Enhancing the Cycling Stability of Sodium Metal Electrodes by

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Enhancing the Cycling Stability of Sodium Metal Electrode by Building an Inorganic/Organic Composite Protective Layer Yun-Jung Kim, Hongkyung Lee, Hyungjun Noh, Jinhong Lee, Seokwoo Kim, Myung-Hyun Ryou, Yong Min Lee, and Hee-Tak Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14437 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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ACS Applied Materials & Interfaces

Enhancing the Cycling Stability of Sodium Metal Electrode by Building an Inorganic/Organic Composite Protective Layer Yun-Jung Kim,a Hongkyung Lee,a Hyungjun Noh,a Jinhong Lee,a Seokwoo Kim,b MyungHyun Ryou,b Yong Min Leeb and Hee-Tak Kima*

a

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of

Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea. b

Department of Chemical and Biological Engineering, Hanbat National University,

Yuseong-gu, Daejeon 34158, Republic of Korea

KEYWORDS: Cycling stability, Dendrite formation, Electrolyte decomposition, Freestanding composite protective layer, Sodium metal electrode 1 ACS Paragon Plus Environment


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ABSTRACT

Owing to the natural abundance of sodium resources and their low price, next-generation batteries employing an Na metal anode, such as Na-O2 and Na-S systems, have attracted a great deal of interest. However, the poor reversibility of an Na metal electrode during repeated electrochemical plating/stripping is a major obstacle to realizing rechargeable sodium metal batteries. It mainly originates from Na dendrite formation and exhaustive electrolyte decomposition due to the high reactivity of Na metal. Herein, we report a freestanding composite protective layer (FCPL) for enhancing the reversibility of an Na metal electrode by mechanically suppressing Na dendritic growth and mitigating the electrolyte decomposition. A systematic variation of the liquid electrolyte uptake of FCPL verifies the existence of a critical shear modulus for suppressing Na dendrite growth, being in good agreement with a linear elastic theory, and emphasizes the importance of the ionic conductivity of FCPL for attaining uniform Na plating/stripping. The Na/Na symmetric cell with an optimized FCPL exhibits a two times longer cycle life compared with that of a bare Na electrode.

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INTRODUCTION Environmental concerns and energy efficiency demands have prompted building grid-scale energy storage systems for the efficient use of renewable energy sources such as solar and wind power.1 As a promising energy storage device, rechargeable lithium ion batteries (LIBs) have been considered owing to their attractive energy density and efficiency, as well as their ease of installation.2-3 Nonetheless, concerns about the sustainability of lithium resources have motivated the development of room-temperature sodium (Na) rechargeable batteries in which the natural abundance and low cost of raw materials are more critical for widespread use.2, 4-5 Indeed, raw materials for sodium-ion batteries (SIBs) are estimated to be about 30 times cheaper than those of LIBs.1, 6 Furthermore, Na-based energy-dense batteries such as Na-sulfur (S) and Na-oxygen (O2) batteries can deliver a higher energy density than LIBs if an Na metal electrode can be successfully employed.7-19 For the above Na batteries, an Na metal electrode is an ideal anode material due to its high theoretical specific capacity (1166 mAh g-1) and low redox potential (-2.70 V versus SHE).1-2, 16

Also, Na metal itself possesses Na ion sources and thus allows a versatile use of un-

sodiated, high capacity cathode materials, including S and O2.10 However, Na metal electrodes have still suffered from inherent problems, as have Li metal electrodes. These include dendritic growth of Na metal during electrochemical deposition and a decomposition of electrolytes due to sodium’s high reactivity which is even more severe than that of Li metal.13, 20-22 In addition, although an Na metal electrode is effective for analyzing Na-based cathodes as a reference and/or counter electrode, its use has been hindered by the aforementioned drawbacks.



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Recently, efforts have been made to understand key factors needed to stabilize an Na metal electrode/electrolyte interface and to explore ways of using metallic Na in advanced battery systems.22-25 Hartmann et al. and Bi et al. revealed that the formation of Na dendrites, which can pierce a separator, leads to an internal short-circuit.20-21 Zhao et al. experimentally confirmed that the Na dendrite formation becomes more severe at a higher depth of discharge and longer cycling.13 The highly reactive nature of Na metal generally imposes more challenging constraints in the design of a solid-electrolyte interphase (SEI) structure.22 An approach to prevent Na dendrite growth and parasitic reaction between Na metal and organic electrolytes is in high demand. In this paper, we present a simple but robust approach to stabilize an Na metal electrode by exploiting an inorganic/organic composite protective layer (CPL), which was originally proposed for Li metal stabilization by our group.26-27 The CPL, consisting of mechanically robust Al2O3 inorganic particles and liquid electrolyte-swollen PVdF-HFP polymers, can suppress Na dendrite formation and consequently improve the cycling stability of an Na metal electrode. The CPL for an Na metal electrode should be redesigned from the previously reported one, in that the current CPL is fabricated in the form of a free-standing film. Due to a rigorous reactivity of Na metal with organic solvents, the direct coating of the CPL slurry on the metal electrode and subsequent drying process, which was previously used for Li metal electrodes, cannot be employed for an Na metal electrode. To overcome the limitation of the previous method, we developed a free-standing CPL (FCPL) and, afterward, integrated it with an Na metal electrode. As a means to modulate the ion conductivity and mechanical strength of the FCPL, propylene carbonate (PC) was introduced to the PVdF-HFP phase, which provides a space for accommodating a liquid electrolyte. Since Na+ conductivity and mechanical property can be tuned by the PC content, the PC-mediated fabrication strategy 4 ACS Paragon Plus Environment

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allows a systematic observation of the effect of the mechanical strength and Na+ conductivity of the FCPL on the cycling stability. By using FCPLs with different properties, we demonstrate that (1) dendritic Na metal growth can be mechanically suppressed when the shear modulus of the FCPL exceeds a critical value as suggested by Newman et al.,28 (2) electrolyte decomposition can be reduced by the FCPL, and (3) the ion conductivity of the FCPL is another key factor to enhance cycling stability once dendrite formation is suppressed. An optimized FCPL exhibits a smaller polarization and a two-fold increase in cycling stability at 0.5 mA cm-2 compared with an unprotected Na metal electrode. The artificial interface engineering with a controlled mechanical strength and ionic conductivity thereby suggests future directions for stabilizing an Na metal electrode.

EXPERIMETAL SECTION Fabrication of FCPL.

FCPL slurry was prepared by stirring 0.8 g of 480 nm-sized Al2O3

(AES-II, lshihara lnc., Japan) and 0.2 g of PVdF-HFP in 1.5 g of a mixture of PC (≥99.7%, Sigma-Aldrich, USA) and dimethylacetamide (DMAc) (≥99.9%, Sigma-Aldrich, USA) for 24 h. The FCPLs were coated on a glass plate by using a doctor blade (blade thickness = 150 µm; Honzo, Japan) and then dried at room temperature for 1 h in a vacuum to allow evaporation of the DMAc. The FCPLs were detached from the glass substrate, and the PC in the FCPLs was extracted in a deionized water bath. Then, the FCPL was dried at 60 oC for 2 days in a vacuum to remove residual water and subsequently stored in an Ar-filled glove. The thickness of FCPL was controlled to be in the range of 17~20 µm. The FCPL was laminated on an Na metal foil by roll-pressing at room temperature. The FCPL-coated Na metal (FCPL/Na) electrode was cut into a disk (Ø 16 or 14 mm) for electrochemical measurements. 5 ACS Paragon Plus Environment


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Measurement of the physical properties of FCPL.

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The ionic conductivity of the liquid

electrolyte-containing FCPL was determined from the AC impedances of the corresponding symmetric SUS cell. 1M NaClO4 EC/PC (1:1 v/v) was used as an electrolyte for the FCPL. (See Figure S1, Supporting information for more information on the ionic conductivity measurement) The shear modulus of the liquid electrolyte-containing FCPLs was measured by using a nano-indentation technique (Nano Indenter XP instrument, MTS, USA), as described elsewhere.27 Electrochemical characterization.

To evaluate the electrochemical performances of the

FCPL/Na electrode, CR2032 coin-type Na/Na symmetric cells were assembled in an Ar-filled glove box, which consisted of a pair of bare or FCPL/Na electrodes, a glass fiber membrane (Whatman, UK) and 200 µl of the electrolyte (1M NaClO4 in EC/PC (50/50 by volume)). A disk-type Na metal electrode was prepared by flattening and punching a cube-type Na-metal (Sigma-Aldrich, USA). Using a battery cycler (WBCS 3000, Wonatech, South Korea), galvanostatic Na stripping/plating tests were conducted at room temperature. For the cells, a single pre-cycle of plating (5h) and stripping (5h) at 0.1 mA cm-2 was conducted to clean the surfaces of the Na electrodes, and an extended cycling at 0.5 mAcm-2 with an areal capacity of 1 mAh cm-2 was carried out. And also, in order to confirm the protective effect of FCPL on Na metal, we assembled full cells by using a Na0.6Mn0.65Ni0.25Co0.10O2 as a cathode with 200 µl of the electrolyte (1M NaClO4 in EC/DMC (50/50 by volume) with fluoroethylene carbonate (FEC) as an additive). All full cells were aged for 12hr before a formation cycling between 2.1 V and 4.3 V (vs. Na/Na+) at a 0.1 C for 1 cycle and 0.2 C for 3 cycles. Successively, an extended cycle performance with or without FCPLs is investigated at a 0.5 C. Electrochemical impedance spectroscopy (EIS) measurements were also made by using a


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Solartron 1255 frequency response analyzer coupled with a Solartron 1287 electrochemical interface over a frequency range of 1 MHz to 0.1 Hz. Surface analysis.

All electrodes taken from disassembled coin cells were rinsed with

anhydrous 1,2-dimethoxy ethane (DME) in an Ar-filled glove box. In order to prevent any contamination and degradation of those samples, they were vacuum-sealed in a polyethylene pouch before surface analysis. The morphology of the Na metal deposits was examined by field emission-scanning electron microscopy (FE-SEM; Sirion, FEI). Also, the chemical nature of the Na metal surface after cell cycling was investigated by using X-ray photoelectron spectroscopy (XPS; Sigma Probe, Thermo VG Scientific) with Mg Ka as the X-ray source. The binding energy (BE) scale was calibrated against the hydrocarbon C 1s peak at 285.0 eV.

RESULTS AND DISCUSSION The chemical compatibility of organic solvents with Na metal stresses the necessity of developing a new CPL fabrication method for Na metal electrodes. As shown in Figure S2, the Na metal foil corroded immediately after contact with the solvents (including DMAc that can dissolve PVdF-HFP). In addition, the Na metal foil showed a much faster and more significant decomposition in dimethylformanide (DMF), dimethyl sulfoxide (DMSO), and DMAc than the Li metal. Due to the highly corrosive nature of Na metal, the direct coating method27 was excluded, and instead, a new fabrication process for an FCPL was developed. The schematic of the FCPL fabrication process is shown in Figure 1. First, the Al2O3/PVdFHFP composite layer was formed on the glass substrate under ambient air. After the subsequent vacuum drying process, the CPL-coated glass substrate was immersed into 7 ACS Paragon Plus Environment


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deionized (DI) water to detach the FCPL from the glass substrate. The as-prepared FCPL was laminated on the Na metal electrode by roll-pressing. Due to the ductility of an Na metal electrode, an FCPL can be tightly attached to an Na metal electrode even at room temperature. The other key feature of the FCPL is the inclusion of PC plasticizer in the slurry. Because PC is compatible with PVdF- HFP, it forms a PC-plasticized PVdF-HFP phase in the FCPL after the casting and drying process. Prior to the roll-pressing step, the residual PC plasticizer was fully extracted from the FCPLs in the DI water bath, forming pores in the FCPL which can accommodate liquid electrolyte. To confirm the complete elimination of PC from the PVdF-HFP phase after the extraction, the FT-IR spectra of the PC-plasticized PVdFHFP membrane before and after DI water treatment were obtained. As shown in Figure S3, the peak at 1750 cm-1 from the carbonyl group of PC disappeared after the extraction, confirming the removal of PC from the PVdF-HFP polymer matrix. The morphology of the FCPL is highly dependent on the PC content. With increasing PC content, the resulting FCPL became more porous, as shown in the cross-sectional SEM images of the FCPLs with different PC content (Figure S4). Therefore, the liquid electrolyte uptake of the FCPL could be tuned by adjusting the PC content. The amount of PC with respect to PVdF-HFP varied from 25 to 500 wt.%; the FCPLs fabricated with PC content of 25, 50, 100, 200, 300, 400 and 500 wt.% are labeled with FCPL-25P, -50P, -100P, -200P, -300P, -400P and -500P, respectively. As expected, the liquid electrolyte uptake of the FCPL gradually increased with the PC content (Table S1), which confirms that the PC provides an extra space for accommodating the Na+ ion-conducting liquid electrolyte (1M NaClO4 in EC/PC (1:1=v/v)).


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Figure 1. Schematic of the FCPL and FCPL/Na metal electrode integration fabrication According to previous studies27, an artificial protective layer for an Li metal electrode should have high mechanical strength as well as sufficiently high ionic conductivity to mechanically suppress Li dendrite formation and promote uniform ion flux to the Li metal surface, respectively. However, for an Na metal electrode, the influences of mechanical strength and ionic conductivity of a protective layer on Na dendrite formation and cycling stability have not been investigated yet. In this regard, the FCPLs provide an effective platform to investigate their influences because the mechanical property and ionic conductivity of the FCPLs can be systematically tuned by varying the PC content. As shown in Figure 2a, the FCPLs exhibit a gradual increase of ionic conductivity with the PC content because of the increased electrolyte uptake (Table S1). On the other hand, as seen in Figure 2b, the shear modulus, which is critical to the mechanical suppression of dendritic metal growth based on the linear elastic model,28 of the FCPL decreased as the PC content increased. It means that the decrease in the volume fraction of Al2O3 weakens the mechanical strength of the FCPLs. It is worth noting that the shear modulus values of all the FCPLs



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except FCPL-500P exceeded the critical shear modulus (6.0 GPa) required for mechanical suppression expected from the linear elastic model.27-28

Figure 2. (A) Ionic conductivity and (B) shear modulus of the as-prepared FCPLs with various ratios of PC and PVdF-HFP The electrochemical performance of the FCPL/Na electrodes was investigated in a galvanostatic cycling test for the corresponding symmetric cells at a current density of 0.5 mA cm-2 (charge/discharge=2 h/2 h). The voltage profiles during the galvanostatic cycling for the various FCPLs are compared in Figure 3 and Figure S5. For the bare Na electrode, charging/discharging overvoltage gradually increased with cycling until the operation stopped at 72 cycles at which point the cell voltage exceeded a safety voltage window (–2 10 ACS Paragon Plus Environment

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5V vs. Na/Na+) (Figure 3a). In general, the increase of overvoltage for the bare Na metal electrode cell originates from continuous electrolyte decomposition at the enlarged Na surface and a successive accumulation of the decomposition products.29 In this work, an excess amount of electrolyte was used to prevent early electrolyte depletion during cell operation to more clearly investigate the interfacial stability of Na metal electrode. As given in the impedance changes with cycle for the bare Na/Na symmetric cell (Figure S6), the change of interfacial resistance with cycling dominates the total impedance change, indicating that the accumulation of the electrolyte decomposition products mainly influences the cell impedance. The comparison of the cycling stabilities for the FCPL/Na metal electrodes showed an interesting feature; the cycling stability was improved with increasing the PC content from 25 to 300%, however, it then lowered above 300%. As shown in Figure 3f, The FCPL-300P/Na metal electrode exhibited the most stable voltage response and longest cycle-life span (127 cycles). In contrast, FCPL-25P and FCPL-500P showed a negative effect on cycling performance in comparison with the bare Na metal electrode. Therefore, shear modulus and ionic conductivity of the FCPLs and symmetrical cell cycling data collectively suggest that the cycling stability of Na metal electrodes becomes poor when either the mechanical property of the FCPL is poor or ionic conductivity is low. In order words, the FCPLs having both high mechanical strength and high ionic conductivity can enhance the cycling stability of Na metal electrodes.



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Figure 3. Time-voltage profiles of the coin-type Na/Na symmetric cells with (A) the bare Na metal and (B-H) the FCPL coated Na electrodes; (B) FCPL-25P, (C) FCPL-50P, (D) FCPL100P, (E) FCPL-200P, (F) FCPL-300P, (G) FCPL-400P and (H) FCPL-500P. All symmetric cells were cycled at 0.5 mA cm-2 (charging/discharging = 2hr/2hr). For a deeper understanding of the above observations, the interfaces between the FCPL and Na metal after 5 cycles at 0.5 mA cm-2 were investigated by using SEM (Figure 4 and Figure S7). For the bare Na metal electrode, a highly uneven and porous electrode surface with granule-like Na deposits was formed (Figure 4a), whereas, for the FCPL/Na electrodes excluding FCPL-500P/Na, a compressed surface morphology was observed (Figure 4b-h). Even for FCPL-25P, of which cycling stability is worse than the bare Na metal, Na dendrite formation was suppressed to some extent. However, the Na metal surface protected by FCPL500P exhibited granular deposits similar to those for the bare Na metal electrode. To our interest, these observations are in good agreement with the prediction from the linear elastic model. The shear modulus of FCPL-500P is lower than the critical value as indicated by the nano-indentation tests (Figure 2b) and, in fact, FCPL-500P did not suppress the growth of 12 ACS Paragon Plus Environment

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Na dendrite. Conversely, the FCPLs with shear modulus higher than the critical one (6.0 GPa) unequivocally suppressed dendritic Na growth. These results are the first experimental verification of the existence of a critical shear modulus for Na dendrite suppression and provide an insight into protection layer design for an Na metal electrode.

Figure 4. The SEM images of (A) the bare Na and (B-H) the FCPL-coated Na electrodes after 5 cycles at 0.5 mA cm-2. The detailed information of the FCPLs is below; (B) FCPL-



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25P, (C) FCPL-50P, (D) FCPL-100P, (E) FCPL-200P, (F) FCPL-300P, (G) FCPL-400P and (H) FCPL-500P. In order to investigate the effect of the FCPL on electrolyte decomposition, the chemical composition of the surface of an Na metal electrode was analyzed for the bare and FCPL/Na electrodes after 5 cycles at 0.5 mA cm-2 by using XPS. For the FCPL/Na electrodes, the FCPLs were detached from the Na electrodes, and the surfaces of the Na electrodes were analyzed. The XPS results for the bare Na and FCPL-25P, FCPL-300P and FCPL-500P/Na electrodes are compared in Figure 5. In the high-resolution C1s spectra of all the electrodes, the peaks corresponding to the –C–O– species, –C=O– species, sodium alkylcarbonate (ROCO2Na) and sodium carbonates (Na2CO3) were observed at 286.8, 288, 289.1 and 290 eV, respectively, indicating that the reductive decomposition of cyclic carbonate electrolyte, 1M NaClO4 EC/PC (1:1=v/v), occurs regardless of the existence of the FCPL on the Na metal electrode (Figure 5a-d).27, 30-31 In the same vein, the peaks of sodium oxide (Na2O), sodium carbonates (Na2CO3), sodium hydroxide (NaOH), –C–O– species and –C=O– species could be observed in every high-resolution O1s spectra at 529.7, 531.6, 534.8 and 534.1 eV, respectively (Figure 5e-h).24, 30-31 For the Na electrodes protected by FCPL-25P and FCPL-300P, which can suppress dendritic Na growth, the peak intensities in the C1s and O1s spectra were significantly reduced compared with those for the bare Na and FCPL-500P/Na electrodes (Figure 5). In particular, such intensity reduction was more pronounced for the peak from Na2CO3, implying that FCPL-25P and -300P can effectively mitigate the reductive decomposition of an EC/PC electrolyte. These behaviours are quite reasonable considering that the decomposition reaction rate can be reduced by lowering the liquid electrolyte content in the FCPL and suppressing the expansion of the Na metal electrode surface. This tendency was identically 14 ACS Paragon Plus Environment

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observed for the Na metal electrodes protected by other FCPLs (FPCL-50P, -100P, -200P and -400P); as shown in Figure S8, a larger degree of electrolyte decomposition was detected for an FCPL with higher electrolyte uptake or lower shear modulus.



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Figure 5. The C1s and O1s XPS spectra of (A, E) the bare Na and (B-D, F-H) the FCPLcoated Na electrodes after 5 cycles at 0.5 mA cm-2. The detailed information of FCPLs is below; (B, F) FCPL-25P, (C, G) FCPL-300P and (D, H) FCPL-500P. From the point of cycling stability, the importance of the ionic conductivity of the FCPL should be noted here. Even though FCPL-25P can efficiently suppress the growth of dendritic 16 ACS Paragon Plus Environment

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Na and mitigate electrolyte decomposition, its cycling stability was even worse than that of the bare Na electrode. This uncommon behaviour could be explained by localized Na ion fluxes through the protective layer for reducing the large polarization from the highly resistive layer. In our previous report on the protection layer made of a hybrid-type single ionic polymer electrolyte, such negative effect was observed;32 the polymeric protection layer with low ionic conductivity exhibited a highly localized Li deposition/stripping at the sites with lower resistance levels. In order to investigate the effect of ionic conductivity on the homogeneity in the Na plating/stripping process, the cross-sectional SEM images of the Na metal electrode were compared for the two FCPLs (FCPL-25P and -300P) of which ionic conductivities are different by 7.5 times. For FCPL-25P, a local detachment of the FCPL from the Na metal electrode was observed after 5 cycles at 0.5 mA cm-2 (Figure S9a), which verifies the occurrence of a concentrated Na plating/stripping. Compared with FCPL-25P, FCPL-300P exhibited a more intimate contact with the Na metal electrode after the repeat cycling, as shown in Figure S9b, indicating a more uniform Na plating/stripping. In addition, considering the progressive electrolyte decomposition which can decrease the ionic conductivity of FCPL, dynamic change in ionic conductivity would be more important in the evolution of the inhomogeneity during the cycling than initial ionic conductivity of FCPL. In this work, the Ohmic resistance of the symmetric cell includes mixed contributions from the FCPL and separator filled with the liquid electrolyte, dynamic ionic conductivities of FCPLs could not be quantified despite their importance. Recently, Wei et al. reported that forming thin Al2O3 on Na metal electrode by atomic layer deposition method can effectively stabilize the Na metal electrode, which demonstrated that the formation of stable and thin inorganic SEI layer on Na metal electrode is quite effective in enhancing the reversibility of Na metal electrode.33 Since the Al2O3 layer is as thin as 2.8 nm, 17 ACS Paragon Plus Environment


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the layer did not impose large ohmic resistance. However, the FCPLs are much thicker than the Al2O3 thin layer, and thus, the liquid electrolyte phase is indispensable for attaining low ohmic polarization. We expect that the FCPLs are not as effective as the Al2O3 thin layer in suppressing the contact of liquid electrolyte to Na metal electrode. However, due to the use of ductile PVdF polymer, the FCPLs would withstand a larger interfacial deformation. Moreover, in terms of large scale manufacturing, our method would be advantageous because FCPLs can be fabricated by simple wet coating process. For a clearer description of the Na plating/stripping behaviours at the FCPL/Na metal interface, the role of the FCPL depending on its physical properties is illustrated in Figure 6. For the FCPL with low liquid electrolyte uptake (Figure 6a), due to its low ionic conductivity, Na+ ion flux is prone to be concentrated at the edge side or defects, resulting in uneven Na metal plating/stripping and consequently a low cycling stability in spite of its mechanical suppression of Na dendrite growth. Conversely, in the case of the FCPL containing excessive empty space for accommodating liquid electrolyte (Figure 6c), Na dendrite could still form, because its mechanical strength is lower than the critical value for suppressing dendritic growth of Na metal. It indicates that this FCPL cannot mechanically protect Na metal during continuous cycling even though the shear modulus of an Al2O3 particle, one of the ingredients of the FCPL, is greatly higher than that of Na metal. In addition, a higher degree of reductive electrolyte decomposition can be resulted from its high electrolyte uptake and expanded reaction surface. However, an optimized FCPL, which has balanced ionic conductivity and mechanical property, can function as an artificial SEI layer, providing a uniform supply of Na+ and mechanical suppression of Na dendrite formation (Figure 6b). Even though the FCPL can mitigate the side reactions between the electrolyte and Na metal, the decomposition of the EC/PC electrolyte could not be completely 18 ACS Paragon Plus Environment

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suppressed due to its extremely high reactivity with the Na metal electrode. Therefore, a combination of an FCPL and a new stable electrolyte23-24 could be a future direction to further improve the reversibility of an Na metal electrode for room temperature Na full cell with Na intercalation-based cathode, Na-O2 and Na-S batteries. In order to confirm the protective effect of FCPL on Na metal, we fabricated full cells with a Na0.6Mn0.65Ni0.25Co0.10O2 cathode and observed the differences of cycling stability between bare Na and FCPL-300P/Na electrodes.34 As shown in Figure S10, the cell with FCPL300P/Na delivered much higher capacity retention (91.6% at 60th cycle) compared with the cell with bare Na anode (76.7% at 60th cycle), which confirms the efficacy of FCPL in stabilizing Na electrode even in a full cell.

Figure 6. Schematics of the morphological changes of the FCPL-coated Na electrodes during the electrochemical plating/stripping cycle with the ratio of PC plasticizer. (A) A plasticizer content is under 100 wt.% indicating that the FCPL does not have enough ionic conductivity even though the mechanical strength is excessively high. (B) The optimized FCPL structure, FCPL-300P, has both sufficient ionic conductivity and shear modulus. (C) A plasticizer ratio 19 ACS Paragon Plus Environment


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is higher than 500 wt.%, which means that the FCPL does not have sufficient mechanical strength to suppress Na dendrite growth. CONCLUSIONS In summary, we report an improved cycling stability of Na metal electrodes by building an FCPL on the surface of an Na metal electrode as a protective layer. From the systematical variation of shear modulus and ionic conductivity of the FCPL, the existence of critical shear modulus for mechanical suppression of Na dendritic growth was experimentally verified for the first time. The FCPLs that can suppress Na dendrite formation also exhibited a significantly reduced electrolyte decomposition. However, the FCPLs with low ionic conductivities could not enhance cycling stability in spite of their Na dendrite suppression function due to localized Na plating/stripping. From these observations, it can be concluded that both high mechanical strength and high ionic conductivities are required for the protective layer. An optimized FCPL (FCPL-300P) which satisfied the two requirements showed almost a two times longer cycle life (127 cycles) than a bare Na cell (72 cycles).

ASSOCIATED CONTENT Supporting Information Schematic configuration of SUS/SUS symmetric cell and Nyquist plot of cell with FCPL300P, digital images of chemically degraded Li and Na metal electrode which soaked into organic solvents, FT-IR spectrum of PVdF-HFP membranes with PC plasticizer (solid lines) and after removal of PC plasticizer by dipping in DI water, areal uptake amount of electrolyte (1M LiClO4 in EC/PC) with various ratio of PC plasticizer against an amount of PVdF-HFP, 20 ACS Paragon Plus Environment

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cross-sectional SEM images of FCPLs, magnified initial time-voltage profiles of the cointype Na/Na symmetric cells with the bare Na metal and the FCPL coated Na electrodes, Nyquist plots of the impedances of the bare Na/Na symmetric cell measured at various cycle numbers, SEM image of bare Na as prepared, SEM images of the bare Na and the FCPLcoated Na electrodes after 5cycles at 0.5 mA cm-2, C1s and O1s XPS spectra of the FCPL coated Na electrodes after 5 cycles at 0.5 mA cm-2, cross-sectional SEM images of (A) FCPL-25P coated Na and (B) FCPL-300P coated Na metal electrodes after 5cycles at 0.5 mA cm-2 (charging/discharging-= 2hr/2hr), cycle retention of Na/Na symmetric cells with bare Na and with FCPL-300P/Na electrodes. This material is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08111.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel.: +82-42-350-3916; Fax: +82-42-350-3910 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by a National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2014R1A1A2056199).

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Enhancing the Cycling Stability of Sodium Metal Electrodes by

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