4.2.1 Design of Advanced Active‐Materials

Generally, developing novel elastic materials and structural designs are main routes to achieving flexible devices. However, developing intrinsically flexible materials is extremely difficult, because it needs to meet many different technical demands [15]. Compared to conventional inorganic materials, organic materials have better stretchability, and possibly lower cost, which enable devices to have some flexibility [16]. Although many works have focused on the design of organic material‐based flexible devices, their electrochemical performance is still incomparable to that of inorganic material‐based devices. Inorganic materials, such as metal oxides, hard carbon, and activated carbon, usually suffer from extremely low elastic limit (usually <0.1%) [15]. They are intrinsically brittle, and are not capable of accommodating a harsh mechanical deformation. Thus, conventional inorganic materials are the major challenge to developing flexible SIBs.

4.2.2 Design of Flexible Substrates and Electrodes

The conventional electrode is usually prepared by using the slurry‐coating technology. The electro‐active materials, such as metal oxides, hard carbon, and other inorganic materials are made into slurry with conductive additives and polymer binders, and further coated on Al or Cu foil current collectors that are rigid. Besides, these electro‐active materials are intrinsically brittle with an extremely low elastic limit [17]. As a result, the electro‐active materials may easily peel off from the Al or Cu foils during repeated mechanical deformation. Currently, flexible electrodes are typically prepared by incorporating electro‐active materials onto flexible or/and conductive substrates [18–20]. For example, the unique physical, chemistry, mechanical, and electrical properties of graphene make it a promising candidate for incorporation in flexible substrates to produce flexible SIBs [21, 22].

Currently, flexible devices can be classified into three categories according to their loading type – bendable, stretchable, and compressible devices [15], as shown in Figure 4.2. Among them, bendable SIBs are the most investigated. The key issue for bendable SIBs is to develop reliable flexible electrodes. Suo investigated the strain in a film coated on a substrate in detail. The strain in the top surface (_top) can be obtained by the equation 23, 24]:

4.1

42

43

where d_f and d_s, Y_f and Y_s are the thickness and Young's moduli of the film (electro‐active materials) and substrate, respectively, and R is the bending radius. Specially, when the flexible film and substrate have the same Young's modulus, the _top can be simplified as follows:

4.4

From the theoretical model, Equations 4.1 and 4.4, we can summarize that depositing nanomaterials with a small thickness onto flexible substrates will be a feasible strategy to developing bendable SIBs.

The stretchable device is another common flexible configuration [25]. However, developing stretchable SIBs is more difficult than developing bendable devices. Designing intrinsically stretchable materials for flexible SIBs is extremely challenging. Another feasible strategy to fabricate stretchable SIBs is using stretchable electrode structures. Three novel structures have been developed and demonstrated in flexible LIBs and supercapacitors: wavy layout, coiled fiber structure, and island‐bridge design [26–28]. Although the stretchability can be obtained by wavy layout and helically coiled design, such structures can only be unidirectionally stretched. The island‐bridge structure circumvents this limitation, but the assembly processes are complicated and the stability obtained is unsatisfactory. In fact, coiled fiber, wavy layout, and island‐bridge structures possess a little compressibility to some extent. However, so far, there has been no relevant works on stretchable SIBs. Cui and coworkers [29] reported a novel method to design stretchable electrodes, by using sugar as a template to generate the highly elastic polydimethylsiloane (PDMS) scaffolds with 3D porous sponge‐like structure. They used the Li₄Ti₅O₁₂ anode and LiFePO₄ cathode and demonstrated the practicability of the stretchable PDMS sponges in LIBs. Actually, fabrication of stretchable electrodes is a versatile strategy [29]. Zhang and coworkers developed a simple strategy to prepare nitrogen‐doped carbon foam (NCF) with compressible 3D electrode architectures by direct carbonization of commercially available melamine foam [30]. They used it as a compressible substrate for supercapacitor applications.

4.2.3 Developing Novel Processing Technologies

Conventional energy storage devices usually have simple shapes, such as coins, cylinders, prism, and pouch types [31, 32]. Stress accumulation caused by the geometrical shape during deformation is another challenge for achieving flexible SIBs. To avoid internal stress between components, the processing technology of conventional energy storage devices should be changed [15]. Therefore, novel processing technologies should be developed to allow flexible SIB operation under repeated mechanical deformation. In previous reports, PDMS is the most widely used flexible polymer for fabricating elastic electrodes and packaging devices due to its unique properties, such as wonderful flexibility, stretchability, and chemical stability [29, 33]. In very recent years, printing technologies, especially 3D printing technology, are drawing much attention owing to their large‐scale and high‐throughput production capabilities [34, 35]. That may be a promising choice to developing flexible SIBs in the near future.

4.3.1 Inorganic Anode Materials for Flexible SIBs

For inorganic anode materials, we herein mainly focus on the recent processes involving carbon materials, alloy anodes, metal oxides/sulfides based on conversion reaction and intercalation‐based titanium‐based compounds as flexible electrodes for flexible SIB applications.

Unlike LIBs, graphite has poor electrochemical performance for sodium storage, which might be related to the larger radius of Na⁺ and also because of thermodynamic issues [36, 37]. Fortunately, disordered carbon, such as hard carbon, soft carbon, and graphene, is particularly attractive due to its disordered structure and larger interlayer distance that could accommodate sodium. Yu and coworkers reported flexible and lightweight porous carbon nanofibers (P‐CNFs) with 3D interconnected structures using simple electro‐spun technology and subsequent pyrolysis process [38]. The P‐CNFs can be directly used as anodes for SIBs, and exhibit a high reversible capacity of 266 mAh g⁻¹ after 100 cycles at a current of 50 mA g⁻¹. Singh and coworker first investigated the electrochemical behavior of reduced graphene oxide (rGO) paper electrode on lithium/sodium storage under different annealing temperatures [39]. They found that the lithium storage capacity increased with increasing thermal treatment temperature in Ar gas from 300 to 900 °C. However, the sodium storage capacity was highest at 500 °C [39], which is due to the decreased interlayer spacing and increasing order of rGO paper during increasing temperatures. Further, they found that both the fracture strength and corresponding strain to failure decreased for rGO paper prepared at increasing treatment temperatures [39]. In order to improve the sodium storage performance of graphene, Shao and coworkers prepared sulfur‐doped flexible graphene films (SFGs) by using thiourea as the reducing agent and sulfurizing agent [40]. The SFGs showed a high reversible capacity of 377 mAh g⁻¹ at 100 mA g⁻¹, which might be related to S doping as it induces larger interspacing layers and creates additional redox sites for reaction with sodium [40].

2D layered transition metal dichalcogenides are promising anodes for energy storage applications [41]. In particular, molybdenum disulfide (MoS₂)has received extraordinary attention in recent years as a new anode for SIBs due to its moderate potential, high specific capacity and highly flexible layered structure 42. The electrochemical reaction of MoS₂ is based on intercalation and conversion, where Na⁺ firstly intercalates into the S–S layered space to form Na_xMoS₂, then the Na_xMoS₂ converts into the Mo and Na₂S phase. David et al. [43] reported a self‐standing flexible electrode composed of acid‐exfoliated few‐layer MoS₂ and rGO flakes for SIBs, which is synthesized through vacuum filtration of homogeneous dispersions of acid‐treated MoS₂ flakes and GO aqueous solution followed by thermal reduction (Figure 4.3a–e). Furthermore, mechanical tests involving static uniaxial tension reveal good mechanical strength of about 2–3 MPa and high failure strain (approximately 2%) of this rGO/MoS₂ composite paper (Figure 4.3f,g). When evaluated as an electrode for SIB, the film shows highly reversible capacity with a stable charge capacity of approximately 230 mAh g⁻¹43. For a cost‐effective large‐scale production method for MoS₂ exfoliation/dispersion, Hu and coworkers [44] reported an efficient method for MoS₂ exfoliation by using a green dispersant, 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO)‐oxidized nanofibrillated cellulose in an aqueous solution. Multifunctional MoS₂ films were fabricated by filtering with carbon nanotubes (CNTs) and were employed as flexible electrodes for Li/SIBs [44]. Gogotsi, Wang, and coworkers designed a freestanding anode material consisting of molybdenum disulfide nanosheets aligned vertically on carbon paper derived from commercial paper towels [45]. The MoS₂ nanosheets were firstly deposited on the flexible substrate, a carbon paper, under hydrothermal process at 180 °C for 24 h, then calcined at 800 °C in Ar atmosphere. The unique structure favors fast electron transfer and electrolyte diffusion, enabling high initial Coulombic efficiency (79.5%), reversible capacity, good cycling performance, and excellent rate capability, delivering a capacity of 205 mAh g⁻¹ at 1000 mA g⁻¹. The reversible phase transition from 2H‐MoS₂ to 1T‐MoS₂ is observed by using in situ Raman spectroscopy during the electrochemical cycling [45]. Self‐standing electrode of MoS₂ nanosheets assembled on graphene foam, which were grown on nickel foam by chemical vapor deposition (CVD) method was achieved by a single microwave‐assisted hydrothermal method. With a high conductive network as well as the interconnected channel properties of the 3D graphene foam, which enable fast electron transport, efficient penetration of electrolyte and short sodium ion diffusion paths, the resulting composites exhibit superior rate performance and high specific capacity of 290 mA h g⁻¹ after 50 cycles at a rate of 0.1 A g⁻¹ in SIBs [46]. As a close analogue to MoS₂, MoSe₂ has smaller band gap and larger layer space, thus showing better electric conductivity and Coulombic efficiency for SIB application. Zhang et al. reported MoSe₂ nanosheets grown on carbon cloth by a solvothermal method as the flexible electrode for SIBs for the first time [47]. When used as a self‐supporting anode directly, the composite delivers high capacity of 452.6 mA h g⁻¹ at 0.2 A g⁻¹, superior rate ability at 5 A g⁻¹, and keeps 85.5% capacity retention after 100 cycles [47].

Layered molybdenum trioxide (MoO₃) has been studied as an attractive electrode in SIBs due to its high specific capacity, low cost, and resource abundance [48]. However, poor cycling stability has limited its application originating from its low electronic/ionic conductivity and irreversible phase transition. Yao and coworkers investigated partially reduced MoO_3−x grown on flexible carbon cloth substrate, where the oxygen vacancy significantly improved the cycling stability and the carbon cloth increased the electronic conductivity of MoO₃ material, as shown in Figure 4.4a. The capacity retention is 92% after 2000 cycles at 1 A g⁻¹49. Compared to a conventional electrode, the above flexible electrode exhibits lighter weight, favorable mechanical strength, and higher electric conductivity without carbon additive, binder, and metal current collector, demonstrating promising perspective and great potential for high energy and power SIB application (Figure 4.4b,c). Similarly, flexible carbon‐coated FeS on carbon cloth substrate was prepared by a facial hydrothermal methodcombined with a carbonization treatment [50]. When used as free‐standing electrode for sodium storage, it could deliver a reversible capacity of 365 mAh g⁻¹ after 100 cycles at 0.15 C [50].

Tin and Tin‐based composites attracted remarkable attention due to their high specific capacity, low cost, and environmental friendliness [51]. However, this kind material usually suffers from deteriorating capacity and cycling stabilities owing to large volume expansion, leading to pulverization of active materials, loss of electrical contact with current collector, and unstable solid electrolyte interphase (SEI) layer [52]. Wang and coworkers [53] reported SnS₂@graphene nanosheet arrays grown on carbon cloth substrate as binder‐free anodes for SIBs. The composite is achieved by using a facile two‐step solvothermal route. The integrated structure facilitates efficient electron transfer and fast kinetics, and improved rate and cycling capability was observed for this composite [53]. Very recently, tin nanodots (1–2 nm) encapsulated in porous nitrogen‐doped carbon nanofibers were synthesized through feasible electro‐spinning techniques and thermal treatment processes using polyacrylonitrile, poly(methyl methacrylate), and SnCl₂ as precursors, as shown in Figure 4.554. The size and content of Sn particles can be controlled by changing their synthesis condition. The as‐obtained nanofibers with flexible membrane could be directly used as binder‐ and current collector‐free anode for SIBs. As expected, the hybrid film exhibits outstanding rate capability and ultralong cycling performance, and a high specific capacity of 450 mAh g⁻¹ can be delivered at a current density of 10 A g⁻¹. When cycling at 2 A g⁻¹ for 1300 cycles, only 10% capacity loss is observed at such a high rate, which is the highest performance reported for tin electrode in SIBs. They further evaluate its practicability in a sodium ion full cell. When coupled with NaVPO₄F/C cathode, the full cell delivers an initial capacity of 540 mAh g⁻¹ with a capacity retention of 85.2% over 100 cycles [54]. Among the reported alloy‐based materials, Sb seems to the most promising anode candidates for SIBs due to its large capacity of 660 mAh g⁻¹ and high electrical conductivity. However, the large volume change during cycling is difficult for the application of Sb‐based materials in SIBs. Wang and coworkers also fabricated antimony/carbon fibers electrode by a simple electro‐spinning method, where ∼30 nm Sb nanoparticles were well dispersed inside the carbon fibers [55]. The binder‐free film exhibits high specific capacity and excellent cycling stability with 92% capacity retention after 100 cycles and 75% over 300 cycles [55].

Phosphorus appears to be the most attractive anode candidate for SIBs because of its ultrahigh theoretical capacity of 2596 mAh g⁻¹, attractive potential plateau, and low cost [56]. The main disadvantage is its low conductivity and rapid capacity degradation caused by large volume expansion (>490%) during cycling. Zhang et al. synthesized amorphous P@N‐doped graphene film via the well‐known “phase‐transformation” approach with superior capacity retention of 85% over 350 cycles and rate capability; a high specific capacity of 809 mAh g⁻¹ can be obtained at 1.5 A g⁻¹, as shown in Figure 4.657. For the metal oxide anode, there is an increasing interest in developing flexible electrodes for SIBs, such as hollow γ‐Fe₂O₃ nanoparticles/CNTs composite film [58], γ‐Fe₂O₃film grown directly on a copper sheet [59], and CuO quantum dots embedded in carbon nanofibers [60].

Due to their low toxicity, low cost, wide abundance, and acceptable voltage, titanium‐based materials including TiO₂ and Na–Ti–O systems are attractive for sodium ion storage. TiO₂ has achieved great success as an anode material for LIBs. Compared to lithium ion storage, the operating potential for TiO₂ must be lowered to more negative values for sodium ion insertion. Mitlin and coworkers [61] successfully employed anatase TiO₂ nanocrystals as anode materials for SIBs for the first time. The mesoporous TiO₂ nanocrystals exhibited a stable reversible sodium ion storage capacity of 150 mAh g⁻¹ at the cycling rate of 50 mA g⁻¹. The redox potential is about 0.60 (cathodic) and 0.95 V (anodic) [61]. Tirado and coworkers [62] reported 1D self‐supported TiO₂ nanotube arrays through oxidation of a titanium substrate at high voltage of 100 V. The TiO₂ nanotube obtained by this method is amorphous at room temperature but it becomes crystallized to anatase or rutile by annealing. The sodium storage mechanism of faradic and pseudocapacitive reaction was studied for anatase TiO₂ nanotube by cyclic voltammetry (CV) technology. According to their investigations, the electrochemical reaction is dominated by pseudocapacitance (surface redox reaction) in the region over 0.5 V and by irreversible faradic reaction below 0.3 V. The pseudocapacitive contribution was also confirmed by ²³Na MAS NMR spectra. In order to enhance the electrochemical performance of TiO₂, Cao and coworkers [63] employed rGO as a flexible and conductive supporter by a facile spray‐drying method. The TiO₂@rGO electrode exhibited high reversible capacity of 170 mAh g⁻¹ at 0.1 C (1 C = 330 mA g⁻¹) and high rate capability of 55 mAh g⁻¹ at 10 C. After 300 cycles at 0.2 C, the capacity is about 87%.

Recently, a few layered (or tunnel structures) titanium‐based oxides, Na–Ti–O compounds, were investigated for SIB anodes utilizing the topotactic intercalation mechanism [8, 64]. Most of these compounds have the general structural features of Na₂Ti₃O₇ and Na₂Ti₆O₁₃ in that they consist of TiO₆ octahedra connected by edges and corners to form either layered or tunnel structures 64. Palacín and coworkers [65] first reported that two sodium ions could be intercalated into the layered Na₂Ti₃O₇ with a theoretical capacity of 200 mAh g⁻¹ at an average insertion potential of 0.3 V. However, Jiao and coworkers revealed that the theoretical capacity of Na₂Ti₃O₇ was 311 mAh g⁻¹ by first‐principles geometry analysis [66]. They presumed that the resultant difference between the practical capacity and simulation values might be due to the charge–discharge current density being too high to allow the complete intercalation reaction of sodium [66]. Recently, Li et al. developed a series of self‐supported Na₂Ti₃O₇ nanotube arrays on titanium foil by hydrothermal reaction and subsequent hydrogenation or sulfidation treatment. These self‐supported materials can be used directly as the working electrode without any conductive agent and binder. Although the electrochemical performance of the self‐supported electrodes is good, the flexibility of the electrodes is defective. In order to fabricate a flexible electrode, Wang and coworkers [67] employed carbon cloth as a flexible and conductive substrate and then designed an ultralong Na₂Ti₃O₇ nanowires/carbon cloth (L‐NTO NWs@CC) electrode by a one‐pot hydrothermal strategy, as shown in Figure 4.7. The interwoven nanowires and carbon networks enable sufficient electro‐active material/electrolyte contact and fast charge transportation. As a self‐supported flexible electrode for SIB, this L‐NTO NWs@CC anode exhibits a high specific capacity of 170 mAh g⁻¹ and outstanding cycling stability (96% of capacity retention after 200 cycles at a current density of 2 C). Even at 3 C (1 C = 177 mA g⁻¹), the discharge capacity still remains as 100.6 mAh g⁻¹ after 300 cycles.

Na₂Ti₆O₁₃ is another most investigated titanium‐based material with a tunnel structure. Two sodium ions are located in each tunnel but there are only three cubic sites in all, suggesting that only one sodium can insert into Na₂Ti₆O₁₃, corresponding to approximately 50 mAh g⁻¹. Brousse and coworkers [68] first reported the electrochemical properties of Na₂Ti₆O₁₃ in sodium storage and found that only half a mole of sodium could effect reversible insertion/extraction. Though the capacity of Na₂Ti₆O₁₃ is quite low, it exhibits impressive cycling performance of over 5000 cycles and high rate capability (over 30 C) [69]. Recent works have shown that a combination of the features of the layered and tunnel‐like structures is expected to enhance the structural stability and rate performance of the Na₂Ti₃O₇ and Na₂Ti₆O₁₃. Wang et al. [70] developed self‐supported sodium titanate (NTO) nanotube arrays on the titanium substrate, as shown in Figure 4.8, which exhibit long cycle life (5000 cycles) with a reversible capacity of 55 mAh g⁻¹70.

4.3.2 Inorganic Cathode Materials for Flexible SIBs

Since the resurgence of research on SIBs in 2010, various positive electrode materials have been prepared and tested for SIB applications. The recent progress in positive materials for SIBs mainly focused on layered transition metal oxide materials (P2‐type, O3‐type layered metal oxide), polyanionic compound (phosphates, fluorophosphates, fluorosulfates, sulfates, pyrophosphates), Prussian blue analogues, and the promising organic cathodes [6]. Nonetheless, studies on flexible SIB cathode are still in their infancy compared with LIB technology. In this section, we briefly present the most recent progress and discovery of promising cathode materials and their sodium storage mechanisms for flexible SIBs as well as the efforts to boost their electrochemical properties.

Flexible and binder‐free electrode consisting of Na_2/3[Ni_1/3Mn_2/3]O₂ cathode and rGO has been fabricated by a simple vacuum filtration process. The ππ bonds of rGO impart strong intercalation between rGO sheets and Na_2/3[Ni_1/3Mn_2/3]O₂ particles, which significantly enhance electrical conductivity of Na_2/3[Ni_1/3Mn_2/3]O₂ particles [71]. The electrode delivered a specific capacity of 86 mAh g⁻¹ at 0.1 C, good excellent stability of 200 cycles and keeps a capacity retention of 68.4% at 10 C current density [71]. Polyanionic compounds are another kind of potential cathode material for SIBs because of their structural robustness and stability. Na_2+2xFe_2−x(SO₄)₃@P‐CNF hybrid films were synthesized by the combined use of electro‐spinning and electro‐spraying techniques and employed as self‐supported and flexible electrodes for SIBs (Figure 4.9) [72]. By combining the merits of the flexible structure with decreased inert components, nanoparticle morphology, highly conductivity, and excellent stability, the free‐standing hybrid film exhibits superior sodium intercalation kinetics and ultralong life of 500 cycles with the capacity retention of >95% at 40 C [72]. Zhang et al. reported the hierarchical mixed polyanion material Na₇V₄(P₂O₇)₄(PO₄)/C nanorod–graphene composite as flexible electrode for lithium and sodium storage (Figure 4.10a) [73]. The composite consists of carbon coated Na₇V₄(P₂O₇)₄(PO₄)/C nanorod wrapped by a rGO sheet, which approaches high initial reversible specific capacities with reasonable discharge potentials over 3.8 V, high rate capabilities, and good cycling properties with capacity retentions of 95% and 83% after 200 cycles at a 1 C rate in sodium and lithium battery systems, respectively. The enhanced electrochemical properties can be contributed to the bicontinuous electron and ion pathways of the open network structure, improved conductivity, and large surface area (Figure 4.9b) [73]. Owing to the large radius of sodium (0.97 vs 0.68 Å for lithium) and heavy atomic mass, the sodium intercalation/de‐intercalation in conventional polyanionic compounds often leads to severe structure degradation and sluggish diffusion kinetics. Vanadium oxides with a layered structure have been recently investigated as promising candidates for SIBs because of their high theoretical specific capacity, large interlayer space, low cost, and abundant sources. Mai's group studied the flexible vanadium oxide hydrate H₂V₃O₈ nanowire membrane synthesized under a hydrothermal and suction filtration process method as binder‐free cathode for SIB application, which shows high specific capacity and long cycling performance for about 280 cycles [74]. They also investigated the sodium storage mechanisms by using CV, ex situ X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X‐ray photoelectron spectroscopy (XPS) measurements. Similarly, free‐standing carbon quantum dot coated VO₂ interwoven nanowires as flexible cathode for lithium and SIBs were presented by Tong's group for the first time [75], where the VO₂ nanowires were firstly directly grown on a carbon cloth substrate and followed by surface engineering using carbon quantum dots. The carbon quantum dots could serve the function of protecting the nanowire surface and play an important role for charge transfer, improving electrical conductivity and exhibiting favorable kinetics during battery operation [75]. Free‐standing bulky paper fabricated by suction filtration of V₂O₅ nanobelts obtained by ambient dissolution–recrystallization process and multi‐walled carbon nanotubes (MWCNTs) also show good electrochemical performance for LIBs and SIBs [76].

Prussian blue and its analogues perform as excellent alkali‐ion rechargeable battery electrodes owing to their inherently open framework structure with large interstitial sites, low cost, and easy accessibility [77, 78]. Importantly, Prussian blue analogues have been demonstrated as potential hosts for reversible di−/tri‐valent ion insertion, such as Mg²⁺, Ca²⁺, Pb²⁺, Zn²⁺, Al³⁺, and Y³⁺ in aqueous/nonaqueous electrolytes. However, their application is hampered by insufficient cycle life and low energy efficiency in nonaqueous electrolytes for SIBs, associated with the low electrical conductivity and structural imperfection. Zhang and coworkers reported a significant advance in the design and preparation of Prussian blue analogue FeFe(CN)₆·xH₂O nanoparticles on a flexible carbon fiber paper substrate as a binder‐free cathode in both organic and aqueous electrolytes by using a simple solution precipitation method [79]. Its sodium storage mechanism and improved electrochemical performance were examined by ex situ ⁵⁷Fe Mossbauer spectroscopy and CV technology. With the merits of flexible structure and high conductivity, the resulting FeFe(CN)₆·xH₂O nanocomposite electrode exhibits a reversible specific capacity of 82 mAh g⁻¹ at 0.2 C, excellent cycling performance for 1000 cycles with 81.2% capacity retention, and good high rate response up to 10 C.

In addition, sodium–air batteries have emerged as potential alternatives to lithium–air batteries owing to resource abundance, cost competition, safety, and environmental friendliness. Kim, Ko, and coworkers synthesized a binder‐free and robust electrode by directly growing hierarchical urchin‐shaped α‐MnO₂ on rGO‐coated carbon microfiber (MGC) and used it as an air electrode for aqueous sodium–air battery [80]. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed urchin‐shaped MnO₂ microspheres composed of radially oriented MnO₂ nanowires with diameters of 20–25 nm uniformly covered on the carbon fibers. Owing to the urchin‐shaped structure of the MnO₂, high surface area and the superior electrocatalytic activity of the MGC, the assembled aqueous Na–air battery exhibited excellent recharge ability up to 20 cycles with a low over potential gap of 0.7 V and high round‐trip efficiency (81%), which is comparable to that of the battery using the noble metal Pt.

4.3.3 Organic Materials for Flexible SIBs

Organic electrode materials are attracting increasing attention as alternatives to conventional inorganic electrode materials due to their sustainability, universal availability, better compatibility with flexible substrates, and simpler packaging, which enable flexible SIBs to have better flexibility [81, 82]. Different from inorganic materials, which is naturally brittle, organic materials possess flexible and soft structures. The volume change of organic materials could be negligible after intercalation of sodium [83]. Although some innovative works have been developed in the design of organic material‐based SIBs, their performance is still far behind that of inorganic material‐based devices, especially in terms of cycling performance. It is very important to highlight that the physical and chemical properties of organic materials can be adjusted by chemical syntheses with different functional groups. The working voltage can be regulated through controlling the lowest unoccupied molecular orbital (LUMO) energies [84]. For example, adding electron‐donating (such as –ONa, –CH₃, –NH₂, etc.) or electron‐withdrawing (such as –F, –Cl, –Br, –NO₂, –OCH₃, etc.) groups, the LUMO energies can be rationally enhanced or lowered, thus leading to reduced or enhanced working voltage. Increasing the active groups can enhance the theoretical specific capacity. The dissolution in electrolyte can be inhibited by formatting salt or/and elevating the intermolecular forces. The conductivity can be enhanced by extending the conjugated structures in the core unit.

Quinone‐based organics is a promising cathode material for flexible SIBs. However, the most quinone‐based cathode usually suffers from low working voltage (under 2.5 V vs Na/Na⁺). Park, Kang, and coworkers reported a quinone‐derived organic cathode, tetrachloro‐1,4‐benzoquinone (C₆Cl₄O₂), by introducing electronegative elements with high energy density of 580 Wh kg⁻¹ and the highest working voltages among organic materials (2.72 V vs Na/Na⁺) [85]. Density functional theory (DFT) calculations demonstrated that the introduction of electronegative elements (here it is Cl) into the quinone structure lowered the LUMO energies of the C₆Cl₄O₂, which in turn increased the sodium storage voltage [85]. To inhibit the dissolution of tetrachloro‐1,4‐benzoquinone, they incorporated C₆Cl₄O₂ molecules into mesoporous carbon (CMK). The C₆Cl₄O₂/CMK delivered higher capacity and longer cycling stability.

Most of the organic electrode materials were tested in a very slow charge/discharge rate (e.g. 10 mA g⁻¹) and were observed to suffer from significant capacity degradation under high rate [86, 87], which is related to the low conductivity of organics. To improve the rate performance, Lei et al. reasonably designed sodium 4,4′‐stilbene‐dicarboxylate (SSDC) for anode material by extension of the π‐conjugated structure 88. As shown in Figure 4.11, the extension of the π‐conjugated system, SSDC, delivered a high reversible capacity of 220 mAh g⁻¹ at a current density of 50 mA g⁻¹. Even at ultrahigh current density of 10 A g⁻¹, a reversible capacity of 72 mAh g⁻¹can be obtained. The capacity retention is over 70% after 400 cycles at 1 A g⁻¹. The wonderful electrochemical performance of SSDC, probably due to the extension of π‐conjugated structure can: (i) enhance the ion/electron transport and (ii) increase the stability of the charged and discharged states [88]. Recently, Lei and coworkers systematically investigated the important factors for high performance organic electrode materials by using disodium rhodizonate salt (DSR) as a model [84]. They found that the solubility, morphology, electrochemical window and binder are very important for high performance SIBs, especially, with high reversible capacity and long‐term cycling stability under fast‐charge/discharge reaction [84].

4.3.4 Other Major Components for Flexible SIBs (Electrolyte, Separators, etc.)

Electrolytes and separators are also very important for flexible SIBs, especially in terms of safety, because flexible devices usually integrate with the human body. In conventional liquid organic electrolytes, sodium dendrite issue is an intractable problem, which will cause disastrous security problems. In this regard, solid electrolytes may be an ideal solution to these crucial issues.

Although solid‐state ceramic electrolytes have been developed for many years and show good safety in LIBs, they are inapplicable for flexible devices due to their rigid nature and low ionic conductivity (<0.1 mS cm⁻¹) [89]. Gel polymer electrolytes (GPEs) may be a better choice for flexible SIBs, which usually consist of conventional liquid electrolytes and polymers with good mechanical strength and high ionic conductivity [90–92]. The polymers usually act as separators, while the liquid electrolytes act as charge carriers. Wu and coworkers [93] reported a sodium ion conducting gel polymer electrolyte (S‐GPE) based on poly(vinylidene difluoride‐co‐hexafluoropropylene), P(VDF‐HFP) porous membrane. Then the P(VDF‐HFP) membrane was soaked in conventional organic electrolyte to obtain the S‐GPE. The GPE possesses high ionic conductivity of 0.60 mS cm⁻¹ at ambient temperature, good mechanical strength and thermostability, and high safety.

In order to further increase the ionic conductivity, developing a polymer/oxide composite membrane separator is an alternative strategy. Archer and coworkers [94] reported a sandwich‐type composite membrane by laminating a nanoporous sheet of γ‐Al₂O₃ with P(VDF‐HFP). Although the membrane had an acceptable ionic conductivity and a good mechanical modulus, it lacked flexibility [94]. Lately, Goodenough and coworkers [95] developed a polymer P(VDF‐HFP)‐polyvinylpyrrolidone (PVP)/Sb₂O₃ composite membrane separator for SIBs with ultraflexible, dendrite‐blocking and low‐cost characteristics (Figure 4.12). The larger particles of 5 μm Sb₂O₃ provide a better flexibility than a nanoporous sheet of γ‐Al₂O₃. The PVP forms pores that are large enough to incorporate an organic electrolyte and are small enough to retain the Sb₂O₃ particles [95]. The composite membrane separator has a wide electrolyte window, and wonderful mechanical and thermal stabilities when soaked with 1 M NaClO₄ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte. The sodium ion conductivity of this S‐GPE is 6.4 mS cm⁻¹ at 25 °C. Even at 250 °C, the P(VDF‐HFP)‐(PVP)/Sb₂O₃ composite membrane is stable.

Although using oxides, such as Al₂O₃, SiO₂, TiO₂, etc., can improve the ionic conductivity of the polymer electrolytes 94, [96–98], this method also causes poor thermal and mechanical stability to some extent. To address this problem, using ionic liquids to replace the conventional metal oxides may be a good strategy due to their wide liquidus range, good electrochemical stability, high ionic conductivity, and excellent thermal stability. Besides, ionic liquids also can act as the supplier of free ion charge carriers. Singh et al. [98] developed a novel S‐GPE membrane based on polymer, poly(ethyleneoxide) (PEO); ionic liquid, 1‐butyl‐3‐methylimidazolium methylsulfate (BMIM‐MS); and sodium salt, sodium methyl sulfate salt (NaMS), by solution cast technique. The as‐prepared polymer electrolyte membranes are free‐standing and flexible with good mechanical stability. This optimized polymer electrolyte shows high electrochemical potential window of 4–5 V with good thermal stability over 300 °C [98].