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Lithium Dendrite Suppression with UVCurable Polysilsesquioxane Separator Binders Wonjun Na, Albert S Lee, Jin Hong Lee, Seung Sang Hwang, Eunkyoung Kim, Soon Man Hong, and Chong Min Koo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02735 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 8, 2016

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ACS Applied Materials & Interfaces

Lithium Dendrite Suppression with UV-Curable Polysilsesquioxane Separator Binders Wonjun Na,†,‡,§ Albert S. Lee,†,§ Jin Hong Lee, † Seung Sang Hwang,†,# Eunkyoung Kim,‡,* Soon Man Hong,†,# and Chong Min Koo†,#,* †

Materials Architecturing Research Center, Korea Institute of Science and Technology,

Hwarangno 14-gil 5, Seong-Buk Gu, Seoul 02792, Republic of Korea ‡

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonseiro,

Seodaemun Gu, Seoul 03722, Republic of Korea #

Nanomaterials Science and Engineering, University of Science and Technology, 217

Gajungro, 176 Gajung-dong, Yuseong-Gu, Daejeon 34113, Republic of Korea

KEYWORDS: lithium metal battery, lithium dendrite, separator, silsesquioxane, thermal stability

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ABSTRACT For the first time, an inorganic-organic hybrid polymer binder was used for the coating of hybrid composites on separators to enhance thermal stability and to prevent formation of lithium dendrite in lithium metal batteries. The fabricated hybrid composite coated separators exhibited minimal thermal shrinkage compared with the previous composite separators (< 5% change in dimension), maintenance of porosity (Gurley number ~ 400 s/100 cm3), and high ionic conductivity (0.82 mS/cm). Lithium metal battery cell examinations with our hybrid composite coated separators revealed excellent C-rate and cyclability performance due to the prevention of lithium dendrite growth on the lithium anode even after 200 cycles under 0.2C5C (charge-discharge) conditions. The mechanism for lithium dendrite prevention was attributed to exceptional nanoscale surface mechanical properties of the hybrid composite coating layer compared with the lithium metal anode, as the elastic modulus of the hybrid composite coated separator far exceeded those of both the lithium metal anode and the required threshold for lithium metal dendrite prevention.

1. INTRODUCTION With ever increasing demands for more efficient and environmentally friendly energy sources, energy storage technology has never been a more important matter to harvest energy for the future.1,2 Lithium ion batteries (LIBs) have been one of the most sought-after technologies for energy storage devices due to their high specific energy and power, as they are now pervasive in our everyday lives, ubiquitously seen in laptops, cellular phones, and other portable devices.3–5 Nevertheless, scaling-up of lithium ion batteries has proved to be a daunting task as lithium ion batteries are hindered by relatively low energy densities.4–6 As such, one of the 2


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most effective ways to improve the energy densities of lithium ion batteries is to substitute graphite (372 mAh/g) with lithium metal (3860 mAh/g) as the anode to give a lithium metal battery with the ultimate electrochemical energy per unit mass.7 Although this substitution of anode seems simple, using lithium metal in batteries gives rise to even greater problems of safety, derived from two different mechanisms. First, during repeated cycles of cell charge-discharge, the lithium dendrite growth through the separator makes short circuits, resulting in cascading fires.8,

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Second, at elevated

temperatures caused by any reasons, thermal shrinkage of polyolefin-based separators leads to internal short circuits, producing sparks from which the organic solvent-based electrolytes may cause the above catastrophic scenario.10 Various strategies have been undertaken to alleviate such problems. Improvements in thermal stability of lithium metal batteries has often been studied through replacement of the organic solvent-based electrolytes with ionic liquids,11,12 development of gel- or fully solidstate polymer electrolytes,13 or coating the separator with inorganic fillers.14–16 Moreover, strategies to prevent or to limit lithium dendrite growth include the development of electrolyte additives,17 supplementing an additional protective layer between electrodes,18 and the use of fully solid-state polymer electrolytes.19 While all of these strategies have been highly effective to improve the thermal stability or prevent lithium dendrite growth, the use of fully solid-state polymer electrolytes has so far been the only seemingly overlapping strategy to improve both thermal stability and to prevent lithium dendrite growth. But, given the low ionic conductivity of solid polymer electrolytes (~10–5 S/cm at room temperature),20 the general consensus among battery engineers and academics is that solid polymer electrolytes fail to meet the minimum requirements for practical device applications (~10–4 3



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S/cm at room temperature).21 Thus, a new strategy to improve both thermal stability and prevent lithium dendrite growth is much to be desired. The coating of polymer composite containing inorganic oxides particles on polyolefin separators has been a wide-spread research field, with extensive studies detailing the effects of particles, binder, and polymer type, to improve the product’s thermal stability.22 Although the vast majority of studies have detailed the effective improvement in bulk thermal shrinkage due to the inorganic filler,23 the role of the binder has often been overlooked. An ideal polymeric binder should not only function to provide good adhesion with polyolefin separator support, but should also improve the interface between electrode and composite coated separator in terms of wettability, thermal stability, and mechanical robustness. According to recent investigations on the nanoscale mechanical properties of a composite separator that prevented lithium dendrite growth,24 we speculated that not only does the inorganic filler improve the mechanical properties, but that the filler-binding polymeric binder plays a more important role to prevent lithium dendrite growth. In this study, we designed a UV-curable inorganic-organic hybrid polymer material, ladder-like structured poly(phenyl-co-methacryloxypropyl)silsesquioxane (LPMA64) as a polymeric binder for the hybrid composite coating on separator.25,26 This fully polymeric inorganic-organic hybrid material, consisting of an Si–O–Si inorganic backbone,27 and phenyl and methacryloxypropyl organic functional pendant groups for thermal stability and UVcuring function, respectively, would provide both the thermal stability and mechanical robustness,26 alleviating the aforementioned safety concerns with lithium metal batteries.

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2. EXPERIMENTAL SECTION 2.1. Materials 3-methacryloxypropyltrimethoxysilane

(98%,

Shin-Etsu

Chemicals,

Japan),

phenyltrimethoxysilane (98%, Shin-Etsu Chemicals, Japan), and tetrahydrofuran (THF, HPLC grade, J.T. Baker Chemicals, USA) were distilled over CaH2 prior to use. Potassium carbonate (Daejung Fine Chemicals, Republic of Korea) was dried at 40 °C. Alumina powder (average particles size 480 nm, Sumitomo Chemicals, Japan), photoinitiator Irgacure 184 (BASF, Germany), and all other chemicals were used as received. Polypropylene (PP) separator was provided by SKC, Republic of Korea. The commercial PP separator, fabricated through biaxial stretching method, had an average pore diameter of 55 nm and porosity of 59%. 2.2. Synthesis of Ladder-Structured Poly(phenyl-co-methacryloxypropyl)silsesquioxane (LPMA64) LPMA64 was synthesized following a reported procedure as described in Scheme 1a.26,28 In a 100 mL round-bottomed flask, deionized water (2.4 g, 0.133 mol) and potassium carbonate (K2CO3) (0.02 g, 0.145 mmol) were charged and stirred for 10 min. Dry THF (4 g, 0.056 mol)

was

added

and

stirred

for

additional

30

min.

Afterwards,

3-

methacryloxypropyltrimethoxysilane (3.97 g, 16 mmol) and phenyltrimethoxysilane (4.76 g, 24 mmol) monomer mixtures were added dropwise via syringe under nitrogen. The reaction was kept under stirring at room temperature for 96 h. Next, the crude reaction mixture was divided into two colorless and cloudy phases. The crude viscous products were obtained through decantation of the colorless mixed solvents, which was extracted several times with MC and deionized water. After collecting the organic layers, drying over anhydrous 5



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magnesium sulfate, filtering, and evaporating the volatiles, LPMA64 was obtained as a white powder

(13.3g,

95%

yield).

1

H

NMR

(CDCl3,

ppm):

0.6–1.1

(t,

Si(CH2CH2CH2OCOCH2CH3, 2H)), 1.4–1.6 Si(CH2CH2CH2OCOCH2CH3, 2H)), 3.2–3.4 Si(CH2CH2CH2OCOCH2CH3, 2H)), 1.3 (s, Si(CH2CH2CH2OCOCH2CH3, 3H)), 5.2–5.4 Si(CH2CH2CH2OCOCH2CH3, 2H)), 7.2–8.0 (m, Si(C6H5), 5H), 29Si NMR (ppm): –64 ~ –70 ppm, , –77 ~ –82 ppm. Mw = 43,000. 2.2. Preparation of Hybrid Composite Coated Separators Hybrid composite coated separators were prepared by first preparing a solution of LPMA64 in THF (2.5 wt %) along with photoinitiator of Irgacure 184 (1 wt % with respect to LPMA64). To this solution, various amounts of alumina were added to make the weight ratio of Al2O3:LPMA64 as 1:2, 1:1, 2:1. The mixed solution was mechanically stirred vigorously for 24 h to yield highly stable suspension solutions. Care was taken to cover the flask to prevent premature UV-crosslinking. Later, the solution was used for gun-spraying on both sides of the PP separators with a Sparmax GP-50 apparatus (Sparmax, Taiwan). The spraying distance was fixed at 15 cm and the injection pressure held constant at 40 psi. The composite coated separators were dried at room temperature and UV-cured with a total UV output energy of 3 J/cm2. All of the hybrid composite coated separators gave thickness of 26 µm, which was 4 µm thicker than 22 µm of pristine PP separator, measured with a micrometer. The hybrid composite coated separators were named as A1L2, A1L1, and A2L1 based on the Al2O3:LPMA64 weight ratio of 1:2, 1:1, 2:1, respectively. 2.3. Electrochemical Characterization The ionic conductivities were determined using a complex impedance analyzer (Bio-Logics, VMP3, France) over frequency range from 1 Hz to 1 MHz at AC amplitude of 10 mV. The electrochemical stability of the hybrid composite coated on the separators was examined with 6


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a linear sweep voltammetry system. In the experiments, a stainless steel working electrode was used with lithium metal as both the counter and reference electrodes. The voltage was swept at a scan rate of 1.0 mV/s. Electrochemical properties examinations of the hybrid composite coated separators were conducted using CR-2032 coin-type cells consisting of a separator, Li metal, and LiFePO4 cathode (90 wt % LiFePO4, 5 wt % carbon black, 5 wt % PVdF), with commercial 1 M LiPF6 in 3:7 v/v ethylene carbonate/diethyl carbonate mixture electrolyte solution. All the cells were assembled in argon-charged glove box. The galvanostatic charge-discharge experiments were carried out in voltage range of 2.5–4.2 V using a battery cycler system WBCS 3000 (WonATech, Republic of Korea) at room temperature. 2.4. Supplementary Characterization The weight average molecular weight (Mw) and molecular weight distributions (Mw/Mn) of polymer samples were determined with a JASCO PU-2080 plus SEC system equipped with RI-2031 and a UV-2075 (254 nm detection wavelength) using THF as an eluent at 40 °C with flow rate of 1 mL/min. The samples were separated through four columns: Shodex-GPC KF802, KF-803, KF-804, and KF-805. PMMA calibration standards were used for GPC analysis. 1H-NMR spectra were taken in CDCl3 at 25 °C on a 400 MHz Bruker system. Differential scanning calorimeter (DSC) was performed with a TA instrument (TA Q20142653) under nitrogen. The surface morphologies of the separators were examined with a field emission scanning electron microscope (FE-SEM, Inspect F-50). The air permeability of separators was measured using a densometer (4110N, Thwing-Albert). Thermal stability of separator membranes were investigated through measurement of their dimensions variations after exposing to heat in an oven at 150 °C for 30 min. Nanoindentation measurements were conducted on a Hysitron TriboIndenter equipped with a Berkovich diamond tip. 7



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Examinations of elastic modulus were performed in a continuous stiffness measurement method as discussed elsewhere.29,30 UV-curing experiments were conducted with a Hitachi Spot UV-cure system.

3. RESULTS AND DISCUSSION 3.1. Fabrication and Characterization of Hybrid Composite Coated Separators As

the

increase

of

phenyl

content

increases

the

mechanical

robustness

and

methacryloxypropyl groups imbuing UV-curing function, ladder-like LPMA64 was fabricated at the optimal balance copolymerization ratio of phenyl:methacryloxypropyl content set at 6:4 as hybrid binder as shown in Scheme 1a.25,26 Characterizations results of LPMA64 are presented in Figures S1 and S2 of the Supplementary Information.

Scheme 1. (a) Synthesis of LPMA64, (b) Schematic depiction for the fabrication of hybrid composite coated separators

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In order to coat the commercial pristine PP separator support with LPMA64/Al2O3 in a fast and facile manner while maintaining feasibility for scaled-up roll-to-roll settings, we chose an automated gun-spray apparatus in which the composite solution was deposited evenly across the PP separator support (Scheme 1b). After room temperature drying, the mild UV-curing process yielded crosslinked LPMA64/Al2O3 coating layer adhered to the PP separator. The UV-curing reaction was monitored with FT-IR spectroscopy (Figure 1). The disappearance of unsaturated C=C bonds at 1638 cm–1 indicated that the UV-curing process was indeed complete. The thickness measurements across the coated separators revealed that the single-side thickness of the coating layer was 2 ± 0.1 µm, giving a total thickness of ~26 µm, when both sides were coated. The hybrid composite separators were named as A1L2, A1L1, and A2L1 for the Al2O3:LPMA64 weight ratios of 1:2, 1:1, 2:1, respectively.

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Figure 1. FT-IR spectra of A2L1 before and after UV-curing process. The morphologies of the pristine PP separator and hybrid composite separators were examined. As the resulting SEM micrographs in Figure 2 show, the PP separator revealed a porous network structure with an average pore diameter of 55 nm. It was noteworthy that the Al2O3/LPMA64 composite did not block the pores of the inner PP separator, as shown by the noticeable retention of pores for A1L2 hybrid composite separator in the background. Moreover, we were able to observe that the crosslinked LPMA64 coating shell on alumina was very smooth with no signs of aggregation. This was attributed to presence of Si–O–Si backbone and Si–OH end groups in LPMA64,31,32 forming electrostatic interactions with the surface of the alumina particles for exceptional binder/filler interface, whereas the high molecular weight (Mw = 43 000) and organic functional group composition allowed impeccable solubility and thus processability. 10


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Figure 2. SEM micrographs of: (a) pristine PP separator, (b) A1L2, (c) A1L1, and (d) A2L1. Figure 3 shows the gurley numbers and room temperature ionic conductivities as a function of alumina composition in the hybrid composite coated separators. First, the air permeability was observed to stay relatively similar despite the Al2O3/LPMA64 composite coating compared with the non-coated neat PP separator. This result indicates that the coating layer rarely blocks the internal pores of the PP separator support. The gun-spraying coating method was indeed viable and the continuous pore structure was maintained after coating with Al2O3/LPMA64 composite. Furthermore, the room temperature ionic conductivities revealed that as the alumina content increased in the hybrid composite coated separator, the ionic conductivity increased as much as 31% over the pristine PP separator. This observation is attributed to the drastically improved surface wetting properties of hybrid composite coated separators due to the incorporation of alumina,33 as shown by the inset images of water 11



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contact angles in Figure 3, exhibiting a gradual increase in hydrophilicity as a function of alumina filler content.

Figure 3. Gurley numbers, room temperature ionic conductivities, and inset photographs of water contact angles of pristine PP separators A1L2, A1L1, and A2L1. 3.2. Thermal Properties Due to the hybrid nature of polysilsesquioxane binder with inorganic backbone, we expected to observe a significant improvement in thermal properties for the prepared hybrid composite coated separators. A comparison of the DSC results for LPMA64 binder, PP separator, and a commonly used binder of PVDF-HFP revealed that while PP and PVDF-HFP gave a melting temperature of 172 and 142 °C, respectively, the LPMA64 hybrid binder prepared in this study did not show any thermal transition up to 250 °C. This high resistance to any thermal transitions,34 was one of the most attractive traits for selection of LPMA64 as hybrid binder. The thermal properties examinations of the separators were performed using two additional different methods. First, examination of thermal shrinkage for the PP separator and hybrid 12


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composite separators, as well as a well-studied Al2O3/PVDF-HFP composite coated separator in an oven preset at 150 °C.35 After 30 min, the values of shrinkage of dimensions were calculated, summarized in Figure 4a and visually shown in Figure 4b. As shown, our hybrid composite separator with the optimal filler/binder balance (A2L1) revealed the least shrinkage (< 5%), whereas the non-coated PP separator (> 10%) and Al2O3/PVDF-HFP composite separator (~15%) exhibited significant thermal shrinkage values. However, as this method did not provide information about the thermal stability under real-life settings, we examined the open circuit voltage (OCV) of Li/separator/LFP cells inside of an oven set at 150 °C for 30 min.16 We observed that the non-coated PP separators could only retain its initial OCV for 2 min, as a precipitous drop and eventual short circuit was observed. Similar results were obtained for the Al2O3/PVDF-HFP separator, as short circuiting behavior was realized only after a mere 2~3 minutes. However, the hybrid composite coated separator sample of A2L1 exhibited a steady preservation of its cell’s electrical integrity. These results corresponded well to the thermal shrinkage tests of the coated separators, our A2L1 hybrid composite separators exhibited the least thermal shrinkage, while the degree of thermal shrinkage of the neat PP separator and Al2O3/PVDF-HFP separator were similar. Moreover, we were able to visualize the stark differences in thermal shrinkage more than the initial thermal stability tests through cell disassembly and observation of the LFP/separator interface after the thermal shock experiments. (Figure 4c inset photograph) This exceptional thermal stability was attributed to the low coefficients of thermal expansion (CTE) values reported for crosslinked ladder-like polysilsesquioxanes (< 50 ppm/K)34 and the electrostatic interactions between Si–OH end groups of the hybrid ladder-like polysilsesquioxanes binder and surface Al–OH groups of Al2O3 filler particles.

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Figure 4. (a) Comparison of thermal shrinkage of separators fabricated in this work, (b) photographs of selected separators after heat treatment at 150 °C for 30 min, and (c) isothermal OCV tests at 150 °C with pristine PP separator Al2O3/PVDF-HFP coated separator and A2L1 sample. 3.3. Electrochemical Properties Before evaluating the cell performances for our hybrid composite coated separators, the electrochemical stability was examined with linear sweep voltammetry technique. As shown in Figure S4 in the supplementary information, our hybrid composite coated separator sample 14


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of A2L1 exhibited electrochemical stability up to 5.0 V vs Li/Li+, which was indistinguishable to the uncoated pristine PP separator. The electrochemical performance values of lithium metal battery cells consisting of a lithium iron phosphate (LFP) cathode, lithium metal anode, and various separators fabricated in this study are shown in Figure 5. As shown, at low discharge current rates C-rates, the discharge capacities were very large at ~158 mAh/g, close to the theoretical discharge capacity of the cathode active material in this potential region (170 mAh/g). It was also noticeable that although the C-rate performance of the discharge capacities for cells fabricated with the pristine PP separator and hybrid composite coated separators with small alumina content were very similar, but the discharge capacities for the hybrid composite coated separator with large alumina content (sample A2L1) showed exceptional superior performance. This phenomenon was especially apparent at high C-rates, as the discharge capacity values for A2L1 and pristine PP separator samples were 75 and 29 mAh/g, respectively, indicative of a ~250% performance boost. The cyclability of the fabricated separators coated with hybrid composite was also examined. As the results in Figure 5b show, similar trends were observed; A2L1 sample exhibited the best performance with minimal depreciation in discharge capacity after 200 cycles under high C-rate conditions of 0.2C-5C (charge-discharge), whereas the pristine noncoated PP separators and hybrid composite separators with small filler content revealed a steeper decline in discharge capacity. Moreover, the coulombic efficiency of the system for A2L1 separators was determined above 99%, with the exceptional of initial cell activation.36 This enhancement in rate capability and cyclability was initially attributed to a combination of conspicuous factors. The improvement surface wettability due the UV-curable 15



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polysilsesquioxane binder/alumina composite coating allowed for improved transport of lithium ions. Also, the improved thermal stability of the composite coatings compared to the neat PP separator allowed for dimensional stability, which in turn allowed for superior high temperature operation at 80 oC. However, we speculated that the degree to which both the rate capability and cyclability performance improved compared to the neat PP separator far exceeded that of the above reasons and that a more indiscreet process was in play.

Figure 5. (a) Rate capabilities and (b) Cycling performance for cells fabricated with pristine PP separators and hybrid composite coated separators at 80 oC with inset discharge profiles for A2L1 sample at various cycles.

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3.4. Mechanical and Interface Properties Although the improvement in lithium metal battery cell performance, especially at high Crates, could be simply related to the improved surface wettability for our hybrid composite separators, we sought out to investigate the hybrid coating layer and electrode interface in more depth. As recent studies have attributed the suppression of lithium dendrite growth in lithium metal batteries when using fully solid-state polymer electrolytes to a mechanical suppressive effect,9,19,37 we stipulated that the mechanical properties of our hybrid composite coated separator would function in the same manner. However, mechanical properties examinations, especially in bulk state, have yet to prove effective in providing comprehensive insight into a quantitative threshold for lithium dendrite prevention. Thus, the nanoscale measurement of mechanical properties of the separators surface was observed as a highly useful tool,38–40 as past studies have shown that highly compressible interfaces are more unstable towards surface roughening.41,42 Figure 6a shows the nanoindentation-derived elastic moduli of various separators reported in literature,40 along with lithium metal, hybrid composite separator prepared in this work, and the theoretical modulus required for the mechanical suppression of dendritic growth.41 As shown, the elastic modulus of our non-coated PP separator (2.1 GPa), PTFE/Al2O3 coated on a PE-PP-PP trilayer separator (1.5 GPa), and PTFE/Al2O3 coated on ultra-high molecular weight polyethylene (UHMWPE) separator (3.8 GPa) all failed to meet the measured elastic modulus of lithium metal electrode (4 GPa) and the minimum required modulus to suppress lithium dendrite growth (6.2 GPa). However, the prepared composite coated separator sample of A2L1 provided an elastic modulus of 7.3 GPa, which is larger than those of both lithium metal and required modulus to suppress lithium dendrite growth. To our knowledge, this is 17



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the first-ever report of a separator to exceed the minimum threshold. We attributed the outstanding mechanical properties of the prepared composite coated separator to the ladderlike structured polysilsesquioxane binder of LPMA64, which after UV-curing provided a well-developed polymeric binder for Al2O3 filler particles. When considering the above literature separators coated with fluorinated polymers such as PVDF and its copolymers,40 it seems that not only the filler, but also the polymer binder reveal a more important role in preventing lithium dendrite growth through binding the fillers to the polymer separator support. In addition, to fully confirm the lithium dendrite preventing effect, the SEM micrographs of lithium metal anode before cycling, after cycling with the pristine PP separator, and after cycling with our hybrid composite coated separator sample of A2L1 were compared (Figure 6b–d). As shown, the neat lithium metal electrode was exceptionally smooth. However, the electrode after 200 cycles with the pristine PP separator under high C-rate conditions of 0.2C5C (charge-discharge) revealed severe formation of lithium dendrites as the surface became very rough, an indication of a ‘ridge and valley’ structure.43 However surprisingly, the lithium metal electrode after cycling with A2L1 hybrid composite coated separator showed a drastically different morphology than that of the electrode cycled with the pristine PP separator. Not only the surface was observed very smooth, but a noticeable ‘pressed’ or ‘squashed’ structure was with no protruding peaks, an indication of a lithium dendrite-free morphology. (Scheme 2) Thus, the flattening of the surface of lithium anodes suggested that the mechanical suppression of lithium dendrite growth can be possible through merely replacing the binder in lithium metal batteries.

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Figure 6. (a) Nanoindentation-derived elastic moduli of pristine PP separator, A2L1 hybrid composite separator, PTFE/Al2O3 coated UHMWPE composite separator40, and PTFE/Al2O3 coated PE-PP-PE/Al2O3 composite separator40; (b)–(d) SEM micrographs of Li metal before cycling, after cycling 200× with pristine PP separator, and after cycling 200× with A2L1 hybrid composite separator, respectively.

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Scheme 2. Schematic Depiction of the Mechanical Suppression Preventing Lithium Dendrite Formation with Hybrid Composite Coated Separators Fabricated in this Work

4. CONCLUSIONS A thermally stable, fully lithium dendrite-free lithium metal battery was fabricated through facile hybrid composite coating on a conventional PP separator. A new UV-curable inorganic-organic hybrid polysilsesquioxane binder was observed highly rigid at the interface of hybrid composite coating layer and lithium metal anode. The surface mechanical properties exceeded the required threshold for mechanical suppression of lithium dendrite growth. The performance of the Li metal battery cell was found greatly exceptional, the cyclability at high C-rates significantly improved, therefore holding great promise for the development and scaling-up of lithium metal batteries.

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ASSOCIATED CONTENT Supporting Information Structural characterization of LPMA64, DSC results, and linear sweep voltammograms (Figures S1–S4). This document is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail [email protected]; Phone +82-2-958-6872; Fax +82-2-958-5309 (C.M.K.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §These authors contributed equally. ACKNOWLEDGMENT This work was supported by Fundamental R&D Program for Core Technology of Materials and the Industrial Strategic Technology Development Program funded by the Ministry of Trade, Industry and Energy, Republic of Korea. Partial funding was given by the Materials Architecturing Research Center of Korea Institute of Science and Technology and the Korea Research Fellowship Program funded by the Ministry of Science, ICT, and Future Planning through the National Research Foundation of Korea.

REFERENCES

(1) Barnhart, C.J.; Benson, S.M. On the Importance of Reducing the Energetic and Material Demands of Electrical Energy Storage. Energy Environ. Sci. 2013, 6, 1083–1092.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

(2) Larcherl, D.; Tarascon, J.-M. Towards Greener and More Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2015, 7, 19–29. (3) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359–367. (4) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652– 657. (5) Scrosati, B.; Garche, J. Lithium Batteries: Status, Prospects and Future. J. Power Sources 2010, 195, 2419–2430. (6) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587–603. (7) Zu, C. X.; Li, H. Thermodynamic Analysis on Energy Densities of Batteries. Energy Environ. Sci. 2011, 4, 2614–2624. (8) Yoshimatsu, I.; Hirai, T.; Yamaki, J. Lithium Electrode Morphology During Cycling in Lithium Cells. J. Electrochem. Soc. 1988, 135, 2422–2427. (9) Khurana, R.; Schaefer, J. L.; Archer, L. A.; Coates, G. W. Suppression of Lithium Dendrite

Growth

Using

Cross-linked

Polyethylene/Poly(ethylene

oxide)

Electrolytes: A New Approach for Practical Lithium-Metal Polymer Batteries. J. Am. Chem. Soc. 2014, 136, 7395–7402. (10) Venugopal, G.; Moore, J.; Howard, J.; Pendalwar, S. Characterization of Microporous Separators for Lithium-Ion Batteries. J. Power Sources 1999, 77, 34– 41. (11) Howlett, P. C.; MacFarlane, D. R.; Hollenkamp, A. F. High Lithium Metal Cycling Efficiency in a Room-Temperature Ionic Liquid. Electrochem. Solid-State Lett. 2004, 7, A97–A101. 22


Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-Liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621–629. (13) Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nanocomposite Polymer Electrolytes for Lithium Batteries. Nature 1998, 394, 456–458. (14) Jeong, H. S.; Kim, J. H.; Lee, S. Y. A Novel Poly (vinylidene fluoridehexafluoropropylene)/Poly(ethylene

terephthalate)

Composite

Nonwoven

Separator With Phase Inversion-Controlled Microporous Structure for a LithiumIon Battery. J. Mater. Chem. 2010, 20, 9180–9186. (15) Zhang, S. S.; Xu, K.; Jow, T. R. An Inorganic Composite Membrane as the Separator of Li-ion Batteries. J. Power Sources 2005, 140, 361–364. (16) Lee, Y.; Lee, H.; Lee, T.; Ryou, M. H.; Lee, Y. M. Synergistic Thermal Stabilization of Ceramic/co-polyimide Coated Polypropylene Separators for Lithium-Ion Batteries. J. Power Sources 2015, 294, 537–544. (17) Li, W.; Yao, H.; Yan, K.; Zheng, G.; Liang, Z.; Chiang, Y.-M.; Cui, Y. The Synergetic Effect of Lithium Polysulfide and Lithium Nitrate to Prevent Lithium Dendrite Growth. Nat. Commun. 2015, 6, 7436. (18) Kim, J.-S.; Kim, D. W.; Jung, H. T.; Choi, J. W. Controlled Lithium Dendrite Growth by a Synergistic Effect of Multilayered Graphene Coating and an Electrolyte Additive. Chem. Mater. 2015, 27, 2780–2787. (19) Agrawal, R. C.; Pandey, G. P. Solid Polymer Electrolytes: Materials Designing and All-Solid-State Battery Applications: An Overview. J. Phys. D Appl. Phys. 2008, 41, 223001.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

(20) Lightfoot, P.; Mehta M. A.; Bruce, P. G. Crystal Structure of the Polymer Electrolyte Poly(ethylene oxide) 3: LiCF3SO3. Science 1993, 262, 883–885. (21) Liu, W.; Liu, N.; Sun, J.; Hsu, P.-C.; Li, Y.; Lee, H.-W.; Cui, Y. Ionic Conductivity Enhancement of Polymer Electrolytes with Ceramic Nanowire Fillers. Nano Lett. 2015, 15, 2740–2745. (22) Lee, H.; Yanilmaz, M.; Toprakci, O.; Fu, K.; Zhang, X. A Review of Recent Developments in Membrane Separators for Rechargeable Lithium-Ion Batteries. Energy Environ. Sci. 2014, 7, 3857–3886. (23) Zhang, S. S. A Review on the Separators of Liquid Electrolyte Li-Ion Batteries. J. Power Sources, 2007, 164, 351–364. (24) Pan, Q.; Smith, D. M.; Qi, H.; Wang, S.; Li, C. Y. Hybrid Electrolytes with Controlled Network Structures for Lithium Metal Batteries. Adv. Mater. 2015, 27, 5995–6001. (25) Lee, A. S.; Jo, Y. Y.; Jeon, H.; Choi, S. S.; Baek, K. Y.; Hwang, S. S. Mechanical

Properties

of

Thiol-ene

UV-Curable

Thermoplastic

Polysilsesquioxanes. Polymer 2015, 68, 140–146. (26) Choi, S.-S.; Lee, A. S.; Lee, H. S.; Jeon, H. Y.; Baek, K.-Y.; Choi, D. H.; Hwang, S.

S.

Synthesis

and

Characterization

of

UV‐Curable

Ladder‐like

Polysilsesquioxane. J. Polym. Sci. Part A Polym. Chem. 2011, 49, 5012–5018 (27) Choi, S.-S.; Lee, A. S.; Hwang, S. S.; Baek, K. Y. Structural Control of Fully Condensed

Polysilsesquioxanes:

Ladderlike

vs

Cage

Structured

Polyphenylsilsesquioxanes. Macromolecules 2015, 48, 6063–6070. (28) Park, S.; Lee, A. S.; Do, Y. S.; Hwang, S. S.; Lee, Y. M.; Lee, J.-H.; Lee, J. S. Rational Molecular Design of PEOlated Ladder-Structured Polysilsesquioxane 24


Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Membranes for High Performance CO2 Removal. Chem. Comm. 2015, 51, 15308– 15311. (29) Cho, K. Y.; Lee, A. S.; Jeon, H.; Park, S.-H.; Jang, M.; Yoon, H. G.; Hong, S. M.; Baek, K.-Y.; Hwang, S. S. Tuning the Interface Between Poly(vinylidene fluoride)/UV-Curable Polysilsesquioxane Hybrid Composites: Compatibility, Thermal, Mechanical, Electrical, and Surface Properties. Polymer 2015, 77, 167– 176. (30) Lee, A. S.; Choi, S.-S.; Oh, S. Y.; Lee, H. S.; Kim, B.; Hwang, S. S.; Baek, K.-Y. Incompletely Condensed POSS-Based Spin-on-Glass Networks for Impeccable Ultra Low-k Integration. J. Mater. Chem. C 2015, 3, 11605–11611. (31) Jeon, H.; Lee, A. S. S.; Kim, H.-J.; Cho, S.-H.; Baek, K. Y.; Hwang, S. S. Preparation

of

Highly

Emissive,

Thermally

Stable,

UV‐Cured

Polysilsesquioxane/ZnO Nanoparticle Composites. J. Appl. Polym. Sci. 2015, 132, 42333. (32) Santhosha, A. L.; Bhattacharyya, A. J. A Few Case Studies on the Correlation of Particle Network and Its Stability on the Ionic Conductivity of Solid–Liquid Composite Electrolytes. J. Phys. Chem. B 2015, 119, 11317–11325. (33) Choi, J. A.; Kim, S. H.; Kim, D. W. Enhancement of Thermal Stability and Cycling Performance in Lithium-Ion Cells Through the Use of Ceramic-Coated Separators. J. Power Sources 2010, 195, 6192–6196. (34) Lee, A. S.; Choi, S. S.; Song, S.-J.; Baek, K.-Y.; Hwang, S. S. Cationically Photopolymerizable Epoxy-Functionalized Thermoplastic Polysilsesquioxanes: Synthesis and Properties. RSC Adv. 2014, 4, 56532–56538.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

(35) Jeong, H. S.; Kim, D. W.; Jeong, Y. U.; Lee, S. Y. Effect of Phase Inversion on Microporous Structure

Development

of Al2

O3/Poly(vinylidene

fluoride-

hexafluoropropylene)-based Ceramic Composite Separators for Lithium-ion Batteries. J. Power Sources 2010, 195, 6116–6121. (36) Lee, J. H.; Lee, A. S.; Lee, J.-C.; Hong, S. M.; Hwang, S. S.; Koo, C. M. Hybrid Ionogel Electrolytes for High Temperature Lithium Batteries. J. Mater. Chem. A. 2015, 3, 2226–2233. (37) Harry, K.J; Hallinan, D.T.; Parkinson, D.Y.; MacDowell, A.A.; Balsara, N.P. Detection of Subsurface Structures Underneath Dendrites Formed on Cycled Lithium Metal Electrodes. Nat. Mater. 2014, 13, 69-73. (38) Ramdon, S.; Bhushan, B. Nanomechanical Characterization and Mechanical Integrity of Unaged and Aged Li-ion Battery Cathodes. J. Power Sources 2014, 246, 219–224. (39) Berla, L. A.; Lee, S. W.; Cui, Y.; Nix, W. D. Mechanical Behavior of Electrochemically Lithiated Silicon. J. Power Sources 2015, 273, 41–51. (40) Halalay, I. C.; Lukitsch, M. J.; Balogh, M. P.; Wong, C. A. Nanoindentation Testing of Separators for Lithium-ion Batteries. J. Power Sources 2013, 238, 469– 477. (41) Monroe, C.; Newman, J. The Impact of Elastic Deformation on Deposition Kinetics at Lithium/Polymer Interfaces. J. Electrochem. Soc. 2005, 152, A396– A404. (42) Lee, H.; Lee, D. J.; Kim, Y.-J., Park, J.-K.; Kim, H.-T. A Simple Composite Protective Layer Coating That Enhances the Cycling Stability of Lithium Metal Batteries. J. Power Sources 2015, 284, 103–108. 26


Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Pacheco, F. A.; Pinnau, I.; Reinhard, M.; Leckie, J. O. Characterization of Isolated Polyamide Thin Films of RO and NF Membranes Using Novel TEM Techniques. J. Membr. Sci. 2010, 358, 51–59.

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