Polymer Molecular Engineering Enables Rapid Electron/Ion Transport in Ultra-Thick Electrode for High-Energy-Density Flexible Lithium-Ion Battery

Flexible lithium-ion batteries (LIBs) with high energy density are of urgent need for the ever-increasing flexible and wearable electronic equipments, but limited by the low areal loading of active materials in traditional electrodes with lamellar structure. It is still a great challenge to solve the sluggish electron/ion transport problem caused by increasing the areal loading of active materials. Herein, a kind of ethylene vinyl acetate copolymer (EVA) is proposed to provide flexible supports and ion channels for ultra-thick flexible LFP/CNT/EVA cathode and LTO/CNT/EVA anode, thereby achieving high energy density and all flexible LIBs. LFP/CNT/EVA shows a ternary homogeneous structure formed by the entanglement of EVA chains and CNT on LFP, which attributes to LFP content up to 80wt% and adjustable thickness from 20 to 460 µm. In sharp contrast to previous studies LFP/CNT/EVA delivers basically the constant specific capacity of ≈160 mAh g⁻¹ at a 0.1 C rate with the thickness increasing, thus achieving ultrahigh areal capacity up to 4.56 mAh cm⁻². A flexible full LIBs based on LFP/CNT/EVA and LTO/CNT/EVA is demonstrated and exhibits favorable cycle performance under an alternant flat and bending state. Those findings are supposed to open new avenues for designing high-energy-density flexible LIBs for future wearable energy storage devices.     

Significance of Current Collectors for High Performance Conventional Lithium-Ion Batteries: A Review

Conventional lithium-ion batteries (LIBs) are constantly evolving to improve their electrochemical performance and safety. In the past decade, research on electrode and electrolyte materials has significantly promoted the development of conventional LIBs. However, the current collector (CC) in conventional LIBs has not received sufficient attention. As a transfer unit that collects and disperses electrons from electrodes and transports them to the load, the performance of the CC significantly affects the performance of LIBs. This article reviews the impact of the CC on the electrochemical performance and safety of conventional LIBs in four aspects: CC requirements, manufacturing process, surface coating modification, and multifunctionalized CCs. Meanwhile, this review provides an objective summary and prospect on the transition from single-functionality to multifunctionality requirements for CC, as well as the future needs for CC development and the technical hurdles that should be overcome. The article focuses on providing a detailed insight into the important impact of CCs on conventional LIBs, which has not received enough attention in the past. It emphasizes the need and urgency for the development of multifunctional CCs to provide new ideas for improving the performance of conventional LIBs.     

Enhanced Variable Stiffness and Variable Stretchability Enabled by Phase‐Changing Particulate Additives

A novel phase‐changing particulate that amplifies a composite's modulus change in response to thermal stimulus is introduced. This particulate additive consists of a low melting point alloy (Field's metal; FM) formed into microparticles using a facile fabrication method, which enables its incorporation into polymer matrices using simple composite manufacturing processes. The effect of the solid–liquid phase change of the FM particles is demonstrated in two host materials: a thermally responsive epoxy and a silicone elastomer. In the epoxy matrix, this thermal response manifests as an amplified change in flexural modulus when heated, which is highly desirable for stiffness‐changing move‐and‐hold applications. In the silicone matrix, the stretchability can be switched depending on the phase of the FM particles. This phenomenon allows the silicone to stretch and hold a strained configuration, and gives rise to mechanically programmable anisotropy through reshaping of the FM inclusions. FM particles present many opportunities where on‐demand tunable modulus is required, and is particularly relevant to soft robotics. Because the melting temperature of FM is near room temperature, triggering the phase change requires low power consumption. The utility of FM particle‐containing composites as variable stiffness and variable stretchability elements for soft robotic applications is demonstrated.     

Review on the Binders for Sustainable High-Energy-Density Lithium Ion Batteries: Status,Solutions, and Prospects

The upsurging demand for electric vehicles and the rapid consumption of lithium-ion batteries (LIBs) calls for LIBs to possess high energy density and resource sustainability. The former requires the usage of electroactive materials with high capacity and the maximum amount within the fixed electrode volume. The latter essentially creates a closed-loop circulation scenario for electroactive materials. In all aspects, binders are of practical significance in bonding electroactive materials, maintaining electrode integrity and detaching electrode slurry from the current collector. Currently, the key role of binders in enhancing the electrochemical behavior of sustainable high-capacity electroactive materials has been recognized. Meanwhile, binders that are designed for easy and cost-effective recycling of electroactive materials are gradually reported. Herein, recently developed binders that hold promises in establishing sustainable high-energy-density LIBs are summarized. The role of binder in facilitating easy separation of electroactive materials are first highlighted. Subsequently, special attention is paid to conductive binders, contributing to less battery chemistries and higher energy density of electrode. Additionally, progress of emerging binders in high-capacity electroactive materials are also reviewed. It is believed that the advances in binders will open up opportunities for establishing a sustainable high-energy-density battery economy.     

Tungsten Boride Stabilized Single-Crystal LiNi0.83Co0.07Mn0.1O2 Cathode for High Energy Density Lithium-Ion Batteries: Performance and Mechanisms

Transition metal doped LiNiO₂ layered compounds have attracted significant interest as cathode materials for lithium-ion batteries (LIBs) in recent years due to their high energy density. However, a critical issue of LiNiO₂-based cathodes is caused particularly at highly delithiated state by irreversible phase transition, initiation/propagation of cracks, and extensive reactions with electrolyte. Herein, a tungsten boride (WB)-doped single-crystalline LiNi_0.83Co_0.07Mn_0.1O₂ (SNCM) cathode is reported that affectively addresses these drawbacks. In situ/ex situ microscopic and spectroscopic evidence that B³⁺ enters the bulk of the SNCM, enlarging the interlayer spacing, thus facilitating Li⁺ diffusion, while W³⁺ forms an amorphous surface layer consisting of Li_xW_yO_z (LWO) and Li_xB_yO_z (LBO), which aids the construction of a robust cathode-electrolyte interphase (CEI) film, are shown. It is also shown that WB doping is effective in controlling the degree of the c-axis contraction and release of oxygen-containing gases at high voltages. The best doping concentration of WB is 0.6 wt.%, at which the capacity retention rate of the SNCM reaches 93.2% after 200 cycles at 2.7–4.3 V, while the morphology and structure of the material remain largely unchanged. The presented modification strategy offers a new way for the design of new stable SNCM cathodes for high-energy-density LIBs.     

Ultra-Fine Nano-Mg(OH)2 Electrodeposited in Flexible Confined Space and its Enhancement of the Performance of LiFePO4 Lithium-Ion Batteries

To improve the Li-ion diffusion and extreme-environment performance of LiFePO₄ (LFP) lithium-ion batteries, a composite cathode material is fabricated using ultra-fine nano-Mg(OH)₂ (MH). First, a flexible confined space is designed in the local area of the cathode surface, through the transition of charged xanthan gum polymer molecules under electric field force and the self-assembly of the xanthan gum network. Then, the 20 nm nano-Mg(OH)₂ is prepared through cathodic electrodeposition within the local flexible confined space, and subsequent in situ surface modification as it traverses the xanthan gum network under gravity. LFP-MH significantly changes the density and homogeneity of the cathode electrolyte interphase film and improves the electrolyte affinity. The Li||LFP-MH half-cell demonstrates excellent rate capability (110 mAh g⁻¹ at 5 C) and long-term cycle performance (116.6 mAh g⁻¹ at 1 C after 1000 cycles), and maintains over 100 mAh g⁻¹ after 150 cycles at 60 °C, as well as no structural collapse of the cathode material after 400 cycles at 5 V high cut-off voltage. The cell also shows an obvious decrease in inner resistance after 100 cycles (99.53/133.12 Ω). This work provides a significant advancement toward LiFePO₄ lithium-ion batteries with excellent electrochemical performance and tolerance to extreme-environment.     

Dynamic Networks of Cellulose Nanofibrils Enable Highly Conductive and Strong Polymer Gel Electrolytes for Lithium-Ion Batteries

Tunable dynamic networks of cellulose nanofibrils (CNFs) are utilized to prepare high-performance polymer gel electrolytes. By swelling an anisotropically dewatered, but never dried, CNF gel in acidic salt solutions, a highly sparse network is constructed with a fraction of CNFs as low as 0.9%, taking advantage of the very high aspect ratio and the ultra-thin thickness of the CNFs (micrometers long and 2–4 nm thick). These CNF networks expose high interfacial areas and can accommodate massive amounts of the ionic conductive liquid polyethylene glycol-based electrolyte into strong homogeneous gel electrolytes. In addition to the reinforced mechanical properties, the presence of the CNFs simultaneously enhances the ionic conductivity due to their excellent strong water-binding capacity according to computational simulations. This strategy renders the electrolyte a room-temperature ionic conductivity of 0.61 ± 0.12 mS cm⁻¹ which is one of the highest among polymer gel electrolytes. The electrolyte shows superior performances as a separator for lithium iron phosphate half-cells in high specific capacity (161 mAh g⁻¹ at 0.1C), excellent rate capability (5C), and cycling stability (94% capacity retention after 300 cycles at 1C) at 60 °C, as well as stable room temperature cycling performance and considerably improved safety compared with commercial liquid electrolyte systems.     

A Highly Flexible and Lightweight MnO2/Graphene Membrane for Superior Zinc-Ion Batteries

Batteries powering next-generation flexible and wearable electronic devices require superior mechanical bendability and foldability. Herein, a self-standing hybrid nanoarchitecture constructed by ultralong MnO₂ nanowires and graphene nanosheets as an advanced and lightweight cathodes for flexible and foldable zinc-ion batteries (ZIBs) is designed and fabricated. The new-designed batteries exhibit not only a high energy density of 436 Wh kg⁻¹ based on the total cathode mass but also good 2000-cycling durability. More importantly, the shape-deformable ZIBs can be operated without any capacity loss under both bent and folded circumstances. The foldable ZIBs with high energy density and long lifetime hold great promise for smart and wearable electronics.     

High Performance Flexible Lithium-Ion Battery Electrodes: Ion Exchange Assisted Fabrication of Carbon Coated Nickel Oxide Nanosheet Arrays on Carbon Cloth

Transition metal oxides (TMOs)-based anode materials of high theoretical capacities have been intensively studied for lithium-ion storage. However, their poor high-rate capability and cycling stability remain to be effectively resolved. Herein, a novel ion exchange (IE)-assisted indirect carbon coating strategy is proposed to realize high performance freestanding TMO-based anodes for flexible lithium-ion batteries (FLIBs). This approach effectively avoids the possible side reaction of oxide reduction, enhances degrees of graphitization of the carbon coating, and preserves advantageous nanostructure of the starting template, leading to enhanced electrical conductivities, alleviated volume variation induced structural instability, fast lithium-ion diffusion pathways and enhanced electron transfer kinetics. As a proof of concept, IE-prepared carbon coated NiO nanosheet arrays with excellent structural and electrochemical stability are developed as freestanding anodes for LIBs and FLIBs, which exhibit outstanding electrochemical performances superior to most state-of-the-art NiO-based anodes reported in recent years. The product anode delivers a high areal capacity (3.08 mAh cm⁻² at 0.25 mA cm⁻²), outstanding high-rate capability (1.78 mAh cm⁻² at 8 mA cm⁻²) and excellent cycling stability (over 300 cycles). Further pouch cell tests confirm the excellent flexibility of the freestanding electrode against mechanical deformation with well-maintained electrochemical performance under folding.     

The First Flexible Dual‐Ion Microbattery Demonstrates Superior Capacity and Ultrahigh Energy Density: Small and Powerful

Humans live today in a high‐tech and informationalized society. With the development of the emerging electronic information age, various electronic systems are inclined to be multifunctional and miniaturized. It is urgent to develop “small and powerful” micro‐batteries with flexibility and high electrochemical performance to meet the diverse needs of microelectronic components. However, low electrochemical performance exists in traditional microenergy storage devices, which fail to satisfy the energy needs for microdevices. Here, for the first time, a planar integrated flexible rechargeable dual‐ion microbattery (DIMB) is reported, which is fabricated from an interdigital pattern of graphite as an electrode and lithium hexafluorophosphate as an electrolyte. As a microbattery, the DIMB exhibits a high reversible capacity of 56.50 mAh cm^?3, and excellent cycle stability with 90% capacity retention after 300 cycles under a high working voltage. The application of DIMB in microdevices, such as light‐emitting diodes (LEDs), digital electronic game consoles, and electrochromic glasses is also investigated, fully demonstrating its “small and powerful” performance. The integrated DIMB is a high‐voltage microdevice that reaches a nonpareil discharge voltage of about 100 V and a charging capacity of 102 mAh g^?1. This dual ion‐based flexible microbattery could become a promising candidate for energy storage and conversion components in next‐generation microelectronic devices and integrated electronic devices.     

Hetero‐Nanonet Rechargeable Paper Batteries: Toward Ultrahigh Energy Density and Origami Foldability

Forthcoming smart energy era is in strong pursuit of full‐fledged rechargeable power sources with reliable electrochemical performances and shape versatility. Here, as a naturally abundant/environmentally friendly cellulose‐mediated cell architecture strategy to address this challenging issue, a new class of hetero‐nanonet (HN) paper batteries based on 1D building blocks of cellulose nanofibrils (CNFs)/multiwall carbon nanotubes (MWNTs) is demonstrated. The HN paper batteries consist of CNF/MWNT‐intermingled heteronets embracing electrode active powders (CM electrodes) and microporous CNF separator membranes. The CNF/MWNT heteronet‐mediated material/structural uniqueness enables the construction of 3D bicontinuous electron/ion transport pathways in the CM electrodes, thus facilitating electrochemical reaction kinetics. Furthermore, the metallic current collectors‐free, CNF/MWNT heteronet architecture allows multiple stacking of CM electrodes in series, eventually leading to user‐tailored, ultrathick (i.e., high‐mass loading) electrodes far beyond those accessible with conventional battery technologies. Notably, the HN battery (multistacked LiNi_0.5Mn_1.5O₄ (cathode)/multistacked graphite (anode)) provides exceptionally high‐energy density (=226 Wh kg^?1 per cell at 400 W kg^?1 per cell), which surpasses the target value (=200 Wh kg^?1 at 400 W kg^?1) of long‐range (=300 miles) electric vehicle batteries. In addition, the heteronet‐enabled mechanical compliance of CM electrodes, in combination with readily deformable CNF separators, allows the fabrication of paper crane batteries via origami folding technique.     

Advances and Prospects in Improving the Utilization Efficiency of Lithium for High Energy Density Lithium Batteries

Lithium-ion batteries have attracted much attention in the field like portable devices and electronic vehicles. Due to growing demands of energy storage systems, lithium metal batteries with higher energy density are promising candidates to replace lithium-ion batteries. However, using excess amounts of lithium can lower the energy density and cause safety risks. To solve these problems, it is crucial to use limited amount of lithium in lithium metal batteries to achieve higher utilization efficiency of lithium, higher energy density, and higher safety. The main reasons for the loss of active lithium are the side reactions between electrolyte and electrode, growth of lithium dendrites, and the volume change of electrode materials during the charge and discharge process. Based on these issues, much effort have been put to improve the utilization efficiency of lithium such as mitigating the side reactions, guiding the uniform lithium deposition, and increasing the adhesion between electrolyte and electrode. In this review, strategies for high utilization efficiency of lithium are presented. Moreover, the remaining challenges and the future perspectives on improving the utilization of lithium are also outlined.     

Spherical Lithium Deposition Enables High Li-Utilization Rate,Low Negative/Positive Ratio,and High Energy Density in Lithium Metal Batteries

Lithium metal battery promises an attractively high energy density. A high Li-utilization rate of Li metal anode is the prerequisite for the high energy density and avoiding a huge waste of the Li resource. However, the dendritic Li deposition gives rise to “dead Li” and parasitic interfacial reactions, resulting in a low Li utilization rate. Herein, Li deposition is regulated to spherical Li by designing an MXene host with an egg-box structure, suitable curvature, and continuous gradient lithiophilic structure. Because the spherical Li greatly reduces the interfacial side reactions and avoids the formation of dead Li, the Li anode affords a high plating/stripping efficiency. Furthermore, the gradient lithiophilic design results in a bottom-up growth of the spherical Li within the host, safely away from the separator. Thus, the spherical Li anode realizes a long life of >3000 h with a high Li-utilization rate of >90%, stable cycling in full cells at an areal capacity up to 5 mAh cm⁻² with a low negative/positive ratio of 0.8, which is critical for high energy density. Such spherical deposition highlights the critical role of the morphological control of alkali metals and provides a viable method to build practical high-energy metal batteries.     

Carbon Nanotube/Hygroscopic Salt Nanocomposites with Dual-Functionality of Effective Cooling and Fire Resistance for Safe and Ultrahigh-Rate Operation of Practical Lithium-Ion Batteries

Fire and explosion accidents and reduced energy utilization due to poor cycling stability of lithium-ion batteries (LIBs) caused by inevitable internal temperature rise during high-rate operations have become a growing concern. Herein, a dual-functional carbon nanotube/hygroscopic salt (DFCNT/HS) film with effective passive cooling performance and fire insulation for the safe usage of practical LIBs under extremely fast discharging conditions is reported. The DFCNT/HS film based on the cooling mechanism of self-adaptive moisture absorption/desorption delivers a high cooling power of 32.9 W m⁻² K⁻¹, which can reduce the maximum temperature of a 18650–3.6 V/2.0 Ah LIB by 11.2 and 17.4 °C at discharging rates of 10 and 15 C, respectively. Covering the cooling film, the battery discharges 23.6 Ah more total capacity at 10 within 500 cycles. What is challenging, almost three-fold extended lifetime of 425 cycles is achieved at 15 C with an extra total capacity of 467.2 Ah. Meanwhile, the developed film also shows an excellent high-temperature resistance up to 540 °C, which can alleviate the devastating fire propagation. The fast heat dissipation and excellent fire insulation as well as the mechanical flexibility and manufacturing scalability make this new material promising for safe usage of high-rate LIBs with zero energy consumption.     

Lithium-Ion and Sodium-Ion Hybrid Capacitors: From Insertion-Type Materials Design to Devices Construction

There is a great appeal to develop an omnipotent player combining lithium-ion batteries (LIBs) with the capacitive storage communities. Hybrid capacitors as a kind of promising energy storage device are attracting increasing attention in the main playground in recent years. Unlike supercapacitors (SCs) and LIBs, hybrid capacitors combine a capacitive electrode with a Faradaic battery electrode. In these hybrid cells, the capacitive electrode brings the power while the energy mainly comes from the Faradaic one. Numerous efforts have been conducted in the past decades; however, the research about hybrid capacitors is still at its infancy stage, and it is not expected to replace LIBs or SCs in the near future utterly. Here, the advances of hybrid capacitors, including insertion-type materials, lithium-ion capacitors, and sodium-ion capacitors, are reviewed. This review aims to offer useful guidance for the design of faradic battery electrodes and hybrid cell construction. Brief challenges and opportunities for future research on hybrid capacitors are finally presented.     

A Materials Perspective on Direct Recycling of Lithium-Ion Batteries: Principles,Challenges and Opportunities

As the dominant means of energy storage technology today, the widespread deployment of lithium-ion batteries (LIBs) would inevitably generate countless spent batteries at their end of life. From the perspectives of environmental protection and resource sustainability, recycling is a necessary strategy to manage end-of-life LIBs. Compared with traditional hydrometallurgical and pyrometallurgical recycling methods, the emerging direct recycling technology, rejuvenating spent electrode materials via a non-destructive way, has attracted rising attention due to its energy efficient processes along with increased economic return and reduced CO₂ footprint. This review investigates the state-of-the-art direct recycling technologies based on effective relithiation through solid-state, aqueous, eutectic solution and ionic liquid mediums and thoroughly discusses the underlying regeneration mechanism of each method regarding different battery chemistries. It is concluded that direct regeneration can be a more energy-efficient, cost-effective, and sustainable way to recycle spent LIBs compared with traditional approaches. Additionally, it is also identified that the direct recycling technology is still in its infancy with several fundamental and technological hurdles such as efficient separation, binder removal and electrolyte recovery. In addressing these remaining challenges, this review proposes an outlook on potential technical avenues to accelerate the development of direct recycling toward industrial applications.     

High Energy Density Solid-State Lithium Metal Batteries Enabled by In Situ Polymerized Integrated Ultrathin Solid Electrolyte/Cathode

Solid-state batteries (SSBs) are regarded as the most promising next-generation energy storage devices due to their potential to achieve higher safety performance and energy density. However, the troubles in the preparation of ultrathin solid-state electrolytes (SEs) as well as the resultant compromise in mechanical strength greatly limit the safety application of SSBs. Herein, a novel in situ polymerized integrated ultrathin SE/cathode design is developed. The ultrathin ceramic layer supported on the cathode serves not only as a rigid scaffold to prevent direct contact between the cathode and anode but also as active inorganic fillers to enhance the mechanical properties of in situ polymerized SE film. The unique Li-ion coordination environments as well as the Li hopping mechanism profoundly promote fast ion transport in composite SEs. The in situ polymerized SEs simultaneously achieve the balance in ultrathin thickness (10 µm), fast ion transport (0.65 mS cm⁻¹), superior Young's modulus (66.8 GPa), and excellent interface contact. The pouch cells with practical Li||LiNi_0.8Co_0.1Mn_0.1O₂ configuration achieve an ultrahigh volumetric energy density of 1018 Wh L⁻¹ and safety performance. The in situ polymerized integrated ultrathin SE/cathode design exhibits great promise for the practical application of SSBs with high energy density and safety performance.     

Engineering of Polyanion Type Cathode Materials for Sodium‐Ion Batteries: Toward Higher Energy/Power Density

The development of high energy/power density sodium‐ion batteries (SIBs) is still challenged by the high redox potential of Na/Na⁺ and large radius of Na⁺ ions, thus requiring extensive further improvement to, in particular, enhance the capacity and voltage of cathode materials. Among the various types of cathodes, the polyanion cathodes have emerged as the most pragmatic option due to their outstanding thermostability, unique inductive effect, and flexible structures. In this Review, a critical overview of the design principles and engineering strategies of polyanion cathodes that could have a pivotal role in developing high energy/power density SIBs are presented. Specifically, the engineering of polyanion cathode materials for higher voltage and specific capacity to increase energy density is discussed. The way in which morphology control, architectural design, and electrode processing have been developed to increase power density for SIBs is also analyzed. Finally, the remaining challenges and the future research direction of this field are presented.     

Antifreezing Hydrogel with High Zinc Reversibility for Flexible and Durable Aqueous Batteries by Cooperative Hydrated Cations

Hydrogels are widely used in flexible aqueous batteries due to their liquid‐like ion transportation abilities and solid‐like mechanical properties. Their potential applications in flexible and wearable electronics introduce a fundamental challenge: how to lower the freezing point of hydrogels to preserve these merits without sacrificing hydrogels' basic advantages in low cost and high safety. Moreover, zinc as an ideal anode in aqueous batteries suffers from low reversibility because of the formation of insulative byproducts, which is mainly caused by hydrogen evolution via extensive hydration of zinc ions. This, in principle, requires the suppression of hydration, which induces an undesirable increase in the freezing point of hydrogels. Here, it is demonstrated that cooperatively hydrated cations, zinc and lithium ions in hydrogels, are very effective in addressing the above challenges. This simple but unique hydrogel not only enables a 98% capacity retention upon cooling down to ?20 °C from room temperature but also allows a near 100% capacity retention with >99.5% Coulombic efficiency over 500 cycles at ?20 °C. In addition, the strengthened mechanical properties of the hydrogel under subzero temperatures result in excellent durability under various harsh deformations after the freezing process.