Overview of Graphene as Anode in Lithium-Ion Batteries

Graphene, as a fabulously new-emerging carbonaceous material with an ideal two-dimensional rigid honeycomb structure, has drawn extensive attention in the field of material science due to extraordinary properties, including mechanical robustness, large specific surface area, desirable flexibility, and high electronic conductivity. In particular, as an auxiliary material of electrode materials, it has the potential to improve the performance of lithium-ion batteries. However, wide utilization of graphene in lithium-ion batteries is not implemented since tremendous challenges and issues, such as quality, quantity, and cost concerns, hinder its commercialization. There remains a debate whether graphene can act as an impetus in the evolution of lithium-ion batteries. In this review, we summarize the desirable properties, several common synthesis methods as well as applications of graphene as the anode in lithium-ion batteries, seeking to provide insightful guidelines for further development of graphene-based lithium-ion batteries.     

Structure-Performance Relationship of Aromatic Polymer Binder for Silicon Anode in Lithium-Ion Batteries

Polymer binders are essential for Silicon (Si) anode-based lithium-ion batteries (LIBs). However, the synthetic guidance for aromatic polymer binder is relatively less explored compared to aliphatic polymer binders. In this study, polyimide-based aromatic polymer binders are developed that have strong binding affinity with Si particles, a conductive agent and copper (Cu) current collector, and they show an improved initial discharge capacity of 2663 mAh g⁻¹, which is 29% higher than that of Kapton-based one (2071 mAh g⁻¹). The copolymerization between “hard” and “soft” segments is crucial to achieve reversible volume expansion/contraction during the repeated charging/discharging process, resulting in the best cycle performance. The new binder ensures both excellent volume retention after full-delithiation and allowed volume expansion at least to some extent upon full-lithiation. This Study finds a power-law relationship between the capacity of Si anode and the mechanical properties of the binder, i.e., the tensile stress (σ) and strain (ɛ). The initial discharge capacity is proportional to σⁿ · ɛ (n = 2.3–2.7). Such an understanding of the relationships between polymer structure, mechanical properties of the polymer and binder performance clearly revealed the importance of the soft-hard polymer structure for aromatic binders used in Si-based high-capacity lithium storage materials.     

Preparation of TiO2(B) Nanosheets by a Hydrothermal Process and Their Application as an Anode for Lithium-Ion Batteries

TiO₂ nanosheets with single monoclinic phase have been synthesized by a hydrothermal method using 6 M NaOH aqueous solution at 180°C. TiO₂ nanosheets exhibited surface area of 100 m² g^?1, which is larger than those obtained by solid-state reaction. The capability of lithium-ion batteries could be strongly enhanced by TiO₂(B) nanosheets to yield discharge capacity higher than 200 mAh g^?1, even upon 25 cycles of 0.1-C-rate discharge–charge operations, showing highly reversible capacity and good cycling stability with excellent capacity retention of 96% with water-based binder. The results suggest that TiO₂(B) nanosheets could be a promising negative electrode material for use in lithium-ion batteries.     

Highly Energy-Dissipative,Fast Self-Healing Binder for Stable Si Anode in Lithium-Ion Batteries

A double-wrapped binder has been rationally designed with high Young's modulus polyacrylic acid (PAA) inside and low Young's modulus bifunctional polyurethane (BFPU) outside to address the large inner stress of silicon anode with drastic volume changes during cycling. Harnessing the “hard to soft” gradient distribution strategy, the rigid PAA acts as a protective layer to dissipate the inner stress first during lithiation, while the elastic binder BFPU serves as a buffer layer to disperse residual stress, and thus avoids structural damage of rigid PAA. Moreover, the introduction of BFPU with fast self-healing ability can dynamically recover the microcracks arising from large stress, further ensuring the integrity of silicon anode. This multifunctional binder with smart design of double-wrapped structure provides enlightenment on enlarging the cycling life of high-energy-density lithium-ion batteries that suffer enormous volume change during the cycling process.     

In Situ Artificial Hybrid SEI Layer Enabled High-Performance Prelithiated SiOx Anode for Lithium-Ion Batteries

Considered the promising anode material for next-generation high-energy lithium-ion batteries, SiO_x has been slow to commercialize due to its low initial Coulombic efficiency (ICE) and unstable solid electrolyte interface (SEI) layer, which leads to reduced full-cell energy density, short cycling lives, and poor rate performance. Herein, a novel strategy is proposed to in situ construct an artificial hybrid SEI layer consisting of LiF and Li₃Sb on a prelithiated SiO_x anode via spontaneous chemical reaction with SbF₃. In addition to the increasing ICE (94.5%), the preformed artificial SEI layer with long-term cycle stability and enhanced Li⁺ transport capability enables a remarkable improvement in capacity retention and rate capability for modified SiO_x. Furthermore, the full cell using Li(Ni_0.8Co_0.1Mn_0.1)O₂ and a pre-treated anode exhibits high ICE (86.0%) and capacity retention (86.6%) after 100 cycles at 0.5 C. This study provides a fresh insight into how to obtain stable interface on a prelithiated SiO_x anode for high energy and long lifespan lithium-ion batteries.     

Conversion‐Type MnO Nanorods as a Surprisingly Stable Anode Framework for Sodium‐Ion Batteries

The emergence of nanomaterials in the past decades has greatly advanced modern energy storage devices. Nanomaterials can offer high capacity and fast kinetics yet are prone to rapid morphological evolution and degradation. As a result, they are often hybridized with a stable framework in order to gain stability and fully utilize its advantages. However, candidates for such framework materials are rather limited, with carbon, conductive polymers, and Ti‐based oxides being the only choices; note these are all inactive or intercalation compounds. Conventionally, alloying‐/conversion‐type electrodes, which are thought to be electrochemically unstable by themselves, have never been considered as framework materials. This concept is challenged. Successful application of conversion‐type MnO nanorod as a anode framework for high‐capacity Mo₂C/MoO_x nanoparticles has been demonstrated in sodium‐ion batteries. Surprisingly, it can stably deliver 110 mAh g^?1 under extremely high rate of 8000 mA g^?1 (≈70 C) over 40 000 cycles with no capacity decay. More generally, this is considered as a proof of concept and much more alloying‐/conversion‐type materials are expected to be explored for such applications.     

Supercritical CO2-Assisted SiOx/Carbon Multi-Layer Coating on Si Anode for Lithium-Ion Batteries

Supercritical CO₂ (SCCO₂), characterized by gas-like diffusivity, low surface tension, and excellent mass transfer properties, is applied to create a SiO_x/carbon multi-layer coating on Si particles. Interaction of SCCO₂ with Si produces a continuous SiO_x layer, which can buffer Si volume change during lithiation/delithiation. In addition, a conformal carbon film is deposited around the Si@SiO_x core. Compared to the carbon film produced via a conventional wet-chemical method, the SCCO₂-deposited carbon has significantly fewer oxygen-containing functional groups and thus higher electronic conductivity. Three types of carbon precursors, namely, glucose, sucrose, and citric acid, in the SCCO₂ syntheses are compared. An eco-friendly, cost-effective, and scalable SCCO₂ process is thus developed for the single-step production of a unique Si@SiO_x@C anode for Li-ion batteries. The sample prepared using the glucose precursor shows the highest tap density, the lowest charge transfer resistance, and the best Li⁺ transport kinetics among the electrodes, resulting in a high specific capacity of 918 mAh g⁻¹ at 5 A g⁻¹. After 300 charge–discharge cycles, the electrode retains its integrity and the accumulation of the solid electrolyte interphase is low. The great potential of the proposed SCCO₂ synthesis and composite anode for Li-ion battery applications is demonstrated.     

Recent Progress in MOF-Derived Porous Materials as Electrodes for High-Performance Lithium-Ion Batteries

In recent years, metal-organic frameworks, especially MOF-based derivatives, have been regarded as one of the best candidate electrode materials for the next generation of advanced materials, due to high porosity, large surface area, modifiable functional groups as well as controllable chemical composition. This review presents the corresponding synthesis methods, structural design, and electrochemical performance of MOF-derived materials, including metal oxides, metal sulfides, metal phosphides, and carbon materials, in high-performance lithium-ion batteries. Subsequently, the problems that exist in the current application of MOF-based derivatives as electrodes in lithium-ion batteries are discussed along with possible and feasible solutions. At last, some reasonable pathways and strategies for the design of MOF derivatives are also suggested.     

Electrochemical Properties of Si‐Ge Heterostructures as an Anode Material for Lithium Ion Batteries

Si‐Ge composites have recently been explored as an anode material for lithium‐ion batteries due to their stable cycle performance and excellent rate capability. Although previous reports show the benefits of Si‐Ge composites on electrochemical performance, the specific mechanism and structural effects have been overlooked. Here, the structural effect of Si‐Ge heterogeneous nanostructures on both mechanics and kinetics is systematically studied through theoretical analysis and detailed experimental results. Si‐Ge and Ge‐Si core–shell nanowires are employed for this study. The Si‐Ge core–shell nanowires show a much improved electrochemical performance, especially cycle performance and rate capability, when compared to those of the Ge‐Si core–shell nanowires electrode. On the basis of the detailed experimental results and associated theoretical analysis, its is demonstrated that the strain distribution and Li diffusivity and/or diffusion path are significantly affected by the Si‐Ge heterostructure, which induce different mechanics and kinetics associated with lithium.     

Synthesis, Structure, and Electrochemistry of Sm-Modified LiMn2O4 Cathode Materials for Lithium-Ion Batteries

Spinel lithium manganese oxide and a series of Sm/LiMn₂O₄ spinels with different Sm additive contents (x = 0.02%, 0.05%) were prepared for the first time via a coprecipitation method for rechargeable lithium-ion batteries. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive analysis of x-rays (EDAX), infrared (IR), and electron spin resonance spectral studies as well as various electrochemical measurements were used to examine the structural and electrochemical characteristics of LiMn_2?x Sm _x O₄ (x = 0.00%, 0.02%, 0.05%). XRD and SEM studies confirmed the nano materials size for all prepared spinels. From cyclic voltammetry studies, in terms of peak splitting, electrochemical active surface area, and intensity of the peaks, the LiMn_1.98Sm_0.02O₄ sample possesses better electrochemical performance compared with the LiMn_1.95Sm_0.05O₄ sample. Hence, limited addition of a rare-earth dopant is preferable to obtain better efficiency. Direct-current (DC) electrical conductivity measurements indicated that these samples are semiconducting and their activation energies decrease with increasing rare-earth Sm³⁺ content.     

In Situ Synthesis of Graphene-Coated Silicon Monoxide Anodes from Coal-Derived Humic Acid for High-Performance Lithium-Ion Batteries

Silicon monoxide (SiO) is attaining extensive interest amongst silicon-based materials due to its high capacity and long cycle life; however, its low intrinsic electrical conductivity and poor coulombic efficiency strictly limit its commercial applications. Here low-cost coal-derived humic acid is used as a feedstock to synthesize in situ graphene-coated disproportionated SiO (D-SiO@G) anode with a facile method. HR-TEM and XRD confirm the well-coated graphene layers on a SiO surface. Scanning transmission X-ray microscopy and X-ray absorption near-edge structure spectra analysis indicate that the graphene coating effectively hinders the side-reactions between the electrolyte and SiO particles. As a result, the D-SiO@G anode presents an initial discharge capacity of 1937.6 mAh g⁻¹ at 0.1 A g⁻¹ and an initial coulombic efficiency of 78.2%. High reversible capacity (1023 mAh g⁻¹ at 2.0 A g⁻¹), excellent cycling performance (72.4% capacity retention after 500 cycles at 2.0 A g⁻¹), and rate capability (774 mAh g⁻¹ at 5 A g⁻¹) results are substantial. Full coin cells assembled with LiFePO₄ electrodes and D-SiO@G electrodes display impressive rate performance. These results indicate promising potential for practical use in high-performance lithium-ion batteries.     

Synthesis of Nanosize Quasispherical MgFe2O4 and Study of Electrochemical Properties as Anode of Lithium Ion Batteries

In this work, nanosize MgFe₂O₄ spinel with quasispherical shape was prepared as anode material for lithium ion batteries by the hydroxide coprecipitation method. The crystal structure, composition, and morphology of the as-prepared powders were characterized by means of x-ray diffraction (XRD) analysis, x-ray photoelectron spectroscopy, and scanning electron microscopy (SEM), respectively. The XRD and SEM data revealed that the material as-prepared at 900°C was of high crystallinity and quasispherical with diameter of about 100 nm. A reaction mechanism is proposed. The?electrochemical properties were evaluated by cyclic voltammetry and galvanostatic charge–discharge studies. The sample calcined at 900°C delivered a higher initial discharge capacity (1200 mAh g^?1) and better cyclability. The enhanced electrochemical behavior was ascribed to the nanosize and the better crystallinity of the spherical powder. All the results suggest that nanosize quasispherical MgFe₂O₄ is a promising candidate anode material for lithium ion batteries.     

Bi Dots Confined by Functional Carbon as High-Performance Anode for Lithium Ion Batteries

Fabrication of Bi/C composites is a common approach to alleviate the severe volume expansion of Bi alloy-based anodes with a high theoretical capacity of 3800 mAh cm⁻³ for lithium ion batteries (LIBs). However, the complicated and tedious synthetic routes restrict its large-scale preparation and practical applications. Herein, a spongiform porous Bi/C composite (marked as Bi@PC) through the carbothermal reduction (CTR) method is constructed. Bi nanodots are in situ confined in a porous carbon substrate activated by the gases produced from the decomposition of the sodium phytate precursor, indicating the feasibility and simplicity of this route. In charge/discharge processes, Bi nanodots embedded in carbon matrix are effective enough to accommodate the strain change and shorten the migration distance. In addition, the porous carbon forms an efficient conductive network for electron shutting. When utilized for lithium storage, a superb capacity of 520 mAh g⁻¹ at 0.2 A g⁻¹ after 100 cycles and a satisfying long cyclic stability of 380 mAh g⁻¹ at 0.5 A g⁻¹ after 500 cycles are achieved. The excellent Li-storage performance and this handy preparation method jointly make this Bi/C composite a potential anode for LIBs, and could inspire the preparation of other alloy-type anodes.     

Synthesis,Structure Transformation,and Electrochemical Properties of Li2MgSi as a Novel Anode for Li‐lon Batteries

In this work, a novel hexagonal Li₂MgSi anode is successfully prepared through a hydrogen‐driven chemical reaction technique. Electrochemical tests indicate significantly improved cycling stability for the as‐synthesized Li₂MgSi compared with that of Mg₂Si. Ball‐milling treatment induces a polymorphic transformation of Li₂MgSi from a hexagonal structure to a cubic structure, suggesting that the cubic Li₂MgSi is a metastable phase. The post‐24‐h‐milled Li₂MgSi delivers a maximum capacity of 807.8 mAh g^?1, which is much higher than that of pristine Li₂MgSi. In particular, the post‐24‐h‐milled Li₂MgSi retains 50% of its capacity after 100 cycles, which is superior to cycling stability of Mg₂Si. XRD analyses correlated with CV measurements do not demonstrate the dissociation of metallic Mg and/or Li–Mg alloy involved in the lithiation of Mg₂Si for the Li₂MgSi anode, which contributes to the improved lithium storage performance of the Li₂MgSi anode. The findings presented in this work are very useful for the design and synthesis of novel intermetallic compounds for lithium storage as anode materials of Li‐ion batteries.     

Engineering Molecular Polymerization for Template-Free SiOx/C Hollow Spheres as Ultrastable Anodes in Lithium-Ion Batteries

SiO_x/C composites with a void-reserving structure are promising anodes for lithium-ion batteries. However, the facile and controllable synthesis of uniformly dispersed SiO_x and carbon components, simultaneously incorporating ample voids, still remains a great challenge. Herein, a molecular polymerization strategy is devised to construct SiO_x/C hollow particles for lithium-ion batteries. 3-aminopropyltriethoxysilane and dialdehyde molecules are judiciously engineered as silicon and carbon precursors to produce the polymer hollow spheres (PHSs) through a one-step aldimine condensation without any template and additive. A range of PHSs is obtained using terephthalaldehyde, glutaraldehyde, and glyoxal as the crosslinkers, demonstrating the high tunability of the strategy. Importantly, in situ pyrolysis of the PHSs warrants the homogeneous incorporation of SiO_x ( < 5 nm) in carbon hollow capsids at a nanocluster scale. The obtained SiO_x/C hollow spheres exhibit excellent Li⁺-ion storage behaviors, including cycling lifespan, coulombic efficiency, and rate performance. The superior performance is attributed to the well-dispersed SiO_x nanoclusters in carbon substrate and the hollow structure. This molecular polymerization approach not only enables Si-based hollow composites effective and scalable anode materials but also opens up a new avenue for the controllable synthesis of template-free hollow architectures.