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

Functionalized Quasi-Solid-State Electrolytes in Aqueous Zn-Ion Batteries for Flexible Devices: Challenges and Strategies

The rapid development of wearable and intelligent flexible devices has posed strict requirements for power sources, including excellent mechanical strength, inherent safety, high energy density, and eco-friendliness. Zn-ion batteries with aqueous quasi-solid-state electrolytes (AQSSEs) with various functional groups that contain electronegative atoms (O/N/F) with tunable electron accumulation states are considered as a promising candidate to power the flexible devices and tremendous progress has been achieved in this prospering area. Herein, this review proposes a comprehensive summary of the recent achievements using the AQSSE in flexible devices by focusing on the significance of different functional groups. The fundamentals and challenges of the ZIBs are introduced from a chemical view in the first place. Then, the mechanism behind the stabilization of the flexible ZIBs with the functionalized AQSSE is summarized and explained in detail. Then the recent progress regarding the enhanced electrochemical stability of the ZIBs with the AQSSE is summarized and classified based on the functional groups on the polymer chain. The advanced characterization methods for the AQSSE are briefly introduced in the following sections. Last but not least, current challenges and future perspectives for this promising area are provided from the authors' point of view.

Multifunctional Human-Computer Interaction System Based on Deep Learning-Assisted Strain Sensing Array

Continuous and reliable monitoring of gait is crucial for health monitoring, such as postoperative recovery of bone joint surgery and early diagnosis of disease. However, existing gait analysis systems often suffer from large volumes and the requirement of special space for setting motion capture systems, limiting their application in daily life. Here, we develop an intelligent gait monitoring and analysis prediction system based on flexible piezoelectric sensors and deep learning neural networks with high sensitivity (241.29 mV/N), quick response (66 ms loading, 87 ms recovery), and excellent stability (R2 = 0.9946). The theoretical simulations and experiments confirm that the sensor provides exceptional signal feedback, which can easily acquire accurate gait data when fitted to shoe soles. By integrating high-quality gait data with a custom-built deep learning model, the system can detect and infer human motion states in real time (the recognition accuracy reaches 94.7%). To further validate the sensor's application in real life, we constructed a flexible wearable recognition system with human-computer interaction interface and a simple operation process for long-term and continuous tracking of athletes' gait, potentially aiding personalized health management, early detection of disease, and remote medical care.

Recent progress of flexible rechargeable batteries

The rapid popularization of wearable electronics, soft robots and implanted medical devices has stimulated extensive research in flexible batteries, which are bendable, foldable, knittable, wearable, and/or stretchable. Benefiting from these distinct characteristics, flexible batteries can be seamlessly integrated into various wearable/implantable devices, such as smart home systems, flexible displays, and implantable sensors. In contrast to conventional lithium-ion batteries necessitating the incorporation of stringent current collectors and packaging layers that are typically rigid, flexible batteries require the flexibility of each component to accommodate diverse shapes or sizes. Accordingly, significant advancements have been achieved in the development of flexible electrodes, current collectors, electrolytes, and flexible structures to uphold superior electrochemical performance and exceptional flexibility. In this review, typical structures of flexible batteries are firstly introduced and classified into mono-dimensional, two-dimensional, and three-dimensional structures according to their configurations. Subsequently, five distinct types of flexible batteries, including flexible lithium-ion batteries, flexible sodium-ion batteries, flexible zinc-ion batteries, flexible lithium/sodium-air batteries, and flexible zinc/magnesium-air batteries, are discussed in detail according to their configurations, respectively. Meanwhile, related comprehensive analysis is introduced to delve into the fundamental design principles pertaining to electrodes, electrolytes, current collectors, and integrated structures for various flexible batteries. Finally, the developments and challenges of flexible batteries are summarized, offering viable guidelines to promote the practical applications in the future.

Recent Advances in Natural-Polymer-Based Hydrogels for Body Movement and Biomedical Monitoring

In recent years, the interest in medical monitoring for human health has been rapidly increasing due to widespread concern. Hydrogels are widely used in medical monitoring and other fields due to their excellent mechanical properties, electrical conductivity and adhesion. However, some of the non-degradable materials in hydrogels may cause some environmental damage and resource waste. Therefore, organic renewable natural polymers with excellent properties of biocompatibility, biodegradability, low cost and non-toxicity are expected to serve as an alternative to those non-degradable materials, and also provide a broad application prospect for the development of natural-polymer-based hydrogels as flexible electronic devices. This paper reviews the progress of research on many different types of natural-polymer-based hydrogels such as proteins and polysaccharides. The applications of natural-polymer-based hydrogels in body movement detection and biomedical monitoring are then discussed. Finally, the present challenges and future prospects of natural polymer-based hydrogels are summarized.

Improvements and Challenges of Hydrogel Polymer Electrolytes for Advanced Zinc Anodes in Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are widely regarded as desirable energy storage devices due to their inherent safety and low cost. Hydrogel polymer electrolytes (HPEs) are cross-linked polymers filled with water and zinc salts. They are not only widely used in flexible batteries but also represent an ideal electrolyte candidate for addressing the issues associated with the Zn anode, including dendrite formation and side reactions. In HPEs, an abundance of hydrophilic groups can form strong hydrogen bonds with water molecules, reducing water activity and inhibiting water decomposition. At the same time, special Zn2+ transport channels can be constructed in HPEs to homogenize the Zn2+ flux and promote uniform Zn deposition. However, HPEs still face issues in practical applications, including poor ionic conductivity, low mechanical strength, poor interface stability, and narrow electrochemical stability windows. This Review discusses the issues associated with HPEs for advanced AZIBs, and the recent progresses are summarized. Finally, the Review outlines the opportunities and challenges for achieving high performance HPEs, facilitating the utilization of HPEs in AZIBs.

Flexible electrochemical energy storage devices and related applications: recent progress and challenges

Given the escalating demand for wearable electronics, there is an urgent need to explore cost-effective and environmentally friendly flexible energy storage devices with exceptional electrochemical properties. However, the existing types of flexible energy storage devices encounter challenges in effectively integrating mechanical and electrochemical performances. This review is intended to provide strategies for the design of components in flexible energy storage devices (electrode materials, gel electrolytes, and separators) with the aim of developing energy storage systems with excellent performance and deformability. Firstly, a concise overview is provided on the structural characteristics and properties of carbon-based materials and conductive polymer materials utilized in flexible energy storage devices. Secondly, the fabrication process and strategies for optimizing their structures are summarized. Subsequently, a comprehensive review is presented regarding the applications of carbon-based materials and conductive polymer materials in various fields of flexible energy storage, such as supercapacitors, lithium-ion batteries, and zinc-ion batteries. Finally, the challenges and future directions for next-generation flexible energy storage systems are proposed.

Self-Healing Ionogel-Enabled Self-Healing and Wide-Temperature Flexible Zinc-Air Batteries with Ultra-Long Cycling Lives

Hydrogel-based zinc-air batteries (ZABs) are promising flexible rechargeable batteries. However, the practical application of hydrogel-based ZABs is limited by their short service life, narrow operating temperature range, and repair difficulty. Herein, a self-healing ionogel is synthesized by the photopolymerization of acrylamide and poly(ethylene glycol) monomethyl ether acrylate in 1-ethyl-3-methylimidazolium dicyanamide with zinc acetate dihydrate and first used as an electrolyte to fabricate self-healing ZABs. The obtained self-healing ionogel has a wide operating temperature range, good environmental and electrochemical stability, high ionic conductivity, satisfactory mechanical strength, repeatable and efficient self-healing properties enabled by the reversibility of hydrogen bonding, and the ability to inhibit the production of dendrites and by-products. Notably, the self-healing ionogel has the highest ionic conductivity and toughness compared to other reported self-healing ionogels. The prepared self-healing ionogel is used to assemble self-healing flexible ZABs with a wide operating temperature range. These ZABs have ultra-long cycling lives and excellent stability under harsh conditions. After being damaged, the ZABs can repeatedly self-heal to recover their battery performance, providing a long-lasting and reliable power supply for wearable devices. This work opens new opportunities for the development of electrolytes for ZABs.

Advances of Nanomaterials for High-Efficiency Zn Metal Anodes in Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) have emerged as one of the most promising candidates for next-generation energy storage devices due to their outstanding safety, cost-effectiveness, and environmental friendliness. However, the practical application of zinc metal anodes (ZMAs) faces significant challenges, such as dendrite growth, hydrogen evolution reaction, corrosion, and passivation. Fortunately, the rapid rise of nanomaterials has inspired solutions for addressing these issues associated with ZMAs. Nanomaterials with unique structural features and multifunctionality can be employed to modify ZMAs, effectively enhancing their interfacial stability and cycling reversibility. Herein, an overview of the failure mechanisms of ZMAs is presented, and the latest research progress of nanomaterials in protecting ZMAs is comprehensively summarized, including electrode structures, interfacial layers, electrolytes, and separators. Finally, a brief summary and optimistic perspective are given on the development of nanomaterials for ZMAs. This review provides a valuable reference for the rational design of efficient ZMAs and the promotion of large-scale application of AZIBs.

An Ionic Liquid Supramolecular Gel Electrolyte with Unique Wide Operating Temperature Range Properties for Zinc-Ion Batteries

Zinc-ion batteries are promising candidates for large-scale energy storage. The side reactions of the hydrogen evolution reaction (HER) and zinc dendrite growth are major challenges for developing high-performance zinc-ion batteries. In this paper, a supramolecular gel electrolyte (BLO-ILZE) was self-assembled in an ionic liquid (EMIMBF4) with zinc tetrafluoroborate (Zn(BF4)2) on the separator in situ to obtain a gel electrolyte used in zinc-ion batteries. BLO-ILZE is demonstrated to significantly enhance conductivity over a broad temperature range between -70 and 100 °C. Interestingly, through testing and fitting, it is found that the supramolecular gel electrolyte satisfies the liquid state law over a wide temperature range, and even achieves high conductivity (2.12 mS cm-1) at -40 °C. It is equivalent to the conductivity of aqueous zinc-ion batteries (ZnSO4/H2O) at -10 °C, which is 2.33 mS cm-1. Moreover, the supramolecular gel electrolyte can effectively inhibit the HER, thus exhibiting a longer lifetime in Zn/Zn cells for 3470 h at 1 mA cm-2 compared to the aqueous zinc-ion batteries with the Zn(BF4)2 aqueous electrolyte (400 h at 1 mA cm-2). The assembled V2O5/BLO-ILZE/Zn full cells also showed cycling performance, with 5000 cycles at 0.5 mA g-1 at room temperature, a capacity of 98%, and a coulombic efficiency of about 100%.

MoNP-doped Defective Carbon Fibers with Bark-like Nanosurface as Effective Bifunctional Electrocatalysts for Zn-air Batteries

The flexible air electrode with high oxygen electrocatalytic performance and outstanding stability under various deformations plays a vital role in high-performance flexible Zn-air batteries (ZABs). Herein, a self-supported Mo, N, and P co-doped carbon cloth (CC) denoted as MoNP@CC with bark-like surface structure is fabricated by a facile two-step approach via a one-pot method and pyrolysis. The surface of the electrode shows a nanoscale "rift valley" and uniformly distributed active sites. Taking advantage of the nano-surface as well as transition metal and heteroatom doping, the self-supported electrocatalysis air electrode exhibits considerable oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) performance in terms of low overpotential (388 mV at 10 mA cm-2) for OER and a much positive potential (0.74 V) at 1.0 mA cm-2 for ORR. Furthermore, MoNP@CC is further used for the flexible ZAB to demonstrate its practical application. The MoNP@CC-based ZAB displays a good cycling performance for 2800 min and an open-circuit voltage of 1.44 V. This work provides a new approach to the construction of a high-performance, self-supported electrocatalysis electrode used for a flexible energy storage device.

Preparation of Gradient Polyurethane and Its Performance for Flexible Sensors

Flexible sensors are prone to the problems of slow recovery rate and large residual strain in practical use. In this paper, a polyurethane functional composite with a gradient change in elastic modulus is proposed as a flexible sensor to meet the recovery rate and residual strain without affecting the motion. Different hard and soft segment ratios are used to synthesize a gradient polyurethane structure. The conductive percolation threshold was obtained between 45 wt% and 50 wt% of flake silver powder. Both gradient polyurethane and gradient polyurethane composites demonstrated that gradient materials can increase the recovery rate and reduce residual strain. The gradient polyurethane composites had a tensile strength of 3.26 MPa, an elastic modulus of 2.58 MPa, an elongation at break of 245%, a sensitivity coefficient of 1.20 at 0-25% deformation, a sensitivity coefficient of 11.38 at 25-75% deformation, a rate of recovery of 1.95 s at a time, and a resistance to fatigue (over 1000 cycles at a fixed strain of 20% showed a stable electrical response). The sensing performance under different cyclic strain frequencies was also investigated. The process has practical applications in the field of wearable skin motion and health monitoring.

Ni Single-Atom Bual Catalytic Electrodes for Long Life and High Energy Efficiency Zinc-Iodine Batteries

Zinc-iodine batteries (Zn-I2) are extremely attractive as the safe and cost-effective scalable energy storage system in the stationary applications. However, the inefficient redox kinetics and "shuttling effect" of iodine species result in unsatisfactory energy efficiency and short cycle life, hindering their commercialization. In this work, Ni single atoms highly dispersed on carbon fibers is designed and synthesized as iodine anchoring sites and dual catalysts for Zn-I2 batteries, and successfully inhibit the iodine species shuttling and boost dual reaction kinetics. Theoretical calculations indicate that the reinforced d-p orbital hybridization and charge interaction between Ni single-atoms and iodine species effectively enhance the confinement of iodine species. Ni single-atoms also accelerate the iodine conversion reactions with tailored bonding structure of I─I bonds and reduced energy barrier for the dual conversion of iodine species. Consequently, the high-rate performance (180 mAh g-1 at 3 A g-1), cycling stability (capacity retention of 74% after 5900 cycles) and high energy efficiency (90% at 3 A g-1) are achieved. The work provides an effective strategy for the development of iodine hosts with high catalytic activity for Zn-I2 batteries.

Asymmetric Coordination of Bimetallic Fe-Co Single-Atom Pairs toward Enhanced Bifunctional Activity for Rechargeable Zinc-Air Batteries

The advancement of rechargeable zinc-air batteries (RZABs) faces challenges from the pronounced polarization and sluggish kinetics of oxygen reduction and evolution reactions (ORR and OER). Single-atom catalysts offer an effective solution, yet their insufficient or singular catalytic activity hinders their development. In this work, a dual single-atom catalyst, FeCo-SAs, was fabricated, featuring atomically dispersed N3-Fe-Co-N4 sites on N-doped graphene nanosheets for bifunctional activity. Introducing Co into Fe single-atoms and secondary pyrolysis altered Fe coordination with N, creating an asymmetric environment that promoted charge transfer and increased the density of states near the Fermi level. This catalyst achieved a narrow potential gap of 0.616 V, with a half-wave potential of 0.884 V for ORR (vs the reversible hydrogen electrode) and a low OER overpotential of 270 mV at 10 mA cm-2. Owing to the superior activity of FeCo-SAs, RZABs exhibited a peak power density of 203.36 mW cm-2 and an extended cycle life of over 550 h, exceeding the commercial Pt/C + IrO2 catalyst. Furthermore, flexible RZABs with FeCo-SAs demonstrated the promising future of bimetallic pairs in wearable energy storage devices.

Mediating Zn Ions Migration Behavior via β-Cyclodextrin Modified Carbon Nanotube Film for High-Performance Zn Anodes

Metallic Zn is considered as a promising anode material because of its abundance, eco-friendliness, and high theoretical capacity. However, the uncontrolled dendrite growth and side reactions restrict its further practical application. Herein, we proposed a β-cyclodextrin-modified multiwalled carbon nanotube (CD-MWCNT) layer for Zn metal anodes. The obtained CD-MWCNT layer with high affinity to Zn can significantly reduce the transfer barrier of Zn2+ at the electrode/electrolyte interface, facilitating the uniform deposition of Zn2+ and suppressing water-caused side reactions. Consequently, the Zn||Zn symmetric cell assembled with CD-MWCNT shows a significantly enhanced cycling durability, maintaining a cycling life exceeding 1000 h even under a high current density of 5 mA cm-2. Furthermore, the full battery equipped with a V2O5 cathode displays an unparalleled long life. This work unveils a promising avenue toward the achievement of high-performance Zn metal anodes.

Universal Sublimation Strategy to Stabilize Single-Metal Sites on Flexible Single-Wall Carbon-Nanotube Films with Strain-Enhanced Activities for Zinc-Air Batteries and Water Splitting

Single-metal-site catalysts have recently aroused extensive research in electrochemical energy fields such as zinc-air batteries and water splitting, but their preparation is still a huge challenge, especially in flexible catalyst films. Herein, we propose a sublimation strategy in which metal phthalocyanine molecules with defined isolated metal-N4 sites are gasified by sublimation and then deposited on flexible single-wall carbon nanotube (SWCNT) films by means of π-π coupling interactions. Specifically, iron phthalocyanine anchored on the SWCNT film prepared was directly used to boost the cathodic oxygen reduction reaction of the zinc-air battery, showing a high peak power density of 247 mW cm-2. Nickel phthalocyanine and cobalt phthalocyanine were, respectively, stabilized on SWCNT films as the anodic and cathodic electrocatalysts for water splitting, showing a low potential of 1.655 V at 10 mA cm-2. In situ Raman spectra and theoretical studies demonstrate that highly efficient activities originate from strain-induced metal phthalocyanine on SWCNTs. This work provides a universal preparation method for single-metal-site catalysts and innovative insights for electrocatalytic mechanisms.

Nanosheet-structured ZnCo-LDH microsphere as active material for rechargeable zinc batteries

We report zinc cobalt-layered double hydroxides (ZnCo-LDH) as the active cathode materials for the development of high-performance Zn-ZnCo batteries. Electrochemical investigations show the battery's capacity increases linearly with increasing the ZnCo-LDH loading (up to 60 mg cm-2). The resulting Zn-ZnCo battery exhibits excellent rate performance and cycle stability, retaining 86% of its capacity even after 5000 cycles of testing. By incorporating ZnCo-LDH with a Pt/C-coated gas diffusion layer to form an integrated multifunctional air-cathode, we demonstrate a hybrid Zn battery, which combines the merits of Zn-ZnCo and Zn-air batteries to show a characteristic two-stage charge-discharge voltage profile. The current work demonstrates the linear relationship between the battery capacity and the active material loading. The results also highlight that a greater battery capacity requires further increasing of loading though very challenging.

Fabrication of Zn-Air Battery with High Output Capacity Under Ultra-Large Current

Zn-air battery (ZAB) is advocated as a more viable option in the new-energy technology. However, the limited-output capacity at a high current density impedes the driving range in power batteries substantially. Here, a novel heterojunction-based graphdiyne (GDY) and Ag29Cu7 alloy quantum dots (Ag29Cu7 QDs/GDY) for constructing a high-performance aqueous ZAB are fabricated. The as-fabricated ZAB achieves discharge at up to 100 mA cm-2 (the highest value ever reported) along with a remarkable output specific capacity of 786.2 mAh g-1 Zn, which is mainly benefitted from the binary-synergistic effect toward a stable triple-phase interface for air electrode induced by the Ag29Cu7 QDs and GDY in harsh base, together with the decreasing reaction energy barrier and polarization. The results outperform the superior reports discharging at low current and will bring breakthrough progress toward the practical applications of ZAB on large power supply facilities.

Toward Simultaneous Dense Zinc Deposition and Broken Side-Reaction Loops in the Zn//V2 O5 System

The Zn//V2 O5 system not only faces the incontrollable growth of zinc (Zn) dendrites, but also withstands the cross-talk effect of by-products produced from the cathode side to the Zn anode, inducing interelectrode talk and aggravating battery failure. To tackle these issues, we construct a rapid Zn2+ -conducting hydrogel electrolyte (R-ZSO) to achieve Zn deposition modulation and side reaction inhibition in Zn//V2 O5 full cells. The polymer matrix and BN exhibit a robust anchoring effect on SO4 2- , accelerating Zn2+ migration and enabling dense Zn deposition behavior. Therefore, the Zn//Zn symmetric cells based on the R-ZSO electrolyte can operate stably for more than 1500 h, which is six times higher than that of cells employing the blank electrolyte. More importantly, the R-ZSO hydrogel electrolyte effectively decouples the cross-talk effects, thus breaking the infinite loop of side reactions. As a result, the Zn//V2 O5 cells using this modified hydrogel electrolyte demonstrate stable operation over 1,000 cycles, with a capacity loss rate of only 0.028 % per cycle. Our study provides a promising gel chemistry, which offers a valuable guide for the construction of high-performance and multifunctional aqueous Zn-ion batteries.

Architecting N-doped Carbon Nanotube-Rich Carbon Nanofibers with Biomimetic Vine-Leaf-Whisker Structure as Robust Bifunctional Electrocatalysts for Rechargeable Zn-Air Batteries

Efficient and durable bifunctional catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are urgently desirable but challenging for rechargeable Zn-air batteries (ZABs), especially flexible wearable ZABs. Inspired by the vine-leaf-whisker structure in nature, we proposed a three-dimensional (3D) hierarchical bifunctional catalyst (denoted as Co-Fe-Zn@N-CNT/CNF) consisting of N-doped carbon nanotubes embedded with abundant CoFe alloy nanoparticles, leaf-shaped N-doped carbon nanoflakes, and porous carbon fibers for rechargeable ZABs. The special biomimetic structure provides a large specific surface area, allowing for high exposure of the active site and ensuring fast mass transport/charge transfer. The close combination of CoFe bimetallic alloys and N-doped carbon nanotubes delivers high electrocatalytic activity, while the coexistence of various active sites such as metal nanoparticles (NPs), metal-Nx, doped N species, and their synergistic interactions endows the catalysts with more active sites. As such, the Co-Fe-Zn@N-CNT/CNF catalyst achieves superior bifunctional catalytic activities for the ORR (a half-wave potential of 0.84 V) and the OER (an overpotential of 326 mV at 10 mA cm-2) in alkaline media, comparable to commercial Pt/C and RuO2. Remarkably, both aqueous and solid-state ZABs assembled with Co-Fe-Zn@N-CNT/CNF catalysts as air electrodes demonstrate excellent charging/discharging performance, high peak power density, and robust long-term cycling stability. More interestingly, the flexible ZAB performs well even under bending conditions, displaying satisfactory device stability and mechanical flexibility. This study presents a new collective morphological-composition-structural engineering strategy for exploiting the efficient bifunctional oxygen electrocatalysts, which is of great significance for high-performance rechargeable ZABs and wearable energy storage devices.

An extended substrate screening strategy enabling a low lattice mismatch for highly reversible zinc anodes

Aqueous zinc batteries possess intrinsic safety and cost-effectiveness, but dendrite growth and side reactions of zinc anodes hinder their practical application. Here, we propose the extended substrate screening strategy for stabilizing zinc anodes and verify its availability (dsubstrate: dZn(002) = 1: 1→dsubstrate: dZn(002)=n:1, n = 1, 2). From a series of calculated phyllosilicates satisfying dsubstrate ≈ 2dZn(002), we select vermiculite, which has the lowest lattice mismatch (0.38%) reported so far, as the model to confirm the effectiveness of "2dZn(002)" substrates for zinc anodes protection. Then, we develop a monolayer porous vermiculite through a large-scale and green preparation as a functional coating for zinc electrodes. Unique "planting Zn(002) seeds" mechanism for "2dZn(002)" substrates is revealed to induce the oriented growth of zinc deposits. Additionally, the coating effectively inhibits side reactions and promotes zinc ion transport. Consequently, the modified symmetric cells operate stably for over 300 h at a high current density of 50 mA cm-2. This work extends the substrate screening strategy and advances the understanding of zinc nucleation mechanism, paving the way for realizing high-rate and stable zinc-metal batteries.

Preparation of Tough and Adhesive PVA/P(AM-AMPS)/Glycerol/Laponite/Na2SO4 Organohydrogels for All-Solid-State Supercapacitors and Self-Powered Wearable Strain Sensors

Hydrogel electrolytes are ideal for flexible wearable electronic devices because of their high ionic conductivity, flexibility, and biocompatibility. However, some problems, such as poor mechanical properties, low conductivity, and lack of adhesivity, are encountered in the process of hydrogel preparation and application, which restrict the further development of hydrogel electrolytes. In this study, PVA was used as the first network, and P(AM-co-AMPS) as the second network to prepare a double-network hydrogel electrolyte. Laponite and Na2SO4 were introduced into the hydrogel during hydrogel formation as the nanofiller and salt with the salting-out effect to enhance its mechanical properties. The hydrogel electrolyte with high toughness (1663 kJ·m-3), adhesivity (77 kPa), and ionic conductivity (1.7 S·m-1) was obtained. In addition, the hydrogel electrolyte also has excellent antifatigue performance. In the 10 consecutive tensile cycles, the tensile strength does not decay. Due to the high adhesivity of the hydrogel electrolyte, a symmetrical all-solid-state supercapacitor was assembled with a tight interface between the hydrogel electrolyte and the AC/CNT composite electrode. The supercapacitor has a high specific capacitance of 186.1 mF·cm-2 at the current density of 1 mA·cm-2. In addition, the capacitor has good flexibility and can withstand bending at various angles. The hydrogel electrolyte also has excellent strain sensing performance, with an ultrafast tensile response time (0.17 s) and high sensitivity factor (GF = 10.01). Finally, the self-powered sensor system composed of a supercapacitor as the power supply device and hydrogel electrolyte as the sensing part was obtained and applied to human motion monitoring, which provides a potential application in the integrated flexible electronic system.

First-Principles Dynamics Investigation of Germanium as an Anode Material in Multivalent-Ion Batteries

Germanium, a promising electrode material for high-capacity lithium ion batteries (LIBs) anodes, attracted much attention because of its large capacity and remarkably fast charge/discharge kinetics. Multivalent-ion batteries are of interest as potential alternatives to LIBs because they have a higher energy density and are less prone to safety hazards. In this study, we probed the potential of amorphous Ge anodes for use in multivalent-ion batteries. Although alloying Al and Zn in Ge anodes is thermodynamically unstable, Mg and Ca alloys with Ge form stable compounds, Mg2.3Ge and Ca2.4Ge that exhibit higher capacities than those obtained by alloying Li, Na, or K with Ge, corresponding to 1697 and 1771 mA·h·g-1, respectively. Despite having a slightly lower capacity than Ca-Ge, Mg-Ge shows an approximately 150% smaller volume expansion ratio (231% vs. 389%) and three orders of magnitude higher ion diffusivity (3.0 × 10-8 vs. 1.1 × 10-11 cm2 s-1) than Ca-Ge. Furthermore, ion diffusion in Mg-Ge occurs at a rate comparable to that of monovalent ions, such as Li+, Na+, and K+. The outstanding performance of the Mg-Ge system may originate from the coordination number of the Ge host atoms and the smaller atomic size of Mg. Therefore, Ge anodes could be applied in multivalent-ion batteries using Mg2+ as the carrier ion because its properties can compete with or surpass monovalent ions. Here, we report that the maximum capacity, volume expansion ratio, and ion diffusivities of the alloying electrode materials can be understood using atomic-scale structural properties, such as the host-host and host-ion coordination numbers, as valuable indicators.

A Minireview of the Solid-State Electrolytes for Zinc Batteries

Aqueous zinc-ion batteries (ZIBs) have gained significant recognition as highly promising rechargeable batteries for the future due to their exceptional safety, low operating costs, and environmental advantages. Nevertheless, the widespread utilization of ZIBs for energy storage has been hindered by inherent challenges associated with aqueous electrolytes, including water decomposition reactions, evaporation, and liquid leakage. Fortunately, recent advances in solid-state electrolyte research have demonstrated great potential in resolving these challenges. Moreover, the flexibility and new chemistry of solid-state electrolytes offer further opportunities for their applications in wearable electronic devices and multifunctional settings. Nonetheless, despite the growing popularity of solid-state electrolyte-based-ZIBs in recent years, the development of solid-state electrolytes is still in its early stages. Bridging the substantial gap that exists is crucial before solid-state ZIBs become a practical reality. This review presents the advancements in various types of solid-state electrolytes for ZIBs, including film separators, inorganic additives, and organic polymers. Furthermore, it discusses the performance and impact of solid-state electrolytes. Finally, it outlines future directions for the development of solid-state ZIBs.

Strategies Toward Stretchable Aqueous Zn-based Batteries for Wearable Electronics from Components to Devices

Recently, aqueous Zn-based batteries (AZBs) are receiving increased attention in wearable and implantable electronics due to the low cost, high safety, high eco-efficiency, and relatively high energy density. However, it is still a big challenge to develop stretchable AZBs (SAZBs) which can be conformally folded, crumpled, and stretched with human body motions. Although a lot of efforts have been dedicated to constructions of SAZBs, a comprehensive review which focuses on summarizing stretchable materials, device configurations and challenges of SAZBs is needed. Herein, this review attempts to critically review the latest developments and progress in stretchable electrodes, electrolytes, packaging materials and device configurations in detail. Furthermore, these challenges and potential future research directions in the field of SAZBs are also discussed.

Salting-Out Effect Realizing High-Strength and Dendrite-Inhibiting Cellulose Hydrogel Electrolyte for Durable Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries are limited by poor Zn stripping/plating reversibility. Not only can hydrogel electrolytes address this issue, but also they are suitable for constructing flexible batteries. However, there exists a contradiction between the mechanical strength and the ionic conductivity for hydrogel electrolytes. Herein, high-concentration kosmotropic ions are introduced into the cellulose hydrogel electrolyte to take advantage of the salting-out effect. This can significantly improve both the mechanical strength and ionic conductivity. Additionally, the obtained cellulose hydrogel electrolyte (denoted as Con-CMC) has strong adhesion, a wide electrochemical stability window, and good water retaining ability. The Con-CMC is also found to accelerate the desolvation process, improve Zn deposition kinetics, promote Zn deposition along the (002) plane, and suppress parasitic reactions. Accordingly, the Zn/Zn cell with Con-CMC demonstrates dendrite-free behavior with prolonged lifespan and can endure extremely large areal capacity of 25 mAh cm-2. The Con-CMC also enables a large average Coulombic efficiency of 99.54% over 500 cycles for the Zn/Cu cell. Furthermore, the assembled pouch-type Zn/polyaniline full battery provides great rate capability, superior cyclability (even with limited Zn anode excess), a slow self-discharge rate, and outstanding affordability to external forces. Overall, this work extends our knowledge of the rational design of hydrogel electrolytes.

Biomass-Derived Flexible Carbon Architectures as Self-Supporting Electrodes for Energy Storage

With the swift advancement of the wearable electronic devices industry, the energy storage components of these devices must possess the capability to maintain stable mechanical and chemical properties after undergoing multiple bending or tensile deformations. This circumstance has expedited research efforts toward novel electrode materials for flexible energy storage devices. Nonetheless, among the numerous materials investigated to date, the incorporation of metal current collectors or insulative adhesives remains requisite, which entails additional costs, unnecessary weight, and high contact resistance. At present, biomass-derived flexible architectures stand out as a promising choice in electrochemical energy device applications. Flexible self-supporting properties impart a heightened mechanical performance, obviating the need for additional binders and lowering the contact resistance. Renewable, earth-abundant biomass endows these materials with cost-effectiveness, diversity, and modulable chemical properties. To fully exploit the application potential in biomass-derived flexible carbon architectures, understanding the latest advancements and the comprehensive foundation behind their synthesis assumes significance. This review delves into the comprehensive analysis of biomass feedstocks and methods employed in the synthesis of flexible self-supporting carbon electrodes. Subsequently, the advancements in their application in energy storage devices are elucidated. Finally, an outlook on the potential of flexible carbon architectures and the challenges they face is provided.

Enhancing Two-Electron Reaction Contribution in MnO2 Cathode Material by Structural Engineering for Stable Cycling in Aqueous Zn Batteries

MnO2 is a promising cathode for aqueous Zn batteries. However, the cycling stability is seriously hindered by active material dissolution, and the pre-addition of Mn2+ salts in electrolytes is widely required. Herein, we propose a structural engineering strategy for MnO2 to enhance the capacity contribution from the reversible two-electron transfer reaction of MnO2/Mn2+ and realize stable cycling in Mn2+-free electrolytes. By compositing with MoO3, MnO2 exhibits weakened Mn-O bonds, more oxygen vacancies, spontaneous generation of structural water, and thus a lowered energy barrier for Mn release during discharge. Meanwhile, the composite material presents stronger electrostatic attractions for dissolved Mn2+, which ensures highly reversible re-deposition during charge. As a result, the mass ratios between materials undergoing reversible two-electron and one-electron transfer reactions increase from 0.85 in MnO2 to 1.68 in the MnO2/MoO3 composite material. In the ZnSO4 electrolyte, the MnO2/MoO3 cathode achieves 92.6% capacity retention after 300 cycles at 0.1 A g-1 (>1900 h), superior to 62.7% for MnO2. MnO2/MoO3 also retains 80.1% capacity after 16 000 cycles at 1 A g-1 (>3200 h). This work presents an effective path to realize stable cycling of MnO2 in Zn batteries.