Facilitated Ion Transport in All-Solid-State Flexible Supercapacitors

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.

Two-dimensional (2-D) materials have been a promising material platform for making powerful energy devices including highly flexible energy storage devices. Owing to their advantageous features of flexibility, large surface area and open ion transport pathway within 2D layers, highly electroactive 2-D transition-metal oxide and phosphate nanosheets materials show promise in flexible supercapacitors with high power and energy densities. Here I will present our recent progress on chemically integrated hybrid 2-D materials to build ultraflexible, high-performance energy storage devices. One example is the building of integrated hybrid vanadyl phosphate VOPO4/graphene ultrathin-film solid-state pseudocapacitors (1). The other is the development of MnO2/graphene hybrid nanostructures based flexible planar supercapacitors (2).Rational material and structural design demonstrated in these studies show hybrid structures not only take advantage of excellent electronic conductivity of graphene nanosheets but also introduce more electrochemically active surfaces for absorption/desorption of electrolyte ions, which can greatly facilitate charge transport during electrochemical processes.References(1) Wu, C.; Lu, X.; Peng, L.; Xu, K.; Peng, X.; Huang, J.; Yu, G.; Xie, Y. Nature Communications 2013, 4, 2431.(2) Peng, L.; Peng, X.; Liu, B.; Wu, C.; Xie, Y.; Yu, G. Nano Letters 2013, 13, 2151.

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 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.

Flexible solid-state supercapacitors are of considerable interest as mobile power supply for future flexible electronics. Graphene or carbon nanotubes based thin films have been used to fabricate flexible solid-state supercapacitors with high gravimetric specific capacitances (80-200 F/g), but usually with a rather low overall or areal specific capacitance (3-50 mF/cm(2)) due to the ultrasmall electrode thickness (typically a few micrometers) and ultralow mass loading, which is not desirable for practical applications. Here we report the exploration of a three-dimensional (3D) graphene hydrogel for the fabrication of high-performance solid-state flexible supercapacitors. With a highly interconnected 3D network structure, graphene hydrogel exhibits exceptional electrical conductivity and mechanical robustness to make it an excellent material for flexible energy storage devices. Our studies demonstrate that flexible supercapacitors with a 120 μm thick graphene hydrogel thin film can exhibit excellent capacitive characteristics, including a high gravimetric specific capacitance of 186 F/g (up to 196 F/g for a 42 μm thick electrode), an unprecedented areal specific capacitance of 372 mF/cm(2) (up to 402 mF/cm(2) for a 185 μm thick electrode), low leakage current (10.6 μA), excellent cycling stability, and extraordinary mechanical flexibility. This study demonstrates the exciting potential of 3D graphene macrostructures for high-performance flexible energy storage devices.

Our modern and technological society requests enhanced energy storage devices to tackle the current necessities. In addition, wearable electronic devices are being demanding because they offer many facilities to the person wearing it. In this manuscript, a historical review is made about the available energy storage devices focusing on super-capacitors and lithium-ion batteries, since they currently are the most present in the industry, and the possible polymeric materials suitable on wearable energy storage devices. Polymers are a suitable option because they not only possess remarkable mechanical resistance, flexibility, long life-times, easy manufacturing techniques and low cost in addition to they can be environmentally friendly, nontoxic, and even biodegradable too. Moreover, the electrical and electrochemical polymer properties can be tunning with suitable fillers giving to versatile conducting polymer composites with a good cost and properties' ratio. Although the advances are promising, there are still many drawbacks that need to be overcome. Future research should focus on improving both the performance of materials and their processability on an industrial scale, where additive manufacturing offers many possibilities. The sustainability of new energy storage devices should not

Energy storage is a field which governs almost all aspects of daily life. Batteries and capacitors are utilised in devices at every scale, from mobile phones all the way up to the national grid. Efficient energy storage requires extensive research into chemistries and configurations, utilising novel materials to assemble devices. These often have a significant effect on features including the energy and power density of a cell. One such property is the pore structure of an electrode. Electrochemical Double-Layer Capacitors, also known as supercapacitors, utilise meso/microporous electrodes with a wide distribution of pore sizes. Larger pores typically facilitate ion diffusion and a high device power density, yet smaller micropores provide a large specific surface area for the electric double layer which facilitates a high device energy density. Recent advancements in post-lithium ion battery technology (such as lithium-sulphur and lithium air batteries) utilise these micropore surface areas for redox reactions. Developing accurate models of energy storage devices allows for the simulation of the processes within the cells without the need for expensive and time-consuming experimental testing. Many models have been developed in the past, however sweeping assumptions are often made with respect to the pore structure. These generally assume a uniform pore size and structure, and do not take into account the effects of different pore sizes present in a true electrode material. Pore network models are able to do this, but require complex or expensive pieces of software and a long simulation time. In this project, a novel continuum model of mass and charge transport has been developed catering for the many pore sizes in an electrode. This model allowed for the simple implementation of a pore size distribution into the simulation of the processes in several supercapacitor configurations, a lithium-sulphur battery, and two lithium-oxygen battery configurations. This novel approach involved characterising an electrode material through analysis of the pore structure and pore size distribution. A novel mathematical transient volume averaged model was developed to solve for the mass transport of species within pores of different sizes. The use of this model in each case demonstrated good agreement with experimental data and allowed for analysis of the effects of different pore structures on the activity of cells. It was found that larger pores (in the macropore region) facilitate mass transport of species, and smaller pores (in the micropore region) have a reduced rate of mass transport and a higher rate of reaction (in batteries) and stern layer formation (in supercapacitors). This novel model demonstrated the importance of modelling ion transport through multiple pore sizes of an electrode material. This model also addresses the need for more complex energy storage device simulation in a reasonable solving time without the need for expensive pieces of software.

Optimizing the electrode structure to achieve efficient material utilization is crucial for high-capacity wearable energy storage devices. Here, a strategy of embedding and covalently grafting polyaniline (PANI) into a three-dimensional porous reduced graphene oxide (RGO)/helical carbon nanotube (HCNT) skeleton was designed to prepare self-supporting flexible supercapacitors (SCs) via an ingenious hydrothermal method, followed by regulation by carbonization. The resulting hybrid aerogel possesses a uniform porous mesh space configuration with excellent flexibility, provides fast ion/electron transmission channels, and maximizes the utilization of pseudocapacitive PANI. Considering the unique spatial configuration of PANI trapped into the porous network, the electrode possesses a remarkable gravimetric capacitance (696.75 F g–1) at 2 A g–1 and an excellent cycling retention (93.57%) after 3500 cycles. Furthermore, the assembled flexible symmetric SC based on carbonized RGO/HCNTs/PANI (CRCP) shows considerable electrochemical performance with a high specific capacitance of 140.1 F g–1 (84.1 F cm–3) at 1 A g–1 (0.6 A cm–3) and a superior energy density of 12.46 W h kg–1 at a power density of 400.36 W kg–1. Moreover, this SC maintains good performance stability at large bending angles. The particular PANI parasitic carbon skeleton-laminated grid design of the CRCP electrode with outstanding capacitance behavior and robust flexibility provides a feasible and efficient preparation technology for fabricating 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.

Graphene-based materials for flexible energy storage devices

Flexible sodium-ion based energy storage devices: Recent progress and challenges

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Recent advance in two-dimensional MXenes: New horizons in flexible batteries and supercapacitors technologies

Co-TCPP/MXene film electrodes and patterned interdigital electrodes with high performance for flexible all-solid supercapacitors

In this chapter the applications of polymer functionalized graphene (PFG) in a solid state battery and supercapacitors are discussed. In a solid state battery the use of PFG as anode, cathode and electrolyte are discussed with some examples. Electrode materials with rapid charge and discharge rates in lithium ion batteries (LIBs) using PFG are noted. They also exhibit long cycle life, high energy density and excellent rate capability, with high reversible Li+ storage capacity and rapid Li+ transport. Solid polymer electrolyte (SPE) materials with reduced crystallinity and higher ionic conductivity is very much necessary and here their development is discussed using PFG. PFG is highly used in flexible supercapacitors because of the large specific surface area, high mechanical stability and good conducting properties of graphene or reduced graphene oxide (RGO). Controlled reduction of GO is found to behave as a good supercapacitor and hence functionalization of GO/RGO with conducting polymers are found to be very useful as electrode material for increasing the specific capacitance of the supercapacitor devices. Here polymer functionalization of GO both by covalent and noncovalent ways are discussed for their use as electrode materials of supercapacitors. The specific capacitance, power density, energy density, cyclic stability, including Ragone plot, etc., for all the systems are discussed making the use of three electrode and two electrode systems during the use of cyclic voltammetry (CV) technique in the presence of an electrolyte which is mostly acids or ionic liquids. Variation of specific capacitance with different current densities are discussed for their utility in practical application. Both symmetric and asymmetric super capacitor devices fabricated with PFG electrodes are also discussed with an aim to improve the specific capacitance of the device and also for their long term stability.