Issues and Challenges Facing Flexible Lithium-Ion Batteries for Practical Application

High-Performance Bifunctional Electrocatalysts for Flexible and Rechargeable Zn-Air Batteries: Recent Advances

Flexible rechargeable Zn-air batteries (FZABs) exhibit high energy density, ultra-thin, lightweight, green, and safe features, and are considered as one of the ideal power sources for flexible wearable electronics. However, the slow and high overpotential oxygen reaction at the air cathode has become one of the key factors restricting the development of FZABs. The improvement of activity and stability of bifunctional catalysts has become a top priority. At the same time, FZABs should maintain the battery performance under different bending and twisting conditions, and the design of the overall structure of FZABs is also important. Based on the understanding of the three typical configurations and working principles of FZABs, this work highlights two common strategies for applying bifunctional catalysts to FZABs: 1) powder-based flexible air cathode and 2) flexible self-supported air cathode. It summarizes the recent advances in bifunctional oxygen electrocatalysts and explores the various types of catalyst structures as well as the related mechanistic understanding. Based on the latest catalyst research advances, this paper introduces and discusses various structure modulation strategies and expects to guide the synthesis and preparation of efficient bifunctional catalysts. Finally, the current status and challenges of bifunctional catalyst research in FZABs are summarized.

Fully-Printed Flexible Aqueous Rechargeable Sodium-Ion Batteries

The flexible aqueous rechargeable sodium-ion batteries (ARSIBs) are a promising portable energy storage system that can meet the flexibility and safety requirements of wearable electronic devices. However, it faces huge challenges in mechanical stability and facile manufacturing processes. Herein, the first fully-printed flexible ARSIBs with appealing mechanical performance by screen-printing technique is prepared, which utilizes Na3V2(PO4)2F3/C (NVPF/C) as the cathode and 2% nitrogenous carbon-loaded Na3MnTi(PO4)3/C (NMTP/C/NC) as the anode. In particular, the organic co-solvent ethylene glycol (EG) is cleverly added to 17 m (mol kg-1) NaClO4 electrolyte to prepare a 17 m NaClO4-EG mixed electrolyte. This mixed electrolyte can withstand low temperatures of -20 °C in practical applications. Encouragingly, the fully-printed flexible ARSIBs (NMTP/C/NC//NVPF/C) exhibit a discharge capacity of 40.1 mAh g-1, an energy density of 40.1 Wh kg-1, and outstanding cycle performance. Moreover, these batteries with various shapes can be used as an energy wristband for an electronic watch in the bending states. The fully-printed flexible ARSIBs in this work are expected to shed light on the development of energy for wearable electronics.

Failure Modes of Flexible LiCoO2 Cathodes Incorporating Polyvinylidene Fluoride Binders with Different Molecular Weights

Understanding the mechanical failure modes of lithium-ion battery [Li-ion batteries (LIBs)] electrodes is exceptionally important for enabling high specific energy and flexible LIB technologies. In this work, the failure modes of lithium cobalt oxide (LCO) cathodes under repeated bending and the role of the polymer binder in improving the mechanical durability of the LCO electrodes for use in flexible LIBs are investigated. Mechanical and electrochemical evaluations of LCO electrodes (areal capacity of ≥2.5 mA h cm-2) employing poly(vinylidene fluoride) (PVDF) binder were carried out, followed by extensive optical and electron microscopies. We find that the molecular weight (MW) of the PVDF significantly influenced the surface and bulk microstructure of the LCO electrodes, particularly the distribution of carbon additive and binder, which plays a crucial role in affecting the mechanical and electrochemical properties of the electrodes. Multiple mechanical failure modes (e.g., surface scratches and microcracks) observed in the LCO electrodes subjected to repeated bending originated from the use of low MW PVDF; these failure modes were successfully mitigated by using a high MW PVDF. Remarkably, the optimized flexible LCO electrode incorporating high MW PVDF showed comparable discharge capacity retention during galvanostatic cycling after repeated bending (7000 cycles at 50 mm bending diameter) to electrodes not subjected to the repeated bending. This study highlights the importance of carrying out a comprehensive investigation of the failure mechanisms in flexible electrodes, which identified the pivotal role of the PVDF MW in the electrode microstructure and its effects on the electrode resilience to failure during repeated bending.

A new Mn-based layered cathode with enlarged interlayer spacing for potassium ion batteries

Layered Mn-based cathode (KxMnO2) has attracted wide attention for potassium ion batteries (PIBs) because of its high specific capacity and energy density. However, the structure and capacity of KxMnO2 cathode are constantly degraded during the cycling due to the strong Jahn-Teller effect of Mn3+ and huge ionic radius of K+. In this work, lithium ion and interlayer water were introduced into Mn layer and K layer in order to suppress the Jahn-Teller effect and expand interlayer spacing, respectively, thus obtaining new types of K0.4Mn1-xLixO2·0.33H2O cathode materials. The interlayer spacing of the K0.4MnO2 increased from 6.34 to 6.93 Å after the interlayer water insertion. X-ray photoelectron spectroscopy studies demonstrated that proper lithium doping can effectively control the ratio of Mn3+/Mn4+ and inhibit the Jahn-Teller effect. In-situ X-ray diffraction exhibited that lithium doping can inhibit the irreversible phase transition and improve the structural stability of materials during cycling. As a result, the optimal K0.4Mn0.9Li0.1O2·0.33H2O not only delivered a higher capacity retention of 84.04 % compared to the value of 28.09 % for K0.4MnO2·0.33H2O, but also maintained a greatly enhanced rate capability. This study provides a new opportunity for designing layered manganese-based cathode materials with high performance for PIBs.

Band engineering enhances the electrochemical properties by constructing TiO2 NRs-MoS2 NSFs flexible electrode

Research and development of flexible electrodes with high performance are crucial to largely determine the performance of flexible lithium-ion batteries (FLIBs) to a large extent. In this work, a flexible anode (TiO2 NRs-MoS2 NSFs/CC) is rationally designed and successfully constructed, in which TiO2 nanorods arrays (NRs) vertically grown on CC as a supporting backbone for MoS2 nanosheets flowers (NSFs) to form a TiO2 NRs-MoS2 NSFs heterostructure. The backbone can not only serve as a mechanical support MoS2 and improve its electronic conductivity, but also limit the dissolution of polysulfides issue during cycling. The density functional theory (DFT) analysis manifests that the obvious interaction between O and S at the interface for the TiO2 NRs-MoS2 NSFs heterostructure changes the electronic structure and reduces the band gap of TiO2 NRs-MoS2 NSFs. The small band gap and high electron state at the Fermi level are both beneficial to the transport of electrons, enhancing the kinetics, and giving the long cycling stability at high density and excellent rate capacity. Furthermore, the assembled TiO2 NRs-MoS2 NSFs/CC//NCM622 full cell delivers superior rate capacity and good cycling stability. Meanwhile, the soft-packed cell shows good mechanical flexibility, which can be lighted up successfully and keep brightness when folding with different angles. This result illustrates that it is a highly potential strategy for constructing flexible electrodes with the controlled electronic structure through band engineering to not only improve the electrochemical performance, but also possibly meet the requirements of high-performance FLIBs.

Multifunctional, Hybrid Materials Design via Spray-Drying: Much more than Just Drying

Spray-drying is a popular and well-known "drying tool" for engineers. This perspective highlights that, beyond this application, spray-drying is a very interesting and powerful tool for materials chemists to enable the design of multifunctional and hybrid materials. Upon spray-drying, the confined space of a liquid droplet is narrowed down, and its ingredients are forced together upon "falling dry." As  detailed in this article, this enables the following material formation strategies either individually or even in combination: nanoparticles and/or molecules can be assembled; precipitation reactions as well as chemical syntheses can be performed; and templated materials can be designed. Beyond this, fragile moieties can be processed, or "precursor materials" be prepared. Post-treatment of spray-dried objects eventually enables the next level in the design of complex materials. Using spray-drying to design (particulate) materials comes with many advantages-but also with many challenges-all of which are outlined here. It is believed that multifunctional, hybrid materials, made via spray-drying, enable very unique property combinations that are particularly highly promising in myriad applications-of which catalysis, diagnostics, purification, storage, and information are highlighted.

Spider Silk-Inspired Binder Design for Flexible Lithium-Ion Battery with High Durability

The development of flexible lithium-ion batteries (LIBs) imposes demands on energy density and high mechanical durability simultaneously. Due to the limited deformability of electrodes, as well as the flat and smooth surface of the metal current collectors, stable/durable/reliable contact between electrode materials and the current collectors remains a challenge, in particular, for electrodes with high loading mass and heavily deformed batteries. Binders play an essential role in binding particles of electrode materials and adhering them to current collectors. Herein, inspired by spider silk, a binder for flexible LIBs is developed, which equips a cross-linked supramolecular poly(urethane-urea) to the polyacrylic acid. The binder imparts super high elastic restorability originating from the meticulously engineered hydrogen-bonding segments as well as extraordinary adhesion. The developed binder provides excellent flexibility and intact electrode morphologies without disintegration even when the electrode is largely deformed, enabling a stable cycling and voltage output even when the batteries are put under tough dynamic deformation tests. The flexible LIBs exhibit a high energy density of 420 Wh L-1 , which is remarkably higher than reported numbers. The unique binder design is greatly promising and offers a valuable material solution for LIBs with high-loading mass and flexible designs.

Hierarchical 3D Electrode Design with High Mass Loading Enabling High-Energy-Density Flexible Lithium-Ion Batteries

Flexible lithium-ion batteries (LIBs) have attracted significant attention owing to their ever-increasing use in flexible and wearable electronic devices. However, the practical application of flexible LIBs in devices has been plagued by the challenge of simultaneously achieving high energy density and high flexibility. Herein, a hierarchical 3D electrode (H3DE) is introduced with high mass loading that can construct highly flexible LIBs with ultrahigh energy density. The H3DE features a bicontinuous structure and the active materials along with conductive agents are uniformly distributed on the 3D framework regardless of the active material type. The bicontinuous electrode/electrolyte integration enables a rapid ion/electron transport, thereby improving the redox kinetics and lowering the internal cell resistance. Moreover, the H3DE exhibits exceptional structural integrity and flexibility during repeated mechanical deformations. Benefiting from the remarkable physicochemical properties, pouch-type flexible LIBs using H3DE demonstrate stable cycling under various bending states, achieving a record-high energy density (438.6 Wh kg-1 and 20.4 mWh cm-2 ), and areal capacity (5.6 mAh cm-2 ), outperforming all previously reported flexible LIBs. This study provides a feasible solution for the preparation of high-energy-density flexible LIBs for various energy storage devices.

Leaf-Inspired Host-Guest Complexation-Dictating Supramolecular Gas Sensors

We report unique conductive leaf-inspired (in particular, stomata-inspired) supramolecular gas sensors in which acetylated cyclodextrin derivatives rule the electric output. The gas sensors consist of polymers bearing acetylated cyclodextrin, adamantane, and carbon black. Host-guest complexes between acetylated cyclodextrin and adamantane corresponding to the closed stomata realize a flexible polymeric matrix. Effective recombination of the cross-links contributes to the robustness. As gas sensors, the supramolecular materials detect ammonia as well as various other gases at 1 ppm in 10 min. The free acetylated cyclodextrin corresponding to open stomata recognized the guest gases to alter the electric resistivity. Interestingly, the conductive device failed to detect ammonia gases at all without acetylated cyclodextrin. The molecular recognition was studied by molecular dynamics simulations. The gas molecules existed stably in the cavity of free acetylated cyclodextrin. These findings show the potential for developing wearable gas sensors.

How Practical Are Fiber Supercapacitors for Wearable Energy Storage Applications?

Future wearable electronics and smart textiles face a major challenge in the development of energy storage devices that are high-performing while still being flexible, lightweight, and safe. Fiber supercapacitors are one of the most promising energy storage technologies for such applications due to their excellent electrochemical characteristics and mechanical flexibility. Over the past decade, researchers have put in tremendous effort and made significant progress on fiber supercapacitors. It is now the time to assess the outcomes to ensure that this kind of energy storage device will be practical for future wearable electronics and smart textiles. While the materials, fabrication methods, and energy storage performance of fiber supercapacitors have been summarized and evaluated in many previous publications, this review paper focuses on two practical questions: Are the reported devices providing sufficient energy and power densities to wearable electronics? Are the reported devices flexible and durable enough to be integrated into smart textiles? To answer the first question, we not only review the electrochemical performance of the reported fiber supercapacitors but also compare them to the power needs of a variety of commercial electronics. To answer the second question, we review the general approaches to assess the flexibility of wearable textiles and suggest standard methods to evaluate the mechanical flexibility and stability of fiber supercapacitors for future studies. Lastly, this article summarizes the challenges for the practical application of fiber supercapacitors and proposes possible solutions.

Densifiable Ink Extrusion for Roll-To-Roll Fiber Lithium-Ion Batteries with Ultra-High Linear and Volumetric Energy Densities

Conventional bulky and rigid planar architecting power systems are difficult to satisfy the growing demand for wearable applications. 1D fiber batteries bearing appealing features of miniaturization, adaptability, and weavability represent a promising solution, yet challenges remain pertaining to energy density and scalability. Herein, an ingenious densifiable functional ink is invented to fabricate scalable, flexible, and high-mass-loading fiber lithium-ion batteries (LIBs) by adopting a fast ink-extrusion technology. In the formulated ink, pyrrole-modified reduced graphene oxide is elaborately introduced and exerts multiple influences; it not only assembles carbon nanotubes and poly(vinylidene fluoride-co-hexafluoropropylene) to compose a sturdy, conductive, and agglomeration-free 3D network that realizes an ultra-high content (75 wt%) of the active materials and endows the electrode excellent flexibility but also serves as a capillary densification inducer, encouraging an extremely large linear mass loading (1.01 mg cm^-1 per fiber) and packing density (782.1 mg cm^-3 ). As a result, the assembled fiber LIBs deliver impressive linear and volumetric energy densities with superb mechanical compliance, demonstrating the best performance among all the reported extruded fiber batteries. This work highlights a highly effective and facile approach to fabricate high-performance fiber energy storage devices for future practical wearable applications.

Flexible All-Solid-State Direct Methanol Fuel Cells with High Specific Power Density

It is vital to create flexible batteries as power sources to suit the needs of flexible electronic devices because they are widely employed in wearable and portable electronics. The direct methanol fuel cell (DMFC) is a desirable alternative portable energy source since it is a clean, safe, and high energy density cell. The traditional DMFC in mechanical assembly and its unbending property, however, prevent it from being employed in flexible electrical devices. In this study, the flexible membrane electrode assembly (MEA) with superior electrical conductivity and nanoscale TiC-modified carbon cloth (TiC/CC) is used as supporting layer. Additionally, solid methanol fuels used in the manufacturing of flexible all-solid-state DMFC have the advantages of being tiny, light, and having high energy density. Furthermore, the DMFC's placement and bending angle have little effect on its performance, suggesting that DMFC is appropriate for flexible portable energy. The flexible all-solid-state DMFC's power density can reach 14.06 mW cm^-2 , and after 50 bends at 60°, its voltage loss can be disregarded. The flexible all-solid DMFC has an energy density that is 777.78 Wh Kg^-1 higher than flexible lithium-ion batteries, which is advantageous for the commercialization of flexible electronic products.

Rational Design of Flexible Zn-Based Batteries for Wearable Electronic Devices

The advent of 5G and the Internet of Things has spawned a demand for wearable electronic devices. However, the lack of a suitable flexible energy storage system has become the "Achilles' Heel" of wearable electronic devices. Additional problems during the transformation of the battery structure from conventional to flexible also present a severe challenge to the battery design. Flexible Zn-based batteries, including Zn-ion batteries and Zn-air batteries, have long been considered promising candidates due to their high safety, eco-efficiency, substantial reserve, and low cost. In the past decade, researchers have come up with elaborate designs for each portion of flexible Zn-based batteries to improve the ionic conductivities, mechanical properties, environment adaptabilities, and scalable productions. It would be helpful to summarize the reported strategies and compare their pros and cons to facilitate further research toward the commercialization of flexible Zn-based batteries. In this review, the current progress in developing flexible Zn-based batteries is comprehensively reviewed, including their electrolytes, cathodes, and anodes, and discussed in terms of their synthesis, characterization, and performance validation. By clarifying the challenges in flexible Zn-based battery design, we summarize the methodology from previous investigations and propose challenges for future development. In the end, a research paradigm of Zn-based batteries is summarized to fit the burgeoning requirement of wearable electronic devices in an iterative process, which will benefit the future development of Zn-based batteries.

Opportunities of Flexible and Portable Electrochemical Devices for Energy Storage: Expanding the Spotlight onto Semi-solid/Solid Electrolytes

The ever-increasing demand for flexible and portable electronics has stimulated research and development in building advanced electrochemical energy devices which are lightweight, ultrathin, small in size, bendable, foldable, knittable, wearable, and/or stretchable. In such flexible and portable devices, semi-solid/solid electrolytes besides anodes and cathodes are the necessary components determining the energy/power performances. By serving as the ion transport channels, such semi-solid/solid electrolytes may be beneficial to resolving the issues of leakage, electrode corrosion, and metal electrode dendrite growth. In this paper, the fundamentals of semi-solid/solid electrolytes (e.g., chemical composition, ionic conductivity, electrochemical window, mechanical strength, thermal stability, and other attractive features), the electrode-electrolyte interfacial properties, and their relationships with the performance of various energy devices (e.g., supercapacitors, secondary ion batteries, metal-sulfur batteries, and metal-air batteries) are comprehensively reviewed in terms of materials synthesis and/or characterization, functional mechanisms, and device assembling for performance validation. The most recent advancements in improving the performance of electrochemical energy devices are summarized with focuses on analyzing the existing technical challenges (e.g., solid electrolyte interphase formation, metal electrode dendrite growth, polysulfide shuttle issue, electrolyte instability in half-open battery structure) and the strategies for overcoming these challenges through modification of semi-solid/solid electrolyte materials. Several possible directions for future research and development are proposed for going beyond existing technological bottlenecks and achieving desirable flexible and portable electrochemical energy devices to fulfill their practical applications. It is expected that this review may provide the readers with a comprehensive cross-technology understanding of the semi-solid/solid electrolytes for facilitating their current and future researches on the flexible and portable electrochemical energy devices.

Kirigami-Inspired Flexible Lithium-Ion Batteries via Transformation of Concentrated Stress into Segmented Strain

Emerging directions in the growing wearable electronics market have spurred the development of flexible energy storage systems that require deformability while maintaining electrochemical performance. However, the traditional fabrication approaches of lithium-ion batteries (LIBs) are challenging to withstand long-cycle bending alternating loads due to the stress concentration caused by the nonuniformity of the actual deformation. Herein, inspired by kirigami, a segmented deformation design of full-cell scale thin-type flexible lithium-ion batteries (FLIBs) with large-scale manufacturing characteristics via the current collector's mechanical blanking process is reported. This strategy allows the battery's elliptical deformation of the actual state to be transformed into the circular strain of the ideal configuration, thereby dispersing the stress concentration on the top of the battery. According to the results, the designed battery maintains >95% capacity after >20 000 harsh in situ dynamic tests. In addition, finite element analysis further reveals the mechanism that the segmented deformation strategy bears the mechanical stress. This work can enlighten the rational design and customization of electrode patterns for high compatibility with various devices, thereby providing potential opportunities for the application of FLIBs.

Developments in Surface/Interface Engineering of Ni-Rich Layered Cathode Materials

Ni-rich layered cathodes with high energy densities reveal an enormous potential for lithium-ion batteries (LIBs), however, their poor stability and reliability have inhibited their application. To ensure their stability over extensive cycles at high voltage, surface/interface modifications are necessary to minimize the adverse reactions at the cathode-electrolyte interface (CEI), which is a critical factor impeding electrode performance. Therefore, this review provides a comprehensive discussion on the surface engineering of Ni-rich cathode materials for enhancing their lithium storage property. Based on the structural characteristics of the Ni-rich cathode, the major failure mechanisms of these structures during synthesis and operation are summarized. Then the existing surface modification techniques are discussed and compared. Recent breakthroughs in various surface coatings and modification strategies are categorized and their unique functionalities in structural protection and performance-enhancing are elaborated. Finally, the challenges and outlook on the Ni-rich cathode materials are also proposed.

Ultrastable Interfacial Contacts Enabling Unimpeded Charge Transfer and Ion Diffusion in Flexible Lithium-Ion Batteries

Deteriorating interfacial contact under mechanical deformation induces large cracks and high charge transfer resistance, resulting in a severe capacity fading of flexible lithium-ion batteries (LIBs). Herein, an oxygen plasma treatment on a polymer separator combined with high-speed centrifugal spraying to construct ultrastable interfacial contacts is reported. With the treatment, abundant hydrophilic oxygen-containing functional groups are produced and ensure strong chemical adhesion between the separator and the active materials. With single walled carbon nanotubes (SWCNTs) sprayed onto the active materials, a dense thin film is formed as the current collector. Meanwhile, the centrifugal force caused by high-speed rotation together with van der Waals forces under fast evaporation produces a much closer interface between the current collector and the active materials. As a result of this ultrastable interfacial interaction, the integrated electrode shows no structural failure after 5000 bending cycles with the charge-transfer resistance as low as 35.8% and a Li-ion diffusion coefficient nearly 19 times of the untreated electrode. Flexible LIBs assembled with these integrated electrodes show excellent structural and electrochemical stability, and can work steadily under various deformed states and repeated bending. This work provides a new technique toward rational design of electrode configuration for flexible LIBs.

Synthesis of V2O5/Single-Walled Carbon Nanotubes Integrated into Nanostructured Composites as Cathode Materials in High Performance Lithium-Ion Batteries

Vanadium pentoxide (V2O5)-anchored single-walled carbon nanotube (SWCNT) composites have been developed through a simple sol–gel process, followed by hydrothermal treatment. The resulting material is suitable for use in flexible ultra-high capacity electrode applications for lithium-ion batteries. The unique combination of V2O5 with 0.2 wt.% of SWCNT offers a highly conductive three-dimensional network. This ultimately alleviates the low lithium-ion intercalation seen in V2O5 itself and facilitates vanadium redox reactions. The integration of SWCNTs into the layered structure of V2O5 leads to a high specific capacity of 390 mAhg−1 at 0.1 C between 1.8 to 3.8 V, which is close to the theoretical capacity of V2O5 (443 mAhg−1). In recent research, most of the V2O5 with carbonaceous materials shows higher specific capacity but limited cyclability and poor rate capability. In this work, good cyclability with only 0.3% per cycle degradation during 200 cycles and enhanced rate capability of 178 mAhg−1 at 10 C have been achieved. The excellent electrochemical kinetics during lithiation/delithiation is attributed to the chemical interaction of SWCNTs entrapped between layers of the V2O5 nanostructured network. Proper dispersion of SWCNTs into the V2O5 structure, and its resulting effects, have been validated by SEM, TEM, XPS, XRD, and electrical resistivity measurements. This innovative hybrid material offers a new direction for the large-scale production of high-performance cathode materials for advanced flexible and structural battery applications.

Bidirectional Planar Flexible Snake-Origami Batteries

With the rapid development of commercial flexible/wearable devices, flexible batteries have attracted great attention as optimal power sources. However, a combination of high energy density and excellent arbitrary deformation ability is still a critical challenge to satisfy practical applications. Inspired by rigid and soft features of chemical molecular structures, novel bidirectional flexible snake-origami lithium-ion batteries (LIBs) with both high energy density and favorable flexibility are designed and fabricated. The flexible snake-origami battery consists of rigid and soft segments, where the former is designed as the energy unit and the latter served as the deformation unit. With the unique features from such design, the as-fabricated battery with calculating all the components exhibits a record-setting energy density of 357 Wh L-1 (133 Wh kg-1 ), compared with the cell-scale flexible LIBs achieved from both academic and industry. Additionally, a design principle is established to verify the validity of utilizing rigid-soft-coupled structure for enduring various deformations, and the intrinsic relationship between battery structure, energy density, and flexibility can be confirmed. The results suggest that the design principle and performance of bidirectional flexible snake-origami batteries will provide a new reliable strategy for achieving high energy flexible batteries for wearable devices.

Flame-Retardant, Highly Conductive, and Low-Temperature-Resistant Organic Gel Electrolyte for High-Performance All-Solid Supercapacitors

Traditional liquid electrolytes are volatile, flammable, and easy to leak, which makes the energy storage device easy to burn and explode in the case of overcharge and short circuit. Here, by utilizing the active P-H bond of a flame retardant (DOPO) to graft onto the polymer chain, flame-retardant organic gel electrolytes were fabricated to address these issues. The gel electrolyte had good ionic conductivity of 4 mS cm^-1 at 20 °C and good flame retardant ability. By changing the molar ratio of the monomers and the salt concentrations, the mechanical strength of the gel electrolyte could be adjusted (maximum stress≈28 KPa, maximum strain≈305 %). The transport mechanism of lithium ions in the gel polymer electrolyte was proposed. The gel electrolyte-assembled supercapacitor (SC) possessed better electrochemical properties than that of SC assembled by liquid electrolyte. Importantly, the gel-based SC remained basically unchanged under multiple bending cycles. Additionally, the gel electrolyte had good low-temperature tolerance (0.1 mS cm^-1 at -40 °C). The gel electrolyte-assembled SC could work normally in the temperature range of -20 to 60 °C. The multiple advantages of gel electrolyte expand the applications in ionic conductor and energy storage devices.

A V3C2MXene/graphene heterostructure as a sustainable electrode material for metal ion batteries

Individually, MXene and graphene based frameworks have been recognized as promising 2D electrode materials for metal ion batteries. Herein, we have engineered a heterostructure of V₃C₂MXene and graphene using computational design. A comprehensive investigation of designed heterostructure has been reported in this work. Simulated heterostructure has been evaluated for various functionalities such as high performance of thermal stability, metal ion intercalation, diffusion energy using density functional theory method. Interestingly, simulation examinations and obtained calculations demonstrate the high storage capacity of Li and Ca (598.63 mAh g^-1), and Na (555.87 mAh g^-1) with the designed V₃C₂/graphene model. Promising diffusion energy barriers for Li (0.11 eV), Na (0.17 eV) and Ca (0.15 eV) ions are also investigated and have explained systematically in the present work. Moreover, we have achieved high capacity and fast charge/discharge rates of V₃C₂/graphene heterostructure indicating its promising electrode potential efficiency for ion batteries especially for Na ion battery. Thus, our investigation demonstrate the advantages of newly designed V₃C₂MXene and graphene heterostructure for advance metal ion batteries.

Hierarchical Triple-Shelled MnCo2 O4 Hollow Microspheres as High-Performance Anode Materials for Potassium-Ion Batteries

Metal oxide anode materials generally possess high theoretical capacities. However, their further development in potassium-ion batteries (KIBs) is limited by self-aggregation and large volume fluctuations during charge/discharge processes. Herein, hierarchical MnCo₂ O₄ hollow microspheres (ts-MCO HSs) with three porous shells that consist of aggregated primary nanoparticles are fabricated as anode materials of KIBs. The porous shells are in favor of reducing the diffusion path of K-ions and electrons, and thus the rate performance can be enhanced. The unique triple-shelled hollow structure is believed to provide sufficient contact between electrolyte and metal oxides, possess additional active storage sites for K-ions, and buffer the volume change during K-ions insertion/extraction. A high specific capacity of 243 mA h g^-1 at 100 mA g^-1 in the 2nd cycle and a highly improved rate performance of 153 mA h g^-1 at 1 A g^-1 are delivered when cycled between 0.01 and 3.0 V. In addition, the transformation of substances during charging/discharging processes are intuitively demonstrated by the in situ X-ray diffraction strategy for the first time, which further proves that the unique structure of ts-MCO HSs with three porous shells can significantly enhance the potassium ions storage performance.

Flexible S@C-CNTs cathodes with robust mechanical strength via blade-coating for lithium-sulfur batteries

Lithium sulfur batteries (LSBs) with high energy density hold some promising applications in the wearable and flexible devices. However, it has been still challenging to develop a simple and feasible approach to prepare flexible LSB cathodes with both robust mechanical strength. Herein, flexible S@C-CNTs cathodes with controllable thicknesses are successfully fabricated via a facile blade-coating method. Due to the strong cohesion among CNTs bundles and the well-designed structure, the flexible S@C-CNTs cathodes are demonstrated to be with a combination of impressive mechanical strength and enhanced electrochemical performance. For the flexible S@C-CNTs cathodes with the sulfur mass loading of 4 mg cm^-2, the areal capacity is close to 3.0 mA h cm^-2, and the breaking stress is up to 5.59 MPa with 7.77% strain. Meanwhile, the pouch cell exhibits excellent cyclic stability at both flat/bent conditions. All demonstrate that the flexible S@C-CNTs cathodes may satisfy the demands of practical application. Moreover, this methodology is suitable for designing other flexible battery electrodes, such as flexible Si@C-CNTs anodes for lithium ion batteries, flexible P@C-CNTs anodes for sodium/potassium ion batteries, etc.

Scalable Binder-Free Freestanding Electrodes Based on a Cellulose Acetate-Assisted Carbon Nanotube Fibrous Network for Practical Flexible Li-Ion Batteries

Herein, a freestanding cellulose acetate-carbon nanotube (CA-CNT) film electrode is presented to achieve highly flexible, high-energy lithium-ion batteries (LIBs). CA serves as a dispersing agent of CNTs and a binder-free network former. A straightforward washing can remove CA in the electrode almost completely, while the fibrous CNT network within the electrode is sustained. Furthermore, the facile fabrication enables the large-scale production of the film electrode because the CA-CNT film is processed by a conventional casting method and not by the area-limited vacuum filtration. The superior electrochemical performance and high flexibility of the full cell assembled with CA-CNT-based electrodes are maintained even at a high active material loading, which has been proven difficult to accomplish in the conventional configuration LIBs. In addition, by simply stacking six sheets of the freestanding film electrode, a capacity as high as 5.4 mA h cm^-2 is achieved. The assembled pouch battery operates stably under extreme deformation. We demonstrate that the rational design of the electrode could extend the flexibility to a higher energy than that achieved with the conventional configuration. We believe that the low production cost, high flexibility, and superior electrochemical performance of the proposed freestanding film electrode can expedite the implementation of wearable gears in daily life.

Review on carbonaceous materials and metal composites in deformable electrodes for flexible lithium-ion batteries

With the rapid propagation of flexible electronic devices, flexible lithium-ion batteries (FLIBs) are emerging as the most promising energy supplier among all of the energy storage devices owing to their high energy and power densities with good cycling stability. As a key component of FLIBs, to date, researchers have tried to develop newly designed high-performance electrochemically and mechanically stable pliable electrodes. To synthesize better quality flexible electrodes, based on high conductivity and mechanical strength of carbonaceous materials and metals, several research studies have been conducted. Despite both materials-based electrodes demonstrating excellent electrochemical and mechanical performances in the laboratory experimental process, they cannot meet the expected demands of stable flexible electrodes with high energy density. After all, various significant issues associated with them need to be overcome, for instance, poor electrochemical performance, the rapid decay of the electrode architecture during deformation, and complicated as well as costly production processes thus limiting their expansive applications. Herein, the recent progression in the exploration of carbonaceous materials and metals based flexible electrode materials are summarized and discussed, with special focus on determining their relative electrochemical performance and structural stability based on recent advancement. Major factors for the future advancement of FLIBs in this field are also discussed.

Effects of Annealing on Electrochemical Properties of Solvothermally Synthesized Cu2SnS3 Anode Nanomaterials

Cu₂SnS₃, as a modified material for high-capacity tin-based anodes, has great potential for lithium-ion battery applications. The solvothermal method is simple, convenient, cost-effective, and easy to scale up, and has thus been widely used for the preparation of nanocrystals. In this work, Cu₂SnS₃ nanoparticles were prepared by the solvothermal method. The effects of high-temperature annealing on the morphology, crystal structure, and electrochemical performance of a Cu₂SnS₃ nano-anode were studied. The experimental results indicate that high-temperature annealing improves the electrochemical performance of Cu₂SnS₃, resulting in higher initial coulombic efficiency and improved cycling and rate characteristics compared with those of the as-prepared sample.

Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid-State Lithium Batteries

In the development of solid-state lithium batteries, solid polymer electrolyte (SPE) has drawn extensive concerns for its thermal and chemical stability, low density, and good processability. Especially SPE efficiently suppresses the formation of lithium dendrite and promotes battery safety. However, most of SPE is derived from the matrix with simple functional group, which suffers from low ionic conductivity, reduced mechanical properties after conductivity modification, bad electrochemical stability, and low lithium-ion transference number. Appling macromolecular design with multiple functional groups to polymer matrix is accepted as a strategy to solve the problems of SPE fundamentally. In this review, macromolecular design based on lithium conducting groups is summarized including copolymerization, network construction, and grafting. Meanwhile, the construction of single-ion conductor polymer is also focused herein. Moreover, synergistic effects between the designed matrix, lithium salt, and fillers are reviewed with the objective to further improve the performance of SPE. At last, future studies on macromolecular design are proposed in the development of SPE for solid-state batteries with high energy density and durability.

DFT-Guided Design and Fabrication of Carbon-Nitride-Based Materials for Energy Storage Devices: A Review

Carbon nitrides (including CN, C₂N, C₃N, C₃N₄, C₄N, and C₅N) are a unique family of nitrogen-rich carbon materials with multiple beneficial properties in crystalline structures, morphologies, and electronic configurations. In this review, we provide a comprehensive review on these materials properties, theoretical advantages, the synthesis and modification strategies of different carbon nitride-based materials (CNBMs) and their application in existing and emerging rechargeable battery systems, such as lithium-ion batteries, sodium and potassium-ion batteries, lithium sulfur batteries, lithium oxygen batteries, lithium metal batteries, zinc-ion batteries, and solid-state batteries. The central theme of this review is to apply the theoretical and computational design to guide the experimental synthesis of CNBMs for energy storage, i.e., facilitate the application of first-principle studies and density functional theory for electrode material design, synthesis, and characterization of different CNBMs for the aforementioned rechargeable batteries. At last, we conclude with the challenges, and prospects of CNBMs, and propose future perspectives and strategies for further advancement of CNBMs for rechargeable batteries.

Respiration-driven triboelectric nanogenerators for biomedical applications

As a fundamental and ubiquitous body motion, respiration offers a large amount of biomechanical energy with an average power up to the Watt level through movements of multiple muscles. The energy from respiration featured with excellent stability, accessibility and continuality inspires the design and engineering of biomechanical energy harvesting devices, such as triboelectric nanogenerators (TENGs), to realize human-powered electronics. This review article is thus dedicated to the emerging respiration-driven TENG technology, covering fundamentals, applications, and perspectives. Specifically, the human breathing mechanics are first introduced serving as the base for the developments of TENG devices with different configurations. Biomedical applications including electrical energy generation, healthcare monitoring, air filtration, gas sensing, electrostimulation, and powering implantable medical devices are then analyzed focusing on the design-application relationships. At last, current developments are summarized and critical challenges for driving these intriguing developments toward practical applications are discussed together with promising solutions.

Electrodeposition Technologies for Li-Based Batteries: New Frontiers of Energy Storage

Electrodeposition induces material syntheses on conductive surfaces, distinguishing it from the widely used solid-state technologies in Li-based batteries. Electrodeposition drives uphill reactions by applying electric energy instead of heating. These features may enable electrodeposition to meet some needs for battery fabrication that conventional technologies can rarely achieve. The latest progress of electrodeposition technologies in Li-based batteries is summarized. Each component of Li-based batteries can be electrodeposited or synthesized with multiple methods. The advantages of electrodeposition are the main focus, and they are discussed in comparison with traditional technologies with the expectation to inspire innovations to build better Li-based batteries. Electrodeposition coats conformal films on surfaces and can control the film thickness, providing an effective approach to enhancing battery performance. Engineering interfaces by electrodeposition can stabilize the solid electrolyte interphase (SEI) and strengthen the adhesion of active materials to substrates, thereby prolonging the battery longevity. Lastly, a perspective of future studies on electrodepositing batteries is provided. The significant merits of electrodeposition should greatly advance the development of Li-based batteries.

Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices

Next-generation wearable technology needs portable flexible energy storage, conversion, and biosensor devices that can be worn on soft and curved surfaces. The conformal integration of these devices requires the use of soft, flexible, light materials, and substrates with similar mechanical properties as well as high performances. In this review, we have collected and discussed the remarkable research contributions of recent years, focusing the attention on the development and arrangement of soft and flexible materials (electrodes, electrolytes, substrates) that allowed traditional power sources and sensors to become viable and compatible with wearable electronics, preserving or improving their conventional performances.

One-pot resource-efficient synthesis of SnSb powders for composite anodes in sodium-ion batteries

SnSb alloy, which can be used as an anode in a sodium-ion cell, was synthesized following a resource-efficient route at low temperature. This one-pot approach greatly reduces the energy consumption and maximizes the efficient use of raw materials. The reaction of elemental tin and antimony in the ionic liquid (IL) trihexyltetradecylphosphonium chloride ([P66614]Cl) at 200 °C led to a microcrystalline powder of single-phase SnSb within 10 h with very high yield (95%). Liquid-state nuclear magnetic resonance spectroscopy revealed that the IL remains essentially stable during the reaction. It was recovered almost quantitatively by distilling off the organic solvent used for product separation. Composites of SnSb powder and carbon nanotubes (CNTs) were fabricated by a simple ball milling process. Electrochemical measurements demonstrate that the Na‖SnSb/CNTs cell retains close to 100% of its initial capacity after 50 cycles at a current of 50 mA g-1, which is much better than the Na‖SnSb cell. The greatly increased capacity retainability can be attributed to the conductive network formed by CNTs inside the SnSb/CNTs electrode, providing 3D effective and fast electronic pathways during sodium intercalation and de-intercalation.

Carbon Fibers Embedded With Iron Selenide (Fe 3 Se 4 ) as Anode for High-Performance Sodium and Potassium Ion Batteries

The development of sodium and potassium ion batteries (SIBs/KIBs) has seen tremendous growth in recent years due to their promising properties as a potential replacement for lithium-ion batteries (LIBs). Here, we report ultrafine iron selenide (Fe₃Se₄) nanoparticles embedded into one-dimensional (1D) carbon fibers (Fe₃Se₄@CFs) as a potential candidate for SIBs/KIBs. The Fe-based metal-organic framework particles (MOFP) are used as a Fe source to obtain highly dispersed Fe₃Se₄ nanoparticles in the product. The Fe₃Se₄@CF consisted of ultrafine particles of Fe₃Se₄ with an average particle size of ~10 nm loaded into CFs with an average diameter of 300 nm. The product exhibited excellent specific activity of ~439 and ~435 mAh/g at the current density of 50 mA/g for SIBs and KIBs, respectively. In addition, the as-prepared anodes (Fe₃Se₄@CFs) exhibited excellent capacity retention up to several hundred cycles (700 cycles for SIBs and 300 cycles for KIBs). The high activity and excellent stability of the developed electrodes make Fe₃Se₄@CFs a promising electrode for next-generation batteries.

Advanced Matrixes for Binder-Free Nanostructured Electrodes in Lithium-Ion Batteries

Commercial lithium-ion batteries (LIBs), limited by their insufficient reversible capacity, short cyclability, and high cost, are facing ever-growing requirements for further increases in power capability, energy density, lifespan, and flexibility. The presence of insulating and electrochemically inactive binders in commercial LIB electrodes causes uneven active material distribution and poor contact of these materials with substrates, reducing battery performance. Thus, nanostructured electrodes with binder-free designs are developed and have numerous advantages including large surface area, robust adhesion to substrates, high areal/specific capacity, fast electron/ion transfer, and free space for alleviating volume expansion, leading to superior battery performance. Herein, recent progress on different kinds of supporting matrixes including metals, carbonaceous materials, and polymers as well as other substrates for binder-free nanostructured electrodes in LIBs are summarized systematically. Furthermore, the potential applications of these binder-free nanostructured electrodes in practical full-cell-configuration LIBs, in particular fully flexible/stretchable LIBs, are outlined in detail. Finally, the future opportunities and challenges for such full-cell LIBs based on binder-free nanostructured electrodes are discussed.

A Flexible Film with SnS2 Nanoparticles Chemically Anchored on 3D-Graphene Framework for High Areal Density and High Rate Sodium Storage

The design and construction of flexible electrodes that can function at high rates and high areal capacities are essential regarding the practical application of flexible sodium-ion batteries (SIBs) and other energy storage devices, which remains significantly challenging by far. Herein, a flexible and 3D porous graphene nanosheet/SnS₂ (3D-GNS/SnS₂ ) film is reported as a high-performance SIB electrode. In this hybrid film, the GNS/SnS₂ microblocks serve as pillars to assemble into a 3D porous and interconnected framework, enabling fast electron/ion transport; while the GNS bridges the GNS/SnS₂ microblocks into a flexible framework to provide satisfactorily mechanical strength and long-range conductivity. Moreover, the SnS₂ nanocrystals, which chemically bond with GNS, provide sufficient active sites for Na storage and ensure the cycling stability. Consequently, this flexible 3D-GNS/SnS₂ film exhibits excellent Na-storage performances, especially in terms of high areal capacity (2.45 mAh cm^-2 ) and high rates with superior stability (385 mAh g^-1 at 1.0 A g^-1 over 1000 cycles with ≈100% retention). A flexible SIB full cell using this anode exhibits high and stable performance under various bending situations. Thus, this work provide a feasible route to prepare flexible electrodes with high practical viability for not only SIBs but also other energy storage devices.

Review of the Design of Current Collectors for Improving the Battery Performance in Lithium-Ion and Post-Lithium-Ion Batteries

Current collectors (CCs) are an important and indispensable constituent of lithium-ion batteries (LIBs) and other batteries. CCs serve a vital bridge function in supporting active materials such as cathode and anode materials, binders, and conductive additives, as well as electrochemically connecting the overall structure of anodes and cathodes with an external circuit. Recently, various factors of CCs such as the thickness, hardness, compositions, coating layers, and structures have been modified to improve aspects of battery performance such as the charge/discharge cyclability, energy density, and the rate performance of a cell. In this paper, the details of interesting and useful attempts of preparing CCs for high battery performance in lithium-ion and post-lithium-ion batteries are reviewed. The advantages and disadvantages of these attempts are discussed.