Qingqing
Zheng
a,
Zewei
Hu
a,
Liyang
Liu
a,
Haiying
Lu
a,
Xin
Wang
a,
Yongpeng
Lei
a,
Chao
Han
b and
Weijie
Li
*a
aState Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China. E-mail: li-306@csu.edu.cn
bSchool of Materials Sciences and Engineering, Central South University, Changsha, 410083, China
First published on 12th July 2024
Flexible zinc ion batteries (FZIBs) have garnered significant attention owing to their cost-effectiveness, environmental friendliness, excellent flexibility and advanced security. Nevertheless, the electrochemical performance of FZIBs, such as energy density and cycling life, has yet to be improved compared to that of conventional rigid zinc-ion batteries (ZIBs). Due to the excellent electrical conductivity and mechanical properties exhibited by advanced one or two dimensional (1D/2D) carbon materials, they are increasingly recognized to play a key role in constructing flexible electrodes and improving the capacity, energy density/power density of FZIBs. However, no review comprehensively summarizes the functions and advances of 1D/2D carbon materials in FZIBs to date. In this review, a comprehensive overview of the development background of 1D/2D carbon materials (carbon nanotubes, graphene, MXenes and carbon fiber), highlighting their great advantages and functions of applications in FZIBs is given. Detailed summaries of recent advancements and the current challenges of 1D/2D carbon materials for high-performance FZIBs along with promising strategies are provided. First, the essential requirements and challenges of FZIBs and the fundamental aspects of 1D/2D carbon materials, including the development background and the unique advantages of these 1D/2D carbon materials applied in FZIBs are summarized. Then, the latest developments of these 1D/2D materials in FZIBs, which could function as active materials, conductive networks, current collectors or Zn hosts in FZIBs are discussed. In addition, the application of 1D/2D carbon materials in separators and gel electrolytes is specially emphasized. Finally, the development prospect of 1D/2D carbon materials used in FZIBs is briefly discussed.
As a resource-rich metal in the world, zinc has attracted a lot of attention due to its cheap price, non-toxic and environmentally friendly nature, low redox potential (−0.76 V versus the standard hydrogen electrode (SHE)), and high theoretical capacity (820 mA h g−1 or 5855 mA h cm−3).9–12 Moreover, compared with traditional liquid electrolytes based on organic solvents, aqueous electrolytes display higher biocompatibility and environmental friendliness, and avoid safety concerns.13,14 Thus, ZIBs are suitable for developing flexible batteries for flexible electronic products. However, the employment of aqueous electrolytes is not suitable in flexible batteries due to the potential liquid leakage under deformation.15 In addition, aqueous electrolytes tend to induce the growth of zinc dendrites and the formation of soluble ZnO2 on the zinc anode, which poison the cathode, resulting in rapid capacity attenuation.16–18 Polymer/hydrogel electrolytes are becoming crucial in addressing specific challenges in flexible battery technology due to their relatively low water content. Furthermore, the formation of a solid–solid interface between the electrodes and polymer/hydrogel electrolytes, rather than a solid–liquid interface, significantly enhances the stability of the battery system.19–21 This alteration mitigates the risks associated with dendrite formation and the degradation of cathode materials. However, polymer/hydrogel electrolytes usually possess low ionic conductivity and high interfacial resistance. Therefore, the batteries consisting of polymer/hydrogel electrolytes exhibit poor rate performance and high resistance.22–24 In addition, conventional metal oxide cathodes suffer from inherently low conductivity, which hinders rapid electron transfer. Their mechanical instability also renders them unable to cope with repeated volume changes during the intercalation/deintercalation cycles.13,25 Moreover, when these cathode materials are directly exposed to mild acid electrolytes, it typically leads to the dissolution of active materials, which accelerates the capacity loss.26 Simultaneously, flexible zinc anodes require robust mechanical stability to ensure the preservation of electrochemical performance and structural integrity during deformation such as bending or stretching. However, flexible zinc anodes are often susceptible to mechanical damage or fatigue.27,28 This problem could cause short circuits of batteries. Collectively, these challenges in both cathode and anode materials contribute to suboptimal zinc storage outcomes, manifesting in lower energy density and a restricted cycling life.
To tackle the mentioned challenges and pursue for the high-performance and excellent mechanical properties of FZIBs, it requires the utilization of flexible battery components (e.g., cathodes, anodes and electrolytes).29,30 Over the past decade, 1D/2D carbon materials have increasingly been applied in FZIBs owing to their superior electrical conductivity, mechanical properties, and adjustable porous structures.31,32 These 1D materials include carbon nanotubes (CNTs), carbon fibers (CFs), and 2D materials include graphene and MXenes.33,34 For example, Zhi et al.35 explained the design principles and device performance of flexible fiber-based batteries. Hu et al.36 introduced the design mechanisms of carbon nanotubes (CNTs), carbon fibers (CFs), and other carbon materials in flexible batteries based on thin films and fibers that can be bent. Moreover, previous reviews on FZIBs have primarily focused on gel electrolytes. For instance, He and colleagues reviewed recent advancements in polymer electrolytes, particularly focusing on the synthesis and characterization of gel electrolytes. Their work aimed to provide insights from laboratory research to commercialization.37 Similarly, Li's team systematically discussed and evaluated the preparation methods of functional gel electrolytes for FZIBs from a fabrication perspective.38 Zhang and colleagues introduced the latest research progress and rational design strategies of flexible quasi-solid-state ZIBs from the perspectives of mechanisms, design principles and applications.39 However, until now, no review has comprehensively reported the progress of application of 1D/2D carbon materials in the cathodes, anodes, gel electrolytes and separators of FZIBs. With the attention to flexible electronics increasing, there is an urgent need for a systematic and timely review of the recent advancements in the use of 1D/2D carbon materials in FZIBs to provide potential directions for the next generation of FZIBs.
In this review, we aim to figure out the development background of 1D/2D carbon materials, highlighting their great advantages and functions of applications in FZIBs. Detailed summaries of recent advancements and the current challenges of 1D/2D carbon materials for high-performance FZIBs along with promising strategies are provided. As depicted in Fig. 1, this review begins by introducing the essential requirements and challenges of FZIBs. Currently summarizing the fundamental aspects of 1D/2D carbon materials (carbon nanotubes, graphene, MXenes and carbon fiber), including the development background and the unique advantages of these 1D/2D carbon materials applied in FZIBs. Then, the latest developments of these 1D/2D materials in FZIBs, which could function as active materials, conductive networks, current collectors or Zn hosts in FZIBs are discussed. In addition, the application of 1D/2D carbon materials in separators and gel electrolytes is specially emphasized. Finally, the development prospect of 1D/2D carbon materials used in FZIBs is briefly discussed. We hope to provide valuable suggestions for the research and development of 1D/2D carbon material applying in FZIBs through this review.
Firstly, there are some essential requirements for the configuration of FZIBs, as summarized in Fig. 2a. (1) Basic configuration: similar to traditional ZIBs, the basic configuration of FZIBs comprises cells assembled with Zn foil anodes and cathodes that include binders, conductive additives, and pure active materials on metal current collectors. During the discharge process, anodic zinc releases electrons and generates Zn2+ ions, which are subsequently dissolved by a Zn2+-containing electrolyte. Currently, the Zn2+ ions present in the electrolyte traverse the separator and insert into the cathode. The flow of electrons from the anode to the cathode through an external circuit provides the power necessary to operate electrical devices.42,43 However, the restricted flexibility of zinc metal is inadequate for wearable or flexible applications. Additionally, zinc metal is susceptible to the shape memory effect, preventing it from enduring continuous bending. In order to achieve a truly flexible zinc anode, two common methods have been employed. The first of these involves the application of coating pastes, comprising Zn powders, conductive and binding additives on flexible and conductive current collectors. Another approach is to directly electrodeposit zinc onto flexible substrates. A comparable methodology for the fabrication of flexible cathodes is to deposit active cathode materials upon flexible substrates. This approach is also applicable to the coating of pastes on flexible substrates, which is effective for powder cathode materials. In addition, inspired by the requirement for leak-proof designs, gel electrolytes are favoured for constructing ZIBs in flexible configurations.44 (2) Flexible device design: the design of the device plays a crucial role in ensuring that the batteries meet the essential requirements for performance and flexibility. Several key aspects of flexible device design include the development of fiber battery design and sandwich structure battery design. Fiber ZIBs exhibit linear shapes and omni-directional flexibility, which can liberate ZIBs from rigid constraints and bestow upon them greater shape versatilities and design freedoms. The sandwich structure adheres to the conventional principles of battery design. The fabrication of sandwiched ZIBs typically involves numerous steps, including the preparation of flexible electrodes, the sandwiching of the electrolyte and separator between two thin-film electrodes, and subsequent packaging processes.44,45 (3) Withstand deformation: FZIBs must withstand deformation while maintaining their electrochemical performance. Therefore, the transition from traditional rigid and brittle materials to flexible alternatives is essential for every component of ZIBs.46–49
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Fig. 2 Essential requirements and challenges of FZIBs. Basic configuration image: reproduced with permission from ref. 50. Copyright 2023, American Chemical Society. Flexible device design image: reproduced with permission from ref. 28. Copyright 2024, Wiley. Withstand deformation image: reproduced with permission from ref. 28 Copyright 2024, Wiley. Lower energy density image: reproduced with permission from ref. 25. Copyright 2022, Wiley. Cycling instability and dendrite formation image: reproduced with permission from ref. 21. Copyright 2023, Wiley. Poor mechanical durability image: reproduced with permission from ref. 43. Copyright 2019, Elsevier. Incompatibility and instability of electrolytes image: reproduced with permission from ref. 24. Copyright 2023, Springer Nature. |
Currently, the challenges of FZIBs can be summarized as follows: (1) lower energy density: while ZIBs offer many advantages, their energy density typically lags behind that of LIBs, which is a significant challenge for many applications where both high energy density and flexibility are required. (2) Cycling instability and dendrite formation: repeated charge and discharge cycles can lead to the formation of Zn dendrites. These dendrites can pierce the separator, causing short circuits and battery failure.50 (3) Poor mechanical durability: the repeated bending and flexing in applications can strain the mechanical integrity of battery components, leading to potential failures in the electrical connections and separator breakdown.41,51 (4) Incompatibility and instability of electrolytes: aqueous electrolytes in ZIBs can be prone to water splitting at high voltages, limiting their voltage window and overall performance, while gel electrolytes could expand the voltage range but may introduce other issues of instability or incompatibility.52
Addressing these challenges requires innovative approaches in materials science and engineering. For instance, exploring advanced 1D/2D carbon materials for electrodes and separators could enhance the mechanical and electrochemical properties of FZIBs. Additionally, developing novel gel electrolytes with improved stability and compatibility will be crucial for the future success of FZIBs.
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Fig. 3 An overview of (a) the advantages and (b) functions of 1D/2D carbon materials in FZIBs. Active materials image: reproduced with permission from ref. 29. Copyright 2020, Wiley. Conductive networks image: reproduced with permission from ref. 34. Copyright 2022, Elsevier. Current collectors or Zn hosts image: reproduced with permission from ref. 33. Copyright 2019, Wiley. Application in gel electrolytes image: reproduced with permission from ref. 31. Copyright 2021, AAAS. Application in separators image: reproduced with permission from ref. 30. Copyright 2020, Wiley. |
Due to the above advantages, 1D/2D carbon materials are excellent candidates for various components in FZIBs, which could be utilized as active materials, conductive networks, current collectors or Zn hosts in FZIBs. Especially, 1D/2D carbon materials are also widely applied in modified separators and gel electrolytes in FZIBs (Fig. 3b). Table 1 summarizes the functions of 1D/2D carbon materials in FZIBs. The details of the functions of 1D/2D carbon materials in FZIBs will be stated in the following text.
Function | Cathode | Anode | Electrolyte | Current density | Cycling number | Capacity retention (%) | Flexibility | Ref. |
---|---|---|---|---|---|---|---|---|
Active materials | ZnMn2O4 | NHVO@Ti3C2Tx | 3 m Zn (CF3SO3)2 | 2 A g−1 | 6000 | 92.1 | Folded | 69 |
V2CTX MXene | Flexible Zn | PAMHS | 0.5 A g−1 | 120 | Twisted | 70 | ||
Conductive networks | α-MnO2@CNT | Zn@CNT | 2 M ZnSO4/0.2 MnSO4/PVA | 32.5C | 1000 | 100 | Bent | 71 |
ZMO/CNT | Zn foil | 1.0 M ZnSO4/0.1 M MnSO4 | 3 A g−1 | 2000 | 97.01 | Bent and twisted | 34 | |
CaVO/CNTs | Zn foil | 3 M Zn (CF3SO3)2 | 10 A g−1 | 3000 | 90.7 | Bent | 72 | |
CVO/RCNTs | Zn foil | 2 M Zn (CF3SO3)2/PVA | 5 A g−1 | 1400 | 61.5 | Bent | 73 | |
KVO/SWCNT | Zn foil | 4 M Zn (CF3SO3)2 | 5 A g−1 | 10![]() |
91 | Bent | 74 | |
MnO2/CNT/PAA | Zn foil | 2 M ZnSO4/0.1 M MnSO4 | 1.5 A g−1 | 1000 | 82 | Bent | 75 | |
PDA/CNT/MnO2 | Zn foil | 3.3 M ZnSO4 | 0.2 A g−1 | 500 | Bent | 76 | ||
MnO2 | Zn power/CNTs | 2 M ZnSO4/0.1 M MnSO4 | 3 A g−1 | 2000 | 91.1 | Bent | 77 | |
CNT/MnO2 | ZCN | 2 M ZnSO4 + 0.2 M MnSO4 | 3 A g−1 | 1000 | 75.3 | Bent | 78 | |
MnO2/rGO | Zn foil | 2 M ZnSO4/0.1 M MnSO4 | 6 A g−1 | 500 | 79 | Bent | 79 | |
MnO2/rGO | Zn foil | 2 M ZnSO4/0.1 M MnSO4 | 2 A g−1 | 2000 | 99.87 | Bent and folded | 80 | |
RGO/NVO | Zn foil | 1 MZnSO4/1 M Na2SO4 | 1 A g−1 | 2000 | 94 | Bent | 81 | |
MnO2/EG | Zn/EG | 2 M ZnSO4/0.1 M MnSO4 | 1C | 480 | Bent | 82 | ||
Ti3C2Tx@MnO2 micro flowers | Zn foil | 2.0 M ZnSO4/0.1 M MnSO4 | 0.5 A g−1 | 2000 | 90.6 | Bent | 83 | |
CC@MnO2@MXene | Zn foil | PVA/ZnSO4/MnSO4 | 1 A g−1 | 800 | 51.4 | Bent | 84 | |
VO2/MXene | Zn foil | PVA/Zn (CF3SO3)2 | 5 A g−1 | 2500 | 72.1 | Bent | 85 | |
ZMO@Ti3C2Tx | Zn foil | Gelatin-based | 1 A g−1 | 5000 | 92.4 | Flat, bent, and twisted | 86 | |
Current collectors or Zn hosts | Co3O4 NSs@CNTF | Zn NSs@CNTF | 2 M ZnSO4/0.0005 M CoSO4 | 5 A g−1 | 10![]() |
97.27 | Bent and twisted | 87 |
CNT/MnO2 foams | Zn foil | 2 M ZnSO4/0.005 M MnSO4 | 10 mA cm−2 | 2500 | 83.5 | Folded | 88 | |
MnO2@CNT | Zn wire | 2 M ZnTFSI/0.2 M MnCl2/PVA | 2 A g−1 | 100 | 98 | Bent and twisted | 89 | |
CNT–MnO2@PEDOT | Zn/CNT | PVA/LiCl/ZnCl2/MnSO4 | 20 mA cm−2 | 1000 | 88.7 | Bent and twisted | 33 | |
FSM@FGF | Zn foil | 2 M ZnSO4 | 1 A g−1 | 300 | 82.7 | Folded | 90 | |
ZOV array | Zn array | Fumed silica/ZnSO4 | 20C | 2000 | 89 | Bent and twisted | 91 | |
MnO2@N-VG@CC | Zn@N-VG@CC | PVA/Zn (CF3SO3)2 | 2 A g−1 | 300 | 80 | Bent and twisted | 92 | |
PANI/SWCNTs | Zn/RGO–SWCNTs | PVA/Zn (CF3SO3)2 | 1 A g−1 | 1000 | 97.3 | Flat, bent, and twisted | 93 | |
N-CNSs@MnO2 | N-CNSs@Zn | PVA | 8 A g−1 | 500 | 76.5 | Bent and twisted | 94 | |
Al2O3@VSe2NSs@N-CNFs | Zn NSs@CNT | CMC/ZnSO4 | 1 A g−1 | 2500 | 86.2 | Bent | 95 | |
V2O5-CFC | Zn foil | 2 M ZnSO4/0.1 M MnSO4 | 0.5 A g−1 | 1000 | Bent | 96 | ||
V2O3@CNFs | Zn@CC | 2 M Zn (CF3SO3)2 | 2 A g−1 | 1000 | Bent and twisted | 97 | ||
MnO2@CC | Zn foil | PAM/ZnSO4/MnSO4 | 0.1 A g−1 | 300 | Bent | 98 | ||
MnO2 | CC@MnO2-UTF@Zn | 2 M ZnSO4/0.2 M MnSO4 | 1 A g−1 | 300 | 81 | Flat and bent | 99 | |
VS2/CC | Zn/CC | PVA/Zn (CH3COO)2/Mn (CH3COO)2· | Flat, bent, and twisted | 100 | ||||
Application in gel electrolytes | MnO2 | Zn foam | PVA/GO | 0.3 A−1 | 1000 | 98 | Flat, bent, and twisted | 31 |
V2O5 | Zn foil | V2O5/GO/PVA | 1 A g−1 | 1000 | 96 | Bent and twisted | 101 | |
V2O5 | Zn foil | PVA/Zn (CF3SO3)2/TiO2 | 0.2 A g−1 | 100 | 99.8 | Flat and bent | 102 | |
Application in separators | V2O5 | Zn foil | 2 M ZnSO4 | 5 A g−1 | 1000 | 75 | Flat and bent | 30 |
V2O5 | Zn foil | 2 M Zn (CF3SO3)2 | 1 A g−1 | 1000 | 78.7 | Bent | 103 |
Modeled on the method of inhibiting dendrite growth in a lithium-ion battery system, the construction of rocking-chair type zinc-free metal anode materials will be an effective way to solve the growth of zinc dendrites.14 MXene is a class of 2D layered transition metal carbides or nitrides, with interlayer spaces capable of accommodating and storing zinc ions, which makes it an ideal zinc ion storage material. As active materials for zinc storage, MXenes offer high capacity, rapid ion transport, and excellent mechanical properties, among other advantages. Yuan et al. synthesized (NH4)2V10O25·8H2O@Ti3C2Tx (NHVO@Ti3C2Tx) films by vacuum filtration of 1D ultrathin NHVO nanobelts and layered Ti3C2Tx nanosheets. The composite film exhibited a low working potential of 0.59 V and could be utilized as an anode for “rocking-chair” type ZIBs. More importantly, 2D Ti3C2Tx not only provided a flexible substrate for NHVO, which could be bent and folded, with an excellent mechanical performance, but also stabilized the structural changes of NHVO nanobelts during charge and discharge. In the meantime, the utilization of MXenes as active materials yielded a specific capacity. Consequently, the NHVO@Ti3C2Tx thin film electrode delivered a remarkable capacity of 514.7 mA h g−1 over 6000 cycles, retaining a capacity of up to 84.2%. The “rocking-chair” Zn-ion full battery constructed by pairing the NHVO@Ti3C2Tx thin film anode and the ZnMn2O4 cathode exhibited a maximum specific capacity of 131.7 mA h g−1 and a maximum energy density of 97.1 W h kg−1, which is better than those of the previously reported “rocking-chair” Zn-ion battery.69 In addition, Li.et al. presented an aqueous zinc hybrid-ion battery (ZHIB) featuring a remarkable capacity enhancement over 18000 cycles, utilizing a 2D layered V2CTX MXene cathode. Here, “TX” denoted the surface functional groups such as –F, –OH, and = O, which significantly differed from those in previously reported ZHIBs. The delamination of V2CTX MXene enabled a continuous exposure of active sites, and the phase transition product in conjunction with V2CTX MXene contributed continuously to capacity, which accounted for the unusual capacity enhancement.70
While MXenes are promising in these diverse applications, several challenges need addressing to fully exploit their potential: (1) MXenes are susceptible to oxidation, which can degrade their properties. Enhancing their stability under environmental conditions is crucial for practical applications. (2) Methods for the synthesis and processing of MXenes need improvement to ensure that they can be produced in large quantities without losing quality. (3) Concerns regarding the toxicity of MXenes and their precursors, particularly those involving hazardous chemicals like hydrofluoric acid, need to be addressed. Developing safer synthesis methods and handling protocols is essential. Addressing these challenges will be key to advancing the use of MXenes as active materials across various domains. With ongoing research and development, MXenes hold the potential to revolutionize multiple industries by providing high-performance alternatives to traditional materials.
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Fig. 4 Carbon nanotubes as conductive networks in FZIBs. (a) Ragone plot illustrating the energy and power densities in comparison with that of other previously reported cathode materials in aqueous ZIBs. (b) The cycling performance and galvanostatic charge/discharge curve of α-MnO2@CNTs at 32.5C. (c) The fabricated flexible quasi-solid-state ZIBs. Reproduced with permission from ref. 71. Copyright 2020, Elsevier. (d) Scheme illustrating the fabrication of ZMO/CNTs. (e) Cycling performance of the ZMO/CNT electrode at 3000 mA g−1. (f) Discharging curves at 100 mA g−1 under different conditions of FZIBs. Reproduced with permission from ref. 34. Copyright 2022, Elsevier. (g) The fabrication process of the free-standing Zn anode. (h) Discharge/charge curves of ZIBs with the freestanding Zn anode and zinc foil at 0.3 A g−1. Reproduced with permission from ref. 77. Copyright 2021, the Royal Society of Chemistry. |
Furthermore, CNTs provide mechanical strength to the cathode structure. Their high tensile strength and flexibility contribute to maintaining the integrity of the electrode even under mechanical stress, which is particularly important for flexible or wearable applications. As depicted in Fig. 4d, Gao et al. synthesized ZMO/CNT cathodes by anchoring ZnMn2O4 particles onto CNTs through a one-step hydrothermal method. Due to robust interfacial interactions (Mn–O–C bond) enhanced both the electron and ion transport pathways, the ZMO/CNT cathode demonstrated high capacity and remarkable cycling life, with a capacity retention of 97.01% even after 2000 cycles (Fig. 4e). In addition, the FZIB was assembled with the ZMO/CNT as the cathode and zinc foil as the anode, which was enough for it to be bent or twisted and the electrochemical impedance in the bent or twisted states was similar to that in the flat state (Fig. 4f).34 Parallelly, CaVO microflowers were successfully synthesized using a hydrothermal method. Subsequently, CaVO/CNT films were effectively prepared using spray printing technology. The CaVO microflowers were enveloped by a network of CNTs, expediting electron transport and enhancing electrical conductivity. Furthermore, the fabricated CaVO/CNT film demonstrated remarkable flexibility, and its electrochemical performance remained stable under different bending conditions.72
By introducing CNTs with various nanostructures and excellent properties, the issues associated with cathodes and anodes in FZIBs can be effectively addressed. When conductive CNTs are combined with low conductivity cathode materials, the resulting hybrid electrodes are expected to exhibit excellent zinc storage performance.42 While CNTs offer significant advantages as conductive networks in both cathodes and anodes of FZIBs, their integration into battery systems also presents challenges. (1) Dispersion and homogeneity: achieving a uniform dispersion of CNTs within electrode materials is challenging due to their tendency to agglomerate. Proper dispersion techniques are essential to ensure that CNTs are evenly distributed, maximizing their beneficial effects on conductivity and mechanical properties. (2) Cost and scalability: the production of CNTs can be costly, and scaling these processes while maintaining quality and performance poses significant challenges. Reducing the cost of CNT production is crucial for their widespread adoption in commercial FZIB applications.
CNTs offer several compelling advantages for both cathodes and anodes in FZIBs, enhancing ionic conductivity, structural integrity and mechanical durability. However, addressing challenges related to dispersion, cost, and material compatibility is crucial for realizing the full potential of CNTs in this promising area of energy storage technology. Ongoing research and development are expected to continue improving the integration of CNTs in FZIBs, paving the way for more reliable, safe, and efficient flexible batteries.
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Fig. 5 Graphene as a conductive network in FZIBs. (a) Schematic illustration of the vacuum filtration on carbon cloth to prepare MnO2/rGO electrodes. (b) Comparison of the charging and discharging profiles of the samples at a current density of 0.3 A g−1. (c) Specific capacities (normalized over the mass of MnO2) of the samples at different current densities. Reproduced with permission from ref. 79. Copyright 2018, Springer nature. (d) The digital photos of the MnO2/rGO nanocomposite membrane demonstrating flexibility under bent, rolled, and folded conditions. (e) Ragone plot of our cells compared with previously reported ZIBs. Reproduced with permission from ref. 80. Copyright 2020, Wiley. |
Similarly, Wan and colleagues fabricated free-standing RGO/NVO films using high aspect ratio NVO nanobelts and RGO through vacuum filtration. Benefiting from the multilayer connection between the RGO sheet and the NVO nanobelt, the RGO/NVO hybrid film not only exhibited excellent mechanical performance, but also much improved electrical conductivity. The manufactured flexible soft-packaged ZIBs were able to operate stably under different conditions and showed no significant capacity decay after 100 cycles.81 Significantly, Dong's group generated transparent graphene nanosheets with a thickness of 14 μm, which were prepared by simple electrochemical exfoliation. The flexible hybrid films were prepared by vacuum filtration of electrochemically exfoliated graphene (EG) nanosheets and ultrathin MnO2 nanosheets. The availability of EG provided free-standing MnO2/EG films with superior flexibility. In addition, the highly flexible EG nanosheets could also serve as a conductive substrate for electrodepositing Zn, which was obtained as a Zn/EG film electrode instead of rigid Zn foil. Owing to the significant effect of EG on slowing down the dissolution of Mn and regulating the volume change during charge/discharge cycles, the MnO2/EG//Zn/EG full battery assembled with MnO2/EG as the cathode and Zn/EG as the anode exhibited a high specific capacity of 300 mA h g−1 at 0.2C. A critical feature of the soft-packaged MnO2/EG//Zn/EG battery was its outstanding flexibility, allowing the battery to maintain the brightness of a small bulb even when bent and deformed. This further validated the importance of EG nanosheets in flexible and high-performance ZIBs.82
In particular, mass loading refers to the amount of active material per unit area in the cathode, which directly influences the energy density and overall performance of batteries. According to recent studies, He's team successfully synthesized a sodium and copper co-intercalated boron titanate manganese oxide (NCMO) cathode. When using TIMCAL graphite & carbon as a conductive material, it exhibited highly reversible Mn deposition and dissolution processes under high-quality loading, attributed not only to the dual-ion co-intercalation increasing the manganese conversion on the cathode surface but also to the stability of the host material during charge/discharge cycles.106 Moreover, in high-loading cathode ZIBs, the use of 1D/2D carbon materials can create a conductive network that improves electron transport pathways and mechanical stability. This network allows for a higher amount of active material to be effectively utilized, thereby increasing the overall capacity and energy density of FZIBs. The high specific surface area and excellent electrical conductivity of these 1D/2D carbon materials facilitate the uniform distribution and robust connection of active materials, thus supporting higher mass loadings without compromising the battery's performance. This network allows for a higher amount of active material to be effectively utilized, thereby increasing the overall capacity and energy density of FZIBs. Furthermore, the structural integrity provided by 1D/2D carbon materials can prevent the detachment of active materials during cycling, enhancing the longevity and cycling stability of FZIBs.
Although graphene offers numerous benefits, its large-scale production remains cost-intensive and technically challenging. Reducing the cost of graphene production through improved synthesis methods is crucial for its broader adoption in commercial applications. Moreover, achieving effective integration of graphene with existing battery materials requires advanced material engineering techniques. What's more, while graphene's general properties are well understood, optimizing these properties for specific battery configurations and applications remains an area of active research. In conclusion, graphene as a conductive network in FZIBs represents a significant advancement in battery technology, particularly for applications requiring high flexibility, durability, and performance. Its ability to enhance both the cathode and anode properties addresses several limitations of traditional battery materials, positioning graphene as a key material in the future advancement of energy storage devices. Continuing advancements in graphene processing and integration will likely unlock further potential of this versatile material in flexible battery applications.
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Fig. 6 MXenes as conductive networks in FZIBs. (a) Schematic depiction of the synthesis process of 3D Ti3C2Tx@MnO2 microflowers. (b) Fabrication procedure for the FZIB. (c) Real-time image demonstrating the FZIB supplying power under both straight and bent conditions. (d) Cycling performance in the straight and bent states at a current density of 100 mA g−1. Reproduced with permission from ref. 83. Copyright 2020, the Royal Society of Chemistry. (e) Schematic representation of intercalation and deintercalation mechanisms of the CC@MnO2@MXene electrode. (f) CV curves under normal and bending conditions. Reproduced with permission from ref. 84 Copyright 2022, Elsevier. (g) Optical images of the VO2/MXene hybrid films. (h) Cycling performance of the VO2/MXene hybrid films at a current density of 5 A g−1. Reproduced with permission from ref. 85. Copyright 2021, Wiley. |
Furthermore, considering the inherent excellent conductivity and mechanical properties of 2D MXenes, Shi et al. fabricated VO2/MXene hybrid films through a simple vacuum filtration process. Within the hybrid film, VO2 nanoparticles were embedded within MXene sheets, establishing a 3D layered conductive network. Optical images revealed that the VO2/MXene hybrid film exhibited high flexibility and remained free of cracks under various bending and folding states (Fig. 6g). The specific capacity of the VO2/MXene hybrid film reached up to 228.5 mA h g−1, with an energy density of up to 197.2 W h kg−1. To demonstrate the application of VO2/MXene hybrid films in flexible energy storage devices, the researchers designed a quasi-solid-state FZIB with the hybrid film as the cathode material. Moreover, the flexible battery exhibited remarkable mechanical flexibility and exceptional Zn2+ storage capacity, maintaining a capacity retention of 72.1% after 2500 cycles at 5 A g−1(Fig. 6h).85 In conclusion, the ability of MXenes to form continuous conductive pathways enhances electron transport efficiency in batteries, leading to improved electrochemical performance. Their high surface area provides ample active sites for electrochemical reactions, increasing the capacity and rate performance of energy storage devices. Additionally, MXenes possess good mechanical flexibility and chemical stability, which are crucial for maintaining the structural integrity and longevity of electrodes during cycling. Furthermore, MXenes can be functionalized to tailor their surface chemistry, allowing for the optimization of their electrochemical properties to meet specific application needs. The combination of these properties positions MXenes as a promising material for advanced conductive networks, contributing to the development of high-performance and durable energy storage systems.
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Fig. 7 CNTs as current collectors or Zn hosts in FZIBs. (a) Schematic depiction of the preparation process of fiber shaped Co3O4 NSs@CNTF and Zn NSs@CNTF electrodes. (b) Schematic diagram of the mechanism of the aqueous ZIB employing Co3O4 NSs@CNTF as the cathode and zinc as the anode. (c) Long-term cycling performance of a fiber-shaped ZIB at a current density of 5 A g−1; comparison of long-term cycling performances for the fiber-shaped ZIB in this work and a previous FZIB and an image of flexible fiber shaped ZIBs integrated into a sweater to charge a smart watch. Reproduced with permission from ref. 87. Copyright 2021, American Chemical Society. (d) Diagram illustrating the MnO2/CNT foam cathode with a reversible chemical conversion and a 3D hierarchical structure for mass and charge transport. (e) Cycling performance of the assembled Zn–MnO2 battery that was tested at 10 mA cm−2. Reproduced with permission from ref. 88. Copyright 2021, Wiley. (f) The schematic illustrations of Zn deposition on CC and CNT electrodes. (g) Cycling performance measured at 20 mA cm−2 of the Zn//MnO2 batteries with Zn/CC and Zn/CNT anodes tested in aqueous electrolyte with a coin cell. (h) Capacity retention of the quasi-solid-state Zn//MnO2 battery with a Zn/CNT anode under different deformation states. Reproduced with permission from ref. 33. Copyright 2019, Wiley. |
In a word, while CNTs have shown great potential in the realm of flexible energy storage devices, the development of CNT-based current collectors is still an evolving field with many challenges to overcome. One major obstacle is the inherently low electrical conductivity of individual CNTs when assembled into macroscopic structures, which limits their performance as current collectors. Furthermore, the mechanical properties of CNTs can degrade over long-term cycling or under repeated mechanical deformation, affecting the durability and reliability of the devices. Advancements in material engineering, surface chemistry, and fabrication techniques are essential to fully realize the potential of CNTs in creating high-performance, durable, and flexible energy storage devices. Future research should focus on addressing these challenges to enable the widespread adoption of CNT-based current collectors in commercial applications.
The application of CNTs as host materials for zinc anodes in FZIBs not only enhances the electrochemical performance and mechanical flexibility of the batteries but also significantly extends their lifespan, indicating broad potential for future applications. Future research should concentrate on optimizing the structure and fabrication processes of carbon nanotubes to further improve the performance and commercial viability of FZIBs.
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Fig. 8 Graphene as a current collector or Zn host in FZIBs. (a) Schematic illustration depicting the selective growth process of free-standing MnO2 on a flexible graphene film for use as a binder-free cathode for FZIBs. (b) Comparative rate performances of commercial MnO2, FSM@FGF-30, FSM@FGF-60, and FSM@FGF-90 at potential and current densities of 1.0–1.9 V and 0.1–2.0 A g−1, respectively. (c) Comparative specific capacity, including those of previously reported ZIB materials. (d) Cycling performance of FSM@FGF for up to 100 cycles at a current density of 0.5 A g−1. Reproduced with permission from ref. 90 Copyright 2022, Elsevier. (e) Schematic illustrations of preparing N-VG@CC and the Zn@N-VG@CC electrode. (f) Rate performance of the coin ZIBs based on Zn@CC and Zn@N-VG@CC anodes. (g) Ragone plot of the quasi-solid-state ZIB compared with other reported energy storage devices. (h) Flexible demonstration of two quasi-solid-state ZIBs connected in series. Reproduced with permission from ref. 92 Copyright 2021, Wiley. |
Graphene-based current collectors have been successfully employed in various cathode materials, including transition metal oxides, vanadium-based materials, and others. The use of graphene has resulted in batteries with higher energy densities, improved cycling stability, and enhanced rate performance compared to conventional current collectors. Future research in this area should focus on optimizing the synthesis and processing methods of graphene to further improve its electrical conductivity, mechanical properties, and compatibility with different cathode materials. Additionally, exploring new graphene-based composite materials and hybrid structures could lead to further advancements in battery performance and applications. In conclusion, graphene-based current collectors hold great promise for improving the electrochemical performance and mechanical flexibility of cathodes in various battery applications, including portable electronics, electric vehicles, and energy storage systems. Continued research and development are expected to accelerate the commercialization of graphene-based battery technologies in the near future.
In conclusion, graphene as a host material for zinc in ZIBs holds great promise for advancing the development of high-performance batteries with enhanced energy density, cycling stability, and safety. Continued research and innovation in this area is anticipated to hasten the adoption of graphene-based materials in next-generation energy storage technologies.
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Fig. 9 CFs as current collectors or Zn hosts in FZIBs. (a) Schematic illustration preparation process of a flexible N-CNSs@MnO2 cathode. (b) Cycling performance of the quasi-solid-state N-CNSs@MnO2//N-CNSs@Zn battery at 8.0 A g−1. (c) Galvanostatic charge/discharge curves obtained at a current density of 1.0 A g−1 for a single quasi-solid-state device and tandem devices consisting of two and three batteries units connected in series. Reproduced with permission from ref. 94 Copyright 2020, Elsevier. (d) Schematic illustration of the preparation process for the Al2O3@VSe2 NSs@N-CNF film. (e) Schematic diagram illustration of the flexible QAZIB. (f) Galvanostatic charge/discharge curves of the flexible QAZIB at different current densities. Reproduced with permission from ref. 95 Copyright 2021, Elsevier. (g) SEM images of the Zn nanowires/CC anode. (h) Optical images of the self-healing flexible Zn-ion battery after bending to different angles. Reproduced with permission from ref. 100 Copyright 2021, Elsevier. |
Moreover, V2O3 nanoparticles (NPs) were embedded into carbon nanofibers by electrospinning to achieve V2O3@Carbon Nanofiber (V2O3@CNF) flexible binder-free ZIB cathode materials. The V2O3@CNF electrodes could withstand bending or twisting and exhibit excellent mechanical performance. After carbonization of the prepared V2O3@CNF cathode material, the pyridine and pyrrole N not only provided electrons to the π bond in the CNFs but also led to a number of structural defects, thus providing a large quantity of active sites. The V2O3@CNFs provided a superior specific capacity of 220 mA h g−1, with a current density of 50 mA g−1.97 Moreover, Cao's group developed a MnO2/CC hybrid electrode by modifying carbon cloth with zeolitic imidazolate frame-work-67 (ZIF-67). The carbon cloth substrate was enveloped by intricately arranged ultra-thin MnO2 nanosheets, forming a distinctive hollow polyhedral structure. This MnO2/CC electrode demonstrated a synergistic Zn2+ and H+ co-intercalation mechanism, attributed to its exceptional reversibility. The substantial surface area of the MnO2-loaded hollow polyhedron significantly enhanced electrolyte infiltration, mitigated volume expansion challenges, and concurrently promoted the conductivity of the underlying CC.98
The use of CFs as cathode current collectors in FZIBs presents a promising approach for improving the performance and durability of these energy storage devices. The integration of advanced materials and innovative fabrication techniques can lead to significant enhancements in electrochemical properties, mechanical flexibility, and overall battery efficiency, making CFs a valuable component in the development of next-generation flexible energy storage solutions.
The scalability and versatility of CF-based zinc anode current collectors open new avenues for the advancement of next-generation flexible, stretchable, and customizable energy storage solutions. This approach not only enhances the performance and durability of ZIBs but also broadens their application potential in wearable electronics and other flexible devices.
Typically, 2D functional fillers provide great SSA, enabling increased number of active interfaces with polymers in composite polymer electrolytes (CPEs). GO, the most typical 2D filler, has been widely adopted in CPEs due to its numerous oxygen-containing functional groups. Specifically, GO possesses a long-range ordered monoatomic layer structure that is expected to create fast ion transport channels at the polymer–GO interface. Xiao et al. introduced GO as a filler into the ZnSO4/MnSO4-PVA-based gel electrolyte. As depicted in Fig. 10a, there were three main steps in the preparation of the GO-containing polymer hydrogel electrolyte (GPHE): (i) mixing PVA and GO, (ii) freeze/thaw processing, and (iii) ZnSO4/MnSO4 solution soaking. The molecular chain of PVA provided the framework for stiffness maintenance, while ions were transported within the ZnSO4/MnSO4 solution. In addition, a detailed finite element analysis (FEA) of the random distribution of GO in the GPHE was conducted using COMSOL Multiphysics, based on Monte Carlo simulations. At high molecular weights, ion transport was primarily achieved through microscopic processes that were independent of the polymer length and viscosity (Fig. 10b). Moreover, the hydrogen bond formed between GO and the PVA polymer affected GO dispersion, enhancing ion mobility and forming a layered structure that bound molecularly to the PVA polymer. The resulting composite polymer electrolyte containing 0.32 wt% GO demonstrated favorable performance with an ionic conductivity of 21 mS cm−1 (Fig. 10c). A rechargeable solid-state zinc-ion fiber battery was developed, maintaining 98.0% capacity after over 1000 charging and discharging cycles (Fig. 10d). To demonstrate practical applications, the solid-state Zn/MnO2 fiber battery successfully powered a textile-based active node (TBAN) for continuous monitoring of various physiological signals and wireless charging (Fig. 10e and f). The lightweight, low-cost, and high-performance Zn/MnO2 fiber battery showed potential for mass production, highlighting its capability to power e-textiles for personalized healthcare.31 Chen and co-workers fabricated V2O5/GO/PVA gel electrolytes with 5 wt% GO, which exhibited about a two orders of magnitude enhancement in mechanical performance compared with pure PVA gel electrolytes.101 As mentioned above, 1D/2D carbon materials as fillers typically possess abundant functional groups. These groups can significantly enhance interactions with polymers and zinc salts, maximizing the anchoring of anions and reducing the affinity between zinc ions and polymers. This accelerates zinc ion transport and improves the compatibility at the filler–polymer interface.
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Fig. 10 Application of 1D/2D carbon materials in gel electrolytes and separators. (a) Schematic illustration of the preparation process of GPHE. (b) Measurement of ion conductivity of GPHEs with different PVA–GO mass ratios and soaking concentrations. (c) Calculation results of ion conductivity of GPHEs with different GO masses. (d) Cycling performance of coin cells with aqueous electrolyte, PVA electrolyte, and GPHE at a current density of 0.3 A g−1. (e) Illustration and design of a flexible ZIB TBAN. (f) PHE-based ZIB fiber maintaining stable capacity output under different bending angles. The inset image shows bending stress and deformation of ZIB fibers. Reproduced with permission from ref. 31 Copyright 2021, AAAS. (g) A Janus separator employing a VG carpet directly grown on one side to reduce local current density and homogenize ion distribution. (h) Current distribution in the 3D scaffold. Electric field distribution of the Janus separator structure and pristine separator structure. Reproduced with permission from ref. 30 Copyright 2020, Wiley. |
Typically, GF separators dominate in aqueous ZIBs due to their hydrophilic nature, high ionic conductivity, and porous structure. In situ modification of GF separators with conductive carbon materials, particularly graphene, presents a promising strategy to enhance electrochemical performance of ZIBs. Li et al. employed plasma-enhanced chemical vapor deposition (PECVD) to grow 3D VG carpets on one side of GF separators, creating Janus separators with an ample porous structure and large SSA to ensure uniform electric field distribution and reduced local current density. Additionally, the introduction of oxygen and nitrogen functional groups enhanced zincophilicity, regulating homogeneous Zn2+ fluxes and stabilizing Zn anodes (Fig. 10g). A simplified 3D model was constructed using the Janus separator structure, and as shown in Fig. 10h, the introduction of a 3D conductive VG scaffold significantly increased the surface area and effectively reduced the current density, outperforming 2D planar graphene. According to the 3D model, reducing the current density delayed the initiation of dendritic growth. The 3D VG scaffold provided a uniform electric field, facilitating the uniform deposition of Zn onto the VG framework. In contrast, the absence of a 3D scaffold in planar configurations easily triggered single nucleation points, leading to an uneven electric field which promoted the formation of Zn dendrites. Consequently, these robust Janus separators facilitated stable cycling of Zn//Zn symmetrical batteries for 300 hours at 0.5 mA cm−2 for 0.5 mA h cm−2. Furthermore, Zn//V2O5 batteries achieved a high energy density of 182 W h kg−1 and withstood deformation without significant electrochemical decay.30 Additionally, to achieve low-cost separators, Wu and colleagues proposed a functional cellulose nanofiber/graphene oxide (CG) separator. This separator featured abundant pores ranging from 10 to 50 nm, a smooth surface, and ample zincophilic oxygen-containing functional groups. These properties facilitated strong interactions with zinc, enabling uniform distribution of Zn2+ ions and promoting dendrite-free Zn anodes.103
While advancements have been notable, carbon separator research remains at an early stage. Presently, the prevalent GF separators suffer from fragility and large pores, prone to issues like zinc dendrite growth and short circuits.120 Additionally, the separator/electrolyte and separator/electrode interface design has been overlooked, crucial for constructing FZIBs. To address this, an integrated system amalgamating all components into a single monolith could effectively stabilize interfaces and structures, ensuring resilience against deformations.
(1) Optimizing the synthesis of 1D/2D carbon materials. Precise synthesis methods allow for the creation of 1D/2D carbon materials with tailored structures and surface chemical properties to fulfill specific functional or component requirements. Although existing 1D/2D carbon materials generally meet the demands of high-performance energy storage devices, the unique structural and surface characteristics of 1D/2D carbon materials, such as high conductivity and hydrophilicity, assume crucial significance when considering the distinctions between FZIBs and other energy storage systems. In this context, precise modulation of the hydrophilic and hydrophobic nature of 1D/2D carbon materials becomes particularly pertinent.
(2) Application of 1D/2D carbon materials for zinc anode protection. Addressing the challenges posed by poor electrochemical and cycling stability of zinc metal anodes has driven researchers to predominantly focus on structural design of the zinc material. This involves modifying the zinc surface through the incorporation of supplementary protective layers, alongside optimizing the electrolyte composition. Within the context of 1D/2D carbon materials, which serve as both the core material and protective layer, the compatibility and intimate contact at the interfaces, namely the 1D/2D carbon material/zinc and/or 1D/2D carbon material/separators, emerge as critical factors for sustaining a stable microstructure and uniform zinc deposition. In essence, ensuring compatibility and close interaction at these interfaces is pivotal for stabilizing the microstructure and promoting homogeneous zinc deposition.
(3) Advancing the development of in situ characterization techniques. Currently, research on the energy storage mechanisms of zinc heavily relies on non-in situ characterization methods, which fall short in accurately revealing the dynamic changes in chemical composition, morphological evolution, oxidation states, and other factors of active materials under real experimental conditions. As a consequence, our understanding of the energy storage mechanisms of active materials and the continuous structural evolution of metal oxides during charge and discharge processes remains incomplete. Furthermore, a more profound investigation is required to discern the impact of introducing 1D/2D carbon materials on energy storage mechanisms. Moreover, a more detailed exploration of the interfacial interactions between 1D/2D carbon materials and metal oxides, as well as their effects on electrochemical performance, is necessary. To tackle these challenges, employing advanced in situ visualization and spectroscopic characterization techniques such as synchrotron X-ray absorption spectroscopy (XAS), transmission X-ray microscopy (TXM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and atomic force microscopy (AFM), plays an indispensable role in resolving these issues and gaining a deeper understanding of the role of 1D/2D carbon materials. Additionally, to achieve mass production of 1D/2D carbon materials, it is crucial to specifically consider costs. Some studies may opt for expensive raw materials, and complex testing techniques also constitute a significant portion of research expenses. Therefore, finding a relative balance between cost and outcomes could accelerate the process of commercial application.
(4) Define suitable standards for assessing the flexibility of FZIBs. Presently, the evaluation of conventional ZIBs typically encompasses parameters such as specific capacity, rate performance, cycling stability, energy density, and power density. Within this context, the discharge depth and current density significantly influence the cycling life of ZIBs. Furthermore, the validation of their exceptional flexibility often involves tests such as bending, twisting, and stretching in FZIBs. However, there currently exists a divergence in experimental methodologies and geometric parameters. This disparity hampers the progression of FZIBs. Consequently, it becomes essential to formulate a set of suitable standards to facilitate the smooth advancement and interdisciplinary application of FZIBs.
(5) Developing novel 1D/2D carbon materials. Although existing 1D/2D carbon materials have been widely used in many fields and have demonstrated great performance, the exploration of novel 1D/2D carbon materials is still an ongoing and evolving process. In addition, by focusing on green synthesis methods and utilizing renewable resources, researchers can minimize the environmental impact of producing 1D/2D carbon materials. Furthermore, it is essential to consider the end-of-life disposal and recyclability of 1D/2D carbon materials. Designing materials that are easily recyclable or biodegradable will ensure that their use does not lead to long-term environmental pollution. This approach aligns with the principles of a circular economy, promoting the reuse and recycling of materials to reduce waste and resource consumption. Integrating these considerations into the development of novel 1D/2D carbon materials will not only enhance their commercial viability but also support broader environmental and societal goals. By prioritizing safety and environmental impact alongside performance and cost, the next generation of 1D/2D carbon materials can meet the demands of both industry and the planet, paving the way for a sustainable energy future.
In conclusion, advanced 1D/2D carbon materials have made significant progress in enhancing the electrochemical performance of FZIBs. However, there are still challenges and limitations that require further research and resolution. The path ahead remains filled with challenges, necessitating ongoing efforts and exploration.
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