Flexible Energy‐Storage Devices: Design Consideration and Recent Progress

Review: 193 refs.

The demands for new energy storage systems capable of providing power for various wearable electronic devices are generating more research interest. Herein, we develop a universal and effective strategy to integrate Zn ion battery (ZIB) and asymmetric supercapacitor (ASC) into one flexible energy storage device, which can switch between high energy density and high-power density without interfering with each other. The NiCo2S4-x nanosheet arrays with sulfur deficiency grown on carbon cloth substrate are used as efficient cathode for ZIB and ASC. The designed flexible ZIB delivers an energy density of up to 464.2 Wh kg−1, while the assemble flexible ASC provides a maximum power density of 8001.2 W kg−1. Subsequently, a flexible dual-functional three-electrode energy storage device (TEESD) is fabricated by a general stacking strategy. As proof-of-concept demonstrations, two flexible dual-functional TEESDs can switch between ZIB and ASC to power a digital watch and light LED thanks to the common cathode. In addition, the integrated TEESDs exhibits superior electrochemical stability and performance durability, which is beneficial for the practical applications in flexible and wearable devices. The general strategy disclosed herein is expected to shine new light into the development of advanced flexible electronics and wearable energy textiles.

With the boom of flexible electronic products and wearable devices, flexible energy storage devices, for example, supercapacitors with high performance, are attracting increasing interest. A flexible water-deactivated polyelectrolyte hydrogel electrolyte with good mechanical properties and high ionic conductivity was prepared by using an anionic polymer, carboxy methyl cellulose, and a cationic monomer, methacrylamidopropyltrimethyl ammonium chloride. It was then applied in a supercapacitor with flexible activated carbon electrodes. This flexible supercapacitor possesses a high operating voltage of 2.1 V owing to the low electrochemical activity for water within the hydrogel as a result of the 'molecular cages' effect and hydrophilic interactions between functional groups and surrounding water molecules. Furthermore, this supercapacitor exhibits good flexibility and tailorability. As the first example of water-deactivated polyelectrolyte hydrogel electrolytes in applications involving flexible high-voltage supercapacitors, this work provides a platform for the design of energy storage devices with high energy density for flexible and wearable electronic devices.

With the rapid development of portable and wearable electronics, the design and fabrication of flexible electrochemical energy storage devices, including batteries and supercapacitors, have attracted tremendous attention among both scientific and industrial fields. Inkjet printing is considered to be a promising technology for manufacturing low‐cost high‐performance flexible energy storage devices because of its material‐saving feature and pattern‐writing flexibility. Very recently, great efforts have been dedicated to adapting inkjet printing for the production of practical flexible energy storage devices. In this review, inkjet printing operation mechanisms, ink properties, and the interaction between the droplet and substrate are first described in detail. Then the development of inkjet‐printed flexible electrochemical energy storage devices in recent years is focused on from the perspective of electrode materials. Next, the application of inkjet‐printed flexible energy storage devices in self‐powered electronic systems is briefly introduced. At last, challenges and future development directions of inkjet‐printed flexible energy storage devices are further discussed.

<p indent="0mm">At present, the flexible electronics industry is in an important transitional period, and various flexible electronic devices with novel properties and enhanced functions are constantly emerging, from limited flexibility to flexible electronic devices with shape adaptability and ductility. This has greatly stimulated demand for flexible electronic devices. The development of manufacturing technologies for flexible circuits with smaller feature sizes and better performance on larger-format substrates at lower costs has attracted increasing attention. Among various technologies, laser direct writing has proven to be an efficient and flexible manufacturing method that can produce large-area flexible electronic circuits. As a nonlithographic, nonvacuum, and online processing technology, laser direct writing has received considerable attention. It can be employed in the production of circuit electrodes of various substrates, including heat-sensitive flexible substrates, and it has huge application prospects in the production of flexible electronic and energy storage devices, sensors, and wearable electronic devices. On this basis, this article summarizes the latest development in the laser direct writing of flexible circuit (LDWFC) technology for manufacturing flexible circuits. It focuses on the material types and characteristics of the conductive inks suitable for LDWFC in the production of flexible circuits. The LDWFC processing technology is introduced in detail from four aspects: laser sintering technology, laser reduction technology, laser-induced modification technology, and laser auxiliary circuit manufacturing technology. In addition, the application of LDWFC in flexible energy storage devices, tenderness sensors, and flexible displays is introduced, and the development prospects of LDWFC technology in flexible circuit manufacturing are highlighted.

Stretchable ionogels: Recent advances in design, toughening mechanisms, material properties and wearable devices applications

Nowadays, researchers have made great efforts on the development of flexible and light weight energy storage devices for their practical applications and the advancement of modern electronic devices. Owing to the promising feathers of high specific power, high rate capability, and long-term cycling life, the supercapacitors (SCs) are considered as highly suitable for various flexible applications. In general, the carbon-based nanomaterials such as carbon nanotubes and graphene nanosheets, exhibit good supercapacitor performance. Manganese dioxide (MnO2) are widely studied for pseudocapacitors owing to their high specific capacitance, high power, and energy density. Thus, MnO2 was applied to increase the supercapacitor performance of carbon materials. However, most of the reported SCs are bulk, heavy, and non-flexible, which are not suitable for wearable energy technology. To overcome these challenges, flexible supercapacitors (FSCs) are developed to meet the wearable electronics. The cotton cloth materials are widely considered for flexible substrates due to inexpensive natural fiber, highly hydrophilic and light weight. In this work, the conductive cotton was successfully prepared by the screen-printing method using the developed ink. The designed textile ink and silver powder mixture demonstrates an outstanding conductivity. The prepared conductive cotton reached a low resistance of less than 15 ohm/cm2. Furthermore, the supercapacitor electrodes were also fabricated by mixing the active materials (CNT, and graphene) into the developed ink. Among these different carbon electrodes, the CNT electrodes show superior electrochemical performance (78.49 mF/cm2 at 0.1 mA/cm2).� To further enhance the specific capacitance, the MnO2 was coated on the electrodes by chemical bath deposition (CBD) using potassium permanganate and sulfuric acid solution. The specific capacitance of as high as 741.83 mF/cm2 at 0.1 mA/cm2 was achieved. Finally, the flexible supercapacitor device was also successfully fabricated, which exhibited a high specific areal capacitance of 677.12 mF/cm2 at 0.0125 mA/cm2 for CNT electrodes. The flexible device also shows excellent rate performance and cyclic stability with capacitance retention of 80% for 3000 cycles, demonstrating that advanced flexible energy storage devices can be achieved.

With the development of flexible and wearable electronic devices, it is a new challenge for polymer hydrogel electrolytes to combine high mechanical flexibility and electrochemical performance into one membrane. In general, the high content of water in hydrogel electrolyte membranes always leads to poor mechanical strength, and limits their applications in flexible energy storage devices. In this work, based on the "salting out" phenomenon in Hofmeister effect, a kind of gelatin-based hydrogel electrolyte membrane is fabricated with high mechanical strength and ionic conductivity by soaking pre-gelated gelatin hydrogel in 2m ZnSO4 aqueous. Among various gelatin-based electrolyte membranes, the gelatin-ZnSO4 electrolyte membrane delivers the "salting out" property of Hofmeister effect, which improves both the mechanical strength and electrochemical performance of gelatin-based electrolyte membranes. The breaking strength reaches 1.5MPa. When applied to supercapacitors and zinc-ion batteries, it can sustain over 7500 and 9300 cycles for repeated charging and discharging processes. This study provides a very simple and universal method to prepare polymer hydrogel electrolytes with high strength, toughness, and stability, and its applications in flexible energy storage devices provide a new idea for the construction of secure and stable flexible and wearable electronic devices.

Chapter 6 - Oxides free materials for flexible and paper-based supercapacitors

Review: 162 refs.

With wearable electronics rapidly coming into fashion, research into flexible energy storage devices and in particular, pliable electrodes, is attracting a lot of attention. Pliable electrodes are usually fabricated by intercalating an active material in a flexible matrix with superior mechano‐electrical properties, and can be grouped either as substrate‐supported or free‐standing. Depending on their mode of deformation the electrodes can be labeled as bendable, compressible or stretchable. Recent progress and challenges in the design and fabrication of pliable electrodes for constructing flexible lithium ion batteries and flexible supercapacitors are highlighted. 2 pliable core‐shell structure electrodes fabricated from: a) carbon nanotube sponge embedded with MoS2 nanoparticles and b) electrospun polyethylene terephthalate fibers impregnated with graphene nanoplatelets are also presented.

Heart failure (HF) is the fastest growing cardiovascular condition globally; associated management costs and hospitalizations place an immense burden on healthcare systems. Wearable electronic devices (WEDs) may be useful tools to enhance HF management and mitigate negative health outcomes. We aimed to perform a systematic review to examine the potential of WEDs to support HF self-care in ambulatory patients at home. Five databases were searched for studies published between 2007 and May 2022, including OVID MEDLINE, EMBASE (OVID), APA PsycINFO (OVID), Cochrane Central Register of Controlled Trials (OVID), and CINAHL Plus with Full Text (Ebsco). After 6210 duplicates were removed, 4045 records were screened and 6 were included for review (2 conference abstracts and 4 full-text citations). All studies used WEDs as 1 component of a larger intervention. Outcome measures included quality of life, physical activity, self-efficacy, self-care, functional status, time to readmission, social isolation, and mood. Studies were of moderate to high quality and mixed findings were reported. Enhanced exercise habits and motivational behavior to exercise, as well as decreased adverse symptoms of fatigue and dyspnea, were identified in 2 studies. However, improvements in exercise capacity and increased motivational behavior did not lead to exercise adherence in another 2 studies. The findings from this review suggest that WEDs may be a viable health behavior improvement strategy for patients with HF. However, studies of higher quality, with the primary intervention being a WED, and consistent outcome measures are needed to replicate the positive findings of studies identified in this review.

With the booming development of portable and wearable electronic devices, flexible energy storage devices have attracted great attention. Among various energy storage devices, aqueous zinc ion batteries (ZIBs) are one of the promising candidates due to their low cost, good safety, high energy and power densities. However, the conventional cathodes of aqueous ZIBs were often prepared by mixing active materials with binders and conductive additives and then coating them onto current collectors. The resultant cathodes often suffer from unsatisfied flexibility. Herein, we fabricated freestanding reduced graphene oxide/NaV3O8•1.5H2O (RGO/NVO) composite films with interlinked multilayered architecture by a vacuum filtrating process. Such composite films exhibit excellent mechanical property and high electronic conductivity. Owing to unique architecture, they display a high capacity of 410 mA h g−1 and excellent cycling performance up to 2000 cycles with a high capacity retention of 94%. Moreover, RGO/NVO composite films can directly serve as the cathodes of flexible aqueous ZIBs. As a proof of concept, flexible ZIBs were assembled based on the composite films. Impressively, they exhibit stable performance at different bending states, demonstrating great potential application in flexible energy storage devices.

The advancement of flexible and efficient energy storage devices is critical for next-generation portable electronics. In this work, we report the in-situ growth of nickel selenide (NiSe) nanoparticles over multi-walled carbon nanotubes (MWCNTs) to fabricate a high-performance flexible micro-supercapacitor (MSC) via a screen-printing technique. A novel conductive ink was formulated using NiSe/MWCNT nanocomposites and an eco-friendly binder system composed of cellulose acetate propionate and diacetone alcohol. The ink exhibited excellent thixotropic behaviour, ensuring optimal printability and film uniformity on PET substrates with silver current collectors. The fabricated symmetric MSC delivered a high areal capacitance of 321.9 $$\text{m}\text{F}{\text{c}\text{m}}^{-2}$$ at a current density of 0.5 $$\text{m}\text{A}{\text{c}\text{m}}^{-2}$$ and retained 93.3% of its initial capacitance after 5000 charge/discharge cycles. Furthermore, the asymmetric MSC (NiSe/MWCNT//AC) exhibited an outstanding areal capacitance of 478.9 $$\text{m}\text{F}{\text{c}\text{m}}^{-2}$$ and excellent cyclic stability. The synergy between the pseudocapacitive behaviour of NiSe and the high conductivity and surface area of MWCNTs significantly improved the electrochemical performance. This cost-effective and scalable screen-printing approach demonstrates strong potential for practical applications in wearable and flexible energy storage devices.

The rapid evolution of wearable and bio-integrated electronics has intensified the demand for high-performance, deformable energy storage systems that can seamlessly conform to the human body while maintaining electrochemical efficiency and mechanical durability. This review critically synthesizes recent advancements in flexible energy storage devices (FESDs), emphasizing cutting-edge developments from 2022 to 2025. It begins by exploring material innovations, including carbon-based nanomaterials like graphene, carbon nanotubes, and MXenes; metal nanowires and oxides; and hybrid composites, detailing their contributions to conductivity, flexibility, and energy storage performance. The discussion progresses to novel device architectures, such as planar, fiber-shaped, and origami-inspired geometries for both supercapacitors and flexible batteries, with special attention to electrode design, substrate selection, and encapsulation techniques that ensure resilience under bending, twisting, and stretching. Integration into real-world applications is analyzed across textile-based platforms, skin-mounted and implantable systems, and self-powered hybrid configurations that combine triboelectric, piezoelectric, or photovoltaic modules for autonomous operation. Experimental validations through real-time use cases in health monitoring, athletic performance, and military wearables underscore the feasibility of these technologies. This review also rigorously evaluates the core challenges impeding widespread adoption, including the trade-off between energy density and flexibility, cycling stability under mechanical stress, safety concerns, toxicity of active materials, and barriers in large-scale manufacturing and cost. Looking ahead, it identifies key research trajectories such as biodegradable electronics, AI-enabled energy systems, and edge-computing integration, and calls for intensified interdisciplinary collaborations spanning materials science, bioengineering, and human–machine interfacing. By articulating both the technological progress and strategic research pathways, this article presents a forward-thinking vision to guide academia and industry toward a new era of smart, energy-autonomous wearable systems.