Flexible Electrochemical Energy Storage Research Articles

The porous spongy nest structure compressible anode fabricated by gas forming technique toward high performance lithium ions batteries

The state-of-the-art electronics promote the development of flexible and deformable batteries, which rely on design of advanced structure batteries and fabrication of suitable electrode materials. The current flexible electronics are generally limited by rigidity and nondeformable electrodes. Herein, this work reports an exceeding compressible spongy carbon nanofibers composite anode which was fabricated by electrospinning and gas-forming techniques. The abundant macro/micro porous and loss structure of spongy layers enable the composite electrode exhibited compressible capability and faster ions infiltration ability. And the nest morphology of spongy carbon nanofibers network promised stable conductivity and superior cycling performance of self-standing anodes. The compressible SnO2@spongy carbon nanofibers and SiO2@spongy carbon nanofibers self-standing anode exhibited outstanding cycling ability before 300 cycles under compressed state, with a capacity of 350 and 398 mA h g−1, respectively. Notably, the stress and strain of compressible spongy composite electrode are 370 kPa and 92%, separately, with recovery ability. The compressible spongy anode is highly recommended for flexible electrochemical energy storage devices and the novel gas-forming technique is a potential method for fabrication of multi morphology electrode.

MOFs-DERIVED METAL OXIDES FOR FLEXIBLE SUPERCAPACITORS

As research on new technologies for the creation of flexible electrochemical energy storage systems continues, they are widely used in industries such as public portable devices, mobile electronics, and electronic products worldwide. Flexible supercapacitors (FSCs) are widely used in wearable electronics and medical fields, with high power density, excellent mechanical integrity and cycle stability. Researchers devoted a lot of time in recent years promoting several kinds of transition metal oxides (TMOs) for SCs. Organometallic frameworks (MOFs) are commonly utilized for the production of metal oxide-based electrodes for high-performance SCs as precursors or templates. These MOF-based metal oxides have a porous and adjustable structure, a large specific surface area, strong electrical conductivity, and good electrochemical stability, and they fulfil the specific capacitance and long-cycle power requirements of electrochemical double-layer capacitors. The latest developments in the production and application of MOF-derived metallic oxide compounds in FSCs are summarized in this study.

Revealing the superlative electrochemical properties of o-B2N2 monolayer in Lithium/Sodium-ion batteries

Promising flexible electrochemical energy storage systems (EESSs) are currently drawing considerable attention for their tremendous prospective end-use in portable self-powered electronic devices, including roll-up displays, and “smart” garments outfitted with piezoelectric patches to harvest energy from body movement. However, the lack of suitable battery electrodes that provides a specific electrochemical performance has made further development of these technologies challenging. Two-dimensional (2D) lightweight and flexible materials with outstanding physical and chemical properties, including mechanical strengths, hydrophilic surfaces, high surface metal diffusivity, and good conductivity, have been identified as a potential prospect for battery electrodes. In this study, taking a new 2D boron nitride allotrope, namely 2D orthorhombic diboron dinitride monolayer (o-B2N2) as representatives, we systematically explored several influencing factors, including electronic, mechanical, and their electrochemical properties (e.g., binding strength, ionic mobility, equilibrium voltage, and theoretical capacity). Considering potential charge-transfer polarization, we employed a charged electrode model to simulate ionic mobility and found ionic mobility has a unique dependence on the surface atomic configuration influenced by bond length, valence electron number, electrical conductivity, excellent ionic mobility, low equilibrium voltage with excellent stability, good flexibility, and extremely superior theoretical capacity, up to 8.7 times higher than that of widely commercialized graphite (3239.74 mAh g−1 Vs 372 mAh g−1) in case of Li-ion batteries and 2159.83 mAh g−1 in case of Na-ion batteries, indicating that the new predicted 2D o-B2N2 monolayer possess the capability to be ideal flexible anode materials for Lithium and Sodium-ion battery. Our finding provides valuable insights for experimental explorations of flexible anode candidates based on 2D o-B2N2 monolayer.

Flexible all solid-state niobium nitride//activated carbon lithium-ion hybrid capacitor with high volumetric power and energy densities

Flexible electrochemical energy storage devices are gaining considerable interest for use in smart portable flexible electronic devices. Herein, a (Niobium nitride) NbN// Activated carbon (AC) flexible (bendable and twistable) solid-state lithium-ion hybrid capacitor (LIHC) enabled by a thin graphite-layer-coated adhesive tape-based flexible current collector is reported. NbN is synthesized by a solvothermal treatment of the niobium pentachloride (NbCl5) precursor and a subsequent nitridation treatment of the intermediate solvothermal product. Microscopic, spectroscopic, and X-ray studies signify the formation of porous 2D nanoflake units with a cubic crystalline phase, which aggregate to form microspheres. The NbN displays a high specific capacity of 347.73 mAh g−1 at 0.2 A g−1 with excellent cyclic stability (98.1% capacity retention after 10,000 cycles). The outstanding cyclic stability is clarified by the analysis of electrochemical impedance spectroscopy (EIS). The designed highly flexible LIHC with a high electrode mass loading of 280 mg cm−3 possesses a high capacitance of 2597 mF cm−3 at a current density of 60 mA cm−3. The device displays a high-energy density of 3.25 mWh cm−3, high power density of 254.2 mW cm−3, and excellent cyclic stability at 133 mA cm−3 (97.7% capacity retention after 10,000 cycles). Therefore, the designed flexible LIHC can be utilized for the development of smart flexible electronic devices.

Mechanically exfoliated graphite paper with layered microstructures for enhancing flexible electrochemical energy storage

This work fabricates mechanically exfoliated graphite paper with layered microstructures, which not only ensures the high flexibility of the resulting electrochemical capacitor, but substantially boosts its electrochemical properties.

Size-refinement enhanced flexibility and electrochemical performance of MXene electrodes for flexible waterproof supercapacitors

Increasing mechanical flexibility without sacrificing electrochemical performance of the electrode material is highly desired in the design of flexible electrochemical energy storage devices. In metal-related materials science, decreasing the grain size introduces more grain boundaries; this stops dislocations and crack propagation under deformation, and results in increased strength and toughness. However, such a size refinement effect has not been considered in the mechanical properties, particle stacking, wetting, and electrochemical performances of flexible supercapacitor electrodes. In this paper, MXene was used as an electrode material to study the size refinement effect of flexible supercapacitors. Size refinement improved the strength and toughness of the MXene electrodes, and this resulted in increased flexibility. Finite elemental analysis provided a theoretical understanding of size refinement-increased flexibility. Moreover, the size refinement also improved the specific surface area, electric conductance, ion transportation, and water wetting properties of the electrode, and the size refinement provided highly increased energy density and power density of the MXene supercapacitors. A highly flexible, water-proof supercapacitor was fabricated using size-refined MXene. The current study provides a new viewpoint for designing tough and flexible energy storage electrodes. The size refinement effect may also be applicable for metal ion batteries and electronic and photo devices composed of MXene and other nanoparticles.

An Overview of Flexible Electrode Materials/Substrates for Flexible Electrochemical Energy Storage/Conversion Devices

AbstractThe rise of portable and wearable electronics has largely stimulated the development of flexible energy storage and conversion devices. As one of the essential parts, the electrode plays critical role in determining the device performance, which required to be highly flexible, light‐weight, and conformable for flexible and wearable applications. However, it remains a formidable challenge in the design of appropriate flexible electrodes. Thus, considerable effort has been making to develop various flexible materials/substrates to fabricate flexible energy devices. Here, this review aims to provide a comprehensive survey on the recently developed free‐standing and flexible electrode materials/substrates for flexible electrochemical energy storage devices, which are categorized into four different types including metal‐based, carbon‐based, polymer‐based, and micro‐patterned flexible electrodes. The specific characteristics, fabrication methods, properties, and pros and cons of each type of materials are thoroughly analysed. Furthermore, challenges and future directions for designing flexible electrode materials are also discussed.

Textile Composite Electrodes: Textile Composite Electrodes for Flexible Batteries and Supercapacitors: Opportunities and Challenges (Adv. Energy Mater. 3/2021)

In article number 2002838, Zijian Zheng and co-workers provide a critical review of the opportunities and challenges in the field of textile composite electrodes (TCEs) in flexible electrochemi cal energy storage devices. TCEs are compared with traditional metal-foil-supported electrodes, focusing particularly on fabrication techniques, electrode structures, and electrochemical and mechanical performance. The layouts of several textile-based electrochemical energy storage devices are outlined and their integration methods for flexible and wearable electronics discussed.

Flexible one-dimensional Zn-based electrochemical energy storage devices: recent progress and future perspectives

This review article summarizes the development of one-dimensional Zn-based electrochemical energy storage devices. The existing obstacles are diagnosed, corresponding solutions are proposed, and future research directions are indicated accordingly.

Electrospun Nanofibers for New Generation Flexible Energy Storage

As one of the essential components for flexible electronics, flexible electrochemical energy storage (EES) has garnered extensive interests at all levels of materials, devices, and systems. The successful implementation of high‐performance flexible EES devices relies on exploring of suitable electrode/electrolyte materials that have both superior electrochemical and mechanical properties. For this function, one‐dimensional electrospun nanofibers have emerged as a class of promising building blocks for the key components of flexible EES devices. In this overview, the fundamental principles and technical advances of electrospinning are examined, for both their successes and challenges in controllable fabrication of nanofibers with the desirable chemical compositions, micro/meso‐/nanostructures, and therefore resultant properties. The advances in applications of electrospun nanofibers for various key flexible EES devices are critically looked into, including those in supercapacitors, metal‐ion batteries, and metal‐air batteries. The existing challenges and prospects of these electrospun nanofiber‐based flexible EES are discussed, aiming to inspire continued efforts in developing the optimal high‐performance and low cost flexible EES devices for long‐awaited practical applications.

Wet-spinning fabrication of multi-walled carbon nanotubes reinforced poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) hybrid fibers for high-performance fiber-shaped supercapacitor

Fiber-shaped supercapacitors with high energy density and power density are promising as energy storage device for flexible electronics because of their one-dimensional distinct structure, excellent flexibility and potential to be integrated. In this study, multi-walled carbon nanotubes (MWCNT)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hybrid fibers with different fraction of MWCNT were fabricated via wet-spinning and post-treatment process. Due to the synergistic effects of two components, the designed hybrid fibers displayed excellent conductivity and mechanical property. Furthermore, the symmetric flexible supercapacitor based on our prepared hybrid fibers possessed an impressive specific capacitance (117.0 mF/cm2 at current density 0.1 mA/cm2), superior energy/power characteristics (16.3 μWh/cm2 at power density of 0.05 mW/cm2), as well as satisfactory cycle stability (90% capacitance retention after 2000 charge/discharge cycles) and flexibility (87% capacitance retention after 1000 bending cycles). Thus, MWCNT/PEDOT:PSS hybrid fibers are a promising candidate for constructing flexible electrochemical energy storage device.

High-Performance Anti-freezing Flexible Zn-MnO2 Battery Based on Polyacrylamide/Graphene Oxide/Ethylene Glycol Gel Electrolyte.

It remains a great challenge for aqueous zinc-ion batteries to work at subzero temperatures, since the water in aqueous electrolytes would freeze and inhibit the transportation of electrolyte ions, inevitably leading to performance deterioration. In this work, we propose an anti-freezing gel electrolyte that contains polyacrylamide, graphene oxide, and ethylene glycol. The graphene oxide can not only enhance the mechanical properties of gel electrolyte but also help construct a three-dimensional macroporous network that facilitates ionic transport, while the ethylene glycol can improve freezing resistance. Due to the synergistic effect, the gel electrolyte exhibits high ionic conductivity (e.g., 14.9 mS cm−1 at −20 °C) and good mechanical properties in comparison with neat polyacrylamide gel electrolyte. Benefiting from that, the assembled flexible quasi-solid-state Zn-MnO2 battery exhibits good electrochemical durability and superior tolerance to extreme working conditions. This work provides new perspectives to develop flexible electrochemical energy storage devices with great environmental adaptability.

An Overview of Bacterial Cellulose in Flexible Electrochemical Energy Storage.

The emerging generation of flexible energy storage devices has accelerated the research pace in terms of new materials, new processing techniques, and new designs that can meet the demands of mechanical stability upon bending or stretching at an acceptable cost, without compromising their electrochemical performance. Among the materials currently explored, biomass-derived materials have received extensive attention, because they are renewable, low in cost, earth-abundant and structurally diverse. This review is focused on fundamentals and applications of the bio-derived material bacterial cellulose (BC) in flexible electrochemical energy storage systems. Specifically, recent advances are summarized in the utilization of BC in stretchable substrates, carbonaceous species, and scaffolds for flexible core component construction. Finally, several perspectives related to BC-based materials for flexible electrochemical energy storages are proposed, aiming to provide possible future research directions in these fields.

Why Cellulose-Based Electrochemical Energy Storage Devices?

Recent findings demonstrate that cellulose, a highly abundant, versatile, sustainable, and inexpensive material, can be used in the preparation of very stable and flexible electrochemical energy storage devices with high energy and power densities by using electrodes with high mass loadings, composed of conducting composites with high surface areas and thin layers of electroactive material, as well as cellulose-based current collectors and functional separators. Close attention should, however, be paid to the properties of the cellulose (e.g., porosity, pore distribution, pore-size distribution, and crystallinity). The manufacturing of cellulose-based electrodes and all-cellulose devices is also well-suited for large-scale production since it can be made using straightforward filtration-based techniques or paper-making approaches, as well as utilizing various printing techniques. Herein, the recent development and possibilities associated with the use of cellulose are discussed, regarding the manufacturing of electrochemical energy storage devices comprising electrodes with high energy and power densities and lightweight current collectors and functional separators.

Single-pot hydrothermal synthesis of manganese phosphate microrods as a cathode material for highly stable flexible solid-state symmetric supercapacitors

Here, we report a binder-free synthesis of microrods-like manganese phosphate thin film over stainless steel substrate by a facile, single-pot hydrothermal method. The XRD analysis reveals that, the formation of manganese phosphate [Mn3(PO4)2] material of a monoclinic crystal structure. From SEM images observed microrods like morphology of manganese phosphate with an average width of ∼25 μm. The manganese phosphate electrode shows a better electrochemical performance with a specific capacitance of 145 F g−1 at 0.2 mA cm-2 current density in 1.0 M Na2SO4 electrolyte. Moreover, the flexible solid-state symmetric electrochemical energy storage device was assembled with PVA-Na2SO4 solid gel-electrolyte consists of manganese phosphate as anode and cathode electrode. The corresponding symmetric supercapacitor achieves a high energy density of 11.7 Wh kg−1 at a high power density of 1.41 kW kg−1 with an excellent specific capacitance of 37 F g−1 at 0.1 mA cm-2 current density. Manganese phosphate shows long-term electrochemical cyclic stability at a current density of 0.8 mA cm-2 for 9000 galvanostatic charge-discharge cycles with excellent capacitance retention (99 %). This excellent capacitive performance confirms that the manganese phosphate is promising material and fabricated flexible solid-state symmetric supercapacitor has high potential in the field of portable and bendable energy storage devices.

A self-healing hydrogel electrolyte for flexible solid-state supercapacitors

Flexible electrochemical energy storage devices have great potential in wearable and portable electronic devices. However, their practical applications have been deferred by mechanical damages such as repeated stretching, bending and squeezing. Self-healing hydrogel electrolytes can automatically repair such physical damages and maintain good electric contact at electrode/electrolyte interface. Herein, we report a hydrogel electrolyte with excellent tensile properties and good conductivity under large strain/stress. A solid state supercapacitor was further demonstrated, showing a high energy density of 21.3 Wh kg-1 at a power density of 350 W kg-1. The as-fabricated supercapacitor possesses good stretchability, with a 68% of retention of its initial capacitance after 40 breaking/healing cycles. The excellent self-healing capability of the hydrogel electrolyte can be attributed to the presence of hydrogen bonds and molecular chain fluidity within its structural motif. Our work provides promising routes towards developing new self-healing solid-state electrolytes for next-generation electrochemical energy storage and conversion applications with good performance and high safety.

Electrospun free-standing FeP@NPC film for flexible sodium ion batteries with remarkable cycling stability

Flexible and wearable portable electronic devices have attracted increasing interest as a promising technology for the growing flexible electrochemical energy storage, which however require excellent mechanical property, fast electrochemical kinetics and good structural stability of the electrode. To-date the reports about flexible sodium ion batteries are still rare owing to the lack of proper electrodes. Herein, we report a new binder-free anode material of electrospun free-standing FeP@NPC film with FeP nanoparticles wrapped in 3D interconnected N, P -codoped carbon fiber for high-performance sodium ion storage. The unique structure design offers various structural advantages: the growth of FeP nanoparticle is confined by the PAN nanofiber during the synthesis, and agglomeration of FeP is alleviated; the 3D connected carbon fiber network serves as electron/ion transport pathway to accelerate the reaction kinetics and meanwhile accommodates the volume expansion of FeP during the sodiation/disodiation process. A high reversible capacity of 557 ​mA ​h g-1 and remarkable cycling life of 1000 cycles are achieved. This research paves a novel strategy to construct the phosphides-based high-performance anode for the potential application in flexible energy storage devices.

Boosting the electrochemical properties of polyaniline by one-step co-doped electrodeposition for high performance flexible supercapacitor applications

A facile one-step co-doped electrodeposition method is proposed to prepare polyaniline (PANI) electrodes with improved supercapacitive properties. During electrodeposition of PANI films, sulfuric acid and perchloric acid simultaneously act as dopants to obtain sulfuric acid and perchloric acid co-doped PANI (SP-PANI) electrode. For comparison, single-doped PANI (i.e., sulfuric acid-doped PANI (S-PANI) and perchloric acid-doped PANI (P-PANI)) electrodes are prepared with the same electrodeposition parameters. Electrochemical measurements indicate that the SP-PANI electrodes exhibit substantially improved electrochemical capacitive properties with respect to single-doped PANI electrodes. This improvement can be attributed to the fact that the co-doping changes the morphology, electrical conductivity, and roughness of the PANI films electrodeposited, thus leading to easy pathway for electron transport and increased electroactive surface area. Moreover, we have demonstrated the feasibility of SP-PANI electrodes for use in high performance flexible solid-state supercapacitors. The assembled supercapacitor with SP-PANI electrodes shows high areal capacitance of 149.3 mF cm−2 at 0.5 mA cm−2. It also features high mechanical flexibility, good electrochemical capacitive properties, and superior deformation and electrochemical stability, making it have high application potential in flexible electrochemical energy storage.

Orderly and highly dense polyaniline nanorod arrays fenced on carbon nanofibers for all-solid-state flexible electrochemical energy storage

Conducting polymer nanoarray structures had attracted increasing attention as potential electrode materials in supercapacitor devices. In this article, the orderly and highly dense polyaniline (i.e., PANI) nanorod arrays had been successfully grown onto the surface of carbon nanofibers(i.e., CNFs) to form PANI@CNFs composite via a one step of facile chemical oxidation method. The specific capacity of the single PANI@CNFs composite electrode was 758 C/g and 720 C/g at current densities of 1.0 and 5.0 A/g, respectively, while the cycling stability could maintain 85% of the initial capacity after 1000 charge-discharge cycles. Furthermore, two pieces of PANI@CNFs composite electrodes were fabricated into a symmetric all-solid-state flexible supercapacitor device to drive small electronic equipments using polyvinyl alcohol-H2SO4 hydrogel as solid electrolyte, while the normal filter paper acted as a separator. The as-prepared symmetric supercapacitor device had excellent electrochemical performance with specific capacitance of 285 F/g at a current density of 0.5 A/g. The maximum energy density was 10.04 Wh/kg at a power density of 225 W/kg and showed no obvious decrease in the performance with a bending angle of 45° or 90°. The as-prepared symmetric all-solid-state flexible supercapacitor device had good promising applications in small wearable electronics.

Designing high electrochemical surface area between polyaniline and hydrogel polymer electrolyte for flexible supercapacitors

High electrochemical surface area (ECSA) is the precondition for high performance of flexible supercapacitors, but remains a considerable obstacle. Because of the lower fluidity of hydrogel polymer electrolyte (HPE, generally utilized to fabricate flexible supercapacitors) than aqueous electrolyte (AE, used in traditional supercapacitors), it is difficult to infill a porous electrode with HPE completely, leading to a low ECSA between electrode materials and HPE. In this work, the structures of the polyaniline nanowires are designed to increase the ECSA. In comparison with nanowire network structure, the patulous structure of the polyaniline nanowire arrays is beneficial to increase the ECSA. The polyaniline nanowire arrays show a specific capacitance of 373.6 F g−1 at 0.1 A g−1 with the HPE, which is comparable to that with the AE at 0.1 A g−1 (376.6 F g−1). Flexible supercapacitors based on the polyaniline nanowire arrays exhibit similar performances under flat, folded, and twisted conditions, demonstrating their high potential application in flexible electrochemical energy storage devices.