One-pot Synthesis of Double-network Hydrogel Electrolyte with

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One-pot Synthesis of Double-network Hydrogel Electrolyte with Extraordinarily Excellent Mechanical Properties for Highly Compressible and Bendable Flexible Supercapacitor Tingrui Lin, Mengni Shi, Furong Huang, Jing Peng, Qingwen Bai, Jiuqiang Li, and Maolin Zhai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11377 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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One-pot Synthesis of Double-Network Hydrogel Electrolyte with Extraordinarily Excellent Mechanical Properties for Highly Compressible and Bendable Flexible Supercapacitor Tingrui Lin,† Mengni Shi,† Furong Huang,† Jing Peng,† Qingwen Bai,‡ Jiuqiang Li,† and Maolin Zhai*,† †

Beijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation Chemistry

Key Laboratory of Fundamental Science, The Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. ‡

Beijing National Laboratory for Molecular Sciences, The Key Laboratory of Polymer

Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. KEYWORDS: radiation synthesis, double network, gel electrolytes, mechanical properties, flexible supercapacitors


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ABSTRACT: High-performance hydrogel electrolytes play a crucial role in flexible supercapacitors (SCs). However, the unsatisfactory mechanical properties of widely-used polyvinyl alcohol-based electrolytes greatly limit their use in the flexible SCs. Here a novel Li2SO4-containing agarose/polyacrylamide

double-network

(Li-AG/PAM

DN)

hydrogel

electrolyte was synthesized by a heating-cooling and subsequent radiation-induced polymerization and crosslinking process. The Li-AG/PAM DN hydrogel electrolyte possesses extremely excellent mechanical properties with a compression strength of 150 MPa, a fracture compression strain of above 99.9%, a tensile strength of 1103 kPa and an elongation at break of 2780%, greatly superior to those have been reported. It also achives a high ionic conductivity of 41 mS cm-1 originating from its interconnected three-dimensional porous network structure that provides a three-dimensional channel for ionic migration. Compared to the SC applying Li2SO4 aqueous solution electrolyte, the corresponding flexible Li-AG/PAM DN hydrogel electrolyteSC presents lower charge transfer resistance, better ionic diffusion, being closer to ideal capacitive behaviors, superior rate capability and better cycling stability, owing to the improved ionic transport in the Li-AG/PAM DN hydrogel electrolyte and electrodes interfaces. Moreover, after test with overcharge, short circuit and high temperature, the capacitance of the Li-AG/PAM DN hydrogel electrolyte-SC can still be well maintained. Furthermore, the electrochemical properties of the Li-AG/PAM DN hydrogel electrolyte-SC remain almost intact under different compression strains/bending angles and even after 1000 compression/bending cycles. It is expected that the Li-AG/PAM DN hydrogel electrolyte may have broad applications in modern flexible and wearable electronics.


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1. Introduction The great market prospect of modern flexible and wearable electronics has aroused great interest in researching compressible, stretchable and bendable energy storage devices.1-4 Among them, the flexible supercapacitor (SC) shows great potential for practical application, due to its high power density, fast rate of charging/discharging, long cycling lifetime and good operational safety.5-8 A flexible SC requires each device component including electrodes, separator and electrolyte to be flexible. Apart from the widely studied flexible electrode of SC9-13, the flexible electrolyte is also crucial in that it can significantly affect the performances of SC, such as rate capability and cycling stability14. Hydrogel electrolyte as a kind of important flexible electrolytes has attracted increasing attention, because of its minimum leakage compared to liquid electrolyte and much higher ionic conductivity compared to solid polymer electrolyte.15-17 Consequently, the researches of hydrogel electrolytes greatly accelerate the development of flexible SCs. Hydrogel electrolytes for SCs are dominantly based on polyvinyl alcohol (PVA) matrix8, such as PVA/H3PO411, 18, PVA/H2SO419-20, PVA/LiCl21-22 and PVA/KOH23-24, due to their flexible adaptability to a wide range of pH values, which is similar to the aqueous electrolyte solution. However, their mechanical properties are insufficient to meet the requirements, greatly limiting their use in the flexible SC. Therefore, improving the mechanical properties of hydrogel electrolytes is of great importance for their application in flexible SCs. Some efforts have been devoted to developing hydrogel electrolytes without PVA for SCs in order to achieve superior mechanical properties. One strategy is to increase the noncovalent interactions between polymer chains to improve the mechanical properties of hydrogel electrolyte. Supramolecular hydrogel electrolytes that are formed by the supramolecular effect


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according

to

this

strategy

chitosan/poly(hydroxyethyl

have

been

studied.

Wang’s

group25

methacrylate)/1-ethyl-3-methylimidazolium

reported

a

chloride

(CS/PHEMA/EMIMCl) supramolecular hydrogel electrolyte, which displayed a tensile strength (σT) of ca. 100 kPa and an elongation at break (λT) of ca. 520%. Guo and co-works26 synthesized a ferric ion crosslinked polyacrylic acid (Fe3+/PAA) supramolecular hydrogel electrolyte with a

σT of ca. 360 kPa and a λT of 380%. The other strategy is to increase the functionalities between two crosslink points to toughen the hydrogel electrolytes. Nanocomposite hydrogel electrolytes, especially crosslinked by silica nanoparticles, are the most typical type, such as the vinyl hybrid silica nanoparticles crosslinked polyacrylamide (VSNPs-PAM) nanocomposite hydrogel electrolyte (σT≈300 kPa and λT=1500%)27, the vinyl group grafted-silica nanoparticles crosslinked polyacrylamide (CH2=CH-SiO2-PAM) nanocomposite hydrogel electrolyte (σT=844 kPa and λT=3400%)28, the hydrogen bonding and vinyl hybrid silica nanoparticles dual crosslinked polyacrylic acid (VSNPs-PAA) nanocomposite hydrogel electrolyte (σT≈120 kPa and λT≈3700%)29, and the bovine serum albumin and silica nanocomposites dual crosslinked poly(N,N-dimethylacrylamide) (BSA-PDMAA-SiO2) nanocomposite hydrogel electrolyte (σT=1098 kPa and λT≈14%)30. Besides, Tang and co-works31 reported an anionic polyurethane acrylates multi-functional crosslinked polyacrylamide (aPUA-PAM) hydrogel electrolyte with a

σT of ca. 80 kPa and a λT of ca. 1000%. All of these hydrogel electrolytes achieve a high λT (more than 500%) while weak in σT (low than 1 MPa), or vice versa, due to the lack of a high-efficient mechanism for energy dissipating. It is well demonstrated that the double network (DN) hydrogel can efficiently disperse the locally applied stress and dissipate the energy though the combination of two networks, thus enhancing


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the hydrogel’s mechanical strength.32-34 Nevertheless, ionic conducting hydrogel electrolytes for SCs based on DN structure, which have great potential to solve the deficiencies of hydrogel electrolyte of SC with satisfactory mechanical properties, have not been reported to the authors’ knowledge. On the other hand, the hydrogel electrolytes without PVA for SCs are often synthesized via a process involving initiator-induced polymerization, drying of hydrogel, and swelling-soaking in ionic conducting medium solution.26-29, 31 The drawbacks of this synthetic procedure are initiatorresidue, time-consuming, and rather hard to regulate the exact concentration of ionic conducting medium. Therefore, it’s of great importance to develop a facile and clean method to synthesize hydrogel electrolytes with excellent mechanical properties. Here we demonstrate a facile one-pot method to prepare a Li2SO4-containing agarose/polyacrylamide (Li-AG/PAM) DN hydrogel electrolyte by a heating-cooling and subsequent radiation-induced polymerization and crosslinking process. The flexible Li-AG/PAM DN hydrogel electrolyte-SC was assembled with the as-prepared hydrogel electrolytes sandwiched by two activated carbon electrodes supported by nickel foam. Its basic electrochemical behaviors, rate capability and cycling stability were analyzed with the SC applying Li2SO4 aqueous solution (Li-AQ) electrolyte as control. The safety, compressibility and bendability of the hydrogel electrolyte-SC were also investigated. 2. METHODS 2.1 Preparation of the Li-AG/PAM DN hydrogel electrolyte. Agarose (AG, 500 mg, electroendosmosis ≤ 0.07, Acros), acrylamide (AM, 4.84 g, J&K), N, N'-methylene-bisacrylamide (MBA, 78.4 μL, 1 wt%, J&K), Li2SO4·H2O (0, 0.80, 1.60, 2.40 and 3.20 g,


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respectively, Beijing Chemical Works) and H2O (20 mL) were fist included in a glass vial. After perfect mixing, the turbid liquid was kept in a 95°C oil bath until it forms transparent and homogeneous solution. Whereafter, the solution was injected into a glass mold, which was composed of two flat glass plates sandwiching a 2 mm-thick silicone gasket, and then cooled to room temperature. As the final step, the glass mold containing reactants was irradiated by a 60Co γ-ray source with a dose rate of 10 Gy min−1 for half an hour in air atmosphere at room temperature to obtain the Li-AG/PAM DN hydrogel electrolyte. For Li2SO4-containing agarose (Li-AG) and Li2SO4-containing polyacrylamide (Li-PAM) hydrogel electrolyte, a similar process was conducted without the addition of AM+MBA and AG, respectively. 2.2 Fabrication of the SCs. Briefly, activated carbon (TF-B520 type, Shanghai Sinotech Co. Ltd), carbon black (Alfa Aesar) and polyvinylidene fluoride (Solvay-1030 type, Solvay Solexis Co. Ltd) with the mass ratio of 8:1:1 were mixed and dispersed in N-methyl-2-pyrrolidone to form a uniform slurry. After stirring for 12 h, the slurry was coated on nickel foam disk with diameter of 15 mm. After vacuum drying at 45 °C for 48 h, the nickel foam disk was pressed into a thin activated carbon electrode with the average activated carbon mass of ca. 15 mg. Finally, the Li-AG/PAM DN hydrogel electrolyte-SC was constructed by two faced activated carbon electrodes coated with nickel foam, sandwiching a piece of as-prepared Li-AG/PAM DN hydrogel electrolyte. The Li-AQ electrolyte-SC was fabricated by the same way, except applying a battery membrane (Whatman Grade GF/D Glass Microfiber Filters, Binder Free) absorbing 0.94 mol kg-1 Li2SO4 aqueous solution (Li-AQ) electrolyte. 2.3 Characterizations of the hydrogel electrolyte. The dried gel electrolyte’s morphology was captured by a field-emission scanning electron microscopy (Hitachi S-4800). Compressive and tensile tests of as-prepared hydrogel electrolytes were performed on an Instron-5969 and an


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Instron-3366 machine, respectively. For compressive test, the hydrogel electrolyte sample was cut into a 7 mm-high cylindrical shape with a diameter of 14 mm, and the compressive rate was 1 mm/min. For tensile test, a 2 mm-wide and 2 mm-thick dumbbell hydrogel electrolyte sample with a gauge length of 13 mm was used, and the tensile rate was 100 mm/min. The ionic conductivity σ (mS cm-1) of hydrogel electrolyte sample was measured by electrochemical impedance spectroscopy (EIS) at an electrochemical workstation (Metrohm Autolab 302N) at 25 °C, and it was calculated according to the following formula: 𝜎𝜎 =

40000𝐻𝐻 𝑅𝑅⋅π⋅𝐷𝐷 2

(1)

where H (mm), D (mm), and R (Ω) are thickness, diameter, and bulk resistance were obtained by the intercept with x-axis in Nyquist plots, respectively. 2.4 Electrochemical measurements of the SCs. All electrochemical measurements of the SCs were performed in a two-electrode system at 25 °C unless mentioned otherwise. Cyclic voltammetry (CV) and EIS were measured on an electrochemical workstation (Metrohm Autolab 302N). All galvanostatic charge-discharge (GCD) data were recorded on a LANDIAN battery testing system (CT2001A, Wuhan LANDIAN Co. Ltd), except that the GCD data of rate capability were recorded on a NEWARE battery testing system (BTS-900, Shenzhen NEWARE Co. Ltd). Potential range set for CV and GCD tests was from 0 to 0.8 V unless mentioned otherwise. EIS was obtained in the frequency range from 106 to 10-2 Hz with a RMS amplitude of 10 mV at open circuit potential. The single-electrode specific capacitance from GCD (Cs, F g−1), single-electrode specific capacitance from CV (𝐶𝐶s′ , F g−1), energy density (E, Wh kg-1), and

power density (P, W/kg) of SCs were estimated individually as the following equations:


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4𝐼𝐼⋅△𝑡𝑡

𝐶𝐶s = △𝑉𝑉⋅𝑚𝑚 𝐶𝐶𝑠𝑠′

(2) 𝑈𝑈

2 ∮𝑈𝑈 max 𝑖𝑖 d𝑢𝑢

(3)

1000

(4)

= 𝜈𝜈⋅𝑚𝑚⋅(𝑈𝑈min −𝑈𝑈 max

1

min )

𝐸𝐸 = 8 𝐶𝐶s ∆𝑉𝑉 2 ∙ 3600 𝐸𝐸

𝑃𝑃 = △𝑡𝑡 ∙ 3600

(5)

where I (A), Δt (s), ΔV (V), Umax (V), Umin (V), i (A), u (V), ν (V s-1) and m (g) are discharge current, discharge time, voltage change after voltage drop during the discharging process in the GCD curve, maximum voltage, minimum voltage, current, voltage, scan rate of the CV curve, and total mass of activated carbon on the two electrodes, respectively. 3. RESULTS AND DISCUSSION 3.1 Preparation and characterizations of the Li-AG/PAM DN hydrogel electrolyte. Using renewable materials to synthesize hydrogel electrolytes is very attractive from the environmental perspective.35 AG, as a typical renewable material, is both low-cost and ecofriendly. However, the mechanical properties of a pure AG hydrogel electrolyte are very weak,36 so PAM is added to form the DN structure that could efficiently dissipate energy as a way to enhance mechanical properties of the AG hydrogel electrolyte. In addition, Li2SO4, a good neutral inorganic salt used in hydrogel electrolytes of flexible SCs,14 is selected as the ionic conducting medium. The preparation of the Li-AG/PAM DN hydrogel electrolyte is based on our previous work.37 As depicted in Figure 1, the Li-AG/PAM DN hydrogel electrolyte was synthesized by a one-pot


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method consisting of heating-cooling and subsequent radiation-induced polymerization and crosslinking process. Briefly, a turbid liquid including AG, AM, MBA and Li2SO4·H2O was heated to form a transparent and uniform solution, resulting from the transformation of AG into soluble linear chains.38 Followed by cooling, the solution turned into a Li2SO4-containing AG single-network (Li-AG SN) hydrogel electrolyte crosslinked physically by AG helix bundles via hydrogen bonding.39 Finally, when the Li-AG SN hydrogel electrolyte consisting of AM and MBA was irradiated by γ ray, free radicals generated and then induced polymerization and crosslinking of AM and MBA to form PAM network inside the Li-AG SN hydrogel electrolyte,37, 40 thus obtaining the Li-AG/PAM DN hydrogel electrolyte. As shown in Figure S1, the Li-AG/PAM DN hydrogel electrolyte has a much higher σT than the hydrogel electrolyte fabricated by the same strategy of the Li-AG/PAM DN hydrogel electrolyte without the addition of MBA crosslinker, indicating the formation of strong chemical crosslinking structure inside the Li-AG/PAM DN hydrogel electrolyte. Additionally, we observed a significant hysteresis that increased strongly with the applied maximum elongation during the cyclic tensile tests of the LiAG/PAM DN hydrogel electrolyte (Figure S2 and Table S1), which is a typical feature of DN hydrogels due to their unique and highly efficient energy dissipation mechanism.41 This suggests the existence of DN structure inside the Li-AG/PAM DN hydrogel electrolyte.


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Figure 1. The synthesis process of the Li-AG/PAM DN hydrogel electrolytes: (a) schematic representation and (b) photos in different steps. As we know, the ionic conductivity of hydrogels without ionic conducting medium is very low, so Li2SO4 is added to the AG/PAM DN hydrogel to form Li-AG/PAM with improved ionic conductivity. The ionic conductivity of the Li-AG/PAM DN hydrogel electrolyte rapidly increases from 0.51 to 45.7 mS cm-1 with the increase of Li2SO4 concentration from 0 to 1.25 mol kg-1 (Figure 2a), due to the increasing number of conductive ions. Besides, Li2SO4 can also affect the mechanical properties of the hydrogel electrolytes. As the Li2SO4 concentration of the hydrogel electrolyte increases from 0 to 0.313 mol kg-1, the compression strength (σC) increases from 140 to 157 MPa, the σT from 1263 to 1399 kPa, and the λT from 3406% to 3608% (Figure 2b, c). The enhanced mechanical properties may result from the enhanced interaction between AG networks and/or PAM networks through Li+ and SO42- serving as bridges. However, when the Li2SO4 concentration increases to 1.25 mol kg-1, the σC decreases to 122 MPa, the σT to 653 kPa, and the λT to 1831%. It may be attributed to the Li2SO4 aggregation in high concentration, leading to the destruction of hydrogen bonds between AG networks and/or PAM networks. The


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effect of the Li2SO4 concentration on the fracture compression strain (εC) is too slight to be precisely measured. It is noteworthy that when the Li2SO4 concentration exceeds 0.94 mol kg-1, the mechanical properties weaken significantly with slow increase in the ionic conductivity. Therefore, the Li-AG/PAM DN hydrogel electrolyte with a Li2SO4 concentration of 0.94 mol kg1

is selected for further investigations considering ionic conductivity and mechanical properties.

Figure 2. Effect of Li2SO4 concentration on the properties of Li-AG/PAM DN hydrogel electrolytes: (a) ionic conductivity, (b) compression properties, and (c) tensile properties at various Li2SO4 concentrations from 0 to 1.25 mol kg-1. Error bars represent standard deviation; sample size n=3. Figure 3a-c display optical images of the Li-AG/PAM DN hydrogel electrolytes. The grid pattern can be seen clearly through the hydrogel electrolyte (a), suggesting good transparency of the hydrogel electrolyte. Moreover, the hydrogel electrolyte can be folded (Figure 3b) and rolled (Figure 3c) without any observable damage, indicating high softness and toughness of the hydrogel electrolyte. In addition, the SEM image exhibits the hydrogel electrolyte’s interconnected three-dimensional (3D) porous network structure (Figure 3d), with an average pore diameter of ca. 150 nm.


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Figure 3. Characterizations of the Li-AG/PAM DN hydrogel electrolytes at a Li2SO4 concentration of 0.94 mol kg-1: (a-c) optical images, (d) SEM image, (e) compression stresscompression strain curve, (f) tensile stress-elongation curve, and (g) ionic conductivity. Error bars represent standard deviation; sample size n=3. The mechanical properties and ionic conductivity of the Li-AG/PAM DN hydrogel electrolyte with a Li2SO4 concentration of 0.94 mol kg-1 were studied in more detail. The hydrogel electrolyte exhibits outstanding compression properties (Figure 3e). The σC reaches a high value of 150±7 MPa, and the εC can even exceed 99.9%. It also exhibits excellent tensile properties (Figure 3f). The σT and λT are 1103±24 kPa and 2780±55%, respectively. Compared to pure Li2SO4-containing agarose (Li-AG) or Li2SO4-containing polyacrylamide (Li-PAM) hydrogel electrolyte (Figure S3 and Table S2), the Li-AG/PAM DN hydrogel electrolyte shows enhanced mechanical properties largely attributed to the effective dispersion of locally applied stress and


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dissipation of the energy though combinations of two networks.32, 37 Furthermore, the hydrogel electrolyte achieves a high ionic conductivity of 41±1 mS cm-1(Figure 3g). This comes from the interconnected 3D porous network structure of the hydrogel electrolyte (demonstrated by the SEM), which provides a 3D channel for ionic migration.26 Table 1 summarizes different hydrogel electrolytes without PVA for SCs recently reported, showing that our DN hydrogel electrolyte possesses not only the highest compression and tensile properties, but also a high ionic conductivity above the average. Table 1. Comparison of different hydrogel electrolytes without PVA for SCs in recent reports.

41

Maximum compression strain of SC 75%

Maximum bending angle of SC 135o

23.3

90%

N/A

This work 25

ca. 70

N/A

N/A

26

ca. 17

50%

N/A

27

ca. 50

N/A

N/A

28

ca. 7.5

80%

N/A

29

8.8

N/A

N/A

30

36

N/A

N/A

31

1.46

N/A

180o

42

N/A

79.4

N/A

N/A

43

N/A N/A N/A N/A

ca. 10 8 N/A N/A

N/A N/A N/A N/A

180o N/A 1.16 cmd N/A

44 45 46 47

Hydrogel electrolytea

Compression propertyb

Tensile propertyc

Conductivity (mS cm-1)

Li-AG/PAM DN

Fe3+/PAA

150 MPa, above 99.9% 1.706 MPa, 95% N/A

VSNPs-PAM

N/A

CH2=CH-SiO2-PAM

N/A

VSNPs-PAA

N/A

BSA-PDMAA-SiO2

N/A

aPUA-PAM

N/A

Aligned ionogel

ca. 0.52 MPa, ca. 95% 0.55 MPa, 95% N/A N/A N/A N/A

1130 kPa, 2780% ca. 100 kPa, ca. 520% ca. 360 kPa, ca. 380% ca. 300 kPa, 1500% 844 kPa, 3400% ca. 120 kPa, ca. 3700% 1098 kPa, ca. 14% ca. 80 kPa, ca. 1000% N/A

CS/PHEMA/EMIMCl

Aligned PA PPDP PAEK-g-PEG Agarose κ-Carrageenan

Ref.

a

PA in this table represents polyampholyte, PPDP propylsulfonate dimethylammonium propylmethacrylamide, and PAEK-g-PEG poly(ethylene glycol)-grafted poly(arylene ether ketone).


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b

σC and εC, respectively.

c

σT and λT, respectively.

d

Bending radius. 3.2 Electrochemical performances of SC applying the Li-AG/PAM DN hydrogel

electrolyte. To demonstrate that the Li-AG/PAM DN hydrogel electrolyte provides enhanced electrochemical performances, we investigated the basic electrochemical behaviors of the SC applying the Li-AG/PAM DN hydrogel electrolyte with the SC applying Li-AQ electrolyte as control. These two SCs were constructed by two faced activated carbon electrodes coated with nickel foam, sandwiching a piece of the Li-AG/PAM DN hydrogel electrolyte and a battery membrane absorbing 0.94 mol kg-1 Li2SO4 aqueous solution electrolyte, respectively (Figure S4). The as-assembled SCs were then packed in CR2025-type coin cells in tests (Figure S5). Figure S6 shows that the thickness of the Li-AG/PAM DN hydrogel electrolyte has almost no effect on the electrochemical performance of corresponding supercapacitor, so the hydrogel electrolyte with a thickness of 2 mm was used for further study as a matter of convenience. Also, the battery membrane used in Li-AQ electrolyte was of the same thickness as control. Cyclic voltammetry (CV) at a scan rate of 10 mV s-1 was first studied (Figure 4a). The CV curve of the Li-AG/PAM DN hydrogel electrolyte-SC is closer to rectangular and symmetrical shapes than that of the Li-AQ electrolyte-SC, demonstrating that the Li-AG/PAM DN hydrogel electrolyte-SC has lower charge transfer resistance and is closer to ideal electric double-layer capacitive behaviors than the Li-AQ electrolyte-SC.26,

48

Moreover, as the scan rate of CV

increases from 5 mV s-1 to 100 mV s-1, the CV of Li-AG/PAM DN hydrogel electrolyte-SC deviates from a rectangular shape far less serious than the Li-AQ electrolyte-SC (Figure 4b, c),


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revealing better ionic diffusion than that of the latter. Then, galvanostatic charge-discharge (GCD) measurements were performed at a current density of 0.5 A g-1 (Figure 4d). The LiAG/PAM DN hydrogel electrolyte-SC shows lower voltage drop (0.057 V) than the Li-AQ electrolyte-SC (0.074 V), indicating the Li-AG/PAM DN hydrogel electrolyte-SC possesses a lower equivalent series resistance (ESR) used to estimate the internal resistance,49 which consists with the CV result. Moreover, compared with that of the Li-AQ electrolyte-SC, the GCD curve of the Li-AG/PAM DN hydrogel electrolyte-SC is closer to symmetric triangular, suggesting that the latter is closer to ideal electric double-layer capacitive behaviors than the former, which was also proved by CV curve.


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Figure 4. Electrochemical performances comparison of SCs applying the Li-AG/PAM DN hydrogel electrolyte and the Li-AQ electrolyte: (a) CV curves at a scan rate of 10 mV s-1, (b, c) CV curves at various scan rates from 5 mV s-1 to 100 mV s-1, (d) GCD curves at a current density of 0.5 A g-1 (Inset is a close-up view of the voltage drop at the beginning of the discharging process), (e) Nyquist plots in the frequency range from 106 to 10-2 Hz (Inset is a close-up view of the left plot in the high to mid-frequency region), (f) specific capacitance obtained from GCD at various current densities from 0.2 to 2.0 A g-1, (g) specific capacitance obtained from CV at various scan rates from 5 mV s-1 to 100 mV s-1, (h) capacitance retention on cycling obtained from GCD at a current density of 0.2 A g-1, and (i) Ragone plots. Furthermore, electrochemical impedance spectroscopy (EIS) tests were carried out (Figure 4e). Both Nyquist plots show a depressed semicircle at high frequencies, a straight line close to 45o at mid frequencies, and a straight line close to 90o at low frequencies. The equivalent circuit model and fitting values of parameters of the Li-AG/PAM DN hydrogel electrolyte-SC and the Li-AQ electrolyte-SC are shown in Figure S7 and Table S3, respectively. At high frequencies, the intersection of the curve at the real part reflects the ohmic resistance (Rs) resulted from the resistance of both electrolyte and electrode materials, and the diameter of depressed semicircle represents the charge transfer resistance (Rct) related to the interfacial resistance between the electrode and electrolyte.50-51 The Li-AG/PAM DN hydrogel electrolyte-SC shows higher Rs (3.33 Ω) than the Li-AQ electrolyte-SC (2.07 Ω) because of the lower ionic conductivity of the Li-AG/PAM DN hydrogel electrolyte than that of the Li-AQ electrolyte. However, the Rct of the Li-AG/PAM DN hydrogel electrolyte-SC (0.58 Ω) is lower than that of the Li-AQ electrolyte-SC (1.08 Ω), revealing that the transport of ions into electrode materials of the former is much better than that of the latter. At mid frequencies, the region of straight line close to 45o denotes the


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Warburg impedance (W) derived from the frequency dependence of ion diffusion from electrolyte to electrode.44, 52 The W of Li-AG/PAM DN hydrogel electrolyte-SC shows higher coefficient Y0 (0.17 Ω-1 s0.5, the Li-AQ electrolyte-SC 0.06 Ω-1 s0.5) (Table S3) due to its better ionic diffusion. At low frequencies, the region of straight line close to 90o presents the capacitive behaviors of the devices.14, 53 Compared with the line of Li-AQ electrolyte-SC, the line of LiAG/PAM DN hydrogel electrolyte-SC is closer to 90o. This suggests that the Li-AG/PAM DN hydrogel electrolyte-SC is closer to ideal electric double-layer capacitive behaviors than the LiAQ electrolyte-SC.28 In addition, the ESR can be obtained by extrapolating the vertical portion of the EIS plot to the x-axis.49, 54-55 Figure S8 shows that the Li-AG/PAM DN hydrogel electrolyteSC exhibits a lower ESR than the Li-AQ electrolyte-SC, indicating that the former displays a much lower charge transfer resistance than the latter, which agrees with the results of GCD. The CV, GCD and EIS results confirm that the Li-AG/PAM DN hydrogel electrolyte-SC possesses lower charge transfer resistance, better ionic diffusion and being closer to ideal electric double-layer capacitive behaviors than the Li-AQ electrolyte-SC. These may be attributed to the strong adhesive force between the electrodes and the Li-AG/PAM DN hydrogel electrolyte, which can facilitate the formation of intimate contacts at the solid/solid electrode-electrolyte interfaces, comparing to the non-ideal wetting behavior of the Li-AQ electrolyte on the activated carbon electrodes.28, 31, 43-44 Rate capability is important for the practical application of SCs. A comparison of the specific capacitances obtained from GCD at various current densities from 0.2 to 2.0 A g-1 between the Li-AG/PAM DN hydrogel electrolyte-SC and the Li-AQ electrolyte-SC is shown in Figure 4f. The Li-AG/PAM DN hydrogel electrolyte-SC exhibits a specific capacitance of 84.7 F g-1 at a current density of 0.2 A g-1, which is slightly lower to the Li-AQ electrolyte-SC (92.1 F g-1).


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However, when the current density is increased from 0.2 to 2 A g-1, the Li-AG/PAM DN hydrogel electrolyte-SC shows a higher specific capacitance and capacitance retention (71.8 F g1

, 85% retention) than the Li-AQ electrolyte-SC (33.8 F g-1, 37% retention). Similarly, the

specific capacitances obtained from CV display that the Li-AG/PAM DN hydrogel electrolyteSC has the nearly same value (79 F g-1) as the Li-AQ electrolyte-SC at a scan rate of 5 mV s-1, but a higher specific capacitance and capacitance retention (27 F g-1, 34% retention) than the LiAQ electrolyte-SC (18 F g-1, 23% retention) as the scan rate is increased from 5 mV s-1 to 100 mV s-1 (Figure 4g). These could be attributed to the better ionic diffusion of the Li-AG/PAM DN hydrogel electrolyte-SC than that of the Li-AQ electrolyte-SC,26, 56 which has been demonstrated by the CV and EIS results previously. More detailed mechanism is described as follows57-59: At a lower current density, the ions can fully diffuse into the interior electrode, leading to more available surface of activated carbon materials that can be used effectively and thus enhancing the specific capacitance. Once the current density increases, the ions can only penetrate into the inner surface of the relatively large pores in electrode, thus reducing active surface area of the activated carbon materials that takes part in due to much lower ion diffusion rate, decreasing the specific capacitance resultantly. In a word, the rate capability is highly dependent on the rate of ion diffusion. Consequently, the Li-AG/PAM DN hydrogel electrolyte-SC presents a superior rate capability than the Li-AQ electrolyte-SC due to its better ionic diffusion. Apart from rate capability, cycling stability is also crucial. The capacitance retention of the LiAG/PAM DN hydrogel electrolyte-SC and the Li-AQ electrolyte-SC during charge/discharge cycles at a current density of 0.2 A g-1 are presented in Figure 4h and Figure S9 for comparison. The Li-AG/PAM DN hydrogel electrolyte-SC exhibits a higher capacitance retention (148%) than the Li-AQ electrolyte-SC (76%) after 5000 charge/discharge cycles. The first observed


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capacitance increment that occurs in both SCs before 500th cycle is ascribed to the activation of the ion transport channel inside the electrode materials, whereas the subsequent capacitance increment that only occurs in the Li-AG/PAM DN hydrogel electrolyte-SC is brought by the activating ion transport channel at the solid/solid electrode-electrolyte interfaces. Besides, the capacitance loss of both SCs is derived from the active materials breaking away from electrodes. The excellent cycling stability of the Li-AG/PAM DN hydrogel electrolyte-SC might be explained as follows: 1) The Li-AG/PAM DN hydrogel electrolyte-SC is closer to ideal reversible capacitive characteristics than the Li-AQ electrolyte-SC. 2) The Li-AG/PAM DN hydrogel electrolyte can act as an elastic shell as to prevent the electrode materials from breaking away from the nickel foam during repeated ion adsorption and desorption.26, 60 3) The strong adhesive force between the electrode materials and the Li-AG/PAM DN hydrogel electrolyte avoids their separation.26, 31 Ragone plots are often used to evaluate the performance of energy storage devices. The LiAG/PAM DN hydrogel electrolyte-SC exhibits a higher energy density and power density compared to the Li-AQ electrolyte-SC (Figure 4i). As shown, three Li-AG/PAM DN hydrogel electrolyte-SCs connected in series can light a red light-emitting diode (LED) over 5 min (Figure 5a). These results indicate that our SC may have a good application in energy storage devices. A series of battery-safety related evaluations including overcharge, short circuit, and high temperature were also carried out on the Li-AG/PAM DN hydrogel electrolyte-SC. As shown in Figure 5b and Figure S10a, although the Li-AG/PAM DN hydrogel electrolyte-SC was overcharged to 1.2 V, it basically maintains the initial capacitance in the subsequent chargedischarge cycles with a normal voltage of 0.8 V. Moreover, the capacitance remains intact on cycling after going through a short circuit test (Figure 5c and Figure S10b). Furthermore, the


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capacitance enhances slightly as the operating temperature rises to 60°C due to the faster ion transfer at higher temperature (Figure 5d and Figure S10c). These results indicate that the LiAG/PAM DN hydrogel electrolyte-SC possesses a superior safety performance.

Figure 5. (a) A digital photograph of three SCs connected in series to light a red LED. Capacity retention on cycling obtained from GCD after (b) over-charge test at a current density of 0.3 A g1

, (c) short circuit test at a current density of 0.3 A g-1, and (d) high temperature of 60 °C test at a

current density of 0.2 A g-1. 3.3 Flexibilities of the SC applying Li-AG/PAM DN hydrogel electrolyte. The Li-AG/PAM DN hydrogel electrolytes are highly compressible, which endows the corresponding SC with outstanding compressibility. For compression tests, the Li-AG/PAM DN hydrogel electrolyte-SC was placed in a height-adjustable stainless steel cylindrical mold


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containing Teflon liners (Figure S11). Figure 6a and Figure S12a presents that the capacitance enhances slightly as the compression strain increases. This may be caused by the increasing ionic transferring between electrode/electrolyte interfaces derived from improved interfacial contact under applied pressure.25,

29, 61

Additionally, the Li-AG/PAM DN hydrogel electrolyte-SC

displayed a 91% capacitance retention after 1000 compression cycles with 50% compression stain applied (Figure 6b and Figure S12b), suggesting that the compression at various cycles has almost no influence on the electrochemical behavior, either. These suggest that the Li-AG/PAM DN hydrogel electrolyte-SC has great electrochemical robustness even in some mechanically extreme conditions.

Figure 6. Compression and bending stabilities of SCs applying the Li-AG/PAM DN hydrogel electrolyte. Capacitance retention obtained from GCD at a current density of 0.2 A g-1 as a function of (a) compression strain, (b) compression cycles, (c) bending angle, and (d) bending


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cycles. Insets are schematics of the supercapacitor applying the Li-AG/PAM DN hydrogel electrolyte undergoing compression or bending. The intrinsically tough property of the Li-AG/PAM DN hydrogel electrolyte endows its SC with a very good bending property. Two pieces of the same stainless steel clip with a certain bending angle were used to clamp the Li-AG/PAM DN hydrogel electrolyte-SC as a way to bend it at a certain angle (Figure S13). The capacitances at various bending angles from 0o to 135o remain almost constant (Figure 6c and Figure S12c), suggesting that the bending at various angles has no influence on its electrochemical behavior. Moreover, the capacitance retention is nearly 100% after 1000 bending cycles at 90o bending (Figure 6d and Figure S12d), suggesting that the bending at various cycles does no harm to the electrochemical behavior. These results demonstrate that the Li-AG/PAM DN hydrogel electrolyte-SC is so flexible that its electrochemical property remains almost intact undergoing bending. Few flexible SCs that are both compressible and bendable have ever been reported (Table 1), although compression and bending are common in their practical use. Benefiting from the extraordinarily excellent mechanical properties and high ionic conductivity of the Li-AG/PAM DN hydrogel electrolyte, its SC can achieve a high maximum compression strain and a high maximum bending angle. Besides, its capacitance is well retained after 1000 successive cyclic compression/bending tests. 4. CONCLUSION A facile one-pot method was developed to prepare a novel Li-AG/PAM DN hydrogel electrolyte by a heating-cooling and subsequent radiation-induced polymerization and crosslinking process. The resultant hydrogel electrolyte owns outstanding compression properties


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(σC=150 MPa and εC>99%) and tensile properties (σT=1103 kPa and λT=2780%). It also has a high ionic conductivity of 41 mS cm-1 originating from its interconnected 3D porous network structure that provides a 3D channel for ionic migration. The corresponding flexible Li-AG/PAM DN hydrogel electrolyte-SC presents lower charge transfer resistance, better ionic diffusion, being closer to ideal electric double-layer capacitive behaviors, superior rate capability and better cycling stability than the Li-AQ electrolyte-SC, due to the improved ionic transport in the interfaces between the Li-AG/PAM DN hydrogel electrolyte and activated carbon electrodes, which comes from the strong stickiness of the hydrogel electrolyte to the electrodes that facilitates the formation of intimate contacts between electrolyte and electrodes. Moreover, the Li-AG/PAM DN hydrogel electrolyte-SC displayed a good capacitance retention after overcharge, short circuit, and high temperature tests. Furthermore, the Li-AG/PAM DN hydrogel electrolyte-SC achieves not only a large compression strain up to 75% and a high bending angle of 135o with no loss in capacitance, but also a good resistance to 1000 compression/bending cycles. This work in synthesizing DN hydrogel electrolyte for highly compressible and bendable SC with the facile and clean one-pot approach paves a road for the development of modern electronics. ASSOCIATED CONTENT Supporting Information. Mechanical properties of hydrogel electrolytes; Assembly process diagram and electrochemical properties of SCs; Photos and electrochemical properties of SCs under compression and bending conditions. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (No. 11375019 & 11575009) is acknowledged. REFERENCES (1) Larson, C.; Peele, B.; Li, S.; Robinson, S.; Totaro, M.; Beccai, L.; Mazzolai, B.; Shepherd, R. Highly Stretchable Electroluminescent Skin for Optical Signaling and Tactile Sensing. Science 2016, 351, 1071-1074. (2) Kim, C.-C.; Lee, H.-H.; Oh, K. H.; Sun, J.-Y. Highly Stretchable, Transparent Ionic Touch Panel. Science 2016, 353, 682-687. (3) Jung, M.-S.; Seo, J.-H.; Moon, M.-W.; Choi, J. W.; Joo, Y.-C.; Choi, I.-S. A Bendable LiIon Battery with a Nano-Hairy Electrode: Direct Integration Scheme on the Polymer Substrate. Adv. Energy Mater. 2015, 5, 1400611. (4) Yousaf, M.; Shi, H. T. H.; Wang, Y.; Chen, Y.; Ma, Z.; Cao, A.; Naguib, H. E.; Han, R. P. S. Novel Pliable Electrodes for Flexible Electrochemical Energy Storage Devices: Recent Progress and Challenges. Adv. Energy Mater. 2016, 6, 1600490.


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Table of Contents


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One-pot Synthesis of Double-network Hydrogel Electrolyte with

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