2D Film of Carbon Nanofibers Elastically Astricted MnO Microparticles: A Flexible Binder-Free Anode for Highly Reversible Lithium Ion Storage

Free-Standing Stable Silicon-Based Anode with Exceptional Flexibility Realized by a Multifunctional Structure Design in Multiple Dimensions

Cyclized polyacrylonitrile (cPAN) with decently flexible, elastic, and conductive properties is a promising substrate or binder material for flexible devices. However, it is infeasible to accommodate the large volume expansion and contribute the exceptional rate capability of silicon anodes in lithium-ion batteries only counting on the limited elasticity and conductivity of cPAN. Herein, we report a robust silicon/carbon-cPAN-graphene (SC-CP-G) composite membrane with excellent flexibility based on a multifunctional structure design in multiple dimensions, which can be used as a free-standing integrated anode for lithium ion batteries. In this integrated electrode, silicon nanoparticles are encapsulated in porous carbon with in situ formed confined space, and the silicon/carbon particles are further embedded in cPAN nanofibers, which are inextricably interwoven with a reduced graphene oxide film, forming an interpenetrating network architecture. The unique hierarchical and functional structure design greatly improves the mechanical performance, cycling stability, and capacity accessibility of silicon electrodes, delivering a specific capacity of 1847 mA h g^-1 at 2 A g^-1 and a capacity retention of 87% after 150 cycles.

MXene nanofibers confining MnOx nanoparticles: a flexible anode for high-speed lithium ion storage networks

The electron and ion conductivities of anode materials such as MnO_x affect critically the properties of anodes in Li-ion batteries. Herein, a three-dimensional (3D) nanofiber network (MnO_x-MXene/CNFs) for high-speed electron and ion transport with a MnO_x surface anchored and embedded inside is designed via electrospinning manganese ion-modified MXene nanosheets and subsequent carbonization. Ion transport analysis reveals improved Li⁺ transport on the MnO_x-MXene/CNF electrode and first-principles density functional theory (DFT) calculation elucidates the Li⁺ adsorption and storage mechanism. The surface-anchored MnO_x nanoparticles form extremely strong bonds with the nanofibers, and the internally embedded MnO_x nanoparticles, due to the fiber confinement effect, ensure the structural stability during charging and discharging, achieving the so-called dual stabilization strategies for cyclic fluctuation. By electrospinning, self-restacking of MXene flakes can be prevented, thereby giving rise to a larger surface area and more accessible active sites on the flexible anode. Benefiting from the 3D network with excellent conductivity and stability, at high current densities, the MnO_x-MXene/CNF anode exhibits outstanding electrochemical characteristics. Even after 2000 cycles, a reversible capacity of 1098 mA h g^-1 can be obtained at 2 A g^-1 with only 0.007208% decay rate. The MnO_x-MXene/CNF anode also shows a significant rate performance such as 1268 mA h g^-1 at 2 A g^-1 and 1137 mA h g^-1 at 5 A g^-1 corresponding to an area specific capacity of 2.536 mA h cm^-2 at 4 mA cm^-2 and 2.274 mA h cm^-2 at 10 mA cm^-2, respectively. The results indicate that the MnO_x-MXene/CNF anode has excellent Li-ion storage properties and great commercial potential.

Commercialization-Driven Electrodes Design for Lithium Batteries: Basic Guidance, Opportunities, and Perspectives

Current lithium-ion battery technology is approaching the theoretical energy density limitation, which is challenged by the increasing requirements of ever-growing energy storage market of electric vehicles, hybrid electric vehicles, and portable electronic devices. Although great progresses are made on tailoring the electrode materials from methodology to mechanism to meet the practical demands, sluggish mass transport, and charge transfer dynamics are the main bottlenecks when increasing the areal/volumetric loading multiple times to commercial level. Thus, this review presents the state-of-the-art developments on rational design of the commercialization-driven electrodes for lithium batteries. First, the basic guidance and challenges (such as electrode mechanical instability, sluggish charge diffusion, deteriorated performance, and safety concerns) on constructing the industry-required high mass loading electrodes toward commercialization are discussed. Second, the corresponding design strategies on cathode/anode electrode materials with high mass loading are proposed to overcome these challenges without compromising energy density and cycling durability, including electrode architecture, integrated configuration, interface engineering, mechanical compression, and Li metal protection. Finally, the future trends and perspectives on commercialization-driven electrodes are offered. These design principles and potential strategies are also promising to be applied in other energy storage and conversion systems, such as supercapacitors, and other metal-ion batteries.

Tin dioxide-based nanomaterials as anodes for lithium-ion batteries

The development of new electrode materials for lithium-ion batteries (LIBs) has attracted significant attention because commercial anode materials in LIBs, like graphite, may not be able to meet the increasing energy demand of new electronic devices. Tin dioxide (SnO2) is considered as a promising alternative to graphite due to its high specific capacity. However, the large volume changes of SnO2 during the lithiation/delithiation process lead to capacity fading and poor cycling performance. In this review, we have summarized the synthesis of SnO2-based nanomaterials with various structures and chemical compositions, and their electrochemical performance as LIB anodes. This review addresses pure SnO2 nanomaterials, the composites of SnO2 and carbonaceous materials, the composites of SnO2 and transition metal oxides, and other hybrid SnO2-based materials. By providing a discussion on the synthesis methods and electrochemistry of some representative SnO2-based nanomaterials, we aim to demonstrate that electrochemical properties can be significantly improved by modifying chemical composition and morphology. By analyzing and summarizing the recent progress in SnO2 anode materials, we hope to show that there is still a long way to go for SnO2 to become a commercial LIB electrode and more research has to be focused on how to enhance the cycling stability.

MOF-Derived CoSe2@N-Doped Carbon Matrix Confined in Hollow Mesoporous Carbon Nanospheres as High-Performance Anodes for Potassium-Ion Batteries

In this work, a novel vacuum-assisted strategy is proposed to homogenously form Metal-organic frameworks within hollow mesoporous carbon nanospheres (HMCSs) via a solid-state reaction. The method is applied to synthesize an ultrafine CoSe2 nanocrystal@N-doped carbon matrix confined within HMCSs (denoted as CoSe2@NC/HMCS) for use as advanced anodes in high-performance potassium-ion batteries (KIBs). The approach involves a solvent-free thermal treatment to form a Co-based zeolitic imidazolate framework (ZIF-67) within the HMCS templates under vacuum conditions and the subsequent selenization. Thermal treatment under vacuum facilitates the infiltration of the cobalt precursor and organic linker into the HMCS and simultaneously transforms them into stable ZIF-67 particles without any solvents. During the subsequent selenization process, the "dual confinement system", composed of both the N-doped carbon matrix derived from the organic linker and the small-sized pores of HMCS, can effectively suppress the overgrowth of CoSe2 nanocrystals. Thus, the resulting uniquely structured composite exhibits a stable cycling performance (442 mAh g-1 at 0.1 A g-1 after 120 cycles) and excellent rate capability (263 mAh g-1 at 2.0 A g-1) as the anode material for KIBs.

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.

Highly Reversible and Rapid Sodium Storage in GeP3 with Synergistic Effect from Outside-In Optimization

The composite GeP₃/C@rGO as a sodium ion battery anode material was fabricated by introducing a carbon matrix into GeP₃ through high-energy ball milling, followed by encapsulating the resultant composite with graphene via a solution-based ultrasonic method. To delineate the individual role of carbon matrix and graphene, material characterization and electrochemical analyses were performed for GeP₃/C@rGO and three other samples: bare GeP₃, GeP₃ with graphene coating (GeP₃@rGO), and GeP₃ with carbon matrix (GeP₃/C). GeP₃/C@rGO exhibits the highest electric conductivity (5.89 × 10^-1 S cm^-1) and the largest surface area (167.85 m² g^-1) among the four samples. The as-prepared GeP₃/C@rGO delivered a reversible high capacity of 1084 mA h g^-1 at 50 mA g^-1, excellent rate capacity (435.4 mA h g^-1 at a high rate of 5 A g^-1), and long-term cycling stability (400 cycles with a reversible capacity of 823.3 mA h g^-1 at 0.2 A g^-1), all of which outperform the other three samples. The kinetics investigation reveals a "pseudocapacitive behavior" in GeP₃/C and GeP₃/C@rGO, where solely faradic reactions took place in bare GeP₃ and GeP₃@rGO with a typical "battery behavior". Based on ex-situ X-ray photoelectron spectroscopy and ex-situ electrochemical impedance spectroscopy, the carbon matrix serves to activate and stabilize the interior of the composite, while the graphene protects and restrains the exterior surface. Benefiting from the synergistic combination of these two components, GeP₃/C@rGO achieved extremely stable cycling stability as well as outstanding rate performance.

Enhanced Li-ion storage performance of novel tube-in-tube structured nanofibers with hollow metal oxide nanospheres covered with a graphitic carbon layer

One-dimensional (1D) nanofibers constructed with structurally stable nano-architecture and highly conductive carbon components can be employed to develop enhanced anodic materials for lithium-ion batteries. However, achieving an intricate combination of well-designed 1D-nanostructural materials and conductive carbon components for excellent lithium-ion storage capacity is a key challenge. In this study, novel and unique tube-in-tube structured nanofibers consisting of hollow metal oxide (CoFe₂O₄) nanospheres covered with a graphitic carbon (GC) layer were feasibly and successfully synthesized. A facile pitch solution infiltration method was applied to provide electrical conductivity in the tube-in-tube structure. Generally, mesophase pitch with liquid characteristics uniformly infiltrates the porous nanocrystals and transforms into graphitic layers around metallic CoFe₂ alloys during the reduction process. The oxidation process that follows produces the hollow CoFe₂O₄ nanosphere by the nanoscale Kirkendall effect and the GC layer by selective decomposition of amorphous carbon layers. Hollow CoFe₂O₄ nanospheres comprising tube-in-tube structured nanofibers and GC layers are formed by pitch-derived carbon; these have improved structural stability and electrical conductivity resulting in excellent cycling and characteristics. Tube-in-tube structured nanofibers consisting of hollow CoFe₂O₄@GC nanospheres showed excellent long-cycle performance (682 mA h g^-1 for the 1400th cycle at a high current density of 3.0 A g^-1) and excellent rate capability (355 mA h g^-1) even at an extremely high current density of 50 A g^-1.

MOF-derived manganese monoxide nanosheet-assembled microflowers for enhanced lithium-ion storage

Achieving high energy density, power density and cycling performance is a great challenge for lithium-ion battery (LIB) anodes. To obtain favorable electrochemical properties, an effective approach for designing composite nanomaterials with good stability and large specific surface area has been reported here. In this work, metal-organic framework (MOF)-derived manganese monoxides with a stable macromolecular framework were synthesized by utilizing the template agent 1,2,3,4-butanetetracarboxylic acid (BTCA) and the organic salt manganese acetylacetone, which possess a compact microflower structure assembled by nanosheets. As a synergistic effect, not only the amorphous carbon derived from MOFs enhances the specific capacity and stability, but also the unique nanosheet exhibits a significant nano-effect and high areal capacity, which is in favour of an electrochemical reaction. For further enhancement of the electrochemical performance, reduced graphene oxide (rGO) was introduced. When tested as a LIB anode, the MnO@rGO composite displays superior reversible capacities (1716 mA h g-1 at 0.1 A g-1 and 930 mA h g-1 at 2 A g-1) and remarkable rate performances. The research results of the composite nanomaterials lay a foundation for the fabrication of high-capacity and stable anode materials in LIBs.

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

With the advent of flexible electronics, lithium-ion batteries have become a key component of high performance energy storage systems. Thus, considerable effort is made to keep up with the development of flexible lithium-ion batteries. To date, many researchers have studied newly designed batteries with flexibility, however, there are several significant challenges that need to be overcome, such as degradation of electrodes under external load, poor battery performance, and complicated cell preparation procedures. In addition, an in-depth understanding of the current challenges for flexible batteries is rarely addressed in a systematical and practical way. Herein, recent progress and current issues of flexible lithium-ion batteries in terms of battery materials and cell designs are reviewed. A critical overview of important issues and challenges for the practical application of flexible lithium-ion batteries is also provided. Finally, the strategies are discussed to overcome current limitations of the practical use of flexible lithium-based batteries, providing a direction for future research.

The Enhanced Lithium-Storage Performance for MnO Nanoparticles Anchored on Electrospun Nitrogen-Doped Carbon Fibers

Manganese monoxide (MnO) is a promising anode material in the lithium-ion battery for its high capacity, low operation potential, and environmental benignity. However, its application is impeded by poor rate capability and rapid capacity fading. In this work, a MnO/carbon hybrid material, in which small-sized MnO nanoparticles are tightly anchored on carbon fibers (denoted as MnO@CFs), was prepared by annealing the electrospun precursor fibers at 650 °C. When applied as the anode material of the Li-ion battery, the small size of MnO shortens the Li-ion diffusion path, and the carbon fibers not only greatly improve the conductivity but also efficiently buffer the MnO structure strain during the charge–discharge process, endowing the MnO@CFs electrode with a good rate capability (185 mAh g⁻¹ at 5 A g⁻¹) and cyclic stability (406 mAh g⁻¹ after 500 cycles at 1.0 A g⁻¹).

Flexible Graphene-Wrapped Carbon Nanotube/Graphene@MnO2 3D Multilevel Porous Film for High-Performance Lithium-Ion Batteries

The ingenious design of a freestanding flexible electrode brings the possibility for power sources in emerging wearable electronic devices. Here, reduced graphene oxide (rGO) wraps carbon nanotubes (CNTs) and rGO tightly surrounded by MnO₂ nanosheets, forming a 3D multilevel porous conductive structure via vacuum freeze-drying. The sandwich-like architecture possesses multiple functions as a flexible anode for lithium-ion batteries. Micrometer-sized pores among the continuously waved rGO layers could extraordinarily improve ion diffusion. Nano-sized pores among the MnO₂ nanosheets and CNT/rGO@MnO₂ particles could provide vast accessible active sites and alleviate volume change. The tight connection between MnO₂ and carbon skeleton could facilitate electron transportation and enhance structural stability. Due to the special structure, the rGO-wrapped CNT/rGO@MnO₂ porous film as an anode shows a high capacity, excellent rate performance, and superior cycling stability (1344.2 mAh g^-1 over 630 cycles at 2 A g^-1 , 608.5 mAh g^-1 over 1000 cycles at 7.5 A g^-1 ). Furthermore, the evolutions of microstructure and chemical valence occurring inside the electrode after cycling are investigated to illuminate the structural superiority for energy storage. The excellent electrochemical performance of this freestanding flexible electrode makes it an attractive candidate for practical application in flexible energy storage.

Preparation of novel two-stage structure MnO micrometer particles as lithium-ion battery anode materials

MnO micrometer particles with a two-stage structure (composed of mass nanoparticles) were produced via a one-step hydrothermal method using histidine and potassium permanganate (KMnO₄) as reagents, with subsequent calcination in a nitrogen (N₂) atmosphere.

Bicontinuous Spider Network Architecture of Free-Standing MnCoO X @NCNF Anode for Li-Ion Battery

Herein, a smart strategy is proposed to tailor unique interwoven nanocable architecture consisting of MnCoO _x nanoparticles embedded in one-dimensional (1D) mesoporous N-doped carbon nanofibers (NCNFs) by using electrospinning technique. The as-prepared network mat of N-doped carbon nanofibers with embedded MnCoO _x nanoparticles (MnCoO _x @NCNFs) is tested as a current collector-free and binder-free flexible anode, which eliminates slurry preparation process during electrode fabrication in the Li-ion battery (LIB). The MnCoO _x @NCNFs possess versatile structural characteristics that can address simultaneously different issues such as poor conductivity, low cycling stability, volume variation, flexibility, and binder issue associate with the metal oxide. The free-standing mat electrode shows not only high initial discharge and charge capacities but also reversible discharge cycling stability of almost 80% retention up to 100 cycles and 60% retention up to 500 cycles at 1.0 A/g. Such high Li storage capacity and excellent cycling stability are attributed to the unique flexible and free-standing spider network-like architecture of the 1D MnCoO _x @NCNFs that provides the platform for bicontinuous electron/ion pathways for superior electrochemical performance. Along with excellent electrochemical performance, simple synthesis procedure of unique binder-free MnCoO _x @NCNFs can achieve cost-effective scalable mass production for practical use in a flexible mode, not merely in LIBs but also in a wide spectrum of energy storage fields.