED CONTENT

S Supporting Information *

Details of methods and supplementary results demonstrating additional structural/membrane properties of IO separators and also electrochemical performance of cells. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (S.Y.L.) [email protected]. Author Contributions

J.H.K. performed the experiments and analyzed the data. J.H.K. and K.H.C. performed the electrochemical analysis. H.K.Y., J.H.K., and J.S.L. participated in characterizing and discussing the ion transport phenomena. S.Y.L. coordinated and supervised the overall project. J.H.K. and S.Y.L. wrote the manuscript and all authors discussed the results and participated in manuscript preparation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2009-C1AAA001-2009-0093307). This work was also supported by the leading industry of Sustainable Energy of the Chungcheong Leading Industry Office of the Korean Ministry of Knowledge Economy. This work was also supported by Ener gy Effi ciency and Resources R&D program (20112010100150) under the Ministry of Knowledge Economy, Republic of Korea. This work was also supported by the BK21 Plus Program (META-material-based Energy Harvest and Storage Technologies, 10Z20130011057) funded by the Ministry of Education (MOE, Korea) and National Research Foundation of Korea (NRF).

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REFERENCES

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