A Highly Compressible Crosslinked Polyacrylamide Hydrogel Enabled

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A Highly Compressible Crosslinked Polyacrylamide Hydrogel Enabled Compressible Zn-MnO2 Battery and A Flexible Battery-Sensor System Zifeng Wang, Funian Mo, Longtao Ma, Qi Yang, Guojin Liang, Zhuoxin Liu, Hongfei Li, Na Li, Haiyan Zhang, and Chunyi Zhi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17607 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 3, 2018

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ACS Applied Materials & Interfaces

A Highly Compressible Crosslinked Polyacrylamide Hydrogel Enabled Compressible Zn-MnO2 Battery and a Flexible Battery-Sensor System Zifeng Wang,a Funian Mo, a Longtao Ma, a Qi Yang, a Guojin Liang, a Zhuoxin Liu, a Hongfei Li a, Na Lib, Haiyan Zhangb and Chunyi Zhia, c* a

Department of Materials Science & Engineering, City University of Hong Kong, 83 Tat

Chee Avenue, Hong Kong SAR, 999077, China b School

of Material and Energy, Guangdong University of Technology, Guangzhou, 510006,

PR China c Shenzhen

Research Institute, City University of Hong Kong, Shenzhen 518000, China

Abstract The fast advancement in flexible and wearable electronics has put up with new requirements on the energy storage device with improved tolerance to deformation apart from offering power output. Despite the tremendous progress in stretchable energy storage devices, the compressional energy storage devices have indeed received limited research attention. In this work, an intrinsically compressible rechargeable battery was proposed using the Zn-MnO2 chemistry and a crosslinked polyacrylamide (PAAm) hydrogel electrolyte. Interestingly, the battery exhibited not only good energy storage performances but also excellent tolerance against large compressional strain without sacrificing the energy storage capability. It was also

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found that the ionic conductivities of the hydrogel increased with the values of the compressional strain, leading to an enhanced electrochemical performance. More importantly, upon dynamic compression, the voltage output of the battery can be very stable and reliable. Consequently, the battery assembled using the hydrogel electrolyte could be used to power a luminescent panel even with a 3 kg load on top of it. It was also demonstrated that the flexible sensor powered by our compressible battery exhibited similar and stable sensory signals compared with the same sensor powered by 2 commercial alkaline batteries. Furthermore, due to the excellent mechanical property of our battery, a smart wristband fabricated by integrating two battery packs and the flexible piezoresistive sensor could be powered and used to monitor the pressure exerted, demonstrating the battery’s potential as the wearable power source for the flexible and wearable devices. Keywords: Rechargeable Zn-MnO2 battery, compressible devices, flexible devices, batterysensor system, polyacrylamide hydrogel electrolyte, multi-functional energy storage devices 1. Introduction The development of the flexible and wearable electronics, especially those with tolerance to various types of deformations, such as stretching, compressing and shearing, without significant loss of performances have been attracting increasingly research attention due to their intriguing applications in different fields.1-8 Integrated power source with high energy storage capability and intrinsic endurance to the exerted strain becomes increasingly important catering for the application of those electronic devices.9-11 Among those, the stretchable energy


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storage devices, represented by the stretchable supercapacitors and rechargeable batteries with various configurations, have been receiving significant research attentions and tremendous achievements have been made in the recent years.12-17 In contrast, however, relatively limited research attention has been attracted on developing highly compressible energy storage devices.18-20 In addition, the insufficient energy density offered by the supercapacitors has indeed affected their applications for powering high-performance electronic devices so that developing alternative energy storage device with both high energy storage capabilities and compressibility is a necessity.21, 22 Polymer hydrogels consisted of elastic crosslinked polymer chains filled with fluidic water solvent are receiving significant research attention in biomedical applications,23-27 ionic conductors,28, 29 optical fibers,30, 31 and sensors,32-34 as well as electrolyte materials for flexible energy storage devices18, 35-37 due to their intriguing properties. Compared with conventional hydrogels, after crosslinking the hydrogels usually gain effective enhancement in their mechanical properties.38,

39

On one hand, due to the synergistic effect between the strong

covalent bonding and the weak reversible interactions, such as ionic bonding and H-bonding, the crosslinked hydrogels are able to exhibit elastomeric-like performance. On the other hand, the superior water absorption capability of the hydrogel materials endows the materials with potentially attractive ionic conductivity when appropriate electrolytic salts are added. Therefore, those intriguing features of crosslinked hydrogels make them ideal electrolyte



materials for flexible and wearable energy storage devices. Additionally, the rechargeable Znion chemistries with various configurations, such as the Zn-MnO240-42 and the Zn-V2O543, 44 systems, are receiving increasing research interests due to their superior safety performance, the use of the low cost and earth abundant electrode materials and high energy storage capabilities. What’s more, by virtue of the aqueous electrolyte utilized, the Zn-ion chemistry shows high compatibility when using hydrogel as electrolyte.45 Herein, a rechargeable Zn-MnO2 battery with good energy storage capability and highly mechanical compressibility benefiting from the use of crosslinked PAAm hydrogel electrolyte was proposed as promising wearable energy storage device. Interestingly, the battery exhibited good tolerance against mechanical compressional strain and the energy storage capability increased with increasing strain due to the improved contact between electrode and electrolyte, as well as the enhanced ionic conductivity of the PAAm hydrogel. More importantly, the compressible battery showed highly stable and durable performances, such as charge/discharge characteristics, specific capacity as well as open circuit voltage, against different types of compression. Even under a heavy load, the battery could work normally to light a luminescent panel. Additionally, by virtue of the superior electrochemical performances and the intrinsic compressibility, when integrated with flexible piezoresistive sensor to fabricate smart wristband, the wearable device could be used to detect and monitoring pressure with different magnitude and frequencies.


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2. Experimental sections 2.1 Fabrication of the crosslinked hydrogel electrolyte In a typical fabrication, 4.0 g of monomer acrylamide and 30.0 mg ammonium persulfate ((NH4)2S2O8) (initiator) together with 5.0 mg N,N′-methylenebis(acrylamide) (crosslinker) were added into 20 ml distilled water under continuous magnetic stirring till the formation of a clear solution. Then the as-prepared solution was transferred into a glass petri dish with desired size wrapped by aluminum foil and then polymerized at 60 oC for 60 min through freeradical polymerization. After successful polymerization, the PAAm hydrogel was taken out from the glass mold and soaked in the electrolytic solution containing 1 mol L-1 ZnSO4 and 0.1 mol L-1 MnSO4 overnight for the completion of ion exchange process. 2.2 Fabrication of the Zn anode material Metallic zinc was electrodeposited on the surface of the graphite paper using three-electrode configuration reported previously.41 In a typical fabrication, Zn was electrodeposited using potentiostatic method at -1.4 V in the aqueous electrolyte containing 0.2 mol L-1 ZnSO4 and 0.5 mol L-1 Na3C6H5O7 with graphite paper, platinum (Pt) and Ag/AgCl as working, counter and reference electrodes, respectively. The mass of the Zn that deposited on the graphite paper current collector could be well-controlled by adjusting the deposition time. After that, the Zn@graphite anode was rinsed by deionized water and then dried prior to use. 2.3 Fabrication of the MnO2 cathode material



Alpha-MnO2 was synthesized using the hydrothermal method reported elsewhere.42 In a typical fabrication, 0.03 mol MnSO4 and 2 ml 0.5 mol L-1 H2SO4 were mixed with 90 ml deionized water under continuous magnetic stirring. After that, 20 ml 0.1 mol L-1 KMnO4 aqueous solution was gradually added into the abovementioned solution and continued stirring for another 2h. Then the dark purple solution was transferred into Teflon-lined autoclave and kept at 120 oC for 12 h. The brown-colored product was collected and rinsed by distilled water through centrifugation till the pH value of the supernatant reaching neutral and then dried in vacuum prior to use. The cathode material was prepared by typical slurry method using asprepared MnO2 mixed in N-Methyl-2-pyrrolidone (NMP) with the conductive acetylene black (AB) and polyvinylidene fluoride (PVDF) as conductive agent and binder at a weight ratio of 7:2:1 and then coated onto the graphite paper. The average loading mass of the active material on the electrode was controlled in the range of 1-3 mg cm-2. After that, the as-prepared cathode material was transferred to oven and dried at 40 oC overnight prior to use. 2.4 Electrochemical test and materials characterization The compressible Zn-MnO2 battery was fabricated by assembling the anode and cathode with the PAAm hydrogel sandwiched in between without the use of separator. The redox electrochemistry and the ionic conductivities of the as-prepared electrode and electrolyte materials were evaluated by the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), respectively, using a CHI760e electrochemical workstation in the either


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aqueous solution containing 1 mol L-1 ZnSO4 and 0.1 mol L-1 MnSO4 or PAAm hydrogel-based electrolyte. Apart from that the charge/discharge characteristics and the cycling stability of the electrode materials as well as the assembled Zn-MnO2 battery were evaluated by the LANHE battery tester. The sensory test was performed on the Stanford Research System MODEL SR 570 Low-Noise Current Preamplifier and viewed via LabVIEWTM system. The field-emission scanning electron microscopy (FESEM Quanta FEG 450) and the transmission electron microscopy (TEM CM20) were used to study the morphological information of the as-synthesized electrode and electrolyte materials. The X-Ray diffractometer (XRD) (Bruker D2 Phaser) was used to collect the crystallographic information of the MnO2 synthesized by hydrothermal method. In addition, the mechanical property of the PAAm hydrogel was tested by the 'MTS, USA' Alliance RT 30kN Electro-Mechanical Materials Tester.



Figure 1. Compressibility of the PAAm gel electrolyte. (a) images showing the elasticity of the PAAm gel under compressional and relaxed states; (b) SEM image showing the hierarchical porous structure of the freeze-dried PAAm gel, scale bar, 20 μm; (c) stressstrain correlation of the PAAm gel electrolyte during 5 consecutive compression cycles to 80% strain value; (d) electrochemical impedance spectra of the PAAm gel electrolyte under different compressional strain values from 0 to 77.8%, respectively; (e) resistance values of the PAAm gel electrolyte under different compressional strain values from 0 to 77.8%, respectively; (f) and (g) pictures showing the conductivity of the PAAm gel electrolyte under relaxed and compressional states with the ability to light a yellow LED bulb.

3. Results & discussion As shown in Figure 1 (a), the crosslinked PAAm hydrogel exhibited an elastomeric behavior when subjected to large compressional strain, in which the hydrogel showed high elasticity upon compression and releasing. This is predominately due to the existence of the crosslinked polymeric chain network and the rich hydrophilic functional groups that trapped the water


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molecules within the framework by H-bonding.38, 39, 46 Due to the porous polymeric framework and the rich surface functional groups, the mobile water molecules tend to bind with the polymers forming hydration state. This will potentially lead to the difference in the osmotic pressure between the mobile water and the bonded water, resulting in its superior water absorption performances. The strong covalent bonding offered mechanical support to the framework and meanwhile the H-bonding provided with recoverability to the hydrogel when subjected to deformations, which are different with many other hydrogels ever reported. The polymeric framework could be clearly observed in the SEM image of the freeze-dried hydrogel in Figure 1 (b), the largely existed hierarchical structural porosity of the hydrogel was favorable for trapping significant amount of water. The superior water absorbency and the crosslinking of PAAm are the major reasons that account for the mechanical elasticity and the good ionic conductivity of the hydrogel. It could be observed in Figure 1 (c) that, when subjected to a compressional strain reciprocating strain from 0 to 80% perpendicular to the plane of material, the stress-strain curves during the five consecutive compressional cycles exactly overlapped, indicating its highly elastic nature. More interestingly, as shown in Figure 1 (d) and (e), the ionic conductivities of the PAAm hydrogel electrolyte showed strong correlation with the applied strain, as revealed by the EIS measurements. The resistance values of the hydrogel decreased from 36.84 to 7.12 Ω with the compressional strain increased from 0 to 77.8%. Furthermore, a threshold value of strain, which potentially causes dramatic decrease of



resistance, exists when the compressional strain increased to 22.2%. It could be seen from the Figure 1 (d) and (e) clearly that the hydrogel under 0 to 22.2% compressional presented comparable value of resistance between 34.88 and 36.84 Ω, whereas the value sharply decreased to 20.13 followed by 13.45 Ω when the compressional strain increased to 33.3 % and 44.4%, respectively. It is believed that the drop of resistance might be mainly caused by the shortened ionic transport distance as the compressional strain increases, which brings about improved ionic migration efficiency favorable for the electrochemical reaction. By virtue of its high ionic conductivity and intrinsic compressibility, when utilized as ionic conductor, as shown in Figure 1 (f) and (g), the hydrogel electrolyte could provide with effective conductive support to power a yellow LED bulb even with heavy load on top of it.


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Figure 2. Electrochemical performances of the rechargeable Zn-MnO2 battery using the asprepared PAAm gel electrolyte. (a) CV of the rechargeable Zn-MnO2 battery at 5 mV s-1 scan rate in PAAm gel electrolyte; (b) the initial 2 cycles of charge-discharge curves of the rechargeable Zn-MnO2 battery in PAAm gel electrolyte at 1 C; (c) the charge-discharge characteristic curves of the battery at different charge/discharge rates from 1 to 5 C, respectively; (d) the cyclic stability test performance of the battery for 1000 cycles at 4 C charge/discharge rate in PAAm gel electrolyte. The electrochemical performances of the rechargeable Zn-MnO2 battery using the asprepared crosslinked PAAm hydrogel electrolyte was presented in Figure 2. The Zn that electrodeposited onto the graphite paper electrode exhibited a nanosheets structure with lateral size about 5–10 μm and thickness of most of the nanosheets less than 100 μm, which could be observed in Figure S1 in the supporting information. The hydrothermal synthesized MnO2 showed nanorod morphology, as presented by the SEM and TEM image in Figure S2 (a) and



(b). XRD pattern in Figure S3 revealed that the as-prepared MnO2 nanorods could be wellindexed to alpha-phase MnO2 (JCPDS: 44-01041). The length of individual nanorod is generally about 3–4 μm with the diameter of each of the nanorod less than 50 nm, which was then mixed thoroughly with the acetylene black and PVDF to form the cathode paste shown in Figure S4. The high aspect ratio Zn nanosheets and its structural porosity may facilitate the electrolyte penetration and offer shorter path for ion transport, whereas the alpha-MnO2 provided with high theoretical capacity of 308 mAh g-1 based on the reaction shown below:42, 47

6MnO2 + 3Zn + H2O + ZnSO4

6MnOOH + ZnSO4[Zn(OH)2]3⋅xH2O

The CV scan at 5 mV s-1 from 0.8–1.9 V of the battery, as shown in Figure 2 (a) revealed two major redox peaks at around 1.78 and 1.3 V accompanied with one minor peak at around 1.1 V with the PAAm hydrogel-based electrolyte. On the other hand, the CV scan in the aqueous electrolyte at 2 mV s-1 in Figure S5 (a) revealed two pairs of redox peaks at 1.7–1.8 V and 1.0– 1.2 V, respectively, due to the slower scan rate and the preferred ionic conductivity in the aqueous solution. Figure 2 (b) and Figure S5 (b) showed the initial two cycles of charge/discharge characteristic curves of the Zn-MnO2 batteries using either the PAAm-based hydrogel or the aqueous solution as electrolytes at 1 C current density, respectively. It was found that two plateaus appeared in the discharge curves at 1.25–1.4 V and 1.15 – 1.25 V, respectively, in both the electrolytes, which were in general consistent with the position of the


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redox peaks obtained in CV studies. Moreover, when charged/discharged at 1 C, the battery delivered a maximum specific capacity of 277.5 mAh g-1 at the 1st cycle in the liquid electrolyte and the maintained 261.1 mAh g-1 at 2 cycle, due to the activation effect. When tested in the hydrogel electrolyte, the specific capacity of the battery maintained to be 230.5 mAh g-1 and 213.4 mAh g-1 at the 1st and 2nd cycles, respectively. In addition to that, it was observed that there were about 300 mV overpotential in both of the charge/discharge curves, which were consistent with the result obtained by Pan et al.42 Figure 2 (c)-(d) and Figure S5 (c)-(d) showed the rate performances and cyclic stability of the battery in the hydrogel and aqueous electrolytes, respectively. It could be observed that, at 1–5 C discharge, the battery using aqueous electrolyte delivered specific capacities of 252.9, 199.2, 175.1, 154.9 and 137 mAh g-1, respectively. Meanwhile, in the hydrogel-based electrolyte, the discharge capacities of the battery obtained 230.5, 161.6, 128.2, 107.9 and 94.8 mAh g-1, respectively. Furthermore, upon cycling at 4 C, initially the battery exhibited a relatively low Coulombic efficiency around 80% due to the activation of electrode materials and the rearrangement of the atoms to accommodate the charge/discharge process and then quickly increased to nearly 100%. It was observed that in the aqueous electrolyte, the battery exhibited an initial discharge capacity of 154.9 mAh g-1 and maintained 90.7 mAh g-1 over 500 cycles of charge/discharge with almost 100% Columbic efficiency throughout the test. When tested using the PAAm hydrogel electrolyte at 4 C, the battery delivered a specific capacity of



136.2 mAh g-1 and maintained 94 mAh g-1 after 1000 cycles of charge/discharge, corresponding to 69.02% capacity retention, surpassing that in the aqueous electrolyte. This result indicated that the use of PAAm hydrogel electrolyte could effectively improve the cyclic stability of the battery. It was found that the PAAm hydrogel can effectively adhere the electrode materials, i.e. the MnO2, which subjects to gradual dissolution in aqueous electrolyte. The corrosion and the detachment of the electrode materials could be also alleviated leading to improved cyclic stability. Figure 3 showed the compressibility of the rechargeable Zn-MnO2 battery using the PAAm hydrogel electrolyte. As shown in Figure 3 (a) that, after getting stabilized within the initial activation cycles, the battery can work normally under 25% strain exerted during 30 cycles of charge/discharge at 1 C while maintaining nearly 100% Columbic efficiency with negligible performance degradation. Moreover, the charge/discharge curves at the 1st, 15th and 30th cycles also revealed that the battery delivered comparable specific capacity about 240 mAh g-1 and the charge/discharge curves remained nearly unchanged. Moreover, under reciprocally compressional strain from 0 to 20% during charge/discharge at 1 C, which could be observed in Figure 3 (c) and (d), the charge/discharge characteristics of the battery remained stable and the specific capacities even slightly increased from 240 to 270 mAh g-1. The phenomenon is further confirmed when the compressional strain exerted increased from 40 to 80%, as shown in Figure 3 (e) and (f), in which the specific capacity increased to nearly 290 mAh g-1 and


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remained highly stable. Giving the EIS measurements, the phenomenon was ascribed to the improved contact between the electrode and electrolyte and greatly reduced internal resistance for ion migration caused by the compressional strain, leading to improved electrochemical



Figure 3. Compressibility of the rechargeable Zn-MnO2 battery with PAAm gel electrolyte. (a) cyclic stability of the battery at 25% compressional strain for 30 cycles tested at 1 C; (b) the charge-discharge curves of the Zn-MnO2 battery at 1st, 15th and 30th cycles under 25% compressional strain at 1 C in PAAm gel electrolyte, respectively; (c) the charge-discharge curves of the battery at cyclic compressional strain of 0 and 20% for 7 consecutive cycles at 1 C, respectively; (d) the specific capacities of the battery under cyclic compressional strain from 0 to 20%; (e) the charge-discharge curves of the battery under strain values increased from 40% to 80% and then return to 40%, respectively; (f) specific capacities of the battery at strain value from 40% to 80% and then return to 40%; (g) the voltage stability of the battery under finger press; (h) voltage stability of the battery under continuous hammering.


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performances.18 The compressibility and the durability of the battery were further evaluated by human finger pressing and hammering on the battery. The open circuit voltage (OCV) was monitored simultaneously, as shown in Figure 3 (g) and (h). It was found that the OCV of a freshly prepared battery remained unaffected by finger pressing and even slightly increased from 1.59 to 1.61 V within 300 seconds. In the magnified curve in the inset figure of Figure 3 (g), it could be noticed that the finger presses only caused small fluctuation of the voltage in the range of ~4 mV and then recovered quickly when the force removed. Moreover, fast hammering on the battery also testified the stability of the OCV, as shown in Figure 3 (h) and its inset figure. It was observed that the OCV of the battery kept highly stable around 1.62 V within 60 seconds and the fast hammering on top of the battery only caused voltage fluctuation in the range of 1 mV, demonstrating it excellent stability against compressional force. To the best of our knowledge, those results are already the best ones among the published results utilizing spongetype electrode materials. What is more, due to the use of intrinsically compressible hydrogel electrolyte, the proposed compressible battery can potentially retain higher energy storage capabilities since the capacity loss resulting from separation of the electrode and electrolyte is negligible.48-51 To further demonstrate the energy storage capability and the compressibility of the assembled battery, two battery packs were fabricated and connected in series to power a



luminescent panel using an inverter (works at about 3V). As shown in Figure 4 (a)–(c), the luminescent panel could be powered using the two assembled battery pack and the picture on the lighted panel could be clearly viewed. Moreover, when a heavy load about 3 kg was placed on top of the battery packs, the batteries could still power the luminescent panel, indicating their stability for practical applications. In addition, a flexible battery-piezoresistive sensor system was developed and utilized to evaluate the stability of our compressible Zn-MnO2 battery. Before being integrated, compared with the commercially available alkaline batteries with the similar OCV, as shown in Figure 4 (d), the sensor powered by 2 compressible batteries connected in series generated similarly stable sensory signals under 2 Hz finger press frequency. When subjected to heavy load on top of the compressible batteries, the signals generated by the flexible sensor maintained invariant within the same testing time, indicating the stable output performance of our compressible Zn-MnO2 battery. By virtue of the excellent mechanical flexibility and compressibility of our battery, two batteries were connected in series and the flexible piezoresistive sensor was adhered on the backside of the two batteries. In this way, the flexible battery-piezoresistive sensor system was developed as a smart wearable wristband for pressure monitoring, as shown in Figure 4 (e)–(g). Figure 4 (e) shows that the as-fabricated wristband could be wore on human wrist without discomfort. What’s more, the flexible piezoresistive sensor powered by the batteries can work properly to detect the pressure exerted on the wristband. The difference in the magnitude of the pressure could be clearly


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resolved as reflected by the magnitude of the current signal, as shown in Figure 4 (f). Apart from that, the piezoresistive sensor could also detect the pressure signal with different frequencies from 0.3 to 4 Hz, respectively, as shown in Figure 4 (g). The performance of the smart wristband could be ascribed to the good energy storage capabilities and the intrinsic compressibility that allow the battery to output power reliably and stably even under



Figure 4. (a) - (c) Optical images showing that two rechargeable Zn-MnO2 battery with PAAm gel electrolytes could be used for powering a luminescent panel under normal condition and with a 3 kg load on it; (d) comparison between the signals generated by the flexible sensor powered by the commercially available alkaline batteries and our compressible batteries without and with load on top of it; (e) flexible smart wristband integrated from two ZIB module and a flexible pressure sensor; (f) sensory signals of the smart wristband generated by human finger touch under different pressures on the device; (g) sensory signals of the smart wristband generated at different frequencies, from 0.3 Hz to 4 Hz respectively, by human finger touch. compressional strain benefiting from the PAAm hydrogel electrolyte utilized. It was also


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demonstrated that the proposed Zn-MnO2 battery with PAAm hydrogel electrolyte could be potentially applied as wearable energy storage device for powering wearable electronics. 4. Conclusions To conclude, in this work, a highly compressible and rechargeable Zn-MnO2 battery was proposed utilizing intrinsically compressible PAAm hydrogel electrolyte, delivering not only superior energy storage performances but also the excellent compressibility and durability. Interestingly, due to the crosslinked polymeric chain and the abundant H-bonding, the hydrogel could absorb significant amount of water and provide high ionic conductivity as well as superior mechanical properties. Consequently, the rechargeable Zn-MnO2 battery assembled using the as-prepared PAAm hydrogel electrolyte delivered a maximum specific capacity of 277.5 mAh g-1 at 1 C and a stable cyclic performance over 1000 times at 4 C. The correlation between the ionic conductivity of the hydrogel electrolyte and the electrochemical performances of the assembled battery showed consistency with the compressional strain. More importantly, the electrochemical performances, such as the charge/discharge characteristics and the discharge capacity as well as the open circuit voltage of the battery, presented high stability and durability against various types of compressional strains. When being used to power a luminescent panel, the two battery packs worked normally even under 3 kg load on top of it. It was also demonstrated that our compressible battery exhibited similarly stable output performances within the time duration of testing compared with the commercially available alkaline battery when it is used to power a flexible piezoresistive sensor. By virtue of



the good energy storage performances and the intrinsic compressibility, a smart wristband assembled with the compressible batteries and the flexible piezoresistive sensor can effectively detect the magnitude and frequencies of the pressure applied. The demonstrated highly durably and stable compressible Zn-MnO2 battery, as well as the battery-pressure sensor flexible wrist band showed promise as wearable electronics in the future. 5. Supporting Information The characterization of electrode materials, such as the electrodeposited Zn and the MnO2 nanorods, including SEM, TEM and XRD. The electrochemical performances of Zn-MnO2 battery in aqueous electrolyte. Molecular structure of crosslinked PAAm hydrogel. Table shown the comparison of electrochemical performances with other recently reported Zn-ion batteries. This information is available free of charge via Internet at http://pubs.acs.org/. 6. Acknowledgement This research was supported by GRF under Project N_CityU 11305218, the Science Technology and Innovation Committee of Shenzhen Municipality (the Grant No. JCYJ20170818103435068) and the Science and Technology Program of Guangdong Province of China (Grant No. 2017B050504004). 7. References 1. Hou, C.; Wang, H.; Zhang, Q.; Li, Y.; Zhu, M., Highly Conductive, Flexible, and Compressible All ‐ Graphene Passive Electronic Skin for Sensing Human Touch. Advanced materials 2014, 26, 5018-5024. 2.

Kim, D.-H.; Ghaffari, R.; Lu, N.; Rogers, J. A., Flexible and stretchable electronics for


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A Highly Compressible Crosslinked Polyacrylamide Hydrogel Enabled

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