Hyperbranched Triphenylamine Polymer for UltraFast Battery Cathode

Jan 30, 2018 - KEYWORDS: triphenylamine polymer, hyperbranched polymer, ultrafast battery, long cycle life, organic−LFP hybrid cathode, organic−in...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6346−6353

Hyperbranched Triphenylamine Polymer for UltraFast Battery Cathode Keiichi Yamamoto,*,† Daichi Suemasa,† Kana Masuda,† Kazunari Aita,† and Takeshi Endo‡ †

Advanced Materials Research Laboratories, JSR Corporation, 100, Kawajiri-cho, Yokkaichi, Mie 510-8552, Japan Molecular Engineering Institute, Kindai University, 11-6, Kayanomori, Iizuka, Fukuoka 820-8555, Japan

‡

S Supporting Information *

ABSTRACT: A novel hyperbranched poly(triphenylamine) (PHTPA) was synthesized, and the electrochemical properties of this material were studied. PHTPA was synthesized by a facile method in a one-step reaction from affordable monomers. Despite all aromatic structures, PHTPA showed good solubility in several organic solvents. The battery performance test of PHTPA showed a high discharge voltage, an ultrafast charge−discharge performance of 100−300 C, and a long cycle life of more than 5000 cycles. Moreover, the addition of the PHTPA to LiFePO4 (LFP) improved the charge-transfer resistance and Warburg coefficient, which is related to the diffusion of lithium ions in LFP, and consequently improved the charge−discharge performance of LFP itself at a high C rate (20−100 C). This behavior is understood to be the result of the organic−inorganic charge transfer. The superior cycle performance of the PHTPA−LFP hybrid cathode was also found. PHTPA will serve as an additive for a high-performance LIB. KEYWORDS: triphenylamine polymer, hyperbranched polymer, ultrafast battery, long cycle life, organic−LFP hybrid cathode, organic−inorganic charge transfer

1. INTRODUCTION Organic cathode materials for a lithium ion battery (LIB) have recently captured attention1−4 because of their high rate performances5−10 and large capacities.11−14 However, there remains incompatibility issue between the high rate and the large capacity. To settle the issue, organic−inorganic hybrid cathodes have been developed.15−23 Vlad et al. demonstrated that the addition of the poly(2,2,6,6-tetramethylpiperidinyloxy4-yl methacrylate) (PTMA) ultrafast organic radical active material16 to LiFePO4 (LFP) enhanced the power capability and investigated three configurations of the hybrid electrodes (h-3D, h-22P, and h-22S).16 On the basis of the results from the literature, it is reasonable to speculate that the contact area between PTMA and LFP will be an important factor for the efficacy of the PTMA additive. A polymer coating on LFP will be more effective, and therefore conducting polymer-coated LFP electrodes have been investigated.17−23 However, their preparation, which is in situ coating polymerization on the inorganic active material particle, left some issues such as difficulty of the process, the limitation of the organic−inorganic ratio, and the degradation of the inorganic materials in water.24 Therefore, a blending method which is easy to use in nonaqueous media with an adjustable organic−inorganic active material ratio was desired. To prepare polymer-coated LFP by the blending method, an organic active material with a high rate performance and good solubility in an organic solvent is required. PTMA was removed from the candidate list because it requires the oxidation process by m-CPBA after polymerization.5 P(NDI2OD-T2), which is © 2018 American Chemical Society

the latest ultrafast organic active material, has been researched by Yao et al.,8 but the material has a low discharge voltage because the active unit is an n-type imide. As one of the highrate organic active materials having high voltage, polytriphenylamine (PTPA) has been researched.25−31 However, the framework of the polymer should be changed to linear or branched to prepare the soluble polymer because the typical PTPA is a networked polymer, which is essentially insoluble. Hartwig et al.39 and Turner et al.40,41 have developed some soluble linear PTPA having aliphatic substituents, independent of each other. However, these polymers left some issues, for example, the monomers are expensive, and these polymers have aliphatic substituents and a biphenylene backbone, both of which reduce the concentration of the redox active nitrogen atom in the polymer structure. Because the concentration of the redox active unit relates to the capacity of the battery, the structure of inactive units should be minimized.28,29 Dendrimers have been known as a macromolecule having excellent solubility.32−34 However, the preparation of the dendrimer requires a multistep reaction; for example, a four-step reaction is required for the third-generation dendrimer;42 therefore, it has been avoided in the industrial production. The hyperbranched polymer has been known as a polymer having properties similar to that of the dendrimer.32 Tanaka et al. have produced a soluble substituent-free hyperbranched triphenylReceived: November 24, 2017 Accepted: January 30, 2018 Published: January 30, 2018 6346

DOI: 10.1021/acsami.7b17943 ACS Appl. Mater. Interfaces 2018, 10, 6346−6353

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis of poly-hyperbranched triphenylamine (PHTPA)

Figure 1. Charge−discharge curves and rate performance of PHTPA (a), typical PTPA (b), and SWCNT-aided PHTPA (c). and toluene of 10 mL were added to a 100 mL flask. The mixture was stirred at rt for 30 min, followed by the addition of bis(tri-tbutylphosphine)palladium(0) of 5 mg (0.01 mmol) as the catalyst. The resulting mixture was allowed to stir at 100 °C for 6 h. After the reaction, the reaction mixture was washed twice with distilled water of 30 mL to remove the byproduct NaBr. The organic phase was separated and was poured into methanol of 50 mL to remove the residual catalyst and monomers. The obtained pale-green powder was dried at 60 °C in vacuum for 24 h. PHTPA dry powder was obtained (3.3 g, 99% yield). Typical PTPA was synthesized by the same method described in the literature.30 2.2. Characterization of PHTPA. The 1H NMR spectrum was recorded on a JEOL 400 MHz spectrometer at room temperature using CDCl3 as the solvent and TMS as the standard. The FT-IR spectrum was recorded on Thermo Scientific Nicolet iS10 FT-IR with diamond ATR. The polymer weight versus polystyrene standard was

amine polymer by C−C coupling in a one-step reaction.43 However, this polymer has a low molecular weight (4 × 103), which would be dissolved into an electrolyte solution in a battery if it was used as a battery material, and also has a biphenylene backbone, which is undesirable as mentioned above. Herein, we report the first demonstration of the hyperbranched monophenylene backbone triphenylamine polymer [hyperbranched poly (triphenylamine) (PHTPA)], which is soluble in several organic solvents and exhibits a high discharge voltage and ultrafast charge−discharge performance.

2. EXPERIMENTAL SECTION 2.1. Synthesis of PHTPA and Typical PTPA. The polymerization was carried out in an inert atmosphere using standard Schlenk techniques. Bis(4-bromophenyl)amine of 3.27 g (10 mmol), diphenylamine of 1.69 g (10 mmol), sodium tert-butoxide of 2.88 g (30 mmol), 6347

DOI: 10.1021/acsami.7b17943 ACS Appl. Mater. Interfaces 2018, 10, 6346−6353

Research Article

ACS Applied Materials & Interfaces Table 1. Rate Performance, DC-IR, and Warburg Coefficient for PHTPA and Typical PTPA electrode composition PHTPA/AB/PVDF = 40:50:10 typical PTPA/AB/PVDF = 40:50:10 PHTPA/AB/SWCNT/PVDF = 80:9:1:10

rate capability (100 C, 200 C) 76% 75% 62%

DC-IR (Ω) (charge, discharge)

Warburg coefficient (Ω/s−1/2) (SOC = 50%)

49, 39 46, 25 31, 26

8.4 22.7 13.9

51% 0% 46%

determined on Tosoh HCL-8320GPC with TSKgel super HM-H as the column using tetrahydrofuran (THF) as the eluent. The mass of the repeat unit was determined on matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS, Shimadzu, AXIMA Confidence) using 1,8,9-anthracenetriol and sodium trifluoroacetate as the matrix and THF as the solvent. 2.3. Cell Fabrication and Electrochemical Measurements. CR2032 coin cells composed of the components described in Figure S5 of the Supporting Information were used for the measurement of the battery characteristics. The cathode was prepared by mixing PHTPA as the active material, acetylene black (AB) as the conductive additive, and polyvinylidene fluoride (PVDF) as the binder in a weight ratio of 40:50:10 unless otherwise noted. The area of the electrode was 1.33 cm2, the loading mass of the active materials was around 0.7 mg for the organic electrode, around 1.5 mg for the LFP or PHTPA−LFP hybrid electrode, and the thickness of the electrode was around 25 μm. The current densities used at each C rate were adopted based on 1 C = 60 mA h/g for PHTPA, 1 C = 150 mA h/g for LFP and hybrid. The charge−discharge performances were measured by TOSCAT 3100 (TOYO-SYSTEM Co., Ltd.) and VMP3 (Bio-Logic Co., Ltd.) at 25 °C. For EIS measurements and Warburg coefficient, VMP3 (Bio-Logic Co., Ltd.) was used at 25 °C. The test cells were newly prepared and were measured after charging to state of charge (SOC) of 50%. The XRD pattern of cathodes was recorded on Rigaku SmartLab with Cu Kα radiation. SEM images were recorded on JSM-6010LA (Hitachi).

of the typical PTPA. Unfortunately, PHTPA exhibited a disappointing discharge capacity of 64 mA h/g at 0.5 C rate, whereas the theoretical capacity of PHTPA is 160 mA h/g. However, high capacity retention was observed at high C rates. The discharge capacity retention versus 0.5 C at each C rate was 86% (50 C), 76% (100 C), 51% (200 C), and 26% (300 C). This impressive rate capability is comparable with that of PTMA and P(NDI2OD-T2), both of which are top-class ultrafast charging materials. Figure 1b shows the comparison of the performance of the typical PTPA. Considering that the typical PTPA exhibited moderate rate capability, the hyperbranched framework of PHTPA would enhance the rate capability of the polymer. Direct current internal resistance (DC-IR) values calculated from the slope of a plot of the current I versus V delta (Figure S8) and the Warburg coefficient (Figure S10) are listed in Table 1. PHTPA would be difficult to consider as high-conducting because DC-IR and charge-transfer resistance (Rct) in EIS (Figure S9) of PHTPA is comparable with that of typical PTPA. However, the Warburg coefficient for PHTPA is lower than that of typical PTPA significantly. It is reasonable that this Warburg coefficient contributes to the high rate performance. We also found that an addition of single-walled carbon nanotubes (SWCNTs) into a cathode composition is effective to reduce the amount of the conductive additive (Figure 1c). The cathode consisting of PHTPA/AB/SWCNT/PVDF = 80:9:1:10 shows similar performance as shown in Figure 1a. This fact means that the high capacity retention in a high-rate region in Figure 1a is not attributed to the intrinsic capacity of AB. The long life cycle performance of PHTPA at the cutoff voltage 4.0−2.8 V was shown in Figure 2. No degradation was observed over 5000 cycles even though it was tested at a high C rate (20 and 100 C). As seen in previous works for nanocrystals of inorganic active materials,44−51 the particle size of the

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of PHTPA. As shown in Scheme 1, PHTPA was synthesized by the Buchwald−Hartwig reaction (C−N coupling)35−38 in a onestep reaction in quantitative yield. The monomers used are affordable and industrially available, which makes it possible for the polymer to be produced industrially. During polymerization, it was observed that the polymer dissolved in toluene which is used as the reaction solvent. After the isolation of the polymer, it was revealed that the polymer powder can be dissolved easily in several organic solvents such as N-methyl-2-pyrrolidone (NMP), THF, chloroform, and toluene even though the polymer has all aromatic structures. As shown in Figures S1−S4 of the Supporting Information, PHTPA obtained was characterized by 1H NMR, FT-IR, GPC, and MALDI-TOFMS. The results of the characterizations were in accord with the structure of PHTPA shown in Scheme 1. The molecular weight of PHTPA was Mw = 1.2 × 104. The feeding ratio of the monomers used in the polymerization (bis(4-bromophenyl)amine/diphenylamine = 1.0) was successful to obtain the polymer with high solubility and high molecular weight. When high or low feeding ratio was used, an insoluble polymer or a low molecular-weight polymer was obtained, respectively. Fortunately, the polymer was insoluble in an electrolyte solution (1 M LiPF6, EC/DEC = 30:70). This result indicates that the polymer can be applied to LIBs. 3.2. Cell Performance of PHTPA. Figure 1a shows the charge−discharge curves and the rate capability of PHTPA. The charge cutoff voltage of 4.2 V was suitable for PHTPA similar to the typical PTPA.25 The mean discharge voltage of PHPTA was 3.4 (V vs Li/Li+), which is slightly lower than that

Figure 2. Long life cycle performance of PHTPA for 20 C (a) and 100 C (b). 6348

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Figure 3. SEM image of the film made from PHTPA and PVDF (PHTPA/PVDF = 40:10) (a) and SEM−EDS analyses of carbon and fluoride atoms (b and c).

Figure 4. SEM images of LFP (a) and PHTPA-coated LFP (PHTPA/LFP/PVDF = 8:32:10) (b).

typical PTPA was applied to the hybrid cathode, LFP would not be coated because of the insolubility of the polymer. The cell performance of the hybrid cathode was measured. Figure 5 and Table 2 show the cell performance of the cathode of which the weight ratio is PHTPA/LFP/1AB/PVDF = 8:32:50:10. It is clear from Figure 5a that the addition of PHTPA improves the cathode performance. At a 20 C rate, the PHTPA−LFP hybrid cathode exhibited 50% capacity retention,

PHTPA would contribute to the high rate performance and the long life cycle. The 5000 cycled cell was opened and the appearance of the cathode was observed. No elution of PHTPA into the electrolyte was observed. The blend film of PHTPA and PVDF (the blend ratio is 4:1) was prepared by casting from NMP, and the SEM image was observed to clarify the reason why PHTPA exhibits the superior performance. The film without AB was used in this observation because AB reduced the contrast of the SEM image. Figure 3 shows the SEM image of the film. An unexpected phaseseparated structure was observed. It was shown by SEM−EDS analysis that the micron-sized microspheres are PHTPA, fibers and films on the microspheres are PVDF, and dark regions are vacancies. In the case of only PHTPA, no phase-separated structure was observed (Figure S7b). These results indicate that the formation of the microsphere of PHTPA would be attributed to the difference in polarity with PVDF, and the low viscosity of the hyperbranched polymer solution7 should reduce the microsphere size. From this morphology, it is likely that the microsphere and the microporous cathode structure including the microspheres of PHTPA would enhance the rate performance and the long life cycle of the cathode.10 Previous ultrafast charging materials such as PTMA and P(NDI2ODT2) might also show the characteristic morphology. 3.3. Cell Performance of Hybrid of LFP and PHTPA. The coating test of a hybrid cathode consisted of PHTPA, inorganic active material LFP, and PVDF (weight ratio is 8:32:10) was carried out by the NMP-aided blending method. As shown in Figure 4b, the microspheres of PHTPA were observed in the coating film, whereas needle-like pristine LFP as seen in Figure 4a was not observed. This result suggests that LFP particles are fully coated with PHTPA in the hybrid cathode, and the contact area between PHTPA and LFP surface is sufficiently large that they can interact with each other. If the

Figure 5. Rate capability (a) and charge−discharge curves (b) for the PHTPA−LFP hybrid (weight ratio = 20:80). 6349

DOI: 10.1021/acsami.7b17943 ACS Appl. Mater. Interfaces 2018, 10, 6346−6353

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ACS Applied Materials & Interfaces Table 2. Rate Performance, DC-IR, Rct, and Warburg Coefficient for LFP and PHTPA−LFP Hybrid active material LFP PHTPA−LFP (20:80)

rate capability (10 C, 20 C) 30% 60%

0% 50%

DC-IR (Ω) (charge,discharge)

Rct (Ω)(SOC = 50%)

Warburg coefficient (Ω/s−1/2) (SOC = 50%)

81, 59 59, 49

45 15

32.7 11.8

Figure 6. (a) The charge-transfer step for high rate charging of the hybrid cathode of PHTPA and LFP. (b−d) Charge−discharge curves for hybrid of PHTPA and LFP (weight ratio = 50:50), for PHTPA and for LFP, respectively. All curves were tested at 1 C rate in this test. The blend ratio of 50:50 was utilized to facilitate the understanding of changes of the curve in the hybrid cathode.

PPy−LFP hybrid gel framework was about four times higher than that of control LFP.23 Wang et al. reported that the addition of the PPy solid to LFP increases the diffusion of lithium ions a little.21 We think that the difference in efficacy between the hybrid gel framework of PPy and solid PPy is attributable to differences in the contact area between the polymer and LFP particles. Therefore, it is understood that PHTPA has characteristics similar to PPy, and a significant improvement of the Warburg coefficient for the PHTPA−LFP hybrid in this work would be attributable to the contact area between the polymer and LFP, which is the result of the solubility of PHTPA. In general, the diffusion of lithium ions is related to the shape or the particle size of the active materials.52−55 However, consistent materials were used in this work, and there should be no change in the particle size or the shape of LFP during the addition of PHTPA into the LFP slurry. It is reasonable to suppose that some kind of effect of PHTPA beside LFP would affect the LFP surface, and then the Li ion inside LFP diffuses to compensate the Li ion on the LFP surface. Figure 6a shows the schematic process of the charge-transfer step for high-rate charging of the hybrid cathodes of PHTPA and LFP. The charge−discharge curve of the hybrid cathode was compared with that of PHTPA and LFP (Figure 6b−d) to investigate the mechanism of the high rate performance of the hybrid cathode. Only the curve derived from PHTPA was observed at the low SOC in the hybrid cathode. At the middle SOC, the plateau derived from LFP appeared, and then the

whereas the cathode that consisted of only LFP did not work well. Figure 5b shows the charge−discharge curves of the hybrid cathode cell. The plateau derived from LFP was observed even in the high C-rate region. This result indicates that not only PHTPA but also LFP are allowed to charge at a high C rate in the hybrid cathode. A similar observation was also found in the hybrid cathode with PTMA and LFP, but PTMA 50 mol% content was needed, and the plateau was largely inclined.16 X-ray diffraction (XRD) of the PHTPA−LFP hybrid cathode was measured to estimate the SOC for LFP in the cathode after 100 C charging. Figure S6 of the Supporting Information shows the XRD pattern of the cathode for uncharged, 100 C charged, and 1 C charged. The XRD pattern clearly showed the presence of FePO4 (FP), which is the charged state of LFP, appearing at 2θ = 18.0, 30.1, 30.9, even though the charge was carried out at the 100 C rate. DC-IR of the PHTPA−LFP hybrid is slightly lower than that of only LFP (Table 2). Surprisingly, the Rct in EIS for the PHTPA−LFP hybrid is significantly lower than that of bare LFP (Figure S9), and the Warburg coefficient for the PHTPA− LFP hybrid is one-third of that of bare LFP (Table 2). This fact suggests that PHTPA enhances the diffusion of lithium ions in LFP, which is the rate-determining step. Shi et al. reported an effect similar to that of the Warburg coefficient of the PPy− Fe3O4 hybrid gel framework, which has polypyrrole (PPy)coated Fe3O4, and was one-tenth of that of LFP.22 Moreover, they reported that the diffusion coefficient of lithium ions of the 6350

DOI: 10.1021/acsami.7b17943 ACS Appl. Mater. Interfaces 2018, 10, 6346−6353

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ACS Applied Materials & Interfaces PHTPA curve appeared again at high SOC. Because the discharge voltage of PHTPA is higher than 3.4 V, which is sufficient to charge LFP, it is reasonable that the organic− inorganic charge transfer occurred at the middle SOC as shown in Figure 6a. As mentioned above, the Rct of the PHTPA−LFP hybrid (Rct(hy)) is very low. The Rct(hy) can be presented as eq 1 shown below, because there should be no charging mode except for the three modes shown in Figure S12a−c. R ct(hy) = xR ct(PHTPA) + (1 − x)(FR ct(LEP) + (1 − F ) R ct(CTS))

(1)

where, Rct(PHTPA) and Rct(LFP) are Rct for PHTPA and LFP shown in Figure S9, respectively. Rct(CTS) is Rct attributed to the charge-transfer step. The Rct(CTS) should be zero because the charge-transfer step needs no external electron or ion (Figure S12c). The x (0.2) is the loading ratio of PHTPA in the hybrid electrode. F is a contribution factor for bare LFP, which means the ratio of LFP direct charge (shown in Figure S12a) and the charge-transfer step (Figure S12c). The F calculated from eq 1 is 0.13, which means that 87% of LFP was charged through the charge-transfer step (Figure S12c). This result is reasonable considering the morphology of the PHTPA−LFP hybrid shown in Figure 4b. We believe that this charge-transfer step reduces Rct, and consequently the attainment of the high rate performance of the hybrid cathode. Vlad et al. also proposed the similar theory in the research for the PTMA−LFP hybrid cathode.16 Overall, we propose the mechanism for high rate charging of the hybrid cathode of PHTPA and LFP as presented in Figures 6 and S12. At first, PHTPA was charged, and PHTPA+PF6− salt was formed at low SOC. Then, the PHTPA+PF6− salt charges neighboring LFP (charge-transfer step, Figure S12c); in other words, PHTPA+PF6− salt behaves like a catalyst, which can charge LFP easily, and finally, the Li ion in LFP diffuses to compensate the Li ion on the LFP surface. However, the information on the mechanism is not enough. Further studies will be needed to yield any findings about the mechanism. The correlation between PHTPA concentrations and their cell performance was investigated. As shown in Figure 7, a high concentration of PHTPA increased the rate performance, whereas it decreased the capacity of the cathode. PHTPA 20 wt % concentration was a good balance, which has the highest rate performance ever reported in the LFP−organic hybrid cathodes16−20,23 and has little decline in the capacity. We also found that the cycle performance of the PHTPA−LFP hybrid cathode is superior to that of LFP (Figure S11). It is likely that the coating of PHTPA on LFP can prevent the degradation of LFP during the charge−discharge cycle. PHTPA will serve effectively as an additive for LIBs in this concentration.

Figure 7. Hybrid ratio dependence of rate performance with previous work for the organic−LFP hybrid cathode (a). In the case of the absence of the data of 0.1 C in previous work, the closer C-rate values were used. Hybrid ratio dependence of specific capacity (b) for the PHTPA−LFP hybrid cathode.

performance and the long life cycle. Moreover, the addition of the PHTPA to LFP improved the Rct and Warburg coefficient, which is related to the diffusion of lithium ions in LFP and consequently improved the charge−discharge performance of LFP itself at a high C rate (20−100 C). This behavior is understood to be the result of the organic− inorganic charge transfer. The superior cycle performance of the PHTPA−LFP hybrid cathode was also found. PHTPA will serve as an additive for a high-performance LIB.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17943. Synthesis and characterization of PHTPA; cell fabrication and charge−discharge test method; XRD pattern and SEM images of cathodes; results of DC-IR measurements, EIS, and Warburg coefficients; cycle performance of PHTPA−LFP hybrid cathode, and schematic reaction equation of charge−discharge reaction on cathode (PDF)

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4. CONCLUSIONS In summary, the novel PHTPA was synthesized by the facile method in a one-step reaction from affordable monomers. Despite all aromatic structures, PHTPA showed good solubility in several organic solvents. The molecular weight of PHTPA was sufficiently high to use as a battery component. PHTPA showed a superior electrochemical performance, including the high discharge voltage, ultrafast charge−discharge rate of 100− 300 C, and a long cycle life of more than 5000 cycles. It is likely that the microporous cathode structure consisting of the microspheres of PHTPA contributes to the high rate

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Keiichi Yamamoto: 0000-0001-9541-1918 Daichi Suemasa: 0000-0002-2771-5167 Takeshi Endo: 0000-0001-6903-8048 Notes

The authors declare no competing financial interest. 6351

DOI: 10.1021/acsami.7b17943 ACS Appl. Mater. Interfaces 2018, 10, 6346−6353

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ACKNOWLEDGMENTS We thank Dr. Masaru Yao from the National Institute of Advanced Industrial Science and Technology (AIST) for the technical support in the cell-performance test.

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DOI: 10.1021/acsami.7b17943 ACS Appl. Mater. Interfaces 2018, 10, 6346−6353