Conjugated Cobalt Polyphthalocyanine as the Elastic and Reprocessable Catalyst for Flexible Li–CO2 Batteries

Li–CO₂ batteries represent an attractive solution for electrochemical energy storage by utilizing atmospheric CO₂ as the energy carrier. However, their practical viability critically depends on the development of efficient and low‐cost cathode catalysts for the reversible formation and decomposition of Li₂CO₃. Here, the great potential of a structurally engineered polymer is demonstrated as the cathode catalyst for rechargeable Li–CO₂ batteries. Conjugated cobalt polyphthalocyanine is prepared via a facile microwave heating method. Due to the crosslinked network, it is intrinsically elastic and has improved chemical, physical, and mechanical stability. Electrochemical measurements show that cobalt polyphthalocyanine facilitates the reversible formation and decomposition of Li₂CO₃, and therefore enables high‐performance Li–CO₂ batteries with large areal capacity and impressive cycling performance. In addition, the elastic and reprocessable property of the polymeric catalyst renders it possible to fabricate flexible batteries.     

Mesoporous and Nanostructured TiO2 layer with Ultra‐High Loading on Nitrogen‐Doped Carbon Foams as Flexible and Free‐Standing Electrodes for Lithium‐Ion Batteries

A simple and green method is developed for the preparation of nanostructured TiO₂ supported on nitrogen‐doped carbon foams (NCFs) as a free‐standing and flexible electrode for lithium‐ion batteries (LIBs), in which the TiO₂ with 2.5–4 times higher loading than the conventional TiO₂‐based flexible electrodes acts as the active material. In addition, the NCFs act as a flexible substrate and efficient conductive networks. The nanocrystalline TiO₂ with a uniform size of ≈10 nm form a mesoporous layer covering the wall of the carbon foam. When used directly as a flexible electrode in a LIB, a capacity of 188 mA h g^?1 is achieved at a current density of 200 mA g^?1 for a potential window of 1.0–3.0 V, and a specific capacity of 149 mA h g^?1 after 100 cycles at a current density of 1000 mA g^?1 is maintained. The highly conductive NCF and flexible network, the mesoporous structure and nanocrystalline size of the TiO₂ phase, the firm adhesion of TiO₂ over the wall of the NCFs, the small volume change in the TiO₂ during the charge/discharge processes, and the high cut‐off potential contribute to the excellent capacity, rate capability, and cycling stability of the TiO₂/NCFs flexible electrode.     

Metal‐Free Carbon Materials for CO2 Electrochemical Reduction

The rapid increase of the CO₂ concentration in the Earth's atmosphere has resulted in numerous environmental issues, such as global warming, ocean acidification, melting of the polar ice, rising sea level, and extinction of species. To search for suitable and capable catalytic systems for CO₂ conversion, electrochemical reduction of CO₂ (CO₂RR) holds great promise. Emerging heterogeneous carbon materials have been considered as promising metal‐free electrocatalysts for the CO₂RR, owing to their abundant natural resources, tailorable porous structures, resistance to acids and bases, high‐temperature stability, and environmental friendliness. They exhibit remarkable CO₂RR properties, including catalytic activity, long durability, and high selectivity. Here, various carbon materials (e.g., carbon fibers, carbon nanotubes, graphene, diamond, nanoporous carbon, and graphene dots) with heteroatom doping (e.g., N, S, and B) that can be used as metal‐free catalysts for the CO₂RR are highlighted. Recent advances regarding the identification of active sites for the CO₂RR and the pathway of reduction of CO₂ to the final product are comprehensively reviewed. Additionally, the emerging challenges and some perspectives on the development of heteroatom‐doped carbon materials as metal‐free electrocatalysts for the CO₂RR are included.     

Metal–Organic Gel‐Derived FexOy/Nitrogen‐Doped Carbon Films for Enhanced Lithium Storage

The development of cost‐effective and flexible electrodes is demanding in the field of energy storage. Herein, flexible Fe_xO_y/nitrogen‐doped carbon films (Fe_xO_y/NC‐MOG) are prepared by facile electrospinning of Fe‐based metal–organic gels (MOGs) followed by high‐temperature carbonization. This approach allows the even mixing of fragile coordination polymers with polyacrylonitrile into flexible films while reserving the structural characteristics of coordination polymers. After thermal treatment, Fe_xO_y/NC‐MOG films possess uniformly distributed Fe_xO_y nanoparticles and larger accessible surface areas than traditional Fe_xO_y‐NC films without MOG. Taking advantage of the unique structure, Fe_xO_y/NC‐MOG exhibits a superior rate performance (449.8 mA h g^?1 at 5000 mA g^–1) and long cycle life (629.3 mA h g^–1 after 500 cycles at 1000 mA g^–1) when used as additive‐free anodes in lithium‐ion batteries.     

Nitrogen‐Doped Carbon Encapsulated Mesoporous Vanadium Nitride Nanowires as Self‐Supported Electrodes for Flexible All‐Solid‐State Supercapacitors

Flexible 3D nanoarchitectures have received tremendous interest recently because of their potential applications in flexible/wearable energy storage devices. Herein, 3D intertwined nitrogen‐doped carbon encapsulated mesoporous vanadium nitride nanowires (MVN@NC NWs) are investigated as thin, lightweight, and self‐supported electrodes for flexible supercapacitors (SCs). The MVN NWs have abundant active sites accessible to charge storage, and the N‐doped carbon shell suppresses electrochemical dissolution of the inner MVN NWs in an alkaline electrolyte, leading to excellent capacitive properties. The flexible MVN@NC NWs film electrode delivers a high areal capacitance of 282 mF cm⁻² and exhibits excellent long‐term stability with 91.8% capacitance retention after 12 000 cycles in a KOH electrolyte. All‐solid‐state flexible SCs assembled by sandwiching two flexible MVN@NC NWs film electrodes with alkaline poly(vinyl alcohol) (PVA), sodium polyacrylate, and KOH gel electrolyte boast a high volumetric capacitance of 10.9 F cm⁻³, an energy density of 0.97 mWh cm⁻³, and a power density of 2.72 W cm⁻³ at a current density of 0.051 A cm⁻³ based on the entire cell. By virtue of the excellent mechanical flexibility, high capacitance, and large energy/power density, the self‐supported MVN@NC NWs paper‐like electrodes have large potential applications in portable and wearable flexible electronics.     

Covalent‐Organic‐Framework‐Based Li–CO2 Batteries

Covalent organic frameworks (COFs) are an emerging class of porous crystalline materials constructed from designer molecular building blocks that are linked and extended periodically via covalent bonds. Their high stability, open channels, and ease of functionalization suggest that they can function as a useful cathode material in reversible lithium batteries. Here, a COF constructed from hydrazone/hydrazide‐containing molecular units, which shows good CO₂ sequestration properties, is reported. The COF is hybridized to Ru‐nanoparticle‐coated carbon nanotubes, and the composite is found to function as highly efficient cathode in a Li–CO₂ battery. The robust 1D channels in the COF serve as CO₂^– and lithium‐ion‐diffusion channels and improve the kinetics of electrochemical reactions. The COF‐based Li–CO₂ battery exhibits an ultrahigh capacity of 27 348 mAh g^?1 at a current density of 200 mA g^?1, and a low cut‐off overpotential of 1.24 V within a limiting capacity of 1000 mAh g^?1. The rate performance of the battery is improved considerably with the use of the COF at the cathode, where the battery shows a slow decay of discharge voltage from a current density of 0.1 to 4 A g^?1. The COF‐based battery runs for 200 cycles when discharged/charged at a high current density of 1 A g^?1.     

Self‐Supported and Flexible Sulfur Cathode Enabled via Synergistic Confinement for High‐Energy‐Density Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries have attracted much attention in the field of electrochemical energy storage due to their high energy density and low cost. However, the “shuttle effect” of the sulfur cathode, resulting in poor cyclic performance, is a big barrier for the development of Li–S batteries. Herein, a novel sulfur cathode integrating sulfur, flexible carbon cloth, and metal–organic framework (MOF)‐derived N‐doped carbon nanoarrays with embedded CoP (CC@CoP/C) is designed. These unique flexible nanoarrays with embedded polar CoP nanoparticles not only offer enough voids for volume expansion to maintain the structural stability during the electrochemical process, but also promote the physical encapsulation and chemical entrapment of all sulfur species. Such designed CC@CoP/C cathodes with synergistic confinement (physical adsorption and chemical interactions) for soluble intermediate lithium polysulfides possess high sulfur loadings (as high as 4.17 mg cm^–2) and exhibit large specific capacities at different C‐rates. Specially, an outstanding long‐term cycling performance can be reached. For example, an ultralow decay of 0.016% per cycle during the whole 600 cycles at a high current density of 2C is displayed. The current work provides a promising design strategy for high‐energy‐density Li–S batteries.     

Efficient Nitrogen‐Doped Carbon for Zinc–Bromine Flow Battery

The zinc–bromine flow battery (ZBFB) is one of the most promising technologies for large‐scale energy storage. Here, nitrogen‐doped carbon is synthesized and investigated as the positive electrode material in ZBFBs. The synthesis includes the carbonization of the glucose precursor and nitrogen doping by etching in ammonia gas. Physicochemical characterizations reveal that the resultant carbon exhibits high electronic conductivity, large specific surface area, and abundant heteroatom‐containing functional groups, which benefit the formation and exposure of the active sites toward the Br₂/Br^? redox couple. As a result, the assembled ZBFB achieves a voltage efficiency of 83.0% and an energy efficiency of 82.5% at a current density of 80 mA cm^?2, which are among the top values in literature. Finally, the ZBFB does not yield any detectable degradation in performance after a 200‐cycle charging/discharging test, revealing its high stability. In summary, this work provides a highly efficient electrode material for the zinc–bromine flow battery.     

A Quasi‐Solid‐State Flexible Fiber‐Shaped Li–CO2 Battery with Low Overpotential and High Energy Efficiency

The rapid development of wearable electronics requires a revolution of power accessories regarding flexibility and energy density. The Li–CO₂ battery was recently proposed as a novel and promising candidate for next‐generation energy‐storage systems. However, the current Li–CO₂ batteries usually suffer from the difficulties of poor stability, low energy efficiency, and leakage of liquid electrolyte, and few flexible Li–CO₂ batteries for wearable electronics have been reported so far. Herein, a quasi‐solid‐state flexible fiber‐shaped Li–CO₂ battery with low overpotential and high energy efficiency, by employing ultrafine Mo₂C nanoparticles anchored on a carbon nanotube (CNT) cloth freestanding hybrid film as the cathode, is demonstrated. Due to the synergistic effects of the CNT substrate and Mo₂C catalyst, it achieves a low charge potential below 3.4 V, a high energy efficiency of ≈80%, and can be reversibly discharged and charged for 40 cycles. Experimental results and theoretical simulation show that the intermediate discharge product Li₂C₂O₄ stabilized by Mo₂C via coordinative electrons transfer should be responsible for the reduction of overpotential. The as‐fabricated quasi‐solid‐state flexible fiber‐shaped Li–CO₂ battery can also keep working normally even under various deformation conditions, giving it great potential of becoming an advanced energy accessory for wearable electronics.     

Dual‐Function,Tunable, Nitrogen‐Doped Carbon for High‐Performance Li Metal–Sulfur Full Cell

Lithium metal–sulfur (Li–S) batteries are attracting broad interest because of their high capacity. However, the batteries experience the polysulfide shuttle effect in cathode and dendrite growth in the Li metal anode. Herein, a bifunctional and tunable mesoporous carbon sphere (MCS) that simultaneously boosts the performance of the sulfur cathode and the Li anode is designed. The MCS homogenizes the flux of Li ions and inhibits the growth of Li dendrites due to its honeycomb structure with high surface area and abundance of nitrogen sites. The Li@MCS cell exhibits a small overpotential of 29 mV and long cycling performance of 350 h under the current density of 1 mA cm^‐2. Upon covering one layer of amorphous carbon on the MCS (CMCS), an individual carbon cage is able to encapsulate sulfur inside and reduce the polysulfide shuttle, which improves the cycling stability of the Li–S battery. As a result, the S@CMCS has a maximum capacity of 411 mAh g^‐1 for 200 cycles at a current density of 3350 mA g^‐1. Based on the excellent performance, the full Li–S cell assembled with Li@MCS anode and S@CMCS cathode shows much higher capacity than a cell assembled with Li@Cu anode and S@CMCS cathode.