Hierarchical Chitin Fibers with Aligned Nanofibrillar Architectures: A Nonwoven-Mat Separator for Lithium Metal Batteries Joong-Kwon Kim,†,∥ Do Hyeong Kim,‡,∥ Se Hun Joo,‡,∥ Byeongwook Choi,† Aming Cha,‡ Kwang Min Kim,§ Tae-Hyuk Kwon,§ Sang Kyu Kwak,*,‡ Seok Ju Kang,*,‡ and Jungho Jin*,† †
School of Materials Science and Engineering, University of Ulsan, Ulsan Metropolitan City 44610, Republic of Korea School of Energy and Chemical Engineering and §Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan Metropolitan City 44919, Republic of Korea
‡
S Supporting Information *
ABSTRACT: Here, we introduce regenerated fibers of chitin (Chiber), the second most abundant biopolymer after cellulose, and propose its utility as a nonwoven fiber separator for lithium metal batteries (LMBs) that exhibits an excellent electrolyte-uptaking capability and Li-dendritemitigating performance. Chiber is produced by a centrifugal jet-spinning technique, which allows a simple and fast production of Chibers consisting of hierarchically aligned self-assembled chitin nanofibers. Following the scrutinization on the Chiber−Li-ion interaction via computational methods, we demonstrate the potential of Chiber as a nonwoven mat-type separator by monitoring it in Li−O2 and Na−O2 cells. KEYWORDS: chitin fiber, separator, Li−O2 battery, Li dendrite, DEMS
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The recent LMBs based on Li−S or Li−O2 system are considered one of the promising next-generation energy storage devices because of their unrivalled energy capacity (∼3500 Wh/kg) in comparison to the present Li-ion batteries (LIBs).19,20 Nevertheless, LMBs have one common yet crucial drawback that originates from using the bulk-metallic Li anode: the uncontrollable growth of Li dendrites, which significantly limits the lifecycle of LMBs and causes serious safety issues due to the internal short circuits.21−24 These protruding Li dendrites arise from an inhomogeneous distribution of Liions at the electrolyte−electrode interface, which inevitably occurs during battery operation.25 A key idea to resolve this issue is alleviating the concentration gradient of Li-ions to mitigate the growth of Li dendrites by, as examples, introducing separators with regular nanopore arrays or creating micropatterns on the Li anode.24,26−30 Meanwhile, other technical strategies have been also intensively developed over the last two decades, such as introducing solid electrolytes, chemical additives, and barrier layers that chemically and physically restrain the dendrite growth.31−34 However, these methods still present a difficulty
ibers derived from natural polymers are an indispensable class of material that has versatility for a wide range of structural and functional applications.1 For example, cellulose (polysaccharide) and silk (protein) have been key ingredients of fibers not only in the classic textile industry but also for state-of-the-art biomedical uses, such as sutures and tissue scaffolds.2−4 Although these biogenic macromolecules naturally occur as fibers that are as serviceable, their semisynthetic versions, i.e., regenerated fibers (RFs), and the material technologies associated with the fiber spinning have led the successful commercialization and elicited scientific interests as well. Viscose rayon and lyocell fibers are good examples of regenerated cellulose fibers.5 In addition, studies on RFs spun from silk fibroin have been numerous as well,6−8 some of which are also in their commercialization stage.9,10 As such, it is of great significance to produce advanced RFs from other natural polymers using simple fiber-spinning processes, and the RFs can offer valuable material options for many engineering applications in diverse fields.11−16 Herein, we introduce a hierarchical RFs made of chitin [poly(β-(1,4)-Nacetyl-D-glucosamine)], the second most abundant natural polysaccharide only after cellulose,17 and suggest its usage as a nonwoven fiber separator for lithium (Li) metal rechargeable batteries (LMBs) through exploiting both the strong affinity to Li-ions and the inertness to the aprotic electrolytes.18 © 2017 American Chemical Society
Received: March 26, 2017 Accepted: May 15, 2017 Published: May 15, 2017 6114
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nozzle as the centrifugal force becomes stronger than the capillary force. Then, these jets can further elongate and solidify to form Chibers when the viscous force acting across the stream of jets dominates the surface tension, overcoming the Rayleigh−Plateau instability.37,42 The formation of continuous fibers can be realized by controlling several process parameters, such as the concentration of the spinning dope (0.4, 0.6, and 0.8 w/v %), rotation speed (3000, 4000, and 5000 rpm), nozzle diameter (28, 30, and 32 gauge), and nozzle-to-collector distance (12, 15, and 18 cm); the values in the parentheses denote the process parameters designed in this study. Carefully examining possible combinations of these parameters, we obtain the optimal Chiberthe one used to manufacture the best performing separator in the following LMB testsunder the following conditions: the chitin/HFIP spinning dope of 0.6 w/v % (15 mL/run), rotation speed of 5000 rpm, nozzle inner diameter of 100 μm (32 gauge), and nozzle-to-collector distance of 18 cm, respectively. Figure 1d shows the photograph of the optimal as-spun Chiber, which is reminiscent of white cotton candy; this is obtained from a single run of CJS using a 30 mL of chitin/HFIP dope, which takes only about 3 min. As visualized in the SEM image (Figure 1e), Chiber has a morphology of randomly entangled fibers, whose average diameter is ∼2 μm with a normal distribution (Figure S1). In contrast, CJS experiments carried out using other sets of parameters result in Chibers with a larger amount of messy defects, which look like irregular ribbons (Figure S1). As in dry spinning, the formation of Chibers via the CJS process occurs by the solvent evaporation from the chitin/HFIP solution, which generally involves the hydrogen-bond-mediated selfassembly of chitin molecules into ultrafine nanofibers (i.e., the evaporation of HFIP, a poor hydrogen-bond-acceptor, causes the revitalization of the chitin’s intermolecular hydrogen bonds inducing chitin molecules to self-assemble into nanofibers).41,43 Thus, it is anticipated that the self-assemblies of chitin nanofibers would be directed along the jet ejection, given that the high-speed centrifugation exerts a strong shear along the nozzle longitudinally while facilitating the evaporation of HFIP. As expected, the topographic AFM analysis of the micron-scale Chiber reveals its hierarchical structure consisting of self-assembled chitin nanofibers, which collectively align along the Chiber axis (Figure S2 for the AFM phase image).44,45 This aligned structural hierarchy of Chiber is further confirmed in the polarization optical microscopy analysis, in which a clear birefringence is observed (Figure S3). Such directed assembly of natural structural polymers forming a multiscale hierarchy is of compelling scientific interest, because it may offer opportunities to construct biocompatible engineering materials with unprecedented structural and functional properties.46 We further investigate the crystalline structure of our aligned Chiber using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and grazing-incidence wide-angle Xray scattering (GIWAXS) (Figure 2). In nature, chitin occurs as supramolecular semicrystalline and exists in two major distinct polymorphs: α-chitin (orthorhombic) and β-chitin (monoclinic). These chitin crystals have different molecular conformations (i.e., antiparallel for α-chitin vs parallel for βchitin) with dissimilar hydrogen-bond densities, which can be intuitively distinguished through FTIR and XRD analyses; each material presents different characteristic peaks.41 As revealed in the FTIR spectra (Figure 2a), β-chitin has a single broad absorption peak at around 1630 cm−1 because of the
of fabrication and mass synthesis. Thus, there is a strong demand for controlling the Li metal in a more practical way. Here, we report on the fabrication of a regenerated chitin fiber (Chiber) and its nonwoven mat-type separator, which is capable of effectively suppressing the growth of Li dendrites that occurs in typical LMBs and improving their Li cycling efficiency (LCE). As a probing study, we perform density functional theory (DFT) calculation and molecular dynamics (MD) simulation, which predict that chitin has a high level of physicochemical affinity to Li cations. The computation result specifically confers an excellent electrolyte-uptaking property on the Chiber separator, which works as one of the key requirements of separators for LMBs. For the fabrication of Chiber, we employ a centrifugal jet-spinning (CJS) technique that closely mimics a method used to make cotton candies.35−38 Within a few minutes, our CJS process allows the production of a large quantity of micron-diameter Chiber, which features a structural hierarchy comprising self-assembled chitin nanofibers that collectively align along the Chiber axis. Following the investigations on the Chiber’s basic structural and functional characteristics, we finally validate the performance of Chiber separator, which is introduced in a Li−O2 battery cell.
RESULTS AND DISCUSSION Figure 1 displays the materials and CJS apparatus used to produce Chiber as well as the photographic, scanning electron
Figure 1. (a) Molecular structure of chitin. (b) A digital photograph of HFIP solvent and β-chitin/HFIP solution (0.6 w/v %). (c) A schematic illustration of the CJS apparatus consisting of (i) a reservoir, (ii) spinneret nozzles, and (iii) collectors. (d) A digital photograph of as-spun Chiber reminiscent of cotton candy and its (e) SEM image. (f) A tapping-mode AFM topographic image of a single strand of Chiber (the arrow indicates the direction of the jet ejection, i.e., Chiber axis).
microscope (SEM), and atomic force microscope (AFM) images of the manufactured Chiber. As for the materials, we use a squid pen extract β-chitin (Figure 1a) and hexafluoroisopropanol (HFIP) solvent to make the spinning dope solutions with varying concentrations (Figure 1b).39−41 The use of HFIP rather than other solvent systems, such as aqueous NaOH/ urea, is beneficial for a simple fiber spinning, as it obviates the need for any coagulant or antisolvent to recrystallize the chitin into fibers. The CJS apparatus consists of (i) a Teflon reservoir, where the spinning dope is to be filled, equipped with (ii) two spinneret nozzles heading oppositely, and (iii) a set of cylindrical-bar collectors surrounding the reservoir, as schematically illustrated in Figure 1c (Video S1). Upon rotating the reservoir, the solution jets initiate and discharge from each 6115
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which arise from the antiparallel molecular arrangement whereby the intramolecular CO···HO hydrogen bonds are simultaneously engaged with the intersheet CO···HN hydrogen bonds. Notably, Chiber has an absorption spectrum intermediate between the two polymorphs. This result indicates that the crystallographic feature of both chitins co-exists in Chiber.41 The XRD profiles also present a similar trend, as shown in Figure 2b. The peak corresponding to the intersheet crystallographic reflection for Chiber (2θ = 9.06°) appears in between those of α-chitin (2θ = 9.34°) and β-chitin (2θ = 8.24°). Furthermore, the high flux two-dimensional (2D) GIWAXS measurement reveals a more detailed insight on the biphasic structure of Chiber as well as the orientation of the chitin crystals (i.e., the alignment of chitin nanofibers along the Chiber axis).47 Figure 2c displays the 2D GIWAXS pattern obtained from Chiber, in which two strong arc-shaped spots noticeably appear at qz values of 1.37 and 0.60 Å−1. These diffraction spots, respectively, correspond to the (100) and (010) plane of both orthorhombic (α-chitin) and monoclinic (β-chitin) crystalline lattice and well match with the 2θ values in the XRD profiles in Figure 2b.48 Thus, these results clearly indicate that the biphasic crystals of Chiber preferentially orient on the substrate, which is in good agreement with the hierarchical structure of Chiber; the preferential out-of-plane orientation of the chitin crystals is thought to be due to the surface-exposed polar hydroxyl and acetylamino groups that interact with the surface hydroxyl groups and dangling bonds of the substrate (Figure S4). In order to check the usage of Chiber as a nonwoven fiber separator for LMBs, we evaluate the porosity, thermal stability, and electrolyte-uptaking property of the Chiber separator, as shown in Figure 3a; for comparison, a conventional polymeric separator (Celgard 2400) and a glass microfiber filter (GF) (Whatman GF/C) separator are chosen as the reference. In order to make the flat and homogeneous Chiber separator, a
Figure 2. (a) FTIR absorbance spectrum and (b) XRD patterns of α-chitin (black line), Chiber (blue line), and β-chitin (red line). (c) 2D GIWAXS pattern of Chiber. The intensified two reflections of (010) and (100) plane are dominantly detected from Chiber crystals. Inset shows the schematic of the GIWAXS setup for the Chiber sample.
intramolecular CO···HO hydrogen bonds. Meanwhile, αchitin is characterized by split peaks at 1620 and 1660 cm−1,
Figure 3. (a) A digital photograph and (b) plane-view SEM images of Chiber separator after ISP process. (c) Plot of air permeability of the Celgard, GF, and Chiber separators from Gurley test. (d) Series photographs of heat deterioration test of Celgard, GF, and Chiber separators. The digital photos were taken at room temperature, 140, and 200 °C, respectively. (e) Time lapse photographs of electrolyte-uptaking test for Celgard, GF, and Chiber separators. The 1 M LiTFSI-DME electrolyte was used for the test. 6116
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ACS Nano fluffy wad of Chiber is shaped into a circular nonwoven fibermat using an isotactic cold pressure (ICP) followed by punch cutting; the average weight and thickness of our Chiber separators are 8 mg and 67 μm, respectively. Figure 3b shows the SEM image of the Chiber separator, which shows that the morphology of Chiber separator is typical of nonwoven fibermats. The porosity of the Chiber separator is assessed by measuring the air permeance using a Gurley densometer as well as by porosimetry. As shown in Figures 3c and S5, the air permeance and average pore size of Chiber separator are superior to the Celgard and similar to those of GF separator. It implies that the Chiber separator can provide smooth physical paths for the transportation of Li-ions, because it is likely that a larger pore size of a porous medium pertains to a smaller tortuosity through a given thickness.49 Moreover, we obtain an elastic modulus of ∼10 MPa and tensile strength of ∼25 MPa for the mechanical toughness of Chiber separator, which is greater than that of a GF separator, as characterized in the tensile testing (Figure S6). Also, as demonstrated in the heat deterioration test (Figures 3d and S7), the Chiber separator exhibits an excellent thermal stability up to 200 °C in terms of both dimension and morphology. Note that the polymer-based Celgard shows severe heat distortion even at 140 °C and becomes totally decomposed at 200 °C in the ambient condition. More importantly, the electrolyte-uptaking property of Chiber separator is monitored in the time-lapse electrolytewetting test using a bis(trifluoromethane)sulphonamide (LiTFSI) lithium salt/dimethoxyethane (DME) electrolyte; these glyme-based electrolytes are suitable for Li−O2 battery application. As shown in Figure 3e (and Figure S8), the Chiber separator exhibits excellent electrolyte-uptaking performances in both vertical and horizontal electrolyte-wetting setups in comparison to the Celgard and GF separators. Given that the Chiber separator has a decent porosity and morphology similar to that of GF separator (Figures 3c and S5), its electrolyteuptaking performance is unequivocally superior. This result suggests that Chiber has an intrinsic physicochemical affinity to the aprotic solvent and Li-ions. To explore the behavior of Li-ions around the Chiber and the Chiber−electrolyte interactions, we perform MD simulations and DFT calculations. Figure 4a shows the MD snapshots of polypropylene (PP) (Celgard), α-chitin, and β-chitin nanofibers immersed in the electrolyte solution at 0 and 10 ns. While the PP nanofiber does not hold Li-ions at its surface, the chitin nanofiber of both polymorphs exhibits a high affinity to Li-ions (at the surface-exposed hydroxyl and acetylamino groups, as shown in Figure S4 and the inset of Figure 4b). Statistically, it is also seen from the radial distribution function (RDF) between the nanofibers and Li-ions (Figure 4b), where the chitin nanofibers of both polymorphs show higher intensities than the PP nanofiber and notably exhibit sharp peaks at r = ∼2 Å. We find that the peaks represent the Li-ions coordinated to hydroxyl and amide functional groups of chitin molecule forming a pocket-like structure, as shown in the inset of Figure 4b. Thus, the functional groups of chitin molecule significantly contribute to the high Li-ion affinity. Furthermore, we estimate the binding energy of Li-ions in [Li(DME)2(PP)]+, [Li(DME)2(chitin)]+, and [Li(DME)3]+ complexes. The binding strength of Li-ions with chitin (i.e., Ebinding = −3.49 eV) is found to be larger than that with PP (i.e., Ebinding = −1.91 eV) but similar to that with DME molecules (i.e., Ebinding= −3.46 eV) (Figure S9). This binding characteristic implies that Li-ions can be bound to chitin nanofiber reversibly. Consequently, the
Figure 4. (a) MD snapshots showing the degree of physicochemical affinity to Li cation with PP(Celgard), α-chitin, and β-chitin. (b) The RDFs of α-chitin, β-chitin, and PP with Li-ions. (c) The normalized adhesion energy of α-chitin and β-chitin to PP in the electrolyte solution.
functional groups of chitin molecule play a significant role in the high Li-ion affinity with reversible binding, and thus a low charge transfer resistance of chitin separator is expected. Separately, we notify that the adhesion energy of the α-chitin or β-chitin nanofiber to the electrolyte solution is estimated to be approximately 3 times larger than that of the PP nanofiber from our MD simulation (Figure 4c). This strong adhesion energy of chitin nanofiber is a telltale signature for the high uptake of electrolyte solution of Chiber separator. In a complementary set of experiments for Chiber, GF, and Celgard separators, we perform an electric measurement by using a Li/Li symmetry cell consisting of 300 μm Li metal/ separator/5 μm Li metal architecture at a galvanostatic current of ±500 μA for 1000 s until the depletion of 5 μm Li metal on a Cu current collector. As expected, the potential vs time plot of Figure 5a exhibits 2 and 3 times longer cycle durability than the GF and Celgard separator, respectively. This is primarily due to the Li affinity in the Chiber separator, as discussed above. Moreover, a smooth and homogeneous surface is clearly visualized in post-mortem SEM characterization of 5 μm Li metal after 10 stripping/plating cycles, while the GF and Celgard show detrimental rough and hetrogeneous Li metal morphologies in Figure 5b (see also Figure S10 for plane-view SEM images). The surface of Celgard and GF separator at the point of failure also exhibits more aggregated, thicker dead Li than the Chiber separator (Figure S11). Thus, the high enhancement of both electric measurement and surface morphology in the Li/Li symmetry cell demonstrates the Chiber separator plays critical roles in increasing Li+ movement, leading to a significantly extended cycle life, and mitigates the dendrite growth on the Li metal surface. To extend our Chiber separator to more realistic LMBs, we introduce it into the metal−O2 batteries, such as Li−O2 and Na−O2 with 1 M LiTFSI (NaOTf for Na−O2 battery) in DME electrolyte. As schematically illustrated in Figure 5c, a 6117
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Figure 5. (a) Chronopotentiometry results of Celgard (red), GF (green), and Chiber (blue) contained coin cell. (b) SEM tilt-view images of 5 μm Li surface of Chiber, Celgard, and GF after 10 times plating/stripping cycle. The insets of (b) display photographs of the 5 μm Li surface. (c) Schematic representation of the Li−O2 cell architecture comprised of three layers: 5 μm Li anode/separator/carbon cathode. (d) The galvanostatic cycle endurance of GF (green) and Chiber (blue) separator contained Li−O2 batteries. (e) Galvanostatic discharge/charge plot and in situ DEMS result of Chiber contained Li−O2 battery. One M LiTFSI-DME electrolyte was used for all electrochemical measurement.
Swagelok-type Li−O2 cell consisting of a 5 μm Li metal anode/ Chiber separator/porous carbon cathode (P50) is used for the cycle test under the same experimental conditions, ± 500 μA for 1000 s, in Figure 5a.50 The constant 1.3 bar O2 gas is injected from the porous carbon cathode side during the cell measurement. The effectiveness of Chiber separators in Li−O2 battery is shown in Figure 5d, where the cycle life is approximately 2 times longer than the conventional GF separator. In particular, the discharge and charge trajectories in the Chiber-containing cell are similar to that of the GF-based reference cell, indicating that the Chiber separator does not cause other types of side reactions during the oxygen reduction reaction and oxygen evolution reaction (OER). It is noted that the inertness of components in the battery cell is important for stable cycling of Li−O2 battery, otherwise it will generate the detrimental gases during the charging process.51,52 In order to more carefully inspect the parasitic reaction in the Chiber-containing Li−O2 cell, we monitor the Li−O2 battery using a differential electrochemical mass spectrometry (DEMS).50 The capacity of 1 mAh is used for the discharge/ charge process at a current of ±200 μA in the DEMS measurement. As shown in Figure 5e, the DEMS results exhibit the OER during the charging process, where the O2 evolution values are in good agreement with those of other literatures.26,51 However, compared to the theoretically calculating values,51 the data show a relatively higher charge overpotential with a lower O2 evolution. A highly probable reason is considered as the inevitable reaction between the unstable nature of the discharge product of lithium peroxide (Li2O2) and carbon cathode surface or aprotic electrolyte, which is unlikely related to our Chiber separator. 51 Furthermore, we access our Chiber separator with another type of alkali metal-based (Na) oxygen battery (Figure S12), Na−O2 battery, into which the Chiber separator can be readily adopted. This result simply suggests that the Chiber separator may work as a potentially viable separator for the Na ion and/ or Na metal-based batteries.
CONCLUSION In conclusion, we have successfully demonstrated a hierarchical regenerated chitin fiber (Chiber), manufactured by a simple CJS technique, working as a nonwoven mat-type Chiber separator that is capable of effectively mitigating Li dendrites in LMBs. In order to find the suitable fiber quality for the Chiber separator, we screened various parameters in the CJS process and found a very homogeneous and biphasic structure with preferential crystal orientation of Chiber. Based on computational support and electrolyte uptake measurement, we found that the Chitin molecule has a stronger Li-ion affinity than conventional battery separators due to unique coordinations with functional groups. In particular, this Li-ion affinity arose from Chiber separators with the binding reversibility allowed to exhibit greatly improved Columbic efficiency with Li metal mitigation not only in the Li/Li symmetry cell but also in the realistic LMBs such as Li−O2 and Na−O2 batteries. EXPERIMENTAL SECTION Preparation of Chitin Fiber (Chiber) Separator. Chitin powder (extracted according to a conventional protocol)18,41,53 and 1,1,1,3,3,3hexafluoro-2-propanol (HFIP, Halocarbon) solvent were used as received without any further purification. The chitin/HFIP solutions for the CJS process were made simply by dissolving the chitin powder in HFIP with vigorous magnetic stirring at room temperature, followed by mechanical homogenization using a high-speed blender (IKA, IKA ULTRA-TURRAX T 25 digital) at 10,000 rpm for 5 min. These spinning dope solutions were put into the reservoir of the CJS apparatus and spun into Chiber under various conditions. The diameters of Chiber were readily controlled by the concentration of solution (0.4, 0.6, and 0.8 w/v %), rotational speed of 3000−5000 rpm, needle gauges (27−32 G), and nozzle-to-collector distance (12− 18 cm), respectively. Finally, the Chibers were carefully collected from the cylinderical bar collectors and then shaped into circular mat-type separators by pressing with an isostatic cold pressure (ICP) (ILSHIN Autoclave, ISA-WIP-45-75-150-AL) at 200 MPa. Characterization. The prepared membranes were analyzed by using Fourier transform infrared-attenuated total reflection (FTIRATR, NICOLET iS50) mode with wavenumber window of 650−4000 6118
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ACS Nano cm−1 and out-of-plane XRD (Bruker D8 Advance). The surface of the chitin fiber was recorded by using SEM (Hitach High-Technologies, S4800) at an acceleration voltage of 3 kV. For SEM images, a thin Pt layer was created (FEI Sputter, Tescan) for increasing the contrast. A mercury porosimeter (Autopore IV9500 model, Micromeritics Inc.) was used to analyze the pore size and porosity generated in the polymer matrix. AFM was also performed with a Nanoscope IVa from Digital Instruments in tapping mode with height and phase contrast. Thermogravimetric analyzer (TGA, Pyris 1 TGA) was conducted to identify the thermal property of chitin α- and β-crystalline phases. GIWAXS was performed on 6D UNIST-PAL line at the POHANG ACCELERATOR LABORATORY (Pohang University of Science and Technology, Korea). The films were placed on an x- and y-axis goniometer and were irradiated with monochromatized X-rays (λ = 0.10608 nm) having grazing incidence angles ranging from 0.09 to 0.15°. The porosity of Chitin membranes was characterized by a paper porosity measurement density meter (GPI, 4340N). Li/Li Symmetry and Metal−O2 Cell Fabrication. For Li/Li symmetry and metal−O2 battery experiments, Li foils (300 μm-thick and 5 μm-thick Li deposited on Cu foil) and Na ingot were purchased from Wellcos Co., Sidrabe Co. and Aldrich Korea, respectively. The battery grade 1 M lithium bis(trifluoromethane)sulfonamide in dimethoxyethane electrolyte was purchased from Wellcos Co. The water content of electrolyte is
