Abstract
1. Introduction
The flexible electronic devices such as artificial electronic skins[
In this review, we consider different design strategies aiming at preparing flexible ZIBs with the outstanding performances based on proposed literatures. Meanwhile, the developments of flexible current collectors, electrode materials and electrolytes are stated. We then discuss various practical application scenarios of flexible ZIBs in recent reports. Furthermore, the performance comparison of various flexible ZIBs is summarized to clearly state the promising directions, facing challenges and improved alternatives. Finally, key scientific problems limiting the further development of flexible ZIBs are clarified and the corresponding solutions are also proposed.
2. Schematic designs of flexible ZIBs
Flexible ZIBs were normally equipped with the property of the resistance to external deformation. What it means was that under external forces from all directions, the electrochemical performance of flexible ZIBs was not affected or very weak. For this purpose, every component of flexible ZIBs including anode, cathode and electrolyte need to exhibit the property of softness. However, parts of traditional batteries generally were constructed employing rigid/fragile materials. Therefore, from electrodes to electrolytes, they were all necessary to design and prepare in reasonable way. In addition, the overall structure of the flexible ZIBs also need to be taken into consideration.
2.1. Flexible electrodes for flexible ZIBs
As for anode of flexible ZIBs, there were two approaches to reach this target. Firstly, thin Zn foil and Zn wire were directly used to be anode of flexible ZIBs[
Figure 1.(a) SEM image of the zinc anode by electrically depositing onto a carbon cloth. Adopted with permission from Ref. [
With respect to cathode of flexible ZIBs, similar to anode bonding with flexible substrates, loading cathode materials on flexible current collectors was a common way to acquire flexible cathodes. In general, conductive (such as carbon black, acetylene black and carbon nano tube) and binding additives (such as carboxymethyl cellulose, polyvinylidene fluoride and polytetrafluoroethylene) were applied to mix with cathode materials. Then, the mixed slurry was coated on the surface of conductive substrates (such as carbon cloth, carbon paper and metal wires) with flexibility. Finally, flexible electrode plate after drying was obtained. Nevertheless, this approach existed some drawbacks. For example, the transport of electrons and ions would be limited due to the existence of conducting additives and binders. Apart from that, under external forces, cathode materials fell off flexible substrates as a result of limited contact area among them. Based on these, many researchers paid attention on in situ growing method. In other words, active materials were in situ formed on the corresponded flexible substrates through hydrothermal method, electrodeposition and so on. Fig. 1(b) displayed the SEM image of the MnO2/rGO sample on carbon cloth by in-situ growing method, revealing excellent flexibility of cathode part[
Recently, researchers tended to use flexible current collectors as flexible basis and then the active materials were integrated on their faces to prepare flexible electrodes. With regard to flexible substrates, there were two choices. The one was metal-based current collectors such as titanium or stainless-steel wires. The other was carbon-based substrates such as carbon cloth, carbon nanotube (CNT) fiber or CNT paper.
2.2. Flexible electrolytes for flexible ZIBs
As a general rule, flexible ZIBs employed hydrogel electrolytes incorporating into different salt solution such as ZnSO4, ZnCl2, Zn(ClO4)2, Zn(NO3)2 and Zn(CF3SO3)2. The hydrogel electrolyte could be classified into two categories: i) natural polymers. ii) synthetic polymers.
2.2.1. Natural polymer hydrogel electrolytes
Natural polymers, e.g., gelatin, cellulose and sodium alginate, had excellent physicochemical properties that afforded fabrication of advanced hydrogel electrolytes for flexible electronic devices: non-toxicity, hydrophilicity, thermodynamic stability, the high capacity for swelling for high ion conductivity and simple craft. Therefore, abundant natural gel electrolytes had been reported to assemble flexible ZIBs. Zhi et al. adopted a gelatin-based natural polymer hydrogel electrolyte to fabricate flexible ZIBs that had excellent resistance to deformation, namely, after cutting 4 times, bending 800 times, hammering 5 times and other deformations, flexible ZIBs still kept excellent electrochemical performance[
Figure 2.(Color online) (a) The process diagram of SA-based hydrogel electrolyte. Adopted with permission from Ref. [
2.2.2. Synthetic polymer hydrogel electrolytes
In fact, most flexible gel electrolytes were chemically synthesized polymer hydrogels. This was ascribed to be able to carry on the structure design reasonably so that flexible hydrogel electrolytes were applied to a variety of scenarios. In addition, chemically synthesized polymer hydrogels could be divided into two categories: i) chemically cross-linked hydrogels; ii) physically cross-linked hydrogels. In general, the use of chemically crosslinked hydrogel electrolytes in flexible ZIBs was a mainstream trend such as common polyacrylamide (PAM)-based hydrogel. On the basis of superior interface compatibility and highly water content, PAM-based polymer electrolytes had been widely used in ZIBs. Numerous amide groups (-CONH2) and network structure were beneficial for ion mobility. Zhi et al. reported a quasi-solid-state washable and tailorable elastic yarn ZIBs on the basis of PAM polymer electrolyte (Fig. 2(b)) with ion conductivity of 17.3 × 10–3 S/cm at room temperature[
Although chemically crosslinked hydrogel electrolytes had met most application scenarios, physically crosslinked hydrogel electrolytes had unique advantages for flexible ZIBs under certain circumstances. Just as its name implies, physically crosslinked hydrogels were synthesized through physical interaction including van der Waals' force, hydrogen bonding and electrostatic interactions. The most common example was poly(vinyl alcohol) (PVA)-based hydrogel. Fig. 2(c) delivered a self-healing PVA hydrogel electrolyte with high ion conductivity for flexible ZIBs. After multiple cutting/self-healing cycles, the flexible ZIBs still exhibited stable specific capacity of 81.4 mA·h/g[
2.3. All-in-one flexible ZIBs
Apart from pursing flexible electrodes and electrolytes, the flexible ZIBs could be fabricated using all-in-one strategy of structural designing. Niu et al. reported a scalable assembly strategy to prepare flexible ultrathin ZIBs via all-in-one integrated architecture. The acquired flexible ultrathin ZIBs could be controllably tailored and edited into desired shapes and structures, and the tailored miniature flexible ZIBs still exhibited great electrochemical performance[
Figure 3.(Color online) (a) The schematic process of design and (b) the cycle performance of ultrathin all-in-one ZIBs. Adopted with permission from Ref. [
3. Functional application of flexible ZIBs
Flexible ZIBs had been reported into applying in many situations including mechanical practicability (stretching, compressing, bending and folding), self-healing, low temperature, smart transformation and others. In these categories, there were many interesting findings found by researchers. The details would be introduced below.
3.1. Mechanical performance
The mechanical practicality was the most basic requirement of flexible ZIBs and researches were widely focused on the mechanical properties in the past few years. Zhi et al. firstly reported good compressive performance of PAM-based hydrogel electrolyte[
Figure 4.(Color online) (a) Illustrations of the Zn-MnO2 battery i) being placed under foot and ii) going through car run-over. (b) Discharge curve of the battery after 2 days' everyday treading. (c) Discharge curve of the battery after 20 times of random run-over by cars on road. All the discharge curves were recorded at 0.924 A/g (3C rate). Adopted with permission from Ref. [
3.2. Self-healing performance
Conventional batteries lost electrochemical performance when damaged. Thus, automatic repair of damaged batteries without affecting the performance of the battery itself was extremely important. Especially for flexible batteries, they were easily damaged when using in many fields. Herein, self-healing performances of various flexible ZIBs were summarized. Zhi et al. employed self-healing carboxylated-polyurethane as the substrate for electrodes to assemble flexible self-healing Zn-MnO2 battery[
Figure 5.(Color online) (a) Cycling performance of the obtained flexible Zn-MnO2 battery before healing and after fourth healing. (b) Demonstration of a self-healing flexible Zn-MnO2 battery powering an electric watch before and after cutting and after healing. Adopted with permission from Ref. [
3.3. Low temperature performance
Due to the freezing, low ion conductivity and slow dynamics of aqueous electrolyte at low temperature, the aqueous metal ion batteries occurred the loss of capacity and power with the drop of temperature. Improving low temperature performance of aqueous batteries including flexible ZIBs had attracted the attention of scientists. Zhi et al. synthesized ethylene glycol-based waterborne anionic polyurethane acrylates (EG-waPUA) and then copolymerized EG-waPUA precursor and AM monomers to fabricate an EG-waPUA/PAM based dual crosslinked hydrogel. Next, the antifreezing Zn-MnO2 batteries (AF-battery) on the basis of the hydrogel were assembled and delivered a high specific capacity of 226 mA·h/g at 0.2 A/g at –20 °C[
Figure 6.(Color online) (a) The demonstration of AF-battery powered a series of electronic devices. Adopted with permission from Ref. [
3.4. Others
Apart from mechanical practicality, self-healing performance and working at low temperature, a series of flexible ZIBs particularly designed for special situations were studied and recorded. This expanded the application range of flexible ZIBs. Zhi et al. firstly reported a smart safe rechargeable flexible ZIBs based on sol-gel transition electrolytes. With the help of thermal-stimulus responsive polymer of poly(N-isopropylacrylamide) (PNIPAM), the synthesized hydrogel delivered a smart reaction of flexible ZIBs, namely, over a critical temperature, the polymer chains precipitated out of solution, resulting in stopping work of flexible ZIBs. Fig. 7(a) exhibited the process of the transformation. When the temperature was lower than the critical temperature, the battery restored its original state and worked again without any changing[
Figure 7.(Color online) (a) The process of the smart reaction of flexible ZIBs when temperature changes. Adopted with permission from Ref. [
4. Challenges and perspectives
From above review, reported flexible ZIBs exhibit superior flexibility, self-repairability, multiple environmental adaptation, intelligence and ideal electrochemical performance including specific capacity and cyclic stability. This lays a solid foundation for practical application of flexible ZIBs in the future. However, the practical application of flexible ZIBs is not yet commercialized owing to several challenges and issues discussed below. In order to develop practical flexible ZIBs that can be fabricated in a large scale, overcoming or alleviating the following difficulties would be especially vital.
(1) High-performing flexible current collectors and electrodes. At present, widespread-usage flexible current collectors are metal foil and mesh including titanium and stainless-steel wires or carbon-based materials including carbon paper and cloth. Nevertheless, because of memory effect of metal materials and finite elongation of carbon materials, the metal- and carbon-based current collectors do not fully meet demand in some situations. Therefore, other flexible current collectors are also researched and developed. Although some flexible conductors such as indium tin oxide (ITO) have been developed[
(2) Flexible hydrogel electrolyte with wide operation potential window. The battery discharging plateau of aqueous ZIBs is confined because of the narrow electrochemically operation potential window of aqueous electrolyte. Therefore, in order to prepare flexible ZIBs with high discharging voltage, the hydrogel electrolyte with wide operation voltage window is inevitable. Up to now, the hydrogel with high concentrated salts contained provides a potential candidate for achieving high voltage[
5. Conclusion
In summary, flexible ZIBs have been fabulously fashionable and generally researched since 2015. The development of flexible ZIBs is still in its infancy even if there are some evolutions to some extent. On the one hand, flexible current collectors or flexible electrodes need to be further developed to make flexible ZIBs suitable for more practical scenarios. On the other hand, flexible hydrogel electrolytes with wide operation potential window are extremely significant to assemble high-voltage flexible battery. Therefore, it is necessary to overcome and battle these key issues. Only if these two challenges are solved, can we carry flexible ZIBs forward further. In this review, we have implemented a brief discussion on the challenges and perspectives existed in the development of flexible ZIBs. We also propose a direction that need to be further researched in the future so that flexible ZIBs can make a step closer to commercial application.
Acknowledgements
This research was supported by the National Key R&D Program of China under Project 2019YFA0705104.
References
[1] Y C Cai, J Shen, C W Yang et al. Mixed-dimensional MXene-hydrogel heterostructures for electronic skin sensors with ultrabroad working range. Sci Adv, 6, eabb5367(2020).
[2] Z Wang, D M Fu, D Z Xie et al. Magnetic helical hydrogel motor for directing T cell chemotaxis. Adv Funct Mater, 31, 2101648(2021).
[3] R D Rodriguez, S Shchadenko, G Murastov et al. Ultra-robust flexible electronics by laser-driven polymer-nanomaterials integration. Adv Funct Mater, 31, 2008818(2021).
[4] K Wu, J H Huang, J Yi et al. Recent advances in polymer electrolytes for zinc ion batteries: Mechanisms, properties, and perspectives. Adv Energy Mater, 10, 1903977(2020).
[5] P Yu, Y X Zeng, H Z Zhang et al. Flexible Zn-ion batteries: Recent progresses and challenges. Small, 15, 1804760(2019).
[6] Q Yang, Y K Wang, X L Li et al. Recent progress of MXene-based nanomaterials in flexible energy storage and electronic devices. Energy Environ Mater, 1, 183(2018).
[7] Z S Song, J Ding, B Liu et al. A rechargeable Zn-air battery with high energy efficiency and long life enabled by a highly water-retentive gel electrolyte with reaction modifier. Adv Mater, 32, 1908127(2020).
[8] F N Mo, Q Li, G J Liang et al. A self-healing crease-free supramolecular all-polymer supercapacitor. Adv Sci, 8, 2100072(2021).
[9] D H Wang, J F Sun, Q Xue et al. A universal method towards conductive textile for flexible batteries with superior softness. Energy Storage Mater, 36, 272(2021).
[10] Y T Xu, J J Zhu, J Z Feng et al. A rechargeable aqueous zinc/sodium manganese oxides battery with robust performance enabled by Na2SO4 electrolyte additive. Energy Storage Mater, 38, 299(2021).
[11] Q Yang, Y Guo, B X Yan et al. Hydrogen-substituted graphdiyne ion tunnels directing concentration redistribution for commercial-grade dendrite-free zinc anodes. Adv Mater, 32, 2001755(2020).
[12] Z H Yi, G Y Chen, F Hou et al. Strategies for the stabilization of Zn metal anodes for Zn-ion batteries. Adv Energy Mater, 11, 2003065(2021).
[13] P H Chen, W Y Zhou, Z J Xiao et al. An integrated configuration with robust interfacial contact for durable and flexible zinc ion batteries. Nano Energy, 74, 104905(2020).
[14] F Wang, O Borodin, T Gao et al. Highly reversible zinc metal anode for aqueous batteries. Nat Mater, 17, 543(2018).
[15] Z W Guo, Y Y Ma, X L Dong et al. An environmentally friendly and flexible aqueous zinc battery using an organic cathode. Angew Chem Int Ed, 57, 11737(2018).
[16] F Wan, L L Zhang, X Dai et al. Aqueous rechargeable zinc/sodium vanadate batteries with enhanced performance from simultaneous insertion of dual carriers. Nat Commun, 9, 1656(2018).
[17] P Tan, B Chen, H R Xu et al. Flexible Zn– and Li–air batteries: Recent advances, challenges, and future perspectives. Energy Environ Sci, 10, 2056(2017).
[18] F N Mo, G J Liang, Q Q Meng et al. A flexible rechargeable aqueous zinc manganese-dioxide battery working at –20 °C. Energy Environ Sci, 12, 706(2019).
[19] Y Huang, J W Liu, Q Y Huang et al. Flexible high energy density zinc-ion batteries enabled by binder-free MnO2/reduced graphene oxide electrode. npj Flex Electron, 2, 21(2018).
[20] H F Li, C P Han, Y Huang et al. An extremely safe and wearable solid-state zinc ion battery based on a hierarchical structured polymer electrolyte. Energy Environ Sci, 11, 941(2018).
[21] D H Wang, H F Li, Z X Liu et al. A nanofibrillated cellulose/polyacrylamide electrolyte-based flexible and sewable high-performance Zn-MnO2 battery with superior shear resistance. Small, 14, 1803978(2018).
[22] Y Tang, C X Liu, H R Zhu et al. Ion-confinement effect enabled by gel electrolyte for highly reversible dendrite-free zinc metal anode. Energy Storage Mater, 27, 109(2020).
[23] H F Li, Z X Liu, G J Liang et al. Waterproof and tailorable elastic rechargeable yarn zinc ion batteries by a cross-linked polyacrylamide electrolyte. ACS Nano, 12, 3140(2018).
[24] S Huang, F Wan, S S Bi et al. A self-healing integrated all-in-one zinc-ion battery. Angew Chem Int Ed, 58, 4313(2019).
[25] M J Yao, Z S Yuan, S S Li et al. Scalable assembly of flexible ultrathin all-in-one zinc-ion batteries with highly stretchable, editable, and customizable functions. Adv Mater, 33, 2008140(2021).
[26] L T Ma, S M Chen, X L Li et al. Liquid-free all-solid-state zinc batteries and encapsulation-free flexible batteries enabled by in situ constructed polymer electrolyte. Angew Chem, 132, 24044(2020).
[27] Z F Wang, F N Mo, L T Ma et al. Highly compressible cross-linked polyacrylamide hydrogel-enabled compressible Zn-MnO2 battery and a flexible battery–sensor system. ACS Appl Mater Interfaces, 10, 44527(2018).
[28] Z X Liu, D H Wang, Z J Tang et al. A mechanically durable and device-level tough Zn-MnO2 battery with high flexibility. Energy Storage Mater, 23, 636(2019).
[29] J Huang, X Chi, J Yang et al. An ultrastable Na-Zn solid-state hybrid battery enabled by a robust dual-cross-linked polymer electrolyte. ACS Appl Mater Interfaces, 12, 17583(2020).
[30] Y Zhang, Q R Wang, S S Bi et al. Flexible all-in-one zinc-ion batteries. Nanoscale, 11, 17630(2019).
[31] J J Wang, J G Wang, H Y Liu et al. A highly flexible and lightweight MnO2/graphene membrane for superior zinc-ion batteries. Adv Funct Mater, 31, 2007397(2021).
[32] D Wang, L Wang, G Liang et al. A superior δ-MnO2 cathode and a self-healing Zn-δ-MnO2 battery. ACS Nano, 13, 10643(2019).
[33] Y Huang, J Liu, J Q Wang et al. An intrinsically self-healing NiCo||Zn rechargeable battery with a self-healable ferric-ion-crosslinking sodium polyacrylate hydrogel electrolyte. Angew Chem Int Ed, 57, 9810(2018).
[34] J Y Liu, J W Long, Z H Shen et al. A self-healing flexible quasi-solid zinc-ion battery using all-in-one electrodes. Adv Sci, 8, 2004689(2021).
[35] Y Quan, M Chen, W Zhou et al. High-performance anti-freezing flexible Zn-MnO2 battery based on polyacrylamide/graphene oxide/ethylene glycol gel electrolyte. Front Chem, 8, 603(2020).
[36] M S Zhu, X J Wang, H M Tang et al. Antifreezing hydrogel with high zinc reversibility for flexible and durable aqueous batteries by cooperative hydrated cations. Adv Funct Mater, 30, 1907218(2020).
[37] F N Mo, H F Li, Z X Pei et al. A smart safe rechargeable zinc ion battery based on Sol-gel transition electrolytes. Sci Bull, 63, 1077(2018).
[38] J C Zhu, M J Yao, S Huang et al. Thermal-gated polymer electrolytes for smart zinc-ion batteries. Angew Chem Int Ed, 59, 16480(2020).
[39] B Wang, J Li, C Hou et al. Stable hydrogel electrolytes for flexible and submarine-use Zn-ion batteries. ACS Appl Mater Interfaces, 12, 46005(2020).
[40] F N Mo, Z Chen, G J Liang et al. Zwitterionic sulfobetaine hydrogel electrolyte building separated positive/negative ion migration channels for aqueous Zn-MnO2 batteries with superior rate capabilities. Adv Energy Mater, 10, 2000035(2020).
[41] J L Wang, Y R Lu, H H Li et al. Large area co-assembly of nanowires for flexible transparent smart windows. J Am Chem Soc, 139, 9921(2017).
[42] X Wang, J H Zhou, Y Zhu et al. Assembly of silver nanowires and PEDOT:PSS with hydrocellulose toward highly flexible, transparent and conductivity-stable conductors. Chem Eng J, 392, 123644(2020).
[43] Y K Wang, F Chen, Z X Liu et al. A highly elastic and reversibly stretchable all-polymer supercapacitor. Angew Chem, 131, 15854(2019).
[44] R B Choudhary, S Ansari, B Purty. Robust electrochemical performance of polypyrrole (PPy) and polyindole (PIn) based hybrid electrode materials for supercapacitor application: A review. J Energy Storage, 29, 101302(2020).
[45] A Jeyaranjan, T S Sakthivel, C J Neal et al. Scalable ternary hierarchical microspheres composed of PANI/rGO/CeO2 for high performance supercapacitor applications. Carbon, 151, 192(2019).
[46] L Li, Z Lou, W Han et al. Highly stretchable micro-supercapacitor arrays with hybrid MWCNT/PANI electrodes. Adv Mater Technol, 2, 1600282(2017).
[47] Y Wang, C Zhu, R Pfattner et al. A highly stretchable, transparent, and conductive polymer. Sci Adv, 3, e1602076(2017).
[48] Y M Liu, I Murtaza, A Shuja et al. Interfacial modification for heightening the interaction between PEDOT and substrate towards enhanced flexible solid supercapacitor performance. Chem Eng J, 379, 122326(2020).
[49] Y B Li, Z Q Zhou, W J Deng et al. A superconcentrated water-in-salt hydrogel electrolyte for high-voltage aqueous potassium-ion batteries. ChemElectroChem, 8, 1451(2021).
[50] Y Deng, H Wang, K Zhang et al. A high-voltage quasi-solid-state flexible supercapacitor with a wide operational temperature range based on a low-cost “water-in-salt” hydrogel electrolyte. Nanoscale, 13, 3010(2021).
[51] Q Liu, J W Zhou, C H Song et al. 2.2V high performance symmetrical fiber-shaped aqueous supercapacitors enabled by “water-in-salt” gel electrolyte and N-Doped graphene fiber. Energy Storage Mater, 24, 495(2020).
[52] Z X Liu, Q Yang, D H Wang et al. A flexible solid-state aqueous zinc hybrid battery with flat and high-voltage discharge plateau. Adv Energy Mater, 9, 1902473(2019).
[53] W D Pan, Y F Wang, X L Zhao et al. High-performance aqueous Na-Zn hybrid ion battery boosted by “water-in-gel” electrolyte. Adv Funct Mater, 31, 2008783(2021).
Set citation alerts for the article
Please enter your email address