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Nanostructured Polyaniline-Cellulose Papers for Solid-State Flexible Aqueous Zn-Ion Battery Yue Ma, Xiuli Xie, Ruihua Lv, Bing Na, Jinbo Ouyang, and Hesheng Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Nanostructured Polyaniline-Cellulose Papers for Solid-State Flexible Aqueous Zn-Ion Battery

Yue Ma, Xiuli Xie, Ruihua Lv, Bing Na*, Jinbo Ouyang, Hesheng Liu Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices, School of Chemistry, Biology and Materials Science, East China University of Technology, No. 418 Guanglan Road, Nanchang, 330013, People’s Republic of China

Abstract:

Solid-state

flexible

aqueous

Zn-ion

battery

was

fabricated

by

nanostructured

polyaniline-cellulose papers as the cathode and Zn-grown graphite papers as the anode. The separator was a flexible gel electrolyte with a high ionic conductivity based on cellulose nanofibers. The Zn-ion battery exhibited an energy density of 117.5 and 67.8 mWh/g at a power density of 0.16 and 3.34 W/g, respectively (estimated from total active mass of both cathode and anode). The energy density of the Zn-ion battery was much higher than that of asymmetric supercapacitors with aqueous electrolytes, while maintaining a comparable power density. Meanwhile, good cyclic stability was achieved with a high capacity retention of 84.7 % after 1000 charge/discharge cycles at a current density of 4 A/g. More importantly, specific capacity changed little under mechanical bending, and there was only 9 % loss after 1000 bending cycles. The solid-state flexible Zn-ion battery has great potentials as energy storage devices for flexible displays and wearable electronics. Keywords: cellulose; aqueous Zn-ion battery; flexible

*

Correspondence author. E-mail address: [email protected], [email protected] ACS Paragon Plus Environment

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Introduction Papers, produced from natural plants, consist of intertwined cellulose microfibers. It imparts papers with high flexibility and good mechanical strength. Together with low-cost and abundance, papers are promisingly applied in the flexible devices for energy storage, such as supercapacitors and batteries.1-5 Paper-based devices can be well operated under bending or folding, thus extending their applications in the wearable electronics where mechanical deformation is involved. Typically, papers serve as flexible substrates to afford electrochemically active materials. In order to improve electron transportation a conductive layer is usually coated on papers before loading of active materials with poor electrical conductivity.1, 2, 6 In addition, it was argued that cellulose microfibers in the papers can act as internal electrolyte reservoirs to facilitate ion diffusion in the electrolytes towards active materials.6 Rechargeable batteries based on aqueous electrolytes have been attracting much attention owing to high safety and low fabrication costs, as compared to popular lithium-ion batteries with organic electrolytes.7-18 Aqueous batteries are categorized into Li-ion, Na-ion, Mg-ion, Zn-ion, and so on, according to the ion type (carriers) in the aqueous electrolytes. Among them, Zn-ion battery can directly use metal Zn as anodes due to its high electrochemical stability in the aqueous electrolytes.19-26 Therefore, fabrication of high-performance cathodes is emphasized in the Zn-ion battery, with further considering high theoretical capacity of metal Zn (820 mAh/g). Inorganic active materials and conductive polymers have been reported as cathodes of Zn-ion battery. For instance, Zn-ion battery with MnO2 as the cathode had a specific capacity of 151 mAh/g at 6.5 C (~2 A/g) and maintained as 135 mAh/g over 2000 cycles in the potential range of 0.8-1.9 V.27 Polyaniline (PANI) as the cathode of Zn-ion battery delivered a specific capacity of 141.2 mAh/g and ACS Paragon Plus Environment

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an energy density of 169.4 mWh/g at a current density of 0.5 mA/cm2, respectively.28 To date, assembly and characterization of flexible Zn-ion battery with gel electrolytes were less concerned. In a recent research, solid-state flexible NiCo//Zn textile battery based on conductive metal yarns was designed and exhibited good electrochemical performance under bending and twisting.29 Herein, we explored nanostructured PANI-cellulose papers and Zn-grown graphite papers as the flexible cathode and anode, respectively. In combination with a gel electrolyte based on cellulose nanofibers, solid-state flexible Zn-ion battery was assembled in a sandwiched configuration. The Zn-ion battery exhibited a specific capacity of 142.3 mAh/g at a current density of 0.2 A/g and a capacity retention of 84.7 % after 1000 charge/discharge cycles. Impressively, specific capacity of our Zn-ion battery was little affected by bending, and maintained as 91 % of its initial value after repeated bending over 1000 cycles. Results and Discussion Flexible cathodes were fabricated by growth of PANI on lens papers via in-situ polymerization (Figures 1a-b). The lens papers grown with PANI, appearing dark green, can be highly bended. It corresponds to high flexibility of PANI-grown lens papers, similar to the original counterparts. Lens papers consist of abundant microfibers stacked together, and among them large voids prevail (Figures 1c and 1e). The PANI is directly grown on the cellulose microfibers of lens papers; and there is no PANI located in the voids among microfibers (Figures 1d and 1f). Good adhesion between PANI and lens papers is expected, beneficial for mechanical deformation without the detachment. Furthermore, grown PANI is mostly in the form of nanorods with a diameter of about 100 nm (the inset in Figure 1f). It suggests that high nucleation density of PANI was induced by hydrophilic cellulose microfibers during in-situ polymerization. The nanostructured PANI, together with its very thin layer on the ACS Paragon Plus Environment

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cellulose microfibers, is helpful for electrolyte infiltration and ion diffusion through large voids of lens papers.

a

b Polymerization

c

d

300 µm

300 µm

e

f 1 µm

20 µm

20 µm

Figure 1 Growth of nanostructured PANI on lens papers. (a, b) optical photographs and (c-f) SEM images of lens papers (a, c, e) before and (b, d, f) after in-situ polymerization, respectively. In (f) an enlarged view is included as an inset. On the other hand, doping of nanostructured PANI by hydrochloric acid was simultaneously achieved during in-situ polymerization, as confirmed by ATR-FTIR spectra (Supporting Information, Figure S1). The IR bands at 1572 and 1492 cm-1 are contributed by C=C stretching of the quinoid ring

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and benzenoid ring in the doped PANI chains, respectively.30-32 Doping of nanostructured PANI is very critical for its electrochemical activity, thus contributing to the performance improvement of the assembled Zn-ion battery. Finally, flexible cathodes were obtained by coating of PANI-grown lens papers with thin conductive layers based on graphite nanoplatelets (Supporting Information, Figure S2). 0.8V

a

+

-

b

before

after

1 M ZnSO4

Zn foil

graphite paper

c

d

5 µm

1 µm

Figure 2 Flexible Zn-deposited graphite papers. (a) schematic diagram of the electrodeposition setup, (b) optical photographs of graphite papers before and after Zn deposition, (c, d) SEM images of Zn nanoflakes deposited on graphite papers at (c) low and (d) high magnifications, respectively. Pure Zn foils are not suitable for flexible anodes owing to their low recovery ability after mechanical deformation such as bending. Thus, we explored graphite papers as flexible substrates. By electrochemical deposition thin layers of metal Zn was grown on the graphite papers, and the color is changed from dark to argenteous (Figures 2a-b). The grown Zn is in the form of isolated nanoflakes and tightly bonded with graphite papers (Figures 2c-d). The isolated Zn nanoflakes make easy electrolyte access, beneficial for ion diffusion towards the anode. The Zn-grown graphite papers are ACS Paragon Plus Environment

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2

1.8

a Potential (V)

Current density (A/g)

highly flexible, and can undergo repeated bending due to isolated Zn nanoflakes.

1 0 -1

b

1.5 0.2 A/g 0.5 A/g 1 A/g 2 A/g 4 A/g

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1000

Cycle number

Figure 3 Electrochemical performance of the Zn-ion battery based on nanostructured PANI. (a) CV curve at a scan rate of 2 mV/s, (b) GCD profiles and (c) specific capacity versus current density, (d) EIS profile (inset is the enlarged view in the high frequency region), (e) Ragon plot of energy density versus power density, (f) cyclic performance over 1000 charge/discharge cycles at a current density of 4 A/g.

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Flexible gel films based on cellulose nanofibers was used as separators to assemble solid-state flexible Zn-ion battery in a sandwiched configuration, in combination with flexible cathodes and anodes. Note that the gel film was prepared by swelling of dry membranes consisting of cellulose nanofibers in a liquid electrolyte, 2 M ZnCl2 and 3 M NH4Cl aqueous solution with PH=4.0 (Supporting Information, Figure S3). Figure 3a shows the CV curve of the solid-state flexible Zn-ion battery at a scan rate of 2 mV/s. There exist two pairs of broad redox peaks, located around 1.1/0.9 and 1.3/1.2 V, respectively. It arises from reversible dedoping/doping of nanostructured PANI upon charge/discharge process. The peak current density reaches about ±1.5 A/g, corresponding to high electrochemical activity of nanostructured PANI. Note that negligible current is contributed by both lens paper and conductive layers during CV measurements (Supporting Information, Figure S4). On the other hand, there is absence of charge/discharge plateaus in the GCD profiles. It suggests that the extent of doping in the nanostructured PANI is not uniform, possibly due to hierarchical structure of PANI chains. The electrochemical activity of nanostructured PANI is weakened with the increasing of current density due to kinetic limitation. It is manifested by the gradual lowering of specific capacity with respect to current densities (Figures 3b-c). The specific capacity is 142.3 mAh/g at a current density of 0.2 A/g, and is decreased to 81.1 mAh/g at a current density of 4 A/g. It is comparable or superior to that of other aqueous Zn-ion batteries with liquid electrolytes based on ZnMn2O4/carbon (~90 mAh/[email protected] A/g),20 LiMn2O4/CNT (116 mAh/[email protected]/g),22 LiFePO4 (~132 mAh/[email protected] A/g),25 PANI (141.2 mAh/[email protected] mA/cm2).28 The capacity retention reaches about 57 % while the current density increased from 0.2 to 4 A/g, as a result of low internal resistance and charge transfer resistance. The internal resistance and charge transfer resistance is about 2.8 and 4.7 Ω for the solid-state Zn-ion ACS Paragon Plus Environment

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battery, respectively, deduced from the high-frequency intercept at real axis of the EIS profile in Figure 3d. Low internal resistance and charge transfer resistance correspond to fast electron and ion transport across the interfaces in the battery. It should be contributed by nanostructured PANI with high surface area and superior ionic conductivity of the gel electrolyte (16.4 mS/cm). Note that the ionic conductivity was deduced from the internal resistance with knowledge of the gel thickness and area (Supporting Information, Figure S5). Correspondingly, high power density of 4.46 W/g is achieved at an energy density of 90.5 mWh/g (Figure 3e). The energy density is 156.7 mWh/g at a power density of 0.22 W/g. Note that energy density was deduced from the area of discharge curves, and power density was obtained via dividing energy density by the discharge time. With further considering the mass participated in the charge/discharge process of Zn anode (at most one-third of that of the cathode), energy density of our solid-state Zn-ion battery is estimated to be 117.5 and 67.8 mWh/g at a power density of 0.16 and 3.34 W/g, respectively (based on total active mass of both cathode and anode). The performance is much higher than that of asymmetric supercapacitors with liquid electrolytes based on [email protected]//AC (43.5 mWh/[email protected] W/g, 11.8 mWh/[email protected] W/g),33 [email protected](OH)2//graphene (43.1 mWh/[email protected] W/g, 19.3 mWh/[email protected] W/g),34 AC//[email protected]α-Ag3VO4 (43.65 mWh/[email protected] W/g, 12.6 mWh/[email protected] W/g)35, respectively. Long-term lifetime is very critical for practical application of the Zn-ion battery. As shown in Figure 3f, the solid-state Zn-ion battery exhibits a high capacity retention of 84.7 % after 1000 charge/discharge cycles at a current density of 4 A/g. In addition, coulombic efficiency reaches about 95 % during the whole cycle life. The decay in the capacity should be related to the slight deterioration of the cathode, and has little to do with the anode. As shown in Figure S6 (Supporting Information), Zn ACS Paragon Plus Environment

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nanoflakes are maintained after 1000 charge/discharge cycles. It corresponds to high reversibility of dissolution/plating of metal Zn during charge/discharge process.

Current density (A/g)

a 1 µm

10 µm

b

2 1 0 -1 -2 0.6

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-Z" (Ω )

Potential (V)

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1.2

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Figure 4 (a) SEM micrograph of PANI powders for coating on lens papers (the inset is the enlarged view), (b-d) electrochemical performance of the Zn-ion battery based on PANI powders: (b) CV curve at a scan rate of 2 mV/s, (c) GCD profile at a current density of 0.2 A/g, (d) EIS profile. The superior battery performance is mostly related to the nanostructured PANI grown on the lens paper. The nanostructured PANI facilitates electron and ion transport as well as its utilization during electrochemical process. To illustrate this, the cathodes were fabricated from PANI powders precipitated during in-situ polymerization. As shown in Figure 4a, PANI powders appear large aggregates with micrometer size. Note that PANI powders were simultaneously synthesized during growth of nanostructured PANI on the lens paper. Thus, it in turn suggests that lens paper can prevent

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aggregation due to anchoring effects, responsible for the formation of nanostructured PANI. The PANI powders were coated on the lens paper, and its mass loading was same to that of nanostructured PANI grown on the lens paper. Same procedures were adopted for the coating of conductive layers and assembly of aqueous Zn-ion battery, as stated in the experimental section. Low current density in the CV curve and inferior specific capacity in the GCD profile are exhibited by the Zn-ion battery based on PANI powders (Figures 4b-c), while compared with that of Zn-ion battery based on nanostructured PANI. For instance, specific capacity is 86.6 mAh/g at a current density of 0.2 A/g for the Zn-ion battery based on PANI powders, far below the value of 142.3 mAh/g for the Zn-ion battery based on nanostructured PANI. It should be caused by large PANI aggregates that hamper electron and ion transport in the battery. This argument is solidified by high internal resistance and charge transfer resistance, as shown by the EIS profile in Figure 4d. The internal resistance and charge transfer resistance is about 4.1 and 5.8 Ω, respectively. Therefore, morphology control of PANI is very critical to improve the performance of Zn-ion battery. The lens paper plays a vital role in tailoring morphology to induce nanostructured PANI. Meanwhile, anchoring of nanostructured PANI on the lens paper is beneficial for high flexibility of the Zn-ion battery. As a result, the solid-state Zn-ion battery can be well operated under bending. A mini mechanical testing machine was used to achieve repeated bending of the Zn-ion battery (Figure 5a). There is only a little change in the CV curves and GCD profiles with respect to bending angle varied from 0 to 180o (Figures 5b-c). Two pairs of redox peaks prevail at each bending angle, accompanied by a slight decrease in the current density. At the same time, specific capacity is maintained as 101.1 mAh/g at a current density of 2 A/g upon bending to 1800, slightly lower than the initial value (103.8 mAh/g) (Figure 5d). ACS Paragon Plus Environment

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Current density (A/g)

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60

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Figure 5 Electrochemical performance of the Zn-ion battery based on nanostructured PANI under bending. (a) an optical photograph of the bended battery, (b) CV curves versus bending angle at a scan rate of 2 mV/s, (c) GCD profiles and (d) specific capacity versus bending angle at a current density of 2 A/g, (e) specific capacity and GCD profiles at a current density of 2 A/g over 1000 bending cycles at an angle of 900, (f) a LED indicator powered by the battery under bending. Furthermore, our flexible Zn-ion battery can undergo repeated bending at an angle of 90o with high capacity retention. After 1000 bending cycles there is only 9% loss in the specific capacity ACS Paragon Plus Environment

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(Figure 5e). Moreover, there is little change in the CV curves with respect to bending cycles (Supporting Information, Figure S7). Note that electrochemical measurements were carried out after every 100 bending cycles. Flexible cathode and anode should be responsible for good electrochemical performance under bending. High flexibility of the Zn-ion battery also suggests good contact between electrodes and the gel electrolyte that facilitates ion transport across solid/liquid interfaces. Together with the solid-state gel electrolyte, our Zn-ion battery has great potentials as power supply for flexible displays and wearable electronics. To illustrate this, the solid-state flexible Zn-ion battery was charged up to 1.7 V and then connected with a red LED indicator. As shown by the optical photograph in Figure 5f, the LED indicator can be efficiently powered by the Zn-ion battery under bending. It is almost same to the lightening of the LED indicator by the Zn-ion battery without bending (Supporting Information, Figure S8). Conclusion Flexible cathodes were produced by in-situ growth of nanostructured PANI on cellulose papers. Together with flexible Zn-grown graphite papers and a gel electrolyte, solid-state flexible aqueous Zn-ion battery was assembled. The Zn-ion battery exhibited good electrochemical performance, comparable or even superior to other aqueous batteries and asymmetric supercapacitors. Impressively, mechanical bending had little influence on the performance of the Zn-ion battery; and a high capacity retention of 91% was exhibited after 1000 bending cycles. The solid-state flexible aqueous Zn-ion battery has great potentials as power supply for flexible electronics where mechanical bending is involved. This research further extends the application of cellulose papers in the aqueous Zn-ion battery as flexible substrates to mediate nanostructured PANI. Experimental Section ACS Paragon Plus Environment

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Materials. Lens papers with a thickness of about 30 µm were commercially available and used as flexible substrates without further treatments. Cellulose nanofibers, suspended in water, had a diameter and length of 4-10 nm and 1-3 µm, respectively. Graphite nanoplatelets had a thickness of 10-30 nm and were supplied as a paste in water. Aniline, hydrochloric acid, ammonium persulfate and other chemicals were analytical reagents and used as-received. Fabrication of flexible cathodes. Nanostructured PANI was grown on lens papers via in-situ polymerization. Two aqueous solutions were individually prepared and kept at 4 ℃: one contained aniline (3.72 g) and hydrochloric acid (2 ml) in 50 ml distilled water, the other consisted of ammonium persulfate (9.12 g) in 50 ml distilled water. Two aqueous solutions were mixed together under stirring, followed by immediate immersion of lens papers. After polymerization at 4 ℃ for 1 h, lens papers grown with PANI were washed by distilled water several times. The mass loading of nanostructured PANI was 0.7 mg/cm2. Thereafter, a mixed suspension of graphite nanoplatelets (95 wt%) and cellulose nanofibers (5 wt%) was coated on both sides of lens papers grown with PANI. After drying conductive layers with a thickness of about 2 µm were produced. The sandwiched papers were cut into strips (2 cm×1 cm) for assembly of Zn-ion battery. Fabrication of flexible anodes. Electrochemical deposition was adopted to prepare Zn-grown graphite papers at a constant voltage of 0.8 V. The Zn foils and graphite papers were connected with the working electrode and the counter/reference electrode of a CS310 electrochemical workstation, respectively. The electrolyte was a 1 M ZnSO4 aqueous solution, and the deposition period was 20 s. The mass loading of Zn on the graphite papers was 0.3 mg/cm2. After washing and drying, Zn-grown graphite papers were cut into strips (2 cm×1cm) for assembly of Zn-ion battery. Assembly of solid-state flexible Zn-ion battery. Two strips of the cathode and anode were stacked ACS Paragon Plus Environment

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together, separated by a dry membrane of cellulose nanofibers with a thickness of about 10 µm. After tight package by a plastic wrap and subsequent hot sealing, a liquid electrolyte, 2 M ZnCl2 and 3 M NH4Cl aqueous solution with PH=4.0, was injected into the package to obtain the gel counterpart by swelling of the membrane. The amount of the liquid electrolyte was controlled to completely swell the membrane of cellulose nanofibers. As a result, solid-state flexible Zn-ion battery in a two-electrode configuration was fabricated. Characterizations. Morphology was observed by a Nova NanoSEM 450 scanning electron microscope. Prior to SEM measurements a thin gold layer was sputtered on the surface of samples. FTIR spectra were collected by a Thermo Nicolet Fourier transform infrared spectrometer with an attenuated total reflection accessory (ATR) at room temperature. The resolution was 4 cm-1, and a total of 64 scans were added. Electrochemical measurements of Zn-ion battery in a two-electrode configuration were carried out by a CS310 electrochemical workstation and a LAND cell testing system at 20 ℃. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were performed over the potential range of 0.7-1.7 V. Electrochemical impedance spectroscopy (EIS) was carried out between 0.01 Hz and 100 kHz.

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (No. 21764001, 21364001) and the project of Jiangxi Province Advantageous Science and Technology Innovation Team (20153BCB24001).

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Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: ATR-FTIR spectra, optical photographs, CV curves, EIS profile and SEM images

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ACS Sustainable Chemistry & Engineering

For table of content use only

Solid-state flexible aqueous Zn-ion battery, based on nanostructured PANI-cellulose papers, shows excellent electrochemical performance even under bending.

Energy density (mWh/g)

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

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100

10

bended 20 µm

1 0.1

1

Power density (W/g)

ACS Paragon Plus Environment

10